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Solutions
1
Types of Mixtures
2
Types of Mixtures
Some materials it is easy to see itrsquos a mixture bc you can see component parts Others like milk do not look like mixtures but they are Can see round droplets Heterogeneous mixture Composition not equal
3
Solutions
NaCl and water form homogeneous mixture Mixture is even throughout
4
Sugar is soluble in water Soluble - ability to be dissolved What happens as sugar dissolves in water
5
Lump of sugar disappears as sugar molecules leave surface of their crystals and mix with water molecules Eventually all sugar molecules are evenly spread through water molecules All visible traces of sugar are gone It becomes solution - homogeneous mixture of two or more substances in a single phase
6
Components of Solutions
Particles of one substance randomly mixed with particles of another Solvent - dissolving medium in a solution Solute - substance dissolved in solution
7
Solute usually less than solvent Dissolved solute particles cannot be seen Stay mixed with solvent forever so long as conditions stay the same Solute-particles from 001-1 nm in diameter
8
Types of Solutions
Solutions can exist as gases liquids or solids
9
Many alloys such as brass (zinc and copper) are sterling silver (silver and copper) are solid solutions Atoms of two or metals are evenly mixed By properly choosing percentages of each metal can get many desirable properties
10
Ex Alloys have higher strength and greater resistance to corrosion than pure metals Pure gold (24K) too soft for jewelry Alloy with silver (14K) increases strength and hardness but keeps appearance of gold
11
SuspensionsSuspension - if particles in solvent are so large they settle out unless mixture constantly stirred Think of sand and water Particles over 1000 nm in diameter
12
Particles that are medium in size between those in solutions and suspensions from mixtures are known as colloids (colloidal dispersions) Particles between 1-1000 nm diameter
13
Colloids
In muddy water large soil particles settle Water still cloudy bc colloidal particles stay dispersed in water Filter Colloidal particles pass through and stays cloudy
14
Particles in colloid small enough to be suspended in solvent by constant movement of surrounding molecules Colloidal particles make up dispersed phase Water is dispersing medium
15
Emulsion and foam are colloids
Mayonnaise is emulsion of oil droplets in water Egg yolk acts as emulsifying agent (keeps oil droplets dispersed)
16
17
18
Class13 of13 Colloid Phases Example
Sol Solid13 dispersed13 in13 liquid Paints13 mud
Gel Solid13 network13 extending13 through13 liquid
Gelatin
Liquid13 emulsion Liquid13 dispersed13 in13 liquid Milk13 mayonnaise
Foam Gas13 dispersed13 in13 liquid Shaving13 cream13 whipped13 cream
Solid13 aerosol Solid13 dispersed13 in13 gas Smoke13 auto13 exhaust
Liquid13 aerosol Liquid13 dispersed13 in13 gas Fog13 mist13 clouds13 aerosol13 spray
Solid13 emulsion Liquid13 dispersed13 in13 solid Cheese13 butter
Tyndall EffectMany colloids appear homogeneous bc individual particles cannot be seen Particles large enough to scatter light In fog can see headlight beam Tyndall effect - occurs when light is scattered by colloidal particles dispersed in transparent medium
19
20
21
Solutions Colloids Suspensions
Homogeneous Heterogeneous Heterogeneous
Particle13 size13 001-shy‐113 nm13 can13 be13 atoms13 ions13 molecules
Particle13 size13 1-shy‐100013 nm13 dispersed13 can13 be13 combined13 or13 large13 molecules
Particle13 size13 over13 100013 nm13 suspended13 can13 be13 large13 particles13 or13 combined13 particles
Do13 not13 separate13 on13 standing Do13 not13 separate13 on13 standing Particles13 settle13 out
Cannot13 be13 separated13 by13 filtration Cannot13 be13 separated13 by13 filtration Can13 be13 separated13 by13 filtration
Do13 not13 scatter13 light Scatter13 light13 (Tyndall13 effect) May13 scatter13 light13 but13 are13 not13 transparent
Electrolytes vs Nonelectrolytes
Substances that dissolve in water are classified according to whether they produce molecules or ions in solution
22
23
When ionic compound dissolves cations and anions separate and are surrounded by water molecules Solute ions free to move making possible to carry electric current
Electrolyte - substance that dissolves in water to give a solution that conducts electric current NaCl ndash electrolyte Usually highly polar compounds become electrolytes
24
Solution containing neutral solute molecules does not conduct current bc it doesnrsquot have mobile charged particles
Nonelectrolyte - substance that dissolves in water to give solution that does not conduct electric current Ex Sugar
25
26
27the solution
process
Factors Affecting Rate of DissolutionEver tried to dissolve sugar in iced tea or coffee You notice temperature has something to do with how quickly solute dissolves What other factors affect how quickly solutes dissolve
28
1 Increasing Surface Area of Solute
bc dissolution process occurs at surface of solute Can increase dissolution by increasing surface area
29
2 Agitating (Stirring) SolutionClose to surface of solute concentration of dissolved solute is high Stirringshaking helps disperse solute particles
30
Brings fresh solvent into contact with solute surface Contact between solvent and solute surface increased
3 Heating a Solvent
Solutes dissolve faster in warmer solvent As temp increases solvent molecules move faster Average kinetic energy increases Collisions between solvent and solute more frequent Helps separate solute molecules and spread them out
32
Solubility
If you add sugar to tea eventually no more sugar dissolves
For every combination of solvent with solute at given temp there is limit to amount of solute that can be dissolved
33
When sugar first dropped into water sugar molecules leave solid surface and move about at random in solvent Some dissolved molecules may collide with crystal and stay there As more solid dissolves and concentration of dissolved molecules increases collisions happen more often
34
Eventually molecules are returning to crystal at same rate they are going into solution Dynamic equilibrium established between dissolution and crystallization Solution equilibrium - physical state in which the opposing processes of dissolution and crystallization of a solute occur at equal rates
35
Saturated vs Unsaturated Solutions
Saturated solution - solution that contains the maximum amount of dissolved solute How do you know if solution is saturated If more NaCl added it falls to bottom and doesnrsquot dissolve Equilibrium already established
36
If more water added to saturated solution more NaCl will dissolve At 20 359 g NaCl is max that will dissolve in 100 g water Unsaturated solution - solution that contains less solute than a saturated solution under the existing conditions
37
Supersaturated Solutions
When saturated solution where solubility increases as temp increases is cooled excess solute usually comes out of solution Sometimes if solution is left to cool the excess solute doesnrsquot separate
38
39
Supersaturated solution - solution that contains more dissolved solute than a saturated solution contains under same conditions
40
Supersaturated solution can remain unchanged over long time if it isnrsquot disturbed Once crystals begin to form continues until equilibrium is recreated at lower temp
41
Example
Sodium thiosulfate Na2S2O3 Solute added to hot water until solution saturated Hot solution filtered Drop small crystal in ldquoseedingrdquo crystallization Continues until equilibrium created
42
Solubility Values
Solubility - amount of substance required to form a saturated solution with specific amount of solvent at specified temp Ex Solubility of sugar = 204 g per 100 g water at 20 Must specify temp bc solubility changes with temp 43
For gases pressure must also be specified Rate at which substance dissolves is unrelated to solubility Max amount of solute that dissolves and reaches equilibrium is always same under same conditions
44
Solute-Solvent Interactions
ldquolike dissolves likerdquo is generally useful rule for predicting whether one substance will dissolve in another Depends on type of bonding polaritynonpolarity and intermolecular forces between solute and solvent
45
Dissolving Ionic Compounds in Aqueous Solution
Polarity of water molecules plays important role in formation of solution of ionic compounds Charged ends of water molecules attract ions and surround them to keep separated from other ions
46
Hydration - solution process with water as solvent Ions said to be hydrated As hydrated ions diffuse into solution other ions exposed and drawn away from crystal surface Entire crystal gradually dissolves
47
When crystallized from aqueous solutions some ionic compounds form crystals that include water molecules The crystalline compounds known as hydrates have specific ratios of water molecules Represented by formulas like CuSO45H2O
48
When crystalline hydrate dissolves in water water of hydration returns to solvent Behavior of solute in hydrated form no different than anhydrous form
49
Nonpolar Solvents
Nonpolar solvents do not attract ions of ionic compound strongly enough to overcome forces holding crystal together
50
Liquid Solutes and Solvents
When you shake bottle of salad dressing oil droplets disperse in water Stop shaking ndash strong attraction between water molecules squeeze out oil and form separate layers Liquid solutes and solvents that are not soluble in each other are immiscible
51
Nonpolar substances (fat oil grease) are generally quite soluble in nonpolar liquids (carbon tetrachloride toluene gasoline)
Only attractions between nonpolar molecules are weak London forces
52
London Dispersion Forces
bc e- are constantly moving at any time there may be uneven distribution of them around a nonpolar atom Temporarily creates positive end and negative end
53
54
London dispersion forces - intermolecular attractions resulting from constant motion of e- and creation of instantaneous dipoles
55
Liquids that dissolve freely in one another in any ratio are said to be completely miscible Benzene and carbon tetrachloride (both nonpolar) are completely miscible
Nonpolar molecules of these two apply no strong forces of attraction or repulsion and molecules mix freely
56
Ethanol and water are also completely miscible -OH group on ethanol is slightly polar
Can form H-bonds with water as well as other ethanol molecules Intermolecular forces in mixture are very similar to those in pure liquids so they are mutually soluble in any amount
57
Gasoline contains mainly nonpolar hydrocarbons
Excellent solvent for fats oils greases
Major intermolecular forces are weak London forces
58
Ethanol less polar than water more than carbon tetrachloride
Better solvent for less-polar substances bc of nonpolar region
59
Effects of Pressure on Solubility
Little effect on solids or liquids Increase pressure - increase solubilities of gases in liquids
60
When gas in contact with surface of liquid gas molecules can enter liquid As amount of dissolve gas increases some molecules start to escape and reenter gas phase
Eq eventually established between rates of enteringleaving gas phase
61
As long as eq undisturbed solubility of gas in liquid unchanged at given pressure
Gas + solvent solution
62
Increasing pressure of solute gas above solution puts stress on eq Molecules collide with surface more often Increase in pressure partially offset by increase in rate of gas molecules entering solution In turn increase in amount of dissolved gas causes increase in rate molecules escape liquid and becomes gas
63
Eventually eq restored at higher gas solubility
Expected from Le Chatelierrsquos principle
Increase in gas pressure causes eq to shift so fewer molecules are in gas phase
64
Henryrsquos Law
Henryrsquos law - solubility of a gas in liquid is directly proportional to the partial pressure of that gas on the surface of the liquid
Applies to gas-liquid solutions at constant temp
65
When mixture of ideal gases is limited in constant volume at constant temp each gas applies same pressure it would apply if it occupied the space alone
Assuming gases do not react each gas dissolves to the extent it would if no other gases were there
66
In carbonated drinks (Coke Pepsi etc) solubility of CO2 increased by increasing pressure
At bottling factory CO2 gas forced into solution of flavored water at pressure of 5-10 atm Gas-in-liquid solution then sealed in bottlescans
67
When cap removed pressure reduced to 1 atm and some CO2 escapes as gas bubbles Effervescence - rapid escape of a gas from a liquid in which it is dissolved
68
Effects of Temperature on Solubility
Letrsquos consider gas solubility Increasing temperature usually decreases gas solubility As temp increases average kinetic energy of molecules in solution increases Greater number of solute molecules escape from attraction of solvent and return to gas phase 69
At higher temps eq is reached with fewer gas molecules in solution Gases generally less soluble
70
Effect of temp on solubility of solids in liquids more difficult to predict
Often increasing temp increases solubility
However the same temp increase can result in large increase in solubility in one case and only slight increase in the next
71
72
enthalpies of Solution
Formation of solution comes with energy change Dissolve KI in water container feels cold Dissolve LiCl in water feels hot Formation of solid-liquid solution can absorb or release heat
73
During formation of solution solvent and solute particles experience changes in forces attracting them to other particles Before dissolving begins solvent molecules held by intermolecular forces (solvent-solvent attraction) In solute molecules held by Ifs (solute-solute attraction)
74
Energy is required to separate solute molecules and solvent molecules from their neighbors
A solute particle that is surrounded by solvent molecules is said to be solvated
75
Step 1 solute particles become separated from solid (energy absorbed) Step 2 solvent particles move apart to allow solute particles to enter liquid (energy absorbed)
76
Step 3 solvent particles attracted to and solvate solute particles (energy released)
77
Heat of solution - net amount of heat energy absorbed or released when a specific amount of solute dissolves in a solvent
Heat of solution negative (heat is released) if sums of energy in steps 1 and 2 is less than step 3
If 1+2 gt 3 HOS is positive (heat absorbed)78
In gaseous state molecules are so far apart there are nearly no forces between them Solute-solute interaction has little effect on heat of solution of a gas Energy released when gas dissolved in liquid bc attraction between solute gas and solvent molecules greater than energy needed to separate solvent molecules
79
Concentration of Solutions80
Concentration - measure of amount of solute in given amount of solvent or solution ldquodiluterdquo just means small amount of solute ldquoconcentratedrdquo just means relatively large amount of solute Unrelated to degree to which solution is saturated Ex Saturated solution of substance not very soluble might be dilute
81
Molarity
Molarity - number of moles of solute in one liter of solution To find molarity must know molar mass A ldquoone molarrdquo solution of NaOH is 1 mole NaOH per liter of solution (1 M)
82
1 mol NaOH = 400 g If this quantity dissolve in enough water to make EXACTLY 100 L solution solution is 1 M If 200 g NaOH dissolve in enough to make 100 L solution what is molarity 0500 M
83
1 M solution is not made by adding 1 mol solute to 1 L solvent That would be more than 1 L Instead 1 mol solute dissolved in less than 1 L Then diluted with more solvent to bring TOTAL VOLUME to 1 L
84
Practice ProblemYou have 340 L of solution that contains 900 g sodium chloride What is the molarity of that solution 0440 M NaCl You have 08 L of a 05 M HCl solution How many moles of HCl does this solution contain 04 mol HCl
85
What is the molarity of a solution composed of 585 g of potassium iodide dissolved in enough water to make a 0125 L of solution 0282 M KI
86
How many moles of H2SO4 are present in 0500 L of a 0150 M H2SO4 solution 00750 mol What volume of 300 M NaCl is needed for reaction that requires 1463 g of NaCl 0834 L
87
Molality
Molality - concentration of solution expressed in moles of solute per kilogram of solvent Solution that contains 1 mol solute (NaOH) dissolved in exactly 1 kg of solvent is ldquoone molalrdquo solution (1 m NaOH)
88
1 mol NaOH = 400 g 400 g NaOH dissolved in 1 kg water is 1 m NaOH
200 g NaOH in 1 kg water = 0500 m NaOH
89
Practice Problem
A solution was prepared by dissolving 171 g sucrose (C12H22O11) in 125 g of water Find the molal concentration of this solution 0400 m C12H22O11
90
A solution of iodine in carbon tetrachloride is used when iodine is needed for certain chemical tests How much iodine must be added to prepare a 0480 m solution of iodine in CCl4 if 1000 g of CCl4 is used 122 g I2
91
What is the molality of a solution composed of 255 g acetone (CH3)2CO dissolved in 200 g of water 22 m acetone What quantity in grams of methanol CH3OH is required to prepare a 0244 m solution in 400 g water 312 g methanol
92
How many grams of AgNO3 are needed to prepare a 0125 m solution in 250 mL of water 531 g AgNO3
What is the molality of a solution containing 182 g HCl and 250 g water 200 m
93
Ions in Aqueous Solutions and Colligative Properties
94
95
compounds in aqueous solutions
DissociationWhen compound made from ions dissolves in water ions separate Dissociation - separation of ions that occurs when an ionic compound dissolves
96
Notice number of ions made per formula unit in equations 1 formula unit of NaCl gives 2 units of ions in solution 1 formula unit of CaCl2 gives 3 units of ions
97
Assuming 100 dissociation solution that contains 1 mol NaCl contains 1 mol Na+ and 1 mol Cl- Can assume 100 dissociation of all soluble ionic compounds
98
Practice Problem
Write the equation for the dissolution of aluminum sulfate in water How many moles of aluminum ions are made by dissolving 1 mol aluminum sulfate What is the total number of moles of ions made by dissolving 1 mol of aluminum sulfate
99
1 AnalyzeGiven Amount of solute = 1 mol Al2(SO4)3 Solvent identity = water Unknown a moles of aluminum ions and sulfate ions Total number of moles of solute ions produced
100
2 Plan
The coefficients in the balanced dissociation equation will tell mole relationships You can use the equation to find out the number of moles of solute ions produced
101
3 Compute
a
b102
Write the equation for the dissolution of each of the following in water and then determine the number of moles of each ions made as well as the total number of ions made a 1 mol ammonium chloride b 1 mol sodium sulfide c 05 mol barium nitrate
103
a 1 mol ammonium chloride
1 mol NH4+ 1 mol Cl- 2 mol total ions
104
b 1 mol sodium sulfide
2 mol Na+ 1 mol S-2 3 mol total ions
105
c 05 mol barium nitrate
05 mol Ba+2 1 mol NO3- 15 mol total ions
106
Precipitation Reactions
Even though no compound is completely insoluble compounds of very low solubility can be considered insoluble for practical purposes There are general guidelines to help predict whether a compound made of certain combination of ions is soluble
107
General Solubility Guidelines
Sodium potassium and ammonium compounds are soluble in water
108
Nitrates acetates and chlorates are soluble
109
110
Most chlorides are soluble except those of silver mercury (I) and lead Lead (II) chloride is soluble in hot water
Most sulfates are soluble except those of barium strontium lead calcium and mercury
111
Most carbonates phosphates and silicates are insoluble except those of sodium potassium and ammonium
112
Most sulfides are insoluble except those of calcium strontium sodium potassium and ammonium
113
Is calcium phosphate Ca3(PO4)2 soluble or insoluble
Not one of the exceptions so it is insoluble
Dissociation equations are not written for insoluble compounds
114
Guidelines also useful in predicting what will happen if 2 solutions of 2 different soluble compounds are mixed
If mixing results in combination of ions that forms insoluble compound a double-replacement and precipitation reaction will happen
115
Will precipitate form when solutions of ammonium sulfide and cadmium nitrate are combined You can tell calcium nitrate is soluble from the guidelines (Most nitrates acetates and chlorates are soluble) You can also tell ammonium sulfide is soluble (Most sulfides are insoluble except those of calcium strontium sodium potassium and ammonium)
116
117
To decide whether precipitate can form must know solubilities of two possible products
Ammonium nitrate ndash soluble (Most nitrates acetates and chlorates are soluble) Cadmium sulfide ndash insoluble (Most sulfides are insoluble except those of calcium strontium sodium potassium and ammonium)
118
bc one product is insoluble double-replacement and precipitation reaction will happen (NH4)2S (aq) + Cd(NO3)2 (aq) rarr NH4NO3 (aq) + CdS (s)
119
Net Ionic Equations
Reactions of ions in aqueous solution usually represented by net ionic equations instead of formula equations Net ionic equation - includes only those compounds and ions that undergo a chemical change in a reaction in an aqueous solution
120
121
To write NIE (net ionic equation) first convert chemical equation into overall ionic equation All soluble ionic compounds shown as dissociated ions in solution Precipitates shown as solids
122
Notice ammonium and nitrate ions appear on both sides They havenrsquot gone through any chemical change Spectator ions - ions that do not take part in a chemical reaction and are found in solution both before and after the reaction
123
To change ionic equation to NIE eliminate spectator ions Cd+2(aq) + S-2(aq) rarr CdS(s)
This NIE applies to any reaction in which precipitate of CdS forms
124
Practice Problem
Identify the precipitate that forms when aqueous solutions of zinc nitrate and ammonium sulfide are combined Write the equation for the possible double-replacement reaction Then write the formula equation overall ionic equation and the net ionic equation
125
1 AnalyzeGiven Identity of reactants zinc nitrate and ammonium sulfide Reaction medium aqueous solution Unknown a equation for the possible double-replacement reaction b identity of the precipitate c formula equation d overall ionic equation e net ionic equation
126
2 PlanWrite the possible double-replacement reaction between Zn(NO3)2 and (NH4)2S Use the rules to determine if any of the products will precipitate Write a formula equation and overall net ionic equation Then cancel spectator ions to make net ionic equation
127
3 Computea possible double-replacement reaction
b rules show that zinc sulfide is not soluble so will precipitate (ammonium nitrate is soluble)
c formula equation128
d overall ionic equation
e net ionic equation129
Will a precipitate form if solutions of potassium sulfate and barium nitrate are combined If so write the net ionic equation for the reaction
Yes Ba+2(aq) + SO4-2(aq) rarr BaSO4(s)
130
Will a precipitate form if solutions of potassium nitrate and magnesium sulfate are combined If so write the net ionic equation for the reaction
No 131
Will a precipitate form if solutions of barium chloride and sodium sulfate are combined If so identify the spectator ions and write the net ionic equation for the reaction
Yes Na+ and Cl- Ba+2(aq) + SO4-2(aq) rarr BaSO4(s)
132
Write the net ionic equation for the precipitation of nickel(II) sulfide
Ni+2(aq) + S-2(aq) rarr NiS(s)
133
IonizationSome molecular compounds can also form ions in solution Usually polar Ionization - ions formed from solute molecules by action of the solvent (creation of ions where there were none) Ionization different from dissociation
134
Dissociation ionic compounds dissolve and ions already present separate Ionization ions formed where non existed before
Ions formed are hydrated Heat released during hydration of ions gives enough energy to break covalent bonds
135
Extent to which solute ionizes depends on strength of bonds within molecules of solute and strength of attraction between solute and solvent molecules
136
HCl ndash molecular compound that ionizes in aqueous solution Contains highly polar bond Attraction between polar HCl molecule and polar water strong enough to break HCl bond
137
The Hydronium Ion
H+ ions from HCl attracts other molecules or ions so strongly that it doesnrsquot normally exist alone Ionization of HCl better described as direct transfer of proton from HCl to H2O forming H3O+
138
139
Hydration of H+ to form hydronium is highly exothermic
Energy released gives a lot of energy needed to ionize a molecular solute
Many molecular compounds that ionize in aqueous solution contain hydrogen and form hydronium ion
140
Strong and Weak Electrolytes
Substances that make ions and conduct current in solution are electrolytes HCl is one of series of compounds that has hydrogen and halogen Hydrogen halides are all molecular compounds with single polar-covalent bonds
141
All are gases Very soluble in water All are electrolytes HCl HBr HI strongly conduct current in solution HF only weakly conducts current at same concentration Strength of conduction related to ability to ionize
142
143
Strong Electrolytes
HCl HBr HI are 100 ionized in solution
Strong electrolyte - any compound whose dilute aqueous solutions conduct electricity well this is due to the presence of all or almost all of the dissolve compound in the form of ions
144
Unique characteristic of strong electrolyte is that they yield only ions
Ex Ionic compound may be highly soluble in water and dissociate into ions (NaCl)
Others may not dissolve much but amount that does dissolve exists only as ions
145
Weak ElectrolytesHF dissolves in water to give acid solution called hydrofluoric acid HF bond stronger than bonds between hydrogen and other halogens When HF dissolves some molecule ionize Reverse reaction also happens HF(aq) + H2O(l) H3O+(aq) + F-(aq)
146
HF(aq) + H2O(l) H3O+
(aq) + F-(aq)Concentration of dissolved unionized HF stays high and concentration of ions stays low
Weak electrolyte - any compound whose aqueous solutions conduct electricity poorly this is due to the presence of small amount of the dissolved compound in the form of ions
147
Different from nonelectrolyte ndash NO ions at all Description of electrolyte as strong or weak not related to concentration of solution Electrolytes differ in degree of ionization not amount of solute dissolved
148
149
Colligative properties of solutions
Presence of solute affects properties of solutions Some properties not dependent on nature of dissolved substance but on how may dissolve particles are present
Colligative properties - properties that depend on the concentration of solute particles but not on their identity
150
Vapor-Pressure Lowering
Boiling and freezing point of solution different from pure solvent Nonvolatile solute raises boiling point and lowers freezing point Nonvolatile substance - one that has little tendency to become gas under existing conditions
151
152
To understand why nonvolatile solute changes boiling and freezing point consider equilibrium vapor pressure
Vapor pressure pressure caused by molecules that have escape liquid phase to gas phase
Can be thought of a measure of tendency of molecules to escape from a liquid
153
Addition of sucrose (nonvolatile solute) lowers concentration of water molecules at surface of liquid This lowers tendency of water molecules to leave solution and enter gas phase Vapor pressure of solution is lower than vapor pressure of pure water
154
Aqueous13 solution13 of13 nonvolatile13 solutePure13 water
Nonelectrolyte solutions of same molality have same concentration of particles
Dilute solutions of same solvent and equal molality of any nonelectrolyte solute lower vapor pressure equally
155
Example
1 m aqueous solution of nonelectrolyte glucose lowers vapor pressure of water 55 x 10-4 atm at 25 1 m aqueous solution of sucrose also lowers water vapor pressure to 55 x 10-4 atm at 25
156
157
bc vapor-pressure lowering depends on concentration of nonelectrolyte solute and doesnrsquot depend on the type of solute it is a colligative proprerty
bc vapor pressure has been lowered solution remains liquid over larger temperature range
Lowers freezing point and raises boiling point
158
Can assume changes in boiling and freezing point also depend on concentration of solute
They are colligative properties
159
Freezing-Point Depression
Freezing point of 1-molal solution of any nonelectrolyte solute in water is found (by experiment) to be 186 lower than freezing point of water When 1 mol of nonelectrolyte solute dissolved in 1 kg water freezing point is -186 not 000 160
When 2 mol nonelectrolyte solute dissolved in 1 kg water freezing point is -372
For any concentration of nonelectrolyte solute in water decrease in freezing point can be determined using value of -186m
161
This value called the molal freezing-point contstant (Kf) is the freezing-point depression of the solvent in a 1-molal solution of a nonvolatile nonelectrolyte solute
162
Each solvent has own characteristic molal freezing-point constant Freezing-point depression (Δtf) the difference between the freezing points of the pure solvent and a solute of nonelectrolyte in that solvent and it is directly proportional to the molal concentration of the solution Δtf = Kfm
163
Δtf = Kfm
Kf expressed as m
m expressed in mol solutekg solvent (molality)
164
Sample Problem
What is the freezing point depression of water in a solution of 171 g of sucrose C12H22O11 and 200 g of water What is the actual freezing point of the solution
165
1 AnalyzeGiven Solute mass and chemical formula 171 g C12H22O11 Solvent mass and identity 2000 g water Unknown a freezing-point depression b freezing point of the solution 166
2 Plan
167
168
Calculate
Unknown a freezing-point depression b freezing point of the solution
169
Practice Problem
A water solution containing an unknown quantity of a nonelectrolyte solute is found to have a freezing point of ndash 023degCWhat is the molal concentration of the solution
012 m170
A solution consists of 103 g of the nonelectrolyte glucose C6H12O6 dissolved in 250 g of water What is the freezing-point depression of the solution
minus0426degC171
In a laboratory experiment the freezing point of an aqueous solution of glucose is found to be minus0325degCWhat is the molal concentration of this solution
0175 m172
If 0500 mol of a nonelectrolyte solute are dissolved in 5000 g of ether what is the freezing point of the solution
minus1181degC173
The freezing point of an aqueous solution that contains a nonelectrolyte is minus90degC aWhat is the freezing-point depression of the solution bWhat is the molal concentration of the solution a minus90degC b 48 m
174
Boiling-Point Elevation
Boiling point is temp at which vapor pressure is equal to atmospheric pressure Change in vapor pressure will cause corresponding change in boiling point
175
176
Vapor pressure of nonvolatile solution is lower than vapor pressure of pure solvent More heat will be required to raise vapor pressure of solution Boiling point of solution is higher than boiling point of pure solvent
Molal boiling-point constant (Kb) - boiling-point elevation of the solvent in a 1-molal solution of a nonvolatile nonelectrolyte solute b-p elevation of any nonelectrolyte in water found by experiment to be 051 So molal b-p constant for water is 051m
177
For different solvents b-p elevations of 1-molal solutions have different values (Table 14-2)
Like freezing-point constants values most accurate for dilute solutions
178
Boiling-point elevation Δtb difference between the boiling points of the pure solvent and a nonelectrolyte solution of that solvent and it is directly proportional to the molal concentration of the solution B-p elevation can be calculated using Δtb = Kbm Δtb (boiling point elevation) expressed in m and m expressed in mol solutekg solvent
Sample Problem
What is the boiling-point elevation of a solution made from 200 g of a nonelectrolyte solute and 4000 g of water The molar mass of the solute is 620 gmol
180
1 AnalyzeGiven solute mass = 200 g solute molar mass = 620 gmol solvent mass and identity = 4000 g of water
Unknown boiling-point elevation
181
2 Plan
182
183
3 Compute
Practice Problem
A solution contains 500 g of sucrose C12H22O11 a nonelectrolyte dissolved in 5000 g of water What is the boiling-point elevation
015degC184
A solution contains 4500 g of sucrose C12H22O11 a nonelectrolyte dissolved in 250 g of waterWhat is the boiling point of the solution
1027degC185
If the boiling point elevation of an aqueous solution containing a nonvolatile electrolyte is 102degC what is the molality of the solution
20 m186
The boiling point of an aqueous solution containing a nonvolatile electrolyte is 10075degC aWhat is the boiling-point elevation bWhat is the molality of the solution a 075degC b 15 m
187
Electrolytes and Colligative PropertiesEarly investigators confused by experiments where certain substances depressed freezing point or elevated boiling point more than expected
188
189
Ex 01 m solution of NaCl lowers freezing point of solvent almost twice as much as 01 m solution of sucrose 01 m CaCl2 lowers fp almost 3 times as much as 01 m solution of sucrose
To understand why this happens contrast behavior or sucrose with NaCl in aqueous solution Each sucrose dissolves to make only 1 particle in solution So 1 mol sucrose dissolves to 1 mol particles 1 mol NaCl dissolves to make 2 moles of particles (1 mol Na+ and 1 mol Cl-)
190
191
Calculated Values for Electrolyte SolutionsColligative properties depend on total concentration of solute particles regardless of their identity Electrolytes cause changes in colligative properties proportional to total molality in terms of all dissolves particles instead of formula units
192
What about barium nitrate Ba(NO3)2 Each mole of barium nitrate yields 3 mol of ions in solution Ba(NO3)2(s) rarr Ba2+(aq) + 2NO3-(aq) You would expect solution of given molality to lower fp of solvent 3 times as much as nonelectrolytic solution of same molality
193
Sample Problem
What is the expected change in the freezing point of water in a solution of 625 g of barium nitrate Ba(NO3)2 in 100 kg of water
194
1 AnalyzeGiven solute mass and formula = 625 g Ba(NO3)2 solvent mass and identity = 100 kg water Δtf = Kfm
Unknown expected freezing-point depression
195
2 PlanThe molality can be calculated by converting the solute mass to moles and then dividing by the number of kilograms of solvent That molality is in terms of formula units of Ba(NO3)2 and must be converted to molality in terms of dissociated ions in solution It must be multiplied by the number of moles of ions produced per mole of formula unit This adjusted molality can then be used to calculate the freezing-point depression
196
197
198
3 Compute
Practice Problem
What is the expected freezing-point depression for a solution that contains 20 mol of magnesium sulfate dissolved in 10 kg of water
minus74degC199
What is the expected boiling-point elevation of water for a solution that contains 150 g of sodium chloride dissolved in 10 kg of water 27degC The freezing point of an aqueous sodium chloride solution is minus200degC What is the molality of the solution 54 m NaCl
200
Actual Values for Electrolyte SolutionsRemember values calculated are only expected values Actual values of colligative properties for all strong electrolytes are almost what would be expected Difference between expected and actual values caused by attractive forces that exist between dissociated ions in aqueous solution
201
Attraction between hydrated ions in solution is small compared with those in crystalline solid However forces of attraction donrsquot interfere with movements of aqueous ions More concentrated solution means ions closer together and attraction is greater Also depends on charge of ions ndash higher charge greater attraction
202
Types of Mixtures
2
Types of Mixtures
Some materials it is easy to see itrsquos a mixture bc you can see component parts Others like milk do not look like mixtures but they are Can see round droplets Heterogeneous mixture Composition not equal
3
Solutions
NaCl and water form homogeneous mixture Mixture is even throughout
4
Sugar is soluble in water Soluble - ability to be dissolved What happens as sugar dissolves in water
5
Lump of sugar disappears as sugar molecules leave surface of their crystals and mix with water molecules Eventually all sugar molecules are evenly spread through water molecules All visible traces of sugar are gone It becomes solution - homogeneous mixture of two or more substances in a single phase
6
Components of Solutions
Particles of one substance randomly mixed with particles of another Solvent - dissolving medium in a solution Solute - substance dissolved in solution
7
Solute usually less than solvent Dissolved solute particles cannot be seen Stay mixed with solvent forever so long as conditions stay the same Solute-particles from 001-1 nm in diameter
8
Types of Solutions
Solutions can exist as gases liquids or solids
9
Many alloys such as brass (zinc and copper) are sterling silver (silver and copper) are solid solutions Atoms of two or metals are evenly mixed By properly choosing percentages of each metal can get many desirable properties
10
Ex Alloys have higher strength and greater resistance to corrosion than pure metals Pure gold (24K) too soft for jewelry Alloy with silver (14K) increases strength and hardness but keeps appearance of gold
11
SuspensionsSuspension - if particles in solvent are so large they settle out unless mixture constantly stirred Think of sand and water Particles over 1000 nm in diameter
12
Particles that are medium in size between those in solutions and suspensions from mixtures are known as colloids (colloidal dispersions) Particles between 1-1000 nm diameter
13
Colloids
In muddy water large soil particles settle Water still cloudy bc colloidal particles stay dispersed in water Filter Colloidal particles pass through and stays cloudy
14
Particles in colloid small enough to be suspended in solvent by constant movement of surrounding molecules Colloidal particles make up dispersed phase Water is dispersing medium
15
Emulsion and foam are colloids
Mayonnaise is emulsion of oil droplets in water Egg yolk acts as emulsifying agent (keeps oil droplets dispersed)
16
17
18
Class13 of13 Colloid Phases Example
Sol Solid13 dispersed13 in13 liquid Paints13 mud
Gel Solid13 network13 extending13 through13 liquid
Gelatin
Liquid13 emulsion Liquid13 dispersed13 in13 liquid Milk13 mayonnaise
Foam Gas13 dispersed13 in13 liquid Shaving13 cream13 whipped13 cream
Solid13 aerosol Solid13 dispersed13 in13 gas Smoke13 auto13 exhaust
Liquid13 aerosol Liquid13 dispersed13 in13 gas Fog13 mist13 clouds13 aerosol13 spray
Solid13 emulsion Liquid13 dispersed13 in13 solid Cheese13 butter
Tyndall EffectMany colloids appear homogeneous bc individual particles cannot be seen Particles large enough to scatter light In fog can see headlight beam Tyndall effect - occurs when light is scattered by colloidal particles dispersed in transparent medium
19
20
21
Solutions Colloids Suspensions
Homogeneous Heterogeneous Heterogeneous
Particle13 size13 001-shy‐113 nm13 can13 be13 atoms13 ions13 molecules
Particle13 size13 1-shy‐100013 nm13 dispersed13 can13 be13 combined13 or13 large13 molecules
Particle13 size13 over13 100013 nm13 suspended13 can13 be13 large13 particles13 or13 combined13 particles
Do13 not13 separate13 on13 standing Do13 not13 separate13 on13 standing Particles13 settle13 out
Cannot13 be13 separated13 by13 filtration Cannot13 be13 separated13 by13 filtration Can13 be13 separated13 by13 filtration
Do13 not13 scatter13 light Scatter13 light13 (Tyndall13 effect) May13 scatter13 light13 but13 are13 not13 transparent
Electrolytes vs Nonelectrolytes
Substances that dissolve in water are classified according to whether they produce molecules or ions in solution
22
23
When ionic compound dissolves cations and anions separate and are surrounded by water molecules Solute ions free to move making possible to carry electric current
Electrolyte - substance that dissolves in water to give a solution that conducts electric current NaCl ndash electrolyte Usually highly polar compounds become electrolytes
24
Solution containing neutral solute molecules does not conduct current bc it doesnrsquot have mobile charged particles
Nonelectrolyte - substance that dissolves in water to give solution that does not conduct electric current Ex Sugar
25
26
27the solution
process
Factors Affecting Rate of DissolutionEver tried to dissolve sugar in iced tea or coffee You notice temperature has something to do with how quickly solute dissolves What other factors affect how quickly solutes dissolve
28
1 Increasing Surface Area of Solute
bc dissolution process occurs at surface of solute Can increase dissolution by increasing surface area
29
2 Agitating (Stirring) SolutionClose to surface of solute concentration of dissolved solute is high Stirringshaking helps disperse solute particles
30
Brings fresh solvent into contact with solute surface Contact between solvent and solute surface increased
3 Heating a Solvent
Solutes dissolve faster in warmer solvent As temp increases solvent molecules move faster Average kinetic energy increases Collisions between solvent and solute more frequent Helps separate solute molecules and spread them out
32
Solubility
If you add sugar to tea eventually no more sugar dissolves
For every combination of solvent with solute at given temp there is limit to amount of solute that can be dissolved
33
When sugar first dropped into water sugar molecules leave solid surface and move about at random in solvent Some dissolved molecules may collide with crystal and stay there As more solid dissolves and concentration of dissolved molecules increases collisions happen more often
34
Eventually molecules are returning to crystal at same rate they are going into solution Dynamic equilibrium established between dissolution and crystallization Solution equilibrium - physical state in which the opposing processes of dissolution and crystallization of a solute occur at equal rates
35
Saturated vs Unsaturated Solutions
Saturated solution - solution that contains the maximum amount of dissolved solute How do you know if solution is saturated If more NaCl added it falls to bottom and doesnrsquot dissolve Equilibrium already established
36
If more water added to saturated solution more NaCl will dissolve At 20 359 g NaCl is max that will dissolve in 100 g water Unsaturated solution - solution that contains less solute than a saturated solution under the existing conditions
37
Supersaturated Solutions
When saturated solution where solubility increases as temp increases is cooled excess solute usually comes out of solution Sometimes if solution is left to cool the excess solute doesnrsquot separate
38
39
Supersaturated solution - solution that contains more dissolved solute than a saturated solution contains under same conditions
40
Supersaturated solution can remain unchanged over long time if it isnrsquot disturbed Once crystals begin to form continues until equilibrium is recreated at lower temp
41
Example
Sodium thiosulfate Na2S2O3 Solute added to hot water until solution saturated Hot solution filtered Drop small crystal in ldquoseedingrdquo crystallization Continues until equilibrium created
42
Solubility Values
Solubility - amount of substance required to form a saturated solution with specific amount of solvent at specified temp Ex Solubility of sugar = 204 g per 100 g water at 20 Must specify temp bc solubility changes with temp 43
For gases pressure must also be specified Rate at which substance dissolves is unrelated to solubility Max amount of solute that dissolves and reaches equilibrium is always same under same conditions
44
Solute-Solvent Interactions
ldquolike dissolves likerdquo is generally useful rule for predicting whether one substance will dissolve in another Depends on type of bonding polaritynonpolarity and intermolecular forces between solute and solvent
45
Dissolving Ionic Compounds in Aqueous Solution
Polarity of water molecules plays important role in formation of solution of ionic compounds Charged ends of water molecules attract ions and surround them to keep separated from other ions
46
Hydration - solution process with water as solvent Ions said to be hydrated As hydrated ions diffuse into solution other ions exposed and drawn away from crystal surface Entire crystal gradually dissolves
47
When crystallized from aqueous solutions some ionic compounds form crystals that include water molecules The crystalline compounds known as hydrates have specific ratios of water molecules Represented by formulas like CuSO45H2O
48
When crystalline hydrate dissolves in water water of hydration returns to solvent Behavior of solute in hydrated form no different than anhydrous form
49
Nonpolar Solvents
Nonpolar solvents do not attract ions of ionic compound strongly enough to overcome forces holding crystal together
50
Liquid Solutes and Solvents
When you shake bottle of salad dressing oil droplets disperse in water Stop shaking ndash strong attraction between water molecules squeeze out oil and form separate layers Liquid solutes and solvents that are not soluble in each other are immiscible
51
Nonpolar substances (fat oil grease) are generally quite soluble in nonpolar liquids (carbon tetrachloride toluene gasoline)
Only attractions between nonpolar molecules are weak London forces
52
London Dispersion Forces
bc e- are constantly moving at any time there may be uneven distribution of them around a nonpolar atom Temporarily creates positive end and negative end
53
54
London dispersion forces - intermolecular attractions resulting from constant motion of e- and creation of instantaneous dipoles
55
Liquids that dissolve freely in one another in any ratio are said to be completely miscible Benzene and carbon tetrachloride (both nonpolar) are completely miscible
Nonpolar molecules of these two apply no strong forces of attraction or repulsion and molecules mix freely
56
Ethanol and water are also completely miscible -OH group on ethanol is slightly polar
Can form H-bonds with water as well as other ethanol molecules Intermolecular forces in mixture are very similar to those in pure liquids so they are mutually soluble in any amount
57
Gasoline contains mainly nonpolar hydrocarbons
Excellent solvent for fats oils greases
Major intermolecular forces are weak London forces
58
Ethanol less polar than water more than carbon tetrachloride
Better solvent for less-polar substances bc of nonpolar region
59
Effects of Pressure on Solubility
Little effect on solids or liquids Increase pressure - increase solubilities of gases in liquids
60
When gas in contact with surface of liquid gas molecules can enter liquid As amount of dissolve gas increases some molecules start to escape and reenter gas phase
Eq eventually established between rates of enteringleaving gas phase
61
As long as eq undisturbed solubility of gas in liquid unchanged at given pressure
Gas + solvent solution
62
Increasing pressure of solute gas above solution puts stress on eq Molecules collide with surface more often Increase in pressure partially offset by increase in rate of gas molecules entering solution In turn increase in amount of dissolved gas causes increase in rate molecules escape liquid and becomes gas
63
Eventually eq restored at higher gas solubility
Expected from Le Chatelierrsquos principle
Increase in gas pressure causes eq to shift so fewer molecules are in gas phase
64
Henryrsquos Law
Henryrsquos law - solubility of a gas in liquid is directly proportional to the partial pressure of that gas on the surface of the liquid
Applies to gas-liquid solutions at constant temp
65
When mixture of ideal gases is limited in constant volume at constant temp each gas applies same pressure it would apply if it occupied the space alone
Assuming gases do not react each gas dissolves to the extent it would if no other gases were there
66
In carbonated drinks (Coke Pepsi etc) solubility of CO2 increased by increasing pressure
At bottling factory CO2 gas forced into solution of flavored water at pressure of 5-10 atm Gas-in-liquid solution then sealed in bottlescans
67
When cap removed pressure reduced to 1 atm and some CO2 escapes as gas bubbles Effervescence - rapid escape of a gas from a liquid in which it is dissolved
68
Effects of Temperature on Solubility
Letrsquos consider gas solubility Increasing temperature usually decreases gas solubility As temp increases average kinetic energy of molecules in solution increases Greater number of solute molecules escape from attraction of solvent and return to gas phase 69
At higher temps eq is reached with fewer gas molecules in solution Gases generally less soluble
70
Effect of temp on solubility of solids in liquids more difficult to predict
Often increasing temp increases solubility
However the same temp increase can result in large increase in solubility in one case and only slight increase in the next
71
72
enthalpies of Solution
Formation of solution comes with energy change Dissolve KI in water container feels cold Dissolve LiCl in water feels hot Formation of solid-liquid solution can absorb or release heat
73
During formation of solution solvent and solute particles experience changes in forces attracting them to other particles Before dissolving begins solvent molecules held by intermolecular forces (solvent-solvent attraction) In solute molecules held by Ifs (solute-solute attraction)
74
Energy is required to separate solute molecules and solvent molecules from their neighbors
A solute particle that is surrounded by solvent molecules is said to be solvated
75
Step 1 solute particles become separated from solid (energy absorbed) Step 2 solvent particles move apart to allow solute particles to enter liquid (energy absorbed)
76
Step 3 solvent particles attracted to and solvate solute particles (energy released)
77
Heat of solution - net amount of heat energy absorbed or released when a specific amount of solute dissolves in a solvent
Heat of solution negative (heat is released) if sums of energy in steps 1 and 2 is less than step 3
If 1+2 gt 3 HOS is positive (heat absorbed)78
In gaseous state molecules are so far apart there are nearly no forces between them Solute-solute interaction has little effect on heat of solution of a gas Energy released when gas dissolved in liquid bc attraction between solute gas and solvent molecules greater than energy needed to separate solvent molecules
79
Concentration of Solutions80
Concentration - measure of amount of solute in given amount of solvent or solution ldquodiluterdquo just means small amount of solute ldquoconcentratedrdquo just means relatively large amount of solute Unrelated to degree to which solution is saturated Ex Saturated solution of substance not very soluble might be dilute
81
Molarity
Molarity - number of moles of solute in one liter of solution To find molarity must know molar mass A ldquoone molarrdquo solution of NaOH is 1 mole NaOH per liter of solution (1 M)
82
1 mol NaOH = 400 g If this quantity dissolve in enough water to make EXACTLY 100 L solution solution is 1 M If 200 g NaOH dissolve in enough to make 100 L solution what is molarity 0500 M
83
1 M solution is not made by adding 1 mol solute to 1 L solvent That would be more than 1 L Instead 1 mol solute dissolved in less than 1 L Then diluted with more solvent to bring TOTAL VOLUME to 1 L
84
Practice ProblemYou have 340 L of solution that contains 900 g sodium chloride What is the molarity of that solution 0440 M NaCl You have 08 L of a 05 M HCl solution How many moles of HCl does this solution contain 04 mol HCl
85
What is the molarity of a solution composed of 585 g of potassium iodide dissolved in enough water to make a 0125 L of solution 0282 M KI
86
How many moles of H2SO4 are present in 0500 L of a 0150 M H2SO4 solution 00750 mol What volume of 300 M NaCl is needed for reaction that requires 1463 g of NaCl 0834 L
87
Molality
Molality - concentration of solution expressed in moles of solute per kilogram of solvent Solution that contains 1 mol solute (NaOH) dissolved in exactly 1 kg of solvent is ldquoone molalrdquo solution (1 m NaOH)
88
1 mol NaOH = 400 g 400 g NaOH dissolved in 1 kg water is 1 m NaOH
200 g NaOH in 1 kg water = 0500 m NaOH
89
Practice Problem
A solution was prepared by dissolving 171 g sucrose (C12H22O11) in 125 g of water Find the molal concentration of this solution 0400 m C12H22O11
90
A solution of iodine in carbon tetrachloride is used when iodine is needed for certain chemical tests How much iodine must be added to prepare a 0480 m solution of iodine in CCl4 if 1000 g of CCl4 is used 122 g I2
91
What is the molality of a solution composed of 255 g acetone (CH3)2CO dissolved in 200 g of water 22 m acetone What quantity in grams of methanol CH3OH is required to prepare a 0244 m solution in 400 g water 312 g methanol
92
How many grams of AgNO3 are needed to prepare a 0125 m solution in 250 mL of water 531 g AgNO3
What is the molality of a solution containing 182 g HCl and 250 g water 200 m
93
Ions in Aqueous Solutions and Colligative Properties
94
95
compounds in aqueous solutions
DissociationWhen compound made from ions dissolves in water ions separate Dissociation - separation of ions that occurs when an ionic compound dissolves
96
Notice number of ions made per formula unit in equations 1 formula unit of NaCl gives 2 units of ions in solution 1 formula unit of CaCl2 gives 3 units of ions
97
Assuming 100 dissociation solution that contains 1 mol NaCl contains 1 mol Na+ and 1 mol Cl- Can assume 100 dissociation of all soluble ionic compounds
98
Practice Problem
Write the equation for the dissolution of aluminum sulfate in water How many moles of aluminum ions are made by dissolving 1 mol aluminum sulfate What is the total number of moles of ions made by dissolving 1 mol of aluminum sulfate
99
1 AnalyzeGiven Amount of solute = 1 mol Al2(SO4)3 Solvent identity = water Unknown a moles of aluminum ions and sulfate ions Total number of moles of solute ions produced
100
2 Plan
The coefficients in the balanced dissociation equation will tell mole relationships You can use the equation to find out the number of moles of solute ions produced
101
3 Compute
a
b102
Write the equation for the dissolution of each of the following in water and then determine the number of moles of each ions made as well as the total number of ions made a 1 mol ammonium chloride b 1 mol sodium sulfide c 05 mol barium nitrate
103
a 1 mol ammonium chloride
1 mol NH4+ 1 mol Cl- 2 mol total ions
104
b 1 mol sodium sulfide
2 mol Na+ 1 mol S-2 3 mol total ions
105
c 05 mol barium nitrate
05 mol Ba+2 1 mol NO3- 15 mol total ions
106
Precipitation Reactions
Even though no compound is completely insoluble compounds of very low solubility can be considered insoluble for practical purposes There are general guidelines to help predict whether a compound made of certain combination of ions is soluble
107
General Solubility Guidelines
Sodium potassium and ammonium compounds are soluble in water
108
Nitrates acetates and chlorates are soluble
109
110
Most chlorides are soluble except those of silver mercury (I) and lead Lead (II) chloride is soluble in hot water
Most sulfates are soluble except those of barium strontium lead calcium and mercury
111
Most carbonates phosphates and silicates are insoluble except those of sodium potassium and ammonium
112
Most sulfides are insoluble except those of calcium strontium sodium potassium and ammonium
113
Is calcium phosphate Ca3(PO4)2 soluble or insoluble
Not one of the exceptions so it is insoluble
Dissociation equations are not written for insoluble compounds
114
Guidelines also useful in predicting what will happen if 2 solutions of 2 different soluble compounds are mixed
If mixing results in combination of ions that forms insoluble compound a double-replacement and precipitation reaction will happen
115
Will precipitate form when solutions of ammonium sulfide and cadmium nitrate are combined You can tell calcium nitrate is soluble from the guidelines (Most nitrates acetates and chlorates are soluble) You can also tell ammonium sulfide is soluble (Most sulfides are insoluble except those of calcium strontium sodium potassium and ammonium)
116
117
To decide whether precipitate can form must know solubilities of two possible products
Ammonium nitrate ndash soluble (Most nitrates acetates and chlorates are soluble) Cadmium sulfide ndash insoluble (Most sulfides are insoluble except those of calcium strontium sodium potassium and ammonium)
118
bc one product is insoluble double-replacement and precipitation reaction will happen (NH4)2S (aq) + Cd(NO3)2 (aq) rarr NH4NO3 (aq) + CdS (s)
119
Net Ionic Equations
Reactions of ions in aqueous solution usually represented by net ionic equations instead of formula equations Net ionic equation - includes only those compounds and ions that undergo a chemical change in a reaction in an aqueous solution
120
121
To write NIE (net ionic equation) first convert chemical equation into overall ionic equation All soluble ionic compounds shown as dissociated ions in solution Precipitates shown as solids
122
Notice ammonium and nitrate ions appear on both sides They havenrsquot gone through any chemical change Spectator ions - ions that do not take part in a chemical reaction and are found in solution both before and after the reaction
123
To change ionic equation to NIE eliminate spectator ions Cd+2(aq) + S-2(aq) rarr CdS(s)
This NIE applies to any reaction in which precipitate of CdS forms
124
Practice Problem
Identify the precipitate that forms when aqueous solutions of zinc nitrate and ammonium sulfide are combined Write the equation for the possible double-replacement reaction Then write the formula equation overall ionic equation and the net ionic equation
125
1 AnalyzeGiven Identity of reactants zinc nitrate and ammonium sulfide Reaction medium aqueous solution Unknown a equation for the possible double-replacement reaction b identity of the precipitate c formula equation d overall ionic equation e net ionic equation
126
2 PlanWrite the possible double-replacement reaction between Zn(NO3)2 and (NH4)2S Use the rules to determine if any of the products will precipitate Write a formula equation and overall net ionic equation Then cancel spectator ions to make net ionic equation
127
3 Computea possible double-replacement reaction
b rules show that zinc sulfide is not soluble so will precipitate (ammonium nitrate is soluble)
c formula equation128
d overall ionic equation
e net ionic equation129
Will a precipitate form if solutions of potassium sulfate and barium nitrate are combined If so write the net ionic equation for the reaction
Yes Ba+2(aq) + SO4-2(aq) rarr BaSO4(s)
130
Will a precipitate form if solutions of potassium nitrate and magnesium sulfate are combined If so write the net ionic equation for the reaction
No 131
Will a precipitate form if solutions of barium chloride and sodium sulfate are combined If so identify the spectator ions and write the net ionic equation for the reaction
Yes Na+ and Cl- Ba+2(aq) + SO4-2(aq) rarr BaSO4(s)
132
Write the net ionic equation for the precipitation of nickel(II) sulfide
Ni+2(aq) + S-2(aq) rarr NiS(s)
133
IonizationSome molecular compounds can also form ions in solution Usually polar Ionization - ions formed from solute molecules by action of the solvent (creation of ions where there were none) Ionization different from dissociation
134
Dissociation ionic compounds dissolve and ions already present separate Ionization ions formed where non existed before
Ions formed are hydrated Heat released during hydration of ions gives enough energy to break covalent bonds
135
Extent to which solute ionizes depends on strength of bonds within molecules of solute and strength of attraction between solute and solvent molecules
136
HCl ndash molecular compound that ionizes in aqueous solution Contains highly polar bond Attraction between polar HCl molecule and polar water strong enough to break HCl bond
137
The Hydronium Ion
H+ ions from HCl attracts other molecules or ions so strongly that it doesnrsquot normally exist alone Ionization of HCl better described as direct transfer of proton from HCl to H2O forming H3O+
138
139
Hydration of H+ to form hydronium is highly exothermic
Energy released gives a lot of energy needed to ionize a molecular solute
Many molecular compounds that ionize in aqueous solution contain hydrogen and form hydronium ion
140
Strong and Weak Electrolytes
Substances that make ions and conduct current in solution are electrolytes HCl is one of series of compounds that has hydrogen and halogen Hydrogen halides are all molecular compounds with single polar-covalent bonds
141
All are gases Very soluble in water All are electrolytes HCl HBr HI strongly conduct current in solution HF only weakly conducts current at same concentration Strength of conduction related to ability to ionize
142
143
Strong Electrolytes
HCl HBr HI are 100 ionized in solution
Strong electrolyte - any compound whose dilute aqueous solutions conduct electricity well this is due to the presence of all or almost all of the dissolve compound in the form of ions
144
Unique characteristic of strong electrolyte is that they yield only ions
Ex Ionic compound may be highly soluble in water and dissociate into ions (NaCl)
Others may not dissolve much but amount that does dissolve exists only as ions
145
Weak ElectrolytesHF dissolves in water to give acid solution called hydrofluoric acid HF bond stronger than bonds between hydrogen and other halogens When HF dissolves some molecule ionize Reverse reaction also happens HF(aq) + H2O(l) H3O+(aq) + F-(aq)
146
HF(aq) + H2O(l) H3O+
(aq) + F-(aq)Concentration of dissolved unionized HF stays high and concentration of ions stays low
Weak electrolyte - any compound whose aqueous solutions conduct electricity poorly this is due to the presence of small amount of the dissolved compound in the form of ions
147
Different from nonelectrolyte ndash NO ions at all Description of electrolyte as strong or weak not related to concentration of solution Electrolytes differ in degree of ionization not amount of solute dissolved
148
149
Colligative properties of solutions
Presence of solute affects properties of solutions Some properties not dependent on nature of dissolved substance but on how may dissolve particles are present
Colligative properties - properties that depend on the concentration of solute particles but not on their identity
150
Vapor-Pressure Lowering
Boiling and freezing point of solution different from pure solvent Nonvolatile solute raises boiling point and lowers freezing point Nonvolatile substance - one that has little tendency to become gas under existing conditions
151
152
To understand why nonvolatile solute changes boiling and freezing point consider equilibrium vapor pressure
Vapor pressure pressure caused by molecules that have escape liquid phase to gas phase
Can be thought of a measure of tendency of molecules to escape from a liquid
153
Addition of sucrose (nonvolatile solute) lowers concentration of water molecules at surface of liquid This lowers tendency of water molecules to leave solution and enter gas phase Vapor pressure of solution is lower than vapor pressure of pure water
154
Aqueous13 solution13 of13 nonvolatile13 solutePure13 water
Nonelectrolyte solutions of same molality have same concentration of particles
Dilute solutions of same solvent and equal molality of any nonelectrolyte solute lower vapor pressure equally
155
Example
1 m aqueous solution of nonelectrolyte glucose lowers vapor pressure of water 55 x 10-4 atm at 25 1 m aqueous solution of sucrose also lowers water vapor pressure to 55 x 10-4 atm at 25
156
157
bc vapor-pressure lowering depends on concentration of nonelectrolyte solute and doesnrsquot depend on the type of solute it is a colligative proprerty
bc vapor pressure has been lowered solution remains liquid over larger temperature range
Lowers freezing point and raises boiling point
158
Can assume changes in boiling and freezing point also depend on concentration of solute
They are colligative properties
159
Freezing-Point Depression
Freezing point of 1-molal solution of any nonelectrolyte solute in water is found (by experiment) to be 186 lower than freezing point of water When 1 mol of nonelectrolyte solute dissolved in 1 kg water freezing point is -186 not 000 160
When 2 mol nonelectrolyte solute dissolved in 1 kg water freezing point is -372
For any concentration of nonelectrolyte solute in water decrease in freezing point can be determined using value of -186m
161
This value called the molal freezing-point contstant (Kf) is the freezing-point depression of the solvent in a 1-molal solution of a nonvolatile nonelectrolyte solute
162
Each solvent has own characteristic molal freezing-point constant Freezing-point depression (Δtf) the difference between the freezing points of the pure solvent and a solute of nonelectrolyte in that solvent and it is directly proportional to the molal concentration of the solution Δtf = Kfm
163
Δtf = Kfm
Kf expressed as m
m expressed in mol solutekg solvent (molality)
164
Sample Problem
What is the freezing point depression of water in a solution of 171 g of sucrose C12H22O11 and 200 g of water What is the actual freezing point of the solution
165
1 AnalyzeGiven Solute mass and chemical formula 171 g C12H22O11 Solvent mass and identity 2000 g water Unknown a freezing-point depression b freezing point of the solution 166
2 Plan
167
168
Calculate
Unknown a freezing-point depression b freezing point of the solution
169
Practice Problem
A water solution containing an unknown quantity of a nonelectrolyte solute is found to have a freezing point of ndash 023degCWhat is the molal concentration of the solution
012 m170
A solution consists of 103 g of the nonelectrolyte glucose C6H12O6 dissolved in 250 g of water What is the freezing-point depression of the solution
minus0426degC171
In a laboratory experiment the freezing point of an aqueous solution of glucose is found to be minus0325degCWhat is the molal concentration of this solution
0175 m172
If 0500 mol of a nonelectrolyte solute are dissolved in 5000 g of ether what is the freezing point of the solution
minus1181degC173
The freezing point of an aqueous solution that contains a nonelectrolyte is minus90degC aWhat is the freezing-point depression of the solution bWhat is the molal concentration of the solution a minus90degC b 48 m
174
Boiling-Point Elevation
Boiling point is temp at which vapor pressure is equal to atmospheric pressure Change in vapor pressure will cause corresponding change in boiling point
175
176
Vapor pressure of nonvolatile solution is lower than vapor pressure of pure solvent More heat will be required to raise vapor pressure of solution Boiling point of solution is higher than boiling point of pure solvent
Molal boiling-point constant (Kb) - boiling-point elevation of the solvent in a 1-molal solution of a nonvolatile nonelectrolyte solute b-p elevation of any nonelectrolyte in water found by experiment to be 051 So molal b-p constant for water is 051m
177
For different solvents b-p elevations of 1-molal solutions have different values (Table 14-2)
Like freezing-point constants values most accurate for dilute solutions
178
Boiling-point elevation Δtb difference between the boiling points of the pure solvent and a nonelectrolyte solution of that solvent and it is directly proportional to the molal concentration of the solution B-p elevation can be calculated using Δtb = Kbm Δtb (boiling point elevation) expressed in m and m expressed in mol solutekg solvent
Sample Problem
What is the boiling-point elevation of a solution made from 200 g of a nonelectrolyte solute and 4000 g of water The molar mass of the solute is 620 gmol
180
1 AnalyzeGiven solute mass = 200 g solute molar mass = 620 gmol solvent mass and identity = 4000 g of water
Unknown boiling-point elevation
181
2 Plan
182
183
3 Compute
Practice Problem
A solution contains 500 g of sucrose C12H22O11 a nonelectrolyte dissolved in 5000 g of water What is the boiling-point elevation
015degC184
A solution contains 4500 g of sucrose C12H22O11 a nonelectrolyte dissolved in 250 g of waterWhat is the boiling point of the solution
1027degC185
If the boiling point elevation of an aqueous solution containing a nonvolatile electrolyte is 102degC what is the molality of the solution
20 m186
The boiling point of an aqueous solution containing a nonvolatile electrolyte is 10075degC aWhat is the boiling-point elevation bWhat is the molality of the solution a 075degC b 15 m
187
Electrolytes and Colligative PropertiesEarly investigators confused by experiments where certain substances depressed freezing point or elevated boiling point more than expected
188
189
Ex 01 m solution of NaCl lowers freezing point of solvent almost twice as much as 01 m solution of sucrose 01 m CaCl2 lowers fp almost 3 times as much as 01 m solution of sucrose
To understand why this happens contrast behavior or sucrose with NaCl in aqueous solution Each sucrose dissolves to make only 1 particle in solution So 1 mol sucrose dissolves to 1 mol particles 1 mol NaCl dissolves to make 2 moles of particles (1 mol Na+ and 1 mol Cl-)
190
191
Calculated Values for Electrolyte SolutionsColligative properties depend on total concentration of solute particles regardless of their identity Electrolytes cause changes in colligative properties proportional to total molality in terms of all dissolves particles instead of formula units
192
What about barium nitrate Ba(NO3)2 Each mole of barium nitrate yields 3 mol of ions in solution Ba(NO3)2(s) rarr Ba2+(aq) + 2NO3-(aq) You would expect solution of given molality to lower fp of solvent 3 times as much as nonelectrolytic solution of same molality
193
Sample Problem
What is the expected change in the freezing point of water in a solution of 625 g of barium nitrate Ba(NO3)2 in 100 kg of water
194
1 AnalyzeGiven solute mass and formula = 625 g Ba(NO3)2 solvent mass and identity = 100 kg water Δtf = Kfm
Unknown expected freezing-point depression
195
2 PlanThe molality can be calculated by converting the solute mass to moles and then dividing by the number of kilograms of solvent That molality is in terms of formula units of Ba(NO3)2 and must be converted to molality in terms of dissociated ions in solution It must be multiplied by the number of moles of ions produced per mole of formula unit This adjusted molality can then be used to calculate the freezing-point depression
196
197
198
3 Compute
Practice Problem
What is the expected freezing-point depression for a solution that contains 20 mol of magnesium sulfate dissolved in 10 kg of water
minus74degC199
What is the expected boiling-point elevation of water for a solution that contains 150 g of sodium chloride dissolved in 10 kg of water 27degC The freezing point of an aqueous sodium chloride solution is minus200degC What is the molality of the solution 54 m NaCl
200
Actual Values for Electrolyte SolutionsRemember values calculated are only expected values Actual values of colligative properties for all strong electrolytes are almost what would be expected Difference between expected and actual values caused by attractive forces that exist between dissociated ions in aqueous solution
201
Attraction between hydrated ions in solution is small compared with those in crystalline solid However forces of attraction donrsquot interfere with movements of aqueous ions More concentrated solution means ions closer together and attraction is greater Also depends on charge of ions ndash higher charge greater attraction
202
Types of Mixtures
Some materials it is easy to see itrsquos a mixture bc you can see component parts Others like milk do not look like mixtures but they are Can see round droplets Heterogeneous mixture Composition not equal
3
Solutions
NaCl and water form homogeneous mixture Mixture is even throughout
4
Sugar is soluble in water Soluble - ability to be dissolved What happens as sugar dissolves in water
5
Lump of sugar disappears as sugar molecules leave surface of their crystals and mix with water molecules Eventually all sugar molecules are evenly spread through water molecules All visible traces of sugar are gone It becomes solution - homogeneous mixture of two or more substances in a single phase
6
Components of Solutions
Particles of one substance randomly mixed with particles of another Solvent - dissolving medium in a solution Solute - substance dissolved in solution
7
Solute usually less than solvent Dissolved solute particles cannot be seen Stay mixed with solvent forever so long as conditions stay the same Solute-particles from 001-1 nm in diameter
8
Types of Solutions
Solutions can exist as gases liquids or solids
9
Many alloys such as brass (zinc and copper) are sterling silver (silver and copper) are solid solutions Atoms of two or metals are evenly mixed By properly choosing percentages of each metal can get many desirable properties
10
Ex Alloys have higher strength and greater resistance to corrosion than pure metals Pure gold (24K) too soft for jewelry Alloy with silver (14K) increases strength and hardness but keeps appearance of gold
11
SuspensionsSuspension - if particles in solvent are so large they settle out unless mixture constantly stirred Think of sand and water Particles over 1000 nm in diameter
12
Particles that are medium in size between those in solutions and suspensions from mixtures are known as colloids (colloidal dispersions) Particles between 1-1000 nm diameter
13
Colloids
In muddy water large soil particles settle Water still cloudy bc colloidal particles stay dispersed in water Filter Colloidal particles pass through and stays cloudy
14
Particles in colloid small enough to be suspended in solvent by constant movement of surrounding molecules Colloidal particles make up dispersed phase Water is dispersing medium
15
Emulsion and foam are colloids
Mayonnaise is emulsion of oil droplets in water Egg yolk acts as emulsifying agent (keeps oil droplets dispersed)
16
17
18
Class13 of13 Colloid Phases Example
Sol Solid13 dispersed13 in13 liquid Paints13 mud
Gel Solid13 network13 extending13 through13 liquid
Gelatin
Liquid13 emulsion Liquid13 dispersed13 in13 liquid Milk13 mayonnaise
Foam Gas13 dispersed13 in13 liquid Shaving13 cream13 whipped13 cream
Solid13 aerosol Solid13 dispersed13 in13 gas Smoke13 auto13 exhaust
Liquid13 aerosol Liquid13 dispersed13 in13 gas Fog13 mist13 clouds13 aerosol13 spray
Solid13 emulsion Liquid13 dispersed13 in13 solid Cheese13 butter
Tyndall EffectMany colloids appear homogeneous bc individual particles cannot be seen Particles large enough to scatter light In fog can see headlight beam Tyndall effect - occurs when light is scattered by colloidal particles dispersed in transparent medium
19
20
21
Solutions Colloids Suspensions
Homogeneous Heterogeneous Heterogeneous
Particle13 size13 001-shy‐113 nm13 can13 be13 atoms13 ions13 molecules
Particle13 size13 1-shy‐100013 nm13 dispersed13 can13 be13 combined13 or13 large13 molecules
Particle13 size13 over13 100013 nm13 suspended13 can13 be13 large13 particles13 or13 combined13 particles
Do13 not13 separate13 on13 standing Do13 not13 separate13 on13 standing Particles13 settle13 out
Cannot13 be13 separated13 by13 filtration Cannot13 be13 separated13 by13 filtration Can13 be13 separated13 by13 filtration
Do13 not13 scatter13 light Scatter13 light13 (Tyndall13 effect) May13 scatter13 light13 but13 are13 not13 transparent
Electrolytes vs Nonelectrolytes
Substances that dissolve in water are classified according to whether they produce molecules or ions in solution
22
23
When ionic compound dissolves cations and anions separate and are surrounded by water molecules Solute ions free to move making possible to carry electric current
Electrolyte - substance that dissolves in water to give a solution that conducts electric current NaCl ndash electrolyte Usually highly polar compounds become electrolytes
24
Solution containing neutral solute molecules does not conduct current bc it doesnrsquot have mobile charged particles
Nonelectrolyte - substance that dissolves in water to give solution that does not conduct electric current Ex Sugar
25
26
27the solution
process
Factors Affecting Rate of DissolutionEver tried to dissolve sugar in iced tea or coffee You notice temperature has something to do with how quickly solute dissolves What other factors affect how quickly solutes dissolve
28
1 Increasing Surface Area of Solute
bc dissolution process occurs at surface of solute Can increase dissolution by increasing surface area
29
2 Agitating (Stirring) SolutionClose to surface of solute concentration of dissolved solute is high Stirringshaking helps disperse solute particles
30
Brings fresh solvent into contact with solute surface Contact between solvent and solute surface increased
3 Heating a Solvent
Solutes dissolve faster in warmer solvent As temp increases solvent molecules move faster Average kinetic energy increases Collisions between solvent and solute more frequent Helps separate solute molecules and spread them out
32
Solubility
If you add sugar to tea eventually no more sugar dissolves
For every combination of solvent with solute at given temp there is limit to amount of solute that can be dissolved
33
When sugar first dropped into water sugar molecules leave solid surface and move about at random in solvent Some dissolved molecules may collide with crystal and stay there As more solid dissolves and concentration of dissolved molecules increases collisions happen more often
34
Eventually molecules are returning to crystal at same rate they are going into solution Dynamic equilibrium established between dissolution and crystallization Solution equilibrium - physical state in which the opposing processes of dissolution and crystallization of a solute occur at equal rates
35
Saturated vs Unsaturated Solutions
Saturated solution - solution that contains the maximum amount of dissolved solute How do you know if solution is saturated If more NaCl added it falls to bottom and doesnrsquot dissolve Equilibrium already established
36
If more water added to saturated solution more NaCl will dissolve At 20 359 g NaCl is max that will dissolve in 100 g water Unsaturated solution - solution that contains less solute than a saturated solution under the existing conditions
37
Supersaturated Solutions
When saturated solution where solubility increases as temp increases is cooled excess solute usually comes out of solution Sometimes if solution is left to cool the excess solute doesnrsquot separate
38
39
Supersaturated solution - solution that contains more dissolved solute than a saturated solution contains under same conditions
40
Supersaturated solution can remain unchanged over long time if it isnrsquot disturbed Once crystals begin to form continues until equilibrium is recreated at lower temp
41
Example
Sodium thiosulfate Na2S2O3 Solute added to hot water until solution saturated Hot solution filtered Drop small crystal in ldquoseedingrdquo crystallization Continues until equilibrium created
42
Solubility Values
Solubility - amount of substance required to form a saturated solution with specific amount of solvent at specified temp Ex Solubility of sugar = 204 g per 100 g water at 20 Must specify temp bc solubility changes with temp 43
For gases pressure must also be specified Rate at which substance dissolves is unrelated to solubility Max amount of solute that dissolves and reaches equilibrium is always same under same conditions
44
Solute-Solvent Interactions
ldquolike dissolves likerdquo is generally useful rule for predicting whether one substance will dissolve in another Depends on type of bonding polaritynonpolarity and intermolecular forces between solute and solvent
45
Dissolving Ionic Compounds in Aqueous Solution
Polarity of water molecules plays important role in formation of solution of ionic compounds Charged ends of water molecules attract ions and surround them to keep separated from other ions
46
Hydration - solution process with water as solvent Ions said to be hydrated As hydrated ions diffuse into solution other ions exposed and drawn away from crystal surface Entire crystal gradually dissolves
47
When crystallized from aqueous solutions some ionic compounds form crystals that include water molecules The crystalline compounds known as hydrates have specific ratios of water molecules Represented by formulas like CuSO45H2O
48
When crystalline hydrate dissolves in water water of hydration returns to solvent Behavior of solute in hydrated form no different than anhydrous form
49
Nonpolar Solvents
Nonpolar solvents do not attract ions of ionic compound strongly enough to overcome forces holding crystal together
50
Liquid Solutes and Solvents
When you shake bottle of salad dressing oil droplets disperse in water Stop shaking ndash strong attraction between water molecules squeeze out oil and form separate layers Liquid solutes and solvents that are not soluble in each other are immiscible
51
Nonpolar substances (fat oil grease) are generally quite soluble in nonpolar liquids (carbon tetrachloride toluene gasoline)
Only attractions between nonpolar molecules are weak London forces
52
London Dispersion Forces
bc e- are constantly moving at any time there may be uneven distribution of them around a nonpolar atom Temporarily creates positive end and negative end
53
54
London dispersion forces - intermolecular attractions resulting from constant motion of e- and creation of instantaneous dipoles
55
Liquids that dissolve freely in one another in any ratio are said to be completely miscible Benzene and carbon tetrachloride (both nonpolar) are completely miscible
Nonpolar molecules of these two apply no strong forces of attraction or repulsion and molecules mix freely
56
Ethanol and water are also completely miscible -OH group on ethanol is slightly polar
Can form H-bonds with water as well as other ethanol molecules Intermolecular forces in mixture are very similar to those in pure liquids so they are mutually soluble in any amount
57
Gasoline contains mainly nonpolar hydrocarbons
Excellent solvent for fats oils greases
Major intermolecular forces are weak London forces
58
Ethanol less polar than water more than carbon tetrachloride
Better solvent for less-polar substances bc of nonpolar region
59
Effects of Pressure on Solubility
Little effect on solids or liquids Increase pressure - increase solubilities of gases in liquids
60
When gas in contact with surface of liquid gas molecules can enter liquid As amount of dissolve gas increases some molecules start to escape and reenter gas phase
Eq eventually established between rates of enteringleaving gas phase
61
As long as eq undisturbed solubility of gas in liquid unchanged at given pressure
Gas + solvent solution
62
Increasing pressure of solute gas above solution puts stress on eq Molecules collide with surface more often Increase in pressure partially offset by increase in rate of gas molecules entering solution In turn increase in amount of dissolved gas causes increase in rate molecules escape liquid and becomes gas
63
Eventually eq restored at higher gas solubility
Expected from Le Chatelierrsquos principle
Increase in gas pressure causes eq to shift so fewer molecules are in gas phase
64
Henryrsquos Law
Henryrsquos law - solubility of a gas in liquid is directly proportional to the partial pressure of that gas on the surface of the liquid
Applies to gas-liquid solutions at constant temp
65
When mixture of ideal gases is limited in constant volume at constant temp each gas applies same pressure it would apply if it occupied the space alone
Assuming gases do not react each gas dissolves to the extent it would if no other gases were there
66
In carbonated drinks (Coke Pepsi etc) solubility of CO2 increased by increasing pressure
At bottling factory CO2 gas forced into solution of flavored water at pressure of 5-10 atm Gas-in-liquid solution then sealed in bottlescans
67
When cap removed pressure reduced to 1 atm and some CO2 escapes as gas bubbles Effervescence - rapid escape of a gas from a liquid in which it is dissolved
68
Effects of Temperature on Solubility
Letrsquos consider gas solubility Increasing temperature usually decreases gas solubility As temp increases average kinetic energy of molecules in solution increases Greater number of solute molecules escape from attraction of solvent and return to gas phase 69
At higher temps eq is reached with fewer gas molecules in solution Gases generally less soluble
70
Effect of temp on solubility of solids in liquids more difficult to predict
Often increasing temp increases solubility
However the same temp increase can result in large increase in solubility in one case and only slight increase in the next
71
72
enthalpies of Solution
Formation of solution comes with energy change Dissolve KI in water container feels cold Dissolve LiCl in water feels hot Formation of solid-liquid solution can absorb or release heat
73
During formation of solution solvent and solute particles experience changes in forces attracting them to other particles Before dissolving begins solvent molecules held by intermolecular forces (solvent-solvent attraction) In solute molecules held by Ifs (solute-solute attraction)
74
Energy is required to separate solute molecules and solvent molecules from their neighbors
A solute particle that is surrounded by solvent molecules is said to be solvated
75
Step 1 solute particles become separated from solid (energy absorbed) Step 2 solvent particles move apart to allow solute particles to enter liquid (energy absorbed)
76
Step 3 solvent particles attracted to and solvate solute particles (energy released)
77
Heat of solution - net amount of heat energy absorbed or released when a specific amount of solute dissolves in a solvent
Heat of solution negative (heat is released) if sums of energy in steps 1 and 2 is less than step 3
If 1+2 gt 3 HOS is positive (heat absorbed)78
In gaseous state molecules are so far apart there are nearly no forces between them Solute-solute interaction has little effect on heat of solution of a gas Energy released when gas dissolved in liquid bc attraction between solute gas and solvent molecules greater than energy needed to separate solvent molecules
79
Concentration of Solutions80
Concentration - measure of amount of solute in given amount of solvent or solution ldquodiluterdquo just means small amount of solute ldquoconcentratedrdquo just means relatively large amount of solute Unrelated to degree to which solution is saturated Ex Saturated solution of substance not very soluble might be dilute
81
Molarity
Molarity - number of moles of solute in one liter of solution To find molarity must know molar mass A ldquoone molarrdquo solution of NaOH is 1 mole NaOH per liter of solution (1 M)
82
1 mol NaOH = 400 g If this quantity dissolve in enough water to make EXACTLY 100 L solution solution is 1 M If 200 g NaOH dissolve in enough to make 100 L solution what is molarity 0500 M
83
1 M solution is not made by adding 1 mol solute to 1 L solvent That would be more than 1 L Instead 1 mol solute dissolved in less than 1 L Then diluted with more solvent to bring TOTAL VOLUME to 1 L
84
Practice ProblemYou have 340 L of solution that contains 900 g sodium chloride What is the molarity of that solution 0440 M NaCl You have 08 L of a 05 M HCl solution How many moles of HCl does this solution contain 04 mol HCl
85
What is the molarity of a solution composed of 585 g of potassium iodide dissolved in enough water to make a 0125 L of solution 0282 M KI
86
How many moles of H2SO4 are present in 0500 L of a 0150 M H2SO4 solution 00750 mol What volume of 300 M NaCl is needed for reaction that requires 1463 g of NaCl 0834 L
87
Molality
Molality - concentration of solution expressed in moles of solute per kilogram of solvent Solution that contains 1 mol solute (NaOH) dissolved in exactly 1 kg of solvent is ldquoone molalrdquo solution (1 m NaOH)
88
1 mol NaOH = 400 g 400 g NaOH dissolved in 1 kg water is 1 m NaOH
200 g NaOH in 1 kg water = 0500 m NaOH
89
Practice Problem
A solution was prepared by dissolving 171 g sucrose (C12H22O11) in 125 g of water Find the molal concentration of this solution 0400 m C12H22O11
90
A solution of iodine in carbon tetrachloride is used when iodine is needed for certain chemical tests How much iodine must be added to prepare a 0480 m solution of iodine in CCl4 if 1000 g of CCl4 is used 122 g I2
91
What is the molality of a solution composed of 255 g acetone (CH3)2CO dissolved in 200 g of water 22 m acetone What quantity in grams of methanol CH3OH is required to prepare a 0244 m solution in 400 g water 312 g methanol
92
How many grams of AgNO3 are needed to prepare a 0125 m solution in 250 mL of water 531 g AgNO3
What is the molality of a solution containing 182 g HCl and 250 g water 200 m
93
Ions in Aqueous Solutions and Colligative Properties
94
95
compounds in aqueous solutions
DissociationWhen compound made from ions dissolves in water ions separate Dissociation - separation of ions that occurs when an ionic compound dissolves
96
Notice number of ions made per formula unit in equations 1 formula unit of NaCl gives 2 units of ions in solution 1 formula unit of CaCl2 gives 3 units of ions
97
Assuming 100 dissociation solution that contains 1 mol NaCl contains 1 mol Na+ and 1 mol Cl- Can assume 100 dissociation of all soluble ionic compounds
98
Practice Problem
Write the equation for the dissolution of aluminum sulfate in water How many moles of aluminum ions are made by dissolving 1 mol aluminum sulfate What is the total number of moles of ions made by dissolving 1 mol of aluminum sulfate
99
1 AnalyzeGiven Amount of solute = 1 mol Al2(SO4)3 Solvent identity = water Unknown a moles of aluminum ions and sulfate ions Total number of moles of solute ions produced
100
2 Plan
The coefficients in the balanced dissociation equation will tell mole relationships You can use the equation to find out the number of moles of solute ions produced
101
3 Compute
a
b102
Write the equation for the dissolution of each of the following in water and then determine the number of moles of each ions made as well as the total number of ions made a 1 mol ammonium chloride b 1 mol sodium sulfide c 05 mol barium nitrate
103
a 1 mol ammonium chloride
1 mol NH4+ 1 mol Cl- 2 mol total ions
104
b 1 mol sodium sulfide
2 mol Na+ 1 mol S-2 3 mol total ions
105
c 05 mol barium nitrate
05 mol Ba+2 1 mol NO3- 15 mol total ions
106
Precipitation Reactions
Even though no compound is completely insoluble compounds of very low solubility can be considered insoluble for practical purposes There are general guidelines to help predict whether a compound made of certain combination of ions is soluble
107
General Solubility Guidelines
Sodium potassium and ammonium compounds are soluble in water
108
Nitrates acetates and chlorates are soluble
109
110
Most chlorides are soluble except those of silver mercury (I) and lead Lead (II) chloride is soluble in hot water
Most sulfates are soluble except those of barium strontium lead calcium and mercury
111
Most carbonates phosphates and silicates are insoluble except those of sodium potassium and ammonium
112
Most sulfides are insoluble except those of calcium strontium sodium potassium and ammonium
113
Is calcium phosphate Ca3(PO4)2 soluble or insoluble
Not one of the exceptions so it is insoluble
Dissociation equations are not written for insoluble compounds
114
Guidelines also useful in predicting what will happen if 2 solutions of 2 different soluble compounds are mixed
If mixing results in combination of ions that forms insoluble compound a double-replacement and precipitation reaction will happen
115
Will precipitate form when solutions of ammonium sulfide and cadmium nitrate are combined You can tell calcium nitrate is soluble from the guidelines (Most nitrates acetates and chlorates are soluble) You can also tell ammonium sulfide is soluble (Most sulfides are insoluble except those of calcium strontium sodium potassium and ammonium)
116
117
To decide whether precipitate can form must know solubilities of two possible products
Ammonium nitrate ndash soluble (Most nitrates acetates and chlorates are soluble) Cadmium sulfide ndash insoluble (Most sulfides are insoluble except those of calcium strontium sodium potassium and ammonium)
118
bc one product is insoluble double-replacement and precipitation reaction will happen (NH4)2S (aq) + Cd(NO3)2 (aq) rarr NH4NO3 (aq) + CdS (s)
119
Net Ionic Equations
Reactions of ions in aqueous solution usually represented by net ionic equations instead of formula equations Net ionic equation - includes only those compounds and ions that undergo a chemical change in a reaction in an aqueous solution
120
121
To write NIE (net ionic equation) first convert chemical equation into overall ionic equation All soluble ionic compounds shown as dissociated ions in solution Precipitates shown as solids
122
Notice ammonium and nitrate ions appear on both sides They havenrsquot gone through any chemical change Spectator ions - ions that do not take part in a chemical reaction and are found in solution both before and after the reaction
123
To change ionic equation to NIE eliminate spectator ions Cd+2(aq) + S-2(aq) rarr CdS(s)
This NIE applies to any reaction in which precipitate of CdS forms
124
Practice Problem
Identify the precipitate that forms when aqueous solutions of zinc nitrate and ammonium sulfide are combined Write the equation for the possible double-replacement reaction Then write the formula equation overall ionic equation and the net ionic equation
125
1 AnalyzeGiven Identity of reactants zinc nitrate and ammonium sulfide Reaction medium aqueous solution Unknown a equation for the possible double-replacement reaction b identity of the precipitate c formula equation d overall ionic equation e net ionic equation
126
2 PlanWrite the possible double-replacement reaction between Zn(NO3)2 and (NH4)2S Use the rules to determine if any of the products will precipitate Write a formula equation and overall net ionic equation Then cancel spectator ions to make net ionic equation
127
3 Computea possible double-replacement reaction
b rules show that zinc sulfide is not soluble so will precipitate (ammonium nitrate is soluble)
c formula equation128
d overall ionic equation
e net ionic equation129
Will a precipitate form if solutions of potassium sulfate and barium nitrate are combined If so write the net ionic equation for the reaction
Yes Ba+2(aq) + SO4-2(aq) rarr BaSO4(s)
130
Will a precipitate form if solutions of potassium nitrate and magnesium sulfate are combined If so write the net ionic equation for the reaction
No 131
Will a precipitate form if solutions of barium chloride and sodium sulfate are combined If so identify the spectator ions and write the net ionic equation for the reaction
Yes Na+ and Cl- Ba+2(aq) + SO4-2(aq) rarr BaSO4(s)
132
Write the net ionic equation for the precipitation of nickel(II) sulfide
Ni+2(aq) + S-2(aq) rarr NiS(s)
133
IonizationSome molecular compounds can also form ions in solution Usually polar Ionization - ions formed from solute molecules by action of the solvent (creation of ions where there were none) Ionization different from dissociation
134
Dissociation ionic compounds dissolve and ions already present separate Ionization ions formed where non existed before
Ions formed are hydrated Heat released during hydration of ions gives enough energy to break covalent bonds
135
Extent to which solute ionizes depends on strength of bonds within molecules of solute and strength of attraction between solute and solvent molecules
136
HCl ndash molecular compound that ionizes in aqueous solution Contains highly polar bond Attraction between polar HCl molecule and polar water strong enough to break HCl bond
137
The Hydronium Ion
H+ ions from HCl attracts other molecules or ions so strongly that it doesnrsquot normally exist alone Ionization of HCl better described as direct transfer of proton from HCl to H2O forming H3O+
138
139
Hydration of H+ to form hydronium is highly exothermic
Energy released gives a lot of energy needed to ionize a molecular solute
Many molecular compounds that ionize in aqueous solution contain hydrogen and form hydronium ion
140
Strong and Weak Electrolytes
Substances that make ions and conduct current in solution are electrolytes HCl is one of series of compounds that has hydrogen and halogen Hydrogen halides are all molecular compounds with single polar-covalent bonds
141
All are gases Very soluble in water All are electrolytes HCl HBr HI strongly conduct current in solution HF only weakly conducts current at same concentration Strength of conduction related to ability to ionize
142
143
Strong Electrolytes
HCl HBr HI are 100 ionized in solution
Strong electrolyte - any compound whose dilute aqueous solutions conduct electricity well this is due to the presence of all or almost all of the dissolve compound in the form of ions
144
Unique characteristic of strong electrolyte is that they yield only ions
Ex Ionic compound may be highly soluble in water and dissociate into ions (NaCl)
Others may not dissolve much but amount that does dissolve exists only as ions
145
Weak ElectrolytesHF dissolves in water to give acid solution called hydrofluoric acid HF bond stronger than bonds between hydrogen and other halogens When HF dissolves some molecule ionize Reverse reaction also happens HF(aq) + H2O(l) H3O+(aq) + F-(aq)
146
HF(aq) + H2O(l) H3O+
(aq) + F-(aq)Concentration of dissolved unionized HF stays high and concentration of ions stays low
Weak electrolyte - any compound whose aqueous solutions conduct electricity poorly this is due to the presence of small amount of the dissolved compound in the form of ions
147
Different from nonelectrolyte ndash NO ions at all Description of electrolyte as strong or weak not related to concentration of solution Electrolytes differ in degree of ionization not amount of solute dissolved
148
149
Colligative properties of solutions
Presence of solute affects properties of solutions Some properties not dependent on nature of dissolved substance but on how may dissolve particles are present
Colligative properties - properties that depend on the concentration of solute particles but not on their identity
150
Vapor-Pressure Lowering
Boiling and freezing point of solution different from pure solvent Nonvolatile solute raises boiling point and lowers freezing point Nonvolatile substance - one that has little tendency to become gas under existing conditions
151
152
To understand why nonvolatile solute changes boiling and freezing point consider equilibrium vapor pressure
Vapor pressure pressure caused by molecules that have escape liquid phase to gas phase
Can be thought of a measure of tendency of molecules to escape from a liquid
153
Addition of sucrose (nonvolatile solute) lowers concentration of water molecules at surface of liquid This lowers tendency of water molecules to leave solution and enter gas phase Vapor pressure of solution is lower than vapor pressure of pure water
154
Aqueous13 solution13 of13 nonvolatile13 solutePure13 water
Nonelectrolyte solutions of same molality have same concentration of particles
Dilute solutions of same solvent and equal molality of any nonelectrolyte solute lower vapor pressure equally
155
Example
1 m aqueous solution of nonelectrolyte glucose lowers vapor pressure of water 55 x 10-4 atm at 25 1 m aqueous solution of sucrose also lowers water vapor pressure to 55 x 10-4 atm at 25
156
157
bc vapor-pressure lowering depends on concentration of nonelectrolyte solute and doesnrsquot depend on the type of solute it is a colligative proprerty
bc vapor pressure has been lowered solution remains liquid over larger temperature range
Lowers freezing point and raises boiling point
158
Can assume changes in boiling and freezing point also depend on concentration of solute
They are colligative properties
159
Freezing-Point Depression
Freezing point of 1-molal solution of any nonelectrolyte solute in water is found (by experiment) to be 186 lower than freezing point of water When 1 mol of nonelectrolyte solute dissolved in 1 kg water freezing point is -186 not 000 160
When 2 mol nonelectrolyte solute dissolved in 1 kg water freezing point is -372
For any concentration of nonelectrolyte solute in water decrease in freezing point can be determined using value of -186m
161
This value called the molal freezing-point contstant (Kf) is the freezing-point depression of the solvent in a 1-molal solution of a nonvolatile nonelectrolyte solute
162
Each solvent has own characteristic molal freezing-point constant Freezing-point depression (Δtf) the difference between the freezing points of the pure solvent and a solute of nonelectrolyte in that solvent and it is directly proportional to the molal concentration of the solution Δtf = Kfm
163
Δtf = Kfm
Kf expressed as m
m expressed in mol solutekg solvent (molality)
164
Sample Problem
What is the freezing point depression of water in a solution of 171 g of sucrose C12H22O11 and 200 g of water What is the actual freezing point of the solution
165
1 AnalyzeGiven Solute mass and chemical formula 171 g C12H22O11 Solvent mass and identity 2000 g water Unknown a freezing-point depression b freezing point of the solution 166
2 Plan
167
168
Calculate
Unknown a freezing-point depression b freezing point of the solution
169
Practice Problem
A water solution containing an unknown quantity of a nonelectrolyte solute is found to have a freezing point of ndash 023degCWhat is the molal concentration of the solution
012 m170
A solution consists of 103 g of the nonelectrolyte glucose C6H12O6 dissolved in 250 g of water What is the freezing-point depression of the solution
minus0426degC171
In a laboratory experiment the freezing point of an aqueous solution of glucose is found to be minus0325degCWhat is the molal concentration of this solution
0175 m172
If 0500 mol of a nonelectrolyte solute are dissolved in 5000 g of ether what is the freezing point of the solution
minus1181degC173
The freezing point of an aqueous solution that contains a nonelectrolyte is minus90degC aWhat is the freezing-point depression of the solution bWhat is the molal concentration of the solution a minus90degC b 48 m
174
Boiling-Point Elevation
Boiling point is temp at which vapor pressure is equal to atmospheric pressure Change in vapor pressure will cause corresponding change in boiling point
175
176
Vapor pressure of nonvolatile solution is lower than vapor pressure of pure solvent More heat will be required to raise vapor pressure of solution Boiling point of solution is higher than boiling point of pure solvent
Molal boiling-point constant (Kb) - boiling-point elevation of the solvent in a 1-molal solution of a nonvolatile nonelectrolyte solute b-p elevation of any nonelectrolyte in water found by experiment to be 051 So molal b-p constant for water is 051m
177
For different solvents b-p elevations of 1-molal solutions have different values (Table 14-2)
Like freezing-point constants values most accurate for dilute solutions
178
Boiling-point elevation Δtb difference between the boiling points of the pure solvent and a nonelectrolyte solution of that solvent and it is directly proportional to the molal concentration of the solution B-p elevation can be calculated using Δtb = Kbm Δtb (boiling point elevation) expressed in m and m expressed in mol solutekg solvent
Sample Problem
What is the boiling-point elevation of a solution made from 200 g of a nonelectrolyte solute and 4000 g of water The molar mass of the solute is 620 gmol
180
1 AnalyzeGiven solute mass = 200 g solute molar mass = 620 gmol solvent mass and identity = 4000 g of water
Unknown boiling-point elevation
181
2 Plan
182
183
3 Compute
Practice Problem
A solution contains 500 g of sucrose C12H22O11 a nonelectrolyte dissolved in 5000 g of water What is the boiling-point elevation
015degC184
A solution contains 4500 g of sucrose C12H22O11 a nonelectrolyte dissolved in 250 g of waterWhat is the boiling point of the solution
1027degC185
If the boiling point elevation of an aqueous solution containing a nonvolatile electrolyte is 102degC what is the molality of the solution
20 m186
The boiling point of an aqueous solution containing a nonvolatile electrolyte is 10075degC aWhat is the boiling-point elevation bWhat is the molality of the solution a 075degC b 15 m
187
Electrolytes and Colligative PropertiesEarly investigators confused by experiments where certain substances depressed freezing point or elevated boiling point more than expected
188
189
Ex 01 m solution of NaCl lowers freezing point of solvent almost twice as much as 01 m solution of sucrose 01 m CaCl2 lowers fp almost 3 times as much as 01 m solution of sucrose
To understand why this happens contrast behavior or sucrose with NaCl in aqueous solution Each sucrose dissolves to make only 1 particle in solution So 1 mol sucrose dissolves to 1 mol particles 1 mol NaCl dissolves to make 2 moles of particles (1 mol Na+ and 1 mol Cl-)
190
191
Calculated Values for Electrolyte SolutionsColligative properties depend on total concentration of solute particles regardless of their identity Electrolytes cause changes in colligative properties proportional to total molality in terms of all dissolves particles instead of formula units
192
What about barium nitrate Ba(NO3)2 Each mole of barium nitrate yields 3 mol of ions in solution Ba(NO3)2(s) rarr Ba2+(aq) + 2NO3-(aq) You would expect solution of given molality to lower fp of solvent 3 times as much as nonelectrolytic solution of same molality
193
Sample Problem
What is the expected change in the freezing point of water in a solution of 625 g of barium nitrate Ba(NO3)2 in 100 kg of water
194
1 AnalyzeGiven solute mass and formula = 625 g Ba(NO3)2 solvent mass and identity = 100 kg water Δtf = Kfm
Unknown expected freezing-point depression
195
2 PlanThe molality can be calculated by converting the solute mass to moles and then dividing by the number of kilograms of solvent That molality is in terms of formula units of Ba(NO3)2 and must be converted to molality in terms of dissociated ions in solution It must be multiplied by the number of moles of ions produced per mole of formula unit This adjusted molality can then be used to calculate the freezing-point depression
196
197
198
3 Compute
Practice Problem
What is the expected freezing-point depression for a solution that contains 20 mol of magnesium sulfate dissolved in 10 kg of water
minus74degC199
What is the expected boiling-point elevation of water for a solution that contains 150 g of sodium chloride dissolved in 10 kg of water 27degC The freezing point of an aqueous sodium chloride solution is minus200degC What is the molality of the solution 54 m NaCl
200
Actual Values for Electrolyte SolutionsRemember values calculated are only expected values Actual values of colligative properties for all strong electrolytes are almost what would be expected Difference between expected and actual values caused by attractive forces that exist between dissociated ions in aqueous solution
201
Attraction between hydrated ions in solution is small compared with those in crystalline solid However forces of attraction donrsquot interfere with movements of aqueous ions More concentrated solution means ions closer together and attraction is greater Also depends on charge of ions ndash higher charge greater attraction
202
Solutions
NaCl and water form homogeneous mixture Mixture is even throughout
4
Sugar is soluble in water Soluble - ability to be dissolved What happens as sugar dissolves in water
5
Lump of sugar disappears as sugar molecules leave surface of their crystals and mix with water molecules Eventually all sugar molecules are evenly spread through water molecules All visible traces of sugar are gone It becomes solution - homogeneous mixture of two or more substances in a single phase
6
Components of Solutions
Particles of one substance randomly mixed with particles of another Solvent - dissolving medium in a solution Solute - substance dissolved in solution
7
Solute usually less than solvent Dissolved solute particles cannot be seen Stay mixed with solvent forever so long as conditions stay the same Solute-particles from 001-1 nm in diameter
8
Types of Solutions
Solutions can exist as gases liquids or solids
9
Many alloys such as brass (zinc and copper) are sterling silver (silver and copper) are solid solutions Atoms of two or metals are evenly mixed By properly choosing percentages of each metal can get many desirable properties
10
Ex Alloys have higher strength and greater resistance to corrosion than pure metals Pure gold (24K) too soft for jewelry Alloy with silver (14K) increases strength and hardness but keeps appearance of gold
11
SuspensionsSuspension - if particles in solvent are so large they settle out unless mixture constantly stirred Think of sand and water Particles over 1000 nm in diameter
12
Particles that are medium in size between those in solutions and suspensions from mixtures are known as colloids (colloidal dispersions) Particles between 1-1000 nm diameter
13
Colloids
In muddy water large soil particles settle Water still cloudy bc colloidal particles stay dispersed in water Filter Colloidal particles pass through and stays cloudy
14
Particles in colloid small enough to be suspended in solvent by constant movement of surrounding molecules Colloidal particles make up dispersed phase Water is dispersing medium
15
Emulsion and foam are colloids
Mayonnaise is emulsion of oil droplets in water Egg yolk acts as emulsifying agent (keeps oil droplets dispersed)
16
17
18
Class13 of13 Colloid Phases Example
Sol Solid13 dispersed13 in13 liquid Paints13 mud
Gel Solid13 network13 extending13 through13 liquid
Gelatin
Liquid13 emulsion Liquid13 dispersed13 in13 liquid Milk13 mayonnaise
Foam Gas13 dispersed13 in13 liquid Shaving13 cream13 whipped13 cream
Solid13 aerosol Solid13 dispersed13 in13 gas Smoke13 auto13 exhaust
Liquid13 aerosol Liquid13 dispersed13 in13 gas Fog13 mist13 clouds13 aerosol13 spray
Solid13 emulsion Liquid13 dispersed13 in13 solid Cheese13 butter
Tyndall EffectMany colloids appear homogeneous bc individual particles cannot be seen Particles large enough to scatter light In fog can see headlight beam Tyndall effect - occurs when light is scattered by colloidal particles dispersed in transparent medium
19
20
21
Solutions Colloids Suspensions
Homogeneous Heterogeneous Heterogeneous
Particle13 size13 001-shy‐113 nm13 can13 be13 atoms13 ions13 molecules
Particle13 size13 1-shy‐100013 nm13 dispersed13 can13 be13 combined13 or13 large13 molecules
Particle13 size13 over13 100013 nm13 suspended13 can13 be13 large13 particles13 or13 combined13 particles
Do13 not13 separate13 on13 standing Do13 not13 separate13 on13 standing Particles13 settle13 out
Cannot13 be13 separated13 by13 filtration Cannot13 be13 separated13 by13 filtration Can13 be13 separated13 by13 filtration
Do13 not13 scatter13 light Scatter13 light13 (Tyndall13 effect) May13 scatter13 light13 but13 are13 not13 transparent
Electrolytes vs Nonelectrolytes
Substances that dissolve in water are classified according to whether they produce molecules or ions in solution
22
23
When ionic compound dissolves cations and anions separate and are surrounded by water molecules Solute ions free to move making possible to carry electric current
Electrolyte - substance that dissolves in water to give a solution that conducts electric current NaCl ndash electrolyte Usually highly polar compounds become electrolytes
24
Solution containing neutral solute molecules does not conduct current bc it doesnrsquot have mobile charged particles
Nonelectrolyte - substance that dissolves in water to give solution that does not conduct electric current Ex Sugar
25
26
27the solution
process
Factors Affecting Rate of DissolutionEver tried to dissolve sugar in iced tea or coffee You notice temperature has something to do with how quickly solute dissolves What other factors affect how quickly solutes dissolve
28
1 Increasing Surface Area of Solute
bc dissolution process occurs at surface of solute Can increase dissolution by increasing surface area
29
2 Agitating (Stirring) SolutionClose to surface of solute concentration of dissolved solute is high Stirringshaking helps disperse solute particles
30
Brings fresh solvent into contact with solute surface Contact between solvent and solute surface increased
3 Heating a Solvent
Solutes dissolve faster in warmer solvent As temp increases solvent molecules move faster Average kinetic energy increases Collisions between solvent and solute more frequent Helps separate solute molecules and spread them out
32
Solubility
If you add sugar to tea eventually no more sugar dissolves
For every combination of solvent with solute at given temp there is limit to amount of solute that can be dissolved
33
When sugar first dropped into water sugar molecules leave solid surface and move about at random in solvent Some dissolved molecules may collide with crystal and stay there As more solid dissolves and concentration of dissolved molecules increases collisions happen more often
34
Eventually molecules are returning to crystal at same rate they are going into solution Dynamic equilibrium established between dissolution and crystallization Solution equilibrium - physical state in which the opposing processes of dissolution and crystallization of a solute occur at equal rates
35
Saturated vs Unsaturated Solutions
Saturated solution - solution that contains the maximum amount of dissolved solute How do you know if solution is saturated If more NaCl added it falls to bottom and doesnrsquot dissolve Equilibrium already established
36
If more water added to saturated solution more NaCl will dissolve At 20 359 g NaCl is max that will dissolve in 100 g water Unsaturated solution - solution that contains less solute than a saturated solution under the existing conditions
37
Supersaturated Solutions
When saturated solution where solubility increases as temp increases is cooled excess solute usually comes out of solution Sometimes if solution is left to cool the excess solute doesnrsquot separate
38
39
Supersaturated solution - solution that contains more dissolved solute than a saturated solution contains under same conditions
40
Supersaturated solution can remain unchanged over long time if it isnrsquot disturbed Once crystals begin to form continues until equilibrium is recreated at lower temp
41
Example
Sodium thiosulfate Na2S2O3 Solute added to hot water until solution saturated Hot solution filtered Drop small crystal in ldquoseedingrdquo crystallization Continues until equilibrium created
42
Solubility Values
Solubility - amount of substance required to form a saturated solution with specific amount of solvent at specified temp Ex Solubility of sugar = 204 g per 100 g water at 20 Must specify temp bc solubility changes with temp 43
For gases pressure must also be specified Rate at which substance dissolves is unrelated to solubility Max amount of solute that dissolves and reaches equilibrium is always same under same conditions
44
Solute-Solvent Interactions
ldquolike dissolves likerdquo is generally useful rule for predicting whether one substance will dissolve in another Depends on type of bonding polaritynonpolarity and intermolecular forces between solute and solvent
45
Dissolving Ionic Compounds in Aqueous Solution
Polarity of water molecules plays important role in formation of solution of ionic compounds Charged ends of water molecules attract ions and surround them to keep separated from other ions
46
Hydration - solution process with water as solvent Ions said to be hydrated As hydrated ions diffuse into solution other ions exposed and drawn away from crystal surface Entire crystal gradually dissolves
47
When crystallized from aqueous solutions some ionic compounds form crystals that include water molecules The crystalline compounds known as hydrates have specific ratios of water molecules Represented by formulas like CuSO45H2O
48
When crystalline hydrate dissolves in water water of hydration returns to solvent Behavior of solute in hydrated form no different than anhydrous form
49
Nonpolar Solvents
Nonpolar solvents do not attract ions of ionic compound strongly enough to overcome forces holding crystal together
50
Liquid Solutes and Solvents
When you shake bottle of salad dressing oil droplets disperse in water Stop shaking ndash strong attraction between water molecules squeeze out oil and form separate layers Liquid solutes and solvents that are not soluble in each other are immiscible
51
Nonpolar substances (fat oil grease) are generally quite soluble in nonpolar liquids (carbon tetrachloride toluene gasoline)
Only attractions between nonpolar molecules are weak London forces
52
London Dispersion Forces
bc e- are constantly moving at any time there may be uneven distribution of them around a nonpolar atom Temporarily creates positive end and negative end
53
54
London dispersion forces - intermolecular attractions resulting from constant motion of e- and creation of instantaneous dipoles
55
Liquids that dissolve freely in one another in any ratio are said to be completely miscible Benzene and carbon tetrachloride (both nonpolar) are completely miscible
Nonpolar molecules of these two apply no strong forces of attraction or repulsion and molecules mix freely
56
Ethanol and water are also completely miscible -OH group on ethanol is slightly polar
Can form H-bonds with water as well as other ethanol molecules Intermolecular forces in mixture are very similar to those in pure liquids so they are mutually soluble in any amount
57
Gasoline contains mainly nonpolar hydrocarbons
Excellent solvent for fats oils greases
Major intermolecular forces are weak London forces
58
Ethanol less polar than water more than carbon tetrachloride
Better solvent for less-polar substances bc of nonpolar region
59
Effects of Pressure on Solubility
Little effect on solids or liquids Increase pressure - increase solubilities of gases in liquids
60
When gas in contact with surface of liquid gas molecules can enter liquid As amount of dissolve gas increases some molecules start to escape and reenter gas phase
Eq eventually established between rates of enteringleaving gas phase
61
As long as eq undisturbed solubility of gas in liquid unchanged at given pressure
Gas + solvent solution
62
Increasing pressure of solute gas above solution puts stress on eq Molecules collide with surface more often Increase in pressure partially offset by increase in rate of gas molecules entering solution In turn increase in amount of dissolved gas causes increase in rate molecules escape liquid and becomes gas
63
Eventually eq restored at higher gas solubility
Expected from Le Chatelierrsquos principle
Increase in gas pressure causes eq to shift so fewer molecules are in gas phase
64
Henryrsquos Law
Henryrsquos law - solubility of a gas in liquid is directly proportional to the partial pressure of that gas on the surface of the liquid
Applies to gas-liquid solutions at constant temp
65
When mixture of ideal gases is limited in constant volume at constant temp each gas applies same pressure it would apply if it occupied the space alone
Assuming gases do not react each gas dissolves to the extent it would if no other gases were there
66
In carbonated drinks (Coke Pepsi etc) solubility of CO2 increased by increasing pressure
At bottling factory CO2 gas forced into solution of flavored water at pressure of 5-10 atm Gas-in-liquid solution then sealed in bottlescans
67
When cap removed pressure reduced to 1 atm and some CO2 escapes as gas bubbles Effervescence - rapid escape of a gas from a liquid in which it is dissolved
68
Effects of Temperature on Solubility
Letrsquos consider gas solubility Increasing temperature usually decreases gas solubility As temp increases average kinetic energy of molecules in solution increases Greater number of solute molecules escape from attraction of solvent and return to gas phase 69
At higher temps eq is reached with fewer gas molecules in solution Gases generally less soluble
70
Effect of temp on solubility of solids in liquids more difficult to predict
Often increasing temp increases solubility
However the same temp increase can result in large increase in solubility in one case and only slight increase in the next
71
72
enthalpies of Solution
Formation of solution comes with energy change Dissolve KI in water container feels cold Dissolve LiCl in water feels hot Formation of solid-liquid solution can absorb or release heat
73
During formation of solution solvent and solute particles experience changes in forces attracting them to other particles Before dissolving begins solvent molecules held by intermolecular forces (solvent-solvent attraction) In solute molecules held by Ifs (solute-solute attraction)
74
Energy is required to separate solute molecules and solvent molecules from their neighbors
A solute particle that is surrounded by solvent molecules is said to be solvated
75
Step 1 solute particles become separated from solid (energy absorbed) Step 2 solvent particles move apart to allow solute particles to enter liquid (energy absorbed)
76
Step 3 solvent particles attracted to and solvate solute particles (energy released)
77
Heat of solution - net amount of heat energy absorbed or released when a specific amount of solute dissolves in a solvent
Heat of solution negative (heat is released) if sums of energy in steps 1 and 2 is less than step 3
If 1+2 gt 3 HOS is positive (heat absorbed)78
In gaseous state molecules are so far apart there are nearly no forces between them Solute-solute interaction has little effect on heat of solution of a gas Energy released when gas dissolved in liquid bc attraction between solute gas and solvent molecules greater than energy needed to separate solvent molecules
79
Concentration of Solutions80
Concentration - measure of amount of solute in given amount of solvent or solution ldquodiluterdquo just means small amount of solute ldquoconcentratedrdquo just means relatively large amount of solute Unrelated to degree to which solution is saturated Ex Saturated solution of substance not very soluble might be dilute
81
Molarity
Molarity - number of moles of solute in one liter of solution To find molarity must know molar mass A ldquoone molarrdquo solution of NaOH is 1 mole NaOH per liter of solution (1 M)
82
1 mol NaOH = 400 g If this quantity dissolve in enough water to make EXACTLY 100 L solution solution is 1 M If 200 g NaOH dissolve in enough to make 100 L solution what is molarity 0500 M
83
1 M solution is not made by adding 1 mol solute to 1 L solvent That would be more than 1 L Instead 1 mol solute dissolved in less than 1 L Then diluted with more solvent to bring TOTAL VOLUME to 1 L
84
Practice ProblemYou have 340 L of solution that contains 900 g sodium chloride What is the molarity of that solution 0440 M NaCl You have 08 L of a 05 M HCl solution How many moles of HCl does this solution contain 04 mol HCl
85
What is the molarity of a solution composed of 585 g of potassium iodide dissolved in enough water to make a 0125 L of solution 0282 M KI
86
How many moles of H2SO4 are present in 0500 L of a 0150 M H2SO4 solution 00750 mol What volume of 300 M NaCl is needed for reaction that requires 1463 g of NaCl 0834 L
87
Molality
Molality - concentration of solution expressed in moles of solute per kilogram of solvent Solution that contains 1 mol solute (NaOH) dissolved in exactly 1 kg of solvent is ldquoone molalrdquo solution (1 m NaOH)
88
1 mol NaOH = 400 g 400 g NaOH dissolved in 1 kg water is 1 m NaOH
200 g NaOH in 1 kg water = 0500 m NaOH
89
Practice Problem
A solution was prepared by dissolving 171 g sucrose (C12H22O11) in 125 g of water Find the molal concentration of this solution 0400 m C12H22O11
90
A solution of iodine in carbon tetrachloride is used when iodine is needed for certain chemical tests How much iodine must be added to prepare a 0480 m solution of iodine in CCl4 if 1000 g of CCl4 is used 122 g I2
91
What is the molality of a solution composed of 255 g acetone (CH3)2CO dissolved in 200 g of water 22 m acetone What quantity in grams of methanol CH3OH is required to prepare a 0244 m solution in 400 g water 312 g methanol
92
How many grams of AgNO3 are needed to prepare a 0125 m solution in 250 mL of water 531 g AgNO3
What is the molality of a solution containing 182 g HCl and 250 g water 200 m
93
Ions in Aqueous Solutions and Colligative Properties
94
95
compounds in aqueous solutions
DissociationWhen compound made from ions dissolves in water ions separate Dissociation - separation of ions that occurs when an ionic compound dissolves
96
Notice number of ions made per formula unit in equations 1 formula unit of NaCl gives 2 units of ions in solution 1 formula unit of CaCl2 gives 3 units of ions
97
Assuming 100 dissociation solution that contains 1 mol NaCl contains 1 mol Na+ and 1 mol Cl- Can assume 100 dissociation of all soluble ionic compounds
98
Practice Problem
Write the equation for the dissolution of aluminum sulfate in water How many moles of aluminum ions are made by dissolving 1 mol aluminum sulfate What is the total number of moles of ions made by dissolving 1 mol of aluminum sulfate
99
1 AnalyzeGiven Amount of solute = 1 mol Al2(SO4)3 Solvent identity = water Unknown a moles of aluminum ions and sulfate ions Total number of moles of solute ions produced
100
2 Plan
The coefficients in the balanced dissociation equation will tell mole relationships You can use the equation to find out the number of moles of solute ions produced
101
3 Compute
a
b102
Write the equation for the dissolution of each of the following in water and then determine the number of moles of each ions made as well as the total number of ions made a 1 mol ammonium chloride b 1 mol sodium sulfide c 05 mol barium nitrate
103
a 1 mol ammonium chloride
1 mol NH4+ 1 mol Cl- 2 mol total ions
104
b 1 mol sodium sulfide
2 mol Na+ 1 mol S-2 3 mol total ions
105
c 05 mol barium nitrate
05 mol Ba+2 1 mol NO3- 15 mol total ions
106
Precipitation Reactions
Even though no compound is completely insoluble compounds of very low solubility can be considered insoluble for practical purposes There are general guidelines to help predict whether a compound made of certain combination of ions is soluble
107
General Solubility Guidelines
Sodium potassium and ammonium compounds are soluble in water
108
Nitrates acetates and chlorates are soluble
109
110
Most chlorides are soluble except those of silver mercury (I) and lead Lead (II) chloride is soluble in hot water
Most sulfates are soluble except those of barium strontium lead calcium and mercury
111
Most carbonates phosphates and silicates are insoluble except those of sodium potassium and ammonium
112
Most sulfides are insoluble except those of calcium strontium sodium potassium and ammonium
113
Is calcium phosphate Ca3(PO4)2 soluble or insoluble
Not one of the exceptions so it is insoluble
Dissociation equations are not written for insoluble compounds
114
Guidelines also useful in predicting what will happen if 2 solutions of 2 different soluble compounds are mixed
If mixing results in combination of ions that forms insoluble compound a double-replacement and precipitation reaction will happen
115
Will precipitate form when solutions of ammonium sulfide and cadmium nitrate are combined You can tell calcium nitrate is soluble from the guidelines (Most nitrates acetates and chlorates are soluble) You can also tell ammonium sulfide is soluble (Most sulfides are insoluble except those of calcium strontium sodium potassium and ammonium)
116
117
To decide whether precipitate can form must know solubilities of two possible products
Ammonium nitrate ndash soluble (Most nitrates acetates and chlorates are soluble) Cadmium sulfide ndash insoluble (Most sulfides are insoluble except those of calcium strontium sodium potassium and ammonium)
118
bc one product is insoluble double-replacement and precipitation reaction will happen (NH4)2S (aq) + Cd(NO3)2 (aq) rarr NH4NO3 (aq) + CdS (s)
119
Net Ionic Equations
Reactions of ions in aqueous solution usually represented by net ionic equations instead of formula equations Net ionic equation - includes only those compounds and ions that undergo a chemical change in a reaction in an aqueous solution
120
121
To write NIE (net ionic equation) first convert chemical equation into overall ionic equation All soluble ionic compounds shown as dissociated ions in solution Precipitates shown as solids
122
Notice ammonium and nitrate ions appear on both sides They havenrsquot gone through any chemical change Spectator ions - ions that do not take part in a chemical reaction and are found in solution both before and after the reaction
123
To change ionic equation to NIE eliminate spectator ions Cd+2(aq) + S-2(aq) rarr CdS(s)
This NIE applies to any reaction in which precipitate of CdS forms
124
Practice Problem
Identify the precipitate that forms when aqueous solutions of zinc nitrate and ammonium sulfide are combined Write the equation for the possible double-replacement reaction Then write the formula equation overall ionic equation and the net ionic equation
125
1 AnalyzeGiven Identity of reactants zinc nitrate and ammonium sulfide Reaction medium aqueous solution Unknown a equation for the possible double-replacement reaction b identity of the precipitate c formula equation d overall ionic equation e net ionic equation
126
2 PlanWrite the possible double-replacement reaction between Zn(NO3)2 and (NH4)2S Use the rules to determine if any of the products will precipitate Write a formula equation and overall net ionic equation Then cancel spectator ions to make net ionic equation
127
3 Computea possible double-replacement reaction
b rules show that zinc sulfide is not soluble so will precipitate (ammonium nitrate is soluble)
c formula equation128
d overall ionic equation
e net ionic equation129
Will a precipitate form if solutions of potassium sulfate and barium nitrate are combined If so write the net ionic equation for the reaction
Yes Ba+2(aq) + SO4-2(aq) rarr BaSO4(s)
130
Will a precipitate form if solutions of potassium nitrate and magnesium sulfate are combined If so write the net ionic equation for the reaction
No 131
Will a precipitate form if solutions of barium chloride and sodium sulfate are combined If so identify the spectator ions and write the net ionic equation for the reaction
Yes Na+ and Cl- Ba+2(aq) + SO4-2(aq) rarr BaSO4(s)
132
Write the net ionic equation for the precipitation of nickel(II) sulfide
Ni+2(aq) + S-2(aq) rarr NiS(s)
133
IonizationSome molecular compounds can also form ions in solution Usually polar Ionization - ions formed from solute molecules by action of the solvent (creation of ions where there were none) Ionization different from dissociation
134
Dissociation ionic compounds dissolve and ions already present separate Ionization ions formed where non existed before
Ions formed are hydrated Heat released during hydration of ions gives enough energy to break covalent bonds
135
Extent to which solute ionizes depends on strength of bonds within molecules of solute and strength of attraction between solute and solvent molecules
136
HCl ndash molecular compound that ionizes in aqueous solution Contains highly polar bond Attraction between polar HCl molecule and polar water strong enough to break HCl bond
137
The Hydronium Ion
H+ ions from HCl attracts other molecules or ions so strongly that it doesnrsquot normally exist alone Ionization of HCl better described as direct transfer of proton from HCl to H2O forming H3O+
138
139
Hydration of H+ to form hydronium is highly exothermic
Energy released gives a lot of energy needed to ionize a molecular solute
Many molecular compounds that ionize in aqueous solution contain hydrogen and form hydronium ion
140
Strong and Weak Electrolytes
Substances that make ions and conduct current in solution are electrolytes HCl is one of series of compounds that has hydrogen and halogen Hydrogen halides are all molecular compounds with single polar-covalent bonds
141
All are gases Very soluble in water All are electrolytes HCl HBr HI strongly conduct current in solution HF only weakly conducts current at same concentration Strength of conduction related to ability to ionize
142
143
Strong Electrolytes
HCl HBr HI are 100 ionized in solution
Strong electrolyte - any compound whose dilute aqueous solutions conduct electricity well this is due to the presence of all or almost all of the dissolve compound in the form of ions
144
Unique characteristic of strong electrolyte is that they yield only ions
Ex Ionic compound may be highly soluble in water and dissociate into ions (NaCl)
Others may not dissolve much but amount that does dissolve exists only as ions
145
Weak ElectrolytesHF dissolves in water to give acid solution called hydrofluoric acid HF bond stronger than bonds between hydrogen and other halogens When HF dissolves some molecule ionize Reverse reaction also happens HF(aq) + H2O(l) H3O+(aq) + F-(aq)
146
HF(aq) + H2O(l) H3O+
(aq) + F-(aq)Concentration of dissolved unionized HF stays high and concentration of ions stays low
Weak electrolyte - any compound whose aqueous solutions conduct electricity poorly this is due to the presence of small amount of the dissolved compound in the form of ions
147
Different from nonelectrolyte ndash NO ions at all Description of electrolyte as strong or weak not related to concentration of solution Electrolytes differ in degree of ionization not amount of solute dissolved
148
149
Colligative properties of solutions
Presence of solute affects properties of solutions Some properties not dependent on nature of dissolved substance but on how may dissolve particles are present
Colligative properties - properties that depend on the concentration of solute particles but not on their identity
150
Vapor-Pressure Lowering
Boiling and freezing point of solution different from pure solvent Nonvolatile solute raises boiling point and lowers freezing point Nonvolatile substance - one that has little tendency to become gas under existing conditions
151
152
To understand why nonvolatile solute changes boiling and freezing point consider equilibrium vapor pressure
Vapor pressure pressure caused by molecules that have escape liquid phase to gas phase
Can be thought of a measure of tendency of molecules to escape from a liquid
153
Addition of sucrose (nonvolatile solute) lowers concentration of water molecules at surface of liquid This lowers tendency of water molecules to leave solution and enter gas phase Vapor pressure of solution is lower than vapor pressure of pure water
154
Aqueous13 solution13 of13 nonvolatile13 solutePure13 water
Nonelectrolyte solutions of same molality have same concentration of particles
Dilute solutions of same solvent and equal molality of any nonelectrolyte solute lower vapor pressure equally
155
Example
1 m aqueous solution of nonelectrolyte glucose lowers vapor pressure of water 55 x 10-4 atm at 25 1 m aqueous solution of sucrose also lowers water vapor pressure to 55 x 10-4 atm at 25
156
157
bc vapor-pressure lowering depends on concentration of nonelectrolyte solute and doesnrsquot depend on the type of solute it is a colligative proprerty
bc vapor pressure has been lowered solution remains liquid over larger temperature range
Lowers freezing point and raises boiling point
158
Can assume changes in boiling and freezing point also depend on concentration of solute
They are colligative properties
159
Freezing-Point Depression
Freezing point of 1-molal solution of any nonelectrolyte solute in water is found (by experiment) to be 186 lower than freezing point of water When 1 mol of nonelectrolyte solute dissolved in 1 kg water freezing point is -186 not 000 160
When 2 mol nonelectrolyte solute dissolved in 1 kg water freezing point is -372
For any concentration of nonelectrolyte solute in water decrease in freezing point can be determined using value of -186m
161
This value called the molal freezing-point contstant (Kf) is the freezing-point depression of the solvent in a 1-molal solution of a nonvolatile nonelectrolyte solute
162
Each solvent has own characteristic molal freezing-point constant Freezing-point depression (Δtf) the difference between the freezing points of the pure solvent and a solute of nonelectrolyte in that solvent and it is directly proportional to the molal concentration of the solution Δtf = Kfm
163
Δtf = Kfm
Kf expressed as m
m expressed in mol solutekg solvent (molality)
164
Sample Problem
What is the freezing point depression of water in a solution of 171 g of sucrose C12H22O11 and 200 g of water What is the actual freezing point of the solution
165
1 AnalyzeGiven Solute mass and chemical formula 171 g C12H22O11 Solvent mass and identity 2000 g water Unknown a freezing-point depression b freezing point of the solution 166
2 Plan
167
168
Calculate
Unknown a freezing-point depression b freezing point of the solution
169
Practice Problem
A water solution containing an unknown quantity of a nonelectrolyte solute is found to have a freezing point of ndash 023degCWhat is the molal concentration of the solution
012 m170
A solution consists of 103 g of the nonelectrolyte glucose C6H12O6 dissolved in 250 g of water What is the freezing-point depression of the solution
minus0426degC171
In a laboratory experiment the freezing point of an aqueous solution of glucose is found to be minus0325degCWhat is the molal concentration of this solution
0175 m172
If 0500 mol of a nonelectrolyte solute are dissolved in 5000 g of ether what is the freezing point of the solution
minus1181degC173
The freezing point of an aqueous solution that contains a nonelectrolyte is minus90degC aWhat is the freezing-point depression of the solution bWhat is the molal concentration of the solution a minus90degC b 48 m
174
Boiling-Point Elevation
Boiling point is temp at which vapor pressure is equal to atmospheric pressure Change in vapor pressure will cause corresponding change in boiling point
175
176
Vapor pressure of nonvolatile solution is lower than vapor pressure of pure solvent More heat will be required to raise vapor pressure of solution Boiling point of solution is higher than boiling point of pure solvent
Molal boiling-point constant (Kb) - boiling-point elevation of the solvent in a 1-molal solution of a nonvolatile nonelectrolyte solute b-p elevation of any nonelectrolyte in water found by experiment to be 051 So molal b-p constant for water is 051m
177
For different solvents b-p elevations of 1-molal solutions have different values (Table 14-2)
Like freezing-point constants values most accurate for dilute solutions
178
Boiling-point elevation Δtb difference between the boiling points of the pure solvent and a nonelectrolyte solution of that solvent and it is directly proportional to the molal concentration of the solution B-p elevation can be calculated using Δtb = Kbm Δtb (boiling point elevation) expressed in m and m expressed in mol solutekg solvent
Sample Problem
What is the boiling-point elevation of a solution made from 200 g of a nonelectrolyte solute and 4000 g of water The molar mass of the solute is 620 gmol
180
1 AnalyzeGiven solute mass = 200 g solute molar mass = 620 gmol solvent mass and identity = 4000 g of water
Unknown boiling-point elevation
181
2 Plan
182
183
3 Compute
Practice Problem
A solution contains 500 g of sucrose C12H22O11 a nonelectrolyte dissolved in 5000 g of water What is the boiling-point elevation
015degC184
A solution contains 4500 g of sucrose C12H22O11 a nonelectrolyte dissolved in 250 g of waterWhat is the boiling point of the solution
1027degC185
If the boiling point elevation of an aqueous solution containing a nonvolatile electrolyte is 102degC what is the molality of the solution
20 m186
The boiling point of an aqueous solution containing a nonvolatile electrolyte is 10075degC aWhat is the boiling-point elevation bWhat is the molality of the solution a 075degC b 15 m
187
Electrolytes and Colligative PropertiesEarly investigators confused by experiments where certain substances depressed freezing point or elevated boiling point more than expected
188
189
Ex 01 m solution of NaCl lowers freezing point of solvent almost twice as much as 01 m solution of sucrose 01 m CaCl2 lowers fp almost 3 times as much as 01 m solution of sucrose
To understand why this happens contrast behavior or sucrose with NaCl in aqueous solution Each sucrose dissolves to make only 1 particle in solution So 1 mol sucrose dissolves to 1 mol particles 1 mol NaCl dissolves to make 2 moles of particles (1 mol Na+ and 1 mol Cl-)
190
191
Calculated Values for Electrolyte SolutionsColligative properties depend on total concentration of solute particles regardless of their identity Electrolytes cause changes in colligative properties proportional to total molality in terms of all dissolves particles instead of formula units
192
What about barium nitrate Ba(NO3)2 Each mole of barium nitrate yields 3 mol of ions in solution Ba(NO3)2(s) rarr Ba2+(aq) + 2NO3-(aq) You would expect solution of given molality to lower fp of solvent 3 times as much as nonelectrolytic solution of same molality
193
Sample Problem
What is the expected change in the freezing point of water in a solution of 625 g of barium nitrate Ba(NO3)2 in 100 kg of water
194
1 AnalyzeGiven solute mass and formula = 625 g Ba(NO3)2 solvent mass and identity = 100 kg water Δtf = Kfm
Unknown expected freezing-point depression
195
2 PlanThe molality can be calculated by converting the solute mass to moles and then dividing by the number of kilograms of solvent That molality is in terms of formula units of Ba(NO3)2 and must be converted to molality in terms of dissociated ions in solution It must be multiplied by the number of moles of ions produced per mole of formula unit This adjusted molality can then be used to calculate the freezing-point depression
196
197
198
3 Compute
Practice Problem
What is the expected freezing-point depression for a solution that contains 20 mol of magnesium sulfate dissolved in 10 kg of water
minus74degC199
What is the expected boiling-point elevation of water for a solution that contains 150 g of sodium chloride dissolved in 10 kg of water 27degC The freezing point of an aqueous sodium chloride solution is minus200degC What is the molality of the solution 54 m NaCl
200
Actual Values for Electrolyte SolutionsRemember values calculated are only expected values Actual values of colligative properties for all strong electrolytes are almost what would be expected Difference between expected and actual values caused by attractive forces that exist between dissociated ions in aqueous solution
201
Attraction between hydrated ions in solution is small compared with those in crystalline solid However forces of attraction donrsquot interfere with movements of aqueous ions More concentrated solution means ions closer together and attraction is greater Also depends on charge of ions ndash higher charge greater attraction
202
Sugar is soluble in water Soluble - ability to be dissolved What happens as sugar dissolves in water
5
Lump of sugar disappears as sugar molecules leave surface of their crystals and mix with water molecules Eventually all sugar molecules are evenly spread through water molecules All visible traces of sugar are gone It becomes solution - homogeneous mixture of two or more substances in a single phase
6
Components of Solutions
Particles of one substance randomly mixed with particles of another Solvent - dissolving medium in a solution Solute - substance dissolved in solution
7
Solute usually less than solvent Dissolved solute particles cannot be seen Stay mixed with solvent forever so long as conditions stay the same Solute-particles from 001-1 nm in diameter
8
Types of Solutions
Solutions can exist as gases liquids or solids
9
Many alloys such as brass (zinc and copper) are sterling silver (silver and copper) are solid solutions Atoms of two or metals are evenly mixed By properly choosing percentages of each metal can get many desirable properties
10
Ex Alloys have higher strength and greater resistance to corrosion than pure metals Pure gold (24K) too soft for jewelry Alloy with silver (14K) increases strength and hardness but keeps appearance of gold
11
SuspensionsSuspension - if particles in solvent are so large they settle out unless mixture constantly stirred Think of sand and water Particles over 1000 nm in diameter
12
Particles that are medium in size between those in solutions and suspensions from mixtures are known as colloids (colloidal dispersions) Particles between 1-1000 nm diameter
13
Colloids
In muddy water large soil particles settle Water still cloudy bc colloidal particles stay dispersed in water Filter Colloidal particles pass through and stays cloudy
14
Particles in colloid small enough to be suspended in solvent by constant movement of surrounding molecules Colloidal particles make up dispersed phase Water is dispersing medium
15
Emulsion and foam are colloids
Mayonnaise is emulsion of oil droplets in water Egg yolk acts as emulsifying agent (keeps oil droplets dispersed)
16
17
18
Class13 of13 Colloid Phases Example
Sol Solid13 dispersed13 in13 liquid Paints13 mud
Gel Solid13 network13 extending13 through13 liquid
Gelatin
Liquid13 emulsion Liquid13 dispersed13 in13 liquid Milk13 mayonnaise
Foam Gas13 dispersed13 in13 liquid Shaving13 cream13 whipped13 cream
Solid13 aerosol Solid13 dispersed13 in13 gas Smoke13 auto13 exhaust
Liquid13 aerosol Liquid13 dispersed13 in13 gas Fog13 mist13 clouds13 aerosol13 spray
Solid13 emulsion Liquid13 dispersed13 in13 solid Cheese13 butter
Tyndall EffectMany colloids appear homogeneous bc individual particles cannot be seen Particles large enough to scatter light In fog can see headlight beam Tyndall effect - occurs when light is scattered by colloidal particles dispersed in transparent medium
19
20
21
Solutions Colloids Suspensions
Homogeneous Heterogeneous Heterogeneous
Particle13 size13 001-shy‐113 nm13 can13 be13 atoms13 ions13 molecules
Particle13 size13 1-shy‐100013 nm13 dispersed13 can13 be13 combined13 or13 large13 molecules
Particle13 size13 over13 100013 nm13 suspended13 can13 be13 large13 particles13 or13 combined13 particles
Do13 not13 separate13 on13 standing Do13 not13 separate13 on13 standing Particles13 settle13 out
Cannot13 be13 separated13 by13 filtration Cannot13 be13 separated13 by13 filtration Can13 be13 separated13 by13 filtration
Do13 not13 scatter13 light Scatter13 light13 (Tyndall13 effect) May13 scatter13 light13 but13 are13 not13 transparent
Electrolytes vs Nonelectrolytes
Substances that dissolve in water are classified according to whether they produce molecules or ions in solution
22
23
When ionic compound dissolves cations and anions separate and are surrounded by water molecules Solute ions free to move making possible to carry electric current
Electrolyte - substance that dissolves in water to give a solution that conducts electric current NaCl ndash electrolyte Usually highly polar compounds become electrolytes
24
Solution containing neutral solute molecules does not conduct current bc it doesnrsquot have mobile charged particles
Nonelectrolyte - substance that dissolves in water to give solution that does not conduct electric current Ex Sugar
25
26
27the solution
process
Factors Affecting Rate of DissolutionEver tried to dissolve sugar in iced tea or coffee You notice temperature has something to do with how quickly solute dissolves What other factors affect how quickly solutes dissolve
28
1 Increasing Surface Area of Solute
bc dissolution process occurs at surface of solute Can increase dissolution by increasing surface area
29
2 Agitating (Stirring) SolutionClose to surface of solute concentration of dissolved solute is high Stirringshaking helps disperse solute particles
30
Brings fresh solvent into contact with solute surface Contact between solvent and solute surface increased
3 Heating a Solvent
Solutes dissolve faster in warmer solvent As temp increases solvent molecules move faster Average kinetic energy increases Collisions between solvent and solute more frequent Helps separate solute molecules and spread them out
32
Solubility
If you add sugar to tea eventually no more sugar dissolves
For every combination of solvent with solute at given temp there is limit to amount of solute that can be dissolved
33
When sugar first dropped into water sugar molecules leave solid surface and move about at random in solvent Some dissolved molecules may collide with crystal and stay there As more solid dissolves and concentration of dissolved molecules increases collisions happen more often
34
Eventually molecules are returning to crystal at same rate they are going into solution Dynamic equilibrium established between dissolution and crystallization Solution equilibrium - physical state in which the opposing processes of dissolution and crystallization of a solute occur at equal rates
35
Saturated vs Unsaturated Solutions
Saturated solution - solution that contains the maximum amount of dissolved solute How do you know if solution is saturated If more NaCl added it falls to bottom and doesnrsquot dissolve Equilibrium already established
36
If more water added to saturated solution more NaCl will dissolve At 20 359 g NaCl is max that will dissolve in 100 g water Unsaturated solution - solution that contains less solute than a saturated solution under the existing conditions
37
Supersaturated Solutions
When saturated solution where solubility increases as temp increases is cooled excess solute usually comes out of solution Sometimes if solution is left to cool the excess solute doesnrsquot separate
38
39
Supersaturated solution - solution that contains more dissolved solute than a saturated solution contains under same conditions
40
Supersaturated solution can remain unchanged over long time if it isnrsquot disturbed Once crystals begin to form continues until equilibrium is recreated at lower temp
41
Example
Sodium thiosulfate Na2S2O3 Solute added to hot water until solution saturated Hot solution filtered Drop small crystal in ldquoseedingrdquo crystallization Continues until equilibrium created
42
Solubility Values
Solubility - amount of substance required to form a saturated solution with specific amount of solvent at specified temp Ex Solubility of sugar = 204 g per 100 g water at 20 Must specify temp bc solubility changes with temp 43
For gases pressure must also be specified Rate at which substance dissolves is unrelated to solubility Max amount of solute that dissolves and reaches equilibrium is always same under same conditions
44
Solute-Solvent Interactions
ldquolike dissolves likerdquo is generally useful rule for predicting whether one substance will dissolve in another Depends on type of bonding polaritynonpolarity and intermolecular forces between solute and solvent
45
Dissolving Ionic Compounds in Aqueous Solution
Polarity of water molecules plays important role in formation of solution of ionic compounds Charged ends of water molecules attract ions and surround them to keep separated from other ions
46
Hydration - solution process with water as solvent Ions said to be hydrated As hydrated ions diffuse into solution other ions exposed and drawn away from crystal surface Entire crystal gradually dissolves
47
When crystallized from aqueous solutions some ionic compounds form crystals that include water molecules The crystalline compounds known as hydrates have specific ratios of water molecules Represented by formulas like CuSO45H2O
48
When crystalline hydrate dissolves in water water of hydration returns to solvent Behavior of solute in hydrated form no different than anhydrous form
49
Nonpolar Solvents
Nonpolar solvents do not attract ions of ionic compound strongly enough to overcome forces holding crystal together
50
Liquid Solutes and Solvents
When you shake bottle of salad dressing oil droplets disperse in water Stop shaking ndash strong attraction between water molecules squeeze out oil and form separate layers Liquid solutes and solvents that are not soluble in each other are immiscible
51
Nonpolar substances (fat oil grease) are generally quite soluble in nonpolar liquids (carbon tetrachloride toluene gasoline)
Only attractions between nonpolar molecules are weak London forces
52
London Dispersion Forces
bc e- are constantly moving at any time there may be uneven distribution of them around a nonpolar atom Temporarily creates positive end and negative end
53
54
London dispersion forces - intermolecular attractions resulting from constant motion of e- and creation of instantaneous dipoles
55
Liquids that dissolve freely in one another in any ratio are said to be completely miscible Benzene and carbon tetrachloride (both nonpolar) are completely miscible
Nonpolar molecules of these two apply no strong forces of attraction or repulsion and molecules mix freely
56
Ethanol and water are also completely miscible -OH group on ethanol is slightly polar
Can form H-bonds with water as well as other ethanol molecules Intermolecular forces in mixture are very similar to those in pure liquids so they are mutually soluble in any amount
57
Gasoline contains mainly nonpolar hydrocarbons
Excellent solvent for fats oils greases
Major intermolecular forces are weak London forces
58
Ethanol less polar than water more than carbon tetrachloride
Better solvent for less-polar substances bc of nonpolar region
59
Effects of Pressure on Solubility
Little effect on solids or liquids Increase pressure - increase solubilities of gases in liquids
60
When gas in contact with surface of liquid gas molecules can enter liquid As amount of dissolve gas increases some molecules start to escape and reenter gas phase
Eq eventually established between rates of enteringleaving gas phase
61
As long as eq undisturbed solubility of gas in liquid unchanged at given pressure
Gas + solvent solution
62
Increasing pressure of solute gas above solution puts stress on eq Molecules collide with surface more often Increase in pressure partially offset by increase in rate of gas molecules entering solution In turn increase in amount of dissolved gas causes increase in rate molecules escape liquid and becomes gas
63
Eventually eq restored at higher gas solubility
Expected from Le Chatelierrsquos principle
Increase in gas pressure causes eq to shift so fewer molecules are in gas phase
64
Henryrsquos Law
Henryrsquos law - solubility of a gas in liquid is directly proportional to the partial pressure of that gas on the surface of the liquid
Applies to gas-liquid solutions at constant temp
65
When mixture of ideal gases is limited in constant volume at constant temp each gas applies same pressure it would apply if it occupied the space alone
Assuming gases do not react each gas dissolves to the extent it would if no other gases were there
66
In carbonated drinks (Coke Pepsi etc) solubility of CO2 increased by increasing pressure
At bottling factory CO2 gas forced into solution of flavored water at pressure of 5-10 atm Gas-in-liquid solution then sealed in bottlescans
67
When cap removed pressure reduced to 1 atm and some CO2 escapes as gas bubbles Effervescence - rapid escape of a gas from a liquid in which it is dissolved
68
Effects of Temperature on Solubility
Letrsquos consider gas solubility Increasing temperature usually decreases gas solubility As temp increases average kinetic energy of molecules in solution increases Greater number of solute molecules escape from attraction of solvent and return to gas phase 69
At higher temps eq is reached with fewer gas molecules in solution Gases generally less soluble
70
Effect of temp on solubility of solids in liquids more difficult to predict
Often increasing temp increases solubility
However the same temp increase can result in large increase in solubility in one case and only slight increase in the next
71
72
enthalpies of Solution
Formation of solution comes with energy change Dissolve KI in water container feels cold Dissolve LiCl in water feels hot Formation of solid-liquid solution can absorb or release heat
73
During formation of solution solvent and solute particles experience changes in forces attracting them to other particles Before dissolving begins solvent molecules held by intermolecular forces (solvent-solvent attraction) In solute molecules held by Ifs (solute-solute attraction)
74
Energy is required to separate solute molecules and solvent molecules from their neighbors
A solute particle that is surrounded by solvent molecules is said to be solvated
75
Step 1 solute particles become separated from solid (energy absorbed) Step 2 solvent particles move apart to allow solute particles to enter liquid (energy absorbed)
76
Step 3 solvent particles attracted to and solvate solute particles (energy released)
77
Heat of solution - net amount of heat energy absorbed or released when a specific amount of solute dissolves in a solvent
Heat of solution negative (heat is released) if sums of energy in steps 1 and 2 is less than step 3
If 1+2 gt 3 HOS is positive (heat absorbed)78
In gaseous state molecules are so far apart there are nearly no forces between them Solute-solute interaction has little effect on heat of solution of a gas Energy released when gas dissolved in liquid bc attraction between solute gas and solvent molecules greater than energy needed to separate solvent molecules
79
Concentration of Solutions80
Concentration - measure of amount of solute in given amount of solvent or solution ldquodiluterdquo just means small amount of solute ldquoconcentratedrdquo just means relatively large amount of solute Unrelated to degree to which solution is saturated Ex Saturated solution of substance not very soluble might be dilute
81
Molarity
Molarity - number of moles of solute in one liter of solution To find molarity must know molar mass A ldquoone molarrdquo solution of NaOH is 1 mole NaOH per liter of solution (1 M)
82
1 mol NaOH = 400 g If this quantity dissolve in enough water to make EXACTLY 100 L solution solution is 1 M If 200 g NaOH dissolve in enough to make 100 L solution what is molarity 0500 M
83
1 M solution is not made by adding 1 mol solute to 1 L solvent That would be more than 1 L Instead 1 mol solute dissolved in less than 1 L Then diluted with more solvent to bring TOTAL VOLUME to 1 L
84
Practice ProblemYou have 340 L of solution that contains 900 g sodium chloride What is the molarity of that solution 0440 M NaCl You have 08 L of a 05 M HCl solution How many moles of HCl does this solution contain 04 mol HCl
85
What is the molarity of a solution composed of 585 g of potassium iodide dissolved in enough water to make a 0125 L of solution 0282 M KI
86
How many moles of H2SO4 are present in 0500 L of a 0150 M H2SO4 solution 00750 mol What volume of 300 M NaCl is needed for reaction that requires 1463 g of NaCl 0834 L
87
Molality
Molality - concentration of solution expressed in moles of solute per kilogram of solvent Solution that contains 1 mol solute (NaOH) dissolved in exactly 1 kg of solvent is ldquoone molalrdquo solution (1 m NaOH)
88
1 mol NaOH = 400 g 400 g NaOH dissolved in 1 kg water is 1 m NaOH
200 g NaOH in 1 kg water = 0500 m NaOH
89
Practice Problem
A solution was prepared by dissolving 171 g sucrose (C12H22O11) in 125 g of water Find the molal concentration of this solution 0400 m C12H22O11
90
A solution of iodine in carbon tetrachloride is used when iodine is needed for certain chemical tests How much iodine must be added to prepare a 0480 m solution of iodine in CCl4 if 1000 g of CCl4 is used 122 g I2
91
What is the molality of a solution composed of 255 g acetone (CH3)2CO dissolved in 200 g of water 22 m acetone What quantity in grams of methanol CH3OH is required to prepare a 0244 m solution in 400 g water 312 g methanol
92
How many grams of AgNO3 are needed to prepare a 0125 m solution in 250 mL of water 531 g AgNO3
What is the molality of a solution containing 182 g HCl and 250 g water 200 m
93
Ions in Aqueous Solutions and Colligative Properties
94
95
compounds in aqueous solutions
DissociationWhen compound made from ions dissolves in water ions separate Dissociation - separation of ions that occurs when an ionic compound dissolves
96
Notice number of ions made per formula unit in equations 1 formula unit of NaCl gives 2 units of ions in solution 1 formula unit of CaCl2 gives 3 units of ions
97
Assuming 100 dissociation solution that contains 1 mol NaCl contains 1 mol Na+ and 1 mol Cl- Can assume 100 dissociation of all soluble ionic compounds
98
Practice Problem
Write the equation for the dissolution of aluminum sulfate in water How many moles of aluminum ions are made by dissolving 1 mol aluminum sulfate What is the total number of moles of ions made by dissolving 1 mol of aluminum sulfate
99
1 AnalyzeGiven Amount of solute = 1 mol Al2(SO4)3 Solvent identity = water Unknown a moles of aluminum ions and sulfate ions Total number of moles of solute ions produced
100
2 Plan
The coefficients in the balanced dissociation equation will tell mole relationships You can use the equation to find out the number of moles of solute ions produced
101
3 Compute
a
b102
Write the equation for the dissolution of each of the following in water and then determine the number of moles of each ions made as well as the total number of ions made a 1 mol ammonium chloride b 1 mol sodium sulfide c 05 mol barium nitrate
103
a 1 mol ammonium chloride
1 mol NH4+ 1 mol Cl- 2 mol total ions
104
b 1 mol sodium sulfide
2 mol Na+ 1 mol S-2 3 mol total ions
105
c 05 mol barium nitrate
05 mol Ba+2 1 mol NO3- 15 mol total ions
106
Precipitation Reactions
Even though no compound is completely insoluble compounds of very low solubility can be considered insoluble for practical purposes There are general guidelines to help predict whether a compound made of certain combination of ions is soluble
107
General Solubility Guidelines
Sodium potassium and ammonium compounds are soluble in water
108
Nitrates acetates and chlorates are soluble
109
110
Most chlorides are soluble except those of silver mercury (I) and lead Lead (II) chloride is soluble in hot water
Most sulfates are soluble except those of barium strontium lead calcium and mercury
111
Most carbonates phosphates and silicates are insoluble except those of sodium potassium and ammonium
112
Most sulfides are insoluble except those of calcium strontium sodium potassium and ammonium
113
Is calcium phosphate Ca3(PO4)2 soluble or insoluble
Not one of the exceptions so it is insoluble
Dissociation equations are not written for insoluble compounds
114
Guidelines also useful in predicting what will happen if 2 solutions of 2 different soluble compounds are mixed
If mixing results in combination of ions that forms insoluble compound a double-replacement and precipitation reaction will happen
115
Will precipitate form when solutions of ammonium sulfide and cadmium nitrate are combined You can tell calcium nitrate is soluble from the guidelines (Most nitrates acetates and chlorates are soluble) You can also tell ammonium sulfide is soluble (Most sulfides are insoluble except those of calcium strontium sodium potassium and ammonium)
116
117
To decide whether precipitate can form must know solubilities of two possible products
Ammonium nitrate ndash soluble (Most nitrates acetates and chlorates are soluble) Cadmium sulfide ndash insoluble (Most sulfides are insoluble except those of calcium strontium sodium potassium and ammonium)
118
bc one product is insoluble double-replacement and precipitation reaction will happen (NH4)2S (aq) + Cd(NO3)2 (aq) rarr NH4NO3 (aq) + CdS (s)
119
Net Ionic Equations
Reactions of ions in aqueous solution usually represented by net ionic equations instead of formula equations Net ionic equation - includes only those compounds and ions that undergo a chemical change in a reaction in an aqueous solution
120
121
To write NIE (net ionic equation) first convert chemical equation into overall ionic equation All soluble ionic compounds shown as dissociated ions in solution Precipitates shown as solids
122
Notice ammonium and nitrate ions appear on both sides They havenrsquot gone through any chemical change Spectator ions - ions that do not take part in a chemical reaction and are found in solution both before and after the reaction
123
To change ionic equation to NIE eliminate spectator ions Cd+2(aq) + S-2(aq) rarr CdS(s)
This NIE applies to any reaction in which precipitate of CdS forms
124
Practice Problem
Identify the precipitate that forms when aqueous solutions of zinc nitrate and ammonium sulfide are combined Write the equation for the possible double-replacement reaction Then write the formula equation overall ionic equation and the net ionic equation
125
1 AnalyzeGiven Identity of reactants zinc nitrate and ammonium sulfide Reaction medium aqueous solution Unknown a equation for the possible double-replacement reaction b identity of the precipitate c formula equation d overall ionic equation e net ionic equation
126
2 PlanWrite the possible double-replacement reaction between Zn(NO3)2 and (NH4)2S Use the rules to determine if any of the products will precipitate Write a formula equation and overall net ionic equation Then cancel spectator ions to make net ionic equation
127
3 Computea possible double-replacement reaction
b rules show that zinc sulfide is not soluble so will precipitate (ammonium nitrate is soluble)
c formula equation128
d overall ionic equation
e net ionic equation129
Will a precipitate form if solutions of potassium sulfate and barium nitrate are combined If so write the net ionic equation for the reaction
Yes Ba+2(aq) + SO4-2(aq) rarr BaSO4(s)
130
Will a precipitate form if solutions of potassium nitrate and magnesium sulfate are combined If so write the net ionic equation for the reaction
No 131
Will a precipitate form if solutions of barium chloride and sodium sulfate are combined If so identify the spectator ions and write the net ionic equation for the reaction
Yes Na+ and Cl- Ba+2(aq) + SO4-2(aq) rarr BaSO4(s)
132
Write the net ionic equation for the precipitation of nickel(II) sulfide
Ni+2(aq) + S-2(aq) rarr NiS(s)
133
IonizationSome molecular compounds can also form ions in solution Usually polar Ionization - ions formed from solute molecules by action of the solvent (creation of ions where there were none) Ionization different from dissociation
134
Dissociation ionic compounds dissolve and ions already present separate Ionization ions formed where non existed before
Ions formed are hydrated Heat released during hydration of ions gives enough energy to break covalent bonds
135
Extent to which solute ionizes depends on strength of bonds within molecules of solute and strength of attraction between solute and solvent molecules
136
HCl ndash molecular compound that ionizes in aqueous solution Contains highly polar bond Attraction between polar HCl molecule and polar water strong enough to break HCl bond
137
The Hydronium Ion
H+ ions from HCl attracts other molecules or ions so strongly that it doesnrsquot normally exist alone Ionization of HCl better described as direct transfer of proton from HCl to H2O forming H3O+
138
139
Hydration of H+ to form hydronium is highly exothermic
Energy released gives a lot of energy needed to ionize a molecular solute
Many molecular compounds that ionize in aqueous solution contain hydrogen and form hydronium ion
140
Strong and Weak Electrolytes
Substances that make ions and conduct current in solution are electrolytes HCl is one of series of compounds that has hydrogen and halogen Hydrogen halides are all molecular compounds with single polar-covalent bonds
141
All are gases Very soluble in water All are electrolytes HCl HBr HI strongly conduct current in solution HF only weakly conducts current at same concentration Strength of conduction related to ability to ionize
142
143
Strong Electrolytes
HCl HBr HI are 100 ionized in solution
Strong electrolyte - any compound whose dilute aqueous solutions conduct electricity well this is due to the presence of all or almost all of the dissolve compound in the form of ions
144
Unique characteristic of strong electrolyte is that they yield only ions
Ex Ionic compound may be highly soluble in water and dissociate into ions (NaCl)
Others may not dissolve much but amount that does dissolve exists only as ions
145
Weak ElectrolytesHF dissolves in water to give acid solution called hydrofluoric acid HF bond stronger than bonds between hydrogen and other halogens When HF dissolves some molecule ionize Reverse reaction also happens HF(aq) + H2O(l) H3O+(aq) + F-(aq)
146
HF(aq) + H2O(l) H3O+
(aq) + F-(aq)Concentration of dissolved unionized HF stays high and concentration of ions stays low
Weak electrolyte - any compound whose aqueous solutions conduct electricity poorly this is due to the presence of small amount of the dissolved compound in the form of ions
147
Different from nonelectrolyte ndash NO ions at all Description of electrolyte as strong or weak not related to concentration of solution Electrolytes differ in degree of ionization not amount of solute dissolved
148
149
Colligative properties of solutions
Presence of solute affects properties of solutions Some properties not dependent on nature of dissolved substance but on how may dissolve particles are present
Colligative properties - properties that depend on the concentration of solute particles but not on their identity
150
Vapor-Pressure Lowering
Boiling and freezing point of solution different from pure solvent Nonvolatile solute raises boiling point and lowers freezing point Nonvolatile substance - one that has little tendency to become gas under existing conditions
151
152
To understand why nonvolatile solute changes boiling and freezing point consider equilibrium vapor pressure
Vapor pressure pressure caused by molecules that have escape liquid phase to gas phase
Can be thought of a measure of tendency of molecules to escape from a liquid
153
Addition of sucrose (nonvolatile solute) lowers concentration of water molecules at surface of liquid This lowers tendency of water molecules to leave solution and enter gas phase Vapor pressure of solution is lower than vapor pressure of pure water
154
Aqueous13 solution13 of13 nonvolatile13 solutePure13 water
Nonelectrolyte solutions of same molality have same concentration of particles
Dilute solutions of same solvent and equal molality of any nonelectrolyte solute lower vapor pressure equally
155
Example
1 m aqueous solution of nonelectrolyte glucose lowers vapor pressure of water 55 x 10-4 atm at 25 1 m aqueous solution of sucrose also lowers water vapor pressure to 55 x 10-4 atm at 25
156
157
bc vapor-pressure lowering depends on concentration of nonelectrolyte solute and doesnrsquot depend on the type of solute it is a colligative proprerty
bc vapor pressure has been lowered solution remains liquid over larger temperature range
Lowers freezing point and raises boiling point
158
Can assume changes in boiling and freezing point also depend on concentration of solute
They are colligative properties
159
Freezing-Point Depression
Freezing point of 1-molal solution of any nonelectrolyte solute in water is found (by experiment) to be 186 lower than freezing point of water When 1 mol of nonelectrolyte solute dissolved in 1 kg water freezing point is -186 not 000 160
When 2 mol nonelectrolyte solute dissolved in 1 kg water freezing point is -372
For any concentration of nonelectrolyte solute in water decrease in freezing point can be determined using value of -186m
161
This value called the molal freezing-point contstant (Kf) is the freezing-point depression of the solvent in a 1-molal solution of a nonvolatile nonelectrolyte solute
162
Each solvent has own characteristic molal freezing-point constant Freezing-point depression (Δtf) the difference between the freezing points of the pure solvent and a solute of nonelectrolyte in that solvent and it is directly proportional to the molal concentration of the solution Δtf = Kfm
163
Δtf = Kfm
Kf expressed as m
m expressed in mol solutekg solvent (molality)
164
Sample Problem
What is the freezing point depression of water in a solution of 171 g of sucrose C12H22O11 and 200 g of water What is the actual freezing point of the solution
165
1 AnalyzeGiven Solute mass and chemical formula 171 g C12H22O11 Solvent mass and identity 2000 g water Unknown a freezing-point depression b freezing point of the solution 166
2 Plan
167
168
Calculate
Unknown a freezing-point depression b freezing point of the solution
169
Practice Problem
A water solution containing an unknown quantity of a nonelectrolyte solute is found to have a freezing point of ndash 023degCWhat is the molal concentration of the solution
012 m170
A solution consists of 103 g of the nonelectrolyte glucose C6H12O6 dissolved in 250 g of water What is the freezing-point depression of the solution
minus0426degC171
In a laboratory experiment the freezing point of an aqueous solution of glucose is found to be minus0325degCWhat is the molal concentration of this solution
0175 m172
If 0500 mol of a nonelectrolyte solute are dissolved in 5000 g of ether what is the freezing point of the solution
minus1181degC173
The freezing point of an aqueous solution that contains a nonelectrolyte is minus90degC aWhat is the freezing-point depression of the solution bWhat is the molal concentration of the solution a minus90degC b 48 m
174
Boiling-Point Elevation
Boiling point is temp at which vapor pressure is equal to atmospheric pressure Change in vapor pressure will cause corresponding change in boiling point
175
176
Vapor pressure of nonvolatile solution is lower than vapor pressure of pure solvent More heat will be required to raise vapor pressure of solution Boiling point of solution is higher than boiling point of pure solvent
Molal boiling-point constant (Kb) - boiling-point elevation of the solvent in a 1-molal solution of a nonvolatile nonelectrolyte solute b-p elevation of any nonelectrolyte in water found by experiment to be 051 So molal b-p constant for water is 051m
177
For different solvents b-p elevations of 1-molal solutions have different values (Table 14-2)
Like freezing-point constants values most accurate for dilute solutions
178
Boiling-point elevation Δtb difference between the boiling points of the pure solvent and a nonelectrolyte solution of that solvent and it is directly proportional to the molal concentration of the solution B-p elevation can be calculated using Δtb = Kbm Δtb (boiling point elevation) expressed in m and m expressed in mol solutekg solvent
Sample Problem
What is the boiling-point elevation of a solution made from 200 g of a nonelectrolyte solute and 4000 g of water The molar mass of the solute is 620 gmol
180
1 AnalyzeGiven solute mass = 200 g solute molar mass = 620 gmol solvent mass and identity = 4000 g of water
Unknown boiling-point elevation
181
2 Plan
182
183
3 Compute
Practice Problem
A solution contains 500 g of sucrose C12H22O11 a nonelectrolyte dissolved in 5000 g of water What is the boiling-point elevation
015degC184
A solution contains 4500 g of sucrose C12H22O11 a nonelectrolyte dissolved in 250 g of waterWhat is the boiling point of the solution
1027degC185
If the boiling point elevation of an aqueous solution containing a nonvolatile electrolyte is 102degC what is the molality of the solution
20 m186
The boiling point of an aqueous solution containing a nonvolatile electrolyte is 10075degC aWhat is the boiling-point elevation bWhat is the molality of the solution a 075degC b 15 m
187
Electrolytes and Colligative PropertiesEarly investigators confused by experiments where certain substances depressed freezing point or elevated boiling point more than expected
188
189
Ex 01 m solution of NaCl lowers freezing point of solvent almost twice as much as 01 m solution of sucrose 01 m CaCl2 lowers fp almost 3 times as much as 01 m solution of sucrose
To understand why this happens contrast behavior or sucrose with NaCl in aqueous solution Each sucrose dissolves to make only 1 particle in solution So 1 mol sucrose dissolves to 1 mol particles 1 mol NaCl dissolves to make 2 moles of particles (1 mol Na+ and 1 mol Cl-)
190
191
Calculated Values for Electrolyte SolutionsColligative properties depend on total concentration of solute particles regardless of their identity Electrolytes cause changes in colligative properties proportional to total molality in terms of all dissolves particles instead of formula units
192
What about barium nitrate Ba(NO3)2 Each mole of barium nitrate yields 3 mol of ions in solution Ba(NO3)2(s) rarr Ba2+(aq) + 2NO3-(aq) You would expect solution of given molality to lower fp of solvent 3 times as much as nonelectrolytic solution of same molality
193
Sample Problem
What is the expected change in the freezing point of water in a solution of 625 g of barium nitrate Ba(NO3)2 in 100 kg of water
194
1 AnalyzeGiven solute mass and formula = 625 g Ba(NO3)2 solvent mass and identity = 100 kg water Δtf = Kfm
Unknown expected freezing-point depression
195
2 PlanThe molality can be calculated by converting the solute mass to moles and then dividing by the number of kilograms of solvent That molality is in terms of formula units of Ba(NO3)2 and must be converted to molality in terms of dissociated ions in solution It must be multiplied by the number of moles of ions produced per mole of formula unit This adjusted molality can then be used to calculate the freezing-point depression
196
197
198
3 Compute
Practice Problem
What is the expected freezing-point depression for a solution that contains 20 mol of magnesium sulfate dissolved in 10 kg of water
minus74degC199
What is the expected boiling-point elevation of water for a solution that contains 150 g of sodium chloride dissolved in 10 kg of water 27degC The freezing point of an aqueous sodium chloride solution is minus200degC What is the molality of the solution 54 m NaCl
200
Actual Values for Electrolyte SolutionsRemember values calculated are only expected values Actual values of colligative properties for all strong electrolytes are almost what would be expected Difference between expected and actual values caused by attractive forces that exist between dissociated ions in aqueous solution
201
Attraction between hydrated ions in solution is small compared with those in crystalline solid However forces of attraction donrsquot interfere with movements of aqueous ions More concentrated solution means ions closer together and attraction is greater Also depends on charge of ions ndash higher charge greater attraction
202
Lump of sugar disappears as sugar molecules leave surface of their crystals and mix with water molecules Eventually all sugar molecules are evenly spread through water molecules All visible traces of sugar are gone It becomes solution - homogeneous mixture of two or more substances in a single phase
6
Components of Solutions
Particles of one substance randomly mixed with particles of another Solvent - dissolving medium in a solution Solute - substance dissolved in solution
7
Solute usually less than solvent Dissolved solute particles cannot be seen Stay mixed with solvent forever so long as conditions stay the same Solute-particles from 001-1 nm in diameter
8
Types of Solutions
Solutions can exist as gases liquids or solids
9
Many alloys such as brass (zinc and copper) are sterling silver (silver and copper) are solid solutions Atoms of two or metals are evenly mixed By properly choosing percentages of each metal can get many desirable properties
10
Ex Alloys have higher strength and greater resistance to corrosion than pure metals Pure gold (24K) too soft for jewelry Alloy with silver (14K) increases strength and hardness but keeps appearance of gold
11
SuspensionsSuspension - if particles in solvent are so large they settle out unless mixture constantly stirred Think of sand and water Particles over 1000 nm in diameter
12
Particles that are medium in size between those in solutions and suspensions from mixtures are known as colloids (colloidal dispersions) Particles between 1-1000 nm diameter
13
Colloids
In muddy water large soil particles settle Water still cloudy bc colloidal particles stay dispersed in water Filter Colloidal particles pass through and stays cloudy
14
Particles in colloid small enough to be suspended in solvent by constant movement of surrounding molecules Colloidal particles make up dispersed phase Water is dispersing medium
15
Emulsion and foam are colloids
Mayonnaise is emulsion of oil droplets in water Egg yolk acts as emulsifying agent (keeps oil droplets dispersed)
16
17
18
Class13 of13 Colloid Phases Example
Sol Solid13 dispersed13 in13 liquid Paints13 mud
Gel Solid13 network13 extending13 through13 liquid
Gelatin
Liquid13 emulsion Liquid13 dispersed13 in13 liquid Milk13 mayonnaise
Foam Gas13 dispersed13 in13 liquid Shaving13 cream13 whipped13 cream
Solid13 aerosol Solid13 dispersed13 in13 gas Smoke13 auto13 exhaust
Liquid13 aerosol Liquid13 dispersed13 in13 gas Fog13 mist13 clouds13 aerosol13 spray
Solid13 emulsion Liquid13 dispersed13 in13 solid Cheese13 butter
Tyndall EffectMany colloids appear homogeneous bc individual particles cannot be seen Particles large enough to scatter light In fog can see headlight beam Tyndall effect - occurs when light is scattered by colloidal particles dispersed in transparent medium
19
20
21
Solutions Colloids Suspensions
Homogeneous Heterogeneous Heterogeneous
Particle13 size13 001-shy‐113 nm13 can13 be13 atoms13 ions13 molecules
Particle13 size13 1-shy‐100013 nm13 dispersed13 can13 be13 combined13 or13 large13 molecules
Particle13 size13 over13 100013 nm13 suspended13 can13 be13 large13 particles13 or13 combined13 particles
Do13 not13 separate13 on13 standing Do13 not13 separate13 on13 standing Particles13 settle13 out
Cannot13 be13 separated13 by13 filtration Cannot13 be13 separated13 by13 filtration Can13 be13 separated13 by13 filtration
Do13 not13 scatter13 light Scatter13 light13 (Tyndall13 effect) May13 scatter13 light13 but13 are13 not13 transparent
Electrolytes vs Nonelectrolytes
Substances that dissolve in water are classified according to whether they produce molecules or ions in solution
22
23
When ionic compound dissolves cations and anions separate and are surrounded by water molecules Solute ions free to move making possible to carry electric current
Electrolyte - substance that dissolves in water to give a solution that conducts electric current NaCl ndash electrolyte Usually highly polar compounds become electrolytes
24
Solution containing neutral solute molecules does not conduct current bc it doesnrsquot have mobile charged particles
Nonelectrolyte - substance that dissolves in water to give solution that does not conduct electric current Ex Sugar
25
26
27the solution
process
Factors Affecting Rate of DissolutionEver tried to dissolve sugar in iced tea or coffee You notice temperature has something to do with how quickly solute dissolves What other factors affect how quickly solutes dissolve
28
1 Increasing Surface Area of Solute
bc dissolution process occurs at surface of solute Can increase dissolution by increasing surface area
29
2 Agitating (Stirring) SolutionClose to surface of solute concentration of dissolved solute is high Stirringshaking helps disperse solute particles
30
Brings fresh solvent into contact with solute surface Contact between solvent and solute surface increased
3 Heating a Solvent
Solutes dissolve faster in warmer solvent As temp increases solvent molecules move faster Average kinetic energy increases Collisions between solvent and solute more frequent Helps separate solute molecules and spread them out
32
Solubility
If you add sugar to tea eventually no more sugar dissolves
For every combination of solvent with solute at given temp there is limit to amount of solute that can be dissolved
33
When sugar first dropped into water sugar molecules leave solid surface and move about at random in solvent Some dissolved molecules may collide with crystal and stay there As more solid dissolves and concentration of dissolved molecules increases collisions happen more often
34
Eventually molecules are returning to crystal at same rate they are going into solution Dynamic equilibrium established between dissolution and crystallization Solution equilibrium - physical state in which the opposing processes of dissolution and crystallization of a solute occur at equal rates
35
Saturated vs Unsaturated Solutions
Saturated solution - solution that contains the maximum amount of dissolved solute How do you know if solution is saturated If more NaCl added it falls to bottom and doesnrsquot dissolve Equilibrium already established
36
If more water added to saturated solution more NaCl will dissolve At 20 359 g NaCl is max that will dissolve in 100 g water Unsaturated solution - solution that contains less solute than a saturated solution under the existing conditions
37
Supersaturated Solutions
When saturated solution where solubility increases as temp increases is cooled excess solute usually comes out of solution Sometimes if solution is left to cool the excess solute doesnrsquot separate
38
39
Supersaturated solution - solution that contains more dissolved solute than a saturated solution contains under same conditions
40
Supersaturated solution can remain unchanged over long time if it isnrsquot disturbed Once crystals begin to form continues until equilibrium is recreated at lower temp
41
Example
Sodium thiosulfate Na2S2O3 Solute added to hot water until solution saturated Hot solution filtered Drop small crystal in ldquoseedingrdquo crystallization Continues until equilibrium created
42
Solubility Values
Solubility - amount of substance required to form a saturated solution with specific amount of solvent at specified temp Ex Solubility of sugar = 204 g per 100 g water at 20 Must specify temp bc solubility changes with temp 43
For gases pressure must also be specified Rate at which substance dissolves is unrelated to solubility Max amount of solute that dissolves and reaches equilibrium is always same under same conditions
44
Solute-Solvent Interactions
ldquolike dissolves likerdquo is generally useful rule for predicting whether one substance will dissolve in another Depends on type of bonding polaritynonpolarity and intermolecular forces between solute and solvent
45
Dissolving Ionic Compounds in Aqueous Solution
Polarity of water molecules plays important role in formation of solution of ionic compounds Charged ends of water molecules attract ions and surround them to keep separated from other ions
46
Hydration - solution process with water as solvent Ions said to be hydrated As hydrated ions diffuse into solution other ions exposed and drawn away from crystal surface Entire crystal gradually dissolves
47
When crystallized from aqueous solutions some ionic compounds form crystals that include water molecules The crystalline compounds known as hydrates have specific ratios of water molecules Represented by formulas like CuSO45H2O
48
When crystalline hydrate dissolves in water water of hydration returns to solvent Behavior of solute in hydrated form no different than anhydrous form
49
Nonpolar Solvents
Nonpolar solvents do not attract ions of ionic compound strongly enough to overcome forces holding crystal together
50
Liquid Solutes and Solvents
When you shake bottle of salad dressing oil droplets disperse in water Stop shaking ndash strong attraction between water molecules squeeze out oil and form separate layers Liquid solutes and solvents that are not soluble in each other are immiscible
51
Nonpolar substances (fat oil grease) are generally quite soluble in nonpolar liquids (carbon tetrachloride toluene gasoline)
Only attractions between nonpolar molecules are weak London forces
52
London Dispersion Forces
bc e- are constantly moving at any time there may be uneven distribution of them around a nonpolar atom Temporarily creates positive end and negative end
53
54
London dispersion forces - intermolecular attractions resulting from constant motion of e- and creation of instantaneous dipoles
55
Liquids that dissolve freely in one another in any ratio are said to be completely miscible Benzene and carbon tetrachloride (both nonpolar) are completely miscible
Nonpolar molecules of these two apply no strong forces of attraction or repulsion and molecules mix freely
56
Ethanol and water are also completely miscible -OH group on ethanol is slightly polar
Can form H-bonds with water as well as other ethanol molecules Intermolecular forces in mixture are very similar to those in pure liquids so they are mutually soluble in any amount
57
Gasoline contains mainly nonpolar hydrocarbons
Excellent solvent for fats oils greases
Major intermolecular forces are weak London forces
58
Ethanol less polar than water more than carbon tetrachloride
Better solvent for less-polar substances bc of nonpolar region
59
Effects of Pressure on Solubility
Little effect on solids or liquids Increase pressure - increase solubilities of gases in liquids
60
When gas in contact with surface of liquid gas molecules can enter liquid As amount of dissolve gas increases some molecules start to escape and reenter gas phase
Eq eventually established between rates of enteringleaving gas phase
61
As long as eq undisturbed solubility of gas in liquid unchanged at given pressure
Gas + solvent solution
62
Increasing pressure of solute gas above solution puts stress on eq Molecules collide with surface more often Increase in pressure partially offset by increase in rate of gas molecules entering solution In turn increase in amount of dissolved gas causes increase in rate molecules escape liquid and becomes gas
63
Eventually eq restored at higher gas solubility
Expected from Le Chatelierrsquos principle
Increase in gas pressure causes eq to shift so fewer molecules are in gas phase
64
Henryrsquos Law
Henryrsquos law - solubility of a gas in liquid is directly proportional to the partial pressure of that gas on the surface of the liquid
Applies to gas-liquid solutions at constant temp
65
When mixture of ideal gases is limited in constant volume at constant temp each gas applies same pressure it would apply if it occupied the space alone
Assuming gases do not react each gas dissolves to the extent it would if no other gases were there
66
In carbonated drinks (Coke Pepsi etc) solubility of CO2 increased by increasing pressure
At bottling factory CO2 gas forced into solution of flavored water at pressure of 5-10 atm Gas-in-liquid solution then sealed in bottlescans
67
When cap removed pressure reduced to 1 atm and some CO2 escapes as gas bubbles Effervescence - rapid escape of a gas from a liquid in which it is dissolved
68
Effects of Temperature on Solubility
Letrsquos consider gas solubility Increasing temperature usually decreases gas solubility As temp increases average kinetic energy of molecules in solution increases Greater number of solute molecules escape from attraction of solvent and return to gas phase 69
At higher temps eq is reached with fewer gas molecules in solution Gases generally less soluble
70
Effect of temp on solubility of solids in liquids more difficult to predict
Often increasing temp increases solubility
However the same temp increase can result in large increase in solubility in one case and only slight increase in the next
71
72
enthalpies of Solution
Formation of solution comes with energy change Dissolve KI in water container feels cold Dissolve LiCl in water feels hot Formation of solid-liquid solution can absorb or release heat
73
During formation of solution solvent and solute particles experience changes in forces attracting them to other particles Before dissolving begins solvent molecules held by intermolecular forces (solvent-solvent attraction) In solute molecules held by Ifs (solute-solute attraction)
74
Energy is required to separate solute molecules and solvent molecules from their neighbors
A solute particle that is surrounded by solvent molecules is said to be solvated
75
Step 1 solute particles become separated from solid (energy absorbed) Step 2 solvent particles move apart to allow solute particles to enter liquid (energy absorbed)
76
Step 3 solvent particles attracted to and solvate solute particles (energy released)
77
Heat of solution - net amount of heat energy absorbed or released when a specific amount of solute dissolves in a solvent
Heat of solution negative (heat is released) if sums of energy in steps 1 and 2 is less than step 3
If 1+2 gt 3 HOS is positive (heat absorbed)78
In gaseous state molecules are so far apart there are nearly no forces between them Solute-solute interaction has little effect on heat of solution of a gas Energy released when gas dissolved in liquid bc attraction between solute gas and solvent molecules greater than energy needed to separate solvent molecules
79
Concentration of Solutions80
Concentration - measure of amount of solute in given amount of solvent or solution ldquodiluterdquo just means small amount of solute ldquoconcentratedrdquo just means relatively large amount of solute Unrelated to degree to which solution is saturated Ex Saturated solution of substance not very soluble might be dilute
81
Molarity
Molarity - number of moles of solute in one liter of solution To find molarity must know molar mass A ldquoone molarrdquo solution of NaOH is 1 mole NaOH per liter of solution (1 M)
82
1 mol NaOH = 400 g If this quantity dissolve in enough water to make EXACTLY 100 L solution solution is 1 M If 200 g NaOH dissolve in enough to make 100 L solution what is molarity 0500 M
83
1 M solution is not made by adding 1 mol solute to 1 L solvent That would be more than 1 L Instead 1 mol solute dissolved in less than 1 L Then diluted with more solvent to bring TOTAL VOLUME to 1 L
84
Practice ProblemYou have 340 L of solution that contains 900 g sodium chloride What is the molarity of that solution 0440 M NaCl You have 08 L of a 05 M HCl solution How many moles of HCl does this solution contain 04 mol HCl
85
What is the molarity of a solution composed of 585 g of potassium iodide dissolved in enough water to make a 0125 L of solution 0282 M KI
86
How many moles of H2SO4 are present in 0500 L of a 0150 M H2SO4 solution 00750 mol What volume of 300 M NaCl is needed for reaction that requires 1463 g of NaCl 0834 L
87
Molality
Molality - concentration of solution expressed in moles of solute per kilogram of solvent Solution that contains 1 mol solute (NaOH) dissolved in exactly 1 kg of solvent is ldquoone molalrdquo solution (1 m NaOH)
88
1 mol NaOH = 400 g 400 g NaOH dissolved in 1 kg water is 1 m NaOH
200 g NaOH in 1 kg water = 0500 m NaOH
89
Practice Problem
A solution was prepared by dissolving 171 g sucrose (C12H22O11) in 125 g of water Find the molal concentration of this solution 0400 m C12H22O11
90
A solution of iodine in carbon tetrachloride is used when iodine is needed for certain chemical tests How much iodine must be added to prepare a 0480 m solution of iodine in CCl4 if 1000 g of CCl4 is used 122 g I2
91
What is the molality of a solution composed of 255 g acetone (CH3)2CO dissolved in 200 g of water 22 m acetone What quantity in grams of methanol CH3OH is required to prepare a 0244 m solution in 400 g water 312 g methanol
92
How many grams of AgNO3 are needed to prepare a 0125 m solution in 250 mL of water 531 g AgNO3
What is the molality of a solution containing 182 g HCl and 250 g water 200 m
93
Ions in Aqueous Solutions and Colligative Properties
94
95
compounds in aqueous solutions
DissociationWhen compound made from ions dissolves in water ions separate Dissociation - separation of ions that occurs when an ionic compound dissolves
96
Notice number of ions made per formula unit in equations 1 formula unit of NaCl gives 2 units of ions in solution 1 formula unit of CaCl2 gives 3 units of ions
97
Assuming 100 dissociation solution that contains 1 mol NaCl contains 1 mol Na+ and 1 mol Cl- Can assume 100 dissociation of all soluble ionic compounds
98
Practice Problem
Write the equation for the dissolution of aluminum sulfate in water How many moles of aluminum ions are made by dissolving 1 mol aluminum sulfate What is the total number of moles of ions made by dissolving 1 mol of aluminum sulfate
99
1 AnalyzeGiven Amount of solute = 1 mol Al2(SO4)3 Solvent identity = water Unknown a moles of aluminum ions and sulfate ions Total number of moles of solute ions produced
100
2 Plan
The coefficients in the balanced dissociation equation will tell mole relationships You can use the equation to find out the number of moles of solute ions produced
101
3 Compute
a
b102
Write the equation for the dissolution of each of the following in water and then determine the number of moles of each ions made as well as the total number of ions made a 1 mol ammonium chloride b 1 mol sodium sulfide c 05 mol barium nitrate
103
a 1 mol ammonium chloride
1 mol NH4+ 1 mol Cl- 2 mol total ions
104
b 1 mol sodium sulfide
2 mol Na+ 1 mol S-2 3 mol total ions
105
c 05 mol barium nitrate
05 mol Ba+2 1 mol NO3- 15 mol total ions
106
Precipitation Reactions
Even though no compound is completely insoluble compounds of very low solubility can be considered insoluble for practical purposes There are general guidelines to help predict whether a compound made of certain combination of ions is soluble
107
General Solubility Guidelines
Sodium potassium and ammonium compounds are soluble in water
108
Nitrates acetates and chlorates are soluble
109
110
Most chlorides are soluble except those of silver mercury (I) and lead Lead (II) chloride is soluble in hot water
Most sulfates are soluble except those of barium strontium lead calcium and mercury
111
Most carbonates phosphates and silicates are insoluble except those of sodium potassium and ammonium
112
Most sulfides are insoluble except those of calcium strontium sodium potassium and ammonium
113
Is calcium phosphate Ca3(PO4)2 soluble or insoluble
Not one of the exceptions so it is insoluble
Dissociation equations are not written for insoluble compounds
114
Guidelines also useful in predicting what will happen if 2 solutions of 2 different soluble compounds are mixed
If mixing results in combination of ions that forms insoluble compound a double-replacement and precipitation reaction will happen
115
Will precipitate form when solutions of ammonium sulfide and cadmium nitrate are combined You can tell calcium nitrate is soluble from the guidelines (Most nitrates acetates and chlorates are soluble) You can also tell ammonium sulfide is soluble (Most sulfides are insoluble except those of calcium strontium sodium potassium and ammonium)
116
117
To decide whether precipitate can form must know solubilities of two possible products
Ammonium nitrate ndash soluble (Most nitrates acetates and chlorates are soluble) Cadmium sulfide ndash insoluble (Most sulfides are insoluble except those of calcium strontium sodium potassium and ammonium)
118
bc one product is insoluble double-replacement and precipitation reaction will happen (NH4)2S (aq) + Cd(NO3)2 (aq) rarr NH4NO3 (aq) + CdS (s)
119
Net Ionic Equations
Reactions of ions in aqueous solution usually represented by net ionic equations instead of formula equations Net ionic equation - includes only those compounds and ions that undergo a chemical change in a reaction in an aqueous solution
120
121
To write NIE (net ionic equation) first convert chemical equation into overall ionic equation All soluble ionic compounds shown as dissociated ions in solution Precipitates shown as solids
122
Notice ammonium and nitrate ions appear on both sides They havenrsquot gone through any chemical change Spectator ions - ions that do not take part in a chemical reaction and are found in solution both before and after the reaction
123
To change ionic equation to NIE eliminate spectator ions Cd+2(aq) + S-2(aq) rarr CdS(s)
This NIE applies to any reaction in which precipitate of CdS forms
124
Practice Problem
Identify the precipitate that forms when aqueous solutions of zinc nitrate and ammonium sulfide are combined Write the equation for the possible double-replacement reaction Then write the formula equation overall ionic equation and the net ionic equation
125
1 AnalyzeGiven Identity of reactants zinc nitrate and ammonium sulfide Reaction medium aqueous solution Unknown a equation for the possible double-replacement reaction b identity of the precipitate c formula equation d overall ionic equation e net ionic equation
126
2 PlanWrite the possible double-replacement reaction between Zn(NO3)2 and (NH4)2S Use the rules to determine if any of the products will precipitate Write a formula equation and overall net ionic equation Then cancel spectator ions to make net ionic equation
127
3 Computea possible double-replacement reaction
b rules show that zinc sulfide is not soluble so will precipitate (ammonium nitrate is soluble)
c formula equation128
d overall ionic equation
e net ionic equation129
Will a precipitate form if solutions of potassium sulfate and barium nitrate are combined If so write the net ionic equation for the reaction
Yes Ba+2(aq) + SO4-2(aq) rarr BaSO4(s)
130
Will a precipitate form if solutions of potassium nitrate and magnesium sulfate are combined If so write the net ionic equation for the reaction
No 131
Will a precipitate form if solutions of barium chloride and sodium sulfate are combined If so identify the spectator ions and write the net ionic equation for the reaction
Yes Na+ and Cl- Ba+2(aq) + SO4-2(aq) rarr BaSO4(s)
132
Write the net ionic equation for the precipitation of nickel(II) sulfide
Ni+2(aq) + S-2(aq) rarr NiS(s)
133
IonizationSome molecular compounds can also form ions in solution Usually polar Ionization - ions formed from solute molecules by action of the solvent (creation of ions where there were none) Ionization different from dissociation
134
Dissociation ionic compounds dissolve and ions already present separate Ionization ions formed where non existed before
Ions formed are hydrated Heat released during hydration of ions gives enough energy to break covalent bonds
135
Extent to which solute ionizes depends on strength of bonds within molecules of solute and strength of attraction between solute and solvent molecules
136
HCl ndash molecular compound that ionizes in aqueous solution Contains highly polar bond Attraction between polar HCl molecule and polar water strong enough to break HCl bond
137
The Hydronium Ion
H+ ions from HCl attracts other molecules or ions so strongly that it doesnrsquot normally exist alone Ionization of HCl better described as direct transfer of proton from HCl to H2O forming H3O+
138
139
Hydration of H+ to form hydronium is highly exothermic
Energy released gives a lot of energy needed to ionize a molecular solute
Many molecular compounds that ionize in aqueous solution contain hydrogen and form hydronium ion
140
Strong and Weak Electrolytes
Substances that make ions and conduct current in solution are electrolytes HCl is one of series of compounds that has hydrogen and halogen Hydrogen halides are all molecular compounds with single polar-covalent bonds
141
All are gases Very soluble in water All are electrolytes HCl HBr HI strongly conduct current in solution HF only weakly conducts current at same concentration Strength of conduction related to ability to ionize
142
143
Strong Electrolytes
HCl HBr HI are 100 ionized in solution
Strong electrolyte - any compound whose dilute aqueous solutions conduct electricity well this is due to the presence of all or almost all of the dissolve compound in the form of ions
144
Unique characteristic of strong electrolyte is that they yield only ions
Ex Ionic compound may be highly soluble in water and dissociate into ions (NaCl)
Others may not dissolve much but amount that does dissolve exists only as ions
145
Weak ElectrolytesHF dissolves in water to give acid solution called hydrofluoric acid HF bond stronger than bonds between hydrogen and other halogens When HF dissolves some molecule ionize Reverse reaction also happens HF(aq) + H2O(l) H3O+(aq) + F-(aq)
146
HF(aq) + H2O(l) H3O+
(aq) + F-(aq)Concentration of dissolved unionized HF stays high and concentration of ions stays low
Weak electrolyte - any compound whose aqueous solutions conduct electricity poorly this is due to the presence of small amount of the dissolved compound in the form of ions
147
Different from nonelectrolyte ndash NO ions at all Description of electrolyte as strong or weak not related to concentration of solution Electrolytes differ in degree of ionization not amount of solute dissolved
148
149
Colligative properties of solutions
Presence of solute affects properties of solutions Some properties not dependent on nature of dissolved substance but on how may dissolve particles are present
Colligative properties - properties that depend on the concentration of solute particles but not on their identity
150
Vapor-Pressure Lowering
Boiling and freezing point of solution different from pure solvent Nonvolatile solute raises boiling point and lowers freezing point Nonvolatile substance - one that has little tendency to become gas under existing conditions
151
152
To understand why nonvolatile solute changes boiling and freezing point consider equilibrium vapor pressure
Vapor pressure pressure caused by molecules that have escape liquid phase to gas phase
Can be thought of a measure of tendency of molecules to escape from a liquid
153
Addition of sucrose (nonvolatile solute) lowers concentration of water molecules at surface of liquid This lowers tendency of water molecules to leave solution and enter gas phase Vapor pressure of solution is lower than vapor pressure of pure water
154
Aqueous13 solution13 of13 nonvolatile13 solutePure13 water
Nonelectrolyte solutions of same molality have same concentration of particles
Dilute solutions of same solvent and equal molality of any nonelectrolyte solute lower vapor pressure equally
155
Example
1 m aqueous solution of nonelectrolyte glucose lowers vapor pressure of water 55 x 10-4 atm at 25 1 m aqueous solution of sucrose also lowers water vapor pressure to 55 x 10-4 atm at 25
156
157
bc vapor-pressure lowering depends on concentration of nonelectrolyte solute and doesnrsquot depend on the type of solute it is a colligative proprerty
bc vapor pressure has been lowered solution remains liquid over larger temperature range
Lowers freezing point and raises boiling point
158
Can assume changes in boiling and freezing point also depend on concentration of solute
They are colligative properties
159
Freezing-Point Depression
Freezing point of 1-molal solution of any nonelectrolyte solute in water is found (by experiment) to be 186 lower than freezing point of water When 1 mol of nonelectrolyte solute dissolved in 1 kg water freezing point is -186 not 000 160
When 2 mol nonelectrolyte solute dissolved in 1 kg water freezing point is -372
For any concentration of nonelectrolyte solute in water decrease in freezing point can be determined using value of -186m
161
This value called the molal freezing-point contstant (Kf) is the freezing-point depression of the solvent in a 1-molal solution of a nonvolatile nonelectrolyte solute
162
Each solvent has own characteristic molal freezing-point constant Freezing-point depression (Δtf) the difference between the freezing points of the pure solvent and a solute of nonelectrolyte in that solvent and it is directly proportional to the molal concentration of the solution Δtf = Kfm
163
Δtf = Kfm
Kf expressed as m
m expressed in mol solutekg solvent (molality)
164
Sample Problem
What is the freezing point depression of water in a solution of 171 g of sucrose C12H22O11 and 200 g of water What is the actual freezing point of the solution
165
1 AnalyzeGiven Solute mass and chemical formula 171 g C12H22O11 Solvent mass and identity 2000 g water Unknown a freezing-point depression b freezing point of the solution 166
2 Plan
167
168
Calculate
Unknown a freezing-point depression b freezing point of the solution
169
Practice Problem
A water solution containing an unknown quantity of a nonelectrolyte solute is found to have a freezing point of ndash 023degCWhat is the molal concentration of the solution
012 m170
A solution consists of 103 g of the nonelectrolyte glucose C6H12O6 dissolved in 250 g of water What is the freezing-point depression of the solution
minus0426degC171
In a laboratory experiment the freezing point of an aqueous solution of glucose is found to be minus0325degCWhat is the molal concentration of this solution
0175 m172
If 0500 mol of a nonelectrolyte solute are dissolved in 5000 g of ether what is the freezing point of the solution
minus1181degC173
The freezing point of an aqueous solution that contains a nonelectrolyte is minus90degC aWhat is the freezing-point depression of the solution bWhat is the molal concentration of the solution a minus90degC b 48 m
174
Boiling-Point Elevation
Boiling point is temp at which vapor pressure is equal to atmospheric pressure Change in vapor pressure will cause corresponding change in boiling point
175
176
Vapor pressure of nonvolatile solution is lower than vapor pressure of pure solvent More heat will be required to raise vapor pressure of solution Boiling point of solution is higher than boiling point of pure solvent
Molal boiling-point constant (Kb) - boiling-point elevation of the solvent in a 1-molal solution of a nonvolatile nonelectrolyte solute b-p elevation of any nonelectrolyte in water found by experiment to be 051 So molal b-p constant for water is 051m
177
For different solvents b-p elevations of 1-molal solutions have different values (Table 14-2)
Like freezing-point constants values most accurate for dilute solutions
178
Boiling-point elevation Δtb difference between the boiling points of the pure solvent and a nonelectrolyte solution of that solvent and it is directly proportional to the molal concentration of the solution B-p elevation can be calculated using Δtb = Kbm Δtb (boiling point elevation) expressed in m and m expressed in mol solutekg solvent
Sample Problem
What is the boiling-point elevation of a solution made from 200 g of a nonelectrolyte solute and 4000 g of water The molar mass of the solute is 620 gmol
180
1 AnalyzeGiven solute mass = 200 g solute molar mass = 620 gmol solvent mass and identity = 4000 g of water
Unknown boiling-point elevation
181
2 Plan
182
183
3 Compute
Practice Problem
A solution contains 500 g of sucrose C12H22O11 a nonelectrolyte dissolved in 5000 g of water What is the boiling-point elevation
015degC184
A solution contains 4500 g of sucrose C12H22O11 a nonelectrolyte dissolved in 250 g of waterWhat is the boiling point of the solution
1027degC185
If the boiling point elevation of an aqueous solution containing a nonvolatile electrolyte is 102degC what is the molality of the solution
20 m186
The boiling point of an aqueous solution containing a nonvolatile electrolyte is 10075degC aWhat is the boiling-point elevation bWhat is the molality of the solution a 075degC b 15 m
187
Electrolytes and Colligative PropertiesEarly investigators confused by experiments where certain substances depressed freezing point or elevated boiling point more than expected
188
189
Ex 01 m solution of NaCl lowers freezing point of solvent almost twice as much as 01 m solution of sucrose 01 m CaCl2 lowers fp almost 3 times as much as 01 m solution of sucrose
To understand why this happens contrast behavior or sucrose with NaCl in aqueous solution Each sucrose dissolves to make only 1 particle in solution So 1 mol sucrose dissolves to 1 mol particles 1 mol NaCl dissolves to make 2 moles of particles (1 mol Na+ and 1 mol Cl-)
190
191
Calculated Values for Electrolyte SolutionsColligative properties depend on total concentration of solute particles regardless of their identity Electrolytes cause changes in colligative properties proportional to total molality in terms of all dissolves particles instead of formula units
192
What about barium nitrate Ba(NO3)2 Each mole of barium nitrate yields 3 mol of ions in solution Ba(NO3)2(s) rarr Ba2+(aq) + 2NO3-(aq) You would expect solution of given molality to lower fp of solvent 3 times as much as nonelectrolytic solution of same molality
193
Sample Problem
What is the expected change in the freezing point of water in a solution of 625 g of barium nitrate Ba(NO3)2 in 100 kg of water
194
1 AnalyzeGiven solute mass and formula = 625 g Ba(NO3)2 solvent mass and identity = 100 kg water Δtf = Kfm
Unknown expected freezing-point depression
195
2 PlanThe molality can be calculated by converting the solute mass to moles and then dividing by the number of kilograms of solvent That molality is in terms of formula units of Ba(NO3)2 and must be converted to molality in terms of dissociated ions in solution It must be multiplied by the number of moles of ions produced per mole of formula unit This adjusted molality can then be used to calculate the freezing-point depression
196
197
198
3 Compute
Practice Problem
What is the expected freezing-point depression for a solution that contains 20 mol of magnesium sulfate dissolved in 10 kg of water
minus74degC199
What is the expected boiling-point elevation of water for a solution that contains 150 g of sodium chloride dissolved in 10 kg of water 27degC The freezing point of an aqueous sodium chloride solution is minus200degC What is the molality of the solution 54 m NaCl
200
Actual Values for Electrolyte SolutionsRemember values calculated are only expected values Actual values of colligative properties for all strong electrolytes are almost what would be expected Difference between expected and actual values caused by attractive forces that exist between dissociated ions in aqueous solution
201
Attraction between hydrated ions in solution is small compared with those in crystalline solid However forces of attraction donrsquot interfere with movements of aqueous ions More concentrated solution means ions closer together and attraction is greater Also depends on charge of ions ndash higher charge greater attraction
202
Components of Solutions
Particles of one substance randomly mixed with particles of another Solvent - dissolving medium in a solution Solute - substance dissolved in solution
7
Solute usually less than solvent Dissolved solute particles cannot be seen Stay mixed with solvent forever so long as conditions stay the same Solute-particles from 001-1 nm in diameter
8
Types of Solutions
Solutions can exist as gases liquids or solids
9
Many alloys such as brass (zinc and copper) are sterling silver (silver and copper) are solid solutions Atoms of two or metals are evenly mixed By properly choosing percentages of each metal can get many desirable properties
10
Ex Alloys have higher strength and greater resistance to corrosion than pure metals Pure gold (24K) too soft for jewelry Alloy with silver (14K) increases strength and hardness but keeps appearance of gold
11
SuspensionsSuspension - if particles in solvent are so large they settle out unless mixture constantly stirred Think of sand and water Particles over 1000 nm in diameter
12
Particles that are medium in size between those in solutions and suspensions from mixtures are known as colloids (colloidal dispersions) Particles between 1-1000 nm diameter
13
Colloids
In muddy water large soil particles settle Water still cloudy bc colloidal particles stay dispersed in water Filter Colloidal particles pass through and stays cloudy
14
Particles in colloid small enough to be suspended in solvent by constant movement of surrounding molecules Colloidal particles make up dispersed phase Water is dispersing medium
15
Emulsion and foam are colloids
Mayonnaise is emulsion of oil droplets in water Egg yolk acts as emulsifying agent (keeps oil droplets dispersed)
16
17
18
Class13 of13 Colloid Phases Example
Sol Solid13 dispersed13 in13 liquid Paints13 mud
Gel Solid13 network13 extending13 through13 liquid
Gelatin
Liquid13 emulsion Liquid13 dispersed13 in13 liquid Milk13 mayonnaise
Foam Gas13 dispersed13 in13 liquid Shaving13 cream13 whipped13 cream
Solid13 aerosol Solid13 dispersed13 in13 gas Smoke13 auto13 exhaust
Liquid13 aerosol Liquid13 dispersed13 in13 gas Fog13 mist13 clouds13 aerosol13 spray
Solid13 emulsion Liquid13 dispersed13 in13 solid Cheese13 butter
Tyndall EffectMany colloids appear homogeneous bc individual particles cannot be seen Particles large enough to scatter light In fog can see headlight beam Tyndall effect - occurs when light is scattered by colloidal particles dispersed in transparent medium
19
20
21
Solutions Colloids Suspensions
Homogeneous Heterogeneous Heterogeneous
Particle13 size13 001-shy‐113 nm13 can13 be13 atoms13 ions13 molecules
Particle13 size13 1-shy‐100013 nm13 dispersed13 can13 be13 combined13 or13 large13 molecules
Particle13 size13 over13 100013 nm13 suspended13 can13 be13 large13 particles13 or13 combined13 particles
Do13 not13 separate13 on13 standing Do13 not13 separate13 on13 standing Particles13 settle13 out
Cannot13 be13 separated13 by13 filtration Cannot13 be13 separated13 by13 filtration Can13 be13 separated13 by13 filtration
Do13 not13 scatter13 light Scatter13 light13 (Tyndall13 effect) May13 scatter13 light13 but13 are13 not13 transparent
Electrolytes vs Nonelectrolytes
Substances that dissolve in water are classified according to whether they produce molecules or ions in solution
22
23
When ionic compound dissolves cations and anions separate and are surrounded by water molecules Solute ions free to move making possible to carry electric current
Electrolyte - substance that dissolves in water to give a solution that conducts electric current NaCl ndash electrolyte Usually highly polar compounds become electrolytes
24
Solution containing neutral solute molecules does not conduct current bc it doesnrsquot have mobile charged particles
Nonelectrolyte - substance that dissolves in water to give solution that does not conduct electric current Ex Sugar
25
26
27the solution
process
Factors Affecting Rate of DissolutionEver tried to dissolve sugar in iced tea or coffee You notice temperature has something to do with how quickly solute dissolves What other factors affect how quickly solutes dissolve
28
1 Increasing Surface Area of Solute
bc dissolution process occurs at surface of solute Can increase dissolution by increasing surface area
29
2 Agitating (Stirring) SolutionClose to surface of solute concentration of dissolved solute is high Stirringshaking helps disperse solute particles
30
Brings fresh solvent into contact with solute surface Contact between solvent and solute surface increased
3 Heating a Solvent
Solutes dissolve faster in warmer solvent As temp increases solvent molecules move faster Average kinetic energy increases Collisions between solvent and solute more frequent Helps separate solute molecules and spread them out
32
Solubility
If you add sugar to tea eventually no more sugar dissolves
For every combination of solvent with solute at given temp there is limit to amount of solute that can be dissolved
33
When sugar first dropped into water sugar molecules leave solid surface and move about at random in solvent Some dissolved molecules may collide with crystal and stay there As more solid dissolves and concentration of dissolved molecules increases collisions happen more often
34
Eventually molecules are returning to crystal at same rate they are going into solution Dynamic equilibrium established between dissolution and crystallization Solution equilibrium - physical state in which the opposing processes of dissolution and crystallization of a solute occur at equal rates
35
Saturated vs Unsaturated Solutions
Saturated solution - solution that contains the maximum amount of dissolved solute How do you know if solution is saturated If more NaCl added it falls to bottom and doesnrsquot dissolve Equilibrium already established
36
If more water added to saturated solution more NaCl will dissolve At 20 359 g NaCl is max that will dissolve in 100 g water Unsaturated solution - solution that contains less solute than a saturated solution under the existing conditions
37
Supersaturated Solutions
When saturated solution where solubility increases as temp increases is cooled excess solute usually comes out of solution Sometimes if solution is left to cool the excess solute doesnrsquot separate
38
39
Supersaturated solution - solution that contains more dissolved solute than a saturated solution contains under same conditions
40
Supersaturated solution can remain unchanged over long time if it isnrsquot disturbed Once crystals begin to form continues until equilibrium is recreated at lower temp
41
Example
Sodium thiosulfate Na2S2O3 Solute added to hot water until solution saturated Hot solution filtered Drop small crystal in ldquoseedingrdquo crystallization Continues until equilibrium created
42
Solubility Values
Solubility - amount of substance required to form a saturated solution with specific amount of solvent at specified temp Ex Solubility of sugar = 204 g per 100 g water at 20 Must specify temp bc solubility changes with temp 43
For gases pressure must also be specified Rate at which substance dissolves is unrelated to solubility Max amount of solute that dissolves and reaches equilibrium is always same under same conditions
44
Solute-Solvent Interactions
ldquolike dissolves likerdquo is generally useful rule for predicting whether one substance will dissolve in another Depends on type of bonding polaritynonpolarity and intermolecular forces between solute and solvent
45
Dissolving Ionic Compounds in Aqueous Solution
Polarity of water molecules plays important role in formation of solution of ionic compounds Charged ends of water molecules attract ions and surround them to keep separated from other ions
46
Hydration - solution process with water as solvent Ions said to be hydrated As hydrated ions diffuse into solution other ions exposed and drawn away from crystal surface Entire crystal gradually dissolves
47
When crystallized from aqueous solutions some ionic compounds form crystals that include water molecules The crystalline compounds known as hydrates have specific ratios of water molecules Represented by formulas like CuSO45H2O
48
When crystalline hydrate dissolves in water water of hydration returns to solvent Behavior of solute in hydrated form no different than anhydrous form
49
Nonpolar Solvents
Nonpolar solvents do not attract ions of ionic compound strongly enough to overcome forces holding crystal together
50
Liquid Solutes and Solvents
When you shake bottle of salad dressing oil droplets disperse in water Stop shaking ndash strong attraction between water molecules squeeze out oil and form separate layers Liquid solutes and solvents that are not soluble in each other are immiscible
51
Nonpolar substances (fat oil grease) are generally quite soluble in nonpolar liquids (carbon tetrachloride toluene gasoline)
Only attractions between nonpolar molecules are weak London forces
52
London Dispersion Forces
bc e- are constantly moving at any time there may be uneven distribution of them around a nonpolar atom Temporarily creates positive end and negative end
53
54
London dispersion forces - intermolecular attractions resulting from constant motion of e- and creation of instantaneous dipoles
55
Liquids that dissolve freely in one another in any ratio are said to be completely miscible Benzene and carbon tetrachloride (both nonpolar) are completely miscible
Nonpolar molecules of these two apply no strong forces of attraction or repulsion and molecules mix freely
56
Ethanol and water are also completely miscible -OH group on ethanol is slightly polar
Can form H-bonds with water as well as other ethanol molecules Intermolecular forces in mixture are very similar to those in pure liquids so they are mutually soluble in any amount
57
Gasoline contains mainly nonpolar hydrocarbons
Excellent solvent for fats oils greases
Major intermolecular forces are weak London forces
58
Ethanol less polar than water more than carbon tetrachloride
Better solvent for less-polar substances bc of nonpolar region
59
Effects of Pressure on Solubility
Little effect on solids or liquids Increase pressure - increase solubilities of gases in liquids
60
When gas in contact with surface of liquid gas molecules can enter liquid As amount of dissolve gas increases some molecules start to escape and reenter gas phase
Eq eventually established between rates of enteringleaving gas phase
61
As long as eq undisturbed solubility of gas in liquid unchanged at given pressure
Gas + solvent solution
62
Increasing pressure of solute gas above solution puts stress on eq Molecules collide with surface more often Increase in pressure partially offset by increase in rate of gas molecules entering solution In turn increase in amount of dissolved gas causes increase in rate molecules escape liquid and becomes gas
63
Eventually eq restored at higher gas solubility
Expected from Le Chatelierrsquos principle
Increase in gas pressure causes eq to shift so fewer molecules are in gas phase
64
Henryrsquos Law
Henryrsquos law - solubility of a gas in liquid is directly proportional to the partial pressure of that gas on the surface of the liquid
Applies to gas-liquid solutions at constant temp
65
When mixture of ideal gases is limited in constant volume at constant temp each gas applies same pressure it would apply if it occupied the space alone
Assuming gases do not react each gas dissolves to the extent it would if no other gases were there
66
In carbonated drinks (Coke Pepsi etc) solubility of CO2 increased by increasing pressure
At bottling factory CO2 gas forced into solution of flavored water at pressure of 5-10 atm Gas-in-liquid solution then sealed in bottlescans
67
When cap removed pressure reduced to 1 atm and some CO2 escapes as gas bubbles Effervescence - rapid escape of a gas from a liquid in which it is dissolved
68
Effects of Temperature on Solubility
Letrsquos consider gas solubility Increasing temperature usually decreases gas solubility As temp increases average kinetic energy of molecules in solution increases Greater number of solute molecules escape from attraction of solvent and return to gas phase 69
At higher temps eq is reached with fewer gas molecules in solution Gases generally less soluble
70
Effect of temp on solubility of solids in liquids more difficult to predict
Often increasing temp increases solubility
However the same temp increase can result in large increase in solubility in one case and only slight increase in the next
71
72
enthalpies of Solution
Formation of solution comes with energy change Dissolve KI in water container feels cold Dissolve LiCl in water feels hot Formation of solid-liquid solution can absorb or release heat
73
During formation of solution solvent and solute particles experience changes in forces attracting them to other particles Before dissolving begins solvent molecules held by intermolecular forces (solvent-solvent attraction) In solute molecules held by Ifs (solute-solute attraction)
74
Energy is required to separate solute molecules and solvent molecules from their neighbors
A solute particle that is surrounded by solvent molecules is said to be solvated
75
Step 1 solute particles become separated from solid (energy absorbed) Step 2 solvent particles move apart to allow solute particles to enter liquid (energy absorbed)
76
Step 3 solvent particles attracted to and solvate solute particles (energy released)
77
Heat of solution - net amount of heat energy absorbed or released when a specific amount of solute dissolves in a solvent
Heat of solution negative (heat is released) if sums of energy in steps 1 and 2 is less than step 3
If 1+2 gt 3 HOS is positive (heat absorbed)78
In gaseous state molecules are so far apart there are nearly no forces between them Solute-solute interaction has little effect on heat of solution of a gas Energy released when gas dissolved in liquid bc attraction between solute gas and solvent molecules greater than energy needed to separate solvent molecules
79
Concentration of Solutions80
Concentration - measure of amount of solute in given amount of solvent or solution ldquodiluterdquo just means small amount of solute ldquoconcentratedrdquo just means relatively large amount of solute Unrelated to degree to which solution is saturated Ex Saturated solution of substance not very soluble might be dilute
81
Molarity
Molarity - number of moles of solute in one liter of solution To find molarity must know molar mass A ldquoone molarrdquo solution of NaOH is 1 mole NaOH per liter of solution (1 M)
82
1 mol NaOH = 400 g If this quantity dissolve in enough water to make EXACTLY 100 L solution solution is 1 M If 200 g NaOH dissolve in enough to make 100 L solution what is molarity 0500 M
83
1 M solution is not made by adding 1 mol solute to 1 L solvent That would be more than 1 L Instead 1 mol solute dissolved in less than 1 L Then diluted with more solvent to bring TOTAL VOLUME to 1 L
84
Practice ProblemYou have 340 L of solution that contains 900 g sodium chloride What is the molarity of that solution 0440 M NaCl You have 08 L of a 05 M HCl solution How many moles of HCl does this solution contain 04 mol HCl
85
What is the molarity of a solution composed of 585 g of potassium iodide dissolved in enough water to make a 0125 L of solution 0282 M KI
86
How many moles of H2SO4 are present in 0500 L of a 0150 M H2SO4 solution 00750 mol What volume of 300 M NaCl is needed for reaction that requires 1463 g of NaCl 0834 L
87
Molality
Molality - concentration of solution expressed in moles of solute per kilogram of solvent Solution that contains 1 mol solute (NaOH) dissolved in exactly 1 kg of solvent is ldquoone molalrdquo solution (1 m NaOH)
88
1 mol NaOH = 400 g 400 g NaOH dissolved in 1 kg water is 1 m NaOH
200 g NaOH in 1 kg water = 0500 m NaOH
89
Practice Problem
A solution was prepared by dissolving 171 g sucrose (C12H22O11) in 125 g of water Find the molal concentration of this solution 0400 m C12H22O11
90
A solution of iodine in carbon tetrachloride is used when iodine is needed for certain chemical tests How much iodine must be added to prepare a 0480 m solution of iodine in CCl4 if 1000 g of CCl4 is used 122 g I2
91
What is the molality of a solution composed of 255 g acetone (CH3)2CO dissolved in 200 g of water 22 m acetone What quantity in grams of methanol CH3OH is required to prepare a 0244 m solution in 400 g water 312 g methanol
92
How many grams of AgNO3 are needed to prepare a 0125 m solution in 250 mL of water 531 g AgNO3
What is the molality of a solution containing 182 g HCl and 250 g water 200 m
93
Ions in Aqueous Solutions and Colligative Properties
94
95
compounds in aqueous solutions
DissociationWhen compound made from ions dissolves in water ions separate Dissociation - separation of ions that occurs when an ionic compound dissolves
96
Notice number of ions made per formula unit in equations 1 formula unit of NaCl gives 2 units of ions in solution 1 formula unit of CaCl2 gives 3 units of ions
97
Assuming 100 dissociation solution that contains 1 mol NaCl contains 1 mol Na+ and 1 mol Cl- Can assume 100 dissociation of all soluble ionic compounds
98
Practice Problem
Write the equation for the dissolution of aluminum sulfate in water How many moles of aluminum ions are made by dissolving 1 mol aluminum sulfate What is the total number of moles of ions made by dissolving 1 mol of aluminum sulfate
99
1 AnalyzeGiven Amount of solute = 1 mol Al2(SO4)3 Solvent identity = water Unknown a moles of aluminum ions and sulfate ions Total number of moles of solute ions produced
100
2 Plan
The coefficients in the balanced dissociation equation will tell mole relationships You can use the equation to find out the number of moles of solute ions produced
101
3 Compute
a
b102
Write the equation for the dissolution of each of the following in water and then determine the number of moles of each ions made as well as the total number of ions made a 1 mol ammonium chloride b 1 mol sodium sulfide c 05 mol barium nitrate
103
a 1 mol ammonium chloride
1 mol NH4+ 1 mol Cl- 2 mol total ions
104
b 1 mol sodium sulfide
2 mol Na+ 1 mol S-2 3 mol total ions
105
c 05 mol barium nitrate
05 mol Ba+2 1 mol NO3- 15 mol total ions
106
Precipitation Reactions
Even though no compound is completely insoluble compounds of very low solubility can be considered insoluble for practical purposes There are general guidelines to help predict whether a compound made of certain combination of ions is soluble
107
General Solubility Guidelines
Sodium potassium and ammonium compounds are soluble in water
108
Nitrates acetates and chlorates are soluble
109
110
Most chlorides are soluble except those of silver mercury (I) and lead Lead (II) chloride is soluble in hot water
Most sulfates are soluble except those of barium strontium lead calcium and mercury
111
Most carbonates phosphates and silicates are insoluble except those of sodium potassium and ammonium
112
Most sulfides are insoluble except those of calcium strontium sodium potassium and ammonium
113
Is calcium phosphate Ca3(PO4)2 soluble or insoluble
Not one of the exceptions so it is insoluble
Dissociation equations are not written for insoluble compounds
114
Guidelines also useful in predicting what will happen if 2 solutions of 2 different soluble compounds are mixed
If mixing results in combination of ions that forms insoluble compound a double-replacement and precipitation reaction will happen
115
Will precipitate form when solutions of ammonium sulfide and cadmium nitrate are combined You can tell calcium nitrate is soluble from the guidelines (Most nitrates acetates and chlorates are soluble) You can also tell ammonium sulfide is soluble (Most sulfides are insoluble except those of calcium strontium sodium potassium and ammonium)
116
117
To decide whether precipitate can form must know solubilities of two possible products
Ammonium nitrate ndash soluble (Most nitrates acetates and chlorates are soluble) Cadmium sulfide ndash insoluble (Most sulfides are insoluble except those of calcium strontium sodium potassium and ammonium)
118
bc one product is insoluble double-replacement and precipitation reaction will happen (NH4)2S (aq) + Cd(NO3)2 (aq) rarr NH4NO3 (aq) + CdS (s)
119
Net Ionic Equations
Reactions of ions in aqueous solution usually represented by net ionic equations instead of formula equations Net ionic equation - includes only those compounds and ions that undergo a chemical change in a reaction in an aqueous solution
120
121
To write NIE (net ionic equation) first convert chemical equation into overall ionic equation All soluble ionic compounds shown as dissociated ions in solution Precipitates shown as solids
122
Notice ammonium and nitrate ions appear on both sides They havenrsquot gone through any chemical change Spectator ions - ions that do not take part in a chemical reaction and are found in solution both before and after the reaction
123
To change ionic equation to NIE eliminate spectator ions Cd+2(aq) + S-2(aq) rarr CdS(s)
This NIE applies to any reaction in which precipitate of CdS forms
124
Practice Problem
Identify the precipitate that forms when aqueous solutions of zinc nitrate and ammonium sulfide are combined Write the equation for the possible double-replacement reaction Then write the formula equation overall ionic equation and the net ionic equation
125
1 AnalyzeGiven Identity of reactants zinc nitrate and ammonium sulfide Reaction medium aqueous solution Unknown a equation for the possible double-replacement reaction b identity of the precipitate c formula equation d overall ionic equation e net ionic equation
126
2 PlanWrite the possible double-replacement reaction between Zn(NO3)2 and (NH4)2S Use the rules to determine if any of the products will precipitate Write a formula equation and overall net ionic equation Then cancel spectator ions to make net ionic equation
127
3 Computea possible double-replacement reaction
b rules show that zinc sulfide is not soluble so will precipitate (ammonium nitrate is soluble)
c formula equation128
d overall ionic equation
e net ionic equation129
Will a precipitate form if solutions of potassium sulfate and barium nitrate are combined If so write the net ionic equation for the reaction
Yes Ba+2(aq) + SO4-2(aq) rarr BaSO4(s)
130
Will a precipitate form if solutions of potassium nitrate and magnesium sulfate are combined If so write the net ionic equation for the reaction
No 131
Will a precipitate form if solutions of barium chloride and sodium sulfate are combined If so identify the spectator ions and write the net ionic equation for the reaction
Yes Na+ and Cl- Ba+2(aq) + SO4-2(aq) rarr BaSO4(s)
132
Write the net ionic equation for the precipitation of nickel(II) sulfide
Ni+2(aq) + S-2(aq) rarr NiS(s)
133
IonizationSome molecular compounds can also form ions in solution Usually polar Ionization - ions formed from solute molecules by action of the solvent (creation of ions where there were none) Ionization different from dissociation
134
Dissociation ionic compounds dissolve and ions already present separate Ionization ions formed where non existed before
Ions formed are hydrated Heat released during hydration of ions gives enough energy to break covalent bonds
135
Extent to which solute ionizes depends on strength of bonds within molecules of solute and strength of attraction between solute and solvent molecules
136
HCl ndash molecular compound that ionizes in aqueous solution Contains highly polar bond Attraction between polar HCl molecule and polar water strong enough to break HCl bond
137
The Hydronium Ion
H+ ions from HCl attracts other molecules or ions so strongly that it doesnrsquot normally exist alone Ionization of HCl better described as direct transfer of proton from HCl to H2O forming H3O+
138
139
Hydration of H+ to form hydronium is highly exothermic
Energy released gives a lot of energy needed to ionize a molecular solute
Many molecular compounds that ionize in aqueous solution contain hydrogen and form hydronium ion
140
Strong and Weak Electrolytes
Substances that make ions and conduct current in solution are electrolytes HCl is one of series of compounds that has hydrogen and halogen Hydrogen halides are all molecular compounds with single polar-covalent bonds
141
All are gases Very soluble in water All are electrolytes HCl HBr HI strongly conduct current in solution HF only weakly conducts current at same concentration Strength of conduction related to ability to ionize
142
143
Strong Electrolytes
HCl HBr HI are 100 ionized in solution
Strong electrolyte - any compound whose dilute aqueous solutions conduct electricity well this is due to the presence of all or almost all of the dissolve compound in the form of ions
144
Unique characteristic of strong electrolyte is that they yield only ions
Ex Ionic compound may be highly soluble in water and dissociate into ions (NaCl)
Others may not dissolve much but amount that does dissolve exists only as ions
145
Weak ElectrolytesHF dissolves in water to give acid solution called hydrofluoric acid HF bond stronger than bonds between hydrogen and other halogens When HF dissolves some molecule ionize Reverse reaction also happens HF(aq) + H2O(l) H3O+(aq) + F-(aq)
146
HF(aq) + H2O(l) H3O+
(aq) + F-(aq)Concentration of dissolved unionized HF stays high and concentration of ions stays low
Weak electrolyte - any compound whose aqueous solutions conduct electricity poorly this is due to the presence of small amount of the dissolved compound in the form of ions
147
Different from nonelectrolyte ndash NO ions at all Description of electrolyte as strong or weak not related to concentration of solution Electrolytes differ in degree of ionization not amount of solute dissolved
148
149
Colligative properties of solutions
Presence of solute affects properties of solutions Some properties not dependent on nature of dissolved substance but on how may dissolve particles are present
Colligative properties - properties that depend on the concentration of solute particles but not on their identity
150
Vapor-Pressure Lowering
Boiling and freezing point of solution different from pure solvent Nonvolatile solute raises boiling point and lowers freezing point Nonvolatile substance - one that has little tendency to become gas under existing conditions
151
152
To understand why nonvolatile solute changes boiling and freezing point consider equilibrium vapor pressure
Vapor pressure pressure caused by molecules that have escape liquid phase to gas phase
Can be thought of a measure of tendency of molecules to escape from a liquid
153
Addition of sucrose (nonvolatile solute) lowers concentration of water molecules at surface of liquid This lowers tendency of water molecules to leave solution and enter gas phase Vapor pressure of solution is lower than vapor pressure of pure water
154
Aqueous13 solution13 of13 nonvolatile13 solutePure13 water
Nonelectrolyte solutions of same molality have same concentration of particles
Dilute solutions of same solvent and equal molality of any nonelectrolyte solute lower vapor pressure equally
155
Example
1 m aqueous solution of nonelectrolyte glucose lowers vapor pressure of water 55 x 10-4 atm at 25 1 m aqueous solution of sucrose also lowers water vapor pressure to 55 x 10-4 atm at 25
156
157
bc vapor-pressure lowering depends on concentration of nonelectrolyte solute and doesnrsquot depend on the type of solute it is a colligative proprerty
bc vapor pressure has been lowered solution remains liquid over larger temperature range
Lowers freezing point and raises boiling point
158
Can assume changes in boiling and freezing point also depend on concentration of solute
They are colligative properties
159
Freezing-Point Depression
Freezing point of 1-molal solution of any nonelectrolyte solute in water is found (by experiment) to be 186 lower than freezing point of water When 1 mol of nonelectrolyte solute dissolved in 1 kg water freezing point is -186 not 000 160
When 2 mol nonelectrolyte solute dissolved in 1 kg water freezing point is -372
For any concentration of nonelectrolyte solute in water decrease in freezing point can be determined using value of -186m
161
This value called the molal freezing-point contstant (Kf) is the freezing-point depression of the solvent in a 1-molal solution of a nonvolatile nonelectrolyte solute
162
Each solvent has own characteristic molal freezing-point constant Freezing-point depression (Δtf) the difference between the freezing points of the pure solvent and a solute of nonelectrolyte in that solvent and it is directly proportional to the molal concentration of the solution Δtf = Kfm
163
Δtf = Kfm
Kf expressed as m
m expressed in mol solutekg solvent (molality)
164
Sample Problem
What is the freezing point depression of water in a solution of 171 g of sucrose C12H22O11 and 200 g of water What is the actual freezing point of the solution
165
1 AnalyzeGiven Solute mass and chemical formula 171 g C12H22O11 Solvent mass and identity 2000 g water Unknown a freezing-point depression b freezing point of the solution 166
2 Plan
167
168
Calculate
Unknown a freezing-point depression b freezing point of the solution
169
Practice Problem
A water solution containing an unknown quantity of a nonelectrolyte solute is found to have a freezing point of ndash 023degCWhat is the molal concentration of the solution
012 m170
A solution consists of 103 g of the nonelectrolyte glucose C6H12O6 dissolved in 250 g of water What is the freezing-point depression of the solution
minus0426degC171
In a laboratory experiment the freezing point of an aqueous solution of glucose is found to be minus0325degCWhat is the molal concentration of this solution
0175 m172
If 0500 mol of a nonelectrolyte solute are dissolved in 5000 g of ether what is the freezing point of the solution
minus1181degC173
The freezing point of an aqueous solution that contains a nonelectrolyte is minus90degC aWhat is the freezing-point depression of the solution bWhat is the molal concentration of the solution a minus90degC b 48 m
174
Boiling-Point Elevation
Boiling point is temp at which vapor pressure is equal to atmospheric pressure Change in vapor pressure will cause corresponding change in boiling point
175
176
Vapor pressure of nonvolatile solution is lower than vapor pressure of pure solvent More heat will be required to raise vapor pressure of solution Boiling point of solution is higher than boiling point of pure solvent
Molal boiling-point constant (Kb) - boiling-point elevation of the solvent in a 1-molal solution of a nonvolatile nonelectrolyte solute b-p elevation of any nonelectrolyte in water found by experiment to be 051 So molal b-p constant for water is 051m
177
For different solvents b-p elevations of 1-molal solutions have different values (Table 14-2)
Like freezing-point constants values most accurate for dilute solutions
178
Boiling-point elevation Δtb difference between the boiling points of the pure solvent and a nonelectrolyte solution of that solvent and it is directly proportional to the molal concentration of the solution B-p elevation can be calculated using Δtb = Kbm Δtb (boiling point elevation) expressed in m and m expressed in mol solutekg solvent
Sample Problem
What is the boiling-point elevation of a solution made from 200 g of a nonelectrolyte solute and 4000 g of water The molar mass of the solute is 620 gmol
180
1 AnalyzeGiven solute mass = 200 g solute molar mass = 620 gmol solvent mass and identity = 4000 g of water
Unknown boiling-point elevation
181
2 Plan
182
183
3 Compute
Practice Problem
A solution contains 500 g of sucrose C12H22O11 a nonelectrolyte dissolved in 5000 g of water What is the boiling-point elevation
015degC184
A solution contains 4500 g of sucrose C12H22O11 a nonelectrolyte dissolved in 250 g of waterWhat is the boiling point of the solution
1027degC185
If the boiling point elevation of an aqueous solution containing a nonvolatile electrolyte is 102degC what is the molality of the solution
20 m186
The boiling point of an aqueous solution containing a nonvolatile electrolyte is 10075degC aWhat is the boiling-point elevation bWhat is the molality of the solution a 075degC b 15 m
187
Electrolytes and Colligative PropertiesEarly investigators confused by experiments where certain substances depressed freezing point or elevated boiling point more than expected
188
189
Ex 01 m solution of NaCl lowers freezing point of solvent almost twice as much as 01 m solution of sucrose 01 m CaCl2 lowers fp almost 3 times as much as 01 m solution of sucrose
To understand why this happens contrast behavior or sucrose with NaCl in aqueous solution Each sucrose dissolves to make only 1 particle in solution So 1 mol sucrose dissolves to 1 mol particles 1 mol NaCl dissolves to make 2 moles of particles (1 mol Na+ and 1 mol Cl-)
190
191
Calculated Values for Electrolyte SolutionsColligative properties depend on total concentration of solute particles regardless of their identity Electrolytes cause changes in colligative properties proportional to total molality in terms of all dissolves particles instead of formula units
192
What about barium nitrate Ba(NO3)2 Each mole of barium nitrate yields 3 mol of ions in solution Ba(NO3)2(s) rarr Ba2+(aq) + 2NO3-(aq) You would expect solution of given molality to lower fp of solvent 3 times as much as nonelectrolytic solution of same molality
193
Sample Problem
What is the expected change in the freezing point of water in a solution of 625 g of barium nitrate Ba(NO3)2 in 100 kg of water
194
1 AnalyzeGiven solute mass and formula = 625 g Ba(NO3)2 solvent mass and identity = 100 kg water Δtf = Kfm
Unknown expected freezing-point depression
195
2 PlanThe molality can be calculated by converting the solute mass to moles and then dividing by the number of kilograms of solvent That molality is in terms of formula units of Ba(NO3)2 and must be converted to molality in terms of dissociated ions in solution It must be multiplied by the number of moles of ions produced per mole of formula unit This adjusted molality can then be used to calculate the freezing-point depression
196
197
198
3 Compute
Practice Problem
What is the expected freezing-point depression for a solution that contains 20 mol of magnesium sulfate dissolved in 10 kg of water
minus74degC199
What is the expected boiling-point elevation of water for a solution that contains 150 g of sodium chloride dissolved in 10 kg of water 27degC The freezing point of an aqueous sodium chloride solution is minus200degC What is the molality of the solution 54 m NaCl
200
Actual Values for Electrolyte SolutionsRemember values calculated are only expected values Actual values of colligative properties for all strong electrolytes are almost what would be expected Difference between expected and actual values caused by attractive forces that exist between dissociated ions in aqueous solution
201
Attraction between hydrated ions in solution is small compared with those in crystalline solid However forces of attraction donrsquot interfere with movements of aqueous ions More concentrated solution means ions closer together and attraction is greater Also depends on charge of ions ndash higher charge greater attraction
202
Solute usually less than solvent Dissolved solute particles cannot be seen Stay mixed with solvent forever so long as conditions stay the same Solute-particles from 001-1 nm in diameter
8
Types of Solutions
Solutions can exist as gases liquids or solids
9
Many alloys such as brass (zinc and copper) are sterling silver (silver and copper) are solid solutions Atoms of two or metals are evenly mixed By properly choosing percentages of each metal can get many desirable properties
10
Ex Alloys have higher strength and greater resistance to corrosion than pure metals Pure gold (24K) too soft for jewelry Alloy with silver (14K) increases strength and hardness but keeps appearance of gold
11
SuspensionsSuspension - if particles in solvent are so large they settle out unless mixture constantly stirred Think of sand and water Particles over 1000 nm in diameter
12
Particles that are medium in size between those in solutions and suspensions from mixtures are known as colloids (colloidal dispersions) Particles between 1-1000 nm diameter
13
Colloids
In muddy water large soil particles settle Water still cloudy bc colloidal particles stay dispersed in water Filter Colloidal particles pass through and stays cloudy
14
Particles in colloid small enough to be suspended in solvent by constant movement of surrounding molecules Colloidal particles make up dispersed phase Water is dispersing medium
15
Emulsion and foam are colloids
Mayonnaise is emulsion of oil droplets in water Egg yolk acts as emulsifying agent (keeps oil droplets dispersed)
16
17
18
Class13 of13 Colloid Phases Example
Sol Solid13 dispersed13 in13 liquid Paints13 mud
Gel Solid13 network13 extending13 through13 liquid
Gelatin
Liquid13 emulsion Liquid13 dispersed13 in13 liquid Milk13 mayonnaise
Foam Gas13 dispersed13 in13 liquid Shaving13 cream13 whipped13 cream
Solid13 aerosol Solid13 dispersed13 in13 gas Smoke13 auto13 exhaust
Liquid13 aerosol Liquid13 dispersed13 in13 gas Fog13 mist13 clouds13 aerosol13 spray
Solid13 emulsion Liquid13 dispersed13 in13 solid Cheese13 butter
Tyndall EffectMany colloids appear homogeneous bc individual particles cannot be seen Particles large enough to scatter light In fog can see headlight beam Tyndall effect - occurs when light is scattered by colloidal particles dispersed in transparent medium
19
20
21
Solutions Colloids Suspensions
Homogeneous Heterogeneous Heterogeneous
Particle13 size13 001-shy‐113 nm13 can13 be13 atoms13 ions13 molecules
Particle13 size13 1-shy‐100013 nm13 dispersed13 can13 be13 combined13 or13 large13 molecules
Particle13 size13 over13 100013 nm13 suspended13 can13 be13 large13 particles13 or13 combined13 particles
Do13 not13 separate13 on13 standing Do13 not13 separate13 on13 standing Particles13 settle13 out
Cannot13 be13 separated13 by13 filtration Cannot13 be13 separated13 by13 filtration Can13 be13 separated13 by13 filtration
Do13 not13 scatter13 light Scatter13 light13 (Tyndall13 effect) May13 scatter13 light13 but13 are13 not13 transparent
Electrolytes vs Nonelectrolytes
Substances that dissolve in water are classified according to whether they produce molecules or ions in solution
22
23
When ionic compound dissolves cations and anions separate and are surrounded by water molecules Solute ions free to move making possible to carry electric current
Electrolyte - substance that dissolves in water to give a solution that conducts electric current NaCl ndash electrolyte Usually highly polar compounds become electrolytes
24
Solution containing neutral solute molecules does not conduct current bc it doesnrsquot have mobile charged particles
Nonelectrolyte - substance that dissolves in water to give solution that does not conduct electric current Ex Sugar
25
26
27the solution
process
Factors Affecting Rate of DissolutionEver tried to dissolve sugar in iced tea or coffee You notice temperature has something to do with how quickly solute dissolves What other factors affect how quickly solutes dissolve
28
1 Increasing Surface Area of Solute
bc dissolution process occurs at surface of solute Can increase dissolution by increasing surface area
29
2 Agitating (Stirring) SolutionClose to surface of solute concentration of dissolved solute is high Stirringshaking helps disperse solute particles
30
Brings fresh solvent into contact with solute surface Contact between solvent and solute surface increased
3 Heating a Solvent
Solutes dissolve faster in warmer solvent As temp increases solvent molecules move faster Average kinetic energy increases Collisions between solvent and solute more frequent Helps separate solute molecules and spread them out
32
Solubility
If you add sugar to tea eventually no more sugar dissolves
For every combination of solvent with solute at given temp there is limit to amount of solute that can be dissolved
33
When sugar first dropped into water sugar molecules leave solid surface and move about at random in solvent Some dissolved molecules may collide with crystal and stay there As more solid dissolves and concentration of dissolved molecules increases collisions happen more often
34
Eventually molecules are returning to crystal at same rate they are going into solution Dynamic equilibrium established between dissolution and crystallization Solution equilibrium - physical state in which the opposing processes of dissolution and crystallization of a solute occur at equal rates
35
Saturated vs Unsaturated Solutions
Saturated solution - solution that contains the maximum amount of dissolved solute How do you know if solution is saturated If more NaCl added it falls to bottom and doesnrsquot dissolve Equilibrium already established
36
If more water added to saturated solution more NaCl will dissolve At 20 359 g NaCl is max that will dissolve in 100 g water Unsaturated solution - solution that contains less solute than a saturated solution under the existing conditions
37
Supersaturated Solutions
When saturated solution where solubility increases as temp increases is cooled excess solute usually comes out of solution Sometimes if solution is left to cool the excess solute doesnrsquot separate
38
39
Supersaturated solution - solution that contains more dissolved solute than a saturated solution contains under same conditions
40
Supersaturated solution can remain unchanged over long time if it isnrsquot disturbed Once crystals begin to form continues until equilibrium is recreated at lower temp
41
Example
Sodium thiosulfate Na2S2O3 Solute added to hot water until solution saturated Hot solution filtered Drop small crystal in ldquoseedingrdquo crystallization Continues until equilibrium created
42
Solubility Values
Solubility - amount of substance required to form a saturated solution with specific amount of solvent at specified temp Ex Solubility of sugar = 204 g per 100 g water at 20 Must specify temp bc solubility changes with temp 43
For gases pressure must also be specified Rate at which substance dissolves is unrelated to solubility Max amount of solute that dissolves and reaches equilibrium is always same under same conditions
44
Solute-Solvent Interactions
ldquolike dissolves likerdquo is generally useful rule for predicting whether one substance will dissolve in another Depends on type of bonding polaritynonpolarity and intermolecular forces between solute and solvent
45
Dissolving Ionic Compounds in Aqueous Solution
Polarity of water molecules plays important role in formation of solution of ionic compounds Charged ends of water molecules attract ions and surround them to keep separated from other ions
46
Hydration - solution process with water as solvent Ions said to be hydrated As hydrated ions diffuse into solution other ions exposed and drawn away from crystal surface Entire crystal gradually dissolves
47
When crystallized from aqueous solutions some ionic compounds form crystals that include water molecules The crystalline compounds known as hydrates have specific ratios of water molecules Represented by formulas like CuSO45H2O
48
When crystalline hydrate dissolves in water water of hydration returns to solvent Behavior of solute in hydrated form no different than anhydrous form
49
Nonpolar Solvents
Nonpolar solvents do not attract ions of ionic compound strongly enough to overcome forces holding crystal together
50
Liquid Solutes and Solvents
When you shake bottle of salad dressing oil droplets disperse in water Stop shaking ndash strong attraction between water molecules squeeze out oil and form separate layers Liquid solutes and solvents that are not soluble in each other are immiscible
51
Nonpolar substances (fat oil grease) are generally quite soluble in nonpolar liquids (carbon tetrachloride toluene gasoline)
Only attractions between nonpolar molecules are weak London forces
52
London Dispersion Forces
bc e- are constantly moving at any time there may be uneven distribution of them around a nonpolar atom Temporarily creates positive end and negative end
53
54
London dispersion forces - intermolecular attractions resulting from constant motion of e- and creation of instantaneous dipoles
55
Liquids that dissolve freely in one another in any ratio are said to be completely miscible Benzene and carbon tetrachloride (both nonpolar) are completely miscible
Nonpolar molecules of these two apply no strong forces of attraction or repulsion and molecules mix freely
56
Ethanol and water are also completely miscible -OH group on ethanol is slightly polar
Can form H-bonds with water as well as other ethanol molecules Intermolecular forces in mixture are very similar to those in pure liquids so they are mutually soluble in any amount
57
Gasoline contains mainly nonpolar hydrocarbons
Excellent solvent for fats oils greases
Major intermolecular forces are weak London forces
58
Ethanol less polar than water more than carbon tetrachloride
Better solvent for less-polar substances bc of nonpolar region
59
Effects of Pressure on Solubility
Little effect on solids or liquids Increase pressure - increase solubilities of gases in liquids
60
When gas in contact with surface of liquid gas molecules can enter liquid As amount of dissolve gas increases some molecules start to escape and reenter gas phase
Eq eventually established between rates of enteringleaving gas phase
61
As long as eq undisturbed solubility of gas in liquid unchanged at given pressure
Gas + solvent solution
62
Increasing pressure of solute gas above solution puts stress on eq Molecules collide with surface more often Increase in pressure partially offset by increase in rate of gas molecules entering solution In turn increase in amount of dissolved gas causes increase in rate molecules escape liquid and becomes gas
63
Eventually eq restored at higher gas solubility
Expected from Le Chatelierrsquos principle
Increase in gas pressure causes eq to shift so fewer molecules are in gas phase
64
Henryrsquos Law
Henryrsquos law - solubility of a gas in liquid is directly proportional to the partial pressure of that gas on the surface of the liquid
Applies to gas-liquid solutions at constant temp
65
When mixture of ideal gases is limited in constant volume at constant temp each gas applies same pressure it would apply if it occupied the space alone
Assuming gases do not react each gas dissolves to the extent it would if no other gases were there
66
In carbonated drinks (Coke Pepsi etc) solubility of CO2 increased by increasing pressure
At bottling factory CO2 gas forced into solution of flavored water at pressure of 5-10 atm Gas-in-liquid solution then sealed in bottlescans
67
When cap removed pressure reduced to 1 atm and some CO2 escapes as gas bubbles Effervescence - rapid escape of a gas from a liquid in which it is dissolved
68
Effects of Temperature on Solubility
Letrsquos consider gas solubility Increasing temperature usually decreases gas solubility As temp increases average kinetic energy of molecules in solution increases Greater number of solute molecules escape from attraction of solvent and return to gas phase 69
At higher temps eq is reached with fewer gas molecules in solution Gases generally less soluble
70
Effect of temp on solubility of solids in liquids more difficult to predict
Often increasing temp increases solubility
However the same temp increase can result in large increase in solubility in one case and only slight increase in the next
71
72
enthalpies of Solution
Formation of solution comes with energy change Dissolve KI in water container feels cold Dissolve LiCl in water feels hot Formation of solid-liquid solution can absorb or release heat
73
During formation of solution solvent and solute particles experience changes in forces attracting them to other particles Before dissolving begins solvent molecules held by intermolecular forces (solvent-solvent attraction) In solute molecules held by Ifs (solute-solute attraction)
74
Energy is required to separate solute molecules and solvent molecules from their neighbors
A solute particle that is surrounded by solvent molecules is said to be solvated
75
Step 1 solute particles become separated from solid (energy absorbed) Step 2 solvent particles move apart to allow solute particles to enter liquid (energy absorbed)
76
Step 3 solvent particles attracted to and solvate solute particles (energy released)
77
Heat of solution - net amount of heat energy absorbed or released when a specific amount of solute dissolves in a solvent
Heat of solution negative (heat is released) if sums of energy in steps 1 and 2 is less than step 3
If 1+2 gt 3 HOS is positive (heat absorbed)78
In gaseous state molecules are so far apart there are nearly no forces between them Solute-solute interaction has little effect on heat of solution of a gas Energy released when gas dissolved in liquid bc attraction between solute gas and solvent molecules greater than energy needed to separate solvent molecules
79
Concentration of Solutions80
Concentration - measure of amount of solute in given amount of solvent or solution ldquodiluterdquo just means small amount of solute ldquoconcentratedrdquo just means relatively large amount of solute Unrelated to degree to which solution is saturated Ex Saturated solution of substance not very soluble might be dilute
81
Molarity
Molarity - number of moles of solute in one liter of solution To find molarity must know molar mass A ldquoone molarrdquo solution of NaOH is 1 mole NaOH per liter of solution (1 M)
82
1 mol NaOH = 400 g If this quantity dissolve in enough water to make EXACTLY 100 L solution solution is 1 M If 200 g NaOH dissolve in enough to make 100 L solution what is molarity 0500 M
83
1 M solution is not made by adding 1 mol solute to 1 L solvent That would be more than 1 L Instead 1 mol solute dissolved in less than 1 L Then diluted with more solvent to bring TOTAL VOLUME to 1 L
84
Practice ProblemYou have 340 L of solution that contains 900 g sodium chloride What is the molarity of that solution 0440 M NaCl You have 08 L of a 05 M HCl solution How many moles of HCl does this solution contain 04 mol HCl
85
What is the molarity of a solution composed of 585 g of potassium iodide dissolved in enough water to make a 0125 L of solution 0282 M KI
86
How many moles of H2SO4 are present in 0500 L of a 0150 M H2SO4 solution 00750 mol What volume of 300 M NaCl is needed for reaction that requires 1463 g of NaCl 0834 L
87
Molality
Molality - concentration of solution expressed in moles of solute per kilogram of solvent Solution that contains 1 mol solute (NaOH) dissolved in exactly 1 kg of solvent is ldquoone molalrdquo solution (1 m NaOH)
88
1 mol NaOH = 400 g 400 g NaOH dissolved in 1 kg water is 1 m NaOH
200 g NaOH in 1 kg water = 0500 m NaOH
89
Practice Problem
A solution was prepared by dissolving 171 g sucrose (C12H22O11) in 125 g of water Find the molal concentration of this solution 0400 m C12H22O11
90
A solution of iodine in carbon tetrachloride is used when iodine is needed for certain chemical tests How much iodine must be added to prepare a 0480 m solution of iodine in CCl4 if 1000 g of CCl4 is used 122 g I2
91
What is the molality of a solution composed of 255 g acetone (CH3)2CO dissolved in 200 g of water 22 m acetone What quantity in grams of methanol CH3OH is required to prepare a 0244 m solution in 400 g water 312 g methanol
92
How many grams of AgNO3 are needed to prepare a 0125 m solution in 250 mL of water 531 g AgNO3
What is the molality of a solution containing 182 g HCl and 250 g water 200 m
93
Ions in Aqueous Solutions and Colligative Properties
94
95
compounds in aqueous solutions
DissociationWhen compound made from ions dissolves in water ions separate Dissociation - separation of ions that occurs when an ionic compound dissolves
96
Notice number of ions made per formula unit in equations 1 formula unit of NaCl gives 2 units of ions in solution 1 formula unit of CaCl2 gives 3 units of ions
97
Assuming 100 dissociation solution that contains 1 mol NaCl contains 1 mol Na+ and 1 mol Cl- Can assume 100 dissociation of all soluble ionic compounds
98
Practice Problem
Write the equation for the dissolution of aluminum sulfate in water How many moles of aluminum ions are made by dissolving 1 mol aluminum sulfate What is the total number of moles of ions made by dissolving 1 mol of aluminum sulfate
99
1 AnalyzeGiven Amount of solute = 1 mol Al2(SO4)3 Solvent identity = water Unknown a moles of aluminum ions and sulfate ions Total number of moles of solute ions produced
100
2 Plan
The coefficients in the balanced dissociation equation will tell mole relationships You can use the equation to find out the number of moles of solute ions produced
101
3 Compute
a
b102
Write the equation for the dissolution of each of the following in water and then determine the number of moles of each ions made as well as the total number of ions made a 1 mol ammonium chloride b 1 mol sodium sulfide c 05 mol barium nitrate
103
a 1 mol ammonium chloride
1 mol NH4+ 1 mol Cl- 2 mol total ions
104
b 1 mol sodium sulfide
2 mol Na+ 1 mol S-2 3 mol total ions
105
c 05 mol barium nitrate
05 mol Ba+2 1 mol NO3- 15 mol total ions
106
Precipitation Reactions
Even though no compound is completely insoluble compounds of very low solubility can be considered insoluble for practical purposes There are general guidelines to help predict whether a compound made of certain combination of ions is soluble
107
General Solubility Guidelines
Sodium potassium and ammonium compounds are soluble in water
108
Nitrates acetates and chlorates are soluble
109
110
Most chlorides are soluble except those of silver mercury (I) and lead Lead (II) chloride is soluble in hot water
Most sulfates are soluble except those of barium strontium lead calcium and mercury
111
Most carbonates phosphates and silicates are insoluble except those of sodium potassium and ammonium
112
Most sulfides are insoluble except those of calcium strontium sodium potassium and ammonium
113
Is calcium phosphate Ca3(PO4)2 soluble or insoluble
Not one of the exceptions so it is insoluble
Dissociation equations are not written for insoluble compounds
114
Guidelines also useful in predicting what will happen if 2 solutions of 2 different soluble compounds are mixed
If mixing results in combination of ions that forms insoluble compound a double-replacement and precipitation reaction will happen
115
Will precipitate form when solutions of ammonium sulfide and cadmium nitrate are combined You can tell calcium nitrate is soluble from the guidelines (Most nitrates acetates and chlorates are soluble) You can also tell ammonium sulfide is soluble (Most sulfides are insoluble except those of calcium strontium sodium potassium and ammonium)
116
117
To decide whether precipitate can form must know solubilities of two possible products
Ammonium nitrate ndash soluble (Most nitrates acetates and chlorates are soluble) Cadmium sulfide ndash insoluble (Most sulfides are insoluble except those of calcium strontium sodium potassium and ammonium)
118
bc one product is insoluble double-replacement and precipitation reaction will happen (NH4)2S (aq) + Cd(NO3)2 (aq) rarr NH4NO3 (aq) + CdS (s)
119
Net Ionic Equations
Reactions of ions in aqueous solution usually represented by net ionic equations instead of formula equations Net ionic equation - includes only those compounds and ions that undergo a chemical change in a reaction in an aqueous solution
120
121
To write NIE (net ionic equation) first convert chemical equation into overall ionic equation All soluble ionic compounds shown as dissociated ions in solution Precipitates shown as solids
122
Notice ammonium and nitrate ions appear on both sides They havenrsquot gone through any chemical change Spectator ions - ions that do not take part in a chemical reaction and are found in solution both before and after the reaction
123
To change ionic equation to NIE eliminate spectator ions Cd+2(aq) + S-2(aq) rarr CdS(s)
This NIE applies to any reaction in which precipitate of CdS forms
124
Practice Problem
Identify the precipitate that forms when aqueous solutions of zinc nitrate and ammonium sulfide are combined Write the equation for the possible double-replacement reaction Then write the formula equation overall ionic equation and the net ionic equation
125
1 AnalyzeGiven Identity of reactants zinc nitrate and ammonium sulfide Reaction medium aqueous solution Unknown a equation for the possible double-replacement reaction b identity of the precipitate c formula equation d overall ionic equation e net ionic equation
126
2 PlanWrite the possible double-replacement reaction between Zn(NO3)2 and (NH4)2S Use the rules to determine if any of the products will precipitate Write a formula equation and overall net ionic equation Then cancel spectator ions to make net ionic equation
127
3 Computea possible double-replacement reaction
b rules show that zinc sulfide is not soluble so will precipitate (ammonium nitrate is soluble)
c formula equation128
d overall ionic equation
e net ionic equation129
Will a precipitate form if solutions of potassium sulfate and barium nitrate are combined If so write the net ionic equation for the reaction
Yes Ba+2(aq) + SO4-2(aq) rarr BaSO4(s)
130
Will a precipitate form if solutions of potassium nitrate and magnesium sulfate are combined If so write the net ionic equation for the reaction
No 131
Will a precipitate form if solutions of barium chloride and sodium sulfate are combined If so identify the spectator ions and write the net ionic equation for the reaction
Yes Na+ and Cl- Ba+2(aq) + SO4-2(aq) rarr BaSO4(s)
132
Write the net ionic equation for the precipitation of nickel(II) sulfide
Ni+2(aq) + S-2(aq) rarr NiS(s)
133
IonizationSome molecular compounds can also form ions in solution Usually polar Ionization - ions formed from solute molecules by action of the solvent (creation of ions where there were none) Ionization different from dissociation
134
Dissociation ionic compounds dissolve and ions already present separate Ionization ions formed where non existed before
Ions formed are hydrated Heat released during hydration of ions gives enough energy to break covalent bonds
135
Extent to which solute ionizes depends on strength of bonds within molecules of solute and strength of attraction between solute and solvent molecules
136
HCl ndash molecular compound that ionizes in aqueous solution Contains highly polar bond Attraction between polar HCl molecule and polar water strong enough to break HCl bond
137
The Hydronium Ion
H+ ions from HCl attracts other molecules or ions so strongly that it doesnrsquot normally exist alone Ionization of HCl better described as direct transfer of proton from HCl to H2O forming H3O+
138
139
Hydration of H+ to form hydronium is highly exothermic
Energy released gives a lot of energy needed to ionize a molecular solute
Many molecular compounds that ionize in aqueous solution contain hydrogen and form hydronium ion
140
Strong and Weak Electrolytes
Substances that make ions and conduct current in solution are electrolytes HCl is one of series of compounds that has hydrogen and halogen Hydrogen halides are all molecular compounds with single polar-covalent bonds
141
All are gases Very soluble in water All are electrolytes HCl HBr HI strongly conduct current in solution HF only weakly conducts current at same concentration Strength of conduction related to ability to ionize
142
143
Strong Electrolytes
HCl HBr HI are 100 ionized in solution
Strong electrolyte - any compound whose dilute aqueous solutions conduct electricity well this is due to the presence of all or almost all of the dissolve compound in the form of ions
144
Unique characteristic of strong electrolyte is that they yield only ions
Ex Ionic compound may be highly soluble in water and dissociate into ions (NaCl)
Others may not dissolve much but amount that does dissolve exists only as ions
145
Weak ElectrolytesHF dissolves in water to give acid solution called hydrofluoric acid HF bond stronger than bonds between hydrogen and other halogens When HF dissolves some molecule ionize Reverse reaction also happens HF(aq) + H2O(l) H3O+(aq) + F-(aq)
146
HF(aq) + H2O(l) H3O+
(aq) + F-(aq)Concentration of dissolved unionized HF stays high and concentration of ions stays low
Weak electrolyte - any compound whose aqueous solutions conduct electricity poorly this is due to the presence of small amount of the dissolved compound in the form of ions
147
Different from nonelectrolyte ndash NO ions at all Description of electrolyte as strong or weak not related to concentration of solution Electrolytes differ in degree of ionization not amount of solute dissolved
148
149
Colligative properties of solutions
Presence of solute affects properties of solutions Some properties not dependent on nature of dissolved substance but on how may dissolve particles are present
Colligative properties - properties that depend on the concentration of solute particles but not on their identity
150
Vapor-Pressure Lowering
Boiling and freezing point of solution different from pure solvent Nonvolatile solute raises boiling point and lowers freezing point Nonvolatile substance - one that has little tendency to become gas under existing conditions
151
152
To understand why nonvolatile solute changes boiling and freezing point consider equilibrium vapor pressure
Vapor pressure pressure caused by molecules that have escape liquid phase to gas phase
Can be thought of a measure of tendency of molecules to escape from a liquid
153
Addition of sucrose (nonvolatile solute) lowers concentration of water molecules at surface of liquid This lowers tendency of water molecules to leave solution and enter gas phase Vapor pressure of solution is lower than vapor pressure of pure water
154
Aqueous13 solution13 of13 nonvolatile13 solutePure13 water
Nonelectrolyte solutions of same molality have same concentration of particles
Dilute solutions of same solvent and equal molality of any nonelectrolyte solute lower vapor pressure equally
155
Example
1 m aqueous solution of nonelectrolyte glucose lowers vapor pressure of water 55 x 10-4 atm at 25 1 m aqueous solution of sucrose also lowers water vapor pressure to 55 x 10-4 atm at 25
156
157
bc vapor-pressure lowering depends on concentration of nonelectrolyte solute and doesnrsquot depend on the type of solute it is a colligative proprerty
bc vapor pressure has been lowered solution remains liquid over larger temperature range
Lowers freezing point and raises boiling point
158
Can assume changes in boiling and freezing point also depend on concentration of solute
They are colligative properties
159
Freezing-Point Depression
Freezing point of 1-molal solution of any nonelectrolyte solute in water is found (by experiment) to be 186 lower than freezing point of water When 1 mol of nonelectrolyte solute dissolved in 1 kg water freezing point is -186 not 000 160
When 2 mol nonelectrolyte solute dissolved in 1 kg water freezing point is -372
For any concentration of nonelectrolyte solute in water decrease in freezing point can be determined using value of -186m
161
This value called the molal freezing-point contstant (Kf) is the freezing-point depression of the solvent in a 1-molal solution of a nonvolatile nonelectrolyte solute
162
Each solvent has own characteristic molal freezing-point constant Freezing-point depression (Δtf) the difference between the freezing points of the pure solvent and a solute of nonelectrolyte in that solvent and it is directly proportional to the molal concentration of the solution Δtf = Kfm
163
Δtf = Kfm
Kf expressed as m
m expressed in mol solutekg solvent (molality)
164
Sample Problem
What is the freezing point depression of water in a solution of 171 g of sucrose C12H22O11 and 200 g of water What is the actual freezing point of the solution
165
1 AnalyzeGiven Solute mass and chemical formula 171 g C12H22O11 Solvent mass and identity 2000 g water Unknown a freezing-point depression b freezing point of the solution 166
2 Plan
167
168
Calculate
Unknown a freezing-point depression b freezing point of the solution
169
Practice Problem
A water solution containing an unknown quantity of a nonelectrolyte solute is found to have a freezing point of ndash 023degCWhat is the molal concentration of the solution
012 m170
A solution consists of 103 g of the nonelectrolyte glucose C6H12O6 dissolved in 250 g of water What is the freezing-point depression of the solution
minus0426degC171
In a laboratory experiment the freezing point of an aqueous solution of glucose is found to be minus0325degCWhat is the molal concentration of this solution
0175 m172
If 0500 mol of a nonelectrolyte solute are dissolved in 5000 g of ether what is the freezing point of the solution
minus1181degC173
The freezing point of an aqueous solution that contains a nonelectrolyte is minus90degC aWhat is the freezing-point depression of the solution bWhat is the molal concentration of the solution a minus90degC b 48 m
174
Boiling-Point Elevation
Boiling point is temp at which vapor pressure is equal to atmospheric pressure Change in vapor pressure will cause corresponding change in boiling point
175
176
Vapor pressure of nonvolatile solution is lower than vapor pressure of pure solvent More heat will be required to raise vapor pressure of solution Boiling point of solution is higher than boiling point of pure solvent
Molal boiling-point constant (Kb) - boiling-point elevation of the solvent in a 1-molal solution of a nonvolatile nonelectrolyte solute b-p elevation of any nonelectrolyte in water found by experiment to be 051 So molal b-p constant for water is 051m
177
For different solvents b-p elevations of 1-molal solutions have different values (Table 14-2)
Like freezing-point constants values most accurate for dilute solutions
178
Boiling-point elevation Δtb difference between the boiling points of the pure solvent and a nonelectrolyte solution of that solvent and it is directly proportional to the molal concentration of the solution B-p elevation can be calculated using Δtb = Kbm Δtb (boiling point elevation) expressed in m and m expressed in mol solutekg solvent
Sample Problem
What is the boiling-point elevation of a solution made from 200 g of a nonelectrolyte solute and 4000 g of water The molar mass of the solute is 620 gmol
180
1 AnalyzeGiven solute mass = 200 g solute molar mass = 620 gmol solvent mass and identity = 4000 g of water
Unknown boiling-point elevation
181
2 Plan
182
183
3 Compute
Practice Problem
A solution contains 500 g of sucrose C12H22O11 a nonelectrolyte dissolved in 5000 g of water What is the boiling-point elevation
015degC184
A solution contains 4500 g of sucrose C12H22O11 a nonelectrolyte dissolved in 250 g of waterWhat is the boiling point of the solution
1027degC185
If the boiling point elevation of an aqueous solution containing a nonvolatile electrolyte is 102degC what is the molality of the solution
20 m186
The boiling point of an aqueous solution containing a nonvolatile electrolyte is 10075degC aWhat is the boiling-point elevation bWhat is the molality of the solution a 075degC b 15 m
187
Electrolytes and Colligative PropertiesEarly investigators confused by experiments where certain substances depressed freezing point or elevated boiling point more than expected
188
189
Ex 01 m solution of NaCl lowers freezing point of solvent almost twice as much as 01 m solution of sucrose 01 m CaCl2 lowers fp almost 3 times as much as 01 m solution of sucrose
To understand why this happens contrast behavior or sucrose with NaCl in aqueous solution Each sucrose dissolves to make only 1 particle in solution So 1 mol sucrose dissolves to 1 mol particles 1 mol NaCl dissolves to make 2 moles of particles (1 mol Na+ and 1 mol Cl-)
190
191
Calculated Values for Electrolyte SolutionsColligative properties depend on total concentration of solute particles regardless of their identity Electrolytes cause changes in colligative properties proportional to total molality in terms of all dissolves particles instead of formula units
192
What about barium nitrate Ba(NO3)2 Each mole of barium nitrate yields 3 mol of ions in solution Ba(NO3)2(s) rarr Ba2+(aq) + 2NO3-(aq) You would expect solution of given molality to lower fp of solvent 3 times as much as nonelectrolytic solution of same molality
193
Sample Problem
What is the expected change in the freezing point of water in a solution of 625 g of barium nitrate Ba(NO3)2 in 100 kg of water
194
1 AnalyzeGiven solute mass and formula = 625 g Ba(NO3)2 solvent mass and identity = 100 kg water Δtf = Kfm
Unknown expected freezing-point depression
195
2 PlanThe molality can be calculated by converting the solute mass to moles and then dividing by the number of kilograms of solvent That molality is in terms of formula units of Ba(NO3)2 and must be converted to molality in terms of dissociated ions in solution It must be multiplied by the number of moles of ions produced per mole of formula unit This adjusted molality can then be used to calculate the freezing-point depression
196
197
198
3 Compute
Practice Problem
What is the expected freezing-point depression for a solution that contains 20 mol of magnesium sulfate dissolved in 10 kg of water
minus74degC199
What is the expected boiling-point elevation of water for a solution that contains 150 g of sodium chloride dissolved in 10 kg of water 27degC The freezing point of an aqueous sodium chloride solution is minus200degC What is the molality of the solution 54 m NaCl
200
Actual Values for Electrolyte SolutionsRemember values calculated are only expected values Actual values of colligative properties for all strong electrolytes are almost what would be expected Difference between expected and actual values caused by attractive forces that exist between dissociated ions in aqueous solution
201
Attraction between hydrated ions in solution is small compared with those in crystalline solid However forces of attraction donrsquot interfere with movements of aqueous ions More concentrated solution means ions closer together and attraction is greater Also depends on charge of ions ndash higher charge greater attraction
202
Types of Solutions
Solutions can exist as gases liquids or solids
9
Many alloys such as brass (zinc and copper) are sterling silver (silver and copper) are solid solutions Atoms of two or metals are evenly mixed By properly choosing percentages of each metal can get many desirable properties
10
Ex Alloys have higher strength and greater resistance to corrosion than pure metals Pure gold (24K) too soft for jewelry Alloy with silver (14K) increases strength and hardness but keeps appearance of gold
11
SuspensionsSuspension - if particles in solvent are so large they settle out unless mixture constantly stirred Think of sand and water Particles over 1000 nm in diameter
12
Particles that are medium in size between those in solutions and suspensions from mixtures are known as colloids (colloidal dispersions) Particles between 1-1000 nm diameter
13
Colloids
In muddy water large soil particles settle Water still cloudy bc colloidal particles stay dispersed in water Filter Colloidal particles pass through and stays cloudy
14
Particles in colloid small enough to be suspended in solvent by constant movement of surrounding molecules Colloidal particles make up dispersed phase Water is dispersing medium
15
Emulsion and foam are colloids
Mayonnaise is emulsion of oil droplets in water Egg yolk acts as emulsifying agent (keeps oil droplets dispersed)
16
17
18
Class13 of13 Colloid Phases Example
Sol Solid13 dispersed13 in13 liquid Paints13 mud
Gel Solid13 network13 extending13 through13 liquid
Gelatin
Liquid13 emulsion Liquid13 dispersed13 in13 liquid Milk13 mayonnaise
Foam Gas13 dispersed13 in13 liquid Shaving13 cream13 whipped13 cream
Solid13 aerosol Solid13 dispersed13 in13 gas Smoke13 auto13 exhaust
Liquid13 aerosol Liquid13 dispersed13 in13 gas Fog13 mist13 clouds13 aerosol13 spray
Solid13 emulsion Liquid13 dispersed13 in13 solid Cheese13 butter
Tyndall EffectMany colloids appear homogeneous bc individual particles cannot be seen Particles large enough to scatter light In fog can see headlight beam Tyndall effect - occurs when light is scattered by colloidal particles dispersed in transparent medium
19
20
21
Solutions Colloids Suspensions
Homogeneous Heterogeneous Heterogeneous
Particle13 size13 001-shy‐113 nm13 can13 be13 atoms13 ions13 molecules
Particle13 size13 1-shy‐100013 nm13 dispersed13 can13 be13 combined13 or13 large13 molecules
Particle13 size13 over13 100013 nm13 suspended13 can13 be13 large13 particles13 or13 combined13 particles
Do13 not13 separate13 on13 standing Do13 not13 separate13 on13 standing Particles13 settle13 out
Cannot13 be13 separated13 by13 filtration Cannot13 be13 separated13 by13 filtration Can13 be13 separated13 by13 filtration
Do13 not13 scatter13 light Scatter13 light13 (Tyndall13 effect) May13 scatter13 light13 but13 are13 not13 transparent
Electrolytes vs Nonelectrolytes
Substances that dissolve in water are classified according to whether they produce molecules or ions in solution
22
23
When ionic compound dissolves cations and anions separate and are surrounded by water molecules Solute ions free to move making possible to carry electric current
Electrolyte - substance that dissolves in water to give a solution that conducts electric current NaCl ndash electrolyte Usually highly polar compounds become electrolytes
24
Solution containing neutral solute molecules does not conduct current bc it doesnrsquot have mobile charged particles
Nonelectrolyte - substance that dissolves in water to give solution that does not conduct electric current Ex Sugar
25
26
27the solution
process
Factors Affecting Rate of DissolutionEver tried to dissolve sugar in iced tea or coffee You notice temperature has something to do with how quickly solute dissolves What other factors affect how quickly solutes dissolve
28
1 Increasing Surface Area of Solute
bc dissolution process occurs at surface of solute Can increase dissolution by increasing surface area
29
2 Agitating (Stirring) SolutionClose to surface of solute concentration of dissolved solute is high Stirringshaking helps disperse solute particles
30
Brings fresh solvent into contact with solute surface Contact between solvent and solute surface increased
3 Heating a Solvent
Solutes dissolve faster in warmer solvent As temp increases solvent molecules move faster Average kinetic energy increases Collisions between solvent and solute more frequent Helps separate solute molecules and spread them out
32
Solubility
If you add sugar to tea eventually no more sugar dissolves
For every combination of solvent with solute at given temp there is limit to amount of solute that can be dissolved
33
When sugar first dropped into water sugar molecules leave solid surface and move about at random in solvent Some dissolved molecules may collide with crystal and stay there As more solid dissolves and concentration of dissolved molecules increases collisions happen more often
34
Eventually molecules are returning to crystal at same rate they are going into solution Dynamic equilibrium established between dissolution and crystallization Solution equilibrium - physical state in which the opposing processes of dissolution and crystallization of a solute occur at equal rates
35
Saturated vs Unsaturated Solutions
Saturated solution - solution that contains the maximum amount of dissolved solute How do you know if solution is saturated If more NaCl added it falls to bottom and doesnrsquot dissolve Equilibrium already established
36
If more water added to saturated solution more NaCl will dissolve At 20 359 g NaCl is max that will dissolve in 100 g water Unsaturated solution - solution that contains less solute than a saturated solution under the existing conditions
37
Supersaturated Solutions
When saturated solution where solubility increases as temp increases is cooled excess solute usually comes out of solution Sometimes if solution is left to cool the excess solute doesnrsquot separate
38
39
Supersaturated solution - solution that contains more dissolved solute than a saturated solution contains under same conditions
40
Supersaturated solution can remain unchanged over long time if it isnrsquot disturbed Once crystals begin to form continues until equilibrium is recreated at lower temp
41
Example
Sodium thiosulfate Na2S2O3 Solute added to hot water until solution saturated Hot solution filtered Drop small crystal in ldquoseedingrdquo crystallization Continues until equilibrium created
42
Solubility Values
Solubility - amount of substance required to form a saturated solution with specific amount of solvent at specified temp Ex Solubility of sugar = 204 g per 100 g water at 20 Must specify temp bc solubility changes with temp 43
For gases pressure must also be specified Rate at which substance dissolves is unrelated to solubility Max amount of solute that dissolves and reaches equilibrium is always same under same conditions
44
Solute-Solvent Interactions
ldquolike dissolves likerdquo is generally useful rule for predicting whether one substance will dissolve in another Depends on type of bonding polaritynonpolarity and intermolecular forces between solute and solvent
45
Dissolving Ionic Compounds in Aqueous Solution
Polarity of water molecules plays important role in formation of solution of ionic compounds Charged ends of water molecules attract ions and surround them to keep separated from other ions
46
Hydration - solution process with water as solvent Ions said to be hydrated As hydrated ions diffuse into solution other ions exposed and drawn away from crystal surface Entire crystal gradually dissolves
47
When crystallized from aqueous solutions some ionic compounds form crystals that include water molecules The crystalline compounds known as hydrates have specific ratios of water molecules Represented by formulas like CuSO45H2O
48
When crystalline hydrate dissolves in water water of hydration returns to solvent Behavior of solute in hydrated form no different than anhydrous form
49
Nonpolar Solvents
Nonpolar solvents do not attract ions of ionic compound strongly enough to overcome forces holding crystal together
50
Liquid Solutes and Solvents
When you shake bottle of salad dressing oil droplets disperse in water Stop shaking ndash strong attraction between water molecules squeeze out oil and form separate layers Liquid solutes and solvents that are not soluble in each other are immiscible
51
Nonpolar substances (fat oil grease) are generally quite soluble in nonpolar liquids (carbon tetrachloride toluene gasoline)
Only attractions between nonpolar molecules are weak London forces
52
London Dispersion Forces
bc e- are constantly moving at any time there may be uneven distribution of them around a nonpolar atom Temporarily creates positive end and negative end
53
54
London dispersion forces - intermolecular attractions resulting from constant motion of e- and creation of instantaneous dipoles
55
Liquids that dissolve freely in one another in any ratio are said to be completely miscible Benzene and carbon tetrachloride (both nonpolar) are completely miscible
Nonpolar molecules of these two apply no strong forces of attraction or repulsion and molecules mix freely
56
Ethanol and water are also completely miscible -OH group on ethanol is slightly polar
Can form H-bonds with water as well as other ethanol molecules Intermolecular forces in mixture are very similar to those in pure liquids so they are mutually soluble in any amount
57
Gasoline contains mainly nonpolar hydrocarbons
Excellent solvent for fats oils greases
Major intermolecular forces are weak London forces
58
Ethanol less polar than water more than carbon tetrachloride
Better solvent for less-polar substances bc of nonpolar region
59
Effects of Pressure on Solubility
Little effect on solids or liquids Increase pressure - increase solubilities of gases in liquids
60
When gas in contact with surface of liquid gas molecules can enter liquid As amount of dissolve gas increases some molecules start to escape and reenter gas phase
Eq eventually established between rates of enteringleaving gas phase
61
As long as eq undisturbed solubility of gas in liquid unchanged at given pressure
Gas + solvent solution
62
Increasing pressure of solute gas above solution puts stress on eq Molecules collide with surface more often Increase in pressure partially offset by increase in rate of gas molecules entering solution In turn increase in amount of dissolved gas causes increase in rate molecules escape liquid and becomes gas
63
Eventually eq restored at higher gas solubility
Expected from Le Chatelierrsquos principle
Increase in gas pressure causes eq to shift so fewer molecules are in gas phase
64
Henryrsquos Law
Henryrsquos law - solubility of a gas in liquid is directly proportional to the partial pressure of that gas on the surface of the liquid
Applies to gas-liquid solutions at constant temp
65
When mixture of ideal gases is limited in constant volume at constant temp each gas applies same pressure it would apply if it occupied the space alone
Assuming gases do not react each gas dissolves to the extent it would if no other gases were there
66
In carbonated drinks (Coke Pepsi etc) solubility of CO2 increased by increasing pressure
At bottling factory CO2 gas forced into solution of flavored water at pressure of 5-10 atm Gas-in-liquid solution then sealed in bottlescans
67
When cap removed pressure reduced to 1 atm and some CO2 escapes as gas bubbles Effervescence - rapid escape of a gas from a liquid in which it is dissolved
68
Effects of Temperature on Solubility
Letrsquos consider gas solubility Increasing temperature usually decreases gas solubility As temp increases average kinetic energy of molecules in solution increases Greater number of solute molecules escape from attraction of solvent and return to gas phase 69
At higher temps eq is reached with fewer gas molecules in solution Gases generally less soluble
70
Effect of temp on solubility of solids in liquids more difficult to predict
Often increasing temp increases solubility
However the same temp increase can result in large increase in solubility in one case and only slight increase in the next
71
72
enthalpies of Solution
Formation of solution comes with energy change Dissolve KI in water container feels cold Dissolve LiCl in water feels hot Formation of solid-liquid solution can absorb or release heat
73
During formation of solution solvent and solute particles experience changes in forces attracting them to other particles Before dissolving begins solvent molecules held by intermolecular forces (solvent-solvent attraction) In solute molecules held by Ifs (solute-solute attraction)
74
Energy is required to separate solute molecules and solvent molecules from their neighbors
A solute particle that is surrounded by solvent molecules is said to be solvated
75
Step 1 solute particles become separated from solid (energy absorbed) Step 2 solvent particles move apart to allow solute particles to enter liquid (energy absorbed)
76
Step 3 solvent particles attracted to and solvate solute particles (energy released)
77
Heat of solution - net amount of heat energy absorbed or released when a specific amount of solute dissolves in a solvent
Heat of solution negative (heat is released) if sums of energy in steps 1 and 2 is less than step 3
If 1+2 gt 3 HOS is positive (heat absorbed)78
In gaseous state molecules are so far apart there are nearly no forces between them Solute-solute interaction has little effect on heat of solution of a gas Energy released when gas dissolved in liquid bc attraction between solute gas and solvent molecules greater than energy needed to separate solvent molecules
79
Concentration of Solutions80
Concentration - measure of amount of solute in given amount of solvent or solution ldquodiluterdquo just means small amount of solute ldquoconcentratedrdquo just means relatively large amount of solute Unrelated to degree to which solution is saturated Ex Saturated solution of substance not very soluble might be dilute
81
Molarity
Molarity - number of moles of solute in one liter of solution To find molarity must know molar mass A ldquoone molarrdquo solution of NaOH is 1 mole NaOH per liter of solution (1 M)
82
1 mol NaOH = 400 g If this quantity dissolve in enough water to make EXACTLY 100 L solution solution is 1 M If 200 g NaOH dissolve in enough to make 100 L solution what is molarity 0500 M
83
1 M solution is not made by adding 1 mol solute to 1 L solvent That would be more than 1 L Instead 1 mol solute dissolved in less than 1 L Then diluted with more solvent to bring TOTAL VOLUME to 1 L
84
Practice ProblemYou have 340 L of solution that contains 900 g sodium chloride What is the molarity of that solution 0440 M NaCl You have 08 L of a 05 M HCl solution How many moles of HCl does this solution contain 04 mol HCl
85
What is the molarity of a solution composed of 585 g of potassium iodide dissolved in enough water to make a 0125 L of solution 0282 M KI
86
How many moles of H2SO4 are present in 0500 L of a 0150 M H2SO4 solution 00750 mol What volume of 300 M NaCl is needed for reaction that requires 1463 g of NaCl 0834 L
87
Molality
Molality - concentration of solution expressed in moles of solute per kilogram of solvent Solution that contains 1 mol solute (NaOH) dissolved in exactly 1 kg of solvent is ldquoone molalrdquo solution (1 m NaOH)
88
1 mol NaOH = 400 g 400 g NaOH dissolved in 1 kg water is 1 m NaOH
200 g NaOH in 1 kg water = 0500 m NaOH
89
Practice Problem
A solution was prepared by dissolving 171 g sucrose (C12H22O11) in 125 g of water Find the molal concentration of this solution 0400 m C12H22O11
90
A solution of iodine in carbon tetrachloride is used when iodine is needed for certain chemical tests How much iodine must be added to prepare a 0480 m solution of iodine in CCl4 if 1000 g of CCl4 is used 122 g I2
91
What is the molality of a solution composed of 255 g acetone (CH3)2CO dissolved in 200 g of water 22 m acetone What quantity in grams of methanol CH3OH is required to prepare a 0244 m solution in 400 g water 312 g methanol
92
How many grams of AgNO3 are needed to prepare a 0125 m solution in 250 mL of water 531 g AgNO3
What is the molality of a solution containing 182 g HCl and 250 g water 200 m
93
Ions in Aqueous Solutions and Colligative Properties
94
95
compounds in aqueous solutions
DissociationWhen compound made from ions dissolves in water ions separate Dissociation - separation of ions that occurs when an ionic compound dissolves
96
Notice number of ions made per formula unit in equations 1 formula unit of NaCl gives 2 units of ions in solution 1 formula unit of CaCl2 gives 3 units of ions
97
Assuming 100 dissociation solution that contains 1 mol NaCl contains 1 mol Na+ and 1 mol Cl- Can assume 100 dissociation of all soluble ionic compounds
98
Practice Problem
Write the equation for the dissolution of aluminum sulfate in water How many moles of aluminum ions are made by dissolving 1 mol aluminum sulfate What is the total number of moles of ions made by dissolving 1 mol of aluminum sulfate
99
1 AnalyzeGiven Amount of solute = 1 mol Al2(SO4)3 Solvent identity = water Unknown a moles of aluminum ions and sulfate ions Total number of moles of solute ions produced
100
2 Plan
The coefficients in the balanced dissociation equation will tell mole relationships You can use the equation to find out the number of moles of solute ions produced
101
3 Compute
a
b102
Write the equation for the dissolution of each of the following in water and then determine the number of moles of each ions made as well as the total number of ions made a 1 mol ammonium chloride b 1 mol sodium sulfide c 05 mol barium nitrate
103
a 1 mol ammonium chloride
1 mol NH4+ 1 mol Cl- 2 mol total ions
104
b 1 mol sodium sulfide
2 mol Na+ 1 mol S-2 3 mol total ions
105
c 05 mol barium nitrate
05 mol Ba+2 1 mol NO3- 15 mol total ions
106
Precipitation Reactions
Even though no compound is completely insoluble compounds of very low solubility can be considered insoluble for practical purposes There are general guidelines to help predict whether a compound made of certain combination of ions is soluble
107
General Solubility Guidelines
Sodium potassium and ammonium compounds are soluble in water
108
Nitrates acetates and chlorates are soluble
109
110
Most chlorides are soluble except those of silver mercury (I) and lead Lead (II) chloride is soluble in hot water
Most sulfates are soluble except those of barium strontium lead calcium and mercury
111
Most carbonates phosphates and silicates are insoluble except those of sodium potassium and ammonium
112
Most sulfides are insoluble except those of calcium strontium sodium potassium and ammonium
113
Is calcium phosphate Ca3(PO4)2 soluble or insoluble
Not one of the exceptions so it is insoluble
Dissociation equations are not written for insoluble compounds
114
Guidelines also useful in predicting what will happen if 2 solutions of 2 different soluble compounds are mixed
If mixing results in combination of ions that forms insoluble compound a double-replacement and precipitation reaction will happen
115
Will precipitate form when solutions of ammonium sulfide and cadmium nitrate are combined You can tell calcium nitrate is soluble from the guidelines (Most nitrates acetates and chlorates are soluble) You can also tell ammonium sulfide is soluble (Most sulfides are insoluble except those of calcium strontium sodium potassium and ammonium)
116
117
To decide whether precipitate can form must know solubilities of two possible products
Ammonium nitrate ndash soluble (Most nitrates acetates and chlorates are soluble) Cadmium sulfide ndash insoluble (Most sulfides are insoluble except those of calcium strontium sodium potassium and ammonium)
118
bc one product is insoluble double-replacement and precipitation reaction will happen (NH4)2S (aq) + Cd(NO3)2 (aq) rarr NH4NO3 (aq) + CdS (s)
119
Net Ionic Equations
Reactions of ions in aqueous solution usually represented by net ionic equations instead of formula equations Net ionic equation - includes only those compounds and ions that undergo a chemical change in a reaction in an aqueous solution
120
121
To write NIE (net ionic equation) first convert chemical equation into overall ionic equation All soluble ionic compounds shown as dissociated ions in solution Precipitates shown as solids
122
Notice ammonium and nitrate ions appear on both sides They havenrsquot gone through any chemical change Spectator ions - ions that do not take part in a chemical reaction and are found in solution both before and after the reaction
123
To change ionic equation to NIE eliminate spectator ions Cd+2(aq) + S-2(aq) rarr CdS(s)
This NIE applies to any reaction in which precipitate of CdS forms
124
Practice Problem
Identify the precipitate that forms when aqueous solutions of zinc nitrate and ammonium sulfide are combined Write the equation for the possible double-replacement reaction Then write the formula equation overall ionic equation and the net ionic equation
125
1 AnalyzeGiven Identity of reactants zinc nitrate and ammonium sulfide Reaction medium aqueous solution Unknown a equation for the possible double-replacement reaction b identity of the precipitate c formula equation d overall ionic equation e net ionic equation
126
2 PlanWrite the possible double-replacement reaction between Zn(NO3)2 and (NH4)2S Use the rules to determine if any of the products will precipitate Write a formula equation and overall net ionic equation Then cancel spectator ions to make net ionic equation
127
3 Computea possible double-replacement reaction
b rules show that zinc sulfide is not soluble so will precipitate (ammonium nitrate is soluble)
c formula equation128
d overall ionic equation
e net ionic equation129
Will a precipitate form if solutions of potassium sulfate and barium nitrate are combined If so write the net ionic equation for the reaction
Yes Ba+2(aq) + SO4-2(aq) rarr BaSO4(s)
130
Will a precipitate form if solutions of potassium nitrate and magnesium sulfate are combined If so write the net ionic equation for the reaction
No 131
Will a precipitate form if solutions of barium chloride and sodium sulfate are combined If so identify the spectator ions and write the net ionic equation for the reaction
Yes Na+ and Cl- Ba+2(aq) + SO4-2(aq) rarr BaSO4(s)
132
Write the net ionic equation for the precipitation of nickel(II) sulfide
Ni+2(aq) + S-2(aq) rarr NiS(s)
133
IonizationSome molecular compounds can also form ions in solution Usually polar Ionization - ions formed from solute molecules by action of the solvent (creation of ions where there were none) Ionization different from dissociation
134
Dissociation ionic compounds dissolve and ions already present separate Ionization ions formed where non existed before
Ions formed are hydrated Heat released during hydration of ions gives enough energy to break covalent bonds
135
Extent to which solute ionizes depends on strength of bonds within molecules of solute and strength of attraction between solute and solvent molecules
136
HCl ndash molecular compound that ionizes in aqueous solution Contains highly polar bond Attraction between polar HCl molecule and polar water strong enough to break HCl bond
137
The Hydronium Ion
H+ ions from HCl attracts other molecules or ions so strongly that it doesnrsquot normally exist alone Ionization of HCl better described as direct transfer of proton from HCl to H2O forming H3O+
138
139
Hydration of H+ to form hydronium is highly exothermic
Energy released gives a lot of energy needed to ionize a molecular solute
Many molecular compounds that ionize in aqueous solution contain hydrogen and form hydronium ion
140
Strong and Weak Electrolytes
Substances that make ions and conduct current in solution are electrolytes HCl is one of series of compounds that has hydrogen and halogen Hydrogen halides are all molecular compounds with single polar-covalent bonds
141
All are gases Very soluble in water All are electrolytes HCl HBr HI strongly conduct current in solution HF only weakly conducts current at same concentration Strength of conduction related to ability to ionize
142
143
Strong Electrolytes
HCl HBr HI are 100 ionized in solution
Strong electrolyte - any compound whose dilute aqueous solutions conduct electricity well this is due to the presence of all or almost all of the dissolve compound in the form of ions
144
Unique characteristic of strong electrolyte is that they yield only ions
Ex Ionic compound may be highly soluble in water and dissociate into ions (NaCl)
Others may not dissolve much but amount that does dissolve exists only as ions
145
Weak ElectrolytesHF dissolves in water to give acid solution called hydrofluoric acid HF bond stronger than bonds between hydrogen and other halogens When HF dissolves some molecule ionize Reverse reaction also happens HF(aq) + H2O(l) H3O+(aq) + F-(aq)
146
HF(aq) + H2O(l) H3O+
(aq) + F-(aq)Concentration of dissolved unionized HF stays high and concentration of ions stays low
Weak electrolyte - any compound whose aqueous solutions conduct electricity poorly this is due to the presence of small amount of the dissolved compound in the form of ions
147
Different from nonelectrolyte ndash NO ions at all Description of electrolyte as strong or weak not related to concentration of solution Electrolytes differ in degree of ionization not amount of solute dissolved
148
149
Colligative properties of solutions
Presence of solute affects properties of solutions Some properties not dependent on nature of dissolved substance but on how may dissolve particles are present
Colligative properties - properties that depend on the concentration of solute particles but not on their identity
150
Vapor-Pressure Lowering
Boiling and freezing point of solution different from pure solvent Nonvolatile solute raises boiling point and lowers freezing point Nonvolatile substance - one that has little tendency to become gas under existing conditions
151
152
To understand why nonvolatile solute changes boiling and freezing point consider equilibrium vapor pressure
Vapor pressure pressure caused by molecules that have escape liquid phase to gas phase
Can be thought of a measure of tendency of molecules to escape from a liquid
153
Addition of sucrose (nonvolatile solute) lowers concentration of water molecules at surface of liquid This lowers tendency of water molecules to leave solution and enter gas phase Vapor pressure of solution is lower than vapor pressure of pure water
154
Aqueous13 solution13 of13 nonvolatile13 solutePure13 water
Nonelectrolyte solutions of same molality have same concentration of particles
Dilute solutions of same solvent and equal molality of any nonelectrolyte solute lower vapor pressure equally
155
Example
1 m aqueous solution of nonelectrolyte glucose lowers vapor pressure of water 55 x 10-4 atm at 25 1 m aqueous solution of sucrose also lowers water vapor pressure to 55 x 10-4 atm at 25
156
157
bc vapor-pressure lowering depends on concentration of nonelectrolyte solute and doesnrsquot depend on the type of solute it is a colligative proprerty
bc vapor pressure has been lowered solution remains liquid over larger temperature range
Lowers freezing point and raises boiling point
158
Can assume changes in boiling and freezing point also depend on concentration of solute
They are colligative properties
159
Freezing-Point Depression
Freezing point of 1-molal solution of any nonelectrolyte solute in water is found (by experiment) to be 186 lower than freezing point of water When 1 mol of nonelectrolyte solute dissolved in 1 kg water freezing point is -186 not 000 160
When 2 mol nonelectrolyte solute dissolved in 1 kg water freezing point is -372
For any concentration of nonelectrolyte solute in water decrease in freezing point can be determined using value of -186m
161
This value called the molal freezing-point contstant (Kf) is the freezing-point depression of the solvent in a 1-molal solution of a nonvolatile nonelectrolyte solute
162
Each solvent has own characteristic molal freezing-point constant Freezing-point depression (Δtf) the difference between the freezing points of the pure solvent and a solute of nonelectrolyte in that solvent and it is directly proportional to the molal concentration of the solution Δtf = Kfm
163
Δtf = Kfm
Kf expressed as m
m expressed in mol solutekg solvent (molality)
164
Sample Problem
What is the freezing point depression of water in a solution of 171 g of sucrose C12H22O11 and 200 g of water What is the actual freezing point of the solution
165
1 AnalyzeGiven Solute mass and chemical formula 171 g C12H22O11 Solvent mass and identity 2000 g water Unknown a freezing-point depression b freezing point of the solution 166
2 Plan
167
168
Calculate
Unknown a freezing-point depression b freezing point of the solution
169
Practice Problem
A water solution containing an unknown quantity of a nonelectrolyte solute is found to have a freezing point of ndash 023degCWhat is the molal concentration of the solution
012 m170
A solution consists of 103 g of the nonelectrolyte glucose C6H12O6 dissolved in 250 g of water What is the freezing-point depression of the solution
minus0426degC171
In a laboratory experiment the freezing point of an aqueous solution of glucose is found to be minus0325degCWhat is the molal concentration of this solution
0175 m172
If 0500 mol of a nonelectrolyte solute are dissolved in 5000 g of ether what is the freezing point of the solution
minus1181degC173
The freezing point of an aqueous solution that contains a nonelectrolyte is minus90degC aWhat is the freezing-point depression of the solution bWhat is the molal concentration of the solution a minus90degC b 48 m
174
Boiling-Point Elevation
Boiling point is temp at which vapor pressure is equal to atmospheric pressure Change in vapor pressure will cause corresponding change in boiling point
175
176
Vapor pressure of nonvolatile solution is lower than vapor pressure of pure solvent More heat will be required to raise vapor pressure of solution Boiling point of solution is higher than boiling point of pure solvent
Molal boiling-point constant (Kb) - boiling-point elevation of the solvent in a 1-molal solution of a nonvolatile nonelectrolyte solute b-p elevation of any nonelectrolyte in water found by experiment to be 051 So molal b-p constant for water is 051m
177
For different solvents b-p elevations of 1-molal solutions have different values (Table 14-2)
Like freezing-point constants values most accurate for dilute solutions
178
Boiling-point elevation Δtb difference between the boiling points of the pure solvent and a nonelectrolyte solution of that solvent and it is directly proportional to the molal concentration of the solution B-p elevation can be calculated using Δtb = Kbm Δtb (boiling point elevation) expressed in m and m expressed in mol solutekg solvent
Sample Problem
What is the boiling-point elevation of a solution made from 200 g of a nonelectrolyte solute and 4000 g of water The molar mass of the solute is 620 gmol
180
1 AnalyzeGiven solute mass = 200 g solute molar mass = 620 gmol solvent mass and identity = 4000 g of water
Unknown boiling-point elevation
181
2 Plan
182
183
3 Compute
Practice Problem
A solution contains 500 g of sucrose C12H22O11 a nonelectrolyte dissolved in 5000 g of water What is the boiling-point elevation
015degC184
A solution contains 4500 g of sucrose C12H22O11 a nonelectrolyte dissolved in 250 g of waterWhat is the boiling point of the solution
1027degC185
If the boiling point elevation of an aqueous solution containing a nonvolatile electrolyte is 102degC what is the molality of the solution
20 m186
The boiling point of an aqueous solution containing a nonvolatile electrolyte is 10075degC aWhat is the boiling-point elevation bWhat is the molality of the solution a 075degC b 15 m
187
Electrolytes and Colligative PropertiesEarly investigators confused by experiments where certain substances depressed freezing point or elevated boiling point more than expected
188
189
Ex 01 m solution of NaCl lowers freezing point of solvent almost twice as much as 01 m solution of sucrose 01 m CaCl2 lowers fp almost 3 times as much as 01 m solution of sucrose
To understand why this happens contrast behavior or sucrose with NaCl in aqueous solution Each sucrose dissolves to make only 1 particle in solution So 1 mol sucrose dissolves to 1 mol particles 1 mol NaCl dissolves to make 2 moles of particles (1 mol Na+ and 1 mol Cl-)
190
191
Calculated Values for Electrolyte SolutionsColligative properties depend on total concentration of solute particles regardless of their identity Electrolytes cause changes in colligative properties proportional to total molality in terms of all dissolves particles instead of formula units
192
What about barium nitrate Ba(NO3)2 Each mole of barium nitrate yields 3 mol of ions in solution Ba(NO3)2(s) rarr Ba2+(aq) + 2NO3-(aq) You would expect solution of given molality to lower fp of solvent 3 times as much as nonelectrolytic solution of same molality
193
Sample Problem
What is the expected change in the freezing point of water in a solution of 625 g of barium nitrate Ba(NO3)2 in 100 kg of water
194
1 AnalyzeGiven solute mass and formula = 625 g Ba(NO3)2 solvent mass and identity = 100 kg water Δtf = Kfm
Unknown expected freezing-point depression
195
2 PlanThe molality can be calculated by converting the solute mass to moles and then dividing by the number of kilograms of solvent That molality is in terms of formula units of Ba(NO3)2 and must be converted to molality in terms of dissociated ions in solution It must be multiplied by the number of moles of ions produced per mole of formula unit This adjusted molality can then be used to calculate the freezing-point depression
196
197
198
3 Compute
Practice Problem
What is the expected freezing-point depression for a solution that contains 20 mol of magnesium sulfate dissolved in 10 kg of water
minus74degC199
What is the expected boiling-point elevation of water for a solution that contains 150 g of sodium chloride dissolved in 10 kg of water 27degC The freezing point of an aqueous sodium chloride solution is minus200degC What is the molality of the solution 54 m NaCl
200
Actual Values for Electrolyte SolutionsRemember values calculated are only expected values Actual values of colligative properties for all strong electrolytes are almost what would be expected Difference between expected and actual values caused by attractive forces that exist between dissociated ions in aqueous solution
201
Attraction between hydrated ions in solution is small compared with those in crystalline solid However forces of attraction donrsquot interfere with movements of aqueous ions More concentrated solution means ions closer together and attraction is greater Also depends on charge of ions ndash higher charge greater attraction
202
Many alloys such as brass (zinc and copper) are sterling silver (silver and copper) are solid solutions Atoms of two or metals are evenly mixed By properly choosing percentages of each metal can get many desirable properties
10
Ex Alloys have higher strength and greater resistance to corrosion than pure metals Pure gold (24K) too soft for jewelry Alloy with silver (14K) increases strength and hardness but keeps appearance of gold
11
SuspensionsSuspension - if particles in solvent are so large they settle out unless mixture constantly stirred Think of sand and water Particles over 1000 nm in diameter
12
Particles that are medium in size between those in solutions and suspensions from mixtures are known as colloids (colloidal dispersions) Particles between 1-1000 nm diameter
13
Colloids
In muddy water large soil particles settle Water still cloudy bc colloidal particles stay dispersed in water Filter Colloidal particles pass through and stays cloudy
14
Particles in colloid small enough to be suspended in solvent by constant movement of surrounding molecules Colloidal particles make up dispersed phase Water is dispersing medium
15
Emulsion and foam are colloids
Mayonnaise is emulsion of oil droplets in water Egg yolk acts as emulsifying agent (keeps oil droplets dispersed)
16
17
18
Class13 of13 Colloid Phases Example
Sol Solid13 dispersed13 in13 liquid Paints13 mud
Gel Solid13 network13 extending13 through13 liquid
Gelatin
Liquid13 emulsion Liquid13 dispersed13 in13 liquid Milk13 mayonnaise
Foam Gas13 dispersed13 in13 liquid Shaving13 cream13 whipped13 cream
Solid13 aerosol Solid13 dispersed13 in13 gas Smoke13 auto13 exhaust
Liquid13 aerosol Liquid13 dispersed13 in13 gas Fog13 mist13 clouds13 aerosol13 spray
Solid13 emulsion Liquid13 dispersed13 in13 solid Cheese13 butter
Tyndall EffectMany colloids appear homogeneous bc individual particles cannot be seen Particles large enough to scatter light In fog can see headlight beam Tyndall effect - occurs when light is scattered by colloidal particles dispersed in transparent medium
19
20
21
Solutions Colloids Suspensions
Homogeneous Heterogeneous Heterogeneous
Particle13 size13 001-shy‐113 nm13 can13 be13 atoms13 ions13 molecules
Particle13 size13 1-shy‐100013 nm13 dispersed13 can13 be13 combined13 or13 large13 molecules
Particle13 size13 over13 100013 nm13 suspended13 can13 be13 large13 particles13 or13 combined13 particles
Do13 not13 separate13 on13 standing Do13 not13 separate13 on13 standing Particles13 settle13 out
Cannot13 be13 separated13 by13 filtration Cannot13 be13 separated13 by13 filtration Can13 be13 separated13 by13 filtration
Do13 not13 scatter13 light Scatter13 light13 (Tyndall13 effect) May13 scatter13 light13 but13 are13 not13 transparent
Electrolytes vs Nonelectrolytes
Substances that dissolve in water are classified according to whether they produce molecules or ions in solution
22
23
When ionic compound dissolves cations and anions separate and are surrounded by water molecules Solute ions free to move making possible to carry electric current
Electrolyte - substance that dissolves in water to give a solution that conducts electric current NaCl ndash electrolyte Usually highly polar compounds become electrolytes
24
Solution containing neutral solute molecules does not conduct current bc it doesnrsquot have mobile charged particles
Nonelectrolyte - substance that dissolves in water to give solution that does not conduct electric current Ex Sugar
25
26
27the solution
process
Factors Affecting Rate of DissolutionEver tried to dissolve sugar in iced tea or coffee You notice temperature has something to do with how quickly solute dissolves What other factors affect how quickly solutes dissolve
28
1 Increasing Surface Area of Solute
bc dissolution process occurs at surface of solute Can increase dissolution by increasing surface area
29
2 Agitating (Stirring) SolutionClose to surface of solute concentration of dissolved solute is high Stirringshaking helps disperse solute particles
30
Brings fresh solvent into contact with solute surface Contact between solvent and solute surface increased
3 Heating a Solvent
Solutes dissolve faster in warmer solvent As temp increases solvent molecules move faster Average kinetic energy increases Collisions between solvent and solute more frequent Helps separate solute molecules and spread them out
32
Solubility
If you add sugar to tea eventually no more sugar dissolves
For every combination of solvent with solute at given temp there is limit to amount of solute that can be dissolved
33
When sugar first dropped into water sugar molecules leave solid surface and move about at random in solvent Some dissolved molecules may collide with crystal and stay there As more solid dissolves and concentration of dissolved molecules increases collisions happen more often
34
Eventually molecules are returning to crystal at same rate they are going into solution Dynamic equilibrium established between dissolution and crystallization Solution equilibrium - physical state in which the opposing processes of dissolution and crystallization of a solute occur at equal rates
35
Saturated vs Unsaturated Solutions
Saturated solution - solution that contains the maximum amount of dissolved solute How do you know if solution is saturated If more NaCl added it falls to bottom and doesnrsquot dissolve Equilibrium already established
36
If more water added to saturated solution more NaCl will dissolve At 20 359 g NaCl is max that will dissolve in 100 g water Unsaturated solution - solution that contains less solute than a saturated solution under the existing conditions
37
Supersaturated Solutions
When saturated solution where solubility increases as temp increases is cooled excess solute usually comes out of solution Sometimes if solution is left to cool the excess solute doesnrsquot separate
38
39
Supersaturated solution - solution that contains more dissolved solute than a saturated solution contains under same conditions
40
Supersaturated solution can remain unchanged over long time if it isnrsquot disturbed Once crystals begin to form continues until equilibrium is recreated at lower temp
41
Example
Sodium thiosulfate Na2S2O3 Solute added to hot water until solution saturated Hot solution filtered Drop small crystal in ldquoseedingrdquo crystallization Continues until equilibrium created
42
Solubility Values
Solubility - amount of substance required to form a saturated solution with specific amount of solvent at specified temp Ex Solubility of sugar = 204 g per 100 g water at 20 Must specify temp bc solubility changes with temp 43
For gases pressure must also be specified Rate at which substance dissolves is unrelated to solubility Max amount of solute that dissolves and reaches equilibrium is always same under same conditions
44
Solute-Solvent Interactions
ldquolike dissolves likerdquo is generally useful rule for predicting whether one substance will dissolve in another Depends on type of bonding polaritynonpolarity and intermolecular forces between solute and solvent
45
Dissolving Ionic Compounds in Aqueous Solution
Polarity of water molecules plays important role in formation of solution of ionic compounds Charged ends of water molecules attract ions and surround them to keep separated from other ions
46
Hydration - solution process with water as solvent Ions said to be hydrated As hydrated ions diffuse into solution other ions exposed and drawn away from crystal surface Entire crystal gradually dissolves
47
When crystallized from aqueous solutions some ionic compounds form crystals that include water molecules The crystalline compounds known as hydrates have specific ratios of water molecules Represented by formulas like CuSO45H2O
48
When crystalline hydrate dissolves in water water of hydration returns to solvent Behavior of solute in hydrated form no different than anhydrous form
49
Nonpolar Solvents
Nonpolar solvents do not attract ions of ionic compound strongly enough to overcome forces holding crystal together
50
Liquid Solutes and Solvents
When you shake bottle of salad dressing oil droplets disperse in water Stop shaking ndash strong attraction between water molecules squeeze out oil and form separate layers Liquid solutes and solvents that are not soluble in each other are immiscible
51
Nonpolar substances (fat oil grease) are generally quite soluble in nonpolar liquids (carbon tetrachloride toluene gasoline)
Only attractions between nonpolar molecules are weak London forces
52
London Dispersion Forces
bc e- are constantly moving at any time there may be uneven distribution of them around a nonpolar atom Temporarily creates positive end and negative end
53
54
London dispersion forces - intermolecular attractions resulting from constant motion of e- and creation of instantaneous dipoles
55
Liquids that dissolve freely in one another in any ratio are said to be completely miscible Benzene and carbon tetrachloride (both nonpolar) are completely miscible
Nonpolar molecules of these two apply no strong forces of attraction or repulsion and molecules mix freely
56
Ethanol and water are also completely miscible -OH group on ethanol is slightly polar
Can form H-bonds with water as well as other ethanol molecules Intermolecular forces in mixture are very similar to those in pure liquids so they are mutually soluble in any amount
57
Gasoline contains mainly nonpolar hydrocarbons
Excellent solvent for fats oils greases
Major intermolecular forces are weak London forces
58
Ethanol less polar than water more than carbon tetrachloride
Better solvent for less-polar substances bc of nonpolar region
59
Effects of Pressure on Solubility
Little effect on solids or liquids Increase pressure - increase solubilities of gases in liquids
60
When gas in contact with surface of liquid gas molecules can enter liquid As amount of dissolve gas increases some molecules start to escape and reenter gas phase
Eq eventually established between rates of enteringleaving gas phase
61
As long as eq undisturbed solubility of gas in liquid unchanged at given pressure
Gas + solvent solution
62
Increasing pressure of solute gas above solution puts stress on eq Molecules collide with surface more often Increase in pressure partially offset by increase in rate of gas molecules entering solution In turn increase in amount of dissolved gas causes increase in rate molecules escape liquid and becomes gas
63
Eventually eq restored at higher gas solubility
Expected from Le Chatelierrsquos principle
Increase in gas pressure causes eq to shift so fewer molecules are in gas phase
64
Henryrsquos Law
Henryrsquos law - solubility of a gas in liquid is directly proportional to the partial pressure of that gas on the surface of the liquid
Applies to gas-liquid solutions at constant temp
65
When mixture of ideal gases is limited in constant volume at constant temp each gas applies same pressure it would apply if it occupied the space alone
Assuming gases do not react each gas dissolves to the extent it would if no other gases were there
66
In carbonated drinks (Coke Pepsi etc) solubility of CO2 increased by increasing pressure
At bottling factory CO2 gas forced into solution of flavored water at pressure of 5-10 atm Gas-in-liquid solution then sealed in bottlescans
67
When cap removed pressure reduced to 1 atm and some CO2 escapes as gas bubbles Effervescence - rapid escape of a gas from a liquid in which it is dissolved
68
Effects of Temperature on Solubility
Letrsquos consider gas solubility Increasing temperature usually decreases gas solubility As temp increases average kinetic energy of molecules in solution increases Greater number of solute molecules escape from attraction of solvent and return to gas phase 69
At higher temps eq is reached with fewer gas molecules in solution Gases generally less soluble
70
Effect of temp on solubility of solids in liquids more difficult to predict
Often increasing temp increases solubility
However the same temp increase can result in large increase in solubility in one case and only slight increase in the next
71
72
enthalpies of Solution
Formation of solution comes with energy change Dissolve KI in water container feels cold Dissolve LiCl in water feels hot Formation of solid-liquid solution can absorb or release heat
73
During formation of solution solvent and solute particles experience changes in forces attracting them to other particles Before dissolving begins solvent molecules held by intermolecular forces (solvent-solvent attraction) In solute molecules held by Ifs (solute-solute attraction)
74
Energy is required to separate solute molecules and solvent molecules from their neighbors
A solute particle that is surrounded by solvent molecules is said to be solvated
75
Step 1 solute particles become separated from solid (energy absorbed) Step 2 solvent particles move apart to allow solute particles to enter liquid (energy absorbed)
76
Step 3 solvent particles attracted to and solvate solute particles (energy released)
77
Heat of solution - net amount of heat energy absorbed or released when a specific amount of solute dissolves in a solvent
Heat of solution negative (heat is released) if sums of energy in steps 1 and 2 is less than step 3
If 1+2 gt 3 HOS is positive (heat absorbed)78
In gaseous state molecules are so far apart there are nearly no forces between them Solute-solute interaction has little effect on heat of solution of a gas Energy released when gas dissolved in liquid bc attraction between solute gas and solvent molecules greater than energy needed to separate solvent molecules
79
Concentration of Solutions80
Concentration - measure of amount of solute in given amount of solvent or solution ldquodiluterdquo just means small amount of solute ldquoconcentratedrdquo just means relatively large amount of solute Unrelated to degree to which solution is saturated Ex Saturated solution of substance not very soluble might be dilute
81
Molarity
Molarity - number of moles of solute in one liter of solution To find molarity must know molar mass A ldquoone molarrdquo solution of NaOH is 1 mole NaOH per liter of solution (1 M)
82
1 mol NaOH = 400 g If this quantity dissolve in enough water to make EXACTLY 100 L solution solution is 1 M If 200 g NaOH dissolve in enough to make 100 L solution what is molarity 0500 M
83
1 M solution is not made by adding 1 mol solute to 1 L solvent That would be more than 1 L Instead 1 mol solute dissolved in less than 1 L Then diluted with more solvent to bring TOTAL VOLUME to 1 L
84
Practice ProblemYou have 340 L of solution that contains 900 g sodium chloride What is the molarity of that solution 0440 M NaCl You have 08 L of a 05 M HCl solution How many moles of HCl does this solution contain 04 mol HCl
85
What is the molarity of a solution composed of 585 g of potassium iodide dissolved in enough water to make a 0125 L of solution 0282 M KI
86
How many moles of H2SO4 are present in 0500 L of a 0150 M H2SO4 solution 00750 mol What volume of 300 M NaCl is needed for reaction that requires 1463 g of NaCl 0834 L
87
Molality
Molality - concentration of solution expressed in moles of solute per kilogram of solvent Solution that contains 1 mol solute (NaOH) dissolved in exactly 1 kg of solvent is ldquoone molalrdquo solution (1 m NaOH)
88
1 mol NaOH = 400 g 400 g NaOH dissolved in 1 kg water is 1 m NaOH
200 g NaOH in 1 kg water = 0500 m NaOH
89
Practice Problem
A solution was prepared by dissolving 171 g sucrose (C12H22O11) in 125 g of water Find the molal concentration of this solution 0400 m C12H22O11
90
A solution of iodine in carbon tetrachloride is used when iodine is needed for certain chemical tests How much iodine must be added to prepare a 0480 m solution of iodine in CCl4 if 1000 g of CCl4 is used 122 g I2
91
What is the molality of a solution composed of 255 g acetone (CH3)2CO dissolved in 200 g of water 22 m acetone What quantity in grams of methanol CH3OH is required to prepare a 0244 m solution in 400 g water 312 g methanol
92
How many grams of AgNO3 are needed to prepare a 0125 m solution in 250 mL of water 531 g AgNO3
What is the molality of a solution containing 182 g HCl and 250 g water 200 m
93
Ions in Aqueous Solutions and Colligative Properties
94
95
compounds in aqueous solutions
DissociationWhen compound made from ions dissolves in water ions separate Dissociation - separation of ions that occurs when an ionic compound dissolves
96
Notice number of ions made per formula unit in equations 1 formula unit of NaCl gives 2 units of ions in solution 1 formula unit of CaCl2 gives 3 units of ions
97
Assuming 100 dissociation solution that contains 1 mol NaCl contains 1 mol Na+ and 1 mol Cl- Can assume 100 dissociation of all soluble ionic compounds
98
Practice Problem
Write the equation for the dissolution of aluminum sulfate in water How many moles of aluminum ions are made by dissolving 1 mol aluminum sulfate What is the total number of moles of ions made by dissolving 1 mol of aluminum sulfate
99
1 AnalyzeGiven Amount of solute = 1 mol Al2(SO4)3 Solvent identity = water Unknown a moles of aluminum ions and sulfate ions Total number of moles of solute ions produced
100
2 Plan
The coefficients in the balanced dissociation equation will tell mole relationships You can use the equation to find out the number of moles of solute ions produced
101
3 Compute
a
b102
Write the equation for the dissolution of each of the following in water and then determine the number of moles of each ions made as well as the total number of ions made a 1 mol ammonium chloride b 1 mol sodium sulfide c 05 mol barium nitrate
103
a 1 mol ammonium chloride
1 mol NH4+ 1 mol Cl- 2 mol total ions
104
b 1 mol sodium sulfide
2 mol Na+ 1 mol S-2 3 mol total ions
105
c 05 mol barium nitrate
05 mol Ba+2 1 mol NO3- 15 mol total ions
106
Precipitation Reactions
Even though no compound is completely insoluble compounds of very low solubility can be considered insoluble for practical purposes There are general guidelines to help predict whether a compound made of certain combination of ions is soluble
107
General Solubility Guidelines
Sodium potassium and ammonium compounds are soluble in water
108
Nitrates acetates and chlorates are soluble
109
110
Most chlorides are soluble except those of silver mercury (I) and lead Lead (II) chloride is soluble in hot water
Most sulfates are soluble except those of barium strontium lead calcium and mercury
111
Most carbonates phosphates and silicates are insoluble except those of sodium potassium and ammonium
112
Most sulfides are insoluble except those of calcium strontium sodium potassium and ammonium
113
Is calcium phosphate Ca3(PO4)2 soluble or insoluble
Not one of the exceptions so it is insoluble
Dissociation equations are not written for insoluble compounds
114
Guidelines also useful in predicting what will happen if 2 solutions of 2 different soluble compounds are mixed
If mixing results in combination of ions that forms insoluble compound a double-replacement and precipitation reaction will happen
115
Will precipitate form when solutions of ammonium sulfide and cadmium nitrate are combined You can tell calcium nitrate is soluble from the guidelines (Most nitrates acetates and chlorates are soluble) You can also tell ammonium sulfide is soluble (Most sulfides are insoluble except those of calcium strontium sodium potassium and ammonium)
116
117
To decide whether precipitate can form must know solubilities of two possible products
Ammonium nitrate ndash soluble (Most nitrates acetates and chlorates are soluble) Cadmium sulfide ndash insoluble (Most sulfides are insoluble except those of calcium strontium sodium potassium and ammonium)
118
bc one product is insoluble double-replacement and precipitation reaction will happen (NH4)2S (aq) + Cd(NO3)2 (aq) rarr NH4NO3 (aq) + CdS (s)
119
Net Ionic Equations
Reactions of ions in aqueous solution usually represented by net ionic equations instead of formula equations Net ionic equation - includes only those compounds and ions that undergo a chemical change in a reaction in an aqueous solution
120
121
To write NIE (net ionic equation) first convert chemical equation into overall ionic equation All soluble ionic compounds shown as dissociated ions in solution Precipitates shown as solids
122
Notice ammonium and nitrate ions appear on both sides They havenrsquot gone through any chemical change Spectator ions - ions that do not take part in a chemical reaction and are found in solution both before and after the reaction
123
To change ionic equation to NIE eliminate spectator ions Cd+2(aq) + S-2(aq) rarr CdS(s)
This NIE applies to any reaction in which precipitate of CdS forms
124
Practice Problem
Identify the precipitate that forms when aqueous solutions of zinc nitrate and ammonium sulfide are combined Write the equation for the possible double-replacement reaction Then write the formula equation overall ionic equation and the net ionic equation
125
1 AnalyzeGiven Identity of reactants zinc nitrate and ammonium sulfide Reaction medium aqueous solution Unknown a equation for the possible double-replacement reaction b identity of the precipitate c formula equation d overall ionic equation e net ionic equation
126
2 PlanWrite the possible double-replacement reaction between Zn(NO3)2 and (NH4)2S Use the rules to determine if any of the products will precipitate Write a formula equation and overall net ionic equation Then cancel spectator ions to make net ionic equation
127
3 Computea possible double-replacement reaction
b rules show that zinc sulfide is not soluble so will precipitate (ammonium nitrate is soluble)
c formula equation128
d overall ionic equation
e net ionic equation129
Will a precipitate form if solutions of potassium sulfate and barium nitrate are combined If so write the net ionic equation for the reaction
Yes Ba+2(aq) + SO4-2(aq) rarr BaSO4(s)
130
Will a precipitate form if solutions of potassium nitrate and magnesium sulfate are combined If so write the net ionic equation for the reaction
No 131
Will a precipitate form if solutions of barium chloride and sodium sulfate are combined If so identify the spectator ions and write the net ionic equation for the reaction
Yes Na+ and Cl- Ba+2(aq) + SO4-2(aq) rarr BaSO4(s)
132
Write the net ionic equation for the precipitation of nickel(II) sulfide
Ni+2(aq) + S-2(aq) rarr NiS(s)
133
IonizationSome molecular compounds can also form ions in solution Usually polar Ionization - ions formed from solute molecules by action of the solvent (creation of ions where there were none) Ionization different from dissociation
134
Dissociation ionic compounds dissolve and ions already present separate Ionization ions formed where non existed before
Ions formed are hydrated Heat released during hydration of ions gives enough energy to break covalent bonds
135
Extent to which solute ionizes depends on strength of bonds within molecules of solute and strength of attraction between solute and solvent molecules
136
HCl ndash molecular compound that ionizes in aqueous solution Contains highly polar bond Attraction between polar HCl molecule and polar water strong enough to break HCl bond
137
The Hydronium Ion
H+ ions from HCl attracts other molecules or ions so strongly that it doesnrsquot normally exist alone Ionization of HCl better described as direct transfer of proton from HCl to H2O forming H3O+
138
139
Hydration of H+ to form hydronium is highly exothermic
Energy released gives a lot of energy needed to ionize a molecular solute
Many molecular compounds that ionize in aqueous solution contain hydrogen and form hydronium ion
140
Strong and Weak Electrolytes
Substances that make ions and conduct current in solution are electrolytes HCl is one of series of compounds that has hydrogen and halogen Hydrogen halides are all molecular compounds with single polar-covalent bonds
141
All are gases Very soluble in water All are electrolytes HCl HBr HI strongly conduct current in solution HF only weakly conducts current at same concentration Strength of conduction related to ability to ionize
142
143
Strong Electrolytes
HCl HBr HI are 100 ionized in solution
Strong electrolyte - any compound whose dilute aqueous solutions conduct electricity well this is due to the presence of all or almost all of the dissolve compound in the form of ions
144
Unique characteristic of strong electrolyte is that they yield only ions
Ex Ionic compound may be highly soluble in water and dissociate into ions (NaCl)
Others may not dissolve much but amount that does dissolve exists only as ions
145
Weak ElectrolytesHF dissolves in water to give acid solution called hydrofluoric acid HF bond stronger than bonds between hydrogen and other halogens When HF dissolves some molecule ionize Reverse reaction also happens HF(aq) + H2O(l) H3O+(aq) + F-(aq)
146
HF(aq) + H2O(l) H3O+
(aq) + F-(aq)Concentration of dissolved unionized HF stays high and concentration of ions stays low
Weak electrolyte - any compound whose aqueous solutions conduct electricity poorly this is due to the presence of small amount of the dissolved compound in the form of ions
147
Different from nonelectrolyte ndash NO ions at all Description of electrolyte as strong or weak not related to concentration of solution Electrolytes differ in degree of ionization not amount of solute dissolved
148
149
Colligative properties of solutions
Presence of solute affects properties of solutions Some properties not dependent on nature of dissolved substance but on how may dissolve particles are present
Colligative properties - properties that depend on the concentration of solute particles but not on their identity
150
Vapor-Pressure Lowering
Boiling and freezing point of solution different from pure solvent Nonvolatile solute raises boiling point and lowers freezing point Nonvolatile substance - one that has little tendency to become gas under existing conditions
151
152
To understand why nonvolatile solute changes boiling and freezing point consider equilibrium vapor pressure
Vapor pressure pressure caused by molecules that have escape liquid phase to gas phase
Can be thought of a measure of tendency of molecules to escape from a liquid
153
Addition of sucrose (nonvolatile solute) lowers concentration of water molecules at surface of liquid This lowers tendency of water molecules to leave solution and enter gas phase Vapor pressure of solution is lower than vapor pressure of pure water
154
Aqueous13 solution13 of13 nonvolatile13 solutePure13 water
Nonelectrolyte solutions of same molality have same concentration of particles
Dilute solutions of same solvent and equal molality of any nonelectrolyte solute lower vapor pressure equally
155
Example
1 m aqueous solution of nonelectrolyte glucose lowers vapor pressure of water 55 x 10-4 atm at 25 1 m aqueous solution of sucrose also lowers water vapor pressure to 55 x 10-4 atm at 25
156
157
bc vapor-pressure lowering depends on concentration of nonelectrolyte solute and doesnrsquot depend on the type of solute it is a colligative proprerty
bc vapor pressure has been lowered solution remains liquid over larger temperature range
Lowers freezing point and raises boiling point
158
Can assume changes in boiling and freezing point also depend on concentration of solute
They are colligative properties
159
Freezing-Point Depression
Freezing point of 1-molal solution of any nonelectrolyte solute in water is found (by experiment) to be 186 lower than freezing point of water When 1 mol of nonelectrolyte solute dissolved in 1 kg water freezing point is -186 not 000 160
When 2 mol nonelectrolyte solute dissolved in 1 kg water freezing point is -372
For any concentration of nonelectrolyte solute in water decrease in freezing point can be determined using value of -186m
161
This value called the molal freezing-point contstant (Kf) is the freezing-point depression of the solvent in a 1-molal solution of a nonvolatile nonelectrolyte solute
162
Each solvent has own characteristic molal freezing-point constant Freezing-point depression (Δtf) the difference between the freezing points of the pure solvent and a solute of nonelectrolyte in that solvent and it is directly proportional to the molal concentration of the solution Δtf = Kfm
163
Δtf = Kfm
Kf expressed as m
m expressed in mol solutekg solvent (molality)
164
Sample Problem
What is the freezing point depression of water in a solution of 171 g of sucrose C12H22O11 and 200 g of water What is the actual freezing point of the solution
165
1 AnalyzeGiven Solute mass and chemical formula 171 g C12H22O11 Solvent mass and identity 2000 g water Unknown a freezing-point depression b freezing point of the solution 166
2 Plan
167
168
Calculate
Unknown a freezing-point depression b freezing point of the solution
169
Practice Problem
A water solution containing an unknown quantity of a nonelectrolyte solute is found to have a freezing point of ndash 023degCWhat is the molal concentration of the solution
012 m170
A solution consists of 103 g of the nonelectrolyte glucose C6H12O6 dissolved in 250 g of water What is the freezing-point depression of the solution
minus0426degC171
In a laboratory experiment the freezing point of an aqueous solution of glucose is found to be minus0325degCWhat is the molal concentration of this solution
0175 m172
If 0500 mol of a nonelectrolyte solute are dissolved in 5000 g of ether what is the freezing point of the solution
minus1181degC173
The freezing point of an aqueous solution that contains a nonelectrolyte is minus90degC aWhat is the freezing-point depression of the solution bWhat is the molal concentration of the solution a minus90degC b 48 m
174
Boiling-Point Elevation
Boiling point is temp at which vapor pressure is equal to atmospheric pressure Change in vapor pressure will cause corresponding change in boiling point
175
176
Vapor pressure of nonvolatile solution is lower than vapor pressure of pure solvent More heat will be required to raise vapor pressure of solution Boiling point of solution is higher than boiling point of pure solvent
Molal boiling-point constant (Kb) - boiling-point elevation of the solvent in a 1-molal solution of a nonvolatile nonelectrolyte solute b-p elevation of any nonelectrolyte in water found by experiment to be 051 So molal b-p constant for water is 051m
177
For different solvents b-p elevations of 1-molal solutions have different values (Table 14-2)
Like freezing-point constants values most accurate for dilute solutions
178
Boiling-point elevation Δtb difference between the boiling points of the pure solvent and a nonelectrolyte solution of that solvent and it is directly proportional to the molal concentration of the solution B-p elevation can be calculated using Δtb = Kbm Δtb (boiling point elevation) expressed in m and m expressed in mol solutekg solvent
Sample Problem
What is the boiling-point elevation of a solution made from 200 g of a nonelectrolyte solute and 4000 g of water The molar mass of the solute is 620 gmol
180
1 AnalyzeGiven solute mass = 200 g solute molar mass = 620 gmol solvent mass and identity = 4000 g of water
Unknown boiling-point elevation
181
2 Plan
182
183
3 Compute
Practice Problem
A solution contains 500 g of sucrose C12H22O11 a nonelectrolyte dissolved in 5000 g of water What is the boiling-point elevation
015degC184
A solution contains 4500 g of sucrose C12H22O11 a nonelectrolyte dissolved in 250 g of waterWhat is the boiling point of the solution
1027degC185
If the boiling point elevation of an aqueous solution containing a nonvolatile electrolyte is 102degC what is the molality of the solution
20 m186
The boiling point of an aqueous solution containing a nonvolatile electrolyte is 10075degC aWhat is the boiling-point elevation bWhat is the molality of the solution a 075degC b 15 m
187
Electrolytes and Colligative PropertiesEarly investigators confused by experiments where certain substances depressed freezing point or elevated boiling point more than expected
188
189
Ex 01 m solution of NaCl lowers freezing point of solvent almost twice as much as 01 m solution of sucrose 01 m CaCl2 lowers fp almost 3 times as much as 01 m solution of sucrose
To understand why this happens contrast behavior or sucrose with NaCl in aqueous solution Each sucrose dissolves to make only 1 particle in solution So 1 mol sucrose dissolves to 1 mol particles 1 mol NaCl dissolves to make 2 moles of particles (1 mol Na+ and 1 mol Cl-)
190
191
Calculated Values for Electrolyte SolutionsColligative properties depend on total concentration of solute particles regardless of their identity Electrolytes cause changes in colligative properties proportional to total molality in terms of all dissolves particles instead of formula units
192
What about barium nitrate Ba(NO3)2 Each mole of barium nitrate yields 3 mol of ions in solution Ba(NO3)2(s) rarr Ba2+(aq) + 2NO3-(aq) You would expect solution of given molality to lower fp of solvent 3 times as much as nonelectrolytic solution of same molality
193
Sample Problem
What is the expected change in the freezing point of water in a solution of 625 g of barium nitrate Ba(NO3)2 in 100 kg of water
194
1 AnalyzeGiven solute mass and formula = 625 g Ba(NO3)2 solvent mass and identity = 100 kg water Δtf = Kfm
Unknown expected freezing-point depression
195
2 PlanThe molality can be calculated by converting the solute mass to moles and then dividing by the number of kilograms of solvent That molality is in terms of formula units of Ba(NO3)2 and must be converted to molality in terms of dissociated ions in solution It must be multiplied by the number of moles of ions produced per mole of formula unit This adjusted molality can then be used to calculate the freezing-point depression
196
197
198
3 Compute
Practice Problem
What is the expected freezing-point depression for a solution that contains 20 mol of magnesium sulfate dissolved in 10 kg of water
minus74degC199
What is the expected boiling-point elevation of water for a solution that contains 150 g of sodium chloride dissolved in 10 kg of water 27degC The freezing point of an aqueous sodium chloride solution is minus200degC What is the molality of the solution 54 m NaCl
200
Actual Values for Electrolyte SolutionsRemember values calculated are only expected values Actual values of colligative properties for all strong electrolytes are almost what would be expected Difference between expected and actual values caused by attractive forces that exist between dissociated ions in aqueous solution
201
Attraction between hydrated ions in solution is small compared with those in crystalline solid However forces of attraction donrsquot interfere with movements of aqueous ions More concentrated solution means ions closer together and attraction is greater Also depends on charge of ions ndash higher charge greater attraction
202
Ex Alloys have higher strength and greater resistance to corrosion than pure metals Pure gold (24K) too soft for jewelry Alloy with silver (14K) increases strength and hardness but keeps appearance of gold
11
SuspensionsSuspension - if particles in solvent are so large they settle out unless mixture constantly stirred Think of sand and water Particles over 1000 nm in diameter
12
Particles that are medium in size between those in solutions and suspensions from mixtures are known as colloids (colloidal dispersions) Particles between 1-1000 nm diameter
13
Colloids
In muddy water large soil particles settle Water still cloudy bc colloidal particles stay dispersed in water Filter Colloidal particles pass through and stays cloudy
14
Particles in colloid small enough to be suspended in solvent by constant movement of surrounding molecules Colloidal particles make up dispersed phase Water is dispersing medium
15
Emulsion and foam are colloids
Mayonnaise is emulsion of oil droplets in water Egg yolk acts as emulsifying agent (keeps oil droplets dispersed)
16
17
18
Class13 of13 Colloid Phases Example
Sol Solid13 dispersed13 in13 liquid Paints13 mud
Gel Solid13 network13 extending13 through13 liquid
Gelatin
Liquid13 emulsion Liquid13 dispersed13 in13 liquid Milk13 mayonnaise
Foam Gas13 dispersed13 in13 liquid Shaving13 cream13 whipped13 cream
Solid13 aerosol Solid13 dispersed13 in13 gas Smoke13 auto13 exhaust
Liquid13 aerosol Liquid13 dispersed13 in13 gas Fog13 mist13 clouds13 aerosol13 spray
Solid13 emulsion Liquid13 dispersed13 in13 solid Cheese13 butter
Tyndall EffectMany colloids appear homogeneous bc individual particles cannot be seen Particles large enough to scatter light In fog can see headlight beam Tyndall effect - occurs when light is scattered by colloidal particles dispersed in transparent medium
19
20
21
Solutions Colloids Suspensions
Homogeneous Heterogeneous Heterogeneous
Particle13 size13 001-shy‐113 nm13 can13 be13 atoms13 ions13 molecules
Particle13 size13 1-shy‐100013 nm13 dispersed13 can13 be13 combined13 or13 large13 molecules
Particle13 size13 over13 100013 nm13 suspended13 can13 be13 large13 particles13 or13 combined13 particles
Do13 not13 separate13 on13 standing Do13 not13 separate13 on13 standing Particles13 settle13 out
Cannot13 be13 separated13 by13 filtration Cannot13 be13 separated13 by13 filtration Can13 be13 separated13 by13 filtration
Do13 not13 scatter13 light Scatter13 light13 (Tyndall13 effect) May13 scatter13 light13 but13 are13 not13 transparent
Electrolytes vs Nonelectrolytes
Substances that dissolve in water are classified according to whether they produce molecules or ions in solution
22
23
When ionic compound dissolves cations and anions separate and are surrounded by water molecules Solute ions free to move making possible to carry electric current
Electrolyte - substance that dissolves in water to give a solution that conducts electric current NaCl ndash electrolyte Usually highly polar compounds become electrolytes
24
Solution containing neutral solute molecules does not conduct current bc it doesnrsquot have mobile charged particles
Nonelectrolyte - substance that dissolves in water to give solution that does not conduct electric current Ex Sugar
25
26
27the solution
process
Factors Affecting Rate of DissolutionEver tried to dissolve sugar in iced tea or coffee You notice temperature has something to do with how quickly solute dissolves What other factors affect how quickly solutes dissolve
28
1 Increasing Surface Area of Solute
bc dissolution process occurs at surface of solute Can increase dissolution by increasing surface area
29
2 Agitating (Stirring) SolutionClose to surface of solute concentration of dissolved solute is high Stirringshaking helps disperse solute particles
30
Brings fresh solvent into contact with solute surface Contact between solvent and solute surface increased
3 Heating a Solvent
Solutes dissolve faster in warmer solvent As temp increases solvent molecules move faster Average kinetic energy increases Collisions between solvent and solute more frequent Helps separate solute molecules and spread them out
32
Solubility
If you add sugar to tea eventually no more sugar dissolves
For every combination of solvent with solute at given temp there is limit to amount of solute that can be dissolved
33
When sugar first dropped into water sugar molecules leave solid surface and move about at random in solvent Some dissolved molecules may collide with crystal and stay there As more solid dissolves and concentration of dissolved molecules increases collisions happen more often
34
Eventually molecules are returning to crystal at same rate they are going into solution Dynamic equilibrium established between dissolution and crystallization Solution equilibrium - physical state in which the opposing processes of dissolution and crystallization of a solute occur at equal rates
35
Saturated vs Unsaturated Solutions
Saturated solution - solution that contains the maximum amount of dissolved solute How do you know if solution is saturated If more NaCl added it falls to bottom and doesnrsquot dissolve Equilibrium already established
36
If more water added to saturated solution more NaCl will dissolve At 20 359 g NaCl is max that will dissolve in 100 g water Unsaturated solution - solution that contains less solute than a saturated solution under the existing conditions
37
Supersaturated Solutions
When saturated solution where solubility increases as temp increases is cooled excess solute usually comes out of solution Sometimes if solution is left to cool the excess solute doesnrsquot separate
38
39
Supersaturated solution - solution that contains more dissolved solute than a saturated solution contains under same conditions
40
Supersaturated solution can remain unchanged over long time if it isnrsquot disturbed Once crystals begin to form continues until equilibrium is recreated at lower temp
41
Example
Sodium thiosulfate Na2S2O3 Solute added to hot water until solution saturated Hot solution filtered Drop small crystal in ldquoseedingrdquo crystallization Continues until equilibrium created
42
Solubility Values
Solubility - amount of substance required to form a saturated solution with specific amount of solvent at specified temp Ex Solubility of sugar = 204 g per 100 g water at 20 Must specify temp bc solubility changes with temp 43
For gases pressure must also be specified Rate at which substance dissolves is unrelated to solubility Max amount of solute that dissolves and reaches equilibrium is always same under same conditions
44
Solute-Solvent Interactions
ldquolike dissolves likerdquo is generally useful rule for predicting whether one substance will dissolve in another Depends on type of bonding polaritynonpolarity and intermolecular forces between solute and solvent
45
Dissolving Ionic Compounds in Aqueous Solution
Polarity of water molecules plays important role in formation of solution of ionic compounds Charged ends of water molecules attract ions and surround them to keep separated from other ions
46
Hydration - solution process with water as solvent Ions said to be hydrated As hydrated ions diffuse into solution other ions exposed and drawn away from crystal surface Entire crystal gradually dissolves
47
When crystallized from aqueous solutions some ionic compounds form crystals that include water molecules The crystalline compounds known as hydrates have specific ratios of water molecules Represented by formulas like CuSO45H2O
48
When crystalline hydrate dissolves in water water of hydration returns to solvent Behavior of solute in hydrated form no different than anhydrous form
49
Nonpolar Solvents
Nonpolar solvents do not attract ions of ionic compound strongly enough to overcome forces holding crystal together
50
Liquid Solutes and Solvents
When you shake bottle of salad dressing oil droplets disperse in water Stop shaking ndash strong attraction between water molecules squeeze out oil and form separate layers Liquid solutes and solvents that are not soluble in each other are immiscible
51
Nonpolar substances (fat oil grease) are generally quite soluble in nonpolar liquids (carbon tetrachloride toluene gasoline)
Only attractions between nonpolar molecules are weak London forces
52
London Dispersion Forces
bc e- are constantly moving at any time there may be uneven distribution of them around a nonpolar atom Temporarily creates positive end and negative end
53
54
London dispersion forces - intermolecular attractions resulting from constant motion of e- and creation of instantaneous dipoles
55
Liquids that dissolve freely in one another in any ratio are said to be completely miscible Benzene and carbon tetrachloride (both nonpolar) are completely miscible
Nonpolar molecules of these two apply no strong forces of attraction or repulsion and molecules mix freely
56
Ethanol and water are also completely miscible -OH group on ethanol is slightly polar
Can form H-bonds with water as well as other ethanol molecules Intermolecular forces in mixture are very similar to those in pure liquids so they are mutually soluble in any amount
57
Gasoline contains mainly nonpolar hydrocarbons
Excellent solvent for fats oils greases
Major intermolecular forces are weak London forces
58
Ethanol less polar than water more than carbon tetrachloride
Better solvent for less-polar substances bc of nonpolar region
59
Effects of Pressure on Solubility
Little effect on solids or liquids Increase pressure - increase solubilities of gases in liquids
60
When gas in contact with surface of liquid gas molecules can enter liquid As amount of dissolve gas increases some molecules start to escape and reenter gas phase
Eq eventually established between rates of enteringleaving gas phase
61
As long as eq undisturbed solubility of gas in liquid unchanged at given pressure
Gas + solvent solution
62
Increasing pressure of solute gas above solution puts stress on eq Molecules collide with surface more often Increase in pressure partially offset by increase in rate of gas molecules entering solution In turn increase in amount of dissolved gas causes increase in rate molecules escape liquid and becomes gas
63
Eventually eq restored at higher gas solubility
Expected from Le Chatelierrsquos principle
Increase in gas pressure causes eq to shift so fewer molecules are in gas phase
64
Henryrsquos Law
Henryrsquos law - solubility of a gas in liquid is directly proportional to the partial pressure of that gas on the surface of the liquid
Applies to gas-liquid solutions at constant temp
65
When mixture of ideal gases is limited in constant volume at constant temp each gas applies same pressure it would apply if it occupied the space alone
Assuming gases do not react each gas dissolves to the extent it would if no other gases were there
66
In carbonated drinks (Coke Pepsi etc) solubility of CO2 increased by increasing pressure
At bottling factory CO2 gas forced into solution of flavored water at pressure of 5-10 atm Gas-in-liquid solution then sealed in bottlescans
67
When cap removed pressure reduced to 1 atm and some CO2 escapes as gas bubbles Effervescence - rapid escape of a gas from a liquid in which it is dissolved
68
Effects of Temperature on Solubility
Letrsquos consider gas solubility Increasing temperature usually decreases gas solubility As temp increases average kinetic energy of molecules in solution increases Greater number of solute molecules escape from attraction of solvent and return to gas phase 69
At higher temps eq is reached with fewer gas molecules in solution Gases generally less soluble
70
Effect of temp on solubility of solids in liquids more difficult to predict
Often increasing temp increases solubility
However the same temp increase can result in large increase in solubility in one case and only slight increase in the next
71
72
enthalpies of Solution
Formation of solution comes with energy change Dissolve KI in water container feels cold Dissolve LiCl in water feels hot Formation of solid-liquid solution can absorb or release heat
73
During formation of solution solvent and solute particles experience changes in forces attracting them to other particles Before dissolving begins solvent molecules held by intermolecular forces (solvent-solvent attraction) In solute molecules held by Ifs (solute-solute attraction)
74
Energy is required to separate solute molecules and solvent molecules from their neighbors
A solute particle that is surrounded by solvent molecules is said to be solvated
75
Step 1 solute particles become separated from solid (energy absorbed) Step 2 solvent particles move apart to allow solute particles to enter liquid (energy absorbed)
76
Step 3 solvent particles attracted to and solvate solute particles (energy released)
77
Heat of solution - net amount of heat energy absorbed or released when a specific amount of solute dissolves in a solvent
Heat of solution negative (heat is released) if sums of energy in steps 1 and 2 is less than step 3
If 1+2 gt 3 HOS is positive (heat absorbed)78
In gaseous state molecules are so far apart there are nearly no forces between them Solute-solute interaction has little effect on heat of solution of a gas Energy released when gas dissolved in liquid bc attraction between solute gas and solvent molecules greater than energy needed to separate solvent molecules
79
Concentration of Solutions80
Concentration - measure of amount of solute in given amount of solvent or solution ldquodiluterdquo just means small amount of solute ldquoconcentratedrdquo just means relatively large amount of solute Unrelated to degree to which solution is saturated Ex Saturated solution of substance not very soluble might be dilute
81
Molarity
Molarity - number of moles of solute in one liter of solution To find molarity must know molar mass A ldquoone molarrdquo solution of NaOH is 1 mole NaOH per liter of solution (1 M)
82
1 mol NaOH = 400 g If this quantity dissolve in enough water to make EXACTLY 100 L solution solution is 1 M If 200 g NaOH dissolve in enough to make 100 L solution what is molarity 0500 M
83
1 M solution is not made by adding 1 mol solute to 1 L solvent That would be more than 1 L Instead 1 mol solute dissolved in less than 1 L Then diluted with more solvent to bring TOTAL VOLUME to 1 L
84
Practice ProblemYou have 340 L of solution that contains 900 g sodium chloride What is the molarity of that solution 0440 M NaCl You have 08 L of a 05 M HCl solution How many moles of HCl does this solution contain 04 mol HCl
85
What is the molarity of a solution composed of 585 g of potassium iodide dissolved in enough water to make a 0125 L of solution 0282 M KI
86
How many moles of H2SO4 are present in 0500 L of a 0150 M H2SO4 solution 00750 mol What volume of 300 M NaCl is needed for reaction that requires 1463 g of NaCl 0834 L
87
Molality
Molality - concentration of solution expressed in moles of solute per kilogram of solvent Solution that contains 1 mol solute (NaOH) dissolved in exactly 1 kg of solvent is ldquoone molalrdquo solution (1 m NaOH)
88
1 mol NaOH = 400 g 400 g NaOH dissolved in 1 kg water is 1 m NaOH
200 g NaOH in 1 kg water = 0500 m NaOH
89
Practice Problem
A solution was prepared by dissolving 171 g sucrose (C12H22O11) in 125 g of water Find the molal concentration of this solution 0400 m C12H22O11
90
A solution of iodine in carbon tetrachloride is used when iodine is needed for certain chemical tests How much iodine must be added to prepare a 0480 m solution of iodine in CCl4 if 1000 g of CCl4 is used 122 g I2
91
What is the molality of a solution composed of 255 g acetone (CH3)2CO dissolved in 200 g of water 22 m acetone What quantity in grams of methanol CH3OH is required to prepare a 0244 m solution in 400 g water 312 g methanol
92
How many grams of AgNO3 are needed to prepare a 0125 m solution in 250 mL of water 531 g AgNO3
What is the molality of a solution containing 182 g HCl and 250 g water 200 m
93
Ions in Aqueous Solutions and Colligative Properties
94
95
compounds in aqueous solutions
DissociationWhen compound made from ions dissolves in water ions separate Dissociation - separation of ions that occurs when an ionic compound dissolves
96
Notice number of ions made per formula unit in equations 1 formula unit of NaCl gives 2 units of ions in solution 1 formula unit of CaCl2 gives 3 units of ions
97
Assuming 100 dissociation solution that contains 1 mol NaCl contains 1 mol Na+ and 1 mol Cl- Can assume 100 dissociation of all soluble ionic compounds
98
Practice Problem
Write the equation for the dissolution of aluminum sulfate in water How many moles of aluminum ions are made by dissolving 1 mol aluminum sulfate What is the total number of moles of ions made by dissolving 1 mol of aluminum sulfate
99
1 AnalyzeGiven Amount of solute = 1 mol Al2(SO4)3 Solvent identity = water Unknown a moles of aluminum ions and sulfate ions Total number of moles of solute ions produced
100
2 Plan
The coefficients in the balanced dissociation equation will tell mole relationships You can use the equation to find out the number of moles of solute ions produced
101
3 Compute
a
b102
Write the equation for the dissolution of each of the following in water and then determine the number of moles of each ions made as well as the total number of ions made a 1 mol ammonium chloride b 1 mol sodium sulfide c 05 mol barium nitrate
103
a 1 mol ammonium chloride
1 mol NH4+ 1 mol Cl- 2 mol total ions
104
b 1 mol sodium sulfide
2 mol Na+ 1 mol S-2 3 mol total ions
105
c 05 mol barium nitrate
05 mol Ba+2 1 mol NO3- 15 mol total ions
106
Precipitation Reactions
Even though no compound is completely insoluble compounds of very low solubility can be considered insoluble for practical purposes There are general guidelines to help predict whether a compound made of certain combination of ions is soluble
107
General Solubility Guidelines
Sodium potassium and ammonium compounds are soluble in water
108
Nitrates acetates and chlorates are soluble
109
110
Most chlorides are soluble except those of silver mercury (I) and lead Lead (II) chloride is soluble in hot water
Most sulfates are soluble except those of barium strontium lead calcium and mercury
111
Most carbonates phosphates and silicates are insoluble except those of sodium potassium and ammonium
112
Most sulfides are insoluble except those of calcium strontium sodium potassium and ammonium
113
Is calcium phosphate Ca3(PO4)2 soluble or insoluble
Not one of the exceptions so it is insoluble
Dissociation equations are not written for insoluble compounds
114
Guidelines also useful in predicting what will happen if 2 solutions of 2 different soluble compounds are mixed
If mixing results in combination of ions that forms insoluble compound a double-replacement and precipitation reaction will happen
115
Will precipitate form when solutions of ammonium sulfide and cadmium nitrate are combined You can tell calcium nitrate is soluble from the guidelines (Most nitrates acetates and chlorates are soluble) You can also tell ammonium sulfide is soluble (Most sulfides are insoluble except those of calcium strontium sodium potassium and ammonium)
116
117
To decide whether precipitate can form must know solubilities of two possible products
Ammonium nitrate ndash soluble (Most nitrates acetates and chlorates are soluble) Cadmium sulfide ndash insoluble (Most sulfides are insoluble except those of calcium strontium sodium potassium and ammonium)
118
bc one product is insoluble double-replacement and precipitation reaction will happen (NH4)2S (aq) + Cd(NO3)2 (aq) rarr NH4NO3 (aq) + CdS (s)
119
Net Ionic Equations
Reactions of ions in aqueous solution usually represented by net ionic equations instead of formula equations Net ionic equation - includes only those compounds and ions that undergo a chemical change in a reaction in an aqueous solution
120
121
To write NIE (net ionic equation) first convert chemical equation into overall ionic equation All soluble ionic compounds shown as dissociated ions in solution Precipitates shown as solids
122
Notice ammonium and nitrate ions appear on both sides They havenrsquot gone through any chemical change Spectator ions - ions that do not take part in a chemical reaction and are found in solution both before and after the reaction
123
To change ionic equation to NIE eliminate spectator ions Cd+2(aq) + S-2(aq) rarr CdS(s)
This NIE applies to any reaction in which precipitate of CdS forms
124
Practice Problem
Identify the precipitate that forms when aqueous solutions of zinc nitrate and ammonium sulfide are combined Write the equation for the possible double-replacement reaction Then write the formula equation overall ionic equation and the net ionic equation
125
1 AnalyzeGiven Identity of reactants zinc nitrate and ammonium sulfide Reaction medium aqueous solution Unknown a equation for the possible double-replacement reaction b identity of the precipitate c formula equation d overall ionic equation e net ionic equation
126
2 PlanWrite the possible double-replacement reaction between Zn(NO3)2 and (NH4)2S Use the rules to determine if any of the products will precipitate Write a formula equation and overall net ionic equation Then cancel spectator ions to make net ionic equation
127
3 Computea possible double-replacement reaction
b rules show that zinc sulfide is not soluble so will precipitate (ammonium nitrate is soluble)
c formula equation128
d overall ionic equation
e net ionic equation129
Will a precipitate form if solutions of potassium sulfate and barium nitrate are combined If so write the net ionic equation for the reaction
Yes Ba+2(aq) + SO4-2(aq) rarr BaSO4(s)
130
Will a precipitate form if solutions of potassium nitrate and magnesium sulfate are combined If so write the net ionic equation for the reaction
No 131
Will a precipitate form if solutions of barium chloride and sodium sulfate are combined If so identify the spectator ions and write the net ionic equation for the reaction
Yes Na+ and Cl- Ba+2(aq) + SO4-2(aq) rarr BaSO4(s)
132
Write the net ionic equation for the precipitation of nickel(II) sulfide
Ni+2(aq) + S-2(aq) rarr NiS(s)
133
IonizationSome molecular compounds can also form ions in solution Usually polar Ionization - ions formed from solute molecules by action of the solvent (creation of ions where there were none) Ionization different from dissociation
134
Dissociation ionic compounds dissolve and ions already present separate Ionization ions formed where non existed before
Ions formed are hydrated Heat released during hydration of ions gives enough energy to break covalent bonds
135
Extent to which solute ionizes depends on strength of bonds within molecules of solute and strength of attraction between solute and solvent molecules
136
HCl ndash molecular compound that ionizes in aqueous solution Contains highly polar bond Attraction between polar HCl molecule and polar water strong enough to break HCl bond
137
The Hydronium Ion
H+ ions from HCl attracts other molecules or ions so strongly that it doesnrsquot normally exist alone Ionization of HCl better described as direct transfer of proton from HCl to H2O forming H3O+
138
139
Hydration of H+ to form hydronium is highly exothermic
Energy released gives a lot of energy needed to ionize a molecular solute
Many molecular compounds that ionize in aqueous solution contain hydrogen and form hydronium ion
140
Strong and Weak Electrolytes
Substances that make ions and conduct current in solution are electrolytes HCl is one of series of compounds that has hydrogen and halogen Hydrogen halides are all molecular compounds with single polar-covalent bonds
141
All are gases Very soluble in water All are electrolytes HCl HBr HI strongly conduct current in solution HF only weakly conducts current at same concentration Strength of conduction related to ability to ionize
142
143
Strong Electrolytes
HCl HBr HI are 100 ionized in solution
Strong electrolyte - any compound whose dilute aqueous solutions conduct electricity well this is due to the presence of all or almost all of the dissolve compound in the form of ions
144
Unique characteristic of strong electrolyte is that they yield only ions
Ex Ionic compound may be highly soluble in water and dissociate into ions (NaCl)
Others may not dissolve much but amount that does dissolve exists only as ions
145
Weak ElectrolytesHF dissolves in water to give acid solution called hydrofluoric acid HF bond stronger than bonds between hydrogen and other halogens When HF dissolves some molecule ionize Reverse reaction also happens HF(aq) + H2O(l) H3O+(aq) + F-(aq)
146
HF(aq) + H2O(l) H3O+
(aq) + F-(aq)Concentration of dissolved unionized HF stays high and concentration of ions stays low
Weak electrolyte - any compound whose aqueous solutions conduct electricity poorly this is due to the presence of small amount of the dissolved compound in the form of ions
147
Different from nonelectrolyte ndash NO ions at all Description of electrolyte as strong or weak not related to concentration of solution Electrolytes differ in degree of ionization not amount of solute dissolved
148
149
Colligative properties of solutions
Presence of solute affects properties of solutions Some properties not dependent on nature of dissolved substance but on how may dissolve particles are present
Colligative properties - properties that depend on the concentration of solute particles but not on their identity
150
Vapor-Pressure Lowering
Boiling and freezing point of solution different from pure solvent Nonvolatile solute raises boiling point and lowers freezing point Nonvolatile substance - one that has little tendency to become gas under existing conditions
151
152
To understand why nonvolatile solute changes boiling and freezing point consider equilibrium vapor pressure
Vapor pressure pressure caused by molecules that have escape liquid phase to gas phase
Can be thought of a measure of tendency of molecules to escape from a liquid
153
Addition of sucrose (nonvolatile solute) lowers concentration of water molecules at surface of liquid This lowers tendency of water molecules to leave solution and enter gas phase Vapor pressure of solution is lower than vapor pressure of pure water
154
Aqueous13 solution13 of13 nonvolatile13 solutePure13 water
Nonelectrolyte solutions of same molality have same concentration of particles
Dilute solutions of same solvent and equal molality of any nonelectrolyte solute lower vapor pressure equally
155
Example
1 m aqueous solution of nonelectrolyte glucose lowers vapor pressure of water 55 x 10-4 atm at 25 1 m aqueous solution of sucrose also lowers water vapor pressure to 55 x 10-4 atm at 25
156
157
bc vapor-pressure lowering depends on concentration of nonelectrolyte solute and doesnrsquot depend on the type of solute it is a colligative proprerty
bc vapor pressure has been lowered solution remains liquid over larger temperature range
Lowers freezing point and raises boiling point
158
Can assume changes in boiling and freezing point also depend on concentration of solute
They are colligative properties
159
Freezing-Point Depression
Freezing point of 1-molal solution of any nonelectrolyte solute in water is found (by experiment) to be 186 lower than freezing point of water When 1 mol of nonelectrolyte solute dissolved in 1 kg water freezing point is -186 not 000 160
When 2 mol nonelectrolyte solute dissolved in 1 kg water freezing point is -372
For any concentration of nonelectrolyte solute in water decrease in freezing point can be determined using value of -186m
161
This value called the molal freezing-point contstant (Kf) is the freezing-point depression of the solvent in a 1-molal solution of a nonvolatile nonelectrolyte solute
162
Each solvent has own characteristic molal freezing-point constant Freezing-point depression (Δtf) the difference between the freezing points of the pure solvent and a solute of nonelectrolyte in that solvent and it is directly proportional to the molal concentration of the solution Δtf = Kfm
163
Δtf = Kfm
Kf expressed as m
m expressed in mol solutekg solvent (molality)
164
Sample Problem
What is the freezing point depression of water in a solution of 171 g of sucrose C12H22O11 and 200 g of water What is the actual freezing point of the solution
165
1 AnalyzeGiven Solute mass and chemical formula 171 g C12H22O11 Solvent mass and identity 2000 g water Unknown a freezing-point depression b freezing point of the solution 166
2 Plan
167
168
Calculate
Unknown a freezing-point depression b freezing point of the solution
169
Practice Problem
A water solution containing an unknown quantity of a nonelectrolyte solute is found to have a freezing point of ndash 023degCWhat is the molal concentration of the solution
012 m170
A solution consists of 103 g of the nonelectrolyte glucose C6H12O6 dissolved in 250 g of water What is the freezing-point depression of the solution
minus0426degC171
In a laboratory experiment the freezing point of an aqueous solution of glucose is found to be minus0325degCWhat is the molal concentration of this solution
0175 m172
If 0500 mol of a nonelectrolyte solute are dissolved in 5000 g of ether what is the freezing point of the solution
minus1181degC173
The freezing point of an aqueous solution that contains a nonelectrolyte is minus90degC aWhat is the freezing-point depression of the solution bWhat is the molal concentration of the solution a minus90degC b 48 m
174
Boiling-Point Elevation
Boiling point is temp at which vapor pressure is equal to atmospheric pressure Change in vapor pressure will cause corresponding change in boiling point
175
176
Vapor pressure of nonvolatile solution is lower than vapor pressure of pure solvent More heat will be required to raise vapor pressure of solution Boiling point of solution is higher than boiling point of pure solvent
Molal boiling-point constant (Kb) - boiling-point elevation of the solvent in a 1-molal solution of a nonvolatile nonelectrolyte solute b-p elevation of any nonelectrolyte in water found by experiment to be 051 So molal b-p constant for water is 051m
177
For different solvents b-p elevations of 1-molal solutions have different values (Table 14-2)
Like freezing-point constants values most accurate for dilute solutions
178
Boiling-point elevation Δtb difference between the boiling points of the pure solvent and a nonelectrolyte solution of that solvent and it is directly proportional to the molal concentration of the solution B-p elevation can be calculated using Δtb = Kbm Δtb (boiling point elevation) expressed in m and m expressed in mol solutekg solvent
Sample Problem
What is the boiling-point elevation of a solution made from 200 g of a nonelectrolyte solute and 4000 g of water The molar mass of the solute is 620 gmol
180
1 AnalyzeGiven solute mass = 200 g solute molar mass = 620 gmol solvent mass and identity = 4000 g of water
Unknown boiling-point elevation
181
2 Plan
182
183
3 Compute
Practice Problem
A solution contains 500 g of sucrose C12H22O11 a nonelectrolyte dissolved in 5000 g of water What is the boiling-point elevation
015degC184
A solution contains 4500 g of sucrose C12H22O11 a nonelectrolyte dissolved in 250 g of waterWhat is the boiling point of the solution
1027degC185
If the boiling point elevation of an aqueous solution containing a nonvolatile electrolyte is 102degC what is the molality of the solution
20 m186
The boiling point of an aqueous solution containing a nonvolatile electrolyte is 10075degC aWhat is the boiling-point elevation bWhat is the molality of the solution a 075degC b 15 m
187
Electrolytes and Colligative PropertiesEarly investigators confused by experiments where certain substances depressed freezing point or elevated boiling point more than expected
188
189
Ex 01 m solution of NaCl lowers freezing point of solvent almost twice as much as 01 m solution of sucrose 01 m CaCl2 lowers fp almost 3 times as much as 01 m solution of sucrose
To understand why this happens contrast behavior or sucrose with NaCl in aqueous solution Each sucrose dissolves to make only 1 particle in solution So 1 mol sucrose dissolves to 1 mol particles 1 mol NaCl dissolves to make 2 moles of particles (1 mol Na+ and 1 mol Cl-)
190
191
Calculated Values for Electrolyte SolutionsColligative properties depend on total concentration of solute particles regardless of their identity Electrolytes cause changes in colligative properties proportional to total molality in terms of all dissolves particles instead of formula units
192
What about barium nitrate Ba(NO3)2 Each mole of barium nitrate yields 3 mol of ions in solution Ba(NO3)2(s) rarr Ba2+(aq) + 2NO3-(aq) You would expect solution of given molality to lower fp of solvent 3 times as much as nonelectrolytic solution of same molality
193
Sample Problem
What is the expected change in the freezing point of water in a solution of 625 g of barium nitrate Ba(NO3)2 in 100 kg of water
194
1 AnalyzeGiven solute mass and formula = 625 g Ba(NO3)2 solvent mass and identity = 100 kg water Δtf = Kfm
Unknown expected freezing-point depression
195
2 PlanThe molality can be calculated by converting the solute mass to moles and then dividing by the number of kilograms of solvent That molality is in terms of formula units of Ba(NO3)2 and must be converted to molality in terms of dissociated ions in solution It must be multiplied by the number of moles of ions produced per mole of formula unit This adjusted molality can then be used to calculate the freezing-point depression
196
197
198
3 Compute
Practice Problem
What is the expected freezing-point depression for a solution that contains 20 mol of magnesium sulfate dissolved in 10 kg of water
minus74degC199
What is the expected boiling-point elevation of water for a solution that contains 150 g of sodium chloride dissolved in 10 kg of water 27degC The freezing point of an aqueous sodium chloride solution is minus200degC What is the molality of the solution 54 m NaCl
200
Actual Values for Electrolyte SolutionsRemember values calculated are only expected values Actual values of colligative properties for all strong electrolytes are almost what would be expected Difference between expected and actual values caused by attractive forces that exist between dissociated ions in aqueous solution
201
Attraction between hydrated ions in solution is small compared with those in crystalline solid However forces of attraction donrsquot interfere with movements of aqueous ions More concentrated solution means ions closer together and attraction is greater Also depends on charge of ions ndash higher charge greater attraction
202
SuspensionsSuspension - if particles in solvent are so large they settle out unless mixture constantly stirred Think of sand and water Particles over 1000 nm in diameter
12
Particles that are medium in size between those in solutions and suspensions from mixtures are known as colloids (colloidal dispersions) Particles between 1-1000 nm diameter
13
Colloids
In muddy water large soil particles settle Water still cloudy bc colloidal particles stay dispersed in water Filter Colloidal particles pass through and stays cloudy
14
Particles in colloid small enough to be suspended in solvent by constant movement of surrounding molecules Colloidal particles make up dispersed phase Water is dispersing medium
15
Emulsion and foam are colloids
Mayonnaise is emulsion of oil droplets in water Egg yolk acts as emulsifying agent (keeps oil droplets dispersed)
16
17
18
Class13 of13 Colloid Phases Example
Sol Solid13 dispersed13 in13 liquid Paints13 mud
Gel Solid13 network13 extending13 through13 liquid
Gelatin
Liquid13 emulsion Liquid13 dispersed13 in13 liquid Milk13 mayonnaise
Foam Gas13 dispersed13 in13 liquid Shaving13 cream13 whipped13 cream
Solid13 aerosol Solid13 dispersed13 in13 gas Smoke13 auto13 exhaust
Liquid13 aerosol Liquid13 dispersed13 in13 gas Fog13 mist13 clouds13 aerosol13 spray
Solid13 emulsion Liquid13 dispersed13 in13 solid Cheese13 butter
Tyndall EffectMany colloids appear homogeneous bc individual particles cannot be seen Particles large enough to scatter light In fog can see headlight beam Tyndall effect - occurs when light is scattered by colloidal particles dispersed in transparent medium
19
20
21
Solutions Colloids Suspensions
Homogeneous Heterogeneous Heterogeneous
Particle13 size13 001-shy‐113 nm13 can13 be13 atoms13 ions13 molecules
Particle13 size13 1-shy‐100013 nm13 dispersed13 can13 be13 combined13 or13 large13 molecules
Particle13 size13 over13 100013 nm13 suspended13 can13 be13 large13 particles13 or13 combined13 particles
Do13 not13 separate13 on13 standing Do13 not13 separate13 on13 standing Particles13 settle13 out
Cannot13 be13 separated13 by13 filtration Cannot13 be13 separated13 by13 filtration Can13 be13 separated13 by13 filtration
Do13 not13 scatter13 light Scatter13 light13 (Tyndall13 effect) May13 scatter13 light13 but13 are13 not13 transparent
Electrolytes vs Nonelectrolytes
Substances that dissolve in water are classified according to whether they produce molecules or ions in solution
22
23
When ionic compound dissolves cations and anions separate and are surrounded by water molecules Solute ions free to move making possible to carry electric current
Electrolyte - substance that dissolves in water to give a solution that conducts electric current NaCl ndash electrolyte Usually highly polar compounds become electrolytes
24
Solution containing neutral solute molecules does not conduct current bc it doesnrsquot have mobile charged particles
Nonelectrolyte - substance that dissolves in water to give solution that does not conduct electric current Ex Sugar
25
26
27the solution
process
Factors Affecting Rate of DissolutionEver tried to dissolve sugar in iced tea or coffee You notice temperature has something to do with how quickly solute dissolves What other factors affect how quickly solutes dissolve
28
1 Increasing Surface Area of Solute
bc dissolution process occurs at surface of solute Can increase dissolution by increasing surface area
29
2 Agitating (Stirring) SolutionClose to surface of solute concentration of dissolved solute is high Stirringshaking helps disperse solute particles
30
Brings fresh solvent into contact with solute surface Contact between solvent and solute surface increased
3 Heating a Solvent
Solutes dissolve faster in warmer solvent As temp increases solvent molecules move faster Average kinetic energy increases Collisions between solvent and solute more frequent Helps separate solute molecules and spread them out
32
Solubility
If you add sugar to tea eventually no more sugar dissolves
For every combination of solvent with solute at given temp there is limit to amount of solute that can be dissolved
33
When sugar first dropped into water sugar molecules leave solid surface and move about at random in solvent Some dissolved molecules may collide with crystal and stay there As more solid dissolves and concentration of dissolved molecules increases collisions happen more often
34
Eventually molecules are returning to crystal at same rate they are going into solution Dynamic equilibrium established between dissolution and crystallization Solution equilibrium - physical state in which the opposing processes of dissolution and crystallization of a solute occur at equal rates
35
Saturated vs Unsaturated Solutions
Saturated solution - solution that contains the maximum amount of dissolved solute How do you know if solution is saturated If more NaCl added it falls to bottom and doesnrsquot dissolve Equilibrium already established
36
If more water added to saturated solution more NaCl will dissolve At 20 359 g NaCl is max that will dissolve in 100 g water Unsaturated solution - solution that contains less solute than a saturated solution under the existing conditions
37
Supersaturated Solutions
When saturated solution where solubility increases as temp increases is cooled excess solute usually comes out of solution Sometimes if solution is left to cool the excess solute doesnrsquot separate
38
39
Supersaturated solution - solution that contains more dissolved solute than a saturated solution contains under same conditions
40
Supersaturated solution can remain unchanged over long time if it isnrsquot disturbed Once crystals begin to form continues until equilibrium is recreated at lower temp
41
Example
Sodium thiosulfate Na2S2O3 Solute added to hot water until solution saturated Hot solution filtered Drop small crystal in ldquoseedingrdquo crystallization Continues until equilibrium created
42
Solubility Values
Solubility - amount of substance required to form a saturated solution with specific amount of solvent at specified temp Ex Solubility of sugar = 204 g per 100 g water at 20 Must specify temp bc solubility changes with temp 43
For gases pressure must also be specified Rate at which substance dissolves is unrelated to solubility Max amount of solute that dissolves and reaches equilibrium is always same under same conditions
44
Solute-Solvent Interactions
ldquolike dissolves likerdquo is generally useful rule for predicting whether one substance will dissolve in another Depends on type of bonding polaritynonpolarity and intermolecular forces between solute and solvent
45
Dissolving Ionic Compounds in Aqueous Solution
Polarity of water molecules plays important role in formation of solution of ionic compounds Charged ends of water molecules attract ions and surround them to keep separated from other ions
46
Hydration - solution process with water as solvent Ions said to be hydrated As hydrated ions diffuse into solution other ions exposed and drawn away from crystal surface Entire crystal gradually dissolves
47
When crystallized from aqueous solutions some ionic compounds form crystals that include water molecules The crystalline compounds known as hydrates have specific ratios of water molecules Represented by formulas like CuSO45H2O
48
When crystalline hydrate dissolves in water water of hydration returns to solvent Behavior of solute in hydrated form no different than anhydrous form
49
Nonpolar Solvents
Nonpolar solvents do not attract ions of ionic compound strongly enough to overcome forces holding crystal together
50
Liquid Solutes and Solvents
When you shake bottle of salad dressing oil droplets disperse in water Stop shaking ndash strong attraction between water molecules squeeze out oil and form separate layers Liquid solutes and solvents that are not soluble in each other are immiscible
51
Nonpolar substances (fat oil grease) are generally quite soluble in nonpolar liquids (carbon tetrachloride toluene gasoline)
Only attractions between nonpolar molecules are weak London forces
52
London Dispersion Forces
bc e- are constantly moving at any time there may be uneven distribution of them around a nonpolar atom Temporarily creates positive end and negative end
53
54
London dispersion forces - intermolecular attractions resulting from constant motion of e- and creation of instantaneous dipoles
55
Liquids that dissolve freely in one another in any ratio are said to be completely miscible Benzene and carbon tetrachloride (both nonpolar) are completely miscible
Nonpolar molecules of these two apply no strong forces of attraction or repulsion and molecules mix freely
56
Ethanol and water are also completely miscible -OH group on ethanol is slightly polar
Can form H-bonds with water as well as other ethanol molecules Intermolecular forces in mixture are very similar to those in pure liquids so they are mutually soluble in any amount
57
Gasoline contains mainly nonpolar hydrocarbons
Excellent solvent for fats oils greases
Major intermolecular forces are weak London forces
58
Ethanol less polar than water more than carbon tetrachloride
Better solvent for less-polar substances bc of nonpolar region
59
Effects of Pressure on Solubility
Little effect on solids or liquids Increase pressure - increase solubilities of gases in liquids
60
When gas in contact with surface of liquid gas molecules can enter liquid As amount of dissolve gas increases some molecules start to escape and reenter gas phase
Eq eventually established between rates of enteringleaving gas phase
61
As long as eq undisturbed solubility of gas in liquid unchanged at given pressure
Gas + solvent solution
62
Increasing pressure of solute gas above solution puts stress on eq Molecules collide with surface more often Increase in pressure partially offset by increase in rate of gas molecules entering solution In turn increase in amount of dissolved gas causes increase in rate molecules escape liquid and becomes gas
63
Eventually eq restored at higher gas solubility
Expected from Le Chatelierrsquos principle
Increase in gas pressure causes eq to shift so fewer molecules are in gas phase
64
Henryrsquos Law
Henryrsquos law - solubility of a gas in liquid is directly proportional to the partial pressure of that gas on the surface of the liquid
Applies to gas-liquid solutions at constant temp
65
When mixture of ideal gases is limited in constant volume at constant temp each gas applies same pressure it would apply if it occupied the space alone
Assuming gases do not react each gas dissolves to the extent it would if no other gases were there
66
In carbonated drinks (Coke Pepsi etc) solubility of CO2 increased by increasing pressure
At bottling factory CO2 gas forced into solution of flavored water at pressure of 5-10 atm Gas-in-liquid solution then sealed in bottlescans
67
When cap removed pressure reduced to 1 atm and some CO2 escapes as gas bubbles Effervescence - rapid escape of a gas from a liquid in which it is dissolved
68
Effects of Temperature on Solubility
Letrsquos consider gas solubility Increasing temperature usually decreases gas solubility As temp increases average kinetic energy of molecules in solution increases Greater number of solute molecules escape from attraction of solvent and return to gas phase 69
At higher temps eq is reached with fewer gas molecules in solution Gases generally less soluble
70
Effect of temp on solubility of solids in liquids more difficult to predict
Often increasing temp increases solubility
However the same temp increase can result in large increase in solubility in one case and only slight increase in the next
71
72
enthalpies of Solution
Formation of solution comes with energy change Dissolve KI in water container feels cold Dissolve LiCl in water feels hot Formation of solid-liquid solution can absorb or release heat
73
During formation of solution solvent and solute particles experience changes in forces attracting them to other particles Before dissolving begins solvent molecules held by intermolecular forces (solvent-solvent attraction) In solute molecules held by Ifs (solute-solute attraction)
74
Energy is required to separate solute molecules and solvent molecules from their neighbors
A solute particle that is surrounded by solvent molecules is said to be solvated
75
Step 1 solute particles become separated from solid (energy absorbed) Step 2 solvent particles move apart to allow solute particles to enter liquid (energy absorbed)
76
Step 3 solvent particles attracted to and solvate solute particles (energy released)
77
Heat of solution - net amount of heat energy absorbed or released when a specific amount of solute dissolves in a solvent
Heat of solution negative (heat is released) if sums of energy in steps 1 and 2 is less than step 3
If 1+2 gt 3 HOS is positive (heat absorbed)78
In gaseous state molecules are so far apart there are nearly no forces between them Solute-solute interaction has little effect on heat of solution of a gas Energy released when gas dissolved in liquid bc attraction between solute gas and solvent molecules greater than energy needed to separate solvent molecules
79
Concentration of Solutions80
Concentration - measure of amount of solute in given amount of solvent or solution ldquodiluterdquo just means small amount of solute ldquoconcentratedrdquo just means relatively large amount of solute Unrelated to degree to which solution is saturated Ex Saturated solution of substance not very soluble might be dilute
81
Molarity
Molarity - number of moles of solute in one liter of solution To find molarity must know molar mass A ldquoone molarrdquo solution of NaOH is 1 mole NaOH per liter of solution (1 M)
82
1 mol NaOH = 400 g If this quantity dissolve in enough water to make EXACTLY 100 L solution solution is 1 M If 200 g NaOH dissolve in enough to make 100 L solution what is molarity 0500 M
83
1 M solution is not made by adding 1 mol solute to 1 L solvent That would be more than 1 L Instead 1 mol solute dissolved in less than 1 L Then diluted with more solvent to bring TOTAL VOLUME to 1 L
84
Practice ProblemYou have 340 L of solution that contains 900 g sodium chloride What is the molarity of that solution 0440 M NaCl You have 08 L of a 05 M HCl solution How many moles of HCl does this solution contain 04 mol HCl
85
What is the molarity of a solution composed of 585 g of potassium iodide dissolved in enough water to make a 0125 L of solution 0282 M KI
86
How many moles of H2SO4 are present in 0500 L of a 0150 M H2SO4 solution 00750 mol What volume of 300 M NaCl is needed for reaction that requires 1463 g of NaCl 0834 L
87
Molality
Molality - concentration of solution expressed in moles of solute per kilogram of solvent Solution that contains 1 mol solute (NaOH) dissolved in exactly 1 kg of solvent is ldquoone molalrdquo solution (1 m NaOH)
88
1 mol NaOH = 400 g 400 g NaOH dissolved in 1 kg water is 1 m NaOH
200 g NaOH in 1 kg water = 0500 m NaOH
89
Practice Problem
A solution was prepared by dissolving 171 g sucrose (C12H22O11) in 125 g of water Find the molal concentration of this solution 0400 m C12H22O11
90
A solution of iodine in carbon tetrachloride is used when iodine is needed for certain chemical tests How much iodine must be added to prepare a 0480 m solution of iodine in CCl4 if 1000 g of CCl4 is used 122 g I2
91
What is the molality of a solution composed of 255 g acetone (CH3)2CO dissolved in 200 g of water 22 m acetone What quantity in grams of methanol CH3OH is required to prepare a 0244 m solution in 400 g water 312 g methanol
92
How many grams of AgNO3 are needed to prepare a 0125 m solution in 250 mL of water 531 g AgNO3
What is the molality of a solution containing 182 g HCl and 250 g water 200 m
93
Ions in Aqueous Solutions and Colligative Properties
94
95
compounds in aqueous solutions
DissociationWhen compound made from ions dissolves in water ions separate Dissociation - separation of ions that occurs when an ionic compound dissolves
96
Notice number of ions made per formula unit in equations 1 formula unit of NaCl gives 2 units of ions in solution 1 formula unit of CaCl2 gives 3 units of ions
97
Assuming 100 dissociation solution that contains 1 mol NaCl contains 1 mol Na+ and 1 mol Cl- Can assume 100 dissociation of all soluble ionic compounds
98
Practice Problem
Write the equation for the dissolution of aluminum sulfate in water How many moles of aluminum ions are made by dissolving 1 mol aluminum sulfate What is the total number of moles of ions made by dissolving 1 mol of aluminum sulfate
99
1 AnalyzeGiven Amount of solute = 1 mol Al2(SO4)3 Solvent identity = water Unknown a moles of aluminum ions and sulfate ions Total number of moles of solute ions produced
100
2 Plan
The coefficients in the balanced dissociation equation will tell mole relationships You can use the equation to find out the number of moles of solute ions produced
101
3 Compute
a
b102
Write the equation for the dissolution of each of the following in water and then determine the number of moles of each ions made as well as the total number of ions made a 1 mol ammonium chloride b 1 mol sodium sulfide c 05 mol barium nitrate
103
a 1 mol ammonium chloride
1 mol NH4+ 1 mol Cl- 2 mol total ions
104
b 1 mol sodium sulfide
2 mol Na+ 1 mol S-2 3 mol total ions
105
c 05 mol barium nitrate
05 mol Ba+2 1 mol NO3- 15 mol total ions
106
Precipitation Reactions
Even though no compound is completely insoluble compounds of very low solubility can be considered insoluble for practical purposes There are general guidelines to help predict whether a compound made of certain combination of ions is soluble
107
General Solubility Guidelines
Sodium potassium and ammonium compounds are soluble in water
108
Nitrates acetates and chlorates are soluble
109
110
Most chlorides are soluble except those of silver mercury (I) and lead Lead (II) chloride is soluble in hot water
Most sulfates are soluble except those of barium strontium lead calcium and mercury
111
Most carbonates phosphates and silicates are insoluble except those of sodium potassium and ammonium
112
Most sulfides are insoluble except those of calcium strontium sodium potassium and ammonium
113
Is calcium phosphate Ca3(PO4)2 soluble or insoluble
Not one of the exceptions so it is insoluble
Dissociation equations are not written for insoluble compounds
114
Guidelines also useful in predicting what will happen if 2 solutions of 2 different soluble compounds are mixed
If mixing results in combination of ions that forms insoluble compound a double-replacement and precipitation reaction will happen
115
Will precipitate form when solutions of ammonium sulfide and cadmium nitrate are combined You can tell calcium nitrate is soluble from the guidelines (Most nitrates acetates and chlorates are soluble) You can also tell ammonium sulfide is soluble (Most sulfides are insoluble except those of calcium strontium sodium potassium and ammonium)
116
117
To decide whether precipitate can form must know solubilities of two possible products
Ammonium nitrate ndash soluble (Most nitrates acetates and chlorates are soluble) Cadmium sulfide ndash insoluble (Most sulfides are insoluble except those of calcium strontium sodium potassium and ammonium)
118
bc one product is insoluble double-replacement and precipitation reaction will happen (NH4)2S (aq) + Cd(NO3)2 (aq) rarr NH4NO3 (aq) + CdS (s)
119
Net Ionic Equations
Reactions of ions in aqueous solution usually represented by net ionic equations instead of formula equations Net ionic equation - includes only those compounds and ions that undergo a chemical change in a reaction in an aqueous solution
120
121
To write NIE (net ionic equation) first convert chemical equation into overall ionic equation All soluble ionic compounds shown as dissociated ions in solution Precipitates shown as solids
122
Notice ammonium and nitrate ions appear on both sides They havenrsquot gone through any chemical change Spectator ions - ions that do not take part in a chemical reaction and are found in solution both before and after the reaction
123
To change ionic equation to NIE eliminate spectator ions Cd+2(aq) + S-2(aq) rarr CdS(s)
This NIE applies to any reaction in which precipitate of CdS forms
124
Practice Problem
Identify the precipitate that forms when aqueous solutions of zinc nitrate and ammonium sulfide are combined Write the equation for the possible double-replacement reaction Then write the formula equation overall ionic equation and the net ionic equation
125
1 AnalyzeGiven Identity of reactants zinc nitrate and ammonium sulfide Reaction medium aqueous solution Unknown a equation for the possible double-replacement reaction b identity of the precipitate c formula equation d overall ionic equation e net ionic equation
126
2 PlanWrite the possible double-replacement reaction between Zn(NO3)2 and (NH4)2S Use the rules to determine if any of the products will precipitate Write a formula equation and overall net ionic equation Then cancel spectator ions to make net ionic equation
127
3 Computea possible double-replacement reaction
b rules show that zinc sulfide is not soluble so will precipitate (ammonium nitrate is soluble)
c formula equation128
d overall ionic equation
e net ionic equation129
Will a precipitate form if solutions of potassium sulfate and barium nitrate are combined If so write the net ionic equation for the reaction
Yes Ba+2(aq) + SO4-2(aq) rarr BaSO4(s)
130
Will a precipitate form if solutions of potassium nitrate and magnesium sulfate are combined If so write the net ionic equation for the reaction
No 131
Will a precipitate form if solutions of barium chloride and sodium sulfate are combined If so identify the spectator ions and write the net ionic equation for the reaction
Yes Na+ and Cl- Ba+2(aq) + SO4-2(aq) rarr BaSO4(s)
132
Write the net ionic equation for the precipitation of nickel(II) sulfide
Ni+2(aq) + S-2(aq) rarr NiS(s)
133
IonizationSome molecular compounds can also form ions in solution Usually polar Ionization - ions formed from solute molecules by action of the solvent (creation of ions where there were none) Ionization different from dissociation
134
Dissociation ionic compounds dissolve and ions already present separate Ionization ions formed where non existed before
Ions formed are hydrated Heat released during hydration of ions gives enough energy to break covalent bonds
135
Extent to which solute ionizes depends on strength of bonds within molecules of solute and strength of attraction between solute and solvent molecules
136
HCl ndash molecular compound that ionizes in aqueous solution Contains highly polar bond Attraction between polar HCl molecule and polar water strong enough to break HCl bond
137
The Hydronium Ion
H+ ions from HCl attracts other molecules or ions so strongly that it doesnrsquot normally exist alone Ionization of HCl better described as direct transfer of proton from HCl to H2O forming H3O+
138
139
Hydration of H+ to form hydronium is highly exothermic
Energy released gives a lot of energy needed to ionize a molecular solute
Many molecular compounds that ionize in aqueous solution contain hydrogen and form hydronium ion
140
Strong and Weak Electrolytes
Substances that make ions and conduct current in solution are electrolytes HCl is one of series of compounds that has hydrogen and halogen Hydrogen halides are all molecular compounds with single polar-covalent bonds
141
All are gases Very soluble in water All are electrolytes HCl HBr HI strongly conduct current in solution HF only weakly conducts current at same concentration Strength of conduction related to ability to ionize
142
143
Strong Electrolytes
HCl HBr HI are 100 ionized in solution
Strong electrolyte - any compound whose dilute aqueous solutions conduct electricity well this is due to the presence of all or almost all of the dissolve compound in the form of ions
144
Unique characteristic of strong electrolyte is that they yield only ions
Ex Ionic compound may be highly soluble in water and dissociate into ions (NaCl)
Others may not dissolve much but amount that does dissolve exists only as ions
145
Weak ElectrolytesHF dissolves in water to give acid solution called hydrofluoric acid HF bond stronger than bonds between hydrogen and other halogens When HF dissolves some molecule ionize Reverse reaction also happens HF(aq) + H2O(l) H3O+(aq) + F-(aq)
146
HF(aq) + H2O(l) H3O+
(aq) + F-(aq)Concentration of dissolved unionized HF stays high and concentration of ions stays low
Weak electrolyte - any compound whose aqueous solutions conduct electricity poorly this is due to the presence of small amount of the dissolved compound in the form of ions
147
Different from nonelectrolyte ndash NO ions at all Description of electrolyte as strong or weak not related to concentration of solution Electrolytes differ in degree of ionization not amount of solute dissolved
148
149
Colligative properties of solutions
Presence of solute affects properties of solutions Some properties not dependent on nature of dissolved substance but on how may dissolve particles are present
Colligative properties - properties that depend on the concentration of solute particles but not on their identity
150
Vapor-Pressure Lowering
Boiling and freezing point of solution different from pure solvent Nonvolatile solute raises boiling point and lowers freezing point Nonvolatile substance - one that has little tendency to become gas under existing conditions
151
152
To understand why nonvolatile solute changes boiling and freezing point consider equilibrium vapor pressure
Vapor pressure pressure caused by molecules that have escape liquid phase to gas phase
Can be thought of a measure of tendency of molecules to escape from a liquid
153
Addition of sucrose (nonvolatile solute) lowers concentration of water molecules at surface of liquid This lowers tendency of water molecules to leave solution and enter gas phase Vapor pressure of solution is lower than vapor pressure of pure water
154
Aqueous13 solution13 of13 nonvolatile13 solutePure13 water
Nonelectrolyte solutions of same molality have same concentration of particles
Dilute solutions of same solvent and equal molality of any nonelectrolyte solute lower vapor pressure equally
155
Example
1 m aqueous solution of nonelectrolyte glucose lowers vapor pressure of water 55 x 10-4 atm at 25 1 m aqueous solution of sucrose also lowers water vapor pressure to 55 x 10-4 atm at 25
156
157
bc vapor-pressure lowering depends on concentration of nonelectrolyte solute and doesnrsquot depend on the type of solute it is a colligative proprerty
bc vapor pressure has been lowered solution remains liquid over larger temperature range
Lowers freezing point and raises boiling point
158
Can assume changes in boiling and freezing point also depend on concentration of solute
They are colligative properties
159
Freezing-Point Depression
Freezing point of 1-molal solution of any nonelectrolyte solute in water is found (by experiment) to be 186 lower than freezing point of water When 1 mol of nonelectrolyte solute dissolved in 1 kg water freezing point is -186 not 000 160
When 2 mol nonelectrolyte solute dissolved in 1 kg water freezing point is -372
For any concentration of nonelectrolyte solute in water decrease in freezing point can be determined using value of -186m
161
This value called the molal freezing-point contstant (Kf) is the freezing-point depression of the solvent in a 1-molal solution of a nonvolatile nonelectrolyte solute
162
Each solvent has own characteristic molal freezing-point constant Freezing-point depression (Δtf) the difference between the freezing points of the pure solvent and a solute of nonelectrolyte in that solvent and it is directly proportional to the molal concentration of the solution Δtf = Kfm
163
Δtf = Kfm
Kf expressed as m
m expressed in mol solutekg solvent (molality)
164
Sample Problem
What is the freezing point depression of water in a solution of 171 g of sucrose C12H22O11 and 200 g of water What is the actual freezing point of the solution
165
1 AnalyzeGiven Solute mass and chemical formula 171 g C12H22O11 Solvent mass and identity 2000 g water Unknown a freezing-point depression b freezing point of the solution 166
2 Plan
167
168
Calculate
Unknown a freezing-point depression b freezing point of the solution
169
Practice Problem
A water solution containing an unknown quantity of a nonelectrolyte solute is found to have a freezing point of ndash 023degCWhat is the molal concentration of the solution
012 m170
A solution consists of 103 g of the nonelectrolyte glucose C6H12O6 dissolved in 250 g of water What is the freezing-point depression of the solution
minus0426degC171
In a laboratory experiment the freezing point of an aqueous solution of glucose is found to be minus0325degCWhat is the molal concentration of this solution
0175 m172
If 0500 mol of a nonelectrolyte solute are dissolved in 5000 g of ether what is the freezing point of the solution
minus1181degC173
The freezing point of an aqueous solution that contains a nonelectrolyte is minus90degC aWhat is the freezing-point depression of the solution bWhat is the molal concentration of the solution a minus90degC b 48 m
174
Boiling-Point Elevation
Boiling point is temp at which vapor pressure is equal to atmospheric pressure Change in vapor pressure will cause corresponding change in boiling point
175
176
Vapor pressure of nonvolatile solution is lower than vapor pressure of pure solvent More heat will be required to raise vapor pressure of solution Boiling point of solution is higher than boiling point of pure solvent
Molal boiling-point constant (Kb) - boiling-point elevation of the solvent in a 1-molal solution of a nonvolatile nonelectrolyte solute b-p elevation of any nonelectrolyte in water found by experiment to be 051 So molal b-p constant for water is 051m
177
For different solvents b-p elevations of 1-molal solutions have different values (Table 14-2)
Like freezing-point constants values most accurate for dilute solutions
178
Boiling-point elevation Δtb difference between the boiling points of the pure solvent and a nonelectrolyte solution of that solvent and it is directly proportional to the molal concentration of the solution B-p elevation can be calculated using Δtb = Kbm Δtb (boiling point elevation) expressed in m and m expressed in mol solutekg solvent
Sample Problem
What is the boiling-point elevation of a solution made from 200 g of a nonelectrolyte solute and 4000 g of water The molar mass of the solute is 620 gmol
180
1 AnalyzeGiven solute mass = 200 g solute molar mass = 620 gmol solvent mass and identity = 4000 g of water
Unknown boiling-point elevation
181
2 Plan
182
183
3 Compute
Practice Problem
A solution contains 500 g of sucrose C12H22O11 a nonelectrolyte dissolved in 5000 g of water What is the boiling-point elevation
015degC184
A solution contains 4500 g of sucrose C12H22O11 a nonelectrolyte dissolved in 250 g of waterWhat is the boiling point of the solution
1027degC185
If the boiling point elevation of an aqueous solution containing a nonvolatile electrolyte is 102degC what is the molality of the solution
20 m186
The boiling point of an aqueous solution containing a nonvolatile electrolyte is 10075degC aWhat is the boiling-point elevation bWhat is the molality of the solution a 075degC b 15 m
187
Electrolytes and Colligative PropertiesEarly investigators confused by experiments where certain substances depressed freezing point or elevated boiling point more than expected
188
189
Ex 01 m solution of NaCl lowers freezing point of solvent almost twice as much as 01 m solution of sucrose 01 m CaCl2 lowers fp almost 3 times as much as 01 m solution of sucrose
To understand why this happens contrast behavior or sucrose with NaCl in aqueous solution Each sucrose dissolves to make only 1 particle in solution So 1 mol sucrose dissolves to 1 mol particles 1 mol NaCl dissolves to make 2 moles of particles (1 mol Na+ and 1 mol Cl-)
190
191
Calculated Values for Electrolyte SolutionsColligative properties depend on total concentration of solute particles regardless of their identity Electrolytes cause changes in colligative properties proportional to total molality in terms of all dissolves particles instead of formula units
192
What about barium nitrate Ba(NO3)2 Each mole of barium nitrate yields 3 mol of ions in solution Ba(NO3)2(s) rarr Ba2+(aq) + 2NO3-(aq) You would expect solution of given molality to lower fp of solvent 3 times as much as nonelectrolytic solution of same molality
193
Sample Problem
What is the expected change in the freezing point of water in a solution of 625 g of barium nitrate Ba(NO3)2 in 100 kg of water
194
1 AnalyzeGiven solute mass and formula = 625 g Ba(NO3)2 solvent mass and identity = 100 kg water Δtf = Kfm
Unknown expected freezing-point depression
195
2 PlanThe molality can be calculated by converting the solute mass to moles and then dividing by the number of kilograms of solvent That molality is in terms of formula units of Ba(NO3)2 and must be converted to molality in terms of dissociated ions in solution It must be multiplied by the number of moles of ions produced per mole of formula unit This adjusted molality can then be used to calculate the freezing-point depression
196
197
198
3 Compute
Practice Problem
What is the expected freezing-point depression for a solution that contains 20 mol of magnesium sulfate dissolved in 10 kg of water
minus74degC199
What is the expected boiling-point elevation of water for a solution that contains 150 g of sodium chloride dissolved in 10 kg of water 27degC The freezing point of an aqueous sodium chloride solution is minus200degC What is the molality of the solution 54 m NaCl
200
Actual Values for Electrolyte SolutionsRemember values calculated are only expected values Actual values of colligative properties for all strong electrolytes are almost what would be expected Difference between expected and actual values caused by attractive forces that exist between dissociated ions in aqueous solution
201
Attraction between hydrated ions in solution is small compared with those in crystalline solid However forces of attraction donrsquot interfere with movements of aqueous ions More concentrated solution means ions closer together and attraction is greater Also depends on charge of ions ndash higher charge greater attraction
202
Particles that are medium in size between those in solutions and suspensions from mixtures are known as colloids (colloidal dispersions) Particles between 1-1000 nm diameter
13
Colloids
In muddy water large soil particles settle Water still cloudy bc colloidal particles stay dispersed in water Filter Colloidal particles pass through and stays cloudy
14
Particles in colloid small enough to be suspended in solvent by constant movement of surrounding molecules Colloidal particles make up dispersed phase Water is dispersing medium
15
Emulsion and foam are colloids
Mayonnaise is emulsion of oil droplets in water Egg yolk acts as emulsifying agent (keeps oil droplets dispersed)
16
17
18
Class13 of13 Colloid Phases Example
Sol Solid13 dispersed13 in13 liquid Paints13 mud
Gel Solid13 network13 extending13 through13 liquid
Gelatin
Liquid13 emulsion Liquid13 dispersed13 in13 liquid Milk13 mayonnaise
Foam Gas13 dispersed13 in13 liquid Shaving13 cream13 whipped13 cream
Solid13 aerosol Solid13 dispersed13 in13 gas Smoke13 auto13 exhaust
Liquid13 aerosol Liquid13 dispersed13 in13 gas Fog13 mist13 clouds13 aerosol13 spray
Solid13 emulsion Liquid13 dispersed13 in13 solid Cheese13 butter
Tyndall EffectMany colloids appear homogeneous bc individual particles cannot be seen Particles large enough to scatter light In fog can see headlight beam Tyndall effect - occurs when light is scattered by colloidal particles dispersed in transparent medium
19
20
21
Solutions Colloids Suspensions
Homogeneous Heterogeneous Heterogeneous
Particle13 size13 001-shy‐113 nm13 can13 be13 atoms13 ions13 molecules
Particle13 size13 1-shy‐100013 nm13 dispersed13 can13 be13 combined13 or13 large13 molecules
Particle13 size13 over13 100013 nm13 suspended13 can13 be13 large13 particles13 or13 combined13 particles
Do13 not13 separate13 on13 standing Do13 not13 separate13 on13 standing Particles13 settle13 out
Cannot13 be13 separated13 by13 filtration Cannot13 be13 separated13 by13 filtration Can13 be13 separated13 by13 filtration
Do13 not13 scatter13 light Scatter13 light13 (Tyndall13 effect) May13 scatter13 light13 but13 are13 not13 transparent
Electrolytes vs Nonelectrolytes
Substances that dissolve in water are classified according to whether they produce molecules or ions in solution
22
23
When ionic compound dissolves cations and anions separate and are surrounded by water molecules Solute ions free to move making possible to carry electric current
Electrolyte - substance that dissolves in water to give a solution that conducts electric current NaCl ndash electrolyte Usually highly polar compounds become electrolytes
24
Solution containing neutral solute molecules does not conduct current bc it doesnrsquot have mobile charged particles
Nonelectrolyte - substance that dissolves in water to give solution that does not conduct electric current Ex Sugar
25
26
27the solution
process
Factors Affecting Rate of DissolutionEver tried to dissolve sugar in iced tea or coffee You notice temperature has something to do with how quickly solute dissolves What other factors affect how quickly solutes dissolve
28
1 Increasing Surface Area of Solute
bc dissolution process occurs at surface of solute Can increase dissolution by increasing surface area
29
2 Agitating (Stirring) SolutionClose to surface of solute concentration of dissolved solute is high Stirringshaking helps disperse solute particles
30
Brings fresh solvent into contact with solute surface Contact between solvent and solute surface increased
3 Heating a Solvent
Solutes dissolve faster in warmer solvent As temp increases solvent molecules move faster Average kinetic energy increases Collisions between solvent and solute more frequent Helps separate solute molecules and spread them out
32
Solubility
If you add sugar to tea eventually no more sugar dissolves
For every combination of solvent with solute at given temp there is limit to amount of solute that can be dissolved
33
When sugar first dropped into water sugar molecules leave solid surface and move about at random in solvent Some dissolved molecules may collide with crystal and stay there As more solid dissolves and concentration of dissolved molecules increases collisions happen more often
34
Eventually molecules are returning to crystal at same rate they are going into solution Dynamic equilibrium established between dissolution and crystallization Solution equilibrium - physical state in which the opposing processes of dissolution and crystallization of a solute occur at equal rates
35
Saturated vs Unsaturated Solutions
Saturated solution - solution that contains the maximum amount of dissolved solute How do you know if solution is saturated If more NaCl added it falls to bottom and doesnrsquot dissolve Equilibrium already established
36
If more water added to saturated solution more NaCl will dissolve At 20 359 g NaCl is max that will dissolve in 100 g water Unsaturated solution - solution that contains less solute than a saturated solution under the existing conditions
37
Supersaturated Solutions
When saturated solution where solubility increases as temp increases is cooled excess solute usually comes out of solution Sometimes if solution is left to cool the excess solute doesnrsquot separate
38
39
Supersaturated solution - solution that contains more dissolved solute than a saturated solution contains under same conditions
40
Supersaturated solution can remain unchanged over long time if it isnrsquot disturbed Once crystals begin to form continues until equilibrium is recreated at lower temp
41
Example
Sodium thiosulfate Na2S2O3 Solute added to hot water until solution saturated Hot solution filtered Drop small crystal in ldquoseedingrdquo crystallization Continues until equilibrium created
42
Solubility Values
Solubility - amount of substance required to form a saturated solution with specific amount of solvent at specified temp Ex Solubility of sugar = 204 g per 100 g water at 20 Must specify temp bc solubility changes with temp 43
For gases pressure must also be specified Rate at which substance dissolves is unrelated to solubility Max amount of solute that dissolves and reaches equilibrium is always same under same conditions
44
Solute-Solvent Interactions
ldquolike dissolves likerdquo is generally useful rule for predicting whether one substance will dissolve in another Depends on type of bonding polaritynonpolarity and intermolecular forces between solute and solvent
45
Dissolving Ionic Compounds in Aqueous Solution
Polarity of water molecules plays important role in formation of solution of ionic compounds Charged ends of water molecules attract ions and surround them to keep separated from other ions
46
Hydration - solution process with water as solvent Ions said to be hydrated As hydrated ions diffuse into solution other ions exposed and drawn away from crystal surface Entire crystal gradually dissolves
47
When crystallized from aqueous solutions some ionic compounds form crystals that include water molecules The crystalline compounds known as hydrates have specific ratios of water molecules Represented by formulas like CuSO45H2O
48
When crystalline hydrate dissolves in water water of hydration returns to solvent Behavior of solute in hydrated form no different than anhydrous form
49
Nonpolar Solvents
Nonpolar solvents do not attract ions of ionic compound strongly enough to overcome forces holding crystal together
50
Liquid Solutes and Solvents
When you shake bottle of salad dressing oil droplets disperse in water Stop shaking ndash strong attraction between water molecules squeeze out oil and form separate layers Liquid solutes and solvents that are not soluble in each other are immiscible
51
Nonpolar substances (fat oil grease) are generally quite soluble in nonpolar liquids (carbon tetrachloride toluene gasoline)
Only attractions between nonpolar molecules are weak London forces
52
London Dispersion Forces
bc e- are constantly moving at any time there may be uneven distribution of them around a nonpolar atom Temporarily creates positive end and negative end
53
54
London dispersion forces - intermolecular attractions resulting from constant motion of e- and creation of instantaneous dipoles
55
Liquids that dissolve freely in one another in any ratio are said to be completely miscible Benzene and carbon tetrachloride (both nonpolar) are completely miscible
Nonpolar molecules of these two apply no strong forces of attraction or repulsion and molecules mix freely
56
Ethanol and water are also completely miscible -OH group on ethanol is slightly polar
Can form H-bonds with water as well as other ethanol molecules Intermolecular forces in mixture are very similar to those in pure liquids so they are mutually soluble in any amount
57
Gasoline contains mainly nonpolar hydrocarbons
Excellent solvent for fats oils greases
Major intermolecular forces are weak London forces
58
Ethanol less polar than water more than carbon tetrachloride
Better solvent for less-polar substances bc of nonpolar region
59
Effects of Pressure on Solubility
Little effect on solids or liquids Increase pressure - increase solubilities of gases in liquids
60
When gas in contact with surface of liquid gas molecules can enter liquid As amount of dissolve gas increases some molecules start to escape and reenter gas phase
Eq eventually established between rates of enteringleaving gas phase
61
As long as eq undisturbed solubility of gas in liquid unchanged at given pressure
Gas + solvent solution
62
Increasing pressure of solute gas above solution puts stress on eq Molecules collide with surface more often Increase in pressure partially offset by increase in rate of gas molecules entering solution In turn increase in amount of dissolved gas causes increase in rate molecules escape liquid and becomes gas
63
Eventually eq restored at higher gas solubility
Expected from Le Chatelierrsquos principle
Increase in gas pressure causes eq to shift so fewer molecules are in gas phase
64
Henryrsquos Law
Henryrsquos law - solubility of a gas in liquid is directly proportional to the partial pressure of that gas on the surface of the liquid
Applies to gas-liquid solutions at constant temp
65
When mixture of ideal gases is limited in constant volume at constant temp each gas applies same pressure it would apply if it occupied the space alone
Assuming gases do not react each gas dissolves to the extent it would if no other gases were there
66
In carbonated drinks (Coke Pepsi etc) solubility of CO2 increased by increasing pressure
At bottling factory CO2 gas forced into solution of flavored water at pressure of 5-10 atm Gas-in-liquid solution then sealed in bottlescans
67
When cap removed pressure reduced to 1 atm and some CO2 escapes as gas bubbles Effervescence - rapid escape of a gas from a liquid in which it is dissolved
68
Effects of Temperature on Solubility
Letrsquos consider gas solubility Increasing temperature usually decreases gas solubility As temp increases average kinetic energy of molecules in solution increases Greater number of solute molecules escape from attraction of solvent and return to gas phase 69
At higher temps eq is reached with fewer gas molecules in solution Gases generally less soluble
70
Effect of temp on solubility of solids in liquids more difficult to predict
Often increasing temp increases solubility
However the same temp increase can result in large increase in solubility in one case and only slight increase in the next
71
72
enthalpies of Solution
Formation of solution comes with energy change Dissolve KI in water container feels cold Dissolve LiCl in water feels hot Formation of solid-liquid solution can absorb or release heat
73
During formation of solution solvent and solute particles experience changes in forces attracting them to other particles Before dissolving begins solvent molecules held by intermolecular forces (solvent-solvent attraction) In solute molecules held by Ifs (solute-solute attraction)
74
Energy is required to separate solute molecules and solvent molecules from their neighbors
A solute particle that is surrounded by solvent molecules is said to be solvated
75
Step 1 solute particles become separated from solid (energy absorbed) Step 2 solvent particles move apart to allow solute particles to enter liquid (energy absorbed)
76
Step 3 solvent particles attracted to and solvate solute particles (energy released)
77
Heat of solution - net amount of heat energy absorbed or released when a specific amount of solute dissolves in a solvent
Heat of solution negative (heat is released) if sums of energy in steps 1 and 2 is less than step 3
If 1+2 gt 3 HOS is positive (heat absorbed)78
In gaseous state molecules are so far apart there are nearly no forces between them Solute-solute interaction has little effect on heat of solution of a gas Energy released when gas dissolved in liquid bc attraction between solute gas and solvent molecules greater than energy needed to separate solvent molecules
79
Concentration of Solutions80
Concentration - measure of amount of solute in given amount of solvent or solution ldquodiluterdquo just means small amount of solute ldquoconcentratedrdquo just means relatively large amount of solute Unrelated to degree to which solution is saturated Ex Saturated solution of substance not very soluble might be dilute
81
Molarity
Molarity - number of moles of solute in one liter of solution To find molarity must know molar mass A ldquoone molarrdquo solution of NaOH is 1 mole NaOH per liter of solution (1 M)
82
1 mol NaOH = 400 g If this quantity dissolve in enough water to make EXACTLY 100 L solution solution is 1 M If 200 g NaOH dissolve in enough to make 100 L solution what is molarity 0500 M
83
1 M solution is not made by adding 1 mol solute to 1 L solvent That would be more than 1 L Instead 1 mol solute dissolved in less than 1 L Then diluted with more solvent to bring TOTAL VOLUME to 1 L
84
Practice ProblemYou have 340 L of solution that contains 900 g sodium chloride What is the molarity of that solution 0440 M NaCl You have 08 L of a 05 M HCl solution How many moles of HCl does this solution contain 04 mol HCl
85
What is the molarity of a solution composed of 585 g of potassium iodide dissolved in enough water to make a 0125 L of solution 0282 M KI
86
How many moles of H2SO4 are present in 0500 L of a 0150 M H2SO4 solution 00750 mol What volume of 300 M NaCl is needed for reaction that requires 1463 g of NaCl 0834 L
87
Molality
Molality - concentration of solution expressed in moles of solute per kilogram of solvent Solution that contains 1 mol solute (NaOH) dissolved in exactly 1 kg of solvent is ldquoone molalrdquo solution (1 m NaOH)
88
1 mol NaOH = 400 g 400 g NaOH dissolved in 1 kg water is 1 m NaOH
200 g NaOH in 1 kg water = 0500 m NaOH
89
Practice Problem
A solution was prepared by dissolving 171 g sucrose (C12H22O11) in 125 g of water Find the molal concentration of this solution 0400 m C12H22O11
90
A solution of iodine in carbon tetrachloride is used when iodine is needed for certain chemical tests How much iodine must be added to prepare a 0480 m solution of iodine in CCl4 if 1000 g of CCl4 is used 122 g I2
91
What is the molality of a solution composed of 255 g acetone (CH3)2CO dissolved in 200 g of water 22 m acetone What quantity in grams of methanol CH3OH is required to prepare a 0244 m solution in 400 g water 312 g methanol
92
How many grams of AgNO3 are needed to prepare a 0125 m solution in 250 mL of water 531 g AgNO3
What is the molality of a solution containing 182 g HCl and 250 g water 200 m
93
Ions in Aqueous Solutions and Colligative Properties
94
95
compounds in aqueous solutions
DissociationWhen compound made from ions dissolves in water ions separate Dissociation - separation of ions that occurs when an ionic compound dissolves
96
Notice number of ions made per formula unit in equations 1 formula unit of NaCl gives 2 units of ions in solution 1 formula unit of CaCl2 gives 3 units of ions
97
Assuming 100 dissociation solution that contains 1 mol NaCl contains 1 mol Na+ and 1 mol Cl- Can assume 100 dissociation of all soluble ionic compounds
98
Practice Problem
Write the equation for the dissolution of aluminum sulfate in water How many moles of aluminum ions are made by dissolving 1 mol aluminum sulfate What is the total number of moles of ions made by dissolving 1 mol of aluminum sulfate
99
1 AnalyzeGiven Amount of solute = 1 mol Al2(SO4)3 Solvent identity = water Unknown a moles of aluminum ions and sulfate ions Total number of moles of solute ions produced
100
2 Plan
The coefficients in the balanced dissociation equation will tell mole relationships You can use the equation to find out the number of moles of solute ions produced
101
3 Compute
a
b102
Write the equation for the dissolution of each of the following in water and then determine the number of moles of each ions made as well as the total number of ions made a 1 mol ammonium chloride b 1 mol sodium sulfide c 05 mol barium nitrate
103
a 1 mol ammonium chloride
1 mol NH4+ 1 mol Cl- 2 mol total ions
104
b 1 mol sodium sulfide
2 mol Na+ 1 mol S-2 3 mol total ions
105
c 05 mol barium nitrate
05 mol Ba+2 1 mol NO3- 15 mol total ions
106
Precipitation Reactions
Even though no compound is completely insoluble compounds of very low solubility can be considered insoluble for practical purposes There are general guidelines to help predict whether a compound made of certain combination of ions is soluble
107
General Solubility Guidelines
Sodium potassium and ammonium compounds are soluble in water
108
Nitrates acetates and chlorates are soluble
109
110
Most chlorides are soluble except those of silver mercury (I) and lead Lead (II) chloride is soluble in hot water
Most sulfates are soluble except those of barium strontium lead calcium and mercury
111
Most carbonates phosphates and silicates are insoluble except those of sodium potassium and ammonium
112
Most sulfides are insoluble except those of calcium strontium sodium potassium and ammonium
113
Is calcium phosphate Ca3(PO4)2 soluble or insoluble
Not one of the exceptions so it is insoluble
Dissociation equations are not written for insoluble compounds
114
Guidelines also useful in predicting what will happen if 2 solutions of 2 different soluble compounds are mixed
If mixing results in combination of ions that forms insoluble compound a double-replacement and precipitation reaction will happen
115
Will precipitate form when solutions of ammonium sulfide and cadmium nitrate are combined You can tell calcium nitrate is soluble from the guidelines (Most nitrates acetates and chlorates are soluble) You can also tell ammonium sulfide is soluble (Most sulfides are insoluble except those of calcium strontium sodium potassium and ammonium)
116
117
To decide whether precipitate can form must know solubilities of two possible products
Ammonium nitrate ndash soluble (Most nitrates acetates and chlorates are soluble) Cadmium sulfide ndash insoluble (Most sulfides are insoluble except those of calcium strontium sodium potassium and ammonium)
118
bc one product is insoluble double-replacement and precipitation reaction will happen (NH4)2S (aq) + Cd(NO3)2 (aq) rarr NH4NO3 (aq) + CdS (s)
119
Net Ionic Equations
Reactions of ions in aqueous solution usually represented by net ionic equations instead of formula equations Net ionic equation - includes only those compounds and ions that undergo a chemical change in a reaction in an aqueous solution
120
121
To write NIE (net ionic equation) first convert chemical equation into overall ionic equation All soluble ionic compounds shown as dissociated ions in solution Precipitates shown as solids
122
Notice ammonium and nitrate ions appear on both sides They havenrsquot gone through any chemical change Spectator ions - ions that do not take part in a chemical reaction and are found in solution both before and after the reaction
123
To change ionic equation to NIE eliminate spectator ions Cd+2(aq) + S-2(aq) rarr CdS(s)
This NIE applies to any reaction in which precipitate of CdS forms
124
Practice Problem
Identify the precipitate that forms when aqueous solutions of zinc nitrate and ammonium sulfide are combined Write the equation for the possible double-replacement reaction Then write the formula equation overall ionic equation and the net ionic equation
125
1 AnalyzeGiven Identity of reactants zinc nitrate and ammonium sulfide Reaction medium aqueous solution Unknown a equation for the possible double-replacement reaction b identity of the precipitate c formula equation d overall ionic equation e net ionic equation
126
2 PlanWrite the possible double-replacement reaction between Zn(NO3)2 and (NH4)2S Use the rules to determine if any of the products will precipitate Write a formula equation and overall net ionic equation Then cancel spectator ions to make net ionic equation
127
3 Computea possible double-replacement reaction
b rules show that zinc sulfide is not soluble so will precipitate (ammonium nitrate is soluble)
c formula equation128
d overall ionic equation
e net ionic equation129
Will a precipitate form if solutions of potassium sulfate and barium nitrate are combined If so write the net ionic equation for the reaction
Yes Ba+2(aq) + SO4-2(aq) rarr BaSO4(s)
130
Will a precipitate form if solutions of potassium nitrate and magnesium sulfate are combined If so write the net ionic equation for the reaction
No 131
Will a precipitate form if solutions of barium chloride and sodium sulfate are combined If so identify the spectator ions and write the net ionic equation for the reaction
Yes Na+ and Cl- Ba+2(aq) + SO4-2(aq) rarr BaSO4(s)
132
Write the net ionic equation for the precipitation of nickel(II) sulfide
Ni+2(aq) + S-2(aq) rarr NiS(s)
133
IonizationSome molecular compounds can also form ions in solution Usually polar Ionization - ions formed from solute molecules by action of the solvent (creation of ions where there were none) Ionization different from dissociation
134
Dissociation ionic compounds dissolve and ions already present separate Ionization ions formed where non existed before
Ions formed are hydrated Heat released during hydration of ions gives enough energy to break covalent bonds
135
Extent to which solute ionizes depends on strength of bonds within molecules of solute and strength of attraction between solute and solvent molecules
136
HCl ndash molecular compound that ionizes in aqueous solution Contains highly polar bond Attraction between polar HCl molecule and polar water strong enough to break HCl bond
137
The Hydronium Ion
H+ ions from HCl attracts other molecules or ions so strongly that it doesnrsquot normally exist alone Ionization of HCl better described as direct transfer of proton from HCl to H2O forming H3O+
138
139
Hydration of H+ to form hydronium is highly exothermic
Energy released gives a lot of energy needed to ionize a molecular solute
Many molecular compounds that ionize in aqueous solution contain hydrogen and form hydronium ion
140
Strong and Weak Electrolytes
Substances that make ions and conduct current in solution are electrolytes HCl is one of series of compounds that has hydrogen and halogen Hydrogen halides are all molecular compounds with single polar-covalent bonds
141
All are gases Very soluble in water All are electrolytes HCl HBr HI strongly conduct current in solution HF only weakly conducts current at same concentration Strength of conduction related to ability to ionize
142
143
Strong Electrolytes
HCl HBr HI are 100 ionized in solution
Strong electrolyte - any compound whose dilute aqueous solutions conduct electricity well this is due to the presence of all or almost all of the dissolve compound in the form of ions
144
Unique characteristic of strong electrolyte is that they yield only ions
Ex Ionic compound may be highly soluble in water and dissociate into ions (NaCl)
Others may not dissolve much but amount that does dissolve exists only as ions
145
Weak ElectrolytesHF dissolves in water to give acid solution called hydrofluoric acid HF bond stronger than bonds between hydrogen and other halogens When HF dissolves some molecule ionize Reverse reaction also happens HF(aq) + H2O(l) H3O+(aq) + F-(aq)
146
HF(aq) + H2O(l) H3O+
(aq) + F-(aq)Concentration of dissolved unionized HF stays high and concentration of ions stays low
Weak electrolyte - any compound whose aqueous solutions conduct electricity poorly this is due to the presence of small amount of the dissolved compound in the form of ions
147
Different from nonelectrolyte ndash NO ions at all Description of electrolyte as strong or weak not related to concentration of solution Electrolytes differ in degree of ionization not amount of solute dissolved
148
149
Colligative properties of solutions
Presence of solute affects properties of solutions Some properties not dependent on nature of dissolved substance but on how may dissolve particles are present
Colligative properties - properties that depend on the concentration of solute particles but not on their identity
150
Vapor-Pressure Lowering
Boiling and freezing point of solution different from pure solvent Nonvolatile solute raises boiling point and lowers freezing point Nonvolatile substance - one that has little tendency to become gas under existing conditions
151
152
To understand why nonvolatile solute changes boiling and freezing point consider equilibrium vapor pressure
Vapor pressure pressure caused by molecules that have escape liquid phase to gas phase
Can be thought of a measure of tendency of molecules to escape from a liquid
153
Addition of sucrose (nonvolatile solute) lowers concentration of water molecules at surface of liquid This lowers tendency of water molecules to leave solution and enter gas phase Vapor pressure of solution is lower than vapor pressure of pure water
154
Aqueous13 solution13 of13 nonvolatile13 solutePure13 water
Nonelectrolyte solutions of same molality have same concentration of particles
Dilute solutions of same solvent and equal molality of any nonelectrolyte solute lower vapor pressure equally
155
Example
1 m aqueous solution of nonelectrolyte glucose lowers vapor pressure of water 55 x 10-4 atm at 25 1 m aqueous solution of sucrose also lowers water vapor pressure to 55 x 10-4 atm at 25
156
157
bc vapor-pressure lowering depends on concentration of nonelectrolyte solute and doesnrsquot depend on the type of solute it is a colligative proprerty
bc vapor pressure has been lowered solution remains liquid over larger temperature range
Lowers freezing point and raises boiling point
158
Can assume changes in boiling and freezing point also depend on concentration of solute
They are colligative properties
159
Freezing-Point Depression
Freezing point of 1-molal solution of any nonelectrolyte solute in water is found (by experiment) to be 186 lower than freezing point of water When 1 mol of nonelectrolyte solute dissolved in 1 kg water freezing point is -186 not 000 160
When 2 mol nonelectrolyte solute dissolved in 1 kg water freezing point is -372
For any concentration of nonelectrolyte solute in water decrease in freezing point can be determined using value of -186m
161
This value called the molal freezing-point contstant (Kf) is the freezing-point depression of the solvent in a 1-molal solution of a nonvolatile nonelectrolyte solute
162
Each solvent has own characteristic molal freezing-point constant Freezing-point depression (Δtf) the difference between the freezing points of the pure solvent and a solute of nonelectrolyte in that solvent and it is directly proportional to the molal concentration of the solution Δtf = Kfm
163
Δtf = Kfm
Kf expressed as m
m expressed in mol solutekg solvent (molality)
164
Sample Problem
What is the freezing point depression of water in a solution of 171 g of sucrose C12H22O11 and 200 g of water What is the actual freezing point of the solution
165
1 AnalyzeGiven Solute mass and chemical formula 171 g C12H22O11 Solvent mass and identity 2000 g water Unknown a freezing-point depression b freezing point of the solution 166
2 Plan
167
168
Calculate
Unknown a freezing-point depression b freezing point of the solution
169
Practice Problem
A water solution containing an unknown quantity of a nonelectrolyte solute is found to have a freezing point of ndash 023degCWhat is the molal concentration of the solution
012 m170
A solution consists of 103 g of the nonelectrolyte glucose C6H12O6 dissolved in 250 g of water What is the freezing-point depression of the solution
minus0426degC171
In a laboratory experiment the freezing point of an aqueous solution of glucose is found to be minus0325degCWhat is the molal concentration of this solution
0175 m172
If 0500 mol of a nonelectrolyte solute are dissolved in 5000 g of ether what is the freezing point of the solution
minus1181degC173
The freezing point of an aqueous solution that contains a nonelectrolyte is minus90degC aWhat is the freezing-point depression of the solution bWhat is the molal concentration of the solution a minus90degC b 48 m
174
Boiling-Point Elevation
Boiling point is temp at which vapor pressure is equal to atmospheric pressure Change in vapor pressure will cause corresponding change in boiling point
175
176
Vapor pressure of nonvolatile solution is lower than vapor pressure of pure solvent More heat will be required to raise vapor pressure of solution Boiling point of solution is higher than boiling point of pure solvent
Molal boiling-point constant (Kb) - boiling-point elevation of the solvent in a 1-molal solution of a nonvolatile nonelectrolyte solute b-p elevation of any nonelectrolyte in water found by experiment to be 051 So molal b-p constant for water is 051m
177
For different solvents b-p elevations of 1-molal solutions have different values (Table 14-2)
Like freezing-point constants values most accurate for dilute solutions
178
Boiling-point elevation Δtb difference between the boiling points of the pure solvent and a nonelectrolyte solution of that solvent and it is directly proportional to the molal concentration of the solution B-p elevation can be calculated using Δtb = Kbm Δtb (boiling point elevation) expressed in m and m expressed in mol solutekg solvent
Sample Problem
What is the boiling-point elevation of a solution made from 200 g of a nonelectrolyte solute and 4000 g of water The molar mass of the solute is 620 gmol
180
1 AnalyzeGiven solute mass = 200 g solute molar mass = 620 gmol solvent mass and identity = 4000 g of water
Unknown boiling-point elevation
181
2 Plan
182
183
3 Compute
Practice Problem
A solution contains 500 g of sucrose C12H22O11 a nonelectrolyte dissolved in 5000 g of water What is the boiling-point elevation
015degC184
A solution contains 4500 g of sucrose C12H22O11 a nonelectrolyte dissolved in 250 g of waterWhat is the boiling point of the solution
1027degC185
If the boiling point elevation of an aqueous solution containing a nonvolatile electrolyte is 102degC what is the molality of the solution
20 m186
The boiling point of an aqueous solution containing a nonvolatile electrolyte is 10075degC aWhat is the boiling-point elevation bWhat is the molality of the solution a 075degC b 15 m
187
Electrolytes and Colligative PropertiesEarly investigators confused by experiments where certain substances depressed freezing point or elevated boiling point more than expected
188
189
Ex 01 m solution of NaCl lowers freezing point of solvent almost twice as much as 01 m solution of sucrose 01 m CaCl2 lowers fp almost 3 times as much as 01 m solution of sucrose
To understand why this happens contrast behavior or sucrose with NaCl in aqueous solution Each sucrose dissolves to make only 1 particle in solution So 1 mol sucrose dissolves to 1 mol particles 1 mol NaCl dissolves to make 2 moles of particles (1 mol Na+ and 1 mol Cl-)
190
191
Calculated Values for Electrolyte SolutionsColligative properties depend on total concentration of solute particles regardless of their identity Electrolytes cause changes in colligative properties proportional to total molality in terms of all dissolves particles instead of formula units
192
What about barium nitrate Ba(NO3)2 Each mole of barium nitrate yields 3 mol of ions in solution Ba(NO3)2(s) rarr Ba2+(aq) + 2NO3-(aq) You would expect solution of given molality to lower fp of solvent 3 times as much as nonelectrolytic solution of same molality
193
Sample Problem
What is the expected change in the freezing point of water in a solution of 625 g of barium nitrate Ba(NO3)2 in 100 kg of water
194
1 AnalyzeGiven solute mass and formula = 625 g Ba(NO3)2 solvent mass and identity = 100 kg water Δtf = Kfm
Unknown expected freezing-point depression
195
2 PlanThe molality can be calculated by converting the solute mass to moles and then dividing by the number of kilograms of solvent That molality is in terms of formula units of Ba(NO3)2 and must be converted to molality in terms of dissociated ions in solution It must be multiplied by the number of moles of ions produced per mole of formula unit This adjusted molality can then be used to calculate the freezing-point depression
196
197
198
3 Compute
Practice Problem
What is the expected freezing-point depression for a solution that contains 20 mol of magnesium sulfate dissolved in 10 kg of water
minus74degC199
What is the expected boiling-point elevation of water for a solution that contains 150 g of sodium chloride dissolved in 10 kg of water 27degC The freezing point of an aqueous sodium chloride solution is minus200degC What is the molality of the solution 54 m NaCl
200
Actual Values for Electrolyte SolutionsRemember values calculated are only expected values Actual values of colligative properties for all strong electrolytes are almost what would be expected Difference between expected and actual values caused by attractive forces that exist between dissociated ions in aqueous solution
201
Attraction between hydrated ions in solution is small compared with those in crystalline solid However forces of attraction donrsquot interfere with movements of aqueous ions More concentrated solution means ions closer together and attraction is greater Also depends on charge of ions ndash higher charge greater attraction
202
In muddy water large soil particles settle Water still cloudy bc colloidal particles stay dispersed in water Filter Colloidal particles pass through and stays cloudy
14
Particles in colloid small enough to be suspended in solvent by constant movement of surrounding molecules Colloidal particles make up dispersed phase Water is dispersing medium
15
Emulsion and foam are colloids
Mayonnaise is emulsion of oil droplets in water Egg yolk acts as emulsifying agent (keeps oil droplets dispersed)
16
17
18
Class13 of13 Colloid Phases Example
Sol Solid13 dispersed13 in13 liquid Paints13 mud
Gel Solid13 network13 extending13 through13 liquid
Gelatin
Liquid13 emulsion Liquid13 dispersed13 in13 liquid Milk13 mayonnaise
Foam Gas13 dispersed13 in13 liquid Shaving13 cream13 whipped13 cream
Solid13 aerosol Solid13 dispersed13 in13 gas Smoke13 auto13 exhaust
Liquid13 aerosol Liquid13 dispersed13 in13 gas Fog13 mist13 clouds13 aerosol13 spray
Solid13 emulsion Liquid13 dispersed13 in13 solid Cheese13 butter
Tyndall EffectMany colloids appear homogeneous bc individual particles cannot be seen Particles large enough to scatter light In fog can see headlight beam Tyndall effect - occurs when light is scattered by colloidal particles dispersed in transparent medium
19
20
21
Solutions Colloids Suspensions
Homogeneous Heterogeneous Heterogeneous
Particle13 size13 001-shy‐113 nm13 can13 be13 atoms13 ions13 molecules
Particle13 size13 1-shy‐100013 nm13 dispersed13 can13 be13 combined13 or13 large13 molecules
Particle13 size13 over13 100013 nm13 suspended13 can13 be13 large13 particles13 or13 combined13 particles
Do13 not13 separate13 on13 standing Do13 not13 separate13 on13 standing Particles13 settle13 out
Cannot13 be13 separated13 by13 filtration Cannot13 be13 separated13 by13 filtration Can13 be13 separated13 by13 filtration
Do13 not13 scatter13 light Scatter13 light13 (Tyndall13 effect) May13 scatter13 light13 but13 are13 not13 transparent
Electrolytes vs Nonelectrolytes
Substances that dissolve in water are classified according to whether they produce molecules or ions in solution
22
23
When ionic compound dissolves cations and anions separate and are surrounded by water molecules Solute ions free to move making possible to carry electric current
Electrolyte - substance that dissolves in water to give a solution that conducts electric current NaCl ndash electrolyte Usually highly polar compounds become electrolytes
24
Solution containing neutral solute molecules does not conduct current bc it doesnrsquot have mobile charged particles
Nonelectrolyte - substance that dissolves in water to give solution that does not conduct electric current Ex Sugar
25
26
27the solution
process
Factors Affecting Rate of DissolutionEver tried to dissolve sugar in iced tea or coffee You notice temperature has something to do with how quickly solute dissolves What other factors affect how quickly solutes dissolve
28
1 Increasing Surface Area of Solute
bc dissolution process occurs at surface of solute Can increase dissolution by increasing surface area
29
2 Agitating (Stirring) SolutionClose to surface of solute concentration of dissolved solute is high Stirringshaking helps disperse solute particles
30
Brings fresh solvent into contact with solute surface Contact between solvent and solute surface increased
3 Heating a Solvent
Solutes dissolve faster in warmer solvent As temp increases solvent molecules move faster Average kinetic energy increases Collisions between solvent and solute more frequent Helps separate solute molecules and spread them out
32
Solubility
If you add sugar to tea eventually no more sugar dissolves
For every combination of solvent with solute at given temp there is limit to amount of solute that can be dissolved
33
When sugar first dropped into water sugar molecules leave solid surface and move about at random in solvent Some dissolved molecules may collide with crystal and stay there As more solid dissolves and concentration of dissolved molecules increases collisions happen more often
34
Eventually molecules are returning to crystal at same rate they are going into solution Dynamic equilibrium established between dissolution and crystallization Solution equilibrium - physical state in which the opposing processes of dissolution and crystallization of a solute occur at equal rates
35
Saturated vs Unsaturated Solutions
Saturated solution - solution that contains the maximum amount of dissolved solute How do you know if solution is saturated If more NaCl added it falls to bottom and doesnrsquot dissolve Equilibrium already established
36
If more water added to saturated solution more NaCl will dissolve At 20 359 g NaCl is max that will dissolve in 100 g water Unsaturated solution - solution that contains less solute than a saturated solution under the existing conditions
37
Supersaturated Solutions
When saturated solution where solubility increases as temp increases is cooled excess solute usually comes out of solution Sometimes if solution is left to cool the excess solute doesnrsquot separate
38
39
Supersaturated solution - solution that contains more dissolved solute than a saturated solution contains under same conditions
40
Supersaturated solution can remain unchanged over long time if it isnrsquot disturbed Once crystals begin to form continues until equilibrium is recreated at lower temp
41
Example
Sodium thiosulfate Na2S2O3 Solute added to hot water until solution saturated Hot solution filtered Drop small crystal in ldquoseedingrdquo crystallization Continues until equilibrium created
42
Solubility Values
Solubility - amount of substance required to form a saturated solution with specific amount of solvent at specified temp Ex Solubility of sugar = 204 g per 100 g water at 20 Must specify temp bc solubility changes with temp 43
For gases pressure must also be specified Rate at which substance dissolves is unrelated to solubility Max amount of solute that dissolves and reaches equilibrium is always same under same conditions
44
Solute-Solvent Interactions
ldquolike dissolves likerdquo is generally useful rule for predicting whether one substance will dissolve in another Depends on type of bonding polaritynonpolarity and intermolecular forces between solute and solvent
45
Dissolving Ionic Compounds in Aqueous Solution
Polarity of water molecules plays important role in formation of solution of ionic compounds Charged ends of water molecules attract ions and surround them to keep separated from other ions
46
Hydration - solution process with water as solvent Ions said to be hydrated As hydrated ions diffuse into solution other ions exposed and drawn away from crystal surface Entire crystal gradually dissolves
47
When crystallized from aqueous solutions some ionic compounds form crystals that include water molecules The crystalline compounds known as hydrates have specific ratios of water molecules Represented by formulas like CuSO45H2O
48
When crystalline hydrate dissolves in water water of hydration returns to solvent Behavior of solute in hydrated form no different than anhydrous form
49
Nonpolar Solvents
Nonpolar solvents do not attract ions of ionic compound strongly enough to overcome forces holding crystal together
50
Liquid Solutes and Solvents
When you shake bottle of salad dressing oil droplets disperse in water Stop shaking ndash strong attraction between water molecules squeeze out oil and form separate layers Liquid solutes and solvents that are not soluble in each other are immiscible
51
Nonpolar substances (fat oil grease) are generally quite soluble in nonpolar liquids (carbon tetrachloride toluene gasoline)
Only attractions between nonpolar molecules are weak London forces
52
London Dispersion Forces
bc e- are constantly moving at any time there may be uneven distribution of them around a nonpolar atom Temporarily creates positive end and negative end
53
54
London dispersion forces - intermolecular attractions resulting from constant motion of e- and creation of instantaneous dipoles
55
Liquids that dissolve freely in one another in any ratio are said to be completely miscible Benzene and carbon tetrachloride (both nonpolar) are completely miscible
Nonpolar molecules of these two apply no strong forces of attraction or repulsion and molecules mix freely
56
Ethanol and water are also completely miscible -OH group on ethanol is slightly polar
Can form H-bonds with water as well as other ethanol molecules Intermolecular forces in mixture are very similar to those in pure liquids so they are mutually soluble in any amount
57
Gasoline contains mainly nonpolar hydrocarbons
Excellent solvent for fats oils greases
Major intermolecular forces are weak London forces
58
Ethanol less polar than water more than carbon tetrachloride
Better solvent for less-polar substances bc of nonpolar region
59
Effects of Pressure on Solubility
Little effect on solids or liquids Increase pressure - increase solubilities of gases in liquids
60
When gas in contact with surface of liquid gas molecules can enter liquid As amount of dissolve gas increases some molecules start to escape and reenter gas phase
Eq eventually established between rates of enteringleaving gas phase
61
As long as eq undisturbed solubility of gas in liquid unchanged at given pressure
Gas + solvent solution
62
Increasing pressure of solute gas above solution puts stress on eq Molecules collide with surface more often Increase in pressure partially offset by increase in rate of gas molecules entering solution In turn increase in amount of dissolved gas causes increase in rate molecules escape liquid and becomes gas
63
Eventually eq restored at higher gas solubility
Expected from Le Chatelierrsquos principle
Increase in gas pressure causes eq to shift so fewer molecules are in gas phase
64
Henryrsquos Law
Henryrsquos law - solubility of a gas in liquid is directly proportional to the partial pressure of that gas on the surface of the liquid
Applies to gas-liquid solutions at constant temp
65
When mixture of ideal gases is limited in constant volume at constant temp each gas applies same pressure it would apply if it occupied the space alone
Assuming gases do not react each gas dissolves to the extent it would if no other gases were there
66
In carbonated drinks (Coke Pepsi etc) solubility of CO2 increased by increasing pressure
At bottling factory CO2 gas forced into solution of flavored water at pressure of 5-10 atm Gas-in-liquid solution then sealed in bottlescans
67
When cap removed pressure reduced to 1 atm and some CO2 escapes as gas bubbles Effervescence - rapid escape of a gas from a liquid in which it is dissolved
68
Effects of Temperature on Solubility
Letrsquos consider gas solubility Increasing temperature usually decreases gas solubility As temp increases average kinetic energy of molecules in solution increases Greater number of solute molecules escape from attraction of solvent and return to gas phase 69
At higher temps eq is reached with fewer gas molecules in solution Gases generally less soluble
70
Effect of temp on solubility of solids in liquids more difficult to predict
Often increasing temp increases solubility
However the same temp increase can result in large increase in solubility in one case and only slight increase in the next
71
72
enthalpies of Solution
Formation of solution comes with energy change Dissolve KI in water container feels cold Dissolve LiCl in water feels hot Formation of solid-liquid solution can absorb or release heat
73
During formation of solution solvent and solute particles experience changes in forces attracting them to other particles Before dissolving begins solvent molecules held by intermolecular forces (solvent-solvent attraction) In solute molecules held by Ifs (solute-solute attraction)
74
Energy is required to separate solute molecules and solvent molecules from their neighbors
A solute particle that is surrounded by solvent molecules is said to be solvated
75
Step 1 solute particles become separated from solid (energy absorbed) Step 2 solvent particles move apart to allow solute particles to enter liquid (energy absorbed)
76
Step 3 solvent particles attracted to and solvate solute particles (energy released)
77
Heat of solution - net amount of heat energy absorbed or released when a specific amount of solute dissolves in a solvent
Heat of solution negative (heat is released) if sums of energy in steps 1 and 2 is less than step 3
If 1+2 gt 3 HOS is positive (heat absorbed)78
In gaseous state molecules are so far apart there are nearly no forces between them Solute-solute interaction has little effect on heat of solution of a gas Energy released when gas dissolved in liquid bc attraction between solute gas and solvent molecules greater than energy needed to separate solvent molecules
79
Concentration of Solutions80
Concentration - measure of amount of solute in given amount of solvent or solution ldquodiluterdquo just means small amount of solute ldquoconcentratedrdquo just means relatively large amount of solute Unrelated to degree to which solution is saturated Ex Saturated solution of substance not very soluble might be dilute
81
Molarity
Molarity - number of moles of solute in one liter of solution To find molarity must know molar mass A ldquoone molarrdquo solution of NaOH is 1 mole NaOH per liter of solution (1 M)
82
1 mol NaOH = 400 g If this quantity dissolve in enough water to make EXACTLY 100 L solution solution is 1 M If 200 g NaOH dissolve in enough to make 100 L solution what is molarity 0500 M
83
1 M solution is not made by adding 1 mol solute to 1 L solvent That would be more than 1 L Instead 1 mol solute dissolved in less than 1 L Then diluted with more solvent to bring TOTAL VOLUME to 1 L
84
Practice ProblemYou have 340 L of solution that contains 900 g sodium chloride What is the molarity of that solution 0440 M NaCl You have 08 L of a 05 M HCl solution How many moles of HCl does this solution contain 04 mol HCl
85
What is the molarity of a solution composed of 585 g of potassium iodide dissolved in enough water to make a 0125 L of solution 0282 M KI
86
How many moles of H2SO4 are present in 0500 L of a 0150 M H2SO4 solution 00750 mol What volume of 300 M NaCl is needed for reaction that requires 1463 g of NaCl 0834 L
87
Molality
Molality - concentration of solution expressed in moles of solute per kilogram of solvent Solution that contains 1 mol solute (NaOH) dissolved in exactly 1 kg of solvent is ldquoone molalrdquo solution (1 m NaOH)
88
1 mol NaOH = 400 g 400 g NaOH dissolved in 1 kg water is 1 m NaOH
200 g NaOH in 1 kg water = 0500 m NaOH
89
Practice Problem
A solution was prepared by dissolving 171 g sucrose (C12H22O11) in 125 g of water Find the molal concentration of this solution 0400 m C12H22O11
90
A solution of iodine in carbon tetrachloride is used when iodine is needed for certain chemical tests How much iodine must be added to prepare a 0480 m solution of iodine in CCl4 if 1000 g of CCl4 is used 122 g I2
91
What is the molality of a solution composed of 255 g acetone (CH3)2CO dissolved in 200 g of water 22 m acetone What quantity in grams of methanol CH3OH is required to prepare a 0244 m solution in 400 g water 312 g methanol
92
How many grams of AgNO3 are needed to prepare a 0125 m solution in 250 mL of water 531 g AgNO3
What is the molality of a solution containing 182 g HCl and 250 g water 200 m
93
Ions in Aqueous Solutions and Colligative Properties
94
95
compounds in aqueous solutions
DissociationWhen compound made from ions dissolves in water ions separate Dissociation - separation of ions that occurs when an ionic compound dissolves
96
Notice number of ions made per formula unit in equations 1 formula unit of NaCl gives 2 units of ions in solution 1 formula unit of CaCl2 gives 3 units of ions
97
Assuming 100 dissociation solution that contains 1 mol NaCl contains 1 mol Na+ and 1 mol Cl- Can assume 100 dissociation of all soluble ionic compounds
98
Practice Problem
Write the equation for the dissolution of aluminum sulfate in water How many moles of aluminum ions are made by dissolving 1 mol aluminum sulfate What is the total number of moles of ions made by dissolving 1 mol of aluminum sulfate
99
1 AnalyzeGiven Amount of solute = 1 mol Al2(SO4)3 Solvent identity = water Unknown a moles of aluminum ions and sulfate ions Total number of moles of solute ions produced
100
2 Plan
The coefficients in the balanced dissociation equation will tell mole relationships You can use the equation to find out the number of moles of solute ions produced
101
3 Compute
a
b102
Write the equation for the dissolution of each of the following in water and then determine the number of moles of each ions made as well as the total number of ions made a 1 mol ammonium chloride b 1 mol sodium sulfide c 05 mol barium nitrate
103
a 1 mol ammonium chloride
1 mol NH4+ 1 mol Cl- 2 mol total ions
104
b 1 mol sodium sulfide
2 mol Na+ 1 mol S-2 3 mol total ions
105
c 05 mol barium nitrate
05 mol Ba+2 1 mol NO3- 15 mol total ions
106
Precipitation Reactions
Even though no compound is completely insoluble compounds of very low solubility can be considered insoluble for practical purposes There are general guidelines to help predict whether a compound made of certain combination of ions is soluble
107
General Solubility Guidelines
Sodium potassium and ammonium compounds are soluble in water
108
Nitrates acetates and chlorates are soluble
109
110
Most chlorides are soluble except those of silver mercury (I) and lead Lead (II) chloride is soluble in hot water
Most sulfates are soluble except those of barium strontium lead calcium and mercury
111
Most carbonates phosphates and silicates are insoluble except those of sodium potassium and ammonium
112
Most sulfides are insoluble except those of calcium strontium sodium potassium and ammonium
113
Is calcium phosphate Ca3(PO4)2 soluble or insoluble
Not one of the exceptions so it is insoluble
Dissociation equations are not written for insoluble compounds
114
Guidelines also useful in predicting what will happen if 2 solutions of 2 different soluble compounds are mixed
If mixing results in combination of ions that forms insoluble compound a double-replacement and precipitation reaction will happen
115
Will precipitate form when solutions of ammonium sulfide and cadmium nitrate are combined You can tell calcium nitrate is soluble from the guidelines (Most nitrates acetates and chlorates are soluble) You can also tell ammonium sulfide is soluble (Most sulfides are insoluble except those of calcium strontium sodium potassium and ammonium)
116
117
To decide whether precipitate can form must know solubilities of two possible products
Ammonium nitrate ndash soluble (Most nitrates acetates and chlorates are soluble) Cadmium sulfide ndash insoluble (Most sulfides are insoluble except those of calcium strontium sodium potassium and ammonium)
118
bc one product is insoluble double-replacement and precipitation reaction will happen (NH4)2S (aq) + Cd(NO3)2 (aq) rarr NH4NO3 (aq) + CdS (s)
119
Net Ionic Equations
Reactions of ions in aqueous solution usually represented by net ionic equations instead of formula equations Net ionic equation - includes only those compounds and ions that undergo a chemical change in a reaction in an aqueous solution
120
121
To write NIE (net ionic equation) first convert chemical equation into overall ionic equation All soluble ionic compounds shown as dissociated ions in solution Precipitates shown as solids
122
Notice ammonium and nitrate ions appear on both sides They havenrsquot gone through any chemical change Spectator ions - ions that do not take part in a chemical reaction and are found in solution both before and after the reaction
123
To change ionic equation to NIE eliminate spectator ions Cd+2(aq) + S-2(aq) rarr CdS(s)
This NIE applies to any reaction in which precipitate of CdS forms
124
Practice Problem
Identify the precipitate that forms when aqueous solutions of zinc nitrate and ammonium sulfide are combined Write the equation for the possible double-replacement reaction Then write the formula equation overall ionic equation and the net ionic equation
125
1 AnalyzeGiven Identity of reactants zinc nitrate and ammonium sulfide Reaction medium aqueous solution Unknown a equation for the possible double-replacement reaction b identity of the precipitate c formula equation d overall ionic equation e net ionic equation
126
2 PlanWrite the possible double-replacement reaction between Zn(NO3)2 and (NH4)2S Use the rules to determine if any of the products will precipitate Write a formula equation and overall net ionic equation Then cancel spectator ions to make net ionic equation
127
3 Computea possible double-replacement reaction
b rules show that zinc sulfide is not soluble so will precipitate (ammonium nitrate is soluble)
c formula equation128
d overall ionic equation
e net ionic equation129
Will a precipitate form if solutions of potassium sulfate and barium nitrate are combined If so write the net ionic equation for the reaction
Yes Ba+2(aq) + SO4-2(aq) rarr BaSO4(s)
130
Will a precipitate form if solutions of potassium nitrate and magnesium sulfate are combined If so write the net ionic equation for the reaction
No 131
Will a precipitate form if solutions of barium chloride and sodium sulfate are combined If so identify the spectator ions and write the net ionic equation for the reaction
Yes Na+ and Cl- Ba+2(aq) + SO4-2(aq) rarr BaSO4(s)
132
Write the net ionic equation for the precipitation of nickel(II) sulfide
Ni+2(aq) + S-2(aq) rarr NiS(s)
133
IonizationSome molecular compounds can also form ions in solution Usually polar Ionization - ions formed from solute molecules by action of the solvent (creation of ions where there were none) Ionization different from dissociation
134
Dissociation ionic compounds dissolve and ions already present separate Ionization ions formed where non existed before
Ions formed are hydrated Heat released during hydration of ions gives enough energy to break covalent bonds
135
Extent to which solute ionizes depends on strength of bonds within molecules of solute and strength of attraction between solute and solvent molecules
136
HCl ndash molecular compound that ionizes in aqueous solution Contains highly polar bond Attraction between polar HCl molecule and polar water strong enough to break HCl bond
137
The Hydronium Ion
H+ ions from HCl attracts other molecules or ions so strongly that it doesnrsquot normally exist alone Ionization of HCl better described as direct transfer of proton from HCl to H2O forming H3O+
138
139
Hydration of H+ to form hydronium is highly exothermic
Energy released gives a lot of energy needed to ionize a molecular solute
Many molecular compounds that ionize in aqueous solution contain hydrogen and form hydronium ion
140
Strong and Weak Electrolytes
Substances that make ions and conduct current in solution are electrolytes HCl is one of series of compounds that has hydrogen and halogen Hydrogen halides are all molecular compounds with single polar-covalent bonds
141
All are gases Very soluble in water All are electrolytes HCl HBr HI strongly conduct current in solution HF only weakly conducts current at same concentration Strength of conduction related to ability to ionize
142
143
Strong Electrolytes
HCl HBr HI are 100 ionized in solution
Strong electrolyte - any compound whose dilute aqueous solutions conduct electricity well this is due to the presence of all or almost all of the dissolve compound in the form of ions
144
Unique characteristic of strong electrolyte is that they yield only ions
Ex Ionic compound may be highly soluble in water and dissociate into ions (NaCl)
Others may not dissolve much but amount that does dissolve exists only as ions
145
Weak ElectrolytesHF dissolves in water to give acid solution called hydrofluoric acid HF bond stronger than bonds between hydrogen and other halogens When HF dissolves some molecule ionize Reverse reaction also happens HF(aq) + H2O(l) H3O+(aq) + F-(aq)
146
HF(aq) + H2O(l) H3O+
(aq) + F-(aq)Concentration of dissolved unionized HF stays high and concentration of ions stays low
Weak electrolyte - any compound whose aqueous solutions conduct electricity poorly this is due to the presence of small amount of the dissolved compound in the form of ions
147
Different from nonelectrolyte ndash NO ions at all Description of electrolyte as strong or weak not related to concentration of solution Electrolytes differ in degree of ionization not amount of solute dissolved
148
149
Colligative properties of solutions
Presence of solute affects properties of solutions Some properties not dependent on nature of dissolved substance but on how may dissolve particles are present
Colligative properties - properties that depend on the concentration of solute particles but not on their identity
150
Vapor-Pressure Lowering
Boiling and freezing point of solution different from pure solvent Nonvolatile solute raises boiling point and lowers freezing point Nonvolatile substance - one that has little tendency to become gas under existing conditions
151
152
To understand why nonvolatile solute changes boiling and freezing point consider equilibrium vapor pressure
Vapor pressure pressure caused by molecules that have escape liquid phase to gas phase
Can be thought of a measure of tendency of molecules to escape from a liquid
153
Addition of sucrose (nonvolatile solute) lowers concentration of water molecules at surface of liquid This lowers tendency of water molecules to leave solution and enter gas phase Vapor pressure of solution is lower than vapor pressure of pure water
154
Aqueous13 solution13 of13 nonvolatile13 solutePure13 water
Nonelectrolyte solutions of same molality have same concentration of particles
Dilute solutions of same solvent and equal molality of any nonelectrolyte solute lower vapor pressure equally
155
Example
1 m aqueous solution of nonelectrolyte glucose lowers vapor pressure of water 55 x 10-4 atm at 25 1 m aqueous solution of sucrose also lowers water vapor pressure to 55 x 10-4 atm at 25
156
157
bc vapor-pressure lowering depends on concentration of nonelectrolyte solute and doesnrsquot depend on the type of solute it is a colligative proprerty
bc vapor pressure has been lowered solution remains liquid over larger temperature range
Lowers freezing point and raises boiling point
158
Can assume changes in boiling and freezing point also depend on concentration of solute
They are colligative properties
159
Freezing-Point Depression
Freezing point of 1-molal solution of any nonelectrolyte solute in water is found (by experiment) to be 186 lower than freezing point of water When 1 mol of nonelectrolyte solute dissolved in 1 kg water freezing point is -186 not 000 160
When 2 mol nonelectrolyte solute dissolved in 1 kg water freezing point is -372
For any concentration of nonelectrolyte solute in water decrease in freezing point can be determined using value of -186m
161
This value called the molal freezing-point contstant (Kf) is the freezing-point depression of the solvent in a 1-molal solution of a nonvolatile nonelectrolyte solute
162
Each solvent has own characteristic molal freezing-point constant Freezing-point depression (Δtf) the difference between the freezing points of the pure solvent and a solute of nonelectrolyte in that solvent and it is directly proportional to the molal concentration of the solution Δtf = Kfm
163
Δtf = Kfm
Kf expressed as m
m expressed in mol solutekg solvent (molality)
164
Sample Problem
What is the freezing point depression of water in a solution of 171 g of sucrose C12H22O11 and 200 g of water What is the actual freezing point of the solution
165
1 AnalyzeGiven Solute mass and chemical formula 171 g C12H22O11 Solvent mass and identity 2000 g water Unknown a freezing-point depression b freezing point of the solution 166
2 Plan
167
168
Calculate
Unknown a freezing-point depression b freezing point of the solution
169
Practice Problem
A water solution containing an unknown quantity of a nonelectrolyte solute is found to have a freezing point of ndash 023degCWhat is the molal concentration of the solution
012 m170
A solution consists of 103 g of the nonelectrolyte glucose C6H12O6 dissolved in 250 g of water What is the freezing-point depression of the solution
minus0426degC171
In a laboratory experiment the freezing point of an aqueous solution of glucose is found to be minus0325degCWhat is the molal concentration of this solution
0175 m172
If 0500 mol of a nonelectrolyte solute are dissolved in 5000 g of ether what is the freezing point of the solution
minus1181degC173
The freezing point of an aqueous solution that contains a nonelectrolyte is minus90degC aWhat is the freezing-point depression of the solution bWhat is the molal concentration of the solution a minus90degC b 48 m
174
Boiling-Point Elevation
Boiling point is temp at which vapor pressure is equal to atmospheric pressure Change in vapor pressure will cause corresponding change in boiling point
175
176
Vapor pressure of nonvolatile solution is lower than vapor pressure of pure solvent More heat will be required to raise vapor pressure of solution Boiling point of solution is higher than boiling point of pure solvent
Molal boiling-point constant (Kb) - boiling-point elevation of the solvent in a 1-molal solution of a nonvolatile nonelectrolyte solute b-p elevation of any nonelectrolyte in water found by experiment to be 051 So molal b-p constant for water is 051m
177
For different solvents b-p elevations of 1-molal solutions have different values (Table 14-2)
Like freezing-point constants values most accurate for dilute solutions
178
Boiling-point elevation Δtb difference between the boiling points of the pure solvent and a nonelectrolyte solution of that solvent and it is directly proportional to the molal concentration of the solution B-p elevation can be calculated using Δtb = Kbm Δtb (boiling point elevation) expressed in m and m expressed in mol solutekg solvent
Sample Problem
What is the boiling-point elevation of a solution made from 200 g of a nonelectrolyte solute and 4000 g of water The molar mass of the solute is 620 gmol
180
1 AnalyzeGiven solute mass = 200 g solute molar mass = 620 gmol solvent mass and identity = 4000 g of water
Unknown boiling-point elevation
181
2 Plan
182
183
3 Compute
Practice Problem
A solution contains 500 g of sucrose C12H22O11 a nonelectrolyte dissolved in 5000 g of water What is the boiling-point elevation
015degC184
A solution contains 4500 g of sucrose C12H22O11 a nonelectrolyte dissolved in 250 g of waterWhat is the boiling point of the solution
1027degC185
If the boiling point elevation of an aqueous solution containing a nonvolatile electrolyte is 102degC what is the molality of the solution
20 m186
The boiling point of an aqueous solution containing a nonvolatile electrolyte is 10075degC aWhat is the boiling-point elevation bWhat is the molality of the solution a 075degC b 15 m
187
Electrolytes and Colligative PropertiesEarly investigators confused by experiments where certain substances depressed freezing point or elevated boiling point more than expected
188
189
Ex 01 m solution of NaCl lowers freezing point of solvent almost twice as much as 01 m solution of sucrose 01 m CaCl2 lowers fp almost 3 times as much as 01 m solution of sucrose
To understand why this happens contrast behavior or sucrose with NaCl in aqueous solution Each sucrose dissolves to make only 1 particle in solution So 1 mol sucrose dissolves to 1 mol particles 1 mol NaCl dissolves to make 2 moles of particles (1 mol Na+ and 1 mol Cl-)
190
191
Calculated Values for Electrolyte SolutionsColligative properties depend on total concentration of solute particles regardless of their identity Electrolytes cause changes in colligative properties proportional to total molality in terms of all dissolves particles instead of formula units
192
What about barium nitrate Ba(NO3)2 Each mole of barium nitrate yields 3 mol of ions in solution Ba(NO3)2(s) rarr Ba2+(aq) + 2NO3-(aq) You would expect solution of given molality to lower fp of solvent 3 times as much as nonelectrolytic solution of same molality
193
Sample Problem
What is the expected change in the freezing point of water in a solution of 625 g of barium nitrate Ba(NO3)2 in 100 kg of water
194
1 AnalyzeGiven solute mass and formula = 625 g Ba(NO3)2 solvent mass and identity = 100 kg water Δtf = Kfm
Unknown expected freezing-point depression
195
2 PlanThe molality can be calculated by converting the solute mass to moles and then dividing by the number of kilograms of solvent That molality is in terms of formula units of Ba(NO3)2 and must be converted to molality in terms of dissociated ions in solution It must be multiplied by the number of moles of ions produced per mole of formula unit This adjusted molality can then be used to calculate the freezing-point depression
196
197
198
3 Compute
Practice Problem
What is the expected freezing-point depression for a solution that contains 20 mol of magnesium sulfate dissolved in 10 kg of water
minus74degC199
What is the expected boiling-point elevation of water for a solution that contains 150 g of sodium chloride dissolved in 10 kg of water 27degC The freezing point of an aqueous sodium chloride solution is minus200degC What is the molality of the solution 54 m NaCl
200
Actual Values for Electrolyte SolutionsRemember values calculated are only expected values Actual values of colligative properties for all strong electrolytes are almost what would be expected Difference between expected and actual values caused by attractive forces that exist between dissociated ions in aqueous solution
201
Attraction between hydrated ions in solution is small compared with those in crystalline solid However forces of attraction donrsquot interfere with movements of aqueous ions More concentrated solution means ions closer together and attraction is greater Also depends on charge of ions ndash higher charge greater attraction
202
Particles in colloid small enough to be suspended in solvent by constant movement of surrounding molecules Colloidal particles make up dispersed phase Water is dispersing medium
15
Emulsion and foam are colloids
Mayonnaise is emulsion of oil droplets in water Egg yolk acts as emulsifying agent (keeps oil droplets dispersed)
16
17
18
Class13 of13 Colloid Phases Example
Sol Solid13 dispersed13 in13 liquid Paints13 mud
Gel Solid13 network13 extending13 through13 liquid
Gelatin
Liquid13 emulsion Liquid13 dispersed13 in13 liquid Milk13 mayonnaise
Foam Gas13 dispersed13 in13 liquid Shaving13 cream13 whipped13 cream
Solid13 aerosol Solid13 dispersed13 in13 gas Smoke13 auto13 exhaust
Liquid13 aerosol Liquid13 dispersed13 in13 gas Fog13 mist13 clouds13 aerosol13 spray
Solid13 emulsion Liquid13 dispersed13 in13 solid Cheese13 butter
Tyndall EffectMany colloids appear homogeneous bc individual particles cannot be seen Particles large enough to scatter light In fog can see headlight beam Tyndall effect - occurs when light is scattered by colloidal particles dispersed in transparent medium
19
20
21
Solutions Colloids Suspensions
Homogeneous Heterogeneous Heterogeneous
Particle13 size13 001-shy‐113 nm13 can13 be13 atoms13 ions13 molecules
Particle13 size13 1-shy‐100013 nm13 dispersed13 can13 be13 combined13 or13 large13 molecules
Particle13 size13 over13 100013 nm13 suspended13 can13 be13 large13 particles13 or13 combined13 particles
Do13 not13 separate13 on13 standing Do13 not13 separate13 on13 standing Particles13 settle13 out
Cannot13 be13 separated13 by13 filtration Cannot13 be13 separated13 by13 filtration Can13 be13 separated13 by13 filtration
Do13 not13 scatter13 light Scatter13 light13 (Tyndall13 effect) May13 scatter13 light13 but13 are13 not13 transparent
Electrolytes vs Nonelectrolytes
Substances that dissolve in water are classified according to whether they produce molecules or ions in solution
22
23
When ionic compound dissolves cations and anions separate and are surrounded by water molecules Solute ions free to move making possible to carry electric current
Electrolyte - substance that dissolves in water to give a solution that conducts electric current NaCl ndash electrolyte Usually highly polar compounds become electrolytes
24
Solution containing neutral solute molecules does not conduct current bc it doesnrsquot have mobile charged particles
Nonelectrolyte - substance that dissolves in water to give solution that does not conduct electric current Ex Sugar
25
26
27the solution
process
Factors Affecting Rate of DissolutionEver tried to dissolve sugar in iced tea or coffee You notice temperature has something to do with how quickly solute dissolves What other factors affect how quickly solutes dissolve
28
1 Increasing Surface Area of Solute
bc dissolution process occurs at surface of solute Can increase dissolution by increasing surface area
29
2 Agitating (Stirring) SolutionClose to surface of solute concentration of dissolved solute is high Stirringshaking helps disperse solute particles
30
Brings fresh solvent into contact with solute surface Contact between solvent and solute surface increased
3 Heating a Solvent
Solutes dissolve faster in warmer solvent As temp increases solvent molecules move faster Average kinetic energy increases Collisions between solvent and solute more frequent Helps separate solute molecules and spread them out
32
Solubility
If you add sugar to tea eventually no more sugar dissolves
For every combination of solvent with solute at given temp there is limit to amount of solute that can be dissolved
33
When sugar first dropped into water sugar molecules leave solid surface and move about at random in solvent Some dissolved molecules may collide with crystal and stay there As more solid dissolves and concentration of dissolved molecules increases collisions happen more often
34
Eventually molecules are returning to crystal at same rate they are going into solution Dynamic equilibrium established between dissolution and crystallization Solution equilibrium - physical state in which the opposing processes of dissolution and crystallization of a solute occur at equal rates
35
Saturated vs Unsaturated Solutions
Saturated solution - solution that contains the maximum amount of dissolved solute How do you know if solution is saturated If more NaCl added it falls to bottom and doesnrsquot dissolve Equilibrium already established
36
If more water added to saturated solution more NaCl will dissolve At 20 359 g NaCl is max that will dissolve in 100 g water Unsaturated solution - solution that contains less solute than a saturated solution under the existing conditions
37
Supersaturated Solutions
When saturated solution where solubility increases as temp increases is cooled excess solute usually comes out of solution Sometimes if solution is left to cool the excess solute doesnrsquot separate
38
39
Supersaturated solution - solution that contains more dissolved solute than a saturated solution contains under same conditions
40
Supersaturated solution can remain unchanged over long time if it isnrsquot disturbed Once crystals begin to form continues until equilibrium is recreated at lower temp
41
Example
Sodium thiosulfate Na2S2O3 Solute added to hot water until solution saturated Hot solution filtered Drop small crystal in ldquoseedingrdquo crystallization Continues until equilibrium created
42
Solubility Values
Solubility - amount of substance required to form a saturated solution with specific amount of solvent at specified temp Ex Solubility of sugar = 204 g per 100 g water at 20 Must specify temp bc solubility changes with temp 43
For gases pressure must also be specified Rate at which substance dissolves is unrelated to solubility Max amount of solute that dissolves and reaches equilibrium is always same under same conditions
44
Solute-Solvent Interactions
ldquolike dissolves likerdquo is generally useful rule for predicting whether one substance will dissolve in another Depends on type of bonding polaritynonpolarity and intermolecular forces between solute and solvent
45
Dissolving Ionic Compounds in Aqueous Solution
Polarity of water molecules plays important role in formation of solution of ionic compounds Charged ends of water molecules attract ions and surround them to keep separated from other ions
46
Hydration - solution process with water as solvent Ions said to be hydrated As hydrated ions diffuse into solution other ions exposed and drawn away from crystal surface Entire crystal gradually dissolves
47
When crystallized from aqueous solutions some ionic compounds form crystals that include water molecules The crystalline compounds known as hydrates have specific ratios of water molecules Represented by formulas like CuSO45H2O
48
When crystalline hydrate dissolves in water water of hydration returns to solvent Behavior of solute in hydrated form no different than anhydrous form
49
Nonpolar Solvents
Nonpolar solvents do not attract ions of ionic compound strongly enough to overcome forces holding crystal together
50
Liquid Solutes and Solvents
When you shake bottle of salad dressing oil droplets disperse in water Stop shaking ndash strong attraction between water molecules squeeze out oil and form separate layers Liquid solutes and solvents that are not soluble in each other are immiscible
51
Nonpolar substances (fat oil grease) are generally quite soluble in nonpolar liquids (carbon tetrachloride toluene gasoline)
Only attractions between nonpolar molecules are weak London forces
52
London Dispersion Forces
bc e- are constantly moving at any time there may be uneven distribution of them around a nonpolar atom Temporarily creates positive end and negative end
53
54
London dispersion forces - intermolecular attractions resulting from constant motion of e- and creation of instantaneous dipoles
55
Liquids that dissolve freely in one another in any ratio are said to be completely miscible Benzene and carbon tetrachloride (both nonpolar) are completely miscible
Nonpolar molecules of these two apply no strong forces of attraction or repulsion and molecules mix freely
56
Ethanol and water are also completely miscible -OH group on ethanol is slightly polar
Can form H-bonds with water as well as other ethanol molecules Intermolecular forces in mixture are very similar to those in pure liquids so they are mutually soluble in any amount
57
Gasoline contains mainly nonpolar hydrocarbons
Excellent solvent for fats oils greases
Major intermolecular forces are weak London forces
58
Ethanol less polar than water more than carbon tetrachloride
Better solvent for less-polar substances bc of nonpolar region
59
Effects of Pressure on Solubility
Little effect on solids or liquids Increase pressure - increase solubilities of gases in liquids
60
When gas in contact with surface of liquid gas molecules can enter liquid As amount of dissolve gas increases some molecules start to escape and reenter gas phase
Eq eventually established between rates of enteringleaving gas phase
61
As long as eq undisturbed solubility of gas in liquid unchanged at given pressure
Gas + solvent solution
62
Increasing pressure of solute gas above solution puts stress on eq Molecules collide with surface more often Increase in pressure partially offset by increase in rate of gas molecules entering solution In turn increase in amount of dissolved gas causes increase in rate molecules escape liquid and becomes gas
63
Eventually eq restored at higher gas solubility
Expected from Le Chatelierrsquos principle
Increase in gas pressure causes eq to shift so fewer molecules are in gas phase
64
Henryrsquos Law
Henryrsquos law - solubility of a gas in liquid is directly proportional to the partial pressure of that gas on the surface of the liquid
Applies to gas-liquid solutions at constant temp
65
When mixture of ideal gases is limited in constant volume at constant temp each gas applies same pressure it would apply if it occupied the space alone
Assuming gases do not react each gas dissolves to the extent it would if no other gases were there
66
In carbonated drinks (Coke Pepsi etc) solubility of CO2 increased by increasing pressure
At bottling factory CO2 gas forced into solution of flavored water at pressure of 5-10 atm Gas-in-liquid solution then sealed in bottlescans
67
When cap removed pressure reduced to 1 atm and some CO2 escapes as gas bubbles Effervescence - rapid escape of a gas from a liquid in which it is dissolved
68
Effects of Temperature on Solubility
Letrsquos consider gas solubility Increasing temperature usually decreases gas solubility As temp increases average kinetic energy of molecules in solution increases Greater number of solute molecules escape from attraction of solvent and return to gas phase 69
At higher temps eq is reached with fewer gas molecules in solution Gases generally less soluble
70
Effect of temp on solubility of solids in liquids more difficult to predict
Often increasing temp increases solubility
However the same temp increase can result in large increase in solubility in one case and only slight increase in the next
71
72
enthalpies of Solution
Formation of solution comes with energy change Dissolve KI in water container feels cold Dissolve LiCl in water feels hot Formation of solid-liquid solution can absorb or release heat
73
During formation of solution solvent and solute particles experience changes in forces attracting them to other particles Before dissolving begins solvent molecules held by intermolecular forces (solvent-solvent attraction) In solute molecules held by Ifs (solute-solute attraction)
74
Energy is required to separate solute molecules and solvent molecules from their neighbors
A solute particle that is surrounded by solvent molecules is said to be solvated
75
Step 1 solute particles become separated from solid (energy absorbed) Step 2 solvent particles move apart to allow solute particles to enter liquid (energy absorbed)
76
Step 3 solvent particles attracted to and solvate solute particles (energy released)
77
Heat of solution - net amount of heat energy absorbed or released when a specific amount of solute dissolves in a solvent
Heat of solution negative (heat is released) if sums of energy in steps 1 and 2 is less than step 3
If 1+2 gt 3 HOS is positive (heat absorbed)78
In gaseous state molecules are so far apart there are nearly no forces between them Solute-solute interaction has little effect on heat of solution of a gas Energy released when gas dissolved in liquid bc attraction between solute gas and solvent molecules greater than energy needed to separate solvent molecules
79
Concentration of Solutions80
Concentration - measure of amount of solute in given amount of solvent or solution ldquodiluterdquo just means small amount of solute ldquoconcentratedrdquo just means relatively large amount of solute Unrelated to degree to which solution is saturated Ex Saturated solution of substance not very soluble might be dilute
81
Molarity
Molarity - number of moles of solute in one liter of solution To find molarity must know molar mass A ldquoone molarrdquo solution of NaOH is 1 mole NaOH per liter of solution (1 M)
82
1 mol NaOH = 400 g If this quantity dissolve in enough water to make EXACTLY 100 L solution solution is 1 M If 200 g NaOH dissolve in enough to make 100 L solution what is molarity 0500 M
83
1 M solution is not made by adding 1 mol solute to 1 L solvent That would be more than 1 L Instead 1 mol solute dissolved in less than 1 L Then diluted with more solvent to bring TOTAL VOLUME to 1 L
84
Practice ProblemYou have 340 L of solution that contains 900 g sodium chloride What is the molarity of that solution 0440 M NaCl You have 08 L of a 05 M HCl solution How many moles of HCl does this solution contain 04 mol HCl
85
What is the molarity of a solution composed of 585 g of potassium iodide dissolved in enough water to make a 0125 L of solution 0282 M KI
86
How many moles of H2SO4 are present in 0500 L of a 0150 M H2SO4 solution 00750 mol What volume of 300 M NaCl is needed for reaction that requires 1463 g of NaCl 0834 L
87
Molality
Molality - concentration of solution expressed in moles of solute per kilogram of solvent Solution that contains 1 mol solute (NaOH) dissolved in exactly 1 kg of solvent is ldquoone molalrdquo solution (1 m NaOH)
88
1 mol NaOH = 400 g 400 g NaOH dissolved in 1 kg water is 1 m NaOH
200 g NaOH in 1 kg water = 0500 m NaOH
89
Practice Problem
A solution was prepared by dissolving 171 g sucrose (C12H22O11) in 125 g of water Find the molal concentration of this solution 0400 m C12H22O11
90
A solution of iodine in carbon tetrachloride is used when iodine is needed for certain chemical tests How much iodine must be added to prepare a 0480 m solution of iodine in CCl4 if 1000 g of CCl4 is used 122 g I2
91
What is the molality of a solution composed of 255 g acetone (CH3)2CO dissolved in 200 g of water 22 m acetone What quantity in grams of methanol CH3OH is required to prepare a 0244 m solution in 400 g water 312 g methanol
92
How many grams of AgNO3 are needed to prepare a 0125 m solution in 250 mL of water 531 g AgNO3
What is the molality of a solution containing 182 g HCl and 250 g water 200 m
93
Ions in Aqueous Solutions and Colligative Properties
94
95
compounds in aqueous solutions
DissociationWhen compound made from ions dissolves in water ions separate Dissociation - separation of ions that occurs when an ionic compound dissolves
96
Notice number of ions made per formula unit in equations 1 formula unit of NaCl gives 2 units of ions in solution 1 formula unit of CaCl2 gives 3 units of ions
97
Assuming 100 dissociation solution that contains 1 mol NaCl contains 1 mol Na+ and 1 mol Cl- Can assume 100 dissociation of all soluble ionic compounds
98
Practice Problem
Write the equation for the dissolution of aluminum sulfate in water How many moles of aluminum ions are made by dissolving 1 mol aluminum sulfate What is the total number of moles of ions made by dissolving 1 mol of aluminum sulfate
99
1 AnalyzeGiven Amount of solute = 1 mol Al2(SO4)3 Solvent identity = water Unknown a moles of aluminum ions and sulfate ions Total number of moles of solute ions produced
100
2 Plan
The coefficients in the balanced dissociation equation will tell mole relationships You can use the equation to find out the number of moles of solute ions produced
101
3 Compute
a
b102
Write the equation for the dissolution of each of the following in water and then determine the number of moles of each ions made as well as the total number of ions made a 1 mol ammonium chloride b 1 mol sodium sulfide c 05 mol barium nitrate
103
a 1 mol ammonium chloride
1 mol NH4+ 1 mol Cl- 2 mol total ions
104
b 1 mol sodium sulfide
2 mol Na+ 1 mol S-2 3 mol total ions
105
c 05 mol barium nitrate
05 mol Ba+2 1 mol NO3- 15 mol total ions
106
Precipitation Reactions
Even though no compound is completely insoluble compounds of very low solubility can be considered insoluble for practical purposes There are general guidelines to help predict whether a compound made of certain combination of ions is soluble
107
General Solubility Guidelines
Sodium potassium and ammonium compounds are soluble in water
108
Nitrates acetates and chlorates are soluble
109
110
Most chlorides are soluble except those of silver mercury (I) and lead Lead (II) chloride is soluble in hot water
Most sulfates are soluble except those of barium strontium lead calcium and mercury
111
Most carbonates phosphates and silicates are insoluble except those of sodium potassium and ammonium
112
Most sulfides are insoluble except those of calcium strontium sodium potassium and ammonium
113
Is calcium phosphate Ca3(PO4)2 soluble or insoluble
Not one of the exceptions so it is insoluble
Dissociation equations are not written for insoluble compounds
114
Guidelines also useful in predicting what will happen if 2 solutions of 2 different soluble compounds are mixed
If mixing results in combination of ions that forms insoluble compound a double-replacement and precipitation reaction will happen
115
Will precipitate form when solutions of ammonium sulfide and cadmium nitrate are combined You can tell calcium nitrate is soluble from the guidelines (Most nitrates acetates and chlorates are soluble) You can also tell ammonium sulfide is soluble (Most sulfides are insoluble except those of calcium strontium sodium potassium and ammonium)
116
117
To decide whether precipitate can form must know solubilities of two possible products
Ammonium nitrate ndash soluble (Most nitrates acetates and chlorates are soluble) Cadmium sulfide ndash insoluble (Most sulfides are insoluble except those of calcium strontium sodium potassium and ammonium)
118
bc one product is insoluble double-replacement and precipitation reaction will happen (NH4)2S (aq) + Cd(NO3)2 (aq) rarr NH4NO3 (aq) + CdS (s)
119
Net Ionic Equations
Reactions of ions in aqueous solution usually represented by net ionic equations instead of formula equations Net ionic equation - includes only those compounds and ions that undergo a chemical change in a reaction in an aqueous solution
120
121
To write NIE (net ionic equation) first convert chemical equation into overall ionic equation All soluble ionic compounds shown as dissociated ions in solution Precipitates shown as solids
122
Notice ammonium and nitrate ions appear on both sides They havenrsquot gone through any chemical change Spectator ions - ions that do not take part in a chemical reaction and are found in solution both before and after the reaction
123
To change ionic equation to NIE eliminate spectator ions Cd+2(aq) + S-2(aq) rarr CdS(s)
This NIE applies to any reaction in which precipitate of CdS forms
124
Practice Problem
Identify the precipitate that forms when aqueous solutions of zinc nitrate and ammonium sulfide are combined Write the equation for the possible double-replacement reaction Then write the formula equation overall ionic equation and the net ionic equation
125
1 AnalyzeGiven Identity of reactants zinc nitrate and ammonium sulfide Reaction medium aqueous solution Unknown a equation for the possible double-replacement reaction b identity of the precipitate c formula equation d overall ionic equation e net ionic equation
126
2 PlanWrite the possible double-replacement reaction between Zn(NO3)2 and (NH4)2S Use the rules to determine if any of the products will precipitate Write a formula equation and overall net ionic equation Then cancel spectator ions to make net ionic equation
127
3 Computea possible double-replacement reaction
b rules show that zinc sulfide is not soluble so will precipitate (ammonium nitrate is soluble)
c formula equation128
d overall ionic equation
e net ionic equation129
Will a precipitate form if solutions of potassium sulfate and barium nitrate are combined If so write the net ionic equation for the reaction
Yes Ba+2(aq) + SO4-2(aq) rarr BaSO4(s)
130
Will a precipitate form if solutions of potassium nitrate and magnesium sulfate are combined If so write the net ionic equation for the reaction
No 131
Will a precipitate form if solutions of barium chloride and sodium sulfate are combined If so identify the spectator ions and write the net ionic equation for the reaction
Yes Na+ and Cl- Ba+2(aq) + SO4-2(aq) rarr BaSO4(s)
132
Write the net ionic equation for the precipitation of nickel(II) sulfide
Ni+2(aq) + S-2(aq) rarr NiS(s)
133
IonizationSome molecular compounds can also form ions in solution Usually polar Ionization - ions formed from solute molecules by action of the solvent (creation of ions where there were none) Ionization different from dissociation
134
Dissociation ionic compounds dissolve and ions already present separate Ionization ions formed where non existed before
Ions formed are hydrated Heat released during hydration of ions gives enough energy to break covalent bonds
135
Extent to which solute ionizes depends on strength of bonds within molecules of solute and strength of attraction between solute and solvent molecules
136
HCl ndash molecular compound that ionizes in aqueous solution Contains highly polar bond Attraction between polar HCl molecule and polar water strong enough to break HCl bond
137
The Hydronium Ion
H+ ions from HCl attracts other molecules or ions so strongly that it doesnrsquot normally exist alone Ionization of HCl better described as direct transfer of proton from HCl to H2O forming H3O+
138
139
Hydration of H+ to form hydronium is highly exothermic
Energy released gives a lot of energy needed to ionize a molecular solute
Many molecular compounds that ionize in aqueous solution contain hydrogen and form hydronium ion
140
Strong and Weak Electrolytes
Substances that make ions and conduct current in solution are electrolytes HCl is one of series of compounds that has hydrogen and halogen Hydrogen halides are all molecular compounds with single polar-covalent bonds
141
All are gases Very soluble in water All are electrolytes HCl HBr HI strongly conduct current in solution HF only weakly conducts current at same concentration Strength of conduction related to ability to ionize
142
143
Strong Electrolytes
HCl HBr HI are 100 ionized in solution
Strong electrolyte - any compound whose dilute aqueous solutions conduct electricity well this is due to the presence of all or almost all of the dissolve compound in the form of ions
144
Unique characteristic of strong electrolyte is that they yield only ions
Ex Ionic compound may be highly soluble in water and dissociate into ions (NaCl)
Others may not dissolve much but amount that does dissolve exists only as ions
145
Weak ElectrolytesHF dissolves in water to give acid solution called hydrofluoric acid HF bond stronger than bonds between hydrogen and other halogens When HF dissolves some molecule ionize Reverse reaction also happens HF(aq) + H2O(l) H3O+(aq) + F-(aq)
146
HF(aq) + H2O(l) H3O+
(aq) + F-(aq)Concentration of dissolved unionized HF stays high and concentration of ions stays low
Weak electrolyte - any compound whose aqueous solutions conduct electricity poorly this is due to the presence of small amount of the dissolved compound in the form of ions
147
Different from nonelectrolyte ndash NO ions at all Description of electrolyte as strong or weak not related to concentration of solution Electrolytes differ in degree of ionization not amount of solute dissolved
148
149
Colligative properties of solutions
Presence of solute affects properties of solutions Some properties not dependent on nature of dissolved substance but on how may dissolve particles are present
Colligative properties - properties that depend on the concentration of solute particles but not on their identity
150
Vapor-Pressure Lowering
Boiling and freezing point of solution different from pure solvent Nonvolatile solute raises boiling point and lowers freezing point Nonvolatile substance - one that has little tendency to become gas under existing conditions
151
152
To understand why nonvolatile solute changes boiling and freezing point consider equilibrium vapor pressure
Vapor pressure pressure caused by molecules that have escape liquid phase to gas phase
Can be thought of a measure of tendency of molecules to escape from a liquid
153
Addition of sucrose (nonvolatile solute) lowers concentration of water molecules at surface of liquid This lowers tendency of water molecules to leave solution and enter gas phase Vapor pressure of solution is lower than vapor pressure of pure water
154
Aqueous13 solution13 of13 nonvolatile13 solutePure13 water
Nonelectrolyte solutions of same molality have same concentration of particles
Dilute solutions of same solvent and equal molality of any nonelectrolyte solute lower vapor pressure equally
155
Example
1 m aqueous solution of nonelectrolyte glucose lowers vapor pressure of water 55 x 10-4 atm at 25 1 m aqueous solution of sucrose also lowers water vapor pressure to 55 x 10-4 atm at 25
156
157
bc vapor-pressure lowering depends on concentration of nonelectrolyte solute and doesnrsquot depend on the type of solute it is a colligative proprerty
bc vapor pressure has been lowered solution remains liquid over larger temperature range
Lowers freezing point and raises boiling point
158
Can assume changes in boiling and freezing point also depend on concentration of solute
They are colligative properties
159
Freezing-Point Depression
Freezing point of 1-molal solution of any nonelectrolyte solute in water is found (by experiment) to be 186 lower than freezing point of water When 1 mol of nonelectrolyte solute dissolved in 1 kg water freezing point is -186 not 000 160
When 2 mol nonelectrolyte solute dissolved in 1 kg water freezing point is -372
For any concentration of nonelectrolyte solute in water decrease in freezing point can be determined using value of -186m
161
This value called the molal freezing-point contstant (Kf) is the freezing-point depression of the solvent in a 1-molal solution of a nonvolatile nonelectrolyte solute
162
Each solvent has own characteristic molal freezing-point constant Freezing-point depression (Δtf) the difference between the freezing points of the pure solvent and a solute of nonelectrolyte in that solvent and it is directly proportional to the molal concentration of the solution Δtf = Kfm
163
Δtf = Kfm
Kf expressed as m
m expressed in mol solutekg solvent (molality)
164
Sample Problem
What is the freezing point depression of water in a solution of 171 g of sucrose C12H22O11 and 200 g of water What is the actual freezing point of the solution
165
1 AnalyzeGiven Solute mass and chemical formula 171 g C12H22O11 Solvent mass and identity 2000 g water Unknown a freezing-point depression b freezing point of the solution 166
2 Plan
167
168
Calculate
Unknown a freezing-point depression b freezing point of the solution
169
Practice Problem
A water solution containing an unknown quantity of a nonelectrolyte solute is found to have a freezing point of ndash 023degCWhat is the molal concentration of the solution
012 m170
A solution consists of 103 g of the nonelectrolyte glucose C6H12O6 dissolved in 250 g of water What is the freezing-point depression of the solution
minus0426degC171
In a laboratory experiment the freezing point of an aqueous solution of glucose is found to be minus0325degCWhat is the molal concentration of this solution
0175 m172
If 0500 mol of a nonelectrolyte solute are dissolved in 5000 g of ether what is the freezing point of the solution
minus1181degC173
The freezing point of an aqueous solution that contains a nonelectrolyte is minus90degC aWhat is the freezing-point depression of the solution bWhat is the molal concentration of the solution a minus90degC b 48 m
174
Boiling-Point Elevation
Boiling point is temp at which vapor pressure is equal to atmospheric pressure Change in vapor pressure will cause corresponding change in boiling point
175
176
Vapor pressure of nonvolatile solution is lower than vapor pressure of pure solvent More heat will be required to raise vapor pressure of solution Boiling point of solution is higher than boiling point of pure solvent
Molal boiling-point constant (Kb) - boiling-point elevation of the solvent in a 1-molal solution of a nonvolatile nonelectrolyte solute b-p elevation of any nonelectrolyte in water found by experiment to be 051 So molal b-p constant for water is 051m
177
For different solvents b-p elevations of 1-molal solutions have different values (Table 14-2)
Like freezing-point constants values most accurate for dilute solutions
178
Boiling-point elevation Δtb difference between the boiling points of the pure solvent and a nonelectrolyte solution of that solvent and it is directly proportional to the molal concentration of the solution B-p elevation can be calculated using Δtb = Kbm Δtb (boiling point elevation) expressed in m and m expressed in mol solutekg solvent
Sample Problem
What is the boiling-point elevation of a solution made from 200 g of a nonelectrolyte solute and 4000 g of water The molar mass of the solute is 620 gmol
180
1 AnalyzeGiven solute mass = 200 g solute molar mass = 620 gmol solvent mass and identity = 4000 g of water
Unknown boiling-point elevation
181
2 Plan
182
183
3 Compute
Practice Problem
A solution contains 500 g of sucrose C12H22O11 a nonelectrolyte dissolved in 5000 g of water What is the boiling-point elevation
015degC184
A solution contains 4500 g of sucrose C12H22O11 a nonelectrolyte dissolved in 250 g of waterWhat is the boiling point of the solution
1027degC185
If the boiling point elevation of an aqueous solution containing a nonvolatile electrolyte is 102degC what is the molality of the solution
20 m186
The boiling point of an aqueous solution containing a nonvolatile electrolyte is 10075degC aWhat is the boiling-point elevation bWhat is the molality of the solution a 075degC b 15 m
187
Electrolytes and Colligative PropertiesEarly investigators confused by experiments where certain substances depressed freezing point or elevated boiling point more than expected
188
189
Ex 01 m solution of NaCl lowers freezing point of solvent almost twice as much as 01 m solution of sucrose 01 m CaCl2 lowers fp almost 3 times as much as 01 m solution of sucrose
To understand why this happens contrast behavior or sucrose with NaCl in aqueous solution Each sucrose dissolves to make only 1 particle in solution So 1 mol sucrose dissolves to 1 mol particles 1 mol NaCl dissolves to make 2 moles of particles (1 mol Na+ and 1 mol Cl-)
190
191
Calculated Values for Electrolyte SolutionsColligative properties depend on total concentration of solute particles regardless of their identity Electrolytes cause changes in colligative properties proportional to total molality in terms of all dissolves particles instead of formula units
192
What about barium nitrate Ba(NO3)2 Each mole of barium nitrate yields 3 mol of ions in solution Ba(NO3)2(s) rarr Ba2+(aq) + 2NO3-(aq) You would expect solution of given molality to lower fp of solvent 3 times as much as nonelectrolytic solution of same molality
193
Sample Problem
What is the expected change in the freezing point of water in a solution of 625 g of barium nitrate Ba(NO3)2 in 100 kg of water
194
1 AnalyzeGiven solute mass and formula = 625 g Ba(NO3)2 solvent mass and identity = 100 kg water Δtf = Kfm
Unknown expected freezing-point depression
195
2 PlanThe molality can be calculated by converting the solute mass to moles and then dividing by the number of kilograms of solvent That molality is in terms of formula units of Ba(NO3)2 and must be converted to molality in terms of dissociated ions in solution It must be multiplied by the number of moles of ions produced per mole of formula unit This adjusted molality can then be used to calculate the freezing-point depression
196
197
198
3 Compute
Practice Problem
What is the expected freezing-point depression for a solution that contains 20 mol of magnesium sulfate dissolved in 10 kg of water
minus74degC199
What is the expected boiling-point elevation of water for a solution that contains 150 g of sodium chloride dissolved in 10 kg of water 27degC The freezing point of an aqueous sodium chloride solution is minus200degC What is the molality of the solution 54 m NaCl
200
Actual Values for Electrolyte SolutionsRemember values calculated are only expected values Actual values of colligative properties for all strong electrolytes are almost what would be expected Difference between expected and actual values caused by attractive forces that exist between dissociated ions in aqueous solution
201
Attraction between hydrated ions in solution is small compared with those in crystalline solid However forces of attraction donrsquot interfere with movements of aqueous ions More concentrated solution means ions closer together and attraction is greater Also depends on charge of ions ndash higher charge greater attraction
202
Emulsion and foam are colloids
Mayonnaise is emulsion of oil droplets in water Egg yolk acts as emulsifying agent (keeps oil droplets dispersed)
16
17
18
Class13 of13 Colloid Phases Example
Sol Solid13 dispersed13 in13 liquid Paints13 mud
Gel Solid13 network13 extending13 through13 liquid
Gelatin
Liquid13 emulsion Liquid13 dispersed13 in13 liquid Milk13 mayonnaise
Foam Gas13 dispersed13 in13 liquid Shaving13 cream13 whipped13 cream
Solid13 aerosol Solid13 dispersed13 in13 gas Smoke13 auto13 exhaust
Liquid13 aerosol Liquid13 dispersed13 in13 gas Fog13 mist13 clouds13 aerosol13 spray
Solid13 emulsion Liquid13 dispersed13 in13 solid Cheese13 butter
Tyndall EffectMany colloids appear homogeneous bc individual particles cannot be seen Particles large enough to scatter light In fog can see headlight beam Tyndall effect - occurs when light is scattered by colloidal particles dispersed in transparent medium
19
20
21
Solutions Colloids Suspensions
Homogeneous Heterogeneous Heterogeneous
Particle13 size13 001-shy‐113 nm13 can13 be13 atoms13 ions13 molecules
Particle13 size13 1-shy‐100013 nm13 dispersed13 can13 be13 combined13 or13 large13 molecules
Particle13 size13 over13 100013 nm13 suspended13 can13 be13 large13 particles13 or13 combined13 particles
Do13 not13 separate13 on13 standing Do13 not13 separate13 on13 standing Particles13 settle13 out
Cannot13 be13 separated13 by13 filtration Cannot13 be13 separated13 by13 filtration Can13 be13 separated13 by13 filtration
Do13 not13 scatter13 light Scatter13 light13 (Tyndall13 effect) May13 scatter13 light13 but13 are13 not13 transparent
Electrolytes vs Nonelectrolytes
Substances that dissolve in water are classified according to whether they produce molecules or ions in solution
22
23
When ionic compound dissolves cations and anions separate and are surrounded by water molecules Solute ions free to move making possible to carry electric current
Electrolyte - substance that dissolves in water to give a solution that conducts electric current NaCl ndash electrolyte Usually highly polar compounds become electrolytes
24
Solution containing neutral solute molecules does not conduct current bc it doesnrsquot have mobile charged particles
Nonelectrolyte - substance that dissolves in water to give solution that does not conduct electric current Ex Sugar
25
26
27the solution
process
Factors Affecting Rate of DissolutionEver tried to dissolve sugar in iced tea or coffee You notice temperature has something to do with how quickly solute dissolves What other factors affect how quickly solutes dissolve
28
1 Increasing Surface Area of Solute
bc dissolution process occurs at surface of solute Can increase dissolution by increasing surface area
29
2 Agitating (Stirring) SolutionClose to surface of solute concentration of dissolved solute is high Stirringshaking helps disperse solute particles
30
Brings fresh solvent into contact with solute surface Contact between solvent and solute surface increased
3 Heating a Solvent
Solutes dissolve faster in warmer solvent As temp increases solvent molecules move faster Average kinetic energy increases Collisions between solvent and solute more frequent Helps separate solute molecules and spread them out
32
Solubility
If you add sugar to tea eventually no more sugar dissolves
For every combination of solvent with solute at given temp there is limit to amount of solute that can be dissolved
33
When sugar first dropped into water sugar molecules leave solid surface and move about at random in solvent Some dissolved molecules may collide with crystal and stay there As more solid dissolves and concentration of dissolved molecules increases collisions happen more often
34
Eventually molecules are returning to crystal at same rate they are going into solution Dynamic equilibrium established between dissolution and crystallization Solution equilibrium - physical state in which the opposing processes of dissolution and crystallization of a solute occur at equal rates
35
Saturated vs Unsaturated Solutions
Saturated solution - solution that contains the maximum amount of dissolved solute How do you know if solution is saturated If more NaCl added it falls to bottom and doesnrsquot dissolve Equilibrium already established
36
If more water added to saturated solution more NaCl will dissolve At 20 359 g NaCl is max that will dissolve in 100 g water Unsaturated solution - solution that contains less solute than a saturated solution under the existing conditions
37
Supersaturated Solutions
When saturated solution where solubility increases as temp increases is cooled excess solute usually comes out of solution Sometimes if solution is left to cool the excess solute doesnrsquot separate
38
39
Supersaturated solution - solution that contains more dissolved solute than a saturated solution contains under same conditions
40
Supersaturated solution can remain unchanged over long time if it isnrsquot disturbed Once crystals begin to form continues until equilibrium is recreated at lower temp
41
Example
Sodium thiosulfate Na2S2O3 Solute added to hot water until solution saturated Hot solution filtered Drop small crystal in ldquoseedingrdquo crystallization Continues until equilibrium created
42
Solubility Values
Solubility - amount of substance required to form a saturated solution with specific amount of solvent at specified temp Ex Solubility of sugar = 204 g per 100 g water at 20 Must specify temp bc solubility changes with temp 43
For gases pressure must also be specified Rate at which substance dissolves is unrelated to solubility Max amount of solute that dissolves and reaches equilibrium is always same under same conditions
44
Solute-Solvent Interactions
ldquolike dissolves likerdquo is generally useful rule for predicting whether one substance will dissolve in another Depends on type of bonding polaritynonpolarity and intermolecular forces between solute and solvent
45
Dissolving Ionic Compounds in Aqueous Solution
Polarity of water molecules plays important role in formation of solution of ionic compounds Charged ends of water molecules attract ions and surround them to keep separated from other ions
46
Hydration - solution process with water as solvent Ions said to be hydrated As hydrated ions diffuse into solution other ions exposed and drawn away from crystal surface Entire crystal gradually dissolves
47
When crystallized from aqueous solutions some ionic compounds form crystals that include water molecules The crystalline compounds known as hydrates have specific ratios of water molecules Represented by formulas like CuSO45H2O
48
When crystalline hydrate dissolves in water water of hydration returns to solvent Behavior of solute in hydrated form no different than anhydrous form
49
Nonpolar Solvents
Nonpolar solvents do not attract ions of ionic compound strongly enough to overcome forces holding crystal together
50
Liquid Solutes and Solvents
When you shake bottle of salad dressing oil droplets disperse in water Stop shaking ndash strong attraction between water molecules squeeze out oil and form separate layers Liquid solutes and solvents that are not soluble in each other are immiscible
51
Nonpolar substances (fat oil grease) are generally quite soluble in nonpolar liquids (carbon tetrachloride toluene gasoline)
Only attractions between nonpolar molecules are weak London forces
52
London Dispersion Forces
bc e- are constantly moving at any time there may be uneven distribution of them around a nonpolar atom Temporarily creates positive end and negative end
53
54
London dispersion forces - intermolecular attractions resulting from constant motion of e- and creation of instantaneous dipoles
55
Liquids that dissolve freely in one another in any ratio are said to be completely miscible Benzene and carbon tetrachloride (both nonpolar) are completely miscible
Nonpolar molecules of these two apply no strong forces of attraction or repulsion and molecules mix freely
56
Ethanol and water are also completely miscible -OH group on ethanol is slightly polar
Can form H-bonds with water as well as other ethanol molecules Intermolecular forces in mixture are very similar to those in pure liquids so they are mutually soluble in any amount
57
Gasoline contains mainly nonpolar hydrocarbons
Excellent solvent for fats oils greases
Major intermolecular forces are weak London forces
58
Ethanol less polar than water more than carbon tetrachloride
Better solvent for less-polar substances bc of nonpolar region
59
Effects of Pressure on Solubility
Little effect on solids or liquids Increase pressure - increase solubilities of gases in liquids
60
When gas in contact with surface of liquid gas molecules can enter liquid As amount of dissolve gas increases some molecules start to escape and reenter gas phase
Eq eventually established between rates of enteringleaving gas phase
61
As long as eq undisturbed solubility of gas in liquid unchanged at given pressure
Gas + solvent solution
62
Increasing pressure of solute gas above solution puts stress on eq Molecules collide with surface more often Increase in pressure partially offset by increase in rate of gas molecules entering solution In turn increase in amount of dissolved gas causes increase in rate molecules escape liquid and becomes gas
63
Eventually eq restored at higher gas solubility
Expected from Le Chatelierrsquos principle
Increase in gas pressure causes eq to shift so fewer molecules are in gas phase
64
Henryrsquos Law
Henryrsquos law - solubility of a gas in liquid is directly proportional to the partial pressure of that gas on the surface of the liquid
Applies to gas-liquid solutions at constant temp
65
When mixture of ideal gases is limited in constant volume at constant temp each gas applies same pressure it would apply if it occupied the space alone
Assuming gases do not react each gas dissolves to the extent it would if no other gases were there
66
In carbonated drinks (Coke Pepsi etc) solubility of CO2 increased by increasing pressure
At bottling factory CO2 gas forced into solution of flavored water at pressure of 5-10 atm Gas-in-liquid solution then sealed in bottlescans
67
When cap removed pressure reduced to 1 atm and some CO2 escapes as gas bubbles Effervescence - rapid escape of a gas from a liquid in which it is dissolved
68
Effects of Temperature on Solubility
Letrsquos consider gas solubility Increasing temperature usually decreases gas solubility As temp increases average kinetic energy of molecules in solution increases Greater number of solute molecules escape from attraction of solvent and return to gas phase 69
At higher temps eq is reached with fewer gas molecules in solution Gases generally less soluble
70
Effect of temp on solubility of solids in liquids more difficult to predict
Often increasing temp increases solubility
However the same temp increase can result in large increase in solubility in one case and only slight increase in the next
71
72
enthalpies of Solution
Formation of solution comes with energy change Dissolve KI in water container feels cold Dissolve LiCl in water feels hot Formation of solid-liquid solution can absorb or release heat
73
During formation of solution solvent and solute particles experience changes in forces attracting them to other particles Before dissolving begins solvent molecules held by intermolecular forces (solvent-solvent attraction) In solute molecules held by Ifs (solute-solute attraction)
74
Energy is required to separate solute molecules and solvent molecules from their neighbors
A solute particle that is surrounded by solvent molecules is said to be solvated
75
Step 1 solute particles become separated from solid (energy absorbed) Step 2 solvent particles move apart to allow solute particles to enter liquid (energy absorbed)
76
Step 3 solvent particles attracted to and solvate solute particles (energy released)
77
Heat of solution - net amount of heat energy absorbed or released when a specific amount of solute dissolves in a solvent
Heat of solution negative (heat is released) if sums of energy in steps 1 and 2 is less than step 3
If 1+2 gt 3 HOS is positive (heat absorbed)78
In gaseous state molecules are so far apart there are nearly no forces between them Solute-solute interaction has little effect on heat of solution of a gas Energy released when gas dissolved in liquid bc attraction between solute gas and solvent molecules greater than energy needed to separate solvent molecules
79
Concentration of Solutions80
Concentration - measure of amount of solute in given amount of solvent or solution ldquodiluterdquo just means small amount of solute ldquoconcentratedrdquo just means relatively large amount of solute Unrelated to degree to which solution is saturated Ex Saturated solution of substance not very soluble might be dilute
81
Molarity
Molarity - number of moles of solute in one liter of solution To find molarity must know molar mass A ldquoone molarrdquo solution of NaOH is 1 mole NaOH per liter of solution (1 M)
82
1 mol NaOH = 400 g If this quantity dissolve in enough water to make EXACTLY 100 L solution solution is 1 M If 200 g NaOH dissolve in enough to make 100 L solution what is molarity 0500 M
83
1 M solution is not made by adding 1 mol solute to 1 L solvent That would be more than 1 L Instead 1 mol solute dissolved in less than 1 L Then diluted with more solvent to bring TOTAL VOLUME to 1 L
84
Practice ProblemYou have 340 L of solution that contains 900 g sodium chloride What is the molarity of that solution 0440 M NaCl You have 08 L of a 05 M HCl solution How many moles of HCl does this solution contain 04 mol HCl
85
What is the molarity of a solution composed of 585 g of potassium iodide dissolved in enough water to make a 0125 L of solution 0282 M KI
86
How many moles of H2SO4 are present in 0500 L of a 0150 M H2SO4 solution 00750 mol What volume of 300 M NaCl is needed for reaction that requires 1463 g of NaCl 0834 L
87
Molality
Molality - concentration of solution expressed in moles of solute per kilogram of solvent Solution that contains 1 mol solute (NaOH) dissolved in exactly 1 kg of solvent is ldquoone molalrdquo solution (1 m NaOH)
88
1 mol NaOH = 400 g 400 g NaOH dissolved in 1 kg water is 1 m NaOH
200 g NaOH in 1 kg water = 0500 m NaOH
89
Practice Problem
A solution was prepared by dissolving 171 g sucrose (C12H22O11) in 125 g of water Find the molal concentration of this solution 0400 m C12H22O11
90
A solution of iodine in carbon tetrachloride is used when iodine is needed for certain chemical tests How much iodine must be added to prepare a 0480 m solution of iodine in CCl4 if 1000 g of CCl4 is used 122 g I2
91
What is the molality of a solution composed of 255 g acetone (CH3)2CO dissolved in 200 g of water 22 m acetone What quantity in grams of methanol CH3OH is required to prepare a 0244 m solution in 400 g water 312 g methanol
92
How many grams of AgNO3 are needed to prepare a 0125 m solution in 250 mL of water 531 g AgNO3
What is the molality of a solution containing 182 g HCl and 250 g water 200 m
93
Ions in Aqueous Solutions and Colligative Properties
94
95
compounds in aqueous solutions
DissociationWhen compound made from ions dissolves in water ions separate Dissociation - separation of ions that occurs when an ionic compound dissolves
96
Notice number of ions made per formula unit in equations 1 formula unit of NaCl gives 2 units of ions in solution 1 formula unit of CaCl2 gives 3 units of ions
97
Assuming 100 dissociation solution that contains 1 mol NaCl contains 1 mol Na+ and 1 mol Cl- Can assume 100 dissociation of all soluble ionic compounds
98
Practice Problem
Write the equation for the dissolution of aluminum sulfate in water How many moles of aluminum ions are made by dissolving 1 mol aluminum sulfate What is the total number of moles of ions made by dissolving 1 mol of aluminum sulfate
99
1 AnalyzeGiven Amount of solute = 1 mol Al2(SO4)3 Solvent identity = water Unknown a moles of aluminum ions and sulfate ions Total number of moles of solute ions produced
100
2 Plan
The coefficients in the balanced dissociation equation will tell mole relationships You can use the equation to find out the number of moles of solute ions produced
101
3 Compute
a
b102
Write the equation for the dissolution of each of the following in water and then determine the number of moles of each ions made as well as the total number of ions made a 1 mol ammonium chloride b 1 mol sodium sulfide c 05 mol barium nitrate
103
a 1 mol ammonium chloride
1 mol NH4+ 1 mol Cl- 2 mol total ions
104
b 1 mol sodium sulfide
2 mol Na+ 1 mol S-2 3 mol total ions
105
c 05 mol barium nitrate
05 mol Ba+2 1 mol NO3- 15 mol total ions
106
Precipitation Reactions
Even though no compound is completely insoluble compounds of very low solubility can be considered insoluble for practical purposes There are general guidelines to help predict whether a compound made of certain combination of ions is soluble
107
General Solubility Guidelines
Sodium potassium and ammonium compounds are soluble in water
108
Nitrates acetates and chlorates are soluble
109
110
Most chlorides are soluble except those of silver mercury (I) and lead Lead (II) chloride is soluble in hot water
Most sulfates are soluble except those of barium strontium lead calcium and mercury
111
Most carbonates phosphates and silicates are insoluble except those of sodium potassium and ammonium
112
Most sulfides are insoluble except those of calcium strontium sodium potassium and ammonium
113
Is calcium phosphate Ca3(PO4)2 soluble or insoluble
Not one of the exceptions so it is insoluble
Dissociation equations are not written for insoluble compounds
114
Guidelines also useful in predicting what will happen if 2 solutions of 2 different soluble compounds are mixed
If mixing results in combination of ions that forms insoluble compound a double-replacement and precipitation reaction will happen
115
Will precipitate form when solutions of ammonium sulfide and cadmium nitrate are combined You can tell calcium nitrate is soluble from the guidelines (Most nitrates acetates and chlorates are soluble) You can also tell ammonium sulfide is soluble (Most sulfides are insoluble except those of calcium strontium sodium potassium and ammonium)
116
117
To decide whether precipitate can form must know solubilities of two possible products
Ammonium nitrate ndash soluble (Most nitrates acetates and chlorates are soluble) Cadmium sulfide ndash insoluble (Most sulfides are insoluble except those of calcium strontium sodium potassium and ammonium)
118
bc one product is insoluble double-replacement and precipitation reaction will happen (NH4)2S (aq) + Cd(NO3)2 (aq) rarr NH4NO3 (aq) + CdS (s)
119
Net Ionic Equations
Reactions of ions in aqueous solution usually represented by net ionic equations instead of formula equations Net ionic equation - includes only those compounds and ions that undergo a chemical change in a reaction in an aqueous solution
120
121
To write NIE (net ionic equation) first convert chemical equation into overall ionic equation All soluble ionic compounds shown as dissociated ions in solution Precipitates shown as solids
122
Notice ammonium and nitrate ions appear on both sides They havenrsquot gone through any chemical change Spectator ions - ions that do not take part in a chemical reaction and are found in solution both before and after the reaction
123
To change ionic equation to NIE eliminate spectator ions Cd+2(aq) + S-2(aq) rarr CdS(s)
This NIE applies to any reaction in which precipitate of CdS forms
124
Practice Problem
Identify the precipitate that forms when aqueous solutions of zinc nitrate and ammonium sulfide are combined Write the equation for the possible double-replacement reaction Then write the formula equation overall ionic equation and the net ionic equation
125
1 AnalyzeGiven Identity of reactants zinc nitrate and ammonium sulfide Reaction medium aqueous solution Unknown a equation for the possible double-replacement reaction b identity of the precipitate c formula equation d overall ionic equation e net ionic equation
126
2 PlanWrite the possible double-replacement reaction between Zn(NO3)2 and (NH4)2S Use the rules to determine if any of the products will precipitate Write a formula equation and overall net ionic equation Then cancel spectator ions to make net ionic equation
127
3 Computea possible double-replacement reaction
b rules show that zinc sulfide is not soluble so will precipitate (ammonium nitrate is soluble)
c formula equation128
d overall ionic equation
e net ionic equation129
Will a precipitate form if solutions of potassium sulfate and barium nitrate are combined If so write the net ionic equation for the reaction
Yes Ba+2(aq) + SO4-2(aq) rarr BaSO4(s)
130
Will a precipitate form if solutions of potassium nitrate and magnesium sulfate are combined If so write the net ionic equation for the reaction
No 131
Will a precipitate form if solutions of barium chloride and sodium sulfate are combined If so identify the spectator ions and write the net ionic equation for the reaction
Yes Na+ and Cl- Ba+2(aq) + SO4-2(aq) rarr BaSO4(s)
132
Write the net ionic equation for the precipitation of nickel(II) sulfide
Ni+2(aq) + S-2(aq) rarr NiS(s)
133
IonizationSome molecular compounds can also form ions in solution Usually polar Ionization - ions formed from solute molecules by action of the solvent (creation of ions where there were none) Ionization different from dissociation
134
Dissociation ionic compounds dissolve and ions already present separate Ionization ions formed where non existed before
Ions formed are hydrated Heat released during hydration of ions gives enough energy to break covalent bonds
135
Extent to which solute ionizes depends on strength of bonds within molecules of solute and strength of attraction between solute and solvent molecules
136
HCl ndash molecular compound that ionizes in aqueous solution Contains highly polar bond Attraction between polar HCl molecule and polar water strong enough to break HCl bond
137
The Hydronium Ion
H+ ions from HCl attracts other molecules or ions so strongly that it doesnrsquot normally exist alone Ionization of HCl better described as direct transfer of proton from HCl to H2O forming H3O+
138
139
Hydration of H+ to form hydronium is highly exothermic
Energy released gives a lot of energy needed to ionize a molecular solute
Many molecular compounds that ionize in aqueous solution contain hydrogen and form hydronium ion
140
Strong and Weak Electrolytes
Substances that make ions and conduct current in solution are electrolytes HCl is one of series of compounds that has hydrogen and halogen Hydrogen halides are all molecular compounds with single polar-covalent bonds
141
All are gases Very soluble in water All are electrolytes HCl HBr HI strongly conduct current in solution HF only weakly conducts current at same concentration Strength of conduction related to ability to ionize
142
143
Strong Electrolytes
HCl HBr HI are 100 ionized in solution
Strong electrolyte - any compound whose dilute aqueous solutions conduct electricity well this is due to the presence of all or almost all of the dissolve compound in the form of ions
144
Unique characteristic of strong electrolyte is that they yield only ions
Ex Ionic compound may be highly soluble in water and dissociate into ions (NaCl)
Others may not dissolve much but amount that does dissolve exists only as ions
145
Weak ElectrolytesHF dissolves in water to give acid solution called hydrofluoric acid HF bond stronger than bonds between hydrogen and other halogens When HF dissolves some molecule ionize Reverse reaction also happens HF(aq) + H2O(l) H3O+(aq) + F-(aq)
146
HF(aq) + H2O(l) H3O+
(aq) + F-(aq)Concentration of dissolved unionized HF stays high and concentration of ions stays low
Weak electrolyte - any compound whose aqueous solutions conduct electricity poorly this is due to the presence of small amount of the dissolved compound in the form of ions
147
Different from nonelectrolyte ndash NO ions at all Description of electrolyte as strong or weak not related to concentration of solution Electrolytes differ in degree of ionization not amount of solute dissolved
148
149
Colligative properties of solutions
Presence of solute affects properties of solutions Some properties not dependent on nature of dissolved substance but on how may dissolve particles are present
Colligative properties - properties that depend on the concentration of solute particles but not on their identity
150
Vapor-Pressure Lowering
Boiling and freezing point of solution different from pure solvent Nonvolatile solute raises boiling point and lowers freezing point Nonvolatile substance - one that has little tendency to become gas under existing conditions
151
152
To understand why nonvolatile solute changes boiling and freezing point consider equilibrium vapor pressure
Vapor pressure pressure caused by molecules that have escape liquid phase to gas phase
Can be thought of a measure of tendency of molecules to escape from a liquid
153
Addition of sucrose (nonvolatile solute) lowers concentration of water molecules at surface of liquid This lowers tendency of water molecules to leave solution and enter gas phase Vapor pressure of solution is lower than vapor pressure of pure water
154
Aqueous13 solution13 of13 nonvolatile13 solutePure13 water
Nonelectrolyte solutions of same molality have same concentration of particles
Dilute solutions of same solvent and equal molality of any nonelectrolyte solute lower vapor pressure equally
155
Example
1 m aqueous solution of nonelectrolyte glucose lowers vapor pressure of water 55 x 10-4 atm at 25 1 m aqueous solution of sucrose also lowers water vapor pressure to 55 x 10-4 atm at 25
156
157
bc vapor-pressure lowering depends on concentration of nonelectrolyte solute and doesnrsquot depend on the type of solute it is a colligative proprerty
bc vapor pressure has been lowered solution remains liquid over larger temperature range
Lowers freezing point and raises boiling point
158
Can assume changes in boiling and freezing point also depend on concentration of solute
They are colligative properties
159
Freezing-Point Depression
Freezing point of 1-molal solution of any nonelectrolyte solute in water is found (by experiment) to be 186 lower than freezing point of water When 1 mol of nonelectrolyte solute dissolved in 1 kg water freezing point is -186 not 000 160
When 2 mol nonelectrolyte solute dissolved in 1 kg water freezing point is -372
For any concentration of nonelectrolyte solute in water decrease in freezing point can be determined using value of -186m
161
This value called the molal freezing-point contstant (Kf) is the freezing-point depression of the solvent in a 1-molal solution of a nonvolatile nonelectrolyte solute
162
Each solvent has own characteristic molal freezing-point constant Freezing-point depression (Δtf) the difference between the freezing points of the pure solvent and a solute of nonelectrolyte in that solvent and it is directly proportional to the molal concentration of the solution Δtf = Kfm
163
Δtf = Kfm
Kf expressed as m
m expressed in mol solutekg solvent (molality)
164
Sample Problem
What is the freezing point depression of water in a solution of 171 g of sucrose C12H22O11 and 200 g of water What is the actual freezing point of the solution
165
1 AnalyzeGiven Solute mass and chemical formula 171 g C12H22O11 Solvent mass and identity 2000 g water Unknown a freezing-point depression b freezing point of the solution 166
2 Plan
167
168
Calculate
Unknown a freezing-point depression b freezing point of the solution
169
Practice Problem
A water solution containing an unknown quantity of a nonelectrolyte solute is found to have a freezing point of ndash 023degCWhat is the molal concentration of the solution
012 m170
A solution consists of 103 g of the nonelectrolyte glucose C6H12O6 dissolved in 250 g of water What is the freezing-point depression of the solution
minus0426degC171
In a laboratory experiment the freezing point of an aqueous solution of glucose is found to be minus0325degCWhat is the molal concentration of this solution
0175 m172
If 0500 mol of a nonelectrolyte solute are dissolved in 5000 g of ether what is the freezing point of the solution
minus1181degC173
The freezing point of an aqueous solution that contains a nonelectrolyte is minus90degC aWhat is the freezing-point depression of the solution bWhat is the molal concentration of the solution a minus90degC b 48 m
174
Boiling-Point Elevation
Boiling point is temp at which vapor pressure is equal to atmospheric pressure Change in vapor pressure will cause corresponding change in boiling point
175
176
Vapor pressure of nonvolatile solution is lower than vapor pressure of pure solvent More heat will be required to raise vapor pressure of solution Boiling point of solution is higher than boiling point of pure solvent
Molal boiling-point constant (Kb) - boiling-point elevation of the solvent in a 1-molal solution of a nonvolatile nonelectrolyte solute b-p elevation of any nonelectrolyte in water found by experiment to be 051 So molal b-p constant for water is 051m
177
For different solvents b-p elevations of 1-molal solutions have different values (Table 14-2)
Like freezing-point constants values most accurate for dilute solutions
178
Boiling-point elevation Δtb difference between the boiling points of the pure solvent and a nonelectrolyte solution of that solvent and it is directly proportional to the molal concentration of the solution B-p elevation can be calculated using Δtb = Kbm Δtb (boiling point elevation) expressed in m and m expressed in mol solutekg solvent
Sample Problem
What is the boiling-point elevation of a solution made from 200 g of a nonelectrolyte solute and 4000 g of water The molar mass of the solute is 620 gmol
180
1 AnalyzeGiven solute mass = 200 g solute molar mass = 620 gmol solvent mass and identity = 4000 g of water
Unknown boiling-point elevation
181
2 Plan
182
183
3 Compute
Practice Problem
A solution contains 500 g of sucrose C12H22O11 a nonelectrolyte dissolved in 5000 g of water What is the boiling-point elevation
015degC184
A solution contains 4500 g of sucrose C12H22O11 a nonelectrolyte dissolved in 250 g of waterWhat is the boiling point of the solution
1027degC185
If the boiling point elevation of an aqueous solution containing a nonvolatile electrolyte is 102degC what is the molality of the solution
20 m186
The boiling point of an aqueous solution containing a nonvolatile electrolyte is 10075degC aWhat is the boiling-point elevation bWhat is the molality of the solution a 075degC b 15 m
187
Electrolytes and Colligative PropertiesEarly investigators confused by experiments where certain substances depressed freezing point or elevated boiling point more than expected
188
189
Ex 01 m solution of NaCl lowers freezing point of solvent almost twice as much as 01 m solution of sucrose 01 m CaCl2 lowers fp almost 3 times as much as 01 m solution of sucrose
To understand why this happens contrast behavior or sucrose with NaCl in aqueous solution Each sucrose dissolves to make only 1 particle in solution So 1 mol sucrose dissolves to 1 mol particles 1 mol NaCl dissolves to make 2 moles of particles (1 mol Na+ and 1 mol Cl-)
190
191
Calculated Values for Electrolyte SolutionsColligative properties depend on total concentration of solute particles regardless of their identity Electrolytes cause changes in colligative properties proportional to total molality in terms of all dissolves particles instead of formula units
192
What about barium nitrate Ba(NO3)2 Each mole of barium nitrate yields 3 mol of ions in solution Ba(NO3)2(s) rarr Ba2+(aq) + 2NO3-(aq) You would expect solution of given molality to lower fp of solvent 3 times as much as nonelectrolytic solution of same molality
193
Sample Problem
What is the expected change in the freezing point of water in a solution of 625 g of barium nitrate Ba(NO3)2 in 100 kg of water
194
1 AnalyzeGiven solute mass and formula = 625 g Ba(NO3)2 solvent mass and identity = 100 kg water Δtf = Kfm
Unknown expected freezing-point depression
195
2 PlanThe molality can be calculated by converting the solute mass to moles and then dividing by the number of kilograms of solvent That molality is in terms of formula units of Ba(NO3)2 and must be converted to molality in terms of dissociated ions in solution It must be multiplied by the number of moles of ions produced per mole of formula unit This adjusted molality can then be used to calculate the freezing-point depression
196
197
198
3 Compute
Practice Problem
What is the expected freezing-point depression for a solution that contains 20 mol of magnesium sulfate dissolved in 10 kg of water
minus74degC199
What is the expected boiling-point elevation of water for a solution that contains 150 g of sodium chloride dissolved in 10 kg of water 27degC The freezing point of an aqueous sodium chloride solution is minus200degC What is the molality of the solution 54 m NaCl
200
Actual Values for Electrolyte SolutionsRemember values calculated are only expected values Actual values of colligative properties for all strong electrolytes are almost what would be expected Difference between expected and actual values caused by attractive forces that exist between dissociated ions in aqueous solution
201
Attraction between hydrated ions in solution is small compared with those in crystalline solid However forces of attraction donrsquot interfere with movements of aqueous ions More concentrated solution means ions closer together and attraction is greater Also depends on charge of ions ndash higher charge greater attraction
202
17
18
Class13 of13 Colloid Phases Example
Sol Solid13 dispersed13 in13 liquid Paints13 mud
Gel Solid13 network13 extending13 through13 liquid
Gelatin
Liquid13 emulsion Liquid13 dispersed13 in13 liquid Milk13 mayonnaise
Foam Gas13 dispersed13 in13 liquid Shaving13 cream13 whipped13 cream
Solid13 aerosol Solid13 dispersed13 in13 gas Smoke13 auto13 exhaust
Liquid13 aerosol Liquid13 dispersed13 in13 gas Fog13 mist13 clouds13 aerosol13 spray
Solid13 emulsion Liquid13 dispersed13 in13 solid Cheese13 butter
Tyndall EffectMany colloids appear homogeneous bc individual particles cannot be seen Particles large enough to scatter light In fog can see headlight beam Tyndall effect - occurs when light is scattered by colloidal particles dispersed in transparent medium
19
20
21
Solutions Colloids Suspensions
Homogeneous Heterogeneous Heterogeneous
Particle13 size13 001-shy‐113 nm13 can13 be13 atoms13 ions13 molecules
Particle13 size13 1-shy‐100013 nm13 dispersed13 can13 be13 combined13 or13 large13 molecules
Particle13 size13 over13 100013 nm13 suspended13 can13 be13 large13 particles13 or13 combined13 particles
Do13 not13 separate13 on13 standing Do13 not13 separate13 on13 standing Particles13 settle13 out
Cannot13 be13 separated13 by13 filtration Cannot13 be13 separated13 by13 filtration Can13 be13 separated13 by13 filtration
Do13 not13 scatter13 light Scatter13 light13 (Tyndall13 effect) May13 scatter13 light13 but13 are13 not13 transparent
Electrolytes vs Nonelectrolytes
Substances that dissolve in water are classified according to whether they produce molecules or ions in solution
22
23
When ionic compound dissolves cations and anions separate and are surrounded by water molecules Solute ions free to move making possible to carry electric current
Electrolyte - substance that dissolves in water to give a solution that conducts electric current NaCl ndash electrolyte Usually highly polar compounds become electrolytes
24
Solution containing neutral solute molecules does not conduct current bc it doesnrsquot have mobile charged particles
Nonelectrolyte - substance that dissolves in water to give solution that does not conduct electric current Ex Sugar
25
26
27the solution
process
Factors Affecting Rate of DissolutionEver tried to dissolve sugar in iced tea or coffee You notice temperature has something to do with how quickly solute dissolves What other factors affect how quickly solutes dissolve
28
1 Increasing Surface Area of Solute
bc dissolution process occurs at surface of solute Can increase dissolution by increasing surface area
29
2 Agitating (Stirring) SolutionClose to surface of solute concentration of dissolved solute is high Stirringshaking helps disperse solute particles
30
Brings fresh solvent into contact with solute surface Contact between solvent and solute surface increased
3 Heating a Solvent
Solutes dissolve faster in warmer solvent As temp increases solvent molecules move faster Average kinetic energy increases Collisions between solvent and solute more frequent Helps separate solute molecules and spread them out
32
Solubility
If you add sugar to tea eventually no more sugar dissolves
For every combination of solvent with solute at given temp there is limit to amount of solute that can be dissolved
33
When sugar first dropped into water sugar molecules leave solid surface and move about at random in solvent Some dissolved molecules may collide with crystal and stay there As more solid dissolves and concentration of dissolved molecules increases collisions happen more often
34
Eventually molecules are returning to crystal at same rate they are going into solution Dynamic equilibrium established between dissolution and crystallization Solution equilibrium - physical state in which the opposing processes of dissolution and crystallization of a solute occur at equal rates
35
Saturated vs Unsaturated Solutions
Saturated solution - solution that contains the maximum amount of dissolved solute How do you know if solution is saturated If more NaCl added it falls to bottom and doesnrsquot dissolve Equilibrium already established
36
If more water added to saturated solution more NaCl will dissolve At 20 359 g NaCl is max that will dissolve in 100 g water Unsaturated solution - solution that contains less solute than a saturated solution under the existing conditions
37
Supersaturated Solutions
When saturated solution where solubility increases as temp increases is cooled excess solute usually comes out of solution Sometimes if solution is left to cool the excess solute doesnrsquot separate
38
39
Supersaturated solution - solution that contains more dissolved solute than a saturated solution contains under same conditions
40
Supersaturated solution can remain unchanged over long time if it isnrsquot disturbed Once crystals begin to form continues until equilibrium is recreated at lower temp
41
Example
Sodium thiosulfate Na2S2O3 Solute added to hot water until solution saturated Hot solution filtered Drop small crystal in ldquoseedingrdquo crystallization Continues until equilibrium created
42
Solubility Values
Solubility - amount of substance required to form a saturated solution with specific amount of solvent at specified temp Ex Solubility of sugar = 204 g per 100 g water at 20 Must specify temp bc solubility changes with temp 43
For gases pressure must also be specified Rate at which substance dissolves is unrelated to solubility Max amount of solute that dissolves and reaches equilibrium is always same under same conditions
44
Solute-Solvent Interactions
ldquolike dissolves likerdquo is generally useful rule for predicting whether one substance will dissolve in another Depends on type of bonding polaritynonpolarity and intermolecular forces between solute and solvent
45
Dissolving Ionic Compounds in Aqueous Solution
Polarity of water molecules plays important role in formation of solution of ionic compounds Charged ends of water molecules attract ions and surround them to keep separated from other ions
46
Hydration - solution process with water as solvent Ions said to be hydrated As hydrated ions diffuse into solution other ions exposed and drawn away from crystal surface Entire crystal gradually dissolves
47
When crystallized from aqueous solutions some ionic compounds form crystals that include water molecules The crystalline compounds known as hydrates have specific ratios of water molecules Represented by formulas like CuSO45H2O
48
When crystalline hydrate dissolves in water water of hydration returns to solvent Behavior of solute in hydrated form no different than anhydrous form
49
Nonpolar Solvents
Nonpolar solvents do not attract ions of ionic compound strongly enough to overcome forces holding crystal together
50
Liquid Solutes and Solvents
When you shake bottle of salad dressing oil droplets disperse in water Stop shaking ndash strong attraction between water molecules squeeze out oil and form separate layers Liquid solutes and solvents that are not soluble in each other are immiscible
51
Nonpolar substances (fat oil grease) are generally quite soluble in nonpolar liquids (carbon tetrachloride toluene gasoline)
Only attractions between nonpolar molecules are weak London forces
52
London Dispersion Forces
bc e- are constantly moving at any time there may be uneven distribution of them around a nonpolar atom Temporarily creates positive end and negative end
53
54
London dispersion forces - intermolecular attractions resulting from constant motion of e- and creation of instantaneous dipoles
55
Liquids that dissolve freely in one another in any ratio are said to be completely miscible Benzene and carbon tetrachloride (both nonpolar) are completely miscible
Nonpolar molecules of these two apply no strong forces of attraction or repulsion and molecules mix freely
56
Ethanol and water are also completely miscible -OH group on ethanol is slightly polar
Can form H-bonds with water as well as other ethanol molecules Intermolecular forces in mixture are very similar to those in pure liquids so they are mutually soluble in any amount
57
Gasoline contains mainly nonpolar hydrocarbons
Excellent solvent for fats oils greases
Major intermolecular forces are weak London forces
58
Ethanol less polar than water more than carbon tetrachloride
Better solvent for less-polar substances bc of nonpolar region
59
Effects of Pressure on Solubility
Little effect on solids or liquids Increase pressure - increase solubilities of gases in liquids
60
When gas in contact with surface of liquid gas molecules can enter liquid As amount of dissolve gas increases some molecules start to escape and reenter gas phase
Eq eventually established between rates of enteringleaving gas phase
61
As long as eq undisturbed solubility of gas in liquid unchanged at given pressure
Gas + solvent solution
62
Increasing pressure of solute gas above solution puts stress on eq Molecules collide with surface more often Increase in pressure partially offset by increase in rate of gas molecules entering solution In turn increase in amount of dissolved gas causes increase in rate molecules escape liquid and becomes gas
63
Eventually eq restored at higher gas solubility
Expected from Le Chatelierrsquos principle
Increase in gas pressure causes eq to shift so fewer molecules are in gas phase
64
Henryrsquos Law
Henryrsquos law - solubility of a gas in liquid is directly proportional to the partial pressure of that gas on the surface of the liquid
Applies to gas-liquid solutions at constant temp
65
When mixture of ideal gases is limited in constant volume at constant temp each gas applies same pressure it would apply if it occupied the space alone
Assuming gases do not react each gas dissolves to the extent it would if no other gases were there
66
In carbonated drinks (Coke Pepsi etc) solubility of CO2 increased by increasing pressure
At bottling factory CO2 gas forced into solution of flavored water at pressure of 5-10 atm Gas-in-liquid solution then sealed in bottlescans
67
When cap removed pressure reduced to 1 atm and some CO2 escapes as gas bubbles Effervescence - rapid escape of a gas from a liquid in which it is dissolved
68
Effects of Temperature on Solubility
Letrsquos consider gas solubility Increasing temperature usually decreases gas solubility As temp increases average kinetic energy of molecules in solution increases Greater number of solute molecules escape from attraction of solvent and return to gas phase 69
At higher temps eq is reached with fewer gas molecules in solution Gases generally less soluble
70
Effect of temp on solubility of solids in liquids more difficult to predict
Often increasing temp increases solubility
However the same temp increase can result in large increase in solubility in one case and only slight increase in the next
71
72
enthalpies of Solution
Formation of solution comes with energy change Dissolve KI in water container feels cold Dissolve LiCl in water feels hot Formation of solid-liquid solution can absorb or release heat
73
During formation of solution solvent and solute particles experience changes in forces attracting them to other particles Before dissolving begins solvent molecules held by intermolecular forces (solvent-solvent attraction) In solute molecules held by Ifs (solute-solute attraction)
74
Energy is required to separate solute molecules and solvent molecules from their neighbors
A solute particle that is surrounded by solvent molecules is said to be solvated
75
Step 1 solute particles become separated from solid (energy absorbed) Step 2 solvent particles move apart to allow solute particles to enter liquid (energy absorbed)
76
Step 3 solvent particles attracted to and solvate solute particles (energy released)
77
Heat of solution - net amount of heat energy absorbed or released when a specific amount of solute dissolves in a solvent
Heat of solution negative (heat is released) if sums of energy in steps 1 and 2 is less than step 3
If 1+2 gt 3 HOS is positive (heat absorbed)78
In gaseous state molecules are so far apart there are nearly no forces between them Solute-solute interaction has little effect on heat of solution of a gas Energy released when gas dissolved in liquid bc attraction between solute gas and solvent molecules greater than energy needed to separate solvent molecules
79
Concentration of Solutions80
Concentration - measure of amount of solute in given amount of solvent or solution ldquodiluterdquo just means small amount of solute ldquoconcentratedrdquo just means relatively large amount of solute Unrelated to degree to which solution is saturated Ex Saturated solution of substance not very soluble might be dilute
81
Molarity
Molarity - number of moles of solute in one liter of solution To find molarity must know molar mass A ldquoone molarrdquo solution of NaOH is 1 mole NaOH per liter of solution (1 M)
82
1 mol NaOH = 400 g If this quantity dissolve in enough water to make EXACTLY 100 L solution solution is 1 M If 200 g NaOH dissolve in enough to make 100 L solution what is molarity 0500 M
83
1 M solution is not made by adding 1 mol solute to 1 L solvent That would be more than 1 L Instead 1 mol solute dissolved in less than 1 L Then diluted with more solvent to bring TOTAL VOLUME to 1 L
84
Practice ProblemYou have 340 L of solution that contains 900 g sodium chloride What is the molarity of that solution 0440 M NaCl You have 08 L of a 05 M HCl solution How many moles of HCl does this solution contain 04 mol HCl
85
What is the molarity of a solution composed of 585 g of potassium iodide dissolved in enough water to make a 0125 L of solution 0282 M KI
86
How many moles of H2SO4 are present in 0500 L of a 0150 M H2SO4 solution 00750 mol What volume of 300 M NaCl is needed for reaction that requires 1463 g of NaCl 0834 L
87
Molality
Molality - concentration of solution expressed in moles of solute per kilogram of solvent Solution that contains 1 mol solute (NaOH) dissolved in exactly 1 kg of solvent is ldquoone molalrdquo solution (1 m NaOH)
88
1 mol NaOH = 400 g 400 g NaOH dissolved in 1 kg water is 1 m NaOH
200 g NaOH in 1 kg water = 0500 m NaOH
89
Practice Problem
A solution was prepared by dissolving 171 g sucrose (C12H22O11) in 125 g of water Find the molal concentration of this solution 0400 m C12H22O11
90
A solution of iodine in carbon tetrachloride is used when iodine is needed for certain chemical tests How much iodine must be added to prepare a 0480 m solution of iodine in CCl4 if 1000 g of CCl4 is used 122 g I2
91
What is the molality of a solution composed of 255 g acetone (CH3)2CO dissolved in 200 g of water 22 m acetone What quantity in grams of methanol CH3OH is required to prepare a 0244 m solution in 400 g water 312 g methanol
92
How many grams of AgNO3 are needed to prepare a 0125 m solution in 250 mL of water 531 g AgNO3
What is the molality of a solution containing 182 g HCl and 250 g water 200 m
93
Ions in Aqueous Solutions and Colligative Properties
94
95
compounds in aqueous solutions
DissociationWhen compound made from ions dissolves in water ions separate Dissociation - separation of ions that occurs when an ionic compound dissolves
96
Notice number of ions made per formula unit in equations 1 formula unit of NaCl gives 2 units of ions in solution 1 formula unit of CaCl2 gives 3 units of ions
97
Assuming 100 dissociation solution that contains 1 mol NaCl contains 1 mol Na+ and 1 mol Cl- Can assume 100 dissociation of all soluble ionic compounds
98
Practice Problem
Write the equation for the dissolution of aluminum sulfate in water How many moles of aluminum ions are made by dissolving 1 mol aluminum sulfate What is the total number of moles of ions made by dissolving 1 mol of aluminum sulfate
99
1 AnalyzeGiven Amount of solute = 1 mol Al2(SO4)3 Solvent identity = water Unknown a moles of aluminum ions and sulfate ions Total number of moles of solute ions produced
100
2 Plan
The coefficients in the balanced dissociation equation will tell mole relationships You can use the equation to find out the number of moles of solute ions produced
101
3 Compute
a
b102
Write the equation for the dissolution of each of the following in water and then determine the number of moles of each ions made as well as the total number of ions made a 1 mol ammonium chloride b 1 mol sodium sulfide c 05 mol barium nitrate
103
a 1 mol ammonium chloride
1 mol NH4+ 1 mol Cl- 2 mol total ions
104
b 1 mol sodium sulfide
2 mol Na+ 1 mol S-2 3 mol total ions
105
c 05 mol barium nitrate
05 mol Ba+2 1 mol NO3- 15 mol total ions
106
Precipitation Reactions
Even though no compound is completely insoluble compounds of very low solubility can be considered insoluble for practical purposes There are general guidelines to help predict whether a compound made of certain combination of ions is soluble
107
General Solubility Guidelines
Sodium potassium and ammonium compounds are soluble in water
108
Nitrates acetates and chlorates are soluble
109
110
Most chlorides are soluble except those of silver mercury (I) and lead Lead (II) chloride is soluble in hot water
Most sulfates are soluble except those of barium strontium lead calcium and mercury
111
Most carbonates phosphates and silicates are insoluble except those of sodium potassium and ammonium
112
Most sulfides are insoluble except those of calcium strontium sodium potassium and ammonium
113
Is calcium phosphate Ca3(PO4)2 soluble or insoluble
Not one of the exceptions so it is insoluble
Dissociation equations are not written for insoluble compounds
114
Guidelines also useful in predicting what will happen if 2 solutions of 2 different soluble compounds are mixed
If mixing results in combination of ions that forms insoluble compound a double-replacement and precipitation reaction will happen
115
Will precipitate form when solutions of ammonium sulfide and cadmium nitrate are combined You can tell calcium nitrate is soluble from the guidelines (Most nitrates acetates and chlorates are soluble) You can also tell ammonium sulfide is soluble (Most sulfides are insoluble except those of calcium strontium sodium potassium and ammonium)
116
117
To decide whether precipitate can form must know solubilities of two possible products
Ammonium nitrate ndash soluble (Most nitrates acetates and chlorates are soluble) Cadmium sulfide ndash insoluble (Most sulfides are insoluble except those of calcium strontium sodium potassium and ammonium)
118
bc one product is insoluble double-replacement and precipitation reaction will happen (NH4)2S (aq) + Cd(NO3)2 (aq) rarr NH4NO3 (aq) + CdS (s)
119
Net Ionic Equations
Reactions of ions in aqueous solution usually represented by net ionic equations instead of formula equations Net ionic equation - includes only those compounds and ions that undergo a chemical change in a reaction in an aqueous solution
120
121
To write NIE (net ionic equation) first convert chemical equation into overall ionic equation All soluble ionic compounds shown as dissociated ions in solution Precipitates shown as solids
122
Notice ammonium and nitrate ions appear on both sides They havenrsquot gone through any chemical change Spectator ions - ions that do not take part in a chemical reaction and are found in solution both before and after the reaction
123
To change ionic equation to NIE eliminate spectator ions Cd+2(aq) + S-2(aq) rarr CdS(s)
This NIE applies to any reaction in which precipitate of CdS forms
124
Practice Problem
Identify the precipitate that forms when aqueous solutions of zinc nitrate and ammonium sulfide are combined Write the equation for the possible double-replacement reaction Then write the formula equation overall ionic equation and the net ionic equation
125
1 AnalyzeGiven Identity of reactants zinc nitrate and ammonium sulfide Reaction medium aqueous solution Unknown a equation for the possible double-replacement reaction b identity of the precipitate c formula equation d overall ionic equation e net ionic equation
126
2 PlanWrite the possible double-replacement reaction between Zn(NO3)2 and (NH4)2S Use the rules to determine if any of the products will precipitate Write a formula equation and overall net ionic equation Then cancel spectator ions to make net ionic equation
127
3 Computea possible double-replacement reaction
b rules show that zinc sulfide is not soluble so will precipitate (ammonium nitrate is soluble)
c formula equation128
d overall ionic equation
e net ionic equation129
Will a precipitate form if solutions of potassium sulfate and barium nitrate are combined If so write the net ionic equation for the reaction
Yes Ba+2(aq) + SO4-2(aq) rarr BaSO4(s)
130
Will a precipitate form if solutions of potassium nitrate and magnesium sulfate are combined If so write the net ionic equation for the reaction
No 131
Will a precipitate form if solutions of barium chloride and sodium sulfate are combined If so identify the spectator ions and write the net ionic equation for the reaction
Yes Na+ and Cl- Ba+2(aq) + SO4-2(aq) rarr BaSO4(s)
132
Write the net ionic equation for the precipitation of nickel(II) sulfide
Ni+2(aq) + S-2(aq) rarr NiS(s)
133
IonizationSome molecular compounds can also form ions in solution Usually polar Ionization - ions formed from solute molecules by action of the solvent (creation of ions where there were none) Ionization different from dissociation
134
Dissociation ionic compounds dissolve and ions already present separate Ionization ions formed where non existed before
Ions formed are hydrated Heat released during hydration of ions gives enough energy to break covalent bonds
135
Extent to which solute ionizes depends on strength of bonds within molecules of solute and strength of attraction between solute and solvent molecules
136
HCl ndash molecular compound that ionizes in aqueous solution Contains highly polar bond Attraction between polar HCl molecule and polar water strong enough to break HCl bond
137
The Hydronium Ion
H+ ions from HCl attracts other molecules or ions so strongly that it doesnrsquot normally exist alone Ionization of HCl better described as direct transfer of proton from HCl to H2O forming H3O+
138
139
Hydration of H+ to form hydronium is highly exothermic
Energy released gives a lot of energy needed to ionize a molecular solute
Many molecular compounds that ionize in aqueous solution contain hydrogen and form hydronium ion
140
Strong and Weak Electrolytes
Substances that make ions and conduct current in solution are electrolytes HCl is one of series of compounds that has hydrogen and halogen Hydrogen halides are all molecular compounds with single polar-covalent bonds
141
All are gases Very soluble in water All are electrolytes HCl HBr HI strongly conduct current in solution HF only weakly conducts current at same concentration Strength of conduction related to ability to ionize
142
143
Strong Electrolytes
HCl HBr HI are 100 ionized in solution
Strong electrolyte - any compound whose dilute aqueous solutions conduct electricity well this is due to the presence of all or almost all of the dissolve compound in the form of ions
144
Unique characteristic of strong electrolyte is that they yield only ions
Ex Ionic compound may be highly soluble in water and dissociate into ions (NaCl)
Others may not dissolve much but amount that does dissolve exists only as ions
145
Weak ElectrolytesHF dissolves in water to give acid solution called hydrofluoric acid HF bond stronger than bonds between hydrogen and other halogens When HF dissolves some molecule ionize Reverse reaction also happens HF(aq) + H2O(l) H3O+(aq) + F-(aq)
146
HF(aq) + H2O(l) H3O+
(aq) + F-(aq)Concentration of dissolved unionized HF stays high and concentration of ions stays low
Weak electrolyte - any compound whose aqueous solutions conduct electricity poorly this is due to the presence of small amount of the dissolved compound in the form of ions
147
Different from nonelectrolyte ndash NO ions at all Description of electrolyte as strong or weak not related to concentration of solution Electrolytes differ in degree of ionization not amount of solute dissolved
148
149
Colligative properties of solutions
Presence of solute affects properties of solutions Some properties not dependent on nature of dissolved substance but on how may dissolve particles are present
Colligative properties - properties that depend on the concentration of solute particles but not on their identity
150
Vapor-Pressure Lowering
Boiling and freezing point of solution different from pure solvent Nonvolatile solute raises boiling point and lowers freezing point Nonvolatile substance - one that has little tendency to become gas under existing conditions
151
152
To understand why nonvolatile solute changes boiling and freezing point consider equilibrium vapor pressure
Vapor pressure pressure caused by molecules that have escape liquid phase to gas phase
Can be thought of a measure of tendency of molecules to escape from a liquid
153
Addition of sucrose (nonvolatile solute) lowers concentration of water molecules at surface of liquid This lowers tendency of water molecules to leave solution and enter gas phase Vapor pressure of solution is lower than vapor pressure of pure water
154
Aqueous13 solution13 of13 nonvolatile13 solutePure13 water
Nonelectrolyte solutions of same molality have same concentration of particles
Dilute solutions of same solvent and equal molality of any nonelectrolyte solute lower vapor pressure equally
155
Example
1 m aqueous solution of nonelectrolyte glucose lowers vapor pressure of water 55 x 10-4 atm at 25 1 m aqueous solution of sucrose also lowers water vapor pressure to 55 x 10-4 atm at 25
156
157
bc vapor-pressure lowering depends on concentration of nonelectrolyte solute and doesnrsquot depend on the type of solute it is a colligative proprerty
bc vapor pressure has been lowered solution remains liquid over larger temperature range
Lowers freezing point and raises boiling point
158
Can assume changes in boiling and freezing point also depend on concentration of solute
They are colligative properties
159
Freezing-Point Depression
Freezing point of 1-molal solution of any nonelectrolyte solute in water is found (by experiment) to be 186 lower than freezing point of water When 1 mol of nonelectrolyte solute dissolved in 1 kg water freezing point is -186 not 000 160
When 2 mol nonelectrolyte solute dissolved in 1 kg water freezing point is -372
For any concentration of nonelectrolyte solute in water decrease in freezing point can be determined using value of -186m
161
This value called the molal freezing-point contstant (Kf) is the freezing-point depression of the solvent in a 1-molal solution of a nonvolatile nonelectrolyte solute
162
Each solvent has own characteristic molal freezing-point constant Freezing-point depression (Δtf) the difference between the freezing points of the pure solvent and a solute of nonelectrolyte in that solvent and it is directly proportional to the molal concentration of the solution Δtf = Kfm
163
Δtf = Kfm
Kf expressed as m
m expressed in mol solutekg solvent (molality)
164
Sample Problem
What is the freezing point depression of water in a solution of 171 g of sucrose C12H22O11 and 200 g of water What is the actual freezing point of the solution
165
1 AnalyzeGiven Solute mass and chemical formula 171 g C12H22O11 Solvent mass and identity 2000 g water Unknown a freezing-point depression b freezing point of the solution 166
2 Plan
167
168
Calculate
Unknown a freezing-point depression b freezing point of the solution
169
Practice Problem
A water solution containing an unknown quantity of a nonelectrolyte solute is found to have a freezing point of ndash 023degCWhat is the molal concentration of the solution
012 m170
A solution consists of 103 g of the nonelectrolyte glucose C6H12O6 dissolved in 250 g of water What is the freezing-point depression of the solution
minus0426degC171
In a laboratory experiment the freezing point of an aqueous solution of glucose is found to be minus0325degCWhat is the molal concentration of this solution
0175 m172
If 0500 mol of a nonelectrolyte solute are dissolved in 5000 g of ether what is the freezing point of the solution
minus1181degC173
The freezing point of an aqueous solution that contains a nonelectrolyte is minus90degC aWhat is the freezing-point depression of the solution bWhat is the molal concentration of the solution a minus90degC b 48 m
174
Boiling-Point Elevation
Boiling point is temp at which vapor pressure is equal to atmospheric pressure Change in vapor pressure will cause corresponding change in boiling point
175
176
Vapor pressure of nonvolatile solution is lower than vapor pressure of pure solvent More heat will be required to raise vapor pressure of solution Boiling point of solution is higher than boiling point of pure solvent
Molal boiling-point constant (Kb) - boiling-point elevation of the solvent in a 1-molal solution of a nonvolatile nonelectrolyte solute b-p elevation of any nonelectrolyte in water found by experiment to be 051 So molal b-p constant for water is 051m
177
For different solvents b-p elevations of 1-molal solutions have different values (Table 14-2)
Like freezing-point constants values most accurate for dilute solutions
178
Boiling-point elevation Δtb difference between the boiling points of the pure solvent and a nonelectrolyte solution of that solvent and it is directly proportional to the molal concentration of the solution B-p elevation can be calculated using Δtb = Kbm Δtb (boiling point elevation) expressed in m and m expressed in mol solutekg solvent
Sample Problem
What is the boiling-point elevation of a solution made from 200 g of a nonelectrolyte solute and 4000 g of water The molar mass of the solute is 620 gmol
180
1 AnalyzeGiven solute mass = 200 g solute molar mass = 620 gmol solvent mass and identity = 4000 g of water
Unknown boiling-point elevation
181
2 Plan
182
183
3 Compute
Practice Problem
A solution contains 500 g of sucrose C12H22O11 a nonelectrolyte dissolved in 5000 g of water What is the boiling-point elevation
015degC184
A solution contains 4500 g of sucrose C12H22O11 a nonelectrolyte dissolved in 250 g of waterWhat is the boiling point of the solution
1027degC185
If the boiling point elevation of an aqueous solution containing a nonvolatile electrolyte is 102degC what is the molality of the solution
20 m186
The boiling point of an aqueous solution containing a nonvolatile electrolyte is 10075degC aWhat is the boiling-point elevation bWhat is the molality of the solution a 075degC b 15 m
187
Electrolytes and Colligative PropertiesEarly investigators confused by experiments where certain substances depressed freezing point or elevated boiling point more than expected
188
189
Ex 01 m solution of NaCl lowers freezing point of solvent almost twice as much as 01 m solution of sucrose 01 m CaCl2 lowers fp almost 3 times as much as 01 m solution of sucrose
To understand why this happens contrast behavior or sucrose with NaCl in aqueous solution Each sucrose dissolves to make only 1 particle in solution So 1 mol sucrose dissolves to 1 mol particles 1 mol NaCl dissolves to make 2 moles of particles (1 mol Na+ and 1 mol Cl-)
190
191
Calculated Values for Electrolyte SolutionsColligative properties depend on total concentration of solute particles regardless of their identity Electrolytes cause changes in colligative properties proportional to total molality in terms of all dissolves particles instead of formula units
192
What about barium nitrate Ba(NO3)2 Each mole of barium nitrate yields 3 mol of ions in solution Ba(NO3)2(s) rarr Ba2+(aq) + 2NO3-(aq) You would expect solution of given molality to lower fp of solvent 3 times as much as nonelectrolytic solution of same molality
193
Sample Problem
What is the expected change in the freezing point of water in a solution of 625 g of barium nitrate Ba(NO3)2 in 100 kg of water
194
1 AnalyzeGiven solute mass and formula = 625 g Ba(NO3)2 solvent mass and identity = 100 kg water Δtf = Kfm
Unknown expected freezing-point depression
195
2 PlanThe molality can be calculated by converting the solute mass to moles and then dividing by the number of kilograms of solvent That molality is in terms of formula units of Ba(NO3)2 and must be converted to molality in terms of dissociated ions in solution It must be multiplied by the number of moles of ions produced per mole of formula unit This adjusted molality can then be used to calculate the freezing-point depression
196
197
198
3 Compute
Practice Problem
What is the expected freezing-point depression for a solution that contains 20 mol of magnesium sulfate dissolved in 10 kg of water
minus74degC199
What is the expected boiling-point elevation of water for a solution that contains 150 g of sodium chloride dissolved in 10 kg of water 27degC The freezing point of an aqueous sodium chloride solution is minus200degC What is the molality of the solution 54 m NaCl
200
Actual Values for Electrolyte SolutionsRemember values calculated are only expected values Actual values of colligative properties for all strong electrolytes are almost what would be expected Difference between expected and actual values caused by attractive forces that exist between dissociated ions in aqueous solution
201
Attraction between hydrated ions in solution is small compared with those in crystalline solid However forces of attraction donrsquot interfere with movements of aqueous ions More concentrated solution means ions closer together and attraction is greater Also depends on charge of ions ndash higher charge greater attraction
202
18
Class13 of13 Colloid Phases Example
Sol Solid13 dispersed13 in13 liquid Paints13 mud
Gel Solid13 network13 extending13 through13 liquid
Gelatin
Liquid13 emulsion Liquid13 dispersed13 in13 liquid Milk13 mayonnaise
Foam Gas13 dispersed13 in13 liquid Shaving13 cream13 whipped13 cream
Solid13 aerosol Solid13 dispersed13 in13 gas Smoke13 auto13 exhaust
Liquid13 aerosol Liquid13 dispersed13 in13 gas Fog13 mist13 clouds13 aerosol13 spray
Solid13 emulsion Liquid13 dispersed13 in13 solid Cheese13 butter
Tyndall EffectMany colloids appear homogeneous bc individual particles cannot be seen Particles large enough to scatter light In fog can see headlight beam Tyndall effect - occurs when light is scattered by colloidal particles dispersed in transparent medium
19
20
21
Solutions Colloids Suspensions
Homogeneous Heterogeneous Heterogeneous
Particle13 size13 001-shy‐113 nm13 can13 be13 atoms13 ions13 molecules
Particle13 size13 1-shy‐100013 nm13 dispersed13 can13 be13 combined13 or13 large13 molecules
Particle13 size13 over13 100013 nm13 suspended13 can13 be13 large13 particles13 or13 combined13 particles
Do13 not13 separate13 on13 standing Do13 not13 separate13 on13 standing Particles13 settle13 out
Cannot13 be13 separated13 by13 filtration Cannot13 be13 separated13 by13 filtration Can13 be13 separated13 by13 filtration
Do13 not13 scatter13 light Scatter13 light13 (Tyndall13 effect) May13 scatter13 light13 but13 are13 not13 transparent
Electrolytes vs Nonelectrolytes
Substances that dissolve in water are classified according to whether they produce molecules or ions in solution
22
23
When ionic compound dissolves cations and anions separate and are surrounded by water molecules Solute ions free to move making possible to carry electric current
Electrolyte - substance that dissolves in water to give a solution that conducts electric current NaCl ndash electrolyte Usually highly polar compounds become electrolytes
24
Solution containing neutral solute molecules does not conduct current bc it doesnrsquot have mobile charged particles
Nonelectrolyte - substance that dissolves in water to give solution that does not conduct electric current Ex Sugar
25
26
27the solution
process
Factors Affecting Rate of DissolutionEver tried to dissolve sugar in iced tea or coffee You notice temperature has something to do with how quickly solute dissolves What other factors affect how quickly solutes dissolve
28
1 Increasing Surface Area of Solute
bc dissolution process occurs at surface of solute Can increase dissolution by increasing surface area
29
2 Agitating (Stirring) SolutionClose to surface of solute concentration of dissolved solute is high Stirringshaking helps disperse solute particles
30
Brings fresh solvent into contact with solute surface Contact between solvent and solute surface increased
3 Heating a Solvent
Solutes dissolve faster in warmer solvent As temp increases solvent molecules move faster Average kinetic energy increases Collisions between solvent and solute more frequent Helps separate solute molecules and spread them out
32
Solubility
If you add sugar to tea eventually no more sugar dissolves
For every combination of solvent with solute at given temp there is limit to amount of solute that can be dissolved
33
When sugar first dropped into water sugar molecules leave solid surface and move about at random in solvent Some dissolved molecules may collide with crystal and stay there As more solid dissolves and concentration of dissolved molecules increases collisions happen more often
34
Eventually molecules are returning to crystal at same rate they are going into solution Dynamic equilibrium established between dissolution and crystallization Solution equilibrium - physical state in which the opposing processes of dissolution and crystallization of a solute occur at equal rates
35
Saturated vs Unsaturated Solutions
Saturated solution - solution that contains the maximum amount of dissolved solute How do you know if solution is saturated If more NaCl added it falls to bottom and doesnrsquot dissolve Equilibrium already established
36
If more water added to saturated solution more NaCl will dissolve At 20 359 g NaCl is max that will dissolve in 100 g water Unsaturated solution - solution that contains less solute than a saturated solution under the existing conditions
37
Supersaturated Solutions
When saturated solution where solubility increases as temp increases is cooled excess solute usually comes out of solution Sometimes if solution is left to cool the excess solute doesnrsquot separate
38
39
Supersaturated solution - solution that contains more dissolved solute than a saturated solution contains under same conditions
40
Supersaturated solution can remain unchanged over long time if it isnrsquot disturbed Once crystals begin to form continues until equilibrium is recreated at lower temp
41
Example
Sodium thiosulfate Na2S2O3 Solute added to hot water until solution saturated Hot solution filtered Drop small crystal in ldquoseedingrdquo crystallization Continues until equilibrium created
42
Solubility Values
Solubility - amount of substance required to form a saturated solution with specific amount of solvent at specified temp Ex Solubility of sugar = 204 g per 100 g water at 20 Must specify temp bc solubility changes with temp 43
For gases pressure must also be specified Rate at which substance dissolves is unrelated to solubility Max amount of solute that dissolves and reaches equilibrium is always same under same conditions
44
Solute-Solvent Interactions
ldquolike dissolves likerdquo is generally useful rule for predicting whether one substance will dissolve in another Depends on type of bonding polaritynonpolarity and intermolecular forces between solute and solvent
45
Dissolving Ionic Compounds in Aqueous Solution
Polarity of water molecules plays important role in formation of solution of ionic compounds Charged ends of water molecules attract ions and surround them to keep separated from other ions
46
Hydration - solution process with water as solvent Ions said to be hydrated As hydrated ions diffuse into solution other ions exposed and drawn away from crystal surface Entire crystal gradually dissolves
47
When crystallized from aqueous solutions some ionic compounds form crystals that include water molecules The crystalline compounds known as hydrates have specific ratios of water molecules Represented by formulas like CuSO45H2O
48
When crystalline hydrate dissolves in water water of hydration returns to solvent Behavior of solute in hydrated form no different than anhydrous form
49
Nonpolar Solvents
Nonpolar solvents do not attract ions of ionic compound strongly enough to overcome forces holding crystal together
50
Liquid Solutes and Solvents
When you shake bottle of salad dressing oil droplets disperse in water Stop shaking ndash strong attraction between water molecules squeeze out oil and form separate layers Liquid solutes and solvents that are not soluble in each other are immiscible
51
Nonpolar substances (fat oil grease) are generally quite soluble in nonpolar liquids (carbon tetrachloride toluene gasoline)
Only attractions between nonpolar molecules are weak London forces
52
London Dispersion Forces
bc e- are constantly moving at any time there may be uneven distribution of them around a nonpolar atom Temporarily creates positive end and negative end
53
54
London dispersion forces - intermolecular attractions resulting from constant motion of e- and creation of instantaneous dipoles
55
Liquids that dissolve freely in one another in any ratio are said to be completely miscible Benzene and carbon tetrachloride (both nonpolar) are completely miscible
Nonpolar molecules of these two apply no strong forces of attraction or repulsion and molecules mix freely
56
Ethanol and water are also completely miscible -OH group on ethanol is slightly polar
Can form H-bonds with water as well as other ethanol molecules Intermolecular forces in mixture are very similar to those in pure liquids so they are mutually soluble in any amount
57
Gasoline contains mainly nonpolar hydrocarbons
Excellent solvent for fats oils greases
Major intermolecular forces are weak London forces
58
Ethanol less polar than water more than carbon tetrachloride
Better solvent for less-polar substances bc of nonpolar region
59
Effects of Pressure on Solubility
Little effect on solids or liquids Increase pressure - increase solubilities of gases in liquids
60
When gas in contact with surface of liquid gas molecules can enter liquid As amount of dissolve gas increases some molecules start to escape and reenter gas phase
Eq eventually established between rates of enteringleaving gas phase
61
As long as eq undisturbed solubility of gas in liquid unchanged at given pressure
Gas + solvent solution
62
Increasing pressure of solute gas above solution puts stress on eq Molecules collide with surface more often Increase in pressure partially offset by increase in rate of gas molecules entering solution In turn increase in amount of dissolved gas causes increase in rate molecules escape liquid and becomes gas
63
Eventually eq restored at higher gas solubility
Expected from Le Chatelierrsquos principle
Increase in gas pressure causes eq to shift so fewer molecules are in gas phase
64
Henryrsquos Law
Henryrsquos law - solubility of a gas in liquid is directly proportional to the partial pressure of that gas on the surface of the liquid
Applies to gas-liquid solutions at constant temp
65
When mixture of ideal gases is limited in constant volume at constant temp each gas applies same pressure it would apply if it occupied the space alone
Assuming gases do not react each gas dissolves to the extent it would if no other gases were there
66
In carbonated drinks (Coke Pepsi etc) solubility of CO2 increased by increasing pressure
At bottling factory CO2 gas forced into solution of flavored water at pressure of 5-10 atm Gas-in-liquid solution then sealed in bottlescans
67
When cap removed pressure reduced to 1 atm and some CO2 escapes as gas bubbles Effervescence - rapid escape of a gas from a liquid in which it is dissolved
68
Effects of Temperature on Solubility
Letrsquos consider gas solubility Increasing temperature usually decreases gas solubility As temp increases average kinetic energy of molecules in solution increases Greater number of solute molecules escape from attraction of solvent and return to gas phase 69
At higher temps eq is reached with fewer gas molecules in solution Gases generally less soluble
70
Effect of temp on solubility of solids in liquids more difficult to predict
Often increasing temp increases solubility
However the same temp increase can result in large increase in solubility in one case and only slight increase in the next
71
72
enthalpies of Solution
Formation of solution comes with energy change Dissolve KI in water container feels cold Dissolve LiCl in water feels hot Formation of solid-liquid solution can absorb or release heat
73
During formation of solution solvent and solute particles experience changes in forces attracting them to other particles Before dissolving begins solvent molecules held by intermolecular forces (solvent-solvent attraction) In solute molecules held by Ifs (solute-solute attraction)
74
Energy is required to separate solute molecules and solvent molecules from their neighbors
A solute particle that is surrounded by solvent molecules is said to be solvated
75
Step 1 solute particles become separated from solid (energy absorbed) Step 2 solvent particles move apart to allow solute particles to enter liquid (energy absorbed)
76
Step 3 solvent particles attracted to and solvate solute particles (energy released)
77
Heat of solution - net amount of heat energy absorbed or released when a specific amount of solute dissolves in a solvent
Heat of solution negative (heat is released) if sums of energy in steps 1 and 2 is less than step 3
If 1+2 gt 3 HOS is positive (heat absorbed)78
In gaseous state molecules are so far apart there are nearly no forces between them Solute-solute interaction has little effect on heat of solution of a gas Energy released when gas dissolved in liquid bc attraction between solute gas and solvent molecules greater than energy needed to separate solvent molecules
79
Concentration of Solutions80
Concentration - measure of amount of solute in given amount of solvent or solution ldquodiluterdquo just means small amount of solute ldquoconcentratedrdquo just means relatively large amount of solute Unrelated to degree to which solution is saturated Ex Saturated solution of substance not very soluble might be dilute
81
Molarity
Molarity - number of moles of solute in one liter of solution To find molarity must know molar mass A ldquoone molarrdquo solution of NaOH is 1 mole NaOH per liter of solution (1 M)
82
1 mol NaOH = 400 g If this quantity dissolve in enough water to make EXACTLY 100 L solution solution is 1 M If 200 g NaOH dissolve in enough to make 100 L solution what is molarity 0500 M
83
1 M solution is not made by adding 1 mol solute to 1 L solvent That would be more than 1 L Instead 1 mol solute dissolved in less than 1 L Then diluted with more solvent to bring TOTAL VOLUME to 1 L
84
Practice ProblemYou have 340 L of solution that contains 900 g sodium chloride What is the molarity of that solution 0440 M NaCl You have 08 L of a 05 M HCl solution How many moles of HCl does this solution contain 04 mol HCl
85
What is the molarity of a solution composed of 585 g of potassium iodide dissolved in enough water to make a 0125 L of solution 0282 M KI
86
How many moles of H2SO4 are present in 0500 L of a 0150 M H2SO4 solution 00750 mol What volume of 300 M NaCl is needed for reaction that requires 1463 g of NaCl 0834 L
87
Molality
Molality - concentration of solution expressed in moles of solute per kilogram of solvent Solution that contains 1 mol solute (NaOH) dissolved in exactly 1 kg of solvent is ldquoone molalrdquo solution (1 m NaOH)
88
1 mol NaOH = 400 g 400 g NaOH dissolved in 1 kg water is 1 m NaOH
200 g NaOH in 1 kg water = 0500 m NaOH
89
Practice Problem
A solution was prepared by dissolving 171 g sucrose (C12H22O11) in 125 g of water Find the molal concentration of this solution 0400 m C12H22O11
90
A solution of iodine in carbon tetrachloride is used when iodine is needed for certain chemical tests How much iodine must be added to prepare a 0480 m solution of iodine in CCl4 if 1000 g of CCl4 is used 122 g I2
91
What is the molality of a solution composed of 255 g acetone (CH3)2CO dissolved in 200 g of water 22 m acetone What quantity in grams of methanol CH3OH is required to prepare a 0244 m solution in 400 g water 312 g methanol
92
How many grams of AgNO3 are needed to prepare a 0125 m solution in 250 mL of water 531 g AgNO3
What is the molality of a solution containing 182 g HCl and 250 g water 200 m
93
Ions in Aqueous Solutions and Colligative Properties
94
95
compounds in aqueous solutions
DissociationWhen compound made from ions dissolves in water ions separate Dissociation - separation of ions that occurs when an ionic compound dissolves
96
Notice number of ions made per formula unit in equations 1 formula unit of NaCl gives 2 units of ions in solution 1 formula unit of CaCl2 gives 3 units of ions
97
Assuming 100 dissociation solution that contains 1 mol NaCl contains 1 mol Na+ and 1 mol Cl- Can assume 100 dissociation of all soluble ionic compounds
98
Practice Problem
Write the equation for the dissolution of aluminum sulfate in water How many moles of aluminum ions are made by dissolving 1 mol aluminum sulfate What is the total number of moles of ions made by dissolving 1 mol of aluminum sulfate
99
1 AnalyzeGiven Amount of solute = 1 mol Al2(SO4)3 Solvent identity = water Unknown a moles of aluminum ions and sulfate ions Total number of moles of solute ions produced
100
2 Plan
The coefficients in the balanced dissociation equation will tell mole relationships You can use the equation to find out the number of moles of solute ions produced
101
3 Compute
a
b102
Write the equation for the dissolution of each of the following in water and then determine the number of moles of each ions made as well as the total number of ions made a 1 mol ammonium chloride b 1 mol sodium sulfide c 05 mol barium nitrate
103
a 1 mol ammonium chloride
1 mol NH4+ 1 mol Cl- 2 mol total ions
104
b 1 mol sodium sulfide
2 mol Na+ 1 mol S-2 3 mol total ions
105
c 05 mol barium nitrate
05 mol Ba+2 1 mol NO3- 15 mol total ions
106
Precipitation Reactions
Even though no compound is completely insoluble compounds of very low solubility can be considered insoluble for practical purposes There are general guidelines to help predict whether a compound made of certain combination of ions is soluble
107
General Solubility Guidelines
Sodium potassium and ammonium compounds are soluble in water
108
Nitrates acetates and chlorates are soluble
109
110
Most chlorides are soluble except those of silver mercury (I) and lead Lead (II) chloride is soluble in hot water
Most sulfates are soluble except those of barium strontium lead calcium and mercury
111
Most carbonates phosphates and silicates are insoluble except those of sodium potassium and ammonium
112
Most sulfides are insoluble except those of calcium strontium sodium potassium and ammonium
113
Is calcium phosphate Ca3(PO4)2 soluble or insoluble
Not one of the exceptions so it is insoluble
Dissociation equations are not written for insoluble compounds
114
Guidelines also useful in predicting what will happen if 2 solutions of 2 different soluble compounds are mixed
If mixing results in combination of ions that forms insoluble compound a double-replacement and precipitation reaction will happen
115
Will precipitate form when solutions of ammonium sulfide and cadmium nitrate are combined You can tell calcium nitrate is soluble from the guidelines (Most nitrates acetates and chlorates are soluble) You can also tell ammonium sulfide is soluble (Most sulfides are insoluble except those of calcium strontium sodium potassium and ammonium)
116
117
To decide whether precipitate can form must know solubilities of two possible products
Ammonium nitrate ndash soluble (Most nitrates acetates and chlorates are soluble) Cadmium sulfide ndash insoluble (Most sulfides are insoluble except those of calcium strontium sodium potassium and ammonium)
118
bc one product is insoluble double-replacement and precipitation reaction will happen (NH4)2S (aq) + Cd(NO3)2 (aq) rarr NH4NO3 (aq) + CdS (s)
119
Net Ionic Equations
Reactions of ions in aqueous solution usually represented by net ionic equations instead of formula equations Net ionic equation - includes only those compounds and ions that undergo a chemical change in a reaction in an aqueous solution
120
121
To write NIE (net ionic equation) first convert chemical equation into overall ionic equation All soluble ionic compounds shown as dissociated ions in solution Precipitates shown as solids
122
Notice ammonium and nitrate ions appear on both sides They havenrsquot gone through any chemical change Spectator ions - ions that do not take part in a chemical reaction and are found in solution both before and after the reaction
123
To change ionic equation to NIE eliminate spectator ions Cd+2(aq) + S-2(aq) rarr CdS(s)
This NIE applies to any reaction in which precipitate of CdS forms
124
Practice Problem
Identify the precipitate that forms when aqueous solutions of zinc nitrate and ammonium sulfide are combined Write the equation for the possible double-replacement reaction Then write the formula equation overall ionic equation and the net ionic equation
125
1 AnalyzeGiven Identity of reactants zinc nitrate and ammonium sulfide Reaction medium aqueous solution Unknown a equation for the possible double-replacement reaction b identity of the precipitate c formula equation d overall ionic equation e net ionic equation
126
2 PlanWrite the possible double-replacement reaction between Zn(NO3)2 and (NH4)2S Use the rules to determine if any of the products will precipitate Write a formula equation and overall net ionic equation Then cancel spectator ions to make net ionic equation
127
3 Computea possible double-replacement reaction
b rules show that zinc sulfide is not soluble so will precipitate (ammonium nitrate is soluble)
c formula equation128
d overall ionic equation
e net ionic equation129
Will a precipitate form if solutions of potassium sulfate and barium nitrate are combined If so write the net ionic equation for the reaction
Yes Ba+2(aq) + SO4-2(aq) rarr BaSO4(s)
130
Will a precipitate form if solutions of potassium nitrate and magnesium sulfate are combined If so write the net ionic equation for the reaction
No 131
Will a precipitate form if solutions of barium chloride and sodium sulfate are combined If so identify the spectator ions and write the net ionic equation for the reaction
Yes Na+ and Cl- Ba+2(aq) + SO4-2(aq) rarr BaSO4(s)
132
Write the net ionic equation for the precipitation of nickel(II) sulfide
Ni+2(aq) + S-2(aq) rarr NiS(s)
133
IonizationSome molecular compounds can also form ions in solution Usually polar Ionization - ions formed from solute molecules by action of the solvent (creation of ions where there were none) Ionization different from dissociation
134
Dissociation ionic compounds dissolve and ions already present separate Ionization ions formed where non existed before
Ions formed are hydrated Heat released during hydration of ions gives enough energy to break covalent bonds
135
Extent to which solute ionizes depends on strength of bonds within molecules of solute and strength of attraction between solute and solvent molecules
136
HCl ndash molecular compound that ionizes in aqueous solution Contains highly polar bond Attraction between polar HCl molecule and polar water strong enough to break HCl bond
137
The Hydronium Ion
H+ ions from HCl attracts other molecules or ions so strongly that it doesnrsquot normally exist alone Ionization of HCl better described as direct transfer of proton from HCl to H2O forming H3O+
138
139
Hydration of H+ to form hydronium is highly exothermic
Energy released gives a lot of energy needed to ionize a molecular solute
Many molecular compounds that ionize in aqueous solution contain hydrogen and form hydronium ion
140
Strong and Weak Electrolytes
Substances that make ions and conduct current in solution are electrolytes HCl is one of series of compounds that has hydrogen and halogen Hydrogen halides are all molecular compounds with single polar-covalent bonds
141
All are gases Very soluble in water All are electrolytes HCl HBr HI strongly conduct current in solution HF only weakly conducts current at same concentration Strength of conduction related to ability to ionize
142
143
Strong Electrolytes
HCl HBr HI are 100 ionized in solution
Strong electrolyte - any compound whose dilute aqueous solutions conduct electricity well this is due to the presence of all or almost all of the dissolve compound in the form of ions
144
Unique characteristic of strong electrolyte is that they yield only ions
Ex Ionic compound may be highly soluble in water and dissociate into ions (NaCl)
Others may not dissolve much but amount that does dissolve exists only as ions
145
Weak ElectrolytesHF dissolves in water to give acid solution called hydrofluoric acid HF bond stronger than bonds between hydrogen and other halogens When HF dissolves some molecule ionize Reverse reaction also happens HF(aq) + H2O(l) H3O+(aq) + F-(aq)
146
HF(aq) + H2O(l) H3O+
(aq) + F-(aq)Concentration of dissolved unionized HF stays high and concentration of ions stays low
Weak electrolyte - any compound whose aqueous solutions conduct electricity poorly this is due to the presence of small amount of the dissolved compound in the form of ions
147
Different from nonelectrolyte ndash NO ions at all Description of electrolyte as strong or weak not related to concentration of solution Electrolytes differ in degree of ionization not amount of solute dissolved
148
149
Colligative properties of solutions
Presence of solute affects properties of solutions Some properties not dependent on nature of dissolved substance but on how may dissolve particles are present
Colligative properties - properties that depend on the concentration of solute particles but not on their identity
150
Vapor-Pressure Lowering
Boiling and freezing point of solution different from pure solvent Nonvolatile solute raises boiling point and lowers freezing point Nonvolatile substance - one that has little tendency to become gas under existing conditions
151
152
To understand why nonvolatile solute changes boiling and freezing point consider equilibrium vapor pressure
Vapor pressure pressure caused by molecules that have escape liquid phase to gas phase
Can be thought of a measure of tendency of molecules to escape from a liquid
153
Addition of sucrose (nonvolatile solute) lowers concentration of water molecules at surface of liquid This lowers tendency of water molecules to leave solution and enter gas phase Vapor pressure of solution is lower than vapor pressure of pure water
154
Aqueous13 solution13 of13 nonvolatile13 solutePure13 water
Nonelectrolyte solutions of same molality have same concentration of particles
Dilute solutions of same solvent and equal molality of any nonelectrolyte solute lower vapor pressure equally
155
Example
1 m aqueous solution of nonelectrolyte glucose lowers vapor pressure of water 55 x 10-4 atm at 25 1 m aqueous solution of sucrose also lowers water vapor pressure to 55 x 10-4 atm at 25
156
157
bc vapor-pressure lowering depends on concentration of nonelectrolyte solute and doesnrsquot depend on the type of solute it is a colligative proprerty
bc vapor pressure has been lowered solution remains liquid over larger temperature range
Lowers freezing point and raises boiling point
158
Can assume changes in boiling and freezing point also depend on concentration of solute
They are colligative properties
159
Freezing-Point Depression
Freezing point of 1-molal solution of any nonelectrolyte solute in water is found (by experiment) to be 186 lower than freezing point of water When 1 mol of nonelectrolyte solute dissolved in 1 kg water freezing point is -186 not 000 160
When 2 mol nonelectrolyte solute dissolved in 1 kg water freezing point is -372
For any concentration of nonelectrolyte solute in water decrease in freezing point can be determined using value of -186m
161
This value called the molal freezing-point contstant (Kf) is the freezing-point depression of the solvent in a 1-molal solution of a nonvolatile nonelectrolyte solute
162
Each solvent has own characteristic molal freezing-point constant Freezing-point depression (Δtf) the difference between the freezing points of the pure solvent and a solute of nonelectrolyte in that solvent and it is directly proportional to the molal concentration of the solution Δtf = Kfm
163
Δtf = Kfm
Kf expressed as m
m expressed in mol solutekg solvent (molality)
164
Sample Problem
What is the freezing point depression of water in a solution of 171 g of sucrose C12H22O11 and 200 g of water What is the actual freezing point of the solution
165
1 AnalyzeGiven Solute mass and chemical formula 171 g C12H22O11 Solvent mass and identity 2000 g water Unknown a freezing-point depression b freezing point of the solution 166
2 Plan
167
168
Calculate
Unknown a freezing-point depression b freezing point of the solution
169
Practice Problem
A water solution containing an unknown quantity of a nonelectrolyte solute is found to have a freezing point of ndash 023degCWhat is the molal concentration of the solution
012 m170
A solution consists of 103 g of the nonelectrolyte glucose C6H12O6 dissolved in 250 g of water What is the freezing-point depression of the solution
minus0426degC171
In a laboratory experiment the freezing point of an aqueous solution of glucose is found to be minus0325degCWhat is the molal concentration of this solution
0175 m172
If 0500 mol of a nonelectrolyte solute are dissolved in 5000 g of ether what is the freezing point of the solution
minus1181degC173
The freezing point of an aqueous solution that contains a nonelectrolyte is minus90degC aWhat is the freezing-point depression of the solution bWhat is the molal concentration of the solution a minus90degC b 48 m
174
Boiling-Point Elevation
Boiling point is temp at which vapor pressure is equal to atmospheric pressure Change in vapor pressure will cause corresponding change in boiling point
175
176
Vapor pressure of nonvolatile solution is lower than vapor pressure of pure solvent More heat will be required to raise vapor pressure of solution Boiling point of solution is higher than boiling point of pure solvent
Molal boiling-point constant (Kb) - boiling-point elevation of the solvent in a 1-molal solution of a nonvolatile nonelectrolyte solute b-p elevation of any nonelectrolyte in water found by experiment to be 051 So molal b-p constant for water is 051m
177
For different solvents b-p elevations of 1-molal solutions have different values (Table 14-2)
Like freezing-point constants values most accurate for dilute solutions
178
Boiling-point elevation Δtb difference between the boiling points of the pure solvent and a nonelectrolyte solution of that solvent and it is directly proportional to the molal concentration of the solution B-p elevation can be calculated using Δtb = Kbm Δtb (boiling point elevation) expressed in m and m expressed in mol solutekg solvent
Sample Problem
What is the boiling-point elevation of a solution made from 200 g of a nonelectrolyte solute and 4000 g of water The molar mass of the solute is 620 gmol
180
1 AnalyzeGiven solute mass = 200 g solute molar mass = 620 gmol solvent mass and identity = 4000 g of water
Unknown boiling-point elevation
181
2 Plan
182
183
3 Compute
Practice Problem
A solution contains 500 g of sucrose C12H22O11 a nonelectrolyte dissolved in 5000 g of water What is the boiling-point elevation
015degC184
A solution contains 4500 g of sucrose C12H22O11 a nonelectrolyte dissolved in 250 g of waterWhat is the boiling point of the solution
1027degC185
If the boiling point elevation of an aqueous solution containing a nonvolatile electrolyte is 102degC what is the molality of the solution
20 m186
The boiling point of an aqueous solution containing a nonvolatile electrolyte is 10075degC aWhat is the boiling-point elevation bWhat is the molality of the solution a 075degC b 15 m
187
Electrolytes and Colligative PropertiesEarly investigators confused by experiments where certain substances depressed freezing point or elevated boiling point more than expected
188
189
Ex 01 m solution of NaCl lowers freezing point of solvent almost twice as much as 01 m solution of sucrose 01 m CaCl2 lowers fp almost 3 times as much as 01 m solution of sucrose
To understand why this happens contrast behavior or sucrose with NaCl in aqueous solution Each sucrose dissolves to make only 1 particle in solution So 1 mol sucrose dissolves to 1 mol particles 1 mol NaCl dissolves to make 2 moles of particles (1 mol Na+ and 1 mol Cl-)
190
191
Calculated Values for Electrolyte SolutionsColligative properties depend on total concentration of solute particles regardless of their identity Electrolytes cause changes in colligative properties proportional to total molality in terms of all dissolves particles instead of formula units
192
What about barium nitrate Ba(NO3)2 Each mole of barium nitrate yields 3 mol of ions in solution Ba(NO3)2(s) rarr Ba2+(aq) + 2NO3-(aq) You would expect solution of given molality to lower fp of solvent 3 times as much as nonelectrolytic solution of same molality
193
Sample Problem
What is the expected change in the freezing point of water in a solution of 625 g of barium nitrate Ba(NO3)2 in 100 kg of water
194
1 AnalyzeGiven solute mass and formula = 625 g Ba(NO3)2 solvent mass and identity = 100 kg water Δtf = Kfm
Unknown expected freezing-point depression
195
2 PlanThe molality can be calculated by converting the solute mass to moles and then dividing by the number of kilograms of solvent That molality is in terms of formula units of Ba(NO3)2 and must be converted to molality in terms of dissociated ions in solution It must be multiplied by the number of moles of ions produced per mole of formula unit This adjusted molality can then be used to calculate the freezing-point depression
196
197
198
3 Compute
Practice Problem
What is the expected freezing-point depression for a solution that contains 20 mol of magnesium sulfate dissolved in 10 kg of water
minus74degC199
What is the expected boiling-point elevation of water for a solution that contains 150 g of sodium chloride dissolved in 10 kg of water 27degC The freezing point of an aqueous sodium chloride solution is minus200degC What is the molality of the solution 54 m NaCl
200
Actual Values for Electrolyte SolutionsRemember values calculated are only expected values Actual values of colligative properties for all strong electrolytes are almost what would be expected Difference between expected and actual values caused by attractive forces that exist between dissociated ions in aqueous solution
201
Attraction between hydrated ions in solution is small compared with those in crystalline solid However forces of attraction donrsquot interfere with movements of aqueous ions More concentrated solution means ions closer together and attraction is greater Also depends on charge of ions ndash higher charge greater attraction
202
Tyndall EffectMany colloids appear homogeneous bc individual particles cannot be seen Particles large enough to scatter light In fog can see headlight beam Tyndall effect - occurs when light is scattered by colloidal particles dispersed in transparent medium
19
20
21
Solutions Colloids Suspensions
Homogeneous Heterogeneous Heterogeneous
Particle13 size13 001-shy‐113 nm13 can13 be13 atoms13 ions13 molecules
Particle13 size13 1-shy‐100013 nm13 dispersed13 can13 be13 combined13 or13 large13 molecules
Particle13 size13 over13 100013 nm13 suspended13 can13 be13 large13 particles13 or13 combined13 particles
Do13 not13 separate13 on13 standing Do13 not13 separate13 on13 standing Particles13 settle13 out
Cannot13 be13 separated13 by13 filtration Cannot13 be13 separated13 by13 filtration Can13 be13 separated13 by13 filtration
Do13 not13 scatter13 light Scatter13 light13 (Tyndall13 effect) May13 scatter13 light13 but13 are13 not13 transparent
Electrolytes vs Nonelectrolytes
Substances that dissolve in water are classified according to whether they produce molecules or ions in solution
22
23
When ionic compound dissolves cations and anions separate and are surrounded by water molecules Solute ions free to move making possible to carry electric current
Electrolyte - substance that dissolves in water to give a solution that conducts electric current NaCl ndash electrolyte Usually highly polar compounds become electrolytes
24
Solution containing neutral solute molecules does not conduct current bc it doesnrsquot have mobile charged particles
Nonelectrolyte - substance that dissolves in water to give solution that does not conduct electric current Ex Sugar
25
26
27the solution
process
Factors Affecting Rate of DissolutionEver tried to dissolve sugar in iced tea or coffee You notice temperature has something to do with how quickly solute dissolves What other factors affect how quickly solutes dissolve
28
1 Increasing Surface Area of Solute
bc dissolution process occurs at surface of solute Can increase dissolution by increasing surface area
29
2 Agitating (Stirring) SolutionClose to surface of solute concentration of dissolved solute is high Stirringshaking helps disperse solute particles
30
Brings fresh solvent into contact with solute surface Contact between solvent and solute surface increased
3 Heating a Solvent
Solutes dissolve faster in warmer solvent As temp increases solvent molecules move faster Average kinetic energy increases Collisions between solvent and solute more frequent Helps separate solute molecules and spread them out
32
Solubility
If you add sugar to tea eventually no more sugar dissolves
For every combination of solvent with solute at given temp there is limit to amount of solute that can be dissolved
33
When sugar first dropped into water sugar molecules leave solid surface and move about at random in solvent Some dissolved molecules may collide with crystal and stay there As more solid dissolves and concentration of dissolved molecules increases collisions happen more often
34
Eventually molecules are returning to crystal at same rate they are going into solution Dynamic equilibrium established between dissolution and crystallization Solution equilibrium - physical state in which the opposing processes of dissolution and crystallization of a solute occur at equal rates
35
Saturated vs Unsaturated Solutions
Saturated solution - solution that contains the maximum amount of dissolved solute How do you know if solution is saturated If more NaCl added it falls to bottom and doesnrsquot dissolve Equilibrium already established
36
If more water added to saturated solution more NaCl will dissolve At 20 359 g NaCl is max that will dissolve in 100 g water Unsaturated solution - solution that contains less solute than a saturated solution under the existing conditions
37
Supersaturated Solutions
When saturated solution where solubility increases as temp increases is cooled excess solute usually comes out of solution Sometimes if solution is left to cool the excess solute doesnrsquot separate
38
39
Supersaturated solution - solution that contains more dissolved solute than a saturated solution contains under same conditions
40
Supersaturated solution can remain unchanged over long time if it isnrsquot disturbed Once crystals begin to form continues until equilibrium is recreated at lower temp
41
Example
Sodium thiosulfate Na2S2O3 Solute added to hot water until solution saturated Hot solution filtered Drop small crystal in ldquoseedingrdquo crystallization Continues until equilibrium created
42
Solubility Values
Solubility - amount of substance required to form a saturated solution with specific amount of solvent at specified temp Ex Solubility of sugar = 204 g per 100 g water at 20 Must specify temp bc solubility changes with temp 43
For gases pressure must also be specified Rate at which substance dissolves is unrelated to solubility Max amount of solute that dissolves and reaches equilibrium is always same under same conditions
44
Solute-Solvent Interactions
ldquolike dissolves likerdquo is generally useful rule for predicting whether one substance will dissolve in another Depends on type of bonding polaritynonpolarity and intermolecular forces between solute and solvent
45
Dissolving Ionic Compounds in Aqueous Solution
Polarity of water molecules plays important role in formation of solution of ionic compounds Charged ends of water molecules attract ions and surround them to keep separated from other ions
46
Hydration - solution process with water as solvent Ions said to be hydrated As hydrated ions diffuse into solution other ions exposed and drawn away from crystal surface Entire crystal gradually dissolves
47
When crystallized from aqueous solutions some ionic compounds form crystals that include water molecules The crystalline compounds known as hydrates have specific ratios of water molecules Represented by formulas like CuSO45H2O
48
When crystalline hydrate dissolves in water water of hydration returns to solvent Behavior of solute in hydrated form no different than anhydrous form
49
Nonpolar Solvents
Nonpolar solvents do not attract ions of ionic compound strongly enough to overcome forces holding crystal together
50
Liquid Solutes and Solvents
When you shake bottle of salad dressing oil droplets disperse in water Stop shaking ndash strong attraction between water molecules squeeze out oil and form separate layers Liquid solutes and solvents that are not soluble in each other are immiscible
51
Nonpolar substances (fat oil grease) are generally quite soluble in nonpolar liquids (carbon tetrachloride toluene gasoline)
Only attractions between nonpolar molecules are weak London forces
52
London Dispersion Forces
bc e- are constantly moving at any time there may be uneven distribution of them around a nonpolar atom Temporarily creates positive end and negative end
53
54
London dispersion forces - intermolecular attractions resulting from constant motion of e- and creation of instantaneous dipoles
55
Liquids that dissolve freely in one another in any ratio are said to be completely miscible Benzene and carbon tetrachloride (both nonpolar) are completely miscible
Nonpolar molecules of these two apply no strong forces of attraction or repulsion and molecules mix freely
56
Ethanol and water are also completely miscible -OH group on ethanol is slightly polar
Can form H-bonds with water as well as other ethanol molecules Intermolecular forces in mixture are very similar to those in pure liquids so they are mutually soluble in any amount
57
Gasoline contains mainly nonpolar hydrocarbons
Excellent solvent for fats oils greases
Major intermolecular forces are weak London forces
58
Ethanol less polar than water more than carbon tetrachloride
Better solvent for less-polar substances bc of nonpolar region
59
Effects of Pressure on Solubility
Little effect on solids or liquids Increase pressure - increase solubilities of gases in liquids
60
When gas in contact with surface of liquid gas molecules can enter liquid As amount of dissolve gas increases some molecules start to escape and reenter gas phase
Eq eventually established between rates of enteringleaving gas phase
61
As long as eq undisturbed solubility of gas in liquid unchanged at given pressure
Gas + solvent solution
62
Increasing pressure of solute gas above solution puts stress on eq Molecules collide with surface more often Increase in pressure partially offset by increase in rate of gas molecules entering solution In turn increase in amount of dissolved gas causes increase in rate molecules escape liquid and becomes gas
63
Eventually eq restored at higher gas solubility
Expected from Le Chatelierrsquos principle
Increase in gas pressure causes eq to shift so fewer molecules are in gas phase
64
Henryrsquos Law
Henryrsquos law - solubility of a gas in liquid is directly proportional to the partial pressure of that gas on the surface of the liquid
Applies to gas-liquid solutions at constant temp
65
When mixture of ideal gases is limited in constant volume at constant temp each gas applies same pressure it would apply if it occupied the space alone
Assuming gases do not react each gas dissolves to the extent it would if no other gases were there
66
In carbonated drinks (Coke Pepsi etc) solubility of CO2 increased by increasing pressure
At bottling factory CO2 gas forced into solution of flavored water at pressure of 5-10 atm Gas-in-liquid solution then sealed in bottlescans
67
When cap removed pressure reduced to 1 atm and some CO2 escapes as gas bubbles Effervescence - rapid escape of a gas from a liquid in which it is dissolved
68
Effects of Temperature on Solubility
Letrsquos consider gas solubility Increasing temperature usually decreases gas solubility As temp increases average kinetic energy of molecules in solution increases Greater number of solute molecules escape from attraction of solvent and return to gas phase 69
At higher temps eq is reached with fewer gas molecules in solution Gases generally less soluble
70
Effect of temp on solubility of solids in liquids more difficult to predict
Often increasing temp increases solubility
However the same temp increase can result in large increase in solubility in one case and only slight increase in the next
71
72
enthalpies of Solution
Formation of solution comes with energy change Dissolve KI in water container feels cold Dissolve LiCl in water feels hot Formation of solid-liquid solution can absorb or release heat
73
During formation of solution solvent and solute particles experience changes in forces attracting them to other particles Before dissolving begins solvent molecules held by intermolecular forces (solvent-solvent attraction) In solute molecules held by Ifs (solute-solute attraction)
74
Energy is required to separate solute molecules and solvent molecules from their neighbors
A solute particle that is surrounded by solvent molecules is said to be solvated
75
Step 1 solute particles become separated from solid (energy absorbed) Step 2 solvent particles move apart to allow solute particles to enter liquid (energy absorbed)
76
Step 3 solvent particles attracted to and solvate solute particles (energy released)
77
Heat of solution - net amount of heat energy absorbed or released when a specific amount of solute dissolves in a solvent
Heat of solution negative (heat is released) if sums of energy in steps 1 and 2 is less than step 3
If 1+2 gt 3 HOS is positive (heat absorbed)78
In gaseous state molecules are so far apart there are nearly no forces between them Solute-solute interaction has little effect on heat of solution of a gas Energy released when gas dissolved in liquid bc attraction between solute gas and solvent molecules greater than energy needed to separate solvent molecules
79
Concentration of Solutions80
Concentration - measure of amount of solute in given amount of solvent or solution ldquodiluterdquo just means small amount of solute ldquoconcentratedrdquo just means relatively large amount of solute Unrelated to degree to which solution is saturated Ex Saturated solution of substance not very soluble might be dilute
81
Molarity
Molarity - number of moles of solute in one liter of solution To find molarity must know molar mass A ldquoone molarrdquo solution of NaOH is 1 mole NaOH per liter of solution (1 M)
82
1 mol NaOH = 400 g If this quantity dissolve in enough water to make EXACTLY 100 L solution solution is 1 M If 200 g NaOH dissolve in enough to make 100 L solution what is molarity 0500 M
83
1 M solution is not made by adding 1 mol solute to 1 L solvent That would be more than 1 L Instead 1 mol solute dissolved in less than 1 L Then diluted with more solvent to bring TOTAL VOLUME to 1 L
84
Practice ProblemYou have 340 L of solution that contains 900 g sodium chloride What is the molarity of that solution 0440 M NaCl You have 08 L of a 05 M HCl solution How many moles of HCl does this solution contain 04 mol HCl
85
What is the molarity of a solution composed of 585 g of potassium iodide dissolved in enough water to make a 0125 L of solution 0282 M KI
86
How many moles of H2SO4 are present in 0500 L of a 0150 M H2SO4 solution 00750 mol What volume of 300 M NaCl is needed for reaction that requires 1463 g of NaCl 0834 L
87
Molality
Molality - concentration of solution expressed in moles of solute per kilogram of solvent Solution that contains 1 mol solute (NaOH) dissolved in exactly 1 kg of solvent is ldquoone molalrdquo solution (1 m NaOH)
88
1 mol NaOH = 400 g 400 g NaOH dissolved in 1 kg water is 1 m NaOH
200 g NaOH in 1 kg water = 0500 m NaOH
89
Practice Problem
A solution was prepared by dissolving 171 g sucrose (C12H22O11) in 125 g of water Find the molal concentration of this solution 0400 m C12H22O11
90
A solution of iodine in carbon tetrachloride is used when iodine is needed for certain chemical tests How much iodine must be added to prepare a 0480 m solution of iodine in CCl4 if 1000 g of CCl4 is used 122 g I2
91
What is the molality of a solution composed of 255 g acetone (CH3)2CO dissolved in 200 g of water 22 m acetone What quantity in grams of methanol CH3OH is required to prepare a 0244 m solution in 400 g water 312 g methanol
92
How many grams of AgNO3 are needed to prepare a 0125 m solution in 250 mL of water 531 g AgNO3
What is the molality of a solution containing 182 g HCl and 250 g water 200 m
93
Ions in Aqueous Solutions and Colligative Properties
94
95
compounds in aqueous solutions
DissociationWhen compound made from ions dissolves in water ions separate Dissociation - separation of ions that occurs when an ionic compound dissolves
96
Notice number of ions made per formula unit in equations 1 formula unit of NaCl gives 2 units of ions in solution 1 formula unit of CaCl2 gives 3 units of ions
97
Assuming 100 dissociation solution that contains 1 mol NaCl contains 1 mol Na+ and 1 mol Cl- Can assume 100 dissociation of all soluble ionic compounds
98
Practice Problem
Write the equation for the dissolution of aluminum sulfate in water How many moles of aluminum ions are made by dissolving 1 mol aluminum sulfate What is the total number of moles of ions made by dissolving 1 mol of aluminum sulfate
99
1 AnalyzeGiven Amount of solute = 1 mol Al2(SO4)3 Solvent identity = water Unknown a moles of aluminum ions and sulfate ions Total number of moles of solute ions produced
100
2 Plan
The coefficients in the balanced dissociation equation will tell mole relationships You can use the equation to find out the number of moles of solute ions produced
101
3 Compute
a
b102
Write the equation for the dissolution of each of the following in water and then determine the number of moles of each ions made as well as the total number of ions made a 1 mol ammonium chloride b 1 mol sodium sulfide c 05 mol barium nitrate
103
a 1 mol ammonium chloride
1 mol NH4+ 1 mol Cl- 2 mol total ions
104
b 1 mol sodium sulfide
2 mol Na+ 1 mol S-2 3 mol total ions
105
c 05 mol barium nitrate
05 mol Ba+2 1 mol NO3- 15 mol total ions
106
Precipitation Reactions
Even though no compound is completely insoluble compounds of very low solubility can be considered insoluble for practical purposes There are general guidelines to help predict whether a compound made of certain combination of ions is soluble
107
General Solubility Guidelines
Sodium potassium and ammonium compounds are soluble in water
108
Nitrates acetates and chlorates are soluble
109
110
Most chlorides are soluble except those of silver mercury (I) and lead Lead (II) chloride is soluble in hot water
Most sulfates are soluble except those of barium strontium lead calcium and mercury
111
Most carbonates phosphates and silicates are insoluble except those of sodium potassium and ammonium
112
Most sulfides are insoluble except those of calcium strontium sodium potassium and ammonium
113
Is calcium phosphate Ca3(PO4)2 soluble or insoluble
Not one of the exceptions so it is insoluble
Dissociation equations are not written for insoluble compounds
114
Guidelines also useful in predicting what will happen if 2 solutions of 2 different soluble compounds are mixed
If mixing results in combination of ions that forms insoluble compound a double-replacement and precipitation reaction will happen
115
Will precipitate form when solutions of ammonium sulfide and cadmium nitrate are combined You can tell calcium nitrate is soluble from the guidelines (Most nitrates acetates and chlorates are soluble) You can also tell ammonium sulfide is soluble (Most sulfides are insoluble except those of calcium strontium sodium potassium and ammonium)
116
117
To decide whether precipitate can form must know solubilities of two possible products
Ammonium nitrate ndash soluble (Most nitrates acetates and chlorates are soluble) Cadmium sulfide ndash insoluble (Most sulfides are insoluble except those of calcium strontium sodium potassium and ammonium)
118
bc one product is insoluble double-replacement and precipitation reaction will happen (NH4)2S (aq) + Cd(NO3)2 (aq) rarr NH4NO3 (aq) + CdS (s)
119
Net Ionic Equations
Reactions of ions in aqueous solution usually represented by net ionic equations instead of formula equations Net ionic equation - includes only those compounds and ions that undergo a chemical change in a reaction in an aqueous solution
120
121
To write NIE (net ionic equation) first convert chemical equation into overall ionic equation All soluble ionic compounds shown as dissociated ions in solution Precipitates shown as solids
122
Notice ammonium and nitrate ions appear on both sides They havenrsquot gone through any chemical change Spectator ions - ions that do not take part in a chemical reaction and are found in solution both before and after the reaction
123
To change ionic equation to NIE eliminate spectator ions Cd+2(aq) + S-2(aq) rarr CdS(s)
This NIE applies to any reaction in which precipitate of CdS forms
124
Practice Problem
Identify the precipitate that forms when aqueous solutions of zinc nitrate and ammonium sulfide are combined Write the equation for the possible double-replacement reaction Then write the formula equation overall ionic equation and the net ionic equation
125
1 AnalyzeGiven Identity of reactants zinc nitrate and ammonium sulfide Reaction medium aqueous solution Unknown a equation for the possible double-replacement reaction b identity of the precipitate c formula equation d overall ionic equation e net ionic equation
126
2 PlanWrite the possible double-replacement reaction between Zn(NO3)2 and (NH4)2S Use the rules to determine if any of the products will precipitate Write a formula equation and overall net ionic equation Then cancel spectator ions to make net ionic equation
127
3 Computea possible double-replacement reaction
b rules show that zinc sulfide is not soluble so will precipitate (ammonium nitrate is soluble)
c formula equation128
d overall ionic equation
e net ionic equation129
Will a precipitate form if solutions of potassium sulfate and barium nitrate are combined If so write the net ionic equation for the reaction
Yes Ba+2(aq) + SO4-2(aq) rarr BaSO4(s)
130
Will a precipitate form if solutions of potassium nitrate and magnesium sulfate are combined If so write the net ionic equation for the reaction
No 131
Will a precipitate form if solutions of barium chloride and sodium sulfate are combined If so identify the spectator ions and write the net ionic equation for the reaction
Yes Na+ and Cl- Ba+2(aq) + SO4-2(aq) rarr BaSO4(s)
132
Write the net ionic equation for the precipitation of nickel(II) sulfide
Ni+2(aq) + S-2(aq) rarr NiS(s)
133
IonizationSome molecular compounds can also form ions in solution Usually polar Ionization - ions formed from solute molecules by action of the solvent (creation of ions where there were none) Ionization different from dissociation
134
Dissociation ionic compounds dissolve and ions already present separate Ionization ions formed where non existed before
Ions formed are hydrated Heat released during hydration of ions gives enough energy to break covalent bonds
135
Extent to which solute ionizes depends on strength of bonds within molecules of solute and strength of attraction between solute and solvent molecules
136
HCl ndash molecular compound that ionizes in aqueous solution Contains highly polar bond Attraction between polar HCl molecule and polar water strong enough to break HCl bond
137
The Hydronium Ion
H+ ions from HCl attracts other molecules or ions so strongly that it doesnrsquot normally exist alone Ionization of HCl better described as direct transfer of proton from HCl to H2O forming H3O+
138
139
Hydration of H+ to form hydronium is highly exothermic
Energy released gives a lot of energy needed to ionize a molecular solute
Many molecular compounds that ionize in aqueous solution contain hydrogen and form hydronium ion
140
Strong and Weak Electrolytes
Substances that make ions and conduct current in solution are electrolytes HCl is one of series of compounds that has hydrogen and halogen Hydrogen halides are all molecular compounds with single polar-covalent bonds
141
All are gases Very soluble in water All are electrolytes HCl HBr HI strongly conduct current in solution HF only weakly conducts current at same concentration Strength of conduction related to ability to ionize
142
143
Strong Electrolytes
HCl HBr HI are 100 ionized in solution
Strong electrolyte - any compound whose dilute aqueous solutions conduct electricity well this is due to the presence of all or almost all of the dissolve compound in the form of ions
144
Unique characteristic of strong electrolyte is that they yield only ions
Ex Ionic compound may be highly soluble in water and dissociate into ions (NaCl)
Others may not dissolve much but amount that does dissolve exists only as ions
145
Weak ElectrolytesHF dissolves in water to give acid solution called hydrofluoric acid HF bond stronger than bonds between hydrogen and other halogens When HF dissolves some molecule ionize Reverse reaction also happens HF(aq) + H2O(l) H3O+(aq) + F-(aq)
146
HF(aq) + H2O(l) H3O+
(aq) + F-(aq)Concentration of dissolved unionized HF stays high and concentration of ions stays low
Weak electrolyte - any compound whose aqueous solutions conduct electricity poorly this is due to the presence of small amount of the dissolved compound in the form of ions
147
Different from nonelectrolyte ndash NO ions at all Description of electrolyte as strong or weak not related to concentration of solution Electrolytes differ in degree of ionization not amount of solute dissolved
148
149
Colligative properties of solutions
Presence of solute affects properties of solutions Some properties not dependent on nature of dissolved substance but on how may dissolve particles are present
Colligative properties - properties that depend on the concentration of solute particles but not on their identity
150
Vapor-Pressure Lowering
Boiling and freezing point of solution different from pure solvent Nonvolatile solute raises boiling point and lowers freezing point Nonvolatile substance - one that has little tendency to become gas under existing conditions
151
152
To understand why nonvolatile solute changes boiling and freezing point consider equilibrium vapor pressure
Vapor pressure pressure caused by molecules that have escape liquid phase to gas phase
Can be thought of a measure of tendency of molecules to escape from a liquid
153
Addition of sucrose (nonvolatile solute) lowers concentration of water molecules at surface of liquid This lowers tendency of water molecules to leave solution and enter gas phase Vapor pressure of solution is lower than vapor pressure of pure water
154
Aqueous13 solution13 of13 nonvolatile13 solutePure13 water
Nonelectrolyte solutions of same molality have same concentration of particles
Dilute solutions of same solvent and equal molality of any nonelectrolyte solute lower vapor pressure equally
155
Example
1 m aqueous solution of nonelectrolyte glucose lowers vapor pressure of water 55 x 10-4 atm at 25 1 m aqueous solution of sucrose also lowers water vapor pressure to 55 x 10-4 atm at 25
156
157
bc vapor-pressure lowering depends on concentration of nonelectrolyte solute and doesnrsquot depend on the type of solute it is a colligative proprerty
bc vapor pressure has been lowered solution remains liquid over larger temperature range
Lowers freezing point and raises boiling point
158
Can assume changes in boiling and freezing point also depend on concentration of solute
They are colligative properties
159
Freezing-Point Depression
Freezing point of 1-molal solution of any nonelectrolyte solute in water is found (by experiment) to be 186 lower than freezing point of water When 1 mol of nonelectrolyte solute dissolved in 1 kg water freezing point is -186 not 000 160
When 2 mol nonelectrolyte solute dissolved in 1 kg water freezing point is -372
For any concentration of nonelectrolyte solute in water decrease in freezing point can be determined using value of -186m
161
This value called the molal freezing-point contstant (Kf) is the freezing-point depression of the solvent in a 1-molal solution of a nonvolatile nonelectrolyte solute
162
Each solvent has own characteristic molal freezing-point constant Freezing-point depression (Δtf) the difference between the freezing points of the pure solvent and a solute of nonelectrolyte in that solvent and it is directly proportional to the molal concentration of the solution Δtf = Kfm
163
Δtf = Kfm
Kf expressed as m
m expressed in mol solutekg solvent (molality)
164
Sample Problem
What is the freezing point depression of water in a solution of 171 g of sucrose C12H22O11 and 200 g of water What is the actual freezing point of the solution
165
1 AnalyzeGiven Solute mass and chemical formula 171 g C12H22O11 Solvent mass and identity 2000 g water Unknown a freezing-point depression b freezing point of the solution 166
2 Plan
167
168
Calculate
Unknown a freezing-point depression b freezing point of the solution
169
Practice Problem
A water solution containing an unknown quantity of a nonelectrolyte solute is found to have a freezing point of ndash 023degCWhat is the molal concentration of the solution
012 m170
A solution consists of 103 g of the nonelectrolyte glucose C6H12O6 dissolved in 250 g of water What is the freezing-point depression of the solution
minus0426degC171
In a laboratory experiment the freezing point of an aqueous solution of glucose is found to be minus0325degCWhat is the molal concentration of this solution
0175 m172
If 0500 mol of a nonelectrolyte solute are dissolved in 5000 g of ether what is the freezing point of the solution
minus1181degC173
The freezing point of an aqueous solution that contains a nonelectrolyte is minus90degC aWhat is the freezing-point depression of the solution bWhat is the molal concentration of the solution a minus90degC b 48 m
174
Boiling-Point Elevation
Boiling point is temp at which vapor pressure is equal to atmospheric pressure Change in vapor pressure will cause corresponding change in boiling point
175
176
Vapor pressure of nonvolatile solution is lower than vapor pressure of pure solvent More heat will be required to raise vapor pressure of solution Boiling point of solution is higher than boiling point of pure solvent
Molal boiling-point constant (Kb) - boiling-point elevation of the solvent in a 1-molal solution of a nonvolatile nonelectrolyte solute b-p elevation of any nonelectrolyte in water found by experiment to be 051 So molal b-p constant for water is 051m
177
For different solvents b-p elevations of 1-molal solutions have different values (Table 14-2)
Like freezing-point constants values most accurate for dilute solutions
178
Boiling-point elevation Δtb difference between the boiling points of the pure solvent and a nonelectrolyte solution of that solvent and it is directly proportional to the molal concentration of the solution B-p elevation can be calculated using Δtb = Kbm Δtb (boiling point elevation) expressed in m and m expressed in mol solutekg solvent
Sample Problem
What is the boiling-point elevation of a solution made from 200 g of a nonelectrolyte solute and 4000 g of water The molar mass of the solute is 620 gmol
180
1 AnalyzeGiven solute mass = 200 g solute molar mass = 620 gmol solvent mass and identity = 4000 g of water
Unknown boiling-point elevation
181
2 Plan
182
183
3 Compute
Practice Problem
A solution contains 500 g of sucrose C12H22O11 a nonelectrolyte dissolved in 5000 g of water What is the boiling-point elevation
015degC184
A solution contains 4500 g of sucrose C12H22O11 a nonelectrolyte dissolved in 250 g of waterWhat is the boiling point of the solution
1027degC185
If the boiling point elevation of an aqueous solution containing a nonvolatile electrolyte is 102degC what is the molality of the solution
20 m186
The boiling point of an aqueous solution containing a nonvolatile electrolyte is 10075degC aWhat is the boiling-point elevation bWhat is the molality of the solution a 075degC b 15 m
187
Electrolytes and Colligative PropertiesEarly investigators confused by experiments where certain substances depressed freezing point or elevated boiling point more than expected
188
189
Ex 01 m solution of NaCl lowers freezing point of solvent almost twice as much as 01 m solution of sucrose 01 m CaCl2 lowers fp almost 3 times as much as 01 m solution of sucrose
To understand why this happens contrast behavior or sucrose with NaCl in aqueous solution Each sucrose dissolves to make only 1 particle in solution So 1 mol sucrose dissolves to 1 mol particles 1 mol NaCl dissolves to make 2 moles of particles (1 mol Na+ and 1 mol Cl-)
190
191
Calculated Values for Electrolyte SolutionsColligative properties depend on total concentration of solute particles regardless of their identity Electrolytes cause changes in colligative properties proportional to total molality in terms of all dissolves particles instead of formula units
192
What about barium nitrate Ba(NO3)2 Each mole of barium nitrate yields 3 mol of ions in solution Ba(NO3)2(s) rarr Ba2+(aq) + 2NO3-(aq) You would expect solution of given molality to lower fp of solvent 3 times as much as nonelectrolytic solution of same molality
193
Sample Problem
What is the expected change in the freezing point of water in a solution of 625 g of barium nitrate Ba(NO3)2 in 100 kg of water
194
1 AnalyzeGiven solute mass and formula = 625 g Ba(NO3)2 solvent mass and identity = 100 kg water Δtf = Kfm
Unknown expected freezing-point depression
195
2 PlanThe molality can be calculated by converting the solute mass to moles and then dividing by the number of kilograms of solvent That molality is in terms of formula units of Ba(NO3)2 and must be converted to molality in terms of dissociated ions in solution It must be multiplied by the number of moles of ions produced per mole of formula unit This adjusted molality can then be used to calculate the freezing-point depression
196
197
198
3 Compute
Practice Problem
What is the expected freezing-point depression for a solution that contains 20 mol of magnesium sulfate dissolved in 10 kg of water
minus74degC199
What is the expected boiling-point elevation of water for a solution that contains 150 g of sodium chloride dissolved in 10 kg of water 27degC The freezing point of an aqueous sodium chloride solution is minus200degC What is the molality of the solution 54 m NaCl
200
Actual Values for Electrolyte SolutionsRemember values calculated are only expected values Actual values of colligative properties for all strong electrolytes are almost what would be expected Difference between expected and actual values caused by attractive forces that exist between dissociated ions in aqueous solution
201
Attraction between hydrated ions in solution is small compared with those in crystalline solid However forces of attraction donrsquot interfere with movements of aqueous ions More concentrated solution means ions closer together and attraction is greater Also depends on charge of ions ndash higher charge greater attraction
202
20
21
Solutions Colloids Suspensions
Homogeneous Heterogeneous Heterogeneous
Particle13 size13 001-shy‐113 nm13 can13 be13 atoms13 ions13 molecules
Particle13 size13 1-shy‐100013 nm13 dispersed13 can13 be13 combined13 or13 large13 molecules
Particle13 size13 over13 100013 nm13 suspended13 can13 be13 large13 particles13 or13 combined13 particles
Do13 not13 separate13 on13 standing Do13 not13 separate13 on13 standing Particles13 settle13 out
Cannot13 be13 separated13 by13 filtration Cannot13 be13 separated13 by13 filtration Can13 be13 separated13 by13 filtration
Do13 not13 scatter13 light Scatter13 light13 (Tyndall13 effect) May13 scatter13 light13 but13 are13 not13 transparent
Electrolytes vs Nonelectrolytes
Substances that dissolve in water are classified according to whether they produce molecules or ions in solution
22
23
When ionic compound dissolves cations and anions separate and are surrounded by water molecules Solute ions free to move making possible to carry electric current
Electrolyte - substance that dissolves in water to give a solution that conducts electric current NaCl ndash electrolyte Usually highly polar compounds become electrolytes
24
Solution containing neutral solute molecules does not conduct current bc it doesnrsquot have mobile charged particles
Nonelectrolyte - substance that dissolves in water to give solution that does not conduct electric current Ex Sugar
25
26
27the solution
process
Factors Affecting Rate of DissolutionEver tried to dissolve sugar in iced tea or coffee You notice temperature has something to do with how quickly solute dissolves What other factors affect how quickly solutes dissolve
28
1 Increasing Surface Area of Solute
bc dissolution process occurs at surface of solute Can increase dissolution by increasing surface area
29
2 Agitating (Stirring) SolutionClose to surface of solute concentration of dissolved solute is high Stirringshaking helps disperse solute particles
30
Brings fresh solvent into contact with solute surface Contact between solvent and solute surface increased
3 Heating a Solvent
Solutes dissolve faster in warmer solvent As temp increases solvent molecules move faster Average kinetic energy increases Collisions between solvent and solute more frequent Helps separate solute molecules and spread them out
32
Solubility
If you add sugar to tea eventually no more sugar dissolves
For every combination of solvent with solute at given temp there is limit to amount of solute that can be dissolved
33
When sugar first dropped into water sugar molecules leave solid surface and move about at random in solvent Some dissolved molecules may collide with crystal and stay there As more solid dissolves and concentration of dissolved molecules increases collisions happen more often
34
Eventually molecules are returning to crystal at same rate they are going into solution Dynamic equilibrium established between dissolution and crystallization Solution equilibrium - physical state in which the opposing processes of dissolution and crystallization of a solute occur at equal rates
35
Saturated vs Unsaturated Solutions
Saturated solution - solution that contains the maximum amount of dissolved solute How do you know if solution is saturated If more NaCl added it falls to bottom and doesnrsquot dissolve Equilibrium already established
36
If more water added to saturated solution more NaCl will dissolve At 20 359 g NaCl is max that will dissolve in 100 g water Unsaturated solution - solution that contains less solute than a saturated solution under the existing conditions
37
Supersaturated Solutions
When saturated solution where solubility increases as temp increases is cooled excess solute usually comes out of solution Sometimes if solution is left to cool the excess solute doesnrsquot separate
38
39
Supersaturated solution - solution that contains more dissolved solute than a saturated solution contains under same conditions
40
Supersaturated solution can remain unchanged over long time if it isnrsquot disturbed Once crystals begin to form continues until equilibrium is recreated at lower temp
41
Example
Sodium thiosulfate Na2S2O3 Solute added to hot water until solution saturated Hot solution filtered Drop small crystal in ldquoseedingrdquo crystallization Continues until equilibrium created
42
Solubility Values
Solubility - amount of substance required to form a saturated solution with specific amount of solvent at specified temp Ex Solubility of sugar = 204 g per 100 g water at 20 Must specify temp bc solubility changes with temp 43
For gases pressure must also be specified Rate at which substance dissolves is unrelated to solubility Max amount of solute that dissolves and reaches equilibrium is always same under same conditions
44
Solute-Solvent Interactions
ldquolike dissolves likerdquo is generally useful rule for predicting whether one substance will dissolve in another Depends on type of bonding polaritynonpolarity and intermolecular forces between solute and solvent
45
Dissolving Ionic Compounds in Aqueous Solution
Polarity of water molecules plays important role in formation of solution of ionic compounds Charged ends of water molecules attract ions and surround them to keep separated from other ions
46
Hydration - solution process with water as solvent Ions said to be hydrated As hydrated ions diffuse into solution other ions exposed and drawn away from crystal surface Entire crystal gradually dissolves
47
When crystallized from aqueous solutions some ionic compounds form crystals that include water molecules The crystalline compounds known as hydrates have specific ratios of water molecules Represented by formulas like CuSO45H2O
48
When crystalline hydrate dissolves in water water of hydration returns to solvent Behavior of solute in hydrated form no different than anhydrous form
49
Nonpolar Solvents
Nonpolar solvents do not attract ions of ionic compound strongly enough to overcome forces holding crystal together
50
Liquid Solutes and Solvents
When you shake bottle of salad dressing oil droplets disperse in water Stop shaking ndash strong attraction between water molecules squeeze out oil and form separate layers Liquid solutes and solvents that are not soluble in each other are immiscible
51
Nonpolar substances (fat oil grease) are generally quite soluble in nonpolar liquids (carbon tetrachloride toluene gasoline)
Only attractions between nonpolar molecules are weak London forces
52
London Dispersion Forces
bc e- are constantly moving at any time there may be uneven distribution of them around a nonpolar atom Temporarily creates positive end and negative end
53
54
London dispersion forces - intermolecular attractions resulting from constant motion of e- and creation of instantaneous dipoles
55
Liquids that dissolve freely in one another in any ratio are said to be completely miscible Benzene and carbon tetrachloride (both nonpolar) are completely miscible
Nonpolar molecules of these two apply no strong forces of attraction or repulsion and molecules mix freely
56
Ethanol and water are also completely miscible -OH group on ethanol is slightly polar
Can form H-bonds with water as well as other ethanol molecules Intermolecular forces in mixture are very similar to those in pure liquids so they are mutually soluble in any amount
57
Gasoline contains mainly nonpolar hydrocarbons
Excellent solvent for fats oils greases
Major intermolecular forces are weak London forces
58
Ethanol less polar than water more than carbon tetrachloride
Better solvent for less-polar substances bc of nonpolar region
59
Effects of Pressure on Solubility
Little effect on solids or liquids Increase pressure - increase solubilities of gases in liquids
60
When gas in contact with surface of liquid gas molecules can enter liquid As amount of dissolve gas increases some molecules start to escape and reenter gas phase
Eq eventually established between rates of enteringleaving gas phase
61
As long as eq undisturbed solubility of gas in liquid unchanged at given pressure
Gas + solvent solution
62
Increasing pressure of solute gas above solution puts stress on eq Molecules collide with surface more often Increase in pressure partially offset by increase in rate of gas molecules entering solution In turn increase in amount of dissolved gas causes increase in rate molecules escape liquid and becomes gas
63
Eventually eq restored at higher gas solubility
Expected from Le Chatelierrsquos principle
Increase in gas pressure causes eq to shift so fewer molecules are in gas phase
64
Henryrsquos Law
Henryrsquos law - solubility of a gas in liquid is directly proportional to the partial pressure of that gas on the surface of the liquid
Applies to gas-liquid solutions at constant temp
65
When mixture of ideal gases is limited in constant volume at constant temp each gas applies same pressure it would apply if it occupied the space alone
Assuming gases do not react each gas dissolves to the extent it would if no other gases were there
66
In carbonated drinks (Coke Pepsi etc) solubility of CO2 increased by increasing pressure
At bottling factory CO2 gas forced into solution of flavored water at pressure of 5-10 atm Gas-in-liquid solution then sealed in bottlescans
67
When cap removed pressure reduced to 1 atm and some CO2 escapes as gas bubbles Effervescence - rapid escape of a gas from a liquid in which it is dissolved
68
Effects of Temperature on Solubility
Letrsquos consider gas solubility Increasing temperature usually decreases gas solubility As temp increases average kinetic energy of molecules in solution increases Greater number of solute molecules escape from attraction of solvent and return to gas phase 69
At higher temps eq is reached with fewer gas molecules in solution Gases generally less soluble
70
Effect of temp on solubility of solids in liquids more difficult to predict
Often increasing temp increases solubility
However the same temp increase can result in large increase in solubility in one case and only slight increase in the next
71
72
enthalpies of Solution
Formation of solution comes with energy change Dissolve KI in water container feels cold Dissolve LiCl in water feels hot Formation of solid-liquid solution can absorb or release heat
73
During formation of solution solvent and solute particles experience changes in forces attracting them to other particles Before dissolving begins solvent molecules held by intermolecular forces (solvent-solvent attraction) In solute molecules held by Ifs (solute-solute attraction)
74
Energy is required to separate solute molecules and solvent molecules from their neighbors
A solute particle that is surrounded by solvent molecules is said to be solvated
75
Step 1 solute particles become separated from solid (energy absorbed) Step 2 solvent particles move apart to allow solute particles to enter liquid (energy absorbed)
76
Step 3 solvent particles attracted to and solvate solute particles (energy released)
77
Heat of solution - net amount of heat energy absorbed or released when a specific amount of solute dissolves in a solvent
Heat of solution negative (heat is released) if sums of energy in steps 1 and 2 is less than step 3
If 1+2 gt 3 HOS is positive (heat absorbed)78
In gaseous state molecules are so far apart there are nearly no forces between them Solute-solute interaction has little effect on heat of solution of a gas Energy released when gas dissolved in liquid bc attraction between solute gas and solvent molecules greater than energy needed to separate solvent molecules
79
Concentration of Solutions80
Concentration - measure of amount of solute in given amount of solvent or solution ldquodiluterdquo just means small amount of solute ldquoconcentratedrdquo just means relatively large amount of solute Unrelated to degree to which solution is saturated Ex Saturated solution of substance not very soluble might be dilute
81
Molarity
Molarity - number of moles of solute in one liter of solution To find molarity must know molar mass A ldquoone molarrdquo solution of NaOH is 1 mole NaOH per liter of solution (1 M)
82
1 mol NaOH = 400 g If this quantity dissolve in enough water to make EXACTLY 100 L solution solution is 1 M If 200 g NaOH dissolve in enough to make 100 L solution what is molarity 0500 M
83
1 M solution is not made by adding 1 mol solute to 1 L solvent That would be more than 1 L Instead 1 mol solute dissolved in less than 1 L Then diluted with more solvent to bring TOTAL VOLUME to 1 L
84
Practice ProblemYou have 340 L of solution that contains 900 g sodium chloride What is the molarity of that solution 0440 M NaCl You have 08 L of a 05 M HCl solution How many moles of HCl does this solution contain 04 mol HCl
85
What is the molarity of a solution composed of 585 g of potassium iodide dissolved in enough water to make a 0125 L of solution 0282 M KI
86
How many moles of H2SO4 are present in 0500 L of a 0150 M H2SO4 solution 00750 mol What volume of 300 M NaCl is needed for reaction that requires 1463 g of NaCl 0834 L
87
Molality
Molality - concentration of solution expressed in moles of solute per kilogram of solvent Solution that contains 1 mol solute (NaOH) dissolved in exactly 1 kg of solvent is ldquoone molalrdquo solution (1 m NaOH)
88
1 mol NaOH = 400 g 400 g NaOH dissolved in 1 kg water is 1 m NaOH
200 g NaOH in 1 kg water = 0500 m NaOH
89
Practice Problem
A solution was prepared by dissolving 171 g sucrose (C12H22O11) in 125 g of water Find the molal concentration of this solution 0400 m C12H22O11
90
A solution of iodine in carbon tetrachloride is used when iodine is needed for certain chemical tests How much iodine must be added to prepare a 0480 m solution of iodine in CCl4 if 1000 g of CCl4 is used 122 g I2
91
What is the molality of a solution composed of 255 g acetone (CH3)2CO dissolved in 200 g of water 22 m acetone What quantity in grams of methanol CH3OH is required to prepare a 0244 m solution in 400 g water 312 g methanol
92
How many grams of AgNO3 are needed to prepare a 0125 m solution in 250 mL of water 531 g AgNO3
What is the molality of a solution containing 182 g HCl and 250 g water 200 m
93
Ions in Aqueous Solutions and Colligative Properties
94
95
compounds in aqueous solutions
DissociationWhen compound made from ions dissolves in water ions separate Dissociation - separation of ions that occurs when an ionic compound dissolves
96
Notice number of ions made per formula unit in equations 1 formula unit of NaCl gives 2 units of ions in solution 1 formula unit of CaCl2 gives 3 units of ions
97
Assuming 100 dissociation solution that contains 1 mol NaCl contains 1 mol Na+ and 1 mol Cl- Can assume 100 dissociation of all soluble ionic compounds
98
Practice Problem
Write the equation for the dissolution of aluminum sulfate in water How many moles of aluminum ions are made by dissolving 1 mol aluminum sulfate What is the total number of moles of ions made by dissolving 1 mol of aluminum sulfate
99
1 AnalyzeGiven Amount of solute = 1 mol Al2(SO4)3 Solvent identity = water Unknown a moles of aluminum ions and sulfate ions Total number of moles of solute ions produced
100
2 Plan
The coefficients in the balanced dissociation equation will tell mole relationships You can use the equation to find out the number of moles of solute ions produced
101
3 Compute
a
b102
Write the equation for the dissolution of each of the following in water and then determine the number of moles of each ions made as well as the total number of ions made a 1 mol ammonium chloride b 1 mol sodium sulfide c 05 mol barium nitrate
103
a 1 mol ammonium chloride
1 mol NH4+ 1 mol Cl- 2 mol total ions
104
b 1 mol sodium sulfide
2 mol Na+ 1 mol S-2 3 mol total ions
105
c 05 mol barium nitrate
05 mol Ba+2 1 mol NO3- 15 mol total ions
106
Precipitation Reactions
Even though no compound is completely insoluble compounds of very low solubility can be considered insoluble for practical purposes There are general guidelines to help predict whether a compound made of certain combination of ions is soluble
107
General Solubility Guidelines
Sodium potassium and ammonium compounds are soluble in water
108
Nitrates acetates and chlorates are soluble
109
110
Most chlorides are soluble except those of silver mercury (I) and lead Lead (II) chloride is soluble in hot water
Most sulfates are soluble except those of barium strontium lead calcium and mercury
111
Most carbonates phosphates and silicates are insoluble except those of sodium potassium and ammonium
112
Most sulfides are insoluble except those of calcium strontium sodium potassium and ammonium
113
Is calcium phosphate Ca3(PO4)2 soluble or insoluble
Not one of the exceptions so it is insoluble
Dissociation equations are not written for insoluble compounds
114
Guidelines also useful in predicting what will happen if 2 solutions of 2 different soluble compounds are mixed
If mixing results in combination of ions that forms insoluble compound a double-replacement and precipitation reaction will happen
115
Will precipitate form when solutions of ammonium sulfide and cadmium nitrate are combined You can tell calcium nitrate is soluble from the guidelines (Most nitrates acetates and chlorates are soluble) You can also tell ammonium sulfide is soluble (Most sulfides are insoluble except those of calcium strontium sodium potassium and ammonium)
116
117
To decide whether precipitate can form must know solubilities of two possible products
Ammonium nitrate ndash soluble (Most nitrates acetates and chlorates are soluble) Cadmium sulfide ndash insoluble (Most sulfides are insoluble except those of calcium strontium sodium potassium and ammonium)
118
bc one product is insoluble double-replacement and precipitation reaction will happen (NH4)2S (aq) + Cd(NO3)2 (aq) rarr NH4NO3 (aq) + CdS (s)
119
Net Ionic Equations
Reactions of ions in aqueous solution usually represented by net ionic equations instead of formula equations Net ionic equation - includes only those compounds and ions that undergo a chemical change in a reaction in an aqueous solution
120
121
To write NIE (net ionic equation) first convert chemical equation into overall ionic equation All soluble ionic compounds shown as dissociated ions in solution Precipitates shown as solids
122
Notice ammonium and nitrate ions appear on both sides They havenrsquot gone through any chemical change Spectator ions - ions that do not take part in a chemical reaction and are found in solution both before and after the reaction
123
To change ionic equation to NIE eliminate spectator ions Cd+2(aq) + S-2(aq) rarr CdS(s)
This NIE applies to any reaction in which precipitate of CdS forms
124
Practice Problem
Identify the precipitate that forms when aqueous solutions of zinc nitrate and ammonium sulfide are combined Write the equation for the possible double-replacement reaction Then write the formula equation overall ionic equation and the net ionic equation
125
1 AnalyzeGiven Identity of reactants zinc nitrate and ammonium sulfide Reaction medium aqueous solution Unknown a equation for the possible double-replacement reaction b identity of the precipitate c formula equation d overall ionic equation e net ionic equation
126
2 PlanWrite the possible double-replacement reaction between Zn(NO3)2 and (NH4)2S Use the rules to determine if any of the products will precipitate Write a formula equation and overall net ionic equation Then cancel spectator ions to make net ionic equation
127
3 Computea possible double-replacement reaction
b rules show that zinc sulfide is not soluble so will precipitate (ammonium nitrate is soluble)
c formula equation128
d overall ionic equation
e net ionic equation129
Will a precipitate form if solutions of potassium sulfate and barium nitrate are combined If so write the net ionic equation for the reaction
Yes Ba+2(aq) + SO4-2(aq) rarr BaSO4(s)
130
Will a precipitate form if solutions of potassium nitrate and magnesium sulfate are combined If so write the net ionic equation for the reaction
No 131
Will a precipitate form if solutions of barium chloride and sodium sulfate are combined If so identify the spectator ions and write the net ionic equation for the reaction
Yes Na+ and Cl- Ba+2(aq) + SO4-2(aq) rarr BaSO4(s)
132
Write the net ionic equation for the precipitation of nickel(II) sulfide
Ni+2(aq) + S-2(aq) rarr NiS(s)
133
IonizationSome molecular compounds can also form ions in solution Usually polar Ionization - ions formed from solute molecules by action of the solvent (creation of ions where there were none) Ionization different from dissociation
134
Dissociation ionic compounds dissolve and ions already present separate Ionization ions formed where non existed before
Ions formed are hydrated Heat released during hydration of ions gives enough energy to break covalent bonds
135
Extent to which solute ionizes depends on strength of bonds within molecules of solute and strength of attraction between solute and solvent molecules
136
HCl ndash molecular compound that ionizes in aqueous solution Contains highly polar bond Attraction between polar HCl molecule and polar water strong enough to break HCl bond
137
The Hydronium Ion
H+ ions from HCl attracts other molecules or ions so strongly that it doesnrsquot normally exist alone Ionization of HCl better described as direct transfer of proton from HCl to H2O forming H3O+
138
139
Hydration of H+ to form hydronium is highly exothermic
Energy released gives a lot of energy needed to ionize a molecular solute
Many molecular compounds that ionize in aqueous solution contain hydrogen and form hydronium ion
140
Strong and Weak Electrolytes
Substances that make ions and conduct current in solution are electrolytes HCl is one of series of compounds that has hydrogen and halogen Hydrogen halides are all molecular compounds with single polar-covalent bonds
141
All are gases Very soluble in water All are electrolytes HCl HBr HI strongly conduct current in solution HF only weakly conducts current at same concentration Strength of conduction related to ability to ionize
142
143
Strong Electrolytes
HCl HBr HI are 100 ionized in solution
Strong electrolyte - any compound whose dilute aqueous solutions conduct electricity well this is due to the presence of all or almost all of the dissolve compound in the form of ions
144
Unique characteristic of strong electrolyte is that they yield only ions
Ex Ionic compound may be highly soluble in water and dissociate into ions (NaCl)
Others may not dissolve much but amount that does dissolve exists only as ions
145
Weak ElectrolytesHF dissolves in water to give acid solution called hydrofluoric acid HF bond stronger than bonds between hydrogen and other halogens When HF dissolves some molecule ionize Reverse reaction also happens HF(aq) + H2O(l) H3O+(aq) + F-(aq)
146
HF(aq) + H2O(l) H3O+
(aq) + F-(aq)Concentration of dissolved unionized HF stays high and concentration of ions stays low
Weak electrolyte - any compound whose aqueous solutions conduct electricity poorly this is due to the presence of small amount of the dissolved compound in the form of ions
147
Different from nonelectrolyte ndash NO ions at all Description of electrolyte as strong or weak not related to concentration of solution Electrolytes differ in degree of ionization not amount of solute dissolved
148
149
Colligative properties of solutions
Presence of solute affects properties of solutions Some properties not dependent on nature of dissolved substance but on how may dissolve particles are present
Colligative properties - properties that depend on the concentration of solute particles but not on their identity
150
Vapor-Pressure Lowering
Boiling and freezing point of solution different from pure solvent Nonvolatile solute raises boiling point and lowers freezing point Nonvolatile substance - one that has little tendency to become gas under existing conditions
151
152
To understand why nonvolatile solute changes boiling and freezing point consider equilibrium vapor pressure
Vapor pressure pressure caused by molecules that have escape liquid phase to gas phase
Can be thought of a measure of tendency of molecules to escape from a liquid
153
Addition of sucrose (nonvolatile solute) lowers concentration of water molecules at surface of liquid This lowers tendency of water molecules to leave solution and enter gas phase Vapor pressure of solution is lower than vapor pressure of pure water
154
Aqueous13 solution13 of13 nonvolatile13 solutePure13 water
Nonelectrolyte solutions of same molality have same concentration of particles
Dilute solutions of same solvent and equal molality of any nonelectrolyte solute lower vapor pressure equally
155
Example
1 m aqueous solution of nonelectrolyte glucose lowers vapor pressure of water 55 x 10-4 atm at 25 1 m aqueous solution of sucrose also lowers water vapor pressure to 55 x 10-4 atm at 25
156
157
bc vapor-pressure lowering depends on concentration of nonelectrolyte solute and doesnrsquot depend on the type of solute it is a colligative proprerty
bc vapor pressure has been lowered solution remains liquid over larger temperature range
Lowers freezing point and raises boiling point
158
Can assume changes in boiling and freezing point also depend on concentration of solute
They are colligative properties
159
Freezing-Point Depression
Freezing point of 1-molal solution of any nonelectrolyte solute in water is found (by experiment) to be 186 lower than freezing point of water When 1 mol of nonelectrolyte solute dissolved in 1 kg water freezing point is -186 not 000 160
When 2 mol nonelectrolyte solute dissolved in 1 kg water freezing point is -372
For any concentration of nonelectrolyte solute in water decrease in freezing point can be determined using value of -186m
161
This value called the molal freezing-point contstant (Kf) is the freezing-point depression of the solvent in a 1-molal solution of a nonvolatile nonelectrolyte solute
162
Each solvent has own characteristic molal freezing-point constant Freezing-point depression (Δtf) the difference between the freezing points of the pure solvent and a solute of nonelectrolyte in that solvent and it is directly proportional to the molal concentration of the solution Δtf = Kfm
163
Δtf = Kfm
Kf expressed as m
m expressed in mol solutekg solvent (molality)
164
Sample Problem
What is the freezing point depression of water in a solution of 171 g of sucrose C12H22O11 and 200 g of water What is the actual freezing point of the solution
165
1 AnalyzeGiven Solute mass and chemical formula 171 g C12H22O11 Solvent mass and identity 2000 g water Unknown a freezing-point depression b freezing point of the solution 166
2 Plan
167
168
Calculate
Unknown a freezing-point depression b freezing point of the solution
169
Practice Problem
A water solution containing an unknown quantity of a nonelectrolyte solute is found to have a freezing point of ndash 023degCWhat is the molal concentration of the solution
012 m170
A solution consists of 103 g of the nonelectrolyte glucose C6H12O6 dissolved in 250 g of water What is the freezing-point depression of the solution
minus0426degC171
In a laboratory experiment the freezing point of an aqueous solution of glucose is found to be minus0325degCWhat is the molal concentration of this solution
0175 m172
If 0500 mol of a nonelectrolyte solute are dissolved in 5000 g of ether what is the freezing point of the solution
minus1181degC173
The freezing point of an aqueous solution that contains a nonelectrolyte is minus90degC aWhat is the freezing-point depression of the solution bWhat is the molal concentration of the solution a minus90degC b 48 m
174
Boiling-Point Elevation
Boiling point is temp at which vapor pressure is equal to atmospheric pressure Change in vapor pressure will cause corresponding change in boiling point
175
176
Vapor pressure of nonvolatile solution is lower than vapor pressure of pure solvent More heat will be required to raise vapor pressure of solution Boiling point of solution is higher than boiling point of pure solvent
Molal boiling-point constant (Kb) - boiling-point elevation of the solvent in a 1-molal solution of a nonvolatile nonelectrolyte solute b-p elevation of any nonelectrolyte in water found by experiment to be 051 So molal b-p constant for water is 051m
177
For different solvents b-p elevations of 1-molal solutions have different values (Table 14-2)
Like freezing-point constants values most accurate for dilute solutions
178
Boiling-point elevation Δtb difference between the boiling points of the pure solvent and a nonelectrolyte solution of that solvent and it is directly proportional to the molal concentration of the solution B-p elevation can be calculated using Δtb = Kbm Δtb (boiling point elevation) expressed in m and m expressed in mol solutekg solvent
Sample Problem
What is the boiling-point elevation of a solution made from 200 g of a nonelectrolyte solute and 4000 g of water The molar mass of the solute is 620 gmol
180
1 AnalyzeGiven solute mass = 200 g solute molar mass = 620 gmol solvent mass and identity = 4000 g of water
Unknown boiling-point elevation
181
2 Plan
182
183
3 Compute
Practice Problem
A solution contains 500 g of sucrose C12H22O11 a nonelectrolyte dissolved in 5000 g of water What is the boiling-point elevation
015degC184
A solution contains 4500 g of sucrose C12H22O11 a nonelectrolyte dissolved in 250 g of waterWhat is the boiling point of the solution
1027degC185
If the boiling point elevation of an aqueous solution containing a nonvolatile electrolyte is 102degC what is the molality of the solution
20 m186
The boiling point of an aqueous solution containing a nonvolatile electrolyte is 10075degC aWhat is the boiling-point elevation bWhat is the molality of the solution a 075degC b 15 m
187
Electrolytes and Colligative PropertiesEarly investigators confused by experiments where certain substances depressed freezing point or elevated boiling point more than expected
188
189
Ex 01 m solution of NaCl lowers freezing point of solvent almost twice as much as 01 m solution of sucrose 01 m CaCl2 lowers fp almost 3 times as much as 01 m solution of sucrose
To understand why this happens contrast behavior or sucrose with NaCl in aqueous solution Each sucrose dissolves to make only 1 particle in solution So 1 mol sucrose dissolves to 1 mol particles 1 mol NaCl dissolves to make 2 moles of particles (1 mol Na+ and 1 mol Cl-)
190
191
Calculated Values for Electrolyte SolutionsColligative properties depend on total concentration of solute particles regardless of their identity Electrolytes cause changes in colligative properties proportional to total molality in terms of all dissolves particles instead of formula units
192
What about barium nitrate Ba(NO3)2 Each mole of barium nitrate yields 3 mol of ions in solution Ba(NO3)2(s) rarr Ba2+(aq) + 2NO3-(aq) You would expect solution of given molality to lower fp of solvent 3 times as much as nonelectrolytic solution of same molality
193
Sample Problem
What is the expected change in the freezing point of water in a solution of 625 g of barium nitrate Ba(NO3)2 in 100 kg of water
194
1 AnalyzeGiven solute mass and formula = 625 g Ba(NO3)2 solvent mass and identity = 100 kg water Δtf = Kfm
Unknown expected freezing-point depression
195
2 PlanThe molality can be calculated by converting the solute mass to moles and then dividing by the number of kilograms of solvent That molality is in terms of formula units of Ba(NO3)2 and must be converted to molality in terms of dissociated ions in solution It must be multiplied by the number of moles of ions produced per mole of formula unit This adjusted molality can then be used to calculate the freezing-point depression
196
197
198
3 Compute
Practice Problem
What is the expected freezing-point depression for a solution that contains 20 mol of magnesium sulfate dissolved in 10 kg of water
minus74degC199
What is the expected boiling-point elevation of water for a solution that contains 150 g of sodium chloride dissolved in 10 kg of water 27degC The freezing point of an aqueous sodium chloride solution is minus200degC What is the molality of the solution 54 m NaCl
200
Actual Values for Electrolyte SolutionsRemember values calculated are only expected values Actual values of colligative properties for all strong electrolytes are almost what would be expected Difference between expected and actual values caused by attractive forces that exist between dissociated ions in aqueous solution
201
Attraction between hydrated ions in solution is small compared with those in crystalline solid However forces of attraction donrsquot interfere with movements of aqueous ions More concentrated solution means ions closer together and attraction is greater Also depends on charge of ions ndash higher charge greater attraction
202
21
Solutions Colloids Suspensions
Homogeneous Heterogeneous Heterogeneous
Particle13 size13 001-shy‐113 nm13 can13 be13 atoms13 ions13 molecules
Particle13 size13 1-shy‐100013 nm13 dispersed13 can13 be13 combined13 or13 large13 molecules
Particle13 size13 over13 100013 nm13 suspended13 can13 be13 large13 particles13 or13 combined13 particles
Do13 not13 separate13 on13 standing Do13 not13 separate13 on13 standing Particles13 settle13 out
Cannot13 be13 separated13 by13 filtration Cannot13 be13 separated13 by13 filtration Can13 be13 separated13 by13 filtration
Do13 not13 scatter13 light Scatter13 light13 (Tyndall13 effect) May13 scatter13 light13 but13 are13 not13 transparent
Electrolytes vs Nonelectrolytes
Substances that dissolve in water are classified according to whether they produce molecules or ions in solution
22
23
When ionic compound dissolves cations and anions separate and are surrounded by water molecules Solute ions free to move making possible to carry electric current
Electrolyte - substance that dissolves in water to give a solution that conducts electric current NaCl ndash electrolyte Usually highly polar compounds become electrolytes
24
Solution containing neutral solute molecules does not conduct current bc it doesnrsquot have mobile charged particles
Nonelectrolyte - substance that dissolves in water to give solution that does not conduct electric current Ex Sugar
25
26
27the solution
process
Factors Affecting Rate of DissolutionEver tried to dissolve sugar in iced tea or coffee You notice temperature has something to do with how quickly solute dissolves What other factors affect how quickly solutes dissolve
28
1 Increasing Surface Area of Solute
bc dissolution process occurs at surface of solute Can increase dissolution by increasing surface area
29
2 Agitating (Stirring) SolutionClose to surface of solute concentration of dissolved solute is high Stirringshaking helps disperse solute particles
30
Brings fresh solvent into contact with solute surface Contact between solvent and solute surface increased
3 Heating a Solvent
Solutes dissolve faster in warmer solvent As temp increases solvent molecules move faster Average kinetic energy increases Collisions between solvent and solute more frequent Helps separate solute molecules and spread them out
32
Solubility
If you add sugar to tea eventually no more sugar dissolves
For every combination of solvent with solute at given temp there is limit to amount of solute that can be dissolved
33
When sugar first dropped into water sugar molecules leave solid surface and move about at random in solvent Some dissolved molecules may collide with crystal and stay there As more solid dissolves and concentration of dissolved molecules increases collisions happen more often
34
Eventually molecules are returning to crystal at same rate they are going into solution Dynamic equilibrium established between dissolution and crystallization Solution equilibrium - physical state in which the opposing processes of dissolution and crystallization of a solute occur at equal rates
35
Saturated vs Unsaturated Solutions
Saturated solution - solution that contains the maximum amount of dissolved solute How do you know if solution is saturated If more NaCl added it falls to bottom and doesnrsquot dissolve Equilibrium already established
36
If more water added to saturated solution more NaCl will dissolve At 20 359 g NaCl is max that will dissolve in 100 g water Unsaturated solution - solution that contains less solute than a saturated solution under the existing conditions
37
Supersaturated Solutions
When saturated solution where solubility increases as temp increases is cooled excess solute usually comes out of solution Sometimes if solution is left to cool the excess solute doesnrsquot separate
38
39
Supersaturated solution - solution that contains more dissolved solute than a saturated solution contains under same conditions
40
Supersaturated solution can remain unchanged over long time if it isnrsquot disturbed Once crystals begin to form continues until equilibrium is recreated at lower temp
41
Example
Sodium thiosulfate Na2S2O3 Solute added to hot water until solution saturated Hot solution filtered Drop small crystal in ldquoseedingrdquo crystallization Continues until equilibrium created
42
Solubility Values
Solubility - amount of substance required to form a saturated solution with specific amount of solvent at specified temp Ex Solubility of sugar = 204 g per 100 g water at 20 Must specify temp bc solubility changes with temp 43
For gases pressure must also be specified Rate at which substance dissolves is unrelated to solubility Max amount of solute that dissolves and reaches equilibrium is always same under same conditions
44
Solute-Solvent Interactions
ldquolike dissolves likerdquo is generally useful rule for predicting whether one substance will dissolve in another Depends on type of bonding polaritynonpolarity and intermolecular forces between solute and solvent
45
Dissolving Ionic Compounds in Aqueous Solution
Polarity of water molecules plays important role in formation of solution of ionic compounds Charged ends of water molecules attract ions and surround them to keep separated from other ions
46
Hydration - solution process with water as solvent Ions said to be hydrated As hydrated ions diffuse into solution other ions exposed and drawn away from crystal surface Entire crystal gradually dissolves
47
When crystallized from aqueous solutions some ionic compounds form crystals that include water molecules The crystalline compounds known as hydrates have specific ratios of water molecules Represented by formulas like CuSO45H2O
48
When crystalline hydrate dissolves in water water of hydration returns to solvent Behavior of solute in hydrated form no different than anhydrous form
49
Nonpolar Solvents
Nonpolar solvents do not attract ions of ionic compound strongly enough to overcome forces holding crystal together
50
Liquid Solutes and Solvents
When you shake bottle of salad dressing oil droplets disperse in water Stop shaking ndash strong attraction between water molecules squeeze out oil and form separate layers Liquid solutes and solvents that are not soluble in each other are immiscible
51
Nonpolar substances (fat oil grease) are generally quite soluble in nonpolar liquids (carbon tetrachloride toluene gasoline)
Only attractions between nonpolar molecules are weak London forces
52
London Dispersion Forces
bc e- are constantly moving at any time there may be uneven distribution of them around a nonpolar atom Temporarily creates positive end and negative end
53
54
London dispersion forces - intermolecular attractions resulting from constant motion of e- and creation of instantaneous dipoles
55
Liquids that dissolve freely in one another in any ratio are said to be completely miscible Benzene and carbon tetrachloride (both nonpolar) are completely miscible
Nonpolar molecules of these two apply no strong forces of attraction or repulsion and molecules mix freely
56
Ethanol and water are also completely miscible -OH group on ethanol is slightly polar
Can form H-bonds with water as well as other ethanol molecules Intermolecular forces in mixture are very similar to those in pure liquids so they are mutually soluble in any amount
57
Gasoline contains mainly nonpolar hydrocarbons
Excellent solvent for fats oils greases
Major intermolecular forces are weak London forces
58
Ethanol less polar than water more than carbon tetrachloride
Better solvent for less-polar substances bc of nonpolar region
59
Effects of Pressure on Solubility
Little effect on solids or liquids Increase pressure - increase solubilities of gases in liquids
60
When gas in contact with surface of liquid gas molecules can enter liquid As amount of dissolve gas increases some molecules start to escape and reenter gas phase
Eq eventually established between rates of enteringleaving gas phase
61
As long as eq undisturbed solubility of gas in liquid unchanged at given pressure
Gas + solvent solution
62
Increasing pressure of solute gas above solution puts stress on eq Molecules collide with surface more often Increase in pressure partially offset by increase in rate of gas molecules entering solution In turn increase in amount of dissolved gas causes increase in rate molecules escape liquid and becomes gas
63
Eventually eq restored at higher gas solubility
Expected from Le Chatelierrsquos principle
Increase in gas pressure causes eq to shift so fewer molecules are in gas phase
64
Henryrsquos Law
Henryrsquos law - solubility of a gas in liquid is directly proportional to the partial pressure of that gas on the surface of the liquid
Applies to gas-liquid solutions at constant temp
65
When mixture of ideal gases is limited in constant volume at constant temp each gas applies same pressure it would apply if it occupied the space alone
Assuming gases do not react each gas dissolves to the extent it would if no other gases were there
66
In carbonated drinks (Coke Pepsi etc) solubility of CO2 increased by increasing pressure
At bottling factory CO2 gas forced into solution of flavored water at pressure of 5-10 atm Gas-in-liquid solution then sealed in bottlescans
67
When cap removed pressure reduced to 1 atm and some CO2 escapes as gas bubbles Effervescence - rapid escape of a gas from a liquid in which it is dissolved
68
Effects of Temperature on Solubility
Letrsquos consider gas solubility Increasing temperature usually decreases gas solubility As temp increases average kinetic energy of molecules in solution increases Greater number of solute molecules escape from attraction of solvent and return to gas phase 69
At higher temps eq is reached with fewer gas molecules in solution Gases generally less soluble
70
Effect of temp on solubility of solids in liquids more difficult to predict
Often increasing temp increases solubility
However the same temp increase can result in large increase in solubility in one case and only slight increase in the next
71
72
enthalpies of Solution
Formation of solution comes with energy change Dissolve KI in water container feels cold Dissolve LiCl in water feels hot Formation of solid-liquid solution can absorb or release heat
73
During formation of solution solvent and solute particles experience changes in forces attracting them to other particles Before dissolving begins solvent molecules held by intermolecular forces (solvent-solvent attraction) In solute molecules held by Ifs (solute-solute attraction)
74
Energy is required to separate solute molecules and solvent molecules from their neighbors
A solute particle that is surrounded by solvent molecules is said to be solvated
75
Step 1 solute particles become separated from solid (energy absorbed) Step 2 solvent particles move apart to allow solute particles to enter liquid (energy absorbed)
76
Step 3 solvent particles attracted to and solvate solute particles (energy released)
77
Heat of solution - net amount of heat energy absorbed or released when a specific amount of solute dissolves in a solvent
Heat of solution negative (heat is released) if sums of energy in steps 1 and 2 is less than step 3
If 1+2 gt 3 HOS is positive (heat absorbed)78
In gaseous state molecules are so far apart there are nearly no forces between them Solute-solute interaction has little effect on heat of solution of a gas Energy released when gas dissolved in liquid bc attraction between solute gas and solvent molecules greater than energy needed to separate solvent molecules
79
Concentration of Solutions80
Concentration - measure of amount of solute in given amount of solvent or solution ldquodiluterdquo just means small amount of solute ldquoconcentratedrdquo just means relatively large amount of solute Unrelated to degree to which solution is saturated Ex Saturated solution of substance not very soluble might be dilute
81
Molarity
Molarity - number of moles of solute in one liter of solution To find molarity must know molar mass A ldquoone molarrdquo solution of NaOH is 1 mole NaOH per liter of solution (1 M)
82
1 mol NaOH = 400 g If this quantity dissolve in enough water to make EXACTLY 100 L solution solution is 1 M If 200 g NaOH dissolve in enough to make 100 L solution what is molarity 0500 M
83
1 M solution is not made by adding 1 mol solute to 1 L solvent That would be more than 1 L Instead 1 mol solute dissolved in less than 1 L Then diluted with more solvent to bring TOTAL VOLUME to 1 L
84
Practice ProblemYou have 340 L of solution that contains 900 g sodium chloride What is the molarity of that solution 0440 M NaCl You have 08 L of a 05 M HCl solution How many moles of HCl does this solution contain 04 mol HCl
85
What is the molarity of a solution composed of 585 g of potassium iodide dissolved in enough water to make a 0125 L of solution 0282 M KI
86
How many moles of H2SO4 are present in 0500 L of a 0150 M H2SO4 solution 00750 mol What volume of 300 M NaCl is needed for reaction that requires 1463 g of NaCl 0834 L
87
Molality
Molality - concentration of solution expressed in moles of solute per kilogram of solvent Solution that contains 1 mol solute (NaOH) dissolved in exactly 1 kg of solvent is ldquoone molalrdquo solution (1 m NaOH)
88
1 mol NaOH = 400 g 400 g NaOH dissolved in 1 kg water is 1 m NaOH
200 g NaOH in 1 kg water = 0500 m NaOH
89
Practice Problem
A solution was prepared by dissolving 171 g sucrose (C12H22O11) in 125 g of water Find the molal concentration of this solution 0400 m C12H22O11
90
A solution of iodine in carbon tetrachloride is used when iodine is needed for certain chemical tests How much iodine must be added to prepare a 0480 m solution of iodine in CCl4 if 1000 g of CCl4 is used 122 g I2
91
What is the molality of a solution composed of 255 g acetone (CH3)2CO dissolved in 200 g of water 22 m acetone What quantity in grams of methanol CH3OH is required to prepare a 0244 m solution in 400 g water 312 g methanol
92
How many grams of AgNO3 are needed to prepare a 0125 m solution in 250 mL of water 531 g AgNO3
What is the molality of a solution containing 182 g HCl and 250 g water 200 m
93
Ions in Aqueous Solutions and Colligative Properties
94
95
compounds in aqueous solutions
DissociationWhen compound made from ions dissolves in water ions separate Dissociation - separation of ions that occurs when an ionic compound dissolves
96
Notice number of ions made per formula unit in equations 1 formula unit of NaCl gives 2 units of ions in solution 1 formula unit of CaCl2 gives 3 units of ions
97
Assuming 100 dissociation solution that contains 1 mol NaCl contains 1 mol Na+ and 1 mol Cl- Can assume 100 dissociation of all soluble ionic compounds
98
Practice Problem
Write the equation for the dissolution of aluminum sulfate in water How many moles of aluminum ions are made by dissolving 1 mol aluminum sulfate What is the total number of moles of ions made by dissolving 1 mol of aluminum sulfate
99
1 AnalyzeGiven Amount of solute = 1 mol Al2(SO4)3 Solvent identity = water Unknown a moles of aluminum ions and sulfate ions Total number of moles of solute ions produced
100
2 Plan
The coefficients in the balanced dissociation equation will tell mole relationships You can use the equation to find out the number of moles of solute ions produced
101
3 Compute
a
b102
Write the equation for the dissolution of each of the following in water and then determine the number of moles of each ions made as well as the total number of ions made a 1 mol ammonium chloride b 1 mol sodium sulfide c 05 mol barium nitrate
103
a 1 mol ammonium chloride
1 mol NH4+ 1 mol Cl- 2 mol total ions
104
b 1 mol sodium sulfide
2 mol Na+ 1 mol S-2 3 mol total ions
105
c 05 mol barium nitrate
05 mol Ba+2 1 mol NO3- 15 mol total ions
106
Precipitation Reactions
Even though no compound is completely insoluble compounds of very low solubility can be considered insoluble for practical purposes There are general guidelines to help predict whether a compound made of certain combination of ions is soluble
107
General Solubility Guidelines
Sodium potassium and ammonium compounds are soluble in water
108
Nitrates acetates and chlorates are soluble
109
110
Most chlorides are soluble except those of silver mercury (I) and lead Lead (II) chloride is soluble in hot water
Most sulfates are soluble except those of barium strontium lead calcium and mercury
111
Most carbonates phosphates and silicates are insoluble except those of sodium potassium and ammonium
112
Most sulfides are insoluble except those of calcium strontium sodium potassium and ammonium
113
Is calcium phosphate Ca3(PO4)2 soluble or insoluble
Not one of the exceptions so it is insoluble
Dissociation equations are not written for insoluble compounds
114
Guidelines also useful in predicting what will happen if 2 solutions of 2 different soluble compounds are mixed
If mixing results in combination of ions that forms insoluble compound a double-replacement and precipitation reaction will happen
115
Will precipitate form when solutions of ammonium sulfide and cadmium nitrate are combined You can tell calcium nitrate is soluble from the guidelines (Most nitrates acetates and chlorates are soluble) You can also tell ammonium sulfide is soluble (Most sulfides are insoluble except those of calcium strontium sodium potassium and ammonium)
116
117
To decide whether precipitate can form must know solubilities of two possible products
Ammonium nitrate ndash soluble (Most nitrates acetates and chlorates are soluble) Cadmium sulfide ndash insoluble (Most sulfides are insoluble except those of calcium strontium sodium potassium and ammonium)
118
bc one product is insoluble double-replacement and precipitation reaction will happen (NH4)2S (aq) + Cd(NO3)2 (aq) rarr NH4NO3 (aq) + CdS (s)
119
Net Ionic Equations
Reactions of ions in aqueous solution usually represented by net ionic equations instead of formula equations Net ionic equation - includes only those compounds and ions that undergo a chemical change in a reaction in an aqueous solution
120
121
To write NIE (net ionic equation) first convert chemical equation into overall ionic equation All soluble ionic compounds shown as dissociated ions in solution Precipitates shown as solids
122
Notice ammonium and nitrate ions appear on both sides They havenrsquot gone through any chemical change Spectator ions - ions that do not take part in a chemical reaction and are found in solution both before and after the reaction
123
To change ionic equation to NIE eliminate spectator ions Cd+2(aq) + S-2(aq) rarr CdS(s)
This NIE applies to any reaction in which precipitate of CdS forms
124
Practice Problem
Identify the precipitate that forms when aqueous solutions of zinc nitrate and ammonium sulfide are combined Write the equation for the possible double-replacement reaction Then write the formula equation overall ionic equation and the net ionic equation
125
1 AnalyzeGiven Identity of reactants zinc nitrate and ammonium sulfide Reaction medium aqueous solution Unknown a equation for the possible double-replacement reaction b identity of the precipitate c formula equation d overall ionic equation e net ionic equation
126
2 PlanWrite the possible double-replacement reaction between Zn(NO3)2 and (NH4)2S Use the rules to determine if any of the products will precipitate Write a formula equation and overall net ionic equation Then cancel spectator ions to make net ionic equation
127
3 Computea possible double-replacement reaction
b rules show that zinc sulfide is not soluble so will precipitate (ammonium nitrate is soluble)
c formula equation128
d overall ionic equation
e net ionic equation129
Will a precipitate form if solutions of potassium sulfate and barium nitrate are combined If so write the net ionic equation for the reaction
Yes Ba+2(aq) + SO4-2(aq) rarr BaSO4(s)
130
Will a precipitate form if solutions of potassium nitrate and magnesium sulfate are combined If so write the net ionic equation for the reaction
No 131
Will a precipitate form if solutions of barium chloride and sodium sulfate are combined If so identify the spectator ions and write the net ionic equation for the reaction
Yes Na+ and Cl- Ba+2(aq) + SO4-2(aq) rarr BaSO4(s)
132
Write the net ionic equation for the precipitation of nickel(II) sulfide
Ni+2(aq) + S-2(aq) rarr NiS(s)
133
IonizationSome molecular compounds can also form ions in solution Usually polar Ionization - ions formed from solute molecules by action of the solvent (creation of ions where there were none) Ionization different from dissociation
134
Dissociation ionic compounds dissolve and ions already present separate Ionization ions formed where non existed before
Ions formed are hydrated Heat released during hydration of ions gives enough energy to break covalent bonds
135
Extent to which solute ionizes depends on strength of bonds within molecules of solute and strength of attraction between solute and solvent molecules
136
HCl ndash molecular compound that ionizes in aqueous solution Contains highly polar bond Attraction between polar HCl molecule and polar water strong enough to break HCl bond
137
The Hydronium Ion
H+ ions from HCl attracts other molecules or ions so strongly that it doesnrsquot normally exist alone Ionization of HCl better described as direct transfer of proton from HCl to H2O forming H3O+
138
139
Hydration of H+ to form hydronium is highly exothermic
Energy released gives a lot of energy needed to ionize a molecular solute
Many molecular compounds that ionize in aqueous solution contain hydrogen and form hydronium ion
140
Strong and Weak Electrolytes
Substances that make ions and conduct current in solution are electrolytes HCl is one of series of compounds that has hydrogen and halogen Hydrogen halides are all molecular compounds with single polar-covalent bonds
141
All are gases Very soluble in water All are electrolytes HCl HBr HI strongly conduct current in solution HF only weakly conducts current at same concentration Strength of conduction related to ability to ionize
142
143
Strong Electrolytes
HCl HBr HI are 100 ionized in solution
Strong electrolyte - any compound whose dilute aqueous solutions conduct electricity well this is due to the presence of all or almost all of the dissolve compound in the form of ions
144
Unique characteristic of strong electrolyte is that they yield only ions
Ex Ionic compound may be highly soluble in water and dissociate into ions (NaCl)
Others may not dissolve much but amount that does dissolve exists only as ions
145
Weak ElectrolytesHF dissolves in water to give acid solution called hydrofluoric acid HF bond stronger than bonds between hydrogen and other halogens When HF dissolves some molecule ionize Reverse reaction also happens HF(aq) + H2O(l) H3O+(aq) + F-(aq)
146
HF(aq) + H2O(l) H3O+
(aq) + F-(aq)Concentration of dissolved unionized HF stays high and concentration of ions stays low
Weak electrolyte - any compound whose aqueous solutions conduct electricity poorly this is due to the presence of small amount of the dissolved compound in the form of ions
147
Different from nonelectrolyte ndash NO ions at all Description of electrolyte as strong or weak not related to concentration of solution Electrolytes differ in degree of ionization not amount of solute dissolved
148
149
Colligative properties of solutions
Presence of solute affects properties of solutions Some properties not dependent on nature of dissolved substance but on how may dissolve particles are present
Colligative properties - properties that depend on the concentration of solute particles but not on their identity
150
Vapor-Pressure Lowering
Boiling and freezing point of solution different from pure solvent Nonvolatile solute raises boiling point and lowers freezing point Nonvolatile substance - one that has little tendency to become gas under existing conditions
151
152
To understand why nonvolatile solute changes boiling and freezing point consider equilibrium vapor pressure
Vapor pressure pressure caused by molecules that have escape liquid phase to gas phase
Can be thought of a measure of tendency of molecules to escape from a liquid
153
Addition of sucrose (nonvolatile solute) lowers concentration of water molecules at surface of liquid This lowers tendency of water molecules to leave solution and enter gas phase Vapor pressure of solution is lower than vapor pressure of pure water
154
Aqueous13 solution13 of13 nonvolatile13 solutePure13 water
Nonelectrolyte solutions of same molality have same concentration of particles
Dilute solutions of same solvent and equal molality of any nonelectrolyte solute lower vapor pressure equally
155
Example
1 m aqueous solution of nonelectrolyte glucose lowers vapor pressure of water 55 x 10-4 atm at 25 1 m aqueous solution of sucrose also lowers water vapor pressure to 55 x 10-4 atm at 25
156
157
bc vapor-pressure lowering depends on concentration of nonelectrolyte solute and doesnrsquot depend on the type of solute it is a colligative proprerty
bc vapor pressure has been lowered solution remains liquid over larger temperature range
Lowers freezing point and raises boiling point
158
Can assume changes in boiling and freezing point also depend on concentration of solute
They are colligative properties
159
Freezing-Point Depression
Freezing point of 1-molal solution of any nonelectrolyte solute in water is found (by experiment) to be 186 lower than freezing point of water When 1 mol of nonelectrolyte solute dissolved in 1 kg water freezing point is -186 not 000 160
When 2 mol nonelectrolyte solute dissolved in 1 kg water freezing point is -372
For any concentration of nonelectrolyte solute in water decrease in freezing point can be determined using value of -186m
161
This value called the molal freezing-point contstant (Kf) is the freezing-point depression of the solvent in a 1-molal solution of a nonvolatile nonelectrolyte solute
162
Each solvent has own characteristic molal freezing-point constant Freezing-point depression (Δtf) the difference between the freezing points of the pure solvent and a solute of nonelectrolyte in that solvent and it is directly proportional to the molal concentration of the solution Δtf = Kfm
163
Δtf = Kfm
Kf expressed as m
m expressed in mol solutekg solvent (molality)
164
Sample Problem
What is the freezing point depression of water in a solution of 171 g of sucrose C12H22O11 and 200 g of water What is the actual freezing point of the solution
165
1 AnalyzeGiven Solute mass and chemical formula 171 g C12H22O11 Solvent mass and identity 2000 g water Unknown a freezing-point depression b freezing point of the solution 166
2 Plan
167
168
Calculate
Unknown a freezing-point depression b freezing point of the solution
169
Practice Problem
A water solution containing an unknown quantity of a nonelectrolyte solute is found to have a freezing point of ndash 023degCWhat is the molal concentration of the solution
012 m170
A solution consists of 103 g of the nonelectrolyte glucose C6H12O6 dissolved in 250 g of water What is the freezing-point depression of the solution
minus0426degC171
In a laboratory experiment the freezing point of an aqueous solution of glucose is found to be minus0325degCWhat is the molal concentration of this solution
0175 m172
If 0500 mol of a nonelectrolyte solute are dissolved in 5000 g of ether what is the freezing point of the solution
minus1181degC173
The freezing point of an aqueous solution that contains a nonelectrolyte is minus90degC aWhat is the freezing-point depression of the solution bWhat is the molal concentration of the solution a minus90degC b 48 m
174
Boiling-Point Elevation
Boiling point is temp at which vapor pressure is equal to atmospheric pressure Change in vapor pressure will cause corresponding change in boiling point
175
176
Vapor pressure of nonvolatile solution is lower than vapor pressure of pure solvent More heat will be required to raise vapor pressure of solution Boiling point of solution is higher than boiling point of pure solvent
Molal boiling-point constant (Kb) - boiling-point elevation of the solvent in a 1-molal solution of a nonvolatile nonelectrolyte solute b-p elevation of any nonelectrolyte in water found by experiment to be 051 So molal b-p constant for water is 051m
177
For different solvents b-p elevations of 1-molal solutions have different values (Table 14-2)
Like freezing-point constants values most accurate for dilute solutions
178
Boiling-point elevation Δtb difference between the boiling points of the pure solvent and a nonelectrolyte solution of that solvent and it is directly proportional to the molal concentration of the solution B-p elevation can be calculated using Δtb = Kbm Δtb (boiling point elevation) expressed in m and m expressed in mol solutekg solvent
Sample Problem
What is the boiling-point elevation of a solution made from 200 g of a nonelectrolyte solute and 4000 g of water The molar mass of the solute is 620 gmol
180
1 AnalyzeGiven solute mass = 200 g solute molar mass = 620 gmol solvent mass and identity = 4000 g of water
Unknown boiling-point elevation
181
2 Plan
182
183
3 Compute
Practice Problem
A solution contains 500 g of sucrose C12H22O11 a nonelectrolyte dissolved in 5000 g of water What is the boiling-point elevation
015degC184
A solution contains 4500 g of sucrose C12H22O11 a nonelectrolyte dissolved in 250 g of waterWhat is the boiling point of the solution
1027degC185
If the boiling point elevation of an aqueous solution containing a nonvolatile electrolyte is 102degC what is the molality of the solution
20 m186
The boiling point of an aqueous solution containing a nonvolatile electrolyte is 10075degC aWhat is the boiling-point elevation bWhat is the molality of the solution a 075degC b 15 m
187
Electrolytes and Colligative PropertiesEarly investigators confused by experiments where certain substances depressed freezing point or elevated boiling point more than expected
188
189
Ex 01 m solution of NaCl lowers freezing point of solvent almost twice as much as 01 m solution of sucrose 01 m CaCl2 lowers fp almost 3 times as much as 01 m solution of sucrose
To understand why this happens contrast behavior or sucrose with NaCl in aqueous solution Each sucrose dissolves to make only 1 particle in solution So 1 mol sucrose dissolves to 1 mol particles 1 mol NaCl dissolves to make 2 moles of particles (1 mol Na+ and 1 mol Cl-)
190
191
Calculated Values for Electrolyte SolutionsColligative properties depend on total concentration of solute particles regardless of their identity Electrolytes cause changes in colligative properties proportional to total molality in terms of all dissolves particles instead of formula units
192
What about barium nitrate Ba(NO3)2 Each mole of barium nitrate yields 3 mol of ions in solution Ba(NO3)2(s) rarr Ba2+(aq) + 2NO3-(aq) You would expect solution of given molality to lower fp of solvent 3 times as much as nonelectrolytic solution of same molality
193
Sample Problem
What is the expected change in the freezing point of water in a solution of 625 g of barium nitrate Ba(NO3)2 in 100 kg of water
194
1 AnalyzeGiven solute mass and formula = 625 g Ba(NO3)2 solvent mass and identity = 100 kg water Δtf = Kfm
Unknown expected freezing-point depression
195
2 PlanThe molality can be calculated by converting the solute mass to moles and then dividing by the number of kilograms of solvent That molality is in terms of formula units of Ba(NO3)2 and must be converted to molality in terms of dissociated ions in solution It must be multiplied by the number of moles of ions produced per mole of formula unit This adjusted molality can then be used to calculate the freezing-point depression
196
197
198
3 Compute
Practice Problem
What is the expected freezing-point depression for a solution that contains 20 mol of magnesium sulfate dissolved in 10 kg of water
minus74degC199
What is the expected boiling-point elevation of water for a solution that contains 150 g of sodium chloride dissolved in 10 kg of water 27degC The freezing point of an aqueous sodium chloride solution is minus200degC What is the molality of the solution 54 m NaCl
200
Actual Values for Electrolyte SolutionsRemember values calculated are only expected values Actual values of colligative properties for all strong electrolytes are almost what would be expected Difference between expected and actual values caused by attractive forces that exist between dissociated ions in aqueous solution
201
Attraction between hydrated ions in solution is small compared with those in crystalline solid However forces of attraction donrsquot interfere with movements of aqueous ions More concentrated solution means ions closer together and attraction is greater Also depends on charge of ions ndash higher charge greater attraction
202
Electrolytes vs Nonelectrolytes
Substances that dissolve in water are classified according to whether they produce molecules or ions in solution
22
23
When ionic compound dissolves cations and anions separate and are surrounded by water molecules Solute ions free to move making possible to carry electric current
Electrolyte - substance that dissolves in water to give a solution that conducts electric current NaCl ndash electrolyte Usually highly polar compounds become electrolytes
24
Solution containing neutral solute molecules does not conduct current bc it doesnrsquot have mobile charged particles
Nonelectrolyte - substance that dissolves in water to give solution that does not conduct electric current Ex Sugar
25
26
27the solution
process
Factors Affecting Rate of DissolutionEver tried to dissolve sugar in iced tea or coffee You notice temperature has something to do with how quickly solute dissolves What other factors affect how quickly solutes dissolve
28
1 Increasing Surface Area of Solute
bc dissolution process occurs at surface of solute Can increase dissolution by increasing surface area
29
2 Agitating (Stirring) SolutionClose to surface of solute concentration of dissolved solute is high Stirringshaking helps disperse solute particles
30
Brings fresh solvent into contact with solute surface Contact between solvent and solute surface increased
3 Heating a Solvent
Solutes dissolve faster in warmer solvent As temp increases solvent molecules move faster Average kinetic energy increases Collisions between solvent and solute more frequent Helps separate solute molecules and spread them out
32
Solubility
If you add sugar to tea eventually no more sugar dissolves
For every combination of solvent with solute at given temp there is limit to amount of solute that can be dissolved
33
When sugar first dropped into water sugar molecules leave solid surface and move about at random in solvent Some dissolved molecules may collide with crystal and stay there As more solid dissolves and concentration of dissolved molecules increases collisions happen more often
34
Eventually molecules are returning to crystal at same rate they are going into solution Dynamic equilibrium established between dissolution and crystallization Solution equilibrium - physical state in which the opposing processes of dissolution and crystallization of a solute occur at equal rates
35
Saturated vs Unsaturated Solutions
Saturated solution - solution that contains the maximum amount of dissolved solute How do you know if solution is saturated If more NaCl added it falls to bottom and doesnrsquot dissolve Equilibrium already established
36
If more water added to saturated solution more NaCl will dissolve At 20 359 g NaCl is max that will dissolve in 100 g water Unsaturated solution - solution that contains less solute than a saturated solution under the existing conditions
37
Supersaturated Solutions
When saturated solution where solubility increases as temp increases is cooled excess solute usually comes out of solution Sometimes if solution is left to cool the excess solute doesnrsquot separate
38
39
Supersaturated solution - solution that contains more dissolved solute than a saturated solution contains under same conditions
40
Supersaturated solution can remain unchanged over long time if it isnrsquot disturbed Once crystals begin to form continues until equilibrium is recreated at lower temp
41
Example
Sodium thiosulfate Na2S2O3 Solute added to hot water until solution saturated Hot solution filtered Drop small crystal in ldquoseedingrdquo crystallization Continues until equilibrium created
42
Solubility Values
Solubility - amount of substance required to form a saturated solution with specific amount of solvent at specified temp Ex Solubility of sugar = 204 g per 100 g water at 20 Must specify temp bc solubility changes with temp 43
For gases pressure must also be specified Rate at which substance dissolves is unrelated to solubility Max amount of solute that dissolves and reaches equilibrium is always same under same conditions
44
Solute-Solvent Interactions
ldquolike dissolves likerdquo is generally useful rule for predicting whether one substance will dissolve in another Depends on type of bonding polaritynonpolarity and intermolecular forces between solute and solvent
45
Dissolving Ionic Compounds in Aqueous Solution
Polarity of water molecules plays important role in formation of solution of ionic compounds Charged ends of water molecules attract ions and surround them to keep separated from other ions
46
Hydration - solution process with water as solvent Ions said to be hydrated As hydrated ions diffuse into solution other ions exposed and drawn away from crystal surface Entire crystal gradually dissolves
47
When crystallized from aqueous solutions some ionic compounds form crystals that include water molecules The crystalline compounds known as hydrates have specific ratios of water molecules Represented by formulas like CuSO45H2O
48
When crystalline hydrate dissolves in water water of hydration returns to solvent Behavior of solute in hydrated form no different than anhydrous form
49
Nonpolar Solvents
Nonpolar solvents do not attract ions of ionic compound strongly enough to overcome forces holding crystal together
50
Liquid Solutes and Solvents
When you shake bottle of salad dressing oil droplets disperse in water Stop shaking ndash strong attraction between water molecules squeeze out oil and form separate layers Liquid solutes and solvents that are not soluble in each other are immiscible
51
Nonpolar substances (fat oil grease) are generally quite soluble in nonpolar liquids (carbon tetrachloride toluene gasoline)
Only attractions between nonpolar molecules are weak London forces
52
London Dispersion Forces
bc e- are constantly moving at any time there may be uneven distribution of them around a nonpolar atom Temporarily creates positive end and negative end
53
54
London dispersion forces - intermolecular attractions resulting from constant motion of e- and creation of instantaneous dipoles
55
Liquids that dissolve freely in one another in any ratio are said to be completely miscible Benzene and carbon tetrachloride (both nonpolar) are completely miscible
Nonpolar molecules of these two apply no strong forces of attraction or repulsion and molecules mix freely
56
Ethanol and water are also completely miscible -OH group on ethanol is slightly polar
Can form H-bonds with water as well as other ethanol molecules Intermolecular forces in mixture are very similar to those in pure liquids so they are mutually soluble in any amount
57
Gasoline contains mainly nonpolar hydrocarbons
Excellent solvent for fats oils greases
Major intermolecular forces are weak London forces
58
Ethanol less polar than water more than carbon tetrachloride
Better solvent for less-polar substances bc of nonpolar region
59
Effects of Pressure on Solubility
Little effect on solids or liquids Increase pressure - increase solubilities of gases in liquids
60
When gas in contact with surface of liquid gas molecules can enter liquid As amount of dissolve gas increases some molecules start to escape and reenter gas phase
Eq eventually established between rates of enteringleaving gas phase
61
As long as eq undisturbed solubility of gas in liquid unchanged at given pressure
Gas + solvent solution
62
Increasing pressure of solute gas above solution puts stress on eq Molecules collide with surface more often Increase in pressure partially offset by increase in rate of gas molecules entering solution In turn increase in amount of dissolved gas causes increase in rate molecules escape liquid and becomes gas
63
Eventually eq restored at higher gas solubility
Expected from Le Chatelierrsquos principle
Increase in gas pressure causes eq to shift so fewer molecules are in gas phase
64
Henryrsquos Law
Henryrsquos law - solubility of a gas in liquid is directly proportional to the partial pressure of that gas on the surface of the liquid
Applies to gas-liquid solutions at constant temp
65
When mixture of ideal gases is limited in constant volume at constant temp each gas applies same pressure it would apply if it occupied the space alone
Assuming gases do not react each gas dissolves to the extent it would if no other gases were there
66
In carbonated drinks (Coke Pepsi etc) solubility of CO2 increased by increasing pressure
At bottling factory CO2 gas forced into solution of flavored water at pressure of 5-10 atm Gas-in-liquid solution then sealed in bottlescans
67
When cap removed pressure reduced to 1 atm and some CO2 escapes as gas bubbles Effervescence - rapid escape of a gas from a liquid in which it is dissolved
68
Effects of Temperature on Solubility
Letrsquos consider gas solubility Increasing temperature usually decreases gas solubility As temp increases average kinetic energy of molecules in solution increases Greater number of solute molecules escape from attraction of solvent and return to gas phase 69
At higher temps eq is reached with fewer gas molecules in solution Gases generally less soluble
70
Effect of temp on solubility of solids in liquids more difficult to predict
Often increasing temp increases solubility
However the same temp increase can result in large increase in solubility in one case and only slight increase in the next
71
72
enthalpies of Solution
Formation of solution comes with energy change Dissolve KI in water container feels cold Dissolve LiCl in water feels hot Formation of solid-liquid solution can absorb or release heat
73
During formation of solution solvent and solute particles experience changes in forces attracting them to other particles Before dissolving begins solvent molecules held by intermolecular forces (solvent-solvent attraction) In solute molecules held by Ifs (solute-solute attraction)
74
Energy is required to separate solute molecules and solvent molecules from their neighbors
A solute particle that is surrounded by solvent molecules is said to be solvated
75
Step 1 solute particles become separated from solid (energy absorbed) Step 2 solvent particles move apart to allow solute particles to enter liquid (energy absorbed)
76
Step 3 solvent particles attracted to and solvate solute particles (energy released)
77
Heat of solution - net amount of heat energy absorbed or released when a specific amount of solute dissolves in a solvent
Heat of solution negative (heat is released) if sums of energy in steps 1 and 2 is less than step 3
If 1+2 gt 3 HOS is positive (heat absorbed)78
In gaseous state molecules are so far apart there are nearly no forces between them Solute-solute interaction has little effect on heat of solution of a gas Energy released when gas dissolved in liquid bc attraction between solute gas and solvent molecules greater than energy needed to separate solvent molecules
79
Concentration of Solutions80
Concentration - measure of amount of solute in given amount of solvent or solution ldquodiluterdquo just means small amount of solute ldquoconcentratedrdquo just means relatively large amount of solute Unrelated to degree to which solution is saturated Ex Saturated solution of substance not very soluble might be dilute
81
Molarity
Molarity - number of moles of solute in one liter of solution To find molarity must know molar mass A ldquoone molarrdquo solution of NaOH is 1 mole NaOH per liter of solution (1 M)
82
1 mol NaOH = 400 g If this quantity dissolve in enough water to make EXACTLY 100 L solution solution is 1 M If 200 g NaOH dissolve in enough to make 100 L solution what is molarity 0500 M
83
1 M solution is not made by adding 1 mol solute to 1 L solvent That would be more than 1 L Instead 1 mol solute dissolved in less than 1 L Then diluted with more solvent to bring TOTAL VOLUME to 1 L
84
Practice ProblemYou have 340 L of solution that contains 900 g sodium chloride What is the molarity of that solution 0440 M NaCl You have 08 L of a 05 M HCl solution How many moles of HCl does this solution contain 04 mol HCl
85
What is the molarity of a solution composed of 585 g of potassium iodide dissolved in enough water to make a 0125 L of solution 0282 M KI
86
How many moles of H2SO4 are present in 0500 L of a 0150 M H2SO4 solution 00750 mol What volume of 300 M NaCl is needed for reaction that requires 1463 g of NaCl 0834 L
87
Molality
Molality - concentration of solution expressed in moles of solute per kilogram of solvent Solution that contains 1 mol solute (NaOH) dissolved in exactly 1 kg of solvent is ldquoone molalrdquo solution (1 m NaOH)
88
1 mol NaOH = 400 g 400 g NaOH dissolved in 1 kg water is 1 m NaOH
200 g NaOH in 1 kg water = 0500 m NaOH
89
Practice Problem
A solution was prepared by dissolving 171 g sucrose (C12H22O11) in 125 g of water Find the molal concentration of this solution 0400 m C12H22O11
90
A solution of iodine in carbon tetrachloride is used when iodine is needed for certain chemical tests How much iodine must be added to prepare a 0480 m solution of iodine in CCl4 if 1000 g of CCl4 is used 122 g I2
91
What is the molality of a solution composed of 255 g acetone (CH3)2CO dissolved in 200 g of water 22 m acetone What quantity in grams of methanol CH3OH is required to prepare a 0244 m solution in 400 g water 312 g methanol
92
How many grams of AgNO3 are needed to prepare a 0125 m solution in 250 mL of water 531 g AgNO3
What is the molality of a solution containing 182 g HCl and 250 g water 200 m
93
Ions in Aqueous Solutions and Colligative Properties
94
95
compounds in aqueous solutions
DissociationWhen compound made from ions dissolves in water ions separate Dissociation - separation of ions that occurs when an ionic compound dissolves
96
Notice number of ions made per formula unit in equations 1 formula unit of NaCl gives 2 units of ions in solution 1 formula unit of CaCl2 gives 3 units of ions
97
Assuming 100 dissociation solution that contains 1 mol NaCl contains 1 mol Na+ and 1 mol Cl- Can assume 100 dissociation of all soluble ionic compounds
98
Practice Problem
Write the equation for the dissolution of aluminum sulfate in water How many moles of aluminum ions are made by dissolving 1 mol aluminum sulfate What is the total number of moles of ions made by dissolving 1 mol of aluminum sulfate
99
1 AnalyzeGiven Amount of solute = 1 mol Al2(SO4)3 Solvent identity = water Unknown a moles of aluminum ions and sulfate ions Total number of moles of solute ions produced
100
2 Plan
The coefficients in the balanced dissociation equation will tell mole relationships You can use the equation to find out the number of moles of solute ions produced
101
3 Compute
a
b102
Write the equation for the dissolution of each of the following in water and then determine the number of moles of each ions made as well as the total number of ions made a 1 mol ammonium chloride b 1 mol sodium sulfide c 05 mol barium nitrate
103
a 1 mol ammonium chloride
1 mol NH4+ 1 mol Cl- 2 mol total ions
104
b 1 mol sodium sulfide
2 mol Na+ 1 mol S-2 3 mol total ions
105
c 05 mol barium nitrate
05 mol Ba+2 1 mol NO3- 15 mol total ions
106
Precipitation Reactions
Even though no compound is completely insoluble compounds of very low solubility can be considered insoluble for practical purposes There are general guidelines to help predict whether a compound made of certain combination of ions is soluble
107
General Solubility Guidelines
Sodium potassium and ammonium compounds are soluble in water
108
Nitrates acetates and chlorates are soluble
109
110
Most chlorides are soluble except those of silver mercury (I) and lead Lead (II) chloride is soluble in hot water
Most sulfates are soluble except those of barium strontium lead calcium and mercury
111
Most carbonates phosphates and silicates are insoluble except those of sodium potassium and ammonium
112
Most sulfides are insoluble except those of calcium strontium sodium potassium and ammonium
113
Is calcium phosphate Ca3(PO4)2 soluble or insoluble
Not one of the exceptions so it is insoluble
Dissociation equations are not written for insoluble compounds
114
Guidelines also useful in predicting what will happen if 2 solutions of 2 different soluble compounds are mixed
If mixing results in combination of ions that forms insoluble compound a double-replacement and precipitation reaction will happen
115
Will precipitate form when solutions of ammonium sulfide and cadmium nitrate are combined You can tell calcium nitrate is soluble from the guidelines (Most nitrates acetates and chlorates are soluble) You can also tell ammonium sulfide is soluble (Most sulfides are insoluble except those of calcium strontium sodium potassium and ammonium)
116
117
To decide whether precipitate can form must know solubilities of two possible products
Ammonium nitrate ndash soluble (Most nitrates acetates and chlorates are soluble) Cadmium sulfide ndash insoluble (Most sulfides are insoluble except those of calcium strontium sodium potassium and ammonium)
118
bc one product is insoluble double-replacement and precipitation reaction will happen (NH4)2S (aq) + Cd(NO3)2 (aq) rarr NH4NO3 (aq) + CdS (s)
119
Net Ionic Equations
Reactions of ions in aqueous solution usually represented by net ionic equations instead of formula equations Net ionic equation - includes only those compounds and ions that undergo a chemical change in a reaction in an aqueous solution
120
121
To write NIE (net ionic equation) first convert chemical equation into overall ionic equation All soluble ionic compounds shown as dissociated ions in solution Precipitates shown as solids
122
Notice ammonium and nitrate ions appear on both sides They havenrsquot gone through any chemical change Spectator ions - ions that do not take part in a chemical reaction and are found in solution both before and after the reaction
123
To change ionic equation to NIE eliminate spectator ions Cd+2(aq) + S-2(aq) rarr CdS(s)
This NIE applies to any reaction in which precipitate of CdS forms
124
Practice Problem
Identify the precipitate that forms when aqueous solutions of zinc nitrate and ammonium sulfide are combined Write the equation for the possible double-replacement reaction Then write the formula equation overall ionic equation and the net ionic equation
125
1 AnalyzeGiven Identity of reactants zinc nitrate and ammonium sulfide Reaction medium aqueous solution Unknown a equation for the possible double-replacement reaction b identity of the precipitate c formula equation d overall ionic equation e net ionic equation
126
2 PlanWrite the possible double-replacement reaction between Zn(NO3)2 and (NH4)2S Use the rules to determine if any of the products will precipitate Write a formula equation and overall net ionic equation Then cancel spectator ions to make net ionic equation
127
3 Computea possible double-replacement reaction
b rules show that zinc sulfide is not soluble so will precipitate (ammonium nitrate is soluble)
c formula equation128
d overall ionic equation
e net ionic equation129
Will a precipitate form if solutions of potassium sulfate and barium nitrate are combined If so write the net ionic equation for the reaction
Yes Ba+2(aq) + SO4-2(aq) rarr BaSO4(s)
130
Will a precipitate form if solutions of potassium nitrate and magnesium sulfate are combined If so write the net ionic equation for the reaction
No 131
Will a precipitate form if solutions of barium chloride and sodium sulfate are combined If so identify the spectator ions and write the net ionic equation for the reaction
Yes Na+ and Cl- Ba+2(aq) + SO4-2(aq) rarr BaSO4(s)
132
Write the net ionic equation for the precipitation of nickel(II) sulfide
Ni+2(aq) + S-2(aq) rarr NiS(s)
133
IonizationSome molecular compounds can also form ions in solution Usually polar Ionization - ions formed from solute molecules by action of the solvent (creation of ions where there were none) Ionization different from dissociation
134
Dissociation ionic compounds dissolve and ions already present separate Ionization ions formed where non existed before
Ions formed are hydrated Heat released during hydration of ions gives enough energy to break covalent bonds
135
Extent to which solute ionizes depends on strength of bonds within molecules of solute and strength of attraction between solute and solvent molecules
136
HCl ndash molecular compound that ionizes in aqueous solution Contains highly polar bond Attraction between polar HCl molecule and polar water strong enough to break HCl bond
137
The Hydronium Ion
H+ ions from HCl attracts other molecules or ions so strongly that it doesnrsquot normally exist alone Ionization of HCl better described as direct transfer of proton from HCl to H2O forming H3O+
138
139
Hydration of H+ to form hydronium is highly exothermic
Energy released gives a lot of energy needed to ionize a molecular solute
Many molecular compounds that ionize in aqueous solution contain hydrogen and form hydronium ion
140
Strong and Weak Electrolytes
Substances that make ions and conduct current in solution are electrolytes HCl is one of series of compounds that has hydrogen and halogen Hydrogen halides are all molecular compounds with single polar-covalent bonds
141
All are gases Very soluble in water All are electrolytes HCl HBr HI strongly conduct current in solution HF only weakly conducts current at same concentration Strength of conduction related to ability to ionize
142
143
Strong Electrolytes
HCl HBr HI are 100 ionized in solution
Strong electrolyte - any compound whose dilute aqueous solutions conduct electricity well this is due to the presence of all or almost all of the dissolve compound in the form of ions
144
Unique characteristic of strong electrolyte is that they yield only ions
Ex Ionic compound may be highly soluble in water and dissociate into ions (NaCl)
Others may not dissolve much but amount that does dissolve exists only as ions
145
Weak ElectrolytesHF dissolves in water to give acid solution called hydrofluoric acid HF bond stronger than bonds between hydrogen and other halogens When HF dissolves some molecule ionize Reverse reaction also happens HF(aq) + H2O(l) H3O+(aq) + F-(aq)
146
HF(aq) + H2O(l) H3O+
(aq) + F-(aq)Concentration of dissolved unionized HF stays high and concentration of ions stays low
Weak electrolyte - any compound whose aqueous solutions conduct electricity poorly this is due to the presence of small amount of the dissolved compound in the form of ions
147
Different from nonelectrolyte ndash NO ions at all Description of electrolyte as strong or weak not related to concentration of solution Electrolytes differ in degree of ionization not amount of solute dissolved
148
149
Colligative properties of solutions
Presence of solute affects properties of solutions Some properties not dependent on nature of dissolved substance but on how may dissolve particles are present
Colligative properties - properties that depend on the concentration of solute particles but not on their identity
150
Vapor-Pressure Lowering
Boiling and freezing point of solution different from pure solvent Nonvolatile solute raises boiling point and lowers freezing point Nonvolatile substance - one that has little tendency to become gas under existing conditions
151
152
To understand why nonvolatile solute changes boiling and freezing point consider equilibrium vapor pressure
Vapor pressure pressure caused by molecules that have escape liquid phase to gas phase
Can be thought of a measure of tendency of molecules to escape from a liquid
153
Addition of sucrose (nonvolatile solute) lowers concentration of water molecules at surface of liquid This lowers tendency of water molecules to leave solution and enter gas phase Vapor pressure of solution is lower than vapor pressure of pure water
154
Aqueous13 solution13 of13 nonvolatile13 solutePure13 water
Nonelectrolyte solutions of same molality have same concentration of particles
Dilute solutions of same solvent and equal molality of any nonelectrolyte solute lower vapor pressure equally
155
Example
1 m aqueous solution of nonelectrolyte glucose lowers vapor pressure of water 55 x 10-4 atm at 25 1 m aqueous solution of sucrose also lowers water vapor pressure to 55 x 10-4 atm at 25
156
157
bc vapor-pressure lowering depends on concentration of nonelectrolyte solute and doesnrsquot depend on the type of solute it is a colligative proprerty
bc vapor pressure has been lowered solution remains liquid over larger temperature range
Lowers freezing point and raises boiling point
158
Can assume changes in boiling and freezing point also depend on concentration of solute
They are colligative properties
159
Freezing-Point Depression
Freezing point of 1-molal solution of any nonelectrolyte solute in water is found (by experiment) to be 186 lower than freezing point of water When 1 mol of nonelectrolyte solute dissolved in 1 kg water freezing point is -186 not 000 160
When 2 mol nonelectrolyte solute dissolved in 1 kg water freezing point is -372
For any concentration of nonelectrolyte solute in water decrease in freezing point can be determined using value of -186m
161
This value called the molal freezing-point contstant (Kf) is the freezing-point depression of the solvent in a 1-molal solution of a nonvolatile nonelectrolyte solute
162
Each solvent has own characteristic molal freezing-point constant Freezing-point depression (Δtf) the difference between the freezing points of the pure solvent and a solute of nonelectrolyte in that solvent and it is directly proportional to the molal concentration of the solution Δtf = Kfm
163
Δtf = Kfm
Kf expressed as m
m expressed in mol solutekg solvent (molality)
164
Sample Problem
What is the freezing point depression of water in a solution of 171 g of sucrose C12H22O11 and 200 g of water What is the actual freezing point of the solution
165
1 AnalyzeGiven Solute mass and chemical formula 171 g C12H22O11 Solvent mass and identity 2000 g water Unknown a freezing-point depression b freezing point of the solution 166
2 Plan
167
168
Calculate
Unknown a freezing-point depression b freezing point of the solution
169
Practice Problem
A water solution containing an unknown quantity of a nonelectrolyte solute is found to have a freezing point of ndash 023degCWhat is the molal concentration of the solution
012 m170
A solution consists of 103 g of the nonelectrolyte glucose C6H12O6 dissolved in 250 g of water What is the freezing-point depression of the solution
minus0426degC171
In a laboratory experiment the freezing point of an aqueous solution of glucose is found to be minus0325degCWhat is the molal concentration of this solution
0175 m172
If 0500 mol of a nonelectrolyte solute are dissolved in 5000 g of ether what is the freezing point of the solution
minus1181degC173
The freezing point of an aqueous solution that contains a nonelectrolyte is minus90degC aWhat is the freezing-point depression of the solution bWhat is the molal concentration of the solution a minus90degC b 48 m
174
Boiling-Point Elevation
Boiling point is temp at which vapor pressure is equal to atmospheric pressure Change in vapor pressure will cause corresponding change in boiling point
175
176
Vapor pressure of nonvolatile solution is lower than vapor pressure of pure solvent More heat will be required to raise vapor pressure of solution Boiling point of solution is higher than boiling point of pure solvent
Molal boiling-point constant (Kb) - boiling-point elevation of the solvent in a 1-molal solution of a nonvolatile nonelectrolyte solute b-p elevation of any nonelectrolyte in water found by experiment to be 051 So molal b-p constant for water is 051m
177
For different solvents b-p elevations of 1-molal solutions have different values (Table 14-2)
Like freezing-point constants values most accurate for dilute solutions
178
Boiling-point elevation Δtb difference between the boiling points of the pure solvent and a nonelectrolyte solution of that solvent and it is directly proportional to the molal concentration of the solution B-p elevation can be calculated using Δtb = Kbm Δtb (boiling point elevation) expressed in m and m expressed in mol solutekg solvent
Sample Problem
What is the boiling-point elevation of a solution made from 200 g of a nonelectrolyte solute and 4000 g of water The molar mass of the solute is 620 gmol
180
1 AnalyzeGiven solute mass = 200 g solute molar mass = 620 gmol solvent mass and identity = 4000 g of water
Unknown boiling-point elevation
181
2 Plan
182
183
3 Compute
Practice Problem
A solution contains 500 g of sucrose C12H22O11 a nonelectrolyte dissolved in 5000 g of water What is the boiling-point elevation
015degC184
A solution contains 4500 g of sucrose C12H22O11 a nonelectrolyte dissolved in 250 g of waterWhat is the boiling point of the solution
1027degC185
If the boiling point elevation of an aqueous solution containing a nonvolatile electrolyte is 102degC what is the molality of the solution
20 m186
The boiling point of an aqueous solution containing a nonvolatile electrolyte is 10075degC aWhat is the boiling-point elevation bWhat is the molality of the solution a 075degC b 15 m
187
Electrolytes and Colligative PropertiesEarly investigators confused by experiments where certain substances depressed freezing point or elevated boiling point more than expected
188
189
Ex 01 m solution of NaCl lowers freezing point of solvent almost twice as much as 01 m solution of sucrose 01 m CaCl2 lowers fp almost 3 times as much as 01 m solution of sucrose
To understand why this happens contrast behavior or sucrose with NaCl in aqueous solution Each sucrose dissolves to make only 1 particle in solution So 1 mol sucrose dissolves to 1 mol particles 1 mol NaCl dissolves to make 2 moles of particles (1 mol Na+ and 1 mol Cl-)
190
191
Calculated Values for Electrolyte SolutionsColligative properties depend on total concentration of solute particles regardless of their identity Electrolytes cause changes in colligative properties proportional to total molality in terms of all dissolves particles instead of formula units
192
What about barium nitrate Ba(NO3)2 Each mole of barium nitrate yields 3 mol of ions in solution Ba(NO3)2(s) rarr Ba2+(aq) + 2NO3-(aq) You would expect solution of given molality to lower fp of solvent 3 times as much as nonelectrolytic solution of same molality
193
Sample Problem
What is the expected change in the freezing point of water in a solution of 625 g of barium nitrate Ba(NO3)2 in 100 kg of water
194
1 AnalyzeGiven solute mass and formula = 625 g Ba(NO3)2 solvent mass and identity = 100 kg water Δtf = Kfm
Unknown expected freezing-point depression
195
2 PlanThe molality can be calculated by converting the solute mass to moles and then dividing by the number of kilograms of solvent That molality is in terms of formula units of Ba(NO3)2 and must be converted to molality in terms of dissociated ions in solution It must be multiplied by the number of moles of ions produced per mole of formula unit This adjusted molality can then be used to calculate the freezing-point depression
196
197
198
3 Compute
Practice Problem
What is the expected freezing-point depression for a solution that contains 20 mol of magnesium sulfate dissolved in 10 kg of water
minus74degC199
What is the expected boiling-point elevation of water for a solution that contains 150 g of sodium chloride dissolved in 10 kg of water 27degC The freezing point of an aqueous sodium chloride solution is minus200degC What is the molality of the solution 54 m NaCl
200
Actual Values for Electrolyte SolutionsRemember values calculated are only expected values Actual values of colligative properties for all strong electrolytes are almost what would be expected Difference between expected and actual values caused by attractive forces that exist between dissociated ions in aqueous solution
201
Attraction between hydrated ions in solution is small compared with those in crystalline solid However forces of attraction donrsquot interfere with movements of aqueous ions More concentrated solution means ions closer together and attraction is greater Also depends on charge of ions ndash higher charge greater attraction
202
23
When ionic compound dissolves cations and anions separate and are surrounded by water molecules Solute ions free to move making possible to carry electric current
Electrolyte - substance that dissolves in water to give a solution that conducts electric current NaCl ndash electrolyte Usually highly polar compounds become electrolytes
24
Solution containing neutral solute molecules does not conduct current bc it doesnrsquot have mobile charged particles
Nonelectrolyte - substance that dissolves in water to give solution that does not conduct electric current Ex Sugar
25
26
27the solution
process
Factors Affecting Rate of DissolutionEver tried to dissolve sugar in iced tea or coffee You notice temperature has something to do with how quickly solute dissolves What other factors affect how quickly solutes dissolve
28
1 Increasing Surface Area of Solute
bc dissolution process occurs at surface of solute Can increase dissolution by increasing surface area
29
2 Agitating (Stirring) SolutionClose to surface of solute concentration of dissolved solute is high Stirringshaking helps disperse solute particles
30
Brings fresh solvent into contact with solute surface Contact between solvent and solute surface increased
3 Heating a Solvent
Solutes dissolve faster in warmer solvent As temp increases solvent molecules move faster Average kinetic energy increases Collisions between solvent and solute more frequent Helps separate solute molecules and spread them out
32
Solubility
If you add sugar to tea eventually no more sugar dissolves
For every combination of solvent with solute at given temp there is limit to amount of solute that can be dissolved
33
When sugar first dropped into water sugar molecules leave solid surface and move about at random in solvent Some dissolved molecules may collide with crystal and stay there As more solid dissolves and concentration of dissolved molecules increases collisions happen more often
34
Eventually molecules are returning to crystal at same rate they are going into solution Dynamic equilibrium established between dissolution and crystallization Solution equilibrium - physical state in which the opposing processes of dissolution and crystallization of a solute occur at equal rates
35
Saturated vs Unsaturated Solutions
Saturated solution - solution that contains the maximum amount of dissolved solute How do you know if solution is saturated If more NaCl added it falls to bottom and doesnrsquot dissolve Equilibrium already established
36
If more water added to saturated solution more NaCl will dissolve At 20 359 g NaCl is max that will dissolve in 100 g water Unsaturated solution - solution that contains less solute than a saturated solution under the existing conditions
37
Supersaturated Solutions
When saturated solution where solubility increases as temp increases is cooled excess solute usually comes out of solution Sometimes if solution is left to cool the excess solute doesnrsquot separate
38
39
Supersaturated solution - solution that contains more dissolved solute than a saturated solution contains under same conditions
40
Supersaturated solution can remain unchanged over long time if it isnrsquot disturbed Once crystals begin to form continues until equilibrium is recreated at lower temp
41
Example
Sodium thiosulfate Na2S2O3 Solute added to hot water until solution saturated Hot solution filtered Drop small crystal in ldquoseedingrdquo crystallization Continues until equilibrium created
42
Solubility Values
Solubility - amount of substance required to form a saturated solution with specific amount of solvent at specified temp Ex Solubility of sugar = 204 g per 100 g water at 20 Must specify temp bc solubility changes with temp 43
For gases pressure must also be specified Rate at which substance dissolves is unrelated to solubility Max amount of solute that dissolves and reaches equilibrium is always same under same conditions
44
Solute-Solvent Interactions
ldquolike dissolves likerdquo is generally useful rule for predicting whether one substance will dissolve in another Depends on type of bonding polaritynonpolarity and intermolecular forces between solute and solvent
45
Dissolving Ionic Compounds in Aqueous Solution
Polarity of water molecules plays important role in formation of solution of ionic compounds Charged ends of water molecules attract ions and surround them to keep separated from other ions
46
Hydration - solution process with water as solvent Ions said to be hydrated As hydrated ions diffuse into solution other ions exposed and drawn away from crystal surface Entire crystal gradually dissolves
47
When crystallized from aqueous solutions some ionic compounds form crystals that include water molecules The crystalline compounds known as hydrates have specific ratios of water molecules Represented by formulas like CuSO45H2O
48
When crystalline hydrate dissolves in water water of hydration returns to solvent Behavior of solute in hydrated form no different than anhydrous form
49
Nonpolar Solvents
Nonpolar solvents do not attract ions of ionic compound strongly enough to overcome forces holding crystal together
50
Liquid Solutes and Solvents
When you shake bottle of salad dressing oil droplets disperse in water Stop shaking ndash strong attraction between water molecules squeeze out oil and form separate layers Liquid solutes and solvents that are not soluble in each other are immiscible
51
Nonpolar substances (fat oil grease) are generally quite soluble in nonpolar liquids (carbon tetrachloride toluene gasoline)
Only attractions between nonpolar molecules are weak London forces
52
London Dispersion Forces
bc e- are constantly moving at any time there may be uneven distribution of them around a nonpolar atom Temporarily creates positive end and negative end
53
54
London dispersion forces - intermolecular attractions resulting from constant motion of e- and creation of instantaneous dipoles
55
Liquids that dissolve freely in one another in any ratio are said to be completely miscible Benzene and carbon tetrachloride (both nonpolar) are completely miscible
Nonpolar molecules of these two apply no strong forces of attraction or repulsion and molecules mix freely
56
Ethanol and water are also completely miscible -OH group on ethanol is slightly polar
Can form H-bonds with water as well as other ethanol molecules Intermolecular forces in mixture are very similar to those in pure liquids so they are mutually soluble in any amount
57
Gasoline contains mainly nonpolar hydrocarbons
Excellent solvent for fats oils greases
Major intermolecular forces are weak London forces
58
Ethanol less polar than water more than carbon tetrachloride
Better solvent for less-polar substances bc of nonpolar region
59
Effects of Pressure on Solubility
Little effect on solids or liquids Increase pressure - increase solubilities of gases in liquids
60
When gas in contact with surface of liquid gas molecules can enter liquid As amount of dissolve gas increases some molecules start to escape and reenter gas phase
Eq eventually established between rates of enteringleaving gas phase
61
As long as eq undisturbed solubility of gas in liquid unchanged at given pressure
Gas + solvent solution
62
Increasing pressure of solute gas above solution puts stress on eq Molecules collide with surface more often Increase in pressure partially offset by increase in rate of gas molecules entering solution In turn increase in amount of dissolved gas causes increase in rate molecules escape liquid and becomes gas
63
Eventually eq restored at higher gas solubility
Expected from Le Chatelierrsquos principle
Increase in gas pressure causes eq to shift so fewer molecules are in gas phase
64
Henryrsquos Law
Henryrsquos law - solubility of a gas in liquid is directly proportional to the partial pressure of that gas on the surface of the liquid
Applies to gas-liquid solutions at constant temp
65
When mixture of ideal gases is limited in constant volume at constant temp each gas applies same pressure it would apply if it occupied the space alone
Assuming gases do not react each gas dissolves to the extent it would if no other gases were there
66
In carbonated drinks (Coke Pepsi etc) solubility of CO2 increased by increasing pressure
At bottling factory CO2 gas forced into solution of flavored water at pressure of 5-10 atm Gas-in-liquid solution then sealed in bottlescans
67
When cap removed pressure reduced to 1 atm and some CO2 escapes as gas bubbles Effervescence - rapid escape of a gas from a liquid in which it is dissolved
68
Effects of Temperature on Solubility
Letrsquos consider gas solubility Increasing temperature usually decreases gas solubility As temp increases average kinetic energy of molecules in solution increases Greater number of solute molecules escape from attraction of solvent and return to gas phase 69
At higher temps eq is reached with fewer gas molecules in solution Gases generally less soluble
70
Effect of temp on solubility of solids in liquids more difficult to predict
Often increasing temp increases solubility
However the same temp increase can result in large increase in solubility in one case and only slight increase in the next
71
72
enthalpies of Solution
Formation of solution comes with energy change Dissolve KI in water container feels cold Dissolve LiCl in water feels hot Formation of solid-liquid solution can absorb or release heat
73
During formation of solution solvent and solute particles experience changes in forces attracting them to other particles Before dissolving begins solvent molecules held by intermolecular forces (solvent-solvent attraction) In solute molecules held by Ifs (solute-solute attraction)
74
Energy is required to separate solute molecules and solvent molecules from their neighbors
A solute particle that is surrounded by solvent molecules is said to be solvated
75
Step 1 solute particles become separated from solid (energy absorbed) Step 2 solvent particles move apart to allow solute particles to enter liquid (energy absorbed)
76
Step 3 solvent particles attracted to and solvate solute particles (energy released)
77
Heat of solution - net amount of heat energy absorbed or released when a specific amount of solute dissolves in a solvent
Heat of solution negative (heat is released) if sums of energy in steps 1 and 2 is less than step 3
If 1+2 gt 3 HOS is positive (heat absorbed)78
In gaseous state molecules are so far apart there are nearly no forces between them Solute-solute interaction has little effect on heat of solution of a gas Energy released when gas dissolved in liquid bc attraction between solute gas and solvent molecules greater than energy needed to separate solvent molecules
79
Concentration of Solutions80
Concentration - measure of amount of solute in given amount of solvent or solution ldquodiluterdquo just means small amount of solute ldquoconcentratedrdquo just means relatively large amount of solute Unrelated to degree to which solution is saturated Ex Saturated solution of substance not very soluble might be dilute
81
Molarity
Molarity - number of moles of solute in one liter of solution To find molarity must know molar mass A ldquoone molarrdquo solution of NaOH is 1 mole NaOH per liter of solution (1 M)
82
1 mol NaOH = 400 g If this quantity dissolve in enough water to make EXACTLY 100 L solution solution is 1 M If 200 g NaOH dissolve in enough to make 100 L solution what is molarity 0500 M
83
1 M solution is not made by adding 1 mol solute to 1 L solvent That would be more than 1 L Instead 1 mol solute dissolved in less than 1 L Then diluted with more solvent to bring TOTAL VOLUME to 1 L
84
Practice ProblemYou have 340 L of solution that contains 900 g sodium chloride What is the molarity of that solution 0440 M NaCl You have 08 L of a 05 M HCl solution How many moles of HCl does this solution contain 04 mol HCl
85
What is the molarity of a solution composed of 585 g of potassium iodide dissolved in enough water to make a 0125 L of solution 0282 M KI
86
How many moles of H2SO4 are present in 0500 L of a 0150 M H2SO4 solution 00750 mol What volume of 300 M NaCl is needed for reaction that requires 1463 g of NaCl 0834 L
87
Molality
Molality - concentration of solution expressed in moles of solute per kilogram of solvent Solution that contains 1 mol solute (NaOH) dissolved in exactly 1 kg of solvent is ldquoone molalrdquo solution (1 m NaOH)
88
1 mol NaOH = 400 g 400 g NaOH dissolved in 1 kg water is 1 m NaOH
200 g NaOH in 1 kg water = 0500 m NaOH
89
Practice Problem
A solution was prepared by dissolving 171 g sucrose (C12H22O11) in 125 g of water Find the molal concentration of this solution 0400 m C12H22O11
90
A solution of iodine in carbon tetrachloride is used when iodine is needed for certain chemical tests How much iodine must be added to prepare a 0480 m solution of iodine in CCl4 if 1000 g of CCl4 is used 122 g I2
91
What is the molality of a solution composed of 255 g acetone (CH3)2CO dissolved in 200 g of water 22 m acetone What quantity in grams of methanol CH3OH is required to prepare a 0244 m solution in 400 g water 312 g methanol
92
How many grams of AgNO3 are needed to prepare a 0125 m solution in 250 mL of water 531 g AgNO3
What is the molality of a solution containing 182 g HCl and 250 g water 200 m
93
Ions in Aqueous Solutions and Colligative Properties
94
95
compounds in aqueous solutions
DissociationWhen compound made from ions dissolves in water ions separate Dissociation - separation of ions that occurs when an ionic compound dissolves
96
Notice number of ions made per formula unit in equations 1 formula unit of NaCl gives 2 units of ions in solution 1 formula unit of CaCl2 gives 3 units of ions
97
Assuming 100 dissociation solution that contains 1 mol NaCl contains 1 mol Na+ and 1 mol Cl- Can assume 100 dissociation of all soluble ionic compounds
98
Practice Problem
Write the equation for the dissolution of aluminum sulfate in water How many moles of aluminum ions are made by dissolving 1 mol aluminum sulfate What is the total number of moles of ions made by dissolving 1 mol of aluminum sulfate
99
1 AnalyzeGiven Amount of solute = 1 mol Al2(SO4)3 Solvent identity = water Unknown a moles of aluminum ions and sulfate ions Total number of moles of solute ions produced
100
2 Plan
The coefficients in the balanced dissociation equation will tell mole relationships You can use the equation to find out the number of moles of solute ions produced
101
3 Compute
a
b102
Write the equation for the dissolution of each of the following in water and then determine the number of moles of each ions made as well as the total number of ions made a 1 mol ammonium chloride b 1 mol sodium sulfide c 05 mol barium nitrate
103
a 1 mol ammonium chloride
1 mol NH4+ 1 mol Cl- 2 mol total ions
104
b 1 mol sodium sulfide
2 mol Na+ 1 mol S-2 3 mol total ions
105
c 05 mol barium nitrate
05 mol Ba+2 1 mol NO3- 15 mol total ions
106
Precipitation Reactions
Even though no compound is completely insoluble compounds of very low solubility can be considered insoluble for practical purposes There are general guidelines to help predict whether a compound made of certain combination of ions is soluble
107
General Solubility Guidelines
Sodium potassium and ammonium compounds are soluble in water
108
Nitrates acetates and chlorates are soluble
109
110
Most chlorides are soluble except those of silver mercury (I) and lead Lead (II) chloride is soluble in hot water
Most sulfates are soluble except those of barium strontium lead calcium and mercury
111
Most carbonates phosphates and silicates are insoluble except those of sodium potassium and ammonium
112
Most sulfides are insoluble except those of calcium strontium sodium potassium and ammonium
113
Is calcium phosphate Ca3(PO4)2 soluble or insoluble
Not one of the exceptions so it is insoluble
Dissociation equations are not written for insoluble compounds
114
Guidelines also useful in predicting what will happen if 2 solutions of 2 different soluble compounds are mixed
If mixing results in combination of ions that forms insoluble compound a double-replacement and precipitation reaction will happen
115
Will precipitate form when solutions of ammonium sulfide and cadmium nitrate are combined You can tell calcium nitrate is soluble from the guidelines (Most nitrates acetates and chlorates are soluble) You can also tell ammonium sulfide is soluble (Most sulfides are insoluble except those of calcium strontium sodium potassium and ammonium)
116
117
To decide whether precipitate can form must know solubilities of two possible products
Ammonium nitrate ndash soluble (Most nitrates acetates and chlorates are soluble) Cadmium sulfide ndash insoluble (Most sulfides are insoluble except those of calcium strontium sodium potassium and ammonium)
118
bc one product is insoluble double-replacement and precipitation reaction will happen (NH4)2S (aq) + Cd(NO3)2 (aq) rarr NH4NO3 (aq) + CdS (s)
119
Net Ionic Equations
Reactions of ions in aqueous solution usually represented by net ionic equations instead of formula equations Net ionic equation - includes only those compounds and ions that undergo a chemical change in a reaction in an aqueous solution
120
121
To write NIE (net ionic equation) first convert chemical equation into overall ionic equation All soluble ionic compounds shown as dissociated ions in solution Precipitates shown as solids
122
Notice ammonium and nitrate ions appear on both sides They havenrsquot gone through any chemical change Spectator ions - ions that do not take part in a chemical reaction and are found in solution both before and after the reaction
123
To change ionic equation to NIE eliminate spectator ions Cd+2(aq) + S-2(aq) rarr CdS(s)
This NIE applies to any reaction in which precipitate of CdS forms
124
Practice Problem
Identify the precipitate that forms when aqueous solutions of zinc nitrate and ammonium sulfide are combined Write the equation for the possible double-replacement reaction Then write the formula equation overall ionic equation and the net ionic equation
125
1 AnalyzeGiven Identity of reactants zinc nitrate and ammonium sulfide Reaction medium aqueous solution Unknown a equation for the possible double-replacement reaction b identity of the precipitate c formula equation d overall ionic equation e net ionic equation
126
2 PlanWrite the possible double-replacement reaction between Zn(NO3)2 and (NH4)2S Use the rules to determine if any of the products will precipitate Write a formula equation and overall net ionic equation Then cancel spectator ions to make net ionic equation
127
3 Computea possible double-replacement reaction
b rules show that zinc sulfide is not soluble so will precipitate (ammonium nitrate is soluble)
c formula equation128
d overall ionic equation
e net ionic equation129
Will a precipitate form if solutions of potassium sulfate and barium nitrate are combined If so write the net ionic equation for the reaction
Yes Ba+2(aq) + SO4-2(aq) rarr BaSO4(s)
130
Will a precipitate form if solutions of potassium nitrate and magnesium sulfate are combined If so write the net ionic equation for the reaction
No 131
Will a precipitate form if solutions of barium chloride and sodium sulfate are combined If so identify the spectator ions and write the net ionic equation for the reaction
Yes Na+ and Cl- Ba+2(aq) + SO4-2(aq) rarr BaSO4(s)
132
Write the net ionic equation for the precipitation of nickel(II) sulfide
Ni+2(aq) + S-2(aq) rarr NiS(s)
133
IonizationSome molecular compounds can also form ions in solution Usually polar Ionization - ions formed from solute molecules by action of the solvent (creation of ions where there were none) Ionization different from dissociation
134
Dissociation ionic compounds dissolve and ions already present separate Ionization ions formed where non existed before
Ions formed are hydrated Heat released during hydration of ions gives enough energy to break covalent bonds
135
Extent to which solute ionizes depends on strength of bonds within molecules of solute and strength of attraction between solute and solvent molecules
136
HCl ndash molecular compound that ionizes in aqueous solution Contains highly polar bond Attraction between polar HCl molecule and polar water strong enough to break HCl bond
137
The Hydronium Ion
H+ ions from HCl attracts other molecules or ions so strongly that it doesnrsquot normally exist alone Ionization of HCl better described as direct transfer of proton from HCl to H2O forming H3O+
138
139
Hydration of H+ to form hydronium is highly exothermic
Energy released gives a lot of energy needed to ionize a molecular solute
Many molecular compounds that ionize in aqueous solution contain hydrogen and form hydronium ion
140
Strong and Weak Electrolytes
Substances that make ions and conduct current in solution are electrolytes HCl is one of series of compounds that has hydrogen and halogen Hydrogen halides are all molecular compounds with single polar-covalent bonds
141
All are gases Very soluble in water All are electrolytes HCl HBr HI strongly conduct current in solution HF only weakly conducts current at same concentration Strength of conduction related to ability to ionize
142
143
Strong Electrolytes
HCl HBr HI are 100 ionized in solution
Strong electrolyte - any compound whose dilute aqueous solutions conduct electricity well this is due to the presence of all or almost all of the dissolve compound in the form of ions
144
Unique characteristic of strong electrolyte is that they yield only ions
Ex Ionic compound may be highly soluble in water and dissociate into ions (NaCl)
Others may not dissolve much but amount that does dissolve exists only as ions
145
Weak ElectrolytesHF dissolves in water to give acid solution called hydrofluoric acid HF bond stronger than bonds between hydrogen and other halogens When HF dissolves some molecule ionize Reverse reaction also happens HF(aq) + H2O(l) H3O+(aq) + F-(aq)
146
HF(aq) + H2O(l) H3O+
(aq) + F-(aq)Concentration of dissolved unionized HF stays high and concentration of ions stays low
Weak electrolyte - any compound whose aqueous solutions conduct electricity poorly this is due to the presence of small amount of the dissolved compound in the form of ions
147
Different from nonelectrolyte ndash NO ions at all Description of electrolyte as strong or weak not related to concentration of solution Electrolytes differ in degree of ionization not amount of solute dissolved
148
149
Colligative properties of solutions
Presence of solute affects properties of solutions Some properties not dependent on nature of dissolved substance but on how may dissolve particles are present
Colligative properties - properties that depend on the concentration of solute particles but not on their identity
150
Vapor-Pressure Lowering
Boiling and freezing point of solution different from pure solvent Nonvolatile solute raises boiling point and lowers freezing point Nonvolatile substance - one that has little tendency to become gas under existing conditions
151
152
To understand why nonvolatile solute changes boiling and freezing point consider equilibrium vapor pressure
Vapor pressure pressure caused by molecules that have escape liquid phase to gas phase
Can be thought of a measure of tendency of molecules to escape from a liquid
153
Addition of sucrose (nonvolatile solute) lowers concentration of water molecules at surface of liquid This lowers tendency of water molecules to leave solution and enter gas phase Vapor pressure of solution is lower than vapor pressure of pure water
154
Aqueous13 solution13 of13 nonvolatile13 solutePure13 water
Nonelectrolyte solutions of same molality have same concentration of particles
Dilute solutions of same solvent and equal molality of any nonelectrolyte solute lower vapor pressure equally
155
Example
1 m aqueous solution of nonelectrolyte glucose lowers vapor pressure of water 55 x 10-4 atm at 25 1 m aqueous solution of sucrose also lowers water vapor pressure to 55 x 10-4 atm at 25
156
157
bc vapor-pressure lowering depends on concentration of nonelectrolyte solute and doesnrsquot depend on the type of solute it is a colligative proprerty
bc vapor pressure has been lowered solution remains liquid over larger temperature range
Lowers freezing point and raises boiling point
158
Can assume changes in boiling and freezing point also depend on concentration of solute
They are colligative properties
159
Freezing-Point Depression
Freezing point of 1-molal solution of any nonelectrolyte solute in water is found (by experiment) to be 186 lower than freezing point of water When 1 mol of nonelectrolyte solute dissolved in 1 kg water freezing point is -186 not 000 160
When 2 mol nonelectrolyte solute dissolved in 1 kg water freezing point is -372
For any concentration of nonelectrolyte solute in water decrease in freezing point can be determined using value of -186m
161
This value called the molal freezing-point contstant (Kf) is the freezing-point depression of the solvent in a 1-molal solution of a nonvolatile nonelectrolyte solute
162
Each solvent has own characteristic molal freezing-point constant Freezing-point depression (Δtf) the difference between the freezing points of the pure solvent and a solute of nonelectrolyte in that solvent and it is directly proportional to the molal concentration of the solution Δtf = Kfm
163
Δtf = Kfm
Kf expressed as m
m expressed in mol solutekg solvent (molality)
164
Sample Problem
What is the freezing point depression of water in a solution of 171 g of sucrose C12H22O11 and 200 g of water What is the actual freezing point of the solution
165
1 AnalyzeGiven Solute mass and chemical formula 171 g C12H22O11 Solvent mass and identity 2000 g water Unknown a freezing-point depression b freezing point of the solution 166
2 Plan
167
168
Calculate
Unknown a freezing-point depression b freezing point of the solution
169
Practice Problem
A water solution containing an unknown quantity of a nonelectrolyte solute is found to have a freezing point of ndash 023degCWhat is the molal concentration of the solution
012 m170
A solution consists of 103 g of the nonelectrolyte glucose C6H12O6 dissolved in 250 g of water What is the freezing-point depression of the solution
minus0426degC171
In a laboratory experiment the freezing point of an aqueous solution of glucose is found to be minus0325degCWhat is the molal concentration of this solution
0175 m172
If 0500 mol of a nonelectrolyte solute are dissolved in 5000 g of ether what is the freezing point of the solution
minus1181degC173
The freezing point of an aqueous solution that contains a nonelectrolyte is minus90degC aWhat is the freezing-point depression of the solution bWhat is the molal concentration of the solution a minus90degC b 48 m
174
Boiling-Point Elevation
Boiling point is temp at which vapor pressure is equal to atmospheric pressure Change in vapor pressure will cause corresponding change in boiling point
175
176
Vapor pressure of nonvolatile solution is lower than vapor pressure of pure solvent More heat will be required to raise vapor pressure of solution Boiling point of solution is higher than boiling point of pure solvent
Molal boiling-point constant (Kb) - boiling-point elevation of the solvent in a 1-molal solution of a nonvolatile nonelectrolyte solute b-p elevation of any nonelectrolyte in water found by experiment to be 051 So molal b-p constant for water is 051m
177
For different solvents b-p elevations of 1-molal solutions have different values (Table 14-2)
Like freezing-point constants values most accurate for dilute solutions
178
Boiling-point elevation Δtb difference between the boiling points of the pure solvent and a nonelectrolyte solution of that solvent and it is directly proportional to the molal concentration of the solution B-p elevation can be calculated using Δtb = Kbm Δtb (boiling point elevation) expressed in m and m expressed in mol solutekg solvent
Sample Problem
What is the boiling-point elevation of a solution made from 200 g of a nonelectrolyte solute and 4000 g of water The molar mass of the solute is 620 gmol
180
1 AnalyzeGiven solute mass = 200 g solute molar mass = 620 gmol solvent mass and identity = 4000 g of water
Unknown boiling-point elevation
181
2 Plan
182
183
3 Compute
Practice Problem
A solution contains 500 g of sucrose C12H22O11 a nonelectrolyte dissolved in 5000 g of water What is the boiling-point elevation
015degC184
A solution contains 4500 g of sucrose C12H22O11 a nonelectrolyte dissolved in 250 g of waterWhat is the boiling point of the solution
1027degC185
If the boiling point elevation of an aqueous solution containing a nonvolatile electrolyte is 102degC what is the molality of the solution
20 m186
The boiling point of an aqueous solution containing a nonvolatile electrolyte is 10075degC aWhat is the boiling-point elevation bWhat is the molality of the solution a 075degC b 15 m
187
Electrolytes and Colligative PropertiesEarly investigators confused by experiments where certain substances depressed freezing point or elevated boiling point more than expected
188
189
Ex 01 m solution of NaCl lowers freezing point of solvent almost twice as much as 01 m solution of sucrose 01 m CaCl2 lowers fp almost 3 times as much as 01 m solution of sucrose
To understand why this happens contrast behavior or sucrose with NaCl in aqueous solution Each sucrose dissolves to make only 1 particle in solution So 1 mol sucrose dissolves to 1 mol particles 1 mol NaCl dissolves to make 2 moles of particles (1 mol Na+ and 1 mol Cl-)
190
191
Calculated Values for Electrolyte SolutionsColligative properties depend on total concentration of solute particles regardless of their identity Electrolytes cause changes in colligative properties proportional to total molality in terms of all dissolves particles instead of formula units
192
What about barium nitrate Ba(NO3)2 Each mole of barium nitrate yields 3 mol of ions in solution Ba(NO3)2(s) rarr Ba2+(aq) + 2NO3-(aq) You would expect solution of given molality to lower fp of solvent 3 times as much as nonelectrolytic solution of same molality
193
Sample Problem
What is the expected change in the freezing point of water in a solution of 625 g of barium nitrate Ba(NO3)2 in 100 kg of water
194
1 AnalyzeGiven solute mass and formula = 625 g Ba(NO3)2 solvent mass and identity = 100 kg water Δtf = Kfm
Unknown expected freezing-point depression
195
2 PlanThe molality can be calculated by converting the solute mass to moles and then dividing by the number of kilograms of solvent That molality is in terms of formula units of Ba(NO3)2 and must be converted to molality in terms of dissociated ions in solution It must be multiplied by the number of moles of ions produced per mole of formula unit This adjusted molality can then be used to calculate the freezing-point depression
196
197
198
3 Compute
Practice Problem
What is the expected freezing-point depression for a solution that contains 20 mol of magnesium sulfate dissolved in 10 kg of water
minus74degC199
What is the expected boiling-point elevation of water for a solution that contains 150 g of sodium chloride dissolved in 10 kg of water 27degC The freezing point of an aqueous sodium chloride solution is minus200degC What is the molality of the solution 54 m NaCl
200
Actual Values for Electrolyte SolutionsRemember values calculated are only expected values Actual values of colligative properties for all strong electrolytes are almost what would be expected Difference between expected and actual values caused by attractive forces that exist between dissociated ions in aqueous solution
201
Attraction between hydrated ions in solution is small compared with those in crystalline solid However forces of attraction donrsquot interfere with movements of aqueous ions More concentrated solution means ions closer together and attraction is greater Also depends on charge of ions ndash higher charge greater attraction
202
Electrolyte - substance that dissolves in water to give a solution that conducts electric current NaCl ndash electrolyte Usually highly polar compounds become electrolytes
24
Solution containing neutral solute molecules does not conduct current bc it doesnrsquot have mobile charged particles
Nonelectrolyte - substance that dissolves in water to give solution that does not conduct electric current Ex Sugar
25
26
27the solution
process
Factors Affecting Rate of DissolutionEver tried to dissolve sugar in iced tea or coffee You notice temperature has something to do with how quickly solute dissolves What other factors affect how quickly solutes dissolve
28
1 Increasing Surface Area of Solute
bc dissolution process occurs at surface of solute Can increase dissolution by increasing surface area
29
2 Agitating (Stirring) SolutionClose to surface of solute concentration of dissolved solute is high Stirringshaking helps disperse solute particles
30
Brings fresh solvent into contact with solute surface Contact between solvent and solute surface increased
3 Heating a Solvent
Solutes dissolve faster in warmer solvent As temp increases solvent molecules move faster Average kinetic energy increases Collisions between solvent and solute more frequent Helps separate solute molecules and spread them out
32
Solubility
If you add sugar to tea eventually no more sugar dissolves
For every combination of solvent with solute at given temp there is limit to amount of solute that can be dissolved
33
When sugar first dropped into water sugar molecules leave solid surface and move about at random in solvent Some dissolved molecules may collide with crystal and stay there As more solid dissolves and concentration of dissolved molecules increases collisions happen more often
34
Eventually molecules are returning to crystal at same rate they are going into solution Dynamic equilibrium established between dissolution and crystallization Solution equilibrium - physical state in which the opposing processes of dissolution and crystallization of a solute occur at equal rates
35
Saturated vs Unsaturated Solutions
Saturated solution - solution that contains the maximum amount of dissolved solute How do you know if solution is saturated If more NaCl added it falls to bottom and doesnrsquot dissolve Equilibrium already established
36
If more water added to saturated solution more NaCl will dissolve At 20 359 g NaCl is max that will dissolve in 100 g water Unsaturated solution - solution that contains less solute than a saturated solution under the existing conditions
37
Supersaturated Solutions
When saturated solution where solubility increases as temp increases is cooled excess solute usually comes out of solution Sometimes if solution is left to cool the excess solute doesnrsquot separate
38
39
Supersaturated solution - solution that contains more dissolved solute than a saturated solution contains under same conditions
40
Supersaturated solution can remain unchanged over long time if it isnrsquot disturbed Once crystals begin to form continues until equilibrium is recreated at lower temp
41
Example
Sodium thiosulfate Na2S2O3 Solute added to hot water until solution saturated Hot solution filtered Drop small crystal in ldquoseedingrdquo crystallization Continues until equilibrium created
42
Solubility Values
Solubility - amount of substance required to form a saturated solution with specific amount of solvent at specified temp Ex Solubility of sugar = 204 g per 100 g water at 20 Must specify temp bc solubility changes with temp 43
For gases pressure must also be specified Rate at which substance dissolves is unrelated to solubility Max amount of solute that dissolves and reaches equilibrium is always same under same conditions
44
Solute-Solvent Interactions
ldquolike dissolves likerdquo is generally useful rule for predicting whether one substance will dissolve in another Depends on type of bonding polaritynonpolarity and intermolecular forces between solute and solvent
45
Dissolving Ionic Compounds in Aqueous Solution
Polarity of water molecules plays important role in formation of solution of ionic compounds Charged ends of water molecules attract ions and surround them to keep separated from other ions
46
Hydration - solution process with water as solvent Ions said to be hydrated As hydrated ions diffuse into solution other ions exposed and drawn away from crystal surface Entire crystal gradually dissolves
47
When crystallized from aqueous solutions some ionic compounds form crystals that include water molecules The crystalline compounds known as hydrates have specific ratios of water molecules Represented by formulas like CuSO45H2O
48
When crystalline hydrate dissolves in water water of hydration returns to solvent Behavior of solute in hydrated form no different than anhydrous form
49
Nonpolar Solvents
Nonpolar solvents do not attract ions of ionic compound strongly enough to overcome forces holding crystal together
50
Liquid Solutes and Solvents
When you shake bottle of salad dressing oil droplets disperse in water Stop shaking ndash strong attraction between water molecules squeeze out oil and form separate layers Liquid solutes and solvents that are not soluble in each other are immiscible
51
Nonpolar substances (fat oil grease) are generally quite soluble in nonpolar liquids (carbon tetrachloride toluene gasoline)
Only attractions between nonpolar molecules are weak London forces
52
London Dispersion Forces
bc e- are constantly moving at any time there may be uneven distribution of them around a nonpolar atom Temporarily creates positive end and negative end
53
54
London dispersion forces - intermolecular attractions resulting from constant motion of e- and creation of instantaneous dipoles
55
Liquids that dissolve freely in one another in any ratio are said to be completely miscible Benzene and carbon tetrachloride (both nonpolar) are completely miscible
Nonpolar molecules of these two apply no strong forces of attraction or repulsion and molecules mix freely
56
Ethanol and water are also completely miscible -OH group on ethanol is slightly polar
Can form H-bonds with water as well as other ethanol molecules Intermolecular forces in mixture are very similar to those in pure liquids so they are mutually soluble in any amount
57
Gasoline contains mainly nonpolar hydrocarbons
Excellent solvent for fats oils greases
Major intermolecular forces are weak London forces
58
Ethanol less polar than water more than carbon tetrachloride
Better solvent for less-polar substances bc of nonpolar region
59
Effects of Pressure on Solubility
Little effect on solids or liquids Increase pressure - increase solubilities of gases in liquids
60
When gas in contact with surface of liquid gas molecules can enter liquid As amount of dissolve gas increases some molecules start to escape and reenter gas phase
Eq eventually established between rates of enteringleaving gas phase
61
As long as eq undisturbed solubility of gas in liquid unchanged at given pressure
Gas + solvent solution
62
Increasing pressure of solute gas above solution puts stress on eq Molecules collide with surface more often Increase in pressure partially offset by increase in rate of gas molecules entering solution In turn increase in amount of dissolved gas causes increase in rate molecules escape liquid and becomes gas
63
Eventually eq restored at higher gas solubility
Expected from Le Chatelierrsquos principle
Increase in gas pressure causes eq to shift so fewer molecules are in gas phase
64
Henryrsquos Law
Henryrsquos law - solubility of a gas in liquid is directly proportional to the partial pressure of that gas on the surface of the liquid
Applies to gas-liquid solutions at constant temp
65
When mixture of ideal gases is limited in constant volume at constant temp each gas applies same pressure it would apply if it occupied the space alone
Assuming gases do not react each gas dissolves to the extent it would if no other gases were there
66
In carbonated drinks (Coke Pepsi etc) solubility of CO2 increased by increasing pressure
At bottling factory CO2 gas forced into solution of flavored water at pressure of 5-10 atm Gas-in-liquid solution then sealed in bottlescans
67
When cap removed pressure reduced to 1 atm and some CO2 escapes as gas bubbles Effervescence - rapid escape of a gas from a liquid in which it is dissolved
68
Effects of Temperature on Solubility
Letrsquos consider gas solubility Increasing temperature usually decreases gas solubility As temp increases average kinetic energy of molecules in solution increases Greater number of solute molecules escape from attraction of solvent and return to gas phase 69
At higher temps eq is reached with fewer gas molecules in solution Gases generally less soluble
70
Effect of temp on solubility of solids in liquids more difficult to predict
Often increasing temp increases solubility
However the same temp increase can result in large increase in solubility in one case and only slight increase in the next
71
72
enthalpies of Solution
Formation of solution comes with energy change Dissolve KI in water container feels cold Dissolve LiCl in water feels hot Formation of solid-liquid solution can absorb or release heat
73
During formation of solution solvent and solute particles experience changes in forces attracting them to other particles Before dissolving begins solvent molecules held by intermolecular forces (solvent-solvent attraction) In solute molecules held by Ifs (solute-solute attraction)
74
Energy is required to separate solute molecules and solvent molecules from their neighbors
A solute particle that is surrounded by solvent molecules is said to be solvated
75
Step 1 solute particles become separated from solid (energy absorbed) Step 2 solvent particles move apart to allow solute particles to enter liquid (energy absorbed)
76
Step 3 solvent particles attracted to and solvate solute particles (energy released)
77
Heat of solution - net amount of heat energy absorbed or released when a specific amount of solute dissolves in a solvent
Heat of solution negative (heat is released) if sums of energy in steps 1 and 2 is less than step 3
If 1+2 gt 3 HOS is positive (heat absorbed)78
In gaseous state molecules are so far apart there are nearly no forces between them Solute-solute interaction has little effect on heat of solution of a gas Energy released when gas dissolved in liquid bc attraction between solute gas and solvent molecules greater than energy needed to separate solvent molecules
79
Concentration of Solutions80
Concentration - measure of amount of solute in given amount of solvent or solution ldquodiluterdquo just means small amount of solute ldquoconcentratedrdquo just means relatively large amount of solute Unrelated to degree to which solution is saturated Ex Saturated solution of substance not very soluble might be dilute
81
Molarity
Molarity - number of moles of solute in one liter of solution To find molarity must know molar mass A ldquoone molarrdquo solution of NaOH is 1 mole NaOH per liter of solution (1 M)
82
1 mol NaOH = 400 g If this quantity dissolve in enough water to make EXACTLY 100 L solution solution is 1 M If 200 g NaOH dissolve in enough to make 100 L solution what is molarity 0500 M
83
1 M solution is not made by adding 1 mol solute to 1 L solvent That would be more than 1 L Instead 1 mol solute dissolved in less than 1 L Then diluted with more solvent to bring TOTAL VOLUME to 1 L
84
Practice ProblemYou have 340 L of solution that contains 900 g sodium chloride What is the molarity of that solution 0440 M NaCl You have 08 L of a 05 M HCl solution How many moles of HCl does this solution contain 04 mol HCl
85
What is the molarity of a solution composed of 585 g of potassium iodide dissolved in enough water to make a 0125 L of solution 0282 M KI
86
How many moles of H2SO4 are present in 0500 L of a 0150 M H2SO4 solution 00750 mol What volume of 300 M NaCl is needed for reaction that requires 1463 g of NaCl 0834 L
87
Molality
Molality - concentration of solution expressed in moles of solute per kilogram of solvent Solution that contains 1 mol solute (NaOH) dissolved in exactly 1 kg of solvent is ldquoone molalrdquo solution (1 m NaOH)
88
1 mol NaOH = 400 g 400 g NaOH dissolved in 1 kg water is 1 m NaOH
200 g NaOH in 1 kg water = 0500 m NaOH
89
Practice Problem
A solution was prepared by dissolving 171 g sucrose (C12H22O11) in 125 g of water Find the molal concentration of this solution 0400 m C12H22O11
90
A solution of iodine in carbon tetrachloride is used when iodine is needed for certain chemical tests How much iodine must be added to prepare a 0480 m solution of iodine in CCl4 if 1000 g of CCl4 is used 122 g I2
91
What is the molality of a solution composed of 255 g acetone (CH3)2CO dissolved in 200 g of water 22 m acetone What quantity in grams of methanol CH3OH is required to prepare a 0244 m solution in 400 g water 312 g methanol
92
How many grams of AgNO3 are needed to prepare a 0125 m solution in 250 mL of water 531 g AgNO3
What is the molality of a solution containing 182 g HCl and 250 g water 200 m
93
Ions in Aqueous Solutions and Colligative Properties
94
95
compounds in aqueous solutions
DissociationWhen compound made from ions dissolves in water ions separate Dissociation - separation of ions that occurs when an ionic compound dissolves
96
Notice number of ions made per formula unit in equations 1 formula unit of NaCl gives 2 units of ions in solution 1 formula unit of CaCl2 gives 3 units of ions
97
Assuming 100 dissociation solution that contains 1 mol NaCl contains 1 mol Na+ and 1 mol Cl- Can assume 100 dissociation of all soluble ionic compounds
98
Practice Problem
Write the equation for the dissolution of aluminum sulfate in water How many moles of aluminum ions are made by dissolving 1 mol aluminum sulfate What is the total number of moles of ions made by dissolving 1 mol of aluminum sulfate
99
1 AnalyzeGiven Amount of solute = 1 mol Al2(SO4)3 Solvent identity = water Unknown a moles of aluminum ions and sulfate ions Total number of moles of solute ions produced
100
2 Plan
The coefficients in the balanced dissociation equation will tell mole relationships You can use the equation to find out the number of moles of solute ions produced
101
3 Compute
a
b102
Write the equation for the dissolution of each of the following in water and then determine the number of moles of each ions made as well as the total number of ions made a 1 mol ammonium chloride b 1 mol sodium sulfide c 05 mol barium nitrate
103
a 1 mol ammonium chloride
1 mol NH4+ 1 mol Cl- 2 mol total ions
104
b 1 mol sodium sulfide
2 mol Na+ 1 mol S-2 3 mol total ions
105
c 05 mol barium nitrate
05 mol Ba+2 1 mol NO3- 15 mol total ions
106
Precipitation Reactions
Even though no compound is completely insoluble compounds of very low solubility can be considered insoluble for practical purposes There are general guidelines to help predict whether a compound made of certain combination of ions is soluble
107
General Solubility Guidelines
Sodium potassium and ammonium compounds are soluble in water
108
Nitrates acetates and chlorates are soluble
109
110
Most chlorides are soluble except those of silver mercury (I) and lead Lead (II) chloride is soluble in hot water
Most sulfates are soluble except those of barium strontium lead calcium and mercury
111
Most carbonates phosphates and silicates are insoluble except those of sodium potassium and ammonium
112
Most sulfides are insoluble except those of calcium strontium sodium potassium and ammonium
113
Is calcium phosphate Ca3(PO4)2 soluble or insoluble
Not one of the exceptions so it is insoluble
Dissociation equations are not written for insoluble compounds
114
Guidelines also useful in predicting what will happen if 2 solutions of 2 different soluble compounds are mixed
If mixing results in combination of ions that forms insoluble compound a double-replacement and precipitation reaction will happen
115
Will precipitate form when solutions of ammonium sulfide and cadmium nitrate are combined You can tell calcium nitrate is soluble from the guidelines (Most nitrates acetates and chlorates are soluble) You can also tell ammonium sulfide is soluble (Most sulfides are insoluble except those of calcium strontium sodium potassium and ammonium)
116
117
To decide whether precipitate can form must know solubilities of two possible products
Ammonium nitrate ndash soluble (Most nitrates acetates and chlorates are soluble) Cadmium sulfide ndash insoluble (Most sulfides are insoluble except those of calcium strontium sodium potassium and ammonium)
118
bc one product is insoluble double-replacement and precipitation reaction will happen (NH4)2S (aq) + Cd(NO3)2 (aq) rarr NH4NO3 (aq) + CdS (s)
119
Net Ionic Equations
Reactions of ions in aqueous solution usually represented by net ionic equations instead of formula equations Net ionic equation - includes only those compounds and ions that undergo a chemical change in a reaction in an aqueous solution
120
121
To write NIE (net ionic equation) first convert chemical equation into overall ionic equation All soluble ionic compounds shown as dissociated ions in solution Precipitates shown as solids
122
Notice ammonium and nitrate ions appear on both sides They havenrsquot gone through any chemical change Spectator ions - ions that do not take part in a chemical reaction and are found in solution both before and after the reaction
123
To change ionic equation to NIE eliminate spectator ions Cd+2(aq) + S-2(aq) rarr CdS(s)
This NIE applies to any reaction in which precipitate of CdS forms
124
Practice Problem
Identify the precipitate that forms when aqueous solutions of zinc nitrate and ammonium sulfide are combined Write the equation for the possible double-replacement reaction Then write the formula equation overall ionic equation and the net ionic equation
125
1 AnalyzeGiven Identity of reactants zinc nitrate and ammonium sulfide Reaction medium aqueous solution Unknown a equation for the possible double-replacement reaction b identity of the precipitate c formula equation d overall ionic equation e net ionic equation
126
2 PlanWrite the possible double-replacement reaction between Zn(NO3)2 and (NH4)2S Use the rules to determine if any of the products will precipitate Write a formula equation and overall net ionic equation Then cancel spectator ions to make net ionic equation
127
3 Computea possible double-replacement reaction
b rules show that zinc sulfide is not soluble so will precipitate (ammonium nitrate is soluble)
c formula equation128
d overall ionic equation
e net ionic equation129
Will a precipitate form if solutions of potassium sulfate and barium nitrate are combined If so write the net ionic equation for the reaction
Yes Ba+2(aq) + SO4-2(aq) rarr BaSO4(s)
130
Will a precipitate form if solutions of potassium nitrate and magnesium sulfate are combined If so write the net ionic equation for the reaction
No 131
Will a precipitate form if solutions of barium chloride and sodium sulfate are combined If so identify the spectator ions and write the net ionic equation for the reaction
Yes Na+ and Cl- Ba+2(aq) + SO4-2(aq) rarr BaSO4(s)
132
Write the net ionic equation for the precipitation of nickel(II) sulfide
Ni+2(aq) + S-2(aq) rarr NiS(s)
133
IonizationSome molecular compounds can also form ions in solution Usually polar Ionization - ions formed from solute molecules by action of the solvent (creation of ions where there were none) Ionization different from dissociation
134
Dissociation ionic compounds dissolve and ions already present separate Ionization ions formed where non existed before
Ions formed are hydrated Heat released during hydration of ions gives enough energy to break covalent bonds
135
Extent to which solute ionizes depends on strength of bonds within molecules of solute and strength of attraction between solute and solvent molecules
136
HCl ndash molecular compound that ionizes in aqueous solution Contains highly polar bond Attraction between polar HCl molecule and polar water strong enough to break HCl bond
137
The Hydronium Ion
H+ ions from HCl attracts other molecules or ions so strongly that it doesnrsquot normally exist alone Ionization of HCl better described as direct transfer of proton from HCl to H2O forming H3O+
138
139
Hydration of H+ to form hydronium is highly exothermic
Energy released gives a lot of energy needed to ionize a molecular solute
Many molecular compounds that ionize in aqueous solution contain hydrogen and form hydronium ion
140
Strong and Weak Electrolytes
Substances that make ions and conduct current in solution are electrolytes HCl is one of series of compounds that has hydrogen and halogen Hydrogen halides are all molecular compounds with single polar-covalent bonds
141
All are gases Very soluble in water All are electrolytes HCl HBr HI strongly conduct current in solution HF only weakly conducts current at same concentration Strength of conduction related to ability to ionize
142
143
Strong Electrolytes
HCl HBr HI are 100 ionized in solution
Strong electrolyte - any compound whose dilute aqueous solutions conduct electricity well this is due to the presence of all or almost all of the dissolve compound in the form of ions
144
Unique characteristic of strong electrolyte is that they yield only ions
Ex Ionic compound may be highly soluble in water and dissociate into ions (NaCl)
Others may not dissolve much but amount that does dissolve exists only as ions
145
Weak ElectrolytesHF dissolves in water to give acid solution called hydrofluoric acid HF bond stronger than bonds between hydrogen and other halogens When HF dissolves some molecule ionize Reverse reaction also happens HF(aq) + H2O(l) H3O+(aq) + F-(aq)
146
HF(aq) + H2O(l) H3O+
(aq) + F-(aq)Concentration of dissolved unionized HF stays high and concentration of ions stays low
Weak electrolyte - any compound whose aqueous solutions conduct electricity poorly this is due to the presence of small amount of the dissolved compound in the form of ions
147
Different from nonelectrolyte ndash NO ions at all Description of electrolyte as strong or weak not related to concentration of solution Electrolytes differ in degree of ionization not amount of solute dissolved
148
149
Colligative properties of solutions
Presence of solute affects properties of solutions Some properties not dependent on nature of dissolved substance but on how may dissolve particles are present
Colligative properties - properties that depend on the concentration of solute particles but not on their identity
150
Vapor-Pressure Lowering
Boiling and freezing point of solution different from pure solvent Nonvolatile solute raises boiling point and lowers freezing point Nonvolatile substance - one that has little tendency to become gas under existing conditions
151
152
To understand why nonvolatile solute changes boiling and freezing point consider equilibrium vapor pressure
Vapor pressure pressure caused by molecules that have escape liquid phase to gas phase
Can be thought of a measure of tendency of molecules to escape from a liquid
153
Addition of sucrose (nonvolatile solute) lowers concentration of water molecules at surface of liquid This lowers tendency of water molecules to leave solution and enter gas phase Vapor pressure of solution is lower than vapor pressure of pure water
154
Aqueous13 solution13 of13 nonvolatile13 solutePure13 water
Nonelectrolyte solutions of same molality have same concentration of particles
Dilute solutions of same solvent and equal molality of any nonelectrolyte solute lower vapor pressure equally
155
Example
1 m aqueous solution of nonelectrolyte glucose lowers vapor pressure of water 55 x 10-4 atm at 25 1 m aqueous solution of sucrose also lowers water vapor pressure to 55 x 10-4 atm at 25
156
157
bc vapor-pressure lowering depends on concentration of nonelectrolyte solute and doesnrsquot depend on the type of solute it is a colligative proprerty
bc vapor pressure has been lowered solution remains liquid over larger temperature range
Lowers freezing point and raises boiling point
158
Can assume changes in boiling and freezing point also depend on concentration of solute
They are colligative properties
159
Freezing-Point Depression
Freezing point of 1-molal solution of any nonelectrolyte solute in water is found (by experiment) to be 186 lower than freezing point of water When 1 mol of nonelectrolyte solute dissolved in 1 kg water freezing point is -186 not 000 160
When 2 mol nonelectrolyte solute dissolved in 1 kg water freezing point is -372
For any concentration of nonelectrolyte solute in water decrease in freezing point can be determined using value of -186m
161
This value called the molal freezing-point contstant (Kf) is the freezing-point depression of the solvent in a 1-molal solution of a nonvolatile nonelectrolyte solute
162
Each solvent has own characteristic molal freezing-point constant Freezing-point depression (Δtf) the difference between the freezing points of the pure solvent and a solute of nonelectrolyte in that solvent and it is directly proportional to the molal concentration of the solution Δtf = Kfm
163
Δtf = Kfm
Kf expressed as m
m expressed in mol solutekg solvent (molality)
164
Sample Problem
What is the freezing point depression of water in a solution of 171 g of sucrose C12H22O11 and 200 g of water What is the actual freezing point of the solution
165
1 AnalyzeGiven Solute mass and chemical formula 171 g C12H22O11 Solvent mass and identity 2000 g water Unknown a freezing-point depression b freezing point of the solution 166
2 Plan
167
168
Calculate
Unknown a freezing-point depression b freezing point of the solution
169
Practice Problem
A water solution containing an unknown quantity of a nonelectrolyte solute is found to have a freezing point of ndash 023degCWhat is the molal concentration of the solution
012 m170
A solution consists of 103 g of the nonelectrolyte glucose C6H12O6 dissolved in 250 g of water What is the freezing-point depression of the solution
minus0426degC171
In a laboratory experiment the freezing point of an aqueous solution of glucose is found to be minus0325degCWhat is the molal concentration of this solution
0175 m172
If 0500 mol of a nonelectrolyte solute are dissolved in 5000 g of ether what is the freezing point of the solution
minus1181degC173
The freezing point of an aqueous solution that contains a nonelectrolyte is minus90degC aWhat is the freezing-point depression of the solution bWhat is the molal concentration of the solution a minus90degC b 48 m
174
Boiling-Point Elevation
Boiling point is temp at which vapor pressure is equal to atmospheric pressure Change in vapor pressure will cause corresponding change in boiling point
175
176
Vapor pressure of nonvolatile solution is lower than vapor pressure of pure solvent More heat will be required to raise vapor pressure of solution Boiling point of solution is higher than boiling point of pure solvent
Molal boiling-point constant (Kb) - boiling-point elevation of the solvent in a 1-molal solution of a nonvolatile nonelectrolyte solute b-p elevation of any nonelectrolyte in water found by experiment to be 051 So molal b-p constant for water is 051m
177
For different solvents b-p elevations of 1-molal solutions have different values (Table 14-2)
Like freezing-point constants values most accurate for dilute solutions
178
Boiling-point elevation Δtb difference between the boiling points of the pure solvent and a nonelectrolyte solution of that solvent and it is directly proportional to the molal concentration of the solution B-p elevation can be calculated using Δtb = Kbm Δtb (boiling point elevation) expressed in m and m expressed in mol solutekg solvent
Sample Problem
What is the boiling-point elevation of a solution made from 200 g of a nonelectrolyte solute and 4000 g of water The molar mass of the solute is 620 gmol
180
1 AnalyzeGiven solute mass = 200 g solute molar mass = 620 gmol solvent mass and identity = 4000 g of water
Unknown boiling-point elevation
181
2 Plan
182
183
3 Compute
Practice Problem
A solution contains 500 g of sucrose C12H22O11 a nonelectrolyte dissolved in 5000 g of water What is the boiling-point elevation
015degC184
A solution contains 4500 g of sucrose C12H22O11 a nonelectrolyte dissolved in 250 g of waterWhat is the boiling point of the solution
1027degC185
If the boiling point elevation of an aqueous solution containing a nonvolatile electrolyte is 102degC what is the molality of the solution
20 m186
The boiling point of an aqueous solution containing a nonvolatile electrolyte is 10075degC aWhat is the boiling-point elevation bWhat is the molality of the solution a 075degC b 15 m
187
Electrolytes and Colligative PropertiesEarly investigators confused by experiments where certain substances depressed freezing point or elevated boiling point more than expected
188
189
Ex 01 m solution of NaCl lowers freezing point of solvent almost twice as much as 01 m solution of sucrose 01 m CaCl2 lowers fp almost 3 times as much as 01 m solution of sucrose
To understand why this happens contrast behavior or sucrose with NaCl in aqueous solution Each sucrose dissolves to make only 1 particle in solution So 1 mol sucrose dissolves to 1 mol particles 1 mol NaCl dissolves to make 2 moles of particles (1 mol Na+ and 1 mol Cl-)
190
191
Calculated Values for Electrolyte SolutionsColligative properties depend on total concentration of solute particles regardless of their identity Electrolytes cause changes in colligative properties proportional to total molality in terms of all dissolves particles instead of formula units
192
What about barium nitrate Ba(NO3)2 Each mole of barium nitrate yields 3 mol of ions in solution Ba(NO3)2(s) rarr Ba2+(aq) + 2NO3-(aq) You would expect solution of given molality to lower fp of solvent 3 times as much as nonelectrolytic solution of same molality
193
Sample Problem
What is the expected change in the freezing point of water in a solution of 625 g of barium nitrate Ba(NO3)2 in 100 kg of water
194
1 AnalyzeGiven solute mass and formula = 625 g Ba(NO3)2 solvent mass and identity = 100 kg water Δtf = Kfm
Unknown expected freezing-point depression
195
2 PlanThe molality can be calculated by converting the solute mass to moles and then dividing by the number of kilograms of solvent That molality is in terms of formula units of Ba(NO3)2 and must be converted to molality in terms of dissociated ions in solution It must be multiplied by the number of moles of ions produced per mole of formula unit This adjusted molality can then be used to calculate the freezing-point depression
196
197
198
3 Compute
Practice Problem
What is the expected freezing-point depression for a solution that contains 20 mol of magnesium sulfate dissolved in 10 kg of water
minus74degC199
What is the expected boiling-point elevation of water for a solution that contains 150 g of sodium chloride dissolved in 10 kg of water 27degC The freezing point of an aqueous sodium chloride solution is minus200degC What is the molality of the solution 54 m NaCl
200
Actual Values for Electrolyte SolutionsRemember values calculated are only expected values Actual values of colligative properties for all strong electrolytes are almost what would be expected Difference between expected and actual values caused by attractive forces that exist between dissociated ions in aqueous solution
201
Attraction between hydrated ions in solution is small compared with those in crystalline solid However forces of attraction donrsquot interfere with movements of aqueous ions More concentrated solution means ions closer together and attraction is greater Also depends on charge of ions ndash higher charge greater attraction
202
Solution containing neutral solute molecules does not conduct current bc it doesnrsquot have mobile charged particles
Nonelectrolyte - substance that dissolves in water to give solution that does not conduct electric current Ex Sugar
25
26
27the solution
process
Factors Affecting Rate of DissolutionEver tried to dissolve sugar in iced tea or coffee You notice temperature has something to do with how quickly solute dissolves What other factors affect how quickly solutes dissolve
28
1 Increasing Surface Area of Solute
bc dissolution process occurs at surface of solute Can increase dissolution by increasing surface area
29
2 Agitating (Stirring) SolutionClose to surface of solute concentration of dissolved solute is high Stirringshaking helps disperse solute particles
30
Brings fresh solvent into contact with solute surface Contact between solvent and solute surface increased
3 Heating a Solvent
Solutes dissolve faster in warmer solvent As temp increases solvent molecules move faster Average kinetic energy increases Collisions between solvent and solute more frequent Helps separate solute molecules and spread them out
32
Solubility
If you add sugar to tea eventually no more sugar dissolves
For every combination of solvent with solute at given temp there is limit to amount of solute that can be dissolved
33
When sugar first dropped into water sugar molecules leave solid surface and move about at random in solvent Some dissolved molecules may collide with crystal and stay there As more solid dissolves and concentration of dissolved molecules increases collisions happen more often
34
Eventually molecules are returning to crystal at same rate they are going into solution Dynamic equilibrium established between dissolution and crystallization Solution equilibrium - physical state in which the opposing processes of dissolution and crystallization of a solute occur at equal rates
35
Saturated vs Unsaturated Solutions
Saturated solution - solution that contains the maximum amount of dissolved solute How do you know if solution is saturated If more NaCl added it falls to bottom and doesnrsquot dissolve Equilibrium already established
36
If more water added to saturated solution more NaCl will dissolve At 20 359 g NaCl is max that will dissolve in 100 g water Unsaturated solution - solution that contains less solute than a saturated solution under the existing conditions
37
Supersaturated Solutions
When saturated solution where solubility increases as temp increases is cooled excess solute usually comes out of solution Sometimes if solution is left to cool the excess solute doesnrsquot separate
38
39
Supersaturated solution - solution that contains more dissolved solute than a saturated solution contains under same conditions
40
Supersaturated solution can remain unchanged over long time if it isnrsquot disturbed Once crystals begin to form continues until equilibrium is recreated at lower temp
41
Example
Sodium thiosulfate Na2S2O3 Solute added to hot water until solution saturated Hot solution filtered Drop small crystal in ldquoseedingrdquo crystallization Continues until equilibrium created
42
Solubility Values
Solubility - amount of substance required to form a saturated solution with specific amount of solvent at specified temp Ex Solubility of sugar = 204 g per 100 g water at 20 Must specify temp bc solubility changes with temp 43
For gases pressure must also be specified Rate at which substance dissolves is unrelated to solubility Max amount of solute that dissolves and reaches equilibrium is always same under same conditions
44
Solute-Solvent Interactions
ldquolike dissolves likerdquo is generally useful rule for predicting whether one substance will dissolve in another Depends on type of bonding polaritynonpolarity and intermolecular forces between solute and solvent
45
Dissolving Ionic Compounds in Aqueous Solution
Polarity of water molecules plays important role in formation of solution of ionic compounds Charged ends of water molecules attract ions and surround them to keep separated from other ions
46
Hydration - solution process with water as solvent Ions said to be hydrated As hydrated ions diffuse into solution other ions exposed and drawn away from crystal surface Entire crystal gradually dissolves
47
When crystallized from aqueous solutions some ionic compounds form crystals that include water molecules The crystalline compounds known as hydrates have specific ratios of water molecules Represented by formulas like CuSO45H2O
48
When crystalline hydrate dissolves in water water of hydration returns to solvent Behavior of solute in hydrated form no different than anhydrous form
49
Nonpolar Solvents
Nonpolar solvents do not attract ions of ionic compound strongly enough to overcome forces holding crystal together
50
Liquid Solutes and Solvents
When you shake bottle of salad dressing oil droplets disperse in water Stop shaking ndash strong attraction between water molecules squeeze out oil and form separate layers Liquid solutes and solvents that are not soluble in each other are immiscible
51
Nonpolar substances (fat oil grease) are generally quite soluble in nonpolar liquids (carbon tetrachloride toluene gasoline)
Only attractions between nonpolar molecules are weak London forces
52
London Dispersion Forces
bc e- are constantly moving at any time there may be uneven distribution of them around a nonpolar atom Temporarily creates positive end and negative end
53
54
London dispersion forces - intermolecular attractions resulting from constant motion of e- and creation of instantaneous dipoles
55
Liquids that dissolve freely in one another in any ratio are said to be completely miscible Benzene and carbon tetrachloride (both nonpolar) are completely miscible
Nonpolar molecules of these two apply no strong forces of attraction or repulsion and molecules mix freely
56
Ethanol and water are also completely miscible -OH group on ethanol is slightly polar
Can form H-bonds with water as well as other ethanol molecules Intermolecular forces in mixture are very similar to those in pure liquids so they are mutually soluble in any amount
57
Gasoline contains mainly nonpolar hydrocarbons
Excellent solvent for fats oils greases
Major intermolecular forces are weak London forces
58
Ethanol less polar than water more than carbon tetrachloride
Better solvent for less-polar substances bc of nonpolar region
59
Effects of Pressure on Solubility
Little effect on solids or liquids Increase pressure - increase solubilities of gases in liquids
60
When gas in contact with surface of liquid gas molecules can enter liquid As amount of dissolve gas increases some molecules start to escape and reenter gas phase
Eq eventually established between rates of enteringleaving gas phase
61
As long as eq undisturbed solubility of gas in liquid unchanged at given pressure
Gas + solvent solution
62
Increasing pressure of solute gas above solution puts stress on eq Molecules collide with surface more often Increase in pressure partially offset by increase in rate of gas molecules entering solution In turn increase in amount of dissolved gas causes increase in rate molecules escape liquid and becomes gas
63
Eventually eq restored at higher gas solubility
Expected from Le Chatelierrsquos principle
Increase in gas pressure causes eq to shift so fewer molecules are in gas phase
64
Henryrsquos Law
Henryrsquos law - solubility of a gas in liquid is directly proportional to the partial pressure of that gas on the surface of the liquid
Applies to gas-liquid solutions at constant temp
65
When mixture of ideal gases is limited in constant volume at constant temp each gas applies same pressure it would apply if it occupied the space alone
Assuming gases do not react each gas dissolves to the extent it would if no other gases were there
66
In carbonated drinks (Coke Pepsi etc) solubility of CO2 increased by increasing pressure
At bottling factory CO2 gas forced into solution of flavored water at pressure of 5-10 atm Gas-in-liquid solution then sealed in bottlescans
67
When cap removed pressure reduced to 1 atm and some CO2 escapes as gas bubbles Effervescence - rapid escape of a gas from a liquid in which it is dissolved
68
Effects of Temperature on Solubility
Letrsquos consider gas solubility Increasing temperature usually decreases gas solubility As temp increases average kinetic energy of molecules in solution increases Greater number of solute molecules escape from attraction of solvent and return to gas phase 69
At higher temps eq is reached with fewer gas molecules in solution Gases generally less soluble
70
Effect of temp on solubility of solids in liquids more difficult to predict
Often increasing temp increases solubility
However the same temp increase can result in large increase in solubility in one case and only slight increase in the next
71
72
enthalpies of Solution
Formation of solution comes with energy change Dissolve KI in water container feels cold Dissolve LiCl in water feels hot Formation of solid-liquid solution can absorb or release heat
73
During formation of solution solvent and solute particles experience changes in forces attracting them to other particles Before dissolving begins solvent molecules held by intermolecular forces (solvent-solvent attraction) In solute molecules held by Ifs (solute-solute attraction)
74
Energy is required to separate solute molecules and solvent molecules from their neighbors
A solute particle that is surrounded by solvent molecules is said to be solvated
75
Step 1 solute particles become separated from solid (energy absorbed) Step 2 solvent particles move apart to allow solute particles to enter liquid (energy absorbed)
76
Step 3 solvent particles attracted to and solvate solute particles (energy released)
77
Heat of solution - net amount of heat energy absorbed or released when a specific amount of solute dissolves in a solvent
Heat of solution negative (heat is released) if sums of energy in steps 1 and 2 is less than step 3
If 1+2 gt 3 HOS is positive (heat absorbed)78
In gaseous state molecules are so far apart there are nearly no forces between them Solute-solute interaction has little effect on heat of solution of a gas Energy released when gas dissolved in liquid bc attraction between solute gas and solvent molecules greater than energy needed to separate solvent molecules
79
Concentration of Solutions80
Concentration - measure of amount of solute in given amount of solvent or solution ldquodiluterdquo just means small amount of solute ldquoconcentratedrdquo just means relatively large amount of solute Unrelated to degree to which solution is saturated Ex Saturated solution of substance not very soluble might be dilute
81
Molarity
Molarity - number of moles of solute in one liter of solution To find molarity must know molar mass A ldquoone molarrdquo solution of NaOH is 1 mole NaOH per liter of solution (1 M)
82
1 mol NaOH = 400 g If this quantity dissolve in enough water to make EXACTLY 100 L solution solution is 1 M If 200 g NaOH dissolve in enough to make 100 L solution what is molarity 0500 M
83
1 M solution is not made by adding 1 mol solute to 1 L solvent That would be more than 1 L Instead 1 mol solute dissolved in less than 1 L Then diluted with more solvent to bring TOTAL VOLUME to 1 L
84
Practice ProblemYou have 340 L of solution that contains 900 g sodium chloride What is the molarity of that solution 0440 M NaCl You have 08 L of a 05 M HCl solution How many moles of HCl does this solution contain 04 mol HCl
85
What is the molarity of a solution composed of 585 g of potassium iodide dissolved in enough water to make a 0125 L of solution 0282 M KI
86
How many moles of H2SO4 are present in 0500 L of a 0150 M H2SO4 solution 00750 mol What volume of 300 M NaCl is needed for reaction that requires 1463 g of NaCl 0834 L
87
Molality
Molality - concentration of solution expressed in moles of solute per kilogram of solvent Solution that contains 1 mol solute (NaOH) dissolved in exactly 1 kg of solvent is ldquoone molalrdquo solution (1 m NaOH)
88
1 mol NaOH = 400 g 400 g NaOH dissolved in 1 kg water is 1 m NaOH
200 g NaOH in 1 kg water = 0500 m NaOH
89
Practice Problem
A solution was prepared by dissolving 171 g sucrose (C12H22O11) in 125 g of water Find the molal concentration of this solution 0400 m C12H22O11
90
A solution of iodine in carbon tetrachloride is used when iodine is needed for certain chemical tests How much iodine must be added to prepare a 0480 m solution of iodine in CCl4 if 1000 g of CCl4 is used 122 g I2
91
What is the molality of a solution composed of 255 g acetone (CH3)2CO dissolved in 200 g of water 22 m acetone What quantity in grams of methanol CH3OH is required to prepare a 0244 m solution in 400 g water 312 g methanol
92
How many grams of AgNO3 are needed to prepare a 0125 m solution in 250 mL of water 531 g AgNO3
What is the molality of a solution containing 182 g HCl and 250 g water 200 m
93
Ions in Aqueous Solutions and Colligative Properties
94
95
compounds in aqueous solutions
DissociationWhen compound made from ions dissolves in water ions separate Dissociation - separation of ions that occurs when an ionic compound dissolves
96
Notice number of ions made per formula unit in equations 1 formula unit of NaCl gives 2 units of ions in solution 1 formula unit of CaCl2 gives 3 units of ions
97
Assuming 100 dissociation solution that contains 1 mol NaCl contains 1 mol Na+ and 1 mol Cl- Can assume 100 dissociation of all soluble ionic compounds
98
Practice Problem
Write the equation for the dissolution of aluminum sulfate in water How many moles of aluminum ions are made by dissolving 1 mol aluminum sulfate What is the total number of moles of ions made by dissolving 1 mol of aluminum sulfate
99
1 AnalyzeGiven Amount of solute = 1 mol Al2(SO4)3 Solvent identity = water Unknown a moles of aluminum ions and sulfate ions Total number of moles of solute ions produced
100
2 Plan
The coefficients in the balanced dissociation equation will tell mole relationships You can use the equation to find out the number of moles of solute ions produced
101
3 Compute
a
b102
Write the equation for the dissolution of each of the following in water and then determine the number of moles of each ions made as well as the total number of ions made a 1 mol ammonium chloride b 1 mol sodium sulfide c 05 mol barium nitrate
103
a 1 mol ammonium chloride
1 mol NH4+ 1 mol Cl- 2 mol total ions
104
b 1 mol sodium sulfide
2 mol Na+ 1 mol S-2 3 mol total ions
105
c 05 mol barium nitrate
05 mol Ba+2 1 mol NO3- 15 mol total ions
106
Precipitation Reactions
Even though no compound is completely insoluble compounds of very low solubility can be considered insoluble for practical purposes There are general guidelines to help predict whether a compound made of certain combination of ions is soluble
107
General Solubility Guidelines
Sodium potassium and ammonium compounds are soluble in water
108
Nitrates acetates and chlorates are soluble
109
110
Most chlorides are soluble except those of silver mercury (I) and lead Lead (II) chloride is soluble in hot water
Most sulfates are soluble except those of barium strontium lead calcium and mercury
111
Most carbonates phosphates and silicates are insoluble except those of sodium potassium and ammonium
112
Most sulfides are insoluble except those of calcium strontium sodium potassium and ammonium
113
Is calcium phosphate Ca3(PO4)2 soluble or insoluble
Not one of the exceptions so it is insoluble
Dissociation equations are not written for insoluble compounds
114
Guidelines also useful in predicting what will happen if 2 solutions of 2 different soluble compounds are mixed
If mixing results in combination of ions that forms insoluble compound a double-replacement and precipitation reaction will happen
115
Will precipitate form when solutions of ammonium sulfide and cadmium nitrate are combined You can tell calcium nitrate is soluble from the guidelines (Most nitrates acetates and chlorates are soluble) You can also tell ammonium sulfide is soluble (Most sulfides are insoluble except those of calcium strontium sodium potassium and ammonium)
116
117
To decide whether precipitate can form must know solubilities of two possible products
Ammonium nitrate ndash soluble (Most nitrates acetates and chlorates are soluble) Cadmium sulfide ndash insoluble (Most sulfides are insoluble except those of calcium strontium sodium potassium and ammonium)
118
bc one product is insoluble double-replacement and precipitation reaction will happen (NH4)2S (aq) + Cd(NO3)2 (aq) rarr NH4NO3 (aq) + CdS (s)
119
Net Ionic Equations
Reactions of ions in aqueous solution usually represented by net ionic equations instead of formula equations Net ionic equation - includes only those compounds and ions that undergo a chemical change in a reaction in an aqueous solution
120
121
To write NIE (net ionic equation) first convert chemical equation into overall ionic equation All soluble ionic compounds shown as dissociated ions in solution Precipitates shown as solids
122
Notice ammonium and nitrate ions appear on both sides They havenrsquot gone through any chemical change Spectator ions - ions that do not take part in a chemical reaction and are found in solution both before and after the reaction
123
To change ionic equation to NIE eliminate spectator ions Cd+2(aq) + S-2(aq) rarr CdS(s)
This NIE applies to any reaction in which precipitate of CdS forms
124
Practice Problem
Identify the precipitate that forms when aqueous solutions of zinc nitrate and ammonium sulfide are combined Write the equation for the possible double-replacement reaction Then write the formula equation overall ionic equation and the net ionic equation
125
1 AnalyzeGiven Identity of reactants zinc nitrate and ammonium sulfide Reaction medium aqueous solution Unknown a equation for the possible double-replacement reaction b identity of the precipitate c formula equation d overall ionic equation e net ionic equation
126
2 PlanWrite the possible double-replacement reaction between Zn(NO3)2 and (NH4)2S Use the rules to determine if any of the products will precipitate Write a formula equation and overall net ionic equation Then cancel spectator ions to make net ionic equation
127
3 Computea possible double-replacement reaction
b rules show that zinc sulfide is not soluble so will precipitate (ammonium nitrate is soluble)
c formula equation128
d overall ionic equation
e net ionic equation129
Will a precipitate form if solutions of potassium sulfate and barium nitrate are combined If so write the net ionic equation for the reaction
Yes Ba+2(aq) + SO4-2(aq) rarr BaSO4(s)
130
Will a precipitate form if solutions of potassium nitrate and magnesium sulfate are combined If so write the net ionic equation for the reaction
No 131
Will a precipitate form if solutions of barium chloride and sodium sulfate are combined If so identify the spectator ions and write the net ionic equation for the reaction
Yes Na+ and Cl- Ba+2(aq) + SO4-2(aq) rarr BaSO4(s)
132
Write the net ionic equation for the precipitation of nickel(II) sulfide
Ni+2(aq) + S-2(aq) rarr NiS(s)
133
IonizationSome molecular compounds can also form ions in solution Usually polar Ionization - ions formed from solute molecules by action of the solvent (creation of ions where there were none) Ionization different from dissociation
134
Dissociation ionic compounds dissolve and ions already present separate Ionization ions formed where non existed before
Ions formed are hydrated Heat released during hydration of ions gives enough energy to break covalent bonds
135
Extent to which solute ionizes depends on strength of bonds within molecules of solute and strength of attraction between solute and solvent molecules
136
HCl ndash molecular compound that ionizes in aqueous solution Contains highly polar bond Attraction between polar HCl molecule and polar water strong enough to break HCl bond
137
The Hydronium Ion
H+ ions from HCl attracts other molecules or ions so strongly that it doesnrsquot normally exist alone Ionization of HCl better described as direct transfer of proton from HCl to H2O forming H3O+
138
139
Hydration of H+ to form hydronium is highly exothermic
Energy released gives a lot of energy needed to ionize a molecular solute
Many molecular compounds that ionize in aqueous solution contain hydrogen and form hydronium ion
140
Strong and Weak Electrolytes
Substances that make ions and conduct current in solution are electrolytes HCl is one of series of compounds that has hydrogen and halogen Hydrogen halides are all molecular compounds with single polar-covalent bonds
141
All are gases Very soluble in water All are electrolytes HCl HBr HI strongly conduct current in solution HF only weakly conducts current at same concentration Strength of conduction related to ability to ionize
142
143
Strong Electrolytes
HCl HBr HI are 100 ionized in solution
Strong electrolyte - any compound whose dilute aqueous solutions conduct electricity well this is due to the presence of all or almost all of the dissolve compound in the form of ions
144
Unique characteristic of strong electrolyte is that they yield only ions
Ex Ionic compound may be highly soluble in water and dissociate into ions (NaCl)
Others may not dissolve much but amount that does dissolve exists only as ions
145
Weak ElectrolytesHF dissolves in water to give acid solution called hydrofluoric acid HF bond stronger than bonds between hydrogen and other halogens When HF dissolves some molecule ionize Reverse reaction also happens HF(aq) + H2O(l) H3O+(aq) + F-(aq)
146
HF(aq) + H2O(l) H3O+
(aq) + F-(aq)Concentration of dissolved unionized HF stays high and concentration of ions stays low
Weak electrolyte - any compound whose aqueous solutions conduct electricity poorly this is due to the presence of small amount of the dissolved compound in the form of ions
147
Different from nonelectrolyte ndash NO ions at all Description of electrolyte as strong or weak not related to concentration of solution Electrolytes differ in degree of ionization not amount of solute dissolved
148
149
Colligative properties of solutions
Presence of solute affects properties of solutions Some properties not dependent on nature of dissolved substance but on how may dissolve particles are present
Colligative properties - properties that depend on the concentration of solute particles but not on their identity
150
Vapor-Pressure Lowering
Boiling and freezing point of solution different from pure solvent Nonvolatile solute raises boiling point and lowers freezing point Nonvolatile substance - one that has little tendency to become gas under existing conditions
151
152
To understand why nonvolatile solute changes boiling and freezing point consider equilibrium vapor pressure
Vapor pressure pressure caused by molecules that have escape liquid phase to gas phase
Can be thought of a measure of tendency of molecules to escape from a liquid
153
Addition of sucrose (nonvolatile solute) lowers concentration of water molecules at surface of liquid This lowers tendency of water molecules to leave solution and enter gas phase Vapor pressure of solution is lower than vapor pressure of pure water
154
Aqueous13 solution13 of13 nonvolatile13 solutePure13 water
Nonelectrolyte solutions of same molality have same concentration of particles
Dilute solutions of same solvent and equal molality of any nonelectrolyte solute lower vapor pressure equally
155
Example
1 m aqueous solution of nonelectrolyte glucose lowers vapor pressure of water 55 x 10-4 atm at 25 1 m aqueous solution of sucrose also lowers water vapor pressure to 55 x 10-4 atm at 25
156
157
bc vapor-pressure lowering depends on concentration of nonelectrolyte solute and doesnrsquot depend on the type of solute it is a colligative proprerty
bc vapor pressure has been lowered solution remains liquid over larger temperature range
Lowers freezing point and raises boiling point
158
Can assume changes in boiling and freezing point also depend on concentration of solute
They are colligative properties
159
Freezing-Point Depression
Freezing point of 1-molal solution of any nonelectrolyte solute in water is found (by experiment) to be 186 lower than freezing point of water When 1 mol of nonelectrolyte solute dissolved in 1 kg water freezing point is -186 not 000 160
When 2 mol nonelectrolyte solute dissolved in 1 kg water freezing point is -372
For any concentration of nonelectrolyte solute in water decrease in freezing point can be determined using value of -186m
161
This value called the molal freezing-point contstant (Kf) is the freezing-point depression of the solvent in a 1-molal solution of a nonvolatile nonelectrolyte solute
162
Each solvent has own characteristic molal freezing-point constant Freezing-point depression (Δtf) the difference between the freezing points of the pure solvent and a solute of nonelectrolyte in that solvent and it is directly proportional to the molal concentration of the solution Δtf = Kfm
163
Δtf = Kfm
Kf expressed as m
m expressed in mol solutekg solvent (molality)
164
Sample Problem
What is the freezing point depression of water in a solution of 171 g of sucrose C12H22O11 and 200 g of water What is the actual freezing point of the solution
165
1 AnalyzeGiven Solute mass and chemical formula 171 g C12H22O11 Solvent mass and identity 2000 g water Unknown a freezing-point depression b freezing point of the solution 166
2 Plan
167
168
Calculate
Unknown a freezing-point depression b freezing point of the solution
169
Practice Problem
A water solution containing an unknown quantity of a nonelectrolyte solute is found to have a freezing point of ndash 023degCWhat is the molal concentration of the solution
012 m170
A solution consists of 103 g of the nonelectrolyte glucose C6H12O6 dissolved in 250 g of water What is the freezing-point depression of the solution
minus0426degC171
In a laboratory experiment the freezing point of an aqueous solution of glucose is found to be minus0325degCWhat is the molal concentration of this solution
0175 m172
If 0500 mol of a nonelectrolyte solute are dissolved in 5000 g of ether what is the freezing point of the solution
minus1181degC173
The freezing point of an aqueous solution that contains a nonelectrolyte is minus90degC aWhat is the freezing-point depression of the solution bWhat is the molal concentration of the solution a minus90degC b 48 m
174
Boiling-Point Elevation
Boiling point is temp at which vapor pressure is equal to atmospheric pressure Change in vapor pressure will cause corresponding change in boiling point
175
176
Vapor pressure of nonvolatile solution is lower than vapor pressure of pure solvent More heat will be required to raise vapor pressure of solution Boiling point of solution is higher than boiling point of pure solvent
Molal boiling-point constant (Kb) - boiling-point elevation of the solvent in a 1-molal solution of a nonvolatile nonelectrolyte solute b-p elevation of any nonelectrolyte in water found by experiment to be 051 So molal b-p constant for water is 051m
177
For different solvents b-p elevations of 1-molal solutions have different values (Table 14-2)
Like freezing-point constants values most accurate for dilute solutions
178
Boiling-point elevation Δtb difference between the boiling points of the pure solvent and a nonelectrolyte solution of that solvent and it is directly proportional to the molal concentration of the solution B-p elevation can be calculated using Δtb = Kbm Δtb (boiling point elevation) expressed in m and m expressed in mol solutekg solvent
Sample Problem
What is the boiling-point elevation of a solution made from 200 g of a nonelectrolyte solute and 4000 g of water The molar mass of the solute is 620 gmol
180
1 AnalyzeGiven solute mass = 200 g solute molar mass = 620 gmol solvent mass and identity = 4000 g of water
Unknown boiling-point elevation
181
2 Plan
182
183
3 Compute
Practice Problem
A solution contains 500 g of sucrose C12H22O11 a nonelectrolyte dissolved in 5000 g of water What is the boiling-point elevation
015degC184
A solution contains 4500 g of sucrose C12H22O11 a nonelectrolyte dissolved in 250 g of waterWhat is the boiling point of the solution
1027degC185
If the boiling point elevation of an aqueous solution containing a nonvolatile electrolyte is 102degC what is the molality of the solution
20 m186
The boiling point of an aqueous solution containing a nonvolatile electrolyte is 10075degC aWhat is the boiling-point elevation bWhat is the molality of the solution a 075degC b 15 m
187
Electrolytes and Colligative PropertiesEarly investigators confused by experiments where certain substances depressed freezing point or elevated boiling point more than expected
188
189
Ex 01 m solution of NaCl lowers freezing point of solvent almost twice as much as 01 m solution of sucrose 01 m CaCl2 lowers fp almost 3 times as much as 01 m solution of sucrose
To understand why this happens contrast behavior or sucrose with NaCl in aqueous solution Each sucrose dissolves to make only 1 particle in solution So 1 mol sucrose dissolves to 1 mol particles 1 mol NaCl dissolves to make 2 moles of particles (1 mol Na+ and 1 mol Cl-)
190
191
Calculated Values for Electrolyte SolutionsColligative properties depend on total concentration of solute particles regardless of their identity Electrolytes cause changes in colligative properties proportional to total molality in terms of all dissolves particles instead of formula units
192
What about barium nitrate Ba(NO3)2 Each mole of barium nitrate yields 3 mol of ions in solution Ba(NO3)2(s) rarr Ba2+(aq) + 2NO3-(aq) You would expect solution of given molality to lower fp of solvent 3 times as much as nonelectrolytic solution of same molality
193
Sample Problem
What is the expected change in the freezing point of water in a solution of 625 g of barium nitrate Ba(NO3)2 in 100 kg of water
194
1 AnalyzeGiven solute mass and formula = 625 g Ba(NO3)2 solvent mass and identity = 100 kg water Δtf = Kfm
Unknown expected freezing-point depression
195
2 PlanThe molality can be calculated by converting the solute mass to moles and then dividing by the number of kilograms of solvent That molality is in terms of formula units of Ba(NO3)2 and must be converted to molality in terms of dissociated ions in solution It must be multiplied by the number of moles of ions produced per mole of formula unit This adjusted molality can then be used to calculate the freezing-point depression
196
197
198
3 Compute
Practice Problem
What is the expected freezing-point depression for a solution that contains 20 mol of magnesium sulfate dissolved in 10 kg of water
minus74degC199
What is the expected boiling-point elevation of water for a solution that contains 150 g of sodium chloride dissolved in 10 kg of water 27degC The freezing point of an aqueous sodium chloride solution is minus200degC What is the molality of the solution 54 m NaCl
200
Actual Values for Electrolyte SolutionsRemember values calculated are only expected values Actual values of colligative properties for all strong electrolytes are almost what would be expected Difference between expected and actual values caused by attractive forces that exist between dissociated ions in aqueous solution
201
Attraction between hydrated ions in solution is small compared with those in crystalline solid However forces of attraction donrsquot interfere with movements of aqueous ions More concentrated solution means ions closer together and attraction is greater Also depends on charge of ions ndash higher charge greater attraction
202
26
27the solution
process
Factors Affecting Rate of DissolutionEver tried to dissolve sugar in iced tea or coffee You notice temperature has something to do with how quickly solute dissolves What other factors affect how quickly solutes dissolve
28
1 Increasing Surface Area of Solute
bc dissolution process occurs at surface of solute Can increase dissolution by increasing surface area
29
2 Agitating (Stirring) SolutionClose to surface of solute concentration of dissolved solute is high Stirringshaking helps disperse solute particles
30
Brings fresh solvent into contact with solute surface Contact between solvent and solute surface increased
3 Heating a Solvent
Solutes dissolve faster in warmer solvent As temp increases solvent molecules move faster Average kinetic energy increases Collisions between solvent and solute more frequent Helps separate solute molecules and spread them out
32
Solubility
If you add sugar to tea eventually no more sugar dissolves
For every combination of solvent with solute at given temp there is limit to amount of solute that can be dissolved
33
When sugar first dropped into water sugar molecules leave solid surface and move about at random in solvent Some dissolved molecules may collide with crystal and stay there As more solid dissolves and concentration of dissolved molecules increases collisions happen more often
34
Eventually molecules are returning to crystal at same rate they are going into solution Dynamic equilibrium established between dissolution and crystallization Solution equilibrium - physical state in which the opposing processes of dissolution and crystallization of a solute occur at equal rates
35
Saturated vs Unsaturated Solutions
Saturated solution - solution that contains the maximum amount of dissolved solute How do you know if solution is saturated If more NaCl added it falls to bottom and doesnrsquot dissolve Equilibrium already established
36
If more water added to saturated solution more NaCl will dissolve At 20 359 g NaCl is max that will dissolve in 100 g water Unsaturated solution - solution that contains less solute than a saturated solution under the existing conditions
37
Supersaturated Solutions
When saturated solution where solubility increases as temp increases is cooled excess solute usually comes out of solution Sometimes if solution is left to cool the excess solute doesnrsquot separate
38
39
Supersaturated solution - solution that contains more dissolved solute than a saturated solution contains under same conditions
40
Supersaturated solution can remain unchanged over long time if it isnrsquot disturbed Once crystals begin to form continues until equilibrium is recreated at lower temp
41
Example
Sodium thiosulfate Na2S2O3 Solute added to hot water until solution saturated Hot solution filtered Drop small crystal in ldquoseedingrdquo crystallization Continues until equilibrium created
42
Solubility Values
Solubility - amount of substance required to form a saturated solution with specific amount of solvent at specified temp Ex Solubility of sugar = 204 g per 100 g water at 20 Must specify temp bc solubility changes with temp 43
For gases pressure must also be specified Rate at which substance dissolves is unrelated to solubility Max amount of solute that dissolves and reaches equilibrium is always same under same conditions
44
Solute-Solvent Interactions
ldquolike dissolves likerdquo is generally useful rule for predicting whether one substance will dissolve in another Depends on type of bonding polaritynonpolarity and intermolecular forces between solute and solvent
45
Dissolving Ionic Compounds in Aqueous Solution
Polarity of water molecules plays important role in formation of solution of ionic compounds Charged ends of water molecules attract ions and surround them to keep separated from other ions
46
Hydration - solution process with water as solvent Ions said to be hydrated As hydrated ions diffuse into solution other ions exposed and drawn away from crystal surface Entire crystal gradually dissolves
47
When crystallized from aqueous solutions some ionic compounds form crystals that include water molecules The crystalline compounds known as hydrates have specific ratios of water molecules Represented by formulas like CuSO45H2O
48
When crystalline hydrate dissolves in water water of hydration returns to solvent Behavior of solute in hydrated form no different than anhydrous form
49
Nonpolar Solvents
Nonpolar solvents do not attract ions of ionic compound strongly enough to overcome forces holding crystal together
50
Liquid Solutes and Solvents
When you shake bottle of salad dressing oil droplets disperse in water Stop shaking ndash strong attraction between water molecules squeeze out oil and form separate layers Liquid solutes and solvents that are not soluble in each other are immiscible
51
Nonpolar substances (fat oil grease) are generally quite soluble in nonpolar liquids (carbon tetrachloride toluene gasoline)
Only attractions between nonpolar molecules are weak London forces
52
London Dispersion Forces
bc e- are constantly moving at any time there may be uneven distribution of them around a nonpolar atom Temporarily creates positive end and negative end
53
54
London dispersion forces - intermolecular attractions resulting from constant motion of e- and creation of instantaneous dipoles
55
Liquids that dissolve freely in one another in any ratio are said to be completely miscible Benzene and carbon tetrachloride (both nonpolar) are completely miscible
Nonpolar molecules of these two apply no strong forces of attraction or repulsion and molecules mix freely
56
Ethanol and water are also completely miscible -OH group on ethanol is slightly polar
Can form H-bonds with water as well as other ethanol molecules Intermolecular forces in mixture are very similar to those in pure liquids so they are mutually soluble in any amount
57
Gasoline contains mainly nonpolar hydrocarbons
Excellent solvent for fats oils greases
Major intermolecular forces are weak London forces
58
Ethanol less polar than water more than carbon tetrachloride
Better solvent for less-polar substances bc of nonpolar region
59
Effects of Pressure on Solubility
Little effect on solids or liquids Increase pressure - increase solubilities of gases in liquids
60
When gas in contact with surface of liquid gas molecules can enter liquid As amount of dissolve gas increases some molecules start to escape and reenter gas phase
Eq eventually established between rates of enteringleaving gas phase
61
As long as eq undisturbed solubility of gas in liquid unchanged at given pressure
Gas + solvent solution
62
Increasing pressure of solute gas above solution puts stress on eq Molecules collide with surface more often Increase in pressure partially offset by increase in rate of gas molecules entering solution In turn increase in amount of dissolved gas causes increase in rate molecules escape liquid and becomes gas
63
Eventually eq restored at higher gas solubility
Expected from Le Chatelierrsquos principle
Increase in gas pressure causes eq to shift so fewer molecules are in gas phase
64
Henryrsquos Law
Henryrsquos law - solubility of a gas in liquid is directly proportional to the partial pressure of that gas on the surface of the liquid
Applies to gas-liquid solutions at constant temp
65
When mixture of ideal gases is limited in constant volume at constant temp each gas applies same pressure it would apply if it occupied the space alone
Assuming gases do not react each gas dissolves to the extent it would if no other gases were there
66
In carbonated drinks (Coke Pepsi etc) solubility of CO2 increased by increasing pressure
At bottling factory CO2 gas forced into solution of flavored water at pressure of 5-10 atm Gas-in-liquid solution then sealed in bottlescans
67
When cap removed pressure reduced to 1 atm and some CO2 escapes as gas bubbles Effervescence - rapid escape of a gas from a liquid in which it is dissolved
68
Effects of Temperature on Solubility
Letrsquos consider gas solubility Increasing temperature usually decreases gas solubility As temp increases average kinetic energy of molecules in solution increases Greater number of solute molecules escape from attraction of solvent and return to gas phase 69
At higher temps eq is reached with fewer gas molecules in solution Gases generally less soluble
70
Effect of temp on solubility of solids in liquids more difficult to predict
Often increasing temp increases solubility
However the same temp increase can result in large increase in solubility in one case and only slight increase in the next
71
72
enthalpies of Solution
Formation of solution comes with energy change Dissolve KI in water container feels cold Dissolve LiCl in water feels hot Formation of solid-liquid solution can absorb or release heat
73
During formation of solution solvent and solute particles experience changes in forces attracting them to other particles Before dissolving begins solvent molecules held by intermolecular forces (solvent-solvent attraction) In solute molecules held by Ifs (solute-solute attraction)
74
Energy is required to separate solute molecules and solvent molecules from their neighbors
A solute particle that is surrounded by solvent molecules is said to be solvated
75
Step 1 solute particles become separated from solid (energy absorbed) Step 2 solvent particles move apart to allow solute particles to enter liquid (energy absorbed)
76
Step 3 solvent particles attracted to and solvate solute particles (energy released)
77
Heat of solution - net amount of heat energy absorbed or released when a specific amount of solute dissolves in a solvent
Heat of solution negative (heat is released) if sums of energy in steps 1 and 2 is less than step 3
If 1+2 gt 3 HOS is positive (heat absorbed)78
In gaseous state molecules are so far apart there are nearly no forces between them Solute-solute interaction has little effect on heat of solution of a gas Energy released when gas dissolved in liquid bc attraction between solute gas and solvent molecules greater than energy needed to separate solvent molecules
79
Concentration of Solutions80
Concentration - measure of amount of solute in given amount of solvent or solution ldquodiluterdquo just means small amount of solute ldquoconcentratedrdquo just means relatively large amount of solute Unrelated to degree to which solution is saturated Ex Saturated solution of substance not very soluble might be dilute
81
Molarity
Molarity - number of moles of solute in one liter of solution To find molarity must know molar mass A ldquoone molarrdquo solution of NaOH is 1 mole NaOH per liter of solution (1 M)
82
1 mol NaOH = 400 g If this quantity dissolve in enough water to make EXACTLY 100 L solution solution is 1 M If 200 g NaOH dissolve in enough to make 100 L solution what is molarity 0500 M
83
1 M solution is not made by adding 1 mol solute to 1 L solvent That would be more than 1 L Instead 1 mol solute dissolved in less than 1 L Then diluted with more solvent to bring TOTAL VOLUME to 1 L
84
Practice ProblemYou have 340 L of solution that contains 900 g sodium chloride What is the molarity of that solution 0440 M NaCl You have 08 L of a 05 M HCl solution How many moles of HCl does this solution contain 04 mol HCl
85
What is the molarity of a solution composed of 585 g of potassium iodide dissolved in enough water to make a 0125 L of solution 0282 M KI
86
How many moles of H2SO4 are present in 0500 L of a 0150 M H2SO4 solution 00750 mol What volume of 300 M NaCl is needed for reaction that requires 1463 g of NaCl 0834 L
87
Molality
Molality - concentration of solution expressed in moles of solute per kilogram of solvent Solution that contains 1 mol solute (NaOH) dissolved in exactly 1 kg of solvent is ldquoone molalrdquo solution (1 m NaOH)
88
1 mol NaOH = 400 g 400 g NaOH dissolved in 1 kg water is 1 m NaOH
200 g NaOH in 1 kg water = 0500 m NaOH
89
Practice Problem
A solution was prepared by dissolving 171 g sucrose (C12H22O11) in 125 g of water Find the molal concentration of this solution 0400 m C12H22O11
90
A solution of iodine in carbon tetrachloride is used when iodine is needed for certain chemical tests How much iodine must be added to prepare a 0480 m solution of iodine in CCl4 if 1000 g of CCl4 is used 122 g I2
91
What is the molality of a solution composed of 255 g acetone (CH3)2CO dissolved in 200 g of water 22 m acetone What quantity in grams of methanol CH3OH is required to prepare a 0244 m solution in 400 g water 312 g methanol
92
How many grams of AgNO3 are needed to prepare a 0125 m solution in 250 mL of water 531 g AgNO3
What is the molality of a solution containing 182 g HCl and 250 g water 200 m
93
Ions in Aqueous Solutions and Colligative Properties
94
95
compounds in aqueous solutions
DissociationWhen compound made from ions dissolves in water ions separate Dissociation - separation of ions that occurs when an ionic compound dissolves
96
Notice number of ions made per formula unit in equations 1 formula unit of NaCl gives 2 units of ions in solution 1 formula unit of CaCl2 gives 3 units of ions
97
Assuming 100 dissociation solution that contains 1 mol NaCl contains 1 mol Na+ and 1 mol Cl- Can assume 100 dissociation of all soluble ionic compounds
98
Practice Problem
Write the equation for the dissolution of aluminum sulfate in water How many moles of aluminum ions are made by dissolving 1 mol aluminum sulfate What is the total number of moles of ions made by dissolving 1 mol of aluminum sulfate
99
1 AnalyzeGiven Amount of solute = 1 mol Al2(SO4)3 Solvent identity = water Unknown a moles of aluminum ions and sulfate ions Total number of moles of solute ions produced
100
2 Plan
The coefficients in the balanced dissociation equation will tell mole relationships You can use the equation to find out the number of moles of solute ions produced
101
3 Compute
a
b102
Write the equation for the dissolution of each of the following in water and then determine the number of moles of each ions made as well as the total number of ions made a 1 mol ammonium chloride b 1 mol sodium sulfide c 05 mol barium nitrate
103
a 1 mol ammonium chloride
1 mol NH4+ 1 mol Cl- 2 mol total ions
104
b 1 mol sodium sulfide
2 mol Na+ 1 mol S-2 3 mol total ions
105
c 05 mol barium nitrate
05 mol Ba+2 1 mol NO3- 15 mol total ions
106
Precipitation Reactions
Even though no compound is completely insoluble compounds of very low solubility can be considered insoluble for practical purposes There are general guidelines to help predict whether a compound made of certain combination of ions is soluble
107
General Solubility Guidelines
Sodium potassium and ammonium compounds are soluble in water
108
Nitrates acetates and chlorates are soluble
109
110
Most chlorides are soluble except those of silver mercury (I) and lead Lead (II) chloride is soluble in hot water
Most sulfates are soluble except those of barium strontium lead calcium and mercury
111
Most carbonates phosphates and silicates are insoluble except those of sodium potassium and ammonium
112
Most sulfides are insoluble except those of calcium strontium sodium potassium and ammonium
113
Is calcium phosphate Ca3(PO4)2 soluble or insoluble
Not one of the exceptions so it is insoluble
Dissociation equations are not written for insoluble compounds
114
Guidelines also useful in predicting what will happen if 2 solutions of 2 different soluble compounds are mixed
If mixing results in combination of ions that forms insoluble compound a double-replacement and precipitation reaction will happen
115
Will precipitate form when solutions of ammonium sulfide and cadmium nitrate are combined You can tell calcium nitrate is soluble from the guidelines (Most nitrates acetates and chlorates are soluble) You can also tell ammonium sulfide is soluble (Most sulfides are insoluble except those of calcium strontium sodium potassium and ammonium)
116
117
To decide whether precipitate can form must know solubilities of two possible products
Ammonium nitrate ndash soluble (Most nitrates acetates and chlorates are soluble) Cadmium sulfide ndash insoluble (Most sulfides are insoluble except those of calcium strontium sodium potassium and ammonium)
118
bc one product is insoluble double-replacement and precipitation reaction will happen (NH4)2S (aq) + Cd(NO3)2 (aq) rarr NH4NO3 (aq) + CdS (s)
119
Net Ionic Equations
Reactions of ions in aqueous solution usually represented by net ionic equations instead of formula equations Net ionic equation - includes only those compounds and ions that undergo a chemical change in a reaction in an aqueous solution
120
121
To write NIE (net ionic equation) first convert chemical equation into overall ionic equation All soluble ionic compounds shown as dissociated ions in solution Precipitates shown as solids
122
Notice ammonium and nitrate ions appear on both sides They havenrsquot gone through any chemical change Spectator ions - ions that do not take part in a chemical reaction and are found in solution both before and after the reaction
123
To change ionic equation to NIE eliminate spectator ions Cd+2(aq) + S-2(aq) rarr CdS(s)
This NIE applies to any reaction in which precipitate of CdS forms
124
Practice Problem
Identify the precipitate that forms when aqueous solutions of zinc nitrate and ammonium sulfide are combined Write the equation for the possible double-replacement reaction Then write the formula equation overall ionic equation and the net ionic equation
125
1 AnalyzeGiven Identity of reactants zinc nitrate and ammonium sulfide Reaction medium aqueous solution Unknown a equation for the possible double-replacement reaction b identity of the precipitate c formula equation d overall ionic equation e net ionic equation
126
2 PlanWrite the possible double-replacement reaction between Zn(NO3)2 and (NH4)2S Use the rules to determine if any of the products will precipitate Write a formula equation and overall net ionic equation Then cancel spectator ions to make net ionic equation
127
3 Computea possible double-replacement reaction
b rules show that zinc sulfide is not soluble so will precipitate (ammonium nitrate is soluble)
c formula equation128
d overall ionic equation
e net ionic equation129
Will a precipitate form if solutions of potassium sulfate and barium nitrate are combined If so write the net ionic equation for the reaction
Yes Ba+2(aq) + SO4-2(aq) rarr BaSO4(s)
130
Will a precipitate form if solutions of potassium nitrate and magnesium sulfate are combined If so write the net ionic equation for the reaction
No 131
Will a precipitate form if solutions of barium chloride and sodium sulfate are combined If so identify the spectator ions and write the net ionic equation for the reaction
Yes Na+ and Cl- Ba+2(aq) + SO4-2(aq) rarr BaSO4(s)
132
Write the net ionic equation for the precipitation of nickel(II) sulfide
Ni+2(aq) + S-2(aq) rarr NiS(s)
133
IonizationSome molecular compounds can also form ions in solution Usually polar Ionization - ions formed from solute molecules by action of the solvent (creation of ions where there were none) Ionization different from dissociation
134
Dissociation ionic compounds dissolve and ions already present separate Ionization ions formed where non existed before
Ions formed are hydrated Heat released during hydration of ions gives enough energy to break covalent bonds
135
Extent to which solute ionizes depends on strength of bonds within molecules of solute and strength of attraction between solute and solvent molecules
136
HCl ndash molecular compound that ionizes in aqueous solution Contains highly polar bond Attraction between polar HCl molecule and polar water strong enough to break HCl bond
137
The Hydronium Ion
H+ ions from HCl attracts other molecules or ions so strongly that it doesnrsquot normally exist alone Ionization of HCl better described as direct transfer of proton from HCl to H2O forming H3O+
138
139
Hydration of H+ to form hydronium is highly exothermic
Energy released gives a lot of energy needed to ionize a molecular solute
Many molecular compounds that ionize in aqueous solution contain hydrogen and form hydronium ion
140
Strong and Weak Electrolytes
Substances that make ions and conduct current in solution are electrolytes HCl is one of series of compounds that has hydrogen and halogen Hydrogen halides are all molecular compounds with single polar-covalent bonds
141
All are gases Very soluble in water All are electrolytes HCl HBr HI strongly conduct current in solution HF only weakly conducts current at same concentration Strength of conduction related to ability to ionize
142
143
Strong Electrolytes
HCl HBr HI are 100 ionized in solution
Strong electrolyte - any compound whose dilute aqueous solutions conduct electricity well this is due to the presence of all or almost all of the dissolve compound in the form of ions
144
Unique characteristic of strong electrolyte is that they yield only ions
Ex Ionic compound may be highly soluble in water and dissociate into ions (NaCl)
Others may not dissolve much but amount that does dissolve exists only as ions
145
Weak ElectrolytesHF dissolves in water to give acid solution called hydrofluoric acid HF bond stronger than bonds between hydrogen and other halogens When HF dissolves some molecule ionize Reverse reaction also happens HF(aq) + H2O(l) H3O+(aq) + F-(aq)
146
HF(aq) + H2O(l) H3O+
(aq) + F-(aq)Concentration of dissolved unionized HF stays high and concentration of ions stays low
Weak electrolyte - any compound whose aqueous solutions conduct electricity poorly this is due to the presence of small amount of the dissolved compound in the form of ions
147
Different from nonelectrolyte ndash NO ions at all Description of electrolyte as strong or weak not related to concentration of solution Electrolytes differ in degree of ionization not amount of solute dissolved
148
149
Colligative properties of solutions
Presence of solute affects properties of solutions Some properties not dependent on nature of dissolved substance but on how may dissolve particles are present
Colligative properties - properties that depend on the concentration of solute particles but not on their identity
150
Vapor-Pressure Lowering
Boiling and freezing point of solution different from pure solvent Nonvolatile solute raises boiling point and lowers freezing point Nonvolatile substance - one that has little tendency to become gas under existing conditions
151
152
To understand why nonvolatile solute changes boiling and freezing point consider equilibrium vapor pressure
Vapor pressure pressure caused by molecules that have escape liquid phase to gas phase
Can be thought of a measure of tendency of molecules to escape from a liquid
153
Addition of sucrose (nonvolatile solute) lowers concentration of water molecules at surface of liquid This lowers tendency of water molecules to leave solution and enter gas phase Vapor pressure of solution is lower than vapor pressure of pure water
154
Aqueous13 solution13 of13 nonvolatile13 solutePure13 water
Nonelectrolyte solutions of same molality have same concentration of particles
Dilute solutions of same solvent and equal molality of any nonelectrolyte solute lower vapor pressure equally
155
Example
1 m aqueous solution of nonelectrolyte glucose lowers vapor pressure of water 55 x 10-4 atm at 25 1 m aqueous solution of sucrose also lowers water vapor pressure to 55 x 10-4 atm at 25
156
157
bc vapor-pressure lowering depends on concentration of nonelectrolyte solute and doesnrsquot depend on the type of solute it is a colligative proprerty
bc vapor pressure has been lowered solution remains liquid over larger temperature range
Lowers freezing point and raises boiling point
158
Can assume changes in boiling and freezing point also depend on concentration of solute
They are colligative properties
159
Freezing-Point Depression
Freezing point of 1-molal solution of any nonelectrolyte solute in water is found (by experiment) to be 186 lower than freezing point of water When 1 mol of nonelectrolyte solute dissolved in 1 kg water freezing point is -186 not 000 160
When 2 mol nonelectrolyte solute dissolved in 1 kg water freezing point is -372
For any concentration of nonelectrolyte solute in water decrease in freezing point can be determined using value of -186m
161
This value called the molal freezing-point contstant (Kf) is the freezing-point depression of the solvent in a 1-molal solution of a nonvolatile nonelectrolyte solute
162
Each solvent has own characteristic molal freezing-point constant Freezing-point depression (Δtf) the difference between the freezing points of the pure solvent and a solute of nonelectrolyte in that solvent and it is directly proportional to the molal concentration of the solution Δtf = Kfm
163
Δtf = Kfm
Kf expressed as m
m expressed in mol solutekg solvent (molality)
164
Sample Problem
What is the freezing point depression of water in a solution of 171 g of sucrose C12H22O11 and 200 g of water What is the actual freezing point of the solution
165
1 AnalyzeGiven Solute mass and chemical formula 171 g C12H22O11 Solvent mass and identity 2000 g water Unknown a freezing-point depression b freezing point of the solution 166
2 Plan
167
168
Calculate
Unknown a freezing-point depression b freezing point of the solution
169
Practice Problem
A water solution containing an unknown quantity of a nonelectrolyte solute is found to have a freezing point of ndash 023degCWhat is the molal concentration of the solution
012 m170
A solution consists of 103 g of the nonelectrolyte glucose C6H12O6 dissolved in 250 g of water What is the freezing-point depression of the solution
minus0426degC171
In a laboratory experiment the freezing point of an aqueous solution of glucose is found to be minus0325degCWhat is the molal concentration of this solution
0175 m172
If 0500 mol of a nonelectrolyte solute are dissolved in 5000 g of ether what is the freezing point of the solution
minus1181degC173
The freezing point of an aqueous solution that contains a nonelectrolyte is minus90degC aWhat is the freezing-point depression of the solution bWhat is the molal concentration of the solution a minus90degC b 48 m
174
Boiling-Point Elevation
Boiling point is temp at which vapor pressure is equal to atmospheric pressure Change in vapor pressure will cause corresponding change in boiling point
175
176
Vapor pressure of nonvolatile solution is lower than vapor pressure of pure solvent More heat will be required to raise vapor pressure of solution Boiling point of solution is higher than boiling point of pure solvent
Molal boiling-point constant (Kb) - boiling-point elevation of the solvent in a 1-molal solution of a nonvolatile nonelectrolyte solute b-p elevation of any nonelectrolyte in water found by experiment to be 051 So molal b-p constant for water is 051m
177
For different solvents b-p elevations of 1-molal solutions have different values (Table 14-2)
Like freezing-point constants values most accurate for dilute solutions
178
Boiling-point elevation Δtb difference between the boiling points of the pure solvent and a nonelectrolyte solution of that solvent and it is directly proportional to the molal concentration of the solution B-p elevation can be calculated using Δtb = Kbm Δtb (boiling point elevation) expressed in m and m expressed in mol solutekg solvent
Sample Problem
What is the boiling-point elevation of a solution made from 200 g of a nonelectrolyte solute and 4000 g of water The molar mass of the solute is 620 gmol
180
1 AnalyzeGiven solute mass = 200 g solute molar mass = 620 gmol solvent mass and identity = 4000 g of water
Unknown boiling-point elevation
181
2 Plan
182
183
3 Compute
Practice Problem
A solution contains 500 g of sucrose C12H22O11 a nonelectrolyte dissolved in 5000 g of water What is the boiling-point elevation
015degC184
A solution contains 4500 g of sucrose C12H22O11 a nonelectrolyte dissolved in 250 g of waterWhat is the boiling point of the solution
1027degC185
If the boiling point elevation of an aqueous solution containing a nonvolatile electrolyte is 102degC what is the molality of the solution
20 m186
The boiling point of an aqueous solution containing a nonvolatile electrolyte is 10075degC aWhat is the boiling-point elevation bWhat is the molality of the solution a 075degC b 15 m
187
Electrolytes and Colligative PropertiesEarly investigators confused by experiments where certain substances depressed freezing point or elevated boiling point more than expected
188
189
Ex 01 m solution of NaCl lowers freezing point of solvent almost twice as much as 01 m solution of sucrose 01 m CaCl2 lowers fp almost 3 times as much as 01 m solution of sucrose
To understand why this happens contrast behavior or sucrose with NaCl in aqueous solution Each sucrose dissolves to make only 1 particle in solution So 1 mol sucrose dissolves to 1 mol particles 1 mol NaCl dissolves to make 2 moles of particles (1 mol Na+ and 1 mol Cl-)
190
191
Calculated Values for Electrolyte SolutionsColligative properties depend on total concentration of solute particles regardless of their identity Electrolytes cause changes in colligative properties proportional to total molality in terms of all dissolves particles instead of formula units
192
What about barium nitrate Ba(NO3)2 Each mole of barium nitrate yields 3 mol of ions in solution Ba(NO3)2(s) rarr Ba2+(aq) + 2NO3-(aq) You would expect solution of given molality to lower fp of solvent 3 times as much as nonelectrolytic solution of same molality
193
Sample Problem
What is the expected change in the freezing point of water in a solution of 625 g of barium nitrate Ba(NO3)2 in 100 kg of water
194
1 AnalyzeGiven solute mass and formula = 625 g Ba(NO3)2 solvent mass and identity = 100 kg water Δtf = Kfm
Unknown expected freezing-point depression
195
2 PlanThe molality can be calculated by converting the solute mass to moles and then dividing by the number of kilograms of solvent That molality is in terms of formula units of Ba(NO3)2 and must be converted to molality in terms of dissociated ions in solution It must be multiplied by the number of moles of ions produced per mole of formula unit This adjusted molality can then be used to calculate the freezing-point depression
196
197
198
3 Compute
Practice Problem
What is the expected freezing-point depression for a solution that contains 20 mol of magnesium sulfate dissolved in 10 kg of water
minus74degC199
What is the expected boiling-point elevation of water for a solution that contains 150 g of sodium chloride dissolved in 10 kg of water 27degC The freezing point of an aqueous sodium chloride solution is minus200degC What is the molality of the solution 54 m NaCl
200
Actual Values for Electrolyte SolutionsRemember values calculated are only expected values Actual values of colligative properties for all strong electrolytes are almost what would be expected Difference between expected and actual values caused by attractive forces that exist between dissociated ions in aqueous solution
201
Attraction between hydrated ions in solution is small compared with those in crystalline solid However forces of attraction donrsquot interfere with movements of aqueous ions More concentrated solution means ions closer together and attraction is greater Also depends on charge of ions ndash higher charge greater attraction
202
27the solution
process
Factors Affecting Rate of DissolutionEver tried to dissolve sugar in iced tea or coffee You notice temperature has something to do with how quickly solute dissolves What other factors affect how quickly solutes dissolve
28
1 Increasing Surface Area of Solute
bc dissolution process occurs at surface of solute Can increase dissolution by increasing surface area
29
2 Agitating (Stirring) SolutionClose to surface of solute concentration of dissolved solute is high Stirringshaking helps disperse solute particles
30
Brings fresh solvent into contact with solute surface Contact between solvent and solute surface increased
3 Heating a Solvent
Solutes dissolve faster in warmer solvent As temp increases solvent molecules move faster Average kinetic energy increases Collisions between solvent and solute more frequent Helps separate solute molecules and spread them out
32
Solubility
If you add sugar to tea eventually no more sugar dissolves
For every combination of solvent with solute at given temp there is limit to amount of solute that can be dissolved
33
When sugar first dropped into water sugar molecules leave solid surface and move about at random in solvent Some dissolved molecules may collide with crystal and stay there As more solid dissolves and concentration of dissolved molecules increases collisions happen more often
34
Eventually molecules are returning to crystal at same rate they are going into solution Dynamic equilibrium established between dissolution and crystallization Solution equilibrium - physical state in which the opposing processes of dissolution and crystallization of a solute occur at equal rates
35
Saturated vs Unsaturated Solutions
Saturated solution - solution that contains the maximum amount of dissolved solute How do you know if solution is saturated If more NaCl added it falls to bottom and doesnrsquot dissolve Equilibrium already established
36
If more water added to saturated solution more NaCl will dissolve At 20 359 g NaCl is max that will dissolve in 100 g water Unsaturated solution - solution that contains less solute than a saturated solution under the existing conditions
37
Supersaturated Solutions
When saturated solution where solubility increases as temp increases is cooled excess solute usually comes out of solution Sometimes if solution is left to cool the excess solute doesnrsquot separate
38
39
Supersaturated solution - solution that contains more dissolved solute than a saturated solution contains under same conditions
40
Supersaturated solution can remain unchanged over long time if it isnrsquot disturbed Once crystals begin to form continues until equilibrium is recreated at lower temp
41
Example
Sodium thiosulfate Na2S2O3 Solute added to hot water until solution saturated Hot solution filtered Drop small crystal in ldquoseedingrdquo crystallization Continues until equilibrium created
42
Solubility Values
Solubility - amount of substance required to form a saturated solution with specific amount of solvent at specified temp Ex Solubility of sugar = 204 g per 100 g water at 20 Must specify temp bc solubility changes with temp 43
For gases pressure must also be specified Rate at which substance dissolves is unrelated to solubility Max amount of solute that dissolves and reaches equilibrium is always same under same conditions
44
Solute-Solvent Interactions
ldquolike dissolves likerdquo is generally useful rule for predicting whether one substance will dissolve in another Depends on type of bonding polaritynonpolarity and intermolecular forces between solute and solvent
45
Dissolving Ionic Compounds in Aqueous Solution
Polarity of water molecules plays important role in formation of solution of ionic compounds Charged ends of water molecules attract ions and surround them to keep separated from other ions
46
Hydration - solution process with water as solvent Ions said to be hydrated As hydrated ions diffuse into solution other ions exposed and drawn away from crystal surface Entire crystal gradually dissolves
47
When crystallized from aqueous solutions some ionic compounds form crystals that include water molecules The crystalline compounds known as hydrates have specific ratios of water molecules Represented by formulas like CuSO45H2O
48
When crystalline hydrate dissolves in water water of hydration returns to solvent Behavior of solute in hydrated form no different than anhydrous form
49
Nonpolar Solvents
Nonpolar solvents do not attract ions of ionic compound strongly enough to overcome forces holding crystal together
50
Liquid Solutes and Solvents
When you shake bottle of salad dressing oil droplets disperse in water Stop shaking ndash strong attraction between water molecules squeeze out oil and form separate layers Liquid solutes and solvents that are not soluble in each other are immiscible
51
Nonpolar substances (fat oil grease) are generally quite soluble in nonpolar liquids (carbon tetrachloride toluene gasoline)
Only attractions between nonpolar molecules are weak London forces
52
London Dispersion Forces
bc e- are constantly moving at any time there may be uneven distribution of them around a nonpolar atom Temporarily creates positive end and negative end
53
54
London dispersion forces - intermolecular attractions resulting from constant motion of e- and creation of instantaneous dipoles
55
Liquids that dissolve freely in one another in any ratio are said to be completely miscible Benzene and carbon tetrachloride (both nonpolar) are completely miscible
Nonpolar molecules of these two apply no strong forces of attraction or repulsion and molecules mix freely
56
Ethanol and water are also completely miscible -OH group on ethanol is slightly polar
Can form H-bonds with water as well as other ethanol molecules Intermolecular forces in mixture are very similar to those in pure liquids so they are mutually soluble in any amount
57
Gasoline contains mainly nonpolar hydrocarbons
Excellent solvent for fats oils greases
Major intermolecular forces are weak London forces
58
Ethanol less polar than water more than carbon tetrachloride
Better solvent for less-polar substances bc of nonpolar region
59
Effects of Pressure on Solubility
Little effect on solids or liquids Increase pressure - increase solubilities of gases in liquids
60
When gas in contact with surface of liquid gas molecules can enter liquid As amount of dissolve gas increases some molecules start to escape and reenter gas phase
Eq eventually established between rates of enteringleaving gas phase
61
As long as eq undisturbed solubility of gas in liquid unchanged at given pressure
Gas + solvent solution
62
Increasing pressure of solute gas above solution puts stress on eq Molecules collide with surface more often Increase in pressure partially offset by increase in rate of gas molecules entering solution In turn increase in amount of dissolved gas causes increase in rate molecules escape liquid and becomes gas
63
Eventually eq restored at higher gas solubility
Expected from Le Chatelierrsquos principle
Increase in gas pressure causes eq to shift so fewer molecules are in gas phase
64
Henryrsquos Law
Henryrsquos law - solubility of a gas in liquid is directly proportional to the partial pressure of that gas on the surface of the liquid
Applies to gas-liquid solutions at constant temp
65
When mixture of ideal gases is limited in constant volume at constant temp each gas applies same pressure it would apply if it occupied the space alone
Assuming gases do not react each gas dissolves to the extent it would if no other gases were there
66
In carbonated drinks (Coke Pepsi etc) solubility of CO2 increased by increasing pressure
At bottling factory CO2 gas forced into solution of flavored water at pressure of 5-10 atm Gas-in-liquid solution then sealed in bottlescans
67
When cap removed pressure reduced to 1 atm and some CO2 escapes as gas bubbles Effervescence - rapid escape of a gas from a liquid in which it is dissolved
68
Effects of Temperature on Solubility
Letrsquos consider gas solubility Increasing temperature usually decreases gas solubility As temp increases average kinetic energy of molecules in solution increases Greater number of solute molecules escape from attraction of solvent and return to gas phase 69
At higher temps eq is reached with fewer gas molecules in solution Gases generally less soluble
70
Effect of temp on solubility of solids in liquids more difficult to predict
Often increasing temp increases solubility
However the same temp increase can result in large increase in solubility in one case and only slight increase in the next
71
72
enthalpies of Solution
Formation of solution comes with energy change Dissolve KI in water container feels cold Dissolve LiCl in water feels hot Formation of solid-liquid solution can absorb or release heat
73
During formation of solution solvent and solute particles experience changes in forces attracting them to other particles Before dissolving begins solvent molecules held by intermolecular forces (solvent-solvent attraction) In solute molecules held by Ifs (solute-solute attraction)
74
Energy is required to separate solute molecules and solvent molecules from their neighbors
A solute particle that is surrounded by solvent molecules is said to be solvated
75
Step 1 solute particles become separated from solid (energy absorbed) Step 2 solvent particles move apart to allow solute particles to enter liquid (energy absorbed)
76
Step 3 solvent particles attracted to and solvate solute particles (energy released)
77
Heat of solution - net amount of heat energy absorbed or released when a specific amount of solute dissolves in a solvent
Heat of solution negative (heat is released) if sums of energy in steps 1 and 2 is less than step 3
If 1+2 gt 3 HOS is positive (heat absorbed)78
In gaseous state molecules are so far apart there are nearly no forces between them Solute-solute interaction has little effect on heat of solution of a gas Energy released when gas dissolved in liquid bc attraction between solute gas and solvent molecules greater than energy needed to separate solvent molecules
79
Concentration of Solutions80
Concentration - measure of amount of solute in given amount of solvent or solution ldquodiluterdquo just means small amount of solute ldquoconcentratedrdquo just means relatively large amount of solute Unrelated to degree to which solution is saturated Ex Saturated solution of substance not very soluble might be dilute
81
Molarity
Molarity - number of moles of solute in one liter of solution To find molarity must know molar mass A ldquoone molarrdquo solution of NaOH is 1 mole NaOH per liter of solution (1 M)
82
1 mol NaOH = 400 g If this quantity dissolve in enough water to make EXACTLY 100 L solution solution is 1 M If 200 g NaOH dissolve in enough to make 100 L solution what is molarity 0500 M
83
1 M solution is not made by adding 1 mol solute to 1 L solvent That would be more than 1 L Instead 1 mol solute dissolved in less than 1 L Then diluted with more solvent to bring TOTAL VOLUME to 1 L
84
Practice ProblemYou have 340 L of solution that contains 900 g sodium chloride What is the molarity of that solution 0440 M NaCl You have 08 L of a 05 M HCl solution How many moles of HCl does this solution contain 04 mol HCl
85
What is the molarity of a solution composed of 585 g of potassium iodide dissolved in enough water to make a 0125 L of solution 0282 M KI
86
How many moles of H2SO4 are present in 0500 L of a 0150 M H2SO4 solution 00750 mol What volume of 300 M NaCl is needed for reaction that requires 1463 g of NaCl 0834 L
87
Molality
Molality - concentration of solution expressed in moles of solute per kilogram of solvent Solution that contains 1 mol solute (NaOH) dissolved in exactly 1 kg of solvent is ldquoone molalrdquo solution (1 m NaOH)
88
1 mol NaOH = 400 g 400 g NaOH dissolved in 1 kg water is 1 m NaOH
200 g NaOH in 1 kg water = 0500 m NaOH
89
Practice Problem
A solution was prepared by dissolving 171 g sucrose (C12H22O11) in 125 g of water Find the molal concentration of this solution 0400 m C12H22O11
90
A solution of iodine in carbon tetrachloride is used when iodine is needed for certain chemical tests How much iodine must be added to prepare a 0480 m solution of iodine in CCl4 if 1000 g of CCl4 is used 122 g I2
91
What is the molality of a solution composed of 255 g acetone (CH3)2CO dissolved in 200 g of water 22 m acetone What quantity in grams of methanol CH3OH is required to prepare a 0244 m solution in 400 g water 312 g methanol
92
How many grams of AgNO3 are needed to prepare a 0125 m solution in 250 mL of water 531 g AgNO3
What is the molality of a solution containing 182 g HCl and 250 g water 200 m
93
Ions in Aqueous Solutions and Colligative Properties
94
95
compounds in aqueous solutions
DissociationWhen compound made from ions dissolves in water ions separate Dissociation - separation of ions that occurs when an ionic compound dissolves
96
Notice number of ions made per formula unit in equations 1 formula unit of NaCl gives 2 units of ions in solution 1 formula unit of CaCl2 gives 3 units of ions
97
Assuming 100 dissociation solution that contains 1 mol NaCl contains 1 mol Na+ and 1 mol Cl- Can assume 100 dissociation of all soluble ionic compounds
98
Practice Problem
Write the equation for the dissolution of aluminum sulfate in water How many moles of aluminum ions are made by dissolving 1 mol aluminum sulfate What is the total number of moles of ions made by dissolving 1 mol of aluminum sulfate
99
1 AnalyzeGiven Amount of solute = 1 mol Al2(SO4)3 Solvent identity = water Unknown a moles of aluminum ions and sulfate ions Total number of moles of solute ions produced
100
2 Plan
The coefficients in the balanced dissociation equation will tell mole relationships You can use the equation to find out the number of moles of solute ions produced
101
3 Compute
a
b102
Write the equation for the dissolution of each of the following in water and then determine the number of moles of each ions made as well as the total number of ions made a 1 mol ammonium chloride b 1 mol sodium sulfide c 05 mol barium nitrate
103
a 1 mol ammonium chloride
1 mol NH4+ 1 mol Cl- 2 mol total ions
104
b 1 mol sodium sulfide
2 mol Na+ 1 mol S-2 3 mol total ions
105
c 05 mol barium nitrate
05 mol Ba+2 1 mol NO3- 15 mol total ions
106
Precipitation Reactions
Even though no compound is completely insoluble compounds of very low solubility can be considered insoluble for practical purposes There are general guidelines to help predict whether a compound made of certain combination of ions is soluble
107
General Solubility Guidelines
Sodium potassium and ammonium compounds are soluble in water
108
Nitrates acetates and chlorates are soluble
109
110
Most chlorides are soluble except those of silver mercury (I) and lead Lead (II) chloride is soluble in hot water
Most sulfates are soluble except those of barium strontium lead calcium and mercury
111
Most carbonates phosphates and silicates are insoluble except those of sodium potassium and ammonium
112
Most sulfides are insoluble except those of calcium strontium sodium potassium and ammonium
113
Is calcium phosphate Ca3(PO4)2 soluble or insoluble
Not one of the exceptions so it is insoluble
Dissociation equations are not written for insoluble compounds
114
Guidelines also useful in predicting what will happen if 2 solutions of 2 different soluble compounds are mixed
If mixing results in combination of ions that forms insoluble compound a double-replacement and precipitation reaction will happen
115
Will precipitate form when solutions of ammonium sulfide and cadmium nitrate are combined You can tell calcium nitrate is soluble from the guidelines (Most nitrates acetates and chlorates are soluble) You can also tell ammonium sulfide is soluble (Most sulfides are insoluble except those of calcium strontium sodium potassium and ammonium)
116
117
To decide whether precipitate can form must know solubilities of two possible products
Ammonium nitrate ndash soluble (Most nitrates acetates and chlorates are soluble) Cadmium sulfide ndash insoluble (Most sulfides are insoluble except those of calcium strontium sodium potassium and ammonium)
118
bc one product is insoluble double-replacement and precipitation reaction will happen (NH4)2S (aq) + Cd(NO3)2 (aq) rarr NH4NO3 (aq) + CdS (s)
119
Net Ionic Equations
Reactions of ions in aqueous solution usually represented by net ionic equations instead of formula equations Net ionic equation - includes only those compounds and ions that undergo a chemical change in a reaction in an aqueous solution
120
121
To write NIE (net ionic equation) first convert chemical equation into overall ionic equation All soluble ionic compounds shown as dissociated ions in solution Precipitates shown as solids
122
Notice ammonium and nitrate ions appear on both sides They havenrsquot gone through any chemical change Spectator ions - ions that do not take part in a chemical reaction and are found in solution both before and after the reaction
123
To change ionic equation to NIE eliminate spectator ions Cd+2(aq) + S-2(aq) rarr CdS(s)
This NIE applies to any reaction in which precipitate of CdS forms
124
Practice Problem
Identify the precipitate that forms when aqueous solutions of zinc nitrate and ammonium sulfide are combined Write the equation for the possible double-replacement reaction Then write the formula equation overall ionic equation and the net ionic equation
125
1 AnalyzeGiven Identity of reactants zinc nitrate and ammonium sulfide Reaction medium aqueous solution Unknown a equation for the possible double-replacement reaction b identity of the precipitate c formula equation d overall ionic equation e net ionic equation
126
2 PlanWrite the possible double-replacement reaction between Zn(NO3)2 and (NH4)2S Use the rules to determine if any of the products will precipitate Write a formula equation and overall net ionic equation Then cancel spectator ions to make net ionic equation
127
3 Computea possible double-replacement reaction
b rules show that zinc sulfide is not soluble so will precipitate (ammonium nitrate is soluble)
c formula equation128
d overall ionic equation
e net ionic equation129
Will a precipitate form if solutions of potassium sulfate and barium nitrate are combined If so write the net ionic equation for the reaction
Yes Ba+2(aq) + SO4-2(aq) rarr BaSO4(s)
130
Will a precipitate form if solutions of potassium nitrate and magnesium sulfate are combined If so write the net ionic equation for the reaction
No 131
Will a precipitate form if solutions of barium chloride and sodium sulfate are combined If so identify the spectator ions and write the net ionic equation for the reaction
Yes Na+ and Cl- Ba+2(aq) + SO4-2(aq) rarr BaSO4(s)
132
Write the net ionic equation for the precipitation of nickel(II) sulfide
Ni+2(aq) + S-2(aq) rarr NiS(s)
133
IonizationSome molecular compounds can also form ions in solution Usually polar Ionization - ions formed from solute molecules by action of the solvent (creation of ions where there were none) Ionization different from dissociation
134
Dissociation ionic compounds dissolve and ions already present separate Ionization ions formed where non existed before
Ions formed are hydrated Heat released during hydration of ions gives enough energy to break covalent bonds
135
Extent to which solute ionizes depends on strength of bonds within molecules of solute and strength of attraction between solute and solvent molecules
136
HCl ndash molecular compound that ionizes in aqueous solution Contains highly polar bond Attraction between polar HCl molecule and polar water strong enough to break HCl bond
137
The Hydronium Ion
H+ ions from HCl attracts other molecules or ions so strongly that it doesnrsquot normally exist alone Ionization of HCl better described as direct transfer of proton from HCl to H2O forming H3O+
138
139
Hydration of H+ to form hydronium is highly exothermic
Energy released gives a lot of energy needed to ionize a molecular solute
Many molecular compounds that ionize in aqueous solution contain hydrogen and form hydronium ion
140
Strong and Weak Electrolytes
Substances that make ions and conduct current in solution are electrolytes HCl is one of series of compounds that has hydrogen and halogen Hydrogen halides are all molecular compounds with single polar-covalent bonds
141
All are gases Very soluble in water All are electrolytes HCl HBr HI strongly conduct current in solution HF only weakly conducts current at same concentration Strength of conduction related to ability to ionize
142
143
Strong Electrolytes
HCl HBr HI are 100 ionized in solution
Strong electrolyte - any compound whose dilute aqueous solutions conduct electricity well this is due to the presence of all or almost all of the dissolve compound in the form of ions
144
Unique characteristic of strong electrolyte is that they yield only ions
Ex Ionic compound may be highly soluble in water and dissociate into ions (NaCl)
Others may not dissolve much but amount that does dissolve exists only as ions
145
Weak ElectrolytesHF dissolves in water to give acid solution called hydrofluoric acid HF bond stronger than bonds between hydrogen and other halogens When HF dissolves some molecule ionize Reverse reaction also happens HF(aq) + H2O(l) H3O+(aq) + F-(aq)
146
HF(aq) + H2O(l) H3O+
(aq) + F-(aq)Concentration of dissolved unionized HF stays high and concentration of ions stays low
Weak electrolyte - any compound whose aqueous solutions conduct electricity poorly this is due to the presence of small amount of the dissolved compound in the form of ions
147
Different from nonelectrolyte ndash NO ions at all Description of electrolyte as strong or weak not related to concentration of solution Electrolytes differ in degree of ionization not amount of solute dissolved
148
149
Colligative properties of solutions
Presence of solute affects properties of solutions Some properties not dependent on nature of dissolved substance but on how may dissolve particles are present
Colligative properties - properties that depend on the concentration of solute particles but not on their identity
150
Vapor-Pressure Lowering
Boiling and freezing point of solution different from pure solvent Nonvolatile solute raises boiling point and lowers freezing point Nonvolatile substance - one that has little tendency to become gas under existing conditions
151
152
To understand why nonvolatile solute changes boiling and freezing point consider equilibrium vapor pressure
Vapor pressure pressure caused by molecules that have escape liquid phase to gas phase
Can be thought of a measure of tendency of molecules to escape from a liquid
153
Addition of sucrose (nonvolatile solute) lowers concentration of water molecules at surface of liquid This lowers tendency of water molecules to leave solution and enter gas phase Vapor pressure of solution is lower than vapor pressure of pure water
154
Aqueous13 solution13 of13 nonvolatile13 solutePure13 water
Nonelectrolyte solutions of same molality have same concentration of particles
Dilute solutions of same solvent and equal molality of any nonelectrolyte solute lower vapor pressure equally
155
Example
1 m aqueous solution of nonelectrolyte glucose lowers vapor pressure of water 55 x 10-4 atm at 25 1 m aqueous solution of sucrose also lowers water vapor pressure to 55 x 10-4 atm at 25
156
157
bc vapor-pressure lowering depends on concentration of nonelectrolyte solute and doesnrsquot depend on the type of solute it is a colligative proprerty
bc vapor pressure has been lowered solution remains liquid over larger temperature range
Lowers freezing point and raises boiling point
158
Can assume changes in boiling and freezing point also depend on concentration of solute
They are colligative properties
159
Freezing-Point Depression
Freezing point of 1-molal solution of any nonelectrolyte solute in water is found (by experiment) to be 186 lower than freezing point of water When 1 mol of nonelectrolyte solute dissolved in 1 kg water freezing point is -186 not 000 160
When 2 mol nonelectrolyte solute dissolved in 1 kg water freezing point is -372
For any concentration of nonelectrolyte solute in water decrease in freezing point can be determined using value of -186m
161
This value called the molal freezing-point contstant (Kf) is the freezing-point depression of the solvent in a 1-molal solution of a nonvolatile nonelectrolyte solute
162
Each solvent has own characteristic molal freezing-point constant Freezing-point depression (Δtf) the difference between the freezing points of the pure solvent and a solute of nonelectrolyte in that solvent and it is directly proportional to the molal concentration of the solution Δtf = Kfm
163
Δtf = Kfm
Kf expressed as m
m expressed in mol solutekg solvent (molality)
164
Sample Problem
What is the freezing point depression of water in a solution of 171 g of sucrose C12H22O11 and 200 g of water What is the actual freezing point of the solution
165
1 AnalyzeGiven Solute mass and chemical formula 171 g C12H22O11 Solvent mass and identity 2000 g water Unknown a freezing-point depression b freezing point of the solution 166
2 Plan
167
168
Calculate
Unknown a freezing-point depression b freezing point of the solution
169
Practice Problem
A water solution containing an unknown quantity of a nonelectrolyte solute is found to have a freezing point of ndash 023degCWhat is the molal concentration of the solution
012 m170
A solution consists of 103 g of the nonelectrolyte glucose C6H12O6 dissolved in 250 g of water What is the freezing-point depression of the solution
minus0426degC171
In a laboratory experiment the freezing point of an aqueous solution of glucose is found to be minus0325degCWhat is the molal concentration of this solution
0175 m172
If 0500 mol of a nonelectrolyte solute are dissolved in 5000 g of ether what is the freezing point of the solution
minus1181degC173
The freezing point of an aqueous solution that contains a nonelectrolyte is minus90degC aWhat is the freezing-point depression of the solution bWhat is the molal concentration of the solution a minus90degC b 48 m
174
Boiling-Point Elevation
Boiling point is temp at which vapor pressure is equal to atmospheric pressure Change in vapor pressure will cause corresponding change in boiling point
175
176
Vapor pressure of nonvolatile solution is lower than vapor pressure of pure solvent More heat will be required to raise vapor pressure of solution Boiling point of solution is higher than boiling point of pure solvent
Molal boiling-point constant (Kb) - boiling-point elevation of the solvent in a 1-molal solution of a nonvolatile nonelectrolyte solute b-p elevation of any nonelectrolyte in water found by experiment to be 051 So molal b-p constant for water is 051m
177
For different solvents b-p elevations of 1-molal solutions have different values (Table 14-2)
Like freezing-point constants values most accurate for dilute solutions
178
Boiling-point elevation Δtb difference between the boiling points of the pure solvent and a nonelectrolyte solution of that solvent and it is directly proportional to the molal concentration of the solution B-p elevation can be calculated using Δtb = Kbm Δtb (boiling point elevation) expressed in m and m expressed in mol solutekg solvent
Sample Problem
What is the boiling-point elevation of a solution made from 200 g of a nonelectrolyte solute and 4000 g of water The molar mass of the solute is 620 gmol
180
1 AnalyzeGiven solute mass = 200 g solute molar mass = 620 gmol solvent mass and identity = 4000 g of water
Unknown boiling-point elevation
181
2 Plan
182
183
3 Compute
Practice Problem
A solution contains 500 g of sucrose C12H22O11 a nonelectrolyte dissolved in 5000 g of water What is the boiling-point elevation
015degC184
A solution contains 4500 g of sucrose C12H22O11 a nonelectrolyte dissolved in 250 g of waterWhat is the boiling point of the solution
1027degC185
If the boiling point elevation of an aqueous solution containing a nonvolatile electrolyte is 102degC what is the molality of the solution
20 m186
The boiling point of an aqueous solution containing a nonvolatile electrolyte is 10075degC aWhat is the boiling-point elevation bWhat is the molality of the solution a 075degC b 15 m
187
Electrolytes and Colligative PropertiesEarly investigators confused by experiments where certain substances depressed freezing point or elevated boiling point more than expected
188
189
Ex 01 m solution of NaCl lowers freezing point of solvent almost twice as much as 01 m solution of sucrose 01 m CaCl2 lowers fp almost 3 times as much as 01 m solution of sucrose
To understand why this happens contrast behavior or sucrose with NaCl in aqueous solution Each sucrose dissolves to make only 1 particle in solution So 1 mol sucrose dissolves to 1 mol particles 1 mol NaCl dissolves to make 2 moles of particles (1 mol Na+ and 1 mol Cl-)
190
191
Calculated Values for Electrolyte SolutionsColligative properties depend on total concentration of solute particles regardless of their identity Electrolytes cause changes in colligative properties proportional to total molality in terms of all dissolves particles instead of formula units
192
What about barium nitrate Ba(NO3)2 Each mole of barium nitrate yields 3 mol of ions in solution Ba(NO3)2(s) rarr Ba2+(aq) + 2NO3-(aq) You would expect solution of given molality to lower fp of solvent 3 times as much as nonelectrolytic solution of same molality
193
Sample Problem
What is the expected change in the freezing point of water in a solution of 625 g of barium nitrate Ba(NO3)2 in 100 kg of water
194
1 AnalyzeGiven solute mass and formula = 625 g Ba(NO3)2 solvent mass and identity = 100 kg water Δtf = Kfm
Unknown expected freezing-point depression
195
2 PlanThe molality can be calculated by converting the solute mass to moles and then dividing by the number of kilograms of solvent That molality is in terms of formula units of Ba(NO3)2 and must be converted to molality in terms of dissociated ions in solution It must be multiplied by the number of moles of ions produced per mole of formula unit This adjusted molality can then be used to calculate the freezing-point depression
196
197
198
3 Compute
Practice Problem
What is the expected freezing-point depression for a solution that contains 20 mol of magnesium sulfate dissolved in 10 kg of water
minus74degC199
What is the expected boiling-point elevation of water for a solution that contains 150 g of sodium chloride dissolved in 10 kg of water 27degC The freezing point of an aqueous sodium chloride solution is minus200degC What is the molality of the solution 54 m NaCl
200
Actual Values for Electrolyte SolutionsRemember values calculated are only expected values Actual values of colligative properties for all strong electrolytes are almost what would be expected Difference between expected and actual values caused by attractive forces that exist between dissociated ions in aqueous solution
201
Attraction between hydrated ions in solution is small compared with those in crystalline solid However forces of attraction donrsquot interfere with movements of aqueous ions More concentrated solution means ions closer together and attraction is greater Also depends on charge of ions ndash higher charge greater attraction
202
Factors Affecting Rate of DissolutionEver tried to dissolve sugar in iced tea or coffee You notice temperature has something to do with how quickly solute dissolves What other factors affect how quickly solutes dissolve
28
1 Increasing Surface Area of Solute
bc dissolution process occurs at surface of solute Can increase dissolution by increasing surface area
29
2 Agitating (Stirring) SolutionClose to surface of solute concentration of dissolved solute is high Stirringshaking helps disperse solute particles
30
Brings fresh solvent into contact with solute surface Contact between solvent and solute surface increased
3 Heating a Solvent
Solutes dissolve faster in warmer solvent As temp increases solvent molecules move faster Average kinetic energy increases Collisions between solvent and solute more frequent Helps separate solute molecules and spread them out
32
Solubility
If you add sugar to tea eventually no more sugar dissolves
For every combination of solvent with solute at given temp there is limit to amount of solute that can be dissolved
33
When sugar first dropped into water sugar molecules leave solid surface and move about at random in solvent Some dissolved molecules may collide with crystal and stay there As more solid dissolves and concentration of dissolved molecules increases collisions happen more often
34
Eventually molecules are returning to crystal at same rate they are going into solution Dynamic equilibrium established between dissolution and crystallization Solution equilibrium - physical state in which the opposing processes of dissolution and crystallization of a solute occur at equal rates
35
Saturated vs Unsaturated Solutions
Saturated solution - solution that contains the maximum amount of dissolved solute How do you know if solution is saturated If more NaCl added it falls to bottom and doesnrsquot dissolve Equilibrium already established
36
If more water added to saturated solution more NaCl will dissolve At 20 359 g NaCl is max that will dissolve in 100 g water Unsaturated solution - solution that contains less solute than a saturated solution under the existing conditions
37
Supersaturated Solutions
When saturated solution where solubility increases as temp increases is cooled excess solute usually comes out of solution Sometimes if solution is left to cool the excess solute doesnrsquot separate
38
39
Supersaturated solution - solution that contains more dissolved solute than a saturated solution contains under same conditions
40
Supersaturated solution can remain unchanged over long time if it isnrsquot disturbed Once crystals begin to form continues until equilibrium is recreated at lower temp
41
Example
Sodium thiosulfate Na2S2O3 Solute added to hot water until solution saturated Hot solution filtered Drop small crystal in ldquoseedingrdquo crystallization Continues until equilibrium created
42
Solubility Values
Solubility - amount of substance required to form a saturated solution with specific amount of solvent at specified temp Ex Solubility of sugar = 204 g per 100 g water at 20 Must specify temp bc solubility changes with temp 43
For gases pressure must also be specified Rate at which substance dissolves is unrelated to solubility Max amount of solute that dissolves and reaches equilibrium is always same under same conditions
44
Solute-Solvent Interactions
ldquolike dissolves likerdquo is generally useful rule for predicting whether one substance will dissolve in another Depends on type of bonding polaritynonpolarity and intermolecular forces between solute and solvent
45
Dissolving Ionic Compounds in Aqueous Solution
Polarity of water molecules plays important role in formation of solution of ionic compounds Charged ends of water molecules attract ions and surround them to keep separated from other ions
46
Hydration - solution process with water as solvent Ions said to be hydrated As hydrated ions diffuse into solution other ions exposed and drawn away from crystal surface Entire crystal gradually dissolves
47
When crystallized from aqueous solutions some ionic compounds form crystals that include water molecules The crystalline compounds known as hydrates have specific ratios of water molecules Represented by formulas like CuSO45H2O
48
When crystalline hydrate dissolves in water water of hydration returns to solvent Behavior of solute in hydrated form no different than anhydrous form
49
Nonpolar Solvents
Nonpolar solvents do not attract ions of ionic compound strongly enough to overcome forces holding crystal together
50
Liquid Solutes and Solvents
When you shake bottle of salad dressing oil droplets disperse in water Stop shaking ndash strong attraction between water molecules squeeze out oil and form separate layers Liquid solutes and solvents that are not soluble in each other are immiscible
51
Nonpolar substances (fat oil grease) are generally quite soluble in nonpolar liquids (carbon tetrachloride toluene gasoline)
Only attractions between nonpolar molecules are weak London forces
52
London Dispersion Forces
bc e- are constantly moving at any time there may be uneven distribution of them around a nonpolar atom Temporarily creates positive end and negative end
53
54
London dispersion forces - intermolecular attractions resulting from constant motion of e- and creation of instantaneous dipoles
55
Liquids that dissolve freely in one another in any ratio are said to be completely miscible Benzene and carbon tetrachloride (both nonpolar) are completely miscible
Nonpolar molecules of these two apply no strong forces of attraction or repulsion and molecules mix freely
56
Ethanol and water are also completely miscible -OH group on ethanol is slightly polar
Can form H-bonds with water as well as other ethanol molecules Intermolecular forces in mixture are very similar to those in pure liquids so they are mutually soluble in any amount
57
Gasoline contains mainly nonpolar hydrocarbons
Excellent solvent for fats oils greases
Major intermolecular forces are weak London forces
58
Ethanol less polar than water more than carbon tetrachloride
Better solvent for less-polar substances bc of nonpolar region
59
Effects of Pressure on Solubility
Little effect on solids or liquids Increase pressure - increase solubilities of gases in liquids
60
When gas in contact with surface of liquid gas molecules can enter liquid As amount of dissolve gas increases some molecules start to escape and reenter gas phase
Eq eventually established between rates of enteringleaving gas phase
61
As long as eq undisturbed solubility of gas in liquid unchanged at given pressure
Gas + solvent solution
62
Increasing pressure of solute gas above solution puts stress on eq Molecules collide with surface more often Increase in pressure partially offset by increase in rate of gas molecules entering solution In turn increase in amount of dissolved gas causes increase in rate molecules escape liquid and becomes gas
63
Eventually eq restored at higher gas solubility
Expected from Le Chatelierrsquos principle
Increase in gas pressure causes eq to shift so fewer molecules are in gas phase
64
Henryrsquos Law
Henryrsquos law - solubility of a gas in liquid is directly proportional to the partial pressure of that gas on the surface of the liquid
Applies to gas-liquid solutions at constant temp
65
When mixture of ideal gases is limited in constant volume at constant temp each gas applies same pressure it would apply if it occupied the space alone
Assuming gases do not react each gas dissolves to the extent it would if no other gases were there
66
In carbonated drinks (Coke Pepsi etc) solubility of CO2 increased by increasing pressure
At bottling factory CO2 gas forced into solution of flavored water at pressure of 5-10 atm Gas-in-liquid solution then sealed in bottlescans
67
When cap removed pressure reduced to 1 atm and some CO2 escapes as gas bubbles Effervescence - rapid escape of a gas from a liquid in which it is dissolved
68
Effects of Temperature on Solubility
Letrsquos consider gas solubility Increasing temperature usually decreases gas solubility As temp increases average kinetic energy of molecules in solution increases Greater number of solute molecules escape from attraction of solvent and return to gas phase 69
At higher temps eq is reached with fewer gas molecules in solution Gases generally less soluble
70
Effect of temp on solubility of solids in liquids more difficult to predict
Often increasing temp increases solubility
However the same temp increase can result in large increase in solubility in one case and only slight increase in the next
71
72
enthalpies of Solution
Formation of solution comes with energy change Dissolve KI in water container feels cold Dissolve LiCl in water feels hot Formation of solid-liquid solution can absorb or release heat
73
During formation of solution solvent and solute particles experience changes in forces attracting them to other particles Before dissolving begins solvent molecules held by intermolecular forces (solvent-solvent attraction) In solute molecules held by Ifs (solute-solute attraction)
74
Energy is required to separate solute molecules and solvent molecules from their neighbors
A solute particle that is surrounded by solvent molecules is said to be solvated
75
Step 1 solute particles become separated from solid (energy absorbed) Step 2 solvent particles move apart to allow solute particles to enter liquid (energy absorbed)
76
Step 3 solvent particles attracted to and solvate solute particles (energy released)
77
Heat of solution - net amount of heat energy absorbed or released when a specific amount of solute dissolves in a solvent
Heat of solution negative (heat is released) if sums of energy in steps 1 and 2 is less than step 3
If 1+2 gt 3 HOS is positive (heat absorbed)78
In gaseous state molecules are so far apart there are nearly no forces between them Solute-solute interaction has little effect on heat of solution of a gas Energy released when gas dissolved in liquid bc attraction between solute gas and solvent molecules greater than energy needed to separate solvent molecules
79
Concentration of Solutions80
Concentration - measure of amount of solute in given amount of solvent or solution ldquodiluterdquo just means small amount of solute ldquoconcentratedrdquo just means relatively large amount of solute Unrelated to degree to which solution is saturated Ex Saturated solution of substance not very soluble might be dilute
81
Molarity
Molarity - number of moles of solute in one liter of solution To find molarity must know molar mass A ldquoone molarrdquo solution of NaOH is 1 mole NaOH per liter of solution (1 M)
82
1 mol NaOH = 400 g If this quantity dissolve in enough water to make EXACTLY 100 L solution solution is 1 M If 200 g NaOH dissolve in enough to make 100 L solution what is molarity 0500 M
83
1 M solution is not made by adding 1 mol solute to 1 L solvent That would be more than 1 L Instead 1 mol solute dissolved in less than 1 L Then diluted with more solvent to bring TOTAL VOLUME to 1 L
84
Practice ProblemYou have 340 L of solution that contains 900 g sodium chloride What is the molarity of that solution 0440 M NaCl You have 08 L of a 05 M HCl solution How many moles of HCl does this solution contain 04 mol HCl
85
What is the molarity of a solution composed of 585 g of potassium iodide dissolved in enough water to make a 0125 L of solution 0282 M KI
86
How many moles of H2SO4 are present in 0500 L of a 0150 M H2SO4 solution 00750 mol What volume of 300 M NaCl is needed for reaction that requires 1463 g of NaCl 0834 L
87
Molality
Molality - concentration of solution expressed in moles of solute per kilogram of solvent Solution that contains 1 mol solute (NaOH) dissolved in exactly 1 kg of solvent is ldquoone molalrdquo solution (1 m NaOH)
88
1 mol NaOH = 400 g 400 g NaOH dissolved in 1 kg water is 1 m NaOH
200 g NaOH in 1 kg water = 0500 m NaOH
89
Practice Problem
A solution was prepared by dissolving 171 g sucrose (C12H22O11) in 125 g of water Find the molal concentration of this solution 0400 m C12H22O11
90
A solution of iodine in carbon tetrachloride is used when iodine is needed for certain chemical tests How much iodine must be added to prepare a 0480 m solution of iodine in CCl4 if 1000 g of CCl4 is used 122 g I2
91
What is the molality of a solution composed of 255 g acetone (CH3)2CO dissolved in 200 g of water 22 m acetone What quantity in grams of methanol CH3OH is required to prepare a 0244 m solution in 400 g water 312 g methanol
92
How many grams of AgNO3 are needed to prepare a 0125 m solution in 250 mL of water 531 g AgNO3
What is the molality of a solution containing 182 g HCl and 250 g water 200 m
93
Ions in Aqueous Solutions and Colligative Properties
94
95
compounds in aqueous solutions
DissociationWhen compound made from ions dissolves in water ions separate Dissociation - separation of ions that occurs when an ionic compound dissolves
96
Notice number of ions made per formula unit in equations 1 formula unit of NaCl gives 2 units of ions in solution 1 formula unit of CaCl2 gives 3 units of ions
97
Assuming 100 dissociation solution that contains 1 mol NaCl contains 1 mol Na+ and 1 mol Cl- Can assume 100 dissociation of all soluble ionic compounds
98
Practice Problem
Write the equation for the dissolution of aluminum sulfate in water How many moles of aluminum ions are made by dissolving 1 mol aluminum sulfate What is the total number of moles of ions made by dissolving 1 mol of aluminum sulfate
99
1 AnalyzeGiven Amount of solute = 1 mol Al2(SO4)3 Solvent identity = water Unknown a moles of aluminum ions and sulfate ions Total number of moles of solute ions produced
100
2 Plan
The coefficients in the balanced dissociation equation will tell mole relationships You can use the equation to find out the number of moles of solute ions produced
101
3 Compute
a
b102
Write the equation for the dissolution of each of the following in water and then determine the number of moles of each ions made as well as the total number of ions made a 1 mol ammonium chloride b 1 mol sodium sulfide c 05 mol barium nitrate
103
a 1 mol ammonium chloride
1 mol NH4+ 1 mol Cl- 2 mol total ions
104
b 1 mol sodium sulfide
2 mol Na+ 1 mol S-2 3 mol total ions
105
c 05 mol barium nitrate
05 mol Ba+2 1 mol NO3- 15 mol total ions
106
Precipitation Reactions
Even though no compound is completely insoluble compounds of very low solubility can be considered insoluble for practical purposes There are general guidelines to help predict whether a compound made of certain combination of ions is soluble
107
General Solubility Guidelines
Sodium potassium and ammonium compounds are soluble in water
108
Nitrates acetates and chlorates are soluble
109
110
Most chlorides are soluble except those of silver mercury (I) and lead Lead (II) chloride is soluble in hot water
Most sulfates are soluble except those of barium strontium lead calcium and mercury
111
Most carbonates phosphates and silicates are insoluble except those of sodium potassium and ammonium
112
Most sulfides are insoluble except those of calcium strontium sodium potassium and ammonium
113
Is calcium phosphate Ca3(PO4)2 soluble or insoluble
Not one of the exceptions so it is insoluble
Dissociation equations are not written for insoluble compounds
114
Guidelines also useful in predicting what will happen if 2 solutions of 2 different soluble compounds are mixed
If mixing results in combination of ions that forms insoluble compound a double-replacement and precipitation reaction will happen
115
Will precipitate form when solutions of ammonium sulfide and cadmium nitrate are combined You can tell calcium nitrate is soluble from the guidelines (Most nitrates acetates and chlorates are soluble) You can also tell ammonium sulfide is soluble (Most sulfides are insoluble except those of calcium strontium sodium potassium and ammonium)
116
117
To decide whether precipitate can form must know solubilities of two possible products
Ammonium nitrate ndash soluble (Most nitrates acetates and chlorates are soluble) Cadmium sulfide ndash insoluble (Most sulfides are insoluble except those of calcium strontium sodium potassium and ammonium)
118
bc one product is insoluble double-replacement and precipitation reaction will happen (NH4)2S (aq) + Cd(NO3)2 (aq) rarr NH4NO3 (aq) + CdS (s)
119
Net Ionic Equations
Reactions of ions in aqueous solution usually represented by net ionic equations instead of formula equations Net ionic equation - includes only those compounds and ions that undergo a chemical change in a reaction in an aqueous solution
120
121
To write NIE (net ionic equation) first convert chemical equation into overall ionic equation All soluble ionic compounds shown as dissociated ions in solution Precipitates shown as solids
122
Notice ammonium and nitrate ions appear on both sides They havenrsquot gone through any chemical change Spectator ions - ions that do not take part in a chemical reaction and are found in solution both before and after the reaction
123
To change ionic equation to NIE eliminate spectator ions Cd+2(aq) + S-2(aq) rarr CdS(s)
This NIE applies to any reaction in which precipitate of CdS forms
124
Practice Problem
Identify the precipitate that forms when aqueous solutions of zinc nitrate and ammonium sulfide are combined Write the equation for the possible double-replacement reaction Then write the formula equation overall ionic equation and the net ionic equation
125
1 AnalyzeGiven Identity of reactants zinc nitrate and ammonium sulfide Reaction medium aqueous solution Unknown a equation for the possible double-replacement reaction b identity of the precipitate c formula equation d overall ionic equation e net ionic equation
126
2 PlanWrite the possible double-replacement reaction between Zn(NO3)2 and (NH4)2S Use the rules to determine if any of the products will precipitate Write a formula equation and overall net ionic equation Then cancel spectator ions to make net ionic equation
127
3 Computea possible double-replacement reaction
b rules show that zinc sulfide is not soluble so will precipitate (ammonium nitrate is soluble)
c formula equation128
d overall ionic equation
e net ionic equation129
Will a precipitate form if solutions of potassium sulfate and barium nitrate are combined If so write the net ionic equation for the reaction
Yes Ba+2(aq) + SO4-2(aq) rarr BaSO4(s)
130
Will a precipitate form if solutions of potassium nitrate and magnesium sulfate are combined If so write the net ionic equation for the reaction
No 131
Will a precipitate form if solutions of barium chloride and sodium sulfate are combined If so identify the spectator ions and write the net ionic equation for the reaction
Yes Na+ and Cl- Ba+2(aq) + SO4-2(aq) rarr BaSO4(s)
132
Write the net ionic equation for the precipitation of nickel(II) sulfide
Ni+2(aq) + S-2(aq) rarr NiS(s)
133
IonizationSome molecular compounds can also form ions in solution Usually polar Ionization - ions formed from solute molecules by action of the solvent (creation of ions where there were none) Ionization different from dissociation
134
Dissociation ionic compounds dissolve and ions already present separate Ionization ions formed where non existed before
Ions formed are hydrated Heat released during hydration of ions gives enough energy to break covalent bonds
135
Extent to which solute ionizes depends on strength of bonds within molecules of solute and strength of attraction between solute and solvent molecules
136
HCl ndash molecular compound that ionizes in aqueous solution Contains highly polar bond Attraction between polar HCl molecule and polar water strong enough to break HCl bond
137
The Hydronium Ion
H+ ions from HCl attracts other molecules or ions so strongly that it doesnrsquot normally exist alone Ionization of HCl better described as direct transfer of proton from HCl to H2O forming H3O+
138
139
Hydration of H+ to form hydronium is highly exothermic
Energy released gives a lot of energy needed to ionize a molecular solute
Many molecular compounds that ionize in aqueous solution contain hydrogen and form hydronium ion
140
Strong and Weak Electrolytes
Substances that make ions and conduct current in solution are electrolytes HCl is one of series of compounds that has hydrogen and halogen Hydrogen halides are all molecular compounds with single polar-covalent bonds
141
All are gases Very soluble in water All are electrolytes HCl HBr HI strongly conduct current in solution HF only weakly conducts current at same concentration Strength of conduction related to ability to ionize
142
143
Strong Electrolytes
HCl HBr HI are 100 ionized in solution
Strong electrolyte - any compound whose dilute aqueous solutions conduct electricity well this is due to the presence of all or almost all of the dissolve compound in the form of ions
144
Unique characteristic of strong electrolyte is that they yield only ions
Ex Ionic compound may be highly soluble in water and dissociate into ions (NaCl)
Others may not dissolve much but amount that does dissolve exists only as ions
145
Weak ElectrolytesHF dissolves in water to give acid solution called hydrofluoric acid HF bond stronger than bonds between hydrogen and other halogens When HF dissolves some molecule ionize Reverse reaction also happens HF(aq) + H2O(l) H3O+(aq) + F-(aq)
146
HF(aq) + H2O(l) H3O+
(aq) + F-(aq)Concentration of dissolved unionized HF stays high and concentration of ions stays low
Weak electrolyte - any compound whose aqueous solutions conduct electricity poorly this is due to the presence of small amount of the dissolved compound in the form of ions
147
Different from nonelectrolyte ndash NO ions at all Description of electrolyte as strong or weak not related to concentration of solution Electrolytes differ in degree of ionization not amount of solute dissolved
148
149
Colligative properties of solutions
Presence of solute affects properties of solutions Some properties not dependent on nature of dissolved substance but on how may dissolve particles are present
Colligative properties - properties that depend on the concentration of solute particles but not on their identity
150
Vapor-Pressure Lowering
Boiling and freezing point of solution different from pure solvent Nonvolatile solute raises boiling point and lowers freezing point Nonvolatile substance - one that has little tendency to become gas under existing conditions
151
152
To understand why nonvolatile solute changes boiling and freezing point consider equilibrium vapor pressure
Vapor pressure pressure caused by molecules that have escape liquid phase to gas phase
Can be thought of a measure of tendency of molecules to escape from a liquid
153
Addition of sucrose (nonvolatile solute) lowers concentration of water molecules at surface of liquid This lowers tendency of water molecules to leave solution and enter gas phase Vapor pressure of solution is lower than vapor pressure of pure water
154
Aqueous13 solution13 of13 nonvolatile13 solutePure13 water
Nonelectrolyte solutions of same molality have same concentration of particles
Dilute solutions of same solvent and equal molality of any nonelectrolyte solute lower vapor pressure equally
155
Example
1 m aqueous solution of nonelectrolyte glucose lowers vapor pressure of water 55 x 10-4 atm at 25 1 m aqueous solution of sucrose also lowers water vapor pressure to 55 x 10-4 atm at 25
156
157
bc vapor-pressure lowering depends on concentration of nonelectrolyte solute and doesnrsquot depend on the type of solute it is a colligative proprerty
bc vapor pressure has been lowered solution remains liquid over larger temperature range
Lowers freezing point and raises boiling point
158
Can assume changes in boiling and freezing point also depend on concentration of solute
They are colligative properties
159
Freezing-Point Depression
Freezing point of 1-molal solution of any nonelectrolyte solute in water is found (by experiment) to be 186 lower than freezing point of water When 1 mol of nonelectrolyte solute dissolved in 1 kg water freezing point is -186 not 000 160
When 2 mol nonelectrolyte solute dissolved in 1 kg water freezing point is -372
For any concentration of nonelectrolyte solute in water decrease in freezing point can be determined using value of -186m
161
This value called the molal freezing-point contstant (Kf) is the freezing-point depression of the solvent in a 1-molal solution of a nonvolatile nonelectrolyte solute
162
Each solvent has own characteristic molal freezing-point constant Freezing-point depression (Δtf) the difference between the freezing points of the pure solvent and a solute of nonelectrolyte in that solvent and it is directly proportional to the molal concentration of the solution Δtf = Kfm
163
Δtf = Kfm
Kf expressed as m
m expressed in mol solutekg solvent (molality)
164
Sample Problem
What is the freezing point depression of water in a solution of 171 g of sucrose C12H22O11 and 200 g of water What is the actual freezing point of the solution
165
1 AnalyzeGiven Solute mass and chemical formula 171 g C12H22O11 Solvent mass and identity 2000 g water Unknown a freezing-point depression b freezing point of the solution 166
2 Plan
167
168
Calculate
Unknown a freezing-point depression b freezing point of the solution
169
Practice Problem
A water solution containing an unknown quantity of a nonelectrolyte solute is found to have a freezing point of ndash 023degCWhat is the molal concentration of the solution
012 m170
A solution consists of 103 g of the nonelectrolyte glucose C6H12O6 dissolved in 250 g of water What is the freezing-point depression of the solution
minus0426degC171
In a laboratory experiment the freezing point of an aqueous solution of glucose is found to be minus0325degCWhat is the molal concentration of this solution
0175 m172
If 0500 mol of a nonelectrolyte solute are dissolved in 5000 g of ether what is the freezing point of the solution
minus1181degC173
The freezing point of an aqueous solution that contains a nonelectrolyte is minus90degC aWhat is the freezing-point depression of the solution bWhat is the molal concentration of the solution a minus90degC b 48 m
174
Boiling-Point Elevation
Boiling point is temp at which vapor pressure is equal to atmospheric pressure Change in vapor pressure will cause corresponding change in boiling point
175
176
Vapor pressure of nonvolatile solution is lower than vapor pressure of pure solvent More heat will be required to raise vapor pressure of solution Boiling point of solution is higher than boiling point of pure solvent
Molal boiling-point constant (Kb) - boiling-point elevation of the solvent in a 1-molal solution of a nonvolatile nonelectrolyte solute b-p elevation of any nonelectrolyte in water found by experiment to be 051 So molal b-p constant for water is 051m
177
For different solvents b-p elevations of 1-molal solutions have different values (Table 14-2)
Like freezing-point constants values most accurate for dilute solutions
178
Boiling-point elevation Δtb difference between the boiling points of the pure solvent and a nonelectrolyte solution of that solvent and it is directly proportional to the molal concentration of the solution B-p elevation can be calculated using Δtb = Kbm Δtb (boiling point elevation) expressed in m and m expressed in mol solutekg solvent
Sample Problem
What is the boiling-point elevation of a solution made from 200 g of a nonelectrolyte solute and 4000 g of water The molar mass of the solute is 620 gmol
180
1 AnalyzeGiven solute mass = 200 g solute molar mass = 620 gmol solvent mass and identity = 4000 g of water
Unknown boiling-point elevation
181
2 Plan
182
183
3 Compute
Practice Problem
A solution contains 500 g of sucrose C12H22O11 a nonelectrolyte dissolved in 5000 g of water What is the boiling-point elevation
015degC184
A solution contains 4500 g of sucrose C12H22O11 a nonelectrolyte dissolved in 250 g of waterWhat is the boiling point of the solution
1027degC185
If the boiling point elevation of an aqueous solution containing a nonvolatile electrolyte is 102degC what is the molality of the solution
20 m186
The boiling point of an aqueous solution containing a nonvolatile electrolyte is 10075degC aWhat is the boiling-point elevation bWhat is the molality of the solution a 075degC b 15 m
187
Electrolytes and Colligative PropertiesEarly investigators confused by experiments where certain substances depressed freezing point or elevated boiling point more than expected
188
189
Ex 01 m solution of NaCl lowers freezing point of solvent almost twice as much as 01 m solution of sucrose 01 m CaCl2 lowers fp almost 3 times as much as 01 m solution of sucrose
To understand why this happens contrast behavior or sucrose with NaCl in aqueous solution Each sucrose dissolves to make only 1 particle in solution So 1 mol sucrose dissolves to 1 mol particles 1 mol NaCl dissolves to make 2 moles of particles (1 mol Na+ and 1 mol Cl-)
190
191
Calculated Values for Electrolyte SolutionsColligative properties depend on total concentration of solute particles regardless of their identity Electrolytes cause changes in colligative properties proportional to total molality in terms of all dissolves particles instead of formula units
192
What about barium nitrate Ba(NO3)2 Each mole of barium nitrate yields 3 mol of ions in solution Ba(NO3)2(s) rarr Ba2+(aq) + 2NO3-(aq) You would expect solution of given molality to lower fp of solvent 3 times as much as nonelectrolytic solution of same molality
193
Sample Problem
What is the expected change in the freezing point of water in a solution of 625 g of barium nitrate Ba(NO3)2 in 100 kg of water
194
1 AnalyzeGiven solute mass and formula = 625 g Ba(NO3)2 solvent mass and identity = 100 kg water Δtf = Kfm
Unknown expected freezing-point depression
195
2 PlanThe molality can be calculated by converting the solute mass to moles and then dividing by the number of kilograms of solvent That molality is in terms of formula units of Ba(NO3)2 and must be converted to molality in terms of dissociated ions in solution It must be multiplied by the number of moles of ions produced per mole of formula unit This adjusted molality can then be used to calculate the freezing-point depression
196
197
198
3 Compute
Practice Problem
What is the expected freezing-point depression for a solution that contains 20 mol of magnesium sulfate dissolved in 10 kg of water
minus74degC199
What is the expected boiling-point elevation of water for a solution that contains 150 g of sodium chloride dissolved in 10 kg of water 27degC The freezing point of an aqueous sodium chloride solution is minus200degC What is the molality of the solution 54 m NaCl
200
Actual Values for Electrolyte SolutionsRemember values calculated are only expected values Actual values of colligative properties for all strong electrolytes are almost what would be expected Difference between expected and actual values caused by attractive forces that exist between dissociated ions in aqueous solution
201
Attraction between hydrated ions in solution is small compared with those in crystalline solid However forces of attraction donrsquot interfere with movements of aqueous ions More concentrated solution means ions closer together and attraction is greater Also depends on charge of ions ndash higher charge greater attraction
202
1 Increasing Surface Area of Solute
bc dissolution process occurs at surface of solute Can increase dissolution by increasing surface area
29
2 Agitating (Stirring) SolutionClose to surface of solute concentration of dissolved solute is high Stirringshaking helps disperse solute particles
30
Brings fresh solvent into contact with solute surface Contact between solvent and solute surface increased
3 Heating a Solvent
Solutes dissolve faster in warmer solvent As temp increases solvent molecules move faster Average kinetic energy increases Collisions between solvent and solute more frequent Helps separate solute molecules and spread them out
32
Solubility
If you add sugar to tea eventually no more sugar dissolves
For every combination of solvent with solute at given temp there is limit to amount of solute that can be dissolved
33
When sugar first dropped into water sugar molecules leave solid surface and move about at random in solvent Some dissolved molecules may collide with crystal and stay there As more solid dissolves and concentration of dissolved molecules increases collisions happen more often
34
Eventually molecules are returning to crystal at same rate they are going into solution Dynamic equilibrium established between dissolution and crystallization Solution equilibrium - physical state in which the opposing processes of dissolution and crystallization of a solute occur at equal rates
35
Saturated vs Unsaturated Solutions
Saturated solution - solution that contains the maximum amount of dissolved solute How do you know if solution is saturated If more NaCl added it falls to bottom and doesnrsquot dissolve Equilibrium already established
36
If more water added to saturated solution more NaCl will dissolve At 20 359 g NaCl is max that will dissolve in 100 g water Unsaturated solution - solution that contains less solute than a saturated solution under the existing conditions
37
Supersaturated Solutions
When saturated solution where solubility increases as temp increases is cooled excess solute usually comes out of solution Sometimes if solution is left to cool the excess solute doesnrsquot separate
38
39
Supersaturated solution - solution that contains more dissolved solute than a saturated solution contains under same conditions
40
Supersaturated solution can remain unchanged over long time if it isnrsquot disturbed Once crystals begin to form continues until equilibrium is recreated at lower temp
41
Example
Sodium thiosulfate Na2S2O3 Solute added to hot water until solution saturated Hot solution filtered Drop small crystal in ldquoseedingrdquo crystallization Continues until equilibrium created
42
Solubility Values
Solubility - amount of substance required to form a saturated solution with specific amount of solvent at specified temp Ex Solubility of sugar = 204 g per 100 g water at 20 Must specify temp bc solubility changes with temp 43
For gases pressure must also be specified Rate at which substance dissolves is unrelated to solubility Max amount of solute that dissolves and reaches equilibrium is always same under same conditions
44
Solute-Solvent Interactions
ldquolike dissolves likerdquo is generally useful rule for predicting whether one substance will dissolve in another Depends on type of bonding polaritynonpolarity and intermolecular forces between solute and solvent
45
Dissolving Ionic Compounds in Aqueous Solution
Polarity of water molecules plays important role in formation of solution of ionic compounds Charged ends of water molecules attract ions and surround them to keep separated from other ions
46
Hydration - solution process with water as solvent Ions said to be hydrated As hydrated ions diffuse into solution other ions exposed and drawn away from crystal surface Entire crystal gradually dissolves
47
When crystallized from aqueous solutions some ionic compounds form crystals that include water molecules The crystalline compounds known as hydrates have specific ratios of water molecules Represented by formulas like CuSO45H2O
48
When crystalline hydrate dissolves in water water of hydration returns to solvent Behavior of solute in hydrated form no different than anhydrous form
49
Nonpolar Solvents
Nonpolar solvents do not attract ions of ionic compound strongly enough to overcome forces holding crystal together
50
Liquid Solutes and Solvents
When you shake bottle of salad dressing oil droplets disperse in water Stop shaking ndash strong attraction between water molecules squeeze out oil and form separate layers Liquid solutes and solvents that are not soluble in each other are immiscible
51
Nonpolar substances (fat oil grease) are generally quite soluble in nonpolar liquids (carbon tetrachloride toluene gasoline)
Only attractions between nonpolar molecules are weak London forces
52
London Dispersion Forces
bc e- are constantly moving at any time there may be uneven distribution of them around a nonpolar atom Temporarily creates positive end and negative end
53
54
London dispersion forces - intermolecular attractions resulting from constant motion of e- and creation of instantaneous dipoles
55
Liquids that dissolve freely in one another in any ratio are said to be completely miscible Benzene and carbon tetrachloride (both nonpolar) are completely miscible
Nonpolar molecules of these two apply no strong forces of attraction or repulsion and molecules mix freely
56
Ethanol and water are also completely miscible -OH group on ethanol is slightly polar
Can form H-bonds with water as well as other ethanol molecules Intermolecular forces in mixture are very similar to those in pure liquids so they are mutually soluble in any amount
57
Gasoline contains mainly nonpolar hydrocarbons
Excellent solvent for fats oils greases
Major intermolecular forces are weak London forces
58
Ethanol less polar than water more than carbon tetrachloride
Better solvent for less-polar substances bc of nonpolar region
59
Effects of Pressure on Solubility
Little effect on solids or liquids Increase pressure - increase solubilities of gases in liquids
60
When gas in contact with surface of liquid gas molecules can enter liquid As amount of dissolve gas increases some molecules start to escape and reenter gas phase
Eq eventually established between rates of enteringleaving gas phase
61
As long as eq undisturbed solubility of gas in liquid unchanged at given pressure
Gas + solvent solution
62
Increasing pressure of solute gas above solution puts stress on eq Molecules collide with surface more often Increase in pressure partially offset by increase in rate of gas molecules entering solution In turn increase in amount of dissolved gas causes increase in rate molecules escape liquid and becomes gas
63
Eventually eq restored at higher gas solubility
Expected from Le Chatelierrsquos principle
Increase in gas pressure causes eq to shift so fewer molecules are in gas phase
64
Henryrsquos Law
Henryrsquos law - solubility of a gas in liquid is directly proportional to the partial pressure of that gas on the surface of the liquid
Applies to gas-liquid solutions at constant temp
65
When mixture of ideal gases is limited in constant volume at constant temp each gas applies same pressure it would apply if it occupied the space alone
Assuming gases do not react each gas dissolves to the extent it would if no other gases were there
66
In carbonated drinks (Coke Pepsi etc) solubility of CO2 increased by increasing pressure
At bottling factory CO2 gas forced into solution of flavored water at pressure of 5-10 atm Gas-in-liquid solution then sealed in bottlescans
67
When cap removed pressure reduced to 1 atm and some CO2 escapes as gas bubbles Effervescence - rapid escape of a gas from a liquid in which it is dissolved
68
Effects of Temperature on Solubility
Letrsquos consider gas solubility Increasing temperature usually decreases gas solubility As temp increases average kinetic energy of molecules in solution increases Greater number of solute molecules escape from attraction of solvent and return to gas phase 69
At higher temps eq is reached with fewer gas molecules in solution Gases generally less soluble
70
Effect of temp on solubility of solids in liquids more difficult to predict
Often increasing temp increases solubility
However the same temp increase can result in large increase in solubility in one case and only slight increase in the next
71
72
enthalpies of Solution
Formation of solution comes with energy change Dissolve KI in water container feels cold Dissolve LiCl in water feels hot Formation of solid-liquid solution can absorb or release heat
73
During formation of solution solvent and solute particles experience changes in forces attracting them to other particles Before dissolving begins solvent molecules held by intermolecular forces (solvent-solvent attraction) In solute molecules held by Ifs (solute-solute attraction)
74
Energy is required to separate solute molecules and solvent molecules from their neighbors
A solute particle that is surrounded by solvent molecules is said to be solvated
75
Step 1 solute particles become separated from solid (energy absorbed) Step 2 solvent particles move apart to allow solute particles to enter liquid (energy absorbed)
76
Step 3 solvent particles attracted to and solvate solute particles (energy released)
77
Heat of solution - net amount of heat energy absorbed or released when a specific amount of solute dissolves in a solvent
Heat of solution negative (heat is released) if sums of energy in steps 1 and 2 is less than step 3
If 1+2 gt 3 HOS is positive (heat absorbed)78
In gaseous state molecules are so far apart there are nearly no forces between them Solute-solute interaction has little effect on heat of solution of a gas Energy released when gas dissolved in liquid bc attraction between solute gas and solvent molecules greater than energy needed to separate solvent molecules
79
Concentration of Solutions80
Concentration - measure of amount of solute in given amount of solvent or solution ldquodiluterdquo just means small amount of solute ldquoconcentratedrdquo just means relatively large amount of solute Unrelated to degree to which solution is saturated Ex Saturated solution of substance not very soluble might be dilute
81
Molarity
Molarity - number of moles of solute in one liter of solution To find molarity must know molar mass A ldquoone molarrdquo solution of NaOH is 1 mole NaOH per liter of solution (1 M)
82
1 mol NaOH = 400 g If this quantity dissolve in enough water to make EXACTLY 100 L solution solution is 1 M If 200 g NaOH dissolve in enough to make 100 L solution what is molarity 0500 M
83
1 M solution is not made by adding 1 mol solute to 1 L solvent That would be more than 1 L Instead 1 mol solute dissolved in less than 1 L Then diluted with more solvent to bring TOTAL VOLUME to 1 L
84
Practice ProblemYou have 340 L of solution that contains 900 g sodium chloride What is the molarity of that solution 0440 M NaCl You have 08 L of a 05 M HCl solution How many moles of HCl does this solution contain 04 mol HCl
85
What is the molarity of a solution composed of 585 g of potassium iodide dissolved in enough water to make a 0125 L of solution 0282 M KI
86
How many moles of H2SO4 are present in 0500 L of a 0150 M H2SO4 solution 00750 mol What volume of 300 M NaCl is needed for reaction that requires 1463 g of NaCl 0834 L
87
Molality
Molality - concentration of solution expressed in moles of solute per kilogram of solvent Solution that contains 1 mol solute (NaOH) dissolved in exactly 1 kg of solvent is ldquoone molalrdquo solution (1 m NaOH)
88
1 mol NaOH = 400 g 400 g NaOH dissolved in 1 kg water is 1 m NaOH
200 g NaOH in 1 kg water = 0500 m NaOH
89
Practice Problem
A solution was prepared by dissolving 171 g sucrose (C12H22O11) in 125 g of water Find the molal concentration of this solution 0400 m C12H22O11
90
A solution of iodine in carbon tetrachloride is used when iodine is needed for certain chemical tests How much iodine must be added to prepare a 0480 m solution of iodine in CCl4 if 1000 g of CCl4 is used 122 g I2
91
What is the molality of a solution composed of 255 g acetone (CH3)2CO dissolved in 200 g of water 22 m acetone What quantity in grams of methanol CH3OH is required to prepare a 0244 m solution in 400 g water 312 g methanol
92
How many grams of AgNO3 are needed to prepare a 0125 m solution in 250 mL of water 531 g AgNO3
What is the molality of a solution containing 182 g HCl and 250 g water 200 m
93
Ions in Aqueous Solutions and Colligative Properties
94
95
compounds in aqueous solutions
DissociationWhen compound made from ions dissolves in water ions separate Dissociation - separation of ions that occurs when an ionic compound dissolves
96
Notice number of ions made per formula unit in equations 1 formula unit of NaCl gives 2 units of ions in solution 1 formula unit of CaCl2 gives 3 units of ions
97
Assuming 100 dissociation solution that contains 1 mol NaCl contains 1 mol Na+ and 1 mol Cl- Can assume 100 dissociation of all soluble ionic compounds
98
Practice Problem
Write the equation for the dissolution of aluminum sulfate in water How many moles of aluminum ions are made by dissolving 1 mol aluminum sulfate What is the total number of moles of ions made by dissolving 1 mol of aluminum sulfate
99
1 AnalyzeGiven Amount of solute = 1 mol Al2(SO4)3 Solvent identity = water Unknown a moles of aluminum ions and sulfate ions Total number of moles of solute ions produced
100
2 Plan
The coefficients in the balanced dissociation equation will tell mole relationships You can use the equation to find out the number of moles of solute ions produced
101
3 Compute
a
b102
Write the equation for the dissolution of each of the following in water and then determine the number of moles of each ions made as well as the total number of ions made a 1 mol ammonium chloride b 1 mol sodium sulfide c 05 mol barium nitrate
103
a 1 mol ammonium chloride
1 mol NH4+ 1 mol Cl- 2 mol total ions
104
b 1 mol sodium sulfide
2 mol Na+ 1 mol S-2 3 mol total ions
105
c 05 mol barium nitrate
05 mol Ba+2 1 mol NO3- 15 mol total ions
106
Precipitation Reactions
Even though no compound is completely insoluble compounds of very low solubility can be considered insoluble for practical purposes There are general guidelines to help predict whether a compound made of certain combination of ions is soluble
107
General Solubility Guidelines
Sodium potassium and ammonium compounds are soluble in water
108
Nitrates acetates and chlorates are soluble
109
110
Most chlorides are soluble except those of silver mercury (I) and lead Lead (II) chloride is soluble in hot water
Most sulfates are soluble except those of barium strontium lead calcium and mercury
111
Most carbonates phosphates and silicates are insoluble except those of sodium potassium and ammonium
112
Most sulfides are insoluble except those of calcium strontium sodium potassium and ammonium
113
Is calcium phosphate Ca3(PO4)2 soluble or insoluble
Not one of the exceptions so it is insoluble
Dissociation equations are not written for insoluble compounds
114
Guidelines also useful in predicting what will happen if 2 solutions of 2 different soluble compounds are mixed
If mixing results in combination of ions that forms insoluble compound a double-replacement and precipitation reaction will happen
115
Will precipitate form when solutions of ammonium sulfide and cadmium nitrate are combined You can tell calcium nitrate is soluble from the guidelines (Most nitrates acetates and chlorates are soluble) You can also tell ammonium sulfide is soluble (Most sulfides are insoluble except those of calcium strontium sodium potassium and ammonium)
116
117
To decide whether precipitate can form must know solubilities of two possible products
Ammonium nitrate ndash soluble (Most nitrates acetates and chlorates are soluble) Cadmium sulfide ndash insoluble (Most sulfides are insoluble except those of calcium strontium sodium potassium and ammonium)
118
bc one product is insoluble double-replacement and precipitation reaction will happen (NH4)2S (aq) + Cd(NO3)2 (aq) rarr NH4NO3 (aq) + CdS (s)
119
Net Ionic Equations
Reactions of ions in aqueous solution usually represented by net ionic equations instead of formula equations Net ionic equation - includes only those compounds and ions that undergo a chemical change in a reaction in an aqueous solution
120
121
To write NIE (net ionic equation) first convert chemical equation into overall ionic equation All soluble ionic compounds shown as dissociated ions in solution Precipitates shown as solids
122
Notice ammonium and nitrate ions appear on both sides They havenrsquot gone through any chemical change Spectator ions - ions that do not take part in a chemical reaction and are found in solution both before and after the reaction
123
To change ionic equation to NIE eliminate spectator ions Cd+2(aq) + S-2(aq) rarr CdS(s)
This NIE applies to any reaction in which precipitate of CdS forms
124
Practice Problem
Identify the precipitate that forms when aqueous solutions of zinc nitrate and ammonium sulfide are combined Write the equation for the possible double-replacement reaction Then write the formula equation overall ionic equation and the net ionic equation
125
1 AnalyzeGiven Identity of reactants zinc nitrate and ammonium sulfide Reaction medium aqueous solution Unknown a equation for the possible double-replacement reaction b identity of the precipitate c formula equation d overall ionic equation e net ionic equation
126
2 PlanWrite the possible double-replacement reaction between Zn(NO3)2 and (NH4)2S Use the rules to determine if any of the products will precipitate Write a formula equation and overall net ionic equation Then cancel spectator ions to make net ionic equation
127
3 Computea possible double-replacement reaction
b rules show that zinc sulfide is not soluble so will precipitate (ammonium nitrate is soluble)
c formula equation128
d overall ionic equation
e net ionic equation129
Will a precipitate form if solutions of potassium sulfate and barium nitrate are combined If so write the net ionic equation for the reaction
Yes Ba+2(aq) + SO4-2(aq) rarr BaSO4(s)
130
Will a precipitate form if solutions of potassium nitrate and magnesium sulfate are combined If so write the net ionic equation for the reaction
No 131
Will a precipitate form if solutions of barium chloride and sodium sulfate are combined If so identify the spectator ions and write the net ionic equation for the reaction
Yes Na+ and Cl- Ba+2(aq) + SO4-2(aq) rarr BaSO4(s)
132
Write the net ionic equation for the precipitation of nickel(II) sulfide
Ni+2(aq) + S-2(aq) rarr NiS(s)
133
IonizationSome molecular compounds can also form ions in solution Usually polar Ionization - ions formed from solute molecules by action of the solvent (creation of ions where there were none) Ionization different from dissociation
134
Dissociation ionic compounds dissolve and ions already present separate Ionization ions formed where non existed before
Ions formed are hydrated Heat released during hydration of ions gives enough energy to break covalent bonds
135
Extent to which solute ionizes depends on strength of bonds within molecules of solute and strength of attraction between solute and solvent molecules
136
HCl ndash molecular compound that ionizes in aqueous solution Contains highly polar bond Attraction between polar HCl molecule and polar water strong enough to break HCl bond
137
The Hydronium Ion
H+ ions from HCl attracts other molecules or ions so strongly that it doesnrsquot normally exist alone Ionization of HCl better described as direct transfer of proton from HCl to H2O forming H3O+
138
139
Hydration of H+ to form hydronium is highly exothermic
Energy released gives a lot of energy needed to ionize a molecular solute
Many molecular compounds that ionize in aqueous solution contain hydrogen and form hydronium ion
140
Strong and Weak Electrolytes
Substances that make ions and conduct current in solution are electrolytes HCl is one of series of compounds that has hydrogen and halogen Hydrogen halides are all molecular compounds with single polar-covalent bonds
141
All are gases Very soluble in water All are electrolytes HCl HBr HI strongly conduct current in solution HF only weakly conducts current at same concentration Strength of conduction related to ability to ionize
142
143
Strong Electrolytes
HCl HBr HI are 100 ionized in solution
Strong electrolyte - any compound whose dilute aqueous solutions conduct electricity well this is due to the presence of all or almost all of the dissolve compound in the form of ions
144
Unique characteristic of strong electrolyte is that they yield only ions
Ex Ionic compound may be highly soluble in water and dissociate into ions (NaCl)
Others may not dissolve much but amount that does dissolve exists only as ions
145
Weak ElectrolytesHF dissolves in water to give acid solution called hydrofluoric acid HF bond stronger than bonds between hydrogen and other halogens When HF dissolves some molecule ionize Reverse reaction also happens HF(aq) + H2O(l) H3O+(aq) + F-(aq)
146
HF(aq) + H2O(l) H3O+
(aq) + F-(aq)Concentration of dissolved unionized HF stays high and concentration of ions stays low
Weak electrolyte - any compound whose aqueous solutions conduct electricity poorly this is due to the presence of small amount of the dissolved compound in the form of ions
147
Different from nonelectrolyte ndash NO ions at all Description of electrolyte as strong or weak not related to concentration of solution Electrolytes differ in degree of ionization not amount of solute dissolved
148
149
Colligative properties of solutions
Presence of solute affects properties of solutions Some properties not dependent on nature of dissolved substance but on how may dissolve particles are present
Colligative properties - properties that depend on the concentration of solute particles but not on their identity
150
Vapor-Pressure Lowering
Boiling and freezing point of solution different from pure solvent Nonvolatile solute raises boiling point and lowers freezing point Nonvolatile substance - one that has little tendency to become gas under existing conditions
151
152
To understand why nonvolatile solute changes boiling and freezing point consider equilibrium vapor pressure
Vapor pressure pressure caused by molecules that have escape liquid phase to gas phase
Can be thought of a measure of tendency of molecules to escape from a liquid
153
Addition of sucrose (nonvolatile solute) lowers concentration of water molecules at surface of liquid This lowers tendency of water molecules to leave solution and enter gas phase Vapor pressure of solution is lower than vapor pressure of pure water
154
Aqueous13 solution13 of13 nonvolatile13 solutePure13 water
Nonelectrolyte solutions of same molality have same concentration of particles
Dilute solutions of same solvent and equal molality of any nonelectrolyte solute lower vapor pressure equally
155
Example
1 m aqueous solution of nonelectrolyte glucose lowers vapor pressure of water 55 x 10-4 atm at 25 1 m aqueous solution of sucrose also lowers water vapor pressure to 55 x 10-4 atm at 25
156
157
bc vapor-pressure lowering depends on concentration of nonelectrolyte solute and doesnrsquot depend on the type of solute it is a colligative proprerty
bc vapor pressure has been lowered solution remains liquid over larger temperature range
Lowers freezing point and raises boiling point
158
Can assume changes in boiling and freezing point also depend on concentration of solute
They are colligative properties
159
Freezing-Point Depression
Freezing point of 1-molal solution of any nonelectrolyte solute in water is found (by experiment) to be 186 lower than freezing point of water When 1 mol of nonelectrolyte solute dissolved in 1 kg water freezing point is -186 not 000 160
When 2 mol nonelectrolyte solute dissolved in 1 kg water freezing point is -372
For any concentration of nonelectrolyte solute in water decrease in freezing point can be determined using value of -186m
161
This value called the molal freezing-point contstant (Kf) is the freezing-point depression of the solvent in a 1-molal solution of a nonvolatile nonelectrolyte solute
162
Each solvent has own characteristic molal freezing-point constant Freezing-point depression (Δtf) the difference between the freezing points of the pure solvent and a solute of nonelectrolyte in that solvent and it is directly proportional to the molal concentration of the solution Δtf = Kfm
163
Δtf = Kfm
Kf expressed as m
m expressed in mol solutekg solvent (molality)
164
Sample Problem
What is the freezing point depression of water in a solution of 171 g of sucrose C12H22O11 and 200 g of water What is the actual freezing point of the solution
165
1 AnalyzeGiven Solute mass and chemical formula 171 g C12H22O11 Solvent mass and identity 2000 g water Unknown a freezing-point depression b freezing point of the solution 166
2 Plan
167
168
Calculate
Unknown a freezing-point depression b freezing point of the solution
169
Practice Problem
A water solution containing an unknown quantity of a nonelectrolyte solute is found to have a freezing point of ndash 023degCWhat is the molal concentration of the solution
012 m170
A solution consists of 103 g of the nonelectrolyte glucose C6H12O6 dissolved in 250 g of water What is the freezing-point depression of the solution
minus0426degC171
In a laboratory experiment the freezing point of an aqueous solution of glucose is found to be minus0325degCWhat is the molal concentration of this solution
0175 m172
If 0500 mol of a nonelectrolyte solute are dissolved in 5000 g of ether what is the freezing point of the solution
minus1181degC173
The freezing point of an aqueous solution that contains a nonelectrolyte is minus90degC aWhat is the freezing-point depression of the solution bWhat is the molal concentration of the solution a minus90degC b 48 m
174
Boiling-Point Elevation
Boiling point is temp at which vapor pressure is equal to atmospheric pressure Change in vapor pressure will cause corresponding change in boiling point
175
176
Vapor pressure of nonvolatile solution is lower than vapor pressure of pure solvent More heat will be required to raise vapor pressure of solution Boiling point of solution is higher than boiling point of pure solvent
Molal boiling-point constant (Kb) - boiling-point elevation of the solvent in a 1-molal solution of a nonvolatile nonelectrolyte solute b-p elevation of any nonelectrolyte in water found by experiment to be 051 So molal b-p constant for water is 051m
177
For different solvents b-p elevations of 1-molal solutions have different values (Table 14-2)
Like freezing-point constants values most accurate for dilute solutions
178
Boiling-point elevation Δtb difference between the boiling points of the pure solvent and a nonelectrolyte solution of that solvent and it is directly proportional to the molal concentration of the solution B-p elevation can be calculated using Δtb = Kbm Δtb (boiling point elevation) expressed in m and m expressed in mol solutekg solvent
Sample Problem
What is the boiling-point elevation of a solution made from 200 g of a nonelectrolyte solute and 4000 g of water The molar mass of the solute is 620 gmol
180
1 AnalyzeGiven solute mass = 200 g solute molar mass = 620 gmol solvent mass and identity = 4000 g of water
Unknown boiling-point elevation
181
2 Plan
182
183
3 Compute
Practice Problem
A solution contains 500 g of sucrose C12H22O11 a nonelectrolyte dissolved in 5000 g of water What is the boiling-point elevation
015degC184
A solution contains 4500 g of sucrose C12H22O11 a nonelectrolyte dissolved in 250 g of waterWhat is the boiling point of the solution
1027degC185
If the boiling point elevation of an aqueous solution containing a nonvolatile electrolyte is 102degC what is the molality of the solution
20 m186
The boiling point of an aqueous solution containing a nonvolatile electrolyte is 10075degC aWhat is the boiling-point elevation bWhat is the molality of the solution a 075degC b 15 m
187
Electrolytes and Colligative PropertiesEarly investigators confused by experiments where certain substances depressed freezing point or elevated boiling point more than expected
188
189
Ex 01 m solution of NaCl lowers freezing point of solvent almost twice as much as 01 m solution of sucrose 01 m CaCl2 lowers fp almost 3 times as much as 01 m solution of sucrose
To understand why this happens contrast behavior or sucrose with NaCl in aqueous solution Each sucrose dissolves to make only 1 particle in solution So 1 mol sucrose dissolves to 1 mol particles 1 mol NaCl dissolves to make 2 moles of particles (1 mol Na+ and 1 mol Cl-)
190
191
Calculated Values for Electrolyte SolutionsColligative properties depend on total concentration of solute particles regardless of their identity Electrolytes cause changes in colligative properties proportional to total molality in terms of all dissolves particles instead of formula units
192
What about barium nitrate Ba(NO3)2 Each mole of barium nitrate yields 3 mol of ions in solution Ba(NO3)2(s) rarr Ba2+(aq) + 2NO3-(aq) You would expect solution of given molality to lower fp of solvent 3 times as much as nonelectrolytic solution of same molality
193
Sample Problem
What is the expected change in the freezing point of water in a solution of 625 g of barium nitrate Ba(NO3)2 in 100 kg of water
194
1 AnalyzeGiven solute mass and formula = 625 g Ba(NO3)2 solvent mass and identity = 100 kg water Δtf = Kfm
Unknown expected freezing-point depression
195
2 PlanThe molality can be calculated by converting the solute mass to moles and then dividing by the number of kilograms of solvent That molality is in terms of formula units of Ba(NO3)2 and must be converted to molality in terms of dissociated ions in solution It must be multiplied by the number of moles of ions produced per mole of formula unit This adjusted molality can then be used to calculate the freezing-point depression
196
197
198
3 Compute
Practice Problem
What is the expected freezing-point depression for a solution that contains 20 mol of magnesium sulfate dissolved in 10 kg of water
minus74degC199
What is the expected boiling-point elevation of water for a solution that contains 150 g of sodium chloride dissolved in 10 kg of water 27degC The freezing point of an aqueous sodium chloride solution is minus200degC What is the molality of the solution 54 m NaCl
200
Actual Values for Electrolyte SolutionsRemember values calculated are only expected values Actual values of colligative properties for all strong electrolytes are almost what would be expected Difference between expected and actual values caused by attractive forces that exist between dissociated ions in aqueous solution
201
Attraction between hydrated ions in solution is small compared with those in crystalline solid However forces of attraction donrsquot interfere with movements of aqueous ions More concentrated solution means ions closer together and attraction is greater Also depends on charge of ions ndash higher charge greater attraction
202
2 Agitating (Stirring) SolutionClose to surface of solute concentration of dissolved solute is high Stirringshaking helps disperse solute particles
30
Brings fresh solvent into contact with solute surface Contact between solvent and solute surface increased
3 Heating a Solvent
Solutes dissolve faster in warmer solvent As temp increases solvent molecules move faster Average kinetic energy increases Collisions between solvent and solute more frequent Helps separate solute molecules and spread them out
32
Solubility
If you add sugar to tea eventually no more sugar dissolves
For every combination of solvent with solute at given temp there is limit to amount of solute that can be dissolved
33
When sugar first dropped into water sugar molecules leave solid surface and move about at random in solvent Some dissolved molecules may collide with crystal and stay there As more solid dissolves and concentration of dissolved molecules increases collisions happen more often
34
Eventually molecules are returning to crystal at same rate they are going into solution Dynamic equilibrium established between dissolution and crystallization Solution equilibrium - physical state in which the opposing processes of dissolution and crystallization of a solute occur at equal rates
35
Saturated vs Unsaturated Solutions
Saturated solution - solution that contains the maximum amount of dissolved solute How do you know if solution is saturated If more NaCl added it falls to bottom and doesnrsquot dissolve Equilibrium already established
36
If more water added to saturated solution more NaCl will dissolve At 20 359 g NaCl is max that will dissolve in 100 g water Unsaturated solution - solution that contains less solute than a saturated solution under the existing conditions
37
Supersaturated Solutions
When saturated solution where solubility increases as temp increases is cooled excess solute usually comes out of solution Sometimes if solution is left to cool the excess solute doesnrsquot separate
38
39
Supersaturated solution - solution that contains more dissolved solute than a saturated solution contains under same conditions
40
Supersaturated solution can remain unchanged over long time if it isnrsquot disturbed Once crystals begin to form continues until equilibrium is recreated at lower temp
41
Example
Sodium thiosulfate Na2S2O3 Solute added to hot water until solution saturated Hot solution filtered Drop small crystal in ldquoseedingrdquo crystallization Continues until equilibrium created
42
Solubility Values
Solubility - amount of substance required to form a saturated solution with specific amount of solvent at specified temp Ex Solubility of sugar = 204 g per 100 g water at 20 Must specify temp bc solubility changes with temp 43
For gases pressure must also be specified Rate at which substance dissolves is unrelated to solubility Max amount of solute that dissolves and reaches equilibrium is always same under same conditions
44
Solute-Solvent Interactions
ldquolike dissolves likerdquo is generally useful rule for predicting whether one substance will dissolve in another Depends on type of bonding polaritynonpolarity and intermolecular forces between solute and solvent
45
Dissolving Ionic Compounds in Aqueous Solution
Polarity of water molecules plays important role in formation of solution of ionic compounds Charged ends of water molecules attract ions and surround them to keep separated from other ions
46
Hydration - solution process with water as solvent Ions said to be hydrated As hydrated ions diffuse into solution other ions exposed and drawn away from crystal surface Entire crystal gradually dissolves
47
When crystallized from aqueous solutions some ionic compounds form crystals that include water molecules The crystalline compounds known as hydrates have specific ratios of water molecules Represented by formulas like CuSO45H2O
48
When crystalline hydrate dissolves in water water of hydration returns to solvent Behavior of solute in hydrated form no different than anhydrous form
49
Nonpolar Solvents
Nonpolar solvents do not attract ions of ionic compound strongly enough to overcome forces holding crystal together
50
Liquid Solutes and Solvents
When you shake bottle of salad dressing oil droplets disperse in water Stop shaking ndash strong attraction between water molecules squeeze out oil and form separate layers Liquid solutes and solvents that are not soluble in each other are immiscible
51
Nonpolar substances (fat oil grease) are generally quite soluble in nonpolar liquids (carbon tetrachloride toluene gasoline)
Only attractions between nonpolar molecules are weak London forces
52
London Dispersion Forces
bc e- are constantly moving at any time there may be uneven distribution of them around a nonpolar atom Temporarily creates positive end and negative end
53
54
London dispersion forces - intermolecular attractions resulting from constant motion of e- and creation of instantaneous dipoles
55
Liquids that dissolve freely in one another in any ratio are said to be completely miscible Benzene and carbon tetrachloride (both nonpolar) are completely miscible
Nonpolar molecules of these two apply no strong forces of attraction or repulsion and molecules mix freely
56
Ethanol and water are also completely miscible -OH group on ethanol is slightly polar
Can form H-bonds with water as well as other ethanol molecules Intermolecular forces in mixture are very similar to those in pure liquids so they are mutually soluble in any amount
57
Gasoline contains mainly nonpolar hydrocarbons
Excellent solvent for fats oils greases
Major intermolecular forces are weak London forces
58
Ethanol less polar than water more than carbon tetrachloride
Better solvent for less-polar substances bc of nonpolar region
59
Effects of Pressure on Solubility
Little effect on solids or liquids Increase pressure - increase solubilities of gases in liquids
60
When gas in contact with surface of liquid gas molecules can enter liquid As amount of dissolve gas increases some molecules start to escape and reenter gas phase
Eq eventually established between rates of enteringleaving gas phase
61
As long as eq undisturbed solubility of gas in liquid unchanged at given pressure
Gas + solvent solution
62
Increasing pressure of solute gas above solution puts stress on eq Molecules collide with surface more often Increase in pressure partially offset by increase in rate of gas molecules entering solution In turn increase in amount of dissolved gas causes increase in rate molecules escape liquid and becomes gas
63
Eventually eq restored at higher gas solubility
Expected from Le Chatelierrsquos principle
Increase in gas pressure causes eq to shift so fewer molecules are in gas phase
64
Henryrsquos Law
Henryrsquos law - solubility of a gas in liquid is directly proportional to the partial pressure of that gas on the surface of the liquid
Applies to gas-liquid solutions at constant temp
65
When mixture of ideal gases is limited in constant volume at constant temp each gas applies same pressure it would apply if it occupied the space alone
Assuming gases do not react each gas dissolves to the extent it would if no other gases were there
66
In carbonated drinks (Coke Pepsi etc) solubility of CO2 increased by increasing pressure
At bottling factory CO2 gas forced into solution of flavored water at pressure of 5-10 atm Gas-in-liquid solution then sealed in bottlescans
67
When cap removed pressure reduced to 1 atm and some CO2 escapes as gas bubbles Effervescence - rapid escape of a gas from a liquid in which it is dissolved
68
Effects of Temperature on Solubility
Letrsquos consider gas solubility Increasing temperature usually decreases gas solubility As temp increases average kinetic energy of molecules in solution increases Greater number of solute molecules escape from attraction of solvent and return to gas phase 69
At higher temps eq is reached with fewer gas molecules in solution Gases generally less soluble
70
Effect of temp on solubility of solids in liquids more difficult to predict
Often increasing temp increases solubility
However the same temp increase can result in large increase in solubility in one case and only slight increase in the next
71
72
enthalpies of Solution
Formation of solution comes with energy change Dissolve KI in water container feels cold Dissolve LiCl in water feels hot Formation of solid-liquid solution can absorb or release heat
73
During formation of solution solvent and solute particles experience changes in forces attracting them to other particles Before dissolving begins solvent molecules held by intermolecular forces (solvent-solvent attraction) In solute molecules held by Ifs (solute-solute attraction)
74
Energy is required to separate solute molecules and solvent molecules from their neighbors
A solute particle that is surrounded by solvent molecules is said to be solvated
75
Step 1 solute particles become separated from solid (energy absorbed) Step 2 solvent particles move apart to allow solute particles to enter liquid (energy absorbed)
76
Step 3 solvent particles attracted to and solvate solute particles (energy released)
77
Heat of solution - net amount of heat energy absorbed or released when a specific amount of solute dissolves in a solvent
Heat of solution negative (heat is released) if sums of energy in steps 1 and 2 is less than step 3
If 1+2 gt 3 HOS is positive (heat absorbed)78
In gaseous state molecules are so far apart there are nearly no forces between them Solute-solute interaction has little effect on heat of solution of a gas Energy released when gas dissolved in liquid bc attraction between solute gas and solvent molecules greater than energy needed to separate solvent molecules
79
Concentration of Solutions80
Concentration - measure of amount of solute in given amount of solvent or solution ldquodiluterdquo just means small amount of solute ldquoconcentratedrdquo just means relatively large amount of solute Unrelated to degree to which solution is saturated Ex Saturated solution of substance not very soluble might be dilute
81
Molarity
Molarity - number of moles of solute in one liter of solution To find molarity must know molar mass A ldquoone molarrdquo solution of NaOH is 1 mole NaOH per liter of solution (1 M)
82
1 mol NaOH = 400 g If this quantity dissolve in enough water to make EXACTLY 100 L solution solution is 1 M If 200 g NaOH dissolve in enough to make 100 L solution what is molarity 0500 M
83
1 M solution is not made by adding 1 mol solute to 1 L solvent That would be more than 1 L Instead 1 mol solute dissolved in less than 1 L Then diluted with more solvent to bring TOTAL VOLUME to 1 L
84
Practice ProblemYou have 340 L of solution that contains 900 g sodium chloride What is the molarity of that solution 0440 M NaCl You have 08 L of a 05 M HCl solution How many moles of HCl does this solution contain 04 mol HCl
85
What is the molarity of a solution composed of 585 g of potassium iodide dissolved in enough water to make a 0125 L of solution 0282 M KI
86
How many moles of H2SO4 are present in 0500 L of a 0150 M H2SO4 solution 00750 mol What volume of 300 M NaCl is needed for reaction that requires 1463 g of NaCl 0834 L
87
Molality
Molality - concentration of solution expressed in moles of solute per kilogram of solvent Solution that contains 1 mol solute (NaOH) dissolved in exactly 1 kg of solvent is ldquoone molalrdquo solution (1 m NaOH)
88
1 mol NaOH = 400 g 400 g NaOH dissolved in 1 kg water is 1 m NaOH
200 g NaOH in 1 kg water = 0500 m NaOH
89
Practice Problem
A solution was prepared by dissolving 171 g sucrose (C12H22O11) in 125 g of water Find the molal concentration of this solution 0400 m C12H22O11
90
A solution of iodine in carbon tetrachloride is used when iodine is needed for certain chemical tests How much iodine must be added to prepare a 0480 m solution of iodine in CCl4 if 1000 g of CCl4 is used 122 g I2
91
What is the molality of a solution composed of 255 g acetone (CH3)2CO dissolved in 200 g of water 22 m acetone What quantity in grams of methanol CH3OH is required to prepare a 0244 m solution in 400 g water 312 g methanol
92
How many grams of AgNO3 are needed to prepare a 0125 m solution in 250 mL of water 531 g AgNO3
What is the molality of a solution containing 182 g HCl and 250 g water 200 m
93
Ions in Aqueous Solutions and Colligative Properties
94
95
compounds in aqueous solutions
DissociationWhen compound made from ions dissolves in water ions separate Dissociation - separation of ions that occurs when an ionic compound dissolves
96
Notice number of ions made per formula unit in equations 1 formula unit of NaCl gives 2 units of ions in solution 1 formula unit of CaCl2 gives 3 units of ions
97
Assuming 100 dissociation solution that contains 1 mol NaCl contains 1 mol Na+ and 1 mol Cl- Can assume 100 dissociation of all soluble ionic compounds
98
Practice Problem
Write the equation for the dissolution of aluminum sulfate in water How many moles of aluminum ions are made by dissolving 1 mol aluminum sulfate What is the total number of moles of ions made by dissolving 1 mol of aluminum sulfate
99
1 AnalyzeGiven Amount of solute = 1 mol Al2(SO4)3 Solvent identity = water Unknown a moles of aluminum ions and sulfate ions Total number of moles of solute ions produced
100
2 Plan
The coefficients in the balanced dissociation equation will tell mole relationships You can use the equation to find out the number of moles of solute ions produced
101
3 Compute
a
b102
Write the equation for the dissolution of each of the following in water and then determine the number of moles of each ions made as well as the total number of ions made a 1 mol ammonium chloride b 1 mol sodium sulfide c 05 mol barium nitrate
103
a 1 mol ammonium chloride
1 mol NH4+ 1 mol Cl- 2 mol total ions
104
b 1 mol sodium sulfide
2 mol Na+ 1 mol S-2 3 mol total ions
105
c 05 mol barium nitrate
05 mol Ba+2 1 mol NO3- 15 mol total ions
106
Precipitation Reactions
Even though no compound is completely insoluble compounds of very low solubility can be considered insoluble for practical purposes There are general guidelines to help predict whether a compound made of certain combination of ions is soluble
107
General Solubility Guidelines
Sodium potassium and ammonium compounds are soluble in water
108
Nitrates acetates and chlorates are soluble
109
110
Most chlorides are soluble except those of silver mercury (I) and lead Lead (II) chloride is soluble in hot water
Most sulfates are soluble except those of barium strontium lead calcium and mercury
111
Most carbonates phosphates and silicates are insoluble except those of sodium potassium and ammonium
112
Most sulfides are insoluble except those of calcium strontium sodium potassium and ammonium
113
Is calcium phosphate Ca3(PO4)2 soluble or insoluble
Not one of the exceptions so it is insoluble
Dissociation equations are not written for insoluble compounds
114
Guidelines also useful in predicting what will happen if 2 solutions of 2 different soluble compounds are mixed
If mixing results in combination of ions that forms insoluble compound a double-replacement and precipitation reaction will happen
115
Will precipitate form when solutions of ammonium sulfide and cadmium nitrate are combined You can tell calcium nitrate is soluble from the guidelines (Most nitrates acetates and chlorates are soluble) You can also tell ammonium sulfide is soluble (Most sulfides are insoluble except those of calcium strontium sodium potassium and ammonium)
116
117
To decide whether precipitate can form must know solubilities of two possible products
Ammonium nitrate ndash soluble (Most nitrates acetates and chlorates are soluble) Cadmium sulfide ndash insoluble (Most sulfides are insoluble except those of calcium strontium sodium potassium and ammonium)
118
bc one product is insoluble double-replacement and precipitation reaction will happen (NH4)2S (aq) + Cd(NO3)2 (aq) rarr NH4NO3 (aq) + CdS (s)
119
Net Ionic Equations
Reactions of ions in aqueous solution usually represented by net ionic equations instead of formula equations Net ionic equation - includes only those compounds and ions that undergo a chemical change in a reaction in an aqueous solution
120
121
To write NIE (net ionic equation) first convert chemical equation into overall ionic equation All soluble ionic compounds shown as dissociated ions in solution Precipitates shown as solids
122
Notice ammonium and nitrate ions appear on both sides They havenrsquot gone through any chemical change Spectator ions - ions that do not take part in a chemical reaction and are found in solution both before and after the reaction
123
To change ionic equation to NIE eliminate spectator ions Cd+2(aq) + S-2(aq) rarr CdS(s)
This NIE applies to any reaction in which precipitate of CdS forms
124
Practice Problem
Identify the precipitate that forms when aqueous solutions of zinc nitrate and ammonium sulfide are combined Write the equation for the possible double-replacement reaction Then write the formula equation overall ionic equation and the net ionic equation
125
1 AnalyzeGiven Identity of reactants zinc nitrate and ammonium sulfide Reaction medium aqueous solution Unknown a equation for the possible double-replacement reaction b identity of the precipitate c formula equation d overall ionic equation e net ionic equation
126
2 PlanWrite the possible double-replacement reaction between Zn(NO3)2 and (NH4)2S Use the rules to determine if any of the products will precipitate Write a formula equation and overall net ionic equation Then cancel spectator ions to make net ionic equation
127
3 Computea possible double-replacement reaction
b rules show that zinc sulfide is not soluble so will precipitate (ammonium nitrate is soluble)
c formula equation128
d overall ionic equation
e net ionic equation129
Will a precipitate form if solutions of potassium sulfate and barium nitrate are combined If so write the net ionic equation for the reaction
Yes Ba+2(aq) + SO4-2(aq) rarr BaSO4(s)
130
Will a precipitate form if solutions of potassium nitrate and magnesium sulfate are combined If so write the net ionic equation for the reaction
No 131
Will a precipitate form if solutions of barium chloride and sodium sulfate are combined If so identify the spectator ions and write the net ionic equation for the reaction
Yes Na+ and Cl- Ba+2(aq) + SO4-2(aq) rarr BaSO4(s)
132
Write the net ionic equation for the precipitation of nickel(II) sulfide
Ni+2(aq) + S-2(aq) rarr NiS(s)
133
IonizationSome molecular compounds can also form ions in solution Usually polar Ionization - ions formed from solute molecules by action of the solvent (creation of ions where there were none) Ionization different from dissociation
134
Dissociation ionic compounds dissolve and ions already present separate Ionization ions formed where non existed before
Ions formed are hydrated Heat released during hydration of ions gives enough energy to break covalent bonds
135
Extent to which solute ionizes depends on strength of bonds within molecules of solute and strength of attraction between solute and solvent molecules
136
HCl ndash molecular compound that ionizes in aqueous solution Contains highly polar bond Attraction between polar HCl molecule and polar water strong enough to break HCl bond
137
The Hydronium Ion
H+ ions from HCl attracts other molecules or ions so strongly that it doesnrsquot normally exist alone Ionization of HCl better described as direct transfer of proton from HCl to H2O forming H3O+
138
139
Hydration of H+ to form hydronium is highly exothermic
Energy released gives a lot of energy needed to ionize a molecular solute
Many molecular compounds that ionize in aqueous solution contain hydrogen and form hydronium ion
140
Strong and Weak Electrolytes
Substances that make ions and conduct current in solution are electrolytes HCl is one of series of compounds that has hydrogen and halogen Hydrogen halides are all molecular compounds with single polar-covalent bonds
141
All are gases Very soluble in water All are electrolytes HCl HBr HI strongly conduct current in solution HF only weakly conducts current at same concentration Strength of conduction related to ability to ionize
142
143
Strong Electrolytes
HCl HBr HI are 100 ionized in solution
Strong electrolyte - any compound whose dilute aqueous solutions conduct electricity well this is due to the presence of all or almost all of the dissolve compound in the form of ions
144
Unique characteristic of strong electrolyte is that they yield only ions
Ex Ionic compound may be highly soluble in water and dissociate into ions (NaCl)
Others may not dissolve much but amount that does dissolve exists only as ions
145
Weak ElectrolytesHF dissolves in water to give acid solution called hydrofluoric acid HF bond stronger than bonds between hydrogen and other halogens When HF dissolves some molecule ionize Reverse reaction also happens HF(aq) + H2O(l) H3O+(aq) + F-(aq)
146
HF(aq) + H2O(l) H3O+
(aq) + F-(aq)Concentration of dissolved unionized HF stays high and concentration of ions stays low
Weak electrolyte - any compound whose aqueous solutions conduct electricity poorly this is due to the presence of small amount of the dissolved compound in the form of ions
147
Different from nonelectrolyte ndash NO ions at all Description of electrolyte as strong or weak not related to concentration of solution Electrolytes differ in degree of ionization not amount of solute dissolved
148
149
Colligative properties of solutions
Presence of solute affects properties of solutions Some properties not dependent on nature of dissolved substance but on how may dissolve particles are present
Colligative properties - properties that depend on the concentration of solute particles but not on their identity
150
Vapor-Pressure Lowering
Boiling and freezing point of solution different from pure solvent Nonvolatile solute raises boiling point and lowers freezing point Nonvolatile substance - one that has little tendency to become gas under existing conditions
151
152
To understand why nonvolatile solute changes boiling and freezing point consider equilibrium vapor pressure
Vapor pressure pressure caused by molecules that have escape liquid phase to gas phase
Can be thought of a measure of tendency of molecules to escape from a liquid
153
Addition of sucrose (nonvolatile solute) lowers concentration of water molecules at surface of liquid This lowers tendency of water molecules to leave solution and enter gas phase Vapor pressure of solution is lower than vapor pressure of pure water
154
Aqueous13 solution13 of13 nonvolatile13 solutePure13 water
Nonelectrolyte solutions of same molality have same concentration of particles
Dilute solutions of same solvent and equal molality of any nonelectrolyte solute lower vapor pressure equally
155
Example
1 m aqueous solution of nonelectrolyte glucose lowers vapor pressure of water 55 x 10-4 atm at 25 1 m aqueous solution of sucrose also lowers water vapor pressure to 55 x 10-4 atm at 25
156
157
bc vapor-pressure lowering depends on concentration of nonelectrolyte solute and doesnrsquot depend on the type of solute it is a colligative proprerty
bc vapor pressure has been lowered solution remains liquid over larger temperature range
Lowers freezing point and raises boiling point
158
Can assume changes in boiling and freezing point also depend on concentration of solute
They are colligative properties
159
Freezing-Point Depression
Freezing point of 1-molal solution of any nonelectrolyte solute in water is found (by experiment) to be 186 lower than freezing point of water When 1 mol of nonelectrolyte solute dissolved in 1 kg water freezing point is -186 not 000 160
When 2 mol nonelectrolyte solute dissolved in 1 kg water freezing point is -372
For any concentration of nonelectrolyte solute in water decrease in freezing point can be determined using value of -186m
161
This value called the molal freezing-point contstant (Kf) is the freezing-point depression of the solvent in a 1-molal solution of a nonvolatile nonelectrolyte solute
162
Each solvent has own characteristic molal freezing-point constant Freezing-point depression (Δtf) the difference between the freezing points of the pure solvent and a solute of nonelectrolyte in that solvent and it is directly proportional to the molal concentration of the solution Δtf = Kfm
163
Δtf = Kfm
Kf expressed as m
m expressed in mol solutekg solvent (molality)
164
Sample Problem
What is the freezing point depression of water in a solution of 171 g of sucrose C12H22O11 and 200 g of water What is the actual freezing point of the solution
165
1 AnalyzeGiven Solute mass and chemical formula 171 g C12H22O11 Solvent mass and identity 2000 g water Unknown a freezing-point depression b freezing point of the solution 166
2 Plan
167
168
Calculate
Unknown a freezing-point depression b freezing point of the solution
169
Practice Problem
A water solution containing an unknown quantity of a nonelectrolyte solute is found to have a freezing point of ndash 023degCWhat is the molal concentration of the solution
012 m170
A solution consists of 103 g of the nonelectrolyte glucose C6H12O6 dissolved in 250 g of water What is the freezing-point depression of the solution
minus0426degC171
In a laboratory experiment the freezing point of an aqueous solution of glucose is found to be minus0325degCWhat is the molal concentration of this solution
0175 m172
If 0500 mol of a nonelectrolyte solute are dissolved in 5000 g of ether what is the freezing point of the solution
minus1181degC173
The freezing point of an aqueous solution that contains a nonelectrolyte is minus90degC aWhat is the freezing-point depression of the solution bWhat is the molal concentration of the solution a minus90degC b 48 m
174
Boiling-Point Elevation
Boiling point is temp at which vapor pressure is equal to atmospheric pressure Change in vapor pressure will cause corresponding change in boiling point
175
176
Vapor pressure of nonvolatile solution is lower than vapor pressure of pure solvent More heat will be required to raise vapor pressure of solution Boiling point of solution is higher than boiling point of pure solvent
Molal boiling-point constant (Kb) - boiling-point elevation of the solvent in a 1-molal solution of a nonvolatile nonelectrolyte solute b-p elevation of any nonelectrolyte in water found by experiment to be 051 So molal b-p constant for water is 051m
177
For different solvents b-p elevations of 1-molal solutions have different values (Table 14-2)
Like freezing-point constants values most accurate for dilute solutions
178
Boiling-point elevation Δtb difference between the boiling points of the pure solvent and a nonelectrolyte solution of that solvent and it is directly proportional to the molal concentration of the solution B-p elevation can be calculated using Δtb = Kbm Δtb (boiling point elevation) expressed in m and m expressed in mol solutekg solvent
Sample Problem
What is the boiling-point elevation of a solution made from 200 g of a nonelectrolyte solute and 4000 g of water The molar mass of the solute is 620 gmol
180
1 AnalyzeGiven solute mass = 200 g solute molar mass = 620 gmol solvent mass and identity = 4000 g of water
Unknown boiling-point elevation
181
2 Plan
182
183
3 Compute
Practice Problem
A solution contains 500 g of sucrose C12H22O11 a nonelectrolyte dissolved in 5000 g of water What is the boiling-point elevation
015degC184
A solution contains 4500 g of sucrose C12H22O11 a nonelectrolyte dissolved in 250 g of waterWhat is the boiling point of the solution
1027degC185
If the boiling point elevation of an aqueous solution containing a nonvolatile electrolyte is 102degC what is the molality of the solution
20 m186
The boiling point of an aqueous solution containing a nonvolatile electrolyte is 10075degC aWhat is the boiling-point elevation bWhat is the molality of the solution a 075degC b 15 m
187
Electrolytes and Colligative PropertiesEarly investigators confused by experiments where certain substances depressed freezing point or elevated boiling point more than expected
188
189
Ex 01 m solution of NaCl lowers freezing point of solvent almost twice as much as 01 m solution of sucrose 01 m CaCl2 lowers fp almost 3 times as much as 01 m solution of sucrose
To understand why this happens contrast behavior or sucrose with NaCl in aqueous solution Each sucrose dissolves to make only 1 particle in solution So 1 mol sucrose dissolves to 1 mol particles 1 mol NaCl dissolves to make 2 moles of particles (1 mol Na+ and 1 mol Cl-)
190
191
Calculated Values for Electrolyte SolutionsColligative properties depend on total concentration of solute particles regardless of their identity Electrolytes cause changes in colligative properties proportional to total molality in terms of all dissolves particles instead of formula units
192
What about barium nitrate Ba(NO3)2 Each mole of barium nitrate yields 3 mol of ions in solution Ba(NO3)2(s) rarr Ba2+(aq) + 2NO3-(aq) You would expect solution of given molality to lower fp of solvent 3 times as much as nonelectrolytic solution of same molality
193
Sample Problem
What is the expected change in the freezing point of water in a solution of 625 g of barium nitrate Ba(NO3)2 in 100 kg of water
194
1 AnalyzeGiven solute mass and formula = 625 g Ba(NO3)2 solvent mass and identity = 100 kg water Δtf = Kfm
Unknown expected freezing-point depression
195
2 PlanThe molality can be calculated by converting the solute mass to moles and then dividing by the number of kilograms of solvent That molality is in terms of formula units of Ba(NO3)2 and must be converted to molality in terms of dissociated ions in solution It must be multiplied by the number of moles of ions produced per mole of formula unit This adjusted molality can then be used to calculate the freezing-point depression
196
197
198
3 Compute
Practice Problem
What is the expected freezing-point depression for a solution that contains 20 mol of magnesium sulfate dissolved in 10 kg of water
minus74degC199
What is the expected boiling-point elevation of water for a solution that contains 150 g of sodium chloride dissolved in 10 kg of water 27degC The freezing point of an aqueous sodium chloride solution is minus200degC What is the molality of the solution 54 m NaCl
200
Actual Values for Electrolyte SolutionsRemember values calculated are only expected values Actual values of colligative properties for all strong electrolytes are almost what would be expected Difference between expected and actual values caused by attractive forces that exist between dissociated ions in aqueous solution
201
Attraction between hydrated ions in solution is small compared with those in crystalline solid However forces of attraction donrsquot interfere with movements of aqueous ions More concentrated solution means ions closer together and attraction is greater Also depends on charge of ions ndash higher charge greater attraction
202
Brings fresh solvent into contact with solute surface Contact between solvent and solute surface increased
3 Heating a Solvent
Solutes dissolve faster in warmer solvent As temp increases solvent molecules move faster Average kinetic energy increases Collisions between solvent and solute more frequent Helps separate solute molecules and spread them out
32
Solubility
If you add sugar to tea eventually no more sugar dissolves
For every combination of solvent with solute at given temp there is limit to amount of solute that can be dissolved
33
When sugar first dropped into water sugar molecules leave solid surface and move about at random in solvent Some dissolved molecules may collide with crystal and stay there As more solid dissolves and concentration of dissolved molecules increases collisions happen more often
34
Eventually molecules are returning to crystal at same rate they are going into solution Dynamic equilibrium established between dissolution and crystallization Solution equilibrium - physical state in which the opposing processes of dissolution and crystallization of a solute occur at equal rates
35
Saturated vs Unsaturated Solutions
Saturated solution - solution that contains the maximum amount of dissolved solute How do you know if solution is saturated If more NaCl added it falls to bottom and doesnrsquot dissolve Equilibrium already established
36
If more water added to saturated solution more NaCl will dissolve At 20 359 g NaCl is max that will dissolve in 100 g water Unsaturated solution - solution that contains less solute than a saturated solution under the existing conditions
37
Supersaturated Solutions
When saturated solution where solubility increases as temp increases is cooled excess solute usually comes out of solution Sometimes if solution is left to cool the excess solute doesnrsquot separate
38
39
Supersaturated solution - solution that contains more dissolved solute than a saturated solution contains under same conditions
40
Supersaturated solution can remain unchanged over long time if it isnrsquot disturbed Once crystals begin to form continues until equilibrium is recreated at lower temp
41
Example
Sodium thiosulfate Na2S2O3 Solute added to hot water until solution saturated Hot solution filtered Drop small crystal in ldquoseedingrdquo crystallization Continues until equilibrium created
42
Solubility Values
Solubility - amount of substance required to form a saturated solution with specific amount of solvent at specified temp Ex Solubility of sugar = 204 g per 100 g water at 20 Must specify temp bc solubility changes with temp 43
For gases pressure must also be specified Rate at which substance dissolves is unrelated to solubility Max amount of solute that dissolves and reaches equilibrium is always same under same conditions
44
Solute-Solvent Interactions
ldquolike dissolves likerdquo is generally useful rule for predicting whether one substance will dissolve in another Depends on type of bonding polaritynonpolarity and intermolecular forces between solute and solvent
45
Dissolving Ionic Compounds in Aqueous Solution
Polarity of water molecules plays important role in formation of solution of ionic compounds Charged ends of water molecules attract ions and surround them to keep separated from other ions
46
Hydration - solution process with water as solvent Ions said to be hydrated As hydrated ions diffuse into solution other ions exposed and drawn away from crystal surface Entire crystal gradually dissolves
47
When crystallized from aqueous solutions some ionic compounds form crystals that include water molecules The crystalline compounds known as hydrates have specific ratios of water molecules Represented by formulas like CuSO45H2O
48
When crystalline hydrate dissolves in water water of hydration returns to solvent Behavior of solute in hydrated form no different than anhydrous form
49
Nonpolar Solvents
Nonpolar solvents do not attract ions of ionic compound strongly enough to overcome forces holding crystal together
50
Liquid Solutes and Solvents
When you shake bottle of salad dressing oil droplets disperse in water Stop shaking ndash strong attraction between water molecules squeeze out oil and form separate layers Liquid solutes and solvents that are not soluble in each other are immiscible
51
Nonpolar substances (fat oil grease) are generally quite soluble in nonpolar liquids (carbon tetrachloride toluene gasoline)
Only attractions between nonpolar molecules are weak London forces
52
London Dispersion Forces
bc e- are constantly moving at any time there may be uneven distribution of them around a nonpolar atom Temporarily creates positive end and negative end
53
54
London dispersion forces - intermolecular attractions resulting from constant motion of e- and creation of instantaneous dipoles
55
Liquids that dissolve freely in one another in any ratio are said to be completely miscible Benzene and carbon tetrachloride (both nonpolar) are completely miscible
Nonpolar molecules of these two apply no strong forces of attraction or repulsion and molecules mix freely
56
Ethanol and water are also completely miscible -OH group on ethanol is slightly polar
Can form H-bonds with water as well as other ethanol molecules Intermolecular forces in mixture are very similar to those in pure liquids so they are mutually soluble in any amount
57
Gasoline contains mainly nonpolar hydrocarbons
Excellent solvent for fats oils greases
Major intermolecular forces are weak London forces
58
Ethanol less polar than water more than carbon tetrachloride
Better solvent for less-polar substances bc of nonpolar region
59
Effects of Pressure on Solubility
Little effect on solids or liquids Increase pressure - increase solubilities of gases in liquids
60
When gas in contact with surface of liquid gas molecules can enter liquid As amount of dissolve gas increases some molecules start to escape and reenter gas phase
Eq eventually established between rates of enteringleaving gas phase
61
As long as eq undisturbed solubility of gas in liquid unchanged at given pressure
Gas + solvent solution
62
Increasing pressure of solute gas above solution puts stress on eq Molecules collide with surface more often Increase in pressure partially offset by increase in rate of gas molecules entering solution In turn increase in amount of dissolved gas causes increase in rate molecules escape liquid and becomes gas
63
Eventually eq restored at higher gas solubility
Expected from Le Chatelierrsquos principle
Increase in gas pressure causes eq to shift so fewer molecules are in gas phase
64
Henryrsquos Law
Henryrsquos law - solubility of a gas in liquid is directly proportional to the partial pressure of that gas on the surface of the liquid
Applies to gas-liquid solutions at constant temp
65
When mixture of ideal gases is limited in constant volume at constant temp each gas applies same pressure it would apply if it occupied the space alone
Assuming gases do not react each gas dissolves to the extent it would if no other gases were there
66
In carbonated drinks (Coke Pepsi etc) solubility of CO2 increased by increasing pressure
At bottling factory CO2 gas forced into solution of flavored water at pressure of 5-10 atm Gas-in-liquid solution then sealed in bottlescans
67
When cap removed pressure reduced to 1 atm and some CO2 escapes as gas bubbles Effervescence - rapid escape of a gas from a liquid in which it is dissolved
68
Effects of Temperature on Solubility
Letrsquos consider gas solubility Increasing temperature usually decreases gas solubility As temp increases average kinetic energy of molecules in solution increases Greater number of solute molecules escape from attraction of solvent and return to gas phase 69
At higher temps eq is reached with fewer gas molecules in solution Gases generally less soluble
70
Effect of temp on solubility of solids in liquids more difficult to predict
Often increasing temp increases solubility
However the same temp increase can result in large increase in solubility in one case and only slight increase in the next
71
72
enthalpies of Solution
Formation of solution comes with energy change Dissolve KI in water container feels cold Dissolve LiCl in water feels hot Formation of solid-liquid solution can absorb or release heat
73
During formation of solution solvent and solute particles experience changes in forces attracting them to other particles Before dissolving begins solvent molecules held by intermolecular forces (solvent-solvent attraction) In solute molecules held by Ifs (solute-solute attraction)
74
Energy is required to separate solute molecules and solvent molecules from their neighbors
A solute particle that is surrounded by solvent molecules is said to be solvated
75
Step 1 solute particles become separated from solid (energy absorbed) Step 2 solvent particles move apart to allow solute particles to enter liquid (energy absorbed)
76
Step 3 solvent particles attracted to and solvate solute particles (energy released)
77
Heat of solution - net amount of heat energy absorbed or released when a specific amount of solute dissolves in a solvent
Heat of solution negative (heat is released) if sums of energy in steps 1 and 2 is less than step 3
If 1+2 gt 3 HOS is positive (heat absorbed)78
In gaseous state molecules are so far apart there are nearly no forces between them Solute-solute interaction has little effect on heat of solution of a gas Energy released when gas dissolved in liquid bc attraction between solute gas and solvent molecules greater than energy needed to separate solvent molecules
79
Concentration of Solutions80
Concentration - measure of amount of solute in given amount of solvent or solution ldquodiluterdquo just means small amount of solute ldquoconcentratedrdquo just means relatively large amount of solute Unrelated to degree to which solution is saturated Ex Saturated solution of substance not very soluble might be dilute
81
Molarity
Molarity - number of moles of solute in one liter of solution To find molarity must know molar mass A ldquoone molarrdquo solution of NaOH is 1 mole NaOH per liter of solution (1 M)
82
1 mol NaOH = 400 g If this quantity dissolve in enough water to make EXACTLY 100 L solution solution is 1 M If 200 g NaOH dissolve in enough to make 100 L solution what is molarity 0500 M
83
1 M solution is not made by adding 1 mol solute to 1 L solvent That would be more than 1 L Instead 1 mol solute dissolved in less than 1 L Then diluted with more solvent to bring TOTAL VOLUME to 1 L
84
Practice ProblemYou have 340 L of solution that contains 900 g sodium chloride What is the molarity of that solution 0440 M NaCl You have 08 L of a 05 M HCl solution How many moles of HCl does this solution contain 04 mol HCl
85
What is the molarity of a solution composed of 585 g of potassium iodide dissolved in enough water to make a 0125 L of solution 0282 M KI
86
How many moles of H2SO4 are present in 0500 L of a 0150 M H2SO4 solution 00750 mol What volume of 300 M NaCl is needed for reaction that requires 1463 g of NaCl 0834 L
87
Molality
Molality - concentration of solution expressed in moles of solute per kilogram of solvent Solution that contains 1 mol solute (NaOH) dissolved in exactly 1 kg of solvent is ldquoone molalrdquo solution (1 m NaOH)
88
1 mol NaOH = 400 g 400 g NaOH dissolved in 1 kg water is 1 m NaOH
200 g NaOH in 1 kg water = 0500 m NaOH
89
Practice Problem
A solution was prepared by dissolving 171 g sucrose (C12H22O11) in 125 g of water Find the molal concentration of this solution 0400 m C12H22O11
90
A solution of iodine in carbon tetrachloride is used when iodine is needed for certain chemical tests How much iodine must be added to prepare a 0480 m solution of iodine in CCl4 if 1000 g of CCl4 is used 122 g I2
91
What is the molality of a solution composed of 255 g acetone (CH3)2CO dissolved in 200 g of water 22 m acetone What quantity in grams of methanol CH3OH is required to prepare a 0244 m solution in 400 g water 312 g methanol
92
How many grams of AgNO3 are needed to prepare a 0125 m solution in 250 mL of water 531 g AgNO3
What is the molality of a solution containing 182 g HCl and 250 g water 200 m
93
Ions in Aqueous Solutions and Colligative Properties
94
95
compounds in aqueous solutions
DissociationWhen compound made from ions dissolves in water ions separate Dissociation - separation of ions that occurs when an ionic compound dissolves
96
Notice number of ions made per formula unit in equations 1 formula unit of NaCl gives 2 units of ions in solution 1 formula unit of CaCl2 gives 3 units of ions
97
Assuming 100 dissociation solution that contains 1 mol NaCl contains 1 mol Na+ and 1 mol Cl- Can assume 100 dissociation of all soluble ionic compounds
98
Practice Problem
Write the equation for the dissolution of aluminum sulfate in water How many moles of aluminum ions are made by dissolving 1 mol aluminum sulfate What is the total number of moles of ions made by dissolving 1 mol of aluminum sulfate
99
1 AnalyzeGiven Amount of solute = 1 mol Al2(SO4)3 Solvent identity = water Unknown a moles of aluminum ions and sulfate ions Total number of moles of solute ions produced
100
2 Plan
The coefficients in the balanced dissociation equation will tell mole relationships You can use the equation to find out the number of moles of solute ions produced
101
3 Compute
a
b102
Write the equation for the dissolution of each of the following in water and then determine the number of moles of each ions made as well as the total number of ions made a 1 mol ammonium chloride b 1 mol sodium sulfide c 05 mol barium nitrate
103
a 1 mol ammonium chloride
1 mol NH4+ 1 mol Cl- 2 mol total ions
104
b 1 mol sodium sulfide
2 mol Na+ 1 mol S-2 3 mol total ions
105
c 05 mol barium nitrate
05 mol Ba+2 1 mol NO3- 15 mol total ions
106
Precipitation Reactions
Even though no compound is completely insoluble compounds of very low solubility can be considered insoluble for practical purposes There are general guidelines to help predict whether a compound made of certain combination of ions is soluble
107
General Solubility Guidelines
Sodium potassium and ammonium compounds are soluble in water
108
Nitrates acetates and chlorates are soluble
109
110
Most chlorides are soluble except those of silver mercury (I) and lead Lead (II) chloride is soluble in hot water
Most sulfates are soluble except those of barium strontium lead calcium and mercury
111
Most carbonates phosphates and silicates are insoluble except those of sodium potassium and ammonium
112
Most sulfides are insoluble except those of calcium strontium sodium potassium and ammonium
113
Is calcium phosphate Ca3(PO4)2 soluble or insoluble
Not one of the exceptions so it is insoluble
Dissociation equations are not written for insoluble compounds
114
Guidelines also useful in predicting what will happen if 2 solutions of 2 different soluble compounds are mixed
If mixing results in combination of ions that forms insoluble compound a double-replacement and precipitation reaction will happen
115
Will precipitate form when solutions of ammonium sulfide and cadmium nitrate are combined You can tell calcium nitrate is soluble from the guidelines (Most nitrates acetates and chlorates are soluble) You can also tell ammonium sulfide is soluble (Most sulfides are insoluble except those of calcium strontium sodium potassium and ammonium)
116
117
To decide whether precipitate can form must know solubilities of two possible products
Ammonium nitrate ndash soluble (Most nitrates acetates and chlorates are soluble) Cadmium sulfide ndash insoluble (Most sulfides are insoluble except those of calcium strontium sodium potassium and ammonium)
118
bc one product is insoluble double-replacement and precipitation reaction will happen (NH4)2S (aq) + Cd(NO3)2 (aq) rarr NH4NO3 (aq) + CdS (s)
119
Net Ionic Equations
Reactions of ions in aqueous solution usually represented by net ionic equations instead of formula equations Net ionic equation - includes only those compounds and ions that undergo a chemical change in a reaction in an aqueous solution
120
121
To write NIE (net ionic equation) first convert chemical equation into overall ionic equation All soluble ionic compounds shown as dissociated ions in solution Precipitates shown as solids
122
Notice ammonium and nitrate ions appear on both sides They havenrsquot gone through any chemical change Spectator ions - ions that do not take part in a chemical reaction and are found in solution both before and after the reaction
123
To change ionic equation to NIE eliminate spectator ions Cd+2(aq) + S-2(aq) rarr CdS(s)
This NIE applies to any reaction in which precipitate of CdS forms
124
Practice Problem
Identify the precipitate that forms when aqueous solutions of zinc nitrate and ammonium sulfide are combined Write the equation for the possible double-replacement reaction Then write the formula equation overall ionic equation and the net ionic equation
125
1 AnalyzeGiven Identity of reactants zinc nitrate and ammonium sulfide Reaction medium aqueous solution Unknown a equation for the possible double-replacement reaction b identity of the precipitate c formula equation d overall ionic equation e net ionic equation
126
2 PlanWrite the possible double-replacement reaction between Zn(NO3)2 and (NH4)2S Use the rules to determine if any of the products will precipitate Write a formula equation and overall net ionic equation Then cancel spectator ions to make net ionic equation
127
3 Computea possible double-replacement reaction
b rules show that zinc sulfide is not soluble so will precipitate (ammonium nitrate is soluble)
c formula equation128
d overall ionic equation
e net ionic equation129
Will a precipitate form if solutions of potassium sulfate and barium nitrate are combined If so write the net ionic equation for the reaction
Yes Ba+2(aq) + SO4-2(aq) rarr BaSO4(s)
130
Will a precipitate form if solutions of potassium nitrate and magnesium sulfate are combined If so write the net ionic equation for the reaction
No 131
Will a precipitate form if solutions of barium chloride and sodium sulfate are combined If so identify the spectator ions and write the net ionic equation for the reaction
Yes Na+ and Cl- Ba+2(aq) + SO4-2(aq) rarr BaSO4(s)
132
Write the net ionic equation for the precipitation of nickel(II) sulfide
Ni+2(aq) + S-2(aq) rarr NiS(s)
133
IonizationSome molecular compounds can also form ions in solution Usually polar Ionization - ions formed from solute molecules by action of the solvent (creation of ions where there were none) Ionization different from dissociation
134
Dissociation ionic compounds dissolve and ions already present separate Ionization ions formed where non existed before
Ions formed are hydrated Heat released during hydration of ions gives enough energy to break covalent bonds
135
Extent to which solute ionizes depends on strength of bonds within molecules of solute and strength of attraction between solute and solvent molecules
136
HCl ndash molecular compound that ionizes in aqueous solution Contains highly polar bond Attraction between polar HCl molecule and polar water strong enough to break HCl bond
137
The Hydronium Ion
H+ ions from HCl attracts other molecules or ions so strongly that it doesnrsquot normally exist alone Ionization of HCl better described as direct transfer of proton from HCl to H2O forming H3O+
138
139
Hydration of H+ to form hydronium is highly exothermic
Energy released gives a lot of energy needed to ionize a molecular solute
Many molecular compounds that ionize in aqueous solution contain hydrogen and form hydronium ion
140
Strong and Weak Electrolytes
Substances that make ions and conduct current in solution are electrolytes HCl is one of series of compounds that has hydrogen and halogen Hydrogen halides are all molecular compounds with single polar-covalent bonds
141
All are gases Very soluble in water All are electrolytes HCl HBr HI strongly conduct current in solution HF only weakly conducts current at same concentration Strength of conduction related to ability to ionize
142
143
Strong Electrolytes
HCl HBr HI are 100 ionized in solution
Strong electrolyte - any compound whose dilute aqueous solutions conduct electricity well this is due to the presence of all or almost all of the dissolve compound in the form of ions
144
Unique characteristic of strong electrolyte is that they yield only ions
Ex Ionic compound may be highly soluble in water and dissociate into ions (NaCl)
Others may not dissolve much but amount that does dissolve exists only as ions
145
Weak ElectrolytesHF dissolves in water to give acid solution called hydrofluoric acid HF bond stronger than bonds between hydrogen and other halogens When HF dissolves some molecule ionize Reverse reaction also happens HF(aq) + H2O(l) H3O+(aq) + F-(aq)
146
HF(aq) + H2O(l) H3O+
(aq) + F-(aq)Concentration of dissolved unionized HF stays high and concentration of ions stays low
Weak electrolyte - any compound whose aqueous solutions conduct electricity poorly this is due to the presence of small amount of the dissolved compound in the form of ions
147
Different from nonelectrolyte ndash NO ions at all Description of electrolyte as strong or weak not related to concentration of solution Electrolytes differ in degree of ionization not amount of solute dissolved
148
149
Colligative properties of solutions
Presence of solute affects properties of solutions Some properties not dependent on nature of dissolved substance but on how may dissolve particles are present
Colligative properties - properties that depend on the concentration of solute particles but not on their identity
150
Vapor-Pressure Lowering
Boiling and freezing point of solution different from pure solvent Nonvolatile solute raises boiling point and lowers freezing point Nonvolatile substance - one that has little tendency to become gas under existing conditions
151
152
To understand why nonvolatile solute changes boiling and freezing point consider equilibrium vapor pressure
Vapor pressure pressure caused by molecules that have escape liquid phase to gas phase
Can be thought of a measure of tendency of molecules to escape from a liquid
153
Addition of sucrose (nonvolatile solute) lowers concentration of water molecules at surface of liquid This lowers tendency of water molecules to leave solution and enter gas phase Vapor pressure of solution is lower than vapor pressure of pure water
154
Aqueous13 solution13 of13 nonvolatile13 solutePure13 water
Nonelectrolyte solutions of same molality have same concentration of particles
Dilute solutions of same solvent and equal molality of any nonelectrolyte solute lower vapor pressure equally
155
Example
1 m aqueous solution of nonelectrolyte glucose lowers vapor pressure of water 55 x 10-4 atm at 25 1 m aqueous solution of sucrose also lowers water vapor pressure to 55 x 10-4 atm at 25
156
157
bc vapor-pressure lowering depends on concentration of nonelectrolyte solute and doesnrsquot depend on the type of solute it is a colligative proprerty
bc vapor pressure has been lowered solution remains liquid over larger temperature range
Lowers freezing point and raises boiling point
158
Can assume changes in boiling and freezing point also depend on concentration of solute
They are colligative properties
159
Freezing-Point Depression
Freezing point of 1-molal solution of any nonelectrolyte solute in water is found (by experiment) to be 186 lower than freezing point of water When 1 mol of nonelectrolyte solute dissolved in 1 kg water freezing point is -186 not 000 160
When 2 mol nonelectrolyte solute dissolved in 1 kg water freezing point is -372
For any concentration of nonelectrolyte solute in water decrease in freezing point can be determined using value of -186m
161
This value called the molal freezing-point contstant (Kf) is the freezing-point depression of the solvent in a 1-molal solution of a nonvolatile nonelectrolyte solute
162
Each solvent has own characteristic molal freezing-point constant Freezing-point depression (Δtf) the difference between the freezing points of the pure solvent and a solute of nonelectrolyte in that solvent and it is directly proportional to the molal concentration of the solution Δtf = Kfm
163
Δtf = Kfm
Kf expressed as m
m expressed in mol solutekg solvent (molality)
164
Sample Problem
What is the freezing point depression of water in a solution of 171 g of sucrose C12H22O11 and 200 g of water What is the actual freezing point of the solution
165
1 AnalyzeGiven Solute mass and chemical formula 171 g C12H22O11 Solvent mass and identity 2000 g water Unknown a freezing-point depression b freezing point of the solution 166
2 Plan
167
168
Calculate
Unknown a freezing-point depression b freezing point of the solution
169
Practice Problem
A water solution containing an unknown quantity of a nonelectrolyte solute is found to have a freezing point of ndash 023degCWhat is the molal concentration of the solution
012 m170
A solution consists of 103 g of the nonelectrolyte glucose C6H12O6 dissolved in 250 g of water What is the freezing-point depression of the solution
minus0426degC171
In a laboratory experiment the freezing point of an aqueous solution of glucose is found to be minus0325degCWhat is the molal concentration of this solution
0175 m172
If 0500 mol of a nonelectrolyte solute are dissolved in 5000 g of ether what is the freezing point of the solution
minus1181degC173
The freezing point of an aqueous solution that contains a nonelectrolyte is minus90degC aWhat is the freezing-point depression of the solution bWhat is the molal concentration of the solution a minus90degC b 48 m
174
Boiling-Point Elevation
Boiling point is temp at which vapor pressure is equal to atmospheric pressure Change in vapor pressure will cause corresponding change in boiling point
175
176
Vapor pressure of nonvolatile solution is lower than vapor pressure of pure solvent More heat will be required to raise vapor pressure of solution Boiling point of solution is higher than boiling point of pure solvent
Molal boiling-point constant (Kb) - boiling-point elevation of the solvent in a 1-molal solution of a nonvolatile nonelectrolyte solute b-p elevation of any nonelectrolyte in water found by experiment to be 051 So molal b-p constant for water is 051m
177
For different solvents b-p elevations of 1-molal solutions have different values (Table 14-2)
Like freezing-point constants values most accurate for dilute solutions
178
Boiling-point elevation Δtb difference between the boiling points of the pure solvent and a nonelectrolyte solution of that solvent and it is directly proportional to the molal concentration of the solution B-p elevation can be calculated using Δtb = Kbm Δtb (boiling point elevation) expressed in m and m expressed in mol solutekg solvent
Sample Problem
What is the boiling-point elevation of a solution made from 200 g of a nonelectrolyte solute and 4000 g of water The molar mass of the solute is 620 gmol
180
1 AnalyzeGiven solute mass = 200 g solute molar mass = 620 gmol solvent mass and identity = 4000 g of water
Unknown boiling-point elevation
181
2 Plan
182
183
3 Compute
Practice Problem
A solution contains 500 g of sucrose C12H22O11 a nonelectrolyte dissolved in 5000 g of water What is the boiling-point elevation
015degC184
A solution contains 4500 g of sucrose C12H22O11 a nonelectrolyte dissolved in 250 g of waterWhat is the boiling point of the solution
1027degC185
If the boiling point elevation of an aqueous solution containing a nonvolatile electrolyte is 102degC what is the molality of the solution
20 m186
The boiling point of an aqueous solution containing a nonvolatile electrolyte is 10075degC aWhat is the boiling-point elevation bWhat is the molality of the solution a 075degC b 15 m
187
Electrolytes and Colligative PropertiesEarly investigators confused by experiments where certain substances depressed freezing point or elevated boiling point more than expected
188
189
Ex 01 m solution of NaCl lowers freezing point of solvent almost twice as much as 01 m solution of sucrose 01 m CaCl2 lowers fp almost 3 times as much as 01 m solution of sucrose
To understand why this happens contrast behavior or sucrose with NaCl in aqueous solution Each sucrose dissolves to make only 1 particle in solution So 1 mol sucrose dissolves to 1 mol particles 1 mol NaCl dissolves to make 2 moles of particles (1 mol Na+ and 1 mol Cl-)
190
191
Calculated Values for Electrolyte SolutionsColligative properties depend on total concentration of solute particles regardless of their identity Electrolytes cause changes in colligative properties proportional to total molality in terms of all dissolves particles instead of formula units
192
What about barium nitrate Ba(NO3)2 Each mole of barium nitrate yields 3 mol of ions in solution Ba(NO3)2(s) rarr Ba2+(aq) + 2NO3-(aq) You would expect solution of given molality to lower fp of solvent 3 times as much as nonelectrolytic solution of same molality
193
Sample Problem
What is the expected change in the freezing point of water in a solution of 625 g of barium nitrate Ba(NO3)2 in 100 kg of water
194
1 AnalyzeGiven solute mass and formula = 625 g Ba(NO3)2 solvent mass and identity = 100 kg water Δtf = Kfm
Unknown expected freezing-point depression
195
2 PlanThe molality can be calculated by converting the solute mass to moles and then dividing by the number of kilograms of solvent That molality is in terms of formula units of Ba(NO3)2 and must be converted to molality in terms of dissociated ions in solution It must be multiplied by the number of moles of ions produced per mole of formula unit This adjusted molality can then be used to calculate the freezing-point depression
196
197
198
3 Compute
Practice Problem
What is the expected freezing-point depression for a solution that contains 20 mol of magnesium sulfate dissolved in 10 kg of water
minus74degC199
What is the expected boiling-point elevation of water for a solution that contains 150 g of sodium chloride dissolved in 10 kg of water 27degC The freezing point of an aqueous sodium chloride solution is minus200degC What is the molality of the solution 54 m NaCl
200
Actual Values for Electrolyte SolutionsRemember values calculated are only expected values Actual values of colligative properties for all strong electrolytes are almost what would be expected Difference between expected and actual values caused by attractive forces that exist between dissociated ions in aqueous solution
201
Attraction between hydrated ions in solution is small compared with those in crystalline solid However forces of attraction donrsquot interfere with movements of aqueous ions More concentrated solution means ions closer together and attraction is greater Also depends on charge of ions ndash higher charge greater attraction
202
3 Heating a Solvent
Solutes dissolve faster in warmer solvent As temp increases solvent molecules move faster Average kinetic energy increases Collisions between solvent and solute more frequent Helps separate solute molecules and spread them out
32
Solubility
If you add sugar to tea eventually no more sugar dissolves
For every combination of solvent with solute at given temp there is limit to amount of solute that can be dissolved
33
When sugar first dropped into water sugar molecules leave solid surface and move about at random in solvent Some dissolved molecules may collide with crystal and stay there As more solid dissolves and concentration of dissolved molecules increases collisions happen more often
34
Eventually molecules are returning to crystal at same rate they are going into solution Dynamic equilibrium established between dissolution and crystallization Solution equilibrium - physical state in which the opposing processes of dissolution and crystallization of a solute occur at equal rates
35
Saturated vs Unsaturated Solutions
Saturated solution - solution that contains the maximum amount of dissolved solute How do you know if solution is saturated If more NaCl added it falls to bottom and doesnrsquot dissolve Equilibrium already established
36
If more water added to saturated solution more NaCl will dissolve At 20 359 g NaCl is max that will dissolve in 100 g water Unsaturated solution - solution that contains less solute than a saturated solution under the existing conditions
37
Supersaturated Solutions
When saturated solution where solubility increases as temp increases is cooled excess solute usually comes out of solution Sometimes if solution is left to cool the excess solute doesnrsquot separate
38
39
Supersaturated solution - solution that contains more dissolved solute than a saturated solution contains under same conditions
40
Supersaturated solution can remain unchanged over long time if it isnrsquot disturbed Once crystals begin to form continues until equilibrium is recreated at lower temp
41
Example
Sodium thiosulfate Na2S2O3 Solute added to hot water until solution saturated Hot solution filtered Drop small crystal in ldquoseedingrdquo crystallization Continues until equilibrium created
42
Solubility Values
Solubility - amount of substance required to form a saturated solution with specific amount of solvent at specified temp Ex Solubility of sugar = 204 g per 100 g water at 20 Must specify temp bc solubility changes with temp 43
For gases pressure must also be specified Rate at which substance dissolves is unrelated to solubility Max amount of solute that dissolves and reaches equilibrium is always same under same conditions
44
Solute-Solvent Interactions
ldquolike dissolves likerdquo is generally useful rule for predicting whether one substance will dissolve in another Depends on type of bonding polaritynonpolarity and intermolecular forces between solute and solvent
45
Dissolving Ionic Compounds in Aqueous Solution
Polarity of water molecules plays important role in formation of solution of ionic compounds Charged ends of water molecules attract ions and surround them to keep separated from other ions
46
Hydration - solution process with water as solvent Ions said to be hydrated As hydrated ions diffuse into solution other ions exposed and drawn away from crystal surface Entire crystal gradually dissolves
47
When crystallized from aqueous solutions some ionic compounds form crystals that include water molecules The crystalline compounds known as hydrates have specific ratios of water molecules Represented by formulas like CuSO45H2O
48
When crystalline hydrate dissolves in water water of hydration returns to solvent Behavior of solute in hydrated form no different than anhydrous form
49
Nonpolar Solvents
Nonpolar solvents do not attract ions of ionic compound strongly enough to overcome forces holding crystal together
50
Liquid Solutes and Solvents
When you shake bottle of salad dressing oil droplets disperse in water Stop shaking ndash strong attraction between water molecules squeeze out oil and form separate layers Liquid solutes and solvents that are not soluble in each other are immiscible
51
Nonpolar substances (fat oil grease) are generally quite soluble in nonpolar liquids (carbon tetrachloride toluene gasoline)
Only attractions between nonpolar molecules are weak London forces
52
London Dispersion Forces
bc e- are constantly moving at any time there may be uneven distribution of them around a nonpolar atom Temporarily creates positive end and negative end
53
54
London dispersion forces - intermolecular attractions resulting from constant motion of e- and creation of instantaneous dipoles
55
Liquids that dissolve freely in one another in any ratio are said to be completely miscible Benzene and carbon tetrachloride (both nonpolar) are completely miscible
Nonpolar molecules of these two apply no strong forces of attraction or repulsion and molecules mix freely
56
Ethanol and water are also completely miscible -OH group on ethanol is slightly polar
Can form H-bonds with water as well as other ethanol molecules Intermolecular forces in mixture are very similar to those in pure liquids so they are mutually soluble in any amount
57
Gasoline contains mainly nonpolar hydrocarbons
Excellent solvent for fats oils greases
Major intermolecular forces are weak London forces
58
Ethanol less polar than water more than carbon tetrachloride
Better solvent for less-polar substances bc of nonpolar region
59
Effects of Pressure on Solubility
Little effect on solids or liquids Increase pressure - increase solubilities of gases in liquids
60
When gas in contact with surface of liquid gas molecules can enter liquid As amount of dissolve gas increases some molecules start to escape and reenter gas phase
Eq eventually established between rates of enteringleaving gas phase
61
As long as eq undisturbed solubility of gas in liquid unchanged at given pressure
Gas + solvent solution
62
Increasing pressure of solute gas above solution puts stress on eq Molecules collide with surface more often Increase in pressure partially offset by increase in rate of gas molecules entering solution In turn increase in amount of dissolved gas causes increase in rate molecules escape liquid and becomes gas
63
Eventually eq restored at higher gas solubility
Expected from Le Chatelierrsquos principle
Increase in gas pressure causes eq to shift so fewer molecules are in gas phase
64
Henryrsquos Law
Henryrsquos law - solubility of a gas in liquid is directly proportional to the partial pressure of that gas on the surface of the liquid
Applies to gas-liquid solutions at constant temp
65
When mixture of ideal gases is limited in constant volume at constant temp each gas applies same pressure it would apply if it occupied the space alone
Assuming gases do not react each gas dissolves to the extent it would if no other gases were there
66
In carbonated drinks (Coke Pepsi etc) solubility of CO2 increased by increasing pressure
At bottling factory CO2 gas forced into solution of flavored water at pressure of 5-10 atm Gas-in-liquid solution then sealed in bottlescans
67
When cap removed pressure reduced to 1 atm and some CO2 escapes as gas bubbles Effervescence - rapid escape of a gas from a liquid in which it is dissolved
68
Effects of Temperature on Solubility
Letrsquos consider gas solubility Increasing temperature usually decreases gas solubility As temp increases average kinetic energy of molecules in solution increases Greater number of solute molecules escape from attraction of solvent and return to gas phase 69
At higher temps eq is reached with fewer gas molecules in solution Gases generally less soluble
70
Effect of temp on solubility of solids in liquids more difficult to predict
Often increasing temp increases solubility
However the same temp increase can result in large increase in solubility in one case and only slight increase in the next
71
72
enthalpies of Solution
Formation of solution comes with energy change Dissolve KI in water container feels cold Dissolve LiCl in water feels hot Formation of solid-liquid solution can absorb or release heat
73
During formation of solution solvent and solute particles experience changes in forces attracting them to other particles Before dissolving begins solvent molecules held by intermolecular forces (solvent-solvent attraction) In solute molecules held by Ifs (solute-solute attraction)
74
Energy is required to separate solute molecules and solvent molecules from their neighbors
A solute particle that is surrounded by solvent molecules is said to be solvated
75
Step 1 solute particles become separated from solid (energy absorbed) Step 2 solvent particles move apart to allow solute particles to enter liquid (energy absorbed)
76
Step 3 solvent particles attracted to and solvate solute particles (energy released)
77
Heat of solution - net amount of heat energy absorbed or released when a specific amount of solute dissolves in a solvent
Heat of solution negative (heat is released) if sums of energy in steps 1 and 2 is less than step 3
If 1+2 gt 3 HOS is positive (heat absorbed)78
In gaseous state molecules are so far apart there are nearly no forces between them Solute-solute interaction has little effect on heat of solution of a gas Energy released when gas dissolved in liquid bc attraction between solute gas and solvent molecules greater than energy needed to separate solvent molecules
79
Concentration of Solutions80
Concentration - measure of amount of solute in given amount of solvent or solution ldquodiluterdquo just means small amount of solute ldquoconcentratedrdquo just means relatively large amount of solute Unrelated to degree to which solution is saturated Ex Saturated solution of substance not very soluble might be dilute
81
Molarity
Molarity - number of moles of solute in one liter of solution To find molarity must know molar mass A ldquoone molarrdquo solution of NaOH is 1 mole NaOH per liter of solution (1 M)
82
1 mol NaOH = 400 g If this quantity dissolve in enough water to make EXACTLY 100 L solution solution is 1 M If 200 g NaOH dissolve in enough to make 100 L solution what is molarity 0500 M
83
1 M solution is not made by adding 1 mol solute to 1 L solvent That would be more than 1 L Instead 1 mol solute dissolved in less than 1 L Then diluted with more solvent to bring TOTAL VOLUME to 1 L
84
Practice ProblemYou have 340 L of solution that contains 900 g sodium chloride What is the molarity of that solution 0440 M NaCl You have 08 L of a 05 M HCl solution How many moles of HCl does this solution contain 04 mol HCl
85
What is the molarity of a solution composed of 585 g of potassium iodide dissolved in enough water to make a 0125 L of solution 0282 M KI
86
How many moles of H2SO4 are present in 0500 L of a 0150 M H2SO4 solution 00750 mol What volume of 300 M NaCl is needed for reaction that requires 1463 g of NaCl 0834 L
87
Molality
Molality - concentration of solution expressed in moles of solute per kilogram of solvent Solution that contains 1 mol solute (NaOH) dissolved in exactly 1 kg of solvent is ldquoone molalrdquo solution (1 m NaOH)
88
1 mol NaOH = 400 g 400 g NaOH dissolved in 1 kg water is 1 m NaOH
200 g NaOH in 1 kg water = 0500 m NaOH
89
Practice Problem
A solution was prepared by dissolving 171 g sucrose (C12H22O11) in 125 g of water Find the molal concentration of this solution 0400 m C12H22O11
90
A solution of iodine in carbon tetrachloride is used when iodine is needed for certain chemical tests How much iodine must be added to prepare a 0480 m solution of iodine in CCl4 if 1000 g of CCl4 is used 122 g I2
91
What is the molality of a solution composed of 255 g acetone (CH3)2CO dissolved in 200 g of water 22 m acetone What quantity in grams of methanol CH3OH is required to prepare a 0244 m solution in 400 g water 312 g methanol
92
How many grams of AgNO3 are needed to prepare a 0125 m solution in 250 mL of water 531 g AgNO3
What is the molality of a solution containing 182 g HCl and 250 g water 200 m
93
Ions in Aqueous Solutions and Colligative Properties
94
95
compounds in aqueous solutions
DissociationWhen compound made from ions dissolves in water ions separate Dissociation - separation of ions that occurs when an ionic compound dissolves
96
Notice number of ions made per formula unit in equations 1 formula unit of NaCl gives 2 units of ions in solution 1 formula unit of CaCl2 gives 3 units of ions
97
Assuming 100 dissociation solution that contains 1 mol NaCl contains 1 mol Na+ and 1 mol Cl- Can assume 100 dissociation of all soluble ionic compounds
98
Practice Problem
Write the equation for the dissolution of aluminum sulfate in water How many moles of aluminum ions are made by dissolving 1 mol aluminum sulfate What is the total number of moles of ions made by dissolving 1 mol of aluminum sulfate
99
1 AnalyzeGiven Amount of solute = 1 mol Al2(SO4)3 Solvent identity = water Unknown a moles of aluminum ions and sulfate ions Total number of moles of solute ions produced
100
2 Plan
The coefficients in the balanced dissociation equation will tell mole relationships You can use the equation to find out the number of moles of solute ions produced
101
3 Compute
a
b102
Write the equation for the dissolution of each of the following in water and then determine the number of moles of each ions made as well as the total number of ions made a 1 mol ammonium chloride b 1 mol sodium sulfide c 05 mol barium nitrate
103
a 1 mol ammonium chloride
1 mol NH4+ 1 mol Cl- 2 mol total ions
104
b 1 mol sodium sulfide
2 mol Na+ 1 mol S-2 3 mol total ions
105
c 05 mol barium nitrate
05 mol Ba+2 1 mol NO3- 15 mol total ions
106
Precipitation Reactions
Even though no compound is completely insoluble compounds of very low solubility can be considered insoluble for practical purposes There are general guidelines to help predict whether a compound made of certain combination of ions is soluble
107
General Solubility Guidelines
Sodium potassium and ammonium compounds are soluble in water
108
Nitrates acetates and chlorates are soluble
109
110
Most chlorides are soluble except those of silver mercury (I) and lead Lead (II) chloride is soluble in hot water
Most sulfates are soluble except those of barium strontium lead calcium and mercury
111
Most carbonates phosphates and silicates are insoluble except those of sodium potassium and ammonium
112
Most sulfides are insoluble except those of calcium strontium sodium potassium and ammonium
113
Is calcium phosphate Ca3(PO4)2 soluble or insoluble
Not one of the exceptions so it is insoluble
Dissociation equations are not written for insoluble compounds
114
Guidelines also useful in predicting what will happen if 2 solutions of 2 different soluble compounds are mixed
If mixing results in combination of ions that forms insoluble compound a double-replacement and precipitation reaction will happen
115
Will precipitate form when solutions of ammonium sulfide and cadmium nitrate are combined You can tell calcium nitrate is soluble from the guidelines (Most nitrates acetates and chlorates are soluble) You can also tell ammonium sulfide is soluble (Most sulfides are insoluble except those of calcium strontium sodium potassium and ammonium)
116
117
To decide whether precipitate can form must know solubilities of two possible products
Ammonium nitrate ndash soluble (Most nitrates acetates and chlorates are soluble) Cadmium sulfide ndash insoluble (Most sulfides are insoluble except those of calcium strontium sodium potassium and ammonium)
118
bc one product is insoluble double-replacement and precipitation reaction will happen (NH4)2S (aq) + Cd(NO3)2 (aq) rarr NH4NO3 (aq) + CdS (s)
119
Net Ionic Equations
Reactions of ions in aqueous solution usually represented by net ionic equations instead of formula equations Net ionic equation - includes only those compounds and ions that undergo a chemical change in a reaction in an aqueous solution
120
121
To write NIE (net ionic equation) first convert chemical equation into overall ionic equation All soluble ionic compounds shown as dissociated ions in solution Precipitates shown as solids
122
Notice ammonium and nitrate ions appear on both sides They havenrsquot gone through any chemical change Spectator ions - ions that do not take part in a chemical reaction and are found in solution both before and after the reaction
123
To change ionic equation to NIE eliminate spectator ions Cd+2(aq) + S-2(aq) rarr CdS(s)
This NIE applies to any reaction in which precipitate of CdS forms
124
Practice Problem
Identify the precipitate that forms when aqueous solutions of zinc nitrate and ammonium sulfide are combined Write the equation for the possible double-replacement reaction Then write the formula equation overall ionic equation and the net ionic equation
125
1 AnalyzeGiven Identity of reactants zinc nitrate and ammonium sulfide Reaction medium aqueous solution Unknown a equation for the possible double-replacement reaction b identity of the precipitate c formula equation d overall ionic equation e net ionic equation
126
2 PlanWrite the possible double-replacement reaction between Zn(NO3)2 and (NH4)2S Use the rules to determine if any of the products will precipitate Write a formula equation and overall net ionic equation Then cancel spectator ions to make net ionic equation
127
3 Computea possible double-replacement reaction
b rules show that zinc sulfide is not soluble so will precipitate (ammonium nitrate is soluble)
c formula equation128
d overall ionic equation
e net ionic equation129
Will a precipitate form if solutions of potassium sulfate and barium nitrate are combined If so write the net ionic equation for the reaction
Yes Ba+2(aq) + SO4-2(aq) rarr BaSO4(s)
130
Will a precipitate form if solutions of potassium nitrate and magnesium sulfate are combined If so write the net ionic equation for the reaction
No 131
Will a precipitate form if solutions of barium chloride and sodium sulfate are combined If so identify the spectator ions and write the net ionic equation for the reaction
Yes Na+ and Cl- Ba+2(aq) + SO4-2(aq) rarr BaSO4(s)
132
Write the net ionic equation for the precipitation of nickel(II) sulfide
Ni+2(aq) + S-2(aq) rarr NiS(s)
133
IonizationSome molecular compounds can also form ions in solution Usually polar Ionization - ions formed from solute molecules by action of the solvent (creation of ions where there were none) Ionization different from dissociation
134
Dissociation ionic compounds dissolve and ions already present separate Ionization ions formed where non existed before
Ions formed are hydrated Heat released during hydration of ions gives enough energy to break covalent bonds
135
Extent to which solute ionizes depends on strength of bonds within molecules of solute and strength of attraction between solute and solvent molecules
136
HCl ndash molecular compound that ionizes in aqueous solution Contains highly polar bond Attraction between polar HCl molecule and polar water strong enough to break HCl bond
137
The Hydronium Ion
H+ ions from HCl attracts other molecules or ions so strongly that it doesnrsquot normally exist alone Ionization of HCl better described as direct transfer of proton from HCl to H2O forming H3O+
138
139
Hydration of H+ to form hydronium is highly exothermic
Energy released gives a lot of energy needed to ionize a molecular solute
Many molecular compounds that ionize in aqueous solution contain hydrogen and form hydronium ion
140
Strong and Weak Electrolytes
Substances that make ions and conduct current in solution are electrolytes HCl is one of series of compounds that has hydrogen and halogen Hydrogen halides are all molecular compounds with single polar-covalent bonds
141
All are gases Very soluble in water All are electrolytes HCl HBr HI strongly conduct current in solution HF only weakly conducts current at same concentration Strength of conduction related to ability to ionize
142
143
Strong Electrolytes
HCl HBr HI are 100 ionized in solution
Strong electrolyte - any compound whose dilute aqueous solutions conduct electricity well this is due to the presence of all or almost all of the dissolve compound in the form of ions
144
Unique characteristic of strong electrolyte is that they yield only ions
Ex Ionic compound may be highly soluble in water and dissociate into ions (NaCl)
Others may not dissolve much but amount that does dissolve exists only as ions
145
Weak ElectrolytesHF dissolves in water to give acid solution called hydrofluoric acid HF bond stronger than bonds between hydrogen and other halogens When HF dissolves some molecule ionize Reverse reaction also happens HF(aq) + H2O(l) H3O+(aq) + F-(aq)
146
HF(aq) + H2O(l) H3O+
(aq) + F-(aq)Concentration of dissolved unionized HF stays high and concentration of ions stays low
Weak electrolyte - any compound whose aqueous solutions conduct electricity poorly this is due to the presence of small amount of the dissolved compound in the form of ions
147
Different from nonelectrolyte ndash NO ions at all Description of electrolyte as strong or weak not related to concentration of solution Electrolytes differ in degree of ionization not amount of solute dissolved
148
149
Colligative properties of solutions
Presence of solute affects properties of solutions Some properties not dependent on nature of dissolved substance but on how may dissolve particles are present
Colligative properties - properties that depend on the concentration of solute particles but not on their identity
150
Vapor-Pressure Lowering
Boiling and freezing point of solution different from pure solvent Nonvolatile solute raises boiling point and lowers freezing point Nonvolatile substance - one that has little tendency to become gas under existing conditions
151
152
To understand why nonvolatile solute changes boiling and freezing point consider equilibrium vapor pressure
Vapor pressure pressure caused by molecules that have escape liquid phase to gas phase
Can be thought of a measure of tendency of molecules to escape from a liquid
153
Addition of sucrose (nonvolatile solute) lowers concentration of water molecules at surface of liquid This lowers tendency of water molecules to leave solution and enter gas phase Vapor pressure of solution is lower than vapor pressure of pure water
154
Aqueous13 solution13 of13 nonvolatile13 solutePure13 water
Nonelectrolyte solutions of same molality have same concentration of particles
Dilute solutions of same solvent and equal molality of any nonelectrolyte solute lower vapor pressure equally
155
Example
1 m aqueous solution of nonelectrolyte glucose lowers vapor pressure of water 55 x 10-4 atm at 25 1 m aqueous solution of sucrose also lowers water vapor pressure to 55 x 10-4 atm at 25
156
157
bc vapor-pressure lowering depends on concentration of nonelectrolyte solute and doesnrsquot depend on the type of solute it is a colligative proprerty
bc vapor pressure has been lowered solution remains liquid over larger temperature range
Lowers freezing point and raises boiling point
158
Can assume changes in boiling and freezing point also depend on concentration of solute
They are colligative properties
159
Freezing-Point Depression
Freezing point of 1-molal solution of any nonelectrolyte solute in water is found (by experiment) to be 186 lower than freezing point of water When 1 mol of nonelectrolyte solute dissolved in 1 kg water freezing point is -186 not 000 160
When 2 mol nonelectrolyte solute dissolved in 1 kg water freezing point is -372
For any concentration of nonelectrolyte solute in water decrease in freezing point can be determined using value of -186m
161
This value called the molal freezing-point contstant (Kf) is the freezing-point depression of the solvent in a 1-molal solution of a nonvolatile nonelectrolyte solute
162
Each solvent has own characteristic molal freezing-point constant Freezing-point depression (Δtf) the difference between the freezing points of the pure solvent and a solute of nonelectrolyte in that solvent and it is directly proportional to the molal concentration of the solution Δtf = Kfm
163
Δtf = Kfm
Kf expressed as m
m expressed in mol solutekg solvent (molality)
164
Sample Problem
What is the freezing point depression of water in a solution of 171 g of sucrose C12H22O11 and 200 g of water What is the actual freezing point of the solution
165
1 AnalyzeGiven Solute mass and chemical formula 171 g C12H22O11 Solvent mass and identity 2000 g water Unknown a freezing-point depression b freezing point of the solution 166
2 Plan
167
168
Calculate
Unknown a freezing-point depression b freezing point of the solution
169
Practice Problem
A water solution containing an unknown quantity of a nonelectrolyte solute is found to have a freezing point of ndash 023degCWhat is the molal concentration of the solution
012 m170
A solution consists of 103 g of the nonelectrolyte glucose C6H12O6 dissolved in 250 g of water What is the freezing-point depression of the solution
minus0426degC171
In a laboratory experiment the freezing point of an aqueous solution of glucose is found to be minus0325degCWhat is the molal concentration of this solution
0175 m172
If 0500 mol of a nonelectrolyte solute are dissolved in 5000 g of ether what is the freezing point of the solution
minus1181degC173
The freezing point of an aqueous solution that contains a nonelectrolyte is minus90degC aWhat is the freezing-point depression of the solution bWhat is the molal concentration of the solution a minus90degC b 48 m
174
Boiling-Point Elevation
Boiling point is temp at which vapor pressure is equal to atmospheric pressure Change in vapor pressure will cause corresponding change in boiling point
175
176
Vapor pressure of nonvolatile solution is lower than vapor pressure of pure solvent More heat will be required to raise vapor pressure of solution Boiling point of solution is higher than boiling point of pure solvent
Molal boiling-point constant (Kb) - boiling-point elevation of the solvent in a 1-molal solution of a nonvolatile nonelectrolyte solute b-p elevation of any nonelectrolyte in water found by experiment to be 051 So molal b-p constant for water is 051m
177
For different solvents b-p elevations of 1-molal solutions have different values (Table 14-2)
Like freezing-point constants values most accurate for dilute solutions
178
Boiling-point elevation Δtb difference between the boiling points of the pure solvent and a nonelectrolyte solution of that solvent and it is directly proportional to the molal concentration of the solution B-p elevation can be calculated using Δtb = Kbm Δtb (boiling point elevation) expressed in m and m expressed in mol solutekg solvent
Sample Problem
What is the boiling-point elevation of a solution made from 200 g of a nonelectrolyte solute and 4000 g of water The molar mass of the solute is 620 gmol
180
1 AnalyzeGiven solute mass = 200 g solute molar mass = 620 gmol solvent mass and identity = 4000 g of water
Unknown boiling-point elevation
181
2 Plan
182
183
3 Compute
Practice Problem
A solution contains 500 g of sucrose C12H22O11 a nonelectrolyte dissolved in 5000 g of water What is the boiling-point elevation
015degC184
A solution contains 4500 g of sucrose C12H22O11 a nonelectrolyte dissolved in 250 g of waterWhat is the boiling point of the solution
1027degC185
If the boiling point elevation of an aqueous solution containing a nonvolatile electrolyte is 102degC what is the molality of the solution
20 m186
The boiling point of an aqueous solution containing a nonvolatile electrolyte is 10075degC aWhat is the boiling-point elevation bWhat is the molality of the solution a 075degC b 15 m
187
Electrolytes and Colligative PropertiesEarly investigators confused by experiments where certain substances depressed freezing point or elevated boiling point more than expected
188
189
Ex 01 m solution of NaCl lowers freezing point of solvent almost twice as much as 01 m solution of sucrose 01 m CaCl2 lowers fp almost 3 times as much as 01 m solution of sucrose
To understand why this happens contrast behavior or sucrose with NaCl in aqueous solution Each sucrose dissolves to make only 1 particle in solution So 1 mol sucrose dissolves to 1 mol particles 1 mol NaCl dissolves to make 2 moles of particles (1 mol Na+ and 1 mol Cl-)
190
191
Calculated Values for Electrolyte SolutionsColligative properties depend on total concentration of solute particles regardless of their identity Electrolytes cause changes in colligative properties proportional to total molality in terms of all dissolves particles instead of formula units
192
What about barium nitrate Ba(NO3)2 Each mole of barium nitrate yields 3 mol of ions in solution Ba(NO3)2(s) rarr Ba2+(aq) + 2NO3-(aq) You would expect solution of given molality to lower fp of solvent 3 times as much as nonelectrolytic solution of same molality
193
Sample Problem
What is the expected change in the freezing point of water in a solution of 625 g of barium nitrate Ba(NO3)2 in 100 kg of water
194
1 AnalyzeGiven solute mass and formula = 625 g Ba(NO3)2 solvent mass and identity = 100 kg water Δtf = Kfm
Unknown expected freezing-point depression
195
2 PlanThe molality can be calculated by converting the solute mass to moles and then dividing by the number of kilograms of solvent That molality is in terms of formula units of Ba(NO3)2 and must be converted to molality in terms of dissociated ions in solution It must be multiplied by the number of moles of ions produced per mole of formula unit This adjusted molality can then be used to calculate the freezing-point depression
196
197
198
3 Compute
Practice Problem
What is the expected freezing-point depression for a solution that contains 20 mol of magnesium sulfate dissolved in 10 kg of water
minus74degC199
What is the expected boiling-point elevation of water for a solution that contains 150 g of sodium chloride dissolved in 10 kg of water 27degC The freezing point of an aqueous sodium chloride solution is minus200degC What is the molality of the solution 54 m NaCl
200
Actual Values for Electrolyte SolutionsRemember values calculated are only expected values Actual values of colligative properties for all strong electrolytes are almost what would be expected Difference between expected and actual values caused by attractive forces that exist between dissociated ions in aqueous solution
201
Attraction between hydrated ions in solution is small compared with those in crystalline solid However forces of attraction donrsquot interfere with movements of aqueous ions More concentrated solution means ions closer together and attraction is greater Also depends on charge of ions ndash higher charge greater attraction
202
Solubility
If you add sugar to tea eventually no more sugar dissolves
For every combination of solvent with solute at given temp there is limit to amount of solute that can be dissolved
33
When sugar first dropped into water sugar molecules leave solid surface and move about at random in solvent Some dissolved molecules may collide with crystal and stay there As more solid dissolves and concentration of dissolved molecules increases collisions happen more often
34
Eventually molecules are returning to crystal at same rate they are going into solution Dynamic equilibrium established between dissolution and crystallization Solution equilibrium - physical state in which the opposing processes of dissolution and crystallization of a solute occur at equal rates
35
Saturated vs Unsaturated Solutions
Saturated solution - solution that contains the maximum amount of dissolved solute How do you know if solution is saturated If more NaCl added it falls to bottom and doesnrsquot dissolve Equilibrium already established
36
If more water added to saturated solution more NaCl will dissolve At 20 359 g NaCl is max that will dissolve in 100 g water Unsaturated solution - solution that contains less solute than a saturated solution under the existing conditions
37
Supersaturated Solutions
When saturated solution where solubility increases as temp increases is cooled excess solute usually comes out of solution Sometimes if solution is left to cool the excess solute doesnrsquot separate
38
39
Supersaturated solution - solution that contains more dissolved solute than a saturated solution contains under same conditions
40
Supersaturated solution can remain unchanged over long time if it isnrsquot disturbed Once crystals begin to form continues until equilibrium is recreated at lower temp
41
Example
Sodium thiosulfate Na2S2O3 Solute added to hot water until solution saturated Hot solution filtered Drop small crystal in ldquoseedingrdquo crystallization Continues until equilibrium created
42
Solubility Values
Solubility - amount of substance required to form a saturated solution with specific amount of solvent at specified temp Ex Solubility of sugar = 204 g per 100 g water at 20 Must specify temp bc solubility changes with temp 43
For gases pressure must also be specified Rate at which substance dissolves is unrelated to solubility Max amount of solute that dissolves and reaches equilibrium is always same under same conditions
44
Solute-Solvent Interactions
ldquolike dissolves likerdquo is generally useful rule for predicting whether one substance will dissolve in another Depends on type of bonding polaritynonpolarity and intermolecular forces between solute and solvent
45
Dissolving Ionic Compounds in Aqueous Solution
Polarity of water molecules plays important role in formation of solution of ionic compounds Charged ends of water molecules attract ions and surround them to keep separated from other ions
46
Hydration - solution process with water as solvent Ions said to be hydrated As hydrated ions diffuse into solution other ions exposed and drawn away from crystal surface Entire crystal gradually dissolves
47
When crystallized from aqueous solutions some ionic compounds form crystals that include water molecules The crystalline compounds known as hydrates have specific ratios of water molecules Represented by formulas like CuSO45H2O
48
When crystalline hydrate dissolves in water water of hydration returns to solvent Behavior of solute in hydrated form no different than anhydrous form
49
Nonpolar Solvents
Nonpolar solvents do not attract ions of ionic compound strongly enough to overcome forces holding crystal together
50
Liquid Solutes and Solvents
When you shake bottle of salad dressing oil droplets disperse in water Stop shaking ndash strong attraction between water molecules squeeze out oil and form separate layers Liquid solutes and solvents that are not soluble in each other are immiscible
51
Nonpolar substances (fat oil grease) are generally quite soluble in nonpolar liquids (carbon tetrachloride toluene gasoline)
Only attractions between nonpolar molecules are weak London forces
52
London Dispersion Forces
bc e- are constantly moving at any time there may be uneven distribution of them around a nonpolar atom Temporarily creates positive end and negative end
53
54
London dispersion forces - intermolecular attractions resulting from constant motion of e- and creation of instantaneous dipoles
55
Liquids that dissolve freely in one another in any ratio are said to be completely miscible Benzene and carbon tetrachloride (both nonpolar) are completely miscible
Nonpolar molecules of these two apply no strong forces of attraction or repulsion and molecules mix freely
56
Ethanol and water are also completely miscible -OH group on ethanol is slightly polar
Can form H-bonds with water as well as other ethanol molecules Intermolecular forces in mixture are very similar to those in pure liquids so they are mutually soluble in any amount
57
Gasoline contains mainly nonpolar hydrocarbons
Excellent solvent for fats oils greases
Major intermolecular forces are weak London forces
58
Ethanol less polar than water more than carbon tetrachloride
Better solvent for less-polar substances bc of nonpolar region
59
Effects of Pressure on Solubility
Little effect on solids or liquids Increase pressure - increase solubilities of gases in liquids
60
When gas in contact with surface of liquid gas molecules can enter liquid As amount of dissolve gas increases some molecules start to escape and reenter gas phase
Eq eventually established between rates of enteringleaving gas phase
61
As long as eq undisturbed solubility of gas in liquid unchanged at given pressure
Gas + solvent solution
62
Increasing pressure of solute gas above solution puts stress on eq Molecules collide with surface more often Increase in pressure partially offset by increase in rate of gas molecules entering solution In turn increase in amount of dissolved gas causes increase in rate molecules escape liquid and becomes gas
63
Eventually eq restored at higher gas solubility
Expected from Le Chatelierrsquos principle
Increase in gas pressure causes eq to shift so fewer molecules are in gas phase
64
Henryrsquos Law
Henryrsquos law - solubility of a gas in liquid is directly proportional to the partial pressure of that gas on the surface of the liquid
Applies to gas-liquid solutions at constant temp
65
When mixture of ideal gases is limited in constant volume at constant temp each gas applies same pressure it would apply if it occupied the space alone
Assuming gases do not react each gas dissolves to the extent it would if no other gases were there
66
In carbonated drinks (Coke Pepsi etc) solubility of CO2 increased by increasing pressure
At bottling factory CO2 gas forced into solution of flavored water at pressure of 5-10 atm Gas-in-liquid solution then sealed in bottlescans
67
When cap removed pressure reduced to 1 atm and some CO2 escapes as gas bubbles Effervescence - rapid escape of a gas from a liquid in which it is dissolved
68
Effects of Temperature on Solubility
Letrsquos consider gas solubility Increasing temperature usually decreases gas solubility As temp increases average kinetic energy of molecules in solution increases Greater number of solute molecules escape from attraction of solvent and return to gas phase 69
At higher temps eq is reached with fewer gas molecules in solution Gases generally less soluble
70
Effect of temp on solubility of solids in liquids more difficult to predict
Often increasing temp increases solubility
However the same temp increase can result in large increase in solubility in one case and only slight increase in the next
71
72
enthalpies of Solution
Formation of solution comes with energy change Dissolve KI in water container feels cold Dissolve LiCl in water feels hot Formation of solid-liquid solution can absorb or release heat
73
During formation of solution solvent and solute particles experience changes in forces attracting them to other particles Before dissolving begins solvent molecules held by intermolecular forces (solvent-solvent attraction) In solute molecules held by Ifs (solute-solute attraction)
74
Energy is required to separate solute molecules and solvent molecules from their neighbors
A solute particle that is surrounded by solvent molecules is said to be solvated
75
Step 1 solute particles become separated from solid (energy absorbed) Step 2 solvent particles move apart to allow solute particles to enter liquid (energy absorbed)
76
Step 3 solvent particles attracted to and solvate solute particles (energy released)
77
Heat of solution - net amount of heat energy absorbed or released when a specific amount of solute dissolves in a solvent
Heat of solution negative (heat is released) if sums of energy in steps 1 and 2 is less than step 3
If 1+2 gt 3 HOS is positive (heat absorbed)78
In gaseous state molecules are so far apart there are nearly no forces between them Solute-solute interaction has little effect on heat of solution of a gas Energy released when gas dissolved in liquid bc attraction between solute gas and solvent molecules greater than energy needed to separate solvent molecules
79
Concentration of Solutions80
Concentration - measure of amount of solute in given amount of solvent or solution ldquodiluterdquo just means small amount of solute ldquoconcentratedrdquo just means relatively large amount of solute Unrelated to degree to which solution is saturated Ex Saturated solution of substance not very soluble might be dilute
81
Molarity
Molarity - number of moles of solute in one liter of solution To find molarity must know molar mass A ldquoone molarrdquo solution of NaOH is 1 mole NaOH per liter of solution (1 M)
82
1 mol NaOH = 400 g If this quantity dissolve in enough water to make EXACTLY 100 L solution solution is 1 M If 200 g NaOH dissolve in enough to make 100 L solution what is molarity 0500 M
83
1 M solution is not made by adding 1 mol solute to 1 L solvent That would be more than 1 L Instead 1 mol solute dissolved in less than 1 L Then diluted with more solvent to bring TOTAL VOLUME to 1 L
84
Practice ProblemYou have 340 L of solution that contains 900 g sodium chloride What is the molarity of that solution 0440 M NaCl You have 08 L of a 05 M HCl solution How many moles of HCl does this solution contain 04 mol HCl
85
What is the molarity of a solution composed of 585 g of potassium iodide dissolved in enough water to make a 0125 L of solution 0282 M KI
86
How many moles of H2SO4 are present in 0500 L of a 0150 M H2SO4 solution 00750 mol What volume of 300 M NaCl is needed for reaction that requires 1463 g of NaCl 0834 L
87
Molality
Molality - concentration of solution expressed in moles of solute per kilogram of solvent Solution that contains 1 mol solute (NaOH) dissolved in exactly 1 kg of solvent is ldquoone molalrdquo solution (1 m NaOH)
88
1 mol NaOH = 400 g 400 g NaOH dissolved in 1 kg water is 1 m NaOH
200 g NaOH in 1 kg water = 0500 m NaOH
89
Practice Problem
A solution was prepared by dissolving 171 g sucrose (C12H22O11) in 125 g of water Find the molal concentration of this solution 0400 m C12H22O11
90
A solution of iodine in carbon tetrachloride is used when iodine is needed for certain chemical tests How much iodine must be added to prepare a 0480 m solution of iodine in CCl4 if 1000 g of CCl4 is used 122 g I2
91
What is the molality of a solution composed of 255 g acetone (CH3)2CO dissolved in 200 g of water 22 m acetone What quantity in grams of methanol CH3OH is required to prepare a 0244 m solution in 400 g water 312 g methanol
92
How many grams of AgNO3 are needed to prepare a 0125 m solution in 250 mL of water 531 g AgNO3
What is the molality of a solution containing 182 g HCl and 250 g water 200 m
93
Ions in Aqueous Solutions and Colligative Properties
94
95
compounds in aqueous solutions
DissociationWhen compound made from ions dissolves in water ions separate Dissociation - separation of ions that occurs when an ionic compound dissolves
96
Notice number of ions made per formula unit in equations 1 formula unit of NaCl gives 2 units of ions in solution 1 formula unit of CaCl2 gives 3 units of ions
97
Assuming 100 dissociation solution that contains 1 mol NaCl contains 1 mol Na+ and 1 mol Cl- Can assume 100 dissociation of all soluble ionic compounds
98
Practice Problem
Write the equation for the dissolution of aluminum sulfate in water How many moles of aluminum ions are made by dissolving 1 mol aluminum sulfate What is the total number of moles of ions made by dissolving 1 mol of aluminum sulfate
99
1 AnalyzeGiven Amount of solute = 1 mol Al2(SO4)3 Solvent identity = water Unknown a moles of aluminum ions and sulfate ions Total number of moles of solute ions produced
100
2 Plan
The coefficients in the balanced dissociation equation will tell mole relationships You can use the equation to find out the number of moles of solute ions produced
101
3 Compute
a
b102
Write the equation for the dissolution of each of the following in water and then determine the number of moles of each ions made as well as the total number of ions made a 1 mol ammonium chloride b 1 mol sodium sulfide c 05 mol barium nitrate
103
a 1 mol ammonium chloride
1 mol NH4+ 1 mol Cl- 2 mol total ions
104
b 1 mol sodium sulfide
2 mol Na+ 1 mol S-2 3 mol total ions
105
c 05 mol barium nitrate
05 mol Ba+2 1 mol NO3- 15 mol total ions
106
Precipitation Reactions
Even though no compound is completely insoluble compounds of very low solubility can be considered insoluble for practical purposes There are general guidelines to help predict whether a compound made of certain combination of ions is soluble
107
General Solubility Guidelines
Sodium potassium and ammonium compounds are soluble in water
108
Nitrates acetates and chlorates are soluble
109
110
Most chlorides are soluble except those of silver mercury (I) and lead Lead (II) chloride is soluble in hot water
Most sulfates are soluble except those of barium strontium lead calcium and mercury
111
Most carbonates phosphates and silicates are insoluble except those of sodium potassium and ammonium
112
Most sulfides are insoluble except those of calcium strontium sodium potassium and ammonium
113
Is calcium phosphate Ca3(PO4)2 soluble or insoluble
Not one of the exceptions so it is insoluble
Dissociation equations are not written for insoluble compounds
114
Guidelines also useful in predicting what will happen if 2 solutions of 2 different soluble compounds are mixed
If mixing results in combination of ions that forms insoluble compound a double-replacement and precipitation reaction will happen
115
Will precipitate form when solutions of ammonium sulfide and cadmium nitrate are combined You can tell calcium nitrate is soluble from the guidelines (Most nitrates acetates and chlorates are soluble) You can also tell ammonium sulfide is soluble (Most sulfides are insoluble except those of calcium strontium sodium potassium and ammonium)
116
117
To decide whether precipitate can form must know solubilities of two possible products
Ammonium nitrate ndash soluble (Most nitrates acetates and chlorates are soluble) Cadmium sulfide ndash insoluble (Most sulfides are insoluble except those of calcium strontium sodium potassium and ammonium)
118
bc one product is insoluble double-replacement and precipitation reaction will happen (NH4)2S (aq) + Cd(NO3)2 (aq) rarr NH4NO3 (aq) + CdS (s)
119
Net Ionic Equations
Reactions of ions in aqueous solution usually represented by net ionic equations instead of formula equations Net ionic equation - includes only those compounds and ions that undergo a chemical change in a reaction in an aqueous solution
120
121
To write NIE (net ionic equation) first convert chemical equation into overall ionic equation All soluble ionic compounds shown as dissociated ions in solution Precipitates shown as solids
122
Notice ammonium and nitrate ions appear on both sides They havenrsquot gone through any chemical change Spectator ions - ions that do not take part in a chemical reaction and are found in solution both before and after the reaction
123
To change ionic equation to NIE eliminate spectator ions Cd+2(aq) + S-2(aq) rarr CdS(s)
This NIE applies to any reaction in which precipitate of CdS forms
124
Practice Problem
Identify the precipitate that forms when aqueous solutions of zinc nitrate and ammonium sulfide are combined Write the equation for the possible double-replacement reaction Then write the formula equation overall ionic equation and the net ionic equation
125
1 AnalyzeGiven Identity of reactants zinc nitrate and ammonium sulfide Reaction medium aqueous solution Unknown a equation for the possible double-replacement reaction b identity of the precipitate c formula equation d overall ionic equation e net ionic equation
126
2 PlanWrite the possible double-replacement reaction between Zn(NO3)2 and (NH4)2S Use the rules to determine if any of the products will precipitate Write a formula equation and overall net ionic equation Then cancel spectator ions to make net ionic equation
127
3 Computea possible double-replacement reaction
b rules show that zinc sulfide is not soluble so will precipitate (ammonium nitrate is soluble)
c formula equation128
d overall ionic equation
e net ionic equation129
Will a precipitate form if solutions of potassium sulfate and barium nitrate are combined If so write the net ionic equation for the reaction
Yes Ba+2(aq) + SO4-2(aq) rarr BaSO4(s)
130
Will a precipitate form if solutions of potassium nitrate and magnesium sulfate are combined If so write the net ionic equation for the reaction
No 131
Will a precipitate form if solutions of barium chloride and sodium sulfate are combined If so identify the spectator ions and write the net ionic equation for the reaction
Yes Na+ and Cl- Ba+2(aq) + SO4-2(aq) rarr BaSO4(s)
132
Write the net ionic equation for the precipitation of nickel(II) sulfide
Ni+2(aq) + S-2(aq) rarr NiS(s)
133
IonizationSome molecular compounds can also form ions in solution Usually polar Ionization - ions formed from solute molecules by action of the solvent (creation of ions where there were none) Ionization different from dissociation
134
Dissociation ionic compounds dissolve and ions already present separate Ionization ions formed where non existed before
Ions formed are hydrated Heat released during hydration of ions gives enough energy to break covalent bonds
135
Extent to which solute ionizes depends on strength of bonds within molecules of solute and strength of attraction between solute and solvent molecules
136
HCl ndash molecular compound that ionizes in aqueous solution Contains highly polar bond Attraction between polar HCl molecule and polar water strong enough to break HCl bond
137
The Hydronium Ion
H+ ions from HCl attracts other molecules or ions so strongly that it doesnrsquot normally exist alone Ionization of HCl better described as direct transfer of proton from HCl to H2O forming H3O+
138
139
Hydration of H+ to form hydronium is highly exothermic
Energy released gives a lot of energy needed to ionize a molecular solute
Many molecular compounds that ionize in aqueous solution contain hydrogen and form hydronium ion
140
Strong and Weak Electrolytes
Substances that make ions and conduct current in solution are electrolytes HCl is one of series of compounds that has hydrogen and halogen Hydrogen halides are all molecular compounds with single polar-covalent bonds
141
All are gases Very soluble in water All are electrolytes HCl HBr HI strongly conduct current in solution HF only weakly conducts current at same concentration Strength of conduction related to ability to ionize
142
143
Strong Electrolytes
HCl HBr HI are 100 ionized in solution
Strong electrolyte - any compound whose dilute aqueous solutions conduct electricity well this is due to the presence of all or almost all of the dissolve compound in the form of ions
144
Unique characteristic of strong electrolyte is that they yield only ions
Ex Ionic compound may be highly soluble in water and dissociate into ions (NaCl)
Others may not dissolve much but amount that does dissolve exists only as ions
145
Weak ElectrolytesHF dissolves in water to give acid solution called hydrofluoric acid HF bond stronger than bonds between hydrogen and other halogens When HF dissolves some molecule ionize Reverse reaction also happens HF(aq) + H2O(l) H3O+(aq) + F-(aq)
146
HF(aq) + H2O(l) H3O+
(aq) + F-(aq)Concentration of dissolved unionized HF stays high and concentration of ions stays low
Weak electrolyte - any compound whose aqueous solutions conduct electricity poorly this is due to the presence of small amount of the dissolved compound in the form of ions
147
Different from nonelectrolyte ndash NO ions at all Description of electrolyte as strong or weak not related to concentration of solution Electrolytes differ in degree of ionization not amount of solute dissolved
148
149
Colligative properties of solutions
Presence of solute affects properties of solutions Some properties not dependent on nature of dissolved substance but on how may dissolve particles are present
Colligative properties - properties that depend on the concentration of solute particles but not on their identity
150
Vapor-Pressure Lowering
Boiling and freezing point of solution different from pure solvent Nonvolatile solute raises boiling point and lowers freezing point Nonvolatile substance - one that has little tendency to become gas under existing conditions
151
152
To understand why nonvolatile solute changes boiling and freezing point consider equilibrium vapor pressure
Vapor pressure pressure caused by molecules that have escape liquid phase to gas phase
Can be thought of a measure of tendency of molecules to escape from a liquid
153
Addition of sucrose (nonvolatile solute) lowers concentration of water molecules at surface of liquid This lowers tendency of water molecules to leave solution and enter gas phase Vapor pressure of solution is lower than vapor pressure of pure water
154
Aqueous13 solution13 of13 nonvolatile13 solutePure13 water
Nonelectrolyte solutions of same molality have same concentration of particles
Dilute solutions of same solvent and equal molality of any nonelectrolyte solute lower vapor pressure equally
155
Example
1 m aqueous solution of nonelectrolyte glucose lowers vapor pressure of water 55 x 10-4 atm at 25 1 m aqueous solution of sucrose also lowers water vapor pressure to 55 x 10-4 atm at 25
156
157
bc vapor-pressure lowering depends on concentration of nonelectrolyte solute and doesnrsquot depend on the type of solute it is a colligative proprerty
bc vapor pressure has been lowered solution remains liquid over larger temperature range
Lowers freezing point and raises boiling point
158
Can assume changes in boiling and freezing point also depend on concentration of solute
They are colligative properties
159
Freezing-Point Depression
Freezing point of 1-molal solution of any nonelectrolyte solute in water is found (by experiment) to be 186 lower than freezing point of water When 1 mol of nonelectrolyte solute dissolved in 1 kg water freezing point is -186 not 000 160
When 2 mol nonelectrolyte solute dissolved in 1 kg water freezing point is -372
For any concentration of nonelectrolyte solute in water decrease in freezing point can be determined using value of -186m
161
This value called the molal freezing-point contstant (Kf) is the freezing-point depression of the solvent in a 1-molal solution of a nonvolatile nonelectrolyte solute
162
Each solvent has own characteristic molal freezing-point constant Freezing-point depression (Δtf) the difference between the freezing points of the pure solvent and a solute of nonelectrolyte in that solvent and it is directly proportional to the molal concentration of the solution Δtf = Kfm
163
Δtf = Kfm
Kf expressed as m
m expressed in mol solutekg solvent (molality)
164
Sample Problem
What is the freezing point depression of water in a solution of 171 g of sucrose C12H22O11 and 200 g of water What is the actual freezing point of the solution
165
1 AnalyzeGiven Solute mass and chemical formula 171 g C12H22O11 Solvent mass and identity 2000 g water Unknown a freezing-point depression b freezing point of the solution 166
2 Plan
167
168
Calculate
Unknown a freezing-point depression b freezing point of the solution
169
Practice Problem
A water solution containing an unknown quantity of a nonelectrolyte solute is found to have a freezing point of ndash 023degCWhat is the molal concentration of the solution
012 m170
A solution consists of 103 g of the nonelectrolyte glucose C6H12O6 dissolved in 250 g of water What is the freezing-point depression of the solution
minus0426degC171
In a laboratory experiment the freezing point of an aqueous solution of glucose is found to be minus0325degCWhat is the molal concentration of this solution
0175 m172
If 0500 mol of a nonelectrolyte solute are dissolved in 5000 g of ether what is the freezing point of the solution
minus1181degC173
The freezing point of an aqueous solution that contains a nonelectrolyte is minus90degC aWhat is the freezing-point depression of the solution bWhat is the molal concentration of the solution a minus90degC b 48 m
174
Boiling-Point Elevation
Boiling point is temp at which vapor pressure is equal to atmospheric pressure Change in vapor pressure will cause corresponding change in boiling point
175
176
Vapor pressure of nonvolatile solution is lower than vapor pressure of pure solvent More heat will be required to raise vapor pressure of solution Boiling point of solution is higher than boiling point of pure solvent
Molal boiling-point constant (Kb) - boiling-point elevation of the solvent in a 1-molal solution of a nonvolatile nonelectrolyte solute b-p elevation of any nonelectrolyte in water found by experiment to be 051 So molal b-p constant for water is 051m
177
For different solvents b-p elevations of 1-molal solutions have different values (Table 14-2)
Like freezing-point constants values most accurate for dilute solutions
178
Boiling-point elevation Δtb difference between the boiling points of the pure solvent and a nonelectrolyte solution of that solvent and it is directly proportional to the molal concentration of the solution B-p elevation can be calculated using Δtb = Kbm Δtb (boiling point elevation) expressed in m and m expressed in mol solutekg solvent
Sample Problem
What is the boiling-point elevation of a solution made from 200 g of a nonelectrolyte solute and 4000 g of water The molar mass of the solute is 620 gmol
180
1 AnalyzeGiven solute mass = 200 g solute molar mass = 620 gmol solvent mass and identity = 4000 g of water
Unknown boiling-point elevation
181
2 Plan
182
183
3 Compute
Practice Problem
A solution contains 500 g of sucrose C12H22O11 a nonelectrolyte dissolved in 5000 g of water What is the boiling-point elevation
015degC184
A solution contains 4500 g of sucrose C12H22O11 a nonelectrolyte dissolved in 250 g of waterWhat is the boiling point of the solution
1027degC185
If the boiling point elevation of an aqueous solution containing a nonvolatile electrolyte is 102degC what is the molality of the solution
20 m186
The boiling point of an aqueous solution containing a nonvolatile electrolyte is 10075degC aWhat is the boiling-point elevation bWhat is the molality of the solution a 075degC b 15 m
187
Electrolytes and Colligative PropertiesEarly investigators confused by experiments where certain substances depressed freezing point or elevated boiling point more than expected
188
189
Ex 01 m solution of NaCl lowers freezing point of solvent almost twice as much as 01 m solution of sucrose 01 m CaCl2 lowers fp almost 3 times as much as 01 m solution of sucrose
To understand why this happens contrast behavior or sucrose with NaCl in aqueous solution Each sucrose dissolves to make only 1 particle in solution So 1 mol sucrose dissolves to 1 mol particles 1 mol NaCl dissolves to make 2 moles of particles (1 mol Na+ and 1 mol Cl-)
190
191
Calculated Values for Electrolyte SolutionsColligative properties depend on total concentration of solute particles regardless of their identity Electrolytes cause changes in colligative properties proportional to total molality in terms of all dissolves particles instead of formula units
192
What about barium nitrate Ba(NO3)2 Each mole of barium nitrate yields 3 mol of ions in solution Ba(NO3)2(s) rarr Ba2+(aq) + 2NO3-(aq) You would expect solution of given molality to lower fp of solvent 3 times as much as nonelectrolytic solution of same molality
193
Sample Problem
What is the expected change in the freezing point of water in a solution of 625 g of barium nitrate Ba(NO3)2 in 100 kg of water
194
1 AnalyzeGiven solute mass and formula = 625 g Ba(NO3)2 solvent mass and identity = 100 kg water Δtf = Kfm
Unknown expected freezing-point depression
195
2 PlanThe molality can be calculated by converting the solute mass to moles and then dividing by the number of kilograms of solvent That molality is in terms of formula units of Ba(NO3)2 and must be converted to molality in terms of dissociated ions in solution It must be multiplied by the number of moles of ions produced per mole of formula unit This adjusted molality can then be used to calculate the freezing-point depression
196
197
198
3 Compute
Practice Problem
What is the expected freezing-point depression for a solution that contains 20 mol of magnesium sulfate dissolved in 10 kg of water
minus74degC199
What is the expected boiling-point elevation of water for a solution that contains 150 g of sodium chloride dissolved in 10 kg of water 27degC The freezing point of an aqueous sodium chloride solution is minus200degC What is the molality of the solution 54 m NaCl
200
Actual Values for Electrolyte SolutionsRemember values calculated are only expected values Actual values of colligative properties for all strong electrolytes are almost what would be expected Difference between expected and actual values caused by attractive forces that exist between dissociated ions in aqueous solution
201
Attraction between hydrated ions in solution is small compared with those in crystalline solid However forces of attraction donrsquot interfere with movements of aqueous ions More concentrated solution means ions closer together and attraction is greater Also depends on charge of ions ndash higher charge greater attraction
202
When sugar first dropped into water sugar molecules leave solid surface and move about at random in solvent Some dissolved molecules may collide with crystal and stay there As more solid dissolves and concentration of dissolved molecules increases collisions happen more often
34
Eventually molecules are returning to crystal at same rate they are going into solution Dynamic equilibrium established between dissolution and crystallization Solution equilibrium - physical state in which the opposing processes of dissolution and crystallization of a solute occur at equal rates
35
Saturated vs Unsaturated Solutions
Saturated solution - solution that contains the maximum amount of dissolved solute How do you know if solution is saturated If more NaCl added it falls to bottom and doesnrsquot dissolve Equilibrium already established
36
If more water added to saturated solution more NaCl will dissolve At 20 359 g NaCl is max that will dissolve in 100 g water Unsaturated solution - solution that contains less solute than a saturated solution under the existing conditions
37
Supersaturated Solutions
When saturated solution where solubility increases as temp increases is cooled excess solute usually comes out of solution Sometimes if solution is left to cool the excess solute doesnrsquot separate
38
39
Supersaturated solution - solution that contains more dissolved solute than a saturated solution contains under same conditions
40
Supersaturated solution can remain unchanged over long time if it isnrsquot disturbed Once crystals begin to form continues until equilibrium is recreated at lower temp
41
Example
Sodium thiosulfate Na2S2O3 Solute added to hot water until solution saturated Hot solution filtered Drop small crystal in ldquoseedingrdquo crystallization Continues until equilibrium created
42
Solubility Values
Solubility - amount of substance required to form a saturated solution with specific amount of solvent at specified temp Ex Solubility of sugar = 204 g per 100 g water at 20 Must specify temp bc solubility changes with temp 43
For gases pressure must also be specified Rate at which substance dissolves is unrelated to solubility Max amount of solute that dissolves and reaches equilibrium is always same under same conditions
44
Solute-Solvent Interactions
ldquolike dissolves likerdquo is generally useful rule for predicting whether one substance will dissolve in another Depends on type of bonding polaritynonpolarity and intermolecular forces between solute and solvent
45
Dissolving Ionic Compounds in Aqueous Solution
Polarity of water molecules plays important role in formation of solution of ionic compounds Charged ends of water molecules attract ions and surround them to keep separated from other ions
46
Hydration - solution process with water as solvent Ions said to be hydrated As hydrated ions diffuse into solution other ions exposed and drawn away from crystal surface Entire crystal gradually dissolves
47
When crystallized from aqueous solutions some ionic compounds form crystals that include water molecules The crystalline compounds known as hydrates have specific ratios of water molecules Represented by formulas like CuSO45H2O
48
When crystalline hydrate dissolves in water water of hydration returns to solvent Behavior of solute in hydrated form no different than anhydrous form
49
Nonpolar Solvents
Nonpolar solvents do not attract ions of ionic compound strongly enough to overcome forces holding crystal together
50
Liquid Solutes and Solvents
When you shake bottle of salad dressing oil droplets disperse in water Stop shaking ndash strong attraction between water molecules squeeze out oil and form separate layers Liquid solutes and solvents that are not soluble in each other are immiscible
51
Nonpolar substances (fat oil grease) are generally quite soluble in nonpolar liquids (carbon tetrachloride toluene gasoline)
Only attractions between nonpolar molecules are weak London forces
52
London Dispersion Forces
bc e- are constantly moving at any time there may be uneven distribution of them around a nonpolar atom Temporarily creates positive end and negative end
53
54
London dispersion forces - intermolecular attractions resulting from constant motion of e- and creation of instantaneous dipoles
55
Liquids that dissolve freely in one another in any ratio are said to be completely miscible Benzene and carbon tetrachloride (both nonpolar) are completely miscible
Nonpolar molecules of these two apply no strong forces of attraction or repulsion and molecules mix freely
56
Ethanol and water are also completely miscible -OH group on ethanol is slightly polar
Can form H-bonds with water as well as other ethanol molecules Intermolecular forces in mixture are very similar to those in pure liquids so they are mutually soluble in any amount
57
Gasoline contains mainly nonpolar hydrocarbons
Excellent solvent for fats oils greases
Major intermolecular forces are weak London forces
58
Ethanol less polar than water more than carbon tetrachloride
Better solvent for less-polar substances bc of nonpolar region
59
Effects of Pressure on Solubility
Little effect on solids or liquids Increase pressure - increase solubilities of gases in liquids
60
When gas in contact with surface of liquid gas molecules can enter liquid As amount of dissolve gas increases some molecules start to escape and reenter gas phase
Eq eventually established between rates of enteringleaving gas phase
61
As long as eq undisturbed solubility of gas in liquid unchanged at given pressure
Gas + solvent solution
62
Increasing pressure of solute gas above solution puts stress on eq Molecules collide with surface more often Increase in pressure partially offset by increase in rate of gas molecules entering solution In turn increase in amount of dissolved gas causes increase in rate molecules escape liquid and becomes gas
63
Eventually eq restored at higher gas solubility
Expected from Le Chatelierrsquos principle
Increase in gas pressure causes eq to shift so fewer molecules are in gas phase
64
Henryrsquos Law
Henryrsquos law - solubility of a gas in liquid is directly proportional to the partial pressure of that gas on the surface of the liquid
Applies to gas-liquid solutions at constant temp
65
When mixture of ideal gases is limited in constant volume at constant temp each gas applies same pressure it would apply if it occupied the space alone
Assuming gases do not react each gas dissolves to the extent it would if no other gases were there
66
In carbonated drinks (Coke Pepsi etc) solubility of CO2 increased by increasing pressure
At bottling factory CO2 gas forced into solution of flavored water at pressure of 5-10 atm Gas-in-liquid solution then sealed in bottlescans
67
When cap removed pressure reduced to 1 atm and some CO2 escapes as gas bubbles Effervescence - rapid escape of a gas from a liquid in which it is dissolved
68
Effects of Temperature on Solubility
Letrsquos consider gas solubility Increasing temperature usually decreases gas solubility As temp increases average kinetic energy of molecules in solution increases Greater number of solute molecules escape from attraction of solvent and return to gas phase 69
At higher temps eq is reached with fewer gas molecules in solution Gases generally less soluble
70
Effect of temp on solubility of solids in liquids more difficult to predict
Often increasing temp increases solubility
However the same temp increase can result in large increase in solubility in one case and only slight increase in the next
71
72
enthalpies of Solution
Formation of solution comes with energy change Dissolve KI in water container feels cold Dissolve LiCl in water feels hot Formation of solid-liquid solution can absorb or release heat
73
During formation of solution solvent and solute particles experience changes in forces attracting them to other particles Before dissolving begins solvent molecules held by intermolecular forces (solvent-solvent attraction) In solute molecules held by Ifs (solute-solute attraction)
74
Energy is required to separate solute molecules and solvent molecules from their neighbors
A solute particle that is surrounded by solvent molecules is said to be solvated
75
Step 1 solute particles become separated from solid (energy absorbed) Step 2 solvent particles move apart to allow solute particles to enter liquid (energy absorbed)
76
Step 3 solvent particles attracted to and solvate solute particles (energy released)
77
Heat of solution - net amount of heat energy absorbed or released when a specific amount of solute dissolves in a solvent
Heat of solution negative (heat is released) if sums of energy in steps 1 and 2 is less than step 3
If 1+2 gt 3 HOS is positive (heat absorbed)78
In gaseous state molecules are so far apart there are nearly no forces between them Solute-solute interaction has little effect on heat of solution of a gas Energy released when gas dissolved in liquid bc attraction between solute gas and solvent molecules greater than energy needed to separate solvent molecules
79
Concentration of Solutions80
Concentration - measure of amount of solute in given amount of solvent or solution ldquodiluterdquo just means small amount of solute ldquoconcentratedrdquo just means relatively large amount of solute Unrelated to degree to which solution is saturated Ex Saturated solution of substance not very soluble might be dilute
81
Molarity
Molarity - number of moles of solute in one liter of solution To find molarity must know molar mass A ldquoone molarrdquo solution of NaOH is 1 mole NaOH per liter of solution (1 M)
82
1 mol NaOH = 400 g If this quantity dissolve in enough water to make EXACTLY 100 L solution solution is 1 M If 200 g NaOH dissolve in enough to make 100 L solution what is molarity 0500 M
83
1 M solution is not made by adding 1 mol solute to 1 L solvent That would be more than 1 L Instead 1 mol solute dissolved in less than 1 L Then diluted with more solvent to bring TOTAL VOLUME to 1 L
84
Practice ProblemYou have 340 L of solution that contains 900 g sodium chloride What is the molarity of that solution 0440 M NaCl You have 08 L of a 05 M HCl solution How many moles of HCl does this solution contain 04 mol HCl
85
What is the molarity of a solution composed of 585 g of potassium iodide dissolved in enough water to make a 0125 L of solution 0282 M KI
86
How many moles of H2SO4 are present in 0500 L of a 0150 M H2SO4 solution 00750 mol What volume of 300 M NaCl is needed for reaction that requires 1463 g of NaCl 0834 L
87
Molality
Molality - concentration of solution expressed in moles of solute per kilogram of solvent Solution that contains 1 mol solute (NaOH) dissolved in exactly 1 kg of solvent is ldquoone molalrdquo solution (1 m NaOH)
88
1 mol NaOH = 400 g 400 g NaOH dissolved in 1 kg water is 1 m NaOH
200 g NaOH in 1 kg water = 0500 m NaOH
89
Practice Problem
A solution was prepared by dissolving 171 g sucrose (C12H22O11) in 125 g of water Find the molal concentration of this solution 0400 m C12H22O11
90
A solution of iodine in carbon tetrachloride is used when iodine is needed for certain chemical tests How much iodine must be added to prepare a 0480 m solution of iodine in CCl4 if 1000 g of CCl4 is used 122 g I2
91
What is the molality of a solution composed of 255 g acetone (CH3)2CO dissolved in 200 g of water 22 m acetone What quantity in grams of methanol CH3OH is required to prepare a 0244 m solution in 400 g water 312 g methanol
92
How many grams of AgNO3 are needed to prepare a 0125 m solution in 250 mL of water 531 g AgNO3
What is the molality of a solution containing 182 g HCl and 250 g water 200 m
93
Ions in Aqueous Solutions and Colligative Properties
94
95
compounds in aqueous solutions
DissociationWhen compound made from ions dissolves in water ions separate Dissociation - separation of ions that occurs when an ionic compound dissolves
96
Notice number of ions made per formula unit in equations 1 formula unit of NaCl gives 2 units of ions in solution 1 formula unit of CaCl2 gives 3 units of ions
97
Assuming 100 dissociation solution that contains 1 mol NaCl contains 1 mol Na+ and 1 mol Cl- Can assume 100 dissociation of all soluble ionic compounds
98
Practice Problem
Write the equation for the dissolution of aluminum sulfate in water How many moles of aluminum ions are made by dissolving 1 mol aluminum sulfate What is the total number of moles of ions made by dissolving 1 mol of aluminum sulfate
99
1 AnalyzeGiven Amount of solute = 1 mol Al2(SO4)3 Solvent identity = water Unknown a moles of aluminum ions and sulfate ions Total number of moles of solute ions produced
100
2 Plan
The coefficients in the balanced dissociation equation will tell mole relationships You can use the equation to find out the number of moles of solute ions produced
101
3 Compute
a
b102
Write the equation for the dissolution of each of the following in water and then determine the number of moles of each ions made as well as the total number of ions made a 1 mol ammonium chloride b 1 mol sodium sulfide c 05 mol barium nitrate
103
a 1 mol ammonium chloride
1 mol NH4+ 1 mol Cl- 2 mol total ions
104
b 1 mol sodium sulfide
2 mol Na+ 1 mol S-2 3 mol total ions
105
c 05 mol barium nitrate
05 mol Ba+2 1 mol NO3- 15 mol total ions
106
Precipitation Reactions
Even though no compound is completely insoluble compounds of very low solubility can be considered insoluble for practical purposes There are general guidelines to help predict whether a compound made of certain combination of ions is soluble
107
General Solubility Guidelines
Sodium potassium and ammonium compounds are soluble in water
108
Nitrates acetates and chlorates are soluble
109
110
Most chlorides are soluble except those of silver mercury (I) and lead Lead (II) chloride is soluble in hot water
Most sulfates are soluble except those of barium strontium lead calcium and mercury
111
Most carbonates phosphates and silicates are insoluble except those of sodium potassium and ammonium
112
Most sulfides are insoluble except those of calcium strontium sodium potassium and ammonium
113
Is calcium phosphate Ca3(PO4)2 soluble or insoluble
Not one of the exceptions so it is insoluble
Dissociation equations are not written for insoluble compounds
114
Guidelines also useful in predicting what will happen if 2 solutions of 2 different soluble compounds are mixed
If mixing results in combination of ions that forms insoluble compound a double-replacement and precipitation reaction will happen
115
Will precipitate form when solutions of ammonium sulfide and cadmium nitrate are combined You can tell calcium nitrate is soluble from the guidelines (Most nitrates acetates and chlorates are soluble) You can also tell ammonium sulfide is soluble (Most sulfides are insoluble except those of calcium strontium sodium potassium and ammonium)
116
117
To decide whether precipitate can form must know solubilities of two possible products
Ammonium nitrate ndash soluble (Most nitrates acetates and chlorates are soluble) Cadmium sulfide ndash insoluble (Most sulfides are insoluble except those of calcium strontium sodium potassium and ammonium)
118
bc one product is insoluble double-replacement and precipitation reaction will happen (NH4)2S (aq) + Cd(NO3)2 (aq) rarr NH4NO3 (aq) + CdS (s)
119
Net Ionic Equations
Reactions of ions in aqueous solution usually represented by net ionic equations instead of formula equations Net ionic equation - includes only those compounds and ions that undergo a chemical change in a reaction in an aqueous solution
120
121
To write NIE (net ionic equation) first convert chemical equation into overall ionic equation All soluble ionic compounds shown as dissociated ions in solution Precipitates shown as solids
122
Notice ammonium and nitrate ions appear on both sides They havenrsquot gone through any chemical change Spectator ions - ions that do not take part in a chemical reaction and are found in solution both before and after the reaction
123
To change ionic equation to NIE eliminate spectator ions Cd+2(aq) + S-2(aq) rarr CdS(s)
This NIE applies to any reaction in which precipitate of CdS forms
124
Practice Problem
Identify the precipitate that forms when aqueous solutions of zinc nitrate and ammonium sulfide are combined Write the equation for the possible double-replacement reaction Then write the formula equation overall ionic equation and the net ionic equation
125
1 AnalyzeGiven Identity of reactants zinc nitrate and ammonium sulfide Reaction medium aqueous solution Unknown a equation for the possible double-replacement reaction b identity of the precipitate c formula equation d overall ionic equation e net ionic equation
126
2 PlanWrite the possible double-replacement reaction between Zn(NO3)2 and (NH4)2S Use the rules to determine if any of the products will precipitate Write a formula equation and overall net ionic equation Then cancel spectator ions to make net ionic equation
127
3 Computea possible double-replacement reaction
b rules show that zinc sulfide is not soluble so will precipitate (ammonium nitrate is soluble)
c formula equation128
d overall ionic equation
e net ionic equation129
Will a precipitate form if solutions of potassium sulfate and barium nitrate are combined If so write the net ionic equation for the reaction
Yes Ba+2(aq) + SO4-2(aq) rarr BaSO4(s)
130
Will a precipitate form if solutions of potassium nitrate and magnesium sulfate are combined If so write the net ionic equation for the reaction
No 131
Will a precipitate form if solutions of barium chloride and sodium sulfate are combined If so identify the spectator ions and write the net ionic equation for the reaction
Yes Na+ and Cl- Ba+2(aq) + SO4-2(aq) rarr BaSO4(s)
132
Write the net ionic equation for the precipitation of nickel(II) sulfide
Ni+2(aq) + S-2(aq) rarr NiS(s)
133
IonizationSome molecular compounds can also form ions in solution Usually polar Ionization - ions formed from solute molecules by action of the solvent (creation of ions where there were none) Ionization different from dissociation
134
Dissociation ionic compounds dissolve and ions already present separate Ionization ions formed where non existed before
Ions formed are hydrated Heat released during hydration of ions gives enough energy to break covalent bonds
135
Extent to which solute ionizes depends on strength of bonds within molecules of solute and strength of attraction between solute and solvent molecules
136
HCl ndash molecular compound that ionizes in aqueous solution Contains highly polar bond Attraction between polar HCl molecule and polar water strong enough to break HCl bond
137
The Hydronium Ion
H+ ions from HCl attracts other molecules or ions so strongly that it doesnrsquot normally exist alone Ionization of HCl better described as direct transfer of proton from HCl to H2O forming H3O+
138
139
Hydration of H+ to form hydronium is highly exothermic
Energy released gives a lot of energy needed to ionize a molecular solute
Many molecular compounds that ionize in aqueous solution contain hydrogen and form hydronium ion
140
Strong and Weak Electrolytes
Substances that make ions and conduct current in solution are electrolytes HCl is one of series of compounds that has hydrogen and halogen Hydrogen halides are all molecular compounds with single polar-covalent bonds
141
All are gases Very soluble in water All are electrolytes HCl HBr HI strongly conduct current in solution HF only weakly conducts current at same concentration Strength of conduction related to ability to ionize
142
143
Strong Electrolytes
HCl HBr HI are 100 ionized in solution
Strong electrolyte - any compound whose dilute aqueous solutions conduct electricity well this is due to the presence of all or almost all of the dissolve compound in the form of ions
144
Unique characteristic of strong electrolyte is that they yield only ions
Ex Ionic compound may be highly soluble in water and dissociate into ions (NaCl)
Others may not dissolve much but amount that does dissolve exists only as ions
145
Weak ElectrolytesHF dissolves in water to give acid solution called hydrofluoric acid HF bond stronger than bonds between hydrogen and other halogens When HF dissolves some molecule ionize Reverse reaction also happens HF(aq) + H2O(l) H3O+(aq) + F-(aq)
146
HF(aq) + H2O(l) H3O+
(aq) + F-(aq)Concentration of dissolved unionized HF stays high and concentration of ions stays low
Weak electrolyte - any compound whose aqueous solutions conduct electricity poorly this is due to the presence of small amount of the dissolved compound in the form of ions
147
Different from nonelectrolyte ndash NO ions at all Description of electrolyte as strong or weak not related to concentration of solution Electrolytes differ in degree of ionization not amount of solute dissolved
148
149
Colligative properties of solutions
Presence of solute affects properties of solutions Some properties not dependent on nature of dissolved substance but on how may dissolve particles are present
Colligative properties - properties that depend on the concentration of solute particles but not on their identity
150
Vapor-Pressure Lowering
Boiling and freezing point of solution different from pure solvent Nonvolatile solute raises boiling point and lowers freezing point Nonvolatile substance - one that has little tendency to become gas under existing conditions
151
152
To understand why nonvolatile solute changes boiling and freezing point consider equilibrium vapor pressure
Vapor pressure pressure caused by molecules that have escape liquid phase to gas phase
Can be thought of a measure of tendency of molecules to escape from a liquid
153
Addition of sucrose (nonvolatile solute) lowers concentration of water molecules at surface of liquid This lowers tendency of water molecules to leave solution and enter gas phase Vapor pressure of solution is lower than vapor pressure of pure water
154
Aqueous13 solution13 of13 nonvolatile13 solutePure13 water
Nonelectrolyte solutions of same molality have same concentration of particles
Dilute solutions of same solvent and equal molality of any nonelectrolyte solute lower vapor pressure equally
155
Example
1 m aqueous solution of nonelectrolyte glucose lowers vapor pressure of water 55 x 10-4 atm at 25 1 m aqueous solution of sucrose also lowers water vapor pressure to 55 x 10-4 atm at 25
156
157
bc vapor-pressure lowering depends on concentration of nonelectrolyte solute and doesnrsquot depend on the type of solute it is a colligative proprerty
bc vapor pressure has been lowered solution remains liquid over larger temperature range
Lowers freezing point and raises boiling point
158
Can assume changes in boiling and freezing point also depend on concentration of solute
They are colligative properties
159
Freezing-Point Depression
Freezing point of 1-molal solution of any nonelectrolyte solute in water is found (by experiment) to be 186 lower than freezing point of water When 1 mol of nonelectrolyte solute dissolved in 1 kg water freezing point is -186 not 000 160
When 2 mol nonelectrolyte solute dissolved in 1 kg water freezing point is -372
For any concentration of nonelectrolyte solute in water decrease in freezing point can be determined using value of -186m
161
This value called the molal freezing-point contstant (Kf) is the freezing-point depression of the solvent in a 1-molal solution of a nonvolatile nonelectrolyte solute
162
Each solvent has own characteristic molal freezing-point constant Freezing-point depression (Δtf) the difference between the freezing points of the pure solvent and a solute of nonelectrolyte in that solvent and it is directly proportional to the molal concentration of the solution Δtf = Kfm
163
Δtf = Kfm
Kf expressed as m
m expressed in mol solutekg solvent (molality)
164
Sample Problem
What is the freezing point depression of water in a solution of 171 g of sucrose C12H22O11 and 200 g of water What is the actual freezing point of the solution
165
1 AnalyzeGiven Solute mass and chemical formula 171 g C12H22O11 Solvent mass and identity 2000 g water Unknown a freezing-point depression b freezing point of the solution 166
2 Plan
167
168
Calculate
Unknown a freezing-point depression b freezing point of the solution
169
Practice Problem
A water solution containing an unknown quantity of a nonelectrolyte solute is found to have a freezing point of ndash 023degCWhat is the molal concentration of the solution
012 m170
A solution consists of 103 g of the nonelectrolyte glucose C6H12O6 dissolved in 250 g of water What is the freezing-point depression of the solution
minus0426degC171
In a laboratory experiment the freezing point of an aqueous solution of glucose is found to be minus0325degCWhat is the molal concentration of this solution
0175 m172
If 0500 mol of a nonelectrolyte solute are dissolved in 5000 g of ether what is the freezing point of the solution
minus1181degC173
The freezing point of an aqueous solution that contains a nonelectrolyte is minus90degC aWhat is the freezing-point depression of the solution bWhat is the molal concentration of the solution a minus90degC b 48 m
174
Boiling-Point Elevation
Boiling point is temp at which vapor pressure is equal to atmospheric pressure Change in vapor pressure will cause corresponding change in boiling point
175
176
Vapor pressure of nonvolatile solution is lower than vapor pressure of pure solvent More heat will be required to raise vapor pressure of solution Boiling point of solution is higher than boiling point of pure solvent
Molal boiling-point constant (Kb) - boiling-point elevation of the solvent in a 1-molal solution of a nonvolatile nonelectrolyte solute b-p elevation of any nonelectrolyte in water found by experiment to be 051 So molal b-p constant for water is 051m
177
For different solvents b-p elevations of 1-molal solutions have different values (Table 14-2)
Like freezing-point constants values most accurate for dilute solutions
178
Boiling-point elevation Δtb difference between the boiling points of the pure solvent and a nonelectrolyte solution of that solvent and it is directly proportional to the molal concentration of the solution B-p elevation can be calculated using Δtb = Kbm Δtb (boiling point elevation) expressed in m and m expressed in mol solutekg solvent
Sample Problem
What is the boiling-point elevation of a solution made from 200 g of a nonelectrolyte solute and 4000 g of water The molar mass of the solute is 620 gmol
180
1 AnalyzeGiven solute mass = 200 g solute molar mass = 620 gmol solvent mass and identity = 4000 g of water
Unknown boiling-point elevation
181
2 Plan
182
183
3 Compute
Practice Problem
A solution contains 500 g of sucrose C12H22O11 a nonelectrolyte dissolved in 5000 g of water What is the boiling-point elevation
015degC184
A solution contains 4500 g of sucrose C12H22O11 a nonelectrolyte dissolved in 250 g of waterWhat is the boiling point of the solution
1027degC185
If the boiling point elevation of an aqueous solution containing a nonvolatile electrolyte is 102degC what is the molality of the solution
20 m186
The boiling point of an aqueous solution containing a nonvolatile electrolyte is 10075degC aWhat is the boiling-point elevation bWhat is the molality of the solution a 075degC b 15 m
187
Electrolytes and Colligative PropertiesEarly investigators confused by experiments where certain substances depressed freezing point or elevated boiling point more than expected
188
189
Ex 01 m solution of NaCl lowers freezing point of solvent almost twice as much as 01 m solution of sucrose 01 m CaCl2 lowers fp almost 3 times as much as 01 m solution of sucrose
To understand why this happens contrast behavior or sucrose with NaCl in aqueous solution Each sucrose dissolves to make only 1 particle in solution So 1 mol sucrose dissolves to 1 mol particles 1 mol NaCl dissolves to make 2 moles of particles (1 mol Na+ and 1 mol Cl-)
190
191
Calculated Values for Electrolyte SolutionsColligative properties depend on total concentration of solute particles regardless of their identity Electrolytes cause changes in colligative properties proportional to total molality in terms of all dissolves particles instead of formula units
192
What about barium nitrate Ba(NO3)2 Each mole of barium nitrate yields 3 mol of ions in solution Ba(NO3)2(s) rarr Ba2+(aq) + 2NO3-(aq) You would expect solution of given molality to lower fp of solvent 3 times as much as nonelectrolytic solution of same molality
193
Sample Problem
What is the expected change in the freezing point of water in a solution of 625 g of barium nitrate Ba(NO3)2 in 100 kg of water
194
1 AnalyzeGiven solute mass and formula = 625 g Ba(NO3)2 solvent mass and identity = 100 kg water Δtf = Kfm
Unknown expected freezing-point depression
195
2 PlanThe molality can be calculated by converting the solute mass to moles and then dividing by the number of kilograms of solvent That molality is in terms of formula units of Ba(NO3)2 and must be converted to molality in terms of dissociated ions in solution It must be multiplied by the number of moles of ions produced per mole of formula unit This adjusted molality can then be used to calculate the freezing-point depression
196
197
198
3 Compute
Practice Problem
What is the expected freezing-point depression for a solution that contains 20 mol of magnesium sulfate dissolved in 10 kg of water
minus74degC199
What is the expected boiling-point elevation of water for a solution that contains 150 g of sodium chloride dissolved in 10 kg of water 27degC The freezing point of an aqueous sodium chloride solution is minus200degC What is the molality of the solution 54 m NaCl
200
Actual Values for Electrolyte SolutionsRemember values calculated are only expected values Actual values of colligative properties for all strong electrolytes are almost what would be expected Difference between expected and actual values caused by attractive forces that exist between dissociated ions in aqueous solution
201
Attraction between hydrated ions in solution is small compared with those in crystalline solid However forces of attraction donrsquot interfere with movements of aqueous ions More concentrated solution means ions closer together and attraction is greater Also depends on charge of ions ndash higher charge greater attraction
202
Eventually molecules are returning to crystal at same rate they are going into solution Dynamic equilibrium established between dissolution and crystallization Solution equilibrium - physical state in which the opposing processes of dissolution and crystallization of a solute occur at equal rates
35
Saturated vs Unsaturated Solutions
Saturated solution - solution that contains the maximum amount of dissolved solute How do you know if solution is saturated If more NaCl added it falls to bottom and doesnrsquot dissolve Equilibrium already established
36
If more water added to saturated solution more NaCl will dissolve At 20 359 g NaCl is max that will dissolve in 100 g water Unsaturated solution - solution that contains less solute than a saturated solution under the existing conditions
37
Supersaturated Solutions
When saturated solution where solubility increases as temp increases is cooled excess solute usually comes out of solution Sometimes if solution is left to cool the excess solute doesnrsquot separate
38
39
Supersaturated solution - solution that contains more dissolved solute than a saturated solution contains under same conditions
40
Supersaturated solution can remain unchanged over long time if it isnrsquot disturbed Once crystals begin to form continues until equilibrium is recreated at lower temp
41
Example
Sodium thiosulfate Na2S2O3 Solute added to hot water until solution saturated Hot solution filtered Drop small crystal in ldquoseedingrdquo crystallization Continues until equilibrium created
42
Solubility Values
Solubility - amount of substance required to form a saturated solution with specific amount of solvent at specified temp Ex Solubility of sugar = 204 g per 100 g water at 20 Must specify temp bc solubility changes with temp 43
For gases pressure must also be specified Rate at which substance dissolves is unrelated to solubility Max amount of solute that dissolves and reaches equilibrium is always same under same conditions
44
Solute-Solvent Interactions
ldquolike dissolves likerdquo is generally useful rule for predicting whether one substance will dissolve in another Depends on type of bonding polaritynonpolarity and intermolecular forces between solute and solvent
45
Dissolving Ionic Compounds in Aqueous Solution
Polarity of water molecules plays important role in formation of solution of ionic compounds Charged ends of water molecules attract ions and surround them to keep separated from other ions
46
Hydration - solution process with water as solvent Ions said to be hydrated As hydrated ions diffuse into solution other ions exposed and drawn away from crystal surface Entire crystal gradually dissolves
47
When crystallized from aqueous solutions some ionic compounds form crystals that include water molecules The crystalline compounds known as hydrates have specific ratios of water molecules Represented by formulas like CuSO45H2O
48
When crystalline hydrate dissolves in water water of hydration returns to solvent Behavior of solute in hydrated form no different than anhydrous form
49
Nonpolar Solvents
Nonpolar solvents do not attract ions of ionic compound strongly enough to overcome forces holding crystal together
50
Liquid Solutes and Solvents
When you shake bottle of salad dressing oil droplets disperse in water Stop shaking ndash strong attraction between water molecules squeeze out oil and form separate layers Liquid solutes and solvents that are not soluble in each other are immiscible
51
Nonpolar substances (fat oil grease) are generally quite soluble in nonpolar liquids (carbon tetrachloride toluene gasoline)
Only attractions between nonpolar molecules are weak London forces
52
London Dispersion Forces
bc e- are constantly moving at any time there may be uneven distribution of them around a nonpolar atom Temporarily creates positive end and negative end
53
54
London dispersion forces - intermolecular attractions resulting from constant motion of e- and creation of instantaneous dipoles
55
Liquids that dissolve freely in one another in any ratio are said to be completely miscible Benzene and carbon tetrachloride (both nonpolar) are completely miscible
Nonpolar molecules of these two apply no strong forces of attraction or repulsion and molecules mix freely
56
Ethanol and water are also completely miscible -OH group on ethanol is slightly polar
Can form H-bonds with water as well as other ethanol molecules Intermolecular forces in mixture are very similar to those in pure liquids so they are mutually soluble in any amount
57
Gasoline contains mainly nonpolar hydrocarbons
Excellent solvent for fats oils greases
Major intermolecular forces are weak London forces
58
Ethanol less polar than water more than carbon tetrachloride
Better solvent for less-polar substances bc of nonpolar region
59
Effects of Pressure on Solubility
Little effect on solids or liquids Increase pressure - increase solubilities of gases in liquids
60
When gas in contact with surface of liquid gas molecules can enter liquid As amount of dissolve gas increases some molecules start to escape and reenter gas phase
Eq eventually established between rates of enteringleaving gas phase
61
As long as eq undisturbed solubility of gas in liquid unchanged at given pressure
Gas + solvent solution
62
Increasing pressure of solute gas above solution puts stress on eq Molecules collide with surface more often Increase in pressure partially offset by increase in rate of gas molecules entering solution In turn increase in amount of dissolved gas causes increase in rate molecules escape liquid and becomes gas
63
Eventually eq restored at higher gas solubility
Expected from Le Chatelierrsquos principle
Increase in gas pressure causes eq to shift so fewer molecules are in gas phase
64
Henryrsquos Law
Henryrsquos law - solubility of a gas in liquid is directly proportional to the partial pressure of that gas on the surface of the liquid
Applies to gas-liquid solutions at constant temp
65
When mixture of ideal gases is limited in constant volume at constant temp each gas applies same pressure it would apply if it occupied the space alone
Assuming gases do not react each gas dissolves to the extent it would if no other gases were there
66
In carbonated drinks (Coke Pepsi etc) solubility of CO2 increased by increasing pressure
At bottling factory CO2 gas forced into solution of flavored water at pressure of 5-10 atm Gas-in-liquid solution then sealed in bottlescans
67
When cap removed pressure reduced to 1 atm and some CO2 escapes as gas bubbles Effervescence - rapid escape of a gas from a liquid in which it is dissolved
68
Effects of Temperature on Solubility
Letrsquos consider gas solubility Increasing temperature usually decreases gas solubility As temp increases average kinetic energy of molecules in solution increases Greater number of solute molecules escape from attraction of solvent and return to gas phase 69
At higher temps eq is reached with fewer gas molecules in solution Gases generally less soluble
70
Effect of temp on solubility of solids in liquids more difficult to predict
Often increasing temp increases solubility
However the same temp increase can result in large increase in solubility in one case and only slight increase in the next
71
72
enthalpies of Solution
Formation of solution comes with energy change Dissolve KI in water container feels cold Dissolve LiCl in water feels hot Formation of solid-liquid solution can absorb or release heat
73
During formation of solution solvent and solute particles experience changes in forces attracting them to other particles Before dissolving begins solvent molecules held by intermolecular forces (solvent-solvent attraction) In solute molecules held by Ifs (solute-solute attraction)
74
Energy is required to separate solute molecules and solvent molecules from their neighbors
A solute particle that is surrounded by solvent molecules is said to be solvated
75
Step 1 solute particles become separated from solid (energy absorbed) Step 2 solvent particles move apart to allow solute particles to enter liquid (energy absorbed)
76
Step 3 solvent particles attracted to and solvate solute particles (energy released)
77
Heat of solution - net amount of heat energy absorbed or released when a specific amount of solute dissolves in a solvent
Heat of solution negative (heat is released) if sums of energy in steps 1 and 2 is less than step 3
If 1+2 gt 3 HOS is positive (heat absorbed)78
In gaseous state molecules are so far apart there are nearly no forces between them Solute-solute interaction has little effect on heat of solution of a gas Energy released when gas dissolved in liquid bc attraction between solute gas and solvent molecules greater than energy needed to separate solvent molecules
79
Concentration of Solutions80
Concentration - measure of amount of solute in given amount of solvent or solution ldquodiluterdquo just means small amount of solute ldquoconcentratedrdquo just means relatively large amount of solute Unrelated to degree to which solution is saturated Ex Saturated solution of substance not very soluble might be dilute
81
Molarity
Molarity - number of moles of solute in one liter of solution To find molarity must know molar mass A ldquoone molarrdquo solution of NaOH is 1 mole NaOH per liter of solution (1 M)
82
1 mol NaOH = 400 g If this quantity dissolve in enough water to make EXACTLY 100 L solution solution is 1 M If 200 g NaOH dissolve in enough to make 100 L solution what is molarity 0500 M
83
1 M solution is not made by adding 1 mol solute to 1 L solvent That would be more than 1 L Instead 1 mol solute dissolved in less than 1 L Then diluted with more solvent to bring TOTAL VOLUME to 1 L
84
Practice ProblemYou have 340 L of solution that contains 900 g sodium chloride What is the molarity of that solution 0440 M NaCl You have 08 L of a 05 M HCl solution How many moles of HCl does this solution contain 04 mol HCl
85
What is the molarity of a solution composed of 585 g of potassium iodide dissolved in enough water to make a 0125 L of solution 0282 M KI
86
How many moles of H2SO4 are present in 0500 L of a 0150 M H2SO4 solution 00750 mol What volume of 300 M NaCl is needed for reaction that requires 1463 g of NaCl 0834 L
87
Molality
Molality - concentration of solution expressed in moles of solute per kilogram of solvent Solution that contains 1 mol solute (NaOH) dissolved in exactly 1 kg of solvent is ldquoone molalrdquo solution (1 m NaOH)
88
1 mol NaOH = 400 g 400 g NaOH dissolved in 1 kg water is 1 m NaOH
200 g NaOH in 1 kg water = 0500 m NaOH
89
Practice Problem
A solution was prepared by dissolving 171 g sucrose (C12H22O11) in 125 g of water Find the molal concentration of this solution 0400 m C12H22O11
90
A solution of iodine in carbon tetrachloride is used when iodine is needed for certain chemical tests How much iodine must be added to prepare a 0480 m solution of iodine in CCl4 if 1000 g of CCl4 is used 122 g I2
91
What is the molality of a solution composed of 255 g acetone (CH3)2CO dissolved in 200 g of water 22 m acetone What quantity in grams of methanol CH3OH is required to prepare a 0244 m solution in 400 g water 312 g methanol
92
How many grams of AgNO3 are needed to prepare a 0125 m solution in 250 mL of water 531 g AgNO3
What is the molality of a solution containing 182 g HCl and 250 g water 200 m
93
Ions in Aqueous Solutions and Colligative Properties
94
95
compounds in aqueous solutions
DissociationWhen compound made from ions dissolves in water ions separate Dissociation - separation of ions that occurs when an ionic compound dissolves
96
Notice number of ions made per formula unit in equations 1 formula unit of NaCl gives 2 units of ions in solution 1 formula unit of CaCl2 gives 3 units of ions
97
Assuming 100 dissociation solution that contains 1 mol NaCl contains 1 mol Na+ and 1 mol Cl- Can assume 100 dissociation of all soluble ionic compounds
98
Practice Problem
Write the equation for the dissolution of aluminum sulfate in water How many moles of aluminum ions are made by dissolving 1 mol aluminum sulfate What is the total number of moles of ions made by dissolving 1 mol of aluminum sulfate
99
1 AnalyzeGiven Amount of solute = 1 mol Al2(SO4)3 Solvent identity = water Unknown a moles of aluminum ions and sulfate ions Total number of moles of solute ions produced
100
2 Plan
The coefficients in the balanced dissociation equation will tell mole relationships You can use the equation to find out the number of moles of solute ions produced
101
3 Compute
a
b102
Write the equation for the dissolution of each of the following in water and then determine the number of moles of each ions made as well as the total number of ions made a 1 mol ammonium chloride b 1 mol sodium sulfide c 05 mol barium nitrate
103
a 1 mol ammonium chloride
1 mol NH4+ 1 mol Cl- 2 mol total ions
104
b 1 mol sodium sulfide
2 mol Na+ 1 mol S-2 3 mol total ions
105
c 05 mol barium nitrate
05 mol Ba+2 1 mol NO3- 15 mol total ions
106
Precipitation Reactions
Even though no compound is completely insoluble compounds of very low solubility can be considered insoluble for practical purposes There are general guidelines to help predict whether a compound made of certain combination of ions is soluble
107
General Solubility Guidelines
Sodium potassium and ammonium compounds are soluble in water
108
Nitrates acetates and chlorates are soluble
109
110
Most chlorides are soluble except those of silver mercury (I) and lead Lead (II) chloride is soluble in hot water
Most sulfates are soluble except those of barium strontium lead calcium and mercury
111
Most carbonates phosphates and silicates are insoluble except those of sodium potassium and ammonium
112
Most sulfides are insoluble except those of calcium strontium sodium potassium and ammonium
113
Is calcium phosphate Ca3(PO4)2 soluble or insoluble
Not one of the exceptions so it is insoluble
Dissociation equations are not written for insoluble compounds
114
Guidelines also useful in predicting what will happen if 2 solutions of 2 different soluble compounds are mixed
If mixing results in combination of ions that forms insoluble compound a double-replacement and precipitation reaction will happen
115
Will precipitate form when solutions of ammonium sulfide and cadmium nitrate are combined You can tell calcium nitrate is soluble from the guidelines (Most nitrates acetates and chlorates are soluble) You can also tell ammonium sulfide is soluble (Most sulfides are insoluble except those of calcium strontium sodium potassium and ammonium)
116
117
To decide whether precipitate can form must know solubilities of two possible products
Ammonium nitrate ndash soluble (Most nitrates acetates and chlorates are soluble) Cadmium sulfide ndash insoluble (Most sulfides are insoluble except those of calcium strontium sodium potassium and ammonium)
118
bc one product is insoluble double-replacement and precipitation reaction will happen (NH4)2S (aq) + Cd(NO3)2 (aq) rarr NH4NO3 (aq) + CdS (s)
119
Net Ionic Equations
Reactions of ions in aqueous solution usually represented by net ionic equations instead of formula equations Net ionic equation - includes only those compounds and ions that undergo a chemical change in a reaction in an aqueous solution
120
121
To write NIE (net ionic equation) first convert chemical equation into overall ionic equation All soluble ionic compounds shown as dissociated ions in solution Precipitates shown as solids
122
Notice ammonium and nitrate ions appear on both sides They havenrsquot gone through any chemical change Spectator ions - ions that do not take part in a chemical reaction and are found in solution both before and after the reaction
123
To change ionic equation to NIE eliminate spectator ions Cd+2(aq) + S-2(aq) rarr CdS(s)
This NIE applies to any reaction in which precipitate of CdS forms
124
Practice Problem
Identify the precipitate that forms when aqueous solutions of zinc nitrate and ammonium sulfide are combined Write the equation for the possible double-replacement reaction Then write the formula equation overall ionic equation and the net ionic equation
125
1 AnalyzeGiven Identity of reactants zinc nitrate and ammonium sulfide Reaction medium aqueous solution Unknown a equation for the possible double-replacement reaction b identity of the precipitate c formula equation d overall ionic equation e net ionic equation
126
2 PlanWrite the possible double-replacement reaction between Zn(NO3)2 and (NH4)2S Use the rules to determine if any of the products will precipitate Write a formula equation and overall net ionic equation Then cancel spectator ions to make net ionic equation
127
3 Computea possible double-replacement reaction
b rules show that zinc sulfide is not soluble so will precipitate (ammonium nitrate is soluble)
c formula equation128
d overall ionic equation
e net ionic equation129
Will a precipitate form if solutions of potassium sulfate and barium nitrate are combined If so write the net ionic equation for the reaction
Yes Ba+2(aq) + SO4-2(aq) rarr BaSO4(s)
130
Will a precipitate form if solutions of potassium nitrate and magnesium sulfate are combined If so write the net ionic equation for the reaction
No 131
Will a precipitate form if solutions of barium chloride and sodium sulfate are combined If so identify the spectator ions and write the net ionic equation for the reaction
Yes Na+ and Cl- Ba+2(aq) + SO4-2(aq) rarr BaSO4(s)
132
Write the net ionic equation for the precipitation of nickel(II) sulfide
Ni+2(aq) + S-2(aq) rarr NiS(s)
133
IonizationSome molecular compounds can also form ions in solution Usually polar Ionization - ions formed from solute molecules by action of the solvent (creation of ions where there were none) Ionization different from dissociation
134
Dissociation ionic compounds dissolve and ions already present separate Ionization ions formed where non existed before
Ions formed are hydrated Heat released during hydration of ions gives enough energy to break covalent bonds
135
Extent to which solute ionizes depends on strength of bonds within molecules of solute and strength of attraction between solute and solvent molecules
136
HCl ndash molecular compound that ionizes in aqueous solution Contains highly polar bond Attraction between polar HCl molecule and polar water strong enough to break HCl bond
137
The Hydronium Ion
H+ ions from HCl attracts other molecules or ions so strongly that it doesnrsquot normally exist alone Ionization of HCl better described as direct transfer of proton from HCl to H2O forming H3O+
138
139
Hydration of H+ to form hydronium is highly exothermic
Energy released gives a lot of energy needed to ionize a molecular solute
Many molecular compounds that ionize in aqueous solution contain hydrogen and form hydronium ion
140
Strong and Weak Electrolytes
Substances that make ions and conduct current in solution are electrolytes HCl is one of series of compounds that has hydrogen and halogen Hydrogen halides are all molecular compounds with single polar-covalent bonds
141
All are gases Very soluble in water All are electrolytes HCl HBr HI strongly conduct current in solution HF only weakly conducts current at same concentration Strength of conduction related to ability to ionize
142
143
Strong Electrolytes
HCl HBr HI are 100 ionized in solution
Strong electrolyte - any compound whose dilute aqueous solutions conduct electricity well this is due to the presence of all or almost all of the dissolve compound in the form of ions
144
Unique characteristic of strong electrolyte is that they yield only ions
Ex Ionic compound may be highly soluble in water and dissociate into ions (NaCl)
Others may not dissolve much but amount that does dissolve exists only as ions
145
Weak ElectrolytesHF dissolves in water to give acid solution called hydrofluoric acid HF bond stronger than bonds between hydrogen and other halogens When HF dissolves some molecule ionize Reverse reaction also happens HF(aq) + H2O(l) H3O+(aq) + F-(aq)
146
HF(aq) + H2O(l) H3O+
(aq) + F-(aq)Concentration of dissolved unionized HF stays high and concentration of ions stays low
Weak electrolyte - any compound whose aqueous solutions conduct electricity poorly this is due to the presence of small amount of the dissolved compound in the form of ions
147
Different from nonelectrolyte ndash NO ions at all Description of electrolyte as strong or weak not related to concentration of solution Electrolytes differ in degree of ionization not amount of solute dissolved
148
149
Colligative properties of solutions
Presence of solute affects properties of solutions Some properties not dependent on nature of dissolved substance but on how may dissolve particles are present
Colligative properties - properties that depend on the concentration of solute particles but not on their identity
150
Vapor-Pressure Lowering
Boiling and freezing point of solution different from pure solvent Nonvolatile solute raises boiling point and lowers freezing point Nonvolatile substance - one that has little tendency to become gas under existing conditions
151
152
To understand why nonvolatile solute changes boiling and freezing point consider equilibrium vapor pressure
Vapor pressure pressure caused by molecules that have escape liquid phase to gas phase
Can be thought of a measure of tendency of molecules to escape from a liquid
153
Addition of sucrose (nonvolatile solute) lowers concentration of water molecules at surface of liquid This lowers tendency of water molecules to leave solution and enter gas phase Vapor pressure of solution is lower than vapor pressure of pure water
154
Aqueous13 solution13 of13 nonvolatile13 solutePure13 water
Nonelectrolyte solutions of same molality have same concentration of particles
Dilute solutions of same solvent and equal molality of any nonelectrolyte solute lower vapor pressure equally
155
Example
1 m aqueous solution of nonelectrolyte glucose lowers vapor pressure of water 55 x 10-4 atm at 25 1 m aqueous solution of sucrose also lowers water vapor pressure to 55 x 10-4 atm at 25
156
157
bc vapor-pressure lowering depends on concentration of nonelectrolyte solute and doesnrsquot depend on the type of solute it is a colligative proprerty
bc vapor pressure has been lowered solution remains liquid over larger temperature range
Lowers freezing point and raises boiling point
158
Can assume changes in boiling and freezing point also depend on concentration of solute
They are colligative properties
159
Freezing-Point Depression
Freezing point of 1-molal solution of any nonelectrolyte solute in water is found (by experiment) to be 186 lower than freezing point of water When 1 mol of nonelectrolyte solute dissolved in 1 kg water freezing point is -186 not 000 160
When 2 mol nonelectrolyte solute dissolved in 1 kg water freezing point is -372
For any concentration of nonelectrolyte solute in water decrease in freezing point can be determined using value of -186m
161
This value called the molal freezing-point contstant (Kf) is the freezing-point depression of the solvent in a 1-molal solution of a nonvolatile nonelectrolyte solute
162
Each solvent has own characteristic molal freezing-point constant Freezing-point depression (Δtf) the difference between the freezing points of the pure solvent and a solute of nonelectrolyte in that solvent and it is directly proportional to the molal concentration of the solution Δtf = Kfm
163
Δtf = Kfm
Kf expressed as m
m expressed in mol solutekg solvent (molality)
164
Sample Problem
What is the freezing point depression of water in a solution of 171 g of sucrose C12H22O11 and 200 g of water What is the actual freezing point of the solution
165
1 AnalyzeGiven Solute mass and chemical formula 171 g C12H22O11 Solvent mass and identity 2000 g water Unknown a freezing-point depression b freezing point of the solution 166
2 Plan
167
168
Calculate
Unknown a freezing-point depression b freezing point of the solution
169
Practice Problem
A water solution containing an unknown quantity of a nonelectrolyte solute is found to have a freezing point of ndash 023degCWhat is the molal concentration of the solution
012 m170
A solution consists of 103 g of the nonelectrolyte glucose C6H12O6 dissolved in 250 g of water What is the freezing-point depression of the solution
minus0426degC171
In a laboratory experiment the freezing point of an aqueous solution of glucose is found to be minus0325degCWhat is the molal concentration of this solution
0175 m172
If 0500 mol of a nonelectrolyte solute are dissolved in 5000 g of ether what is the freezing point of the solution
minus1181degC173
The freezing point of an aqueous solution that contains a nonelectrolyte is minus90degC aWhat is the freezing-point depression of the solution bWhat is the molal concentration of the solution a minus90degC b 48 m
174
Boiling-Point Elevation
Boiling point is temp at which vapor pressure is equal to atmospheric pressure Change in vapor pressure will cause corresponding change in boiling point
175
176
Vapor pressure of nonvolatile solution is lower than vapor pressure of pure solvent More heat will be required to raise vapor pressure of solution Boiling point of solution is higher than boiling point of pure solvent
Molal boiling-point constant (Kb) - boiling-point elevation of the solvent in a 1-molal solution of a nonvolatile nonelectrolyte solute b-p elevation of any nonelectrolyte in water found by experiment to be 051 So molal b-p constant for water is 051m
177
For different solvents b-p elevations of 1-molal solutions have different values (Table 14-2)
Like freezing-point constants values most accurate for dilute solutions
178
Boiling-point elevation Δtb difference between the boiling points of the pure solvent and a nonelectrolyte solution of that solvent and it is directly proportional to the molal concentration of the solution B-p elevation can be calculated using Δtb = Kbm Δtb (boiling point elevation) expressed in m and m expressed in mol solutekg solvent
Sample Problem
What is the boiling-point elevation of a solution made from 200 g of a nonelectrolyte solute and 4000 g of water The molar mass of the solute is 620 gmol
180
1 AnalyzeGiven solute mass = 200 g solute molar mass = 620 gmol solvent mass and identity = 4000 g of water
Unknown boiling-point elevation
181
2 Plan
182
183
3 Compute
Practice Problem
A solution contains 500 g of sucrose C12H22O11 a nonelectrolyte dissolved in 5000 g of water What is the boiling-point elevation
015degC184
A solution contains 4500 g of sucrose C12H22O11 a nonelectrolyte dissolved in 250 g of waterWhat is the boiling point of the solution
1027degC185
If the boiling point elevation of an aqueous solution containing a nonvolatile electrolyte is 102degC what is the molality of the solution
20 m186
The boiling point of an aqueous solution containing a nonvolatile electrolyte is 10075degC aWhat is the boiling-point elevation bWhat is the molality of the solution a 075degC b 15 m
187
Electrolytes and Colligative PropertiesEarly investigators confused by experiments where certain substances depressed freezing point or elevated boiling point more than expected
188
189
Ex 01 m solution of NaCl lowers freezing point of solvent almost twice as much as 01 m solution of sucrose 01 m CaCl2 lowers fp almost 3 times as much as 01 m solution of sucrose
To understand why this happens contrast behavior or sucrose with NaCl in aqueous solution Each sucrose dissolves to make only 1 particle in solution So 1 mol sucrose dissolves to 1 mol particles 1 mol NaCl dissolves to make 2 moles of particles (1 mol Na+ and 1 mol Cl-)
190
191
Calculated Values for Electrolyte SolutionsColligative properties depend on total concentration of solute particles regardless of their identity Electrolytes cause changes in colligative properties proportional to total molality in terms of all dissolves particles instead of formula units
192
What about barium nitrate Ba(NO3)2 Each mole of barium nitrate yields 3 mol of ions in solution Ba(NO3)2(s) rarr Ba2+(aq) + 2NO3-(aq) You would expect solution of given molality to lower fp of solvent 3 times as much as nonelectrolytic solution of same molality
193
Sample Problem
What is the expected change in the freezing point of water in a solution of 625 g of barium nitrate Ba(NO3)2 in 100 kg of water
194
1 AnalyzeGiven solute mass and formula = 625 g Ba(NO3)2 solvent mass and identity = 100 kg water Δtf = Kfm
Unknown expected freezing-point depression
195
2 PlanThe molality can be calculated by converting the solute mass to moles and then dividing by the number of kilograms of solvent That molality is in terms of formula units of Ba(NO3)2 and must be converted to molality in terms of dissociated ions in solution It must be multiplied by the number of moles of ions produced per mole of formula unit This adjusted molality can then be used to calculate the freezing-point depression
196
197
198
3 Compute
Practice Problem
What is the expected freezing-point depression for a solution that contains 20 mol of magnesium sulfate dissolved in 10 kg of water
minus74degC199
What is the expected boiling-point elevation of water for a solution that contains 150 g of sodium chloride dissolved in 10 kg of water 27degC The freezing point of an aqueous sodium chloride solution is minus200degC What is the molality of the solution 54 m NaCl
200
Actual Values for Electrolyte SolutionsRemember values calculated are only expected values Actual values of colligative properties for all strong electrolytes are almost what would be expected Difference between expected and actual values caused by attractive forces that exist between dissociated ions in aqueous solution
201
Attraction between hydrated ions in solution is small compared with those in crystalline solid However forces of attraction donrsquot interfere with movements of aqueous ions More concentrated solution means ions closer together and attraction is greater Also depends on charge of ions ndash higher charge greater attraction
202
Saturated vs Unsaturated Solutions
Saturated solution - solution that contains the maximum amount of dissolved solute How do you know if solution is saturated If more NaCl added it falls to bottom and doesnrsquot dissolve Equilibrium already established
36
If more water added to saturated solution more NaCl will dissolve At 20 359 g NaCl is max that will dissolve in 100 g water Unsaturated solution - solution that contains less solute than a saturated solution under the existing conditions
37
Supersaturated Solutions
When saturated solution where solubility increases as temp increases is cooled excess solute usually comes out of solution Sometimes if solution is left to cool the excess solute doesnrsquot separate
38
39
Supersaturated solution - solution that contains more dissolved solute than a saturated solution contains under same conditions
40
Supersaturated solution can remain unchanged over long time if it isnrsquot disturbed Once crystals begin to form continues until equilibrium is recreated at lower temp
41
Example
Sodium thiosulfate Na2S2O3 Solute added to hot water until solution saturated Hot solution filtered Drop small crystal in ldquoseedingrdquo crystallization Continues until equilibrium created
42
Solubility Values
Solubility - amount of substance required to form a saturated solution with specific amount of solvent at specified temp Ex Solubility of sugar = 204 g per 100 g water at 20 Must specify temp bc solubility changes with temp 43
For gases pressure must also be specified Rate at which substance dissolves is unrelated to solubility Max amount of solute that dissolves and reaches equilibrium is always same under same conditions
44
Solute-Solvent Interactions
ldquolike dissolves likerdquo is generally useful rule for predicting whether one substance will dissolve in another Depends on type of bonding polaritynonpolarity and intermolecular forces between solute and solvent
45
Dissolving Ionic Compounds in Aqueous Solution
Polarity of water molecules plays important role in formation of solution of ionic compounds Charged ends of water molecules attract ions and surround them to keep separated from other ions
46
Hydration - solution process with water as solvent Ions said to be hydrated As hydrated ions diffuse into solution other ions exposed and drawn away from crystal surface Entire crystal gradually dissolves
47
When crystallized from aqueous solutions some ionic compounds form crystals that include water molecules The crystalline compounds known as hydrates have specific ratios of water molecules Represented by formulas like CuSO45H2O
48
When crystalline hydrate dissolves in water water of hydration returns to solvent Behavior of solute in hydrated form no different than anhydrous form
49
Nonpolar Solvents
Nonpolar solvents do not attract ions of ionic compound strongly enough to overcome forces holding crystal together
50
Liquid Solutes and Solvents
When you shake bottle of salad dressing oil droplets disperse in water Stop shaking ndash strong attraction between water molecules squeeze out oil and form separate layers Liquid solutes and solvents that are not soluble in each other are immiscible
51
Nonpolar substances (fat oil grease) are generally quite soluble in nonpolar liquids (carbon tetrachloride toluene gasoline)
Only attractions between nonpolar molecules are weak London forces
52
London Dispersion Forces
bc e- are constantly moving at any time there may be uneven distribution of them around a nonpolar atom Temporarily creates positive end and negative end
53
54
London dispersion forces - intermolecular attractions resulting from constant motion of e- and creation of instantaneous dipoles
55
Liquids that dissolve freely in one another in any ratio are said to be completely miscible Benzene and carbon tetrachloride (both nonpolar) are completely miscible
Nonpolar molecules of these two apply no strong forces of attraction or repulsion and molecules mix freely
56
Ethanol and water are also completely miscible -OH group on ethanol is slightly polar
Can form H-bonds with water as well as other ethanol molecules Intermolecular forces in mixture are very similar to those in pure liquids so they are mutually soluble in any amount
57
Gasoline contains mainly nonpolar hydrocarbons
Excellent solvent for fats oils greases
Major intermolecular forces are weak London forces
58
Ethanol less polar than water more than carbon tetrachloride
Better solvent for less-polar substances bc of nonpolar region
59
Effects of Pressure on Solubility
Little effect on solids or liquids Increase pressure - increase solubilities of gases in liquids
60
When gas in contact with surface of liquid gas molecules can enter liquid As amount of dissolve gas increases some molecules start to escape and reenter gas phase
Eq eventually established between rates of enteringleaving gas phase
61
As long as eq undisturbed solubility of gas in liquid unchanged at given pressure
Gas + solvent solution
62
Increasing pressure of solute gas above solution puts stress on eq Molecules collide with surface more often Increase in pressure partially offset by increase in rate of gas molecules entering solution In turn increase in amount of dissolved gas causes increase in rate molecules escape liquid and becomes gas
63
Eventually eq restored at higher gas solubility
Expected from Le Chatelierrsquos principle
Increase in gas pressure causes eq to shift so fewer molecules are in gas phase
64
Henryrsquos Law
Henryrsquos law - solubility of a gas in liquid is directly proportional to the partial pressure of that gas on the surface of the liquid
Applies to gas-liquid solutions at constant temp
65
When mixture of ideal gases is limited in constant volume at constant temp each gas applies same pressure it would apply if it occupied the space alone
Assuming gases do not react each gas dissolves to the extent it would if no other gases were there
66
In carbonated drinks (Coke Pepsi etc) solubility of CO2 increased by increasing pressure
At bottling factory CO2 gas forced into solution of flavored water at pressure of 5-10 atm Gas-in-liquid solution then sealed in bottlescans
67
When cap removed pressure reduced to 1 atm and some CO2 escapes as gas bubbles Effervescence - rapid escape of a gas from a liquid in which it is dissolved
68
Effects of Temperature on Solubility
Letrsquos consider gas solubility Increasing temperature usually decreases gas solubility As temp increases average kinetic energy of molecules in solution increases Greater number of solute molecules escape from attraction of solvent and return to gas phase 69
At higher temps eq is reached with fewer gas molecules in solution Gases generally less soluble
70
Effect of temp on solubility of solids in liquids more difficult to predict
Often increasing temp increases solubility
However the same temp increase can result in large increase in solubility in one case and only slight increase in the next
71
72
enthalpies of Solution
Formation of solution comes with energy change Dissolve KI in water container feels cold Dissolve LiCl in water feels hot Formation of solid-liquid solution can absorb or release heat
73
During formation of solution solvent and solute particles experience changes in forces attracting them to other particles Before dissolving begins solvent molecules held by intermolecular forces (solvent-solvent attraction) In solute molecules held by Ifs (solute-solute attraction)
74
Energy is required to separate solute molecules and solvent molecules from their neighbors
A solute particle that is surrounded by solvent molecules is said to be solvated
75
Step 1 solute particles become separated from solid (energy absorbed) Step 2 solvent particles move apart to allow solute particles to enter liquid (energy absorbed)
76
Step 3 solvent particles attracted to and solvate solute particles (energy released)
77
Heat of solution - net amount of heat energy absorbed or released when a specific amount of solute dissolves in a solvent
Heat of solution negative (heat is released) if sums of energy in steps 1 and 2 is less than step 3
If 1+2 gt 3 HOS is positive (heat absorbed)78
In gaseous state molecules are so far apart there are nearly no forces between them Solute-solute interaction has little effect on heat of solution of a gas Energy released when gas dissolved in liquid bc attraction between solute gas and solvent molecules greater than energy needed to separate solvent molecules
79
Concentration of Solutions80
Concentration - measure of amount of solute in given amount of solvent or solution ldquodiluterdquo just means small amount of solute ldquoconcentratedrdquo just means relatively large amount of solute Unrelated to degree to which solution is saturated Ex Saturated solution of substance not very soluble might be dilute
81
Molarity
Molarity - number of moles of solute in one liter of solution To find molarity must know molar mass A ldquoone molarrdquo solution of NaOH is 1 mole NaOH per liter of solution (1 M)
82
1 mol NaOH = 400 g If this quantity dissolve in enough water to make EXACTLY 100 L solution solution is 1 M If 200 g NaOH dissolve in enough to make 100 L solution what is molarity 0500 M
83
1 M solution is not made by adding 1 mol solute to 1 L solvent That would be more than 1 L Instead 1 mol solute dissolved in less than 1 L Then diluted with more solvent to bring TOTAL VOLUME to 1 L
84
Practice ProblemYou have 340 L of solution that contains 900 g sodium chloride What is the molarity of that solution 0440 M NaCl You have 08 L of a 05 M HCl solution How many moles of HCl does this solution contain 04 mol HCl
85
What is the molarity of a solution composed of 585 g of potassium iodide dissolved in enough water to make a 0125 L of solution 0282 M KI
86
How many moles of H2SO4 are present in 0500 L of a 0150 M H2SO4 solution 00750 mol What volume of 300 M NaCl is needed for reaction that requires 1463 g of NaCl 0834 L
87
Molality
Molality - concentration of solution expressed in moles of solute per kilogram of solvent Solution that contains 1 mol solute (NaOH) dissolved in exactly 1 kg of solvent is ldquoone molalrdquo solution (1 m NaOH)
88
1 mol NaOH = 400 g 400 g NaOH dissolved in 1 kg water is 1 m NaOH
200 g NaOH in 1 kg water = 0500 m NaOH
89
Practice Problem
A solution was prepared by dissolving 171 g sucrose (C12H22O11) in 125 g of water Find the molal concentration of this solution 0400 m C12H22O11
90
A solution of iodine in carbon tetrachloride is used when iodine is needed for certain chemical tests How much iodine must be added to prepare a 0480 m solution of iodine in CCl4 if 1000 g of CCl4 is used 122 g I2
91
What is the molality of a solution composed of 255 g acetone (CH3)2CO dissolved in 200 g of water 22 m acetone What quantity in grams of methanol CH3OH is required to prepare a 0244 m solution in 400 g water 312 g methanol
92
How many grams of AgNO3 are needed to prepare a 0125 m solution in 250 mL of water 531 g AgNO3
What is the molality of a solution containing 182 g HCl and 250 g water 200 m
93
Ions in Aqueous Solutions and Colligative Properties
94
95
compounds in aqueous solutions
DissociationWhen compound made from ions dissolves in water ions separate Dissociation - separation of ions that occurs when an ionic compound dissolves
96
Notice number of ions made per formula unit in equations 1 formula unit of NaCl gives 2 units of ions in solution 1 formula unit of CaCl2 gives 3 units of ions
97
Assuming 100 dissociation solution that contains 1 mol NaCl contains 1 mol Na+ and 1 mol Cl- Can assume 100 dissociation of all soluble ionic compounds
98
Practice Problem
Write the equation for the dissolution of aluminum sulfate in water How many moles of aluminum ions are made by dissolving 1 mol aluminum sulfate What is the total number of moles of ions made by dissolving 1 mol of aluminum sulfate
99
1 AnalyzeGiven Amount of solute = 1 mol Al2(SO4)3 Solvent identity = water Unknown a moles of aluminum ions and sulfate ions Total number of moles of solute ions produced
100
2 Plan
The coefficients in the balanced dissociation equation will tell mole relationships You can use the equation to find out the number of moles of solute ions produced
101
3 Compute
a
b102
Write the equation for the dissolution of each of the following in water and then determine the number of moles of each ions made as well as the total number of ions made a 1 mol ammonium chloride b 1 mol sodium sulfide c 05 mol barium nitrate
103
a 1 mol ammonium chloride
1 mol NH4+ 1 mol Cl- 2 mol total ions
104
b 1 mol sodium sulfide
2 mol Na+ 1 mol S-2 3 mol total ions
105
c 05 mol barium nitrate
05 mol Ba+2 1 mol NO3- 15 mol total ions
106
Precipitation Reactions
Even though no compound is completely insoluble compounds of very low solubility can be considered insoluble for practical purposes There are general guidelines to help predict whether a compound made of certain combination of ions is soluble
107
General Solubility Guidelines
Sodium potassium and ammonium compounds are soluble in water
108
Nitrates acetates and chlorates are soluble
109
110
Most chlorides are soluble except those of silver mercury (I) and lead Lead (II) chloride is soluble in hot water
Most sulfates are soluble except those of barium strontium lead calcium and mercury
111
Most carbonates phosphates and silicates are insoluble except those of sodium potassium and ammonium
112
Most sulfides are insoluble except those of calcium strontium sodium potassium and ammonium
113
Is calcium phosphate Ca3(PO4)2 soluble or insoluble
Not one of the exceptions so it is insoluble
Dissociation equations are not written for insoluble compounds
114
Guidelines also useful in predicting what will happen if 2 solutions of 2 different soluble compounds are mixed
If mixing results in combination of ions that forms insoluble compound a double-replacement and precipitation reaction will happen
115
Will precipitate form when solutions of ammonium sulfide and cadmium nitrate are combined You can tell calcium nitrate is soluble from the guidelines (Most nitrates acetates and chlorates are soluble) You can also tell ammonium sulfide is soluble (Most sulfides are insoluble except those of calcium strontium sodium potassium and ammonium)
116
117
To decide whether precipitate can form must know solubilities of two possible products
Ammonium nitrate ndash soluble (Most nitrates acetates and chlorates are soluble) Cadmium sulfide ndash insoluble (Most sulfides are insoluble except those of calcium strontium sodium potassium and ammonium)
118
bc one product is insoluble double-replacement and precipitation reaction will happen (NH4)2S (aq) + Cd(NO3)2 (aq) rarr NH4NO3 (aq) + CdS (s)
119
Net Ionic Equations
Reactions of ions in aqueous solution usually represented by net ionic equations instead of formula equations Net ionic equation - includes only those compounds and ions that undergo a chemical change in a reaction in an aqueous solution
120
121
To write NIE (net ionic equation) first convert chemical equation into overall ionic equation All soluble ionic compounds shown as dissociated ions in solution Precipitates shown as solids
122
Notice ammonium and nitrate ions appear on both sides They havenrsquot gone through any chemical change Spectator ions - ions that do not take part in a chemical reaction and are found in solution both before and after the reaction
123
To change ionic equation to NIE eliminate spectator ions Cd+2(aq) + S-2(aq) rarr CdS(s)
This NIE applies to any reaction in which precipitate of CdS forms
124
Practice Problem
Identify the precipitate that forms when aqueous solutions of zinc nitrate and ammonium sulfide are combined Write the equation for the possible double-replacement reaction Then write the formula equation overall ionic equation and the net ionic equation
125
1 AnalyzeGiven Identity of reactants zinc nitrate and ammonium sulfide Reaction medium aqueous solution Unknown a equation for the possible double-replacement reaction b identity of the precipitate c formula equation d overall ionic equation e net ionic equation
126
2 PlanWrite the possible double-replacement reaction between Zn(NO3)2 and (NH4)2S Use the rules to determine if any of the products will precipitate Write a formula equation and overall net ionic equation Then cancel spectator ions to make net ionic equation
127
3 Computea possible double-replacement reaction
b rules show that zinc sulfide is not soluble so will precipitate (ammonium nitrate is soluble)
c formula equation128
d overall ionic equation
e net ionic equation129
Will a precipitate form if solutions of potassium sulfate and barium nitrate are combined If so write the net ionic equation for the reaction
Yes Ba+2(aq) + SO4-2(aq) rarr BaSO4(s)
130
Will a precipitate form if solutions of potassium nitrate and magnesium sulfate are combined If so write the net ionic equation for the reaction
No 131
Will a precipitate form if solutions of barium chloride and sodium sulfate are combined If so identify the spectator ions and write the net ionic equation for the reaction
Yes Na+ and Cl- Ba+2(aq) + SO4-2(aq) rarr BaSO4(s)
132
Write the net ionic equation for the precipitation of nickel(II) sulfide
Ni+2(aq) + S-2(aq) rarr NiS(s)
133
IonizationSome molecular compounds can also form ions in solution Usually polar Ionization - ions formed from solute molecules by action of the solvent (creation of ions where there were none) Ionization different from dissociation
134
Dissociation ionic compounds dissolve and ions already present separate Ionization ions formed where non existed before
Ions formed are hydrated Heat released during hydration of ions gives enough energy to break covalent bonds
135
Extent to which solute ionizes depends on strength of bonds within molecules of solute and strength of attraction between solute and solvent molecules
136
HCl ndash molecular compound that ionizes in aqueous solution Contains highly polar bond Attraction between polar HCl molecule and polar water strong enough to break HCl bond
137
The Hydronium Ion
H+ ions from HCl attracts other molecules or ions so strongly that it doesnrsquot normally exist alone Ionization of HCl better described as direct transfer of proton from HCl to H2O forming H3O+
138
139
Hydration of H+ to form hydronium is highly exothermic
Energy released gives a lot of energy needed to ionize a molecular solute
Many molecular compounds that ionize in aqueous solution contain hydrogen and form hydronium ion
140
Strong and Weak Electrolytes
Substances that make ions and conduct current in solution are electrolytes HCl is one of series of compounds that has hydrogen and halogen Hydrogen halides are all molecular compounds with single polar-covalent bonds
141
All are gases Very soluble in water All are electrolytes HCl HBr HI strongly conduct current in solution HF only weakly conducts current at same concentration Strength of conduction related to ability to ionize
142
143
Strong Electrolytes
HCl HBr HI are 100 ionized in solution
Strong electrolyte - any compound whose dilute aqueous solutions conduct electricity well this is due to the presence of all or almost all of the dissolve compound in the form of ions
144
Unique characteristic of strong electrolyte is that they yield only ions
Ex Ionic compound may be highly soluble in water and dissociate into ions (NaCl)
Others may not dissolve much but amount that does dissolve exists only as ions
145
Weak ElectrolytesHF dissolves in water to give acid solution called hydrofluoric acid HF bond stronger than bonds between hydrogen and other halogens When HF dissolves some molecule ionize Reverse reaction also happens HF(aq) + H2O(l) H3O+(aq) + F-(aq)
146
HF(aq) + H2O(l) H3O+
(aq) + F-(aq)Concentration of dissolved unionized HF stays high and concentration of ions stays low
Weak electrolyte - any compound whose aqueous solutions conduct electricity poorly this is due to the presence of small amount of the dissolved compound in the form of ions
147
Different from nonelectrolyte ndash NO ions at all Description of electrolyte as strong or weak not related to concentration of solution Electrolytes differ in degree of ionization not amount of solute dissolved
148
149
Colligative properties of solutions
Presence of solute affects properties of solutions Some properties not dependent on nature of dissolved substance but on how may dissolve particles are present
Colligative properties - properties that depend on the concentration of solute particles but not on their identity
150
Vapor-Pressure Lowering
Boiling and freezing point of solution different from pure solvent Nonvolatile solute raises boiling point and lowers freezing point Nonvolatile substance - one that has little tendency to become gas under existing conditions
151
152
To understand why nonvolatile solute changes boiling and freezing point consider equilibrium vapor pressure
Vapor pressure pressure caused by molecules that have escape liquid phase to gas phase
Can be thought of a measure of tendency of molecules to escape from a liquid
153
Addition of sucrose (nonvolatile solute) lowers concentration of water molecules at surface of liquid This lowers tendency of water molecules to leave solution and enter gas phase Vapor pressure of solution is lower than vapor pressure of pure water
154
Aqueous13 solution13 of13 nonvolatile13 solutePure13 water
Nonelectrolyte solutions of same molality have same concentration of particles
Dilute solutions of same solvent and equal molality of any nonelectrolyte solute lower vapor pressure equally
155
Example
1 m aqueous solution of nonelectrolyte glucose lowers vapor pressure of water 55 x 10-4 atm at 25 1 m aqueous solution of sucrose also lowers water vapor pressure to 55 x 10-4 atm at 25
156
157
bc vapor-pressure lowering depends on concentration of nonelectrolyte solute and doesnrsquot depend on the type of solute it is a colligative proprerty
bc vapor pressure has been lowered solution remains liquid over larger temperature range
Lowers freezing point and raises boiling point
158
Can assume changes in boiling and freezing point also depend on concentration of solute
They are colligative properties
159
Freezing-Point Depression
Freezing point of 1-molal solution of any nonelectrolyte solute in water is found (by experiment) to be 186 lower than freezing point of water When 1 mol of nonelectrolyte solute dissolved in 1 kg water freezing point is -186 not 000 160
When 2 mol nonelectrolyte solute dissolved in 1 kg water freezing point is -372
For any concentration of nonelectrolyte solute in water decrease in freezing point can be determined using value of -186m
161
This value called the molal freezing-point contstant (Kf) is the freezing-point depression of the solvent in a 1-molal solution of a nonvolatile nonelectrolyte solute
162
Each solvent has own characteristic molal freezing-point constant Freezing-point depression (Δtf) the difference between the freezing points of the pure solvent and a solute of nonelectrolyte in that solvent and it is directly proportional to the molal concentration of the solution Δtf = Kfm
163
Δtf = Kfm
Kf expressed as m
m expressed in mol solutekg solvent (molality)
164
Sample Problem
What is the freezing point depression of water in a solution of 171 g of sucrose C12H22O11 and 200 g of water What is the actual freezing point of the solution
165
1 AnalyzeGiven Solute mass and chemical formula 171 g C12H22O11 Solvent mass and identity 2000 g water Unknown a freezing-point depression b freezing point of the solution 166
2 Plan
167
168
Calculate
Unknown a freezing-point depression b freezing point of the solution
169
Practice Problem
A water solution containing an unknown quantity of a nonelectrolyte solute is found to have a freezing point of ndash 023degCWhat is the molal concentration of the solution
012 m170
A solution consists of 103 g of the nonelectrolyte glucose C6H12O6 dissolved in 250 g of water What is the freezing-point depression of the solution
minus0426degC171
In a laboratory experiment the freezing point of an aqueous solution of glucose is found to be minus0325degCWhat is the molal concentration of this solution
0175 m172
If 0500 mol of a nonelectrolyte solute are dissolved in 5000 g of ether what is the freezing point of the solution
minus1181degC173
The freezing point of an aqueous solution that contains a nonelectrolyte is minus90degC aWhat is the freezing-point depression of the solution bWhat is the molal concentration of the solution a minus90degC b 48 m
174
Boiling-Point Elevation
Boiling point is temp at which vapor pressure is equal to atmospheric pressure Change in vapor pressure will cause corresponding change in boiling point
175
176
Vapor pressure of nonvolatile solution is lower than vapor pressure of pure solvent More heat will be required to raise vapor pressure of solution Boiling point of solution is higher than boiling point of pure solvent
Molal boiling-point constant (Kb) - boiling-point elevation of the solvent in a 1-molal solution of a nonvolatile nonelectrolyte solute b-p elevation of any nonelectrolyte in water found by experiment to be 051 So molal b-p constant for water is 051m
177
For different solvents b-p elevations of 1-molal solutions have different values (Table 14-2)
Like freezing-point constants values most accurate for dilute solutions
178
Boiling-point elevation Δtb difference between the boiling points of the pure solvent and a nonelectrolyte solution of that solvent and it is directly proportional to the molal concentration of the solution B-p elevation can be calculated using Δtb = Kbm Δtb (boiling point elevation) expressed in m and m expressed in mol solutekg solvent
Sample Problem
What is the boiling-point elevation of a solution made from 200 g of a nonelectrolyte solute and 4000 g of water The molar mass of the solute is 620 gmol
180
1 AnalyzeGiven solute mass = 200 g solute molar mass = 620 gmol solvent mass and identity = 4000 g of water
Unknown boiling-point elevation
181
2 Plan
182
183
3 Compute
Practice Problem
A solution contains 500 g of sucrose C12H22O11 a nonelectrolyte dissolved in 5000 g of water What is the boiling-point elevation
015degC184
A solution contains 4500 g of sucrose C12H22O11 a nonelectrolyte dissolved in 250 g of waterWhat is the boiling point of the solution
1027degC185
If the boiling point elevation of an aqueous solution containing a nonvolatile electrolyte is 102degC what is the molality of the solution
20 m186
The boiling point of an aqueous solution containing a nonvolatile electrolyte is 10075degC aWhat is the boiling-point elevation bWhat is the molality of the solution a 075degC b 15 m
187
Electrolytes and Colligative PropertiesEarly investigators confused by experiments where certain substances depressed freezing point or elevated boiling point more than expected
188
189
Ex 01 m solution of NaCl lowers freezing point of solvent almost twice as much as 01 m solution of sucrose 01 m CaCl2 lowers fp almost 3 times as much as 01 m solution of sucrose
To understand why this happens contrast behavior or sucrose with NaCl in aqueous solution Each sucrose dissolves to make only 1 particle in solution So 1 mol sucrose dissolves to 1 mol particles 1 mol NaCl dissolves to make 2 moles of particles (1 mol Na+ and 1 mol Cl-)
190
191
Calculated Values for Electrolyte SolutionsColligative properties depend on total concentration of solute particles regardless of their identity Electrolytes cause changes in colligative properties proportional to total molality in terms of all dissolves particles instead of formula units
192
What about barium nitrate Ba(NO3)2 Each mole of barium nitrate yields 3 mol of ions in solution Ba(NO3)2(s) rarr Ba2+(aq) + 2NO3-(aq) You would expect solution of given molality to lower fp of solvent 3 times as much as nonelectrolytic solution of same molality
193
Sample Problem
What is the expected change in the freezing point of water in a solution of 625 g of barium nitrate Ba(NO3)2 in 100 kg of water
194
1 AnalyzeGiven solute mass and formula = 625 g Ba(NO3)2 solvent mass and identity = 100 kg water Δtf = Kfm
Unknown expected freezing-point depression
195
2 PlanThe molality can be calculated by converting the solute mass to moles and then dividing by the number of kilograms of solvent That molality is in terms of formula units of Ba(NO3)2 and must be converted to molality in terms of dissociated ions in solution It must be multiplied by the number of moles of ions produced per mole of formula unit This adjusted molality can then be used to calculate the freezing-point depression
196
197
198
3 Compute
Practice Problem
What is the expected freezing-point depression for a solution that contains 20 mol of magnesium sulfate dissolved in 10 kg of water
minus74degC199
What is the expected boiling-point elevation of water for a solution that contains 150 g of sodium chloride dissolved in 10 kg of water 27degC The freezing point of an aqueous sodium chloride solution is minus200degC What is the molality of the solution 54 m NaCl
200
Actual Values for Electrolyte SolutionsRemember values calculated are only expected values Actual values of colligative properties for all strong electrolytes are almost what would be expected Difference between expected and actual values caused by attractive forces that exist between dissociated ions in aqueous solution
201
Attraction between hydrated ions in solution is small compared with those in crystalline solid However forces of attraction donrsquot interfere with movements of aqueous ions More concentrated solution means ions closer together and attraction is greater Also depends on charge of ions ndash higher charge greater attraction
202
If more water added to saturated solution more NaCl will dissolve At 20 359 g NaCl is max that will dissolve in 100 g water Unsaturated solution - solution that contains less solute than a saturated solution under the existing conditions
37
Supersaturated Solutions
When saturated solution where solubility increases as temp increases is cooled excess solute usually comes out of solution Sometimes if solution is left to cool the excess solute doesnrsquot separate
38
39
Supersaturated solution - solution that contains more dissolved solute than a saturated solution contains under same conditions
40
Supersaturated solution can remain unchanged over long time if it isnrsquot disturbed Once crystals begin to form continues until equilibrium is recreated at lower temp
41
Example
Sodium thiosulfate Na2S2O3 Solute added to hot water until solution saturated Hot solution filtered Drop small crystal in ldquoseedingrdquo crystallization Continues until equilibrium created
42
Solubility Values
Solubility - amount of substance required to form a saturated solution with specific amount of solvent at specified temp Ex Solubility of sugar = 204 g per 100 g water at 20 Must specify temp bc solubility changes with temp 43
For gases pressure must also be specified Rate at which substance dissolves is unrelated to solubility Max amount of solute that dissolves and reaches equilibrium is always same under same conditions
44
Solute-Solvent Interactions
ldquolike dissolves likerdquo is generally useful rule for predicting whether one substance will dissolve in another Depends on type of bonding polaritynonpolarity and intermolecular forces between solute and solvent
45
Dissolving Ionic Compounds in Aqueous Solution
Polarity of water molecules plays important role in formation of solution of ionic compounds Charged ends of water molecules attract ions and surround them to keep separated from other ions
46
Hydration - solution process with water as solvent Ions said to be hydrated As hydrated ions diffuse into solution other ions exposed and drawn away from crystal surface Entire crystal gradually dissolves
47
When crystallized from aqueous solutions some ionic compounds form crystals that include water molecules The crystalline compounds known as hydrates have specific ratios of water molecules Represented by formulas like CuSO45H2O
48
When crystalline hydrate dissolves in water water of hydration returns to solvent Behavior of solute in hydrated form no different than anhydrous form
49
Nonpolar Solvents
Nonpolar solvents do not attract ions of ionic compound strongly enough to overcome forces holding crystal together
50
Liquid Solutes and Solvents
When you shake bottle of salad dressing oil droplets disperse in water Stop shaking ndash strong attraction between water molecules squeeze out oil and form separate layers Liquid solutes and solvents that are not soluble in each other are immiscible
51
Nonpolar substances (fat oil grease) are generally quite soluble in nonpolar liquids (carbon tetrachloride toluene gasoline)
Only attractions between nonpolar molecules are weak London forces
52
London Dispersion Forces
bc e- are constantly moving at any time there may be uneven distribution of them around a nonpolar atom Temporarily creates positive end and negative end
53
54
London dispersion forces - intermolecular attractions resulting from constant motion of e- and creation of instantaneous dipoles
55
Liquids that dissolve freely in one another in any ratio are said to be completely miscible Benzene and carbon tetrachloride (both nonpolar) are completely miscible
Nonpolar molecules of these two apply no strong forces of attraction or repulsion and molecules mix freely
56
Ethanol and water are also completely miscible -OH group on ethanol is slightly polar
Can form H-bonds with water as well as other ethanol molecules Intermolecular forces in mixture are very similar to those in pure liquids so they are mutually soluble in any amount
57
Gasoline contains mainly nonpolar hydrocarbons
Excellent solvent for fats oils greases
Major intermolecular forces are weak London forces
58
Ethanol less polar than water more than carbon tetrachloride
Better solvent for less-polar substances bc of nonpolar region
59
Effects of Pressure on Solubility
Little effect on solids or liquids Increase pressure - increase solubilities of gases in liquids
60
When gas in contact with surface of liquid gas molecules can enter liquid As amount of dissolve gas increases some molecules start to escape and reenter gas phase
Eq eventually established between rates of enteringleaving gas phase
61
As long as eq undisturbed solubility of gas in liquid unchanged at given pressure
Gas + solvent solution
62
Increasing pressure of solute gas above solution puts stress on eq Molecules collide with surface more often Increase in pressure partially offset by increase in rate of gas molecules entering solution In turn increase in amount of dissolved gas causes increase in rate molecules escape liquid and becomes gas
63
Eventually eq restored at higher gas solubility
Expected from Le Chatelierrsquos principle
Increase in gas pressure causes eq to shift so fewer molecules are in gas phase
64
Henryrsquos Law
Henryrsquos law - solubility of a gas in liquid is directly proportional to the partial pressure of that gas on the surface of the liquid
Applies to gas-liquid solutions at constant temp
65
When mixture of ideal gases is limited in constant volume at constant temp each gas applies same pressure it would apply if it occupied the space alone
Assuming gases do not react each gas dissolves to the extent it would if no other gases were there
66
In carbonated drinks (Coke Pepsi etc) solubility of CO2 increased by increasing pressure
At bottling factory CO2 gas forced into solution of flavored water at pressure of 5-10 atm Gas-in-liquid solution then sealed in bottlescans
67
When cap removed pressure reduced to 1 atm and some CO2 escapes as gas bubbles Effervescence - rapid escape of a gas from a liquid in which it is dissolved
68
Effects of Temperature on Solubility
Letrsquos consider gas solubility Increasing temperature usually decreases gas solubility As temp increases average kinetic energy of molecules in solution increases Greater number of solute molecules escape from attraction of solvent and return to gas phase 69
At higher temps eq is reached with fewer gas molecules in solution Gases generally less soluble
70
Effect of temp on solubility of solids in liquids more difficult to predict
Often increasing temp increases solubility
However the same temp increase can result in large increase in solubility in one case and only slight increase in the next
71
72
enthalpies of Solution
Formation of solution comes with energy change Dissolve KI in water container feels cold Dissolve LiCl in water feels hot Formation of solid-liquid solution can absorb or release heat
73
During formation of solution solvent and solute particles experience changes in forces attracting them to other particles Before dissolving begins solvent molecules held by intermolecular forces (solvent-solvent attraction) In solute molecules held by Ifs (solute-solute attraction)
74
Energy is required to separate solute molecules and solvent molecules from their neighbors
A solute particle that is surrounded by solvent molecules is said to be solvated
75
Step 1 solute particles become separated from solid (energy absorbed) Step 2 solvent particles move apart to allow solute particles to enter liquid (energy absorbed)
76
Step 3 solvent particles attracted to and solvate solute particles (energy released)
77
Heat of solution - net amount of heat energy absorbed or released when a specific amount of solute dissolves in a solvent
Heat of solution negative (heat is released) if sums of energy in steps 1 and 2 is less than step 3
If 1+2 gt 3 HOS is positive (heat absorbed)78
In gaseous state molecules are so far apart there are nearly no forces between them Solute-solute interaction has little effect on heat of solution of a gas Energy released when gas dissolved in liquid bc attraction between solute gas and solvent molecules greater than energy needed to separate solvent molecules
79
Concentration of Solutions80
Concentration - measure of amount of solute in given amount of solvent or solution ldquodiluterdquo just means small amount of solute ldquoconcentratedrdquo just means relatively large amount of solute Unrelated to degree to which solution is saturated Ex Saturated solution of substance not very soluble might be dilute
81
Molarity
Molarity - number of moles of solute in one liter of solution To find molarity must know molar mass A ldquoone molarrdquo solution of NaOH is 1 mole NaOH per liter of solution (1 M)
82
1 mol NaOH = 400 g If this quantity dissolve in enough water to make EXACTLY 100 L solution solution is 1 M If 200 g NaOH dissolve in enough to make 100 L solution what is molarity 0500 M
83
1 M solution is not made by adding 1 mol solute to 1 L solvent That would be more than 1 L Instead 1 mol solute dissolved in less than 1 L Then diluted with more solvent to bring TOTAL VOLUME to 1 L
84
Practice ProblemYou have 340 L of solution that contains 900 g sodium chloride What is the molarity of that solution 0440 M NaCl You have 08 L of a 05 M HCl solution How many moles of HCl does this solution contain 04 mol HCl
85
What is the molarity of a solution composed of 585 g of potassium iodide dissolved in enough water to make a 0125 L of solution 0282 M KI
86
How many moles of H2SO4 are present in 0500 L of a 0150 M H2SO4 solution 00750 mol What volume of 300 M NaCl is needed for reaction that requires 1463 g of NaCl 0834 L
87
Molality
Molality - concentration of solution expressed in moles of solute per kilogram of solvent Solution that contains 1 mol solute (NaOH) dissolved in exactly 1 kg of solvent is ldquoone molalrdquo solution (1 m NaOH)
88
1 mol NaOH = 400 g 400 g NaOH dissolved in 1 kg water is 1 m NaOH
200 g NaOH in 1 kg water = 0500 m NaOH
89
Practice Problem
A solution was prepared by dissolving 171 g sucrose (C12H22O11) in 125 g of water Find the molal concentration of this solution 0400 m C12H22O11
90
A solution of iodine in carbon tetrachloride is used when iodine is needed for certain chemical tests How much iodine must be added to prepare a 0480 m solution of iodine in CCl4 if 1000 g of CCl4 is used 122 g I2
91
What is the molality of a solution composed of 255 g acetone (CH3)2CO dissolved in 200 g of water 22 m acetone What quantity in grams of methanol CH3OH is required to prepare a 0244 m solution in 400 g water 312 g methanol
92
How many grams of AgNO3 are needed to prepare a 0125 m solution in 250 mL of water 531 g AgNO3
What is the molality of a solution containing 182 g HCl and 250 g water 200 m
93
Ions in Aqueous Solutions and Colligative Properties
94
95
compounds in aqueous solutions
DissociationWhen compound made from ions dissolves in water ions separate Dissociation - separation of ions that occurs when an ionic compound dissolves
96
Notice number of ions made per formula unit in equations 1 formula unit of NaCl gives 2 units of ions in solution 1 formula unit of CaCl2 gives 3 units of ions
97
Assuming 100 dissociation solution that contains 1 mol NaCl contains 1 mol Na+ and 1 mol Cl- Can assume 100 dissociation of all soluble ionic compounds
98
Practice Problem
Write the equation for the dissolution of aluminum sulfate in water How many moles of aluminum ions are made by dissolving 1 mol aluminum sulfate What is the total number of moles of ions made by dissolving 1 mol of aluminum sulfate
99
1 AnalyzeGiven Amount of solute = 1 mol Al2(SO4)3 Solvent identity = water Unknown a moles of aluminum ions and sulfate ions Total number of moles of solute ions produced
100
2 Plan
The coefficients in the balanced dissociation equation will tell mole relationships You can use the equation to find out the number of moles of solute ions produced
101
3 Compute
a
b102
Write the equation for the dissolution of each of the following in water and then determine the number of moles of each ions made as well as the total number of ions made a 1 mol ammonium chloride b 1 mol sodium sulfide c 05 mol barium nitrate
103
a 1 mol ammonium chloride
1 mol NH4+ 1 mol Cl- 2 mol total ions
104
b 1 mol sodium sulfide
2 mol Na+ 1 mol S-2 3 mol total ions
105
c 05 mol barium nitrate
05 mol Ba+2 1 mol NO3- 15 mol total ions
106
Precipitation Reactions
Even though no compound is completely insoluble compounds of very low solubility can be considered insoluble for practical purposes There are general guidelines to help predict whether a compound made of certain combination of ions is soluble
107
General Solubility Guidelines
Sodium potassium and ammonium compounds are soluble in water
108
Nitrates acetates and chlorates are soluble
109
110
Most chlorides are soluble except those of silver mercury (I) and lead Lead (II) chloride is soluble in hot water
Most sulfates are soluble except those of barium strontium lead calcium and mercury
111
Most carbonates phosphates and silicates are insoluble except those of sodium potassium and ammonium
112
Most sulfides are insoluble except those of calcium strontium sodium potassium and ammonium
113
Is calcium phosphate Ca3(PO4)2 soluble or insoluble
Not one of the exceptions so it is insoluble
Dissociation equations are not written for insoluble compounds
114
Guidelines also useful in predicting what will happen if 2 solutions of 2 different soluble compounds are mixed
If mixing results in combination of ions that forms insoluble compound a double-replacement and precipitation reaction will happen
115
Will precipitate form when solutions of ammonium sulfide and cadmium nitrate are combined You can tell calcium nitrate is soluble from the guidelines (Most nitrates acetates and chlorates are soluble) You can also tell ammonium sulfide is soluble (Most sulfides are insoluble except those of calcium strontium sodium potassium and ammonium)
116
117
To decide whether precipitate can form must know solubilities of two possible products
Ammonium nitrate ndash soluble (Most nitrates acetates and chlorates are soluble) Cadmium sulfide ndash insoluble (Most sulfides are insoluble except those of calcium strontium sodium potassium and ammonium)
118
bc one product is insoluble double-replacement and precipitation reaction will happen (NH4)2S (aq) + Cd(NO3)2 (aq) rarr NH4NO3 (aq) + CdS (s)
119
Net Ionic Equations
Reactions of ions in aqueous solution usually represented by net ionic equations instead of formula equations Net ionic equation - includes only those compounds and ions that undergo a chemical change in a reaction in an aqueous solution
120
121
To write NIE (net ionic equation) first convert chemical equation into overall ionic equation All soluble ionic compounds shown as dissociated ions in solution Precipitates shown as solids
122
Notice ammonium and nitrate ions appear on both sides They havenrsquot gone through any chemical change Spectator ions - ions that do not take part in a chemical reaction and are found in solution both before and after the reaction
123
To change ionic equation to NIE eliminate spectator ions Cd+2(aq) + S-2(aq) rarr CdS(s)
This NIE applies to any reaction in which precipitate of CdS forms
124
Practice Problem
Identify the precipitate that forms when aqueous solutions of zinc nitrate and ammonium sulfide are combined Write the equation for the possible double-replacement reaction Then write the formula equation overall ionic equation and the net ionic equation
125
1 AnalyzeGiven Identity of reactants zinc nitrate and ammonium sulfide Reaction medium aqueous solution Unknown a equation for the possible double-replacement reaction b identity of the precipitate c formula equation d overall ionic equation e net ionic equation
126
2 PlanWrite the possible double-replacement reaction between Zn(NO3)2 and (NH4)2S Use the rules to determine if any of the products will precipitate Write a formula equation and overall net ionic equation Then cancel spectator ions to make net ionic equation
127
3 Computea possible double-replacement reaction
b rules show that zinc sulfide is not soluble so will precipitate (ammonium nitrate is soluble)
c formula equation128
d overall ionic equation
e net ionic equation129
Will a precipitate form if solutions of potassium sulfate and barium nitrate are combined If so write the net ionic equation for the reaction
Yes Ba+2(aq) + SO4-2(aq) rarr BaSO4(s)
130
Will a precipitate form if solutions of potassium nitrate and magnesium sulfate are combined If so write the net ionic equation for the reaction
No 131
Will a precipitate form if solutions of barium chloride and sodium sulfate are combined If so identify the spectator ions and write the net ionic equation for the reaction
Yes Na+ and Cl- Ba+2(aq) + SO4-2(aq) rarr BaSO4(s)
132
Write the net ionic equation for the precipitation of nickel(II) sulfide
Ni+2(aq) + S-2(aq) rarr NiS(s)
133
IonizationSome molecular compounds can also form ions in solution Usually polar Ionization - ions formed from solute molecules by action of the solvent (creation of ions where there were none) Ionization different from dissociation
134
Dissociation ionic compounds dissolve and ions already present separate Ionization ions formed where non existed before
Ions formed are hydrated Heat released during hydration of ions gives enough energy to break covalent bonds
135
Extent to which solute ionizes depends on strength of bonds within molecules of solute and strength of attraction between solute and solvent molecules
136
HCl ndash molecular compound that ionizes in aqueous solution Contains highly polar bond Attraction between polar HCl molecule and polar water strong enough to break HCl bond
137
The Hydronium Ion
H+ ions from HCl attracts other molecules or ions so strongly that it doesnrsquot normally exist alone Ionization of HCl better described as direct transfer of proton from HCl to H2O forming H3O+
138
139
Hydration of H+ to form hydronium is highly exothermic
Energy released gives a lot of energy needed to ionize a molecular solute
Many molecular compounds that ionize in aqueous solution contain hydrogen and form hydronium ion
140
Strong and Weak Electrolytes
Substances that make ions and conduct current in solution are electrolytes HCl is one of series of compounds that has hydrogen and halogen Hydrogen halides are all molecular compounds with single polar-covalent bonds
141
All are gases Very soluble in water All are electrolytes HCl HBr HI strongly conduct current in solution HF only weakly conducts current at same concentration Strength of conduction related to ability to ionize
142
143
Strong Electrolytes
HCl HBr HI are 100 ionized in solution
Strong electrolyte - any compound whose dilute aqueous solutions conduct electricity well this is due to the presence of all or almost all of the dissolve compound in the form of ions
144
Unique characteristic of strong electrolyte is that they yield only ions
Ex Ionic compound may be highly soluble in water and dissociate into ions (NaCl)
Others may not dissolve much but amount that does dissolve exists only as ions
145
Weak ElectrolytesHF dissolves in water to give acid solution called hydrofluoric acid HF bond stronger than bonds between hydrogen and other halogens When HF dissolves some molecule ionize Reverse reaction also happens HF(aq) + H2O(l) H3O+(aq) + F-(aq)
146
HF(aq) + H2O(l) H3O+
(aq) + F-(aq)Concentration of dissolved unionized HF stays high and concentration of ions stays low
Weak electrolyte - any compound whose aqueous solutions conduct electricity poorly this is due to the presence of small amount of the dissolved compound in the form of ions
147
Different from nonelectrolyte ndash NO ions at all Description of electrolyte as strong or weak not related to concentration of solution Electrolytes differ in degree of ionization not amount of solute dissolved
148
149
Colligative properties of solutions
Presence of solute affects properties of solutions Some properties not dependent on nature of dissolved substance but on how may dissolve particles are present
Colligative properties - properties that depend on the concentration of solute particles but not on their identity
150
Vapor-Pressure Lowering
Boiling and freezing point of solution different from pure solvent Nonvolatile solute raises boiling point and lowers freezing point Nonvolatile substance - one that has little tendency to become gas under existing conditions
151
152
To understand why nonvolatile solute changes boiling and freezing point consider equilibrium vapor pressure
Vapor pressure pressure caused by molecules that have escape liquid phase to gas phase
Can be thought of a measure of tendency of molecules to escape from a liquid
153
Addition of sucrose (nonvolatile solute) lowers concentration of water molecules at surface of liquid This lowers tendency of water molecules to leave solution and enter gas phase Vapor pressure of solution is lower than vapor pressure of pure water
154
Aqueous13 solution13 of13 nonvolatile13 solutePure13 water
Nonelectrolyte solutions of same molality have same concentration of particles
Dilute solutions of same solvent and equal molality of any nonelectrolyte solute lower vapor pressure equally
155
Example
1 m aqueous solution of nonelectrolyte glucose lowers vapor pressure of water 55 x 10-4 atm at 25 1 m aqueous solution of sucrose also lowers water vapor pressure to 55 x 10-4 atm at 25
156
157
bc vapor-pressure lowering depends on concentration of nonelectrolyte solute and doesnrsquot depend on the type of solute it is a colligative proprerty
bc vapor pressure has been lowered solution remains liquid over larger temperature range
Lowers freezing point and raises boiling point
158
Can assume changes in boiling and freezing point also depend on concentration of solute
They are colligative properties
159
Freezing-Point Depression
Freezing point of 1-molal solution of any nonelectrolyte solute in water is found (by experiment) to be 186 lower than freezing point of water When 1 mol of nonelectrolyte solute dissolved in 1 kg water freezing point is -186 not 000 160
When 2 mol nonelectrolyte solute dissolved in 1 kg water freezing point is -372
For any concentration of nonelectrolyte solute in water decrease in freezing point can be determined using value of -186m
161
This value called the molal freezing-point contstant (Kf) is the freezing-point depression of the solvent in a 1-molal solution of a nonvolatile nonelectrolyte solute
162
Each solvent has own characteristic molal freezing-point constant Freezing-point depression (Δtf) the difference between the freezing points of the pure solvent and a solute of nonelectrolyte in that solvent and it is directly proportional to the molal concentration of the solution Δtf = Kfm
163
Δtf = Kfm
Kf expressed as m
m expressed in mol solutekg solvent (molality)
164
Sample Problem
What is the freezing point depression of water in a solution of 171 g of sucrose C12H22O11 and 200 g of water What is the actual freezing point of the solution
165
1 AnalyzeGiven Solute mass and chemical formula 171 g C12H22O11 Solvent mass and identity 2000 g water Unknown a freezing-point depression b freezing point of the solution 166
2 Plan
167
168
Calculate
Unknown a freezing-point depression b freezing point of the solution
169
Practice Problem
A water solution containing an unknown quantity of a nonelectrolyte solute is found to have a freezing point of ndash 023degCWhat is the molal concentration of the solution
012 m170
A solution consists of 103 g of the nonelectrolyte glucose C6H12O6 dissolved in 250 g of water What is the freezing-point depression of the solution
minus0426degC171
In a laboratory experiment the freezing point of an aqueous solution of glucose is found to be minus0325degCWhat is the molal concentration of this solution
0175 m172
If 0500 mol of a nonelectrolyte solute are dissolved in 5000 g of ether what is the freezing point of the solution
minus1181degC173
The freezing point of an aqueous solution that contains a nonelectrolyte is minus90degC aWhat is the freezing-point depression of the solution bWhat is the molal concentration of the solution a minus90degC b 48 m
174
Boiling-Point Elevation
Boiling point is temp at which vapor pressure is equal to atmospheric pressure Change in vapor pressure will cause corresponding change in boiling point
175
176
Vapor pressure of nonvolatile solution is lower than vapor pressure of pure solvent More heat will be required to raise vapor pressure of solution Boiling point of solution is higher than boiling point of pure solvent
Molal boiling-point constant (Kb) - boiling-point elevation of the solvent in a 1-molal solution of a nonvolatile nonelectrolyte solute b-p elevation of any nonelectrolyte in water found by experiment to be 051 So molal b-p constant for water is 051m
177
For different solvents b-p elevations of 1-molal solutions have different values (Table 14-2)
Like freezing-point constants values most accurate for dilute solutions
178
Boiling-point elevation Δtb difference between the boiling points of the pure solvent and a nonelectrolyte solution of that solvent and it is directly proportional to the molal concentration of the solution B-p elevation can be calculated using Δtb = Kbm Δtb (boiling point elevation) expressed in m and m expressed in mol solutekg solvent
Sample Problem
What is the boiling-point elevation of a solution made from 200 g of a nonelectrolyte solute and 4000 g of water The molar mass of the solute is 620 gmol
180
1 AnalyzeGiven solute mass = 200 g solute molar mass = 620 gmol solvent mass and identity = 4000 g of water
Unknown boiling-point elevation
181
2 Plan
182
183
3 Compute
Practice Problem
A solution contains 500 g of sucrose C12H22O11 a nonelectrolyte dissolved in 5000 g of water What is the boiling-point elevation
015degC184
A solution contains 4500 g of sucrose C12H22O11 a nonelectrolyte dissolved in 250 g of waterWhat is the boiling point of the solution
1027degC185
If the boiling point elevation of an aqueous solution containing a nonvolatile electrolyte is 102degC what is the molality of the solution
20 m186
The boiling point of an aqueous solution containing a nonvolatile electrolyte is 10075degC aWhat is the boiling-point elevation bWhat is the molality of the solution a 075degC b 15 m
187
Electrolytes and Colligative PropertiesEarly investigators confused by experiments where certain substances depressed freezing point or elevated boiling point more than expected
188
189
Ex 01 m solution of NaCl lowers freezing point of solvent almost twice as much as 01 m solution of sucrose 01 m CaCl2 lowers fp almost 3 times as much as 01 m solution of sucrose
To understand why this happens contrast behavior or sucrose with NaCl in aqueous solution Each sucrose dissolves to make only 1 particle in solution So 1 mol sucrose dissolves to 1 mol particles 1 mol NaCl dissolves to make 2 moles of particles (1 mol Na+ and 1 mol Cl-)
190
191
Calculated Values for Electrolyte SolutionsColligative properties depend on total concentration of solute particles regardless of their identity Electrolytes cause changes in colligative properties proportional to total molality in terms of all dissolves particles instead of formula units
192
What about barium nitrate Ba(NO3)2 Each mole of barium nitrate yields 3 mol of ions in solution Ba(NO3)2(s) rarr Ba2+(aq) + 2NO3-(aq) You would expect solution of given molality to lower fp of solvent 3 times as much as nonelectrolytic solution of same molality
193
Sample Problem
What is the expected change in the freezing point of water in a solution of 625 g of barium nitrate Ba(NO3)2 in 100 kg of water
194
1 AnalyzeGiven solute mass and formula = 625 g Ba(NO3)2 solvent mass and identity = 100 kg water Δtf = Kfm
Unknown expected freezing-point depression
195
2 PlanThe molality can be calculated by converting the solute mass to moles and then dividing by the number of kilograms of solvent That molality is in terms of formula units of Ba(NO3)2 and must be converted to molality in terms of dissociated ions in solution It must be multiplied by the number of moles of ions produced per mole of formula unit This adjusted molality can then be used to calculate the freezing-point depression
196
197
198
3 Compute
Practice Problem
What is the expected freezing-point depression for a solution that contains 20 mol of magnesium sulfate dissolved in 10 kg of water
minus74degC199
What is the expected boiling-point elevation of water for a solution that contains 150 g of sodium chloride dissolved in 10 kg of water 27degC The freezing point of an aqueous sodium chloride solution is minus200degC What is the molality of the solution 54 m NaCl
200
Actual Values for Electrolyte SolutionsRemember values calculated are only expected values Actual values of colligative properties for all strong electrolytes are almost what would be expected Difference between expected and actual values caused by attractive forces that exist between dissociated ions in aqueous solution
201
Attraction between hydrated ions in solution is small compared with those in crystalline solid However forces of attraction donrsquot interfere with movements of aqueous ions More concentrated solution means ions closer together and attraction is greater Also depends on charge of ions ndash higher charge greater attraction
202
Supersaturated Solutions
When saturated solution where solubility increases as temp increases is cooled excess solute usually comes out of solution Sometimes if solution is left to cool the excess solute doesnrsquot separate
38
39
Supersaturated solution - solution that contains more dissolved solute than a saturated solution contains under same conditions
40
Supersaturated solution can remain unchanged over long time if it isnrsquot disturbed Once crystals begin to form continues until equilibrium is recreated at lower temp
41
Example
Sodium thiosulfate Na2S2O3 Solute added to hot water until solution saturated Hot solution filtered Drop small crystal in ldquoseedingrdquo crystallization Continues until equilibrium created
42
Solubility Values
Solubility - amount of substance required to form a saturated solution with specific amount of solvent at specified temp Ex Solubility of sugar = 204 g per 100 g water at 20 Must specify temp bc solubility changes with temp 43
For gases pressure must also be specified Rate at which substance dissolves is unrelated to solubility Max amount of solute that dissolves and reaches equilibrium is always same under same conditions
44
Solute-Solvent Interactions
ldquolike dissolves likerdquo is generally useful rule for predicting whether one substance will dissolve in another Depends on type of bonding polaritynonpolarity and intermolecular forces between solute and solvent
45
Dissolving Ionic Compounds in Aqueous Solution
Polarity of water molecules plays important role in formation of solution of ionic compounds Charged ends of water molecules attract ions and surround them to keep separated from other ions
46
Hydration - solution process with water as solvent Ions said to be hydrated As hydrated ions diffuse into solution other ions exposed and drawn away from crystal surface Entire crystal gradually dissolves
47
When crystallized from aqueous solutions some ionic compounds form crystals that include water molecules The crystalline compounds known as hydrates have specific ratios of water molecules Represented by formulas like CuSO45H2O
48
When crystalline hydrate dissolves in water water of hydration returns to solvent Behavior of solute in hydrated form no different than anhydrous form
49
Nonpolar Solvents
Nonpolar solvents do not attract ions of ionic compound strongly enough to overcome forces holding crystal together
50
Liquid Solutes and Solvents
When you shake bottle of salad dressing oil droplets disperse in water Stop shaking ndash strong attraction between water molecules squeeze out oil and form separate layers Liquid solutes and solvents that are not soluble in each other are immiscible
51
Nonpolar substances (fat oil grease) are generally quite soluble in nonpolar liquids (carbon tetrachloride toluene gasoline)
Only attractions between nonpolar molecules are weak London forces
52
London Dispersion Forces
bc e- are constantly moving at any time there may be uneven distribution of them around a nonpolar atom Temporarily creates positive end and negative end
53
54
London dispersion forces - intermolecular attractions resulting from constant motion of e- and creation of instantaneous dipoles
55
Liquids that dissolve freely in one another in any ratio are said to be completely miscible Benzene and carbon tetrachloride (both nonpolar) are completely miscible
Nonpolar molecules of these two apply no strong forces of attraction or repulsion and molecules mix freely
56
Ethanol and water are also completely miscible -OH group on ethanol is slightly polar
Can form H-bonds with water as well as other ethanol molecules Intermolecular forces in mixture are very similar to those in pure liquids so they are mutually soluble in any amount
57
Gasoline contains mainly nonpolar hydrocarbons
Excellent solvent for fats oils greases
Major intermolecular forces are weak London forces
58
Ethanol less polar than water more than carbon tetrachloride
Better solvent for less-polar substances bc of nonpolar region
59
Effects of Pressure on Solubility
Little effect on solids or liquids Increase pressure - increase solubilities of gases in liquids
60
When gas in contact with surface of liquid gas molecules can enter liquid As amount of dissolve gas increases some molecules start to escape and reenter gas phase
Eq eventually established between rates of enteringleaving gas phase
61
As long as eq undisturbed solubility of gas in liquid unchanged at given pressure
Gas + solvent solution
62
Increasing pressure of solute gas above solution puts stress on eq Molecules collide with surface more often Increase in pressure partially offset by increase in rate of gas molecules entering solution In turn increase in amount of dissolved gas causes increase in rate molecules escape liquid and becomes gas
63
Eventually eq restored at higher gas solubility
Expected from Le Chatelierrsquos principle
Increase in gas pressure causes eq to shift so fewer molecules are in gas phase
64
Henryrsquos Law
Henryrsquos law - solubility of a gas in liquid is directly proportional to the partial pressure of that gas on the surface of the liquid
Applies to gas-liquid solutions at constant temp
65
When mixture of ideal gases is limited in constant volume at constant temp each gas applies same pressure it would apply if it occupied the space alone
Assuming gases do not react each gas dissolves to the extent it would if no other gases were there
66
In carbonated drinks (Coke Pepsi etc) solubility of CO2 increased by increasing pressure
At bottling factory CO2 gas forced into solution of flavored water at pressure of 5-10 atm Gas-in-liquid solution then sealed in bottlescans
67
When cap removed pressure reduced to 1 atm and some CO2 escapes as gas bubbles Effervescence - rapid escape of a gas from a liquid in which it is dissolved
68
Effects of Temperature on Solubility
Letrsquos consider gas solubility Increasing temperature usually decreases gas solubility As temp increases average kinetic energy of molecules in solution increases Greater number of solute molecules escape from attraction of solvent and return to gas phase 69
At higher temps eq is reached with fewer gas molecules in solution Gases generally less soluble
70
Effect of temp on solubility of solids in liquids more difficult to predict
Often increasing temp increases solubility
However the same temp increase can result in large increase in solubility in one case and only slight increase in the next
71
72
enthalpies of Solution
Formation of solution comes with energy change Dissolve KI in water container feels cold Dissolve LiCl in water feels hot Formation of solid-liquid solution can absorb or release heat
73
During formation of solution solvent and solute particles experience changes in forces attracting them to other particles Before dissolving begins solvent molecules held by intermolecular forces (solvent-solvent attraction) In solute molecules held by Ifs (solute-solute attraction)
74
Energy is required to separate solute molecules and solvent molecules from their neighbors
A solute particle that is surrounded by solvent molecules is said to be solvated
75
Step 1 solute particles become separated from solid (energy absorbed) Step 2 solvent particles move apart to allow solute particles to enter liquid (energy absorbed)
76
Step 3 solvent particles attracted to and solvate solute particles (energy released)
77
Heat of solution - net amount of heat energy absorbed or released when a specific amount of solute dissolves in a solvent
Heat of solution negative (heat is released) if sums of energy in steps 1 and 2 is less than step 3
If 1+2 gt 3 HOS is positive (heat absorbed)78
In gaseous state molecules are so far apart there are nearly no forces between them Solute-solute interaction has little effect on heat of solution of a gas Energy released when gas dissolved in liquid bc attraction between solute gas and solvent molecules greater than energy needed to separate solvent molecules
79
Concentration of Solutions80
Concentration - measure of amount of solute in given amount of solvent or solution ldquodiluterdquo just means small amount of solute ldquoconcentratedrdquo just means relatively large amount of solute Unrelated to degree to which solution is saturated Ex Saturated solution of substance not very soluble might be dilute
81
Molarity
Molarity - number of moles of solute in one liter of solution To find molarity must know molar mass A ldquoone molarrdquo solution of NaOH is 1 mole NaOH per liter of solution (1 M)
82
1 mol NaOH = 400 g If this quantity dissolve in enough water to make EXACTLY 100 L solution solution is 1 M If 200 g NaOH dissolve in enough to make 100 L solution what is molarity 0500 M
83
1 M solution is not made by adding 1 mol solute to 1 L solvent That would be more than 1 L Instead 1 mol solute dissolved in less than 1 L Then diluted with more solvent to bring TOTAL VOLUME to 1 L
84
Practice ProblemYou have 340 L of solution that contains 900 g sodium chloride What is the molarity of that solution 0440 M NaCl You have 08 L of a 05 M HCl solution How many moles of HCl does this solution contain 04 mol HCl
85
What is the molarity of a solution composed of 585 g of potassium iodide dissolved in enough water to make a 0125 L of solution 0282 M KI
86
How many moles of H2SO4 are present in 0500 L of a 0150 M H2SO4 solution 00750 mol What volume of 300 M NaCl is needed for reaction that requires 1463 g of NaCl 0834 L
87
Molality
Molality - concentration of solution expressed in moles of solute per kilogram of solvent Solution that contains 1 mol solute (NaOH) dissolved in exactly 1 kg of solvent is ldquoone molalrdquo solution (1 m NaOH)
88
1 mol NaOH = 400 g 400 g NaOH dissolved in 1 kg water is 1 m NaOH
200 g NaOH in 1 kg water = 0500 m NaOH
89
Practice Problem
A solution was prepared by dissolving 171 g sucrose (C12H22O11) in 125 g of water Find the molal concentration of this solution 0400 m C12H22O11
90
A solution of iodine in carbon tetrachloride is used when iodine is needed for certain chemical tests How much iodine must be added to prepare a 0480 m solution of iodine in CCl4 if 1000 g of CCl4 is used 122 g I2
91
What is the molality of a solution composed of 255 g acetone (CH3)2CO dissolved in 200 g of water 22 m acetone What quantity in grams of methanol CH3OH is required to prepare a 0244 m solution in 400 g water 312 g methanol
92
How many grams of AgNO3 are needed to prepare a 0125 m solution in 250 mL of water 531 g AgNO3
What is the molality of a solution containing 182 g HCl and 250 g water 200 m
93
Ions in Aqueous Solutions and Colligative Properties
94
95
compounds in aqueous solutions
DissociationWhen compound made from ions dissolves in water ions separate Dissociation - separation of ions that occurs when an ionic compound dissolves
96
Notice number of ions made per formula unit in equations 1 formula unit of NaCl gives 2 units of ions in solution 1 formula unit of CaCl2 gives 3 units of ions
97
Assuming 100 dissociation solution that contains 1 mol NaCl contains 1 mol Na+ and 1 mol Cl- Can assume 100 dissociation of all soluble ionic compounds
98
Practice Problem
Write the equation for the dissolution of aluminum sulfate in water How many moles of aluminum ions are made by dissolving 1 mol aluminum sulfate What is the total number of moles of ions made by dissolving 1 mol of aluminum sulfate
99
1 AnalyzeGiven Amount of solute = 1 mol Al2(SO4)3 Solvent identity = water Unknown a moles of aluminum ions and sulfate ions Total number of moles of solute ions produced
100
2 Plan
The coefficients in the balanced dissociation equation will tell mole relationships You can use the equation to find out the number of moles of solute ions produced
101
3 Compute
a
b102
Write the equation for the dissolution of each of the following in water and then determine the number of moles of each ions made as well as the total number of ions made a 1 mol ammonium chloride b 1 mol sodium sulfide c 05 mol barium nitrate
103
a 1 mol ammonium chloride
1 mol NH4+ 1 mol Cl- 2 mol total ions
104
b 1 mol sodium sulfide
2 mol Na+ 1 mol S-2 3 mol total ions
105
c 05 mol barium nitrate
05 mol Ba+2 1 mol NO3- 15 mol total ions
106
Precipitation Reactions
Even though no compound is completely insoluble compounds of very low solubility can be considered insoluble for practical purposes There are general guidelines to help predict whether a compound made of certain combination of ions is soluble
107
General Solubility Guidelines
Sodium potassium and ammonium compounds are soluble in water
108
Nitrates acetates and chlorates are soluble
109
110
Most chlorides are soluble except those of silver mercury (I) and lead Lead (II) chloride is soluble in hot water
Most sulfates are soluble except those of barium strontium lead calcium and mercury
111
Most carbonates phosphates and silicates are insoluble except those of sodium potassium and ammonium
112
Most sulfides are insoluble except those of calcium strontium sodium potassium and ammonium
113
Is calcium phosphate Ca3(PO4)2 soluble or insoluble
Not one of the exceptions so it is insoluble
Dissociation equations are not written for insoluble compounds
114
Guidelines also useful in predicting what will happen if 2 solutions of 2 different soluble compounds are mixed
If mixing results in combination of ions that forms insoluble compound a double-replacement and precipitation reaction will happen
115
Will precipitate form when solutions of ammonium sulfide and cadmium nitrate are combined You can tell calcium nitrate is soluble from the guidelines (Most nitrates acetates and chlorates are soluble) You can also tell ammonium sulfide is soluble (Most sulfides are insoluble except those of calcium strontium sodium potassium and ammonium)
116
117
To decide whether precipitate can form must know solubilities of two possible products
Ammonium nitrate ndash soluble (Most nitrates acetates and chlorates are soluble) Cadmium sulfide ndash insoluble (Most sulfides are insoluble except those of calcium strontium sodium potassium and ammonium)
118
bc one product is insoluble double-replacement and precipitation reaction will happen (NH4)2S (aq) + Cd(NO3)2 (aq) rarr NH4NO3 (aq) + CdS (s)
119
Net Ionic Equations
Reactions of ions in aqueous solution usually represented by net ionic equations instead of formula equations Net ionic equation - includes only those compounds and ions that undergo a chemical change in a reaction in an aqueous solution
120
121
To write NIE (net ionic equation) first convert chemical equation into overall ionic equation All soluble ionic compounds shown as dissociated ions in solution Precipitates shown as solids
122
Notice ammonium and nitrate ions appear on both sides They havenrsquot gone through any chemical change Spectator ions - ions that do not take part in a chemical reaction and are found in solution both before and after the reaction
123
To change ionic equation to NIE eliminate spectator ions Cd+2(aq) + S-2(aq) rarr CdS(s)
This NIE applies to any reaction in which precipitate of CdS forms
124
Practice Problem
Identify the precipitate that forms when aqueous solutions of zinc nitrate and ammonium sulfide are combined Write the equation for the possible double-replacement reaction Then write the formula equation overall ionic equation and the net ionic equation
125
1 AnalyzeGiven Identity of reactants zinc nitrate and ammonium sulfide Reaction medium aqueous solution Unknown a equation for the possible double-replacement reaction b identity of the precipitate c formula equation d overall ionic equation e net ionic equation
126
2 PlanWrite the possible double-replacement reaction between Zn(NO3)2 and (NH4)2S Use the rules to determine if any of the products will precipitate Write a formula equation and overall net ionic equation Then cancel spectator ions to make net ionic equation
127
3 Computea possible double-replacement reaction
b rules show that zinc sulfide is not soluble so will precipitate (ammonium nitrate is soluble)
c formula equation128
d overall ionic equation
e net ionic equation129
Will a precipitate form if solutions of potassium sulfate and barium nitrate are combined If so write the net ionic equation for the reaction
Yes Ba+2(aq) + SO4-2(aq) rarr BaSO4(s)
130
Will a precipitate form if solutions of potassium nitrate and magnesium sulfate are combined If so write the net ionic equation for the reaction
No 131
Will a precipitate form if solutions of barium chloride and sodium sulfate are combined If so identify the spectator ions and write the net ionic equation for the reaction
Yes Na+ and Cl- Ba+2(aq) + SO4-2(aq) rarr BaSO4(s)
132
Write the net ionic equation for the precipitation of nickel(II) sulfide
Ni+2(aq) + S-2(aq) rarr NiS(s)
133
IonizationSome molecular compounds can also form ions in solution Usually polar Ionization - ions formed from solute molecules by action of the solvent (creation of ions where there were none) Ionization different from dissociation
134
Dissociation ionic compounds dissolve and ions already present separate Ionization ions formed where non existed before
Ions formed are hydrated Heat released during hydration of ions gives enough energy to break covalent bonds
135
Extent to which solute ionizes depends on strength of bonds within molecules of solute and strength of attraction between solute and solvent molecules
136
HCl ndash molecular compound that ionizes in aqueous solution Contains highly polar bond Attraction between polar HCl molecule and polar water strong enough to break HCl bond
137
The Hydronium Ion
H+ ions from HCl attracts other molecules or ions so strongly that it doesnrsquot normally exist alone Ionization of HCl better described as direct transfer of proton from HCl to H2O forming H3O+
138
139
Hydration of H+ to form hydronium is highly exothermic
Energy released gives a lot of energy needed to ionize a molecular solute
Many molecular compounds that ionize in aqueous solution contain hydrogen and form hydronium ion
140
Strong and Weak Electrolytes
Substances that make ions and conduct current in solution are electrolytes HCl is one of series of compounds that has hydrogen and halogen Hydrogen halides are all molecular compounds with single polar-covalent bonds
141
All are gases Very soluble in water All are electrolytes HCl HBr HI strongly conduct current in solution HF only weakly conducts current at same concentration Strength of conduction related to ability to ionize
142
143
Strong Electrolytes
HCl HBr HI are 100 ionized in solution
Strong electrolyte - any compound whose dilute aqueous solutions conduct electricity well this is due to the presence of all or almost all of the dissolve compound in the form of ions
144
Unique characteristic of strong electrolyte is that they yield only ions
Ex Ionic compound may be highly soluble in water and dissociate into ions (NaCl)
Others may not dissolve much but amount that does dissolve exists only as ions
145
Weak ElectrolytesHF dissolves in water to give acid solution called hydrofluoric acid HF bond stronger than bonds between hydrogen and other halogens When HF dissolves some molecule ionize Reverse reaction also happens HF(aq) + H2O(l) H3O+(aq) + F-(aq)
146
HF(aq) + H2O(l) H3O+
(aq) + F-(aq)Concentration of dissolved unionized HF stays high and concentration of ions stays low
Weak electrolyte - any compound whose aqueous solutions conduct electricity poorly this is due to the presence of small amount of the dissolved compound in the form of ions
147
Different from nonelectrolyte ndash NO ions at all Description of electrolyte as strong or weak not related to concentration of solution Electrolytes differ in degree of ionization not amount of solute dissolved
148
149
Colligative properties of solutions
Presence of solute affects properties of solutions Some properties not dependent on nature of dissolved substance but on how may dissolve particles are present
Colligative properties - properties that depend on the concentration of solute particles but not on their identity
150
Vapor-Pressure Lowering
Boiling and freezing point of solution different from pure solvent Nonvolatile solute raises boiling point and lowers freezing point Nonvolatile substance - one that has little tendency to become gas under existing conditions
151
152
To understand why nonvolatile solute changes boiling and freezing point consider equilibrium vapor pressure
Vapor pressure pressure caused by molecules that have escape liquid phase to gas phase
Can be thought of a measure of tendency of molecules to escape from a liquid
153
Addition of sucrose (nonvolatile solute) lowers concentration of water molecules at surface of liquid This lowers tendency of water molecules to leave solution and enter gas phase Vapor pressure of solution is lower than vapor pressure of pure water
154
Aqueous13 solution13 of13 nonvolatile13 solutePure13 water
Nonelectrolyte solutions of same molality have same concentration of particles
Dilute solutions of same solvent and equal molality of any nonelectrolyte solute lower vapor pressure equally
155
Example
1 m aqueous solution of nonelectrolyte glucose lowers vapor pressure of water 55 x 10-4 atm at 25 1 m aqueous solution of sucrose also lowers water vapor pressure to 55 x 10-4 atm at 25
156
157
bc vapor-pressure lowering depends on concentration of nonelectrolyte solute and doesnrsquot depend on the type of solute it is a colligative proprerty
bc vapor pressure has been lowered solution remains liquid over larger temperature range
Lowers freezing point and raises boiling point
158
Can assume changes in boiling and freezing point also depend on concentration of solute
They are colligative properties
159
Freezing-Point Depression
Freezing point of 1-molal solution of any nonelectrolyte solute in water is found (by experiment) to be 186 lower than freezing point of water When 1 mol of nonelectrolyte solute dissolved in 1 kg water freezing point is -186 not 000 160
When 2 mol nonelectrolyte solute dissolved in 1 kg water freezing point is -372
For any concentration of nonelectrolyte solute in water decrease in freezing point can be determined using value of -186m
161
This value called the molal freezing-point contstant (Kf) is the freezing-point depression of the solvent in a 1-molal solution of a nonvolatile nonelectrolyte solute
162
Each solvent has own characteristic molal freezing-point constant Freezing-point depression (Δtf) the difference between the freezing points of the pure solvent and a solute of nonelectrolyte in that solvent and it is directly proportional to the molal concentration of the solution Δtf = Kfm
163
Δtf = Kfm
Kf expressed as m
m expressed in mol solutekg solvent (molality)
164
Sample Problem
What is the freezing point depression of water in a solution of 171 g of sucrose C12H22O11 and 200 g of water What is the actual freezing point of the solution
165
1 AnalyzeGiven Solute mass and chemical formula 171 g C12H22O11 Solvent mass and identity 2000 g water Unknown a freezing-point depression b freezing point of the solution 166
2 Plan
167
168
Calculate
Unknown a freezing-point depression b freezing point of the solution
169
Practice Problem
A water solution containing an unknown quantity of a nonelectrolyte solute is found to have a freezing point of ndash 023degCWhat is the molal concentration of the solution
012 m170
A solution consists of 103 g of the nonelectrolyte glucose C6H12O6 dissolved in 250 g of water What is the freezing-point depression of the solution
minus0426degC171
In a laboratory experiment the freezing point of an aqueous solution of glucose is found to be minus0325degCWhat is the molal concentration of this solution
0175 m172
If 0500 mol of a nonelectrolyte solute are dissolved in 5000 g of ether what is the freezing point of the solution
minus1181degC173
The freezing point of an aqueous solution that contains a nonelectrolyte is minus90degC aWhat is the freezing-point depression of the solution bWhat is the molal concentration of the solution a minus90degC b 48 m
174
Boiling-Point Elevation
Boiling point is temp at which vapor pressure is equal to atmospheric pressure Change in vapor pressure will cause corresponding change in boiling point
175
176
Vapor pressure of nonvolatile solution is lower than vapor pressure of pure solvent More heat will be required to raise vapor pressure of solution Boiling point of solution is higher than boiling point of pure solvent
Molal boiling-point constant (Kb) - boiling-point elevation of the solvent in a 1-molal solution of a nonvolatile nonelectrolyte solute b-p elevation of any nonelectrolyte in water found by experiment to be 051 So molal b-p constant for water is 051m
177
For different solvents b-p elevations of 1-molal solutions have different values (Table 14-2)
Like freezing-point constants values most accurate for dilute solutions
178
Boiling-point elevation Δtb difference between the boiling points of the pure solvent and a nonelectrolyte solution of that solvent and it is directly proportional to the molal concentration of the solution B-p elevation can be calculated using Δtb = Kbm Δtb (boiling point elevation) expressed in m and m expressed in mol solutekg solvent
Sample Problem
What is the boiling-point elevation of a solution made from 200 g of a nonelectrolyte solute and 4000 g of water The molar mass of the solute is 620 gmol
180
1 AnalyzeGiven solute mass = 200 g solute molar mass = 620 gmol solvent mass and identity = 4000 g of water
Unknown boiling-point elevation
181
2 Plan
182
183
3 Compute
Practice Problem
A solution contains 500 g of sucrose C12H22O11 a nonelectrolyte dissolved in 5000 g of water What is the boiling-point elevation
015degC184
A solution contains 4500 g of sucrose C12H22O11 a nonelectrolyte dissolved in 250 g of waterWhat is the boiling point of the solution
1027degC185
If the boiling point elevation of an aqueous solution containing a nonvolatile electrolyte is 102degC what is the molality of the solution
20 m186
The boiling point of an aqueous solution containing a nonvolatile electrolyte is 10075degC aWhat is the boiling-point elevation bWhat is the molality of the solution a 075degC b 15 m
187
Electrolytes and Colligative PropertiesEarly investigators confused by experiments where certain substances depressed freezing point or elevated boiling point more than expected
188
189
Ex 01 m solution of NaCl lowers freezing point of solvent almost twice as much as 01 m solution of sucrose 01 m CaCl2 lowers fp almost 3 times as much as 01 m solution of sucrose
To understand why this happens contrast behavior or sucrose with NaCl in aqueous solution Each sucrose dissolves to make only 1 particle in solution So 1 mol sucrose dissolves to 1 mol particles 1 mol NaCl dissolves to make 2 moles of particles (1 mol Na+ and 1 mol Cl-)
190
191
Calculated Values for Electrolyte SolutionsColligative properties depend on total concentration of solute particles regardless of their identity Electrolytes cause changes in colligative properties proportional to total molality in terms of all dissolves particles instead of formula units
192
What about barium nitrate Ba(NO3)2 Each mole of barium nitrate yields 3 mol of ions in solution Ba(NO3)2(s) rarr Ba2+(aq) + 2NO3-(aq) You would expect solution of given molality to lower fp of solvent 3 times as much as nonelectrolytic solution of same molality
193
Sample Problem
What is the expected change in the freezing point of water in a solution of 625 g of barium nitrate Ba(NO3)2 in 100 kg of water
194
1 AnalyzeGiven solute mass and formula = 625 g Ba(NO3)2 solvent mass and identity = 100 kg water Δtf = Kfm
Unknown expected freezing-point depression
195
2 PlanThe molality can be calculated by converting the solute mass to moles and then dividing by the number of kilograms of solvent That molality is in terms of formula units of Ba(NO3)2 and must be converted to molality in terms of dissociated ions in solution It must be multiplied by the number of moles of ions produced per mole of formula unit This adjusted molality can then be used to calculate the freezing-point depression
196
197
198
3 Compute
Practice Problem
What is the expected freezing-point depression for a solution that contains 20 mol of magnesium sulfate dissolved in 10 kg of water
minus74degC199
What is the expected boiling-point elevation of water for a solution that contains 150 g of sodium chloride dissolved in 10 kg of water 27degC The freezing point of an aqueous sodium chloride solution is minus200degC What is the molality of the solution 54 m NaCl
200
Actual Values for Electrolyte SolutionsRemember values calculated are only expected values Actual values of colligative properties for all strong electrolytes are almost what would be expected Difference between expected and actual values caused by attractive forces that exist between dissociated ions in aqueous solution
201
Attraction between hydrated ions in solution is small compared with those in crystalline solid However forces of attraction donrsquot interfere with movements of aqueous ions More concentrated solution means ions closer together and attraction is greater Also depends on charge of ions ndash higher charge greater attraction
202
39
Supersaturated solution - solution that contains more dissolved solute than a saturated solution contains under same conditions
40
Supersaturated solution can remain unchanged over long time if it isnrsquot disturbed Once crystals begin to form continues until equilibrium is recreated at lower temp
41
Example
Sodium thiosulfate Na2S2O3 Solute added to hot water until solution saturated Hot solution filtered Drop small crystal in ldquoseedingrdquo crystallization Continues until equilibrium created
42
Solubility Values
Solubility - amount of substance required to form a saturated solution with specific amount of solvent at specified temp Ex Solubility of sugar = 204 g per 100 g water at 20 Must specify temp bc solubility changes with temp 43
For gases pressure must also be specified Rate at which substance dissolves is unrelated to solubility Max amount of solute that dissolves and reaches equilibrium is always same under same conditions
44
Solute-Solvent Interactions
ldquolike dissolves likerdquo is generally useful rule for predicting whether one substance will dissolve in another Depends on type of bonding polaritynonpolarity and intermolecular forces between solute and solvent
45
Dissolving Ionic Compounds in Aqueous Solution
Polarity of water molecules plays important role in formation of solution of ionic compounds Charged ends of water molecules attract ions and surround them to keep separated from other ions
46
Hydration - solution process with water as solvent Ions said to be hydrated As hydrated ions diffuse into solution other ions exposed and drawn away from crystal surface Entire crystal gradually dissolves
47
When crystallized from aqueous solutions some ionic compounds form crystals that include water molecules The crystalline compounds known as hydrates have specific ratios of water molecules Represented by formulas like CuSO45H2O
48
When crystalline hydrate dissolves in water water of hydration returns to solvent Behavior of solute in hydrated form no different than anhydrous form
49
Nonpolar Solvents
Nonpolar solvents do not attract ions of ionic compound strongly enough to overcome forces holding crystal together
50
Liquid Solutes and Solvents
When you shake bottle of salad dressing oil droplets disperse in water Stop shaking ndash strong attraction between water molecules squeeze out oil and form separate layers Liquid solutes and solvents that are not soluble in each other are immiscible
51
Nonpolar substances (fat oil grease) are generally quite soluble in nonpolar liquids (carbon tetrachloride toluene gasoline)
Only attractions between nonpolar molecules are weak London forces
52
London Dispersion Forces
bc e- are constantly moving at any time there may be uneven distribution of them around a nonpolar atom Temporarily creates positive end and negative end
53
54
London dispersion forces - intermolecular attractions resulting from constant motion of e- and creation of instantaneous dipoles
55
Liquids that dissolve freely in one another in any ratio are said to be completely miscible Benzene and carbon tetrachloride (both nonpolar) are completely miscible
Nonpolar molecules of these two apply no strong forces of attraction or repulsion and molecules mix freely
56
Ethanol and water are also completely miscible -OH group on ethanol is slightly polar
Can form H-bonds with water as well as other ethanol molecules Intermolecular forces in mixture are very similar to those in pure liquids so they are mutually soluble in any amount
57
Gasoline contains mainly nonpolar hydrocarbons
Excellent solvent for fats oils greases
Major intermolecular forces are weak London forces
58
Ethanol less polar than water more than carbon tetrachloride
Better solvent for less-polar substances bc of nonpolar region
59
Effects of Pressure on Solubility
Little effect on solids or liquids Increase pressure - increase solubilities of gases in liquids
60
When gas in contact with surface of liquid gas molecules can enter liquid As amount of dissolve gas increases some molecules start to escape and reenter gas phase
Eq eventually established between rates of enteringleaving gas phase
61
As long as eq undisturbed solubility of gas in liquid unchanged at given pressure
Gas + solvent solution
62
Increasing pressure of solute gas above solution puts stress on eq Molecules collide with surface more often Increase in pressure partially offset by increase in rate of gas molecules entering solution In turn increase in amount of dissolved gas causes increase in rate molecules escape liquid and becomes gas
63
Eventually eq restored at higher gas solubility
Expected from Le Chatelierrsquos principle
Increase in gas pressure causes eq to shift so fewer molecules are in gas phase
64
Henryrsquos Law
Henryrsquos law - solubility of a gas in liquid is directly proportional to the partial pressure of that gas on the surface of the liquid
Applies to gas-liquid solutions at constant temp
65
When mixture of ideal gases is limited in constant volume at constant temp each gas applies same pressure it would apply if it occupied the space alone
Assuming gases do not react each gas dissolves to the extent it would if no other gases were there
66
In carbonated drinks (Coke Pepsi etc) solubility of CO2 increased by increasing pressure
At bottling factory CO2 gas forced into solution of flavored water at pressure of 5-10 atm Gas-in-liquid solution then sealed in bottlescans
67
When cap removed pressure reduced to 1 atm and some CO2 escapes as gas bubbles Effervescence - rapid escape of a gas from a liquid in which it is dissolved
68
Effects of Temperature on Solubility
Letrsquos consider gas solubility Increasing temperature usually decreases gas solubility As temp increases average kinetic energy of molecules in solution increases Greater number of solute molecules escape from attraction of solvent and return to gas phase 69
At higher temps eq is reached with fewer gas molecules in solution Gases generally less soluble
70
Effect of temp on solubility of solids in liquids more difficult to predict
Often increasing temp increases solubility
However the same temp increase can result in large increase in solubility in one case and only slight increase in the next
71
72
enthalpies of Solution
Formation of solution comes with energy change Dissolve KI in water container feels cold Dissolve LiCl in water feels hot Formation of solid-liquid solution can absorb or release heat
73
During formation of solution solvent and solute particles experience changes in forces attracting them to other particles Before dissolving begins solvent molecules held by intermolecular forces (solvent-solvent attraction) In solute molecules held by Ifs (solute-solute attraction)
74
Energy is required to separate solute molecules and solvent molecules from their neighbors
A solute particle that is surrounded by solvent molecules is said to be solvated
75
Step 1 solute particles become separated from solid (energy absorbed) Step 2 solvent particles move apart to allow solute particles to enter liquid (energy absorbed)
76
Step 3 solvent particles attracted to and solvate solute particles (energy released)
77
Heat of solution - net amount of heat energy absorbed or released when a specific amount of solute dissolves in a solvent
Heat of solution negative (heat is released) if sums of energy in steps 1 and 2 is less than step 3
If 1+2 gt 3 HOS is positive (heat absorbed)78
In gaseous state molecules are so far apart there are nearly no forces between them Solute-solute interaction has little effect on heat of solution of a gas Energy released when gas dissolved in liquid bc attraction between solute gas and solvent molecules greater than energy needed to separate solvent molecules
79
Concentration of Solutions80
Concentration - measure of amount of solute in given amount of solvent or solution ldquodiluterdquo just means small amount of solute ldquoconcentratedrdquo just means relatively large amount of solute Unrelated to degree to which solution is saturated Ex Saturated solution of substance not very soluble might be dilute
81
Molarity
Molarity - number of moles of solute in one liter of solution To find molarity must know molar mass A ldquoone molarrdquo solution of NaOH is 1 mole NaOH per liter of solution (1 M)
82
1 mol NaOH = 400 g If this quantity dissolve in enough water to make EXACTLY 100 L solution solution is 1 M If 200 g NaOH dissolve in enough to make 100 L solution what is molarity 0500 M
83
1 M solution is not made by adding 1 mol solute to 1 L solvent That would be more than 1 L Instead 1 mol solute dissolved in less than 1 L Then diluted with more solvent to bring TOTAL VOLUME to 1 L
84
Practice ProblemYou have 340 L of solution that contains 900 g sodium chloride What is the molarity of that solution 0440 M NaCl You have 08 L of a 05 M HCl solution How many moles of HCl does this solution contain 04 mol HCl
85
What is the molarity of a solution composed of 585 g of potassium iodide dissolved in enough water to make a 0125 L of solution 0282 M KI
86
How many moles of H2SO4 are present in 0500 L of a 0150 M H2SO4 solution 00750 mol What volume of 300 M NaCl is needed for reaction that requires 1463 g of NaCl 0834 L
87
Molality
Molality - concentration of solution expressed in moles of solute per kilogram of solvent Solution that contains 1 mol solute (NaOH) dissolved in exactly 1 kg of solvent is ldquoone molalrdquo solution (1 m NaOH)
88
1 mol NaOH = 400 g 400 g NaOH dissolved in 1 kg water is 1 m NaOH
200 g NaOH in 1 kg water = 0500 m NaOH
89
Practice Problem
A solution was prepared by dissolving 171 g sucrose (C12H22O11) in 125 g of water Find the molal concentration of this solution 0400 m C12H22O11
90
A solution of iodine in carbon tetrachloride is used when iodine is needed for certain chemical tests How much iodine must be added to prepare a 0480 m solution of iodine in CCl4 if 1000 g of CCl4 is used 122 g I2
91
What is the molality of a solution composed of 255 g acetone (CH3)2CO dissolved in 200 g of water 22 m acetone What quantity in grams of methanol CH3OH is required to prepare a 0244 m solution in 400 g water 312 g methanol
92
How many grams of AgNO3 are needed to prepare a 0125 m solution in 250 mL of water 531 g AgNO3
What is the molality of a solution containing 182 g HCl and 250 g water 200 m
93
Ions in Aqueous Solutions and Colligative Properties
94
95
compounds in aqueous solutions
DissociationWhen compound made from ions dissolves in water ions separate Dissociation - separation of ions that occurs when an ionic compound dissolves
96
Notice number of ions made per formula unit in equations 1 formula unit of NaCl gives 2 units of ions in solution 1 formula unit of CaCl2 gives 3 units of ions
97
Assuming 100 dissociation solution that contains 1 mol NaCl contains 1 mol Na+ and 1 mol Cl- Can assume 100 dissociation of all soluble ionic compounds
98
Practice Problem
Write the equation for the dissolution of aluminum sulfate in water How many moles of aluminum ions are made by dissolving 1 mol aluminum sulfate What is the total number of moles of ions made by dissolving 1 mol of aluminum sulfate
99
1 AnalyzeGiven Amount of solute = 1 mol Al2(SO4)3 Solvent identity = water Unknown a moles of aluminum ions and sulfate ions Total number of moles of solute ions produced
100
2 Plan
The coefficients in the balanced dissociation equation will tell mole relationships You can use the equation to find out the number of moles of solute ions produced
101
3 Compute
a
b102
Write the equation for the dissolution of each of the following in water and then determine the number of moles of each ions made as well as the total number of ions made a 1 mol ammonium chloride b 1 mol sodium sulfide c 05 mol barium nitrate
103
a 1 mol ammonium chloride
1 mol NH4+ 1 mol Cl- 2 mol total ions
104
b 1 mol sodium sulfide
2 mol Na+ 1 mol S-2 3 mol total ions
105
c 05 mol barium nitrate
05 mol Ba+2 1 mol NO3- 15 mol total ions
106
Precipitation Reactions
Even though no compound is completely insoluble compounds of very low solubility can be considered insoluble for practical purposes There are general guidelines to help predict whether a compound made of certain combination of ions is soluble
107
General Solubility Guidelines
Sodium potassium and ammonium compounds are soluble in water
108
Nitrates acetates and chlorates are soluble
109
110
Most chlorides are soluble except those of silver mercury (I) and lead Lead (II) chloride is soluble in hot water
Most sulfates are soluble except those of barium strontium lead calcium and mercury
111
Most carbonates phosphates and silicates are insoluble except those of sodium potassium and ammonium
112
Most sulfides are insoluble except those of calcium strontium sodium potassium and ammonium
113
Is calcium phosphate Ca3(PO4)2 soluble or insoluble
Not one of the exceptions so it is insoluble
Dissociation equations are not written for insoluble compounds
114
Guidelines also useful in predicting what will happen if 2 solutions of 2 different soluble compounds are mixed
If mixing results in combination of ions that forms insoluble compound a double-replacement and precipitation reaction will happen
115
Will precipitate form when solutions of ammonium sulfide and cadmium nitrate are combined You can tell calcium nitrate is soluble from the guidelines (Most nitrates acetates and chlorates are soluble) You can also tell ammonium sulfide is soluble (Most sulfides are insoluble except those of calcium strontium sodium potassium and ammonium)
116
117
To decide whether precipitate can form must know solubilities of two possible products
Ammonium nitrate ndash soluble (Most nitrates acetates and chlorates are soluble) Cadmium sulfide ndash insoluble (Most sulfides are insoluble except those of calcium strontium sodium potassium and ammonium)
118
bc one product is insoluble double-replacement and precipitation reaction will happen (NH4)2S (aq) + Cd(NO3)2 (aq) rarr NH4NO3 (aq) + CdS (s)
119
Net Ionic Equations
Reactions of ions in aqueous solution usually represented by net ionic equations instead of formula equations Net ionic equation - includes only those compounds and ions that undergo a chemical change in a reaction in an aqueous solution
120
121
To write NIE (net ionic equation) first convert chemical equation into overall ionic equation All soluble ionic compounds shown as dissociated ions in solution Precipitates shown as solids
122
Notice ammonium and nitrate ions appear on both sides They havenrsquot gone through any chemical change Spectator ions - ions that do not take part in a chemical reaction and are found in solution both before and after the reaction
123
To change ionic equation to NIE eliminate spectator ions Cd+2(aq) + S-2(aq) rarr CdS(s)
This NIE applies to any reaction in which precipitate of CdS forms
124
Practice Problem
Identify the precipitate that forms when aqueous solutions of zinc nitrate and ammonium sulfide are combined Write the equation for the possible double-replacement reaction Then write the formula equation overall ionic equation and the net ionic equation
125
1 AnalyzeGiven Identity of reactants zinc nitrate and ammonium sulfide Reaction medium aqueous solution Unknown a equation for the possible double-replacement reaction b identity of the precipitate c formula equation d overall ionic equation e net ionic equation
126
2 PlanWrite the possible double-replacement reaction between Zn(NO3)2 and (NH4)2S Use the rules to determine if any of the products will precipitate Write a formula equation and overall net ionic equation Then cancel spectator ions to make net ionic equation
127
3 Computea possible double-replacement reaction
b rules show that zinc sulfide is not soluble so will precipitate (ammonium nitrate is soluble)
c formula equation128
d overall ionic equation
e net ionic equation129
Will a precipitate form if solutions of potassium sulfate and barium nitrate are combined If so write the net ionic equation for the reaction
Yes Ba+2(aq) + SO4-2(aq) rarr BaSO4(s)
130
Will a precipitate form if solutions of potassium nitrate and magnesium sulfate are combined If so write the net ionic equation for the reaction
No 131
Will a precipitate form if solutions of barium chloride and sodium sulfate are combined If so identify the spectator ions and write the net ionic equation for the reaction
Yes Na+ and Cl- Ba+2(aq) + SO4-2(aq) rarr BaSO4(s)
132
Write the net ionic equation for the precipitation of nickel(II) sulfide
Ni+2(aq) + S-2(aq) rarr NiS(s)
133
IonizationSome molecular compounds can also form ions in solution Usually polar Ionization - ions formed from solute molecules by action of the solvent (creation of ions where there were none) Ionization different from dissociation
134
Dissociation ionic compounds dissolve and ions already present separate Ionization ions formed where non existed before
Ions formed are hydrated Heat released during hydration of ions gives enough energy to break covalent bonds
135
Extent to which solute ionizes depends on strength of bonds within molecules of solute and strength of attraction between solute and solvent molecules
136
HCl ndash molecular compound that ionizes in aqueous solution Contains highly polar bond Attraction between polar HCl molecule and polar water strong enough to break HCl bond
137
The Hydronium Ion
H+ ions from HCl attracts other molecules or ions so strongly that it doesnrsquot normally exist alone Ionization of HCl better described as direct transfer of proton from HCl to H2O forming H3O+
138
139
Hydration of H+ to form hydronium is highly exothermic
Energy released gives a lot of energy needed to ionize a molecular solute
Many molecular compounds that ionize in aqueous solution contain hydrogen and form hydronium ion
140
Strong and Weak Electrolytes
Substances that make ions and conduct current in solution are electrolytes HCl is one of series of compounds that has hydrogen and halogen Hydrogen halides are all molecular compounds with single polar-covalent bonds
141
All are gases Very soluble in water All are electrolytes HCl HBr HI strongly conduct current in solution HF only weakly conducts current at same concentration Strength of conduction related to ability to ionize
142
143
Strong Electrolytes
HCl HBr HI are 100 ionized in solution
Strong electrolyte - any compound whose dilute aqueous solutions conduct electricity well this is due to the presence of all or almost all of the dissolve compound in the form of ions
144
Unique characteristic of strong electrolyte is that they yield only ions
Ex Ionic compound may be highly soluble in water and dissociate into ions (NaCl)
Others may not dissolve much but amount that does dissolve exists only as ions
145
Weak ElectrolytesHF dissolves in water to give acid solution called hydrofluoric acid HF bond stronger than bonds between hydrogen and other halogens When HF dissolves some molecule ionize Reverse reaction also happens HF(aq) + H2O(l) H3O+(aq) + F-(aq)
146
HF(aq) + H2O(l) H3O+
(aq) + F-(aq)Concentration of dissolved unionized HF stays high and concentration of ions stays low
Weak electrolyte - any compound whose aqueous solutions conduct electricity poorly this is due to the presence of small amount of the dissolved compound in the form of ions
147
Different from nonelectrolyte ndash NO ions at all Description of electrolyte as strong or weak not related to concentration of solution Electrolytes differ in degree of ionization not amount of solute dissolved
148
149
Colligative properties of solutions
Presence of solute affects properties of solutions Some properties not dependent on nature of dissolved substance but on how may dissolve particles are present
Colligative properties - properties that depend on the concentration of solute particles but not on their identity
150
Vapor-Pressure Lowering
Boiling and freezing point of solution different from pure solvent Nonvolatile solute raises boiling point and lowers freezing point Nonvolatile substance - one that has little tendency to become gas under existing conditions
151
152
To understand why nonvolatile solute changes boiling and freezing point consider equilibrium vapor pressure
Vapor pressure pressure caused by molecules that have escape liquid phase to gas phase
Can be thought of a measure of tendency of molecules to escape from a liquid
153
Addition of sucrose (nonvolatile solute) lowers concentration of water molecules at surface of liquid This lowers tendency of water molecules to leave solution and enter gas phase Vapor pressure of solution is lower than vapor pressure of pure water
154
Aqueous13 solution13 of13 nonvolatile13 solutePure13 water
Nonelectrolyte solutions of same molality have same concentration of particles
Dilute solutions of same solvent and equal molality of any nonelectrolyte solute lower vapor pressure equally
155
Example
1 m aqueous solution of nonelectrolyte glucose lowers vapor pressure of water 55 x 10-4 atm at 25 1 m aqueous solution of sucrose also lowers water vapor pressure to 55 x 10-4 atm at 25
156
157
bc vapor-pressure lowering depends on concentration of nonelectrolyte solute and doesnrsquot depend on the type of solute it is a colligative proprerty
bc vapor pressure has been lowered solution remains liquid over larger temperature range
Lowers freezing point and raises boiling point
158
Can assume changes in boiling and freezing point also depend on concentration of solute
They are colligative properties
159
Freezing-Point Depression
Freezing point of 1-molal solution of any nonelectrolyte solute in water is found (by experiment) to be 186 lower than freezing point of water When 1 mol of nonelectrolyte solute dissolved in 1 kg water freezing point is -186 not 000 160
When 2 mol nonelectrolyte solute dissolved in 1 kg water freezing point is -372
For any concentration of nonelectrolyte solute in water decrease in freezing point can be determined using value of -186m
161
This value called the molal freezing-point contstant (Kf) is the freezing-point depression of the solvent in a 1-molal solution of a nonvolatile nonelectrolyte solute
162
Each solvent has own characteristic molal freezing-point constant Freezing-point depression (Δtf) the difference between the freezing points of the pure solvent and a solute of nonelectrolyte in that solvent and it is directly proportional to the molal concentration of the solution Δtf = Kfm
163
Δtf = Kfm
Kf expressed as m
m expressed in mol solutekg solvent (molality)
164
Sample Problem
What is the freezing point depression of water in a solution of 171 g of sucrose C12H22O11 and 200 g of water What is the actual freezing point of the solution
165
1 AnalyzeGiven Solute mass and chemical formula 171 g C12H22O11 Solvent mass and identity 2000 g water Unknown a freezing-point depression b freezing point of the solution 166
2 Plan
167
168
Calculate
Unknown a freezing-point depression b freezing point of the solution
169
Practice Problem
A water solution containing an unknown quantity of a nonelectrolyte solute is found to have a freezing point of ndash 023degCWhat is the molal concentration of the solution
012 m170
A solution consists of 103 g of the nonelectrolyte glucose C6H12O6 dissolved in 250 g of water What is the freezing-point depression of the solution
minus0426degC171
In a laboratory experiment the freezing point of an aqueous solution of glucose is found to be minus0325degCWhat is the molal concentration of this solution
0175 m172
If 0500 mol of a nonelectrolyte solute are dissolved in 5000 g of ether what is the freezing point of the solution
minus1181degC173
The freezing point of an aqueous solution that contains a nonelectrolyte is minus90degC aWhat is the freezing-point depression of the solution bWhat is the molal concentration of the solution a minus90degC b 48 m
174
Boiling-Point Elevation
Boiling point is temp at which vapor pressure is equal to atmospheric pressure Change in vapor pressure will cause corresponding change in boiling point
175
176
Vapor pressure of nonvolatile solution is lower than vapor pressure of pure solvent More heat will be required to raise vapor pressure of solution Boiling point of solution is higher than boiling point of pure solvent
Molal boiling-point constant (Kb) - boiling-point elevation of the solvent in a 1-molal solution of a nonvolatile nonelectrolyte solute b-p elevation of any nonelectrolyte in water found by experiment to be 051 So molal b-p constant for water is 051m
177
For different solvents b-p elevations of 1-molal solutions have different values (Table 14-2)
Like freezing-point constants values most accurate for dilute solutions
178
Boiling-point elevation Δtb difference between the boiling points of the pure solvent and a nonelectrolyte solution of that solvent and it is directly proportional to the molal concentration of the solution B-p elevation can be calculated using Δtb = Kbm Δtb (boiling point elevation) expressed in m and m expressed in mol solutekg solvent
Sample Problem
What is the boiling-point elevation of a solution made from 200 g of a nonelectrolyte solute and 4000 g of water The molar mass of the solute is 620 gmol
180
1 AnalyzeGiven solute mass = 200 g solute molar mass = 620 gmol solvent mass and identity = 4000 g of water
Unknown boiling-point elevation
181
2 Plan
182
183
3 Compute
Practice Problem
A solution contains 500 g of sucrose C12H22O11 a nonelectrolyte dissolved in 5000 g of water What is the boiling-point elevation
015degC184
A solution contains 4500 g of sucrose C12H22O11 a nonelectrolyte dissolved in 250 g of waterWhat is the boiling point of the solution
1027degC185
If the boiling point elevation of an aqueous solution containing a nonvolatile electrolyte is 102degC what is the molality of the solution
20 m186
The boiling point of an aqueous solution containing a nonvolatile electrolyte is 10075degC aWhat is the boiling-point elevation bWhat is the molality of the solution a 075degC b 15 m
187
Electrolytes and Colligative PropertiesEarly investigators confused by experiments where certain substances depressed freezing point or elevated boiling point more than expected
188
189
Ex 01 m solution of NaCl lowers freezing point of solvent almost twice as much as 01 m solution of sucrose 01 m CaCl2 lowers fp almost 3 times as much as 01 m solution of sucrose
To understand why this happens contrast behavior or sucrose with NaCl in aqueous solution Each sucrose dissolves to make only 1 particle in solution So 1 mol sucrose dissolves to 1 mol particles 1 mol NaCl dissolves to make 2 moles of particles (1 mol Na+ and 1 mol Cl-)
190
191
Calculated Values for Electrolyte SolutionsColligative properties depend on total concentration of solute particles regardless of their identity Electrolytes cause changes in colligative properties proportional to total molality in terms of all dissolves particles instead of formula units
192
What about barium nitrate Ba(NO3)2 Each mole of barium nitrate yields 3 mol of ions in solution Ba(NO3)2(s) rarr Ba2+(aq) + 2NO3-(aq) You would expect solution of given molality to lower fp of solvent 3 times as much as nonelectrolytic solution of same molality
193
Sample Problem
What is the expected change in the freezing point of water in a solution of 625 g of barium nitrate Ba(NO3)2 in 100 kg of water
194
1 AnalyzeGiven solute mass and formula = 625 g Ba(NO3)2 solvent mass and identity = 100 kg water Δtf = Kfm
Unknown expected freezing-point depression
195
2 PlanThe molality can be calculated by converting the solute mass to moles and then dividing by the number of kilograms of solvent That molality is in terms of formula units of Ba(NO3)2 and must be converted to molality in terms of dissociated ions in solution It must be multiplied by the number of moles of ions produced per mole of formula unit This adjusted molality can then be used to calculate the freezing-point depression
196
197
198
3 Compute
Practice Problem
What is the expected freezing-point depression for a solution that contains 20 mol of magnesium sulfate dissolved in 10 kg of water
minus74degC199
What is the expected boiling-point elevation of water for a solution that contains 150 g of sodium chloride dissolved in 10 kg of water 27degC The freezing point of an aqueous sodium chloride solution is minus200degC What is the molality of the solution 54 m NaCl
200
Actual Values for Electrolyte SolutionsRemember values calculated are only expected values Actual values of colligative properties for all strong electrolytes are almost what would be expected Difference between expected and actual values caused by attractive forces that exist between dissociated ions in aqueous solution
201
Attraction between hydrated ions in solution is small compared with those in crystalline solid However forces of attraction donrsquot interfere with movements of aqueous ions More concentrated solution means ions closer together and attraction is greater Also depends on charge of ions ndash higher charge greater attraction
202
40
Supersaturated solution can remain unchanged over long time if it isnrsquot disturbed Once crystals begin to form continues until equilibrium is recreated at lower temp
41
Example
Sodium thiosulfate Na2S2O3 Solute added to hot water until solution saturated Hot solution filtered Drop small crystal in ldquoseedingrdquo crystallization Continues until equilibrium created
42
Solubility Values
Solubility - amount of substance required to form a saturated solution with specific amount of solvent at specified temp Ex Solubility of sugar = 204 g per 100 g water at 20 Must specify temp bc solubility changes with temp 43
For gases pressure must also be specified Rate at which substance dissolves is unrelated to solubility Max amount of solute that dissolves and reaches equilibrium is always same under same conditions
44
Solute-Solvent Interactions
ldquolike dissolves likerdquo is generally useful rule for predicting whether one substance will dissolve in another Depends on type of bonding polaritynonpolarity and intermolecular forces between solute and solvent
45
Dissolving Ionic Compounds in Aqueous Solution
Polarity of water molecules plays important role in formation of solution of ionic compounds Charged ends of water molecules attract ions and surround them to keep separated from other ions
46
Hydration - solution process with water as solvent Ions said to be hydrated As hydrated ions diffuse into solution other ions exposed and drawn away from crystal surface Entire crystal gradually dissolves
47
When crystallized from aqueous solutions some ionic compounds form crystals that include water molecules The crystalline compounds known as hydrates have specific ratios of water molecules Represented by formulas like CuSO45H2O
48
When crystalline hydrate dissolves in water water of hydration returns to solvent Behavior of solute in hydrated form no different than anhydrous form
49
Nonpolar Solvents
Nonpolar solvents do not attract ions of ionic compound strongly enough to overcome forces holding crystal together
50
Liquid Solutes and Solvents
When you shake bottle of salad dressing oil droplets disperse in water Stop shaking ndash strong attraction between water molecules squeeze out oil and form separate layers Liquid solutes and solvents that are not soluble in each other are immiscible
51
Nonpolar substances (fat oil grease) are generally quite soluble in nonpolar liquids (carbon tetrachloride toluene gasoline)
Only attractions between nonpolar molecules are weak London forces
52
London Dispersion Forces
bc e- are constantly moving at any time there may be uneven distribution of them around a nonpolar atom Temporarily creates positive end and negative end
53
54
London dispersion forces - intermolecular attractions resulting from constant motion of e- and creation of instantaneous dipoles
55
Liquids that dissolve freely in one another in any ratio are said to be completely miscible Benzene and carbon tetrachloride (both nonpolar) are completely miscible
Nonpolar molecules of these two apply no strong forces of attraction or repulsion and molecules mix freely
56
Ethanol and water are also completely miscible -OH group on ethanol is slightly polar
Can form H-bonds with water as well as other ethanol molecules Intermolecular forces in mixture are very similar to those in pure liquids so they are mutually soluble in any amount
57
Gasoline contains mainly nonpolar hydrocarbons
Excellent solvent for fats oils greases
Major intermolecular forces are weak London forces
58
Ethanol less polar than water more than carbon tetrachloride
Better solvent for less-polar substances bc of nonpolar region
59
Effects of Pressure on Solubility
Little effect on solids or liquids Increase pressure - increase solubilities of gases in liquids
60
When gas in contact with surface of liquid gas molecules can enter liquid As amount of dissolve gas increases some molecules start to escape and reenter gas phase
Eq eventually established between rates of enteringleaving gas phase
61
As long as eq undisturbed solubility of gas in liquid unchanged at given pressure
Gas + solvent solution
62
Increasing pressure of solute gas above solution puts stress on eq Molecules collide with surface more often Increase in pressure partially offset by increase in rate of gas molecules entering solution In turn increase in amount of dissolved gas causes increase in rate molecules escape liquid and becomes gas
63
Eventually eq restored at higher gas solubility
Expected from Le Chatelierrsquos principle
Increase in gas pressure causes eq to shift so fewer molecules are in gas phase
64
Henryrsquos Law
Henryrsquos law - solubility of a gas in liquid is directly proportional to the partial pressure of that gas on the surface of the liquid
Applies to gas-liquid solutions at constant temp
65
When mixture of ideal gases is limited in constant volume at constant temp each gas applies same pressure it would apply if it occupied the space alone
Assuming gases do not react each gas dissolves to the extent it would if no other gases were there
66
In carbonated drinks (Coke Pepsi etc) solubility of CO2 increased by increasing pressure
At bottling factory CO2 gas forced into solution of flavored water at pressure of 5-10 atm Gas-in-liquid solution then sealed in bottlescans
67
When cap removed pressure reduced to 1 atm and some CO2 escapes as gas bubbles Effervescence - rapid escape of a gas from a liquid in which it is dissolved
68
Effects of Temperature on Solubility
Letrsquos consider gas solubility Increasing temperature usually decreases gas solubility As temp increases average kinetic energy of molecules in solution increases Greater number of solute molecules escape from attraction of solvent and return to gas phase 69
At higher temps eq is reached with fewer gas molecules in solution Gases generally less soluble
70
Effect of temp on solubility of solids in liquids more difficult to predict
Often increasing temp increases solubility
However the same temp increase can result in large increase in solubility in one case and only slight increase in the next
71
72
enthalpies of Solution
Formation of solution comes with energy change Dissolve KI in water container feels cold Dissolve LiCl in water feels hot Formation of solid-liquid solution can absorb or release heat
73
During formation of solution solvent and solute particles experience changes in forces attracting them to other particles Before dissolving begins solvent molecules held by intermolecular forces (solvent-solvent attraction) In solute molecules held by Ifs (solute-solute attraction)
74
Energy is required to separate solute molecules and solvent molecules from their neighbors
A solute particle that is surrounded by solvent molecules is said to be solvated
75
Step 1 solute particles become separated from solid (energy absorbed) Step 2 solvent particles move apart to allow solute particles to enter liquid (energy absorbed)
76
Step 3 solvent particles attracted to and solvate solute particles (energy released)
77
Heat of solution - net amount of heat energy absorbed or released when a specific amount of solute dissolves in a solvent
Heat of solution negative (heat is released) if sums of energy in steps 1 and 2 is less than step 3
If 1+2 gt 3 HOS is positive (heat absorbed)78
In gaseous state molecules are so far apart there are nearly no forces between them Solute-solute interaction has little effect on heat of solution of a gas Energy released when gas dissolved in liquid bc attraction between solute gas and solvent molecules greater than energy needed to separate solvent molecules
79
Concentration of Solutions80
Concentration - measure of amount of solute in given amount of solvent or solution ldquodiluterdquo just means small amount of solute ldquoconcentratedrdquo just means relatively large amount of solute Unrelated to degree to which solution is saturated Ex Saturated solution of substance not very soluble might be dilute
81
Molarity
Molarity - number of moles of solute in one liter of solution To find molarity must know molar mass A ldquoone molarrdquo solution of NaOH is 1 mole NaOH per liter of solution (1 M)
82
1 mol NaOH = 400 g If this quantity dissolve in enough water to make EXACTLY 100 L solution solution is 1 M If 200 g NaOH dissolve in enough to make 100 L solution what is molarity 0500 M
83
1 M solution is not made by adding 1 mol solute to 1 L solvent That would be more than 1 L Instead 1 mol solute dissolved in less than 1 L Then diluted with more solvent to bring TOTAL VOLUME to 1 L
84
Practice ProblemYou have 340 L of solution that contains 900 g sodium chloride What is the molarity of that solution 0440 M NaCl You have 08 L of a 05 M HCl solution How many moles of HCl does this solution contain 04 mol HCl
85
What is the molarity of a solution composed of 585 g of potassium iodide dissolved in enough water to make a 0125 L of solution 0282 M KI
86
How many moles of H2SO4 are present in 0500 L of a 0150 M H2SO4 solution 00750 mol What volume of 300 M NaCl is needed for reaction that requires 1463 g of NaCl 0834 L
87
Molality
Molality - concentration of solution expressed in moles of solute per kilogram of solvent Solution that contains 1 mol solute (NaOH) dissolved in exactly 1 kg of solvent is ldquoone molalrdquo solution (1 m NaOH)
88
1 mol NaOH = 400 g 400 g NaOH dissolved in 1 kg water is 1 m NaOH
200 g NaOH in 1 kg water = 0500 m NaOH
89
Practice Problem
A solution was prepared by dissolving 171 g sucrose (C12H22O11) in 125 g of water Find the molal concentration of this solution 0400 m C12H22O11
90
A solution of iodine in carbon tetrachloride is used when iodine is needed for certain chemical tests How much iodine must be added to prepare a 0480 m solution of iodine in CCl4 if 1000 g of CCl4 is used 122 g I2
91
What is the molality of a solution composed of 255 g acetone (CH3)2CO dissolved in 200 g of water 22 m acetone What quantity in grams of methanol CH3OH is required to prepare a 0244 m solution in 400 g water 312 g methanol
92
How many grams of AgNO3 are needed to prepare a 0125 m solution in 250 mL of water 531 g AgNO3
What is the molality of a solution containing 182 g HCl and 250 g water 200 m
93
Ions in Aqueous Solutions and Colligative Properties
94
95
compounds in aqueous solutions
DissociationWhen compound made from ions dissolves in water ions separate Dissociation - separation of ions that occurs when an ionic compound dissolves
96
Notice number of ions made per formula unit in equations 1 formula unit of NaCl gives 2 units of ions in solution 1 formula unit of CaCl2 gives 3 units of ions
97
Assuming 100 dissociation solution that contains 1 mol NaCl contains 1 mol Na+ and 1 mol Cl- Can assume 100 dissociation of all soluble ionic compounds
98
Practice Problem
Write the equation for the dissolution of aluminum sulfate in water How many moles of aluminum ions are made by dissolving 1 mol aluminum sulfate What is the total number of moles of ions made by dissolving 1 mol of aluminum sulfate
99
1 AnalyzeGiven Amount of solute = 1 mol Al2(SO4)3 Solvent identity = water Unknown a moles of aluminum ions and sulfate ions Total number of moles of solute ions produced
100
2 Plan
The coefficients in the balanced dissociation equation will tell mole relationships You can use the equation to find out the number of moles of solute ions produced
101
3 Compute
a
b102
Write the equation for the dissolution of each of the following in water and then determine the number of moles of each ions made as well as the total number of ions made a 1 mol ammonium chloride b 1 mol sodium sulfide c 05 mol barium nitrate
103
a 1 mol ammonium chloride
1 mol NH4+ 1 mol Cl- 2 mol total ions
104
b 1 mol sodium sulfide
2 mol Na+ 1 mol S-2 3 mol total ions
105
c 05 mol barium nitrate
05 mol Ba+2 1 mol NO3- 15 mol total ions
106
Precipitation Reactions
Even though no compound is completely insoluble compounds of very low solubility can be considered insoluble for practical purposes There are general guidelines to help predict whether a compound made of certain combination of ions is soluble
107
General Solubility Guidelines
Sodium potassium and ammonium compounds are soluble in water
108
Nitrates acetates and chlorates are soluble
109
110
Most chlorides are soluble except those of silver mercury (I) and lead Lead (II) chloride is soluble in hot water
Most sulfates are soluble except those of barium strontium lead calcium and mercury
111
Most carbonates phosphates and silicates are insoluble except those of sodium potassium and ammonium
112
Most sulfides are insoluble except those of calcium strontium sodium potassium and ammonium
113
Is calcium phosphate Ca3(PO4)2 soluble or insoluble
Not one of the exceptions so it is insoluble
Dissociation equations are not written for insoluble compounds
114
Guidelines also useful in predicting what will happen if 2 solutions of 2 different soluble compounds are mixed
If mixing results in combination of ions that forms insoluble compound a double-replacement and precipitation reaction will happen
115
Will precipitate form when solutions of ammonium sulfide and cadmium nitrate are combined You can tell calcium nitrate is soluble from the guidelines (Most nitrates acetates and chlorates are soluble) You can also tell ammonium sulfide is soluble (Most sulfides are insoluble except those of calcium strontium sodium potassium and ammonium)
116
117
To decide whether precipitate can form must know solubilities of two possible products
Ammonium nitrate ndash soluble (Most nitrates acetates and chlorates are soluble) Cadmium sulfide ndash insoluble (Most sulfides are insoluble except those of calcium strontium sodium potassium and ammonium)
118
bc one product is insoluble double-replacement and precipitation reaction will happen (NH4)2S (aq) + Cd(NO3)2 (aq) rarr NH4NO3 (aq) + CdS (s)
119
Net Ionic Equations
Reactions of ions in aqueous solution usually represented by net ionic equations instead of formula equations Net ionic equation - includes only those compounds and ions that undergo a chemical change in a reaction in an aqueous solution
120
121
To write NIE (net ionic equation) first convert chemical equation into overall ionic equation All soluble ionic compounds shown as dissociated ions in solution Precipitates shown as solids
122
Notice ammonium and nitrate ions appear on both sides They havenrsquot gone through any chemical change Spectator ions - ions that do not take part in a chemical reaction and are found in solution both before and after the reaction
123
To change ionic equation to NIE eliminate spectator ions Cd+2(aq) + S-2(aq) rarr CdS(s)
This NIE applies to any reaction in which precipitate of CdS forms
124
Practice Problem
Identify the precipitate that forms when aqueous solutions of zinc nitrate and ammonium sulfide are combined Write the equation for the possible double-replacement reaction Then write the formula equation overall ionic equation and the net ionic equation
125
1 AnalyzeGiven Identity of reactants zinc nitrate and ammonium sulfide Reaction medium aqueous solution Unknown a equation for the possible double-replacement reaction b identity of the precipitate c formula equation d overall ionic equation e net ionic equation
126
2 PlanWrite the possible double-replacement reaction between Zn(NO3)2 and (NH4)2S Use the rules to determine if any of the products will precipitate Write a formula equation and overall net ionic equation Then cancel spectator ions to make net ionic equation
127
3 Computea possible double-replacement reaction
b rules show that zinc sulfide is not soluble so will precipitate (ammonium nitrate is soluble)
c formula equation128
d overall ionic equation
e net ionic equation129
Will a precipitate form if solutions of potassium sulfate and barium nitrate are combined If so write the net ionic equation for the reaction
Yes Ba+2(aq) + SO4-2(aq) rarr BaSO4(s)
130
Will a precipitate form if solutions of potassium nitrate and magnesium sulfate are combined If so write the net ionic equation for the reaction
No 131
Will a precipitate form if solutions of barium chloride and sodium sulfate are combined If so identify the spectator ions and write the net ionic equation for the reaction
Yes Na+ and Cl- Ba+2(aq) + SO4-2(aq) rarr BaSO4(s)
132
Write the net ionic equation for the precipitation of nickel(II) sulfide
Ni+2(aq) + S-2(aq) rarr NiS(s)
133
IonizationSome molecular compounds can also form ions in solution Usually polar Ionization - ions formed from solute molecules by action of the solvent (creation of ions where there were none) Ionization different from dissociation
134
Dissociation ionic compounds dissolve and ions already present separate Ionization ions formed where non existed before
Ions formed are hydrated Heat released during hydration of ions gives enough energy to break covalent bonds
135
Extent to which solute ionizes depends on strength of bonds within molecules of solute and strength of attraction between solute and solvent molecules
136
HCl ndash molecular compound that ionizes in aqueous solution Contains highly polar bond Attraction between polar HCl molecule and polar water strong enough to break HCl bond
137
The Hydronium Ion
H+ ions from HCl attracts other molecules or ions so strongly that it doesnrsquot normally exist alone Ionization of HCl better described as direct transfer of proton from HCl to H2O forming H3O+
138
139
Hydration of H+ to form hydronium is highly exothermic
Energy released gives a lot of energy needed to ionize a molecular solute
Many molecular compounds that ionize in aqueous solution contain hydrogen and form hydronium ion
140
Strong and Weak Electrolytes
Substances that make ions and conduct current in solution are electrolytes HCl is one of series of compounds that has hydrogen and halogen Hydrogen halides are all molecular compounds with single polar-covalent bonds
141
All are gases Very soluble in water All are electrolytes HCl HBr HI strongly conduct current in solution HF only weakly conducts current at same concentration Strength of conduction related to ability to ionize
142
143
Strong Electrolytes
HCl HBr HI are 100 ionized in solution
Strong electrolyte - any compound whose dilute aqueous solutions conduct electricity well this is due to the presence of all or almost all of the dissolve compound in the form of ions
144
Unique characteristic of strong electrolyte is that they yield only ions
Ex Ionic compound may be highly soluble in water and dissociate into ions (NaCl)
Others may not dissolve much but amount that does dissolve exists only as ions
145
Weak ElectrolytesHF dissolves in water to give acid solution called hydrofluoric acid HF bond stronger than bonds between hydrogen and other halogens When HF dissolves some molecule ionize Reverse reaction also happens HF(aq) + H2O(l) H3O+(aq) + F-(aq)
146
HF(aq) + H2O(l) H3O+
(aq) + F-(aq)Concentration of dissolved unionized HF stays high and concentration of ions stays low
Weak electrolyte - any compound whose aqueous solutions conduct electricity poorly this is due to the presence of small amount of the dissolved compound in the form of ions
147
Different from nonelectrolyte ndash NO ions at all Description of electrolyte as strong or weak not related to concentration of solution Electrolytes differ in degree of ionization not amount of solute dissolved
148
149
Colligative properties of solutions
Presence of solute affects properties of solutions Some properties not dependent on nature of dissolved substance but on how may dissolve particles are present
Colligative properties - properties that depend on the concentration of solute particles but not on their identity
150
Vapor-Pressure Lowering
Boiling and freezing point of solution different from pure solvent Nonvolatile solute raises boiling point and lowers freezing point Nonvolatile substance - one that has little tendency to become gas under existing conditions
151
152
To understand why nonvolatile solute changes boiling and freezing point consider equilibrium vapor pressure
Vapor pressure pressure caused by molecules that have escape liquid phase to gas phase
Can be thought of a measure of tendency of molecules to escape from a liquid
153
Addition of sucrose (nonvolatile solute) lowers concentration of water molecules at surface of liquid This lowers tendency of water molecules to leave solution and enter gas phase Vapor pressure of solution is lower than vapor pressure of pure water
154
Aqueous13 solution13 of13 nonvolatile13 solutePure13 water
Nonelectrolyte solutions of same molality have same concentration of particles
Dilute solutions of same solvent and equal molality of any nonelectrolyte solute lower vapor pressure equally
155
Example
1 m aqueous solution of nonelectrolyte glucose lowers vapor pressure of water 55 x 10-4 atm at 25 1 m aqueous solution of sucrose also lowers water vapor pressure to 55 x 10-4 atm at 25
156
157
bc vapor-pressure lowering depends on concentration of nonelectrolyte solute and doesnrsquot depend on the type of solute it is a colligative proprerty
bc vapor pressure has been lowered solution remains liquid over larger temperature range
Lowers freezing point and raises boiling point
158
Can assume changes in boiling and freezing point also depend on concentration of solute
They are colligative properties
159
Freezing-Point Depression
Freezing point of 1-molal solution of any nonelectrolyte solute in water is found (by experiment) to be 186 lower than freezing point of water When 1 mol of nonelectrolyte solute dissolved in 1 kg water freezing point is -186 not 000 160
When 2 mol nonelectrolyte solute dissolved in 1 kg water freezing point is -372
For any concentration of nonelectrolyte solute in water decrease in freezing point can be determined using value of -186m
161
This value called the molal freezing-point contstant (Kf) is the freezing-point depression of the solvent in a 1-molal solution of a nonvolatile nonelectrolyte solute
162
Each solvent has own characteristic molal freezing-point constant Freezing-point depression (Δtf) the difference between the freezing points of the pure solvent and a solute of nonelectrolyte in that solvent and it is directly proportional to the molal concentration of the solution Δtf = Kfm
163
Δtf = Kfm
Kf expressed as m
m expressed in mol solutekg solvent (molality)
164
Sample Problem
What is the freezing point depression of water in a solution of 171 g of sucrose C12H22O11 and 200 g of water What is the actual freezing point of the solution
165
1 AnalyzeGiven Solute mass and chemical formula 171 g C12H22O11 Solvent mass and identity 2000 g water Unknown a freezing-point depression b freezing point of the solution 166
2 Plan
167
168
Calculate
Unknown a freezing-point depression b freezing point of the solution
169
Practice Problem
A water solution containing an unknown quantity of a nonelectrolyte solute is found to have a freezing point of ndash 023degCWhat is the molal concentration of the solution
012 m170
A solution consists of 103 g of the nonelectrolyte glucose C6H12O6 dissolved in 250 g of water What is the freezing-point depression of the solution
minus0426degC171
In a laboratory experiment the freezing point of an aqueous solution of glucose is found to be minus0325degCWhat is the molal concentration of this solution
0175 m172
If 0500 mol of a nonelectrolyte solute are dissolved in 5000 g of ether what is the freezing point of the solution
minus1181degC173
The freezing point of an aqueous solution that contains a nonelectrolyte is minus90degC aWhat is the freezing-point depression of the solution bWhat is the molal concentration of the solution a minus90degC b 48 m
174
Boiling-Point Elevation
Boiling point is temp at which vapor pressure is equal to atmospheric pressure Change in vapor pressure will cause corresponding change in boiling point
175
176
Vapor pressure of nonvolatile solution is lower than vapor pressure of pure solvent More heat will be required to raise vapor pressure of solution Boiling point of solution is higher than boiling point of pure solvent
Molal boiling-point constant (Kb) - boiling-point elevation of the solvent in a 1-molal solution of a nonvolatile nonelectrolyte solute b-p elevation of any nonelectrolyte in water found by experiment to be 051 So molal b-p constant for water is 051m
177
For different solvents b-p elevations of 1-molal solutions have different values (Table 14-2)
Like freezing-point constants values most accurate for dilute solutions
178
Boiling-point elevation Δtb difference between the boiling points of the pure solvent and a nonelectrolyte solution of that solvent and it is directly proportional to the molal concentration of the solution B-p elevation can be calculated using Δtb = Kbm Δtb (boiling point elevation) expressed in m and m expressed in mol solutekg solvent
Sample Problem
What is the boiling-point elevation of a solution made from 200 g of a nonelectrolyte solute and 4000 g of water The molar mass of the solute is 620 gmol
180
1 AnalyzeGiven solute mass = 200 g solute molar mass = 620 gmol solvent mass and identity = 4000 g of water
Unknown boiling-point elevation
181
2 Plan
182
183
3 Compute
Practice Problem
A solution contains 500 g of sucrose C12H22O11 a nonelectrolyte dissolved in 5000 g of water What is the boiling-point elevation
015degC184
A solution contains 4500 g of sucrose C12H22O11 a nonelectrolyte dissolved in 250 g of waterWhat is the boiling point of the solution
1027degC185
If the boiling point elevation of an aqueous solution containing a nonvolatile electrolyte is 102degC what is the molality of the solution
20 m186
The boiling point of an aqueous solution containing a nonvolatile electrolyte is 10075degC aWhat is the boiling-point elevation bWhat is the molality of the solution a 075degC b 15 m
187
Electrolytes and Colligative PropertiesEarly investigators confused by experiments where certain substances depressed freezing point or elevated boiling point more than expected
188
189
Ex 01 m solution of NaCl lowers freezing point of solvent almost twice as much as 01 m solution of sucrose 01 m CaCl2 lowers fp almost 3 times as much as 01 m solution of sucrose
To understand why this happens contrast behavior or sucrose with NaCl in aqueous solution Each sucrose dissolves to make only 1 particle in solution So 1 mol sucrose dissolves to 1 mol particles 1 mol NaCl dissolves to make 2 moles of particles (1 mol Na+ and 1 mol Cl-)
190
191
Calculated Values for Electrolyte SolutionsColligative properties depend on total concentration of solute particles regardless of their identity Electrolytes cause changes in colligative properties proportional to total molality in terms of all dissolves particles instead of formula units
192
What about barium nitrate Ba(NO3)2 Each mole of barium nitrate yields 3 mol of ions in solution Ba(NO3)2(s) rarr Ba2+(aq) + 2NO3-(aq) You would expect solution of given molality to lower fp of solvent 3 times as much as nonelectrolytic solution of same molality
193
Sample Problem
What is the expected change in the freezing point of water in a solution of 625 g of barium nitrate Ba(NO3)2 in 100 kg of water
194
1 AnalyzeGiven solute mass and formula = 625 g Ba(NO3)2 solvent mass and identity = 100 kg water Δtf = Kfm
Unknown expected freezing-point depression
195
2 PlanThe molality can be calculated by converting the solute mass to moles and then dividing by the number of kilograms of solvent That molality is in terms of formula units of Ba(NO3)2 and must be converted to molality in terms of dissociated ions in solution It must be multiplied by the number of moles of ions produced per mole of formula unit This adjusted molality can then be used to calculate the freezing-point depression
196
197
198
3 Compute
Practice Problem
What is the expected freezing-point depression for a solution that contains 20 mol of magnesium sulfate dissolved in 10 kg of water
minus74degC199
What is the expected boiling-point elevation of water for a solution that contains 150 g of sodium chloride dissolved in 10 kg of water 27degC The freezing point of an aqueous sodium chloride solution is minus200degC What is the molality of the solution 54 m NaCl
200
Actual Values for Electrolyte SolutionsRemember values calculated are only expected values Actual values of colligative properties for all strong electrolytes are almost what would be expected Difference between expected and actual values caused by attractive forces that exist between dissociated ions in aqueous solution
201
Attraction between hydrated ions in solution is small compared with those in crystalline solid However forces of attraction donrsquot interfere with movements of aqueous ions More concentrated solution means ions closer together and attraction is greater Also depends on charge of ions ndash higher charge greater attraction
202
Supersaturated solution can remain unchanged over long time if it isnrsquot disturbed Once crystals begin to form continues until equilibrium is recreated at lower temp
41
Example
Sodium thiosulfate Na2S2O3 Solute added to hot water until solution saturated Hot solution filtered Drop small crystal in ldquoseedingrdquo crystallization Continues until equilibrium created
42
Solubility Values
Solubility - amount of substance required to form a saturated solution with specific amount of solvent at specified temp Ex Solubility of sugar = 204 g per 100 g water at 20 Must specify temp bc solubility changes with temp 43
For gases pressure must also be specified Rate at which substance dissolves is unrelated to solubility Max amount of solute that dissolves and reaches equilibrium is always same under same conditions
44
Solute-Solvent Interactions
ldquolike dissolves likerdquo is generally useful rule for predicting whether one substance will dissolve in another Depends on type of bonding polaritynonpolarity and intermolecular forces between solute and solvent
45
Dissolving Ionic Compounds in Aqueous Solution
Polarity of water molecules plays important role in formation of solution of ionic compounds Charged ends of water molecules attract ions and surround them to keep separated from other ions
46
Hydration - solution process with water as solvent Ions said to be hydrated As hydrated ions diffuse into solution other ions exposed and drawn away from crystal surface Entire crystal gradually dissolves
47
When crystallized from aqueous solutions some ionic compounds form crystals that include water molecules The crystalline compounds known as hydrates have specific ratios of water molecules Represented by formulas like CuSO45H2O
48
When crystalline hydrate dissolves in water water of hydration returns to solvent Behavior of solute in hydrated form no different than anhydrous form
49
Nonpolar Solvents
Nonpolar solvents do not attract ions of ionic compound strongly enough to overcome forces holding crystal together
50
Liquid Solutes and Solvents
When you shake bottle of salad dressing oil droplets disperse in water Stop shaking ndash strong attraction between water molecules squeeze out oil and form separate layers Liquid solutes and solvents that are not soluble in each other are immiscible
51
Nonpolar substances (fat oil grease) are generally quite soluble in nonpolar liquids (carbon tetrachloride toluene gasoline)
Only attractions between nonpolar molecules are weak London forces
52
London Dispersion Forces
bc e- are constantly moving at any time there may be uneven distribution of them around a nonpolar atom Temporarily creates positive end and negative end
53
54
London dispersion forces - intermolecular attractions resulting from constant motion of e- and creation of instantaneous dipoles
55
Liquids that dissolve freely in one another in any ratio are said to be completely miscible Benzene and carbon tetrachloride (both nonpolar) are completely miscible
Nonpolar molecules of these two apply no strong forces of attraction or repulsion and molecules mix freely
56
Ethanol and water are also completely miscible -OH group on ethanol is slightly polar
Can form H-bonds with water as well as other ethanol molecules Intermolecular forces in mixture are very similar to those in pure liquids so they are mutually soluble in any amount
57
Gasoline contains mainly nonpolar hydrocarbons
Excellent solvent for fats oils greases
Major intermolecular forces are weak London forces
58
Ethanol less polar than water more than carbon tetrachloride
Better solvent for less-polar substances bc of nonpolar region
59
Effects of Pressure on Solubility
Little effect on solids or liquids Increase pressure - increase solubilities of gases in liquids
60
When gas in contact with surface of liquid gas molecules can enter liquid As amount of dissolve gas increases some molecules start to escape and reenter gas phase
Eq eventually established between rates of enteringleaving gas phase
61
As long as eq undisturbed solubility of gas in liquid unchanged at given pressure
Gas + solvent solution
62
Increasing pressure of solute gas above solution puts stress on eq Molecules collide with surface more often Increase in pressure partially offset by increase in rate of gas molecules entering solution In turn increase in amount of dissolved gas causes increase in rate molecules escape liquid and becomes gas
63
Eventually eq restored at higher gas solubility
Expected from Le Chatelierrsquos principle
Increase in gas pressure causes eq to shift so fewer molecules are in gas phase
64
Henryrsquos Law
Henryrsquos law - solubility of a gas in liquid is directly proportional to the partial pressure of that gas on the surface of the liquid
Applies to gas-liquid solutions at constant temp
65
When mixture of ideal gases is limited in constant volume at constant temp each gas applies same pressure it would apply if it occupied the space alone
Assuming gases do not react each gas dissolves to the extent it would if no other gases were there
66
In carbonated drinks (Coke Pepsi etc) solubility of CO2 increased by increasing pressure
At bottling factory CO2 gas forced into solution of flavored water at pressure of 5-10 atm Gas-in-liquid solution then sealed in bottlescans
67
When cap removed pressure reduced to 1 atm and some CO2 escapes as gas bubbles Effervescence - rapid escape of a gas from a liquid in which it is dissolved
68
Effects of Temperature on Solubility
Letrsquos consider gas solubility Increasing temperature usually decreases gas solubility As temp increases average kinetic energy of molecules in solution increases Greater number of solute molecules escape from attraction of solvent and return to gas phase 69
At higher temps eq is reached with fewer gas molecules in solution Gases generally less soluble
70
Effect of temp on solubility of solids in liquids more difficult to predict
Often increasing temp increases solubility
However the same temp increase can result in large increase in solubility in one case and only slight increase in the next
71
72
enthalpies of Solution
Formation of solution comes with energy change Dissolve KI in water container feels cold Dissolve LiCl in water feels hot Formation of solid-liquid solution can absorb or release heat
73
During formation of solution solvent and solute particles experience changes in forces attracting them to other particles Before dissolving begins solvent molecules held by intermolecular forces (solvent-solvent attraction) In solute molecules held by Ifs (solute-solute attraction)
74
Energy is required to separate solute molecules and solvent molecules from their neighbors
A solute particle that is surrounded by solvent molecules is said to be solvated
75
Step 1 solute particles become separated from solid (energy absorbed) Step 2 solvent particles move apart to allow solute particles to enter liquid (energy absorbed)
76
Step 3 solvent particles attracted to and solvate solute particles (energy released)
77
Heat of solution - net amount of heat energy absorbed or released when a specific amount of solute dissolves in a solvent
Heat of solution negative (heat is released) if sums of energy in steps 1 and 2 is less than step 3
If 1+2 gt 3 HOS is positive (heat absorbed)78
In gaseous state molecules are so far apart there are nearly no forces between them Solute-solute interaction has little effect on heat of solution of a gas Energy released when gas dissolved in liquid bc attraction between solute gas and solvent molecules greater than energy needed to separate solvent molecules
79
Concentration of Solutions80
Concentration - measure of amount of solute in given amount of solvent or solution ldquodiluterdquo just means small amount of solute ldquoconcentratedrdquo just means relatively large amount of solute Unrelated to degree to which solution is saturated Ex Saturated solution of substance not very soluble might be dilute
81
Molarity
Molarity - number of moles of solute in one liter of solution To find molarity must know molar mass A ldquoone molarrdquo solution of NaOH is 1 mole NaOH per liter of solution (1 M)
82
1 mol NaOH = 400 g If this quantity dissolve in enough water to make EXACTLY 100 L solution solution is 1 M If 200 g NaOH dissolve in enough to make 100 L solution what is molarity 0500 M
83
1 M solution is not made by adding 1 mol solute to 1 L solvent That would be more than 1 L Instead 1 mol solute dissolved in less than 1 L Then diluted with more solvent to bring TOTAL VOLUME to 1 L
84
Practice ProblemYou have 340 L of solution that contains 900 g sodium chloride What is the molarity of that solution 0440 M NaCl You have 08 L of a 05 M HCl solution How many moles of HCl does this solution contain 04 mol HCl
85
What is the molarity of a solution composed of 585 g of potassium iodide dissolved in enough water to make a 0125 L of solution 0282 M KI
86
How many moles of H2SO4 are present in 0500 L of a 0150 M H2SO4 solution 00750 mol What volume of 300 M NaCl is needed for reaction that requires 1463 g of NaCl 0834 L
87
Molality
Molality - concentration of solution expressed in moles of solute per kilogram of solvent Solution that contains 1 mol solute (NaOH) dissolved in exactly 1 kg of solvent is ldquoone molalrdquo solution (1 m NaOH)
88
1 mol NaOH = 400 g 400 g NaOH dissolved in 1 kg water is 1 m NaOH
200 g NaOH in 1 kg water = 0500 m NaOH
89
Practice Problem
A solution was prepared by dissolving 171 g sucrose (C12H22O11) in 125 g of water Find the molal concentration of this solution 0400 m C12H22O11
90
A solution of iodine in carbon tetrachloride is used when iodine is needed for certain chemical tests How much iodine must be added to prepare a 0480 m solution of iodine in CCl4 if 1000 g of CCl4 is used 122 g I2
91
What is the molality of a solution composed of 255 g acetone (CH3)2CO dissolved in 200 g of water 22 m acetone What quantity in grams of methanol CH3OH is required to prepare a 0244 m solution in 400 g water 312 g methanol
92
How many grams of AgNO3 are needed to prepare a 0125 m solution in 250 mL of water 531 g AgNO3
What is the molality of a solution containing 182 g HCl and 250 g water 200 m
93
Ions in Aqueous Solutions and Colligative Properties
94
95
compounds in aqueous solutions
DissociationWhen compound made from ions dissolves in water ions separate Dissociation - separation of ions that occurs when an ionic compound dissolves
96
Notice number of ions made per formula unit in equations 1 formula unit of NaCl gives 2 units of ions in solution 1 formula unit of CaCl2 gives 3 units of ions
97
Assuming 100 dissociation solution that contains 1 mol NaCl contains 1 mol Na+ and 1 mol Cl- Can assume 100 dissociation of all soluble ionic compounds
98
Practice Problem
Write the equation for the dissolution of aluminum sulfate in water How many moles of aluminum ions are made by dissolving 1 mol aluminum sulfate What is the total number of moles of ions made by dissolving 1 mol of aluminum sulfate
99
1 AnalyzeGiven Amount of solute = 1 mol Al2(SO4)3 Solvent identity = water Unknown a moles of aluminum ions and sulfate ions Total number of moles of solute ions produced
100
2 Plan
The coefficients in the balanced dissociation equation will tell mole relationships You can use the equation to find out the number of moles of solute ions produced
101
3 Compute
a
b102
Write the equation for the dissolution of each of the following in water and then determine the number of moles of each ions made as well as the total number of ions made a 1 mol ammonium chloride b 1 mol sodium sulfide c 05 mol barium nitrate
103
a 1 mol ammonium chloride
1 mol NH4+ 1 mol Cl- 2 mol total ions
104
b 1 mol sodium sulfide
2 mol Na+ 1 mol S-2 3 mol total ions
105
c 05 mol barium nitrate
05 mol Ba+2 1 mol NO3- 15 mol total ions
106
Precipitation Reactions
Even though no compound is completely insoluble compounds of very low solubility can be considered insoluble for practical purposes There are general guidelines to help predict whether a compound made of certain combination of ions is soluble
107
General Solubility Guidelines
Sodium potassium and ammonium compounds are soluble in water
108
Nitrates acetates and chlorates are soluble
109
110
Most chlorides are soluble except those of silver mercury (I) and lead Lead (II) chloride is soluble in hot water
Most sulfates are soluble except those of barium strontium lead calcium and mercury
111
Most carbonates phosphates and silicates are insoluble except those of sodium potassium and ammonium
112
Most sulfides are insoluble except those of calcium strontium sodium potassium and ammonium
113
Is calcium phosphate Ca3(PO4)2 soluble or insoluble
Not one of the exceptions so it is insoluble
Dissociation equations are not written for insoluble compounds
114
Guidelines also useful in predicting what will happen if 2 solutions of 2 different soluble compounds are mixed
If mixing results in combination of ions that forms insoluble compound a double-replacement and precipitation reaction will happen
115
Will precipitate form when solutions of ammonium sulfide and cadmium nitrate are combined You can tell calcium nitrate is soluble from the guidelines (Most nitrates acetates and chlorates are soluble) You can also tell ammonium sulfide is soluble (Most sulfides are insoluble except those of calcium strontium sodium potassium and ammonium)
116
117
To decide whether precipitate can form must know solubilities of two possible products
Ammonium nitrate ndash soluble (Most nitrates acetates and chlorates are soluble) Cadmium sulfide ndash insoluble (Most sulfides are insoluble except those of calcium strontium sodium potassium and ammonium)
118
bc one product is insoluble double-replacement and precipitation reaction will happen (NH4)2S (aq) + Cd(NO3)2 (aq) rarr NH4NO3 (aq) + CdS (s)
119
Net Ionic Equations
Reactions of ions in aqueous solution usually represented by net ionic equations instead of formula equations Net ionic equation - includes only those compounds and ions that undergo a chemical change in a reaction in an aqueous solution
120
121
To write NIE (net ionic equation) first convert chemical equation into overall ionic equation All soluble ionic compounds shown as dissociated ions in solution Precipitates shown as solids
122
Notice ammonium and nitrate ions appear on both sides They havenrsquot gone through any chemical change Spectator ions - ions that do not take part in a chemical reaction and are found in solution both before and after the reaction
123
To change ionic equation to NIE eliminate spectator ions Cd+2(aq) + S-2(aq) rarr CdS(s)
This NIE applies to any reaction in which precipitate of CdS forms
124
Practice Problem
Identify the precipitate that forms when aqueous solutions of zinc nitrate and ammonium sulfide are combined Write the equation for the possible double-replacement reaction Then write the formula equation overall ionic equation and the net ionic equation
125
1 AnalyzeGiven Identity of reactants zinc nitrate and ammonium sulfide Reaction medium aqueous solution Unknown a equation for the possible double-replacement reaction b identity of the precipitate c formula equation d overall ionic equation e net ionic equation
126
2 PlanWrite the possible double-replacement reaction between Zn(NO3)2 and (NH4)2S Use the rules to determine if any of the products will precipitate Write a formula equation and overall net ionic equation Then cancel spectator ions to make net ionic equation
127
3 Computea possible double-replacement reaction
b rules show that zinc sulfide is not soluble so will precipitate (ammonium nitrate is soluble)
c formula equation128
d overall ionic equation
e net ionic equation129
Will a precipitate form if solutions of potassium sulfate and barium nitrate are combined If so write the net ionic equation for the reaction
Yes Ba+2(aq) + SO4-2(aq) rarr BaSO4(s)
130
Will a precipitate form if solutions of potassium nitrate and magnesium sulfate are combined If so write the net ionic equation for the reaction
No 131
Will a precipitate form if solutions of barium chloride and sodium sulfate are combined If so identify the spectator ions and write the net ionic equation for the reaction
Yes Na+ and Cl- Ba+2(aq) + SO4-2(aq) rarr BaSO4(s)
132
Write the net ionic equation for the precipitation of nickel(II) sulfide
Ni+2(aq) + S-2(aq) rarr NiS(s)
133
IonizationSome molecular compounds can also form ions in solution Usually polar Ionization - ions formed from solute molecules by action of the solvent (creation of ions where there were none) Ionization different from dissociation
134
Dissociation ionic compounds dissolve and ions already present separate Ionization ions formed where non existed before
Ions formed are hydrated Heat released during hydration of ions gives enough energy to break covalent bonds
135
Extent to which solute ionizes depends on strength of bonds within molecules of solute and strength of attraction between solute and solvent molecules
136
HCl ndash molecular compound that ionizes in aqueous solution Contains highly polar bond Attraction between polar HCl molecule and polar water strong enough to break HCl bond
137
The Hydronium Ion
H+ ions from HCl attracts other molecules or ions so strongly that it doesnrsquot normally exist alone Ionization of HCl better described as direct transfer of proton from HCl to H2O forming H3O+
138
139
Hydration of H+ to form hydronium is highly exothermic
Energy released gives a lot of energy needed to ionize a molecular solute
Many molecular compounds that ionize in aqueous solution contain hydrogen and form hydronium ion
140
Strong and Weak Electrolytes
Substances that make ions and conduct current in solution are electrolytes HCl is one of series of compounds that has hydrogen and halogen Hydrogen halides are all molecular compounds with single polar-covalent bonds
141
All are gases Very soluble in water All are electrolytes HCl HBr HI strongly conduct current in solution HF only weakly conducts current at same concentration Strength of conduction related to ability to ionize
142
143
Strong Electrolytes
HCl HBr HI are 100 ionized in solution
Strong electrolyte - any compound whose dilute aqueous solutions conduct electricity well this is due to the presence of all or almost all of the dissolve compound in the form of ions
144
Unique characteristic of strong electrolyte is that they yield only ions
Ex Ionic compound may be highly soluble in water and dissociate into ions (NaCl)
Others may not dissolve much but amount that does dissolve exists only as ions
145
Weak ElectrolytesHF dissolves in water to give acid solution called hydrofluoric acid HF bond stronger than bonds between hydrogen and other halogens When HF dissolves some molecule ionize Reverse reaction also happens HF(aq) + H2O(l) H3O+(aq) + F-(aq)
146
HF(aq) + H2O(l) H3O+
(aq) + F-(aq)Concentration of dissolved unionized HF stays high and concentration of ions stays low
Weak electrolyte - any compound whose aqueous solutions conduct electricity poorly this is due to the presence of small amount of the dissolved compound in the form of ions
147
Different from nonelectrolyte ndash NO ions at all Description of electrolyte as strong or weak not related to concentration of solution Electrolytes differ in degree of ionization not amount of solute dissolved
148
149
Colligative properties of solutions
Presence of solute affects properties of solutions Some properties not dependent on nature of dissolved substance but on how may dissolve particles are present
Colligative properties - properties that depend on the concentration of solute particles but not on their identity
150
Vapor-Pressure Lowering
Boiling and freezing point of solution different from pure solvent Nonvolatile solute raises boiling point and lowers freezing point Nonvolatile substance - one that has little tendency to become gas under existing conditions
151
152
To understand why nonvolatile solute changes boiling and freezing point consider equilibrium vapor pressure
Vapor pressure pressure caused by molecules that have escape liquid phase to gas phase
Can be thought of a measure of tendency of molecules to escape from a liquid
153
Addition of sucrose (nonvolatile solute) lowers concentration of water molecules at surface of liquid This lowers tendency of water molecules to leave solution and enter gas phase Vapor pressure of solution is lower than vapor pressure of pure water
154
Aqueous13 solution13 of13 nonvolatile13 solutePure13 water
Nonelectrolyte solutions of same molality have same concentration of particles
Dilute solutions of same solvent and equal molality of any nonelectrolyte solute lower vapor pressure equally
155
Example
1 m aqueous solution of nonelectrolyte glucose lowers vapor pressure of water 55 x 10-4 atm at 25 1 m aqueous solution of sucrose also lowers water vapor pressure to 55 x 10-4 atm at 25
156
157
bc vapor-pressure lowering depends on concentration of nonelectrolyte solute and doesnrsquot depend on the type of solute it is a colligative proprerty
bc vapor pressure has been lowered solution remains liquid over larger temperature range
Lowers freezing point and raises boiling point
158
Can assume changes in boiling and freezing point also depend on concentration of solute
They are colligative properties
159
Freezing-Point Depression
Freezing point of 1-molal solution of any nonelectrolyte solute in water is found (by experiment) to be 186 lower than freezing point of water When 1 mol of nonelectrolyte solute dissolved in 1 kg water freezing point is -186 not 000 160
When 2 mol nonelectrolyte solute dissolved in 1 kg water freezing point is -372
For any concentration of nonelectrolyte solute in water decrease in freezing point can be determined using value of -186m
161
This value called the molal freezing-point contstant (Kf) is the freezing-point depression of the solvent in a 1-molal solution of a nonvolatile nonelectrolyte solute
162
Each solvent has own characteristic molal freezing-point constant Freezing-point depression (Δtf) the difference between the freezing points of the pure solvent and a solute of nonelectrolyte in that solvent and it is directly proportional to the molal concentration of the solution Δtf = Kfm
163
Δtf = Kfm
Kf expressed as m
m expressed in mol solutekg solvent (molality)
164
Sample Problem
What is the freezing point depression of water in a solution of 171 g of sucrose C12H22O11 and 200 g of water What is the actual freezing point of the solution
165
1 AnalyzeGiven Solute mass and chemical formula 171 g C12H22O11 Solvent mass and identity 2000 g water Unknown a freezing-point depression b freezing point of the solution 166
2 Plan
167
168
Calculate
Unknown a freezing-point depression b freezing point of the solution
169
Practice Problem
A water solution containing an unknown quantity of a nonelectrolyte solute is found to have a freezing point of ndash 023degCWhat is the molal concentration of the solution
012 m170
A solution consists of 103 g of the nonelectrolyte glucose C6H12O6 dissolved in 250 g of water What is the freezing-point depression of the solution
minus0426degC171
In a laboratory experiment the freezing point of an aqueous solution of glucose is found to be minus0325degCWhat is the molal concentration of this solution
0175 m172
If 0500 mol of a nonelectrolyte solute are dissolved in 5000 g of ether what is the freezing point of the solution
minus1181degC173
The freezing point of an aqueous solution that contains a nonelectrolyte is minus90degC aWhat is the freezing-point depression of the solution bWhat is the molal concentration of the solution a minus90degC b 48 m
174
Boiling-Point Elevation
Boiling point is temp at which vapor pressure is equal to atmospheric pressure Change in vapor pressure will cause corresponding change in boiling point
175
176
Vapor pressure of nonvolatile solution is lower than vapor pressure of pure solvent More heat will be required to raise vapor pressure of solution Boiling point of solution is higher than boiling point of pure solvent
Molal boiling-point constant (Kb) - boiling-point elevation of the solvent in a 1-molal solution of a nonvolatile nonelectrolyte solute b-p elevation of any nonelectrolyte in water found by experiment to be 051 So molal b-p constant for water is 051m
177
For different solvents b-p elevations of 1-molal solutions have different values (Table 14-2)
Like freezing-point constants values most accurate for dilute solutions
178
Boiling-point elevation Δtb difference between the boiling points of the pure solvent and a nonelectrolyte solution of that solvent and it is directly proportional to the molal concentration of the solution B-p elevation can be calculated using Δtb = Kbm Δtb (boiling point elevation) expressed in m and m expressed in mol solutekg solvent
Sample Problem
What is the boiling-point elevation of a solution made from 200 g of a nonelectrolyte solute and 4000 g of water The molar mass of the solute is 620 gmol
180
1 AnalyzeGiven solute mass = 200 g solute molar mass = 620 gmol solvent mass and identity = 4000 g of water
Unknown boiling-point elevation
181
2 Plan
182
183
3 Compute
Practice Problem
A solution contains 500 g of sucrose C12H22O11 a nonelectrolyte dissolved in 5000 g of water What is the boiling-point elevation
015degC184
A solution contains 4500 g of sucrose C12H22O11 a nonelectrolyte dissolved in 250 g of waterWhat is the boiling point of the solution
1027degC185
If the boiling point elevation of an aqueous solution containing a nonvolatile electrolyte is 102degC what is the molality of the solution
20 m186
The boiling point of an aqueous solution containing a nonvolatile electrolyte is 10075degC aWhat is the boiling-point elevation bWhat is the molality of the solution a 075degC b 15 m
187
Electrolytes and Colligative PropertiesEarly investigators confused by experiments where certain substances depressed freezing point or elevated boiling point more than expected
188
189
Ex 01 m solution of NaCl lowers freezing point of solvent almost twice as much as 01 m solution of sucrose 01 m CaCl2 lowers fp almost 3 times as much as 01 m solution of sucrose
To understand why this happens contrast behavior or sucrose with NaCl in aqueous solution Each sucrose dissolves to make only 1 particle in solution So 1 mol sucrose dissolves to 1 mol particles 1 mol NaCl dissolves to make 2 moles of particles (1 mol Na+ and 1 mol Cl-)
190
191
Calculated Values for Electrolyte SolutionsColligative properties depend on total concentration of solute particles regardless of their identity Electrolytes cause changes in colligative properties proportional to total molality in terms of all dissolves particles instead of formula units
192
What about barium nitrate Ba(NO3)2 Each mole of barium nitrate yields 3 mol of ions in solution Ba(NO3)2(s) rarr Ba2+(aq) + 2NO3-(aq) You would expect solution of given molality to lower fp of solvent 3 times as much as nonelectrolytic solution of same molality
193
Sample Problem
What is the expected change in the freezing point of water in a solution of 625 g of barium nitrate Ba(NO3)2 in 100 kg of water
194
1 AnalyzeGiven solute mass and formula = 625 g Ba(NO3)2 solvent mass and identity = 100 kg water Δtf = Kfm
Unknown expected freezing-point depression
195
2 PlanThe molality can be calculated by converting the solute mass to moles and then dividing by the number of kilograms of solvent That molality is in terms of formula units of Ba(NO3)2 and must be converted to molality in terms of dissociated ions in solution It must be multiplied by the number of moles of ions produced per mole of formula unit This adjusted molality can then be used to calculate the freezing-point depression
196
197
198
3 Compute
Practice Problem
What is the expected freezing-point depression for a solution that contains 20 mol of magnesium sulfate dissolved in 10 kg of water
minus74degC199
What is the expected boiling-point elevation of water for a solution that contains 150 g of sodium chloride dissolved in 10 kg of water 27degC The freezing point of an aqueous sodium chloride solution is minus200degC What is the molality of the solution 54 m NaCl
200
Actual Values for Electrolyte SolutionsRemember values calculated are only expected values Actual values of colligative properties for all strong electrolytes are almost what would be expected Difference between expected and actual values caused by attractive forces that exist between dissociated ions in aqueous solution
201
Attraction between hydrated ions in solution is small compared with those in crystalline solid However forces of attraction donrsquot interfere with movements of aqueous ions More concentrated solution means ions closer together and attraction is greater Also depends on charge of ions ndash higher charge greater attraction
202
Example
Sodium thiosulfate Na2S2O3 Solute added to hot water until solution saturated Hot solution filtered Drop small crystal in ldquoseedingrdquo crystallization Continues until equilibrium created
42
Solubility Values
Solubility - amount of substance required to form a saturated solution with specific amount of solvent at specified temp Ex Solubility of sugar = 204 g per 100 g water at 20 Must specify temp bc solubility changes with temp 43
For gases pressure must also be specified Rate at which substance dissolves is unrelated to solubility Max amount of solute that dissolves and reaches equilibrium is always same under same conditions
44
Solute-Solvent Interactions
ldquolike dissolves likerdquo is generally useful rule for predicting whether one substance will dissolve in another Depends on type of bonding polaritynonpolarity and intermolecular forces between solute and solvent
45
Dissolving Ionic Compounds in Aqueous Solution
Polarity of water molecules plays important role in formation of solution of ionic compounds Charged ends of water molecules attract ions and surround them to keep separated from other ions
46
Hydration - solution process with water as solvent Ions said to be hydrated As hydrated ions diffuse into solution other ions exposed and drawn away from crystal surface Entire crystal gradually dissolves
47
When crystallized from aqueous solutions some ionic compounds form crystals that include water molecules The crystalline compounds known as hydrates have specific ratios of water molecules Represented by formulas like CuSO45H2O
48
When crystalline hydrate dissolves in water water of hydration returns to solvent Behavior of solute in hydrated form no different than anhydrous form
49
Nonpolar Solvents
Nonpolar solvents do not attract ions of ionic compound strongly enough to overcome forces holding crystal together
50
Liquid Solutes and Solvents
When you shake bottle of salad dressing oil droplets disperse in water Stop shaking ndash strong attraction between water molecules squeeze out oil and form separate layers Liquid solutes and solvents that are not soluble in each other are immiscible
51
Nonpolar substances (fat oil grease) are generally quite soluble in nonpolar liquids (carbon tetrachloride toluene gasoline)
Only attractions between nonpolar molecules are weak London forces
52
London Dispersion Forces
bc e- are constantly moving at any time there may be uneven distribution of them around a nonpolar atom Temporarily creates positive end and negative end
53
54
London dispersion forces - intermolecular attractions resulting from constant motion of e- and creation of instantaneous dipoles
55
Liquids that dissolve freely in one another in any ratio are said to be completely miscible Benzene and carbon tetrachloride (both nonpolar) are completely miscible
Nonpolar molecules of these two apply no strong forces of attraction or repulsion and molecules mix freely
56
Ethanol and water are also completely miscible -OH group on ethanol is slightly polar
Can form H-bonds with water as well as other ethanol molecules Intermolecular forces in mixture are very similar to those in pure liquids so they are mutually soluble in any amount
57
Gasoline contains mainly nonpolar hydrocarbons
Excellent solvent for fats oils greases
Major intermolecular forces are weak London forces
58
Ethanol less polar than water more than carbon tetrachloride
Better solvent for less-polar substances bc of nonpolar region
59
Effects of Pressure on Solubility
Little effect on solids or liquids Increase pressure - increase solubilities of gases in liquids
60
When gas in contact with surface of liquid gas molecules can enter liquid As amount of dissolve gas increases some molecules start to escape and reenter gas phase
Eq eventually established between rates of enteringleaving gas phase
61
As long as eq undisturbed solubility of gas in liquid unchanged at given pressure
Gas + solvent solution
62
Increasing pressure of solute gas above solution puts stress on eq Molecules collide with surface more often Increase in pressure partially offset by increase in rate of gas molecules entering solution In turn increase in amount of dissolved gas causes increase in rate molecules escape liquid and becomes gas
63
Eventually eq restored at higher gas solubility
Expected from Le Chatelierrsquos principle
Increase in gas pressure causes eq to shift so fewer molecules are in gas phase
64
Henryrsquos Law
Henryrsquos law - solubility of a gas in liquid is directly proportional to the partial pressure of that gas on the surface of the liquid
Applies to gas-liquid solutions at constant temp
65
When mixture of ideal gases is limited in constant volume at constant temp each gas applies same pressure it would apply if it occupied the space alone
Assuming gases do not react each gas dissolves to the extent it would if no other gases were there
66
In carbonated drinks (Coke Pepsi etc) solubility of CO2 increased by increasing pressure
At bottling factory CO2 gas forced into solution of flavored water at pressure of 5-10 atm Gas-in-liquid solution then sealed in bottlescans
67
When cap removed pressure reduced to 1 atm and some CO2 escapes as gas bubbles Effervescence - rapid escape of a gas from a liquid in which it is dissolved
68
Effects of Temperature on Solubility
Letrsquos consider gas solubility Increasing temperature usually decreases gas solubility As temp increases average kinetic energy of molecules in solution increases Greater number of solute molecules escape from attraction of solvent and return to gas phase 69
At higher temps eq is reached with fewer gas molecules in solution Gases generally less soluble
70
Effect of temp on solubility of solids in liquids more difficult to predict
Often increasing temp increases solubility
However the same temp increase can result in large increase in solubility in one case and only slight increase in the next
71
72
enthalpies of Solution
Formation of solution comes with energy change Dissolve KI in water container feels cold Dissolve LiCl in water feels hot Formation of solid-liquid solution can absorb or release heat
73
During formation of solution solvent and solute particles experience changes in forces attracting them to other particles Before dissolving begins solvent molecules held by intermolecular forces (solvent-solvent attraction) In solute molecules held by Ifs (solute-solute attraction)
74
Energy is required to separate solute molecules and solvent molecules from their neighbors
A solute particle that is surrounded by solvent molecules is said to be solvated
75
Step 1 solute particles become separated from solid (energy absorbed) Step 2 solvent particles move apart to allow solute particles to enter liquid (energy absorbed)
76
Step 3 solvent particles attracted to and solvate solute particles (energy released)
77
Heat of solution - net amount of heat energy absorbed or released when a specific amount of solute dissolves in a solvent
Heat of solution negative (heat is released) if sums of energy in steps 1 and 2 is less than step 3
If 1+2 gt 3 HOS is positive (heat absorbed)78
In gaseous state molecules are so far apart there are nearly no forces between them Solute-solute interaction has little effect on heat of solution of a gas Energy released when gas dissolved in liquid bc attraction between solute gas and solvent molecules greater than energy needed to separate solvent molecules
79
Concentration of Solutions80
Concentration - measure of amount of solute in given amount of solvent or solution ldquodiluterdquo just means small amount of solute ldquoconcentratedrdquo just means relatively large amount of solute Unrelated to degree to which solution is saturated Ex Saturated solution of substance not very soluble might be dilute
81
Molarity
Molarity - number of moles of solute in one liter of solution To find molarity must know molar mass A ldquoone molarrdquo solution of NaOH is 1 mole NaOH per liter of solution (1 M)
82
1 mol NaOH = 400 g If this quantity dissolve in enough water to make EXACTLY 100 L solution solution is 1 M If 200 g NaOH dissolve in enough to make 100 L solution what is molarity 0500 M
83
1 M solution is not made by adding 1 mol solute to 1 L solvent That would be more than 1 L Instead 1 mol solute dissolved in less than 1 L Then diluted with more solvent to bring TOTAL VOLUME to 1 L
84
Practice ProblemYou have 340 L of solution that contains 900 g sodium chloride What is the molarity of that solution 0440 M NaCl You have 08 L of a 05 M HCl solution How many moles of HCl does this solution contain 04 mol HCl
85
What is the molarity of a solution composed of 585 g of potassium iodide dissolved in enough water to make a 0125 L of solution 0282 M KI
86
How many moles of H2SO4 are present in 0500 L of a 0150 M H2SO4 solution 00750 mol What volume of 300 M NaCl is needed for reaction that requires 1463 g of NaCl 0834 L
87
Molality
Molality - concentration of solution expressed in moles of solute per kilogram of solvent Solution that contains 1 mol solute (NaOH) dissolved in exactly 1 kg of solvent is ldquoone molalrdquo solution (1 m NaOH)
88
1 mol NaOH = 400 g 400 g NaOH dissolved in 1 kg water is 1 m NaOH
200 g NaOH in 1 kg water = 0500 m NaOH
89
Practice Problem
A solution was prepared by dissolving 171 g sucrose (C12H22O11) in 125 g of water Find the molal concentration of this solution 0400 m C12H22O11
90
A solution of iodine in carbon tetrachloride is used when iodine is needed for certain chemical tests How much iodine must be added to prepare a 0480 m solution of iodine in CCl4 if 1000 g of CCl4 is used 122 g I2
91
What is the molality of a solution composed of 255 g acetone (CH3)2CO dissolved in 200 g of water 22 m acetone What quantity in grams of methanol CH3OH is required to prepare a 0244 m solution in 400 g water 312 g methanol
92
How many grams of AgNO3 are needed to prepare a 0125 m solution in 250 mL of water 531 g AgNO3
What is the molality of a solution containing 182 g HCl and 250 g water 200 m
93
Ions in Aqueous Solutions and Colligative Properties
94
95
compounds in aqueous solutions
DissociationWhen compound made from ions dissolves in water ions separate Dissociation - separation of ions that occurs when an ionic compound dissolves
96
Notice number of ions made per formula unit in equations 1 formula unit of NaCl gives 2 units of ions in solution 1 formula unit of CaCl2 gives 3 units of ions
97
Assuming 100 dissociation solution that contains 1 mol NaCl contains 1 mol Na+ and 1 mol Cl- Can assume 100 dissociation of all soluble ionic compounds
98
Practice Problem
Write the equation for the dissolution of aluminum sulfate in water How many moles of aluminum ions are made by dissolving 1 mol aluminum sulfate What is the total number of moles of ions made by dissolving 1 mol of aluminum sulfate
99
1 AnalyzeGiven Amount of solute = 1 mol Al2(SO4)3 Solvent identity = water Unknown a moles of aluminum ions and sulfate ions Total number of moles of solute ions produced
100
2 Plan
The coefficients in the balanced dissociation equation will tell mole relationships You can use the equation to find out the number of moles of solute ions produced
101
3 Compute
a
b102
Write the equation for the dissolution of each of the following in water and then determine the number of moles of each ions made as well as the total number of ions made a 1 mol ammonium chloride b 1 mol sodium sulfide c 05 mol barium nitrate
103
a 1 mol ammonium chloride
1 mol NH4+ 1 mol Cl- 2 mol total ions
104
b 1 mol sodium sulfide
2 mol Na+ 1 mol S-2 3 mol total ions
105
c 05 mol barium nitrate
05 mol Ba+2 1 mol NO3- 15 mol total ions
106
Precipitation Reactions
Even though no compound is completely insoluble compounds of very low solubility can be considered insoluble for practical purposes There are general guidelines to help predict whether a compound made of certain combination of ions is soluble
107
General Solubility Guidelines
Sodium potassium and ammonium compounds are soluble in water
108
Nitrates acetates and chlorates are soluble
109
110
Most chlorides are soluble except those of silver mercury (I) and lead Lead (II) chloride is soluble in hot water
Most sulfates are soluble except those of barium strontium lead calcium and mercury
111
Most carbonates phosphates and silicates are insoluble except those of sodium potassium and ammonium
112
Most sulfides are insoluble except those of calcium strontium sodium potassium and ammonium
113
Is calcium phosphate Ca3(PO4)2 soluble or insoluble
Not one of the exceptions so it is insoluble
Dissociation equations are not written for insoluble compounds
114
Guidelines also useful in predicting what will happen if 2 solutions of 2 different soluble compounds are mixed
If mixing results in combination of ions that forms insoluble compound a double-replacement and precipitation reaction will happen
115
Will precipitate form when solutions of ammonium sulfide and cadmium nitrate are combined You can tell calcium nitrate is soluble from the guidelines (Most nitrates acetates and chlorates are soluble) You can also tell ammonium sulfide is soluble (Most sulfides are insoluble except those of calcium strontium sodium potassium and ammonium)
116
117
To decide whether precipitate can form must know solubilities of two possible products
Ammonium nitrate ndash soluble (Most nitrates acetates and chlorates are soluble) Cadmium sulfide ndash insoluble (Most sulfides are insoluble except those of calcium strontium sodium potassium and ammonium)
118
bc one product is insoluble double-replacement and precipitation reaction will happen (NH4)2S (aq) + Cd(NO3)2 (aq) rarr NH4NO3 (aq) + CdS (s)
119
Net Ionic Equations
Reactions of ions in aqueous solution usually represented by net ionic equations instead of formula equations Net ionic equation - includes only those compounds and ions that undergo a chemical change in a reaction in an aqueous solution
120
121
To write NIE (net ionic equation) first convert chemical equation into overall ionic equation All soluble ionic compounds shown as dissociated ions in solution Precipitates shown as solids
122
Notice ammonium and nitrate ions appear on both sides They havenrsquot gone through any chemical change Spectator ions - ions that do not take part in a chemical reaction and are found in solution both before and after the reaction
123
To change ionic equation to NIE eliminate spectator ions Cd+2(aq) + S-2(aq) rarr CdS(s)
This NIE applies to any reaction in which precipitate of CdS forms
124
Practice Problem
Identify the precipitate that forms when aqueous solutions of zinc nitrate and ammonium sulfide are combined Write the equation for the possible double-replacement reaction Then write the formula equation overall ionic equation and the net ionic equation
125
1 AnalyzeGiven Identity of reactants zinc nitrate and ammonium sulfide Reaction medium aqueous solution Unknown a equation for the possible double-replacement reaction b identity of the precipitate c formula equation d overall ionic equation e net ionic equation
126
2 PlanWrite the possible double-replacement reaction between Zn(NO3)2 and (NH4)2S Use the rules to determine if any of the products will precipitate Write a formula equation and overall net ionic equation Then cancel spectator ions to make net ionic equation
127
3 Computea possible double-replacement reaction
b rules show that zinc sulfide is not soluble so will precipitate (ammonium nitrate is soluble)
c formula equation128
d overall ionic equation
e net ionic equation129
Will a precipitate form if solutions of potassium sulfate and barium nitrate are combined If so write the net ionic equation for the reaction
Yes Ba+2(aq) + SO4-2(aq) rarr BaSO4(s)
130
Will a precipitate form if solutions of potassium nitrate and magnesium sulfate are combined If so write the net ionic equation for the reaction
No 131
Will a precipitate form if solutions of barium chloride and sodium sulfate are combined If so identify the spectator ions and write the net ionic equation for the reaction
Yes Na+ and Cl- Ba+2(aq) + SO4-2(aq) rarr BaSO4(s)
132
Write the net ionic equation for the precipitation of nickel(II) sulfide
Ni+2(aq) + S-2(aq) rarr NiS(s)
133
IonizationSome molecular compounds can also form ions in solution Usually polar Ionization - ions formed from solute molecules by action of the solvent (creation of ions where there were none) Ionization different from dissociation
134
Dissociation ionic compounds dissolve and ions already present separate Ionization ions formed where non existed before
Ions formed are hydrated Heat released during hydration of ions gives enough energy to break covalent bonds
135
Extent to which solute ionizes depends on strength of bonds within molecules of solute and strength of attraction between solute and solvent molecules
136
HCl ndash molecular compound that ionizes in aqueous solution Contains highly polar bond Attraction between polar HCl molecule and polar water strong enough to break HCl bond
137
The Hydronium Ion
H+ ions from HCl attracts other molecules or ions so strongly that it doesnrsquot normally exist alone Ionization of HCl better described as direct transfer of proton from HCl to H2O forming H3O+
138
139
Hydration of H+ to form hydronium is highly exothermic
Energy released gives a lot of energy needed to ionize a molecular solute
Many molecular compounds that ionize in aqueous solution contain hydrogen and form hydronium ion
140
Strong and Weak Electrolytes
Substances that make ions and conduct current in solution are electrolytes HCl is one of series of compounds that has hydrogen and halogen Hydrogen halides are all molecular compounds with single polar-covalent bonds
141
All are gases Very soluble in water All are electrolytes HCl HBr HI strongly conduct current in solution HF only weakly conducts current at same concentration Strength of conduction related to ability to ionize
142
143
Strong Electrolytes
HCl HBr HI are 100 ionized in solution
Strong electrolyte - any compound whose dilute aqueous solutions conduct electricity well this is due to the presence of all or almost all of the dissolve compound in the form of ions
144
Unique characteristic of strong electrolyte is that they yield only ions
Ex Ionic compound may be highly soluble in water and dissociate into ions (NaCl)
Others may not dissolve much but amount that does dissolve exists only as ions
145
Weak ElectrolytesHF dissolves in water to give acid solution called hydrofluoric acid HF bond stronger than bonds between hydrogen and other halogens When HF dissolves some molecule ionize Reverse reaction also happens HF(aq) + H2O(l) H3O+(aq) + F-(aq)
146
HF(aq) + H2O(l) H3O+
(aq) + F-(aq)Concentration of dissolved unionized HF stays high and concentration of ions stays low
Weak electrolyte - any compound whose aqueous solutions conduct electricity poorly this is due to the presence of small amount of the dissolved compound in the form of ions
147
Different from nonelectrolyte ndash NO ions at all Description of electrolyte as strong or weak not related to concentration of solution Electrolytes differ in degree of ionization not amount of solute dissolved
148
149
Colligative properties of solutions
Presence of solute affects properties of solutions Some properties not dependent on nature of dissolved substance but on how may dissolve particles are present
Colligative properties - properties that depend on the concentration of solute particles but not on their identity
150
Vapor-Pressure Lowering
Boiling and freezing point of solution different from pure solvent Nonvolatile solute raises boiling point and lowers freezing point Nonvolatile substance - one that has little tendency to become gas under existing conditions
151
152
To understand why nonvolatile solute changes boiling and freezing point consider equilibrium vapor pressure
Vapor pressure pressure caused by molecules that have escape liquid phase to gas phase
Can be thought of a measure of tendency of molecules to escape from a liquid
153
Addition of sucrose (nonvolatile solute) lowers concentration of water molecules at surface of liquid This lowers tendency of water molecules to leave solution and enter gas phase Vapor pressure of solution is lower than vapor pressure of pure water
154
Aqueous13 solution13 of13 nonvolatile13 solutePure13 water
Nonelectrolyte solutions of same molality have same concentration of particles
Dilute solutions of same solvent and equal molality of any nonelectrolyte solute lower vapor pressure equally
155
Example
1 m aqueous solution of nonelectrolyte glucose lowers vapor pressure of water 55 x 10-4 atm at 25 1 m aqueous solution of sucrose also lowers water vapor pressure to 55 x 10-4 atm at 25
156
157
bc vapor-pressure lowering depends on concentration of nonelectrolyte solute and doesnrsquot depend on the type of solute it is a colligative proprerty
bc vapor pressure has been lowered solution remains liquid over larger temperature range
Lowers freezing point and raises boiling point
158
Can assume changes in boiling and freezing point also depend on concentration of solute
They are colligative properties
159
Freezing-Point Depression
Freezing point of 1-molal solution of any nonelectrolyte solute in water is found (by experiment) to be 186 lower than freezing point of water When 1 mol of nonelectrolyte solute dissolved in 1 kg water freezing point is -186 not 000 160
When 2 mol nonelectrolyte solute dissolved in 1 kg water freezing point is -372
For any concentration of nonelectrolyte solute in water decrease in freezing point can be determined using value of -186m
161
This value called the molal freezing-point contstant (Kf) is the freezing-point depression of the solvent in a 1-molal solution of a nonvolatile nonelectrolyte solute
162
Each solvent has own characteristic molal freezing-point constant Freezing-point depression (Δtf) the difference between the freezing points of the pure solvent and a solute of nonelectrolyte in that solvent and it is directly proportional to the molal concentration of the solution Δtf = Kfm
163
Δtf = Kfm
Kf expressed as m
m expressed in mol solutekg solvent (molality)
164
Sample Problem
What is the freezing point depression of water in a solution of 171 g of sucrose C12H22O11 and 200 g of water What is the actual freezing point of the solution
165
1 AnalyzeGiven Solute mass and chemical formula 171 g C12H22O11 Solvent mass and identity 2000 g water Unknown a freezing-point depression b freezing point of the solution 166
2 Plan
167
168
Calculate
Unknown a freezing-point depression b freezing point of the solution
169
Practice Problem
A water solution containing an unknown quantity of a nonelectrolyte solute is found to have a freezing point of ndash 023degCWhat is the molal concentration of the solution
012 m170
A solution consists of 103 g of the nonelectrolyte glucose C6H12O6 dissolved in 250 g of water What is the freezing-point depression of the solution
minus0426degC171
In a laboratory experiment the freezing point of an aqueous solution of glucose is found to be minus0325degCWhat is the molal concentration of this solution
0175 m172
If 0500 mol of a nonelectrolyte solute are dissolved in 5000 g of ether what is the freezing point of the solution
minus1181degC173
The freezing point of an aqueous solution that contains a nonelectrolyte is minus90degC aWhat is the freezing-point depression of the solution bWhat is the molal concentration of the solution a minus90degC b 48 m
174
Boiling-Point Elevation
Boiling point is temp at which vapor pressure is equal to atmospheric pressure Change in vapor pressure will cause corresponding change in boiling point
175
176
Vapor pressure of nonvolatile solution is lower than vapor pressure of pure solvent More heat will be required to raise vapor pressure of solution Boiling point of solution is higher than boiling point of pure solvent
Molal boiling-point constant (Kb) - boiling-point elevation of the solvent in a 1-molal solution of a nonvolatile nonelectrolyte solute b-p elevation of any nonelectrolyte in water found by experiment to be 051 So molal b-p constant for water is 051m
177
For different solvents b-p elevations of 1-molal solutions have different values (Table 14-2)
Like freezing-point constants values most accurate for dilute solutions
178
Boiling-point elevation Δtb difference between the boiling points of the pure solvent and a nonelectrolyte solution of that solvent and it is directly proportional to the molal concentration of the solution B-p elevation can be calculated using Δtb = Kbm Δtb (boiling point elevation) expressed in m and m expressed in mol solutekg solvent
Sample Problem
What is the boiling-point elevation of a solution made from 200 g of a nonelectrolyte solute and 4000 g of water The molar mass of the solute is 620 gmol
180
1 AnalyzeGiven solute mass = 200 g solute molar mass = 620 gmol solvent mass and identity = 4000 g of water
Unknown boiling-point elevation
181
2 Plan
182
183
3 Compute
Practice Problem
A solution contains 500 g of sucrose C12H22O11 a nonelectrolyte dissolved in 5000 g of water What is the boiling-point elevation
015degC184
A solution contains 4500 g of sucrose C12H22O11 a nonelectrolyte dissolved in 250 g of waterWhat is the boiling point of the solution
1027degC185
If the boiling point elevation of an aqueous solution containing a nonvolatile electrolyte is 102degC what is the molality of the solution
20 m186
The boiling point of an aqueous solution containing a nonvolatile electrolyte is 10075degC aWhat is the boiling-point elevation bWhat is the molality of the solution a 075degC b 15 m
187
Electrolytes and Colligative PropertiesEarly investigators confused by experiments where certain substances depressed freezing point or elevated boiling point more than expected
188
189
Ex 01 m solution of NaCl lowers freezing point of solvent almost twice as much as 01 m solution of sucrose 01 m CaCl2 lowers fp almost 3 times as much as 01 m solution of sucrose
To understand why this happens contrast behavior or sucrose with NaCl in aqueous solution Each sucrose dissolves to make only 1 particle in solution So 1 mol sucrose dissolves to 1 mol particles 1 mol NaCl dissolves to make 2 moles of particles (1 mol Na+ and 1 mol Cl-)
190
191
Calculated Values for Electrolyte SolutionsColligative properties depend on total concentration of solute particles regardless of their identity Electrolytes cause changes in colligative properties proportional to total molality in terms of all dissolves particles instead of formula units
192
What about barium nitrate Ba(NO3)2 Each mole of barium nitrate yields 3 mol of ions in solution Ba(NO3)2(s) rarr Ba2+(aq) + 2NO3-(aq) You would expect solution of given molality to lower fp of solvent 3 times as much as nonelectrolytic solution of same molality
193
Sample Problem
What is the expected change in the freezing point of water in a solution of 625 g of barium nitrate Ba(NO3)2 in 100 kg of water
194
1 AnalyzeGiven solute mass and formula = 625 g Ba(NO3)2 solvent mass and identity = 100 kg water Δtf = Kfm
Unknown expected freezing-point depression
195
2 PlanThe molality can be calculated by converting the solute mass to moles and then dividing by the number of kilograms of solvent That molality is in terms of formula units of Ba(NO3)2 and must be converted to molality in terms of dissociated ions in solution It must be multiplied by the number of moles of ions produced per mole of formula unit This adjusted molality can then be used to calculate the freezing-point depression
196
197
198
3 Compute
Practice Problem
What is the expected freezing-point depression for a solution that contains 20 mol of magnesium sulfate dissolved in 10 kg of water
minus74degC199
What is the expected boiling-point elevation of water for a solution that contains 150 g of sodium chloride dissolved in 10 kg of water 27degC The freezing point of an aqueous sodium chloride solution is minus200degC What is the molality of the solution 54 m NaCl
200
Actual Values for Electrolyte SolutionsRemember values calculated are only expected values Actual values of colligative properties for all strong electrolytes are almost what would be expected Difference between expected and actual values caused by attractive forces that exist between dissociated ions in aqueous solution
201
Attraction between hydrated ions in solution is small compared with those in crystalline solid However forces of attraction donrsquot interfere with movements of aqueous ions More concentrated solution means ions closer together and attraction is greater Also depends on charge of ions ndash higher charge greater attraction
202
Solubility Values
Solubility - amount of substance required to form a saturated solution with specific amount of solvent at specified temp Ex Solubility of sugar = 204 g per 100 g water at 20 Must specify temp bc solubility changes with temp 43
For gases pressure must also be specified Rate at which substance dissolves is unrelated to solubility Max amount of solute that dissolves and reaches equilibrium is always same under same conditions
44
Solute-Solvent Interactions
ldquolike dissolves likerdquo is generally useful rule for predicting whether one substance will dissolve in another Depends on type of bonding polaritynonpolarity and intermolecular forces between solute and solvent
45
Dissolving Ionic Compounds in Aqueous Solution
Polarity of water molecules plays important role in formation of solution of ionic compounds Charged ends of water molecules attract ions and surround them to keep separated from other ions
46
Hydration - solution process with water as solvent Ions said to be hydrated As hydrated ions diffuse into solution other ions exposed and drawn away from crystal surface Entire crystal gradually dissolves
47
When crystallized from aqueous solutions some ionic compounds form crystals that include water molecules The crystalline compounds known as hydrates have specific ratios of water molecules Represented by formulas like CuSO45H2O
48
When crystalline hydrate dissolves in water water of hydration returns to solvent Behavior of solute in hydrated form no different than anhydrous form
49
Nonpolar Solvents
Nonpolar solvents do not attract ions of ionic compound strongly enough to overcome forces holding crystal together
50
Liquid Solutes and Solvents
When you shake bottle of salad dressing oil droplets disperse in water Stop shaking ndash strong attraction between water molecules squeeze out oil and form separate layers Liquid solutes and solvents that are not soluble in each other are immiscible
51
Nonpolar substances (fat oil grease) are generally quite soluble in nonpolar liquids (carbon tetrachloride toluene gasoline)
Only attractions between nonpolar molecules are weak London forces
52
London Dispersion Forces
bc e- are constantly moving at any time there may be uneven distribution of them around a nonpolar atom Temporarily creates positive end and negative end
53
54
London dispersion forces - intermolecular attractions resulting from constant motion of e- and creation of instantaneous dipoles
55
Liquids that dissolve freely in one another in any ratio are said to be completely miscible Benzene and carbon tetrachloride (both nonpolar) are completely miscible
Nonpolar molecules of these two apply no strong forces of attraction or repulsion and molecules mix freely
56
Ethanol and water are also completely miscible -OH group on ethanol is slightly polar
Can form H-bonds with water as well as other ethanol molecules Intermolecular forces in mixture are very similar to those in pure liquids so they are mutually soluble in any amount
57
Gasoline contains mainly nonpolar hydrocarbons
Excellent solvent for fats oils greases
Major intermolecular forces are weak London forces
58
Ethanol less polar than water more than carbon tetrachloride
Better solvent for less-polar substances bc of nonpolar region
59
Effects of Pressure on Solubility
Little effect on solids or liquids Increase pressure - increase solubilities of gases in liquids
60
When gas in contact with surface of liquid gas molecules can enter liquid As amount of dissolve gas increases some molecules start to escape and reenter gas phase
Eq eventually established between rates of enteringleaving gas phase
61
As long as eq undisturbed solubility of gas in liquid unchanged at given pressure
Gas + solvent solution
62
Increasing pressure of solute gas above solution puts stress on eq Molecules collide with surface more often Increase in pressure partially offset by increase in rate of gas molecules entering solution In turn increase in amount of dissolved gas causes increase in rate molecules escape liquid and becomes gas
63
Eventually eq restored at higher gas solubility
Expected from Le Chatelierrsquos principle
Increase in gas pressure causes eq to shift so fewer molecules are in gas phase
64
Henryrsquos Law
Henryrsquos law - solubility of a gas in liquid is directly proportional to the partial pressure of that gas on the surface of the liquid
Applies to gas-liquid solutions at constant temp
65
When mixture of ideal gases is limited in constant volume at constant temp each gas applies same pressure it would apply if it occupied the space alone
Assuming gases do not react each gas dissolves to the extent it would if no other gases were there
66
In carbonated drinks (Coke Pepsi etc) solubility of CO2 increased by increasing pressure
At bottling factory CO2 gas forced into solution of flavored water at pressure of 5-10 atm Gas-in-liquid solution then sealed in bottlescans
67
When cap removed pressure reduced to 1 atm and some CO2 escapes as gas bubbles Effervescence - rapid escape of a gas from a liquid in which it is dissolved
68
Effects of Temperature on Solubility
Letrsquos consider gas solubility Increasing temperature usually decreases gas solubility As temp increases average kinetic energy of molecules in solution increases Greater number of solute molecules escape from attraction of solvent and return to gas phase 69
At higher temps eq is reached with fewer gas molecules in solution Gases generally less soluble
70
Effect of temp on solubility of solids in liquids more difficult to predict
Often increasing temp increases solubility
However the same temp increase can result in large increase in solubility in one case and only slight increase in the next
71
72
enthalpies of Solution
Formation of solution comes with energy change Dissolve KI in water container feels cold Dissolve LiCl in water feels hot Formation of solid-liquid solution can absorb or release heat
73
During formation of solution solvent and solute particles experience changes in forces attracting them to other particles Before dissolving begins solvent molecules held by intermolecular forces (solvent-solvent attraction) In solute molecules held by Ifs (solute-solute attraction)
74
Energy is required to separate solute molecules and solvent molecules from their neighbors
A solute particle that is surrounded by solvent molecules is said to be solvated
75
Step 1 solute particles become separated from solid (energy absorbed) Step 2 solvent particles move apart to allow solute particles to enter liquid (energy absorbed)
76
Step 3 solvent particles attracted to and solvate solute particles (energy released)
77
Heat of solution - net amount of heat energy absorbed or released when a specific amount of solute dissolves in a solvent
Heat of solution negative (heat is released) if sums of energy in steps 1 and 2 is less than step 3
If 1+2 gt 3 HOS is positive (heat absorbed)78
In gaseous state molecules are so far apart there are nearly no forces between them Solute-solute interaction has little effect on heat of solution of a gas Energy released when gas dissolved in liquid bc attraction between solute gas and solvent molecules greater than energy needed to separate solvent molecules
79
Concentration of Solutions80
Concentration - measure of amount of solute in given amount of solvent or solution ldquodiluterdquo just means small amount of solute ldquoconcentratedrdquo just means relatively large amount of solute Unrelated to degree to which solution is saturated Ex Saturated solution of substance not very soluble might be dilute
81
Molarity
Molarity - number of moles of solute in one liter of solution To find molarity must know molar mass A ldquoone molarrdquo solution of NaOH is 1 mole NaOH per liter of solution (1 M)
82
1 mol NaOH = 400 g If this quantity dissolve in enough water to make EXACTLY 100 L solution solution is 1 M If 200 g NaOH dissolve in enough to make 100 L solution what is molarity 0500 M
83
1 M solution is not made by adding 1 mol solute to 1 L solvent That would be more than 1 L Instead 1 mol solute dissolved in less than 1 L Then diluted with more solvent to bring TOTAL VOLUME to 1 L
84
Practice ProblemYou have 340 L of solution that contains 900 g sodium chloride What is the molarity of that solution 0440 M NaCl You have 08 L of a 05 M HCl solution How many moles of HCl does this solution contain 04 mol HCl
85
What is the molarity of a solution composed of 585 g of potassium iodide dissolved in enough water to make a 0125 L of solution 0282 M KI
86
How many moles of H2SO4 are present in 0500 L of a 0150 M H2SO4 solution 00750 mol What volume of 300 M NaCl is needed for reaction that requires 1463 g of NaCl 0834 L
87
Molality
Molality - concentration of solution expressed in moles of solute per kilogram of solvent Solution that contains 1 mol solute (NaOH) dissolved in exactly 1 kg of solvent is ldquoone molalrdquo solution (1 m NaOH)
88
1 mol NaOH = 400 g 400 g NaOH dissolved in 1 kg water is 1 m NaOH
200 g NaOH in 1 kg water = 0500 m NaOH
89
Practice Problem
A solution was prepared by dissolving 171 g sucrose (C12H22O11) in 125 g of water Find the molal concentration of this solution 0400 m C12H22O11
90
A solution of iodine in carbon tetrachloride is used when iodine is needed for certain chemical tests How much iodine must be added to prepare a 0480 m solution of iodine in CCl4 if 1000 g of CCl4 is used 122 g I2
91
What is the molality of a solution composed of 255 g acetone (CH3)2CO dissolved in 200 g of water 22 m acetone What quantity in grams of methanol CH3OH is required to prepare a 0244 m solution in 400 g water 312 g methanol
92
How many grams of AgNO3 are needed to prepare a 0125 m solution in 250 mL of water 531 g AgNO3
What is the molality of a solution containing 182 g HCl and 250 g water 200 m
93
Ions in Aqueous Solutions and Colligative Properties
94
95
compounds in aqueous solutions
DissociationWhen compound made from ions dissolves in water ions separate Dissociation - separation of ions that occurs when an ionic compound dissolves
96
Notice number of ions made per formula unit in equations 1 formula unit of NaCl gives 2 units of ions in solution 1 formula unit of CaCl2 gives 3 units of ions
97
Assuming 100 dissociation solution that contains 1 mol NaCl contains 1 mol Na+ and 1 mol Cl- Can assume 100 dissociation of all soluble ionic compounds
98
Practice Problem
Write the equation for the dissolution of aluminum sulfate in water How many moles of aluminum ions are made by dissolving 1 mol aluminum sulfate What is the total number of moles of ions made by dissolving 1 mol of aluminum sulfate
99
1 AnalyzeGiven Amount of solute = 1 mol Al2(SO4)3 Solvent identity = water Unknown a moles of aluminum ions and sulfate ions Total number of moles of solute ions produced
100
2 Plan
The coefficients in the balanced dissociation equation will tell mole relationships You can use the equation to find out the number of moles of solute ions produced
101
3 Compute
a
b102
Write the equation for the dissolution of each of the following in water and then determine the number of moles of each ions made as well as the total number of ions made a 1 mol ammonium chloride b 1 mol sodium sulfide c 05 mol barium nitrate
103
a 1 mol ammonium chloride
1 mol NH4+ 1 mol Cl- 2 mol total ions
104
b 1 mol sodium sulfide
2 mol Na+ 1 mol S-2 3 mol total ions
105
c 05 mol barium nitrate
05 mol Ba+2 1 mol NO3- 15 mol total ions
106
Precipitation Reactions
Even though no compound is completely insoluble compounds of very low solubility can be considered insoluble for practical purposes There are general guidelines to help predict whether a compound made of certain combination of ions is soluble
107
General Solubility Guidelines
Sodium potassium and ammonium compounds are soluble in water
108
Nitrates acetates and chlorates are soluble
109
110
Most chlorides are soluble except those of silver mercury (I) and lead Lead (II) chloride is soluble in hot water
Most sulfates are soluble except those of barium strontium lead calcium and mercury
111
Most carbonates phosphates and silicates are insoluble except those of sodium potassium and ammonium
112
Most sulfides are insoluble except those of calcium strontium sodium potassium and ammonium
113
Is calcium phosphate Ca3(PO4)2 soluble or insoluble
Not one of the exceptions so it is insoluble
Dissociation equations are not written for insoluble compounds
114
Guidelines also useful in predicting what will happen if 2 solutions of 2 different soluble compounds are mixed
If mixing results in combination of ions that forms insoluble compound a double-replacement and precipitation reaction will happen
115
Will precipitate form when solutions of ammonium sulfide and cadmium nitrate are combined You can tell calcium nitrate is soluble from the guidelines (Most nitrates acetates and chlorates are soluble) You can also tell ammonium sulfide is soluble (Most sulfides are insoluble except those of calcium strontium sodium potassium and ammonium)
116
117
To decide whether precipitate can form must know solubilities of two possible products
Ammonium nitrate ndash soluble (Most nitrates acetates and chlorates are soluble) Cadmium sulfide ndash insoluble (Most sulfides are insoluble except those of calcium strontium sodium potassium and ammonium)
118
bc one product is insoluble double-replacement and precipitation reaction will happen (NH4)2S (aq) + Cd(NO3)2 (aq) rarr NH4NO3 (aq) + CdS (s)
119
Net Ionic Equations
Reactions of ions in aqueous solution usually represented by net ionic equations instead of formula equations Net ionic equation - includes only those compounds and ions that undergo a chemical change in a reaction in an aqueous solution
120
121
To write NIE (net ionic equation) first convert chemical equation into overall ionic equation All soluble ionic compounds shown as dissociated ions in solution Precipitates shown as solids
122
Notice ammonium and nitrate ions appear on both sides They havenrsquot gone through any chemical change Spectator ions - ions that do not take part in a chemical reaction and are found in solution both before and after the reaction
123
To change ionic equation to NIE eliminate spectator ions Cd+2(aq) + S-2(aq) rarr CdS(s)
This NIE applies to any reaction in which precipitate of CdS forms
124
Practice Problem
Identify the precipitate that forms when aqueous solutions of zinc nitrate and ammonium sulfide are combined Write the equation for the possible double-replacement reaction Then write the formula equation overall ionic equation and the net ionic equation
125
1 AnalyzeGiven Identity of reactants zinc nitrate and ammonium sulfide Reaction medium aqueous solution Unknown a equation for the possible double-replacement reaction b identity of the precipitate c formula equation d overall ionic equation e net ionic equation
126
2 PlanWrite the possible double-replacement reaction between Zn(NO3)2 and (NH4)2S Use the rules to determine if any of the products will precipitate Write a formula equation and overall net ionic equation Then cancel spectator ions to make net ionic equation
127
3 Computea possible double-replacement reaction
b rules show that zinc sulfide is not soluble so will precipitate (ammonium nitrate is soluble)
c formula equation128
d overall ionic equation
e net ionic equation129
Will a precipitate form if solutions of potassium sulfate and barium nitrate are combined If so write the net ionic equation for the reaction
Yes Ba+2(aq) + SO4-2(aq) rarr BaSO4(s)
130
Will a precipitate form if solutions of potassium nitrate and magnesium sulfate are combined If so write the net ionic equation for the reaction
No 131
Will a precipitate form if solutions of barium chloride and sodium sulfate are combined If so identify the spectator ions and write the net ionic equation for the reaction
Yes Na+ and Cl- Ba+2(aq) + SO4-2(aq) rarr BaSO4(s)
132
Write the net ionic equation for the precipitation of nickel(II) sulfide
Ni+2(aq) + S-2(aq) rarr NiS(s)
133
IonizationSome molecular compounds can also form ions in solution Usually polar Ionization - ions formed from solute molecules by action of the solvent (creation of ions where there were none) Ionization different from dissociation
134
Dissociation ionic compounds dissolve and ions already present separate Ionization ions formed where non existed before
Ions formed are hydrated Heat released during hydration of ions gives enough energy to break covalent bonds
135
Extent to which solute ionizes depends on strength of bonds within molecules of solute and strength of attraction between solute and solvent molecules
136
HCl ndash molecular compound that ionizes in aqueous solution Contains highly polar bond Attraction between polar HCl molecule and polar water strong enough to break HCl bond
137
The Hydronium Ion
H+ ions from HCl attracts other molecules or ions so strongly that it doesnrsquot normally exist alone Ionization of HCl better described as direct transfer of proton from HCl to H2O forming H3O+
138
139
Hydration of H+ to form hydronium is highly exothermic
Energy released gives a lot of energy needed to ionize a molecular solute
Many molecular compounds that ionize in aqueous solution contain hydrogen and form hydronium ion
140
Strong and Weak Electrolytes
Substances that make ions and conduct current in solution are electrolytes HCl is one of series of compounds that has hydrogen and halogen Hydrogen halides are all molecular compounds with single polar-covalent bonds
141
All are gases Very soluble in water All are electrolytes HCl HBr HI strongly conduct current in solution HF only weakly conducts current at same concentration Strength of conduction related to ability to ionize
142
143
Strong Electrolytes
HCl HBr HI are 100 ionized in solution
Strong electrolyte - any compound whose dilute aqueous solutions conduct electricity well this is due to the presence of all or almost all of the dissolve compound in the form of ions
144
Unique characteristic of strong electrolyte is that they yield only ions
Ex Ionic compound may be highly soluble in water and dissociate into ions (NaCl)
Others may not dissolve much but amount that does dissolve exists only as ions
145
Weak ElectrolytesHF dissolves in water to give acid solution called hydrofluoric acid HF bond stronger than bonds between hydrogen and other halogens When HF dissolves some molecule ionize Reverse reaction also happens HF(aq) + H2O(l) H3O+(aq) + F-(aq)
146
HF(aq) + H2O(l) H3O+
(aq) + F-(aq)Concentration of dissolved unionized HF stays high and concentration of ions stays low
Weak electrolyte - any compound whose aqueous solutions conduct electricity poorly this is due to the presence of small amount of the dissolved compound in the form of ions
147
Different from nonelectrolyte ndash NO ions at all Description of electrolyte as strong or weak not related to concentration of solution Electrolytes differ in degree of ionization not amount of solute dissolved
148
149
Colligative properties of solutions
Presence of solute affects properties of solutions Some properties not dependent on nature of dissolved substance but on how may dissolve particles are present
Colligative properties - properties that depend on the concentration of solute particles but not on their identity
150
Vapor-Pressure Lowering
Boiling and freezing point of solution different from pure solvent Nonvolatile solute raises boiling point and lowers freezing point Nonvolatile substance - one that has little tendency to become gas under existing conditions
151
152
To understand why nonvolatile solute changes boiling and freezing point consider equilibrium vapor pressure
Vapor pressure pressure caused by molecules that have escape liquid phase to gas phase
Can be thought of a measure of tendency of molecules to escape from a liquid
153
Addition of sucrose (nonvolatile solute) lowers concentration of water molecules at surface of liquid This lowers tendency of water molecules to leave solution and enter gas phase Vapor pressure of solution is lower than vapor pressure of pure water
154
Aqueous13 solution13 of13 nonvolatile13 solutePure13 water
Nonelectrolyte solutions of same molality have same concentration of particles
Dilute solutions of same solvent and equal molality of any nonelectrolyte solute lower vapor pressure equally
155
Example
1 m aqueous solution of nonelectrolyte glucose lowers vapor pressure of water 55 x 10-4 atm at 25 1 m aqueous solution of sucrose also lowers water vapor pressure to 55 x 10-4 atm at 25
156
157
bc vapor-pressure lowering depends on concentration of nonelectrolyte solute and doesnrsquot depend on the type of solute it is a colligative proprerty
bc vapor pressure has been lowered solution remains liquid over larger temperature range
Lowers freezing point and raises boiling point
158
Can assume changes in boiling and freezing point also depend on concentration of solute
They are colligative properties
159
Freezing-Point Depression
Freezing point of 1-molal solution of any nonelectrolyte solute in water is found (by experiment) to be 186 lower than freezing point of water When 1 mol of nonelectrolyte solute dissolved in 1 kg water freezing point is -186 not 000 160
When 2 mol nonelectrolyte solute dissolved in 1 kg water freezing point is -372
For any concentration of nonelectrolyte solute in water decrease in freezing point can be determined using value of -186m
161
This value called the molal freezing-point contstant (Kf) is the freezing-point depression of the solvent in a 1-molal solution of a nonvolatile nonelectrolyte solute
162
Each solvent has own characteristic molal freezing-point constant Freezing-point depression (Δtf) the difference between the freezing points of the pure solvent and a solute of nonelectrolyte in that solvent and it is directly proportional to the molal concentration of the solution Δtf = Kfm
163
Δtf = Kfm
Kf expressed as m
m expressed in mol solutekg solvent (molality)
164
Sample Problem
What is the freezing point depression of water in a solution of 171 g of sucrose C12H22O11 and 200 g of water What is the actual freezing point of the solution
165
1 AnalyzeGiven Solute mass and chemical formula 171 g C12H22O11 Solvent mass and identity 2000 g water Unknown a freezing-point depression b freezing point of the solution 166
2 Plan
167
168
Calculate
Unknown a freezing-point depression b freezing point of the solution
169
Practice Problem
A water solution containing an unknown quantity of a nonelectrolyte solute is found to have a freezing point of ndash 023degCWhat is the molal concentration of the solution
012 m170
A solution consists of 103 g of the nonelectrolyte glucose C6H12O6 dissolved in 250 g of water What is the freezing-point depression of the solution
minus0426degC171
In a laboratory experiment the freezing point of an aqueous solution of glucose is found to be minus0325degCWhat is the molal concentration of this solution
0175 m172
If 0500 mol of a nonelectrolyte solute are dissolved in 5000 g of ether what is the freezing point of the solution
minus1181degC173
The freezing point of an aqueous solution that contains a nonelectrolyte is minus90degC aWhat is the freezing-point depression of the solution bWhat is the molal concentration of the solution a minus90degC b 48 m
174
Boiling-Point Elevation
Boiling point is temp at which vapor pressure is equal to atmospheric pressure Change in vapor pressure will cause corresponding change in boiling point
175
176
Vapor pressure of nonvolatile solution is lower than vapor pressure of pure solvent More heat will be required to raise vapor pressure of solution Boiling point of solution is higher than boiling point of pure solvent
Molal boiling-point constant (Kb) - boiling-point elevation of the solvent in a 1-molal solution of a nonvolatile nonelectrolyte solute b-p elevation of any nonelectrolyte in water found by experiment to be 051 So molal b-p constant for water is 051m
177
For different solvents b-p elevations of 1-molal solutions have different values (Table 14-2)
Like freezing-point constants values most accurate for dilute solutions
178
Boiling-point elevation Δtb difference between the boiling points of the pure solvent and a nonelectrolyte solution of that solvent and it is directly proportional to the molal concentration of the solution B-p elevation can be calculated using Δtb = Kbm Δtb (boiling point elevation) expressed in m and m expressed in mol solutekg solvent
Sample Problem
What is the boiling-point elevation of a solution made from 200 g of a nonelectrolyte solute and 4000 g of water The molar mass of the solute is 620 gmol
180
1 AnalyzeGiven solute mass = 200 g solute molar mass = 620 gmol solvent mass and identity = 4000 g of water
Unknown boiling-point elevation
181
2 Plan
182
183
3 Compute
Practice Problem
A solution contains 500 g of sucrose C12H22O11 a nonelectrolyte dissolved in 5000 g of water What is the boiling-point elevation
015degC184
A solution contains 4500 g of sucrose C12H22O11 a nonelectrolyte dissolved in 250 g of waterWhat is the boiling point of the solution
1027degC185
If the boiling point elevation of an aqueous solution containing a nonvolatile electrolyte is 102degC what is the molality of the solution
20 m186
The boiling point of an aqueous solution containing a nonvolatile electrolyte is 10075degC aWhat is the boiling-point elevation bWhat is the molality of the solution a 075degC b 15 m
187
Electrolytes and Colligative PropertiesEarly investigators confused by experiments where certain substances depressed freezing point or elevated boiling point more than expected
188
189
Ex 01 m solution of NaCl lowers freezing point of solvent almost twice as much as 01 m solution of sucrose 01 m CaCl2 lowers fp almost 3 times as much as 01 m solution of sucrose
To understand why this happens contrast behavior or sucrose with NaCl in aqueous solution Each sucrose dissolves to make only 1 particle in solution So 1 mol sucrose dissolves to 1 mol particles 1 mol NaCl dissolves to make 2 moles of particles (1 mol Na+ and 1 mol Cl-)
190
191
Calculated Values for Electrolyte SolutionsColligative properties depend on total concentration of solute particles regardless of their identity Electrolytes cause changes in colligative properties proportional to total molality in terms of all dissolves particles instead of formula units
192
What about barium nitrate Ba(NO3)2 Each mole of barium nitrate yields 3 mol of ions in solution Ba(NO3)2(s) rarr Ba2+(aq) + 2NO3-(aq) You would expect solution of given molality to lower fp of solvent 3 times as much as nonelectrolytic solution of same molality
193
Sample Problem
What is the expected change in the freezing point of water in a solution of 625 g of barium nitrate Ba(NO3)2 in 100 kg of water
194
1 AnalyzeGiven solute mass and formula = 625 g Ba(NO3)2 solvent mass and identity = 100 kg water Δtf = Kfm
Unknown expected freezing-point depression
195
2 PlanThe molality can be calculated by converting the solute mass to moles and then dividing by the number of kilograms of solvent That molality is in terms of formula units of Ba(NO3)2 and must be converted to molality in terms of dissociated ions in solution It must be multiplied by the number of moles of ions produced per mole of formula unit This adjusted molality can then be used to calculate the freezing-point depression
196
197
198
3 Compute
Practice Problem
What is the expected freezing-point depression for a solution that contains 20 mol of magnesium sulfate dissolved in 10 kg of water
minus74degC199
What is the expected boiling-point elevation of water for a solution that contains 150 g of sodium chloride dissolved in 10 kg of water 27degC The freezing point of an aqueous sodium chloride solution is minus200degC What is the molality of the solution 54 m NaCl
200
Actual Values for Electrolyte SolutionsRemember values calculated are only expected values Actual values of colligative properties for all strong electrolytes are almost what would be expected Difference between expected and actual values caused by attractive forces that exist between dissociated ions in aqueous solution
201
Attraction between hydrated ions in solution is small compared with those in crystalline solid However forces of attraction donrsquot interfere with movements of aqueous ions More concentrated solution means ions closer together and attraction is greater Also depends on charge of ions ndash higher charge greater attraction
202
For gases pressure must also be specified Rate at which substance dissolves is unrelated to solubility Max amount of solute that dissolves and reaches equilibrium is always same under same conditions
44
Solute-Solvent Interactions
ldquolike dissolves likerdquo is generally useful rule for predicting whether one substance will dissolve in another Depends on type of bonding polaritynonpolarity and intermolecular forces between solute and solvent
45
Dissolving Ionic Compounds in Aqueous Solution
Polarity of water molecules plays important role in formation of solution of ionic compounds Charged ends of water molecules attract ions and surround them to keep separated from other ions
46
Hydration - solution process with water as solvent Ions said to be hydrated As hydrated ions diffuse into solution other ions exposed and drawn away from crystal surface Entire crystal gradually dissolves
47
When crystallized from aqueous solutions some ionic compounds form crystals that include water molecules The crystalline compounds known as hydrates have specific ratios of water molecules Represented by formulas like CuSO45H2O
48
When crystalline hydrate dissolves in water water of hydration returns to solvent Behavior of solute in hydrated form no different than anhydrous form
49
Nonpolar Solvents
Nonpolar solvents do not attract ions of ionic compound strongly enough to overcome forces holding crystal together
50
Liquid Solutes and Solvents
When you shake bottle of salad dressing oil droplets disperse in water Stop shaking ndash strong attraction between water molecules squeeze out oil and form separate layers Liquid solutes and solvents that are not soluble in each other are immiscible
51
Nonpolar substances (fat oil grease) are generally quite soluble in nonpolar liquids (carbon tetrachloride toluene gasoline)
Only attractions between nonpolar molecules are weak London forces
52
London Dispersion Forces
bc e- are constantly moving at any time there may be uneven distribution of them around a nonpolar atom Temporarily creates positive end and negative end
53
54
London dispersion forces - intermolecular attractions resulting from constant motion of e- and creation of instantaneous dipoles
55
Liquids that dissolve freely in one another in any ratio are said to be completely miscible Benzene and carbon tetrachloride (both nonpolar) are completely miscible
Nonpolar molecules of these two apply no strong forces of attraction or repulsion and molecules mix freely
56
Ethanol and water are also completely miscible -OH group on ethanol is slightly polar
Can form H-bonds with water as well as other ethanol molecules Intermolecular forces in mixture are very similar to those in pure liquids so they are mutually soluble in any amount
57
Gasoline contains mainly nonpolar hydrocarbons
Excellent solvent for fats oils greases
Major intermolecular forces are weak London forces
58
Ethanol less polar than water more than carbon tetrachloride
Better solvent for less-polar substances bc of nonpolar region
59
Effects of Pressure on Solubility
Little effect on solids or liquids Increase pressure - increase solubilities of gases in liquids
60
When gas in contact with surface of liquid gas molecules can enter liquid As amount of dissolve gas increases some molecules start to escape and reenter gas phase
Eq eventually established between rates of enteringleaving gas phase
61
As long as eq undisturbed solubility of gas in liquid unchanged at given pressure
Gas + solvent solution
62
Increasing pressure of solute gas above solution puts stress on eq Molecules collide with surface more often Increase in pressure partially offset by increase in rate of gas molecules entering solution In turn increase in amount of dissolved gas causes increase in rate molecules escape liquid and becomes gas
63
Eventually eq restored at higher gas solubility
Expected from Le Chatelierrsquos principle
Increase in gas pressure causes eq to shift so fewer molecules are in gas phase
64
Henryrsquos Law
Henryrsquos law - solubility of a gas in liquid is directly proportional to the partial pressure of that gas on the surface of the liquid
Applies to gas-liquid solutions at constant temp
65
When mixture of ideal gases is limited in constant volume at constant temp each gas applies same pressure it would apply if it occupied the space alone
Assuming gases do not react each gas dissolves to the extent it would if no other gases were there
66
In carbonated drinks (Coke Pepsi etc) solubility of CO2 increased by increasing pressure
At bottling factory CO2 gas forced into solution of flavored water at pressure of 5-10 atm Gas-in-liquid solution then sealed in bottlescans
67
When cap removed pressure reduced to 1 atm and some CO2 escapes as gas bubbles Effervescence - rapid escape of a gas from a liquid in which it is dissolved
68
Effects of Temperature on Solubility
Letrsquos consider gas solubility Increasing temperature usually decreases gas solubility As temp increases average kinetic energy of molecules in solution increases Greater number of solute molecules escape from attraction of solvent and return to gas phase 69
At higher temps eq is reached with fewer gas molecules in solution Gases generally less soluble
70
Effect of temp on solubility of solids in liquids more difficult to predict
Often increasing temp increases solubility
However the same temp increase can result in large increase in solubility in one case and only slight increase in the next
71
72
enthalpies of Solution
Formation of solution comes with energy change Dissolve KI in water container feels cold Dissolve LiCl in water feels hot Formation of solid-liquid solution can absorb or release heat
73
During formation of solution solvent and solute particles experience changes in forces attracting them to other particles Before dissolving begins solvent molecules held by intermolecular forces (solvent-solvent attraction) In solute molecules held by Ifs (solute-solute attraction)
74
Energy is required to separate solute molecules and solvent molecules from their neighbors
A solute particle that is surrounded by solvent molecules is said to be solvated
75
Step 1 solute particles become separated from solid (energy absorbed) Step 2 solvent particles move apart to allow solute particles to enter liquid (energy absorbed)
76
Step 3 solvent particles attracted to and solvate solute particles (energy released)
77
Heat of solution - net amount of heat energy absorbed or released when a specific amount of solute dissolves in a solvent
Heat of solution negative (heat is released) if sums of energy in steps 1 and 2 is less than step 3
If 1+2 gt 3 HOS is positive (heat absorbed)78
In gaseous state molecules are so far apart there are nearly no forces between them Solute-solute interaction has little effect on heat of solution of a gas Energy released when gas dissolved in liquid bc attraction between solute gas and solvent molecules greater than energy needed to separate solvent molecules
79
Concentration of Solutions80
Concentration - measure of amount of solute in given amount of solvent or solution ldquodiluterdquo just means small amount of solute ldquoconcentratedrdquo just means relatively large amount of solute Unrelated to degree to which solution is saturated Ex Saturated solution of substance not very soluble might be dilute
81
Molarity
Molarity - number of moles of solute in one liter of solution To find molarity must know molar mass A ldquoone molarrdquo solution of NaOH is 1 mole NaOH per liter of solution (1 M)
82
1 mol NaOH = 400 g If this quantity dissolve in enough water to make EXACTLY 100 L solution solution is 1 M If 200 g NaOH dissolve in enough to make 100 L solution what is molarity 0500 M
83
1 M solution is not made by adding 1 mol solute to 1 L solvent That would be more than 1 L Instead 1 mol solute dissolved in less than 1 L Then diluted with more solvent to bring TOTAL VOLUME to 1 L
84
Practice ProblemYou have 340 L of solution that contains 900 g sodium chloride What is the molarity of that solution 0440 M NaCl You have 08 L of a 05 M HCl solution How many moles of HCl does this solution contain 04 mol HCl
85
What is the molarity of a solution composed of 585 g of potassium iodide dissolved in enough water to make a 0125 L of solution 0282 M KI
86
How many moles of H2SO4 are present in 0500 L of a 0150 M H2SO4 solution 00750 mol What volume of 300 M NaCl is needed for reaction that requires 1463 g of NaCl 0834 L
87
Molality
Molality - concentration of solution expressed in moles of solute per kilogram of solvent Solution that contains 1 mol solute (NaOH) dissolved in exactly 1 kg of solvent is ldquoone molalrdquo solution (1 m NaOH)
88
1 mol NaOH = 400 g 400 g NaOH dissolved in 1 kg water is 1 m NaOH
200 g NaOH in 1 kg water = 0500 m NaOH
89
Practice Problem
A solution was prepared by dissolving 171 g sucrose (C12H22O11) in 125 g of water Find the molal concentration of this solution 0400 m C12H22O11
90
A solution of iodine in carbon tetrachloride is used when iodine is needed for certain chemical tests How much iodine must be added to prepare a 0480 m solution of iodine in CCl4 if 1000 g of CCl4 is used 122 g I2
91
What is the molality of a solution composed of 255 g acetone (CH3)2CO dissolved in 200 g of water 22 m acetone What quantity in grams of methanol CH3OH is required to prepare a 0244 m solution in 400 g water 312 g methanol
92
How many grams of AgNO3 are needed to prepare a 0125 m solution in 250 mL of water 531 g AgNO3
What is the molality of a solution containing 182 g HCl and 250 g water 200 m
93
Ions in Aqueous Solutions and Colligative Properties
94
95
compounds in aqueous solutions
DissociationWhen compound made from ions dissolves in water ions separate Dissociation - separation of ions that occurs when an ionic compound dissolves
96
Notice number of ions made per formula unit in equations 1 formula unit of NaCl gives 2 units of ions in solution 1 formula unit of CaCl2 gives 3 units of ions
97
Assuming 100 dissociation solution that contains 1 mol NaCl contains 1 mol Na+ and 1 mol Cl- Can assume 100 dissociation of all soluble ionic compounds
98
Practice Problem
Write the equation for the dissolution of aluminum sulfate in water How many moles of aluminum ions are made by dissolving 1 mol aluminum sulfate What is the total number of moles of ions made by dissolving 1 mol of aluminum sulfate
99
1 AnalyzeGiven Amount of solute = 1 mol Al2(SO4)3 Solvent identity = water Unknown a moles of aluminum ions and sulfate ions Total number of moles of solute ions produced
100
2 Plan
The coefficients in the balanced dissociation equation will tell mole relationships You can use the equation to find out the number of moles of solute ions produced
101
3 Compute
a
b102
Write the equation for the dissolution of each of the following in water and then determine the number of moles of each ions made as well as the total number of ions made a 1 mol ammonium chloride b 1 mol sodium sulfide c 05 mol barium nitrate
103
a 1 mol ammonium chloride
1 mol NH4+ 1 mol Cl- 2 mol total ions
104
b 1 mol sodium sulfide
2 mol Na+ 1 mol S-2 3 mol total ions
105
c 05 mol barium nitrate
05 mol Ba+2 1 mol NO3- 15 mol total ions
106
Precipitation Reactions
Even though no compound is completely insoluble compounds of very low solubility can be considered insoluble for practical purposes There are general guidelines to help predict whether a compound made of certain combination of ions is soluble
107
General Solubility Guidelines
Sodium potassium and ammonium compounds are soluble in water
108
Nitrates acetates and chlorates are soluble
109
110
Most chlorides are soluble except those of silver mercury (I) and lead Lead (II) chloride is soluble in hot water
Most sulfates are soluble except those of barium strontium lead calcium and mercury
111
Most carbonates phosphates and silicates are insoluble except those of sodium potassium and ammonium
112
Most sulfides are insoluble except those of calcium strontium sodium potassium and ammonium
113
Is calcium phosphate Ca3(PO4)2 soluble or insoluble
Not one of the exceptions so it is insoluble
Dissociation equations are not written for insoluble compounds
114
Guidelines also useful in predicting what will happen if 2 solutions of 2 different soluble compounds are mixed
If mixing results in combination of ions that forms insoluble compound a double-replacement and precipitation reaction will happen
115
Will precipitate form when solutions of ammonium sulfide and cadmium nitrate are combined You can tell calcium nitrate is soluble from the guidelines (Most nitrates acetates and chlorates are soluble) You can also tell ammonium sulfide is soluble (Most sulfides are insoluble except those of calcium strontium sodium potassium and ammonium)
116
117
To decide whether precipitate can form must know solubilities of two possible products
Ammonium nitrate ndash soluble (Most nitrates acetates and chlorates are soluble) Cadmium sulfide ndash insoluble (Most sulfides are insoluble except those of calcium strontium sodium potassium and ammonium)
118
bc one product is insoluble double-replacement and precipitation reaction will happen (NH4)2S (aq) + Cd(NO3)2 (aq) rarr NH4NO3 (aq) + CdS (s)
119
Net Ionic Equations
Reactions of ions in aqueous solution usually represented by net ionic equations instead of formula equations Net ionic equation - includes only those compounds and ions that undergo a chemical change in a reaction in an aqueous solution
120
121
To write NIE (net ionic equation) first convert chemical equation into overall ionic equation All soluble ionic compounds shown as dissociated ions in solution Precipitates shown as solids
122
Notice ammonium and nitrate ions appear on both sides They havenrsquot gone through any chemical change Spectator ions - ions that do not take part in a chemical reaction and are found in solution both before and after the reaction
123
To change ionic equation to NIE eliminate spectator ions Cd+2(aq) + S-2(aq) rarr CdS(s)
This NIE applies to any reaction in which precipitate of CdS forms
124
Practice Problem
Identify the precipitate that forms when aqueous solutions of zinc nitrate and ammonium sulfide are combined Write the equation for the possible double-replacement reaction Then write the formula equation overall ionic equation and the net ionic equation
125
1 AnalyzeGiven Identity of reactants zinc nitrate and ammonium sulfide Reaction medium aqueous solution Unknown a equation for the possible double-replacement reaction b identity of the precipitate c formula equation d overall ionic equation e net ionic equation
126
2 PlanWrite the possible double-replacement reaction between Zn(NO3)2 and (NH4)2S Use the rules to determine if any of the products will precipitate Write a formula equation and overall net ionic equation Then cancel spectator ions to make net ionic equation
127
3 Computea possible double-replacement reaction
b rules show that zinc sulfide is not soluble so will precipitate (ammonium nitrate is soluble)
c formula equation128
d overall ionic equation
e net ionic equation129
Will a precipitate form if solutions of potassium sulfate and barium nitrate are combined If so write the net ionic equation for the reaction
Yes Ba+2(aq) + SO4-2(aq) rarr BaSO4(s)
130
Will a precipitate form if solutions of potassium nitrate and magnesium sulfate are combined If so write the net ionic equation for the reaction
No 131
Will a precipitate form if solutions of barium chloride and sodium sulfate are combined If so identify the spectator ions and write the net ionic equation for the reaction
Yes Na+ and Cl- Ba+2(aq) + SO4-2(aq) rarr BaSO4(s)
132
Write the net ionic equation for the precipitation of nickel(II) sulfide
Ni+2(aq) + S-2(aq) rarr NiS(s)
133
IonizationSome molecular compounds can also form ions in solution Usually polar Ionization - ions formed from solute molecules by action of the solvent (creation of ions where there were none) Ionization different from dissociation
134
Dissociation ionic compounds dissolve and ions already present separate Ionization ions formed where non existed before
Ions formed are hydrated Heat released during hydration of ions gives enough energy to break covalent bonds
135
Extent to which solute ionizes depends on strength of bonds within molecules of solute and strength of attraction between solute and solvent molecules
136
HCl ndash molecular compound that ionizes in aqueous solution Contains highly polar bond Attraction between polar HCl molecule and polar water strong enough to break HCl bond
137
The Hydronium Ion
H+ ions from HCl attracts other molecules or ions so strongly that it doesnrsquot normally exist alone Ionization of HCl better described as direct transfer of proton from HCl to H2O forming H3O+
138
139
Hydration of H+ to form hydronium is highly exothermic
Energy released gives a lot of energy needed to ionize a molecular solute
Many molecular compounds that ionize in aqueous solution contain hydrogen and form hydronium ion
140
Strong and Weak Electrolytes
Substances that make ions and conduct current in solution are electrolytes HCl is one of series of compounds that has hydrogen and halogen Hydrogen halides are all molecular compounds with single polar-covalent bonds
141
All are gases Very soluble in water All are electrolytes HCl HBr HI strongly conduct current in solution HF only weakly conducts current at same concentration Strength of conduction related to ability to ionize
142
143
Strong Electrolytes
HCl HBr HI are 100 ionized in solution
Strong electrolyte - any compound whose dilute aqueous solutions conduct electricity well this is due to the presence of all or almost all of the dissolve compound in the form of ions
144
Unique characteristic of strong electrolyte is that they yield only ions
Ex Ionic compound may be highly soluble in water and dissociate into ions (NaCl)
Others may not dissolve much but amount that does dissolve exists only as ions
145
Weak ElectrolytesHF dissolves in water to give acid solution called hydrofluoric acid HF bond stronger than bonds between hydrogen and other halogens When HF dissolves some molecule ionize Reverse reaction also happens HF(aq) + H2O(l) H3O+(aq) + F-(aq)
146
HF(aq) + H2O(l) H3O+
(aq) + F-(aq)Concentration of dissolved unionized HF stays high and concentration of ions stays low
Weak electrolyte - any compound whose aqueous solutions conduct electricity poorly this is due to the presence of small amount of the dissolved compound in the form of ions
147
Different from nonelectrolyte ndash NO ions at all Description of electrolyte as strong or weak not related to concentration of solution Electrolytes differ in degree of ionization not amount of solute dissolved
148
149
Colligative properties of solutions
Presence of solute affects properties of solutions Some properties not dependent on nature of dissolved substance but on how may dissolve particles are present
Colligative properties - properties that depend on the concentration of solute particles but not on their identity
150
Vapor-Pressure Lowering
Boiling and freezing point of solution different from pure solvent Nonvolatile solute raises boiling point and lowers freezing point Nonvolatile substance - one that has little tendency to become gas under existing conditions
151
152
To understand why nonvolatile solute changes boiling and freezing point consider equilibrium vapor pressure
Vapor pressure pressure caused by molecules that have escape liquid phase to gas phase
Can be thought of a measure of tendency of molecules to escape from a liquid
153
Addition of sucrose (nonvolatile solute) lowers concentration of water molecules at surface of liquid This lowers tendency of water molecules to leave solution and enter gas phase Vapor pressure of solution is lower than vapor pressure of pure water
154
Aqueous13 solution13 of13 nonvolatile13 solutePure13 water
Nonelectrolyte solutions of same molality have same concentration of particles
Dilute solutions of same solvent and equal molality of any nonelectrolyte solute lower vapor pressure equally
155
Example
1 m aqueous solution of nonelectrolyte glucose lowers vapor pressure of water 55 x 10-4 atm at 25 1 m aqueous solution of sucrose also lowers water vapor pressure to 55 x 10-4 atm at 25
156
157
bc vapor-pressure lowering depends on concentration of nonelectrolyte solute and doesnrsquot depend on the type of solute it is a colligative proprerty
bc vapor pressure has been lowered solution remains liquid over larger temperature range
Lowers freezing point and raises boiling point
158
Can assume changes in boiling and freezing point also depend on concentration of solute
They are colligative properties
159
Freezing-Point Depression
Freezing point of 1-molal solution of any nonelectrolyte solute in water is found (by experiment) to be 186 lower than freezing point of water When 1 mol of nonelectrolyte solute dissolved in 1 kg water freezing point is -186 not 000 160
When 2 mol nonelectrolyte solute dissolved in 1 kg water freezing point is -372
For any concentration of nonelectrolyte solute in water decrease in freezing point can be determined using value of -186m
161
This value called the molal freezing-point contstant (Kf) is the freezing-point depression of the solvent in a 1-molal solution of a nonvolatile nonelectrolyte solute
162
Each solvent has own characteristic molal freezing-point constant Freezing-point depression (Δtf) the difference between the freezing points of the pure solvent and a solute of nonelectrolyte in that solvent and it is directly proportional to the molal concentration of the solution Δtf = Kfm
163
Δtf = Kfm
Kf expressed as m
m expressed in mol solutekg solvent (molality)
164
Sample Problem
What is the freezing point depression of water in a solution of 171 g of sucrose C12H22O11 and 200 g of water What is the actual freezing point of the solution
165
1 AnalyzeGiven Solute mass and chemical formula 171 g C12H22O11 Solvent mass and identity 2000 g water Unknown a freezing-point depression b freezing point of the solution 166
2 Plan
167
168
Calculate
Unknown a freezing-point depression b freezing point of the solution
169
Practice Problem
A water solution containing an unknown quantity of a nonelectrolyte solute is found to have a freezing point of ndash 023degCWhat is the molal concentration of the solution
012 m170
A solution consists of 103 g of the nonelectrolyte glucose C6H12O6 dissolved in 250 g of water What is the freezing-point depression of the solution
minus0426degC171
In a laboratory experiment the freezing point of an aqueous solution of glucose is found to be minus0325degCWhat is the molal concentration of this solution
0175 m172
If 0500 mol of a nonelectrolyte solute are dissolved in 5000 g of ether what is the freezing point of the solution
minus1181degC173
The freezing point of an aqueous solution that contains a nonelectrolyte is minus90degC aWhat is the freezing-point depression of the solution bWhat is the molal concentration of the solution a minus90degC b 48 m
174
Boiling-Point Elevation
Boiling point is temp at which vapor pressure is equal to atmospheric pressure Change in vapor pressure will cause corresponding change in boiling point
175
176
Vapor pressure of nonvolatile solution is lower than vapor pressure of pure solvent More heat will be required to raise vapor pressure of solution Boiling point of solution is higher than boiling point of pure solvent
Molal boiling-point constant (Kb) - boiling-point elevation of the solvent in a 1-molal solution of a nonvolatile nonelectrolyte solute b-p elevation of any nonelectrolyte in water found by experiment to be 051 So molal b-p constant for water is 051m
177
For different solvents b-p elevations of 1-molal solutions have different values (Table 14-2)
Like freezing-point constants values most accurate for dilute solutions
178
Boiling-point elevation Δtb difference between the boiling points of the pure solvent and a nonelectrolyte solution of that solvent and it is directly proportional to the molal concentration of the solution B-p elevation can be calculated using Δtb = Kbm Δtb (boiling point elevation) expressed in m and m expressed in mol solutekg solvent
Sample Problem
What is the boiling-point elevation of a solution made from 200 g of a nonelectrolyte solute and 4000 g of water The molar mass of the solute is 620 gmol
180
1 AnalyzeGiven solute mass = 200 g solute molar mass = 620 gmol solvent mass and identity = 4000 g of water
Unknown boiling-point elevation
181
2 Plan
182
183
3 Compute
Practice Problem
A solution contains 500 g of sucrose C12H22O11 a nonelectrolyte dissolved in 5000 g of water What is the boiling-point elevation
015degC184
A solution contains 4500 g of sucrose C12H22O11 a nonelectrolyte dissolved in 250 g of waterWhat is the boiling point of the solution
1027degC185
If the boiling point elevation of an aqueous solution containing a nonvolatile electrolyte is 102degC what is the molality of the solution
20 m186
The boiling point of an aqueous solution containing a nonvolatile electrolyte is 10075degC aWhat is the boiling-point elevation bWhat is the molality of the solution a 075degC b 15 m
187
Electrolytes and Colligative PropertiesEarly investigators confused by experiments where certain substances depressed freezing point or elevated boiling point more than expected
188
189
Ex 01 m solution of NaCl lowers freezing point of solvent almost twice as much as 01 m solution of sucrose 01 m CaCl2 lowers fp almost 3 times as much as 01 m solution of sucrose
To understand why this happens contrast behavior or sucrose with NaCl in aqueous solution Each sucrose dissolves to make only 1 particle in solution So 1 mol sucrose dissolves to 1 mol particles 1 mol NaCl dissolves to make 2 moles of particles (1 mol Na+ and 1 mol Cl-)
190
191
Calculated Values for Electrolyte SolutionsColligative properties depend on total concentration of solute particles regardless of their identity Electrolytes cause changes in colligative properties proportional to total molality in terms of all dissolves particles instead of formula units
192
What about barium nitrate Ba(NO3)2 Each mole of barium nitrate yields 3 mol of ions in solution Ba(NO3)2(s) rarr Ba2+(aq) + 2NO3-(aq) You would expect solution of given molality to lower fp of solvent 3 times as much as nonelectrolytic solution of same molality
193
Sample Problem
What is the expected change in the freezing point of water in a solution of 625 g of barium nitrate Ba(NO3)2 in 100 kg of water
194
1 AnalyzeGiven solute mass and formula = 625 g Ba(NO3)2 solvent mass and identity = 100 kg water Δtf = Kfm
Unknown expected freezing-point depression
195
2 PlanThe molality can be calculated by converting the solute mass to moles and then dividing by the number of kilograms of solvent That molality is in terms of formula units of Ba(NO3)2 and must be converted to molality in terms of dissociated ions in solution It must be multiplied by the number of moles of ions produced per mole of formula unit This adjusted molality can then be used to calculate the freezing-point depression
196
197
198
3 Compute
Practice Problem
What is the expected freezing-point depression for a solution that contains 20 mol of magnesium sulfate dissolved in 10 kg of water
minus74degC199
What is the expected boiling-point elevation of water for a solution that contains 150 g of sodium chloride dissolved in 10 kg of water 27degC The freezing point of an aqueous sodium chloride solution is minus200degC What is the molality of the solution 54 m NaCl
200
Actual Values for Electrolyte SolutionsRemember values calculated are only expected values Actual values of colligative properties for all strong electrolytes are almost what would be expected Difference between expected and actual values caused by attractive forces that exist between dissociated ions in aqueous solution
201
Attraction between hydrated ions in solution is small compared with those in crystalline solid However forces of attraction donrsquot interfere with movements of aqueous ions More concentrated solution means ions closer together and attraction is greater Also depends on charge of ions ndash higher charge greater attraction
202
Solute-Solvent Interactions
ldquolike dissolves likerdquo is generally useful rule for predicting whether one substance will dissolve in another Depends on type of bonding polaritynonpolarity and intermolecular forces between solute and solvent
45
Dissolving Ionic Compounds in Aqueous Solution
Polarity of water molecules plays important role in formation of solution of ionic compounds Charged ends of water molecules attract ions and surround them to keep separated from other ions
46
Hydration - solution process with water as solvent Ions said to be hydrated As hydrated ions diffuse into solution other ions exposed and drawn away from crystal surface Entire crystal gradually dissolves
47
When crystallized from aqueous solutions some ionic compounds form crystals that include water molecules The crystalline compounds known as hydrates have specific ratios of water molecules Represented by formulas like CuSO45H2O
48
When crystalline hydrate dissolves in water water of hydration returns to solvent Behavior of solute in hydrated form no different than anhydrous form
49
Nonpolar Solvents
Nonpolar solvents do not attract ions of ionic compound strongly enough to overcome forces holding crystal together
50
Liquid Solutes and Solvents
When you shake bottle of salad dressing oil droplets disperse in water Stop shaking ndash strong attraction between water molecules squeeze out oil and form separate layers Liquid solutes and solvents that are not soluble in each other are immiscible
51
Nonpolar substances (fat oil grease) are generally quite soluble in nonpolar liquids (carbon tetrachloride toluene gasoline)
Only attractions between nonpolar molecules are weak London forces
52
London Dispersion Forces
bc e- are constantly moving at any time there may be uneven distribution of them around a nonpolar atom Temporarily creates positive end and negative end
53
54
London dispersion forces - intermolecular attractions resulting from constant motion of e- and creation of instantaneous dipoles
55
Liquids that dissolve freely in one another in any ratio are said to be completely miscible Benzene and carbon tetrachloride (both nonpolar) are completely miscible
Nonpolar molecules of these two apply no strong forces of attraction or repulsion and molecules mix freely
56
Ethanol and water are also completely miscible -OH group on ethanol is slightly polar
Can form H-bonds with water as well as other ethanol molecules Intermolecular forces in mixture are very similar to those in pure liquids so they are mutually soluble in any amount
57
Gasoline contains mainly nonpolar hydrocarbons
Excellent solvent for fats oils greases
Major intermolecular forces are weak London forces
58
Ethanol less polar than water more than carbon tetrachloride
Better solvent for less-polar substances bc of nonpolar region
59
Effects of Pressure on Solubility
Little effect on solids or liquids Increase pressure - increase solubilities of gases in liquids
60
When gas in contact with surface of liquid gas molecules can enter liquid As amount of dissolve gas increases some molecules start to escape and reenter gas phase
Eq eventually established between rates of enteringleaving gas phase
61
As long as eq undisturbed solubility of gas in liquid unchanged at given pressure
Gas + solvent solution
62
Increasing pressure of solute gas above solution puts stress on eq Molecules collide with surface more often Increase in pressure partially offset by increase in rate of gas molecules entering solution In turn increase in amount of dissolved gas causes increase in rate molecules escape liquid and becomes gas
63
Eventually eq restored at higher gas solubility
Expected from Le Chatelierrsquos principle
Increase in gas pressure causes eq to shift so fewer molecules are in gas phase
64
Henryrsquos Law
Henryrsquos law - solubility of a gas in liquid is directly proportional to the partial pressure of that gas on the surface of the liquid
Applies to gas-liquid solutions at constant temp
65
When mixture of ideal gases is limited in constant volume at constant temp each gas applies same pressure it would apply if it occupied the space alone
Assuming gases do not react each gas dissolves to the extent it would if no other gases were there
66
In carbonated drinks (Coke Pepsi etc) solubility of CO2 increased by increasing pressure
At bottling factory CO2 gas forced into solution of flavored water at pressure of 5-10 atm Gas-in-liquid solution then sealed in bottlescans
67
When cap removed pressure reduced to 1 atm and some CO2 escapes as gas bubbles Effervescence - rapid escape of a gas from a liquid in which it is dissolved
68
Effects of Temperature on Solubility
Letrsquos consider gas solubility Increasing temperature usually decreases gas solubility As temp increases average kinetic energy of molecules in solution increases Greater number of solute molecules escape from attraction of solvent and return to gas phase 69
At higher temps eq is reached with fewer gas molecules in solution Gases generally less soluble
70
Effect of temp on solubility of solids in liquids more difficult to predict
Often increasing temp increases solubility
However the same temp increase can result in large increase in solubility in one case and only slight increase in the next
71
72
enthalpies of Solution
Formation of solution comes with energy change Dissolve KI in water container feels cold Dissolve LiCl in water feels hot Formation of solid-liquid solution can absorb or release heat
73
During formation of solution solvent and solute particles experience changes in forces attracting them to other particles Before dissolving begins solvent molecules held by intermolecular forces (solvent-solvent attraction) In solute molecules held by Ifs (solute-solute attraction)
74
Energy is required to separate solute molecules and solvent molecules from their neighbors
A solute particle that is surrounded by solvent molecules is said to be solvated
75
Step 1 solute particles become separated from solid (energy absorbed) Step 2 solvent particles move apart to allow solute particles to enter liquid (energy absorbed)
76
Step 3 solvent particles attracted to and solvate solute particles (energy released)
77
Heat of solution - net amount of heat energy absorbed or released when a specific amount of solute dissolves in a solvent
Heat of solution negative (heat is released) if sums of energy in steps 1 and 2 is less than step 3
If 1+2 gt 3 HOS is positive (heat absorbed)78
In gaseous state molecules are so far apart there are nearly no forces between them Solute-solute interaction has little effect on heat of solution of a gas Energy released when gas dissolved in liquid bc attraction between solute gas and solvent molecules greater than energy needed to separate solvent molecules
79
Concentration of Solutions80
Concentration - measure of amount of solute in given amount of solvent or solution ldquodiluterdquo just means small amount of solute ldquoconcentratedrdquo just means relatively large amount of solute Unrelated to degree to which solution is saturated Ex Saturated solution of substance not very soluble might be dilute
81
Molarity
Molarity - number of moles of solute in one liter of solution To find molarity must know molar mass A ldquoone molarrdquo solution of NaOH is 1 mole NaOH per liter of solution (1 M)
82
1 mol NaOH = 400 g If this quantity dissolve in enough water to make EXACTLY 100 L solution solution is 1 M If 200 g NaOH dissolve in enough to make 100 L solution what is molarity 0500 M
83
1 M solution is not made by adding 1 mol solute to 1 L solvent That would be more than 1 L Instead 1 mol solute dissolved in less than 1 L Then diluted with more solvent to bring TOTAL VOLUME to 1 L
84
Practice ProblemYou have 340 L of solution that contains 900 g sodium chloride What is the molarity of that solution 0440 M NaCl You have 08 L of a 05 M HCl solution How many moles of HCl does this solution contain 04 mol HCl
85
What is the molarity of a solution composed of 585 g of potassium iodide dissolved in enough water to make a 0125 L of solution 0282 M KI
86
How many moles of H2SO4 are present in 0500 L of a 0150 M H2SO4 solution 00750 mol What volume of 300 M NaCl is needed for reaction that requires 1463 g of NaCl 0834 L
87
Molality
Molality - concentration of solution expressed in moles of solute per kilogram of solvent Solution that contains 1 mol solute (NaOH) dissolved in exactly 1 kg of solvent is ldquoone molalrdquo solution (1 m NaOH)
88
1 mol NaOH = 400 g 400 g NaOH dissolved in 1 kg water is 1 m NaOH
200 g NaOH in 1 kg water = 0500 m NaOH
89
Practice Problem
A solution was prepared by dissolving 171 g sucrose (C12H22O11) in 125 g of water Find the molal concentration of this solution 0400 m C12H22O11
90
A solution of iodine in carbon tetrachloride is used when iodine is needed for certain chemical tests How much iodine must be added to prepare a 0480 m solution of iodine in CCl4 if 1000 g of CCl4 is used 122 g I2
91
What is the molality of a solution composed of 255 g acetone (CH3)2CO dissolved in 200 g of water 22 m acetone What quantity in grams of methanol CH3OH is required to prepare a 0244 m solution in 400 g water 312 g methanol
92
How many grams of AgNO3 are needed to prepare a 0125 m solution in 250 mL of water 531 g AgNO3
What is the molality of a solution containing 182 g HCl and 250 g water 200 m
93
Ions in Aqueous Solutions and Colligative Properties
94
95
compounds in aqueous solutions
DissociationWhen compound made from ions dissolves in water ions separate Dissociation - separation of ions that occurs when an ionic compound dissolves
96
Notice number of ions made per formula unit in equations 1 formula unit of NaCl gives 2 units of ions in solution 1 formula unit of CaCl2 gives 3 units of ions
97
Assuming 100 dissociation solution that contains 1 mol NaCl contains 1 mol Na+ and 1 mol Cl- Can assume 100 dissociation of all soluble ionic compounds
98
Practice Problem
Write the equation for the dissolution of aluminum sulfate in water How many moles of aluminum ions are made by dissolving 1 mol aluminum sulfate What is the total number of moles of ions made by dissolving 1 mol of aluminum sulfate
99
1 AnalyzeGiven Amount of solute = 1 mol Al2(SO4)3 Solvent identity = water Unknown a moles of aluminum ions and sulfate ions Total number of moles of solute ions produced
100
2 Plan
The coefficients in the balanced dissociation equation will tell mole relationships You can use the equation to find out the number of moles of solute ions produced
101
3 Compute
a
b102
Write the equation for the dissolution of each of the following in water and then determine the number of moles of each ions made as well as the total number of ions made a 1 mol ammonium chloride b 1 mol sodium sulfide c 05 mol barium nitrate
103
a 1 mol ammonium chloride
1 mol NH4+ 1 mol Cl- 2 mol total ions
104
b 1 mol sodium sulfide
2 mol Na+ 1 mol S-2 3 mol total ions
105
c 05 mol barium nitrate
05 mol Ba+2 1 mol NO3- 15 mol total ions
106
Precipitation Reactions
Even though no compound is completely insoluble compounds of very low solubility can be considered insoluble for practical purposes There are general guidelines to help predict whether a compound made of certain combination of ions is soluble
107
General Solubility Guidelines
Sodium potassium and ammonium compounds are soluble in water
108
Nitrates acetates and chlorates are soluble
109
110
Most chlorides are soluble except those of silver mercury (I) and lead Lead (II) chloride is soluble in hot water
Most sulfates are soluble except those of barium strontium lead calcium and mercury
111
Most carbonates phosphates and silicates are insoluble except those of sodium potassium and ammonium
112
Most sulfides are insoluble except those of calcium strontium sodium potassium and ammonium
113
Is calcium phosphate Ca3(PO4)2 soluble or insoluble
Not one of the exceptions so it is insoluble
Dissociation equations are not written for insoluble compounds
114
Guidelines also useful in predicting what will happen if 2 solutions of 2 different soluble compounds are mixed
If mixing results in combination of ions that forms insoluble compound a double-replacement and precipitation reaction will happen
115
Will precipitate form when solutions of ammonium sulfide and cadmium nitrate are combined You can tell calcium nitrate is soluble from the guidelines (Most nitrates acetates and chlorates are soluble) You can also tell ammonium sulfide is soluble (Most sulfides are insoluble except those of calcium strontium sodium potassium and ammonium)
116
117
To decide whether precipitate can form must know solubilities of two possible products
Ammonium nitrate ndash soluble (Most nitrates acetates and chlorates are soluble) Cadmium sulfide ndash insoluble (Most sulfides are insoluble except those of calcium strontium sodium potassium and ammonium)
118
bc one product is insoluble double-replacement and precipitation reaction will happen (NH4)2S (aq) + Cd(NO3)2 (aq) rarr NH4NO3 (aq) + CdS (s)
119
Net Ionic Equations
Reactions of ions in aqueous solution usually represented by net ionic equations instead of formula equations Net ionic equation - includes only those compounds and ions that undergo a chemical change in a reaction in an aqueous solution
120
121
To write NIE (net ionic equation) first convert chemical equation into overall ionic equation All soluble ionic compounds shown as dissociated ions in solution Precipitates shown as solids
122
Notice ammonium and nitrate ions appear on both sides They havenrsquot gone through any chemical change Spectator ions - ions that do not take part in a chemical reaction and are found in solution both before and after the reaction
123
To change ionic equation to NIE eliminate spectator ions Cd+2(aq) + S-2(aq) rarr CdS(s)
This NIE applies to any reaction in which precipitate of CdS forms
124
Practice Problem
Identify the precipitate that forms when aqueous solutions of zinc nitrate and ammonium sulfide are combined Write the equation for the possible double-replacement reaction Then write the formula equation overall ionic equation and the net ionic equation
125
1 AnalyzeGiven Identity of reactants zinc nitrate and ammonium sulfide Reaction medium aqueous solution Unknown a equation for the possible double-replacement reaction b identity of the precipitate c formula equation d overall ionic equation e net ionic equation
126
2 PlanWrite the possible double-replacement reaction between Zn(NO3)2 and (NH4)2S Use the rules to determine if any of the products will precipitate Write a formula equation and overall net ionic equation Then cancel spectator ions to make net ionic equation
127
3 Computea possible double-replacement reaction
b rules show that zinc sulfide is not soluble so will precipitate (ammonium nitrate is soluble)
c formula equation128
d overall ionic equation
e net ionic equation129
Will a precipitate form if solutions of potassium sulfate and barium nitrate are combined If so write the net ionic equation for the reaction
Yes Ba+2(aq) + SO4-2(aq) rarr BaSO4(s)
130
Will a precipitate form if solutions of potassium nitrate and magnesium sulfate are combined If so write the net ionic equation for the reaction
No 131
Will a precipitate form if solutions of barium chloride and sodium sulfate are combined If so identify the spectator ions and write the net ionic equation for the reaction
Yes Na+ and Cl- Ba+2(aq) + SO4-2(aq) rarr BaSO4(s)
132
Write the net ionic equation for the precipitation of nickel(II) sulfide
Ni+2(aq) + S-2(aq) rarr NiS(s)
133
IonizationSome molecular compounds can also form ions in solution Usually polar Ionization - ions formed from solute molecules by action of the solvent (creation of ions where there were none) Ionization different from dissociation
134
Dissociation ionic compounds dissolve and ions already present separate Ionization ions formed where non existed before
Ions formed are hydrated Heat released during hydration of ions gives enough energy to break covalent bonds
135
Extent to which solute ionizes depends on strength of bonds within molecules of solute and strength of attraction between solute and solvent molecules
136
HCl ndash molecular compound that ionizes in aqueous solution Contains highly polar bond Attraction between polar HCl molecule and polar water strong enough to break HCl bond
137
The Hydronium Ion
H+ ions from HCl attracts other molecules or ions so strongly that it doesnrsquot normally exist alone Ionization of HCl better described as direct transfer of proton from HCl to H2O forming H3O+
138
139
Hydration of H+ to form hydronium is highly exothermic
Energy released gives a lot of energy needed to ionize a molecular solute
Many molecular compounds that ionize in aqueous solution contain hydrogen and form hydronium ion
140
Strong and Weak Electrolytes
Substances that make ions and conduct current in solution are electrolytes HCl is one of series of compounds that has hydrogen and halogen Hydrogen halides are all molecular compounds with single polar-covalent bonds
141
All are gases Very soluble in water All are electrolytes HCl HBr HI strongly conduct current in solution HF only weakly conducts current at same concentration Strength of conduction related to ability to ionize
142
143
Strong Electrolytes
HCl HBr HI are 100 ionized in solution
Strong electrolyte - any compound whose dilute aqueous solutions conduct electricity well this is due to the presence of all or almost all of the dissolve compound in the form of ions
144
Unique characteristic of strong electrolyte is that they yield only ions
Ex Ionic compound may be highly soluble in water and dissociate into ions (NaCl)
Others may not dissolve much but amount that does dissolve exists only as ions
145
Weak ElectrolytesHF dissolves in water to give acid solution called hydrofluoric acid HF bond stronger than bonds between hydrogen and other halogens When HF dissolves some molecule ionize Reverse reaction also happens HF(aq) + H2O(l) H3O+(aq) + F-(aq)
146
HF(aq) + H2O(l) H3O+
(aq) + F-(aq)Concentration of dissolved unionized HF stays high and concentration of ions stays low
Weak electrolyte - any compound whose aqueous solutions conduct electricity poorly this is due to the presence of small amount of the dissolved compound in the form of ions
147
Different from nonelectrolyte ndash NO ions at all Description of electrolyte as strong or weak not related to concentration of solution Electrolytes differ in degree of ionization not amount of solute dissolved
148
149
Colligative properties of solutions
Presence of solute affects properties of solutions Some properties not dependent on nature of dissolved substance but on how may dissolve particles are present
Colligative properties - properties that depend on the concentration of solute particles but not on their identity
150
Vapor-Pressure Lowering
Boiling and freezing point of solution different from pure solvent Nonvolatile solute raises boiling point and lowers freezing point Nonvolatile substance - one that has little tendency to become gas under existing conditions
151
152
To understand why nonvolatile solute changes boiling and freezing point consider equilibrium vapor pressure
Vapor pressure pressure caused by molecules that have escape liquid phase to gas phase
Can be thought of a measure of tendency of molecules to escape from a liquid
153
Addition of sucrose (nonvolatile solute) lowers concentration of water molecules at surface of liquid This lowers tendency of water molecules to leave solution and enter gas phase Vapor pressure of solution is lower than vapor pressure of pure water
154
Aqueous13 solution13 of13 nonvolatile13 solutePure13 water
Nonelectrolyte solutions of same molality have same concentration of particles
Dilute solutions of same solvent and equal molality of any nonelectrolyte solute lower vapor pressure equally
155
Example
1 m aqueous solution of nonelectrolyte glucose lowers vapor pressure of water 55 x 10-4 atm at 25 1 m aqueous solution of sucrose also lowers water vapor pressure to 55 x 10-4 atm at 25
156
157
bc vapor-pressure lowering depends on concentration of nonelectrolyte solute and doesnrsquot depend on the type of solute it is a colligative proprerty
bc vapor pressure has been lowered solution remains liquid over larger temperature range
Lowers freezing point and raises boiling point
158
Can assume changes in boiling and freezing point also depend on concentration of solute
They are colligative properties
159
Freezing-Point Depression
Freezing point of 1-molal solution of any nonelectrolyte solute in water is found (by experiment) to be 186 lower than freezing point of water When 1 mol of nonelectrolyte solute dissolved in 1 kg water freezing point is -186 not 000 160
When 2 mol nonelectrolyte solute dissolved in 1 kg water freezing point is -372
For any concentration of nonelectrolyte solute in water decrease in freezing point can be determined using value of -186m
161
This value called the molal freezing-point contstant (Kf) is the freezing-point depression of the solvent in a 1-molal solution of a nonvolatile nonelectrolyte solute
162
Each solvent has own characteristic molal freezing-point constant Freezing-point depression (Δtf) the difference between the freezing points of the pure solvent and a solute of nonelectrolyte in that solvent and it is directly proportional to the molal concentration of the solution Δtf = Kfm
163
Δtf = Kfm
Kf expressed as m
m expressed in mol solutekg solvent (molality)
164
Sample Problem
What is the freezing point depression of water in a solution of 171 g of sucrose C12H22O11 and 200 g of water What is the actual freezing point of the solution
165
1 AnalyzeGiven Solute mass and chemical formula 171 g C12H22O11 Solvent mass and identity 2000 g water Unknown a freezing-point depression b freezing point of the solution 166
2 Plan
167
168
Calculate
Unknown a freezing-point depression b freezing point of the solution
169
Practice Problem
A water solution containing an unknown quantity of a nonelectrolyte solute is found to have a freezing point of ndash 023degCWhat is the molal concentration of the solution
012 m170
A solution consists of 103 g of the nonelectrolyte glucose C6H12O6 dissolved in 250 g of water What is the freezing-point depression of the solution
minus0426degC171
In a laboratory experiment the freezing point of an aqueous solution of glucose is found to be minus0325degCWhat is the molal concentration of this solution
0175 m172
If 0500 mol of a nonelectrolyte solute are dissolved in 5000 g of ether what is the freezing point of the solution
minus1181degC173
The freezing point of an aqueous solution that contains a nonelectrolyte is minus90degC aWhat is the freezing-point depression of the solution bWhat is the molal concentration of the solution a minus90degC b 48 m
174
Boiling-Point Elevation
Boiling point is temp at which vapor pressure is equal to atmospheric pressure Change in vapor pressure will cause corresponding change in boiling point
175
176
Vapor pressure of nonvolatile solution is lower than vapor pressure of pure solvent More heat will be required to raise vapor pressure of solution Boiling point of solution is higher than boiling point of pure solvent
Molal boiling-point constant (Kb) - boiling-point elevation of the solvent in a 1-molal solution of a nonvolatile nonelectrolyte solute b-p elevation of any nonelectrolyte in water found by experiment to be 051 So molal b-p constant for water is 051m
177
For different solvents b-p elevations of 1-molal solutions have different values (Table 14-2)
Like freezing-point constants values most accurate for dilute solutions
178
Boiling-point elevation Δtb difference between the boiling points of the pure solvent and a nonelectrolyte solution of that solvent and it is directly proportional to the molal concentration of the solution B-p elevation can be calculated using Δtb = Kbm Δtb (boiling point elevation) expressed in m and m expressed in mol solutekg solvent
Sample Problem
What is the boiling-point elevation of a solution made from 200 g of a nonelectrolyte solute and 4000 g of water The molar mass of the solute is 620 gmol
180
1 AnalyzeGiven solute mass = 200 g solute molar mass = 620 gmol solvent mass and identity = 4000 g of water
Unknown boiling-point elevation
181
2 Plan
182
183
3 Compute
Practice Problem
A solution contains 500 g of sucrose C12H22O11 a nonelectrolyte dissolved in 5000 g of water What is the boiling-point elevation
015degC184
A solution contains 4500 g of sucrose C12H22O11 a nonelectrolyte dissolved in 250 g of waterWhat is the boiling point of the solution
1027degC185
If the boiling point elevation of an aqueous solution containing a nonvolatile electrolyte is 102degC what is the molality of the solution
20 m186
The boiling point of an aqueous solution containing a nonvolatile electrolyte is 10075degC aWhat is the boiling-point elevation bWhat is the molality of the solution a 075degC b 15 m
187
Electrolytes and Colligative PropertiesEarly investigators confused by experiments where certain substances depressed freezing point or elevated boiling point more than expected
188
189
Ex 01 m solution of NaCl lowers freezing point of solvent almost twice as much as 01 m solution of sucrose 01 m CaCl2 lowers fp almost 3 times as much as 01 m solution of sucrose
To understand why this happens contrast behavior or sucrose with NaCl in aqueous solution Each sucrose dissolves to make only 1 particle in solution So 1 mol sucrose dissolves to 1 mol particles 1 mol NaCl dissolves to make 2 moles of particles (1 mol Na+ and 1 mol Cl-)
190
191
Calculated Values for Electrolyte SolutionsColligative properties depend on total concentration of solute particles regardless of their identity Electrolytes cause changes in colligative properties proportional to total molality in terms of all dissolves particles instead of formula units
192
What about barium nitrate Ba(NO3)2 Each mole of barium nitrate yields 3 mol of ions in solution Ba(NO3)2(s) rarr Ba2+(aq) + 2NO3-(aq) You would expect solution of given molality to lower fp of solvent 3 times as much as nonelectrolytic solution of same molality
193
Sample Problem
What is the expected change in the freezing point of water in a solution of 625 g of barium nitrate Ba(NO3)2 in 100 kg of water
194
1 AnalyzeGiven solute mass and formula = 625 g Ba(NO3)2 solvent mass and identity = 100 kg water Δtf = Kfm
Unknown expected freezing-point depression
195
2 PlanThe molality can be calculated by converting the solute mass to moles and then dividing by the number of kilograms of solvent That molality is in terms of formula units of Ba(NO3)2 and must be converted to molality in terms of dissociated ions in solution It must be multiplied by the number of moles of ions produced per mole of formula unit This adjusted molality can then be used to calculate the freezing-point depression
196
197
198
3 Compute
Practice Problem
What is the expected freezing-point depression for a solution that contains 20 mol of magnesium sulfate dissolved in 10 kg of water
minus74degC199
What is the expected boiling-point elevation of water for a solution that contains 150 g of sodium chloride dissolved in 10 kg of water 27degC The freezing point of an aqueous sodium chloride solution is minus200degC What is the molality of the solution 54 m NaCl
200
Actual Values for Electrolyte SolutionsRemember values calculated are only expected values Actual values of colligative properties for all strong electrolytes are almost what would be expected Difference between expected and actual values caused by attractive forces that exist between dissociated ions in aqueous solution
201
Attraction between hydrated ions in solution is small compared with those in crystalline solid However forces of attraction donrsquot interfere with movements of aqueous ions More concentrated solution means ions closer together and attraction is greater Also depends on charge of ions ndash higher charge greater attraction
202
Dissolving Ionic Compounds in Aqueous Solution
Polarity of water molecules plays important role in formation of solution of ionic compounds Charged ends of water molecules attract ions and surround them to keep separated from other ions
46
Hydration - solution process with water as solvent Ions said to be hydrated As hydrated ions diffuse into solution other ions exposed and drawn away from crystal surface Entire crystal gradually dissolves
47
When crystallized from aqueous solutions some ionic compounds form crystals that include water molecules The crystalline compounds known as hydrates have specific ratios of water molecules Represented by formulas like CuSO45H2O
48
When crystalline hydrate dissolves in water water of hydration returns to solvent Behavior of solute in hydrated form no different than anhydrous form
49
Nonpolar Solvents
Nonpolar solvents do not attract ions of ionic compound strongly enough to overcome forces holding crystal together
50
Liquid Solutes and Solvents
When you shake bottle of salad dressing oil droplets disperse in water Stop shaking ndash strong attraction between water molecules squeeze out oil and form separate layers Liquid solutes and solvents that are not soluble in each other are immiscible
51
Nonpolar substances (fat oil grease) are generally quite soluble in nonpolar liquids (carbon tetrachloride toluene gasoline)
Only attractions between nonpolar molecules are weak London forces
52
London Dispersion Forces
bc e- are constantly moving at any time there may be uneven distribution of them around a nonpolar atom Temporarily creates positive end and negative end
53
54
London dispersion forces - intermolecular attractions resulting from constant motion of e- and creation of instantaneous dipoles
55
Liquids that dissolve freely in one another in any ratio are said to be completely miscible Benzene and carbon tetrachloride (both nonpolar) are completely miscible
Nonpolar molecules of these two apply no strong forces of attraction or repulsion and molecules mix freely
56
Ethanol and water are also completely miscible -OH group on ethanol is slightly polar
Can form H-bonds with water as well as other ethanol molecules Intermolecular forces in mixture are very similar to those in pure liquids so they are mutually soluble in any amount
57
Gasoline contains mainly nonpolar hydrocarbons
Excellent solvent for fats oils greases
Major intermolecular forces are weak London forces
58
Ethanol less polar than water more than carbon tetrachloride
Better solvent for less-polar substances bc of nonpolar region
59
Effects of Pressure on Solubility
Little effect on solids or liquids Increase pressure - increase solubilities of gases in liquids
60
When gas in contact with surface of liquid gas molecules can enter liquid As amount of dissolve gas increases some molecules start to escape and reenter gas phase
Eq eventually established between rates of enteringleaving gas phase
61
As long as eq undisturbed solubility of gas in liquid unchanged at given pressure
Gas + solvent solution
62
Increasing pressure of solute gas above solution puts stress on eq Molecules collide with surface more often Increase in pressure partially offset by increase in rate of gas molecules entering solution In turn increase in amount of dissolved gas causes increase in rate molecules escape liquid and becomes gas
63
Eventually eq restored at higher gas solubility
Expected from Le Chatelierrsquos principle
Increase in gas pressure causes eq to shift so fewer molecules are in gas phase
64
Henryrsquos Law
Henryrsquos law - solubility of a gas in liquid is directly proportional to the partial pressure of that gas on the surface of the liquid
Applies to gas-liquid solutions at constant temp
65
When mixture of ideal gases is limited in constant volume at constant temp each gas applies same pressure it would apply if it occupied the space alone
Assuming gases do not react each gas dissolves to the extent it would if no other gases were there
66
In carbonated drinks (Coke Pepsi etc) solubility of CO2 increased by increasing pressure
At bottling factory CO2 gas forced into solution of flavored water at pressure of 5-10 atm Gas-in-liquid solution then sealed in bottlescans
67
When cap removed pressure reduced to 1 atm and some CO2 escapes as gas bubbles Effervescence - rapid escape of a gas from a liquid in which it is dissolved
68
Effects of Temperature on Solubility
Letrsquos consider gas solubility Increasing temperature usually decreases gas solubility As temp increases average kinetic energy of molecules in solution increases Greater number of solute molecules escape from attraction of solvent and return to gas phase 69
At higher temps eq is reached with fewer gas molecules in solution Gases generally less soluble
70
Effect of temp on solubility of solids in liquids more difficult to predict
Often increasing temp increases solubility
However the same temp increase can result in large increase in solubility in one case and only slight increase in the next
71
72
enthalpies of Solution
Formation of solution comes with energy change Dissolve KI in water container feels cold Dissolve LiCl in water feels hot Formation of solid-liquid solution can absorb or release heat
73
During formation of solution solvent and solute particles experience changes in forces attracting them to other particles Before dissolving begins solvent molecules held by intermolecular forces (solvent-solvent attraction) In solute molecules held by Ifs (solute-solute attraction)
74
Energy is required to separate solute molecules and solvent molecules from their neighbors
A solute particle that is surrounded by solvent molecules is said to be solvated
75
Step 1 solute particles become separated from solid (energy absorbed) Step 2 solvent particles move apart to allow solute particles to enter liquid (energy absorbed)
76
Step 3 solvent particles attracted to and solvate solute particles (energy released)
77
Heat of solution - net amount of heat energy absorbed or released when a specific amount of solute dissolves in a solvent
Heat of solution negative (heat is released) if sums of energy in steps 1 and 2 is less than step 3
If 1+2 gt 3 HOS is positive (heat absorbed)78
In gaseous state molecules are so far apart there are nearly no forces between them Solute-solute interaction has little effect on heat of solution of a gas Energy released when gas dissolved in liquid bc attraction between solute gas and solvent molecules greater than energy needed to separate solvent molecules
79
Concentration of Solutions80
Concentration - measure of amount of solute in given amount of solvent or solution ldquodiluterdquo just means small amount of solute ldquoconcentratedrdquo just means relatively large amount of solute Unrelated to degree to which solution is saturated Ex Saturated solution of substance not very soluble might be dilute
81
Molarity
Molarity - number of moles of solute in one liter of solution To find molarity must know molar mass A ldquoone molarrdquo solution of NaOH is 1 mole NaOH per liter of solution (1 M)
82
1 mol NaOH = 400 g If this quantity dissolve in enough water to make EXACTLY 100 L solution solution is 1 M If 200 g NaOH dissolve in enough to make 100 L solution what is molarity 0500 M
83
1 M solution is not made by adding 1 mol solute to 1 L solvent That would be more than 1 L Instead 1 mol solute dissolved in less than 1 L Then diluted with more solvent to bring TOTAL VOLUME to 1 L
84
Practice ProblemYou have 340 L of solution that contains 900 g sodium chloride What is the molarity of that solution 0440 M NaCl You have 08 L of a 05 M HCl solution How many moles of HCl does this solution contain 04 mol HCl
85
What is the molarity of a solution composed of 585 g of potassium iodide dissolved in enough water to make a 0125 L of solution 0282 M KI
86
How many moles of H2SO4 are present in 0500 L of a 0150 M H2SO4 solution 00750 mol What volume of 300 M NaCl is needed for reaction that requires 1463 g of NaCl 0834 L
87
Molality
Molality - concentration of solution expressed in moles of solute per kilogram of solvent Solution that contains 1 mol solute (NaOH) dissolved in exactly 1 kg of solvent is ldquoone molalrdquo solution (1 m NaOH)
88
1 mol NaOH = 400 g 400 g NaOH dissolved in 1 kg water is 1 m NaOH
200 g NaOH in 1 kg water = 0500 m NaOH
89
Practice Problem
A solution was prepared by dissolving 171 g sucrose (C12H22O11) in 125 g of water Find the molal concentration of this solution 0400 m C12H22O11
90
A solution of iodine in carbon tetrachloride is used when iodine is needed for certain chemical tests How much iodine must be added to prepare a 0480 m solution of iodine in CCl4 if 1000 g of CCl4 is used 122 g I2
91
What is the molality of a solution composed of 255 g acetone (CH3)2CO dissolved in 200 g of water 22 m acetone What quantity in grams of methanol CH3OH is required to prepare a 0244 m solution in 400 g water 312 g methanol
92
How many grams of AgNO3 are needed to prepare a 0125 m solution in 250 mL of water 531 g AgNO3
What is the molality of a solution containing 182 g HCl and 250 g water 200 m
93
Ions in Aqueous Solutions and Colligative Properties
94
95
compounds in aqueous solutions
DissociationWhen compound made from ions dissolves in water ions separate Dissociation - separation of ions that occurs when an ionic compound dissolves
96
Notice number of ions made per formula unit in equations 1 formula unit of NaCl gives 2 units of ions in solution 1 formula unit of CaCl2 gives 3 units of ions
97
Assuming 100 dissociation solution that contains 1 mol NaCl contains 1 mol Na+ and 1 mol Cl- Can assume 100 dissociation of all soluble ionic compounds
98
Practice Problem
Write the equation for the dissolution of aluminum sulfate in water How many moles of aluminum ions are made by dissolving 1 mol aluminum sulfate What is the total number of moles of ions made by dissolving 1 mol of aluminum sulfate
99
1 AnalyzeGiven Amount of solute = 1 mol Al2(SO4)3 Solvent identity = water Unknown a moles of aluminum ions and sulfate ions Total number of moles of solute ions produced
100
2 Plan
The coefficients in the balanced dissociation equation will tell mole relationships You can use the equation to find out the number of moles of solute ions produced
101
3 Compute
a
b102
Write the equation for the dissolution of each of the following in water and then determine the number of moles of each ions made as well as the total number of ions made a 1 mol ammonium chloride b 1 mol sodium sulfide c 05 mol barium nitrate
103
a 1 mol ammonium chloride
1 mol NH4+ 1 mol Cl- 2 mol total ions
104
b 1 mol sodium sulfide
2 mol Na+ 1 mol S-2 3 mol total ions
105
c 05 mol barium nitrate
05 mol Ba+2 1 mol NO3- 15 mol total ions
106
Precipitation Reactions
Even though no compound is completely insoluble compounds of very low solubility can be considered insoluble for practical purposes There are general guidelines to help predict whether a compound made of certain combination of ions is soluble
107
General Solubility Guidelines
Sodium potassium and ammonium compounds are soluble in water
108
Nitrates acetates and chlorates are soluble
109
110
Most chlorides are soluble except those of silver mercury (I) and lead Lead (II) chloride is soluble in hot water
Most sulfates are soluble except those of barium strontium lead calcium and mercury
111
Most carbonates phosphates and silicates are insoluble except those of sodium potassium and ammonium
112
Most sulfides are insoluble except those of calcium strontium sodium potassium and ammonium
113
Is calcium phosphate Ca3(PO4)2 soluble or insoluble
Not one of the exceptions so it is insoluble
Dissociation equations are not written for insoluble compounds
114
Guidelines also useful in predicting what will happen if 2 solutions of 2 different soluble compounds are mixed
If mixing results in combination of ions that forms insoluble compound a double-replacement and precipitation reaction will happen
115
Will precipitate form when solutions of ammonium sulfide and cadmium nitrate are combined You can tell calcium nitrate is soluble from the guidelines (Most nitrates acetates and chlorates are soluble) You can also tell ammonium sulfide is soluble (Most sulfides are insoluble except those of calcium strontium sodium potassium and ammonium)
116
117
To decide whether precipitate can form must know solubilities of two possible products
Ammonium nitrate ndash soluble (Most nitrates acetates and chlorates are soluble) Cadmium sulfide ndash insoluble (Most sulfides are insoluble except those of calcium strontium sodium potassium and ammonium)
118
bc one product is insoluble double-replacement and precipitation reaction will happen (NH4)2S (aq) + Cd(NO3)2 (aq) rarr NH4NO3 (aq) + CdS (s)
119
Net Ionic Equations
Reactions of ions in aqueous solution usually represented by net ionic equations instead of formula equations Net ionic equation - includes only those compounds and ions that undergo a chemical change in a reaction in an aqueous solution
120
121
To write NIE (net ionic equation) first convert chemical equation into overall ionic equation All soluble ionic compounds shown as dissociated ions in solution Precipitates shown as solids
122
Notice ammonium and nitrate ions appear on both sides They havenrsquot gone through any chemical change Spectator ions - ions that do not take part in a chemical reaction and are found in solution both before and after the reaction
123
To change ionic equation to NIE eliminate spectator ions Cd+2(aq) + S-2(aq) rarr CdS(s)
This NIE applies to any reaction in which precipitate of CdS forms
124
Practice Problem
Identify the precipitate that forms when aqueous solutions of zinc nitrate and ammonium sulfide are combined Write the equation for the possible double-replacement reaction Then write the formula equation overall ionic equation and the net ionic equation
125
1 AnalyzeGiven Identity of reactants zinc nitrate and ammonium sulfide Reaction medium aqueous solution Unknown a equation for the possible double-replacement reaction b identity of the precipitate c formula equation d overall ionic equation e net ionic equation
126
2 PlanWrite the possible double-replacement reaction between Zn(NO3)2 and (NH4)2S Use the rules to determine if any of the products will precipitate Write a formula equation and overall net ionic equation Then cancel spectator ions to make net ionic equation
127
3 Computea possible double-replacement reaction
b rules show that zinc sulfide is not soluble so will precipitate (ammonium nitrate is soluble)
c formula equation128
d overall ionic equation
e net ionic equation129
Will a precipitate form if solutions of potassium sulfate and barium nitrate are combined If so write the net ionic equation for the reaction
Yes Ba+2(aq) + SO4-2(aq) rarr BaSO4(s)
130
Will a precipitate form if solutions of potassium nitrate and magnesium sulfate are combined If so write the net ionic equation for the reaction
No 131
Will a precipitate form if solutions of barium chloride and sodium sulfate are combined If so identify the spectator ions and write the net ionic equation for the reaction
Yes Na+ and Cl- Ba+2(aq) + SO4-2(aq) rarr BaSO4(s)
132
Write the net ionic equation for the precipitation of nickel(II) sulfide
Ni+2(aq) + S-2(aq) rarr NiS(s)
133
IonizationSome molecular compounds can also form ions in solution Usually polar Ionization - ions formed from solute molecules by action of the solvent (creation of ions where there were none) Ionization different from dissociation
134
Dissociation ionic compounds dissolve and ions already present separate Ionization ions formed where non existed before
Ions formed are hydrated Heat released during hydration of ions gives enough energy to break covalent bonds
135
Extent to which solute ionizes depends on strength of bonds within molecules of solute and strength of attraction between solute and solvent molecules
136
HCl ndash molecular compound that ionizes in aqueous solution Contains highly polar bond Attraction between polar HCl molecule and polar water strong enough to break HCl bond
137
The Hydronium Ion
H+ ions from HCl attracts other molecules or ions so strongly that it doesnrsquot normally exist alone Ionization of HCl better described as direct transfer of proton from HCl to H2O forming H3O+
138
139
Hydration of H+ to form hydronium is highly exothermic
Energy released gives a lot of energy needed to ionize a molecular solute
Many molecular compounds that ionize in aqueous solution contain hydrogen and form hydronium ion
140
Strong and Weak Electrolytes
Substances that make ions and conduct current in solution are electrolytes HCl is one of series of compounds that has hydrogen and halogen Hydrogen halides are all molecular compounds with single polar-covalent bonds
141
All are gases Very soluble in water All are electrolytes HCl HBr HI strongly conduct current in solution HF only weakly conducts current at same concentration Strength of conduction related to ability to ionize
142
143
Strong Electrolytes
HCl HBr HI are 100 ionized in solution
Strong electrolyte - any compound whose dilute aqueous solutions conduct electricity well this is due to the presence of all or almost all of the dissolve compound in the form of ions
144
Unique characteristic of strong electrolyte is that they yield only ions
Ex Ionic compound may be highly soluble in water and dissociate into ions (NaCl)
Others may not dissolve much but amount that does dissolve exists only as ions
145
Weak ElectrolytesHF dissolves in water to give acid solution called hydrofluoric acid HF bond stronger than bonds between hydrogen and other halogens When HF dissolves some molecule ionize Reverse reaction also happens HF(aq) + H2O(l) H3O+(aq) + F-(aq)
146
HF(aq) + H2O(l) H3O+
(aq) + F-(aq)Concentration of dissolved unionized HF stays high and concentration of ions stays low
Weak electrolyte - any compound whose aqueous solutions conduct electricity poorly this is due to the presence of small amount of the dissolved compound in the form of ions
147
Different from nonelectrolyte ndash NO ions at all Description of electrolyte as strong or weak not related to concentration of solution Electrolytes differ in degree of ionization not amount of solute dissolved
148
149
Colligative properties of solutions
Presence of solute affects properties of solutions Some properties not dependent on nature of dissolved substance but on how may dissolve particles are present
Colligative properties - properties that depend on the concentration of solute particles but not on their identity
150
Vapor-Pressure Lowering
Boiling and freezing point of solution different from pure solvent Nonvolatile solute raises boiling point and lowers freezing point Nonvolatile substance - one that has little tendency to become gas under existing conditions
151
152
To understand why nonvolatile solute changes boiling and freezing point consider equilibrium vapor pressure
Vapor pressure pressure caused by molecules that have escape liquid phase to gas phase
Can be thought of a measure of tendency of molecules to escape from a liquid
153
Addition of sucrose (nonvolatile solute) lowers concentration of water molecules at surface of liquid This lowers tendency of water molecules to leave solution and enter gas phase Vapor pressure of solution is lower than vapor pressure of pure water
154
Aqueous13 solution13 of13 nonvolatile13 solutePure13 water
Nonelectrolyte solutions of same molality have same concentration of particles
Dilute solutions of same solvent and equal molality of any nonelectrolyte solute lower vapor pressure equally
155
Example
1 m aqueous solution of nonelectrolyte glucose lowers vapor pressure of water 55 x 10-4 atm at 25 1 m aqueous solution of sucrose also lowers water vapor pressure to 55 x 10-4 atm at 25
156
157
bc vapor-pressure lowering depends on concentration of nonelectrolyte solute and doesnrsquot depend on the type of solute it is a colligative proprerty
bc vapor pressure has been lowered solution remains liquid over larger temperature range
Lowers freezing point and raises boiling point
158
Can assume changes in boiling and freezing point also depend on concentration of solute
They are colligative properties
159
Freezing-Point Depression
Freezing point of 1-molal solution of any nonelectrolyte solute in water is found (by experiment) to be 186 lower than freezing point of water When 1 mol of nonelectrolyte solute dissolved in 1 kg water freezing point is -186 not 000 160
When 2 mol nonelectrolyte solute dissolved in 1 kg water freezing point is -372
For any concentration of nonelectrolyte solute in water decrease in freezing point can be determined using value of -186m
161
This value called the molal freezing-point contstant (Kf) is the freezing-point depression of the solvent in a 1-molal solution of a nonvolatile nonelectrolyte solute
162
Each solvent has own characteristic molal freezing-point constant Freezing-point depression (Δtf) the difference between the freezing points of the pure solvent and a solute of nonelectrolyte in that solvent and it is directly proportional to the molal concentration of the solution Δtf = Kfm
163
Δtf = Kfm
Kf expressed as m
m expressed in mol solutekg solvent (molality)
164
Sample Problem
What is the freezing point depression of water in a solution of 171 g of sucrose C12H22O11 and 200 g of water What is the actual freezing point of the solution
165
1 AnalyzeGiven Solute mass and chemical formula 171 g C12H22O11 Solvent mass and identity 2000 g water Unknown a freezing-point depression b freezing point of the solution 166
2 Plan
167
168
Calculate
Unknown a freezing-point depression b freezing point of the solution
169
Practice Problem
A water solution containing an unknown quantity of a nonelectrolyte solute is found to have a freezing point of ndash 023degCWhat is the molal concentration of the solution
012 m170
A solution consists of 103 g of the nonelectrolyte glucose C6H12O6 dissolved in 250 g of water What is the freezing-point depression of the solution
minus0426degC171
In a laboratory experiment the freezing point of an aqueous solution of glucose is found to be minus0325degCWhat is the molal concentration of this solution
0175 m172
If 0500 mol of a nonelectrolyte solute are dissolved in 5000 g of ether what is the freezing point of the solution
minus1181degC173
The freezing point of an aqueous solution that contains a nonelectrolyte is minus90degC aWhat is the freezing-point depression of the solution bWhat is the molal concentration of the solution a minus90degC b 48 m
174
Boiling-Point Elevation
Boiling point is temp at which vapor pressure is equal to atmospheric pressure Change in vapor pressure will cause corresponding change in boiling point
175
176
Vapor pressure of nonvolatile solution is lower than vapor pressure of pure solvent More heat will be required to raise vapor pressure of solution Boiling point of solution is higher than boiling point of pure solvent
Molal boiling-point constant (Kb) - boiling-point elevation of the solvent in a 1-molal solution of a nonvolatile nonelectrolyte solute b-p elevation of any nonelectrolyte in water found by experiment to be 051 So molal b-p constant for water is 051m
177
For different solvents b-p elevations of 1-molal solutions have different values (Table 14-2)
Like freezing-point constants values most accurate for dilute solutions
178
Boiling-point elevation Δtb difference between the boiling points of the pure solvent and a nonelectrolyte solution of that solvent and it is directly proportional to the molal concentration of the solution B-p elevation can be calculated using Δtb = Kbm Δtb (boiling point elevation) expressed in m and m expressed in mol solutekg solvent
Sample Problem
What is the boiling-point elevation of a solution made from 200 g of a nonelectrolyte solute and 4000 g of water The molar mass of the solute is 620 gmol
180
1 AnalyzeGiven solute mass = 200 g solute molar mass = 620 gmol solvent mass and identity = 4000 g of water
Unknown boiling-point elevation
181
2 Plan
182
183
3 Compute
Practice Problem
A solution contains 500 g of sucrose C12H22O11 a nonelectrolyte dissolved in 5000 g of water What is the boiling-point elevation
015degC184
A solution contains 4500 g of sucrose C12H22O11 a nonelectrolyte dissolved in 250 g of waterWhat is the boiling point of the solution
1027degC185
If the boiling point elevation of an aqueous solution containing a nonvolatile electrolyte is 102degC what is the molality of the solution
20 m186
The boiling point of an aqueous solution containing a nonvolatile electrolyte is 10075degC aWhat is the boiling-point elevation bWhat is the molality of the solution a 075degC b 15 m
187
Electrolytes and Colligative PropertiesEarly investigators confused by experiments where certain substances depressed freezing point or elevated boiling point more than expected
188
189
Ex 01 m solution of NaCl lowers freezing point of solvent almost twice as much as 01 m solution of sucrose 01 m CaCl2 lowers fp almost 3 times as much as 01 m solution of sucrose
To understand why this happens contrast behavior or sucrose with NaCl in aqueous solution Each sucrose dissolves to make only 1 particle in solution So 1 mol sucrose dissolves to 1 mol particles 1 mol NaCl dissolves to make 2 moles of particles (1 mol Na+ and 1 mol Cl-)
190
191
Calculated Values for Electrolyte SolutionsColligative properties depend on total concentration of solute particles regardless of their identity Electrolytes cause changes in colligative properties proportional to total molality in terms of all dissolves particles instead of formula units
192
What about barium nitrate Ba(NO3)2 Each mole of barium nitrate yields 3 mol of ions in solution Ba(NO3)2(s) rarr Ba2+(aq) + 2NO3-(aq) You would expect solution of given molality to lower fp of solvent 3 times as much as nonelectrolytic solution of same molality
193
Sample Problem
What is the expected change in the freezing point of water in a solution of 625 g of barium nitrate Ba(NO3)2 in 100 kg of water
194
1 AnalyzeGiven solute mass and formula = 625 g Ba(NO3)2 solvent mass and identity = 100 kg water Δtf = Kfm
Unknown expected freezing-point depression
195
2 PlanThe molality can be calculated by converting the solute mass to moles and then dividing by the number of kilograms of solvent That molality is in terms of formula units of Ba(NO3)2 and must be converted to molality in terms of dissociated ions in solution It must be multiplied by the number of moles of ions produced per mole of formula unit This adjusted molality can then be used to calculate the freezing-point depression
196
197
198
3 Compute
Practice Problem
What is the expected freezing-point depression for a solution that contains 20 mol of magnesium sulfate dissolved in 10 kg of water
minus74degC199
What is the expected boiling-point elevation of water for a solution that contains 150 g of sodium chloride dissolved in 10 kg of water 27degC The freezing point of an aqueous sodium chloride solution is minus200degC What is the molality of the solution 54 m NaCl
200
Actual Values for Electrolyte SolutionsRemember values calculated are only expected values Actual values of colligative properties for all strong electrolytes are almost what would be expected Difference between expected and actual values caused by attractive forces that exist between dissociated ions in aqueous solution
201
Attraction between hydrated ions in solution is small compared with those in crystalline solid However forces of attraction donrsquot interfere with movements of aqueous ions More concentrated solution means ions closer together and attraction is greater Also depends on charge of ions ndash higher charge greater attraction
202
Hydration - solution process with water as solvent Ions said to be hydrated As hydrated ions diffuse into solution other ions exposed and drawn away from crystal surface Entire crystal gradually dissolves
47
When crystallized from aqueous solutions some ionic compounds form crystals that include water molecules The crystalline compounds known as hydrates have specific ratios of water molecules Represented by formulas like CuSO45H2O
48
When crystalline hydrate dissolves in water water of hydration returns to solvent Behavior of solute in hydrated form no different than anhydrous form
49
Nonpolar Solvents
Nonpolar solvents do not attract ions of ionic compound strongly enough to overcome forces holding crystal together
50
Liquid Solutes and Solvents
When you shake bottle of salad dressing oil droplets disperse in water Stop shaking ndash strong attraction between water molecules squeeze out oil and form separate layers Liquid solutes and solvents that are not soluble in each other are immiscible
51
Nonpolar substances (fat oil grease) are generally quite soluble in nonpolar liquids (carbon tetrachloride toluene gasoline)
Only attractions between nonpolar molecules are weak London forces
52
London Dispersion Forces
bc e- are constantly moving at any time there may be uneven distribution of them around a nonpolar atom Temporarily creates positive end and negative end
53
54
London dispersion forces - intermolecular attractions resulting from constant motion of e- and creation of instantaneous dipoles
55
Liquids that dissolve freely in one another in any ratio are said to be completely miscible Benzene and carbon tetrachloride (both nonpolar) are completely miscible
Nonpolar molecules of these two apply no strong forces of attraction or repulsion and molecules mix freely
56
Ethanol and water are also completely miscible -OH group on ethanol is slightly polar
Can form H-bonds with water as well as other ethanol molecules Intermolecular forces in mixture are very similar to those in pure liquids so they are mutually soluble in any amount
57
Gasoline contains mainly nonpolar hydrocarbons
Excellent solvent for fats oils greases
Major intermolecular forces are weak London forces
58
Ethanol less polar than water more than carbon tetrachloride
Better solvent for less-polar substances bc of nonpolar region
59
Effects of Pressure on Solubility
Little effect on solids or liquids Increase pressure - increase solubilities of gases in liquids
60
When gas in contact with surface of liquid gas molecules can enter liquid As amount of dissolve gas increases some molecules start to escape and reenter gas phase
Eq eventually established between rates of enteringleaving gas phase
61
As long as eq undisturbed solubility of gas in liquid unchanged at given pressure
Gas + solvent solution
62
Increasing pressure of solute gas above solution puts stress on eq Molecules collide with surface more often Increase in pressure partially offset by increase in rate of gas molecules entering solution In turn increase in amount of dissolved gas causes increase in rate molecules escape liquid and becomes gas
63
Eventually eq restored at higher gas solubility
Expected from Le Chatelierrsquos principle
Increase in gas pressure causes eq to shift so fewer molecules are in gas phase
64
Henryrsquos Law
Henryrsquos law - solubility of a gas in liquid is directly proportional to the partial pressure of that gas on the surface of the liquid
Applies to gas-liquid solutions at constant temp
65
When mixture of ideal gases is limited in constant volume at constant temp each gas applies same pressure it would apply if it occupied the space alone
Assuming gases do not react each gas dissolves to the extent it would if no other gases were there
66
In carbonated drinks (Coke Pepsi etc) solubility of CO2 increased by increasing pressure
At bottling factory CO2 gas forced into solution of flavored water at pressure of 5-10 atm Gas-in-liquid solution then sealed in bottlescans
67
When cap removed pressure reduced to 1 atm and some CO2 escapes as gas bubbles Effervescence - rapid escape of a gas from a liquid in which it is dissolved
68
Effects of Temperature on Solubility
Letrsquos consider gas solubility Increasing temperature usually decreases gas solubility As temp increases average kinetic energy of molecules in solution increases Greater number of solute molecules escape from attraction of solvent and return to gas phase 69
At higher temps eq is reached with fewer gas molecules in solution Gases generally less soluble
70
Effect of temp on solubility of solids in liquids more difficult to predict
Often increasing temp increases solubility
However the same temp increase can result in large increase in solubility in one case and only slight increase in the next
71
72
enthalpies of Solution
Formation of solution comes with energy change Dissolve KI in water container feels cold Dissolve LiCl in water feels hot Formation of solid-liquid solution can absorb or release heat
73
During formation of solution solvent and solute particles experience changes in forces attracting them to other particles Before dissolving begins solvent molecules held by intermolecular forces (solvent-solvent attraction) In solute molecules held by Ifs (solute-solute attraction)
74
Energy is required to separate solute molecules and solvent molecules from their neighbors
A solute particle that is surrounded by solvent molecules is said to be solvated
75
Step 1 solute particles become separated from solid (energy absorbed) Step 2 solvent particles move apart to allow solute particles to enter liquid (energy absorbed)
76
Step 3 solvent particles attracted to and solvate solute particles (energy released)
77
Heat of solution - net amount of heat energy absorbed or released when a specific amount of solute dissolves in a solvent
Heat of solution negative (heat is released) if sums of energy in steps 1 and 2 is less than step 3
If 1+2 gt 3 HOS is positive (heat absorbed)78
In gaseous state molecules are so far apart there are nearly no forces between them Solute-solute interaction has little effect on heat of solution of a gas Energy released when gas dissolved in liquid bc attraction between solute gas and solvent molecules greater than energy needed to separate solvent molecules
79
Concentration of Solutions80
Concentration - measure of amount of solute in given amount of solvent or solution ldquodiluterdquo just means small amount of solute ldquoconcentratedrdquo just means relatively large amount of solute Unrelated to degree to which solution is saturated Ex Saturated solution of substance not very soluble might be dilute
81
Molarity
Molarity - number of moles of solute in one liter of solution To find molarity must know molar mass A ldquoone molarrdquo solution of NaOH is 1 mole NaOH per liter of solution (1 M)
82
1 mol NaOH = 400 g If this quantity dissolve in enough water to make EXACTLY 100 L solution solution is 1 M If 200 g NaOH dissolve in enough to make 100 L solution what is molarity 0500 M
83
1 M solution is not made by adding 1 mol solute to 1 L solvent That would be more than 1 L Instead 1 mol solute dissolved in less than 1 L Then diluted with more solvent to bring TOTAL VOLUME to 1 L
84
Practice ProblemYou have 340 L of solution that contains 900 g sodium chloride What is the molarity of that solution 0440 M NaCl You have 08 L of a 05 M HCl solution How many moles of HCl does this solution contain 04 mol HCl
85
What is the molarity of a solution composed of 585 g of potassium iodide dissolved in enough water to make a 0125 L of solution 0282 M KI
86
How many moles of H2SO4 are present in 0500 L of a 0150 M H2SO4 solution 00750 mol What volume of 300 M NaCl is needed for reaction that requires 1463 g of NaCl 0834 L
87
Molality
Molality - concentration of solution expressed in moles of solute per kilogram of solvent Solution that contains 1 mol solute (NaOH) dissolved in exactly 1 kg of solvent is ldquoone molalrdquo solution (1 m NaOH)
88
1 mol NaOH = 400 g 400 g NaOH dissolved in 1 kg water is 1 m NaOH
200 g NaOH in 1 kg water = 0500 m NaOH
89
Practice Problem
A solution was prepared by dissolving 171 g sucrose (C12H22O11) in 125 g of water Find the molal concentration of this solution 0400 m C12H22O11
90
A solution of iodine in carbon tetrachloride is used when iodine is needed for certain chemical tests How much iodine must be added to prepare a 0480 m solution of iodine in CCl4 if 1000 g of CCl4 is used 122 g I2
91
What is the molality of a solution composed of 255 g acetone (CH3)2CO dissolved in 200 g of water 22 m acetone What quantity in grams of methanol CH3OH is required to prepare a 0244 m solution in 400 g water 312 g methanol
92
How many grams of AgNO3 are needed to prepare a 0125 m solution in 250 mL of water 531 g AgNO3
What is the molality of a solution containing 182 g HCl and 250 g water 200 m
93
Ions in Aqueous Solutions and Colligative Properties
94
95
compounds in aqueous solutions
DissociationWhen compound made from ions dissolves in water ions separate Dissociation - separation of ions that occurs when an ionic compound dissolves
96
Notice number of ions made per formula unit in equations 1 formula unit of NaCl gives 2 units of ions in solution 1 formula unit of CaCl2 gives 3 units of ions
97
Assuming 100 dissociation solution that contains 1 mol NaCl contains 1 mol Na+ and 1 mol Cl- Can assume 100 dissociation of all soluble ionic compounds
98
Practice Problem
Write the equation for the dissolution of aluminum sulfate in water How many moles of aluminum ions are made by dissolving 1 mol aluminum sulfate What is the total number of moles of ions made by dissolving 1 mol of aluminum sulfate
99
1 AnalyzeGiven Amount of solute = 1 mol Al2(SO4)3 Solvent identity = water Unknown a moles of aluminum ions and sulfate ions Total number of moles of solute ions produced
100
2 Plan
The coefficients in the balanced dissociation equation will tell mole relationships You can use the equation to find out the number of moles of solute ions produced
101
3 Compute
a
b102
Write the equation for the dissolution of each of the following in water and then determine the number of moles of each ions made as well as the total number of ions made a 1 mol ammonium chloride b 1 mol sodium sulfide c 05 mol barium nitrate
103
a 1 mol ammonium chloride
1 mol NH4+ 1 mol Cl- 2 mol total ions
104
b 1 mol sodium sulfide
2 mol Na+ 1 mol S-2 3 mol total ions
105
c 05 mol barium nitrate
05 mol Ba+2 1 mol NO3- 15 mol total ions
106
Precipitation Reactions
Even though no compound is completely insoluble compounds of very low solubility can be considered insoluble for practical purposes There are general guidelines to help predict whether a compound made of certain combination of ions is soluble
107
General Solubility Guidelines
Sodium potassium and ammonium compounds are soluble in water
108
Nitrates acetates and chlorates are soluble
109
110
Most chlorides are soluble except those of silver mercury (I) and lead Lead (II) chloride is soluble in hot water
Most sulfates are soluble except those of barium strontium lead calcium and mercury
111
Most carbonates phosphates and silicates are insoluble except those of sodium potassium and ammonium
112
Most sulfides are insoluble except those of calcium strontium sodium potassium and ammonium
113
Is calcium phosphate Ca3(PO4)2 soluble or insoluble
Not one of the exceptions so it is insoluble
Dissociation equations are not written for insoluble compounds
114
Guidelines also useful in predicting what will happen if 2 solutions of 2 different soluble compounds are mixed
If mixing results in combination of ions that forms insoluble compound a double-replacement and precipitation reaction will happen
115
Will precipitate form when solutions of ammonium sulfide and cadmium nitrate are combined You can tell calcium nitrate is soluble from the guidelines (Most nitrates acetates and chlorates are soluble) You can also tell ammonium sulfide is soluble (Most sulfides are insoluble except those of calcium strontium sodium potassium and ammonium)
116
117
To decide whether precipitate can form must know solubilities of two possible products
Ammonium nitrate ndash soluble (Most nitrates acetates and chlorates are soluble) Cadmium sulfide ndash insoluble (Most sulfides are insoluble except those of calcium strontium sodium potassium and ammonium)
118
bc one product is insoluble double-replacement and precipitation reaction will happen (NH4)2S (aq) + Cd(NO3)2 (aq) rarr NH4NO3 (aq) + CdS (s)
119
Net Ionic Equations
Reactions of ions in aqueous solution usually represented by net ionic equations instead of formula equations Net ionic equation - includes only those compounds and ions that undergo a chemical change in a reaction in an aqueous solution
120
121
To write NIE (net ionic equation) first convert chemical equation into overall ionic equation All soluble ionic compounds shown as dissociated ions in solution Precipitates shown as solids
122
Notice ammonium and nitrate ions appear on both sides They havenrsquot gone through any chemical change Spectator ions - ions that do not take part in a chemical reaction and are found in solution both before and after the reaction
123
To change ionic equation to NIE eliminate spectator ions Cd+2(aq) + S-2(aq) rarr CdS(s)
This NIE applies to any reaction in which precipitate of CdS forms
124
Practice Problem
Identify the precipitate that forms when aqueous solutions of zinc nitrate and ammonium sulfide are combined Write the equation for the possible double-replacement reaction Then write the formula equation overall ionic equation and the net ionic equation
125
1 AnalyzeGiven Identity of reactants zinc nitrate and ammonium sulfide Reaction medium aqueous solution Unknown a equation for the possible double-replacement reaction b identity of the precipitate c formula equation d overall ionic equation e net ionic equation
126
2 PlanWrite the possible double-replacement reaction between Zn(NO3)2 and (NH4)2S Use the rules to determine if any of the products will precipitate Write a formula equation and overall net ionic equation Then cancel spectator ions to make net ionic equation
127
3 Computea possible double-replacement reaction
b rules show that zinc sulfide is not soluble so will precipitate (ammonium nitrate is soluble)
c formula equation128
d overall ionic equation
e net ionic equation129
Will a precipitate form if solutions of potassium sulfate and barium nitrate are combined If so write the net ionic equation for the reaction
Yes Ba+2(aq) + SO4-2(aq) rarr BaSO4(s)
130
Will a precipitate form if solutions of potassium nitrate and magnesium sulfate are combined If so write the net ionic equation for the reaction
No 131
Will a precipitate form if solutions of barium chloride and sodium sulfate are combined If so identify the spectator ions and write the net ionic equation for the reaction
Yes Na+ and Cl- Ba+2(aq) + SO4-2(aq) rarr BaSO4(s)
132
Write the net ionic equation for the precipitation of nickel(II) sulfide
Ni+2(aq) + S-2(aq) rarr NiS(s)
133
IonizationSome molecular compounds can also form ions in solution Usually polar Ionization - ions formed from solute molecules by action of the solvent (creation of ions where there were none) Ionization different from dissociation
134
Dissociation ionic compounds dissolve and ions already present separate Ionization ions formed where non existed before
Ions formed are hydrated Heat released during hydration of ions gives enough energy to break covalent bonds
135
Extent to which solute ionizes depends on strength of bonds within molecules of solute and strength of attraction between solute and solvent molecules
136
HCl ndash molecular compound that ionizes in aqueous solution Contains highly polar bond Attraction between polar HCl molecule and polar water strong enough to break HCl bond
137
The Hydronium Ion
H+ ions from HCl attracts other molecules or ions so strongly that it doesnrsquot normally exist alone Ionization of HCl better described as direct transfer of proton from HCl to H2O forming H3O+
138
139
Hydration of H+ to form hydronium is highly exothermic
Energy released gives a lot of energy needed to ionize a molecular solute
Many molecular compounds that ionize in aqueous solution contain hydrogen and form hydronium ion
140
Strong and Weak Electrolytes
Substances that make ions and conduct current in solution are electrolytes HCl is one of series of compounds that has hydrogen and halogen Hydrogen halides are all molecular compounds with single polar-covalent bonds
141
All are gases Very soluble in water All are electrolytes HCl HBr HI strongly conduct current in solution HF only weakly conducts current at same concentration Strength of conduction related to ability to ionize
142
143
Strong Electrolytes
HCl HBr HI are 100 ionized in solution
Strong electrolyte - any compound whose dilute aqueous solutions conduct electricity well this is due to the presence of all or almost all of the dissolve compound in the form of ions
144
Unique characteristic of strong electrolyte is that they yield only ions
Ex Ionic compound may be highly soluble in water and dissociate into ions (NaCl)
Others may not dissolve much but amount that does dissolve exists only as ions
145
Weak ElectrolytesHF dissolves in water to give acid solution called hydrofluoric acid HF bond stronger than bonds between hydrogen and other halogens When HF dissolves some molecule ionize Reverse reaction also happens HF(aq) + H2O(l) H3O+(aq) + F-(aq)
146
HF(aq) + H2O(l) H3O+
(aq) + F-(aq)Concentration of dissolved unionized HF stays high and concentration of ions stays low
Weak electrolyte - any compound whose aqueous solutions conduct electricity poorly this is due to the presence of small amount of the dissolved compound in the form of ions
147
Different from nonelectrolyte ndash NO ions at all Description of electrolyte as strong or weak not related to concentration of solution Electrolytes differ in degree of ionization not amount of solute dissolved
148
149
Colligative properties of solutions
Presence of solute affects properties of solutions Some properties not dependent on nature of dissolved substance but on how may dissolve particles are present
Colligative properties - properties that depend on the concentration of solute particles but not on their identity
150
Vapor-Pressure Lowering
Boiling and freezing point of solution different from pure solvent Nonvolatile solute raises boiling point and lowers freezing point Nonvolatile substance - one that has little tendency to become gas under existing conditions
151
152
To understand why nonvolatile solute changes boiling and freezing point consider equilibrium vapor pressure
Vapor pressure pressure caused by molecules that have escape liquid phase to gas phase
Can be thought of a measure of tendency of molecules to escape from a liquid
153
Addition of sucrose (nonvolatile solute) lowers concentration of water molecules at surface of liquid This lowers tendency of water molecules to leave solution and enter gas phase Vapor pressure of solution is lower than vapor pressure of pure water
154
Aqueous13 solution13 of13 nonvolatile13 solutePure13 water
Nonelectrolyte solutions of same molality have same concentration of particles
Dilute solutions of same solvent and equal molality of any nonelectrolyte solute lower vapor pressure equally
155
Example
1 m aqueous solution of nonelectrolyte glucose lowers vapor pressure of water 55 x 10-4 atm at 25 1 m aqueous solution of sucrose also lowers water vapor pressure to 55 x 10-4 atm at 25
156
157
bc vapor-pressure lowering depends on concentration of nonelectrolyte solute and doesnrsquot depend on the type of solute it is a colligative proprerty
bc vapor pressure has been lowered solution remains liquid over larger temperature range
Lowers freezing point and raises boiling point
158
Can assume changes in boiling and freezing point also depend on concentration of solute
They are colligative properties
159
Freezing-Point Depression
Freezing point of 1-molal solution of any nonelectrolyte solute in water is found (by experiment) to be 186 lower than freezing point of water When 1 mol of nonelectrolyte solute dissolved in 1 kg water freezing point is -186 not 000 160
When 2 mol nonelectrolyte solute dissolved in 1 kg water freezing point is -372
For any concentration of nonelectrolyte solute in water decrease in freezing point can be determined using value of -186m
161
This value called the molal freezing-point contstant (Kf) is the freezing-point depression of the solvent in a 1-molal solution of a nonvolatile nonelectrolyte solute
162
Each solvent has own characteristic molal freezing-point constant Freezing-point depression (Δtf) the difference between the freezing points of the pure solvent and a solute of nonelectrolyte in that solvent and it is directly proportional to the molal concentration of the solution Δtf = Kfm
163
Δtf = Kfm
Kf expressed as m
m expressed in mol solutekg solvent (molality)
164
Sample Problem
What is the freezing point depression of water in a solution of 171 g of sucrose C12H22O11 and 200 g of water What is the actual freezing point of the solution
165
1 AnalyzeGiven Solute mass and chemical formula 171 g C12H22O11 Solvent mass and identity 2000 g water Unknown a freezing-point depression b freezing point of the solution 166
2 Plan
167
168
Calculate
Unknown a freezing-point depression b freezing point of the solution
169
Practice Problem
A water solution containing an unknown quantity of a nonelectrolyte solute is found to have a freezing point of ndash 023degCWhat is the molal concentration of the solution
012 m170
A solution consists of 103 g of the nonelectrolyte glucose C6H12O6 dissolved in 250 g of water What is the freezing-point depression of the solution
minus0426degC171
In a laboratory experiment the freezing point of an aqueous solution of glucose is found to be minus0325degCWhat is the molal concentration of this solution
0175 m172
If 0500 mol of a nonelectrolyte solute are dissolved in 5000 g of ether what is the freezing point of the solution
minus1181degC173
The freezing point of an aqueous solution that contains a nonelectrolyte is minus90degC aWhat is the freezing-point depression of the solution bWhat is the molal concentration of the solution a minus90degC b 48 m
174
Boiling-Point Elevation
Boiling point is temp at which vapor pressure is equal to atmospheric pressure Change in vapor pressure will cause corresponding change in boiling point
175
176
Vapor pressure of nonvolatile solution is lower than vapor pressure of pure solvent More heat will be required to raise vapor pressure of solution Boiling point of solution is higher than boiling point of pure solvent
Molal boiling-point constant (Kb) - boiling-point elevation of the solvent in a 1-molal solution of a nonvolatile nonelectrolyte solute b-p elevation of any nonelectrolyte in water found by experiment to be 051 So molal b-p constant for water is 051m
177
For different solvents b-p elevations of 1-molal solutions have different values (Table 14-2)
Like freezing-point constants values most accurate for dilute solutions
178
Boiling-point elevation Δtb difference between the boiling points of the pure solvent and a nonelectrolyte solution of that solvent and it is directly proportional to the molal concentration of the solution B-p elevation can be calculated using Δtb = Kbm Δtb (boiling point elevation) expressed in m and m expressed in mol solutekg solvent
Sample Problem
What is the boiling-point elevation of a solution made from 200 g of a nonelectrolyte solute and 4000 g of water The molar mass of the solute is 620 gmol
180
1 AnalyzeGiven solute mass = 200 g solute molar mass = 620 gmol solvent mass and identity = 4000 g of water
Unknown boiling-point elevation
181
2 Plan
182
183
3 Compute
Practice Problem
A solution contains 500 g of sucrose C12H22O11 a nonelectrolyte dissolved in 5000 g of water What is the boiling-point elevation
015degC184
A solution contains 4500 g of sucrose C12H22O11 a nonelectrolyte dissolved in 250 g of waterWhat is the boiling point of the solution
1027degC185
If the boiling point elevation of an aqueous solution containing a nonvolatile electrolyte is 102degC what is the molality of the solution
20 m186
The boiling point of an aqueous solution containing a nonvolatile electrolyte is 10075degC aWhat is the boiling-point elevation bWhat is the molality of the solution a 075degC b 15 m
187
Electrolytes and Colligative PropertiesEarly investigators confused by experiments where certain substances depressed freezing point or elevated boiling point more than expected
188
189
Ex 01 m solution of NaCl lowers freezing point of solvent almost twice as much as 01 m solution of sucrose 01 m CaCl2 lowers fp almost 3 times as much as 01 m solution of sucrose
To understand why this happens contrast behavior or sucrose with NaCl in aqueous solution Each sucrose dissolves to make only 1 particle in solution So 1 mol sucrose dissolves to 1 mol particles 1 mol NaCl dissolves to make 2 moles of particles (1 mol Na+ and 1 mol Cl-)
190
191
Calculated Values for Electrolyte SolutionsColligative properties depend on total concentration of solute particles regardless of their identity Electrolytes cause changes in colligative properties proportional to total molality in terms of all dissolves particles instead of formula units
192
What about barium nitrate Ba(NO3)2 Each mole of barium nitrate yields 3 mol of ions in solution Ba(NO3)2(s) rarr Ba2+(aq) + 2NO3-(aq) You would expect solution of given molality to lower fp of solvent 3 times as much as nonelectrolytic solution of same molality
193
Sample Problem
What is the expected change in the freezing point of water in a solution of 625 g of barium nitrate Ba(NO3)2 in 100 kg of water
194
1 AnalyzeGiven solute mass and formula = 625 g Ba(NO3)2 solvent mass and identity = 100 kg water Δtf = Kfm
Unknown expected freezing-point depression
195
2 PlanThe molality can be calculated by converting the solute mass to moles and then dividing by the number of kilograms of solvent That molality is in terms of formula units of Ba(NO3)2 and must be converted to molality in terms of dissociated ions in solution It must be multiplied by the number of moles of ions produced per mole of formula unit This adjusted molality can then be used to calculate the freezing-point depression
196
197
198
3 Compute
Practice Problem
What is the expected freezing-point depression for a solution that contains 20 mol of magnesium sulfate dissolved in 10 kg of water
minus74degC199
What is the expected boiling-point elevation of water for a solution that contains 150 g of sodium chloride dissolved in 10 kg of water 27degC The freezing point of an aqueous sodium chloride solution is minus200degC What is the molality of the solution 54 m NaCl
200
Actual Values for Electrolyte SolutionsRemember values calculated are only expected values Actual values of colligative properties for all strong electrolytes are almost what would be expected Difference between expected and actual values caused by attractive forces that exist between dissociated ions in aqueous solution
201
Attraction between hydrated ions in solution is small compared with those in crystalline solid However forces of attraction donrsquot interfere with movements of aqueous ions More concentrated solution means ions closer together and attraction is greater Also depends on charge of ions ndash higher charge greater attraction
202
When crystallized from aqueous solutions some ionic compounds form crystals that include water molecules The crystalline compounds known as hydrates have specific ratios of water molecules Represented by formulas like CuSO45H2O
48
When crystalline hydrate dissolves in water water of hydration returns to solvent Behavior of solute in hydrated form no different than anhydrous form
49
Nonpolar Solvents
Nonpolar solvents do not attract ions of ionic compound strongly enough to overcome forces holding crystal together
50
Liquid Solutes and Solvents
When you shake bottle of salad dressing oil droplets disperse in water Stop shaking ndash strong attraction between water molecules squeeze out oil and form separate layers Liquid solutes and solvents that are not soluble in each other are immiscible
51
Nonpolar substances (fat oil grease) are generally quite soluble in nonpolar liquids (carbon tetrachloride toluene gasoline)
Only attractions between nonpolar molecules are weak London forces
52
London Dispersion Forces
bc e- are constantly moving at any time there may be uneven distribution of them around a nonpolar atom Temporarily creates positive end and negative end
53
54
London dispersion forces - intermolecular attractions resulting from constant motion of e- and creation of instantaneous dipoles
55
Liquids that dissolve freely in one another in any ratio are said to be completely miscible Benzene and carbon tetrachloride (both nonpolar) are completely miscible
Nonpolar molecules of these two apply no strong forces of attraction or repulsion and molecules mix freely
56
Ethanol and water are also completely miscible -OH group on ethanol is slightly polar
Can form H-bonds with water as well as other ethanol molecules Intermolecular forces in mixture are very similar to those in pure liquids so they are mutually soluble in any amount
57
Gasoline contains mainly nonpolar hydrocarbons
Excellent solvent for fats oils greases
Major intermolecular forces are weak London forces
58
Ethanol less polar than water more than carbon tetrachloride
Better solvent for less-polar substances bc of nonpolar region
59
Effects of Pressure on Solubility
Little effect on solids or liquids Increase pressure - increase solubilities of gases in liquids
60
When gas in contact with surface of liquid gas molecules can enter liquid As amount of dissolve gas increases some molecules start to escape and reenter gas phase
Eq eventually established between rates of enteringleaving gas phase
61
As long as eq undisturbed solubility of gas in liquid unchanged at given pressure
Gas + solvent solution
62
Increasing pressure of solute gas above solution puts stress on eq Molecules collide with surface more often Increase in pressure partially offset by increase in rate of gas molecules entering solution In turn increase in amount of dissolved gas causes increase in rate molecules escape liquid and becomes gas
63
Eventually eq restored at higher gas solubility
Expected from Le Chatelierrsquos principle
Increase in gas pressure causes eq to shift so fewer molecules are in gas phase
64
Henryrsquos Law
Henryrsquos law - solubility of a gas in liquid is directly proportional to the partial pressure of that gas on the surface of the liquid
Applies to gas-liquid solutions at constant temp
65
When mixture of ideal gases is limited in constant volume at constant temp each gas applies same pressure it would apply if it occupied the space alone
Assuming gases do not react each gas dissolves to the extent it would if no other gases were there
66
In carbonated drinks (Coke Pepsi etc) solubility of CO2 increased by increasing pressure
At bottling factory CO2 gas forced into solution of flavored water at pressure of 5-10 atm Gas-in-liquid solution then sealed in bottlescans
67
When cap removed pressure reduced to 1 atm and some CO2 escapes as gas bubbles Effervescence - rapid escape of a gas from a liquid in which it is dissolved
68
Effects of Temperature on Solubility
Letrsquos consider gas solubility Increasing temperature usually decreases gas solubility As temp increases average kinetic energy of molecules in solution increases Greater number of solute molecules escape from attraction of solvent and return to gas phase 69
At higher temps eq is reached with fewer gas molecules in solution Gases generally less soluble
70
Effect of temp on solubility of solids in liquids more difficult to predict
Often increasing temp increases solubility
However the same temp increase can result in large increase in solubility in one case and only slight increase in the next
71
72
enthalpies of Solution
Formation of solution comes with energy change Dissolve KI in water container feels cold Dissolve LiCl in water feels hot Formation of solid-liquid solution can absorb or release heat
73
During formation of solution solvent and solute particles experience changes in forces attracting them to other particles Before dissolving begins solvent molecules held by intermolecular forces (solvent-solvent attraction) In solute molecules held by Ifs (solute-solute attraction)
74
Energy is required to separate solute molecules and solvent molecules from their neighbors
A solute particle that is surrounded by solvent molecules is said to be solvated
75
Step 1 solute particles become separated from solid (energy absorbed) Step 2 solvent particles move apart to allow solute particles to enter liquid (energy absorbed)
76
Step 3 solvent particles attracted to and solvate solute particles (energy released)
77
Heat of solution - net amount of heat energy absorbed or released when a specific amount of solute dissolves in a solvent
Heat of solution negative (heat is released) if sums of energy in steps 1 and 2 is less than step 3
If 1+2 gt 3 HOS is positive (heat absorbed)78
In gaseous state molecules are so far apart there are nearly no forces between them Solute-solute interaction has little effect on heat of solution of a gas Energy released when gas dissolved in liquid bc attraction between solute gas and solvent molecules greater than energy needed to separate solvent molecules
79
Concentration of Solutions80
Concentration - measure of amount of solute in given amount of solvent or solution ldquodiluterdquo just means small amount of solute ldquoconcentratedrdquo just means relatively large amount of solute Unrelated to degree to which solution is saturated Ex Saturated solution of substance not very soluble might be dilute
81
Molarity
Molarity - number of moles of solute in one liter of solution To find molarity must know molar mass A ldquoone molarrdquo solution of NaOH is 1 mole NaOH per liter of solution (1 M)
82
1 mol NaOH = 400 g If this quantity dissolve in enough water to make EXACTLY 100 L solution solution is 1 M If 200 g NaOH dissolve in enough to make 100 L solution what is molarity 0500 M
83
1 M solution is not made by adding 1 mol solute to 1 L solvent That would be more than 1 L Instead 1 mol solute dissolved in less than 1 L Then diluted with more solvent to bring TOTAL VOLUME to 1 L
84
Practice ProblemYou have 340 L of solution that contains 900 g sodium chloride What is the molarity of that solution 0440 M NaCl You have 08 L of a 05 M HCl solution How many moles of HCl does this solution contain 04 mol HCl
85
What is the molarity of a solution composed of 585 g of potassium iodide dissolved in enough water to make a 0125 L of solution 0282 M KI
86
How many moles of H2SO4 are present in 0500 L of a 0150 M H2SO4 solution 00750 mol What volume of 300 M NaCl is needed for reaction that requires 1463 g of NaCl 0834 L
87
Molality
Molality - concentration of solution expressed in moles of solute per kilogram of solvent Solution that contains 1 mol solute (NaOH) dissolved in exactly 1 kg of solvent is ldquoone molalrdquo solution (1 m NaOH)
88
1 mol NaOH = 400 g 400 g NaOH dissolved in 1 kg water is 1 m NaOH
200 g NaOH in 1 kg water = 0500 m NaOH
89
Practice Problem
A solution was prepared by dissolving 171 g sucrose (C12H22O11) in 125 g of water Find the molal concentration of this solution 0400 m C12H22O11
90
A solution of iodine in carbon tetrachloride is used when iodine is needed for certain chemical tests How much iodine must be added to prepare a 0480 m solution of iodine in CCl4 if 1000 g of CCl4 is used 122 g I2
91
What is the molality of a solution composed of 255 g acetone (CH3)2CO dissolved in 200 g of water 22 m acetone What quantity in grams of methanol CH3OH is required to prepare a 0244 m solution in 400 g water 312 g methanol
92
How many grams of AgNO3 are needed to prepare a 0125 m solution in 250 mL of water 531 g AgNO3
What is the molality of a solution containing 182 g HCl and 250 g water 200 m
93
Ions in Aqueous Solutions and Colligative Properties
94
95
compounds in aqueous solutions
DissociationWhen compound made from ions dissolves in water ions separate Dissociation - separation of ions that occurs when an ionic compound dissolves
96
Notice number of ions made per formula unit in equations 1 formula unit of NaCl gives 2 units of ions in solution 1 formula unit of CaCl2 gives 3 units of ions
97
Assuming 100 dissociation solution that contains 1 mol NaCl contains 1 mol Na+ and 1 mol Cl- Can assume 100 dissociation of all soluble ionic compounds
98
Practice Problem
Write the equation for the dissolution of aluminum sulfate in water How many moles of aluminum ions are made by dissolving 1 mol aluminum sulfate What is the total number of moles of ions made by dissolving 1 mol of aluminum sulfate
99
1 AnalyzeGiven Amount of solute = 1 mol Al2(SO4)3 Solvent identity = water Unknown a moles of aluminum ions and sulfate ions Total number of moles of solute ions produced
100
2 Plan
The coefficients in the balanced dissociation equation will tell mole relationships You can use the equation to find out the number of moles of solute ions produced
101
3 Compute
a
b102
Write the equation for the dissolution of each of the following in water and then determine the number of moles of each ions made as well as the total number of ions made a 1 mol ammonium chloride b 1 mol sodium sulfide c 05 mol barium nitrate
103
a 1 mol ammonium chloride
1 mol NH4+ 1 mol Cl- 2 mol total ions
104
b 1 mol sodium sulfide
2 mol Na+ 1 mol S-2 3 mol total ions
105
c 05 mol barium nitrate
05 mol Ba+2 1 mol NO3- 15 mol total ions
106
Precipitation Reactions
Even though no compound is completely insoluble compounds of very low solubility can be considered insoluble for practical purposes There are general guidelines to help predict whether a compound made of certain combination of ions is soluble
107
General Solubility Guidelines
Sodium potassium and ammonium compounds are soluble in water
108
Nitrates acetates and chlorates are soluble
109
110
Most chlorides are soluble except those of silver mercury (I) and lead Lead (II) chloride is soluble in hot water
Most sulfates are soluble except those of barium strontium lead calcium and mercury
111
Most carbonates phosphates and silicates are insoluble except those of sodium potassium and ammonium
112
Most sulfides are insoluble except those of calcium strontium sodium potassium and ammonium
113
Is calcium phosphate Ca3(PO4)2 soluble or insoluble
Not one of the exceptions so it is insoluble
Dissociation equations are not written for insoluble compounds
114
Guidelines also useful in predicting what will happen if 2 solutions of 2 different soluble compounds are mixed
If mixing results in combination of ions that forms insoluble compound a double-replacement and precipitation reaction will happen
115
Will precipitate form when solutions of ammonium sulfide and cadmium nitrate are combined You can tell calcium nitrate is soluble from the guidelines (Most nitrates acetates and chlorates are soluble) You can also tell ammonium sulfide is soluble (Most sulfides are insoluble except those of calcium strontium sodium potassium and ammonium)
116
117
To decide whether precipitate can form must know solubilities of two possible products
Ammonium nitrate ndash soluble (Most nitrates acetates and chlorates are soluble) Cadmium sulfide ndash insoluble (Most sulfides are insoluble except those of calcium strontium sodium potassium and ammonium)
118
bc one product is insoluble double-replacement and precipitation reaction will happen (NH4)2S (aq) + Cd(NO3)2 (aq) rarr NH4NO3 (aq) + CdS (s)
119
Net Ionic Equations
Reactions of ions in aqueous solution usually represented by net ionic equations instead of formula equations Net ionic equation - includes only those compounds and ions that undergo a chemical change in a reaction in an aqueous solution
120
121
To write NIE (net ionic equation) first convert chemical equation into overall ionic equation All soluble ionic compounds shown as dissociated ions in solution Precipitates shown as solids
122
Notice ammonium and nitrate ions appear on both sides They havenrsquot gone through any chemical change Spectator ions - ions that do not take part in a chemical reaction and are found in solution both before and after the reaction
123
To change ionic equation to NIE eliminate spectator ions Cd+2(aq) + S-2(aq) rarr CdS(s)
This NIE applies to any reaction in which precipitate of CdS forms
124
Practice Problem
Identify the precipitate that forms when aqueous solutions of zinc nitrate and ammonium sulfide are combined Write the equation for the possible double-replacement reaction Then write the formula equation overall ionic equation and the net ionic equation
125
1 AnalyzeGiven Identity of reactants zinc nitrate and ammonium sulfide Reaction medium aqueous solution Unknown a equation for the possible double-replacement reaction b identity of the precipitate c formula equation d overall ionic equation e net ionic equation
126
2 PlanWrite the possible double-replacement reaction between Zn(NO3)2 and (NH4)2S Use the rules to determine if any of the products will precipitate Write a formula equation and overall net ionic equation Then cancel spectator ions to make net ionic equation
127
3 Computea possible double-replacement reaction
b rules show that zinc sulfide is not soluble so will precipitate (ammonium nitrate is soluble)
c formula equation128
d overall ionic equation
e net ionic equation129
Will a precipitate form if solutions of potassium sulfate and barium nitrate are combined If so write the net ionic equation for the reaction
Yes Ba+2(aq) + SO4-2(aq) rarr BaSO4(s)
130
Will a precipitate form if solutions of potassium nitrate and magnesium sulfate are combined If so write the net ionic equation for the reaction
No 131
Will a precipitate form if solutions of barium chloride and sodium sulfate are combined If so identify the spectator ions and write the net ionic equation for the reaction
Yes Na+ and Cl- Ba+2(aq) + SO4-2(aq) rarr BaSO4(s)
132
Write the net ionic equation for the precipitation of nickel(II) sulfide
Ni+2(aq) + S-2(aq) rarr NiS(s)
133
IonizationSome molecular compounds can also form ions in solution Usually polar Ionization - ions formed from solute molecules by action of the solvent (creation of ions where there were none) Ionization different from dissociation
134
Dissociation ionic compounds dissolve and ions already present separate Ionization ions formed where non existed before
Ions formed are hydrated Heat released during hydration of ions gives enough energy to break covalent bonds
135
Extent to which solute ionizes depends on strength of bonds within molecules of solute and strength of attraction between solute and solvent molecules
136
HCl ndash molecular compound that ionizes in aqueous solution Contains highly polar bond Attraction between polar HCl molecule and polar water strong enough to break HCl bond
137
The Hydronium Ion
H+ ions from HCl attracts other molecules or ions so strongly that it doesnrsquot normally exist alone Ionization of HCl better described as direct transfer of proton from HCl to H2O forming H3O+
138
139
Hydration of H+ to form hydronium is highly exothermic
Energy released gives a lot of energy needed to ionize a molecular solute
Many molecular compounds that ionize in aqueous solution contain hydrogen and form hydronium ion
140
Strong and Weak Electrolytes
Substances that make ions and conduct current in solution are electrolytes HCl is one of series of compounds that has hydrogen and halogen Hydrogen halides are all molecular compounds with single polar-covalent bonds
141
All are gases Very soluble in water All are electrolytes HCl HBr HI strongly conduct current in solution HF only weakly conducts current at same concentration Strength of conduction related to ability to ionize
142
143
Strong Electrolytes
HCl HBr HI are 100 ionized in solution
Strong electrolyte - any compound whose dilute aqueous solutions conduct electricity well this is due to the presence of all or almost all of the dissolve compound in the form of ions
144
Unique characteristic of strong electrolyte is that they yield only ions
Ex Ionic compound may be highly soluble in water and dissociate into ions (NaCl)
Others may not dissolve much but amount that does dissolve exists only as ions
145
Weak ElectrolytesHF dissolves in water to give acid solution called hydrofluoric acid HF bond stronger than bonds between hydrogen and other halogens When HF dissolves some molecule ionize Reverse reaction also happens HF(aq) + H2O(l) H3O+(aq) + F-(aq)
146
HF(aq) + H2O(l) H3O+
(aq) + F-(aq)Concentration of dissolved unionized HF stays high and concentration of ions stays low
Weak electrolyte - any compound whose aqueous solutions conduct electricity poorly this is due to the presence of small amount of the dissolved compound in the form of ions
147
Different from nonelectrolyte ndash NO ions at all Description of electrolyte as strong or weak not related to concentration of solution Electrolytes differ in degree of ionization not amount of solute dissolved
148
149
Colligative properties of solutions
Presence of solute affects properties of solutions Some properties not dependent on nature of dissolved substance but on how may dissolve particles are present
Colligative properties - properties that depend on the concentration of solute particles but not on their identity
150
Vapor-Pressure Lowering
Boiling and freezing point of solution different from pure solvent Nonvolatile solute raises boiling point and lowers freezing point Nonvolatile substance - one that has little tendency to become gas under existing conditions
151
152
To understand why nonvolatile solute changes boiling and freezing point consider equilibrium vapor pressure
Vapor pressure pressure caused by molecules that have escape liquid phase to gas phase
Can be thought of a measure of tendency of molecules to escape from a liquid
153
Addition of sucrose (nonvolatile solute) lowers concentration of water molecules at surface of liquid This lowers tendency of water molecules to leave solution and enter gas phase Vapor pressure of solution is lower than vapor pressure of pure water
154
Aqueous13 solution13 of13 nonvolatile13 solutePure13 water
Nonelectrolyte solutions of same molality have same concentration of particles
Dilute solutions of same solvent and equal molality of any nonelectrolyte solute lower vapor pressure equally
155
Example
1 m aqueous solution of nonelectrolyte glucose lowers vapor pressure of water 55 x 10-4 atm at 25 1 m aqueous solution of sucrose also lowers water vapor pressure to 55 x 10-4 atm at 25
156
157
bc vapor-pressure lowering depends on concentration of nonelectrolyte solute and doesnrsquot depend on the type of solute it is a colligative proprerty
bc vapor pressure has been lowered solution remains liquid over larger temperature range
Lowers freezing point and raises boiling point
158
Can assume changes in boiling and freezing point also depend on concentration of solute
They are colligative properties
159
Freezing-Point Depression
Freezing point of 1-molal solution of any nonelectrolyte solute in water is found (by experiment) to be 186 lower than freezing point of water When 1 mol of nonelectrolyte solute dissolved in 1 kg water freezing point is -186 not 000 160
When 2 mol nonelectrolyte solute dissolved in 1 kg water freezing point is -372
For any concentration of nonelectrolyte solute in water decrease in freezing point can be determined using value of -186m
161
This value called the molal freezing-point contstant (Kf) is the freezing-point depression of the solvent in a 1-molal solution of a nonvolatile nonelectrolyte solute
162
Each solvent has own characteristic molal freezing-point constant Freezing-point depression (Δtf) the difference between the freezing points of the pure solvent and a solute of nonelectrolyte in that solvent and it is directly proportional to the molal concentration of the solution Δtf = Kfm
163
Δtf = Kfm
Kf expressed as m
m expressed in mol solutekg solvent (molality)
164
Sample Problem
What is the freezing point depression of water in a solution of 171 g of sucrose C12H22O11 and 200 g of water What is the actual freezing point of the solution
165
1 AnalyzeGiven Solute mass and chemical formula 171 g C12H22O11 Solvent mass and identity 2000 g water Unknown a freezing-point depression b freezing point of the solution 166
2 Plan
167
168
Calculate
Unknown a freezing-point depression b freezing point of the solution
169
Practice Problem
A water solution containing an unknown quantity of a nonelectrolyte solute is found to have a freezing point of ndash 023degCWhat is the molal concentration of the solution
012 m170
A solution consists of 103 g of the nonelectrolyte glucose C6H12O6 dissolved in 250 g of water What is the freezing-point depression of the solution
minus0426degC171
In a laboratory experiment the freezing point of an aqueous solution of glucose is found to be minus0325degCWhat is the molal concentration of this solution
0175 m172
If 0500 mol of a nonelectrolyte solute are dissolved in 5000 g of ether what is the freezing point of the solution
minus1181degC173
The freezing point of an aqueous solution that contains a nonelectrolyte is minus90degC aWhat is the freezing-point depression of the solution bWhat is the molal concentration of the solution a minus90degC b 48 m
174
Boiling-Point Elevation
Boiling point is temp at which vapor pressure is equal to atmospheric pressure Change in vapor pressure will cause corresponding change in boiling point
175
176
Vapor pressure of nonvolatile solution is lower than vapor pressure of pure solvent More heat will be required to raise vapor pressure of solution Boiling point of solution is higher than boiling point of pure solvent
Molal boiling-point constant (Kb) - boiling-point elevation of the solvent in a 1-molal solution of a nonvolatile nonelectrolyte solute b-p elevation of any nonelectrolyte in water found by experiment to be 051 So molal b-p constant for water is 051m
177
For different solvents b-p elevations of 1-molal solutions have different values (Table 14-2)
Like freezing-point constants values most accurate for dilute solutions
178
Boiling-point elevation Δtb difference between the boiling points of the pure solvent and a nonelectrolyte solution of that solvent and it is directly proportional to the molal concentration of the solution B-p elevation can be calculated using Δtb = Kbm Δtb (boiling point elevation) expressed in m and m expressed in mol solutekg solvent
Sample Problem
What is the boiling-point elevation of a solution made from 200 g of a nonelectrolyte solute and 4000 g of water The molar mass of the solute is 620 gmol
180
1 AnalyzeGiven solute mass = 200 g solute molar mass = 620 gmol solvent mass and identity = 4000 g of water
Unknown boiling-point elevation
181
2 Plan
182
183
3 Compute
Practice Problem
A solution contains 500 g of sucrose C12H22O11 a nonelectrolyte dissolved in 5000 g of water What is the boiling-point elevation
015degC184
A solution contains 4500 g of sucrose C12H22O11 a nonelectrolyte dissolved in 250 g of waterWhat is the boiling point of the solution
1027degC185
If the boiling point elevation of an aqueous solution containing a nonvolatile electrolyte is 102degC what is the molality of the solution
20 m186
The boiling point of an aqueous solution containing a nonvolatile electrolyte is 10075degC aWhat is the boiling-point elevation bWhat is the molality of the solution a 075degC b 15 m
187
Electrolytes and Colligative PropertiesEarly investigators confused by experiments where certain substances depressed freezing point or elevated boiling point more than expected
188
189
Ex 01 m solution of NaCl lowers freezing point of solvent almost twice as much as 01 m solution of sucrose 01 m CaCl2 lowers fp almost 3 times as much as 01 m solution of sucrose
To understand why this happens contrast behavior or sucrose with NaCl in aqueous solution Each sucrose dissolves to make only 1 particle in solution So 1 mol sucrose dissolves to 1 mol particles 1 mol NaCl dissolves to make 2 moles of particles (1 mol Na+ and 1 mol Cl-)
190
191
Calculated Values for Electrolyte SolutionsColligative properties depend on total concentration of solute particles regardless of their identity Electrolytes cause changes in colligative properties proportional to total molality in terms of all dissolves particles instead of formula units
192
What about barium nitrate Ba(NO3)2 Each mole of barium nitrate yields 3 mol of ions in solution Ba(NO3)2(s) rarr Ba2+(aq) + 2NO3-(aq) You would expect solution of given molality to lower fp of solvent 3 times as much as nonelectrolytic solution of same molality
193
Sample Problem
What is the expected change in the freezing point of water in a solution of 625 g of barium nitrate Ba(NO3)2 in 100 kg of water
194
1 AnalyzeGiven solute mass and formula = 625 g Ba(NO3)2 solvent mass and identity = 100 kg water Δtf = Kfm
Unknown expected freezing-point depression
195
2 PlanThe molality can be calculated by converting the solute mass to moles and then dividing by the number of kilograms of solvent That molality is in terms of formula units of Ba(NO3)2 and must be converted to molality in terms of dissociated ions in solution It must be multiplied by the number of moles of ions produced per mole of formula unit This adjusted molality can then be used to calculate the freezing-point depression
196
197
198
3 Compute
Practice Problem
What is the expected freezing-point depression for a solution that contains 20 mol of magnesium sulfate dissolved in 10 kg of water
minus74degC199
What is the expected boiling-point elevation of water for a solution that contains 150 g of sodium chloride dissolved in 10 kg of water 27degC The freezing point of an aqueous sodium chloride solution is minus200degC What is the molality of the solution 54 m NaCl
200
Actual Values for Electrolyte SolutionsRemember values calculated are only expected values Actual values of colligative properties for all strong electrolytes are almost what would be expected Difference between expected and actual values caused by attractive forces that exist between dissociated ions in aqueous solution
201
Attraction between hydrated ions in solution is small compared with those in crystalline solid However forces of attraction donrsquot interfere with movements of aqueous ions More concentrated solution means ions closer together and attraction is greater Also depends on charge of ions ndash higher charge greater attraction
202
When crystalline hydrate dissolves in water water of hydration returns to solvent Behavior of solute in hydrated form no different than anhydrous form
49
Nonpolar Solvents
Nonpolar solvents do not attract ions of ionic compound strongly enough to overcome forces holding crystal together
50
Liquid Solutes and Solvents
When you shake bottle of salad dressing oil droplets disperse in water Stop shaking ndash strong attraction between water molecules squeeze out oil and form separate layers Liquid solutes and solvents that are not soluble in each other are immiscible
51
Nonpolar substances (fat oil grease) are generally quite soluble in nonpolar liquids (carbon tetrachloride toluene gasoline)
Only attractions between nonpolar molecules are weak London forces
52
London Dispersion Forces
bc e- are constantly moving at any time there may be uneven distribution of them around a nonpolar atom Temporarily creates positive end and negative end
53
54
London dispersion forces - intermolecular attractions resulting from constant motion of e- and creation of instantaneous dipoles
55
Liquids that dissolve freely in one another in any ratio are said to be completely miscible Benzene and carbon tetrachloride (both nonpolar) are completely miscible
Nonpolar molecules of these two apply no strong forces of attraction or repulsion and molecules mix freely
56
Ethanol and water are also completely miscible -OH group on ethanol is slightly polar
Can form H-bonds with water as well as other ethanol molecules Intermolecular forces in mixture are very similar to those in pure liquids so they are mutually soluble in any amount
57
Gasoline contains mainly nonpolar hydrocarbons
Excellent solvent for fats oils greases
Major intermolecular forces are weak London forces
58
Ethanol less polar than water more than carbon tetrachloride
Better solvent for less-polar substances bc of nonpolar region
59
Effects of Pressure on Solubility
Little effect on solids or liquids Increase pressure - increase solubilities of gases in liquids
60
When gas in contact with surface of liquid gas molecules can enter liquid As amount of dissolve gas increases some molecules start to escape and reenter gas phase
Eq eventually established between rates of enteringleaving gas phase
61
As long as eq undisturbed solubility of gas in liquid unchanged at given pressure
Gas + solvent solution
62
Increasing pressure of solute gas above solution puts stress on eq Molecules collide with surface more often Increase in pressure partially offset by increase in rate of gas molecules entering solution In turn increase in amount of dissolved gas causes increase in rate molecules escape liquid and becomes gas
63
Eventually eq restored at higher gas solubility
Expected from Le Chatelierrsquos principle
Increase in gas pressure causes eq to shift so fewer molecules are in gas phase
64
Henryrsquos Law
Henryrsquos law - solubility of a gas in liquid is directly proportional to the partial pressure of that gas on the surface of the liquid
Applies to gas-liquid solutions at constant temp
65
When mixture of ideal gases is limited in constant volume at constant temp each gas applies same pressure it would apply if it occupied the space alone
Assuming gases do not react each gas dissolves to the extent it would if no other gases were there
66
In carbonated drinks (Coke Pepsi etc) solubility of CO2 increased by increasing pressure
At bottling factory CO2 gas forced into solution of flavored water at pressure of 5-10 atm Gas-in-liquid solution then sealed in bottlescans
67
When cap removed pressure reduced to 1 atm and some CO2 escapes as gas bubbles Effervescence - rapid escape of a gas from a liquid in which it is dissolved
68
Effects of Temperature on Solubility
Letrsquos consider gas solubility Increasing temperature usually decreases gas solubility As temp increases average kinetic energy of molecules in solution increases Greater number of solute molecules escape from attraction of solvent and return to gas phase 69
At higher temps eq is reached with fewer gas molecules in solution Gases generally less soluble
70
Effect of temp on solubility of solids in liquids more difficult to predict
Often increasing temp increases solubility
However the same temp increase can result in large increase in solubility in one case and only slight increase in the next
71
72
enthalpies of Solution
Formation of solution comes with energy change Dissolve KI in water container feels cold Dissolve LiCl in water feels hot Formation of solid-liquid solution can absorb or release heat
73
During formation of solution solvent and solute particles experience changes in forces attracting them to other particles Before dissolving begins solvent molecules held by intermolecular forces (solvent-solvent attraction) In solute molecules held by Ifs (solute-solute attraction)
74
Energy is required to separate solute molecules and solvent molecules from their neighbors
A solute particle that is surrounded by solvent molecules is said to be solvated
75
Step 1 solute particles become separated from solid (energy absorbed) Step 2 solvent particles move apart to allow solute particles to enter liquid (energy absorbed)
76
Step 3 solvent particles attracted to and solvate solute particles (energy released)
77
Heat of solution - net amount of heat energy absorbed or released when a specific amount of solute dissolves in a solvent
Heat of solution negative (heat is released) if sums of energy in steps 1 and 2 is less than step 3
If 1+2 gt 3 HOS is positive (heat absorbed)78
In gaseous state molecules are so far apart there are nearly no forces between them Solute-solute interaction has little effect on heat of solution of a gas Energy released when gas dissolved in liquid bc attraction between solute gas and solvent molecules greater than energy needed to separate solvent molecules
79
Concentration of Solutions80
Concentration - measure of amount of solute in given amount of solvent or solution ldquodiluterdquo just means small amount of solute ldquoconcentratedrdquo just means relatively large amount of solute Unrelated to degree to which solution is saturated Ex Saturated solution of substance not very soluble might be dilute
81
Molarity
Molarity - number of moles of solute in one liter of solution To find molarity must know molar mass A ldquoone molarrdquo solution of NaOH is 1 mole NaOH per liter of solution (1 M)
82
1 mol NaOH = 400 g If this quantity dissolve in enough water to make EXACTLY 100 L solution solution is 1 M If 200 g NaOH dissolve in enough to make 100 L solution what is molarity 0500 M
83
1 M solution is not made by adding 1 mol solute to 1 L solvent That would be more than 1 L Instead 1 mol solute dissolved in less than 1 L Then diluted with more solvent to bring TOTAL VOLUME to 1 L
84
Practice ProblemYou have 340 L of solution that contains 900 g sodium chloride What is the molarity of that solution 0440 M NaCl You have 08 L of a 05 M HCl solution How many moles of HCl does this solution contain 04 mol HCl
85
What is the molarity of a solution composed of 585 g of potassium iodide dissolved in enough water to make a 0125 L of solution 0282 M KI
86
How many moles of H2SO4 are present in 0500 L of a 0150 M H2SO4 solution 00750 mol What volume of 300 M NaCl is needed for reaction that requires 1463 g of NaCl 0834 L
87
Molality
Molality - concentration of solution expressed in moles of solute per kilogram of solvent Solution that contains 1 mol solute (NaOH) dissolved in exactly 1 kg of solvent is ldquoone molalrdquo solution (1 m NaOH)
88
1 mol NaOH = 400 g 400 g NaOH dissolved in 1 kg water is 1 m NaOH
200 g NaOH in 1 kg water = 0500 m NaOH
89
Practice Problem
A solution was prepared by dissolving 171 g sucrose (C12H22O11) in 125 g of water Find the molal concentration of this solution 0400 m C12H22O11
90
A solution of iodine in carbon tetrachloride is used when iodine is needed for certain chemical tests How much iodine must be added to prepare a 0480 m solution of iodine in CCl4 if 1000 g of CCl4 is used 122 g I2
91
What is the molality of a solution composed of 255 g acetone (CH3)2CO dissolved in 200 g of water 22 m acetone What quantity in grams of methanol CH3OH is required to prepare a 0244 m solution in 400 g water 312 g methanol
92
How many grams of AgNO3 are needed to prepare a 0125 m solution in 250 mL of water 531 g AgNO3
What is the molality of a solution containing 182 g HCl and 250 g water 200 m
93
Ions in Aqueous Solutions and Colligative Properties
94
95
compounds in aqueous solutions
DissociationWhen compound made from ions dissolves in water ions separate Dissociation - separation of ions that occurs when an ionic compound dissolves
96
Notice number of ions made per formula unit in equations 1 formula unit of NaCl gives 2 units of ions in solution 1 formula unit of CaCl2 gives 3 units of ions
97
Assuming 100 dissociation solution that contains 1 mol NaCl contains 1 mol Na+ and 1 mol Cl- Can assume 100 dissociation of all soluble ionic compounds
98
Practice Problem
Write the equation for the dissolution of aluminum sulfate in water How many moles of aluminum ions are made by dissolving 1 mol aluminum sulfate What is the total number of moles of ions made by dissolving 1 mol of aluminum sulfate
99
1 AnalyzeGiven Amount of solute = 1 mol Al2(SO4)3 Solvent identity = water Unknown a moles of aluminum ions and sulfate ions Total number of moles of solute ions produced
100
2 Plan
The coefficients in the balanced dissociation equation will tell mole relationships You can use the equation to find out the number of moles of solute ions produced
101
3 Compute
a
b102
Write the equation for the dissolution of each of the following in water and then determine the number of moles of each ions made as well as the total number of ions made a 1 mol ammonium chloride b 1 mol sodium sulfide c 05 mol barium nitrate
103
a 1 mol ammonium chloride
1 mol NH4+ 1 mol Cl- 2 mol total ions
104
b 1 mol sodium sulfide
2 mol Na+ 1 mol S-2 3 mol total ions
105
c 05 mol barium nitrate
05 mol Ba+2 1 mol NO3- 15 mol total ions
106
Precipitation Reactions
Even though no compound is completely insoluble compounds of very low solubility can be considered insoluble for practical purposes There are general guidelines to help predict whether a compound made of certain combination of ions is soluble
107
General Solubility Guidelines
Sodium potassium and ammonium compounds are soluble in water
108
Nitrates acetates and chlorates are soluble
109
110
Most chlorides are soluble except those of silver mercury (I) and lead Lead (II) chloride is soluble in hot water
Most sulfates are soluble except those of barium strontium lead calcium and mercury
111
Most carbonates phosphates and silicates are insoluble except those of sodium potassium and ammonium
112
Most sulfides are insoluble except those of calcium strontium sodium potassium and ammonium
113
Is calcium phosphate Ca3(PO4)2 soluble or insoluble
Not one of the exceptions so it is insoluble
Dissociation equations are not written for insoluble compounds
114
Guidelines also useful in predicting what will happen if 2 solutions of 2 different soluble compounds are mixed
If mixing results in combination of ions that forms insoluble compound a double-replacement and precipitation reaction will happen
115
Will precipitate form when solutions of ammonium sulfide and cadmium nitrate are combined You can tell calcium nitrate is soluble from the guidelines (Most nitrates acetates and chlorates are soluble) You can also tell ammonium sulfide is soluble (Most sulfides are insoluble except those of calcium strontium sodium potassium and ammonium)
116
117
To decide whether precipitate can form must know solubilities of two possible products
Ammonium nitrate ndash soluble (Most nitrates acetates and chlorates are soluble) Cadmium sulfide ndash insoluble (Most sulfides are insoluble except those of calcium strontium sodium potassium and ammonium)
118
bc one product is insoluble double-replacement and precipitation reaction will happen (NH4)2S (aq) + Cd(NO3)2 (aq) rarr NH4NO3 (aq) + CdS (s)
119
Net Ionic Equations
Reactions of ions in aqueous solution usually represented by net ionic equations instead of formula equations Net ionic equation - includes only those compounds and ions that undergo a chemical change in a reaction in an aqueous solution
120
121
To write NIE (net ionic equation) first convert chemical equation into overall ionic equation All soluble ionic compounds shown as dissociated ions in solution Precipitates shown as solids
122
Notice ammonium and nitrate ions appear on both sides They havenrsquot gone through any chemical change Spectator ions - ions that do not take part in a chemical reaction and are found in solution both before and after the reaction
123
To change ionic equation to NIE eliminate spectator ions Cd+2(aq) + S-2(aq) rarr CdS(s)
This NIE applies to any reaction in which precipitate of CdS forms
124
Practice Problem
Identify the precipitate that forms when aqueous solutions of zinc nitrate and ammonium sulfide are combined Write the equation for the possible double-replacement reaction Then write the formula equation overall ionic equation and the net ionic equation
125
1 AnalyzeGiven Identity of reactants zinc nitrate and ammonium sulfide Reaction medium aqueous solution Unknown a equation for the possible double-replacement reaction b identity of the precipitate c formula equation d overall ionic equation e net ionic equation
126
2 PlanWrite the possible double-replacement reaction between Zn(NO3)2 and (NH4)2S Use the rules to determine if any of the products will precipitate Write a formula equation and overall net ionic equation Then cancel spectator ions to make net ionic equation
127
3 Computea possible double-replacement reaction
b rules show that zinc sulfide is not soluble so will precipitate (ammonium nitrate is soluble)
c formula equation128
d overall ionic equation
e net ionic equation129
Will a precipitate form if solutions of potassium sulfate and barium nitrate are combined If so write the net ionic equation for the reaction
Yes Ba+2(aq) + SO4-2(aq) rarr BaSO4(s)
130
Will a precipitate form if solutions of potassium nitrate and magnesium sulfate are combined If so write the net ionic equation for the reaction
No 131
Will a precipitate form if solutions of barium chloride and sodium sulfate are combined If so identify the spectator ions and write the net ionic equation for the reaction
Yes Na+ and Cl- Ba+2(aq) + SO4-2(aq) rarr BaSO4(s)
132
Write the net ionic equation for the precipitation of nickel(II) sulfide
Ni+2(aq) + S-2(aq) rarr NiS(s)
133
IonizationSome molecular compounds can also form ions in solution Usually polar Ionization - ions formed from solute molecules by action of the solvent (creation of ions where there were none) Ionization different from dissociation
134
Dissociation ionic compounds dissolve and ions already present separate Ionization ions formed where non existed before
Ions formed are hydrated Heat released during hydration of ions gives enough energy to break covalent bonds
135
Extent to which solute ionizes depends on strength of bonds within molecules of solute and strength of attraction between solute and solvent molecules
136
HCl ndash molecular compound that ionizes in aqueous solution Contains highly polar bond Attraction between polar HCl molecule and polar water strong enough to break HCl bond
137
The Hydronium Ion
H+ ions from HCl attracts other molecules or ions so strongly that it doesnrsquot normally exist alone Ionization of HCl better described as direct transfer of proton from HCl to H2O forming H3O+
138
139
Hydration of H+ to form hydronium is highly exothermic
Energy released gives a lot of energy needed to ionize a molecular solute
Many molecular compounds that ionize in aqueous solution contain hydrogen and form hydronium ion
140
Strong and Weak Electrolytes
Substances that make ions and conduct current in solution are electrolytes HCl is one of series of compounds that has hydrogen and halogen Hydrogen halides are all molecular compounds with single polar-covalent bonds
141
All are gases Very soluble in water All are electrolytes HCl HBr HI strongly conduct current in solution HF only weakly conducts current at same concentration Strength of conduction related to ability to ionize
142
143
Strong Electrolytes
HCl HBr HI are 100 ionized in solution
Strong electrolyte - any compound whose dilute aqueous solutions conduct electricity well this is due to the presence of all or almost all of the dissolve compound in the form of ions
144
Unique characteristic of strong electrolyte is that they yield only ions
Ex Ionic compound may be highly soluble in water and dissociate into ions (NaCl)
Others may not dissolve much but amount that does dissolve exists only as ions
145
Weak ElectrolytesHF dissolves in water to give acid solution called hydrofluoric acid HF bond stronger than bonds between hydrogen and other halogens When HF dissolves some molecule ionize Reverse reaction also happens HF(aq) + H2O(l) H3O+(aq) + F-(aq)
146
HF(aq) + H2O(l) H3O+
(aq) + F-(aq)Concentration of dissolved unionized HF stays high and concentration of ions stays low
Weak electrolyte - any compound whose aqueous solutions conduct electricity poorly this is due to the presence of small amount of the dissolved compound in the form of ions
147
Different from nonelectrolyte ndash NO ions at all Description of electrolyte as strong or weak not related to concentration of solution Electrolytes differ in degree of ionization not amount of solute dissolved
148
149
Colligative properties of solutions
Presence of solute affects properties of solutions Some properties not dependent on nature of dissolved substance but on how may dissolve particles are present
Colligative properties - properties that depend on the concentration of solute particles but not on their identity
150
Vapor-Pressure Lowering
Boiling and freezing point of solution different from pure solvent Nonvolatile solute raises boiling point and lowers freezing point Nonvolatile substance - one that has little tendency to become gas under existing conditions
151
152
To understand why nonvolatile solute changes boiling and freezing point consider equilibrium vapor pressure
Vapor pressure pressure caused by molecules that have escape liquid phase to gas phase
Can be thought of a measure of tendency of molecules to escape from a liquid
153
Addition of sucrose (nonvolatile solute) lowers concentration of water molecules at surface of liquid This lowers tendency of water molecules to leave solution and enter gas phase Vapor pressure of solution is lower than vapor pressure of pure water
154
Aqueous13 solution13 of13 nonvolatile13 solutePure13 water
Nonelectrolyte solutions of same molality have same concentration of particles
Dilute solutions of same solvent and equal molality of any nonelectrolyte solute lower vapor pressure equally
155
Example
1 m aqueous solution of nonelectrolyte glucose lowers vapor pressure of water 55 x 10-4 atm at 25 1 m aqueous solution of sucrose also lowers water vapor pressure to 55 x 10-4 atm at 25
156
157
bc vapor-pressure lowering depends on concentration of nonelectrolyte solute and doesnrsquot depend on the type of solute it is a colligative proprerty
bc vapor pressure has been lowered solution remains liquid over larger temperature range
Lowers freezing point and raises boiling point
158
Can assume changes in boiling and freezing point also depend on concentration of solute
They are colligative properties
159
Freezing-Point Depression
Freezing point of 1-molal solution of any nonelectrolyte solute in water is found (by experiment) to be 186 lower than freezing point of water When 1 mol of nonelectrolyte solute dissolved in 1 kg water freezing point is -186 not 000 160
When 2 mol nonelectrolyte solute dissolved in 1 kg water freezing point is -372
For any concentration of nonelectrolyte solute in water decrease in freezing point can be determined using value of -186m
161
This value called the molal freezing-point contstant (Kf) is the freezing-point depression of the solvent in a 1-molal solution of a nonvolatile nonelectrolyte solute
162
Each solvent has own characteristic molal freezing-point constant Freezing-point depression (Δtf) the difference between the freezing points of the pure solvent and a solute of nonelectrolyte in that solvent and it is directly proportional to the molal concentration of the solution Δtf = Kfm
163
Δtf = Kfm
Kf expressed as m
m expressed in mol solutekg solvent (molality)
164
Sample Problem
What is the freezing point depression of water in a solution of 171 g of sucrose C12H22O11 and 200 g of water What is the actual freezing point of the solution
165
1 AnalyzeGiven Solute mass and chemical formula 171 g C12H22O11 Solvent mass and identity 2000 g water Unknown a freezing-point depression b freezing point of the solution 166
2 Plan
167
168
Calculate
Unknown a freezing-point depression b freezing point of the solution
169
Practice Problem
A water solution containing an unknown quantity of a nonelectrolyte solute is found to have a freezing point of ndash 023degCWhat is the molal concentration of the solution
012 m170
A solution consists of 103 g of the nonelectrolyte glucose C6H12O6 dissolved in 250 g of water What is the freezing-point depression of the solution
minus0426degC171
In a laboratory experiment the freezing point of an aqueous solution of glucose is found to be minus0325degCWhat is the molal concentration of this solution
0175 m172
If 0500 mol of a nonelectrolyte solute are dissolved in 5000 g of ether what is the freezing point of the solution
minus1181degC173
The freezing point of an aqueous solution that contains a nonelectrolyte is minus90degC aWhat is the freezing-point depression of the solution bWhat is the molal concentration of the solution a minus90degC b 48 m
174
Boiling-Point Elevation
Boiling point is temp at which vapor pressure is equal to atmospheric pressure Change in vapor pressure will cause corresponding change in boiling point
175
176
Vapor pressure of nonvolatile solution is lower than vapor pressure of pure solvent More heat will be required to raise vapor pressure of solution Boiling point of solution is higher than boiling point of pure solvent
Molal boiling-point constant (Kb) - boiling-point elevation of the solvent in a 1-molal solution of a nonvolatile nonelectrolyte solute b-p elevation of any nonelectrolyte in water found by experiment to be 051 So molal b-p constant for water is 051m
177
For different solvents b-p elevations of 1-molal solutions have different values (Table 14-2)
Like freezing-point constants values most accurate for dilute solutions
178
Boiling-point elevation Δtb difference between the boiling points of the pure solvent and a nonelectrolyte solution of that solvent and it is directly proportional to the molal concentration of the solution B-p elevation can be calculated using Δtb = Kbm Δtb (boiling point elevation) expressed in m and m expressed in mol solutekg solvent
Sample Problem
What is the boiling-point elevation of a solution made from 200 g of a nonelectrolyte solute and 4000 g of water The molar mass of the solute is 620 gmol
180
1 AnalyzeGiven solute mass = 200 g solute molar mass = 620 gmol solvent mass and identity = 4000 g of water
Unknown boiling-point elevation
181
2 Plan
182
183
3 Compute
Practice Problem
A solution contains 500 g of sucrose C12H22O11 a nonelectrolyte dissolved in 5000 g of water What is the boiling-point elevation
015degC184
A solution contains 4500 g of sucrose C12H22O11 a nonelectrolyte dissolved in 250 g of waterWhat is the boiling point of the solution
1027degC185
If the boiling point elevation of an aqueous solution containing a nonvolatile electrolyte is 102degC what is the molality of the solution
20 m186
The boiling point of an aqueous solution containing a nonvolatile electrolyte is 10075degC aWhat is the boiling-point elevation bWhat is the molality of the solution a 075degC b 15 m
187
Electrolytes and Colligative PropertiesEarly investigators confused by experiments where certain substances depressed freezing point or elevated boiling point more than expected
188
189
Ex 01 m solution of NaCl lowers freezing point of solvent almost twice as much as 01 m solution of sucrose 01 m CaCl2 lowers fp almost 3 times as much as 01 m solution of sucrose
To understand why this happens contrast behavior or sucrose with NaCl in aqueous solution Each sucrose dissolves to make only 1 particle in solution So 1 mol sucrose dissolves to 1 mol particles 1 mol NaCl dissolves to make 2 moles of particles (1 mol Na+ and 1 mol Cl-)
190
191
Calculated Values for Electrolyte SolutionsColligative properties depend on total concentration of solute particles regardless of their identity Electrolytes cause changes in colligative properties proportional to total molality in terms of all dissolves particles instead of formula units
192
What about barium nitrate Ba(NO3)2 Each mole of barium nitrate yields 3 mol of ions in solution Ba(NO3)2(s) rarr Ba2+(aq) + 2NO3-(aq) You would expect solution of given molality to lower fp of solvent 3 times as much as nonelectrolytic solution of same molality
193
Sample Problem
What is the expected change in the freezing point of water in a solution of 625 g of barium nitrate Ba(NO3)2 in 100 kg of water
194
1 AnalyzeGiven solute mass and formula = 625 g Ba(NO3)2 solvent mass and identity = 100 kg water Δtf = Kfm
Unknown expected freezing-point depression
195
2 PlanThe molality can be calculated by converting the solute mass to moles and then dividing by the number of kilograms of solvent That molality is in terms of formula units of Ba(NO3)2 and must be converted to molality in terms of dissociated ions in solution It must be multiplied by the number of moles of ions produced per mole of formula unit This adjusted molality can then be used to calculate the freezing-point depression
196
197
198
3 Compute
Practice Problem
What is the expected freezing-point depression for a solution that contains 20 mol of magnesium sulfate dissolved in 10 kg of water
minus74degC199
What is the expected boiling-point elevation of water for a solution that contains 150 g of sodium chloride dissolved in 10 kg of water 27degC The freezing point of an aqueous sodium chloride solution is minus200degC What is the molality of the solution 54 m NaCl
200
Actual Values for Electrolyte SolutionsRemember values calculated are only expected values Actual values of colligative properties for all strong electrolytes are almost what would be expected Difference between expected and actual values caused by attractive forces that exist between dissociated ions in aqueous solution
201
Attraction between hydrated ions in solution is small compared with those in crystalline solid However forces of attraction donrsquot interfere with movements of aqueous ions More concentrated solution means ions closer together and attraction is greater Also depends on charge of ions ndash higher charge greater attraction
202
Nonpolar Solvents
Nonpolar solvents do not attract ions of ionic compound strongly enough to overcome forces holding crystal together
50
Liquid Solutes and Solvents
When you shake bottle of salad dressing oil droplets disperse in water Stop shaking ndash strong attraction between water molecules squeeze out oil and form separate layers Liquid solutes and solvents that are not soluble in each other are immiscible
51
Nonpolar substances (fat oil grease) are generally quite soluble in nonpolar liquids (carbon tetrachloride toluene gasoline)
Only attractions between nonpolar molecules are weak London forces
52
London Dispersion Forces
bc e- are constantly moving at any time there may be uneven distribution of them around a nonpolar atom Temporarily creates positive end and negative end
53
54
London dispersion forces - intermolecular attractions resulting from constant motion of e- and creation of instantaneous dipoles
55
Liquids that dissolve freely in one another in any ratio are said to be completely miscible Benzene and carbon tetrachloride (both nonpolar) are completely miscible
Nonpolar molecules of these two apply no strong forces of attraction or repulsion and molecules mix freely
56
Ethanol and water are also completely miscible -OH group on ethanol is slightly polar
Can form H-bonds with water as well as other ethanol molecules Intermolecular forces in mixture are very similar to those in pure liquids so they are mutually soluble in any amount
57
Gasoline contains mainly nonpolar hydrocarbons
Excellent solvent for fats oils greases
Major intermolecular forces are weak London forces
58
Ethanol less polar than water more than carbon tetrachloride
Better solvent for less-polar substances bc of nonpolar region
59
Effects of Pressure on Solubility
Little effect on solids or liquids Increase pressure - increase solubilities of gases in liquids
60
When gas in contact with surface of liquid gas molecules can enter liquid As amount of dissolve gas increases some molecules start to escape and reenter gas phase
Eq eventually established between rates of enteringleaving gas phase
61
As long as eq undisturbed solubility of gas in liquid unchanged at given pressure
Gas + solvent solution
62
Increasing pressure of solute gas above solution puts stress on eq Molecules collide with surface more often Increase in pressure partially offset by increase in rate of gas molecules entering solution In turn increase in amount of dissolved gas causes increase in rate molecules escape liquid and becomes gas
63
Eventually eq restored at higher gas solubility
Expected from Le Chatelierrsquos principle
Increase in gas pressure causes eq to shift so fewer molecules are in gas phase
64
Henryrsquos Law
Henryrsquos law - solubility of a gas in liquid is directly proportional to the partial pressure of that gas on the surface of the liquid
Applies to gas-liquid solutions at constant temp
65
When mixture of ideal gases is limited in constant volume at constant temp each gas applies same pressure it would apply if it occupied the space alone
Assuming gases do not react each gas dissolves to the extent it would if no other gases were there
66
In carbonated drinks (Coke Pepsi etc) solubility of CO2 increased by increasing pressure
At bottling factory CO2 gas forced into solution of flavored water at pressure of 5-10 atm Gas-in-liquid solution then sealed in bottlescans
67
When cap removed pressure reduced to 1 atm and some CO2 escapes as gas bubbles Effervescence - rapid escape of a gas from a liquid in which it is dissolved
68
Effects of Temperature on Solubility
Letrsquos consider gas solubility Increasing temperature usually decreases gas solubility As temp increases average kinetic energy of molecules in solution increases Greater number of solute molecules escape from attraction of solvent and return to gas phase 69
At higher temps eq is reached with fewer gas molecules in solution Gases generally less soluble
70
Effect of temp on solubility of solids in liquids more difficult to predict
Often increasing temp increases solubility
However the same temp increase can result in large increase in solubility in one case and only slight increase in the next
71
72
enthalpies of Solution
Formation of solution comes with energy change Dissolve KI in water container feels cold Dissolve LiCl in water feels hot Formation of solid-liquid solution can absorb or release heat
73
During formation of solution solvent and solute particles experience changes in forces attracting them to other particles Before dissolving begins solvent molecules held by intermolecular forces (solvent-solvent attraction) In solute molecules held by Ifs (solute-solute attraction)
74
Energy is required to separate solute molecules and solvent molecules from their neighbors
A solute particle that is surrounded by solvent molecules is said to be solvated
75
Step 1 solute particles become separated from solid (energy absorbed) Step 2 solvent particles move apart to allow solute particles to enter liquid (energy absorbed)
76
Step 3 solvent particles attracted to and solvate solute particles (energy released)
77
Heat of solution - net amount of heat energy absorbed or released when a specific amount of solute dissolves in a solvent
Heat of solution negative (heat is released) if sums of energy in steps 1 and 2 is less than step 3
If 1+2 gt 3 HOS is positive (heat absorbed)78
In gaseous state molecules are so far apart there are nearly no forces between them Solute-solute interaction has little effect on heat of solution of a gas Energy released when gas dissolved in liquid bc attraction between solute gas and solvent molecules greater than energy needed to separate solvent molecules
79
Concentration of Solutions80
Concentration - measure of amount of solute in given amount of solvent or solution ldquodiluterdquo just means small amount of solute ldquoconcentratedrdquo just means relatively large amount of solute Unrelated to degree to which solution is saturated Ex Saturated solution of substance not very soluble might be dilute
81
Molarity
Molarity - number of moles of solute in one liter of solution To find molarity must know molar mass A ldquoone molarrdquo solution of NaOH is 1 mole NaOH per liter of solution (1 M)
82
1 mol NaOH = 400 g If this quantity dissolve in enough water to make EXACTLY 100 L solution solution is 1 M If 200 g NaOH dissolve in enough to make 100 L solution what is molarity 0500 M
83
1 M solution is not made by adding 1 mol solute to 1 L solvent That would be more than 1 L Instead 1 mol solute dissolved in less than 1 L Then diluted with more solvent to bring TOTAL VOLUME to 1 L
84
Practice ProblemYou have 340 L of solution that contains 900 g sodium chloride What is the molarity of that solution 0440 M NaCl You have 08 L of a 05 M HCl solution How many moles of HCl does this solution contain 04 mol HCl
85
What is the molarity of a solution composed of 585 g of potassium iodide dissolved in enough water to make a 0125 L of solution 0282 M KI
86
How many moles of H2SO4 are present in 0500 L of a 0150 M H2SO4 solution 00750 mol What volume of 300 M NaCl is needed for reaction that requires 1463 g of NaCl 0834 L
87
Molality
Molality - concentration of solution expressed in moles of solute per kilogram of solvent Solution that contains 1 mol solute (NaOH) dissolved in exactly 1 kg of solvent is ldquoone molalrdquo solution (1 m NaOH)
88
1 mol NaOH = 400 g 400 g NaOH dissolved in 1 kg water is 1 m NaOH
200 g NaOH in 1 kg water = 0500 m NaOH
89
Practice Problem
A solution was prepared by dissolving 171 g sucrose (C12H22O11) in 125 g of water Find the molal concentration of this solution 0400 m C12H22O11
90
A solution of iodine in carbon tetrachloride is used when iodine is needed for certain chemical tests How much iodine must be added to prepare a 0480 m solution of iodine in CCl4 if 1000 g of CCl4 is used 122 g I2
91
What is the molality of a solution composed of 255 g acetone (CH3)2CO dissolved in 200 g of water 22 m acetone What quantity in grams of methanol CH3OH is required to prepare a 0244 m solution in 400 g water 312 g methanol
92
How many grams of AgNO3 are needed to prepare a 0125 m solution in 250 mL of water 531 g AgNO3
What is the molality of a solution containing 182 g HCl and 250 g water 200 m
93
Ions in Aqueous Solutions and Colligative Properties
94
95
compounds in aqueous solutions
DissociationWhen compound made from ions dissolves in water ions separate Dissociation - separation of ions that occurs when an ionic compound dissolves
96
Notice number of ions made per formula unit in equations 1 formula unit of NaCl gives 2 units of ions in solution 1 formula unit of CaCl2 gives 3 units of ions
97
Assuming 100 dissociation solution that contains 1 mol NaCl contains 1 mol Na+ and 1 mol Cl- Can assume 100 dissociation of all soluble ionic compounds
98
Practice Problem
Write the equation for the dissolution of aluminum sulfate in water How many moles of aluminum ions are made by dissolving 1 mol aluminum sulfate What is the total number of moles of ions made by dissolving 1 mol of aluminum sulfate
99
1 AnalyzeGiven Amount of solute = 1 mol Al2(SO4)3 Solvent identity = water Unknown a moles of aluminum ions and sulfate ions Total number of moles of solute ions produced
100
2 Plan
The coefficients in the balanced dissociation equation will tell mole relationships You can use the equation to find out the number of moles of solute ions produced
101
3 Compute
a
b102
Write the equation for the dissolution of each of the following in water and then determine the number of moles of each ions made as well as the total number of ions made a 1 mol ammonium chloride b 1 mol sodium sulfide c 05 mol barium nitrate
103
a 1 mol ammonium chloride
1 mol NH4+ 1 mol Cl- 2 mol total ions
104
b 1 mol sodium sulfide
2 mol Na+ 1 mol S-2 3 mol total ions
105
c 05 mol barium nitrate
05 mol Ba+2 1 mol NO3- 15 mol total ions
106
Precipitation Reactions
Even though no compound is completely insoluble compounds of very low solubility can be considered insoluble for practical purposes There are general guidelines to help predict whether a compound made of certain combination of ions is soluble
107
General Solubility Guidelines
Sodium potassium and ammonium compounds are soluble in water
108
Nitrates acetates and chlorates are soluble
109
110
Most chlorides are soluble except those of silver mercury (I) and lead Lead (II) chloride is soluble in hot water
Most sulfates are soluble except those of barium strontium lead calcium and mercury
111
Most carbonates phosphates and silicates are insoluble except those of sodium potassium and ammonium
112
Most sulfides are insoluble except those of calcium strontium sodium potassium and ammonium
113
Is calcium phosphate Ca3(PO4)2 soluble or insoluble
Not one of the exceptions so it is insoluble
Dissociation equations are not written for insoluble compounds
114
Guidelines also useful in predicting what will happen if 2 solutions of 2 different soluble compounds are mixed
If mixing results in combination of ions that forms insoluble compound a double-replacement and precipitation reaction will happen
115
Will precipitate form when solutions of ammonium sulfide and cadmium nitrate are combined You can tell calcium nitrate is soluble from the guidelines (Most nitrates acetates and chlorates are soluble) You can also tell ammonium sulfide is soluble (Most sulfides are insoluble except those of calcium strontium sodium potassium and ammonium)
116
117
To decide whether precipitate can form must know solubilities of two possible products
Ammonium nitrate ndash soluble (Most nitrates acetates and chlorates are soluble) Cadmium sulfide ndash insoluble (Most sulfides are insoluble except those of calcium strontium sodium potassium and ammonium)
118
bc one product is insoluble double-replacement and precipitation reaction will happen (NH4)2S (aq) + Cd(NO3)2 (aq) rarr NH4NO3 (aq) + CdS (s)
119
Net Ionic Equations
Reactions of ions in aqueous solution usually represented by net ionic equations instead of formula equations Net ionic equation - includes only those compounds and ions that undergo a chemical change in a reaction in an aqueous solution
120
121
To write NIE (net ionic equation) first convert chemical equation into overall ionic equation All soluble ionic compounds shown as dissociated ions in solution Precipitates shown as solids
122
Notice ammonium and nitrate ions appear on both sides They havenrsquot gone through any chemical change Spectator ions - ions that do not take part in a chemical reaction and are found in solution both before and after the reaction
123
To change ionic equation to NIE eliminate spectator ions Cd+2(aq) + S-2(aq) rarr CdS(s)
This NIE applies to any reaction in which precipitate of CdS forms
124
Practice Problem
Identify the precipitate that forms when aqueous solutions of zinc nitrate and ammonium sulfide are combined Write the equation for the possible double-replacement reaction Then write the formula equation overall ionic equation and the net ionic equation
125
1 AnalyzeGiven Identity of reactants zinc nitrate and ammonium sulfide Reaction medium aqueous solution Unknown a equation for the possible double-replacement reaction b identity of the precipitate c formula equation d overall ionic equation e net ionic equation
126
2 PlanWrite the possible double-replacement reaction between Zn(NO3)2 and (NH4)2S Use the rules to determine if any of the products will precipitate Write a formula equation and overall net ionic equation Then cancel spectator ions to make net ionic equation
127
3 Computea possible double-replacement reaction
b rules show that zinc sulfide is not soluble so will precipitate (ammonium nitrate is soluble)
c formula equation128
d overall ionic equation
e net ionic equation129
Will a precipitate form if solutions of potassium sulfate and barium nitrate are combined If so write the net ionic equation for the reaction
Yes Ba+2(aq) + SO4-2(aq) rarr BaSO4(s)
130
Will a precipitate form if solutions of potassium nitrate and magnesium sulfate are combined If so write the net ionic equation for the reaction
No 131
Will a precipitate form if solutions of barium chloride and sodium sulfate are combined If so identify the spectator ions and write the net ionic equation for the reaction
Yes Na+ and Cl- Ba+2(aq) + SO4-2(aq) rarr BaSO4(s)
132
Write the net ionic equation for the precipitation of nickel(II) sulfide
Ni+2(aq) + S-2(aq) rarr NiS(s)
133
IonizationSome molecular compounds can also form ions in solution Usually polar Ionization - ions formed from solute molecules by action of the solvent (creation of ions where there were none) Ionization different from dissociation
134
Dissociation ionic compounds dissolve and ions already present separate Ionization ions formed where non existed before
Ions formed are hydrated Heat released during hydration of ions gives enough energy to break covalent bonds
135
Extent to which solute ionizes depends on strength of bonds within molecules of solute and strength of attraction between solute and solvent molecules
136
HCl ndash molecular compound that ionizes in aqueous solution Contains highly polar bond Attraction between polar HCl molecule and polar water strong enough to break HCl bond
137
The Hydronium Ion
H+ ions from HCl attracts other molecules or ions so strongly that it doesnrsquot normally exist alone Ionization of HCl better described as direct transfer of proton from HCl to H2O forming H3O+
138
139
Hydration of H+ to form hydronium is highly exothermic
Energy released gives a lot of energy needed to ionize a molecular solute
Many molecular compounds that ionize in aqueous solution contain hydrogen and form hydronium ion
140
Strong and Weak Electrolytes
Substances that make ions and conduct current in solution are electrolytes HCl is one of series of compounds that has hydrogen and halogen Hydrogen halides are all molecular compounds with single polar-covalent bonds
141
All are gases Very soluble in water All are electrolytes HCl HBr HI strongly conduct current in solution HF only weakly conducts current at same concentration Strength of conduction related to ability to ionize
142
143
Strong Electrolytes
HCl HBr HI are 100 ionized in solution
Strong electrolyte - any compound whose dilute aqueous solutions conduct electricity well this is due to the presence of all or almost all of the dissolve compound in the form of ions
144
Unique characteristic of strong electrolyte is that they yield only ions
Ex Ionic compound may be highly soluble in water and dissociate into ions (NaCl)
Others may not dissolve much but amount that does dissolve exists only as ions
145
Weak ElectrolytesHF dissolves in water to give acid solution called hydrofluoric acid HF bond stronger than bonds between hydrogen and other halogens When HF dissolves some molecule ionize Reverse reaction also happens HF(aq) + H2O(l) H3O+(aq) + F-(aq)
146
HF(aq) + H2O(l) H3O+
(aq) + F-(aq)Concentration of dissolved unionized HF stays high and concentration of ions stays low
Weak electrolyte - any compound whose aqueous solutions conduct electricity poorly this is due to the presence of small amount of the dissolved compound in the form of ions
147
Different from nonelectrolyte ndash NO ions at all Description of electrolyte as strong or weak not related to concentration of solution Electrolytes differ in degree of ionization not amount of solute dissolved
148
149
Colligative properties of solutions
Presence of solute affects properties of solutions Some properties not dependent on nature of dissolved substance but on how may dissolve particles are present
Colligative properties - properties that depend on the concentration of solute particles but not on their identity
150
Vapor-Pressure Lowering
Boiling and freezing point of solution different from pure solvent Nonvolatile solute raises boiling point and lowers freezing point Nonvolatile substance - one that has little tendency to become gas under existing conditions
151
152
To understand why nonvolatile solute changes boiling and freezing point consider equilibrium vapor pressure
Vapor pressure pressure caused by molecules that have escape liquid phase to gas phase
Can be thought of a measure of tendency of molecules to escape from a liquid
153
Addition of sucrose (nonvolatile solute) lowers concentration of water molecules at surface of liquid This lowers tendency of water molecules to leave solution and enter gas phase Vapor pressure of solution is lower than vapor pressure of pure water
154
Aqueous13 solution13 of13 nonvolatile13 solutePure13 water
Nonelectrolyte solutions of same molality have same concentration of particles
Dilute solutions of same solvent and equal molality of any nonelectrolyte solute lower vapor pressure equally
155
Example
1 m aqueous solution of nonelectrolyte glucose lowers vapor pressure of water 55 x 10-4 atm at 25 1 m aqueous solution of sucrose also lowers water vapor pressure to 55 x 10-4 atm at 25
156
157
bc vapor-pressure lowering depends on concentration of nonelectrolyte solute and doesnrsquot depend on the type of solute it is a colligative proprerty
bc vapor pressure has been lowered solution remains liquid over larger temperature range
Lowers freezing point and raises boiling point
158
Can assume changes in boiling and freezing point also depend on concentration of solute
They are colligative properties
159
Freezing-Point Depression
Freezing point of 1-molal solution of any nonelectrolyte solute in water is found (by experiment) to be 186 lower than freezing point of water When 1 mol of nonelectrolyte solute dissolved in 1 kg water freezing point is -186 not 000 160
When 2 mol nonelectrolyte solute dissolved in 1 kg water freezing point is -372
For any concentration of nonelectrolyte solute in water decrease in freezing point can be determined using value of -186m
161
This value called the molal freezing-point contstant (Kf) is the freezing-point depression of the solvent in a 1-molal solution of a nonvolatile nonelectrolyte solute
162
Each solvent has own characteristic molal freezing-point constant Freezing-point depression (Δtf) the difference between the freezing points of the pure solvent and a solute of nonelectrolyte in that solvent and it is directly proportional to the molal concentration of the solution Δtf = Kfm
163
Δtf = Kfm
Kf expressed as m
m expressed in mol solutekg solvent (molality)
164
Sample Problem
What is the freezing point depression of water in a solution of 171 g of sucrose C12H22O11 and 200 g of water What is the actual freezing point of the solution
165
1 AnalyzeGiven Solute mass and chemical formula 171 g C12H22O11 Solvent mass and identity 2000 g water Unknown a freezing-point depression b freezing point of the solution 166
2 Plan
167
168
Calculate
Unknown a freezing-point depression b freezing point of the solution
169
Practice Problem
A water solution containing an unknown quantity of a nonelectrolyte solute is found to have a freezing point of ndash 023degCWhat is the molal concentration of the solution
012 m170
A solution consists of 103 g of the nonelectrolyte glucose C6H12O6 dissolved in 250 g of water What is the freezing-point depression of the solution
minus0426degC171
In a laboratory experiment the freezing point of an aqueous solution of glucose is found to be minus0325degCWhat is the molal concentration of this solution
0175 m172
If 0500 mol of a nonelectrolyte solute are dissolved in 5000 g of ether what is the freezing point of the solution
minus1181degC173
The freezing point of an aqueous solution that contains a nonelectrolyte is minus90degC aWhat is the freezing-point depression of the solution bWhat is the molal concentration of the solution a minus90degC b 48 m
174
Boiling-Point Elevation
Boiling point is temp at which vapor pressure is equal to atmospheric pressure Change in vapor pressure will cause corresponding change in boiling point
175
176
Vapor pressure of nonvolatile solution is lower than vapor pressure of pure solvent More heat will be required to raise vapor pressure of solution Boiling point of solution is higher than boiling point of pure solvent
Molal boiling-point constant (Kb) - boiling-point elevation of the solvent in a 1-molal solution of a nonvolatile nonelectrolyte solute b-p elevation of any nonelectrolyte in water found by experiment to be 051 So molal b-p constant for water is 051m
177
For different solvents b-p elevations of 1-molal solutions have different values (Table 14-2)
Like freezing-point constants values most accurate for dilute solutions
178
Boiling-point elevation Δtb difference between the boiling points of the pure solvent and a nonelectrolyte solution of that solvent and it is directly proportional to the molal concentration of the solution B-p elevation can be calculated using Δtb = Kbm Δtb (boiling point elevation) expressed in m and m expressed in mol solutekg solvent
Sample Problem
What is the boiling-point elevation of a solution made from 200 g of a nonelectrolyte solute and 4000 g of water The molar mass of the solute is 620 gmol
180
1 AnalyzeGiven solute mass = 200 g solute molar mass = 620 gmol solvent mass and identity = 4000 g of water
Unknown boiling-point elevation
181
2 Plan
182
183
3 Compute
Practice Problem
A solution contains 500 g of sucrose C12H22O11 a nonelectrolyte dissolved in 5000 g of water What is the boiling-point elevation
015degC184
A solution contains 4500 g of sucrose C12H22O11 a nonelectrolyte dissolved in 250 g of waterWhat is the boiling point of the solution
1027degC185
If the boiling point elevation of an aqueous solution containing a nonvolatile electrolyte is 102degC what is the molality of the solution
20 m186
The boiling point of an aqueous solution containing a nonvolatile electrolyte is 10075degC aWhat is the boiling-point elevation bWhat is the molality of the solution a 075degC b 15 m
187
Electrolytes and Colligative PropertiesEarly investigators confused by experiments where certain substances depressed freezing point or elevated boiling point more than expected
188
189
Ex 01 m solution of NaCl lowers freezing point of solvent almost twice as much as 01 m solution of sucrose 01 m CaCl2 lowers fp almost 3 times as much as 01 m solution of sucrose
To understand why this happens contrast behavior or sucrose with NaCl in aqueous solution Each sucrose dissolves to make only 1 particle in solution So 1 mol sucrose dissolves to 1 mol particles 1 mol NaCl dissolves to make 2 moles of particles (1 mol Na+ and 1 mol Cl-)
190
191
Calculated Values for Electrolyte SolutionsColligative properties depend on total concentration of solute particles regardless of their identity Electrolytes cause changes in colligative properties proportional to total molality in terms of all dissolves particles instead of formula units
192
What about barium nitrate Ba(NO3)2 Each mole of barium nitrate yields 3 mol of ions in solution Ba(NO3)2(s) rarr Ba2+(aq) + 2NO3-(aq) You would expect solution of given molality to lower fp of solvent 3 times as much as nonelectrolytic solution of same molality
193
Sample Problem
What is the expected change in the freezing point of water in a solution of 625 g of barium nitrate Ba(NO3)2 in 100 kg of water
194
1 AnalyzeGiven solute mass and formula = 625 g Ba(NO3)2 solvent mass and identity = 100 kg water Δtf = Kfm
Unknown expected freezing-point depression
195
2 PlanThe molality can be calculated by converting the solute mass to moles and then dividing by the number of kilograms of solvent That molality is in terms of formula units of Ba(NO3)2 and must be converted to molality in terms of dissociated ions in solution It must be multiplied by the number of moles of ions produced per mole of formula unit This adjusted molality can then be used to calculate the freezing-point depression
196
197
198
3 Compute
Practice Problem
What is the expected freezing-point depression for a solution that contains 20 mol of magnesium sulfate dissolved in 10 kg of water
minus74degC199
What is the expected boiling-point elevation of water for a solution that contains 150 g of sodium chloride dissolved in 10 kg of water 27degC The freezing point of an aqueous sodium chloride solution is minus200degC What is the molality of the solution 54 m NaCl
200
Actual Values for Electrolyte SolutionsRemember values calculated are only expected values Actual values of colligative properties for all strong electrolytes are almost what would be expected Difference between expected and actual values caused by attractive forces that exist between dissociated ions in aqueous solution
201
Attraction between hydrated ions in solution is small compared with those in crystalline solid However forces of attraction donrsquot interfere with movements of aqueous ions More concentrated solution means ions closer together and attraction is greater Also depends on charge of ions ndash higher charge greater attraction
202
Liquid Solutes and Solvents
When you shake bottle of salad dressing oil droplets disperse in water Stop shaking ndash strong attraction between water molecules squeeze out oil and form separate layers Liquid solutes and solvents that are not soluble in each other are immiscible
51
Nonpolar substances (fat oil grease) are generally quite soluble in nonpolar liquids (carbon tetrachloride toluene gasoline)
Only attractions between nonpolar molecules are weak London forces
52
London Dispersion Forces
bc e- are constantly moving at any time there may be uneven distribution of them around a nonpolar atom Temporarily creates positive end and negative end
53
54
London dispersion forces - intermolecular attractions resulting from constant motion of e- and creation of instantaneous dipoles
55
Liquids that dissolve freely in one another in any ratio are said to be completely miscible Benzene and carbon tetrachloride (both nonpolar) are completely miscible
Nonpolar molecules of these two apply no strong forces of attraction or repulsion and molecules mix freely
56
Ethanol and water are also completely miscible -OH group on ethanol is slightly polar
Can form H-bonds with water as well as other ethanol molecules Intermolecular forces in mixture are very similar to those in pure liquids so they are mutually soluble in any amount
57
Gasoline contains mainly nonpolar hydrocarbons
Excellent solvent for fats oils greases
Major intermolecular forces are weak London forces
58
Ethanol less polar than water more than carbon tetrachloride
Better solvent for less-polar substances bc of nonpolar region
59
Effects of Pressure on Solubility
Little effect on solids or liquids Increase pressure - increase solubilities of gases in liquids
60
When gas in contact with surface of liquid gas molecules can enter liquid As amount of dissolve gas increases some molecules start to escape and reenter gas phase
Eq eventually established between rates of enteringleaving gas phase
61
As long as eq undisturbed solubility of gas in liquid unchanged at given pressure
Gas + solvent solution
62
Increasing pressure of solute gas above solution puts stress on eq Molecules collide with surface more often Increase in pressure partially offset by increase in rate of gas molecules entering solution In turn increase in amount of dissolved gas causes increase in rate molecules escape liquid and becomes gas
63
Eventually eq restored at higher gas solubility
Expected from Le Chatelierrsquos principle
Increase in gas pressure causes eq to shift so fewer molecules are in gas phase
64
Henryrsquos Law
Henryrsquos law - solubility of a gas in liquid is directly proportional to the partial pressure of that gas on the surface of the liquid
Applies to gas-liquid solutions at constant temp
65
When mixture of ideal gases is limited in constant volume at constant temp each gas applies same pressure it would apply if it occupied the space alone
Assuming gases do not react each gas dissolves to the extent it would if no other gases were there
66
In carbonated drinks (Coke Pepsi etc) solubility of CO2 increased by increasing pressure
At bottling factory CO2 gas forced into solution of flavored water at pressure of 5-10 atm Gas-in-liquid solution then sealed in bottlescans
67
When cap removed pressure reduced to 1 atm and some CO2 escapes as gas bubbles Effervescence - rapid escape of a gas from a liquid in which it is dissolved
68
Effects of Temperature on Solubility
Letrsquos consider gas solubility Increasing temperature usually decreases gas solubility As temp increases average kinetic energy of molecules in solution increases Greater number of solute molecules escape from attraction of solvent and return to gas phase 69
At higher temps eq is reached with fewer gas molecules in solution Gases generally less soluble
70
Effect of temp on solubility of solids in liquids more difficult to predict
Often increasing temp increases solubility
However the same temp increase can result in large increase in solubility in one case and only slight increase in the next
71
72
enthalpies of Solution
Formation of solution comes with energy change Dissolve KI in water container feels cold Dissolve LiCl in water feels hot Formation of solid-liquid solution can absorb or release heat
73
During formation of solution solvent and solute particles experience changes in forces attracting them to other particles Before dissolving begins solvent molecules held by intermolecular forces (solvent-solvent attraction) In solute molecules held by Ifs (solute-solute attraction)
74
Energy is required to separate solute molecules and solvent molecules from their neighbors
A solute particle that is surrounded by solvent molecules is said to be solvated
75
Step 1 solute particles become separated from solid (energy absorbed) Step 2 solvent particles move apart to allow solute particles to enter liquid (energy absorbed)
76
Step 3 solvent particles attracted to and solvate solute particles (energy released)
77
Heat of solution - net amount of heat energy absorbed or released when a specific amount of solute dissolves in a solvent
Heat of solution negative (heat is released) if sums of energy in steps 1 and 2 is less than step 3
If 1+2 gt 3 HOS is positive (heat absorbed)78
In gaseous state molecules are so far apart there are nearly no forces between them Solute-solute interaction has little effect on heat of solution of a gas Energy released when gas dissolved in liquid bc attraction between solute gas and solvent molecules greater than energy needed to separate solvent molecules
79
Concentration of Solutions80
Concentration - measure of amount of solute in given amount of solvent or solution ldquodiluterdquo just means small amount of solute ldquoconcentratedrdquo just means relatively large amount of solute Unrelated to degree to which solution is saturated Ex Saturated solution of substance not very soluble might be dilute
81
Molarity
Molarity - number of moles of solute in one liter of solution To find molarity must know molar mass A ldquoone molarrdquo solution of NaOH is 1 mole NaOH per liter of solution (1 M)
82
1 mol NaOH = 400 g If this quantity dissolve in enough water to make EXACTLY 100 L solution solution is 1 M If 200 g NaOH dissolve in enough to make 100 L solution what is molarity 0500 M
83
1 M solution is not made by adding 1 mol solute to 1 L solvent That would be more than 1 L Instead 1 mol solute dissolved in less than 1 L Then diluted with more solvent to bring TOTAL VOLUME to 1 L
84
Practice ProblemYou have 340 L of solution that contains 900 g sodium chloride What is the molarity of that solution 0440 M NaCl You have 08 L of a 05 M HCl solution How many moles of HCl does this solution contain 04 mol HCl
85
What is the molarity of a solution composed of 585 g of potassium iodide dissolved in enough water to make a 0125 L of solution 0282 M KI
86
How many moles of H2SO4 are present in 0500 L of a 0150 M H2SO4 solution 00750 mol What volume of 300 M NaCl is needed for reaction that requires 1463 g of NaCl 0834 L
87
Molality
Molality - concentration of solution expressed in moles of solute per kilogram of solvent Solution that contains 1 mol solute (NaOH) dissolved in exactly 1 kg of solvent is ldquoone molalrdquo solution (1 m NaOH)
88
1 mol NaOH = 400 g 400 g NaOH dissolved in 1 kg water is 1 m NaOH
200 g NaOH in 1 kg water = 0500 m NaOH
89
Practice Problem
A solution was prepared by dissolving 171 g sucrose (C12H22O11) in 125 g of water Find the molal concentration of this solution 0400 m C12H22O11
90
A solution of iodine in carbon tetrachloride is used when iodine is needed for certain chemical tests How much iodine must be added to prepare a 0480 m solution of iodine in CCl4 if 1000 g of CCl4 is used 122 g I2
91
What is the molality of a solution composed of 255 g acetone (CH3)2CO dissolved in 200 g of water 22 m acetone What quantity in grams of methanol CH3OH is required to prepare a 0244 m solution in 400 g water 312 g methanol
92
How many grams of AgNO3 are needed to prepare a 0125 m solution in 250 mL of water 531 g AgNO3
What is the molality of a solution containing 182 g HCl and 250 g water 200 m
93
Ions in Aqueous Solutions and Colligative Properties
94
95
compounds in aqueous solutions
DissociationWhen compound made from ions dissolves in water ions separate Dissociation - separation of ions that occurs when an ionic compound dissolves
96
Notice number of ions made per formula unit in equations 1 formula unit of NaCl gives 2 units of ions in solution 1 formula unit of CaCl2 gives 3 units of ions
97
Assuming 100 dissociation solution that contains 1 mol NaCl contains 1 mol Na+ and 1 mol Cl- Can assume 100 dissociation of all soluble ionic compounds
98
Practice Problem
Write the equation for the dissolution of aluminum sulfate in water How many moles of aluminum ions are made by dissolving 1 mol aluminum sulfate What is the total number of moles of ions made by dissolving 1 mol of aluminum sulfate
99
1 AnalyzeGiven Amount of solute = 1 mol Al2(SO4)3 Solvent identity = water Unknown a moles of aluminum ions and sulfate ions Total number of moles of solute ions produced
100
2 Plan
The coefficients in the balanced dissociation equation will tell mole relationships You can use the equation to find out the number of moles of solute ions produced
101
3 Compute
a
b102
Write the equation for the dissolution of each of the following in water and then determine the number of moles of each ions made as well as the total number of ions made a 1 mol ammonium chloride b 1 mol sodium sulfide c 05 mol barium nitrate
103
a 1 mol ammonium chloride
1 mol NH4+ 1 mol Cl- 2 mol total ions
104
b 1 mol sodium sulfide
2 mol Na+ 1 mol S-2 3 mol total ions
105
c 05 mol barium nitrate
05 mol Ba+2 1 mol NO3- 15 mol total ions
106
Precipitation Reactions
Even though no compound is completely insoluble compounds of very low solubility can be considered insoluble for practical purposes There are general guidelines to help predict whether a compound made of certain combination of ions is soluble
107
General Solubility Guidelines
Sodium potassium and ammonium compounds are soluble in water
108
Nitrates acetates and chlorates are soluble
109
110
Most chlorides are soluble except those of silver mercury (I) and lead Lead (II) chloride is soluble in hot water
Most sulfates are soluble except those of barium strontium lead calcium and mercury
111
Most carbonates phosphates and silicates are insoluble except those of sodium potassium and ammonium
112
Most sulfides are insoluble except those of calcium strontium sodium potassium and ammonium
113
Is calcium phosphate Ca3(PO4)2 soluble or insoluble
Not one of the exceptions so it is insoluble
Dissociation equations are not written for insoluble compounds
114
Guidelines also useful in predicting what will happen if 2 solutions of 2 different soluble compounds are mixed
If mixing results in combination of ions that forms insoluble compound a double-replacement and precipitation reaction will happen
115
Will precipitate form when solutions of ammonium sulfide and cadmium nitrate are combined You can tell calcium nitrate is soluble from the guidelines (Most nitrates acetates and chlorates are soluble) You can also tell ammonium sulfide is soluble (Most sulfides are insoluble except those of calcium strontium sodium potassium and ammonium)
116
117
To decide whether precipitate can form must know solubilities of two possible products
Ammonium nitrate ndash soluble (Most nitrates acetates and chlorates are soluble) Cadmium sulfide ndash insoluble (Most sulfides are insoluble except those of calcium strontium sodium potassium and ammonium)
118
bc one product is insoluble double-replacement and precipitation reaction will happen (NH4)2S (aq) + Cd(NO3)2 (aq) rarr NH4NO3 (aq) + CdS (s)
119
Net Ionic Equations
Reactions of ions in aqueous solution usually represented by net ionic equations instead of formula equations Net ionic equation - includes only those compounds and ions that undergo a chemical change in a reaction in an aqueous solution
120
121
To write NIE (net ionic equation) first convert chemical equation into overall ionic equation All soluble ionic compounds shown as dissociated ions in solution Precipitates shown as solids
122
Notice ammonium and nitrate ions appear on both sides They havenrsquot gone through any chemical change Spectator ions - ions that do not take part in a chemical reaction and are found in solution both before and after the reaction
123
To change ionic equation to NIE eliminate spectator ions Cd+2(aq) + S-2(aq) rarr CdS(s)
This NIE applies to any reaction in which precipitate of CdS forms
124
Practice Problem
Identify the precipitate that forms when aqueous solutions of zinc nitrate and ammonium sulfide are combined Write the equation for the possible double-replacement reaction Then write the formula equation overall ionic equation and the net ionic equation
125
1 AnalyzeGiven Identity of reactants zinc nitrate and ammonium sulfide Reaction medium aqueous solution Unknown a equation for the possible double-replacement reaction b identity of the precipitate c formula equation d overall ionic equation e net ionic equation
126
2 PlanWrite the possible double-replacement reaction between Zn(NO3)2 and (NH4)2S Use the rules to determine if any of the products will precipitate Write a formula equation and overall net ionic equation Then cancel spectator ions to make net ionic equation
127
3 Computea possible double-replacement reaction
b rules show that zinc sulfide is not soluble so will precipitate (ammonium nitrate is soluble)
c formula equation128
d overall ionic equation
e net ionic equation129
Will a precipitate form if solutions of potassium sulfate and barium nitrate are combined If so write the net ionic equation for the reaction
Yes Ba+2(aq) + SO4-2(aq) rarr BaSO4(s)
130
Will a precipitate form if solutions of potassium nitrate and magnesium sulfate are combined If so write the net ionic equation for the reaction
No 131
Will a precipitate form if solutions of barium chloride and sodium sulfate are combined If so identify the spectator ions and write the net ionic equation for the reaction
Yes Na+ and Cl- Ba+2(aq) + SO4-2(aq) rarr BaSO4(s)
132
Write the net ionic equation for the precipitation of nickel(II) sulfide
Ni+2(aq) + S-2(aq) rarr NiS(s)
133
IonizationSome molecular compounds can also form ions in solution Usually polar Ionization - ions formed from solute molecules by action of the solvent (creation of ions where there were none) Ionization different from dissociation
134
Dissociation ionic compounds dissolve and ions already present separate Ionization ions formed where non existed before
Ions formed are hydrated Heat released during hydration of ions gives enough energy to break covalent bonds
135
Extent to which solute ionizes depends on strength of bonds within molecules of solute and strength of attraction between solute and solvent molecules
136
HCl ndash molecular compound that ionizes in aqueous solution Contains highly polar bond Attraction between polar HCl molecule and polar water strong enough to break HCl bond
137
The Hydronium Ion
H+ ions from HCl attracts other molecules or ions so strongly that it doesnrsquot normally exist alone Ionization of HCl better described as direct transfer of proton from HCl to H2O forming H3O+
138
139
Hydration of H+ to form hydronium is highly exothermic
Energy released gives a lot of energy needed to ionize a molecular solute
Many molecular compounds that ionize in aqueous solution contain hydrogen and form hydronium ion
140
Strong and Weak Electrolytes
Substances that make ions and conduct current in solution are electrolytes HCl is one of series of compounds that has hydrogen and halogen Hydrogen halides are all molecular compounds with single polar-covalent bonds
141
All are gases Very soluble in water All are electrolytes HCl HBr HI strongly conduct current in solution HF only weakly conducts current at same concentration Strength of conduction related to ability to ionize
142
143
Strong Electrolytes
HCl HBr HI are 100 ionized in solution
Strong electrolyte - any compound whose dilute aqueous solutions conduct electricity well this is due to the presence of all or almost all of the dissolve compound in the form of ions
144
Unique characteristic of strong electrolyte is that they yield only ions
Ex Ionic compound may be highly soluble in water and dissociate into ions (NaCl)
Others may not dissolve much but amount that does dissolve exists only as ions
145
Weak ElectrolytesHF dissolves in water to give acid solution called hydrofluoric acid HF bond stronger than bonds between hydrogen and other halogens When HF dissolves some molecule ionize Reverse reaction also happens HF(aq) + H2O(l) H3O+(aq) + F-(aq)
146
HF(aq) + H2O(l) H3O+
(aq) + F-(aq)Concentration of dissolved unionized HF stays high and concentration of ions stays low
Weak electrolyte - any compound whose aqueous solutions conduct electricity poorly this is due to the presence of small amount of the dissolved compound in the form of ions
147
Different from nonelectrolyte ndash NO ions at all Description of electrolyte as strong or weak not related to concentration of solution Electrolytes differ in degree of ionization not amount of solute dissolved
148
149
Colligative properties of solutions
Presence of solute affects properties of solutions Some properties not dependent on nature of dissolved substance but on how may dissolve particles are present
Colligative properties - properties that depend on the concentration of solute particles but not on their identity
150
Vapor-Pressure Lowering
Boiling and freezing point of solution different from pure solvent Nonvolatile solute raises boiling point and lowers freezing point Nonvolatile substance - one that has little tendency to become gas under existing conditions
151
152
To understand why nonvolatile solute changes boiling and freezing point consider equilibrium vapor pressure
Vapor pressure pressure caused by molecules that have escape liquid phase to gas phase
Can be thought of a measure of tendency of molecules to escape from a liquid
153
Addition of sucrose (nonvolatile solute) lowers concentration of water molecules at surface of liquid This lowers tendency of water molecules to leave solution and enter gas phase Vapor pressure of solution is lower than vapor pressure of pure water
154
Aqueous13 solution13 of13 nonvolatile13 solutePure13 water
Nonelectrolyte solutions of same molality have same concentration of particles
Dilute solutions of same solvent and equal molality of any nonelectrolyte solute lower vapor pressure equally
155
Example
1 m aqueous solution of nonelectrolyte glucose lowers vapor pressure of water 55 x 10-4 atm at 25 1 m aqueous solution of sucrose also lowers water vapor pressure to 55 x 10-4 atm at 25
156
157
bc vapor-pressure lowering depends on concentration of nonelectrolyte solute and doesnrsquot depend on the type of solute it is a colligative proprerty
bc vapor pressure has been lowered solution remains liquid over larger temperature range
Lowers freezing point and raises boiling point
158
Can assume changes in boiling and freezing point also depend on concentration of solute
They are colligative properties
159
Freezing-Point Depression
Freezing point of 1-molal solution of any nonelectrolyte solute in water is found (by experiment) to be 186 lower than freezing point of water When 1 mol of nonelectrolyte solute dissolved in 1 kg water freezing point is -186 not 000 160
When 2 mol nonelectrolyte solute dissolved in 1 kg water freezing point is -372
For any concentration of nonelectrolyte solute in water decrease in freezing point can be determined using value of -186m
161
This value called the molal freezing-point contstant (Kf) is the freezing-point depression of the solvent in a 1-molal solution of a nonvolatile nonelectrolyte solute
162
Each solvent has own characteristic molal freezing-point constant Freezing-point depression (Δtf) the difference between the freezing points of the pure solvent and a solute of nonelectrolyte in that solvent and it is directly proportional to the molal concentration of the solution Δtf = Kfm
163
Δtf = Kfm
Kf expressed as m
m expressed in mol solutekg solvent (molality)
164
Sample Problem
What is the freezing point depression of water in a solution of 171 g of sucrose C12H22O11 and 200 g of water What is the actual freezing point of the solution
165
1 AnalyzeGiven Solute mass and chemical formula 171 g C12H22O11 Solvent mass and identity 2000 g water Unknown a freezing-point depression b freezing point of the solution 166
2 Plan
167
168
Calculate
Unknown a freezing-point depression b freezing point of the solution
169
Practice Problem
A water solution containing an unknown quantity of a nonelectrolyte solute is found to have a freezing point of ndash 023degCWhat is the molal concentration of the solution
012 m170
A solution consists of 103 g of the nonelectrolyte glucose C6H12O6 dissolved in 250 g of water What is the freezing-point depression of the solution
minus0426degC171
In a laboratory experiment the freezing point of an aqueous solution of glucose is found to be minus0325degCWhat is the molal concentration of this solution
0175 m172
If 0500 mol of a nonelectrolyte solute are dissolved in 5000 g of ether what is the freezing point of the solution
minus1181degC173
The freezing point of an aqueous solution that contains a nonelectrolyte is minus90degC aWhat is the freezing-point depression of the solution bWhat is the molal concentration of the solution a minus90degC b 48 m
174
Boiling-Point Elevation
Boiling point is temp at which vapor pressure is equal to atmospheric pressure Change in vapor pressure will cause corresponding change in boiling point
175
176
Vapor pressure of nonvolatile solution is lower than vapor pressure of pure solvent More heat will be required to raise vapor pressure of solution Boiling point of solution is higher than boiling point of pure solvent
Molal boiling-point constant (Kb) - boiling-point elevation of the solvent in a 1-molal solution of a nonvolatile nonelectrolyte solute b-p elevation of any nonelectrolyte in water found by experiment to be 051 So molal b-p constant for water is 051m
177
For different solvents b-p elevations of 1-molal solutions have different values (Table 14-2)
Like freezing-point constants values most accurate for dilute solutions
178
Boiling-point elevation Δtb difference between the boiling points of the pure solvent and a nonelectrolyte solution of that solvent and it is directly proportional to the molal concentration of the solution B-p elevation can be calculated using Δtb = Kbm Δtb (boiling point elevation) expressed in m and m expressed in mol solutekg solvent
Sample Problem
What is the boiling-point elevation of a solution made from 200 g of a nonelectrolyte solute and 4000 g of water The molar mass of the solute is 620 gmol
180
1 AnalyzeGiven solute mass = 200 g solute molar mass = 620 gmol solvent mass and identity = 4000 g of water
Unknown boiling-point elevation
181
2 Plan
182
183
3 Compute
Practice Problem
A solution contains 500 g of sucrose C12H22O11 a nonelectrolyte dissolved in 5000 g of water What is the boiling-point elevation
015degC184
A solution contains 4500 g of sucrose C12H22O11 a nonelectrolyte dissolved in 250 g of waterWhat is the boiling point of the solution
1027degC185
If the boiling point elevation of an aqueous solution containing a nonvolatile electrolyte is 102degC what is the molality of the solution
20 m186
The boiling point of an aqueous solution containing a nonvolatile electrolyte is 10075degC aWhat is the boiling-point elevation bWhat is the molality of the solution a 075degC b 15 m
187
Electrolytes and Colligative PropertiesEarly investigators confused by experiments where certain substances depressed freezing point or elevated boiling point more than expected
188
189
Ex 01 m solution of NaCl lowers freezing point of solvent almost twice as much as 01 m solution of sucrose 01 m CaCl2 lowers fp almost 3 times as much as 01 m solution of sucrose
To understand why this happens contrast behavior or sucrose with NaCl in aqueous solution Each sucrose dissolves to make only 1 particle in solution So 1 mol sucrose dissolves to 1 mol particles 1 mol NaCl dissolves to make 2 moles of particles (1 mol Na+ and 1 mol Cl-)
190
191
Calculated Values for Electrolyte SolutionsColligative properties depend on total concentration of solute particles regardless of their identity Electrolytes cause changes in colligative properties proportional to total molality in terms of all dissolves particles instead of formula units
192
What about barium nitrate Ba(NO3)2 Each mole of barium nitrate yields 3 mol of ions in solution Ba(NO3)2(s) rarr Ba2+(aq) + 2NO3-(aq) You would expect solution of given molality to lower fp of solvent 3 times as much as nonelectrolytic solution of same molality
193
Sample Problem
What is the expected change in the freezing point of water in a solution of 625 g of barium nitrate Ba(NO3)2 in 100 kg of water
194
1 AnalyzeGiven solute mass and formula = 625 g Ba(NO3)2 solvent mass and identity = 100 kg water Δtf = Kfm
Unknown expected freezing-point depression
195
2 PlanThe molality can be calculated by converting the solute mass to moles and then dividing by the number of kilograms of solvent That molality is in terms of formula units of Ba(NO3)2 and must be converted to molality in terms of dissociated ions in solution It must be multiplied by the number of moles of ions produced per mole of formula unit This adjusted molality can then be used to calculate the freezing-point depression
196
197
198
3 Compute
Practice Problem
What is the expected freezing-point depression for a solution that contains 20 mol of magnesium sulfate dissolved in 10 kg of water
minus74degC199
What is the expected boiling-point elevation of water for a solution that contains 150 g of sodium chloride dissolved in 10 kg of water 27degC The freezing point of an aqueous sodium chloride solution is minus200degC What is the molality of the solution 54 m NaCl
200
Actual Values for Electrolyte SolutionsRemember values calculated are only expected values Actual values of colligative properties for all strong electrolytes are almost what would be expected Difference between expected and actual values caused by attractive forces that exist between dissociated ions in aqueous solution
201
Attraction between hydrated ions in solution is small compared with those in crystalline solid However forces of attraction donrsquot interfere with movements of aqueous ions More concentrated solution means ions closer together and attraction is greater Also depends on charge of ions ndash higher charge greater attraction
202
Nonpolar substances (fat oil grease) are generally quite soluble in nonpolar liquids (carbon tetrachloride toluene gasoline)
Only attractions between nonpolar molecules are weak London forces
52
London Dispersion Forces
bc e- are constantly moving at any time there may be uneven distribution of them around a nonpolar atom Temporarily creates positive end and negative end
53
54
London dispersion forces - intermolecular attractions resulting from constant motion of e- and creation of instantaneous dipoles
55
Liquids that dissolve freely in one another in any ratio are said to be completely miscible Benzene and carbon tetrachloride (both nonpolar) are completely miscible
Nonpolar molecules of these two apply no strong forces of attraction or repulsion and molecules mix freely
56
Ethanol and water are also completely miscible -OH group on ethanol is slightly polar
Can form H-bonds with water as well as other ethanol molecules Intermolecular forces in mixture are very similar to those in pure liquids so they are mutually soluble in any amount
57
Gasoline contains mainly nonpolar hydrocarbons
Excellent solvent for fats oils greases
Major intermolecular forces are weak London forces
58
Ethanol less polar than water more than carbon tetrachloride
Better solvent for less-polar substances bc of nonpolar region
59
Effects of Pressure on Solubility
Little effect on solids or liquids Increase pressure - increase solubilities of gases in liquids
60
When gas in contact with surface of liquid gas molecules can enter liquid As amount of dissolve gas increases some molecules start to escape and reenter gas phase
Eq eventually established between rates of enteringleaving gas phase
61
As long as eq undisturbed solubility of gas in liquid unchanged at given pressure
Gas + solvent solution
62
Increasing pressure of solute gas above solution puts stress on eq Molecules collide with surface more often Increase in pressure partially offset by increase in rate of gas molecules entering solution In turn increase in amount of dissolved gas causes increase in rate molecules escape liquid and becomes gas
63
Eventually eq restored at higher gas solubility
Expected from Le Chatelierrsquos principle
Increase in gas pressure causes eq to shift so fewer molecules are in gas phase
64
Henryrsquos Law
Henryrsquos law - solubility of a gas in liquid is directly proportional to the partial pressure of that gas on the surface of the liquid
Applies to gas-liquid solutions at constant temp
65
When mixture of ideal gases is limited in constant volume at constant temp each gas applies same pressure it would apply if it occupied the space alone
Assuming gases do not react each gas dissolves to the extent it would if no other gases were there
66
In carbonated drinks (Coke Pepsi etc) solubility of CO2 increased by increasing pressure
At bottling factory CO2 gas forced into solution of flavored water at pressure of 5-10 atm Gas-in-liquid solution then sealed in bottlescans
67
When cap removed pressure reduced to 1 atm and some CO2 escapes as gas bubbles Effervescence - rapid escape of a gas from a liquid in which it is dissolved
68
Effects of Temperature on Solubility
Letrsquos consider gas solubility Increasing temperature usually decreases gas solubility As temp increases average kinetic energy of molecules in solution increases Greater number of solute molecules escape from attraction of solvent and return to gas phase 69
At higher temps eq is reached with fewer gas molecules in solution Gases generally less soluble
70
Effect of temp on solubility of solids in liquids more difficult to predict
Often increasing temp increases solubility
However the same temp increase can result in large increase in solubility in one case and only slight increase in the next
71
72
enthalpies of Solution
Formation of solution comes with energy change Dissolve KI in water container feels cold Dissolve LiCl in water feels hot Formation of solid-liquid solution can absorb or release heat
73
During formation of solution solvent and solute particles experience changes in forces attracting them to other particles Before dissolving begins solvent molecules held by intermolecular forces (solvent-solvent attraction) In solute molecules held by Ifs (solute-solute attraction)
74
Energy is required to separate solute molecules and solvent molecules from their neighbors
A solute particle that is surrounded by solvent molecules is said to be solvated
75
Step 1 solute particles become separated from solid (energy absorbed) Step 2 solvent particles move apart to allow solute particles to enter liquid (energy absorbed)
76
Step 3 solvent particles attracted to and solvate solute particles (energy released)
77
Heat of solution - net amount of heat energy absorbed or released when a specific amount of solute dissolves in a solvent
Heat of solution negative (heat is released) if sums of energy in steps 1 and 2 is less than step 3
If 1+2 gt 3 HOS is positive (heat absorbed)78
In gaseous state molecules are so far apart there are nearly no forces between them Solute-solute interaction has little effect on heat of solution of a gas Energy released when gas dissolved in liquid bc attraction between solute gas and solvent molecules greater than energy needed to separate solvent molecules
79
Concentration of Solutions80
Concentration - measure of amount of solute in given amount of solvent or solution ldquodiluterdquo just means small amount of solute ldquoconcentratedrdquo just means relatively large amount of solute Unrelated to degree to which solution is saturated Ex Saturated solution of substance not very soluble might be dilute
81
Molarity
Molarity - number of moles of solute in one liter of solution To find molarity must know molar mass A ldquoone molarrdquo solution of NaOH is 1 mole NaOH per liter of solution (1 M)
82
1 mol NaOH = 400 g If this quantity dissolve in enough water to make EXACTLY 100 L solution solution is 1 M If 200 g NaOH dissolve in enough to make 100 L solution what is molarity 0500 M
83
1 M solution is not made by adding 1 mol solute to 1 L solvent That would be more than 1 L Instead 1 mol solute dissolved in less than 1 L Then diluted with more solvent to bring TOTAL VOLUME to 1 L
84
Practice ProblemYou have 340 L of solution that contains 900 g sodium chloride What is the molarity of that solution 0440 M NaCl You have 08 L of a 05 M HCl solution How many moles of HCl does this solution contain 04 mol HCl
85
What is the molarity of a solution composed of 585 g of potassium iodide dissolved in enough water to make a 0125 L of solution 0282 M KI
86
How many moles of H2SO4 are present in 0500 L of a 0150 M H2SO4 solution 00750 mol What volume of 300 M NaCl is needed for reaction that requires 1463 g of NaCl 0834 L
87
Molality
Molality - concentration of solution expressed in moles of solute per kilogram of solvent Solution that contains 1 mol solute (NaOH) dissolved in exactly 1 kg of solvent is ldquoone molalrdquo solution (1 m NaOH)
88
1 mol NaOH = 400 g 400 g NaOH dissolved in 1 kg water is 1 m NaOH
200 g NaOH in 1 kg water = 0500 m NaOH
89
Practice Problem
A solution was prepared by dissolving 171 g sucrose (C12H22O11) in 125 g of water Find the molal concentration of this solution 0400 m C12H22O11
90
A solution of iodine in carbon tetrachloride is used when iodine is needed for certain chemical tests How much iodine must be added to prepare a 0480 m solution of iodine in CCl4 if 1000 g of CCl4 is used 122 g I2
91
What is the molality of a solution composed of 255 g acetone (CH3)2CO dissolved in 200 g of water 22 m acetone What quantity in grams of methanol CH3OH is required to prepare a 0244 m solution in 400 g water 312 g methanol
92
How many grams of AgNO3 are needed to prepare a 0125 m solution in 250 mL of water 531 g AgNO3
What is the molality of a solution containing 182 g HCl and 250 g water 200 m
93
Ions in Aqueous Solutions and Colligative Properties
94
95
compounds in aqueous solutions
DissociationWhen compound made from ions dissolves in water ions separate Dissociation - separation of ions that occurs when an ionic compound dissolves
96
Notice number of ions made per formula unit in equations 1 formula unit of NaCl gives 2 units of ions in solution 1 formula unit of CaCl2 gives 3 units of ions
97
Assuming 100 dissociation solution that contains 1 mol NaCl contains 1 mol Na+ and 1 mol Cl- Can assume 100 dissociation of all soluble ionic compounds
98
Practice Problem
Write the equation for the dissolution of aluminum sulfate in water How many moles of aluminum ions are made by dissolving 1 mol aluminum sulfate What is the total number of moles of ions made by dissolving 1 mol of aluminum sulfate
99
1 AnalyzeGiven Amount of solute = 1 mol Al2(SO4)3 Solvent identity = water Unknown a moles of aluminum ions and sulfate ions Total number of moles of solute ions produced
100
2 Plan
The coefficients in the balanced dissociation equation will tell mole relationships You can use the equation to find out the number of moles of solute ions produced
101
3 Compute
a
b102
Write the equation for the dissolution of each of the following in water and then determine the number of moles of each ions made as well as the total number of ions made a 1 mol ammonium chloride b 1 mol sodium sulfide c 05 mol barium nitrate
103
a 1 mol ammonium chloride
1 mol NH4+ 1 mol Cl- 2 mol total ions
104
b 1 mol sodium sulfide
2 mol Na+ 1 mol S-2 3 mol total ions
105
c 05 mol barium nitrate
05 mol Ba+2 1 mol NO3- 15 mol total ions
106
Precipitation Reactions
Even though no compound is completely insoluble compounds of very low solubility can be considered insoluble for practical purposes There are general guidelines to help predict whether a compound made of certain combination of ions is soluble
107
General Solubility Guidelines
Sodium potassium and ammonium compounds are soluble in water
108
Nitrates acetates and chlorates are soluble
109
110
Most chlorides are soluble except those of silver mercury (I) and lead Lead (II) chloride is soluble in hot water
Most sulfates are soluble except those of barium strontium lead calcium and mercury
111
Most carbonates phosphates and silicates are insoluble except those of sodium potassium and ammonium
112
Most sulfides are insoluble except those of calcium strontium sodium potassium and ammonium
113
Is calcium phosphate Ca3(PO4)2 soluble or insoluble
Not one of the exceptions so it is insoluble
Dissociation equations are not written for insoluble compounds
114
Guidelines also useful in predicting what will happen if 2 solutions of 2 different soluble compounds are mixed
If mixing results in combination of ions that forms insoluble compound a double-replacement and precipitation reaction will happen
115
Will precipitate form when solutions of ammonium sulfide and cadmium nitrate are combined You can tell calcium nitrate is soluble from the guidelines (Most nitrates acetates and chlorates are soluble) You can also tell ammonium sulfide is soluble (Most sulfides are insoluble except those of calcium strontium sodium potassium and ammonium)
116
117
To decide whether precipitate can form must know solubilities of two possible products
Ammonium nitrate ndash soluble (Most nitrates acetates and chlorates are soluble) Cadmium sulfide ndash insoluble (Most sulfides are insoluble except those of calcium strontium sodium potassium and ammonium)
118
bc one product is insoluble double-replacement and precipitation reaction will happen (NH4)2S (aq) + Cd(NO3)2 (aq) rarr NH4NO3 (aq) + CdS (s)
119
Net Ionic Equations
Reactions of ions in aqueous solution usually represented by net ionic equations instead of formula equations Net ionic equation - includes only those compounds and ions that undergo a chemical change in a reaction in an aqueous solution
120
121
To write NIE (net ionic equation) first convert chemical equation into overall ionic equation All soluble ionic compounds shown as dissociated ions in solution Precipitates shown as solids
122
Notice ammonium and nitrate ions appear on both sides They havenrsquot gone through any chemical change Spectator ions - ions that do not take part in a chemical reaction and are found in solution both before and after the reaction
123
To change ionic equation to NIE eliminate spectator ions Cd+2(aq) + S-2(aq) rarr CdS(s)
This NIE applies to any reaction in which precipitate of CdS forms
124
Practice Problem
Identify the precipitate that forms when aqueous solutions of zinc nitrate and ammonium sulfide are combined Write the equation for the possible double-replacement reaction Then write the formula equation overall ionic equation and the net ionic equation
125
1 AnalyzeGiven Identity of reactants zinc nitrate and ammonium sulfide Reaction medium aqueous solution Unknown a equation for the possible double-replacement reaction b identity of the precipitate c formula equation d overall ionic equation e net ionic equation
126
2 PlanWrite the possible double-replacement reaction between Zn(NO3)2 and (NH4)2S Use the rules to determine if any of the products will precipitate Write a formula equation and overall net ionic equation Then cancel spectator ions to make net ionic equation
127
3 Computea possible double-replacement reaction
b rules show that zinc sulfide is not soluble so will precipitate (ammonium nitrate is soluble)
c formula equation128
d overall ionic equation
e net ionic equation129
Will a precipitate form if solutions of potassium sulfate and barium nitrate are combined If so write the net ionic equation for the reaction
Yes Ba+2(aq) + SO4-2(aq) rarr BaSO4(s)
130
Will a precipitate form if solutions of potassium nitrate and magnesium sulfate are combined If so write the net ionic equation for the reaction
No 131
Will a precipitate form if solutions of barium chloride and sodium sulfate are combined If so identify the spectator ions and write the net ionic equation for the reaction
Yes Na+ and Cl- Ba+2(aq) + SO4-2(aq) rarr BaSO4(s)
132
Write the net ionic equation for the precipitation of nickel(II) sulfide
Ni+2(aq) + S-2(aq) rarr NiS(s)
133
IonizationSome molecular compounds can also form ions in solution Usually polar Ionization - ions formed from solute molecules by action of the solvent (creation of ions where there were none) Ionization different from dissociation
134
Dissociation ionic compounds dissolve and ions already present separate Ionization ions formed where non existed before
Ions formed are hydrated Heat released during hydration of ions gives enough energy to break covalent bonds
135
Extent to which solute ionizes depends on strength of bonds within molecules of solute and strength of attraction between solute and solvent molecules
136
HCl ndash molecular compound that ionizes in aqueous solution Contains highly polar bond Attraction between polar HCl molecule and polar water strong enough to break HCl bond
137
The Hydronium Ion
H+ ions from HCl attracts other molecules or ions so strongly that it doesnrsquot normally exist alone Ionization of HCl better described as direct transfer of proton from HCl to H2O forming H3O+
138
139
Hydration of H+ to form hydronium is highly exothermic
Energy released gives a lot of energy needed to ionize a molecular solute
Many molecular compounds that ionize in aqueous solution contain hydrogen and form hydronium ion
140
Strong and Weak Electrolytes
Substances that make ions and conduct current in solution are electrolytes HCl is one of series of compounds that has hydrogen and halogen Hydrogen halides are all molecular compounds with single polar-covalent bonds
141
All are gases Very soluble in water All are electrolytes HCl HBr HI strongly conduct current in solution HF only weakly conducts current at same concentration Strength of conduction related to ability to ionize
142
143
Strong Electrolytes
HCl HBr HI are 100 ionized in solution
Strong electrolyte - any compound whose dilute aqueous solutions conduct electricity well this is due to the presence of all or almost all of the dissolve compound in the form of ions
144
Unique characteristic of strong electrolyte is that they yield only ions
Ex Ionic compound may be highly soluble in water and dissociate into ions (NaCl)
Others may not dissolve much but amount that does dissolve exists only as ions
145
Weak ElectrolytesHF dissolves in water to give acid solution called hydrofluoric acid HF bond stronger than bonds between hydrogen and other halogens When HF dissolves some molecule ionize Reverse reaction also happens HF(aq) + H2O(l) H3O+(aq) + F-(aq)
146
HF(aq) + H2O(l) H3O+
(aq) + F-(aq)Concentration of dissolved unionized HF stays high and concentration of ions stays low
Weak electrolyte - any compound whose aqueous solutions conduct electricity poorly this is due to the presence of small amount of the dissolved compound in the form of ions
147
Different from nonelectrolyte ndash NO ions at all Description of electrolyte as strong or weak not related to concentration of solution Electrolytes differ in degree of ionization not amount of solute dissolved
148
149
Colligative properties of solutions
Presence of solute affects properties of solutions Some properties not dependent on nature of dissolved substance but on how may dissolve particles are present
Colligative properties - properties that depend on the concentration of solute particles but not on their identity
150
Vapor-Pressure Lowering
Boiling and freezing point of solution different from pure solvent Nonvolatile solute raises boiling point and lowers freezing point Nonvolatile substance - one that has little tendency to become gas under existing conditions
151
152
To understand why nonvolatile solute changes boiling and freezing point consider equilibrium vapor pressure
Vapor pressure pressure caused by molecules that have escape liquid phase to gas phase
Can be thought of a measure of tendency of molecules to escape from a liquid
153
Addition of sucrose (nonvolatile solute) lowers concentration of water molecules at surface of liquid This lowers tendency of water molecules to leave solution and enter gas phase Vapor pressure of solution is lower than vapor pressure of pure water
154
Aqueous13 solution13 of13 nonvolatile13 solutePure13 water
Nonelectrolyte solutions of same molality have same concentration of particles
Dilute solutions of same solvent and equal molality of any nonelectrolyte solute lower vapor pressure equally
155
Example
1 m aqueous solution of nonelectrolyte glucose lowers vapor pressure of water 55 x 10-4 atm at 25 1 m aqueous solution of sucrose also lowers water vapor pressure to 55 x 10-4 atm at 25
156
157
bc vapor-pressure lowering depends on concentration of nonelectrolyte solute and doesnrsquot depend on the type of solute it is a colligative proprerty
bc vapor pressure has been lowered solution remains liquid over larger temperature range
Lowers freezing point and raises boiling point
158
Can assume changes in boiling and freezing point also depend on concentration of solute
They are colligative properties
159
Freezing-Point Depression
Freezing point of 1-molal solution of any nonelectrolyte solute in water is found (by experiment) to be 186 lower than freezing point of water When 1 mol of nonelectrolyte solute dissolved in 1 kg water freezing point is -186 not 000 160
When 2 mol nonelectrolyte solute dissolved in 1 kg water freezing point is -372
For any concentration of nonelectrolyte solute in water decrease in freezing point can be determined using value of -186m
161
This value called the molal freezing-point contstant (Kf) is the freezing-point depression of the solvent in a 1-molal solution of a nonvolatile nonelectrolyte solute
162
Each solvent has own characteristic molal freezing-point constant Freezing-point depression (Δtf) the difference between the freezing points of the pure solvent and a solute of nonelectrolyte in that solvent and it is directly proportional to the molal concentration of the solution Δtf = Kfm
163
Δtf = Kfm
Kf expressed as m
m expressed in mol solutekg solvent (molality)
164
Sample Problem
What is the freezing point depression of water in a solution of 171 g of sucrose C12H22O11 and 200 g of water What is the actual freezing point of the solution
165
1 AnalyzeGiven Solute mass and chemical formula 171 g C12H22O11 Solvent mass and identity 2000 g water Unknown a freezing-point depression b freezing point of the solution 166
2 Plan
167
168
Calculate
Unknown a freezing-point depression b freezing point of the solution
169
Practice Problem
A water solution containing an unknown quantity of a nonelectrolyte solute is found to have a freezing point of ndash 023degCWhat is the molal concentration of the solution
012 m170
A solution consists of 103 g of the nonelectrolyte glucose C6H12O6 dissolved in 250 g of water What is the freezing-point depression of the solution
minus0426degC171
In a laboratory experiment the freezing point of an aqueous solution of glucose is found to be minus0325degCWhat is the molal concentration of this solution
0175 m172
If 0500 mol of a nonelectrolyte solute are dissolved in 5000 g of ether what is the freezing point of the solution
minus1181degC173
The freezing point of an aqueous solution that contains a nonelectrolyte is minus90degC aWhat is the freezing-point depression of the solution bWhat is the molal concentration of the solution a minus90degC b 48 m
174
Boiling-Point Elevation
Boiling point is temp at which vapor pressure is equal to atmospheric pressure Change in vapor pressure will cause corresponding change in boiling point
175
176
Vapor pressure of nonvolatile solution is lower than vapor pressure of pure solvent More heat will be required to raise vapor pressure of solution Boiling point of solution is higher than boiling point of pure solvent
Molal boiling-point constant (Kb) - boiling-point elevation of the solvent in a 1-molal solution of a nonvolatile nonelectrolyte solute b-p elevation of any nonelectrolyte in water found by experiment to be 051 So molal b-p constant for water is 051m
177
For different solvents b-p elevations of 1-molal solutions have different values (Table 14-2)
Like freezing-point constants values most accurate for dilute solutions
178
Boiling-point elevation Δtb difference between the boiling points of the pure solvent and a nonelectrolyte solution of that solvent and it is directly proportional to the molal concentration of the solution B-p elevation can be calculated using Δtb = Kbm Δtb (boiling point elevation) expressed in m and m expressed in mol solutekg solvent
Sample Problem
What is the boiling-point elevation of a solution made from 200 g of a nonelectrolyte solute and 4000 g of water The molar mass of the solute is 620 gmol
180
1 AnalyzeGiven solute mass = 200 g solute molar mass = 620 gmol solvent mass and identity = 4000 g of water
Unknown boiling-point elevation
181
2 Plan
182
183
3 Compute
Practice Problem
A solution contains 500 g of sucrose C12H22O11 a nonelectrolyte dissolved in 5000 g of water What is the boiling-point elevation
015degC184
A solution contains 4500 g of sucrose C12H22O11 a nonelectrolyte dissolved in 250 g of waterWhat is the boiling point of the solution
1027degC185
If the boiling point elevation of an aqueous solution containing a nonvolatile electrolyte is 102degC what is the molality of the solution
20 m186
The boiling point of an aqueous solution containing a nonvolatile electrolyte is 10075degC aWhat is the boiling-point elevation bWhat is the molality of the solution a 075degC b 15 m
187
Electrolytes and Colligative PropertiesEarly investigators confused by experiments where certain substances depressed freezing point or elevated boiling point more than expected
188
189
Ex 01 m solution of NaCl lowers freezing point of solvent almost twice as much as 01 m solution of sucrose 01 m CaCl2 lowers fp almost 3 times as much as 01 m solution of sucrose
To understand why this happens contrast behavior or sucrose with NaCl in aqueous solution Each sucrose dissolves to make only 1 particle in solution So 1 mol sucrose dissolves to 1 mol particles 1 mol NaCl dissolves to make 2 moles of particles (1 mol Na+ and 1 mol Cl-)
190
191
Calculated Values for Electrolyte SolutionsColligative properties depend on total concentration of solute particles regardless of their identity Electrolytes cause changes in colligative properties proportional to total molality in terms of all dissolves particles instead of formula units
192
What about barium nitrate Ba(NO3)2 Each mole of barium nitrate yields 3 mol of ions in solution Ba(NO3)2(s) rarr Ba2+(aq) + 2NO3-(aq) You would expect solution of given molality to lower fp of solvent 3 times as much as nonelectrolytic solution of same molality
193
Sample Problem
What is the expected change in the freezing point of water in a solution of 625 g of barium nitrate Ba(NO3)2 in 100 kg of water
194
1 AnalyzeGiven solute mass and formula = 625 g Ba(NO3)2 solvent mass and identity = 100 kg water Δtf = Kfm
Unknown expected freezing-point depression
195
2 PlanThe molality can be calculated by converting the solute mass to moles and then dividing by the number of kilograms of solvent That molality is in terms of formula units of Ba(NO3)2 and must be converted to molality in terms of dissociated ions in solution It must be multiplied by the number of moles of ions produced per mole of formula unit This adjusted molality can then be used to calculate the freezing-point depression
196
197
198
3 Compute
Practice Problem
What is the expected freezing-point depression for a solution that contains 20 mol of magnesium sulfate dissolved in 10 kg of water
minus74degC199
What is the expected boiling-point elevation of water for a solution that contains 150 g of sodium chloride dissolved in 10 kg of water 27degC The freezing point of an aqueous sodium chloride solution is minus200degC What is the molality of the solution 54 m NaCl
200
Actual Values for Electrolyte SolutionsRemember values calculated are only expected values Actual values of colligative properties for all strong electrolytes are almost what would be expected Difference between expected and actual values caused by attractive forces that exist between dissociated ions in aqueous solution
201
Attraction between hydrated ions in solution is small compared with those in crystalline solid However forces of attraction donrsquot interfere with movements of aqueous ions More concentrated solution means ions closer together and attraction is greater Also depends on charge of ions ndash higher charge greater attraction
202
London Dispersion Forces
bc e- are constantly moving at any time there may be uneven distribution of them around a nonpolar atom Temporarily creates positive end and negative end
53
54
London dispersion forces - intermolecular attractions resulting from constant motion of e- and creation of instantaneous dipoles
55
Liquids that dissolve freely in one another in any ratio are said to be completely miscible Benzene and carbon tetrachloride (both nonpolar) are completely miscible
Nonpolar molecules of these two apply no strong forces of attraction or repulsion and molecules mix freely
56
Ethanol and water are also completely miscible -OH group on ethanol is slightly polar
Can form H-bonds with water as well as other ethanol molecules Intermolecular forces in mixture are very similar to those in pure liquids so they are mutually soluble in any amount
57
Gasoline contains mainly nonpolar hydrocarbons
Excellent solvent for fats oils greases
Major intermolecular forces are weak London forces
58
Ethanol less polar than water more than carbon tetrachloride
Better solvent for less-polar substances bc of nonpolar region
59
Effects of Pressure on Solubility
Little effect on solids or liquids Increase pressure - increase solubilities of gases in liquids
60
When gas in contact with surface of liquid gas molecules can enter liquid As amount of dissolve gas increases some molecules start to escape and reenter gas phase
Eq eventually established between rates of enteringleaving gas phase
61
As long as eq undisturbed solubility of gas in liquid unchanged at given pressure
Gas + solvent solution
62
Increasing pressure of solute gas above solution puts stress on eq Molecules collide with surface more often Increase in pressure partially offset by increase in rate of gas molecules entering solution In turn increase in amount of dissolved gas causes increase in rate molecules escape liquid and becomes gas
63
Eventually eq restored at higher gas solubility
Expected from Le Chatelierrsquos principle
Increase in gas pressure causes eq to shift so fewer molecules are in gas phase
64
Henryrsquos Law
Henryrsquos law - solubility of a gas in liquid is directly proportional to the partial pressure of that gas on the surface of the liquid
Applies to gas-liquid solutions at constant temp
65
When mixture of ideal gases is limited in constant volume at constant temp each gas applies same pressure it would apply if it occupied the space alone
Assuming gases do not react each gas dissolves to the extent it would if no other gases were there
66
In carbonated drinks (Coke Pepsi etc) solubility of CO2 increased by increasing pressure
At bottling factory CO2 gas forced into solution of flavored water at pressure of 5-10 atm Gas-in-liquid solution then sealed in bottlescans
67
When cap removed pressure reduced to 1 atm and some CO2 escapes as gas bubbles Effervescence - rapid escape of a gas from a liquid in which it is dissolved
68
Effects of Temperature on Solubility
Letrsquos consider gas solubility Increasing temperature usually decreases gas solubility As temp increases average kinetic energy of molecules in solution increases Greater number of solute molecules escape from attraction of solvent and return to gas phase 69
At higher temps eq is reached with fewer gas molecules in solution Gases generally less soluble
70
Effect of temp on solubility of solids in liquids more difficult to predict
Often increasing temp increases solubility
However the same temp increase can result in large increase in solubility in one case and only slight increase in the next
71
72
enthalpies of Solution
Formation of solution comes with energy change Dissolve KI in water container feels cold Dissolve LiCl in water feels hot Formation of solid-liquid solution can absorb or release heat
73
During formation of solution solvent and solute particles experience changes in forces attracting them to other particles Before dissolving begins solvent molecules held by intermolecular forces (solvent-solvent attraction) In solute molecules held by Ifs (solute-solute attraction)
74
Energy is required to separate solute molecules and solvent molecules from their neighbors
A solute particle that is surrounded by solvent molecules is said to be solvated
75
Step 1 solute particles become separated from solid (energy absorbed) Step 2 solvent particles move apart to allow solute particles to enter liquid (energy absorbed)
76
Step 3 solvent particles attracted to and solvate solute particles (energy released)
77
Heat of solution - net amount of heat energy absorbed or released when a specific amount of solute dissolves in a solvent
Heat of solution negative (heat is released) if sums of energy in steps 1 and 2 is less than step 3
If 1+2 gt 3 HOS is positive (heat absorbed)78
In gaseous state molecules are so far apart there are nearly no forces between them Solute-solute interaction has little effect on heat of solution of a gas Energy released when gas dissolved in liquid bc attraction between solute gas and solvent molecules greater than energy needed to separate solvent molecules
79
Concentration of Solutions80
Concentration - measure of amount of solute in given amount of solvent or solution ldquodiluterdquo just means small amount of solute ldquoconcentratedrdquo just means relatively large amount of solute Unrelated to degree to which solution is saturated Ex Saturated solution of substance not very soluble might be dilute
81
Molarity
Molarity - number of moles of solute in one liter of solution To find molarity must know molar mass A ldquoone molarrdquo solution of NaOH is 1 mole NaOH per liter of solution (1 M)
82
1 mol NaOH = 400 g If this quantity dissolve in enough water to make EXACTLY 100 L solution solution is 1 M If 200 g NaOH dissolve in enough to make 100 L solution what is molarity 0500 M
83
1 M solution is not made by adding 1 mol solute to 1 L solvent That would be more than 1 L Instead 1 mol solute dissolved in less than 1 L Then diluted with more solvent to bring TOTAL VOLUME to 1 L
84
Practice ProblemYou have 340 L of solution that contains 900 g sodium chloride What is the molarity of that solution 0440 M NaCl You have 08 L of a 05 M HCl solution How many moles of HCl does this solution contain 04 mol HCl
85
What is the molarity of a solution composed of 585 g of potassium iodide dissolved in enough water to make a 0125 L of solution 0282 M KI
86
How many moles of H2SO4 are present in 0500 L of a 0150 M H2SO4 solution 00750 mol What volume of 300 M NaCl is needed for reaction that requires 1463 g of NaCl 0834 L
87
Molality
Molality - concentration of solution expressed in moles of solute per kilogram of solvent Solution that contains 1 mol solute (NaOH) dissolved in exactly 1 kg of solvent is ldquoone molalrdquo solution (1 m NaOH)
88
1 mol NaOH = 400 g 400 g NaOH dissolved in 1 kg water is 1 m NaOH
200 g NaOH in 1 kg water = 0500 m NaOH
89
Practice Problem
A solution was prepared by dissolving 171 g sucrose (C12H22O11) in 125 g of water Find the molal concentration of this solution 0400 m C12H22O11
90
A solution of iodine in carbon tetrachloride is used when iodine is needed for certain chemical tests How much iodine must be added to prepare a 0480 m solution of iodine in CCl4 if 1000 g of CCl4 is used 122 g I2
91
What is the molality of a solution composed of 255 g acetone (CH3)2CO dissolved in 200 g of water 22 m acetone What quantity in grams of methanol CH3OH is required to prepare a 0244 m solution in 400 g water 312 g methanol
92
How many grams of AgNO3 are needed to prepare a 0125 m solution in 250 mL of water 531 g AgNO3
What is the molality of a solution containing 182 g HCl and 250 g water 200 m
93
Ions in Aqueous Solutions and Colligative Properties
94
95
compounds in aqueous solutions
DissociationWhen compound made from ions dissolves in water ions separate Dissociation - separation of ions that occurs when an ionic compound dissolves
96
Notice number of ions made per formula unit in equations 1 formula unit of NaCl gives 2 units of ions in solution 1 formula unit of CaCl2 gives 3 units of ions
97
Assuming 100 dissociation solution that contains 1 mol NaCl contains 1 mol Na+ and 1 mol Cl- Can assume 100 dissociation of all soluble ionic compounds
98
Practice Problem
Write the equation for the dissolution of aluminum sulfate in water How many moles of aluminum ions are made by dissolving 1 mol aluminum sulfate What is the total number of moles of ions made by dissolving 1 mol of aluminum sulfate
99
1 AnalyzeGiven Amount of solute = 1 mol Al2(SO4)3 Solvent identity = water Unknown a moles of aluminum ions and sulfate ions Total number of moles of solute ions produced
100
2 Plan
The coefficients in the balanced dissociation equation will tell mole relationships You can use the equation to find out the number of moles of solute ions produced
101
3 Compute
a
b102
Write the equation for the dissolution of each of the following in water and then determine the number of moles of each ions made as well as the total number of ions made a 1 mol ammonium chloride b 1 mol sodium sulfide c 05 mol barium nitrate
103
a 1 mol ammonium chloride
1 mol NH4+ 1 mol Cl- 2 mol total ions
104
b 1 mol sodium sulfide
2 mol Na+ 1 mol S-2 3 mol total ions
105
c 05 mol barium nitrate
05 mol Ba+2 1 mol NO3- 15 mol total ions
106
Precipitation Reactions
Even though no compound is completely insoluble compounds of very low solubility can be considered insoluble for practical purposes There are general guidelines to help predict whether a compound made of certain combination of ions is soluble
107
General Solubility Guidelines
Sodium potassium and ammonium compounds are soluble in water
108
Nitrates acetates and chlorates are soluble
109
110
Most chlorides are soluble except those of silver mercury (I) and lead Lead (II) chloride is soluble in hot water
Most sulfates are soluble except those of barium strontium lead calcium and mercury
111
Most carbonates phosphates and silicates are insoluble except those of sodium potassium and ammonium
112
Most sulfides are insoluble except those of calcium strontium sodium potassium and ammonium
113
Is calcium phosphate Ca3(PO4)2 soluble or insoluble
Not one of the exceptions so it is insoluble
Dissociation equations are not written for insoluble compounds
114
Guidelines also useful in predicting what will happen if 2 solutions of 2 different soluble compounds are mixed
If mixing results in combination of ions that forms insoluble compound a double-replacement and precipitation reaction will happen
115
Will precipitate form when solutions of ammonium sulfide and cadmium nitrate are combined You can tell calcium nitrate is soluble from the guidelines (Most nitrates acetates and chlorates are soluble) You can also tell ammonium sulfide is soluble (Most sulfides are insoluble except those of calcium strontium sodium potassium and ammonium)
116
117
To decide whether precipitate can form must know solubilities of two possible products
Ammonium nitrate ndash soluble (Most nitrates acetates and chlorates are soluble) Cadmium sulfide ndash insoluble (Most sulfides are insoluble except those of calcium strontium sodium potassium and ammonium)
118
bc one product is insoluble double-replacement and precipitation reaction will happen (NH4)2S (aq) + Cd(NO3)2 (aq) rarr NH4NO3 (aq) + CdS (s)
119
Net Ionic Equations
Reactions of ions in aqueous solution usually represented by net ionic equations instead of formula equations Net ionic equation - includes only those compounds and ions that undergo a chemical change in a reaction in an aqueous solution
120
121
To write NIE (net ionic equation) first convert chemical equation into overall ionic equation All soluble ionic compounds shown as dissociated ions in solution Precipitates shown as solids
122
Notice ammonium and nitrate ions appear on both sides They havenrsquot gone through any chemical change Spectator ions - ions that do not take part in a chemical reaction and are found in solution both before and after the reaction
123
To change ionic equation to NIE eliminate spectator ions Cd+2(aq) + S-2(aq) rarr CdS(s)
This NIE applies to any reaction in which precipitate of CdS forms
124
Practice Problem
Identify the precipitate that forms when aqueous solutions of zinc nitrate and ammonium sulfide are combined Write the equation for the possible double-replacement reaction Then write the formula equation overall ionic equation and the net ionic equation
125
1 AnalyzeGiven Identity of reactants zinc nitrate and ammonium sulfide Reaction medium aqueous solution Unknown a equation for the possible double-replacement reaction b identity of the precipitate c formula equation d overall ionic equation e net ionic equation
126
2 PlanWrite the possible double-replacement reaction between Zn(NO3)2 and (NH4)2S Use the rules to determine if any of the products will precipitate Write a formula equation and overall net ionic equation Then cancel spectator ions to make net ionic equation
127
3 Computea possible double-replacement reaction
b rules show that zinc sulfide is not soluble so will precipitate (ammonium nitrate is soluble)
c formula equation128
d overall ionic equation
e net ionic equation129
Will a precipitate form if solutions of potassium sulfate and barium nitrate are combined If so write the net ionic equation for the reaction
Yes Ba+2(aq) + SO4-2(aq) rarr BaSO4(s)
130
Will a precipitate form if solutions of potassium nitrate and magnesium sulfate are combined If so write the net ionic equation for the reaction
No 131
Will a precipitate form if solutions of barium chloride and sodium sulfate are combined If so identify the spectator ions and write the net ionic equation for the reaction
Yes Na+ and Cl- Ba+2(aq) + SO4-2(aq) rarr BaSO4(s)
132
Write the net ionic equation for the precipitation of nickel(II) sulfide
Ni+2(aq) + S-2(aq) rarr NiS(s)
133
IonizationSome molecular compounds can also form ions in solution Usually polar Ionization - ions formed from solute molecules by action of the solvent (creation of ions where there were none) Ionization different from dissociation
134
Dissociation ionic compounds dissolve and ions already present separate Ionization ions formed where non existed before
Ions formed are hydrated Heat released during hydration of ions gives enough energy to break covalent bonds
135
Extent to which solute ionizes depends on strength of bonds within molecules of solute and strength of attraction between solute and solvent molecules
136
HCl ndash molecular compound that ionizes in aqueous solution Contains highly polar bond Attraction between polar HCl molecule and polar water strong enough to break HCl bond
137
The Hydronium Ion
H+ ions from HCl attracts other molecules or ions so strongly that it doesnrsquot normally exist alone Ionization of HCl better described as direct transfer of proton from HCl to H2O forming H3O+
138
139
Hydration of H+ to form hydronium is highly exothermic
Energy released gives a lot of energy needed to ionize a molecular solute
Many molecular compounds that ionize in aqueous solution contain hydrogen and form hydronium ion
140
Strong and Weak Electrolytes
Substances that make ions and conduct current in solution are electrolytes HCl is one of series of compounds that has hydrogen and halogen Hydrogen halides are all molecular compounds with single polar-covalent bonds
141
All are gases Very soluble in water All are electrolytes HCl HBr HI strongly conduct current in solution HF only weakly conducts current at same concentration Strength of conduction related to ability to ionize
142
143
Strong Electrolytes
HCl HBr HI are 100 ionized in solution
Strong electrolyte - any compound whose dilute aqueous solutions conduct electricity well this is due to the presence of all or almost all of the dissolve compound in the form of ions
144
Unique characteristic of strong electrolyte is that they yield only ions
Ex Ionic compound may be highly soluble in water and dissociate into ions (NaCl)
Others may not dissolve much but amount that does dissolve exists only as ions
145
Weak ElectrolytesHF dissolves in water to give acid solution called hydrofluoric acid HF bond stronger than bonds between hydrogen and other halogens When HF dissolves some molecule ionize Reverse reaction also happens HF(aq) + H2O(l) H3O+(aq) + F-(aq)
146
HF(aq) + H2O(l) H3O+
(aq) + F-(aq)Concentration of dissolved unionized HF stays high and concentration of ions stays low
Weak electrolyte - any compound whose aqueous solutions conduct electricity poorly this is due to the presence of small amount of the dissolved compound in the form of ions
147
Different from nonelectrolyte ndash NO ions at all Description of electrolyte as strong or weak not related to concentration of solution Electrolytes differ in degree of ionization not amount of solute dissolved
148
149
Colligative properties of solutions
Presence of solute affects properties of solutions Some properties not dependent on nature of dissolved substance but on how may dissolve particles are present
Colligative properties - properties that depend on the concentration of solute particles but not on their identity
150
Vapor-Pressure Lowering
Boiling and freezing point of solution different from pure solvent Nonvolatile solute raises boiling point and lowers freezing point Nonvolatile substance - one that has little tendency to become gas under existing conditions
151
152
To understand why nonvolatile solute changes boiling and freezing point consider equilibrium vapor pressure
Vapor pressure pressure caused by molecules that have escape liquid phase to gas phase
Can be thought of a measure of tendency of molecules to escape from a liquid
153
Addition of sucrose (nonvolatile solute) lowers concentration of water molecules at surface of liquid This lowers tendency of water molecules to leave solution and enter gas phase Vapor pressure of solution is lower than vapor pressure of pure water
154
Aqueous13 solution13 of13 nonvolatile13 solutePure13 water
Nonelectrolyte solutions of same molality have same concentration of particles
Dilute solutions of same solvent and equal molality of any nonelectrolyte solute lower vapor pressure equally
155
Example
1 m aqueous solution of nonelectrolyte glucose lowers vapor pressure of water 55 x 10-4 atm at 25 1 m aqueous solution of sucrose also lowers water vapor pressure to 55 x 10-4 atm at 25
156
157
bc vapor-pressure lowering depends on concentration of nonelectrolyte solute and doesnrsquot depend on the type of solute it is a colligative proprerty
bc vapor pressure has been lowered solution remains liquid over larger temperature range
Lowers freezing point and raises boiling point
158
Can assume changes in boiling and freezing point also depend on concentration of solute
They are colligative properties
159
Freezing-Point Depression
Freezing point of 1-molal solution of any nonelectrolyte solute in water is found (by experiment) to be 186 lower than freezing point of water When 1 mol of nonelectrolyte solute dissolved in 1 kg water freezing point is -186 not 000 160
When 2 mol nonelectrolyte solute dissolved in 1 kg water freezing point is -372
For any concentration of nonelectrolyte solute in water decrease in freezing point can be determined using value of -186m
161
This value called the molal freezing-point contstant (Kf) is the freezing-point depression of the solvent in a 1-molal solution of a nonvolatile nonelectrolyte solute
162
Each solvent has own characteristic molal freezing-point constant Freezing-point depression (Δtf) the difference between the freezing points of the pure solvent and a solute of nonelectrolyte in that solvent and it is directly proportional to the molal concentration of the solution Δtf = Kfm
163
Δtf = Kfm
Kf expressed as m
m expressed in mol solutekg solvent (molality)
164
Sample Problem
What is the freezing point depression of water in a solution of 171 g of sucrose C12H22O11 and 200 g of water What is the actual freezing point of the solution
165
1 AnalyzeGiven Solute mass and chemical formula 171 g C12H22O11 Solvent mass and identity 2000 g water Unknown a freezing-point depression b freezing point of the solution 166
2 Plan
167
168
Calculate
Unknown a freezing-point depression b freezing point of the solution
169
Practice Problem
A water solution containing an unknown quantity of a nonelectrolyte solute is found to have a freezing point of ndash 023degCWhat is the molal concentration of the solution
012 m170
A solution consists of 103 g of the nonelectrolyte glucose C6H12O6 dissolved in 250 g of water What is the freezing-point depression of the solution
minus0426degC171
In a laboratory experiment the freezing point of an aqueous solution of glucose is found to be minus0325degCWhat is the molal concentration of this solution
0175 m172
If 0500 mol of a nonelectrolyte solute are dissolved in 5000 g of ether what is the freezing point of the solution
minus1181degC173
The freezing point of an aqueous solution that contains a nonelectrolyte is minus90degC aWhat is the freezing-point depression of the solution bWhat is the molal concentration of the solution a minus90degC b 48 m
174
Boiling-Point Elevation
Boiling point is temp at which vapor pressure is equal to atmospheric pressure Change in vapor pressure will cause corresponding change in boiling point
175
176
Vapor pressure of nonvolatile solution is lower than vapor pressure of pure solvent More heat will be required to raise vapor pressure of solution Boiling point of solution is higher than boiling point of pure solvent
Molal boiling-point constant (Kb) - boiling-point elevation of the solvent in a 1-molal solution of a nonvolatile nonelectrolyte solute b-p elevation of any nonelectrolyte in water found by experiment to be 051 So molal b-p constant for water is 051m
177
For different solvents b-p elevations of 1-molal solutions have different values (Table 14-2)
Like freezing-point constants values most accurate for dilute solutions
178
Boiling-point elevation Δtb difference between the boiling points of the pure solvent and a nonelectrolyte solution of that solvent and it is directly proportional to the molal concentration of the solution B-p elevation can be calculated using Δtb = Kbm Δtb (boiling point elevation) expressed in m and m expressed in mol solutekg solvent
Sample Problem
What is the boiling-point elevation of a solution made from 200 g of a nonelectrolyte solute and 4000 g of water The molar mass of the solute is 620 gmol
180
1 AnalyzeGiven solute mass = 200 g solute molar mass = 620 gmol solvent mass and identity = 4000 g of water
Unknown boiling-point elevation
181
2 Plan
182
183
3 Compute
Practice Problem
A solution contains 500 g of sucrose C12H22O11 a nonelectrolyte dissolved in 5000 g of water What is the boiling-point elevation
015degC184
A solution contains 4500 g of sucrose C12H22O11 a nonelectrolyte dissolved in 250 g of waterWhat is the boiling point of the solution
1027degC185
If the boiling point elevation of an aqueous solution containing a nonvolatile electrolyte is 102degC what is the molality of the solution
20 m186
The boiling point of an aqueous solution containing a nonvolatile electrolyte is 10075degC aWhat is the boiling-point elevation bWhat is the molality of the solution a 075degC b 15 m
187
Electrolytes and Colligative PropertiesEarly investigators confused by experiments where certain substances depressed freezing point or elevated boiling point more than expected
188
189
Ex 01 m solution of NaCl lowers freezing point of solvent almost twice as much as 01 m solution of sucrose 01 m CaCl2 lowers fp almost 3 times as much as 01 m solution of sucrose
To understand why this happens contrast behavior or sucrose with NaCl in aqueous solution Each sucrose dissolves to make only 1 particle in solution So 1 mol sucrose dissolves to 1 mol particles 1 mol NaCl dissolves to make 2 moles of particles (1 mol Na+ and 1 mol Cl-)
190
191
Calculated Values for Electrolyte SolutionsColligative properties depend on total concentration of solute particles regardless of their identity Electrolytes cause changes in colligative properties proportional to total molality in terms of all dissolves particles instead of formula units
192
What about barium nitrate Ba(NO3)2 Each mole of barium nitrate yields 3 mol of ions in solution Ba(NO3)2(s) rarr Ba2+(aq) + 2NO3-(aq) You would expect solution of given molality to lower fp of solvent 3 times as much as nonelectrolytic solution of same molality
193
Sample Problem
What is the expected change in the freezing point of water in a solution of 625 g of barium nitrate Ba(NO3)2 in 100 kg of water
194
1 AnalyzeGiven solute mass and formula = 625 g Ba(NO3)2 solvent mass and identity = 100 kg water Δtf = Kfm
Unknown expected freezing-point depression
195
2 PlanThe molality can be calculated by converting the solute mass to moles and then dividing by the number of kilograms of solvent That molality is in terms of formula units of Ba(NO3)2 and must be converted to molality in terms of dissociated ions in solution It must be multiplied by the number of moles of ions produced per mole of formula unit This adjusted molality can then be used to calculate the freezing-point depression
196
197
198
3 Compute
Practice Problem
What is the expected freezing-point depression for a solution that contains 20 mol of magnesium sulfate dissolved in 10 kg of water
minus74degC199
What is the expected boiling-point elevation of water for a solution that contains 150 g of sodium chloride dissolved in 10 kg of water 27degC The freezing point of an aqueous sodium chloride solution is minus200degC What is the molality of the solution 54 m NaCl
200
Actual Values for Electrolyte SolutionsRemember values calculated are only expected values Actual values of colligative properties for all strong electrolytes are almost what would be expected Difference between expected and actual values caused by attractive forces that exist between dissociated ions in aqueous solution
201
Attraction between hydrated ions in solution is small compared with those in crystalline solid However forces of attraction donrsquot interfere with movements of aqueous ions More concentrated solution means ions closer together and attraction is greater Also depends on charge of ions ndash higher charge greater attraction
202
54
London dispersion forces - intermolecular attractions resulting from constant motion of e- and creation of instantaneous dipoles
55
Liquids that dissolve freely in one another in any ratio are said to be completely miscible Benzene and carbon tetrachloride (both nonpolar) are completely miscible
Nonpolar molecules of these two apply no strong forces of attraction or repulsion and molecules mix freely
56
Ethanol and water are also completely miscible -OH group on ethanol is slightly polar
Can form H-bonds with water as well as other ethanol molecules Intermolecular forces in mixture are very similar to those in pure liquids so they are mutually soluble in any amount
57
Gasoline contains mainly nonpolar hydrocarbons
Excellent solvent for fats oils greases
Major intermolecular forces are weak London forces
58
Ethanol less polar than water more than carbon tetrachloride
Better solvent for less-polar substances bc of nonpolar region
59
Effects of Pressure on Solubility
Little effect on solids or liquids Increase pressure - increase solubilities of gases in liquids
60
When gas in contact with surface of liquid gas molecules can enter liquid As amount of dissolve gas increases some molecules start to escape and reenter gas phase
Eq eventually established between rates of enteringleaving gas phase
61
As long as eq undisturbed solubility of gas in liquid unchanged at given pressure
Gas + solvent solution
62
Increasing pressure of solute gas above solution puts stress on eq Molecules collide with surface more often Increase in pressure partially offset by increase in rate of gas molecules entering solution In turn increase in amount of dissolved gas causes increase in rate molecules escape liquid and becomes gas
63
Eventually eq restored at higher gas solubility
Expected from Le Chatelierrsquos principle
Increase in gas pressure causes eq to shift so fewer molecules are in gas phase
64
Henryrsquos Law
Henryrsquos law - solubility of a gas in liquid is directly proportional to the partial pressure of that gas on the surface of the liquid
Applies to gas-liquid solutions at constant temp
65
When mixture of ideal gases is limited in constant volume at constant temp each gas applies same pressure it would apply if it occupied the space alone
Assuming gases do not react each gas dissolves to the extent it would if no other gases were there
66
In carbonated drinks (Coke Pepsi etc) solubility of CO2 increased by increasing pressure
At bottling factory CO2 gas forced into solution of flavored water at pressure of 5-10 atm Gas-in-liquid solution then sealed in bottlescans
67
When cap removed pressure reduced to 1 atm and some CO2 escapes as gas bubbles Effervescence - rapid escape of a gas from a liquid in which it is dissolved
68
Effects of Temperature on Solubility
Letrsquos consider gas solubility Increasing temperature usually decreases gas solubility As temp increases average kinetic energy of molecules in solution increases Greater number of solute molecules escape from attraction of solvent and return to gas phase 69
At higher temps eq is reached with fewer gas molecules in solution Gases generally less soluble
70
Effect of temp on solubility of solids in liquids more difficult to predict
Often increasing temp increases solubility
However the same temp increase can result in large increase in solubility in one case and only slight increase in the next
71
72
enthalpies of Solution
Formation of solution comes with energy change Dissolve KI in water container feels cold Dissolve LiCl in water feels hot Formation of solid-liquid solution can absorb or release heat
73
During formation of solution solvent and solute particles experience changes in forces attracting them to other particles Before dissolving begins solvent molecules held by intermolecular forces (solvent-solvent attraction) In solute molecules held by Ifs (solute-solute attraction)
74
Energy is required to separate solute molecules and solvent molecules from their neighbors
A solute particle that is surrounded by solvent molecules is said to be solvated
75
Step 1 solute particles become separated from solid (energy absorbed) Step 2 solvent particles move apart to allow solute particles to enter liquid (energy absorbed)
76
Step 3 solvent particles attracted to and solvate solute particles (energy released)
77
Heat of solution - net amount of heat energy absorbed or released when a specific amount of solute dissolves in a solvent
Heat of solution negative (heat is released) if sums of energy in steps 1 and 2 is less than step 3
If 1+2 gt 3 HOS is positive (heat absorbed)78
In gaseous state molecules are so far apart there are nearly no forces between them Solute-solute interaction has little effect on heat of solution of a gas Energy released when gas dissolved in liquid bc attraction between solute gas and solvent molecules greater than energy needed to separate solvent molecules
79
Concentration of Solutions80
Concentration - measure of amount of solute in given amount of solvent or solution ldquodiluterdquo just means small amount of solute ldquoconcentratedrdquo just means relatively large amount of solute Unrelated to degree to which solution is saturated Ex Saturated solution of substance not very soluble might be dilute
81
Molarity
Molarity - number of moles of solute in one liter of solution To find molarity must know molar mass A ldquoone molarrdquo solution of NaOH is 1 mole NaOH per liter of solution (1 M)
82
1 mol NaOH = 400 g If this quantity dissolve in enough water to make EXACTLY 100 L solution solution is 1 M If 200 g NaOH dissolve in enough to make 100 L solution what is molarity 0500 M
83
1 M solution is not made by adding 1 mol solute to 1 L solvent That would be more than 1 L Instead 1 mol solute dissolved in less than 1 L Then diluted with more solvent to bring TOTAL VOLUME to 1 L
84
Practice ProblemYou have 340 L of solution that contains 900 g sodium chloride What is the molarity of that solution 0440 M NaCl You have 08 L of a 05 M HCl solution How many moles of HCl does this solution contain 04 mol HCl
85
What is the molarity of a solution composed of 585 g of potassium iodide dissolved in enough water to make a 0125 L of solution 0282 M KI
86
How many moles of H2SO4 are present in 0500 L of a 0150 M H2SO4 solution 00750 mol What volume of 300 M NaCl is needed for reaction that requires 1463 g of NaCl 0834 L
87
Molality
Molality - concentration of solution expressed in moles of solute per kilogram of solvent Solution that contains 1 mol solute (NaOH) dissolved in exactly 1 kg of solvent is ldquoone molalrdquo solution (1 m NaOH)
88
1 mol NaOH = 400 g 400 g NaOH dissolved in 1 kg water is 1 m NaOH
200 g NaOH in 1 kg water = 0500 m NaOH
89
Practice Problem
A solution was prepared by dissolving 171 g sucrose (C12H22O11) in 125 g of water Find the molal concentration of this solution 0400 m C12H22O11
90
A solution of iodine in carbon tetrachloride is used when iodine is needed for certain chemical tests How much iodine must be added to prepare a 0480 m solution of iodine in CCl4 if 1000 g of CCl4 is used 122 g I2
91
What is the molality of a solution composed of 255 g acetone (CH3)2CO dissolved in 200 g of water 22 m acetone What quantity in grams of methanol CH3OH is required to prepare a 0244 m solution in 400 g water 312 g methanol
92
How many grams of AgNO3 are needed to prepare a 0125 m solution in 250 mL of water 531 g AgNO3
What is the molality of a solution containing 182 g HCl and 250 g water 200 m
93
Ions in Aqueous Solutions and Colligative Properties
94
95
compounds in aqueous solutions
DissociationWhen compound made from ions dissolves in water ions separate Dissociation - separation of ions that occurs when an ionic compound dissolves
96
Notice number of ions made per formula unit in equations 1 formula unit of NaCl gives 2 units of ions in solution 1 formula unit of CaCl2 gives 3 units of ions
97
Assuming 100 dissociation solution that contains 1 mol NaCl contains 1 mol Na+ and 1 mol Cl- Can assume 100 dissociation of all soluble ionic compounds
98
Practice Problem
Write the equation for the dissolution of aluminum sulfate in water How many moles of aluminum ions are made by dissolving 1 mol aluminum sulfate What is the total number of moles of ions made by dissolving 1 mol of aluminum sulfate
99
1 AnalyzeGiven Amount of solute = 1 mol Al2(SO4)3 Solvent identity = water Unknown a moles of aluminum ions and sulfate ions Total number of moles of solute ions produced
100
2 Plan
The coefficients in the balanced dissociation equation will tell mole relationships You can use the equation to find out the number of moles of solute ions produced
101
3 Compute
a
b102
Write the equation for the dissolution of each of the following in water and then determine the number of moles of each ions made as well as the total number of ions made a 1 mol ammonium chloride b 1 mol sodium sulfide c 05 mol barium nitrate
103
a 1 mol ammonium chloride
1 mol NH4+ 1 mol Cl- 2 mol total ions
104
b 1 mol sodium sulfide
2 mol Na+ 1 mol S-2 3 mol total ions
105
c 05 mol barium nitrate
05 mol Ba+2 1 mol NO3- 15 mol total ions
106
Precipitation Reactions
Even though no compound is completely insoluble compounds of very low solubility can be considered insoluble for practical purposes There are general guidelines to help predict whether a compound made of certain combination of ions is soluble
107
General Solubility Guidelines
Sodium potassium and ammonium compounds are soluble in water
108
Nitrates acetates and chlorates are soluble
109
110
Most chlorides are soluble except those of silver mercury (I) and lead Lead (II) chloride is soluble in hot water
Most sulfates are soluble except those of barium strontium lead calcium and mercury
111
Most carbonates phosphates and silicates are insoluble except those of sodium potassium and ammonium
112
Most sulfides are insoluble except those of calcium strontium sodium potassium and ammonium
113
Is calcium phosphate Ca3(PO4)2 soluble or insoluble
Not one of the exceptions so it is insoluble
Dissociation equations are not written for insoluble compounds
114
Guidelines also useful in predicting what will happen if 2 solutions of 2 different soluble compounds are mixed
If mixing results in combination of ions that forms insoluble compound a double-replacement and precipitation reaction will happen
115
Will precipitate form when solutions of ammonium sulfide and cadmium nitrate are combined You can tell calcium nitrate is soluble from the guidelines (Most nitrates acetates and chlorates are soluble) You can also tell ammonium sulfide is soluble (Most sulfides are insoluble except those of calcium strontium sodium potassium and ammonium)
116
117
To decide whether precipitate can form must know solubilities of two possible products
Ammonium nitrate ndash soluble (Most nitrates acetates and chlorates are soluble) Cadmium sulfide ndash insoluble (Most sulfides are insoluble except those of calcium strontium sodium potassium and ammonium)
118
bc one product is insoluble double-replacement and precipitation reaction will happen (NH4)2S (aq) + Cd(NO3)2 (aq) rarr NH4NO3 (aq) + CdS (s)
119
Net Ionic Equations
Reactions of ions in aqueous solution usually represented by net ionic equations instead of formula equations Net ionic equation - includes only those compounds and ions that undergo a chemical change in a reaction in an aqueous solution
120
121
To write NIE (net ionic equation) first convert chemical equation into overall ionic equation All soluble ionic compounds shown as dissociated ions in solution Precipitates shown as solids
122
Notice ammonium and nitrate ions appear on both sides They havenrsquot gone through any chemical change Spectator ions - ions that do not take part in a chemical reaction and are found in solution both before and after the reaction
123
To change ionic equation to NIE eliminate spectator ions Cd+2(aq) + S-2(aq) rarr CdS(s)
This NIE applies to any reaction in which precipitate of CdS forms
124
Practice Problem
Identify the precipitate that forms when aqueous solutions of zinc nitrate and ammonium sulfide are combined Write the equation for the possible double-replacement reaction Then write the formula equation overall ionic equation and the net ionic equation
125
1 AnalyzeGiven Identity of reactants zinc nitrate and ammonium sulfide Reaction medium aqueous solution Unknown a equation for the possible double-replacement reaction b identity of the precipitate c formula equation d overall ionic equation e net ionic equation
126
2 PlanWrite the possible double-replacement reaction between Zn(NO3)2 and (NH4)2S Use the rules to determine if any of the products will precipitate Write a formula equation and overall net ionic equation Then cancel spectator ions to make net ionic equation
127
3 Computea possible double-replacement reaction
b rules show that zinc sulfide is not soluble so will precipitate (ammonium nitrate is soluble)
c formula equation128
d overall ionic equation
e net ionic equation129
Will a precipitate form if solutions of potassium sulfate and barium nitrate are combined If so write the net ionic equation for the reaction
Yes Ba+2(aq) + SO4-2(aq) rarr BaSO4(s)
130
Will a precipitate form if solutions of potassium nitrate and magnesium sulfate are combined If so write the net ionic equation for the reaction
No 131
Will a precipitate form if solutions of barium chloride and sodium sulfate are combined If so identify the spectator ions and write the net ionic equation for the reaction
Yes Na+ and Cl- Ba+2(aq) + SO4-2(aq) rarr BaSO4(s)
132
Write the net ionic equation for the precipitation of nickel(II) sulfide
Ni+2(aq) + S-2(aq) rarr NiS(s)
133
IonizationSome molecular compounds can also form ions in solution Usually polar Ionization - ions formed from solute molecules by action of the solvent (creation of ions where there were none) Ionization different from dissociation
134
Dissociation ionic compounds dissolve and ions already present separate Ionization ions formed where non existed before
Ions formed are hydrated Heat released during hydration of ions gives enough energy to break covalent bonds
135
Extent to which solute ionizes depends on strength of bonds within molecules of solute and strength of attraction between solute and solvent molecules
136
HCl ndash molecular compound that ionizes in aqueous solution Contains highly polar bond Attraction between polar HCl molecule and polar water strong enough to break HCl bond
137
The Hydronium Ion
H+ ions from HCl attracts other molecules or ions so strongly that it doesnrsquot normally exist alone Ionization of HCl better described as direct transfer of proton from HCl to H2O forming H3O+
138
139
Hydration of H+ to form hydronium is highly exothermic
Energy released gives a lot of energy needed to ionize a molecular solute
Many molecular compounds that ionize in aqueous solution contain hydrogen and form hydronium ion
140
Strong and Weak Electrolytes
Substances that make ions and conduct current in solution are electrolytes HCl is one of series of compounds that has hydrogen and halogen Hydrogen halides are all molecular compounds with single polar-covalent bonds
141
All are gases Very soluble in water All are electrolytes HCl HBr HI strongly conduct current in solution HF only weakly conducts current at same concentration Strength of conduction related to ability to ionize
142
143
Strong Electrolytes
HCl HBr HI are 100 ionized in solution
Strong electrolyte - any compound whose dilute aqueous solutions conduct electricity well this is due to the presence of all or almost all of the dissolve compound in the form of ions
144
Unique characteristic of strong electrolyte is that they yield only ions
Ex Ionic compound may be highly soluble in water and dissociate into ions (NaCl)
Others may not dissolve much but amount that does dissolve exists only as ions
145
Weak ElectrolytesHF dissolves in water to give acid solution called hydrofluoric acid HF bond stronger than bonds between hydrogen and other halogens When HF dissolves some molecule ionize Reverse reaction also happens HF(aq) + H2O(l) H3O+(aq) + F-(aq)
146
HF(aq) + H2O(l) H3O+
(aq) + F-(aq)Concentration of dissolved unionized HF stays high and concentration of ions stays low
Weak electrolyte - any compound whose aqueous solutions conduct electricity poorly this is due to the presence of small amount of the dissolved compound in the form of ions
147
Different from nonelectrolyte ndash NO ions at all Description of electrolyte as strong or weak not related to concentration of solution Electrolytes differ in degree of ionization not amount of solute dissolved
148
149
Colligative properties of solutions
Presence of solute affects properties of solutions Some properties not dependent on nature of dissolved substance but on how may dissolve particles are present
Colligative properties - properties that depend on the concentration of solute particles but not on their identity
150
Vapor-Pressure Lowering
Boiling and freezing point of solution different from pure solvent Nonvolatile solute raises boiling point and lowers freezing point Nonvolatile substance - one that has little tendency to become gas under existing conditions
151
152
To understand why nonvolatile solute changes boiling and freezing point consider equilibrium vapor pressure
Vapor pressure pressure caused by molecules that have escape liquid phase to gas phase
Can be thought of a measure of tendency of molecules to escape from a liquid
153
Addition of sucrose (nonvolatile solute) lowers concentration of water molecules at surface of liquid This lowers tendency of water molecules to leave solution and enter gas phase Vapor pressure of solution is lower than vapor pressure of pure water
154
Aqueous13 solution13 of13 nonvolatile13 solutePure13 water
Nonelectrolyte solutions of same molality have same concentration of particles
Dilute solutions of same solvent and equal molality of any nonelectrolyte solute lower vapor pressure equally
155
Example
1 m aqueous solution of nonelectrolyte glucose lowers vapor pressure of water 55 x 10-4 atm at 25 1 m aqueous solution of sucrose also lowers water vapor pressure to 55 x 10-4 atm at 25
156
157
bc vapor-pressure lowering depends on concentration of nonelectrolyte solute and doesnrsquot depend on the type of solute it is a colligative proprerty
bc vapor pressure has been lowered solution remains liquid over larger temperature range
Lowers freezing point and raises boiling point
158
Can assume changes in boiling and freezing point also depend on concentration of solute
They are colligative properties
159
Freezing-Point Depression
Freezing point of 1-molal solution of any nonelectrolyte solute in water is found (by experiment) to be 186 lower than freezing point of water When 1 mol of nonelectrolyte solute dissolved in 1 kg water freezing point is -186 not 000 160
When 2 mol nonelectrolyte solute dissolved in 1 kg water freezing point is -372
For any concentration of nonelectrolyte solute in water decrease in freezing point can be determined using value of -186m
161
This value called the molal freezing-point contstant (Kf) is the freezing-point depression of the solvent in a 1-molal solution of a nonvolatile nonelectrolyte solute
162
Each solvent has own characteristic molal freezing-point constant Freezing-point depression (Δtf) the difference between the freezing points of the pure solvent and a solute of nonelectrolyte in that solvent and it is directly proportional to the molal concentration of the solution Δtf = Kfm
163
Δtf = Kfm
Kf expressed as m
m expressed in mol solutekg solvent (molality)
164
Sample Problem
What is the freezing point depression of water in a solution of 171 g of sucrose C12H22O11 and 200 g of water What is the actual freezing point of the solution
165
1 AnalyzeGiven Solute mass and chemical formula 171 g C12H22O11 Solvent mass and identity 2000 g water Unknown a freezing-point depression b freezing point of the solution 166
2 Plan
167
168
Calculate
Unknown a freezing-point depression b freezing point of the solution
169
Practice Problem
A water solution containing an unknown quantity of a nonelectrolyte solute is found to have a freezing point of ndash 023degCWhat is the molal concentration of the solution
012 m170
A solution consists of 103 g of the nonelectrolyte glucose C6H12O6 dissolved in 250 g of water What is the freezing-point depression of the solution
minus0426degC171
In a laboratory experiment the freezing point of an aqueous solution of glucose is found to be minus0325degCWhat is the molal concentration of this solution
0175 m172
If 0500 mol of a nonelectrolyte solute are dissolved in 5000 g of ether what is the freezing point of the solution
minus1181degC173
The freezing point of an aqueous solution that contains a nonelectrolyte is minus90degC aWhat is the freezing-point depression of the solution bWhat is the molal concentration of the solution a minus90degC b 48 m
174
Boiling-Point Elevation
Boiling point is temp at which vapor pressure is equal to atmospheric pressure Change in vapor pressure will cause corresponding change in boiling point
175
176
Vapor pressure of nonvolatile solution is lower than vapor pressure of pure solvent More heat will be required to raise vapor pressure of solution Boiling point of solution is higher than boiling point of pure solvent
Molal boiling-point constant (Kb) - boiling-point elevation of the solvent in a 1-molal solution of a nonvolatile nonelectrolyte solute b-p elevation of any nonelectrolyte in water found by experiment to be 051 So molal b-p constant for water is 051m
177
For different solvents b-p elevations of 1-molal solutions have different values (Table 14-2)
Like freezing-point constants values most accurate for dilute solutions
178
Boiling-point elevation Δtb difference between the boiling points of the pure solvent and a nonelectrolyte solution of that solvent and it is directly proportional to the molal concentration of the solution B-p elevation can be calculated using Δtb = Kbm Δtb (boiling point elevation) expressed in m and m expressed in mol solutekg solvent
Sample Problem
What is the boiling-point elevation of a solution made from 200 g of a nonelectrolyte solute and 4000 g of water The molar mass of the solute is 620 gmol
180
1 AnalyzeGiven solute mass = 200 g solute molar mass = 620 gmol solvent mass and identity = 4000 g of water
Unknown boiling-point elevation
181
2 Plan
182
183
3 Compute
Practice Problem
A solution contains 500 g of sucrose C12H22O11 a nonelectrolyte dissolved in 5000 g of water What is the boiling-point elevation
015degC184
A solution contains 4500 g of sucrose C12H22O11 a nonelectrolyte dissolved in 250 g of waterWhat is the boiling point of the solution
1027degC185
If the boiling point elevation of an aqueous solution containing a nonvolatile electrolyte is 102degC what is the molality of the solution
20 m186
The boiling point of an aqueous solution containing a nonvolatile electrolyte is 10075degC aWhat is the boiling-point elevation bWhat is the molality of the solution a 075degC b 15 m
187
Electrolytes and Colligative PropertiesEarly investigators confused by experiments where certain substances depressed freezing point or elevated boiling point more than expected
188
189
Ex 01 m solution of NaCl lowers freezing point of solvent almost twice as much as 01 m solution of sucrose 01 m CaCl2 lowers fp almost 3 times as much as 01 m solution of sucrose
To understand why this happens contrast behavior or sucrose with NaCl in aqueous solution Each sucrose dissolves to make only 1 particle in solution So 1 mol sucrose dissolves to 1 mol particles 1 mol NaCl dissolves to make 2 moles of particles (1 mol Na+ and 1 mol Cl-)
190
191
Calculated Values for Electrolyte SolutionsColligative properties depend on total concentration of solute particles regardless of their identity Electrolytes cause changes in colligative properties proportional to total molality in terms of all dissolves particles instead of formula units
192
What about barium nitrate Ba(NO3)2 Each mole of barium nitrate yields 3 mol of ions in solution Ba(NO3)2(s) rarr Ba2+(aq) + 2NO3-(aq) You would expect solution of given molality to lower fp of solvent 3 times as much as nonelectrolytic solution of same molality
193
Sample Problem
What is the expected change in the freezing point of water in a solution of 625 g of barium nitrate Ba(NO3)2 in 100 kg of water
194
1 AnalyzeGiven solute mass and formula = 625 g Ba(NO3)2 solvent mass and identity = 100 kg water Δtf = Kfm
Unknown expected freezing-point depression
195
2 PlanThe molality can be calculated by converting the solute mass to moles and then dividing by the number of kilograms of solvent That molality is in terms of formula units of Ba(NO3)2 and must be converted to molality in terms of dissociated ions in solution It must be multiplied by the number of moles of ions produced per mole of formula unit This adjusted molality can then be used to calculate the freezing-point depression
196
197
198
3 Compute
Practice Problem
What is the expected freezing-point depression for a solution that contains 20 mol of magnesium sulfate dissolved in 10 kg of water
minus74degC199
What is the expected boiling-point elevation of water for a solution that contains 150 g of sodium chloride dissolved in 10 kg of water 27degC The freezing point of an aqueous sodium chloride solution is minus200degC What is the molality of the solution 54 m NaCl
200
Actual Values for Electrolyte SolutionsRemember values calculated are only expected values Actual values of colligative properties for all strong electrolytes are almost what would be expected Difference between expected and actual values caused by attractive forces that exist between dissociated ions in aqueous solution
201
Attraction between hydrated ions in solution is small compared with those in crystalline solid However forces of attraction donrsquot interfere with movements of aqueous ions More concentrated solution means ions closer together and attraction is greater Also depends on charge of ions ndash higher charge greater attraction
202
55
Liquids that dissolve freely in one another in any ratio are said to be completely miscible Benzene and carbon tetrachloride (both nonpolar) are completely miscible
Nonpolar molecules of these two apply no strong forces of attraction or repulsion and molecules mix freely
56
Ethanol and water are also completely miscible -OH group on ethanol is slightly polar
Can form H-bonds with water as well as other ethanol molecules Intermolecular forces in mixture are very similar to those in pure liquids so they are mutually soluble in any amount
57
Gasoline contains mainly nonpolar hydrocarbons
Excellent solvent for fats oils greases
Major intermolecular forces are weak London forces
58
Ethanol less polar than water more than carbon tetrachloride
Better solvent for less-polar substances bc of nonpolar region
59
Effects of Pressure on Solubility
Little effect on solids or liquids Increase pressure - increase solubilities of gases in liquids
60
When gas in contact with surface of liquid gas molecules can enter liquid As amount of dissolve gas increases some molecules start to escape and reenter gas phase
Eq eventually established between rates of enteringleaving gas phase
61
As long as eq undisturbed solubility of gas in liquid unchanged at given pressure
Gas + solvent solution
62
Increasing pressure of solute gas above solution puts stress on eq Molecules collide with surface more often Increase in pressure partially offset by increase in rate of gas molecules entering solution In turn increase in amount of dissolved gas causes increase in rate molecules escape liquid and becomes gas
63
Eventually eq restored at higher gas solubility
Expected from Le Chatelierrsquos principle
Increase in gas pressure causes eq to shift so fewer molecules are in gas phase
64
Henryrsquos Law
Henryrsquos law - solubility of a gas in liquid is directly proportional to the partial pressure of that gas on the surface of the liquid
Applies to gas-liquid solutions at constant temp
65
When mixture of ideal gases is limited in constant volume at constant temp each gas applies same pressure it would apply if it occupied the space alone
Assuming gases do not react each gas dissolves to the extent it would if no other gases were there
66
In carbonated drinks (Coke Pepsi etc) solubility of CO2 increased by increasing pressure
At bottling factory CO2 gas forced into solution of flavored water at pressure of 5-10 atm Gas-in-liquid solution then sealed in bottlescans
67
When cap removed pressure reduced to 1 atm and some CO2 escapes as gas bubbles Effervescence - rapid escape of a gas from a liquid in which it is dissolved
68
Effects of Temperature on Solubility
Letrsquos consider gas solubility Increasing temperature usually decreases gas solubility As temp increases average kinetic energy of molecules in solution increases Greater number of solute molecules escape from attraction of solvent and return to gas phase 69
At higher temps eq is reached with fewer gas molecules in solution Gases generally less soluble
70
Effect of temp on solubility of solids in liquids more difficult to predict
Often increasing temp increases solubility
However the same temp increase can result in large increase in solubility in one case and only slight increase in the next
71
72
enthalpies of Solution
Formation of solution comes with energy change Dissolve KI in water container feels cold Dissolve LiCl in water feels hot Formation of solid-liquid solution can absorb or release heat
73
During formation of solution solvent and solute particles experience changes in forces attracting them to other particles Before dissolving begins solvent molecules held by intermolecular forces (solvent-solvent attraction) In solute molecules held by Ifs (solute-solute attraction)
74
Energy is required to separate solute molecules and solvent molecules from their neighbors
A solute particle that is surrounded by solvent molecules is said to be solvated
75
Step 1 solute particles become separated from solid (energy absorbed) Step 2 solvent particles move apart to allow solute particles to enter liquid (energy absorbed)
76
Step 3 solvent particles attracted to and solvate solute particles (energy released)
77
Heat of solution - net amount of heat energy absorbed or released when a specific amount of solute dissolves in a solvent
Heat of solution negative (heat is released) if sums of energy in steps 1 and 2 is less than step 3
If 1+2 gt 3 HOS is positive (heat absorbed)78
In gaseous state molecules are so far apart there are nearly no forces between them Solute-solute interaction has little effect on heat of solution of a gas Energy released when gas dissolved in liquid bc attraction between solute gas and solvent molecules greater than energy needed to separate solvent molecules
79
Concentration of Solutions80
Concentration - measure of amount of solute in given amount of solvent or solution ldquodiluterdquo just means small amount of solute ldquoconcentratedrdquo just means relatively large amount of solute Unrelated to degree to which solution is saturated Ex Saturated solution of substance not very soluble might be dilute
81
Molarity
Molarity - number of moles of solute in one liter of solution To find molarity must know molar mass A ldquoone molarrdquo solution of NaOH is 1 mole NaOH per liter of solution (1 M)
82
1 mol NaOH = 400 g If this quantity dissolve in enough water to make EXACTLY 100 L solution solution is 1 M If 200 g NaOH dissolve in enough to make 100 L solution what is molarity 0500 M
83
1 M solution is not made by adding 1 mol solute to 1 L solvent That would be more than 1 L Instead 1 mol solute dissolved in less than 1 L Then diluted with more solvent to bring TOTAL VOLUME to 1 L
84
Practice ProblemYou have 340 L of solution that contains 900 g sodium chloride What is the molarity of that solution 0440 M NaCl You have 08 L of a 05 M HCl solution How many moles of HCl does this solution contain 04 mol HCl
85
What is the molarity of a solution composed of 585 g of potassium iodide dissolved in enough water to make a 0125 L of solution 0282 M KI
86
How many moles of H2SO4 are present in 0500 L of a 0150 M H2SO4 solution 00750 mol What volume of 300 M NaCl is needed for reaction that requires 1463 g of NaCl 0834 L
87
Molality
Molality - concentration of solution expressed in moles of solute per kilogram of solvent Solution that contains 1 mol solute (NaOH) dissolved in exactly 1 kg of solvent is ldquoone molalrdquo solution (1 m NaOH)
88
1 mol NaOH = 400 g 400 g NaOH dissolved in 1 kg water is 1 m NaOH
200 g NaOH in 1 kg water = 0500 m NaOH
89
Practice Problem
A solution was prepared by dissolving 171 g sucrose (C12H22O11) in 125 g of water Find the molal concentration of this solution 0400 m C12H22O11
90
A solution of iodine in carbon tetrachloride is used when iodine is needed for certain chemical tests How much iodine must be added to prepare a 0480 m solution of iodine in CCl4 if 1000 g of CCl4 is used 122 g I2
91
What is the molality of a solution composed of 255 g acetone (CH3)2CO dissolved in 200 g of water 22 m acetone What quantity in grams of methanol CH3OH is required to prepare a 0244 m solution in 400 g water 312 g methanol
92
How many grams of AgNO3 are needed to prepare a 0125 m solution in 250 mL of water 531 g AgNO3
What is the molality of a solution containing 182 g HCl and 250 g water 200 m
93
Ions in Aqueous Solutions and Colligative Properties
94
95
compounds in aqueous solutions
DissociationWhen compound made from ions dissolves in water ions separate Dissociation - separation of ions that occurs when an ionic compound dissolves
96
Notice number of ions made per formula unit in equations 1 formula unit of NaCl gives 2 units of ions in solution 1 formula unit of CaCl2 gives 3 units of ions
97
Assuming 100 dissociation solution that contains 1 mol NaCl contains 1 mol Na+ and 1 mol Cl- Can assume 100 dissociation of all soluble ionic compounds
98
Practice Problem
Write the equation for the dissolution of aluminum sulfate in water How many moles of aluminum ions are made by dissolving 1 mol aluminum sulfate What is the total number of moles of ions made by dissolving 1 mol of aluminum sulfate
99
1 AnalyzeGiven Amount of solute = 1 mol Al2(SO4)3 Solvent identity = water Unknown a moles of aluminum ions and sulfate ions Total number of moles of solute ions produced
100
2 Plan
The coefficients in the balanced dissociation equation will tell mole relationships You can use the equation to find out the number of moles of solute ions produced
101
3 Compute
a
b102
Write the equation for the dissolution of each of the following in water and then determine the number of moles of each ions made as well as the total number of ions made a 1 mol ammonium chloride b 1 mol sodium sulfide c 05 mol barium nitrate
103
a 1 mol ammonium chloride
1 mol NH4+ 1 mol Cl- 2 mol total ions
104
b 1 mol sodium sulfide
2 mol Na+ 1 mol S-2 3 mol total ions
105
c 05 mol barium nitrate
05 mol Ba+2 1 mol NO3- 15 mol total ions
106
Precipitation Reactions
Even though no compound is completely insoluble compounds of very low solubility can be considered insoluble for practical purposes There are general guidelines to help predict whether a compound made of certain combination of ions is soluble
107
General Solubility Guidelines
Sodium potassium and ammonium compounds are soluble in water
108
Nitrates acetates and chlorates are soluble
109
110
Most chlorides are soluble except those of silver mercury (I) and lead Lead (II) chloride is soluble in hot water
Most sulfates are soluble except those of barium strontium lead calcium and mercury
111
Most carbonates phosphates and silicates are insoluble except those of sodium potassium and ammonium
112
Most sulfides are insoluble except those of calcium strontium sodium potassium and ammonium
113
Is calcium phosphate Ca3(PO4)2 soluble or insoluble
Not one of the exceptions so it is insoluble
Dissociation equations are not written for insoluble compounds
114
Guidelines also useful in predicting what will happen if 2 solutions of 2 different soluble compounds are mixed
If mixing results in combination of ions that forms insoluble compound a double-replacement and precipitation reaction will happen
115
Will precipitate form when solutions of ammonium sulfide and cadmium nitrate are combined You can tell calcium nitrate is soluble from the guidelines (Most nitrates acetates and chlorates are soluble) You can also tell ammonium sulfide is soluble (Most sulfides are insoluble except those of calcium strontium sodium potassium and ammonium)
116
117
To decide whether precipitate can form must know solubilities of two possible products
Ammonium nitrate ndash soluble (Most nitrates acetates and chlorates are soluble) Cadmium sulfide ndash insoluble (Most sulfides are insoluble except those of calcium strontium sodium potassium and ammonium)
118
bc one product is insoluble double-replacement and precipitation reaction will happen (NH4)2S (aq) + Cd(NO3)2 (aq) rarr NH4NO3 (aq) + CdS (s)
119
Net Ionic Equations
Reactions of ions in aqueous solution usually represented by net ionic equations instead of formula equations Net ionic equation - includes only those compounds and ions that undergo a chemical change in a reaction in an aqueous solution
120
121
To write NIE (net ionic equation) first convert chemical equation into overall ionic equation All soluble ionic compounds shown as dissociated ions in solution Precipitates shown as solids
122
Notice ammonium and nitrate ions appear on both sides They havenrsquot gone through any chemical change Spectator ions - ions that do not take part in a chemical reaction and are found in solution both before and after the reaction
123
To change ionic equation to NIE eliminate spectator ions Cd+2(aq) + S-2(aq) rarr CdS(s)
This NIE applies to any reaction in which precipitate of CdS forms
124
Practice Problem
Identify the precipitate that forms when aqueous solutions of zinc nitrate and ammonium sulfide are combined Write the equation for the possible double-replacement reaction Then write the formula equation overall ionic equation and the net ionic equation
125
1 AnalyzeGiven Identity of reactants zinc nitrate and ammonium sulfide Reaction medium aqueous solution Unknown a equation for the possible double-replacement reaction b identity of the precipitate c formula equation d overall ionic equation e net ionic equation
126
2 PlanWrite the possible double-replacement reaction between Zn(NO3)2 and (NH4)2S Use the rules to determine if any of the products will precipitate Write a formula equation and overall net ionic equation Then cancel spectator ions to make net ionic equation
127
3 Computea possible double-replacement reaction
b rules show that zinc sulfide is not soluble so will precipitate (ammonium nitrate is soluble)
c formula equation128
d overall ionic equation
e net ionic equation129
Will a precipitate form if solutions of potassium sulfate and barium nitrate are combined If so write the net ionic equation for the reaction
Yes Ba+2(aq) + SO4-2(aq) rarr BaSO4(s)
130
Will a precipitate form if solutions of potassium nitrate and magnesium sulfate are combined If so write the net ionic equation for the reaction
No 131
Will a precipitate form if solutions of barium chloride and sodium sulfate are combined If so identify the spectator ions and write the net ionic equation for the reaction
Yes Na+ and Cl- Ba+2(aq) + SO4-2(aq) rarr BaSO4(s)
132
Write the net ionic equation for the precipitation of nickel(II) sulfide
Ni+2(aq) + S-2(aq) rarr NiS(s)
133
IonizationSome molecular compounds can also form ions in solution Usually polar Ionization - ions formed from solute molecules by action of the solvent (creation of ions where there were none) Ionization different from dissociation
134
Dissociation ionic compounds dissolve and ions already present separate Ionization ions formed where non existed before
Ions formed are hydrated Heat released during hydration of ions gives enough energy to break covalent bonds
135
Extent to which solute ionizes depends on strength of bonds within molecules of solute and strength of attraction between solute and solvent molecules
136
HCl ndash molecular compound that ionizes in aqueous solution Contains highly polar bond Attraction between polar HCl molecule and polar water strong enough to break HCl bond
137
The Hydronium Ion
H+ ions from HCl attracts other molecules or ions so strongly that it doesnrsquot normally exist alone Ionization of HCl better described as direct transfer of proton from HCl to H2O forming H3O+
138
139
Hydration of H+ to form hydronium is highly exothermic
Energy released gives a lot of energy needed to ionize a molecular solute
Many molecular compounds that ionize in aqueous solution contain hydrogen and form hydronium ion
140
Strong and Weak Electrolytes
Substances that make ions and conduct current in solution are electrolytes HCl is one of series of compounds that has hydrogen and halogen Hydrogen halides are all molecular compounds with single polar-covalent bonds
141
All are gases Very soluble in water All are electrolytes HCl HBr HI strongly conduct current in solution HF only weakly conducts current at same concentration Strength of conduction related to ability to ionize
142
143
Strong Electrolytes
HCl HBr HI are 100 ionized in solution
Strong electrolyte - any compound whose dilute aqueous solutions conduct electricity well this is due to the presence of all or almost all of the dissolve compound in the form of ions
144
Unique characteristic of strong electrolyte is that they yield only ions
Ex Ionic compound may be highly soluble in water and dissociate into ions (NaCl)
Others may not dissolve much but amount that does dissolve exists only as ions
145
Weak ElectrolytesHF dissolves in water to give acid solution called hydrofluoric acid HF bond stronger than bonds between hydrogen and other halogens When HF dissolves some molecule ionize Reverse reaction also happens HF(aq) + H2O(l) H3O+(aq) + F-(aq)
146
HF(aq) + H2O(l) H3O+
(aq) + F-(aq)Concentration of dissolved unionized HF stays high and concentration of ions stays low
Weak electrolyte - any compound whose aqueous solutions conduct electricity poorly this is due to the presence of small amount of the dissolved compound in the form of ions
147
Different from nonelectrolyte ndash NO ions at all Description of electrolyte as strong or weak not related to concentration of solution Electrolytes differ in degree of ionization not amount of solute dissolved
148
149
Colligative properties of solutions
Presence of solute affects properties of solutions Some properties not dependent on nature of dissolved substance but on how may dissolve particles are present
Colligative properties - properties that depend on the concentration of solute particles but not on their identity
150
Vapor-Pressure Lowering
Boiling and freezing point of solution different from pure solvent Nonvolatile solute raises boiling point and lowers freezing point Nonvolatile substance - one that has little tendency to become gas under existing conditions
151
152
To understand why nonvolatile solute changes boiling and freezing point consider equilibrium vapor pressure
Vapor pressure pressure caused by molecules that have escape liquid phase to gas phase
Can be thought of a measure of tendency of molecules to escape from a liquid
153
Addition of sucrose (nonvolatile solute) lowers concentration of water molecules at surface of liquid This lowers tendency of water molecules to leave solution and enter gas phase Vapor pressure of solution is lower than vapor pressure of pure water
154
Aqueous13 solution13 of13 nonvolatile13 solutePure13 water
Nonelectrolyte solutions of same molality have same concentration of particles
Dilute solutions of same solvent and equal molality of any nonelectrolyte solute lower vapor pressure equally
155
Example
1 m aqueous solution of nonelectrolyte glucose lowers vapor pressure of water 55 x 10-4 atm at 25 1 m aqueous solution of sucrose also lowers water vapor pressure to 55 x 10-4 atm at 25
156
157
bc vapor-pressure lowering depends on concentration of nonelectrolyte solute and doesnrsquot depend on the type of solute it is a colligative proprerty
bc vapor pressure has been lowered solution remains liquid over larger temperature range
Lowers freezing point and raises boiling point
158
Can assume changes in boiling and freezing point also depend on concentration of solute
They are colligative properties
159
Freezing-Point Depression
Freezing point of 1-molal solution of any nonelectrolyte solute in water is found (by experiment) to be 186 lower than freezing point of water When 1 mol of nonelectrolyte solute dissolved in 1 kg water freezing point is -186 not 000 160
When 2 mol nonelectrolyte solute dissolved in 1 kg water freezing point is -372
For any concentration of nonelectrolyte solute in water decrease in freezing point can be determined using value of -186m
161
This value called the molal freezing-point contstant (Kf) is the freezing-point depression of the solvent in a 1-molal solution of a nonvolatile nonelectrolyte solute
162
Each solvent has own characteristic molal freezing-point constant Freezing-point depression (Δtf) the difference between the freezing points of the pure solvent and a solute of nonelectrolyte in that solvent and it is directly proportional to the molal concentration of the solution Δtf = Kfm
163
Δtf = Kfm
Kf expressed as m
m expressed in mol solutekg solvent (molality)
164
Sample Problem
What is the freezing point depression of water in a solution of 171 g of sucrose C12H22O11 and 200 g of water What is the actual freezing point of the solution
165
1 AnalyzeGiven Solute mass and chemical formula 171 g C12H22O11 Solvent mass and identity 2000 g water Unknown a freezing-point depression b freezing point of the solution 166
2 Plan
167
168
Calculate
Unknown a freezing-point depression b freezing point of the solution
169
Practice Problem
A water solution containing an unknown quantity of a nonelectrolyte solute is found to have a freezing point of ndash 023degCWhat is the molal concentration of the solution
012 m170
A solution consists of 103 g of the nonelectrolyte glucose C6H12O6 dissolved in 250 g of water What is the freezing-point depression of the solution
minus0426degC171
In a laboratory experiment the freezing point of an aqueous solution of glucose is found to be minus0325degCWhat is the molal concentration of this solution
0175 m172
If 0500 mol of a nonelectrolyte solute are dissolved in 5000 g of ether what is the freezing point of the solution
minus1181degC173
The freezing point of an aqueous solution that contains a nonelectrolyte is minus90degC aWhat is the freezing-point depression of the solution bWhat is the molal concentration of the solution a minus90degC b 48 m
174
Boiling-Point Elevation
Boiling point is temp at which vapor pressure is equal to atmospheric pressure Change in vapor pressure will cause corresponding change in boiling point
175
176
Vapor pressure of nonvolatile solution is lower than vapor pressure of pure solvent More heat will be required to raise vapor pressure of solution Boiling point of solution is higher than boiling point of pure solvent
Molal boiling-point constant (Kb) - boiling-point elevation of the solvent in a 1-molal solution of a nonvolatile nonelectrolyte solute b-p elevation of any nonelectrolyte in water found by experiment to be 051 So molal b-p constant for water is 051m
177
For different solvents b-p elevations of 1-molal solutions have different values (Table 14-2)
Like freezing-point constants values most accurate for dilute solutions
178
Boiling-point elevation Δtb difference between the boiling points of the pure solvent and a nonelectrolyte solution of that solvent and it is directly proportional to the molal concentration of the solution B-p elevation can be calculated using Δtb = Kbm Δtb (boiling point elevation) expressed in m and m expressed in mol solutekg solvent
Sample Problem
What is the boiling-point elevation of a solution made from 200 g of a nonelectrolyte solute and 4000 g of water The molar mass of the solute is 620 gmol
180
1 AnalyzeGiven solute mass = 200 g solute molar mass = 620 gmol solvent mass and identity = 4000 g of water
Unknown boiling-point elevation
181
2 Plan
182
183
3 Compute
Practice Problem
A solution contains 500 g of sucrose C12H22O11 a nonelectrolyte dissolved in 5000 g of water What is the boiling-point elevation
015degC184
A solution contains 4500 g of sucrose C12H22O11 a nonelectrolyte dissolved in 250 g of waterWhat is the boiling point of the solution
1027degC185
If the boiling point elevation of an aqueous solution containing a nonvolatile electrolyte is 102degC what is the molality of the solution
20 m186
The boiling point of an aqueous solution containing a nonvolatile electrolyte is 10075degC aWhat is the boiling-point elevation bWhat is the molality of the solution a 075degC b 15 m
187
Electrolytes and Colligative PropertiesEarly investigators confused by experiments where certain substances depressed freezing point or elevated boiling point more than expected
188
189
Ex 01 m solution of NaCl lowers freezing point of solvent almost twice as much as 01 m solution of sucrose 01 m CaCl2 lowers fp almost 3 times as much as 01 m solution of sucrose
To understand why this happens contrast behavior or sucrose with NaCl in aqueous solution Each sucrose dissolves to make only 1 particle in solution So 1 mol sucrose dissolves to 1 mol particles 1 mol NaCl dissolves to make 2 moles of particles (1 mol Na+ and 1 mol Cl-)
190
191
Calculated Values for Electrolyte SolutionsColligative properties depend on total concentration of solute particles regardless of their identity Electrolytes cause changes in colligative properties proportional to total molality in terms of all dissolves particles instead of formula units
192
What about barium nitrate Ba(NO3)2 Each mole of barium nitrate yields 3 mol of ions in solution Ba(NO3)2(s) rarr Ba2+(aq) + 2NO3-(aq) You would expect solution of given molality to lower fp of solvent 3 times as much as nonelectrolytic solution of same molality
193
Sample Problem
What is the expected change in the freezing point of water in a solution of 625 g of barium nitrate Ba(NO3)2 in 100 kg of water
194
1 AnalyzeGiven solute mass and formula = 625 g Ba(NO3)2 solvent mass and identity = 100 kg water Δtf = Kfm
Unknown expected freezing-point depression
195
2 PlanThe molality can be calculated by converting the solute mass to moles and then dividing by the number of kilograms of solvent That molality is in terms of formula units of Ba(NO3)2 and must be converted to molality in terms of dissociated ions in solution It must be multiplied by the number of moles of ions produced per mole of formula unit This adjusted molality can then be used to calculate the freezing-point depression
196
197
198
3 Compute
Practice Problem
What is the expected freezing-point depression for a solution that contains 20 mol of magnesium sulfate dissolved in 10 kg of water
minus74degC199
What is the expected boiling-point elevation of water for a solution that contains 150 g of sodium chloride dissolved in 10 kg of water 27degC The freezing point of an aqueous sodium chloride solution is minus200degC What is the molality of the solution 54 m NaCl
200
Actual Values for Electrolyte SolutionsRemember values calculated are only expected values Actual values of colligative properties for all strong electrolytes are almost what would be expected Difference between expected and actual values caused by attractive forces that exist between dissociated ions in aqueous solution
201
Attraction between hydrated ions in solution is small compared with those in crystalline solid However forces of attraction donrsquot interfere with movements of aqueous ions More concentrated solution means ions closer together and attraction is greater Also depends on charge of ions ndash higher charge greater attraction
202
Liquids that dissolve freely in one another in any ratio are said to be completely miscible Benzene and carbon tetrachloride (both nonpolar) are completely miscible
Nonpolar molecules of these two apply no strong forces of attraction or repulsion and molecules mix freely
56
Ethanol and water are also completely miscible -OH group on ethanol is slightly polar
Can form H-bonds with water as well as other ethanol molecules Intermolecular forces in mixture are very similar to those in pure liquids so they are mutually soluble in any amount
57
Gasoline contains mainly nonpolar hydrocarbons
Excellent solvent for fats oils greases
Major intermolecular forces are weak London forces
58
Ethanol less polar than water more than carbon tetrachloride
Better solvent for less-polar substances bc of nonpolar region
59
Effects of Pressure on Solubility
Little effect on solids or liquids Increase pressure - increase solubilities of gases in liquids
60
When gas in contact with surface of liquid gas molecules can enter liquid As amount of dissolve gas increases some molecules start to escape and reenter gas phase
Eq eventually established between rates of enteringleaving gas phase
61
As long as eq undisturbed solubility of gas in liquid unchanged at given pressure
Gas + solvent solution
62
Increasing pressure of solute gas above solution puts stress on eq Molecules collide with surface more often Increase in pressure partially offset by increase in rate of gas molecules entering solution In turn increase in amount of dissolved gas causes increase in rate molecules escape liquid and becomes gas
63
Eventually eq restored at higher gas solubility
Expected from Le Chatelierrsquos principle
Increase in gas pressure causes eq to shift so fewer molecules are in gas phase
64
Henryrsquos Law
Henryrsquos law - solubility of a gas in liquid is directly proportional to the partial pressure of that gas on the surface of the liquid
Applies to gas-liquid solutions at constant temp
65
When mixture of ideal gases is limited in constant volume at constant temp each gas applies same pressure it would apply if it occupied the space alone
Assuming gases do not react each gas dissolves to the extent it would if no other gases were there
66
In carbonated drinks (Coke Pepsi etc) solubility of CO2 increased by increasing pressure
At bottling factory CO2 gas forced into solution of flavored water at pressure of 5-10 atm Gas-in-liquid solution then sealed in bottlescans
67
When cap removed pressure reduced to 1 atm and some CO2 escapes as gas bubbles Effervescence - rapid escape of a gas from a liquid in which it is dissolved
68
Effects of Temperature on Solubility
Letrsquos consider gas solubility Increasing temperature usually decreases gas solubility As temp increases average kinetic energy of molecules in solution increases Greater number of solute molecules escape from attraction of solvent and return to gas phase 69
At higher temps eq is reached with fewer gas molecules in solution Gases generally less soluble
70
Effect of temp on solubility of solids in liquids more difficult to predict
Often increasing temp increases solubility
However the same temp increase can result in large increase in solubility in one case and only slight increase in the next
71
72
enthalpies of Solution
Formation of solution comes with energy change Dissolve KI in water container feels cold Dissolve LiCl in water feels hot Formation of solid-liquid solution can absorb or release heat
73
During formation of solution solvent and solute particles experience changes in forces attracting them to other particles Before dissolving begins solvent molecules held by intermolecular forces (solvent-solvent attraction) In solute molecules held by Ifs (solute-solute attraction)
74
Energy is required to separate solute molecules and solvent molecules from their neighbors
A solute particle that is surrounded by solvent molecules is said to be solvated
75
Step 1 solute particles become separated from solid (energy absorbed) Step 2 solvent particles move apart to allow solute particles to enter liquid (energy absorbed)
76
Step 3 solvent particles attracted to and solvate solute particles (energy released)
77
Heat of solution - net amount of heat energy absorbed or released when a specific amount of solute dissolves in a solvent
Heat of solution negative (heat is released) if sums of energy in steps 1 and 2 is less than step 3
If 1+2 gt 3 HOS is positive (heat absorbed)78
In gaseous state molecules are so far apart there are nearly no forces between them Solute-solute interaction has little effect on heat of solution of a gas Energy released when gas dissolved in liquid bc attraction between solute gas and solvent molecules greater than energy needed to separate solvent molecules
79
Concentration of Solutions80
Concentration - measure of amount of solute in given amount of solvent or solution ldquodiluterdquo just means small amount of solute ldquoconcentratedrdquo just means relatively large amount of solute Unrelated to degree to which solution is saturated Ex Saturated solution of substance not very soluble might be dilute
81
Molarity
Molarity - number of moles of solute in one liter of solution To find molarity must know molar mass A ldquoone molarrdquo solution of NaOH is 1 mole NaOH per liter of solution (1 M)
82
1 mol NaOH = 400 g If this quantity dissolve in enough water to make EXACTLY 100 L solution solution is 1 M If 200 g NaOH dissolve in enough to make 100 L solution what is molarity 0500 M
83
1 M solution is not made by adding 1 mol solute to 1 L solvent That would be more than 1 L Instead 1 mol solute dissolved in less than 1 L Then diluted with more solvent to bring TOTAL VOLUME to 1 L
84
Practice ProblemYou have 340 L of solution that contains 900 g sodium chloride What is the molarity of that solution 0440 M NaCl You have 08 L of a 05 M HCl solution How many moles of HCl does this solution contain 04 mol HCl
85
What is the molarity of a solution composed of 585 g of potassium iodide dissolved in enough water to make a 0125 L of solution 0282 M KI
86
How many moles of H2SO4 are present in 0500 L of a 0150 M H2SO4 solution 00750 mol What volume of 300 M NaCl is needed for reaction that requires 1463 g of NaCl 0834 L
87
Molality
Molality - concentration of solution expressed in moles of solute per kilogram of solvent Solution that contains 1 mol solute (NaOH) dissolved in exactly 1 kg of solvent is ldquoone molalrdquo solution (1 m NaOH)
88
1 mol NaOH = 400 g 400 g NaOH dissolved in 1 kg water is 1 m NaOH
200 g NaOH in 1 kg water = 0500 m NaOH
89
Practice Problem
A solution was prepared by dissolving 171 g sucrose (C12H22O11) in 125 g of water Find the molal concentration of this solution 0400 m C12H22O11
90
A solution of iodine in carbon tetrachloride is used when iodine is needed for certain chemical tests How much iodine must be added to prepare a 0480 m solution of iodine in CCl4 if 1000 g of CCl4 is used 122 g I2
91
What is the molality of a solution composed of 255 g acetone (CH3)2CO dissolved in 200 g of water 22 m acetone What quantity in grams of methanol CH3OH is required to prepare a 0244 m solution in 400 g water 312 g methanol
92
How many grams of AgNO3 are needed to prepare a 0125 m solution in 250 mL of water 531 g AgNO3
What is the molality of a solution containing 182 g HCl and 250 g water 200 m
93
Ions in Aqueous Solutions and Colligative Properties
94
95
compounds in aqueous solutions
DissociationWhen compound made from ions dissolves in water ions separate Dissociation - separation of ions that occurs when an ionic compound dissolves
96
Notice number of ions made per formula unit in equations 1 formula unit of NaCl gives 2 units of ions in solution 1 formula unit of CaCl2 gives 3 units of ions
97
Assuming 100 dissociation solution that contains 1 mol NaCl contains 1 mol Na+ and 1 mol Cl- Can assume 100 dissociation of all soluble ionic compounds
98
Practice Problem
Write the equation for the dissolution of aluminum sulfate in water How many moles of aluminum ions are made by dissolving 1 mol aluminum sulfate What is the total number of moles of ions made by dissolving 1 mol of aluminum sulfate
99
1 AnalyzeGiven Amount of solute = 1 mol Al2(SO4)3 Solvent identity = water Unknown a moles of aluminum ions and sulfate ions Total number of moles of solute ions produced
100
2 Plan
The coefficients in the balanced dissociation equation will tell mole relationships You can use the equation to find out the number of moles of solute ions produced
101
3 Compute
a
b102
Write the equation for the dissolution of each of the following in water and then determine the number of moles of each ions made as well as the total number of ions made a 1 mol ammonium chloride b 1 mol sodium sulfide c 05 mol barium nitrate
103
a 1 mol ammonium chloride
1 mol NH4+ 1 mol Cl- 2 mol total ions
104
b 1 mol sodium sulfide
2 mol Na+ 1 mol S-2 3 mol total ions
105
c 05 mol barium nitrate
05 mol Ba+2 1 mol NO3- 15 mol total ions
106
Precipitation Reactions
Even though no compound is completely insoluble compounds of very low solubility can be considered insoluble for practical purposes There are general guidelines to help predict whether a compound made of certain combination of ions is soluble
107
General Solubility Guidelines
Sodium potassium and ammonium compounds are soluble in water
108
Nitrates acetates and chlorates are soluble
109
110
Most chlorides are soluble except those of silver mercury (I) and lead Lead (II) chloride is soluble in hot water
Most sulfates are soluble except those of barium strontium lead calcium and mercury
111
Most carbonates phosphates and silicates are insoluble except those of sodium potassium and ammonium
112
Most sulfides are insoluble except those of calcium strontium sodium potassium and ammonium
113
Is calcium phosphate Ca3(PO4)2 soluble or insoluble
Not one of the exceptions so it is insoluble
Dissociation equations are not written for insoluble compounds
114
Guidelines also useful in predicting what will happen if 2 solutions of 2 different soluble compounds are mixed
If mixing results in combination of ions that forms insoluble compound a double-replacement and precipitation reaction will happen
115
Will precipitate form when solutions of ammonium sulfide and cadmium nitrate are combined You can tell calcium nitrate is soluble from the guidelines (Most nitrates acetates and chlorates are soluble) You can also tell ammonium sulfide is soluble (Most sulfides are insoluble except those of calcium strontium sodium potassium and ammonium)
116
117
To decide whether precipitate can form must know solubilities of two possible products
Ammonium nitrate ndash soluble (Most nitrates acetates and chlorates are soluble) Cadmium sulfide ndash insoluble (Most sulfides are insoluble except those of calcium strontium sodium potassium and ammonium)
118
bc one product is insoluble double-replacement and precipitation reaction will happen (NH4)2S (aq) + Cd(NO3)2 (aq) rarr NH4NO3 (aq) + CdS (s)
119
Net Ionic Equations
Reactions of ions in aqueous solution usually represented by net ionic equations instead of formula equations Net ionic equation - includes only those compounds and ions that undergo a chemical change in a reaction in an aqueous solution
120
121
To write NIE (net ionic equation) first convert chemical equation into overall ionic equation All soluble ionic compounds shown as dissociated ions in solution Precipitates shown as solids
122
Notice ammonium and nitrate ions appear on both sides They havenrsquot gone through any chemical change Spectator ions - ions that do not take part in a chemical reaction and are found in solution both before and after the reaction
123
To change ionic equation to NIE eliminate spectator ions Cd+2(aq) + S-2(aq) rarr CdS(s)
This NIE applies to any reaction in which precipitate of CdS forms
124
Practice Problem
Identify the precipitate that forms when aqueous solutions of zinc nitrate and ammonium sulfide are combined Write the equation for the possible double-replacement reaction Then write the formula equation overall ionic equation and the net ionic equation
125
1 AnalyzeGiven Identity of reactants zinc nitrate and ammonium sulfide Reaction medium aqueous solution Unknown a equation for the possible double-replacement reaction b identity of the precipitate c formula equation d overall ionic equation e net ionic equation
126
2 PlanWrite the possible double-replacement reaction between Zn(NO3)2 and (NH4)2S Use the rules to determine if any of the products will precipitate Write a formula equation and overall net ionic equation Then cancel spectator ions to make net ionic equation
127
3 Computea possible double-replacement reaction
b rules show that zinc sulfide is not soluble so will precipitate (ammonium nitrate is soluble)
c formula equation128
d overall ionic equation
e net ionic equation129
Will a precipitate form if solutions of potassium sulfate and barium nitrate are combined If so write the net ionic equation for the reaction
Yes Ba+2(aq) + SO4-2(aq) rarr BaSO4(s)
130
Will a precipitate form if solutions of potassium nitrate and magnesium sulfate are combined If so write the net ionic equation for the reaction
No 131
Will a precipitate form if solutions of barium chloride and sodium sulfate are combined If so identify the spectator ions and write the net ionic equation for the reaction
Yes Na+ and Cl- Ba+2(aq) + SO4-2(aq) rarr BaSO4(s)
132
Write the net ionic equation for the precipitation of nickel(II) sulfide
Ni+2(aq) + S-2(aq) rarr NiS(s)
133
IonizationSome molecular compounds can also form ions in solution Usually polar Ionization - ions formed from solute molecules by action of the solvent (creation of ions where there were none) Ionization different from dissociation
134
Dissociation ionic compounds dissolve and ions already present separate Ionization ions formed where non existed before
Ions formed are hydrated Heat released during hydration of ions gives enough energy to break covalent bonds
135
Extent to which solute ionizes depends on strength of bonds within molecules of solute and strength of attraction between solute and solvent molecules
136
HCl ndash molecular compound that ionizes in aqueous solution Contains highly polar bond Attraction between polar HCl molecule and polar water strong enough to break HCl bond
137
The Hydronium Ion
H+ ions from HCl attracts other molecules or ions so strongly that it doesnrsquot normally exist alone Ionization of HCl better described as direct transfer of proton from HCl to H2O forming H3O+
138
139
Hydration of H+ to form hydronium is highly exothermic
Energy released gives a lot of energy needed to ionize a molecular solute
Many molecular compounds that ionize in aqueous solution contain hydrogen and form hydronium ion
140
Strong and Weak Electrolytes
Substances that make ions and conduct current in solution are electrolytes HCl is one of series of compounds that has hydrogen and halogen Hydrogen halides are all molecular compounds with single polar-covalent bonds
141
All are gases Very soluble in water All are electrolytes HCl HBr HI strongly conduct current in solution HF only weakly conducts current at same concentration Strength of conduction related to ability to ionize
142
143
Strong Electrolytes
HCl HBr HI are 100 ionized in solution
Strong electrolyte - any compound whose dilute aqueous solutions conduct electricity well this is due to the presence of all or almost all of the dissolve compound in the form of ions
144
Unique characteristic of strong electrolyte is that they yield only ions
Ex Ionic compound may be highly soluble in water and dissociate into ions (NaCl)
Others may not dissolve much but amount that does dissolve exists only as ions
145
Weak ElectrolytesHF dissolves in water to give acid solution called hydrofluoric acid HF bond stronger than bonds between hydrogen and other halogens When HF dissolves some molecule ionize Reverse reaction also happens HF(aq) + H2O(l) H3O+(aq) + F-(aq)
146
HF(aq) + H2O(l) H3O+
(aq) + F-(aq)Concentration of dissolved unionized HF stays high and concentration of ions stays low
Weak electrolyte - any compound whose aqueous solutions conduct electricity poorly this is due to the presence of small amount of the dissolved compound in the form of ions
147
Different from nonelectrolyte ndash NO ions at all Description of electrolyte as strong or weak not related to concentration of solution Electrolytes differ in degree of ionization not amount of solute dissolved
148
149
Colligative properties of solutions
Presence of solute affects properties of solutions Some properties not dependent on nature of dissolved substance but on how may dissolve particles are present
Colligative properties - properties that depend on the concentration of solute particles but not on their identity
150
Vapor-Pressure Lowering
Boiling and freezing point of solution different from pure solvent Nonvolatile solute raises boiling point and lowers freezing point Nonvolatile substance - one that has little tendency to become gas under existing conditions
151
152
To understand why nonvolatile solute changes boiling and freezing point consider equilibrium vapor pressure
Vapor pressure pressure caused by molecules that have escape liquid phase to gas phase
Can be thought of a measure of tendency of molecules to escape from a liquid
153
Addition of sucrose (nonvolatile solute) lowers concentration of water molecules at surface of liquid This lowers tendency of water molecules to leave solution and enter gas phase Vapor pressure of solution is lower than vapor pressure of pure water
154
Aqueous13 solution13 of13 nonvolatile13 solutePure13 water
Nonelectrolyte solutions of same molality have same concentration of particles
Dilute solutions of same solvent and equal molality of any nonelectrolyte solute lower vapor pressure equally
155
Example
1 m aqueous solution of nonelectrolyte glucose lowers vapor pressure of water 55 x 10-4 atm at 25 1 m aqueous solution of sucrose also lowers water vapor pressure to 55 x 10-4 atm at 25
156
157
bc vapor-pressure lowering depends on concentration of nonelectrolyte solute and doesnrsquot depend on the type of solute it is a colligative proprerty
bc vapor pressure has been lowered solution remains liquid over larger temperature range
Lowers freezing point and raises boiling point
158
Can assume changes in boiling and freezing point also depend on concentration of solute
They are colligative properties
159
Freezing-Point Depression
Freezing point of 1-molal solution of any nonelectrolyte solute in water is found (by experiment) to be 186 lower than freezing point of water When 1 mol of nonelectrolyte solute dissolved in 1 kg water freezing point is -186 not 000 160
When 2 mol nonelectrolyte solute dissolved in 1 kg water freezing point is -372
For any concentration of nonelectrolyte solute in water decrease in freezing point can be determined using value of -186m
161
This value called the molal freezing-point contstant (Kf) is the freezing-point depression of the solvent in a 1-molal solution of a nonvolatile nonelectrolyte solute
162
Each solvent has own characteristic molal freezing-point constant Freezing-point depression (Δtf) the difference between the freezing points of the pure solvent and a solute of nonelectrolyte in that solvent and it is directly proportional to the molal concentration of the solution Δtf = Kfm
163
Δtf = Kfm
Kf expressed as m
m expressed in mol solutekg solvent (molality)
164
Sample Problem
What is the freezing point depression of water in a solution of 171 g of sucrose C12H22O11 and 200 g of water What is the actual freezing point of the solution
165
1 AnalyzeGiven Solute mass and chemical formula 171 g C12H22O11 Solvent mass and identity 2000 g water Unknown a freezing-point depression b freezing point of the solution 166
2 Plan
167
168
Calculate
Unknown a freezing-point depression b freezing point of the solution
169
Practice Problem
A water solution containing an unknown quantity of a nonelectrolyte solute is found to have a freezing point of ndash 023degCWhat is the molal concentration of the solution
012 m170
A solution consists of 103 g of the nonelectrolyte glucose C6H12O6 dissolved in 250 g of water What is the freezing-point depression of the solution
minus0426degC171
In a laboratory experiment the freezing point of an aqueous solution of glucose is found to be minus0325degCWhat is the molal concentration of this solution
0175 m172
If 0500 mol of a nonelectrolyte solute are dissolved in 5000 g of ether what is the freezing point of the solution
minus1181degC173
The freezing point of an aqueous solution that contains a nonelectrolyte is minus90degC aWhat is the freezing-point depression of the solution bWhat is the molal concentration of the solution a minus90degC b 48 m
174
Boiling-Point Elevation
Boiling point is temp at which vapor pressure is equal to atmospheric pressure Change in vapor pressure will cause corresponding change in boiling point
175
176
Vapor pressure of nonvolatile solution is lower than vapor pressure of pure solvent More heat will be required to raise vapor pressure of solution Boiling point of solution is higher than boiling point of pure solvent
Molal boiling-point constant (Kb) - boiling-point elevation of the solvent in a 1-molal solution of a nonvolatile nonelectrolyte solute b-p elevation of any nonelectrolyte in water found by experiment to be 051 So molal b-p constant for water is 051m
177
For different solvents b-p elevations of 1-molal solutions have different values (Table 14-2)
Like freezing-point constants values most accurate for dilute solutions
178
Boiling-point elevation Δtb difference between the boiling points of the pure solvent and a nonelectrolyte solution of that solvent and it is directly proportional to the molal concentration of the solution B-p elevation can be calculated using Δtb = Kbm Δtb (boiling point elevation) expressed in m and m expressed in mol solutekg solvent
Sample Problem
What is the boiling-point elevation of a solution made from 200 g of a nonelectrolyte solute and 4000 g of water The molar mass of the solute is 620 gmol
180
1 AnalyzeGiven solute mass = 200 g solute molar mass = 620 gmol solvent mass and identity = 4000 g of water
Unknown boiling-point elevation
181
2 Plan
182
183
3 Compute
Practice Problem
A solution contains 500 g of sucrose C12H22O11 a nonelectrolyte dissolved in 5000 g of water What is the boiling-point elevation
015degC184
A solution contains 4500 g of sucrose C12H22O11 a nonelectrolyte dissolved in 250 g of waterWhat is the boiling point of the solution
1027degC185
If the boiling point elevation of an aqueous solution containing a nonvolatile electrolyte is 102degC what is the molality of the solution
20 m186
The boiling point of an aqueous solution containing a nonvolatile electrolyte is 10075degC aWhat is the boiling-point elevation bWhat is the molality of the solution a 075degC b 15 m
187
Electrolytes and Colligative PropertiesEarly investigators confused by experiments where certain substances depressed freezing point or elevated boiling point more than expected
188
189
Ex 01 m solution of NaCl lowers freezing point of solvent almost twice as much as 01 m solution of sucrose 01 m CaCl2 lowers fp almost 3 times as much as 01 m solution of sucrose
To understand why this happens contrast behavior or sucrose with NaCl in aqueous solution Each sucrose dissolves to make only 1 particle in solution So 1 mol sucrose dissolves to 1 mol particles 1 mol NaCl dissolves to make 2 moles of particles (1 mol Na+ and 1 mol Cl-)
190
191
Calculated Values for Electrolyte SolutionsColligative properties depend on total concentration of solute particles regardless of their identity Electrolytes cause changes in colligative properties proportional to total molality in terms of all dissolves particles instead of formula units
192
What about barium nitrate Ba(NO3)2 Each mole of barium nitrate yields 3 mol of ions in solution Ba(NO3)2(s) rarr Ba2+(aq) + 2NO3-(aq) You would expect solution of given molality to lower fp of solvent 3 times as much as nonelectrolytic solution of same molality
193
Sample Problem
What is the expected change in the freezing point of water in a solution of 625 g of barium nitrate Ba(NO3)2 in 100 kg of water
194
1 AnalyzeGiven solute mass and formula = 625 g Ba(NO3)2 solvent mass and identity = 100 kg water Δtf = Kfm
Unknown expected freezing-point depression
195
2 PlanThe molality can be calculated by converting the solute mass to moles and then dividing by the number of kilograms of solvent That molality is in terms of formula units of Ba(NO3)2 and must be converted to molality in terms of dissociated ions in solution It must be multiplied by the number of moles of ions produced per mole of formula unit This adjusted molality can then be used to calculate the freezing-point depression
196
197
198
3 Compute
Practice Problem
What is the expected freezing-point depression for a solution that contains 20 mol of magnesium sulfate dissolved in 10 kg of water
minus74degC199
What is the expected boiling-point elevation of water for a solution that contains 150 g of sodium chloride dissolved in 10 kg of water 27degC The freezing point of an aqueous sodium chloride solution is minus200degC What is the molality of the solution 54 m NaCl
200
Actual Values for Electrolyte SolutionsRemember values calculated are only expected values Actual values of colligative properties for all strong electrolytes are almost what would be expected Difference between expected and actual values caused by attractive forces that exist between dissociated ions in aqueous solution
201
Attraction between hydrated ions in solution is small compared with those in crystalline solid However forces of attraction donrsquot interfere with movements of aqueous ions More concentrated solution means ions closer together and attraction is greater Also depends on charge of ions ndash higher charge greater attraction
202
Ethanol and water are also completely miscible -OH group on ethanol is slightly polar
Can form H-bonds with water as well as other ethanol molecules Intermolecular forces in mixture are very similar to those in pure liquids so they are mutually soluble in any amount
57
Gasoline contains mainly nonpolar hydrocarbons
Excellent solvent for fats oils greases
Major intermolecular forces are weak London forces
58
Ethanol less polar than water more than carbon tetrachloride
Better solvent for less-polar substances bc of nonpolar region
59
Effects of Pressure on Solubility
Little effect on solids or liquids Increase pressure - increase solubilities of gases in liquids
60
When gas in contact with surface of liquid gas molecules can enter liquid As amount of dissolve gas increases some molecules start to escape and reenter gas phase
Eq eventually established between rates of enteringleaving gas phase
61
As long as eq undisturbed solubility of gas in liquid unchanged at given pressure
Gas + solvent solution
62
Increasing pressure of solute gas above solution puts stress on eq Molecules collide with surface more often Increase in pressure partially offset by increase in rate of gas molecules entering solution In turn increase in amount of dissolved gas causes increase in rate molecules escape liquid and becomes gas
63
Eventually eq restored at higher gas solubility
Expected from Le Chatelierrsquos principle
Increase in gas pressure causes eq to shift so fewer molecules are in gas phase
64
Henryrsquos Law
Henryrsquos law - solubility of a gas in liquid is directly proportional to the partial pressure of that gas on the surface of the liquid
Applies to gas-liquid solutions at constant temp
65
When mixture of ideal gases is limited in constant volume at constant temp each gas applies same pressure it would apply if it occupied the space alone
Assuming gases do not react each gas dissolves to the extent it would if no other gases were there
66
In carbonated drinks (Coke Pepsi etc) solubility of CO2 increased by increasing pressure
At bottling factory CO2 gas forced into solution of flavored water at pressure of 5-10 atm Gas-in-liquid solution then sealed in bottlescans
67
When cap removed pressure reduced to 1 atm and some CO2 escapes as gas bubbles Effervescence - rapid escape of a gas from a liquid in which it is dissolved
68
Effects of Temperature on Solubility
Letrsquos consider gas solubility Increasing temperature usually decreases gas solubility As temp increases average kinetic energy of molecules in solution increases Greater number of solute molecules escape from attraction of solvent and return to gas phase 69
At higher temps eq is reached with fewer gas molecules in solution Gases generally less soluble
70
Effect of temp on solubility of solids in liquids more difficult to predict
Often increasing temp increases solubility
However the same temp increase can result in large increase in solubility in one case and only slight increase in the next
71
72
enthalpies of Solution
Formation of solution comes with energy change Dissolve KI in water container feels cold Dissolve LiCl in water feels hot Formation of solid-liquid solution can absorb or release heat
73
During formation of solution solvent and solute particles experience changes in forces attracting them to other particles Before dissolving begins solvent molecules held by intermolecular forces (solvent-solvent attraction) In solute molecules held by Ifs (solute-solute attraction)
74
Energy is required to separate solute molecules and solvent molecules from their neighbors
A solute particle that is surrounded by solvent molecules is said to be solvated
75
Step 1 solute particles become separated from solid (energy absorbed) Step 2 solvent particles move apart to allow solute particles to enter liquid (energy absorbed)
76
Step 3 solvent particles attracted to and solvate solute particles (energy released)
77
Heat of solution - net amount of heat energy absorbed or released when a specific amount of solute dissolves in a solvent
Heat of solution negative (heat is released) if sums of energy in steps 1 and 2 is less than step 3
If 1+2 gt 3 HOS is positive (heat absorbed)78
In gaseous state molecules are so far apart there are nearly no forces between them Solute-solute interaction has little effect on heat of solution of a gas Energy released when gas dissolved in liquid bc attraction between solute gas and solvent molecules greater than energy needed to separate solvent molecules
79
Concentration of Solutions80
Concentration - measure of amount of solute in given amount of solvent or solution ldquodiluterdquo just means small amount of solute ldquoconcentratedrdquo just means relatively large amount of solute Unrelated to degree to which solution is saturated Ex Saturated solution of substance not very soluble might be dilute
81
Molarity
Molarity - number of moles of solute in one liter of solution To find molarity must know molar mass A ldquoone molarrdquo solution of NaOH is 1 mole NaOH per liter of solution (1 M)
82
1 mol NaOH = 400 g If this quantity dissolve in enough water to make EXACTLY 100 L solution solution is 1 M If 200 g NaOH dissolve in enough to make 100 L solution what is molarity 0500 M
83
1 M solution is not made by adding 1 mol solute to 1 L solvent That would be more than 1 L Instead 1 mol solute dissolved in less than 1 L Then diluted with more solvent to bring TOTAL VOLUME to 1 L
84
Practice ProblemYou have 340 L of solution that contains 900 g sodium chloride What is the molarity of that solution 0440 M NaCl You have 08 L of a 05 M HCl solution How many moles of HCl does this solution contain 04 mol HCl
85
What is the molarity of a solution composed of 585 g of potassium iodide dissolved in enough water to make a 0125 L of solution 0282 M KI
86
How many moles of H2SO4 are present in 0500 L of a 0150 M H2SO4 solution 00750 mol What volume of 300 M NaCl is needed for reaction that requires 1463 g of NaCl 0834 L
87
Molality
Molality - concentration of solution expressed in moles of solute per kilogram of solvent Solution that contains 1 mol solute (NaOH) dissolved in exactly 1 kg of solvent is ldquoone molalrdquo solution (1 m NaOH)
88
1 mol NaOH = 400 g 400 g NaOH dissolved in 1 kg water is 1 m NaOH
200 g NaOH in 1 kg water = 0500 m NaOH
89
Practice Problem
A solution was prepared by dissolving 171 g sucrose (C12H22O11) in 125 g of water Find the molal concentration of this solution 0400 m C12H22O11
90
A solution of iodine in carbon tetrachloride is used when iodine is needed for certain chemical tests How much iodine must be added to prepare a 0480 m solution of iodine in CCl4 if 1000 g of CCl4 is used 122 g I2
91
What is the molality of a solution composed of 255 g acetone (CH3)2CO dissolved in 200 g of water 22 m acetone What quantity in grams of methanol CH3OH is required to prepare a 0244 m solution in 400 g water 312 g methanol
92
How many grams of AgNO3 are needed to prepare a 0125 m solution in 250 mL of water 531 g AgNO3
What is the molality of a solution containing 182 g HCl and 250 g water 200 m
93
Ions in Aqueous Solutions and Colligative Properties
94
95
compounds in aqueous solutions
DissociationWhen compound made from ions dissolves in water ions separate Dissociation - separation of ions that occurs when an ionic compound dissolves
96
Notice number of ions made per formula unit in equations 1 formula unit of NaCl gives 2 units of ions in solution 1 formula unit of CaCl2 gives 3 units of ions
97
Assuming 100 dissociation solution that contains 1 mol NaCl contains 1 mol Na+ and 1 mol Cl- Can assume 100 dissociation of all soluble ionic compounds
98
Practice Problem
Write the equation for the dissolution of aluminum sulfate in water How many moles of aluminum ions are made by dissolving 1 mol aluminum sulfate What is the total number of moles of ions made by dissolving 1 mol of aluminum sulfate
99
1 AnalyzeGiven Amount of solute = 1 mol Al2(SO4)3 Solvent identity = water Unknown a moles of aluminum ions and sulfate ions Total number of moles of solute ions produced
100
2 Plan
The coefficients in the balanced dissociation equation will tell mole relationships You can use the equation to find out the number of moles of solute ions produced
101
3 Compute
a
b102
Write the equation for the dissolution of each of the following in water and then determine the number of moles of each ions made as well as the total number of ions made a 1 mol ammonium chloride b 1 mol sodium sulfide c 05 mol barium nitrate
103
a 1 mol ammonium chloride
1 mol NH4+ 1 mol Cl- 2 mol total ions
104
b 1 mol sodium sulfide
2 mol Na+ 1 mol S-2 3 mol total ions
105
c 05 mol barium nitrate
05 mol Ba+2 1 mol NO3- 15 mol total ions
106
Precipitation Reactions
Even though no compound is completely insoluble compounds of very low solubility can be considered insoluble for practical purposes There are general guidelines to help predict whether a compound made of certain combination of ions is soluble
107
General Solubility Guidelines
Sodium potassium and ammonium compounds are soluble in water
108
Nitrates acetates and chlorates are soluble
109
110
Most chlorides are soluble except those of silver mercury (I) and lead Lead (II) chloride is soluble in hot water
Most sulfates are soluble except those of barium strontium lead calcium and mercury
111
Most carbonates phosphates and silicates are insoluble except those of sodium potassium and ammonium
112
Most sulfides are insoluble except those of calcium strontium sodium potassium and ammonium
113
Is calcium phosphate Ca3(PO4)2 soluble or insoluble
Not one of the exceptions so it is insoluble
Dissociation equations are not written for insoluble compounds
114
Guidelines also useful in predicting what will happen if 2 solutions of 2 different soluble compounds are mixed
If mixing results in combination of ions that forms insoluble compound a double-replacement and precipitation reaction will happen
115
Will precipitate form when solutions of ammonium sulfide and cadmium nitrate are combined You can tell calcium nitrate is soluble from the guidelines (Most nitrates acetates and chlorates are soluble) You can also tell ammonium sulfide is soluble (Most sulfides are insoluble except those of calcium strontium sodium potassium and ammonium)
116
117
To decide whether precipitate can form must know solubilities of two possible products
Ammonium nitrate ndash soluble (Most nitrates acetates and chlorates are soluble) Cadmium sulfide ndash insoluble (Most sulfides are insoluble except those of calcium strontium sodium potassium and ammonium)
118
bc one product is insoluble double-replacement and precipitation reaction will happen (NH4)2S (aq) + Cd(NO3)2 (aq) rarr NH4NO3 (aq) + CdS (s)
119
Net Ionic Equations
Reactions of ions in aqueous solution usually represented by net ionic equations instead of formula equations Net ionic equation - includes only those compounds and ions that undergo a chemical change in a reaction in an aqueous solution
120
121
To write NIE (net ionic equation) first convert chemical equation into overall ionic equation All soluble ionic compounds shown as dissociated ions in solution Precipitates shown as solids
122
Notice ammonium and nitrate ions appear on both sides They havenrsquot gone through any chemical change Spectator ions - ions that do not take part in a chemical reaction and are found in solution both before and after the reaction
123
To change ionic equation to NIE eliminate spectator ions Cd+2(aq) + S-2(aq) rarr CdS(s)
This NIE applies to any reaction in which precipitate of CdS forms
124
Practice Problem
Identify the precipitate that forms when aqueous solutions of zinc nitrate and ammonium sulfide are combined Write the equation for the possible double-replacement reaction Then write the formula equation overall ionic equation and the net ionic equation
125
1 AnalyzeGiven Identity of reactants zinc nitrate and ammonium sulfide Reaction medium aqueous solution Unknown a equation for the possible double-replacement reaction b identity of the precipitate c formula equation d overall ionic equation e net ionic equation
126
2 PlanWrite the possible double-replacement reaction between Zn(NO3)2 and (NH4)2S Use the rules to determine if any of the products will precipitate Write a formula equation and overall net ionic equation Then cancel spectator ions to make net ionic equation
127
3 Computea possible double-replacement reaction
b rules show that zinc sulfide is not soluble so will precipitate (ammonium nitrate is soluble)
c formula equation128
d overall ionic equation
e net ionic equation129
Will a precipitate form if solutions of potassium sulfate and barium nitrate are combined If so write the net ionic equation for the reaction
Yes Ba+2(aq) + SO4-2(aq) rarr BaSO4(s)
130
Will a precipitate form if solutions of potassium nitrate and magnesium sulfate are combined If so write the net ionic equation for the reaction
No 131
Will a precipitate form if solutions of barium chloride and sodium sulfate are combined If so identify the spectator ions and write the net ionic equation for the reaction
Yes Na+ and Cl- Ba+2(aq) + SO4-2(aq) rarr BaSO4(s)
132
Write the net ionic equation for the precipitation of nickel(II) sulfide
Ni+2(aq) + S-2(aq) rarr NiS(s)
133
IonizationSome molecular compounds can also form ions in solution Usually polar Ionization - ions formed from solute molecules by action of the solvent (creation of ions where there were none) Ionization different from dissociation
134
Dissociation ionic compounds dissolve and ions already present separate Ionization ions formed where non existed before
Ions formed are hydrated Heat released during hydration of ions gives enough energy to break covalent bonds
135
Extent to which solute ionizes depends on strength of bonds within molecules of solute and strength of attraction between solute and solvent molecules
136
HCl ndash molecular compound that ionizes in aqueous solution Contains highly polar bond Attraction between polar HCl molecule and polar water strong enough to break HCl bond
137
The Hydronium Ion
H+ ions from HCl attracts other molecules or ions so strongly that it doesnrsquot normally exist alone Ionization of HCl better described as direct transfer of proton from HCl to H2O forming H3O+
138
139
Hydration of H+ to form hydronium is highly exothermic
Energy released gives a lot of energy needed to ionize a molecular solute
Many molecular compounds that ionize in aqueous solution contain hydrogen and form hydronium ion
140
Strong and Weak Electrolytes
Substances that make ions and conduct current in solution are electrolytes HCl is one of series of compounds that has hydrogen and halogen Hydrogen halides are all molecular compounds with single polar-covalent bonds
141
All are gases Very soluble in water All are electrolytes HCl HBr HI strongly conduct current in solution HF only weakly conducts current at same concentration Strength of conduction related to ability to ionize
142
143
Strong Electrolytes
HCl HBr HI are 100 ionized in solution
Strong electrolyte - any compound whose dilute aqueous solutions conduct electricity well this is due to the presence of all or almost all of the dissolve compound in the form of ions
144
Unique characteristic of strong electrolyte is that they yield only ions
Ex Ionic compound may be highly soluble in water and dissociate into ions (NaCl)
Others may not dissolve much but amount that does dissolve exists only as ions
145
Weak ElectrolytesHF dissolves in water to give acid solution called hydrofluoric acid HF bond stronger than bonds between hydrogen and other halogens When HF dissolves some molecule ionize Reverse reaction also happens HF(aq) + H2O(l) H3O+(aq) + F-(aq)
146
HF(aq) + H2O(l) H3O+
(aq) + F-(aq)Concentration of dissolved unionized HF stays high and concentration of ions stays low
Weak electrolyte - any compound whose aqueous solutions conduct electricity poorly this is due to the presence of small amount of the dissolved compound in the form of ions
147
Different from nonelectrolyte ndash NO ions at all Description of electrolyte as strong or weak not related to concentration of solution Electrolytes differ in degree of ionization not amount of solute dissolved
148
149
Colligative properties of solutions
Presence of solute affects properties of solutions Some properties not dependent on nature of dissolved substance but on how may dissolve particles are present
Colligative properties - properties that depend on the concentration of solute particles but not on their identity
150
Vapor-Pressure Lowering
Boiling and freezing point of solution different from pure solvent Nonvolatile solute raises boiling point and lowers freezing point Nonvolatile substance - one that has little tendency to become gas under existing conditions
151
152
To understand why nonvolatile solute changes boiling and freezing point consider equilibrium vapor pressure
Vapor pressure pressure caused by molecules that have escape liquid phase to gas phase
Can be thought of a measure of tendency of molecules to escape from a liquid
153
Addition of sucrose (nonvolatile solute) lowers concentration of water molecules at surface of liquid This lowers tendency of water molecules to leave solution and enter gas phase Vapor pressure of solution is lower than vapor pressure of pure water
154
Aqueous13 solution13 of13 nonvolatile13 solutePure13 water
Nonelectrolyte solutions of same molality have same concentration of particles
Dilute solutions of same solvent and equal molality of any nonelectrolyte solute lower vapor pressure equally
155
Example
1 m aqueous solution of nonelectrolyte glucose lowers vapor pressure of water 55 x 10-4 atm at 25 1 m aqueous solution of sucrose also lowers water vapor pressure to 55 x 10-4 atm at 25
156
157
bc vapor-pressure lowering depends on concentration of nonelectrolyte solute and doesnrsquot depend on the type of solute it is a colligative proprerty
bc vapor pressure has been lowered solution remains liquid over larger temperature range
Lowers freezing point and raises boiling point
158
Can assume changes in boiling and freezing point also depend on concentration of solute
They are colligative properties
159
Freezing-Point Depression
Freezing point of 1-molal solution of any nonelectrolyte solute in water is found (by experiment) to be 186 lower than freezing point of water When 1 mol of nonelectrolyte solute dissolved in 1 kg water freezing point is -186 not 000 160
When 2 mol nonelectrolyte solute dissolved in 1 kg water freezing point is -372
For any concentration of nonelectrolyte solute in water decrease in freezing point can be determined using value of -186m
161
This value called the molal freezing-point contstant (Kf) is the freezing-point depression of the solvent in a 1-molal solution of a nonvolatile nonelectrolyte solute
162
Each solvent has own characteristic molal freezing-point constant Freezing-point depression (Δtf) the difference between the freezing points of the pure solvent and a solute of nonelectrolyte in that solvent and it is directly proportional to the molal concentration of the solution Δtf = Kfm
163
Δtf = Kfm
Kf expressed as m
m expressed in mol solutekg solvent (molality)
164
Sample Problem
What is the freezing point depression of water in a solution of 171 g of sucrose C12H22O11 and 200 g of water What is the actual freezing point of the solution
165
1 AnalyzeGiven Solute mass and chemical formula 171 g C12H22O11 Solvent mass and identity 2000 g water Unknown a freezing-point depression b freezing point of the solution 166
2 Plan
167
168
Calculate
Unknown a freezing-point depression b freezing point of the solution
169
Practice Problem
A water solution containing an unknown quantity of a nonelectrolyte solute is found to have a freezing point of ndash 023degCWhat is the molal concentration of the solution
012 m170
A solution consists of 103 g of the nonelectrolyte glucose C6H12O6 dissolved in 250 g of water What is the freezing-point depression of the solution
minus0426degC171
In a laboratory experiment the freezing point of an aqueous solution of glucose is found to be minus0325degCWhat is the molal concentration of this solution
0175 m172
If 0500 mol of a nonelectrolyte solute are dissolved in 5000 g of ether what is the freezing point of the solution
minus1181degC173
The freezing point of an aqueous solution that contains a nonelectrolyte is minus90degC aWhat is the freezing-point depression of the solution bWhat is the molal concentration of the solution a minus90degC b 48 m
174
Boiling-Point Elevation
Boiling point is temp at which vapor pressure is equal to atmospheric pressure Change in vapor pressure will cause corresponding change in boiling point
175
176
Vapor pressure of nonvolatile solution is lower than vapor pressure of pure solvent More heat will be required to raise vapor pressure of solution Boiling point of solution is higher than boiling point of pure solvent
Molal boiling-point constant (Kb) - boiling-point elevation of the solvent in a 1-molal solution of a nonvolatile nonelectrolyte solute b-p elevation of any nonelectrolyte in water found by experiment to be 051 So molal b-p constant for water is 051m
177
For different solvents b-p elevations of 1-molal solutions have different values (Table 14-2)
Like freezing-point constants values most accurate for dilute solutions
178
Boiling-point elevation Δtb difference between the boiling points of the pure solvent and a nonelectrolyte solution of that solvent and it is directly proportional to the molal concentration of the solution B-p elevation can be calculated using Δtb = Kbm Δtb (boiling point elevation) expressed in m and m expressed in mol solutekg solvent
Sample Problem
What is the boiling-point elevation of a solution made from 200 g of a nonelectrolyte solute and 4000 g of water The molar mass of the solute is 620 gmol
180
1 AnalyzeGiven solute mass = 200 g solute molar mass = 620 gmol solvent mass and identity = 4000 g of water
Unknown boiling-point elevation
181
2 Plan
182
183
3 Compute
Practice Problem
A solution contains 500 g of sucrose C12H22O11 a nonelectrolyte dissolved in 5000 g of water What is the boiling-point elevation
015degC184
A solution contains 4500 g of sucrose C12H22O11 a nonelectrolyte dissolved in 250 g of waterWhat is the boiling point of the solution
1027degC185
If the boiling point elevation of an aqueous solution containing a nonvolatile electrolyte is 102degC what is the molality of the solution
20 m186
The boiling point of an aqueous solution containing a nonvolatile electrolyte is 10075degC aWhat is the boiling-point elevation bWhat is the molality of the solution a 075degC b 15 m
187
Electrolytes and Colligative PropertiesEarly investigators confused by experiments where certain substances depressed freezing point or elevated boiling point more than expected
188
189
Ex 01 m solution of NaCl lowers freezing point of solvent almost twice as much as 01 m solution of sucrose 01 m CaCl2 lowers fp almost 3 times as much as 01 m solution of sucrose
To understand why this happens contrast behavior or sucrose with NaCl in aqueous solution Each sucrose dissolves to make only 1 particle in solution So 1 mol sucrose dissolves to 1 mol particles 1 mol NaCl dissolves to make 2 moles of particles (1 mol Na+ and 1 mol Cl-)
190
191
Calculated Values for Electrolyte SolutionsColligative properties depend on total concentration of solute particles regardless of their identity Electrolytes cause changes in colligative properties proportional to total molality in terms of all dissolves particles instead of formula units
192
What about barium nitrate Ba(NO3)2 Each mole of barium nitrate yields 3 mol of ions in solution Ba(NO3)2(s) rarr Ba2+(aq) + 2NO3-(aq) You would expect solution of given molality to lower fp of solvent 3 times as much as nonelectrolytic solution of same molality
193
Sample Problem
What is the expected change in the freezing point of water in a solution of 625 g of barium nitrate Ba(NO3)2 in 100 kg of water
194
1 AnalyzeGiven solute mass and formula = 625 g Ba(NO3)2 solvent mass and identity = 100 kg water Δtf = Kfm
Unknown expected freezing-point depression
195
2 PlanThe molality can be calculated by converting the solute mass to moles and then dividing by the number of kilograms of solvent That molality is in terms of formula units of Ba(NO3)2 and must be converted to molality in terms of dissociated ions in solution It must be multiplied by the number of moles of ions produced per mole of formula unit This adjusted molality can then be used to calculate the freezing-point depression
196
197
198
3 Compute
Practice Problem
What is the expected freezing-point depression for a solution that contains 20 mol of magnesium sulfate dissolved in 10 kg of water
minus74degC199
What is the expected boiling-point elevation of water for a solution that contains 150 g of sodium chloride dissolved in 10 kg of water 27degC The freezing point of an aqueous sodium chloride solution is minus200degC What is the molality of the solution 54 m NaCl
200
Actual Values for Electrolyte SolutionsRemember values calculated are only expected values Actual values of colligative properties for all strong electrolytes are almost what would be expected Difference between expected and actual values caused by attractive forces that exist between dissociated ions in aqueous solution
201
Attraction between hydrated ions in solution is small compared with those in crystalline solid However forces of attraction donrsquot interfere with movements of aqueous ions More concentrated solution means ions closer together and attraction is greater Also depends on charge of ions ndash higher charge greater attraction
202
Gasoline contains mainly nonpolar hydrocarbons
Excellent solvent for fats oils greases
Major intermolecular forces are weak London forces
58
Ethanol less polar than water more than carbon tetrachloride
Better solvent for less-polar substances bc of nonpolar region
59
Effects of Pressure on Solubility
Little effect on solids or liquids Increase pressure - increase solubilities of gases in liquids
60
When gas in contact with surface of liquid gas molecules can enter liquid As amount of dissolve gas increases some molecules start to escape and reenter gas phase
Eq eventually established between rates of enteringleaving gas phase
61
As long as eq undisturbed solubility of gas in liquid unchanged at given pressure
Gas + solvent solution
62
Increasing pressure of solute gas above solution puts stress on eq Molecules collide with surface more often Increase in pressure partially offset by increase in rate of gas molecules entering solution In turn increase in amount of dissolved gas causes increase in rate molecules escape liquid and becomes gas
63
Eventually eq restored at higher gas solubility
Expected from Le Chatelierrsquos principle
Increase in gas pressure causes eq to shift so fewer molecules are in gas phase
64
Henryrsquos Law
Henryrsquos law - solubility of a gas in liquid is directly proportional to the partial pressure of that gas on the surface of the liquid
Applies to gas-liquid solutions at constant temp
65
When mixture of ideal gases is limited in constant volume at constant temp each gas applies same pressure it would apply if it occupied the space alone
Assuming gases do not react each gas dissolves to the extent it would if no other gases were there
66
In carbonated drinks (Coke Pepsi etc) solubility of CO2 increased by increasing pressure
At bottling factory CO2 gas forced into solution of flavored water at pressure of 5-10 atm Gas-in-liquid solution then sealed in bottlescans
67
When cap removed pressure reduced to 1 atm and some CO2 escapes as gas bubbles Effervescence - rapid escape of a gas from a liquid in which it is dissolved
68
Effects of Temperature on Solubility
Letrsquos consider gas solubility Increasing temperature usually decreases gas solubility As temp increases average kinetic energy of molecules in solution increases Greater number of solute molecules escape from attraction of solvent and return to gas phase 69
At higher temps eq is reached with fewer gas molecules in solution Gases generally less soluble
70
Effect of temp on solubility of solids in liquids more difficult to predict
Often increasing temp increases solubility
However the same temp increase can result in large increase in solubility in one case and only slight increase in the next
71
72
enthalpies of Solution
Formation of solution comes with energy change Dissolve KI in water container feels cold Dissolve LiCl in water feels hot Formation of solid-liquid solution can absorb or release heat
73
During formation of solution solvent and solute particles experience changes in forces attracting them to other particles Before dissolving begins solvent molecules held by intermolecular forces (solvent-solvent attraction) In solute molecules held by Ifs (solute-solute attraction)
74
Energy is required to separate solute molecules and solvent molecules from their neighbors
A solute particle that is surrounded by solvent molecules is said to be solvated
75
Step 1 solute particles become separated from solid (energy absorbed) Step 2 solvent particles move apart to allow solute particles to enter liquid (energy absorbed)
76
Step 3 solvent particles attracted to and solvate solute particles (energy released)
77
Heat of solution - net amount of heat energy absorbed or released when a specific amount of solute dissolves in a solvent
Heat of solution negative (heat is released) if sums of energy in steps 1 and 2 is less than step 3
If 1+2 gt 3 HOS is positive (heat absorbed)78
In gaseous state molecules are so far apart there are nearly no forces between them Solute-solute interaction has little effect on heat of solution of a gas Energy released when gas dissolved in liquid bc attraction between solute gas and solvent molecules greater than energy needed to separate solvent molecules
79
Concentration of Solutions80
Concentration - measure of amount of solute in given amount of solvent or solution ldquodiluterdquo just means small amount of solute ldquoconcentratedrdquo just means relatively large amount of solute Unrelated to degree to which solution is saturated Ex Saturated solution of substance not very soluble might be dilute
81
Molarity
Molarity - number of moles of solute in one liter of solution To find molarity must know molar mass A ldquoone molarrdquo solution of NaOH is 1 mole NaOH per liter of solution (1 M)
82
1 mol NaOH = 400 g If this quantity dissolve in enough water to make EXACTLY 100 L solution solution is 1 M If 200 g NaOH dissolve in enough to make 100 L solution what is molarity 0500 M
83
1 M solution is not made by adding 1 mol solute to 1 L solvent That would be more than 1 L Instead 1 mol solute dissolved in less than 1 L Then diluted with more solvent to bring TOTAL VOLUME to 1 L
84
Practice ProblemYou have 340 L of solution that contains 900 g sodium chloride What is the molarity of that solution 0440 M NaCl You have 08 L of a 05 M HCl solution How many moles of HCl does this solution contain 04 mol HCl
85
What is the molarity of a solution composed of 585 g of potassium iodide dissolved in enough water to make a 0125 L of solution 0282 M KI
86
How many moles of H2SO4 are present in 0500 L of a 0150 M H2SO4 solution 00750 mol What volume of 300 M NaCl is needed for reaction that requires 1463 g of NaCl 0834 L
87
Molality
Molality - concentration of solution expressed in moles of solute per kilogram of solvent Solution that contains 1 mol solute (NaOH) dissolved in exactly 1 kg of solvent is ldquoone molalrdquo solution (1 m NaOH)
88
1 mol NaOH = 400 g 400 g NaOH dissolved in 1 kg water is 1 m NaOH
200 g NaOH in 1 kg water = 0500 m NaOH
89
Practice Problem
A solution was prepared by dissolving 171 g sucrose (C12H22O11) in 125 g of water Find the molal concentration of this solution 0400 m C12H22O11
90
A solution of iodine in carbon tetrachloride is used when iodine is needed for certain chemical tests How much iodine must be added to prepare a 0480 m solution of iodine in CCl4 if 1000 g of CCl4 is used 122 g I2
91
What is the molality of a solution composed of 255 g acetone (CH3)2CO dissolved in 200 g of water 22 m acetone What quantity in grams of methanol CH3OH is required to prepare a 0244 m solution in 400 g water 312 g methanol
92
How many grams of AgNO3 are needed to prepare a 0125 m solution in 250 mL of water 531 g AgNO3
What is the molality of a solution containing 182 g HCl and 250 g water 200 m
93
Ions in Aqueous Solutions and Colligative Properties
94
95
compounds in aqueous solutions
DissociationWhen compound made from ions dissolves in water ions separate Dissociation - separation of ions that occurs when an ionic compound dissolves
96
Notice number of ions made per formula unit in equations 1 formula unit of NaCl gives 2 units of ions in solution 1 formula unit of CaCl2 gives 3 units of ions
97
Assuming 100 dissociation solution that contains 1 mol NaCl contains 1 mol Na+ and 1 mol Cl- Can assume 100 dissociation of all soluble ionic compounds
98
Practice Problem
Write the equation for the dissolution of aluminum sulfate in water How many moles of aluminum ions are made by dissolving 1 mol aluminum sulfate What is the total number of moles of ions made by dissolving 1 mol of aluminum sulfate
99
1 AnalyzeGiven Amount of solute = 1 mol Al2(SO4)3 Solvent identity = water Unknown a moles of aluminum ions and sulfate ions Total number of moles of solute ions produced
100
2 Plan
The coefficients in the balanced dissociation equation will tell mole relationships You can use the equation to find out the number of moles of solute ions produced
101
3 Compute
a
b102
Write the equation for the dissolution of each of the following in water and then determine the number of moles of each ions made as well as the total number of ions made a 1 mol ammonium chloride b 1 mol sodium sulfide c 05 mol barium nitrate
103
a 1 mol ammonium chloride
1 mol NH4+ 1 mol Cl- 2 mol total ions
104
b 1 mol sodium sulfide
2 mol Na+ 1 mol S-2 3 mol total ions
105
c 05 mol barium nitrate
05 mol Ba+2 1 mol NO3- 15 mol total ions
106
Precipitation Reactions
Even though no compound is completely insoluble compounds of very low solubility can be considered insoluble for practical purposes There are general guidelines to help predict whether a compound made of certain combination of ions is soluble
107
General Solubility Guidelines
Sodium potassium and ammonium compounds are soluble in water
108
Nitrates acetates and chlorates are soluble
109
110
Most chlorides are soluble except those of silver mercury (I) and lead Lead (II) chloride is soluble in hot water
Most sulfates are soluble except those of barium strontium lead calcium and mercury
111
Most carbonates phosphates and silicates are insoluble except those of sodium potassium and ammonium
112
Most sulfides are insoluble except those of calcium strontium sodium potassium and ammonium
113
Is calcium phosphate Ca3(PO4)2 soluble or insoluble
Not one of the exceptions so it is insoluble
Dissociation equations are not written for insoluble compounds
114
Guidelines also useful in predicting what will happen if 2 solutions of 2 different soluble compounds are mixed
If mixing results in combination of ions that forms insoluble compound a double-replacement and precipitation reaction will happen
115
Will precipitate form when solutions of ammonium sulfide and cadmium nitrate are combined You can tell calcium nitrate is soluble from the guidelines (Most nitrates acetates and chlorates are soluble) You can also tell ammonium sulfide is soluble (Most sulfides are insoluble except those of calcium strontium sodium potassium and ammonium)
116
117
To decide whether precipitate can form must know solubilities of two possible products
Ammonium nitrate ndash soluble (Most nitrates acetates and chlorates are soluble) Cadmium sulfide ndash insoluble (Most sulfides are insoluble except those of calcium strontium sodium potassium and ammonium)
118
bc one product is insoluble double-replacement and precipitation reaction will happen (NH4)2S (aq) + Cd(NO3)2 (aq) rarr NH4NO3 (aq) + CdS (s)
119
Net Ionic Equations
Reactions of ions in aqueous solution usually represented by net ionic equations instead of formula equations Net ionic equation - includes only those compounds and ions that undergo a chemical change in a reaction in an aqueous solution
120
121
To write NIE (net ionic equation) first convert chemical equation into overall ionic equation All soluble ionic compounds shown as dissociated ions in solution Precipitates shown as solids
122
Notice ammonium and nitrate ions appear on both sides They havenrsquot gone through any chemical change Spectator ions - ions that do not take part in a chemical reaction and are found in solution both before and after the reaction
123
To change ionic equation to NIE eliminate spectator ions Cd+2(aq) + S-2(aq) rarr CdS(s)
This NIE applies to any reaction in which precipitate of CdS forms
124
Practice Problem
Identify the precipitate that forms when aqueous solutions of zinc nitrate and ammonium sulfide are combined Write the equation for the possible double-replacement reaction Then write the formula equation overall ionic equation and the net ionic equation
125
1 AnalyzeGiven Identity of reactants zinc nitrate and ammonium sulfide Reaction medium aqueous solution Unknown a equation for the possible double-replacement reaction b identity of the precipitate c formula equation d overall ionic equation e net ionic equation
126
2 PlanWrite the possible double-replacement reaction between Zn(NO3)2 and (NH4)2S Use the rules to determine if any of the products will precipitate Write a formula equation and overall net ionic equation Then cancel spectator ions to make net ionic equation
127
3 Computea possible double-replacement reaction
b rules show that zinc sulfide is not soluble so will precipitate (ammonium nitrate is soluble)
c formula equation128
d overall ionic equation
e net ionic equation129
Will a precipitate form if solutions of potassium sulfate and barium nitrate are combined If so write the net ionic equation for the reaction
Yes Ba+2(aq) + SO4-2(aq) rarr BaSO4(s)
130
Will a precipitate form if solutions of potassium nitrate and magnesium sulfate are combined If so write the net ionic equation for the reaction
No 131
Will a precipitate form if solutions of barium chloride and sodium sulfate are combined If so identify the spectator ions and write the net ionic equation for the reaction
Yes Na+ and Cl- Ba+2(aq) + SO4-2(aq) rarr BaSO4(s)
132
Write the net ionic equation for the precipitation of nickel(II) sulfide
Ni+2(aq) + S-2(aq) rarr NiS(s)
133
IonizationSome molecular compounds can also form ions in solution Usually polar Ionization - ions formed from solute molecules by action of the solvent (creation of ions where there were none) Ionization different from dissociation
134
Dissociation ionic compounds dissolve and ions already present separate Ionization ions formed where non existed before
Ions formed are hydrated Heat released during hydration of ions gives enough energy to break covalent bonds
135
Extent to which solute ionizes depends on strength of bonds within molecules of solute and strength of attraction between solute and solvent molecules
136
HCl ndash molecular compound that ionizes in aqueous solution Contains highly polar bond Attraction between polar HCl molecule and polar water strong enough to break HCl bond
137
The Hydronium Ion
H+ ions from HCl attracts other molecules or ions so strongly that it doesnrsquot normally exist alone Ionization of HCl better described as direct transfer of proton from HCl to H2O forming H3O+
138
139
Hydration of H+ to form hydronium is highly exothermic
Energy released gives a lot of energy needed to ionize a molecular solute
Many molecular compounds that ionize in aqueous solution contain hydrogen and form hydronium ion
140
Strong and Weak Electrolytes
Substances that make ions and conduct current in solution are electrolytes HCl is one of series of compounds that has hydrogen and halogen Hydrogen halides are all molecular compounds with single polar-covalent bonds
141
All are gases Very soluble in water All are electrolytes HCl HBr HI strongly conduct current in solution HF only weakly conducts current at same concentration Strength of conduction related to ability to ionize
142
143
Strong Electrolytes
HCl HBr HI are 100 ionized in solution
Strong electrolyte - any compound whose dilute aqueous solutions conduct electricity well this is due to the presence of all or almost all of the dissolve compound in the form of ions
144
Unique characteristic of strong electrolyte is that they yield only ions
Ex Ionic compound may be highly soluble in water and dissociate into ions (NaCl)
Others may not dissolve much but amount that does dissolve exists only as ions
145
Weak ElectrolytesHF dissolves in water to give acid solution called hydrofluoric acid HF bond stronger than bonds between hydrogen and other halogens When HF dissolves some molecule ionize Reverse reaction also happens HF(aq) + H2O(l) H3O+(aq) + F-(aq)
146
HF(aq) + H2O(l) H3O+
(aq) + F-(aq)Concentration of dissolved unionized HF stays high and concentration of ions stays low
Weak electrolyte - any compound whose aqueous solutions conduct electricity poorly this is due to the presence of small amount of the dissolved compound in the form of ions
147
Different from nonelectrolyte ndash NO ions at all Description of electrolyte as strong or weak not related to concentration of solution Electrolytes differ in degree of ionization not amount of solute dissolved
148
149
Colligative properties of solutions
Presence of solute affects properties of solutions Some properties not dependent on nature of dissolved substance but on how may dissolve particles are present
Colligative properties - properties that depend on the concentration of solute particles but not on their identity
150
Vapor-Pressure Lowering
Boiling and freezing point of solution different from pure solvent Nonvolatile solute raises boiling point and lowers freezing point Nonvolatile substance - one that has little tendency to become gas under existing conditions
151
152
To understand why nonvolatile solute changes boiling and freezing point consider equilibrium vapor pressure
Vapor pressure pressure caused by molecules that have escape liquid phase to gas phase
Can be thought of a measure of tendency of molecules to escape from a liquid
153
Addition of sucrose (nonvolatile solute) lowers concentration of water molecules at surface of liquid This lowers tendency of water molecules to leave solution and enter gas phase Vapor pressure of solution is lower than vapor pressure of pure water
154
Aqueous13 solution13 of13 nonvolatile13 solutePure13 water
Nonelectrolyte solutions of same molality have same concentration of particles
Dilute solutions of same solvent and equal molality of any nonelectrolyte solute lower vapor pressure equally
155
Example
1 m aqueous solution of nonelectrolyte glucose lowers vapor pressure of water 55 x 10-4 atm at 25 1 m aqueous solution of sucrose also lowers water vapor pressure to 55 x 10-4 atm at 25
156
157
bc vapor-pressure lowering depends on concentration of nonelectrolyte solute and doesnrsquot depend on the type of solute it is a colligative proprerty
bc vapor pressure has been lowered solution remains liquid over larger temperature range
Lowers freezing point and raises boiling point
158
Can assume changes in boiling and freezing point also depend on concentration of solute
They are colligative properties
159
Freezing-Point Depression
Freezing point of 1-molal solution of any nonelectrolyte solute in water is found (by experiment) to be 186 lower than freezing point of water When 1 mol of nonelectrolyte solute dissolved in 1 kg water freezing point is -186 not 000 160
When 2 mol nonelectrolyte solute dissolved in 1 kg water freezing point is -372
For any concentration of nonelectrolyte solute in water decrease in freezing point can be determined using value of -186m
161
This value called the molal freezing-point contstant (Kf) is the freezing-point depression of the solvent in a 1-molal solution of a nonvolatile nonelectrolyte solute
162
Each solvent has own characteristic molal freezing-point constant Freezing-point depression (Δtf) the difference between the freezing points of the pure solvent and a solute of nonelectrolyte in that solvent and it is directly proportional to the molal concentration of the solution Δtf = Kfm
163
Δtf = Kfm
Kf expressed as m
m expressed in mol solutekg solvent (molality)
164
Sample Problem
What is the freezing point depression of water in a solution of 171 g of sucrose C12H22O11 and 200 g of water What is the actual freezing point of the solution
165
1 AnalyzeGiven Solute mass and chemical formula 171 g C12H22O11 Solvent mass and identity 2000 g water Unknown a freezing-point depression b freezing point of the solution 166
2 Plan
167
168
Calculate
Unknown a freezing-point depression b freezing point of the solution
169
Practice Problem
A water solution containing an unknown quantity of a nonelectrolyte solute is found to have a freezing point of ndash 023degCWhat is the molal concentration of the solution
012 m170
A solution consists of 103 g of the nonelectrolyte glucose C6H12O6 dissolved in 250 g of water What is the freezing-point depression of the solution
minus0426degC171
In a laboratory experiment the freezing point of an aqueous solution of glucose is found to be minus0325degCWhat is the molal concentration of this solution
0175 m172
If 0500 mol of a nonelectrolyte solute are dissolved in 5000 g of ether what is the freezing point of the solution
minus1181degC173
The freezing point of an aqueous solution that contains a nonelectrolyte is minus90degC aWhat is the freezing-point depression of the solution bWhat is the molal concentration of the solution a minus90degC b 48 m
174
Boiling-Point Elevation
Boiling point is temp at which vapor pressure is equal to atmospheric pressure Change in vapor pressure will cause corresponding change in boiling point
175
176
Vapor pressure of nonvolatile solution is lower than vapor pressure of pure solvent More heat will be required to raise vapor pressure of solution Boiling point of solution is higher than boiling point of pure solvent
Molal boiling-point constant (Kb) - boiling-point elevation of the solvent in a 1-molal solution of a nonvolatile nonelectrolyte solute b-p elevation of any nonelectrolyte in water found by experiment to be 051 So molal b-p constant for water is 051m
177
For different solvents b-p elevations of 1-molal solutions have different values (Table 14-2)
Like freezing-point constants values most accurate for dilute solutions
178
Boiling-point elevation Δtb difference between the boiling points of the pure solvent and a nonelectrolyte solution of that solvent and it is directly proportional to the molal concentration of the solution B-p elevation can be calculated using Δtb = Kbm Δtb (boiling point elevation) expressed in m and m expressed in mol solutekg solvent
Sample Problem
What is the boiling-point elevation of a solution made from 200 g of a nonelectrolyte solute and 4000 g of water The molar mass of the solute is 620 gmol
180
1 AnalyzeGiven solute mass = 200 g solute molar mass = 620 gmol solvent mass and identity = 4000 g of water
Unknown boiling-point elevation
181
2 Plan
182
183
3 Compute
Practice Problem
A solution contains 500 g of sucrose C12H22O11 a nonelectrolyte dissolved in 5000 g of water What is the boiling-point elevation
015degC184
A solution contains 4500 g of sucrose C12H22O11 a nonelectrolyte dissolved in 250 g of waterWhat is the boiling point of the solution
1027degC185
If the boiling point elevation of an aqueous solution containing a nonvolatile electrolyte is 102degC what is the molality of the solution
20 m186
The boiling point of an aqueous solution containing a nonvolatile electrolyte is 10075degC aWhat is the boiling-point elevation bWhat is the molality of the solution a 075degC b 15 m
187
Electrolytes and Colligative PropertiesEarly investigators confused by experiments where certain substances depressed freezing point or elevated boiling point more than expected
188
189
Ex 01 m solution of NaCl lowers freezing point of solvent almost twice as much as 01 m solution of sucrose 01 m CaCl2 lowers fp almost 3 times as much as 01 m solution of sucrose
To understand why this happens contrast behavior or sucrose with NaCl in aqueous solution Each sucrose dissolves to make only 1 particle in solution So 1 mol sucrose dissolves to 1 mol particles 1 mol NaCl dissolves to make 2 moles of particles (1 mol Na+ and 1 mol Cl-)
190
191
Calculated Values for Electrolyte SolutionsColligative properties depend on total concentration of solute particles regardless of their identity Electrolytes cause changes in colligative properties proportional to total molality in terms of all dissolves particles instead of formula units
192
What about barium nitrate Ba(NO3)2 Each mole of barium nitrate yields 3 mol of ions in solution Ba(NO3)2(s) rarr Ba2+(aq) + 2NO3-(aq) You would expect solution of given molality to lower fp of solvent 3 times as much as nonelectrolytic solution of same molality
193
Sample Problem
What is the expected change in the freezing point of water in a solution of 625 g of barium nitrate Ba(NO3)2 in 100 kg of water
194
1 AnalyzeGiven solute mass and formula = 625 g Ba(NO3)2 solvent mass and identity = 100 kg water Δtf = Kfm
Unknown expected freezing-point depression
195
2 PlanThe molality can be calculated by converting the solute mass to moles and then dividing by the number of kilograms of solvent That molality is in terms of formula units of Ba(NO3)2 and must be converted to molality in terms of dissociated ions in solution It must be multiplied by the number of moles of ions produced per mole of formula unit This adjusted molality can then be used to calculate the freezing-point depression
196
197
198
3 Compute
Practice Problem
What is the expected freezing-point depression for a solution that contains 20 mol of magnesium sulfate dissolved in 10 kg of water
minus74degC199
What is the expected boiling-point elevation of water for a solution that contains 150 g of sodium chloride dissolved in 10 kg of water 27degC The freezing point of an aqueous sodium chloride solution is minus200degC What is the molality of the solution 54 m NaCl
200
Actual Values for Electrolyte SolutionsRemember values calculated are only expected values Actual values of colligative properties for all strong electrolytes are almost what would be expected Difference between expected and actual values caused by attractive forces that exist between dissociated ions in aqueous solution
201
Attraction between hydrated ions in solution is small compared with those in crystalline solid However forces of attraction donrsquot interfere with movements of aqueous ions More concentrated solution means ions closer together and attraction is greater Also depends on charge of ions ndash higher charge greater attraction
202
Ethanol less polar than water more than carbon tetrachloride
Better solvent for less-polar substances bc of nonpolar region
59
Effects of Pressure on Solubility
Little effect on solids or liquids Increase pressure - increase solubilities of gases in liquids
60
When gas in contact with surface of liquid gas molecules can enter liquid As amount of dissolve gas increases some molecules start to escape and reenter gas phase
Eq eventually established between rates of enteringleaving gas phase
61
As long as eq undisturbed solubility of gas in liquid unchanged at given pressure
Gas + solvent solution
62
Increasing pressure of solute gas above solution puts stress on eq Molecules collide with surface more often Increase in pressure partially offset by increase in rate of gas molecules entering solution In turn increase in amount of dissolved gas causes increase in rate molecules escape liquid and becomes gas
63
Eventually eq restored at higher gas solubility
Expected from Le Chatelierrsquos principle
Increase in gas pressure causes eq to shift so fewer molecules are in gas phase
64
Henryrsquos Law
Henryrsquos law - solubility of a gas in liquid is directly proportional to the partial pressure of that gas on the surface of the liquid
Applies to gas-liquid solutions at constant temp
65
When mixture of ideal gases is limited in constant volume at constant temp each gas applies same pressure it would apply if it occupied the space alone
Assuming gases do not react each gas dissolves to the extent it would if no other gases were there
66
In carbonated drinks (Coke Pepsi etc) solubility of CO2 increased by increasing pressure
At bottling factory CO2 gas forced into solution of flavored water at pressure of 5-10 atm Gas-in-liquid solution then sealed in bottlescans
67
When cap removed pressure reduced to 1 atm and some CO2 escapes as gas bubbles Effervescence - rapid escape of a gas from a liquid in which it is dissolved
68
Effects of Temperature on Solubility
Letrsquos consider gas solubility Increasing temperature usually decreases gas solubility As temp increases average kinetic energy of molecules in solution increases Greater number of solute molecules escape from attraction of solvent and return to gas phase 69
At higher temps eq is reached with fewer gas molecules in solution Gases generally less soluble
70
Effect of temp on solubility of solids in liquids more difficult to predict
Often increasing temp increases solubility
However the same temp increase can result in large increase in solubility in one case and only slight increase in the next
71
72
enthalpies of Solution
Formation of solution comes with energy change Dissolve KI in water container feels cold Dissolve LiCl in water feels hot Formation of solid-liquid solution can absorb or release heat
73
During formation of solution solvent and solute particles experience changes in forces attracting them to other particles Before dissolving begins solvent molecules held by intermolecular forces (solvent-solvent attraction) In solute molecules held by Ifs (solute-solute attraction)
74
Energy is required to separate solute molecules and solvent molecules from their neighbors
A solute particle that is surrounded by solvent molecules is said to be solvated
75
Step 1 solute particles become separated from solid (energy absorbed) Step 2 solvent particles move apart to allow solute particles to enter liquid (energy absorbed)
76
Step 3 solvent particles attracted to and solvate solute particles (energy released)
77
Heat of solution - net amount of heat energy absorbed or released when a specific amount of solute dissolves in a solvent
Heat of solution negative (heat is released) if sums of energy in steps 1 and 2 is less than step 3
If 1+2 gt 3 HOS is positive (heat absorbed)78
In gaseous state molecules are so far apart there are nearly no forces between them Solute-solute interaction has little effect on heat of solution of a gas Energy released when gas dissolved in liquid bc attraction between solute gas and solvent molecules greater than energy needed to separate solvent molecules
79
Concentration of Solutions80
Concentration - measure of amount of solute in given amount of solvent or solution ldquodiluterdquo just means small amount of solute ldquoconcentratedrdquo just means relatively large amount of solute Unrelated to degree to which solution is saturated Ex Saturated solution of substance not very soluble might be dilute
81
Molarity
Molarity - number of moles of solute in one liter of solution To find molarity must know molar mass A ldquoone molarrdquo solution of NaOH is 1 mole NaOH per liter of solution (1 M)
82
1 mol NaOH = 400 g If this quantity dissolve in enough water to make EXACTLY 100 L solution solution is 1 M If 200 g NaOH dissolve in enough to make 100 L solution what is molarity 0500 M
83
1 M solution is not made by adding 1 mol solute to 1 L solvent That would be more than 1 L Instead 1 mol solute dissolved in less than 1 L Then diluted with more solvent to bring TOTAL VOLUME to 1 L
84
Practice ProblemYou have 340 L of solution that contains 900 g sodium chloride What is the molarity of that solution 0440 M NaCl You have 08 L of a 05 M HCl solution How many moles of HCl does this solution contain 04 mol HCl
85
What is the molarity of a solution composed of 585 g of potassium iodide dissolved in enough water to make a 0125 L of solution 0282 M KI
86
How many moles of H2SO4 are present in 0500 L of a 0150 M H2SO4 solution 00750 mol What volume of 300 M NaCl is needed for reaction that requires 1463 g of NaCl 0834 L
87
Molality
Molality - concentration of solution expressed in moles of solute per kilogram of solvent Solution that contains 1 mol solute (NaOH) dissolved in exactly 1 kg of solvent is ldquoone molalrdquo solution (1 m NaOH)
88
1 mol NaOH = 400 g 400 g NaOH dissolved in 1 kg water is 1 m NaOH
200 g NaOH in 1 kg water = 0500 m NaOH
89
Practice Problem
A solution was prepared by dissolving 171 g sucrose (C12H22O11) in 125 g of water Find the molal concentration of this solution 0400 m C12H22O11
90
A solution of iodine in carbon tetrachloride is used when iodine is needed for certain chemical tests How much iodine must be added to prepare a 0480 m solution of iodine in CCl4 if 1000 g of CCl4 is used 122 g I2
91
What is the molality of a solution composed of 255 g acetone (CH3)2CO dissolved in 200 g of water 22 m acetone What quantity in grams of methanol CH3OH is required to prepare a 0244 m solution in 400 g water 312 g methanol
92
How many grams of AgNO3 are needed to prepare a 0125 m solution in 250 mL of water 531 g AgNO3
What is the molality of a solution containing 182 g HCl and 250 g water 200 m
93
Ions in Aqueous Solutions and Colligative Properties
94
95
compounds in aqueous solutions
DissociationWhen compound made from ions dissolves in water ions separate Dissociation - separation of ions that occurs when an ionic compound dissolves
96
Notice number of ions made per formula unit in equations 1 formula unit of NaCl gives 2 units of ions in solution 1 formula unit of CaCl2 gives 3 units of ions
97
Assuming 100 dissociation solution that contains 1 mol NaCl contains 1 mol Na+ and 1 mol Cl- Can assume 100 dissociation of all soluble ionic compounds
98
Practice Problem
Write the equation for the dissolution of aluminum sulfate in water How many moles of aluminum ions are made by dissolving 1 mol aluminum sulfate What is the total number of moles of ions made by dissolving 1 mol of aluminum sulfate
99
1 AnalyzeGiven Amount of solute = 1 mol Al2(SO4)3 Solvent identity = water Unknown a moles of aluminum ions and sulfate ions Total number of moles of solute ions produced
100
2 Plan
The coefficients in the balanced dissociation equation will tell mole relationships You can use the equation to find out the number of moles of solute ions produced
101
3 Compute
a
b102
Write the equation for the dissolution of each of the following in water and then determine the number of moles of each ions made as well as the total number of ions made a 1 mol ammonium chloride b 1 mol sodium sulfide c 05 mol barium nitrate
103
a 1 mol ammonium chloride
1 mol NH4+ 1 mol Cl- 2 mol total ions
104
b 1 mol sodium sulfide
2 mol Na+ 1 mol S-2 3 mol total ions
105
c 05 mol barium nitrate
05 mol Ba+2 1 mol NO3- 15 mol total ions
106
Precipitation Reactions
Even though no compound is completely insoluble compounds of very low solubility can be considered insoluble for practical purposes There are general guidelines to help predict whether a compound made of certain combination of ions is soluble
107
General Solubility Guidelines
Sodium potassium and ammonium compounds are soluble in water
108
Nitrates acetates and chlorates are soluble
109
110
Most chlorides are soluble except those of silver mercury (I) and lead Lead (II) chloride is soluble in hot water
Most sulfates are soluble except those of barium strontium lead calcium and mercury
111
Most carbonates phosphates and silicates are insoluble except those of sodium potassium and ammonium
112
Most sulfides are insoluble except those of calcium strontium sodium potassium and ammonium
113
Is calcium phosphate Ca3(PO4)2 soluble or insoluble
Not one of the exceptions so it is insoluble
Dissociation equations are not written for insoluble compounds
114
Guidelines also useful in predicting what will happen if 2 solutions of 2 different soluble compounds are mixed
If mixing results in combination of ions that forms insoluble compound a double-replacement and precipitation reaction will happen
115
Will precipitate form when solutions of ammonium sulfide and cadmium nitrate are combined You can tell calcium nitrate is soluble from the guidelines (Most nitrates acetates and chlorates are soluble) You can also tell ammonium sulfide is soluble (Most sulfides are insoluble except those of calcium strontium sodium potassium and ammonium)
116
117
To decide whether precipitate can form must know solubilities of two possible products
Ammonium nitrate ndash soluble (Most nitrates acetates and chlorates are soluble) Cadmium sulfide ndash insoluble (Most sulfides are insoluble except those of calcium strontium sodium potassium and ammonium)
118
bc one product is insoluble double-replacement and precipitation reaction will happen (NH4)2S (aq) + Cd(NO3)2 (aq) rarr NH4NO3 (aq) + CdS (s)
119
Net Ionic Equations
Reactions of ions in aqueous solution usually represented by net ionic equations instead of formula equations Net ionic equation - includes only those compounds and ions that undergo a chemical change in a reaction in an aqueous solution
120
121
To write NIE (net ionic equation) first convert chemical equation into overall ionic equation All soluble ionic compounds shown as dissociated ions in solution Precipitates shown as solids
122
Notice ammonium and nitrate ions appear on both sides They havenrsquot gone through any chemical change Spectator ions - ions that do not take part in a chemical reaction and are found in solution both before and after the reaction
123
To change ionic equation to NIE eliminate spectator ions Cd+2(aq) + S-2(aq) rarr CdS(s)
This NIE applies to any reaction in which precipitate of CdS forms
124
Practice Problem
Identify the precipitate that forms when aqueous solutions of zinc nitrate and ammonium sulfide are combined Write the equation for the possible double-replacement reaction Then write the formula equation overall ionic equation and the net ionic equation
125
1 AnalyzeGiven Identity of reactants zinc nitrate and ammonium sulfide Reaction medium aqueous solution Unknown a equation for the possible double-replacement reaction b identity of the precipitate c formula equation d overall ionic equation e net ionic equation
126
2 PlanWrite the possible double-replacement reaction between Zn(NO3)2 and (NH4)2S Use the rules to determine if any of the products will precipitate Write a formula equation and overall net ionic equation Then cancel spectator ions to make net ionic equation
127
3 Computea possible double-replacement reaction
b rules show that zinc sulfide is not soluble so will precipitate (ammonium nitrate is soluble)
c formula equation128
d overall ionic equation
e net ionic equation129
Will a precipitate form if solutions of potassium sulfate and barium nitrate are combined If so write the net ionic equation for the reaction
Yes Ba+2(aq) + SO4-2(aq) rarr BaSO4(s)
130
Will a precipitate form if solutions of potassium nitrate and magnesium sulfate are combined If so write the net ionic equation for the reaction
No 131
Will a precipitate form if solutions of barium chloride and sodium sulfate are combined If so identify the spectator ions and write the net ionic equation for the reaction
Yes Na+ and Cl- Ba+2(aq) + SO4-2(aq) rarr BaSO4(s)
132
Write the net ionic equation for the precipitation of nickel(II) sulfide
Ni+2(aq) + S-2(aq) rarr NiS(s)
133
IonizationSome molecular compounds can also form ions in solution Usually polar Ionization - ions formed from solute molecules by action of the solvent (creation of ions where there were none) Ionization different from dissociation
134
Dissociation ionic compounds dissolve and ions already present separate Ionization ions formed where non existed before
Ions formed are hydrated Heat released during hydration of ions gives enough energy to break covalent bonds
135
Extent to which solute ionizes depends on strength of bonds within molecules of solute and strength of attraction between solute and solvent molecules
136
HCl ndash molecular compound that ionizes in aqueous solution Contains highly polar bond Attraction between polar HCl molecule and polar water strong enough to break HCl bond
137
The Hydronium Ion
H+ ions from HCl attracts other molecules or ions so strongly that it doesnrsquot normally exist alone Ionization of HCl better described as direct transfer of proton from HCl to H2O forming H3O+
138
139
Hydration of H+ to form hydronium is highly exothermic
Energy released gives a lot of energy needed to ionize a molecular solute
Many molecular compounds that ionize in aqueous solution contain hydrogen and form hydronium ion
140
Strong and Weak Electrolytes
Substances that make ions and conduct current in solution are electrolytes HCl is one of series of compounds that has hydrogen and halogen Hydrogen halides are all molecular compounds with single polar-covalent bonds
141
All are gases Very soluble in water All are electrolytes HCl HBr HI strongly conduct current in solution HF only weakly conducts current at same concentration Strength of conduction related to ability to ionize
142
143
Strong Electrolytes
HCl HBr HI are 100 ionized in solution
Strong electrolyte - any compound whose dilute aqueous solutions conduct electricity well this is due to the presence of all or almost all of the dissolve compound in the form of ions
144
Unique characteristic of strong electrolyte is that they yield only ions
Ex Ionic compound may be highly soluble in water and dissociate into ions (NaCl)
Others may not dissolve much but amount that does dissolve exists only as ions
145
Weak ElectrolytesHF dissolves in water to give acid solution called hydrofluoric acid HF bond stronger than bonds between hydrogen and other halogens When HF dissolves some molecule ionize Reverse reaction also happens HF(aq) + H2O(l) H3O+(aq) + F-(aq)
146
HF(aq) + H2O(l) H3O+
(aq) + F-(aq)Concentration of dissolved unionized HF stays high and concentration of ions stays low
Weak electrolyte - any compound whose aqueous solutions conduct electricity poorly this is due to the presence of small amount of the dissolved compound in the form of ions
147
Different from nonelectrolyte ndash NO ions at all Description of electrolyte as strong or weak not related to concentration of solution Electrolytes differ in degree of ionization not amount of solute dissolved
148
149
Colligative properties of solutions
Presence of solute affects properties of solutions Some properties not dependent on nature of dissolved substance but on how may dissolve particles are present
Colligative properties - properties that depend on the concentration of solute particles but not on their identity
150
Vapor-Pressure Lowering
Boiling and freezing point of solution different from pure solvent Nonvolatile solute raises boiling point and lowers freezing point Nonvolatile substance - one that has little tendency to become gas under existing conditions
151
152
To understand why nonvolatile solute changes boiling and freezing point consider equilibrium vapor pressure
Vapor pressure pressure caused by molecules that have escape liquid phase to gas phase
Can be thought of a measure of tendency of molecules to escape from a liquid
153
Addition of sucrose (nonvolatile solute) lowers concentration of water molecules at surface of liquid This lowers tendency of water molecules to leave solution and enter gas phase Vapor pressure of solution is lower than vapor pressure of pure water
154
Aqueous13 solution13 of13 nonvolatile13 solutePure13 water
Nonelectrolyte solutions of same molality have same concentration of particles
Dilute solutions of same solvent and equal molality of any nonelectrolyte solute lower vapor pressure equally
155
Example
1 m aqueous solution of nonelectrolyte glucose lowers vapor pressure of water 55 x 10-4 atm at 25 1 m aqueous solution of sucrose also lowers water vapor pressure to 55 x 10-4 atm at 25
156
157
bc vapor-pressure lowering depends on concentration of nonelectrolyte solute and doesnrsquot depend on the type of solute it is a colligative proprerty
bc vapor pressure has been lowered solution remains liquid over larger temperature range
Lowers freezing point and raises boiling point
158
Can assume changes in boiling and freezing point also depend on concentration of solute
They are colligative properties
159
Freezing-Point Depression
Freezing point of 1-molal solution of any nonelectrolyte solute in water is found (by experiment) to be 186 lower than freezing point of water When 1 mol of nonelectrolyte solute dissolved in 1 kg water freezing point is -186 not 000 160
When 2 mol nonelectrolyte solute dissolved in 1 kg water freezing point is -372
For any concentration of nonelectrolyte solute in water decrease in freezing point can be determined using value of -186m
161
This value called the molal freezing-point contstant (Kf) is the freezing-point depression of the solvent in a 1-molal solution of a nonvolatile nonelectrolyte solute
162
Each solvent has own characteristic molal freezing-point constant Freezing-point depression (Δtf) the difference between the freezing points of the pure solvent and a solute of nonelectrolyte in that solvent and it is directly proportional to the molal concentration of the solution Δtf = Kfm
163
Δtf = Kfm
Kf expressed as m
m expressed in mol solutekg solvent (molality)
164
Sample Problem
What is the freezing point depression of water in a solution of 171 g of sucrose C12H22O11 and 200 g of water What is the actual freezing point of the solution
165
1 AnalyzeGiven Solute mass and chemical formula 171 g C12H22O11 Solvent mass and identity 2000 g water Unknown a freezing-point depression b freezing point of the solution 166
2 Plan
167
168
Calculate
Unknown a freezing-point depression b freezing point of the solution
169
Practice Problem
A water solution containing an unknown quantity of a nonelectrolyte solute is found to have a freezing point of ndash 023degCWhat is the molal concentration of the solution
012 m170
A solution consists of 103 g of the nonelectrolyte glucose C6H12O6 dissolved in 250 g of water What is the freezing-point depression of the solution
minus0426degC171
In a laboratory experiment the freezing point of an aqueous solution of glucose is found to be minus0325degCWhat is the molal concentration of this solution
0175 m172
If 0500 mol of a nonelectrolyte solute are dissolved in 5000 g of ether what is the freezing point of the solution
minus1181degC173
The freezing point of an aqueous solution that contains a nonelectrolyte is minus90degC aWhat is the freezing-point depression of the solution bWhat is the molal concentration of the solution a minus90degC b 48 m
174
Boiling-Point Elevation
Boiling point is temp at which vapor pressure is equal to atmospheric pressure Change in vapor pressure will cause corresponding change in boiling point
175
176
Vapor pressure of nonvolatile solution is lower than vapor pressure of pure solvent More heat will be required to raise vapor pressure of solution Boiling point of solution is higher than boiling point of pure solvent
Molal boiling-point constant (Kb) - boiling-point elevation of the solvent in a 1-molal solution of a nonvolatile nonelectrolyte solute b-p elevation of any nonelectrolyte in water found by experiment to be 051 So molal b-p constant for water is 051m
177
For different solvents b-p elevations of 1-molal solutions have different values (Table 14-2)
Like freezing-point constants values most accurate for dilute solutions
178
Boiling-point elevation Δtb difference between the boiling points of the pure solvent and a nonelectrolyte solution of that solvent and it is directly proportional to the molal concentration of the solution B-p elevation can be calculated using Δtb = Kbm Δtb (boiling point elevation) expressed in m and m expressed in mol solutekg solvent
Sample Problem
What is the boiling-point elevation of a solution made from 200 g of a nonelectrolyte solute and 4000 g of water The molar mass of the solute is 620 gmol
180
1 AnalyzeGiven solute mass = 200 g solute molar mass = 620 gmol solvent mass and identity = 4000 g of water
Unknown boiling-point elevation
181
2 Plan
182
183
3 Compute
Practice Problem
A solution contains 500 g of sucrose C12H22O11 a nonelectrolyte dissolved in 5000 g of water What is the boiling-point elevation
015degC184
A solution contains 4500 g of sucrose C12H22O11 a nonelectrolyte dissolved in 250 g of waterWhat is the boiling point of the solution
1027degC185
If the boiling point elevation of an aqueous solution containing a nonvolatile electrolyte is 102degC what is the molality of the solution
20 m186
The boiling point of an aqueous solution containing a nonvolatile electrolyte is 10075degC aWhat is the boiling-point elevation bWhat is the molality of the solution a 075degC b 15 m
187
Electrolytes and Colligative PropertiesEarly investigators confused by experiments where certain substances depressed freezing point or elevated boiling point more than expected
188
189
Ex 01 m solution of NaCl lowers freezing point of solvent almost twice as much as 01 m solution of sucrose 01 m CaCl2 lowers fp almost 3 times as much as 01 m solution of sucrose
To understand why this happens contrast behavior or sucrose with NaCl in aqueous solution Each sucrose dissolves to make only 1 particle in solution So 1 mol sucrose dissolves to 1 mol particles 1 mol NaCl dissolves to make 2 moles of particles (1 mol Na+ and 1 mol Cl-)
190
191
Calculated Values for Electrolyte SolutionsColligative properties depend on total concentration of solute particles regardless of their identity Electrolytes cause changes in colligative properties proportional to total molality in terms of all dissolves particles instead of formula units
192
What about barium nitrate Ba(NO3)2 Each mole of barium nitrate yields 3 mol of ions in solution Ba(NO3)2(s) rarr Ba2+(aq) + 2NO3-(aq) You would expect solution of given molality to lower fp of solvent 3 times as much as nonelectrolytic solution of same molality
193
Sample Problem
What is the expected change in the freezing point of water in a solution of 625 g of barium nitrate Ba(NO3)2 in 100 kg of water
194
1 AnalyzeGiven solute mass and formula = 625 g Ba(NO3)2 solvent mass and identity = 100 kg water Δtf = Kfm
Unknown expected freezing-point depression
195
2 PlanThe molality can be calculated by converting the solute mass to moles and then dividing by the number of kilograms of solvent That molality is in terms of formula units of Ba(NO3)2 and must be converted to molality in terms of dissociated ions in solution It must be multiplied by the number of moles of ions produced per mole of formula unit This adjusted molality can then be used to calculate the freezing-point depression
196
197
198
3 Compute
Practice Problem
What is the expected freezing-point depression for a solution that contains 20 mol of magnesium sulfate dissolved in 10 kg of water
minus74degC199
What is the expected boiling-point elevation of water for a solution that contains 150 g of sodium chloride dissolved in 10 kg of water 27degC The freezing point of an aqueous sodium chloride solution is minus200degC What is the molality of the solution 54 m NaCl
200
Actual Values for Electrolyte SolutionsRemember values calculated are only expected values Actual values of colligative properties for all strong electrolytes are almost what would be expected Difference between expected and actual values caused by attractive forces that exist between dissociated ions in aqueous solution
201
Attraction between hydrated ions in solution is small compared with those in crystalline solid However forces of attraction donrsquot interfere with movements of aqueous ions More concentrated solution means ions closer together and attraction is greater Also depends on charge of ions ndash higher charge greater attraction
202
Effects of Pressure on Solubility
Little effect on solids or liquids Increase pressure - increase solubilities of gases in liquids
60
When gas in contact with surface of liquid gas molecules can enter liquid As amount of dissolve gas increases some molecules start to escape and reenter gas phase
Eq eventually established between rates of enteringleaving gas phase
61
As long as eq undisturbed solubility of gas in liquid unchanged at given pressure
Gas + solvent solution
62
Increasing pressure of solute gas above solution puts stress on eq Molecules collide with surface more often Increase in pressure partially offset by increase in rate of gas molecules entering solution In turn increase in amount of dissolved gas causes increase in rate molecules escape liquid and becomes gas
63
Eventually eq restored at higher gas solubility
Expected from Le Chatelierrsquos principle
Increase in gas pressure causes eq to shift so fewer molecules are in gas phase
64
Henryrsquos Law
Henryrsquos law - solubility of a gas in liquid is directly proportional to the partial pressure of that gas on the surface of the liquid
Applies to gas-liquid solutions at constant temp
65
When mixture of ideal gases is limited in constant volume at constant temp each gas applies same pressure it would apply if it occupied the space alone
Assuming gases do not react each gas dissolves to the extent it would if no other gases were there
66
In carbonated drinks (Coke Pepsi etc) solubility of CO2 increased by increasing pressure
At bottling factory CO2 gas forced into solution of flavored water at pressure of 5-10 atm Gas-in-liquid solution then sealed in bottlescans
67
When cap removed pressure reduced to 1 atm and some CO2 escapes as gas bubbles Effervescence - rapid escape of a gas from a liquid in which it is dissolved
68
Effects of Temperature on Solubility
Letrsquos consider gas solubility Increasing temperature usually decreases gas solubility As temp increases average kinetic energy of molecules in solution increases Greater number of solute molecules escape from attraction of solvent and return to gas phase 69
At higher temps eq is reached with fewer gas molecules in solution Gases generally less soluble
70
Effect of temp on solubility of solids in liquids more difficult to predict
Often increasing temp increases solubility
However the same temp increase can result in large increase in solubility in one case and only slight increase in the next
71
72
enthalpies of Solution
Formation of solution comes with energy change Dissolve KI in water container feels cold Dissolve LiCl in water feels hot Formation of solid-liquid solution can absorb or release heat
73
During formation of solution solvent and solute particles experience changes in forces attracting them to other particles Before dissolving begins solvent molecules held by intermolecular forces (solvent-solvent attraction) In solute molecules held by Ifs (solute-solute attraction)
74
Energy is required to separate solute molecules and solvent molecules from their neighbors
A solute particle that is surrounded by solvent molecules is said to be solvated
75
Step 1 solute particles become separated from solid (energy absorbed) Step 2 solvent particles move apart to allow solute particles to enter liquid (energy absorbed)
76
Step 3 solvent particles attracted to and solvate solute particles (energy released)
77
Heat of solution - net amount of heat energy absorbed or released when a specific amount of solute dissolves in a solvent
Heat of solution negative (heat is released) if sums of energy in steps 1 and 2 is less than step 3
If 1+2 gt 3 HOS is positive (heat absorbed)78
In gaseous state molecules are so far apart there are nearly no forces between them Solute-solute interaction has little effect on heat of solution of a gas Energy released when gas dissolved in liquid bc attraction between solute gas and solvent molecules greater than energy needed to separate solvent molecules
79
Concentration of Solutions80
Concentration - measure of amount of solute in given amount of solvent or solution ldquodiluterdquo just means small amount of solute ldquoconcentratedrdquo just means relatively large amount of solute Unrelated to degree to which solution is saturated Ex Saturated solution of substance not very soluble might be dilute
81
Molarity
Molarity - number of moles of solute in one liter of solution To find molarity must know molar mass A ldquoone molarrdquo solution of NaOH is 1 mole NaOH per liter of solution (1 M)
82
1 mol NaOH = 400 g If this quantity dissolve in enough water to make EXACTLY 100 L solution solution is 1 M If 200 g NaOH dissolve in enough to make 100 L solution what is molarity 0500 M
83
1 M solution is not made by adding 1 mol solute to 1 L solvent That would be more than 1 L Instead 1 mol solute dissolved in less than 1 L Then diluted with more solvent to bring TOTAL VOLUME to 1 L
84
Practice ProblemYou have 340 L of solution that contains 900 g sodium chloride What is the molarity of that solution 0440 M NaCl You have 08 L of a 05 M HCl solution How many moles of HCl does this solution contain 04 mol HCl
85
What is the molarity of a solution composed of 585 g of potassium iodide dissolved in enough water to make a 0125 L of solution 0282 M KI
86
How many moles of H2SO4 are present in 0500 L of a 0150 M H2SO4 solution 00750 mol What volume of 300 M NaCl is needed for reaction that requires 1463 g of NaCl 0834 L
87
Molality
Molality - concentration of solution expressed in moles of solute per kilogram of solvent Solution that contains 1 mol solute (NaOH) dissolved in exactly 1 kg of solvent is ldquoone molalrdquo solution (1 m NaOH)
88
1 mol NaOH = 400 g 400 g NaOH dissolved in 1 kg water is 1 m NaOH
200 g NaOH in 1 kg water = 0500 m NaOH
89
Practice Problem
A solution was prepared by dissolving 171 g sucrose (C12H22O11) in 125 g of water Find the molal concentration of this solution 0400 m C12H22O11
90
A solution of iodine in carbon tetrachloride is used when iodine is needed for certain chemical tests How much iodine must be added to prepare a 0480 m solution of iodine in CCl4 if 1000 g of CCl4 is used 122 g I2
91
What is the molality of a solution composed of 255 g acetone (CH3)2CO dissolved in 200 g of water 22 m acetone What quantity in grams of methanol CH3OH is required to prepare a 0244 m solution in 400 g water 312 g methanol
92
How many grams of AgNO3 are needed to prepare a 0125 m solution in 250 mL of water 531 g AgNO3
What is the molality of a solution containing 182 g HCl and 250 g water 200 m
93
Ions in Aqueous Solutions and Colligative Properties
94
95
compounds in aqueous solutions
DissociationWhen compound made from ions dissolves in water ions separate Dissociation - separation of ions that occurs when an ionic compound dissolves
96
Notice number of ions made per formula unit in equations 1 formula unit of NaCl gives 2 units of ions in solution 1 formula unit of CaCl2 gives 3 units of ions
97
Assuming 100 dissociation solution that contains 1 mol NaCl contains 1 mol Na+ and 1 mol Cl- Can assume 100 dissociation of all soluble ionic compounds
98
Practice Problem
Write the equation for the dissolution of aluminum sulfate in water How many moles of aluminum ions are made by dissolving 1 mol aluminum sulfate What is the total number of moles of ions made by dissolving 1 mol of aluminum sulfate
99
1 AnalyzeGiven Amount of solute = 1 mol Al2(SO4)3 Solvent identity = water Unknown a moles of aluminum ions and sulfate ions Total number of moles of solute ions produced
100
2 Plan
The coefficients in the balanced dissociation equation will tell mole relationships You can use the equation to find out the number of moles of solute ions produced
101
3 Compute
a
b102
Write the equation for the dissolution of each of the following in water and then determine the number of moles of each ions made as well as the total number of ions made a 1 mol ammonium chloride b 1 mol sodium sulfide c 05 mol barium nitrate
103
a 1 mol ammonium chloride
1 mol NH4+ 1 mol Cl- 2 mol total ions
104
b 1 mol sodium sulfide
2 mol Na+ 1 mol S-2 3 mol total ions
105
c 05 mol barium nitrate
05 mol Ba+2 1 mol NO3- 15 mol total ions
106
Precipitation Reactions
Even though no compound is completely insoluble compounds of very low solubility can be considered insoluble for practical purposes There are general guidelines to help predict whether a compound made of certain combination of ions is soluble
107
General Solubility Guidelines
Sodium potassium and ammonium compounds are soluble in water
108
Nitrates acetates and chlorates are soluble
109
110
Most chlorides are soluble except those of silver mercury (I) and lead Lead (II) chloride is soluble in hot water
Most sulfates are soluble except those of barium strontium lead calcium and mercury
111
Most carbonates phosphates and silicates are insoluble except those of sodium potassium and ammonium
112
Most sulfides are insoluble except those of calcium strontium sodium potassium and ammonium
113
Is calcium phosphate Ca3(PO4)2 soluble or insoluble
Not one of the exceptions so it is insoluble
Dissociation equations are not written for insoluble compounds
114
Guidelines also useful in predicting what will happen if 2 solutions of 2 different soluble compounds are mixed
If mixing results in combination of ions that forms insoluble compound a double-replacement and precipitation reaction will happen
115
Will precipitate form when solutions of ammonium sulfide and cadmium nitrate are combined You can tell calcium nitrate is soluble from the guidelines (Most nitrates acetates and chlorates are soluble) You can also tell ammonium sulfide is soluble (Most sulfides are insoluble except those of calcium strontium sodium potassium and ammonium)
116
117
To decide whether precipitate can form must know solubilities of two possible products
Ammonium nitrate ndash soluble (Most nitrates acetates and chlorates are soluble) Cadmium sulfide ndash insoluble (Most sulfides are insoluble except those of calcium strontium sodium potassium and ammonium)
118
bc one product is insoluble double-replacement and precipitation reaction will happen (NH4)2S (aq) + Cd(NO3)2 (aq) rarr NH4NO3 (aq) + CdS (s)
119
Net Ionic Equations
Reactions of ions in aqueous solution usually represented by net ionic equations instead of formula equations Net ionic equation - includes only those compounds and ions that undergo a chemical change in a reaction in an aqueous solution
120
121
To write NIE (net ionic equation) first convert chemical equation into overall ionic equation All soluble ionic compounds shown as dissociated ions in solution Precipitates shown as solids
122
Notice ammonium and nitrate ions appear on both sides They havenrsquot gone through any chemical change Spectator ions - ions that do not take part in a chemical reaction and are found in solution both before and after the reaction
123
To change ionic equation to NIE eliminate spectator ions Cd+2(aq) + S-2(aq) rarr CdS(s)
This NIE applies to any reaction in which precipitate of CdS forms
124
Practice Problem
Identify the precipitate that forms when aqueous solutions of zinc nitrate and ammonium sulfide are combined Write the equation for the possible double-replacement reaction Then write the formula equation overall ionic equation and the net ionic equation
125
1 AnalyzeGiven Identity of reactants zinc nitrate and ammonium sulfide Reaction medium aqueous solution Unknown a equation for the possible double-replacement reaction b identity of the precipitate c formula equation d overall ionic equation e net ionic equation
126
2 PlanWrite the possible double-replacement reaction between Zn(NO3)2 and (NH4)2S Use the rules to determine if any of the products will precipitate Write a formula equation and overall net ionic equation Then cancel spectator ions to make net ionic equation
127
3 Computea possible double-replacement reaction
b rules show that zinc sulfide is not soluble so will precipitate (ammonium nitrate is soluble)
c formula equation128
d overall ionic equation
e net ionic equation129
Will a precipitate form if solutions of potassium sulfate and barium nitrate are combined If so write the net ionic equation for the reaction
Yes Ba+2(aq) + SO4-2(aq) rarr BaSO4(s)
130
Will a precipitate form if solutions of potassium nitrate and magnesium sulfate are combined If so write the net ionic equation for the reaction
No 131
Will a precipitate form if solutions of barium chloride and sodium sulfate are combined If so identify the spectator ions and write the net ionic equation for the reaction
Yes Na+ and Cl- Ba+2(aq) + SO4-2(aq) rarr BaSO4(s)
132
Write the net ionic equation for the precipitation of nickel(II) sulfide
Ni+2(aq) + S-2(aq) rarr NiS(s)
133
IonizationSome molecular compounds can also form ions in solution Usually polar Ionization - ions formed from solute molecules by action of the solvent (creation of ions where there were none) Ionization different from dissociation
134
Dissociation ionic compounds dissolve and ions already present separate Ionization ions formed where non existed before
Ions formed are hydrated Heat released during hydration of ions gives enough energy to break covalent bonds
135
Extent to which solute ionizes depends on strength of bonds within molecules of solute and strength of attraction between solute and solvent molecules
136
HCl ndash molecular compound that ionizes in aqueous solution Contains highly polar bond Attraction between polar HCl molecule and polar water strong enough to break HCl bond
137
The Hydronium Ion
H+ ions from HCl attracts other molecules or ions so strongly that it doesnrsquot normally exist alone Ionization of HCl better described as direct transfer of proton from HCl to H2O forming H3O+
138
139
Hydration of H+ to form hydronium is highly exothermic
Energy released gives a lot of energy needed to ionize a molecular solute
Many molecular compounds that ionize in aqueous solution contain hydrogen and form hydronium ion
140
Strong and Weak Electrolytes
Substances that make ions and conduct current in solution are electrolytes HCl is one of series of compounds that has hydrogen and halogen Hydrogen halides are all molecular compounds with single polar-covalent bonds
141
All are gases Very soluble in water All are electrolytes HCl HBr HI strongly conduct current in solution HF only weakly conducts current at same concentration Strength of conduction related to ability to ionize
142
143
Strong Electrolytes
HCl HBr HI are 100 ionized in solution
Strong electrolyte - any compound whose dilute aqueous solutions conduct electricity well this is due to the presence of all or almost all of the dissolve compound in the form of ions
144
Unique characteristic of strong electrolyte is that they yield only ions
Ex Ionic compound may be highly soluble in water and dissociate into ions (NaCl)
Others may not dissolve much but amount that does dissolve exists only as ions
145
Weak ElectrolytesHF dissolves in water to give acid solution called hydrofluoric acid HF bond stronger than bonds between hydrogen and other halogens When HF dissolves some molecule ionize Reverse reaction also happens HF(aq) + H2O(l) H3O+(aq) + F-(aq)
146
HF(aq) + H2O(l) H3O+
(aq) + F-(aq)Concentration of dissolved unionized HF stays high and concentration of ions stays low
Weak electrolyte - any compound whose aqueous solutions conduct electricity poorly this is due to the presence of small amount of the dissolved compound in the form of ions
147
Different from nonelectrolyte ndash NO ions at all Description of electrolyte as strong or weak not related to concentration of solution Electrolytes differ in degree of ionization not amount of solute dissolved
148
149
Colligative properties of solutions
Presence of solute affects properties of solutions Some properties not dependent on nature of dissolved substance but on how may dissolve particles are present
Colligative properties - properties that depend on the concentration of solute particles but not on their identity
150
Vapor-Pressure Lowering
Boiling and freezing point of solution different from pure solvent Nonvolatile solute raises boiling point and lowers freezing point Nonvolatile substance - one that has little tendency to become gas under existing conditions
151
152
To understand why nonvolatile solute changes boiling and freezing point consider equilibrium vapor pressure
Vapor pressure pressure caused by molecules that have escape liquid phase to gas phase
Can be thought of a measure of tendency of molecules to escape from a liquid
153
Addition of sucrose (nonvolatile solute) lowers concentration of water molecules at surface of liquid This lowers tendency of water molecules to leave solution and enter gas phase Vapor pressure of solution is lower than vapor pressure of pure water
154
Aqueous13 solution13 of13 nonvolatile13 solutePure13 water
Nonelectrolyte solutions of same molality have same concentration of particles
Dilute solutions of same solvent and equal molality of any nonelectrolyte solute lower vapor pressure equally
155
Example
1 m aqueous solution of nonelectrolyte glucose lowers vapor pressure of water 55 x 10-4 atm at 25 1 m aqueous solution of sucrose also lowers water vapor pressure to 55 x 10-4 atm at 25
156
157
bc vapor-pressure lowering depends on concentration of nonelectrolyte solute and doesnrsquot depend on the type of solute it is a colligative proprerty
bc vapor pressure has been lowered solution remains liquid over larger temperature range
Lowers freezing point and raises boiling point
158
Can assume changes in boiling and freezing point also depend on concentration of solute
They are colligative properties
159
Freezing-Point Depression
Freezing point of 1-molal solution of any nonelectrolyte solute in water is found (by experiment) to be 186 lower than freezing point of water When 1 mol of nonelectrolyte solute dissolved in 1 kg water freezing point is -186 not 000 160
When 2 mol nonelectrolyte solute dissolved in 1 kg water freezing point is -372
For any concentration of nonelectrolyte solute in water decrease in freezing point can be determined using value of -186m
161
This value called the molal freezing-point contstant (Kf) is the freezing-point depression of the solvent in a 1-molal solution of a nonvolatile nonelectrolyte solute
162
Each solvent has own characteristic molal freezing-point constant Freezing-point depression (Δtf) the difference between the freezing points of the pure solvent and a solute of nonelectrolyte in that solvent and it is directly proportional to the molal concentration of the solution Δtf = Kfm
163
Δtf = Kfm
Kf expressed as m
m expressed in mol solutekg solvent (molality)
164
Sample Problem
What is the freezing point depression of water in a solution of 171 g of sucrose C12H22O11 and 200 g of water What is the actual freezing point of the solution
165
1 AnalyzeGiven Solute mass and chemical formula 171 g C12H22O11 Solvent mass and identity 2000 g water Unknown a freezing-point depression b freezing point of the solution 166
2 Plan
167
168
Calculate
Unknown a freezing-point depression b freezing point of the solution
169
Practice Problem
A water solution containing an unknown quantity of a nonelectrolyte solute is found to have a freezing point of ndash 023degCWhat is the molal concentration of the solution
012 m170
A solution consists of 103 g of the nonelectrolyte glucose C6H12O6 dissolved in 250 g of water What is the freezing-point depression of the solution
minus0426degC171
In a laboratory experiment the freezing point of an aqueous solution of glucose is found to be minus0325degCWhat is the molal concentration of this solution
0175 m172
If 0500 mol of a nonelectrolyte solute are dissolved in 5000 g of ether what is the freezing point of the solution
minus1181degC173
The freezing point of an aqueous solution that contains a nonelectrolyte is minus90degC aWhat is the freezing-point depression of the solution bWhat is the molal concentration of the solution a minus90degC b 48 m
174
Boiling-Point Elevation
Boiling point is temp at which vapor pressure is equal to atmospheric pressure Change in vapor pressure will cause corresponding change in boiling point
175
176
Vapor pressure of nonvolatile solution is lower than vapor pressure of pure solvent More heat will be required to raise vapor pressure of solution Boiling point of solution is higher than boiling point of pure solvent
Molal boiling-point constant (Kb) - boiling-point elevation of the solvent in a 1-molal solution of a nonvolatile nonelectrolyte solute b-p elevation of any nonelectrolyte in water found by experiment to be 051 So molal b-p constant for water is 051m
177
For different solvents b-p elevations of 1-molal solutions have different values (Table 14-2)
Like freezing-point constants values most accurate for dilute solutions
178
Boiling-point elevation Δtb difference between the boiling points of the pure solvent and a nonelectrolyte solution of that solvent and it is directly proportional to the molal concentration of the solution B-p elevation can be calculated using Δtb = Kbm Δtb (boiling point elevation) expressed in m and m expressed in mol solutekg solvent
Sample Problem
What is the boiling-point elevation of a solution made from 200 g of a nonelectrolyte solute and 4000 g of water The molar mass of the solute is 620 gmol
180
1 AnalyzeGiven solute mass = 200 g solute molar mass = 620 gmol solvent mass and identity = 4000 g of water
Unknown boiling-point elevation
181
2 Plan
182
183
3 Compute
Practice Problem
A solution contains 500 g of sucrose C12H22O11 a nonelectrolyte dissolved in 5000 g of water What is the boiling-point elevation
015degC184
A solution contains 4500 g of sucrose C12H22O11 a nonelectrolyte dissolved in 250 g of waterWhat is the boiling point of the solution
1027degC185
If the boiling point elevation of an aqueous solution containing a nonvolatile electrolyte is 102degC what is the molality of the solution
20 m186
The boiling point of an aqueous solution containing a nonvolatile electrolyte is 10075degC aWhat is the boiling-point elevation bWhat is the molality of the solution a 075degC b 15 m
187
Electrolytes and Colligative PropertiesEarly investigators confused by experiments where certain substances depressed freezing point or elevated boiling point more than expected
188
189
Ex 01 m solution of NaCl lowers freezing point of solvent almost twice as much as 01 m solution of sucrose 01 m CaCl2 lowers fp almost 3 times as much as 01 m solution of sucrose
To understand why this happens contrast behavior or sucrose with NaCl in aqueous solution Each sucrose dissolves to make only 1 particle in solution So 1 mol sucrose dissolves to 1 mol particles 1 mol NaCl dissolves to make 2 moles of particles (1 mol Na+ and 1 mol Cl-)
190
191
Calculated Values for Electrolyte SolutionsColligative properties depend on total concentration of solute particles regardless of their identity Electrolytes cause changes in colligative properties proportional to total molality in terms of all dissolves particles instead of formula units
192
What about barium nitrate Ba(NO3)2 Each mole of barium nitrate yields 3 mol of ions in solution Ba(NO3)2(s) rarr Ba2+(aq) + 2NO3-(aq) You would expect solution of given molality to lower fp of solvent 3 times as much as nonelectrolytic solution of same molality
193
Sample Problem
What is the expected change in the freezing point of water in a solution of 625 g of barium nitrate Ba(NO3)2 in 100 kg of water
194
1 AnalyzeGiven solute mass and formula = 625 g Ba(NO3)2 solvent mass and identity = 100 kg water Δtf = Kfm
Unknown expected freezing-point depression
195
2 PlanThe molality can be calculated by converting the solute mass to moles and then dividing by the number of kilograms of solvent That molality is in terms of formula units of Ba(NO3)2 and must be converted to molality in terms of dissociated ions in solution It must be multiplied by the number of moles of ions produced per mole of formula unit This adjusted molality can then be used to calculate the freezing-point depression
196
197
198
3 Compute
Practice Problem
What is the expected freezing-point depression for a solution that contains 20 mol of magnesium sulfate dissolved in 10 kg of water
minus74degC199
What is the expected boiling-point elevation of water for a solution that contains 150 g of sodium chloride dissolved in 10 kg of water 27degC The freezing point of an aqueous sodium chloride solution is minus200degC What is the molality of the solution 54 m NaCl
200
Actual Values for Electrolyte SolutionsRemember values calculated are only expected values Actual values of colligative properties for all strong electrolytes are almost what would be expected Difference between expected and actual values caused by attractive forces that exist between dissociated ions in aqueous solution
201
Attraction between hydrated ions in solution is small compared with those in crystalline solid However forces of attraction donrsquot interfere with movements of aqueous ions More concentrated solution means ions closer together and attraction is greater Also depends on charge of ions ndash higher charge greater attraction
202
When gas in contact with surface of liquid gas molecules can enter liquid As amount of dissolve gas increases some molecules start to escape and reenter gas phase
Eq eventually established between rates of enteringleaving gas phase
61
As long as eq undisturbed solubility of gas in liquid unchanged at given pressure
Gas + solvent solution
62
Increasing pressure of solute gas above solution puts stress on eq Molecules collide with surface more often Increase in pressure partially offset by increase in rate of gas molecules entering solution In turn increase in amount of dissolved gas causes increase in rate molecules escape liquid and becomes gas
63
Eventually eq restored at higher gas solubility
Expected from Le Chatelierrsquos principle
Increase in gas pressure causes eq to shift so fewer molecules are in gas phase
64
Henryrsquos Law
Henryrsquos law - solubility of a gas in liquid is directly proportional to the partial pressure of that gas on the surface of the liquid
Applies to gas-liquid solutions at constant temp
65
When mixture of ideal gases is limited in constant volume at constant temp each gas applies same pressure it would apply if it occupied the space alone
Assuming gases do not react each gas dissolves to the extent it would if no other gases were there
66
In carbonated drinks (Coke Pepsi etc) solubility of CO2 increased by increasing pressure
At bottling factory CO2 gas forced into solution of flavored water at pressure of 5-10 atm Gas-in-liquid solution then sealed in bottlescans
67
When cap removed pressure reduced to 1 atm and some CO2 escapes as gas bubbles Effervescence - rapid escape of a gas from a liquid in which it is dissolved
68
Effects of Temperature on Solubility
Letrsquos consider gas solubility Increasing temperature usually decreases gas solubility As temp increases average kinetic energy of molecules in solution increases Greater number of solute molecules escape from attraction of solvent and return to gas phase 69
At higher temps eq is reached with fewer gas molecules in solution Gases generally less soluble
70
Effect of temp on solubility of solids in liquids more difficult to predict
Often increasing temp increases solubility
However the same temp increase can result in large increase in solubility in one case and only slight increase in the next
71
72
enthalpies of Solution
Formation of solution comes with energy change Dissolve KI in water container feels cold Dissolve LiCl in water feels hot Formation of solid-liquid solution can absorb or release heat
73
During formation of solution solvent and solute particles experience changes in forces attracting them to other particles Before dissolving begins solvent molecules held by intermolecular forces (solvent-solvent attraction) In solute molecules held by Ifs (solute-solute attraction)
74
Energy is required to separate solute molecules and solvent molecules from their neighbors
A solute particle that is surrounded by solvent molecules is said to be solvated
75
Step 1 solute particles become separated from solid (energy absorbed) Step 2 solvent particles move apart to allow solute particles to enter liquid (energy absorbed)
76
Step 3 solvent particles attracted to and solvate solute particles (energy released)
77
Heat of solution - net amount of heat energy absorbed or released when a specific amount of solute dissolves in a solvent
Heat of solution negative (heat is released) if sums of energy in steps 1 and 2 is less than step 3
If 1+2 gt 3 HOS is positive (heat absorbed)78
In gaseous state molecules are so far apart there are nearly no forces between them Solute-solute interaction has little effect on heat of solution of a gas Energy released when gas dissolved in liquid bc attraction between solute gas and solvent molecules greater than energy needed to separate solvent molecules
79
Concentration of Solutions80
Concentration - measure of amount of solute in given amount of solvent or solution ldquodiluterdquo just means small amount of solute ldquoconcentratedrdquo just means relatively large amount of solute Unrelated to degree to which solution is saturated Ex Saturated solution of substance not very soluble might be dilute
81
Molarity
Molarity - number of moles of solute in one liter of solution To find molarity must know molar mass A ldquoone molarrdquo solution of NaOH is 1 mole NaOH per liter of solution (1 M)
82
1 mol NaOH = 400 g If this quantity dissolve in enough water to make EXACTLY 100 L solution solution is 1 M If 200 g NaOH dissolve in enough to make 100 L solution what is molarity 0500 M
83
1 M solution is not made by adding 1 mol solute to 1 L solvent That would be more than 1 L Instead 1 mol solute dissolved in less than 1 L Then diluted with more solvent to bring TOTAL VOLUME to 1 L
84
Practice ProblemYou have 340 L of solution that contains 900 g sodium chloride What is the molarity of that solution 0440 M NaCl You have 08 L of a 05 M HCl solution How many moles of HCl does this solution contain 04 mol HCl
85
What is the molarity of a solution composed of 585 g of potassium iodide dissolved in enough water to make a 0125 L of solution 0282 M KI
86
How many moles of H2SO4 are present in 0500 L of a 0150 M H2SO4 solution 00750 mol What volume of 300 M NaCl is needed for reaction that requires 1463 g of NaCl 0834 L
87
Molality
Molality - concentration of solution expressed in moles of solute per kilogram of solvent Solution that contains 1 mol solute (NaOH) dissolved in exactly 1 kg of solvent is ldquoone molalrdquo solution (1 m NaOH)
88
1 mol NaOH = 400 g 400 g NaOH dissolved in 1 kg water is 1 m NaOH
200 g NaOH in 1 kg water = 0500 m NaOH
89
Practice Problem
A solution was prepared by dissolving 171 g sucrose (C12H22O11) in 125 g of water Find the molal concentration of this solution 0400 m C12H22O11
90
A solution of iodine in carbon tetrachloride is used when iodine is needed for certain chemical tests How much iodine must be added to prepare a 0480 m solution of iodine in CCl4 if 1000 g of CCl4 is used 122 g I2
91
What is the molality of a solution composed of 255 g acetone (CH3)2CO dissolved in 200 g of water 22 m acetone What quantity in grams of methanol CH3OH is required to prepare a 0244 m solution in 400 g water 312 g methanol
92
How many grams of AgNO3 are needed to prepare a 0125 m solution in 250 mL of water 531 g AgNO3
What is the molality of a solution containing 182 g HCl and 250 g water 200 m
93
Ions in Aqueous Solutions and Colligative Properties
94
95
compounds in aqueous solutions
DissociationWhen compound made from ions dissolves in water ions separate Dissociation - separation of ions that occurs when an ionic compound dissolves
96
Notice number of ions made per formula unit in equations 1 formula unit of NaCl gives 2 units of ions in solution 1 formula unit of CaCl2 gives 3 units of ions
97
Assuming 100 dissociation solution that contains 1 mol NaCl contains 1 mol Na+ and 1 mol Cl- Can assume 100 dissociation of all soluble ionic compounds
98
Practice Problem
Write the equation for the dissolution of aluminum sulfate in water How many moles of aluminum ions are made by dissolving 1 mol aluminum sulfate What is the total number of moles of ions made by dissolving 1 mol of aluminum sulfate
99
1 AnalyzeGiven Amount of solute = 1 mol Al2(SO4)3 Solvent identity = water Unknown a moles of aluminum ions and sulfate ions Total number of moles of solute ions produced
100
2 Plan
The coefficients in the balanced dissociation equation will tell mole relationships You can use the equation to find out the number of moles of solute ions produced
101
3 Compute
a
b102
Write the equation for the dissolution of each of the following in water and then determine the number of moles of each ions made as well as the total number of ions made a 1 mol ammonium chloride b 1 mol sodium sulfide c 05 mol barium nitrate
103
a 1 mol ammonium chloride
1 mol NH4+ 1 mol Cl- 2 mol total ions
104
b 1 mol sodium sulfide
2 mol Na+ 1 mol S-2 3 mol total ions
105
c 05 mol barium nitrate
05 mol Ba+2 1 mol NO3- 15 mol total ions
106
Precipitation Reactions
Even though no compound is completely insoluble compounds of very low solubility can be considered insoluble for practical purposes There are general guidelines to help predict whether a compound made of certain combination of ions is soluble
107
General Solubility Guidelines
Sodium potassium and ammonium compounds are soluble in water
108
Nitrates acetates and chlorates are soluble
109
110
Most chlorides are soluble except those of silver mercury (I) and lead Lead (II) chloride is soluble in hot water
Most sulfates are soluble except those of barium strontium lead calcium and mercury
111
Most carbonates phosphates and silicates are insoluble except those of sodium potassium and ammonium
112
Most sulfides are insoluble except those of calcium strontium sodium potassium and ammonium
113
Is calcium phosphate Ca3(PO4)2 soluble or insoluble
Not one of the exceptions so it is insoluble
Dissociation equations are not written for insoluble compounds
114
Guidelines also useful in predicting what will happen if 2 solutions of 2 different soluble compounds are mixed
If mixing results in combination of ions that forms insoluble compound a double-replacement and precipitation reaction will happen
115
Will precipitate form when solutions of ammonium sulfide and cadmium nitrate are combined You can tell calcium nitrate is soluble from the guidelines (Most nitrates acetates and chlorates are soluble) You can also tell ammonium sulfide is soluble (Most sulfides are insoluble except those of calcium strontium sodium potassium and ammonium)
116
117
To decide whether precipitate can form must know solubilities of two possible products
Ammonium nitrate ndash soluble (Most nitrates acetates and chlorates are soluble) Cadmium sulfide ndash insoluble (Most sulfides are insoluble except those of calcium strontium sodium potassium and ammonium)
118
bc one product is insoluble double-replacement and precipitation reaction will happen (NH4)2S (aq) + Cd(NO3)2 (aq) rarr NH4NO3 (aq) + CdS (s)
119
Net Ionic Equations
Reactions of ions in aqueous solution usually represented by net ionic equations instead of formula equations Net ionic equation - includes only those compounds and ions that undergo a chemical change in a reaction in an aqueous solution
120
121
To write NIE (net ionic equation) first convert chemical equation into overall ionic equation All soluble ionic compounds shown as dissociated ions in solution Precipitates shown as solids
122
Notice ammonium and nitrate ions appear on both sides They havenrsquot gone through any chemical change Spectator ions - ions that do not take part in a chemical reaction and are found in solution both before and after the reaction
123
To change ionic equation to NIE eliminate spectator ions Cd+2(aq) + S-2(aq) rarr CdS(s)
This NIE applies to any reaction in which precipitate of CdS forms
124
Practice Problem
Identify the precipitate that forms when aqueous solutions of zinc nitrate and ammonium sulfide are combined Write the equation for the possible double-replacement reaction Then write the formula equation overall ionic equation and the net ionic equation
125
1 AnalyzeGiven Identity of reactants zinc nitrate and ammonium sulfide Reaction medium aqueous solution Unknown a equation for the possible double-replacement reaction b identity of the precipitate c formula equation d overall ionic equation e net ionic equation
126
2 PlanWrite the possible double-replacement reaction between Zn(NO3)2 and (NH4)2S Use the rules to determine if any of the products will precipitate Write a formula equation and overall net ionic equation Then cancel spectator ions to make net ionic equation
127
3 Computea possible double-replacement reaction
b rules show that zinc sulfide is not soluble so will precipitate (ammonium nitrate is soluble)
c formula equation128
d overall ionic equation
e net ionic equation129
Will a precipitate form if solutions of potassium sulfate and barium nitrate are combined If so write the net ionic equation for the reaction
Yes Ba+2(aq) + SO4-2(aq) rarr BaSO4(s)
130
Will a precipitate form if solutions of potassium nitrate and magnesium sulfate are combined If so write the net ionic equation for the reaction
No 131
Will a precipitate form if solutions of barium chloride and sodium sulfate are combined If so identify the spectator ions and write the net ionic equation for the reaction
Yes Na+ and Cl- Ba+2(aq) + SO4-2(aq) rarr BaSO4(s)
132
Write the net ionic equation for the precipitation of nickel(II) sulfide
Ni+2(aq) + S-2(aq) rarr NiS(s)
133
IonizationSome molecular compounds can also form ions in solution Usually polar Ionization - ions formed from solute molecules by action of the solvent (creation of ions where there were none) Ionization different from dissociation
134
Dissociation ionic compounds dissolve and ions already present separate Ionization ions formed where non existed before
Ions formed are hydrated Heat released during hydration of ions gives enough energy to break covalent bonds
135
Extent to which solute ionizes depends on strength of bonds within molecules of solute and strength of attraction between solute and solvent molecules
136
HCl ndash molecular compound that ionizes in aqueous solution Contains highly polar bond Attraction between polar HCl molecule and polar water strong enough to break HCl bond
137
The Hydronium Ion
H+ ions from HCl attracts other molecules or ions so strongly that it doesnrsquot normally exist alone Ionization of HCl better described as direct transfer of proton from HCl to H2O forming H3O+
138
139
Hydration of H+ to form hydronium is highly exothermic
Energy released gives a lot of energy needed to ionize a molecular solute
Many molecular compounds that ionize in aqueous solution contain hydrogen and form hydronium ion
140
Strong and Weak Electrolytes
Substances that make ions and conduct current in solution are electrolytes HCl is one of series of compounds that has hydrogen and halogen Hydrogen halides are all molecular compounds with single polar-covalent bonds
141
All are gases Very soluble in water All are electrolytes HCl HBr HI strongly conduct current in solution HF only weakly conducts current at same concentration Strength of conduction related to ability to ionize
142
143
Strong Electrolytes
HCl HBr HI are 100 ionized in solution
Strong electrolyte - any compound whose dilute aqueous solutions conduct electricity well this is due to the presence of all or almost all of the dissolve compound in the form of ions
144
Unique characteristic of strong electrolyte is that they yield only ions
Ex Ionic compound may be highly soluble in water and dissociate into ions (NaCl)
Others may not dissolve much but amount that does dissolve exists only as ions
145
Weak ElectrolytesHF dissolves in water to give acid solution called hydrofluoric acid HF bond stronger than bonds between hydrogen and other halogens When HF dissolves some molecule ionize Reverse reaction also happens HF(aq) + H2O(l) H3O+(aq) + F-(aq)
146
HF(aq) + H2O(l) H3O+
(aq) + F-(aq)Concentration of dissolved unionized HF stays high and concentration of ions stays low
Weak electrolyte - any compound whose aqueous solutions conduct electricity poorly this is due to the presence of small amount of the dissolved compound in the form of ions
147
Different from nonelectrolyte ndash NO ions at all Description of electrolyte as strong or weak not related to concentration of solution Electrolytes differ in degree of ionization not amount of solute dissolved
148
149
Colligative properties of solutions
Presence of solute affects properties of solutions Some properties not dependent on nature of dissolved substance but on how may dissolve particles are present
Colligative properties - properties that depend on the concentration of solute particles but not on their identity
150
Vapor-Pressure Lowering
Boiling and freezing point of solution different from pure solvent Nonvolatile solute raises boiling point and lowers freezing point Nonvolatile substance - one that has little tendency to become gas under existing conditions
151
152
To understand why nonvolatile solute changes boiling and freezing point consider equilibrium vapor pressure
Vapor pressure pressure caused by molecules that have escape liquid phase to gas phase
Can be thought of a measure of tendency of molecules to escape from a liquid
153
Addition of sucrose (nonvolatile solute) lowers concentration of water molecules at surface of liquid This lowers tendency of water molecules to leave solution and enter gas phase Vapor pressure of solution is lower than vapor pressure of pure water
154
Aqueous13 solution13 of13 nonvolatile13 solutePure13 water
Nonelectrolyte solutions of same molality have same concentration of particles
Dilute solutions of same solvent and equal molality of any nonelectrolyte solute lower vapor pressure equally
155
Example
1 m aqueous solution of nonelectrolyte glucose lowers vapor pressure of water 55 x 10-4 atm at 25 1 m aqueous solution of sucrose also lowers water vapor pressure to 55 x 10-4 atm at 25
156
157
bc vapor-pressure lowering depends on concentration of nonelectrolyte solute and doesnrsquot depend on the type of solute it is a colligative proprerty
bc vapor pressure has been lowered solution remains liquid over larger temperature range
Lowers freezing point and raises boiling point
158
Can assume changes in boiling and freezing point also depend on concentration of solute
They are colligative properties
159
Freezing-Point Depression
Freezing point of 1-molal solution of any nonelectrolyte solute in water is found (by experiment) to be 186 lower than freezing point of water When 1 mol of nonelectrolyte solute dissolved in 1 kg water freezing point is -186 not 000 160
When 2 mol nonelectrolyte solute dissolved in 1 kg water freezing point is -372
For any concentration of nonelectrolyte solute in water decrease in freezing point can be determined using value of -186m
161
This value called the molal freezing-point contstant (Kf) is the freezing-point depression of the solvent in a 1-molal solution of a nonvolatile nonelectrolyte solute
162
Each solvent has own characteristic molal freezing-point constant Freezing-point depression (Δtf) the difference between the freezing points of the pure solvent and a solute of nonelectrolyte in that solvent and it is directly proportional to the molal concentration of the solution Δtf = Kfm
163
Δtf = Kfm
Kf expressed as m
m expressed in mol solutekg solvent (molality)
164
Sample Problem
What is the freezing point depression of water in a solution of 171 g of sucrose C12H22O11 and 200 g of water What is the actual freezing point of the solution
165
1 AnalyzeGiven Solute mass and chemical formula 171 g C12H22O11 Solvent mass and identity 2000 g water Unknown a freezing-point depression b freezing point of the solution 166
2 Plan
167
168
Calculate
Unknown a freezing-point depression b freezing point of the solution
169
Practice Problem
A water solution containing an unknown quantity of a nonelectrolyte solute is found to have a freezing point of ndash 023degCWhat is the molal concentration of the solution
012 m170
A solution consists of 103 g of the nonelectrolyte glucose C6H12O6 dissolved in 250 g of water What is the freezing-point depression of the solution
minus0426degC171
In a laboratory experiment the freezing point of an aqueous solution of glucose is found to be minus0325degCWhat is the molal concentration of this solution
0175 m172
If 0500 mol of a nonelectrolyte solute are dissolved in 5000 g of ether what is the freezing point of the solution
minus1181degC173
The freezing point of an aqueous solution that contains a nonelectrolyte is minus90degC aWhat is the freezing-point depression of the solution bWhat is the molal concentration of the solution a minus90degC b 48 m
174
Boiling-Point Elevation
Boiling point is temp at which vapor pressure is equal to atmospheric pressure Change in vapor pressure will cause corresponding change in boiling point
175
176
Vapor pressure of nonvolatile solution is lower than vapor pressure of pure solvent More heat will be required to raise vapor pressure of solution Boiling point of solution is higher than boiling point of pure solvent
Molal boiling-point constant (Kb) - boiling-point elevation of the solvent in a 1-molal solution of a nonvolatile nonelectrolyte solute b-p elevation of any nonelectrolyte in water found by experiment to be 051 So molal b-p constant for water is 051m
177
For different solvents b-p elevations of 1-molal solutions have different values (Table 14-2)
Like freezing-point constants values most accurate for dilute solutions
178
Boiling-point elevation Δtb difference between the boiling points of the pure solvent and a nonelectrolyte solution of that solvent and it is directly proportional to the molal concentration of the solution B-p elevation can be calculated using Δtb = Kbm Δtb (boiling point elevation) expressed in m and m expressed in mol solutekg solvent
Sample Problem
What is the boiling-point elevation of a solution made from 200 g of a nonelectrolyte solute and 4000 g of water The molar mass of the solute is 620 gmol
180
1 AnalyzeGiven solute mass = 200 g solute molar mass = 620 gmol solvent mass and identity = 4000 g of water
Unknown boiling-point elevation
181
2 Plan
182
183
3 Compute
Practice Problem
A solution contains 500 g of sucrose C12H22O11 a nonelectrolyte dissolved in 5000 g of water What is the boiling-point elevation
015degC184
A solution contains 4500 g of sucrose C12H22O11 a nonelectrolyte dissolved in 250 g of waterWhat is the boiling point of the solution
1027degC185
If the boiling point elevation of an aqueous solution containing a nonvolatile electrolyte is 102degC what is the molality of the solution
20 m186
The boiling point of an aqueous solution containing a nonvolatile electrolyte is 10075degC aWhat is the boiling-point elevation bWhat is the molality of the solution a 075degC b 15 m
187
Electrolytes and Colligative PropertiesEarly investigators confused by experiments where certain substances depressed freezing point or elevated boiling point more than expected
188
189
Ex 01 m solution of NaCl lowers freezing point of solvent almost twice as much as 01 m solution of sucrose 01 m CaCl2 lowers fp almost 3 times as much as 01 m solution of sucrose
To understand why this happens contrast behavior or sucrose with NaCl in aqueous solution Each sucrose dissolves to make only 1 particle in solution So 1 mol sucrose dissolves to 1 mol particles 1 mol NaCl dissolves to make 2 moles of particles (1 mol Na+ and 1 mol Cl-)
190
191
Calculated Values for Electrolyte SolutionsColligative properties depend on total concentration of solute particles regardless of their identity Electrolytes cause changes in colligative properties proportional to total molality in terms of all dissolves particles instead of formula units
192
What about barium nitrate Ba(NO3)2 Each mole of barium nitrate yields 3 mol of ions in solution Ba(NO3)2(s) rarr Ba2+(aq) + 2NO3-(aq) You would expect solution of given molality to lower fp of solvent 3 times as much as nonelectrolytic solution of same molality
193
Sample Problem
What is the expected change in the freezing point of water in a solution of 625 g of barium nitrate Ba(NO3)2 in 100 kg of water
194
1 AnalyzeGiven solute mass and formula = 625 g Ba(NO3)2 solvent mass and identity = 100 kg water Δtf = Kfm
Unknown expected freezing-point depression
195
2 PlanThe molality can be calculated by converting the solute mass to moles and then dividing by the number of kilograms of solvent That molality is in terms of formula units of Ba(NO3)2 and must be converted to molality in terms of dissociated ions in solution It must be multiplied by the number of moles of ions produced per mole of formula unit This adjusted molality can then be used to calculate the freezing-point depression
196
197
198
3 Compute
Practice Problem
What is the expected freezing-point depression for a solution that contains 20 mol of magnesium sulfate dissolved in 10 kg of water
minus74degC199
What is the expected boiling-point elevation of water for a solution that contains 150 g of sodium chloride dissolved in 10 kg of water 27degC The freezing point of an aqueous sodium chloride solution is minus200degC What is the molality of the solution 54 m NaCl
200
Actual Values for Electrolyte SolutionsRemember values calculated are only expected values Actual values of colligative properties for all strong electrolytes are almost what would be expected Difference between expected and actual values caused by attractive forces that exist between dissociated ions in aqueous solution
201
Attraction between hydrated ions in solution is small compared with those in crystalline solid However forces of attraction donrsquot interfere with movements of aqueous ions More concentrated solution means ions closer together and attraction is greater Also depends on charge of ions ndash higher charge greater attraction
202
As long as eq undisturbed solubility of gas in liquid unchanged at given pressure
Gas + solvent solution
62
Increasing pressure of solute gas above solution puts stress on eq Molecules collide with surface more often Increase in pressure partially offset by increase in rate of gas molecules entering solution In turn increase in amount of dissolved gas causes increase in rate molecules escape liquid and becomes gas
63
Eventually eq restored at higher gas solubility
Expected from Le Chatelierrsquos principle
Increase in gas pressure causes eq to shift so fewer molecules are in gas phase
64
Henryrsquos Law
Henryrsquos law - solubility of a gas in liquid is directly proportional to the partial pressure of that gas on the surface of the liquid
Applies to gas-liquid solutions at constant temp
65
When mixture of ideal gases is limited in constant volume at constant temp each gas applies same pressure it would apply if it occupied the space alone
Assuming gases do not react each gas dissolves to the extent it would if no other gases were there
66
In carbonated drinks (Coke Pepsi etc) solubility of CO2 increased by increasing pressure
At bottling factory CO2 gas forced into solution of flavored water at pressure of 5-10 atm Gas-in-liquid solution then sealed in bottlescans
67
When cap removed pressure reduced to 1 atm and some CO2 escapes as gas bubbles Effervescence - rapid escape of a gas from a liquid in which it is dissolved
68
Effects of Temperature on Solubility
Letrsquos consider gas solubility Increasing temperature usually decreases gas solubility As temp increases average kinetic energy of molecules in solution increases Greater number of solute molecules escape from attraction of solvent and return to gas phase 69
At higher temps eq is reached with fewer gas molecules in solution Gases generally less soluble
70
Effect of temp on solubility of solids in liquids more difficult to predict
Often increasing temp increases solubility
However the same temp increase can result in large increase in solubility in one case and only slight increase in the next
71
72
enthalpies of Solution
Formation of solution comes with energy change Dissolve KI in water container feels cold Dissolve LiCl in water feels hot Formation of solid-liquid solution can absorb or release heat
73
During formation of solution solvent and solute particles experience changes in forces attracting them to other particles Before dissolving begins solvent molecules held by intermolecular forces (solvent-solvent attraction) In solute molecules held by Ifs (solute-solute attraction)
74
Energy is required to separate solute molecules and solvent molecules from their neighbors
A solute particle that is surrounded by solvent molecules is said to be solvated
75
Step 1 solute particles become separated from solid (energy absorbed) Step 2 solvent particles move apart to allow solute particles to enter liquid (energy absorbed)
76
Step 3 solvent particles attracted to and solvate solute particles (energy released)
77
Heat of solution - net amount of heat energy absorbed or released when a specific amount of solute dissolves in a solvent
Heat of solution negative (heat is released) if sums of energy in steps 1 and 2 is less than step 3
If 1+2 gt 3 HOS is positive (heat absorbed)78
In gaseous state molecules are so far apart there are nearly no forces between them Solute-solute interaction has little effect on heat of solution of a gas Energy released when gas dissolved in liquid bc attraction between solute gas and solvent molecules greater than energy needed to separate solvent molecules
79
Concentration of Solutions80
Concentration - measure of amount of solute in given amount of solvent or solution ldquodiluterdquo just means small amount of solute ldquoconcentratedrdquo just means relatively large amount of solute Unrelated to degree to which solution is saturated Ex Saturated solution of substance not very soluble might be dilute
81
Molarity
Molarity - number of moles of solute in one liter of solution To find molarity must know molar mass A ldquoone molarrdquo solution of NaOH is 1 mole NaOH per liter of solution (1 M)
82
1 mol NaOH = 400 g If this quantity dissolve in enough water to make EXACTLY 100 L solution solution is 1 M If 200 g NaOH dissolve in enough to make 100 L solution what is molarity 0500 M
83
1 M solution is not made by adding 1 mol solute to 1 L solvent That would be more than 1 L Instead 1 mol solute dissolved in less than 1 L Then diluted with more solvent to bring TOTAL VOLUME to 1 L
84
Practice ProblemYou have 340 L of solution that contains 900 g sodium chloride What is the molarity of that solution 0440 M NaCl You have 08 L of a 05 M HCl solution How many moles of HCl does this solution contain 04 mol HCl
85
What is the molarity of a solution composed of 585 g of potassium iodide dissolved in enough water to make a 0125 L of solution 0282 M KI
86
How many moles of H2SO4 are present in 0500 L of a 0150 M H2SO4 solution 00750 mol What volume of 300 M NaCl is needed for reaction that requires 1463 g of NaCl 0834 L
87
Molality
Molality - concentration of solution expressed in moles of solute per kilogram of solvent Solution that contains 1 mol solute (NaOH) dissolved in exactly 1 kg of solvent is ldquoone molalrdquo solution (1 m NaOH)
88
1 mol NaOH = 400 g 400 g NaOH dissolved in 1 kg water is 1 m NaOH
200 g NaOH in 1 kg water = 0500 m NaOH
89
Practice Problem
A solution was prepared by dissolving 171 g sucrose (C12H22O11) in 125 g of water Find the molal concentration of this solution 0400 m C12H22O11
90
A solution of iodine in carbon tetrachloride is used when iodine is needed for certain chemical tests How much iodine must be added to prepare a 0480 m solution of iodine in CCl4 if 1000 g of CCl4 is used 122 g I2
91
What is the molality of a solution composed of 255 g acetone (CH3)2CO dissolved in 200 g of water 22 m acetone What quantity in grams of methanol CH3OH is required to prepare a 0244 m solution in 400 g water 312 g methanol
92
How many grams of AgNO3 are needed to prepare a 0125 m solution in 250 mL of water 531 g AgNO3
What is the molality of a solution containing 182 g HCl and 250 g water 200 m
93
Ions in Aqueous Solutions and Colligative Properties
94
95
compounds in aqueous solutions
DissociationWhen compound made from ions dissolves in water ions separate Dissociation - separation of ions that occurs when an ionic compound dissolves
96
Notice number of ions made per formula unit in equations 1 formula unit of NaCl gives 2 units of ions in solution 1 formula unit of CaCl2 gives 3 units of ions
97
Assuming 100 dissociation solution that contains 1 mol NaCl contains 1 mol Na+ and 1 mol Cl- Can assume 100 dissociation of all soluble ionic compounds
98
Practice Problem
Write the equation for the dissolution of aluminum sulfate in water How many moles of aluminum ions are made by dissolving 1 mol aluminum sulfate What is the total number of moles of ions made by dissolving 1 mol of aluminum sulfate
99
1 AnalyzeGiven Amount of solute = 1 mol Al2(SO4)3 Solvent identity = water Unknown a moles of aluminum ions and sulfate ions Total number of moles of solute ions produced
100
2 Plan
The coefficients in the balanced dissociation equation will tell mole relationships You can use the equation to find out the number of moles of solute ions produced
101
3 Compute
a
b102
Write the equation for the dissolution of each of the following in water and then determine the number of moles of each ions made as well as the total number of ions made a 1 mol ammonium chloride b 1 mol sodium sulfide c 05 mol barium nitrate
103
a 1 mol ammonium chloride
1 mol NH4+ 1 mol Cl- 2 mol total ions
104
b 1 mol sodium sulfide
2 mol Na+ 1 mol S-2 3 mol total ions
105
c 05 mol barium nitrate
05 mol Ba+2 1 mol NO3- 15 mol total ions
106
Precipitation Reactions
Even though no compound is completely insoluble compounds of very low solubility can be considered insoluble for practical purposes There are general guidelines to help predict whether a compound made of certain combination of ions is soluble
107
General Solubility Guidelines
Sodium potassium and ammonium compounds are soluble in water
108
Nitrates acetates and chlorates are soluble
109
110
Most chlorides are soluble except those of silver mercury (I) and lead Lead (II) chloride is soluble in hot water
Most sulfates are soluble except those of barium strontium lead calcium and mercury
111
Most carbonates phosphates and silicates are insoluble except those of sodium potassium and ammonium
112
Most sulfides are insoluble except those of calcium strontium sodium potassium and ammonium
113
Is calcium phosphate Ca3(PO4)2 soluble or insoluble
Not one of the exceptions so it is insoluble
Dissociation equations are not written for insoluble compounds
114
Guidelines also useful in predicting what will happen if 2 solutions of 2 different soluble compounds are mixed
If mixing results in combination of ions that forms insoluble compound a double-replacement and precipitation reaction will happen
115
Will precipitate form when solutions of ammonium sulfide and cadmium nitrate are combined You can tell calcium nitrate is soluble from the guidelines (Most nitrates acetates and chlorates are soluble) You can also tell ammonium sulfide is soluble (Most sulfides are insoluble except those of calcium strontium sodium potassium and ammonium)
116
117
To decide whether precipitate can form must know solubilities of two possible products
Ammonium nitrate ndash soluble (Most nitrates acetates and chlorates are soluble) Cadmium sulfide ndash insoluble (Most sulfides are insoluble except those of calcium strontium sodium potassium and ammonium)
118
bc one product is insoluble double-replacement and precipitation reaction will happen (NH4)2S (aq) + Cd(NO3)2 (aq) rarr NH4NO3 (aq) + CdS (s)
119
Net Ionic Equations
Reactions of ions in aqueous solution usually represented by net ionic equations instead of formula equations Net ionic equation - includes only those compounds and ions that undergo a chemical change in a reaction in an aqueous solution
120
121
To write NIE (net ionic equation) first convert chemical equation into overall ionic equation All soluble ionic compounds shown as dissociated ions in solution Precipitates shown as solids
122
Notice ammonium and nitrate ions appear on both sides They havenrsquot gone through any chemical change Spectator ions - ions that do not take part in a chemical reaction and are found in solution both before and after the reaction
123
To change ionic equation to NIE eliminate spectator ions Cd+2(aq) + S-2(aq) rarr CdS(s)
This NIE applies to any reaction in which precipitate of CdS forms
124
Practice Problem
Identify the precipitate that forms when aqueous solutions of zinc nitrate and ammonium sulfide are combined Write the equation for the possible double-replacement reaction Then write the formula equation overall ionic equation and the net ionic equation
125
1 AnalyzeGiven Identity of reactants zinc nitrate and ammonium sulfide Reaction medium aqueous solution Unknown a equation for the possible double-replacement reaction b identity of the precipitate c formula equation d overall ionic equation e net ionic equation
126
2 PlanWrite the possible double-replacement reaction between Zn(NO3)2 and (NH4)2S Use the rules to determine if any of the products will precipitate Write a formula equation and overall net ionic equation Then cancel spectator ions to make net ionic equation
127
3 Computea possible double-replacement reaction
b rules show that zinc sulfide is not soluble so will precipitate (ammonium nitrate is soluble)
c formula equation128
d overall ionic equation
e net ionic equation129
Will a precipitate form if solutions of potassium sulfate and barium nitrate are combined If so write the net ionic equation for the reaction
Yes Ba+2(aq) + SO4-2(aq) rarr BaSO4(s)
130
Will a precipitate form if solutions of potassium nitrate and magnesium sulfate are combined If so write the net ionic equation for the reaction
No 131
Will a precipitate form if solutions of barium chloride and sodium sulfate are combined If so identify the spectator ions and write the net ionic equation for the reaction
Yes Na+ and Cl- Ba+2(aq) + SO4-2(aq) rarr BaSO4(s)
132
Write the net ionic equation for the precipitation of nickel(II) sulfide
Ni+2(aq) + S-2(aq) rarr NiS(s)
133
IonizationSome molecular compounds can also form ions in solution Usually polar Ionization - ions formed from solute molecules by action of the solvent (creation of ions where there were none) Ionization different from dissociation
134
Dissociation ionic compounds dissolve and ions already present separate Ionization ions formed where non existed before
Ions formed are hydrated Heat released during hydration of ions gives enough energy to break covalent bonds
135
Extent to which solute ionizes depends on strength of bonds within molecules of solute and strength of attraction between solute and solvent molecules
136
HCl ndash molecular compound that ionizes in aqueous solution Contains highly polar bond Attraction between polar HCl molecule and polar water strong enough to break HCl bond
137
The Hydronium Ion
H+ ions from HCl attracts other molecules or ions so strongly that it doesnrsquot normally exist alone Ionization of HCl better described as direct transfer of proton from HCl to H2O forming H3O+
138
139
Hydration of H+ to form hydronium is highly exothermic
Energy released gives a lot of energy needed to ionize a molecular solute
Many molecular compounds that ionize in aqueous solution contain hydrogen and form hydronium ion
140
Strong and Weak Electrolytes
Substances that make ions and conduct current in solution are electrolytes HCl is one of series of compounds that has hydrogen and halogen Hydrogen halides are all molecular compounds with single polar-covalent bonds
141
All are gases Very soluble in water All are electrolytes HCl HBr HI strongly conduct current in solution HF only weakly conducts current at same concentration Strength of conduction related to ability to ionize
142
143
Strong Electrolytes
HCl HBr HI are 100 ionized in solution
Strong electrolyte - any compound whose dilute aqueous solutions conduct electricity well this is due to the presence of all or almost all of the dissolve compound in the form of ions
144
Unique characteristic of strong electrolyte is that they yield only ions
Ex Ionic compound may be highly soluble in water and dissociate into ions (NaCl)
Others may not dissolve much but amount that does dissolve exists only as ions
145
Weak ElectrolytesHF dissolves in water to give acid solution called hydrofluoric acid HF bond stronger than bonds between hydrogen and other halogens When HF dissolves some molecule ionize Reverse reaction also happens HF(aq) + H2O(l) H3O+(aq) + F-(aq)
146
HF(aq) + H2O(l) H3O+
(aq) + F-(aq)Concentration of dissolved unionized HF stays high and concentration of ions stays low
Weak electrolyte - any compound whose aqueous solutions conduct electricity poorly this is due to the presence of small amount of the dissolved compound in the form of ions
147
Different from nonelectrolyte ndash NO ions at all Description of electrolyte as strong or weak not related to concentration of solution Electrolytes differ in degree of ionization not amount of solute dissolved
148
149
Colligative properties of solutions
Presence of solute affects properties of solutions Some properties not dependent on nature of dissolved substance but on how may dissolve particles are present
Colligative properties - properties that depend on the concentration of solute particles but not on their identity
150
Vapor-Pressure Lowering
Boiling and freezing point of solution different from pure solvent Nonvolatile solute raises boiling point and lowers freezing point Nonvolatile substance - one that has little tendency to become gas under existing conditions
151
152
To understand why nonvolatile solute changes boiling and freezing point consider equilibrium vapor pressure
Vapor pressure pressure caused by molecules that have escape liquid phase to gas phase
Can be thought of a measure of tendency of molecules to escape from a liquid
153
Addition of sucrose (nonvolatile solute) lowers concentration of water molecules at surface of liquid This lowers tendency of water molecules to leave solution and enter gas phase Vapor pressure of solution is lower than vapor pressure of pure water
154
Aqueous13 solution13 of13 nonvolatile13 solutePure13 water
Nonelectrolyte solutions of same molality have same concentration of particles
Dilute solutions of same solvent and equal molality of any nonelectrolyte solute lower vapor pressure equally
155
Example
1 m aqueous solution of nonelectrolyte glucose lowers vapor pressure of water 55 x 10-4 atm at 25 1 m aqueous solution of sucrose also lowers water vapor pressure to 55 x 10-4 atm at 25
156
157
bc vapor-pressure lowering depends on concentration of nonelectrolyte solute and doesnrsquot depend on the type of solute it is a colligative proprerty
bc vapor pressure has been lowered solution remains liquid over larger temperature range
Lowers freezing point and raises boiling point
158
Can assume changes in boiling and freezing point also depend on concentration of solute
They are colligative properties
159
Freezing-Point Depression
Freezing point of 1-molal solution of any nonelectrolyte solute in water is found (by experiment) to be 186 lower than freezing point of water When 1 mol of nonelectrolyte solute dissolved in 1 kg water freezing point is -186 not 000 160
When 2 mol nonelectrolyte solute dissolved in 1 kg water freezing point is -372
For any concentration of nonelectrolyte solute in water decrease in freezing point can be determined using value of -186m
161
This value called the molal freezing-point contstant (Kf) is the freezing-point depression of the solvent in a 1-molal solution of a nonvolatile nonelectrolyte solute
162
Each solvent has own characteristic molal freezing-point constant Freezing-point depression (Δtf) the difference between the freezing points of the pure solvent and a solute of nonelectrolyte in that solvent and it is directly proportional to the molal concentration of the solution Δtf = Kfm
163
Δtf = Kfm
Kf expressed as m
m expressed in mol solutekg solvent (molality)
164
Sample Problem
What is the freezing point depression of water in a solution of 171 g of sucrose C12H22O11 and 200 g of water What is the actual freezing point of the solution
165
1 AnalyzeGiven Solute mass and chemical formula 171 g C12H22O11 Solvent mass and identity 2000 g water Unknown a freezing-point depression b freezing point of the solution 166
2 Plan
167
168
Calculate
Unknown a freezing-point depression b freezing point of the solution
169
Practice Problem
A water solution containing an unknown quantity of a nonelectrolyte solute is found to have a freezing point of ndash 023degCWhat is the molal concentration of the solution
012 m170
A solution consists of 103 g of the nonelectrolyte glucose C6H12O6 dissolved in 250 g of water What is the freezing-point depression of the solution
minus0426degC171
In a laboratory experiment the freezing point of an aqueous solution of glucose is found to be minus0325degCWhat is the molal concentration of this solution
0175 m172
If 0500 mol of a nonelectrolyte solute are dissolved in 5000 g of ether what is the freezing point of the solution
minus1181degC173
The freezing point of an aqueous solution that contains a nonelectrolyte is minus90degC aWhat is the freezing-point depression of the solution bWhat is the molal concentration of the solution a minus90degC b 48 m
174
Boiling-Point Elevation
Boiling point is temp at which vapor pressure is equal to atmospheric pressure Change in vapor pressure will cause corresponding change in boiling point
175
176
Vapor pressure of nonvolatile solution is lower than vapor pressure of pure solvent More heat will be required to raise vapor pressure of solution Boiling point of solution is higher than boiling point of pure solvent
Molal boiling-point constant (Kb) - boiling-point elevation of the solvent in a 1-molal solution of a nonvolatile nonelectrolyte solute b-p elevation of any nonelectrolyte in water found by experiment to be 051 So molal b-p constant for water is 051m
177
For different solvents b-p elevations of 1-molal solutions have different values (Table 14-2)
Like freezing-point constants values most accurate for dilute solutions
178
Boiling-point elevation Δtb difference between the boiling points of the pure solvent and a nonelectrolyte solution of that solvent and it is directly proportional to the molal concentration of the solution B-p elevation can be calculated using Δtb = Kbm Δtb (boiling point elevation) expressed in m and m expressed in mol solutekg solvent
Sample Problem
What is the boiling-point elevation of a solution made from 200 g of a nonelectrolyte solute and 4000 g of water The molar mass of the solute is 620 gmol
180
1 AnalyzeGiven solute mass = 200 g solute molar mass = 620 gmol solvent mass and identity = 4000 g of water
Unknown boiling-point elevation
181
2 Plan
182
183
3 Compute
Practice Problem
A solution contains 500 g of sucrose C12H22O11 a nonelectrolyte dissolved in 5000 g of water What is the boiling-point elevation
015degC184
A solution contains 4500 g of sucrose C12H22O11 a nonelectrolyte dissolved in 250 g of waterWhat is the boiling point of the solution
1027degC185
If the boiling point elevation of an aqueous solution containing a nonvolatile electrolyte is 102degC what is the molality of the solution
20 m186
The boiling point of an aqueous solution containing a nonvolatile electrolyte is 10075degC aWhat is the boiling-point elevation bWhat is the molality of the solution a 075degC b 15 m
187
Electrolytes and Colligative PropertiesEarly investigators confused by experiments where certain substances depressed freezing point or elevated boiling point more than expected
188
189
Ex 01 m solution of NaCl lowers freezing point of solvent almost twice as much as 01 m solution of sucrose 01 m CaCl2 lowers fp almost 3 times as much as 01 m solution of sucrose
To understand why this happens contrast behavior or sucrose with NaCl in aqueous solution Each sucrose dissolves to make only 1 particle in solution So 1 mol sucrose dissolves to 1 mol particles 1 mol NaCl dissolves to make 2 moles of particles (1 mol Na+ and 1 mol Cl-)
190
191
Calculated Values for Electrolyte SolutionsColligative properties depend on total concentration of solute particles regardless of their identity Electrolytes cause changes in colligative properties proportional to total molality in terms of all dissolves particles instead of formula units
192
What about barium nitrate Ba(NO3)2 Each mole of barium nitrate yields 3 mol of ions in solution Ba(NO3)2(s) rarr Ba2+(aq) + 2NO3-(aq) You would expect solution of given molality to lower fp of solvent 3 times as much as nonelectrolytic solution of same molality
193
Sample Problem
What is the expected change in the freezing point of water in a solution of 625 g of barium nitrate Ba(NO3)2 in 100 kg of water
194
1 AnalyzeGiven solute mass and formula = 625 g Ba(NO3)2 solvent mass and identity = 100 kg water Δtf = Kfm
Unknown expected freezing-point depression
195
2 PlanThe molality can be calculated by converting the solute mass to moles and then dividing by the number of kilograms of solvent That molality is in terms of formula units of Ba(NO3)2 and must be converted to molality in terms of dissociated ions in solution It must be multiplied by the number of moles of ions produced per mole of formula unit This adjusted molality can then be used to calculate the freezing-point depression
196
197
198
3 Compute
Practice Problem
What is the expected freezing-point depression for a solution that contains 20 mol of magnesium sulfate dissolved in 10 kg of water
minus74degC199
What is the expected boiling-point elevation of water for a solution that contains 150 g of sodium chloride dissolved in 10 kg of water 27degC The freezing point of an aqueous sodium chloride solution is minus200degC What is the molality of the solution 54 m NaCl
200
Actual Values for Electrolyte SolutionsRemember values calculated are only expected values Actual values of colligative properties for all strong electrolytes are almost what would be expected Difference between expected and actual values caused by attractive forces that exist between dissociated ions in aqueous solution
201
Attraction between hydrated ions in solution is small compared with those in crystalline solid However forces of attraction donrsquot interfere with movements of aqueous ions More concentrated solution means ions closer together and attraction is greater Also depends on charge of ions ndash higher charge greater attraction
202
Increasing pressure of solute gas above solution puts stress on eq Molecules collide with surface more often Increase in pressure partially offset by increase in rate of gas molecules entering solution In turn increase in amount of dissolved gas causes increase in rate molecules escape liquid and becomes gas
63
Eventually eq restored at higher gas solubility
Expected from Le Chatelierrsquos principle
Increase in gas pressure causes eq to shift so fewer molecules are in gas phase
64
Henryrsquos Law
Henryrsquos law - solubility of a gas in liquid is directly proportional to the partial pressure of that gas on the surface of the liquid
Applies to gas-liquid solutions at constant temp
65
When mixture of ideal gases is limited in constant volume at constant temp each gas applies same pressure it would apply if it occupied the space alone
Assuming gases do not react each gas dissolves to the extent it would if no other gases were there
66
In carbonated drinks (Coke Pepsi etc) solubility of CO2 increased by increasing pressure
At bottling factory CO2 gas forced into solution of flavored water at pressure of 5-10 atm Gas-in-liquid solution then sealed in bottlescans
67
When cap removed pressure reduced to 1 atm and some CO2 escapes as gas bubbles Effervescence - rapid escape of a gas from a liquid in which it is dissolved
68
Effects of Temperature on Solubility
Letrsquos consider gas solubility Increasing temperature usually decreases gas solubility As temp increases average kinetic energy of molecules in solution increases Greater number of solute molecules escape from attraction of solvent and return to gas phase 69
At higher temps eq is reached with fewer gas molecules in solution Gases generally less soluble
70
Effect of temp on solubility of solids in liquids more difficult to predict
Often increasing temp increases solubility
However the same temp increase can result in large increase in solubility in one case and only slight increase in the next
71
72
enthalpies of Solution
Formation of solution comes with energy change Dissolve KI in water container feels cold Dissolve LiCl in water feels hot Formation of solid-liquid solution can absorb or release heat
73
During formation of solution solvent and solute particles experience changes in forces attracting them to other particles Before dissolving begins solvent molecules held by intermolecular forces (solvent-solvent attraction) In solute molecules held by Ifs (solute-solute attraction)
74
Energy is required to separate solute molecules and solvent molecules from their neighbors
A solute particle that is surrounded by solvent molecules is said to be solvated
75
Step 1 solute particles become separated from solid (energy absorbed) Step 2 solvent particles move apart to allow solute particles to enter liquid (energy absorbed)
76
Step 3 solvent particles attracted to and solvate solute particles (energy released)
77
Heat of solution - net amount of heat energy absorbed or released when a specific amount of solute dissolves in a solvent
Heat of solution negative (heat is released) if sums of energy in steps 1 and 2 is less than step 3
If 1+2 gt 3 HOS is positive (heat absorbed)78
In gaseous state molecules are so far apart there are nearly no forces between them Solute-solute interaction has little effect on heat of solution of a gas Energy released when gas dissolved in liquid bc attraction between solute gas and solvent molecules greater than energy needed to separate solvent molecules
79
Concentration of Solutions80
Concentration - measure of amount of solute in given amount of solvent or solution ldquodiluterdquo just means small amount of solute ldquoconcentratedrdquo just means relatively large amount of solute Unrelated to degree to which solution is saturated Ex Saturated solution of substance not very soluble might be dilute
81
Molarity
Molarity - number of moles of solute in one liter of solution To find molarity must know molar mass A ldquoone molarrdquo solution of NaOH is 1 mole NaOH per liter of solution (1 M)
82
1 mol NaOH = 400 g If this quantity dissolve in enough water to make EXACTLY 100 L solution solution is 1 M If 200 g NaOH dissolve in enough to make 100 L solution what is molarity 0500 M
83
1 M solution is not made by adding 1 mol solute to 1 L solvent That would be more than 1 L Instead 1 mol solute dissolved in less than 1 L Then diluted with more solvent to bring TOTAL VOLUME to 1 L
84
Practice ProblemYou have 340 L of solution that contains 900 g sodium chloride What is the molarity of that solution 0440 M NaCl You have 08 L of a 05 M HCl solution How many moles of HCl does this solution contain 04 mol HCl
85
What is the molarity of a solution composed of 585 g of potassium iodide dissolved in enough water to make a 0125 L of solution 0282 M KI
86
How many moles of H2SO4 are present in 0500 L of a 0150 M H2SO4 solution 00750 mol What volume of 300 M NaCl is needed for reaction that requires 1463 g of NaCl 0834 L
87
Molality
Molality - concentration of solution expressed in moles of solute per kilogram of solvent Solution that contains 1 mol solute (NaOH) dissolved in exactly 1 kg of solvent is ldquoone molalrdquo solution (1 m NaOH)
88
1 mol NaOH = 400 g 400 g NaOH dissolved in 1 kg water is 1 m NaOH
200 g NaOH in 1 kg water = 0500 m NaOH
89
Practice Problem
A solution was prepared by dissolving 171 g sucrose (C12H22O11) in 125 g of water Find the molal concentration of this solution 0400 m C12H22O11
90
A solution of iodine in carbon tetrachloride is used when iodine is needed for certain chemical tests How much iodine must be added to prepare a 0480 m solution of iodine in CCl4 if 1000 g of CCl4 is used 122 g I2
91
What is the molality of a solution composed of 255 g acetone (CH3)2CO dissolved in 200 g of water 22 m acetone What quantity in grams of methanol CH3OH is required to prepare a 0244 m solution in 400 g water 312 g methanol
92
How many grams of AgNO3 are needed to prepare a 0125 m solution in 250 mL of water 531 g AgNO3
What is the molality of a solution containing 182 g HCl and 250 g water 200 m
93
Ions in Aqueous Solutions and Colligative Properties
94
95
compounds in aqueous solutions
DissociationWhen compound made from ions dissolves in water ions separate Dissociation - separation of ions that occurs when an ionic compound dissolves
96
Notice number of ions made per formula unit in equations 1 formula unit of NaCl gives 2 units of ions in solution 1 formula unit of CaCl2 gives 3 units of ions
97
Assuming 100 dissociation solution that contains 1 mol NaCl contains 1 mol Na+ and 1 mol Cl- Can assume 100 dissociation of all soluble ionic compounds
98
Practice Problem
Write the equation for the dissolution of aluminum sulfate in water How many moles of aluminum ions are made by dissolving 1 mol aluminum sulfate What is the total number of moles of ions made by dissolving 1 mol of aluminum sulfate
99
1 AnalyzeGiven Amount of solute = 1 mol Al2(SO4)3 Solvent identity = water Unknown a moles of aluminum ions and sulfate ions Total number of moles of solute ions produced
100
2 Plan
The coefficients in the balanced dissociation equation will tell mole relationships You can use the equation to find out the number of moles of solute ions produced
101
3 Compute
a
b102
Write the equation for the dissolution of each of the following in water and then determine the number of moles of each ions made as well as the total number of ions made a 1 mol ammonium chloride b 1 mol sodium sulfide c 05 mol barium nitrate
103
a 1 mol ammonium chloride
1 mol NH4+ 1 mol Cl- 2 mol total ions
104
b 1 mol sodium sulfide
2 mol Na+ 1 mol S-2 3 mol total ions
105
c 05 mol barium nitrate
05 mol Ba+2 1 mol NO3- 15 mol total ions
106
Precipitation Reactions
Even though no compound is completely insoluble compounds of very low solubility can be considered insoluble for practical purposes There are general guidelines to help predict whether a compound made of certain combination of ions is soluble
107
General Solubility Guidelines
Sodium potassium and ammonium compounds are soluble in water
108
Nitrates acetates and chlorates are soluble
109
110
Most chlorides are soluble except those of silver mercury (I) and lead Lead (II) chloride is soluble in hot water
Most sulfates are soluble except those of barium strontium lead calcium and mercury
111
Most carbonates phosphates and silicates are insoluble except those of sodium potassium and ammonium
112
Most sulfides are insoluble except those of calcium strontium sodium potassium and ammonium
113
Is calcium phosphate Ca3(PO4)2 soluble or insoluble
Not one of the exceptions so it is insoluble
Dissociation equations are not written for insoluble compounds
114
Guidelines also useful in predicting what will happen if 2 solutions of 2 different soluble compounds are mixed
If mixing results in combination of ions that forms insoluble compound a double-replacement and precipitation reaction will happen
115
Will precipitate form when solutions of ammonium sulfide and cadmium nitrate are combined You can tell calcium nitrate is soluble from the guidelines (Most nitrates acetates and chlorates are soluble) You can also tell ammonium sulfide is soluble (Most sulfides are insoluble except those of calcium strontium sodium potassium and ammonium)
116
117
To decide whether precipitate can form must know solubilities of two possible products
Ammonium nitrate ndash soluble (Most nitrates acetates and chlorates are soluble) Cadmium sulfide ndash insoluble (Most sulfides are insoluble except those of calcium strontium sodium potassium and ammonium)
118
bc one product is insoluble double-replacement and precipitation reaction will happen (NH4)2S (aq) + Cd(NO3)2 (aq) rarr NH4NO3 (aq) + CdS (s)
119
Net Ionic Equations
Reactions of ions in aqueous solution usually represented by net ionic equations instead of formula equations Net ionic equation - includes only those compounds and ions that undergo a chemical change in a reaction in an aqueous solution
120
121
To write NIE (net ionic equation) first convert chemical equation into overall ionic equation All soluble ionic compounds shown as dissociated ions in solution Precipitates shown as solids
122
Notice ammonium and nitrate ions appear on both sides They havenrsquot gone through any chemical change Spectator ions - ions that do not take part in a chemical reaction and are found in solution both before and after the reaction
123
To change ionic equation to NIE eliminate spectator ions Cd+2(aq) + S-2(aq) rarr CdS(s)
This NIE applies to any reaction in which precipitate of CdS forms
124
Practice Problem
Identify the precipitate that forms when aqueous solutions of zinc nitrate and ammonium sulfide are combined Write the equation for the possible double-replacement reaction Then write the formula equation overall ionic equation and the net ionic equation
125
1 AnalyzeGiven Identity of reactants zinc nitrate and ammonium sulfide Reaction medium aqueous solution Unknown a equation for the possible double-replacement reaction b identity of the precipitate c formula equation d overall ionic equation e net ionic equation
126
2 PlanWrite the possible double-replacement reaction between Zn(NO3)2 and (NH4)2S Use the rules to determine if any of the products will precipitate Write a formula equation and overall net ionic equation Then cancel spectator ions to make net ionic equation
127
3 Computea possible double-replacement reaction
b rules show that zinc sulfide is not soluble so will precipitate (ammonium nitrate is soluble)
c formula equation128
d overall ionic equation
e net ionic equation129
Will a precipitate form if solutions of potassium sulfate and barium nitrate are combined If so write the net ionic equation for the reaction
Yes Ba+2(aq) + SO4-2(aq) rarr BaSO4(s)
130
Will a precipitate form if solutions of potassium nitrate and magnesium sulfate are combined If so write the net ionic equation for the reaction
No 131
Will a precipitate form if solutions of barium chloride and sodium sulfate are combined If so identify the spectator ions and write the net ionic equation for the reaction
Yes Na+ and Cl- Ba+2(aq) + SO4-2(aq) rarr BaSO4(s)
132
Write the net ionic equation for the precipitation of nickel(II) sulfide
Ni+2(aq) + S-2(aq) rarr NiS(s)
133
IonizationSome molecular compounds can also form ions in solution Usually polar Ionization - ions formed from solute molecules by action of the solvent (creation of ions where there were none) Ionization different from dissociation
134
Dissociation ionic compounds dissolve and ions already present separate Ionization ions formed where non existed before
Ions formed are hydrated Heat released during hydration of ions gives enough energy to break covalent bonds
135
Extent to which solute ionizes depends on strength of bonds within molecules of solute and strength of attraction between solute and solvent molecules
136
HCl ndash molecular compound that ionizes in aqueous solution Contains highly polar bond Attraction between polar HCl molecule and polar water strong enough to break HCl bond
137
The Hydronium Ion
H+ ions from HCl attracts other molecules or ions so strongly that it doesnrsquot normally exist alone Ionization of HCl better described as direct transfer of proton from HCl to H2O forming H3O+
138
139
Hydration of H+ to form hydronium is highly exothermic
Energy released gives a lot of energy needed to ionize a molecular solute
Many molecular compounds that ionize in aqueous solution contain hydrogen and form hydronium ion
140
Strong and Weak Electrolytes
Substances that make ions and conduct current in solution are electrolytes HCl is one of series of compounds that has hydrogen and halogen Hydrogen halides are all molecular compounds with single polar-covalent bonds
141
All are gases Very soluble in water All are electrolytes HCl HBr HI strongly conduct current in solution HF only weakly conducts current at same concentration Strength of conduction related to ability to ionize
142
143
Strong Electrolytes
HCl HBr HI are 100 ionized in solution
Strong electrolyte - any compound whose dilute aqueous solutions conduct electricity well this is due to the presence of all or almost all of the dissolve compound in the form of ions
144
Unique characteristic of strong electrolyte is that they yield only ions
Ex Ionic compound may be highly soluble in water and dissociate into ions (NaCl)
Others may not dissolve much but amount that does dissolve exists only as ions
145
Weak ElectrolytesHF dissolves in water to give acid solution called hydrofluoric acid HF bond stronger than bonds between hydrogen and other halogens When HF dissolves some molecule ionize Reverse reaction also happens HF(aq) + H2O(l) H3O+(aq) + F-(aq)
146
HF(aq) + H2O(l) H3O+
(aq) + F-(aq)Concentration of dissolved unionized HF stays high and concentration of ions stays low
Weak electrolyte - any compound whose aqueous solutions conduct electricity poorly this is due to the presence of small amount of the dissolved compound in the form of ions
147
Different from nonelectrolyte ndash NO ions at all Description of electrolyte as strong or weak not related to concentration of solution Electrolytes differ in degree of ionization not amount of solute dissolved
148
149
Colligative properties of solutions
Presence of solute affects properties of solutions Some properties not dependent on nature of dissolved substance but on how may dissolve particles are present
Colligative properties - properties that depend on the concentration of solute particles but not on their identity
150
Vapor-Pressure Lowering
Boiling and freezing point of solution different from pure solvent Nonvolatile solute raises boiling point and lowers freezing point Nonvolatile substance - one that has little tendency to become gas under existing conditions
151
152
To understand why nonvolatile solute changes boiling and freezing point consider equilibrium vapor pressure
Vapor pressure pressure caused by molecules that have escape liquid phase to gas phase
Can be thought of a measure of tendency of molecules to escape from a liquid
153
Addition of sucrose (nonvolatile solute) lowers concentration of water molecules at surface of liquid This lowers tendency of water molecules to leave solution and enter gas phase Vapor pressure of solution is lower than vapor pressure of pure water
154
Aqueous13 solution13 of13 nonvolatile13 solutePure13 water
Nonelectrolyte solutions of same molality have same concentration of particles
Dilute solutions of same solvent and equal molality of any nonelectrolyte solute lower vapor pressure equally
155
Example
1 m aqueous solution of nonelectrolyte glucose lowers vapor pressure of water 55 x 10-4 atm at 25 1 m aqueous solution of sucrose also lowers water vapor pressure to 55 x 10-4 atm at 25
156
157
bc vapor-pressure lowering depends on concentration of nonelectrolyte solute and doesnrsquot depend on the type of solute it is a colligative proprerty
bc vapor pressure has been lowered solution remains liquid over larger temperature range
Lowers freezing point and raises boiling point
158
Can assume changes in boiling and freezing point also depend on concentration of solute
They are colligative properties
159
Freezing-Point Depression
Freezing point of 1-molal solution of any nonelectrolyte solute in water is found (by experiment) to be 186 lower than freezing point of water When 1 mol of nonelectrolyte solute dissolved in 1 kg water freezing point is -186 not 000 160
When 2 mol nonelectrolyte solute dissolved in 1 kg water freezing point is -372
For any concentration of nonelectrolyte solute in water decrease in freezing point can be determined using value of -186m
161
This value called the molal freezing-point contstant (Kf) is the freezing-point depression of the solvent in a 1-molal solution of a nonvolatile nonelectrolyte solute
162
Each solvent has own characteristic molal freezing-point constant Freezing-point depression (Δtf) the difference between the freezing points of the pure solvent and a solute of nonelectrolyte in that solvent and it is directly proportional to the molal concentration of the solution Δtf = Kfm
163
Δtf = Kfm
Kf expressed as m
m expressed in mol solutekg solvent (molality)
164
Sample Problem
What is the freezing point depression of water in a solution of 171 g of sucrose C12H22O11 and 200 g of water What is the actual freezing point of the solution
165
1 AnalyzeGiven Solute mass and chemical formula 171 g C12H22O11 Solvent mass and identity 2000 g water Unknown a freezing-point depression b freezing point of the solution 166
2 Plan
167
168
Calculate
Unknown a freezing-point depression b freezing point of the solution
169
Practice Problem
A water solution containing an unknown quantity of a nonelectrolyte solute is found to have a freezing point of ndash 023degCWhat is the molal concentration of the solution
012 m170
A solution consists of 103 g of the nonelectrolyte glucose C6H12O6 dissolved in 250 g of water What is the freezing-point depression of the solution
minus0426degC171
In a laboratory experiment the freezing point of an aqueous solution of glucose is found to be minus0325degCWhat is the molal concentration of this solution
0175 m172
If 0500 mol of a nonelectrolyte solute are dissolved in 5000 g of ether what is the freezing point of the solution
minus1181degC173
The freezing point of an aqueous solution that contains a nonelectrolyte is minus90degC aWhat is the freezing-point depression of the solution bWhat is the molal concentration of the solution a minus90degC b 48 m
174
Boiling-Point Elevation
Boiling point is temp at which vapor pressure is equal to atmospheric pressure Change in vapor pressure will cause corresponding change in boiling point
175
176
Vapor pressure of nonvolatile solution is lower than vapor pressure of pure solvent More heat will be required to raise vapor pressure of solution Boiling point of solution is higher than boiling point of pure solvent
Molal boiling-point constant (Kb) - boiling-point elevation of the solvent in a 1-molal solution of a nonvolatile nonelectrolyte solute b-p elevation of any nonelectrolyte in water found by experiment to be 051 So molal b-p constant for water is 051m
177
For different solvents b-p elevations of 1-molal solutions have different values (Table 14-2)
Like freezing-point constants values most accurate for dilute solutions
178
Boiling-point elevation Δtb difference between the boiling points of the pure solvent and a nonelectrolyte solution of that solvent and it is directly proportional to the molal concentration of the solution B-p elevation can be calculated using Δtb = Kbm Δtb (boiling point elevation) expressed in m and m expressed in mol solutekg solvent
Sample Problem
What is the boiling-point elevation of a solution made from 200 g of a nonelectrolyte solute and 4000 g of water The molar mass of the solute is 620 gmol
180
1 AnalyzeGiven solute mass = 200 g solute molar mass = 620 gmol solvent mass and identity = 4000 g of water
Unknown boiling-point elevation
181
2 Plan
182
183
3 Compute
Practice Problem
A solution contains 500 g of sucrose C12H22O11 a nonelectrolyte dissolved in 5000 g of water What is the boiling-point elevation
015degC184
A solution contains 4500 g of sucrose C12H22O11 a nonelectrolyte dissolved in 250 g of waterWhat is the boiling point of the solution
1027degC185
If the boiling point elevation of an aqueous solution containing a nonvolatile electrolyte is 102degC what is the molality of the solution
20 m186
The boiling point of an aqueous solution containing a nonvolatile electrolyte is 10075degC aWhat is the boiling-point elevation bWhat is the molality of the solution a 075degC b 15 m
187
Electrolytes and Colligative PropertiesEarly investigators confused by experiments where certain substances depressed freezing point or elevated boiling point more than expected
188
189
Ex 01 m solution of NaCl lowers freezing point of solvent almost twice as much as 01 m solution of sucrose 01 m CaCl2 lowers fp almost 3 times as much as 01 m solution of sucrose
To understand why this happens contrast behavior or sucrose with NaCl in aqueous solution Each sucrose dissolves to make only 1 particle in solution So 1 mol sucrose dissolves to 1 mol particles 1 mol NaCl dissolves to make 2 moles of particles (1 mol Na+ and 1 mol Cl-)
190
191
Calculated Values for Electrolyte SolutionsColligative properties depend on total concentration of solute particles regardless of their identity Electrolytes cause changes in colligative properties proportional to total molality in terms of all dissolves particles instead of formula units
192
What about barium nitrate Ba(NO3)2 Each mole of barium nitrate yields 3 mol of ions in solution Ba(NO3)2(s) rarr Ba2+(aq) + 2NO3-(aq) You would expect solution of given molality to lower fp of solvent 3 times as much as nonelectrolytic solution of same molality
193
Sample Problem
What is the expected change in the freezing point of water in a solution of 625 g of barium nitrate Ba(NO3)2 in 100 kg of water
194
1 AnalyzeGiven solute mass and formula = 625 g Ba(NO3)2 solvent mass and identity = 100 kg water Δtf = Kfm
Unknown expected freezing-point depression
195
2 PlanThe molality can be calculated by converting the solute mass to moles and then dividing by the number of kilograms of solvent That molality is in terms of formula units of Ba(NO3)2 and must be converted to molality in terms of dissociated ions in solution It must be multiplied by the number of moles of ions produced per mole of formula unit This adjusted molality can then be used to calculate the freezing-point depression
196
197
198
3 Compute
Practice Problem
What is the expected freezing-point depression for a solution that contains 20 mol of magnesium sulfate dissolved in 10 kg of water
minus74degC199
What is the expected boiling-point elevation of water for a solution that contains 150 g of sodium chloride dissolved in 10 kg of water 27degC The freezing point of an aqueous sodium chloride solution is minus200degC What is the molality of the solution 54 m NaCl
200
Actual Values for Electrolyte SolutionsRemember values calculated are only expected values Actual values of colligative properties for all strong electrolytes are almost what would be expected Difference between expected and actual values caused by attractive forces that exist between dissociated ions in aqueous solution
201
Attraction between hydrated ions in solution is small compared with those in crystalline solid However forces of attraction donrsquot interfere with movements of aqueous ions More concentrated solution means ions closer together and attraction is greater Also depends on charge of ions ndash higher charge greater attraction
202
Eventually eq restored at higher gas solubility
Expected from Le Chatelierrsquos principle
Increase in gas pressure causes eq to shift so fewer molecules are in gas phase
64
Henryrsquos Law
Henryrsquos law - solubility of a gas in liquid is directly proportional to the partial pressure of that gas on the surface of the liquid
Applies to gas-liquid solutions at constant temp
65
When mixture of ideal gases is limited in constant volume at constant temp each gas applies same pressure it would apply if it occupied the space alone
Assuming gases do not react each gas dissolves to the extent it would if no other gases were there
66
In carbonated drinks (Coke Pepsi etc) solubility of CO2 increased by increasing pressure
At bottling factory CO2 gas forced into solution of flavored water at pressure of 5-10 atm Gas-in-liquid solution then sealed in bottlescans
67
When cap removed pressure reduced to 1 atm and some CO2 escapes as gas bubbles Effervescence - rapid escape of a gas from a liquid in which it is dissolved
68
Effects of Temperature on Solubility
Letrsquos consider gas solubility Increasing temperature usually decreases gas solubility As temp increases average kinetic energy of molecules in solution increases Greater number of solute molecules escape from attraction of solvent and return to gas phase 69
At higher temps eq is reached with fewer gas molecules in solution Gases generally less soluble
70
Effect of temp on solubility of solids in liquids more difficult to predict
Often increasing temp increases solubility
However the same temp increase can result in large increase in solubility in one case and only slight increase in the next
71
72
enthalpies of Solution
Formation of solution comes with energy change Dissolve KI in water container feels cold Dissolve LiCl in water feels hot Formation of solid-liquid solution can absorb or release heat
73
During formation of solution solvent and solute particles experience changes in forces attracting them to other particles Before dissolving begins solvent molecules held by intermolecular forces (solvent-solvent attraction) In solute molecules held by Ifs (solute-solute attraction)
74
Energy is required to separate solute molecules and solvent molecules from their neighbors
A solute particle that is surrounded by solvent molecules is said to be solvated
75
Step 1 solute particles become separated from solid (energy absorbed) Step 2 solvent particles move apart to allow solute particles to enter liquid (energy absorbed)
76
Step 3 solvent particles attracted to and solvate solute particles (energy released)
77
Heat of solution - net amount of heat energy absorbed or released when a specific amount of solute dissolves in a solvent
Heat of solution negative (heat is released) if sums of energy in steps 1 and 2 is less than step 3
If 1+2 gt 3 HOS is positive (heat absorbed)78
In gaseous state molecules are so far apart there are nearly no forces between them Solute-solute interaction has little effect on heat of solution of a gas Energy released when gas dissolved in liquid bc attraction between solute gas and solvent molecules greater than energy needed to separate solvent molecules
79
Concentration of Solutions80
Concentration - measure of amount of solute in given amount of solvent or solution ldquodiluterdquo just means small amount of solute ldquoconcentratedrdquo just means relatively large amount of solute Unrelated to degree to which solution is saturated Ex Saturated solution of substance not very soluble might be dilute
81
Molarity
Molarity - number of moles of solute in one liter of solution To find molarity must know molar mass A ldquoone molarrdquo solution of NaOH is 1 mole NaOH per liter of solution (1 M)
82
1 mol NaOH = 400 g If this quantity dissolve in enough water to make EXACTLY 100 L solution solution is 1 M If 200 g NaOH dissolve in enough to make 100 L solution what is molarity 0500 M
83
1 M solution is not made by adding 1 mol solute to 1 L solvent That would be more than 1 L Instead 1 mol solute dissolved in less than 1 L Then diluted with more solvent to bring TOTAL VOLUME to 1 L
84
Practice ProblemYou have 340 L of solution that contains 900 g sodium chloride What is the molarity of that solution 0440 M NaCl You have 08 L of a 05 M HCl solution How many moles of HCl does this solution contain 04 mol HCl
85
What is the molarity of a solution composed of 585 g of potassium iodide dissolved in enough water to make a 0125 L of solution 0282 M KI
86
How many moles of H2SO4 are present in 0500 L of a 0150 M H2SO4 solution 00750 mol What volume of 300 M NaCl is needed for reaction that requires 1463 g of NaCl 0834 L
87
Molality
Molality - concentration of solution expressed in moles of solute per kilogram of solvent Solution that contains 1 mol solute (NaOH) dissolved in exactly 1 kg of solvent is ldquoone molalrdquo solution (1 m NaOH)
88
1 mol NaOH = 400 g 400 g NaOH dissolved in 1 kg water is 1 m NaOH
200 g NaOH in 1 kg water = 0500 m NaOH
89
Practice Problem
A solution was prepared by dissolving 171 g sucrose (C12H22O11) in 125 g of water Find the molal concentration of this solution 0400 m C12H22O11
90
A solution of iodine in carbon tetrachloride is used when iodine is needed for certain chemical tests How much iodine must be added to prepare a 0480 m solution of iodine in CCl4 if 1000 g of CCl4 is used 122 g I2
91
What is the molality of a solution composed of 255 g acetone (CH3)2CO dissolved in 200 g of water 22 m acetone What quantity in grams of methanol CH3OH is required to prepare a 0244 m solution in 400 g water 312 g methanol
92
How many grams of AgNO3 are needed to prepare a 0125 m solution in 250 mL of water 531 g AgNO3
What is the molality of a solution containing 182 g HCl and 250 g water 200 m
93
Ions in Aqueous Solutions and Colligative Properties
94
95
compounds in aqueous solutions
DissociationWhen compound made from ions dissolves in water ions separate Dissociation - separation of ions that occurs when an ionic compound dissolves
96
Notice number of ions made per formula unit in equations 1 formula unit of NaCl gives 2 units of ions in solution 1 formula unit of CaCl2 gives 3 units of ions
97
Assuming 100 dissociation solution that contains 1 mol NaCl contains 1 mol Na+ and 1 mol Cl- Can assume 100 dissociation of all soluble ionic compounds
98
Practice Problem
Write the equation for the dissolution of aluminum sulfate in water How many moles of aluminum ions are made by dissolving 1 mol aluminum sulfate What is the total number of moles of ions made by dissolving 1 mol of aluminum sulfate
99
1 AnalyzeGiven Amount of solute = 1 mol Al2(SO4)3 Solvent identity = water Unknown a moles of aluminum ions and sulfate ions Total number of moles of solute ions produced
100
2 Plan
The coefficients in the balanced dissociation equation will tell mole relationships You can use the equation to find out the number of moles of solute ions produced
101
3 Compute
a
b102
Write the equation for the dissolution of each of the following in water and then determine the number of moles of each ions made as well as the total number of ions made a 1 mol ammonium chloride b 1 mol sodium sulfide c 05 mol barium nitrate
103
a 1 mol ammonium chloride
1 mol NH4+ 1 mol Cl- 2 mol total ions
104
b 1 mol sodium sulfide
2 mol Na+ 1 mol S-2 3 mol total ions
105
c 05 mol barium nitrate
05 mol Ba+2 1 mol NO3- 15 mol total ions
106
Precipitation Reactions
Even though no compound is completely insoluble compounds of very low solubility can be considered insoluble for practical purposes There are general guidelines to help predict whether a compound made of certain combination of ions is soluble
107
General Solubility Guidelines
Sodium potassium and ammonium compounds are soluble in water
108
Nitrates acetates and chlorates are soluble
109
110
Most chlorides are soluble except those of silver mercury (I) and lead Lead (II) chloride is soluble in hot water
Most sulfates are soluble except those of barium strontium lead calcium and mercury
111
Most carbonates phosphates and silicates are insoluble except those of sodium potassium and ammonium
112
Most sulfides are insoluble except those of calcium strontium sodium potassium and ammonium
113
Is calcium phosphate Ca3(PO4)2 soluble or insoluble
Not one of the exceptions so it is insoluble
Dissociation equations are not written for insoluble compounds
114
Guidelines also useful in predicting what will happen if 2 solutions of 2 different soluble compounds are mixed
If mixing results in combination of ions that forms insoluble compound a double-replacement and precipitation reaction will happen
115
Will precipitate form when solutions of ammonium sulfide and cadmium nitrate are combined You can tell calcium nitrate is soluble from the guidelines (Most nitrates acetates and chlorates are soluble) You can also tell ammonium sulfide is soluble (Most sulfides are insoluble except those of calcium strontium sodium potassium and ammonium)
116
117
To decide whether precipitate can form must know solubilities of two possible products
Ammonium nitrate ndash soluble (Most nitrates acetates and chlorates are soluble) Cadmium sulfide ndash insoluble (Most sulfides are insoluble except those of calcium strontium sodium potassium and ammonium)
118
bc one product is insoluble double-replacement and precipitation reaction will happen (NH4)2S (aq) + Cd(NO3)2 (aq) rarr NH4NO3 (aq) + CdS (s)
119
Net Ionic Equations
Reactions of ions in aqueous solution usually represented by net ionic equations instead of formula equations Net ionic equation - includes only those compounds and ions that undergo a chemical change in a reaction in an aqueous solution
120
121
To write NIE (net ionic equation) first convert chemical equation into overall ionic equation All soluble ionic compounds shown as dissociated ions in solution Precipitates shown as solids
122
Notice ammonium and nitrate ions appear on both sides They havenrsquot gone through any chemical change Spectator ions - ions that do not take part in a chemical reaction and are found in solution both before and after the reaction
123
To change ionic equation to NIE eliminate spectator ions Cd+2(aq) + S-2(aq) rarr CdS(s)
This NIE applies to any reaction in which precipitate of CdS forms
124
Practice Problem
Identify the precipitate that forms when aqueous solutions of zinc nitrate and ammonium sulfide are combined Write the equation for the possible double-replacement reaction Then write the formula equation overall ionic equation and the net ionic equation
125
1 AnalyzeGiven Identity of reactants zinc nitrate and ammonium sulfide Reaction medium aqueous solution Unknown a equation for the possible double-replacement reaction b identity of the precipitate c formula equation d overall ionic equation e net ionic equation
126
2 PlanWrite the possible double-replacement reaction between Zn(NO3)2 and (NH4)2S Use the rules to determine if any of the products will precipitate Write a formula equation and overall net ionic equation Then cancel spectator ions to make net ionic equation
127
3 Computea possible double-replacement reaction
b rules show that zinc sulfide is not soluble so will precipitate (ammonium nitrate is soluble)
c formula equation128
d overall ionic equation
e net ionic equation129
Will a precipitate form if solutions of potassium sulfate and barium nitrate are combined If so write the net ionic equation for the reaction
Yes Ba+2(aq) + SO4-2(aq) rarr BaSO4(s)
130
Will a precipitate form if solutions of potassium nitrate and magnesium sulfate are combined If so write the net ionic equation for the reaction
No 131
Will a precipitate form if solutions of barium chloride and sodium sulfate are combined If so identify the spectator ions and write the net ionic equation for the reaction
Yes Na+ and Cl- Ba+2(aq) + SO4-2(aq) rarr BaSO4(s)
132
Write the net ionic equation for the precipitation of nickel(II) sulfide
Ni+2(aq) + S-2(aq) rarr NiS(s)
133
IonizationSome molecular compounds can also form ions in solution Usually polar Ionization - ions formed from solute molecules by action of the solvent (creation of ions where there were none) Ionization different from dissociation
134
Dissociation ionic compounds dissolve and ions already present separate Ionization ions formed where non existed before
Ions formed are hydrated Heat released during hydration of ions gives enough energy to break covalent bonds
135
Extent to which solute ionizes depends on strength of bonds within molecules of solute and strength of attraction between solute and solvent molecules
136
HCl ndash molecular compound that ionizes in aqueous solution Contains highly polar bond Attraction between polar HCl molecule and polar water strong enough to break HCl bond
137
The Hydronium Ion
H+ ions from HCl attracts other molecules or ions so strongly that it doesnrsquot normally exist alone Ionization of HCl better described as direct transfer of proton from HCl to H2O forming H3O+
138
139
Hydration of H+ to form hydronium is highly exothermic
Energy released gives a lot of energy needed to ionize a molecular solute
Many molecular compounds that ionize in aqueous solution contain hydrogen and form hydronium ion
140
Strong and Weak Electrolytes
Substances that make ions and conduct current in solution are electrolytes HCl is one of series of compounds that has hydrogen and halogen Hydrogen halides are all molecular compounds with single polar-covalent bonds
141
All are gases Very soluble in water All are electrolytes HCl HBr HI strongly conduct current in solution HF only weakly conducts current at same concentration Strength of conduction related to ability to ionize
142
143
Strong Electrolytes
HCl HBr HI are 100 ionized in solution
Strong electrolyte - any compound whose dilute aqueous solutions conduct electricity well this is due to the presence of all or almost all of the dissolve compound in the form of ions
144
Unique characteristic of strong electrolyte is that they yield only ions
Ex Ionic compound may be highly soluble in water and dissociate into ions (NaCl)
Others may not dissolve much but amount that does dissolve exists only as ions
145
Weak ElectrolytesHF dissolves in water to give acid solution called hydrofluoric acid HF bond stronger than bonds between hydrogen and other halogens When HF dissolves some molecule ionize Reverse reaction also happens HF(aq) + H2O(l) H3O+(aq) + F-(aq)
146
HF(aq) + H2O(l) H3O+
(aq) + F-(aq)Concentration of dissolved unionized HF stays high and concentration of ions stays low
Weak electrolyte - any compound whose aqueous solutions conduct electricity poorly this is due to the presence of small amount of the dissolved compound in the form of ions
147
Different from nonelectrolyte ndash NO ions at all Description of electrolyte as strong or weak not related to concentration of solution Electrolytes differ in degree of ionization not amount of solute dissolved
148
149
Colligative properties of solutions
Presence of solute affects properties of solutions Some properties not dependent on nature of dissolved substance but on how may dissolve particles are present
Colligative properties - properties that depend on the concentration of solute particles but not on their identity
150
Vapor-Pressure Lowering
Boiling and freezing point of solution different from pure solvent Nonvolatile solute raises boiling point and lowers freezing point Nonvolatile substance - one that has little tendency to become gas under existing conditions
151
152
To understand why nonvolatile solute changes boiling and freezing point consider equilibrium vapor pressure
Vapor pressure pressure caused by molecules that have escape liquid phase to gas phase
Can be thought of a measure of tendency of molecules to escape from a liquid
153
Addition of sucrose (nonvolatile solute) lowers concentration of water molecules at surface of liquid This lowers tendency of water molecules to leave solution and enter gas phase Vapor pressure of solution is lower than vapor pressure of pure water
154
Aqueous13 solution13 of13 nonvolatile13 solutePure13 water
Nonelectrolyte solutions of same molality have same concentration of particles
Dilute solutions of same solvent and equal molality of any nonelectrolyte solute lower vapor pressure equally
155
Example
1 m aqueous solution of nonelectrolyte glucose lowers vapor pressure of water 55 x 10-4 atm at 25 1 m aqueous solution of sucrose also lowers water vapor pressure to 55 x 10-4 atm at 25
156
157
bc vapor-pressure lowering depends on concentration of nonelectrolyte solute and doesnrsquot depend on the type of solute it is a colligative proprerty
bc vapor pressure has been lowered solution remains liquid over larger temperature range
Lowers freezing point and raises boiling point
158
Can assume changes in boiling and freezing point also depend on concentration of solute
They are colligative properties
159
Freezing-Point Depression
Freezing point of 1-molal solution of any nonelectrolyte solute in water is found (by experiment) to be 186 lower than freezing point of water When 1 mol of nonelectrolyte solute dissolved in 1 kg water freezing point is -186 not 000 160
When 2 mol nonelectrolyte solute dissolved in 1 kg water freezing point is -372
For any concentration of nonelectrolyte solute in water decrease in freezing point can be determined using value of -186m
161
This value called the molal freezing-point contstant (Kf) is the freezing-point depression of the solvent in a 1-molal solution of a nonvolatile nonelectrolyte solute
162
Each solvent has own characteristic molal freezing-point constant Freezing-point depression (Δtf) the difference between the freezing points of the pure solvent and a solute of nonelectrolyte in that solvent and it is directly proportional to the molal concentration of the solution Δtf = Kfm
163
Δtf = Kfm
Kf expressed as m
m expressed in mol solutekg solvent (molality)
164
Sample Problem
What is the freezing point depression of water in a solution of 171 g of sucrose C12H22O11 and 200 g of water What is the actual freezing point of the solution
165
1 AnalyzeGiven Solute mass and chemical formula 171 g C12H22O11 Solvent mass and identity 2000 g water Unknown a freezing-point depression b freezing point of the solution 166
2 Plan
167
168
Calculate
Unknown a freezing-point depression b freezing point of the solution
169
Practice Problem
A water solution containing an unknown quantity of a nonelectrolyte solute is found to have a freezing point of ndash 023degCWhat is the molal concentration of the solution
012 m170
A solution consists of 103 g of the nonelectrolyte glucose C6H12O6 dissolved in 250 g of water What is the freezing-point depression of the solution
minus0426degC171
In a laboratory experiment the freezing point of an aqueous solution of glucose is found to be minus0325degCWhat is the molal concentration of this solution
0175 m172
If 0500 mol of a nonelectrolyte solute are dissolved in 5000 g of ether what is the freezing point of the solution
minus1181degC173
The freezing point of an aqueous solution that contains a nonelectrolyte is minus90degC aWhat is the freezing-point depression of the solution bWhat is the molal concentration of the solution a minus90degC b 48 m
174
Boiling-Point Elevation
Boiling point is temp at which vapor pressure is equal to atmospheric pressure Change in vapor pressure will cause corresponding change in boiling point
175
176
Vapor pressure of nonvolatile solution is lower than vapor pressure of pure solvent More heat will be required to raise vapor pressure of solution Boiling point of solution is higher than boiling point of pure solvent
Molal boiling-point constant (Kb) - boiling-point elevation of the solvent in a 1-molal solution of a nonvolatile nonelectrolyte solute b-p elevation of any nonelectrolyte in water found by experiment to be 051 So molal b-p constant for water is 051m
177
For different solvents b-p elevations of 1-molal solutions have different values (Table 14-2)
Like freezing-point constants values most accurate for dilute solutions
178
Boiling-point elevation Δtb difference between the boiling points of the pure solvent and a nonelectrolyte solution of that solvent and it is directly proportional to the molal concentration of the solution B-p elevation can be calculated using Δtb = Kbm Δtb (boiling point elevation) expressed in m and m expressed in mol solutekg solvent
Sample Problem
What is the boiling-point elevation of a solution made from 200 g of a nonelectrolyte solute and 4000 g of water The molar mass of the solute is 620 gmol
180
1 AnalyzeGiven solute mass = 200 g solute molar mass = 620 gmol solvent mass and identity = 4000 g of water
Unknown boiling-point elevation
181
2 Plan
182
183
3 Compute
Practice Problem
A solution contains 500 g of sucrose C12H22O11 a nonelectrolyte dissolved in 5000 g of water What is the boiling-point elevation
015degC184
A solution contains 4500 g of sucrose C12H22O11 a nonelectrolyte dissolved in 250 g of waterWhat is the boiling point of the solution
1027degC185
If the boiling point elevation of an aqueous solution containing a nonvolatile electrolyte is 102degC what is the molality of the solution
20 m186
The boiling point of an aqueous solution containing a nonvolatile electrolyte is 10075degC aWhat is the boiling-point elevation bWhat is the molality of the solution a 075degC b 15 m
187
Electrolytes and Colligative PropertiesEarly investigators confused by experiments where certain substances depressed freezing point or elevated boiling point more than expected
188
189
Ex 01 m solution of NaCl lowers freezing point of solvent almost twice as much as 01 m solution of sucrose 01 m CaCl2 lowers fp almost 3 times as much as 01 m solution of sucrose
To understand why this happens contrast behavior or sucrose with NaCl in aqueous solution Each sucrose dissolves to make only 1 particle in solution So 1 mol sucrose dissolves to 1 mol particles 1 mol NaCl dissolves to make 2 moles of particles (1 mol Na+ and 1 mol Cl-)
190
191
Calculated Values for Electrolyte SolutionsColligative properties depend on total concentration of solute particles regardless of their identity Electrolytes cause changes in colligative properties proportional to total molality in terms of all dissolves particles instead of formula units
192
What about barium nitrate Ba(NO3)2 Each mole of barium nitrate yields 3 mol of ions in solution Ba(NO3)2(s) rarr Ba2+(aq) + 2NO3-(aq) You would expect solution of given molality to lower fp of solvent 3 times as much as nonelectrolytic solution of same molality
193
Sample Problem
What is the expected change in the freezing point of water in a solution of 625 g of barium nitrate Ba(NO3)2 in 100 kg of water
194
1 AnalyzeGiven solute mass and formula = 625 g Ba(NO3)2 solvent mass and identity = 100 kg water Δtf = Kfm
Unknown expected freezing-point depression
195
2 PlanThe molality can be calculated by converting the solute mass to moles and then dividing by the number of kilograms of solvent That molality is in terms of formula units of Ba(NO3)2 and must be converted to molality in terms of dissociated ions in solution It must be multiplied by the number of moles of ions produced per mole of formula unit This adjusted molality can then be used to calculate the freezing-point depression
196
197
198
3 Compute
Practice Problem
What is the expected freezing-point depression for a solution that contains 20 mol of magnesium sulfate dissolved in 10 kg of water
minus74degC199
What is the expected boiling-point elevation of water for a solution that contains 150 g of sodium chloride dissolved in 10 kg of water 27degC The freezing point of an aqueous sodium chloride solution is minus200degC What is the molality of the solution 54 m NaCl
200
Actual Values for Electrolyte SolutionsRemember values calculated are only expected values Actual values of colligative properties for all strong electrolytes are almost what would be expected Difference between expected and actual values caused by attractive forces that exist between dissociated ions in aqueous solution
201
Attraction between hydrated ions in solution is small compared with those in crystalline solid However forces of attraction donrsquot interfere with movements of aqueous ions More concentrated solution means ions closer together and attraction is greater Also depends on charge of ions ndash higher charge greater attraction
202
Henryrsquos Law
Henryrsquos law - solubility of a gas in liquid is directly proportional to the partial pressure of that gas on the surface of the liquid
Applies to gas-liquid solutions at constant temp
65
When mixture of ideal gases is limited in constant volume at constant temp each gas applies same pressure it would apply if it occupied the space alone
Assuming gases do not react each gas dissolves to the extent it would if no other gases were there
66
In carbonated drinks (Coke Pepsi etc) solubility of CO2 increased by increasing pressure
At bottling factory CO2 gas forced into solution of flavored water at pressure of 5-10 atm Gas-in-liquid solution then sealed in bottlescans
67
When cap removed pressure reduced to 1 atm and some CO2 escapes as gas bubbles Effervescence - rapid escape of a gas from a liquid in which it is dissolved
68
Effects of Temperature on Solubility
Letrsquos consider gas solubility Increasing temperature usually decreases gas solubility As temp increases average kinetic energy of molecules in solution increases Greater number of solute molecules escape from attraction of solvent and return to gas phase 69
At higher temps eq is reached with fewer gas molecules in solution Gases generally less soluble
70
Effect of temp on solubility of solids in liquids more difficult to predict
Often increasing temp increases solubility
However the same temp increase can result in large increase in solubility in one case and only slight increase in the next
71
72
enthalpies of Solution
Formation of solution comes with energy change Dissolve KI in water container feels cold Dissolve LiCl in water feels hot Formation of solid-liquid solution can absorb or release heat
73
During formation of solution solvent and solute particles experience changes in forces attracting them to other particles Before dissolving begins solvent molecules held by intermolecular forces (solvent-solvent attraction) In solute molecules held by Ifs (solute-solute attraction)
74
Energy is required to separate solute molecules and solvent molecules from their neighbors
A solute particle that is surrounded by solvent molecules is said to be solvated
75
Step 1 solute particles become separated from solid (energy absorbed) Step 2 solvent particles move apart to allow solute particles to enter liquid (energy absorbed)
76
Step 3 solvent particles attracted to and solvate solute particles (energy released)
77
Heat of solution - net amount of heat energy absorbed or released when a specific amount of solute dissolves in a solvent
Heat of solution negative (heat is released) if sums of energy in steps 1 and 2 is less than step 3
If 1+2 gt 3 HOS is positive (heat absorbed)78
In gaseous state molecules are so far apart there are nearly no forces between them Solute-solute interaction has little effect on heat of solution of a gas Energy released when gas dissolved in liquid bc attraction between solute gas and solvent molecules greater than energy needed to separate solvent molecules
79
Concentration of Solutions80
Concentration - measure of amount of solute in given amount of solvent or solution ldquodiluterdquo just means small amount of solute ldquoconcentratedrdquo just means relatively large amount of solute Unrelated to degree to which solution is saturated Ex Saturated solution of substance not very soluble might be dilute
81
Molarity
Molarity - number of moles of solute in one liter of solution To find molarity must know molar mass A ldquoone molarrdquo solution of NaOH is 1 mole NaOH per liter of solution (1 M)
82
1 mol NaOH = 400 g If this quantity dissolve in enough water to make EXACTLY 100 L solution solution is 1 M If 200 g NaOH dissolve in enough to make 100 L solution what is molarity 0500 M
83
1 M solution is not made by adding 1 mol solute to 1 L solvent That would be more than 1 L Instead 1 mol solute dissolved in less than 1 L Then diluted with more solvent to bring TOTAL VOLUME to 1 L
84
Practice ProblemYou have 340 L of solution that contains 900 g sodium chloride What is the molarity of that solution 0440 M NaCl You have 08 L of a 05 M HCl solution How many moles of HCl does this solution contain 04 mol HCl
85
What is the molarity of a solution composed of 585 g of potassium iodide dissolved in enough water to make a 0125 L of solution 0282 M KI
86
How many moles of H2SO4 are present in 0500 L of a 0150 M H2SO4 solution 00750 mol What volume of 300 M NaCl is needed for reaction that requires 1463 g of NaCl 0834 L
87
Molality
Molality - concentration of solution expressed in moles of solute per kilogram of solvent Solution that contains 1 mol solute (NaOH) dissolved in exactly 1 kg of solvent is ldquoone molalrdquo solution (1 m NaOH)
88
1 mol NaOH = 400 g 400 g NaOH dissolved in 1 kg water is 1 m NaOH
200 g NaOH in 1 kg water = 0500 m NaOH
89
Practice Problem
A solution was prepared by dissolving 171 g sucrose (C12H22O11) in 125 g of water Find the molal concentration of this solution 0400 m C12H22O11
90
A solution of iodine in carbon tetrachloride is used when iodine is needed for certain chemical tests How much iodine must be added to prepare a 0480 m solution of iodine in CCl4 if 1000 g of CCl4 is used 122 g I2
91
What is the molality of a solution composed of 255 g acetone (CH3)2CO dissolved in 200 g of water 22 m acetone What quantity in grams of methanol CH3OH is required to prepare a 0244 m solution in 400 g water 312 g methanol
92
How many grams of AgNO3 are needed to prepare a 0125 m solution in 250 mL of water 531 g AgNO3
What is the molality of a solution containing 182 g HCl and 250 g water 200 m
93
Ions in Aqueous Solutions and Colligative Properties
94
95
compounds in aqueous solutions
DissociationWhen compound made from ions dissolves in water ions separate Dissociation - separation of ions that occurs when an ionic compound dissolves
96
Notice number of ions made per formula unit in equations 1 formula unit of NaCl gives 2 units of ions in solution 1 formula unit of CaCl2 gives 3 units of ions
97
Assuming 100 dissociation solution that contains 1 mol NaCl contains 1 mol Na+ and 1 mol Cl- Can assume 100 dissociation of all soluble ionic compounds
98
Practice Problem
Write the equation for the dissolution of aluminum sulfate in water How many moles of aluminum ions are made by dissolving 1 mol aluminum sulfate What is the total number of moles of ions made by dissolving 1 mol of aluminum sulfate
99
1 AnalyzeGiven Amount of solute = 1 mol Al2(SO4)3 Solvent identity = water Unknown a moles of aluminum ions and sulfate ions Total number of moles of solute ions produced
100
2 Plan
The coefficients in the balanced dissociation equation will tell mole relationships You can use the equation to find out the number of moles of solute ions produced
101
3 Compute
a
b102
Write the equation for the dissolution of each of the following in water and then determine the number of moles of each ions made as well as the total number of ions made a 1 mol ammonium chloride b 1 mol sodium sulfide c 05 mol barium nitrate
103
a 1 mol ammonium chloride
1 mol NH4+ 1 mol Cl- 2 mol total ions
104
b 1 mol sodium sulfide
2 mol Na+ 1 mol S-2 3 mol total ions
105
c 05 mol barium nitrate
05 mol Ba+2 1 mol NO3- 15 mol total ions
106
Precipitation Reactions
Even though no compound is completely insoluble compounds of very low solubility can be considered insoluble for practical purposes There are general guidelines to help predict whether a compound made of certain combination of ions is soluble
107
General Solubility Guidelines
Sodium potassium and ammonium compounds are soluble in water
108
Nitrates acetates and chlorates are soluble
109
110
Most chlorides are soluble except those of silver mercury (I) and lead Lead (II) chloride is soluble in hot water
Most sulfates are soluble except those of barium strontium lead calcium and mercury
111
Most carbonates phosphates and silicates are insoluble except those of sodium potassium and ammonium
112
Most sulfides are insoluble except those of calcium strontium sodium potassium and ammonium
113
Is calcium phosphate Ca3(PO4)2 soluble or insoluble
Not one of the exceptions so it is insoluble
Dissociation equations are not written for insoluble compounds
114
Guidelines also useful in predicting what will happen if 2 solutions of 2 different soluble compounds are mixed
If mixing results in combination of ions that forms insoluble compound a double-replacement and precipitation reaction will happen
115
Will precipitate form when solutions of ammonium sulfide and cadmium nitrate are combined You can tell calcium nitrate is soluble from the guidelines (Most nitrates acetates and chlorates are soluble) You can also tell ammonium sulfide is soluble (Most sulfides are insoluble except those of calcium strontium sodium potassium and ammonium)
116
117
To decide whether precipitate can form must know solubilities of two possible products
Ammonium nitrate ndash soluble (Most nitrates acetates and chlorates are soluble) Cadmium sulfide ndash insoluble (Most sulfides are insoluble except those of calcium strontium sodium potassium and ammonium)
118
bc one product is insoluble double-replacement and precipitation reaction will happen (NH4)2S (aq) + Cd(NO3)2 (aq) rarr NH4NO3 (aq) + CdS (s)
119
Net Ionic Equations
Reactions of ions in aqueous solution usually represented by net ionic equations instead of formula equations Net ionic equation - includes only those compounds and ions that undergo a chemical change in a reaction in an aqueous solution
120
121
To write NIE (net ionic equation) first convert chemical equation into overall ionic equation All soluble ionic compounds shown as dissociated ions in solution Precipitates shown as solids
122
Notice ammonium and nitrate ions appear on both sides They havenrsquot gone through any chemical change Spectator ions - ions that do not take part in a chemical reaction and are found in solution both before and after the reaction
123
To change ionic equation to NIE eliminate spectator ions Cd+2(aq) + S-2(aq) rarr CdS(s)
This NIE applies to any reaction in which precipitate of CdS forms
124
Practice Problem
Identify the precipitate that forms when aqueous solutions of zinc nitrate and ammonium sulfide are combined Write the equation for the possible double-replacement reaction Then write the formula equation overall ionic equation and the net ionic equation
125
1 AnalyzeGiven Identity of reactants zinc nitrate and ammonium sulfide Reaction medium aqueous solution Unknown a equation for the possible double-replacement reaction b identity of the precipitate c formula equation d overall ionic equation e net ionic equation
126
2 PlanWrite the possible double-replacement reaction between Zn(NO3)2 and (NH4)2S Use the rules to determine if any of the products will precipitate Write a formula equation and overall net ionic equation Then cancel spectator ions to make net ionic equation
127
3 Computea possible double-replacement reaction
b rules show that zinc sulfide is not soluble so will precipitate (ammonium nitrate is soluble)
c formula equation128
d overall ionic equation
e net ionic equation129
Will a precipitate form if solutions of potassium sulfate and barium nitrate are combined If so write the net ionic equation for the reaction
Yes Ba+2(aq) + SO4-2(aq) rarr BaSO4(s)
130
Will a precipitate form if solutions of potassium nitrate and magnesium sulfate are combined If so write the net ionic equation for the reaction
No 131
Will a precipitate form if solutions of barium chloride and sodium sulfate are combined If so identify the spectator ions and write the net ionic equation for the reaction
Yes Na+ and Cl- Ba+2(aq) + SO4-2(aq) rarr BaSO4(s)
132
Write the net ionic equation for the precipitation of nickel(II) sulfide
Ni+2(aq) + S-2(aq) rarr NiS(s)
133
IonizationSome molecular compounds can also form ions in solution Usually polar Ionization - ions formed from solute molecules by action of the solvent (creation of ions where there were none) Ionization different from dissociation
134
Dissociation ionic compounds dissolve and ions already present separate Ionization ions formed where non existed before
Ions formed are hydrated Heat released during hydration of ions gives enough energy to break covalent bonds
135
Extent to which solute ionizes depends on strength of bonds within molecules of solute and strength of attraction between solute and solvent molecules
136
HCl ndash molecular compound that ionizes in aqueous solution Contains highly polar bond Attraction between polar HCl molecule and polar water strong enough to break HCl bond
137
The Hydronium Ion
H+ ions from HCl attracts other molecules or ions so strongly that it doesnrsquot normally exist alone Ionization of HCl better described as direct transfer of proton from HCl to H2O forming H3O+
138
139
Hydration of H+ to form hydronium is highly exothermic
Energy released gives a lot of energy needed to ionize a molecular solute
Many molecular compounds that ionize in aqueous solution contain hydrogen and form hydronium ion
140
Strong and Weak Electrolytes
Substances that make ions and conduct current in solution are electrolytes HCl is one of series of compounds that has hydrogen and halogen Hydrogen halides are all molecular compounds with single polar-covalent bonds
141
All are gases Very soluble in water All are electrolytes HCl HBr HI strongly conduct current in solution HF only weakly conducts current at same concentration Strength of conduction related to ability to ionize
142
143
Strong Electrolytes
HCl HBr HI are 100 ionized in solution
Strong electrolyte - any compound whose dilute aqueous solutions conduct electricity well this is due to the presence of all or almost all of the dissolve compound in the form of ions
144
Unique characteristic of strong electrolyte is that they yield only ions
Ex Ionic compound may be highly soluble in water and dissociate into ions (NaCl)
Others may not dissolve much but amount that does dissolve exists only as ions
145
Weak ElectrolytesHF dissolves in water to give acid solution called hydrofluoric acid HF bond stronger than bonds between hydrogen and other halogens When HF dissolves some molecule ionize Reverse reaction also happens HF(aq) + H2O(l) H3O+(aq) + F-(aq)
146
HF(aq) + H2O(l) H3O+
(aq) + F-(aq)Concentration of dissolved unionized HF stays high and concentration of ions stays low
Weak electrolyte - any compound whose aqueous solutions conduct electricity poorly this is due to the presence of small amount of the dissolved compound in the form of ions
147
Different from nonelectrolyte ndash NO ions at all Description of electrolyte as strong or weak not related to concentration of solution Electrolytes differ in degree of ionization not amount of solute dissolved
148
149
Colligative properties of solutions
Presence of solute affects properties of solutions Some properties not dependent on nature of dissolved substance but on how may dissolve particles are present
Colligative properties - properties that depend on the concentration of solute particles but not on their identity
150
Vapor-Pressure Lowering
Boiling and freezing point of solution different from pure solvent Nonvolatile solute raises boiling point and lowers freezing point Nonvolatile substance - one that has little tendency to become gas under existing conditions
151
152
To understand why nonvolatile solute changes boiling and freezing point consider equilibrium vapor pressure
Vapor pressure pressure caused by molecules that have escape liquid phase to gas phase
Can be thought of a measure of tendency of molecules to escape from a liquid
153
Addition of sucrose (nonvolatile solute) lowers concentration of water molecules at surface of liquid This lowers tendency of water molecules to leave solution and enter gas phase Vapor pressure of solution is lower than vapor pressure of pure water
154
Aqueous13 solution13 of13 nonvolatile13 solutePure13 water
Nonelectrolyte solutions of same molality have same concentration of particles
Dilute solutions of same solvent and equal molality of any nonelectrolyte solute lower vapor pressure equally
155
Example
1 m aqueous solution of nonelectrolyte glucose lowers vapor pressure of water 55 x 10-4 atm at 25 1 m aqueous solution of sucrose also lowers water vapor pressure to 55 x 10-4 atm at 25
156
157
bc vapor-pressure lowering depends on concentration of nonelectrolyte solute and doesnrsquot depend on the type of solute it is a colligative proprerty
bc vapor pressure has been lowered solution remains liquid over larger temperature range
Lowers freezing point and raises boiling point
158
Can assume changes in boiling and freezing point also depend on concentration of solute
They are colligative properties
159
Freezing-Point Depression
Freezing point of 1-molal solution of any nonelectrolyte solute in water is found (by experiment) to be 186 lower than freezing point of water When 1 mol of nonelectrolyte solute dissolved in 1 kg water freezing point is -186 not 000 160
When 2 mol nonelectrolyte solute dissolved in 1 kg water freezing point is -372
For any concentration of nonelectrolyte solute in water decrease in freezing point can be determined using value of -186m
161
This value called the molal freezing-point contstant (Kf) is the freezing-point depression of the solvent in a 1-molal solution of a nonvolatile nonelectrolyte solute
162
Each solvent has own characteristic molal freezing-point constant Freezing-point depression (Δtf) the difference between the freezing points of the pure solvent and a solute of nonelectrolyte in that solvent and it is directly proportional to the molal concentration of the solution Δtf = Kfm
163
Δtf = Kfm
Kf expressed as m
m expressed in mol solutekg solvent (molality)
164
Sample Problem
What is the freezing point depression of water in a solution of 171 g of sucrose C12H22O11 and 200 g of water What is the actual freezing point of the solution
165
1 AnalyzeGiven Solute mass and chemical formula 171 g C12H22O11 Solvent mass and identity 2000 g water Unknown a freezing-point depression b freezing point of the solution 166
2 Plan
167
168
Calculate
Unknown a freezing-point depression b freezing point of the solution
169
Practice Problem
A water solution containing an unknown quantity of a nonelectrolyte solute is found to have a freezing point of ndash 023degCWhat is the molal concentration of the solution
012 m170
A solution consists of 103 g of the nonelectrolyte glucose C6H12O6 dissolved in 250 g of water What is the freezing-point depression of the solution
minus0426degC171
In a laboratory experiment the freezing point of an aqueous solution of glucose is found to be minus0325degCWhat is the molal concentration of this solution
0175 m172
If 0500 mol of a nonelectrolyte solute are dissolved in 5000 g of ether what is the freezing point of the solution
minus1181degC173
The freezing point of an aqueous solution that contains a nonelectrolyte is minus90degC aWhat is the freezing-point depression of the solution bWhat is the molal concentration of the solution a minus90degC b 48 m
174
Boiling-Point Elevation
Boiling point is temp at which vapor pressure is equal to atmospheric pressure Change in vapor pressure will cause corresponding change in boiling point
175
176
Vapor pressure of nonvolatile solution is lower than vapor pressure of pure solvent More heat will be required to raise vapor pressure of solution Boiling point of solution is higher than boiling point of pure solvent
Molal boiling-point constant (Kb) - boiling-point elevation of the solvent in a 1-molal solution of a nonvolatile nonelectrolyte solute b-p elevation of any nonelectrolyte in water found by experiment to be 051 So molal b-p constant for water is 051m
177
For different solvents b-p elevations of 1-molal solutions have different values (Table 14-2)
Like freezing-point constants values most accurate for dilute solutions
178
Boiling-point elevation Δtb difference between the boiling points of the pure solvent and a nonelectrolyte solution of that solvent and it is directly proportional to the molal concentration of the solution B-p elevation can be calculated using Δtb = Kbm Δtb (boiling point elevation) expressed in m and m expressed in mol solutekg solvent
Sample Problem
What is the boiling-point elevation of a solution made from 200 g of a nonelectrolyte solute and 4000 g of water The molar mass of the solute is 620 gmol
180
1 AnalyzeGiven solute mass = 200 g solute molar mass = 620 gmol solvent mass and identity = 4000 g of water
Unknown boiling-point elevation
181
2 Plan
182
183
3 Compute
Practice Problem
A solution contains 500 g of sucrose C12H22O11 a nonelectrolyte dissolved in 5000 g of water What is the boiling-point elevation
015degC184
A solution contains 4500 g of sucrose C12H22O11 a nonelectrolyte dissolved in 250 g of waterWhat is the boiling point of the solution
1027degC185
If the boiling point elevation of an aqueous solution containing a nonvolatile electrolyte is 102degC what is the molality of the solution
20 m186
The boiling point of an aqueous solution containing a nonvolatile electrolyte is 10075degC aWhat is the boiling-point elevation bWhat is the molality of the solution a 075degC b 15 m
187
Electrolytes and Colligative PropertiesEarly investigators confused by experiments where certain substances depressed freezing point or elevated boiling point more than expected
188
189
Ex 01 m solution of NaCl lowers freezing point of solvent almost twice as much as 01 m solution of sucrose 01 m CaCl2 lowers fp almost 3 times as much as 01 m solution of sucrose
To understand why this happens contrast behavior or sucrose with NaCl in aqueous solution Each sucrose dissolves to make only 1 particle in solution So 1 mol sucrose dissolves to 1 mol particles 1 mol NaCl dissolves to make 2 moles of particles (1 mol Na+ and 1 mol Cl-)
190
191
Calculated Values for Electrolyte SolutionsColligative properties depend on total concentration of solute particles regardless of their identity Electrolytes cause changes in colligative properties proportional to total molality in terms of all dissolves particles instead of formula units
192
What about barium nitrate Ba(NO3)2 Each mole of barium nitrate yields 3 mol of ions in solution Ba(NO3)2(s) rarr Ba2+(aq) + 2NO3-(aq) You would expect solution of given molality to lower fp of solvent 3 times as much as nonelectrolytic solution of same molality
193
Sample Problem
What is the expected change in the freezing point of water in a solution of 625 g of barium nitrate Ba(NO3)2 in 100 kg of water
194
1 AnalyzeGiven solute mass and formula = 625 g Ba(NO3)2 solvent mass and identity = 100 kg water Δtf = Kfm
Unknown expected freezing-point depression
195
2 PlanThe molality can be calculated by converting the solute mass to moles and then dividing by the number of kilograms of solvent That molality is in terms of formula units of Ba(NO3)2 and must be converted to molality in terms of dissociated ions in solution It must be multiplied by the number of moles of ions produced per mole of formula unit This adjusted molality can then be used to calculate the freezing-point depression
196
197
198
3 Compute
Practice Problem
What is the expected freezing-point depression for a solution that contains 20 mol of magnesium sulfate dissolved in 10 kg of water
minus74degC199
What is the expected boiling-point elevation of water for a solution that contains 150 g of sodium chloride dissolved in 10 kg of water 27degC The freezing point of an aqueous sodium chloride solution is minus200degC What is the molality of the solution 54 m NaCl
200
Actual Values for Electrolyte SolutionsRemember values calculated are only expected values Actual values of colligative properties for all strong electrolytes are almost what would be expected Difference between expected and actual values caused by attractive forces that exist between dissociated ions in aqueous solution
201
Attraction between hydrated ions in solution is small compared with those in crystalline solid However forces of attraction donrsquot interfere with movements of aqueous ions More concentrated solution means ions closer together and attraction is greater Also depends on charge of ions ndash higher charge greater attraction
202
When mixture of ideal gases is limited in constant volume at constant temp each gas applies same pressure it would apply if it occupied the space alone
Assuming gases do not react each gas dissolves to the extent it would if no other gases were there
66
In carbonated drinks (Coke Pepsi etc) solubility of CO2 increased by increasing pressure
At bottling factory CO2 gas forced into solution of flavored water at pressure of 5-10 atm Gas-in-liquid solution then sealed in bottlescans
67
When cap removed pressure reduced to 1 atm and some CO2 escapes as gas bubbles Effervescence - rapid escape of a gas from a liquid in which it is dissolved
68
Effects of Temperature on Solubility
Letrsquos consider gas solubility Increasing temperature usually decreases gas solubility As temp increases average kinetic energy of molecules in solution increases Greater number of solute molecules escape from attraction of solvent and return to gas phase 69
At higher temps eq is reached with fewer gas molecules in solution Gases generally less soluble
70
Effect of temp on solubility of solids in liquids more difficult to predict
Often increasing temp increases solubility
However the same temp increase can result in large increase in solubility in one case and only slight increase in the next
71
72
enthalpies of Solution
Formation of solution comes with energy change Dissolve KI in water container feels cold Dissolve LiCl in water feels hot Formation of solid-liquid solution can absorb or release heat
73
During formation of solution solvent and solute particles experience changes in forces attracting them to other particles Before dissolving begins solvent molecules held by intermolecular forces (solvent-solvent attraction) In solute molecules held by Ifs (solute-solute attraction)
74
Energy is required to separate solute molecules and solvent molecules from their neighbors
A solute particle that is surrounded by solvent molecules is said to be solvated
75
Step 1 solute particles become separated from solid (energy absorbed) Step 2 solvent particles move apart to allow solute particles to enter liquid (energy absorbed)
76
Step 3 solvent particles attracted to and solvate solute particles (energy released)
77
Heat of solution - net amount of heat energy absorbed or released when a specific amount of solute dissolves in a solvent
Heat of solution negative (heat is released) if sums of energy in steps 1 and 2 is less than step 3
If 1+2 gt 3 HOS is positive (heat absorbed)78
In gaseous state molecules are so far apart there are nearly no forces between them Solute-solute interaction has little effect on heat of solution of a gas Energy released when gas dissolved in liquid bc attraction between solute gas and solvent molecules greater than energy needed to separate solvent molecules
79
Concentration of Solutions80
Concentration - measure of amount of solute in given amount of solvent or solution ldquodiluterdquo just means small amount of solute ldquoconcentratedrdquo just means relatively large amount of solute Unrelated to degree to which solution is saturated Ex Saturated solution of substance not very soluble might be dilute
81
Molarity
Molarity - number of moles of solute in one liter of solution To find molarity must know molar mass A ldquoone molarrdquo solution of NaOH is 1 mole NaOH per liter of solution (1 M)
82
1 mol NaOH = 400 g If this quantity dissolve in enough water to make EXACTLY 100 L solution solution is 1 M If 200 g NaOH dissolve in enough to make 100 L solution what is molarity 0500 M
83
1 M solution is not made by adding 1 mol solute to 1 L solvent That would be more than 1 L Instead 1 mol solute dissolved in less than 1 L Then diluted with more solvent to bring TOTAL VOLUME to 1 L
84
Practice ProblemYou have 340 L of solution that contains 900 g sodium chloride What is the molarity of that solution 0440 M NaCl You have 08 L of a 05 M HCl solution How many moles of HCl does this solution contain 04 mol HCl
85
What is the molarity of a solution composed of 585 g of potassium iodide dissolved in enough water to make a 0125 L of solution 0282 M KI
86
How many moles of H2SO4 are present in 0500 L of a 0150 M H2SO4 solution 00750 mol What volume of 300 M NaCl is needed for reaction that requires 1463 g of NaCl 0834 L
87
Molality
Molality - concentration of solution expressed in moles of solute per kilogram of solvent Solution that contains 1 mol solute (NaOH) dissolved in exactly 1 kg of solvent is ldquoone molalrdquo solution (1 m NaOH)
88
1 mol NaOH = 400 g 400 g NaOH dissolved in 1 kg water is 1 m NaOH
200 g NaOH in 1 kg water = 0500 m NaOH
89
Practice Problem
A solution was prepared by dissolving 171 g sucrose (C12H22O11) in 125 g of water Find the molal concentration of this solution 0400 m C12H22O11
90
A solution of iodine in carbon tetrachloride is used when iodine is needed for certain chemical tests How much iodine must be added to prepare a 0480 m solution of iodine in CCl4 if 1000 g of CCl4 is used 122 g I2
91
What is the molality of a solution composed of 255 g acetone (CH3)2CO dissolved in 200 g of water 22 m acetone What quantity in grams of methanol CH3OH is required to prepare a 0244 m solution in 400 g water 312 g methanol
92
How many grams of AgNO3 are needed to prepare a 0125 m solution in 250 mL of water 531 g AgNO3
What is the molality of a solution containing 182 g HCl and 250 g water 200 m
93
Ions in Aqueous Solutions and Colligative Properties
94
95
compounds in aqueous solutions
DissociationWhen compound made from ions dissolves in water ions separate Dissociation - separation of ions that occurs when an ionic compound dissolves
96
Notice number of ions made per formula unit in equations 1 formula unit of NaCl gives 2 units of ions in solution 1 formula unit of CaCl2 gives 3 units of ions
97
Assuming 100 dissociation solution that contains 1 mol NaCl contains 1 mol Na+ and 1 mol Cl- Can assume 100 dissociation of all soluble ionic compounds
98
Practice Problem
Write the equation for the dissolution of aluminum sulfate in water How many moles of aluminum ions are made by dissolving 1 mol aluminum sulfate What is the total number of moles of ions made by dissolving 1 mol of aluminum sulfate
99
1 AnalyzeGiven Amount of solute = 1 mol Al2(SO4)3 Solvent identity = water Unknown a moles of aluminum ions and sulfate ions Total number of moles of solute ions produced
100
2 Plan
The coefficients in the balanced dissociation equation will tell mole relationships You can use the equation to find out the number of moles of solute ions produced
101
3 Compute
a
b102
Write the equation for the dissolution of each of the following in water and then determine the number of moles of each ions made as well as the total number of ions made a 1 mol ammonium chloride b 1 mol sodium sulfide c 05 mol barium nitrate
103
a 1 mol ammonium chloride
1 mol NH4+ 1 mol Cl- 2 mol total ions
104
b 1 mol sodium sulfide
2 mol Na+ 1 mol S-2 3 mol total ions
105
c 05 mol barium nitrate
05 mol Ba+2 1 mol NO3- 15 mol total ions
106
Precipitation Reactions
Even though no compound is completely insoluble compounds of very low solubility can be considered insoluble for practical purposes There are general guidelines to help predict whether a compound made of certain combination of ions is soluble
107
General Solubility Guidelines
Sodium potassium and ammonium compounds are soluble in water
108
Nitrates acetates and chlorates are soluble
109
110
Most chlorides are soluble except those of silver mercury (I) and lead Lead (II) chloride is soluble in hot water
Most sulfates are soluble except those of barium strontium lead calcium and mercury
111
Most carbonates phosphates and silicates are insoluble except those of sodium potassium and ammonium
112
Most sulfides are insoluble except those of calcium strontium sodium potassium and ammonium
113
Is calcium phosphate Ca3(PO4)2 soluble or insoluble
Not one of the exceptions so it is insoluble
Dissociation equations are not written for insoluble compounds
114
Guidelines also useful in predicting what will happen if 2 solutions of 2 different soluble compounds are mixed
If mixing results in combination of ions that forms insoluble compound a double-replacement and precipitation reaction will happen
115
Will precipitate form when solutions of ammonium sulfide and cadmium nitrate are combined You can tell calcium nitrate is soluble from the guidelines (Most nitrates acetates and chlorates are soluble) You can also tell ammonium sulfide is soluble (Most sulfides are insoluble except those of calcium strontium sodium potassium and ammonium)
116
117
To decide whether precipitate can form must know solubilities of two possible products
Ammonium nitrate ndash soluble (Most nitrates acetates and chlorates are soluble) Cadmium sulfide ndash insoluble (Most sulfides are insoluble except those of calcium strontium sodium potassium and ammonium)
118
bc one product is insoluble double-replacement and precipitation reaction will happen (NH4)2S (aq) + Cd(NO3)2 (aq) rarr NH4NO3 (aq) + CdS (s)
119
Net Ionic Equations
Reactions of ions in aqueous solution usually represented by net ionic equations instead of formula equations Net ionic equation - includes only those compounds and ions that undergo a chemical change in a reaction in an aqueous solution
120
121
To write NIE (net ionic equation) first convert chemical equation into overall ionic equation All soluble ionic compounds shown as dissociated ions in solution Precipitates shown as solids
122
Notice ammonium and nitrate ions appear on both sides They havenrsquot gone through any chemical change Spectator ions - ions that do not take part in a chemical reaction and are found in solution both before and after the reaction
123
To change ionic equation to NIE eliminate spectator ions Cd+2(aq) + S-2(aq) rarr CdS(s)
This NIE applies to any reaction in which precipitate of CdS forms
124
Practice Problem
Identify the precipitate that forms when aqueous solutions of zinc nitrate and ammonium sulfide are combined Write the equation for the possible double-replacement reaction Then write the formula equation overall ionic equation and the net ionic equation
125
1 AnalyzeGiven Identity of reactants zinc nitrate and ammonium sulfide Reaction medium aqueous solution Unknown a equation for the possible double-replacement reaction b identity of the precipitate c formula equation d overall ionic equation e net ionic equation
126
2 PlanWrite the possible double-replacement reaction between Zn(NO3)2 and (NH4)2S Use the rules to determine if any of the products will precipitate Write a formula equation and overall net ionic equation Then cancel spectator ions to make net ionic equation
127
3 Computea possible double-replacement reaction
b rules show that zinc sulfide is not soluble so will precipitate (ammonium nitrate is soluble)
c formula equation128
d overall ionic equation
e net ionic equation129
Will a precipitate form if solutions of potassium sulfate and barium nitrate are combined If so write the net ionic equation for the reaction
Yes Ba+2(aq) + SO4-2(aq) rarr BaSO4(s)
130
Will a precipitate form if solutions of potassium nitrate and magnesium sulfate are combined If so write the net ionic equation for the reaction
No 131
Will a precipitate form if solutions of barium chloride and sodium sulfate are combined If so identify the spectator ions and write the net ionic equation for the reaction
Yes Na+ and Cl- Ba+2(aq) + SO4-2(aq) rarr BaSO4(s)
132
Write the net ionic equation for the precipitation of nickel(II) sulfide
Ni+2(aq) + S-2(aq) rarr NiS(s)
133
IonizationSome molecular compounds can also form ions in solution Usually polar Ionization - ions formed from solute molecules by action of the solvent (creation of ions where there were none) Ionization different from dissociation
134
Dissociation ionic compounds dissolve and ions already present separate Ionization ions formed where non existed before
Ions formed are hydrated Heat released during hydration of ions gives enough energy to break covalent bonds
135
Extent to which solute ionizes depends on strength of bonds within molecules of solute and strength of attraction between solute and solvent molecules
136
HCl ndash molecular compound that ionizes in aqueous solution Contains highly polar bond Attraction between polar HCl molecule and polar water strong enough to break HCl bond
137
The Hydronium Ion
H+ ions from HCl attracts other molecules or ions so strongly that it doesnrsquot normally exist alone Ionization of HCl better described as direct transfer of proton from HCl to H2O forming H3O+
138
139
Hydration of H+ to form hydronium is highly exothermic
Energy released gives a lot of energy needed to ionize a molecular solute
Many molecular compounds that ionize in aqueous solution contain hydrogen and form hydronium ion
140
Strong and Weak Electrolytes
Substances that make ions and conduct current in solution are electrolytes HCl is one of series of compounds that has hydrogen and halogen Hydrogen halides are all molecular compounds with single polar-covalent bonds
141
All are gases Very soluble in water All are electrolytes HCl HBr HI strongly conduct current in solution HF only weakly conducts current at same concentration Strength of conduction related to ability to ionize
142
143
Strong Electrolytes
HCl HBr HI are 100 ionized in solution
Strong electrolyte - any compound whose dilute aqueous solutions conduct electricity well this is due to the presence of all or almost all of the dissolve compound in the form of ions
144
Unique characteristic of strong electrolyte is that they yield only ions
Ex Ionic compound may be highly soluble in water and dissociate into ions (NaCl)
Others may not dissolve much but amount that does dissolve exists only as ions
145
Weak ElectrolytesHF dissolves in water to give acid solution called hydrofluoric acid HF bond stronger than bonds between hydrogen and other halogens When HF dissolves some molecule ionize Reverse reaction also happens HF(aq) + H2O(l) H3O+(aq) + F-(aq)
146
HF(aq) + H2O(l) H3O+
(aq) + F-(aq)Concentration of dissolved unionized HF stays high and concentration of ions stays low
Weak electrolyte - any compound whose aqueous solutions conduct electricity poorly this is due to the presence of small amount of the dissolved compound in the form of ions
147
Different from nonelectrolyte ndash NO ions at all Description of electrolyte as strong or weak not related to concentration of solution Electrolytes differ in degree of ionization not amount of solute dissolved
148
149
Colligative properties of solutions
Presence of solute affects properties of solutions Some properties not dependent on nature of dissolved substance but on how may dissolve particles are present
Colligative properties - properties that depend on the concentration of solute particles but not on their identity
150
Vapor-Pressure Lowering
Boiling and freezing point of solution different from pure solvent Nonvolatile solute raises boiling point and lowers freezing point Nonvolatile substance - one that has little tendency to become gas under existing conditions
151
152
To understand why nonvolatile solute changes boiling and freezing point consider equilibrium vapor pressure
Vapor pressure pressure caused by molecules that have escape liquid phase to gas phase
Can be thought of a measure of tendency of molecules to escape from a liquid
153
Addition of sucrose (nonvolatile solute) lowers concentration of water molecules at surface of liquid This lowers tendency of water molecules to leave solution and enter gas phase Vapor pressure of solution is lower than vapor pressure of pure water
154
Aqueous13 solution13 of13 nonvolatile13 solutePure13 water
Nonelectrolyte solutions of same molality have same concentration of particles
Dilute solutions of same solvent and equal molality of any nonelectrolyte solute lower vapor pressure equally
155
Example
1 m aqueous solution of nonelectrolyte glucose lowers vapor pressure of water 55 x 10-4 atm at 25 1 m aqueous solution of sucrose also lowers water vapor pressure to 55 x 10-4 atm at 25
156
157
bc vapor-pressure lowering depends on concentration of nonelectrolyte solute and doesnrsquot depend on the type of solute it is a colligative proprerty
bc vapor pressure has been lowered solution remains liquid over larger temperature range
Lowers freezing point and raises boiling point
158
Can assume changes in boiling and freezing point also depend on concentration of solute
They are colligative properties
159
Freezing-Point Depression
Freezing point of 1-molal solution of any nonelectrolyte solute in water is found (by experiment) to be 186 lower than freezing point of water When 1 mol of nonelectrolyte solute dissolved in 1 kg water freezing point is -186 not 000 160
When 2 mol nonelectrolyte solute dissolved in 1 kg water freezing point is -372
For any concentration of nonelectrolyte solute in water decrease in freezing point can be determined using value of -186m
161
This value called the molal freezing-point contstant (Kf) is the freezing-point depression of the solvent in a 1-molal solution of a nonvolatile nonelectrolyte solute
162
Each solvent has own characteristic molal freezing-point constant Freezing-point depression (Δtf) the difference between the freezing points of the pure solvent and a solute of nonelectrolyte in that solvent and it is directly proportional to the molal concentration of the solution Δtf = Kfm
163
Δtf = Kfm
Kf expressed as m
m expressed in mol solutekg solvent (molality)
164
Sample Problem
What is the freezing point depression of water in a solution of 171 g of sucrose C12H22O11 and 200 g of water What is the actual freezing point of the solution
165
1 AnalyzeGiven Solute mass and chemical formula 171 g C12H22O11 Solvent mass and identity 2000 g water Unknown a freezing-point depression b freezing point of the solution 166
2 Plan
167
168
Calculate
Unknown a freezing-point depression b freezing point of the solution
169
Practice Problem
A water solution containing an unknown quantity of a nonelectrolyte solute is found to have a freezing point of ndash 023degCWhat is the molal concentration of the solution
012 m170
A solution consists of 103 g of the nonelectrolyte glucose C6H12O6 dissolved in 250 g of water What is the freezing-point depression of the solution
minus0426degC171
In a laboratory experiment the freezing point of an aqueous solution of glucose is found to be minus0325degCWhat is the molal concentration of this solution
0175 m172
If 0500 mol of a nonelectrolyte solute are dissolved in 5000 g of ether what is the freezing point of the solution
minus1181degC173
The freezing point of an aqueous solution that contains a nonelectrolyte is minus90degC aWhat is the freezing-point depression of the solution bWhat is the molal concentration of the solution a minus90degC b 48 m
174
Boiling-Point Elevation
Boiling point is temp at which vapor pressure is equal to atmospheric pressure Change in vapor pressure will cause corresponding change in boiling point
175
176
Vapor pressure of nonvolatile solution is lower than vapor pressure of pure solvent More heat will be required to raise vapor pressure of solution Boiling point of solution is higher than boiling point of pure solvent
Molal boiling-point constant (Kb) - boiling-point elevation of the solvent in a 1-molal solution of a nonvolatile nonelectrolyte solute b-p elevation of any nonelectrolyte in water found by experiment to be 051 So molal b-p constant for water is 051m
177
For different solvents b-p elevations of 1-molal solutions have different values (Table 14-2)
Like freezing-point constants values most accurate for dilute solutions
178
Boiling-point elevation Δtb difference between the boiling points of the pure solvent and a nonelectrolyte solution of that solvent and it is directly proportional to the molal concentration of the solution B-p elevation can be calculated using Δtb = Kbm Δtb (boiling point elevation) expressed in m and m expressed in mol solutekg solvent
Sample Problem
What is the boiling-point elevation of a solution made from 200 g of a nonelectrolyte solute and 4000 g of water The molar mass of the solute is 620 gmol
180
1 AnalyzeGiven solute mass = 200 g solute molar mass = 620 gmol solvent mass and identity = 4000 g of water
Unknown boiling-point elevation
181
2 Plan
182
183
3 Compute
Practice Problem
A solution contains 500 g of sucrose C12H22O11 a nonelectrolyte dissolved in 5000 g of water What is the boiling-point elevation
015degC184
A solution contains 4500 g of sucrose C12H22O11 a nonelectrolyte dissolved in 250 g of waterWhat is the boiling point of the solution
1027degC185
If the boiling point elevation of an aqueous solution containing a nonvolatile electrolyte is 102degC what is the molality of the solution
20 m186
The boiling point of an aqueous solution containing a nonvolatile electrolyte is 10075degC aWhat is the boiling-point elevation bWhat is the molality of the solution a 075degC b 15 m
187
Electrolytes and Colligative PropertiesEarly investigators confused by experiments where certain substances depressed freezing point or elevated boiling point more than expected
188
189
Ex 01 m solution of NaCl lowers freezing point of solvent almost twice as much as 01 m solution of sucrose 01 m CaCl2 lowers fp almost 3 times as much as 01 m solution of sucrose
To understand why this happens contrast behavior or sucrose with NaCl in aqueous solution Each sucrose dissolves to make only 1 particle in solution So 1 mol sucrose dissolves to 1 mol particles 1 mol NaCl dissolves to make 2 moles of particles (1 mol Na+ and 1 mol Cl-)
190
191
Calculated Values for Electrolyte SolutionsColligative properties depend on total concentration of solute particles regardless of their identity Electrolytes cause changes in colligative properties proportional to total molality in terms of all dissolves particles instead of formula units
192
What about barium nitrate Ba(NO3)2 Each mole of barium nitrate yields 3 mol of ions in solution Ba(NO3)2(s) rarr Ba2+(aq) + 2NO3-(aq) You would expect solution of given molality to lower fp of solvent 3 times as much as nonelectrolytic solution of same molality
193
Sample Problem
What is the expected change in the freezing point of water in a solution of 625 g of barium nitrate Ba(NO3)2 in 100 kg of water
194
1 AnalyzeGiven solute mass and formula = 625 g Ba(NO3)2 solvent mass and identity = 100 kg water Δtf = Kfm
Unknown expected freezing-point depression
195
2 PlanThe molality can be calculated by converting the solute mass to moles and then dividing by the number of kilograms of solvent That molality is in terms of formula units of Ba(NO3)2 and must be converted to molality in terms of dissociated ions in solution It must be multiplied by the number of moles of ions produced per mole of formula unit This adjusted molality can then be used to calculate the freezing-point depression
196
197
198
3 Compute
Practice Problem
What is the expected freezing-point depression for a solution that contains 20 mol of magnesium sulfate dissolved in 10 kg of water
minus74degC199
What is the expected boiling-point elevation of water for a solution that contains 150 g of sodium chloride dissolved in 10 kg of water 27degC The freezing point of an aqueous sodium chloride solution is minus200degC What is the molality of the solution 54 m NaCl
200
Actual Values for Electrolyte SolutionsRemember values calculated are only expected values Actual values of colligative properties for all strong electrolytes are almost what would be expected Difference between expected and actual values caused by attractive forces that exist between dissociated ions in aqueous solution
201
Attraction between hydrated ions in solution is small compared with those in crystalline solid However forces of attraction donrsquot interfere with movements of aqueous ions More concentrated solution means ions closer together and attraction is greater Also depends on charge of ions ndash higher charge greater attraction
202
In carbonated drinks (Coke Pepsi etc) solubility of CO2 increased by increasing pressure
At bottling factory CO2 gas forced into solution of flavored water at pressure of 5-10 atm Gas-in-liquid solution then sealed in bottlescans
67
When cap removed pressure reduced to 1 atm and some CO2 escapes as gas bubbles Effervescence - rapid escape of a gas from a liquid in which it is dissolved
68
Effects of Temperature on Solubility
Letrsquos consider gas solubility Increasing temperature usually decreases gas solubility As temp increases average kinetic energy of molecules in solution increases Greater number of solute molecules escape from attraction of solvent and return to gas phase 69
At higher temps eq is reached with fewer gas molecules in solution Gases generally less soluble
70
Effect of temp on solubility of solids in liquids more difficult to predict
Often increasing temp increases solubility
However the same temp increase can result in large increase in solubility in one case and only slight increase in the next
71
72
enthalpies of Solution
Formation of solution comes with energy change Dissolve KI in water container feels cold Dissolve LiCl in water feels hot Formation of solid-liquid solution can absorb or release heat
73
During formation of solution solvent and solute particles experience changes in forces attracting them to other particles Before dissolving begins solvent molecules held by intermolecular forces (solvent-solvent attraction) In solute molecules held by Ifs (solute-solute attraction)
74
Energy is required to separate solute molecules and solvent molecules from their neighbors
A solute particle that is surrounded by solvent molecules is said to be solvated
75
Step 1 solute particles become separated from solid (energy absorbed) Step 2 solvent particles move apart to allow solute particles to enter liquid (energy absorbed)
76
Step 3 solvent particles attracted to and solvate solute particles (energy released)
77
Heat of solution - net amount of heat energy absorbed or released when a specific amount of solute dissolves in a solvent
Heat of solution negative (heat is released) if sums of energy in steps 1 and 2 is less than step 3
If 1+2 gt 3 HOS is positive (heat absorbed)78
In gaseous state molecules are so far apart there are nearly no forces between them Solute-solute interaction has little effect on heat of solution of a gas Energy released when gas dissolved in liquid bc attraction between solute gas and solvent molecules greater than energy needed to separate solvent molecules
79
Concentration of Solutions80
Concentration - measure of amount of solute in given amount of solvent or solution ldquodiluterdquo just means small amount of solute ldquoconcentratedrdquo just means relatively large amount of solute Unrelated to degree to which solution is saturated Ex Saturated solution of substance not very soluble might be dilute
81
Molarity
Molarity - number of moles of solute in one liter of solution To find molarity must know molar mass A ldquoone molarrdquo solution of NaOH is 1 mole NaOH per liter of solution (1 M)
82
1 mol NaOH = 400 g If this quantity dissolve in enough water to make EXACTLY 100 L solution solution is 1 M If 200 g NaOH dissolve in enough to make 100 L solution what is molarity 0500 M
83
1 M solution is not made by adding 1 mol solute to 1 L solvent That would be more than 1 L Instead 1 mol solute dissolved in less than 1 L Then diluted with more solvent to bring TOTAL VOLUME to 1 L
84
Practice ProblemYou have 340 L of solution that contains 900 g sodium chloride What is the molarity of that solution 0440 M NaCl You have 08 L of a 05 M HCl solution How many moles of HCl does this solution contain 04 mol HCl
85
What is the molarity of a solution composed of 585 g of potassium iodide dissolved in enough water to make a 0125 L of solution 0282 M KI
86
How many moles of H2SO4 are present in 0500 L of a 0150 M H2SO4 solution 00750 mol What volume of 300 M NaCl is needed for reaction that requires 1463 g of NaCl 0834 L
87
Molality
Molality - concentration of solution expressed in moles of solute per kilogram of solvent Solution that contains 1 mol solute (NaOH) dissolved in exactly 1 kg of solvent is ldquoone molalrdquo solution (1 m NaOH)
88
1 mol NaOH = 400 g 400 g NaOH dissolved in 1 kg water is 1 m NaOH
200 g NaOH in 1 kg water = 0500 m NaOH
89
Practice Problem
A solution was prepared by dissolving 171 g sucrose (C12H22O11) in 125 g of water Find the molal concentration of this solution 0400 m C12H22O11
90
A solution of iodine in carbon tetrachloride is used when iodine is needed for certain chemical tests How much iodine must be added to prepare a 0480 m solution of iodine in CCl4 if 1000 g of CCl4 is used 122 g I2
91
What is the molality of a solution composed of 255 g acetone (CH3)2CO dissolved in 200 g of water 22 m acetone What quantity in grams of methanol CH3OH is required to prepare a 0244 m solution in 400 g water 312 g methanol
92
How many grams of AgNO3 are needed to prepare a 0125 m solution in 250 mL of water 531 g AgNO3
What is the molality of a solution containing 182 g HCl and 250 g water 200 m
93
Ions in Aqueous Solutions and Colligative Properties
94
95
compounds in aqueous solutions
DissociationWhen compound made from ions dissolves in water ions separate Dissociation - separation of ions that occurs when an ionic compound dissolves
96
Notice number of ions made per formula unit in equations 1 formula unit of NaCl gives 2 units of ions in solution 1 formula unit of CaCl2 gives 3 units of ions
97
Assuming 100 dissociation solution that contains 1 mol NaCl contains 1 mol Na+ and 1 mol Cl- Can assume 100 dissociation of all soluble ionic compounds
98
Practice Problem
Write the equation for the dissolution of aluminum sulfate in water How many moles of aluminum ions are made by dissolving 1 mol aluminum sulfate What is the total number of moles of ions made by dissolving 1 mol of aluminum sulfate
99
1 AnalyzeGiven Amount of solute = 1 mol Al2(SO4)3 Solvent identity = water Unknown a moles of aluminum ions and sulfate ions Total number of moles of solute ions produced
100
2 Plan
The coefficients in the balanced dissociation equation will tell mole relationships You can use the equation to find out the number of moles of solute ions produced
101
3 Compute
a
b102
Write the equation for the dissolution of each of the following in water and then determine the number of moles of each ions made as well as the total number of ions made a 1 mol ammonium chloride b 1 mol sodium sulfide c 05 mol barium nitrate
103
a 1 mol ammonium chloride
1 mol NH4+ 1 mol Cl- 2 mol total ions
104
b 1 mol sodium sulfide
2 mol Na+ 1 mol S-2 3 mol total ions
105
c 05 mol barium nitrate
05 mol Ba+2 1 mol NO3- 15 mol total ions
106
Precipitation Reactions
Even though no compound is completely insoluble compounds of very low solubility can be considered insoluble for practical purposes There are general guidelines to help predict whether a compound made of certain combination of ions is soluble
107
General Solubility Guidelines
Sodium potassium and ammonium compounds are soluble in water
108
Nitrates acetates and chlorates are soluble
109
110
Most chlorides are soluble except those of silver mercury (I) and lead Lead (II) chloride is soluble in hot water
Most sulfates are soluble except those of barium strontium lead calcium and mercury
111
Most carbonates phosphates and silicates are insoluble except those of sodium potassium and ammonium
112
Most sulfides are insoluble except those of calcium strontium sodium potassium and ammonium
113
Is calcium phosphate Ca3(PO4)2 soluble or insoluble
Not one of the exceptions so it is insoluble
Dissociation equations are not written for insoluble compounds
114
Guidelines also useful in predicting what will happen if 2 solutions of 2 different soluble compounds are mixed
If mixing results in combination of ions that forms insoluble compound a double-replacement and precipitation reaction will happen
115
Will precipitate form when solutions of ammonium sulfide and cadmium nitrate are combined You can tell calcium nitrate is soluble from the guidelines (Most nitrates acetates and chlorates are soluble) You can also tell ammonium sulfide is soluble (Most sulfides are insoluble except those of calcium strontium sodium potassium and ammonium)
116
117
To decide whether precipitate can form must know solubilities of two possible products
Ammonium nitrate ndash soluble (Most nitrates acetates and chlorates are soluble) Cadmium sulfide ndash insoluble (Most sulfides are insoluble except those of calcium strontium sodium potassium and ammonium)
118
bc one product is insoluble double-replacement and precipitation reaction will happen (NH4)2S (aq) + Cd(NO3)2 (aq) rarr NH4NO3 (aq) + CdS (s)
119
Net Ionic Equations
Reactions of ions in aqueous solution usually represented by net ionic equations instead of formula equations Net ionic equation - includes only those compounds and ions that undergo a chemical change in a reaction in an aqueous solution
120
121
To write NIE (net ionic equation) first convert chemical equation into overall ionic equation All soluble ionic compounds shown as dissociated ions in solution Precipitates shown as solids
122
Notice ammonium and nitrate ions appear on both sides They havenrsquot gone through any chemical change Spectator ions - ions that do not take part in a chemical reaction and are found in solution both before and after the reaction
123
To change ionic equation to NIE eliminate spectator ions Cd+2(aq) + S-2(aq) rarr CdS(s)
This NIE applies to any reaction in which precipitate of CdS forms
124
Practice Problem
Identify the precipitate that forms when aqueous solutions of zinc nitrate and ammonium sulfide are combined Write the equation for the possible double-replacement reaction Then write the formula equation overall ionic equation and the net ionic equation
125
1 AnalyzeGiven Identity of reactants zinc nitrate and ammonium sulfide Reaction medium aqueous solution Unknown a equation for the possible double-replacement reaction b identity of the precipitate c formula equation d overall ionic equation e net ionic equation
126
2 PlanWrite the possible double-replacement reaction between Zn(NO3)2 and (NH4)2S Use the rules to determine if any of the products will precipitate Write a formula equation and overall net ionic equation Then cancel spectator ions to make net ionic equation
127
3 Computea possible double-replacement reaction
b rules show that zinc sulfide is not soluble so will precipitate (ammonium nitrate is soluble)
c formula equation128
d overall ionic equation
e net ionic equation129
Will a precipitate form if solutions of potassium sulfate and barium nitrate are combined If so write the net ionic equation for the reaction
Yes Ba+2(aq) + SO4-2(aq) rarr BaSO4(s)
130
Will a precipitate form if solutions of potassium nitrate and magnesium sulfate are combined If so write the net ionic equation for the reaction
No 131
Will a precipitate form if solutions of barium chloride and sodium sulfate are combined If so identify the spectator ions and write the net ionic equation for the reaction
Yes Na+ and Cl- Ba+2(aq) + SO4-2(aq) rarr BaSO4(s)
132
Write the net ionic equation for the precipitation of nickel(II) sulfide
Ni+2(aq) + S-2(aq) rarr NiS(s)
133
IonizationSome molecular compounds can also form ions in solution Usually polar Ionization - ions formed from solute molecules by action of the solvent (creation of ions where there were none) Ionization different from dissociation
134
Dissociation ionic compounds dissolve and ions already present separate Ionization ions formed where non existed before
Ions formed are hydrated Heat released during hydration of ions gives enough energy to break covalent bonds
135
Extent to which solute ionizes depends on strength of bonds within molecules of solute and strength of attraction between solute and solvent molecules
136
HCl ndash molecular compound that ionizes in aqueous solution Contains highly polar bond Attraction between polar HCl molecule and polar water strong enough to break HCl bond
137
The Hydronium Ion
H+ ions from HCl attracts other molecules or ions so strongly that it doesnrsquot normally exist alone Ionization of HCl better described as direct transfer of proton from HCl to H2O forming H3O+
138
139
Hydration of H+ to form hydronium is highly exothermic
Energy released gives a lot of energy needed to ionize a molecular solute
Many molecular compounds that ionize in aqueous solution contain hydrogen and form hydronium ion
140
Strong and Weak Electrolytes
Substances that make ions and conduct current in solution are electrolytes HCl is one of series of compounds that has hydrogen and halogen Hydrogen halides are all molecular compounds with single polar-covalent bonds
141
All are gases Very soluble in water All are electrolytes HCl HBr HI strongly conduct current in solution HF only weakly conducts current at same concentration Strength of conduction related to ability to ionize
142
143
Strong Electrolytes
HCl HBr HI are 100 ionized in solution
Strong electrolyte - any compound whose dilute aqueous solutions conduct electricity well this is due to the presence of all or almost all of the dissolve compound in the form of ions
144
Unique characteristic of strong electrolyte is that they yield only ions
Ex Ionic compound may be highly soluble in water and dissociate into ions (NaCl)
Others may not dissolve much but amount that does dissolve exists only as ions
145
Weak ElectrolytesHF dissolves in water to give acid solution called hydrofluoric acid HF bond stronger than bonds between hydrogen and other halogens When HF dissolves some molecule ionize Reverse reaction also happens HF(aq) + H2O(l) H3O+(aq) + F-(aq)
146
HF(aq) + H2O(l) H3O+
(aq) + F-(aq)Concentration of dissolved unionized HF stays high and concentration of ions stays low
Weak electrolyte - any compound whose aqueous solutions conduct electricity poorly this is due to the presence of small amount of the dissolved compound in the form of ions
147
Different from nonelectrolyte ndash NO ions at all Description of electrolyte as strong or weak not related to concentration of solution Electrolytes differ in degree of ionization not amount of solute dissolved
148
149
Colligative properties of solutions
Presence of solute affects properties of solutions Some properties not dependent on nature of dissolved substance but on how may dissolve particles are present
Colligative properties - properties that depend on the concentration of solute particles but not on their identity
150
Vapor-Pressure Lowering
Boiling and freezing point of solution different from pure solvent Nonvolatile solute raises boiling point and lowers freezing point Nonvolatile substance - one that has little tendency to become gas under existing conditions
151
152
To understand why nonvolatile solute changes boiling and freezing point consider equilibrium vapor pressure
Vapor pressure pressure caused by molecules that have escape liquid phase to gas phase
Can be thought of a measure of tendency of molecules to escape from a liquid
153
Addition of sucrose (nonvolatile solute) lowers concentration of water molecules at surface of liquid This lowers tendency of water molecules to leave solution and enter gas phase Vapor pressure of solution is lower than vapor pressure of pure water
154
Aqueous13 solution13 of13 nonvolatile13 solutePure13 water
Nonelectrolyte solutions of same molality have same concentration of particles
Dilute solutions of same solvent and equal molality of any nonelectrolyte solute lower vapor pressure equally
155
Example
1 m aqueous solution of nonelectrolyte glucose lowers vapor pressure of water 55 x 10-4 atm at 25 1 m aqueous solution of sucrose also lowers water vapor pressure to 55 x 10-4 atm at 25
156
157
bc vapor-pressure lowering depends on concentration of nonelectrolyte solute and doesnrsquot depend on the type of solute it is a colligative proprerty
bc vapor pressure has been lowered solution remains liquid over larger temperature range
Lowers freezing point and raises boiling point
158
Can assume changes in boiling and freezing point also depend on concentration of solute
They are colligative properties
159
Freezing-Point Depression
Freezing point of 1-molal solution of any nonelectrolyte solute in water is found (by experiment) to be 186 lower than freezing point of water When 1 mol of nonelectrolyte solute dissolved in 1 kg water freezing point is -186 not 000 160
When 2 mol nonelectrolyte solute dissolved in 1 kg water freezing point is -372
For any concentration of nonelectrolyte solute in water decrease in freezing point can be determined using value of -186m
161
This value called the molal freezing-point contstant (Kf) is the freezing-point depression of the solvent in a 1-molal solution of a nonvolatile nonelectrolyte solute
162
Each solvent has own characteristic molal freezing-point constant Freezing-point depression (Δtf) the difference between the freezing points of the pure solvent and a solute of nonelectrolyte in that solvent and it is directly proportional to the molal concentration of the solution Δtf = Kfm
163
Δtf = Kfm
Kf expressed as m
m expressed in mol solutekg solvent (molality)
164
Sample Problem
What is the freezing point depression of water in a solution of 171 g of sucrose C12H22O11 and 200 g of water What is the actual freezing point of the solution
165
1 AnalyzeGiven Solute mass and chemical formula 171 g C12H22O11 Solvent mass and identity 2000 g water Unknown a freezing-point depression b freezing point of the solution 166
2 Plan
167
168
Calculate
Unknown a freezing-point depression b freezing point of the solution
169
Practice Problem
A water solution containing an unknown quantity of a nonelectrolyte solute is found to have a freezing point of ndash 023degCWhat is the molal concentration of the solution
012 m170
A solution consists of 103 g of the nonelectrolyte glucose C6H12O6 dissolved in 250 g of water What is the freezing-point depression of the solution
minus0426degC171
In a laboratory experiment the freezing point of an aqueous solution of glucose is found to be minus0325degCWhat is the molal concentration of this solution
0175 m172
If 0500 mol of a nonelectrolyte solute are dissolved in 5000 g of ether what is the freezing point of the solution
minus1181degC173
The freezing point of an aqueous solution that contains a nonelectrolyte is minus90degC aWhat is the freezing-point depression of the solution bWhat is the molal concentration of the solution a minus90degC b 48 m
174
Boiling-Point Elevation
Boiling point is temp at which vapor pressure is equal to atmospheric pressure Change in vapor pressure will cause corresponding change in boiling point
175
176
Vapor pressure of nonvolatile solution is lower than vapor pressure of pure solvent More heat will be required to raise vapor pressure of solution Boiling point of solution is higher than boiling point of pure solvent
Molal boiling-point constant (Kb) - boiling-point elevation of the solvent in a 1-molal solution of a nonvolatile nonelectrolyte solute b-p elevation of any nonelectrolyte in water found by experiment to be 051 So molal b-p constant for water is 051m
177
For different solvents b-p elevations of 1-molal solutions have different values (Table 14-2)
Like freezing-point constants values most accurate for dilute solutions
178
Boiling-point elevation Δtb difference between the boiling points of the pure solvent and a nonelectrolyte solution of that solvent and it is directly proportional to the molal concentration of the solution B-p elevation can be calculated using Δtb = Kbm Δtb (boiling point elevation) expressed in m and m expressed in mol solutekg solvent
Sample Problem
What is the boiling-point elevation of a solution made from 200 g of a nonelectrolyte solute and 4000 g of water The molar mass of the solute is 620 gmol
180
1 AnalyzeGiven solute mass = 200 g solute molar mass = 620 gmol solvent mass and identity = 4000 g of water
Unknown boiling-point elevation
181
2 Plan
182
183
3 Compute
Practice Problem
A solution contains 500 g of sucrose C12H22O11 a nonelectrolyte dissolved in 5000 g of water What is the boiling-point elevation
015degC184
A solution contains 4500 g of sucrose C12H22O11 a nonelectrolyte dissolved in 250 g of waterWhat is the boiling point of the solution
1027degC185
If the boiling point elevation of an aqueous solution containing a nonvolatile electrolyte is 102degC what is the molality of the solution
20 m186
The boiling point of an aqueous solution containing a nonvolatile electrolyte is 10075degC aWhat is the boiling-point elevation bWhat is the molality of the solution a 075degC b 15 m
187
Electrolytes and Colligative PropertiesEarly investigators confused by experiments where certain substances depressed freezing point or elevated boiling point more than expected
188
189
Ex 01 m solution of NaCl lowers freezing point of solvent almost twice as much as 01 m solution of sucrose 01 m CaCl2 lowers fp almost 3 times as much as 01 m solution of sucrose
To understand why this happens contrast behavior or sucrose with NaCl in aqueous solution Each sucrose dissolves to make only 1 particle in solution So 1 mol sucrose dissolves to 1 mol particles 1 mol NaCl dissolves to make 2 moles of particles (1 mol Na+ and 1 mol Cl-)
190
191
Calculated Values for Electrolyte SolutionsColligative properties depend on total concentration of solute particles regardless of their identity Electrolytes cause changes in colligative properties proportional to total molality in terms of all dissolves particles instead of formula units
192
What about barium nitrate Ba(NO3)2 Each mole of barium nitrate yields 3 mol of ions in solution Ba(NO3)2(s) rarr Ba2+(aq) + 2NO3-(aq) You would expect solution of given molality to lower fp of solvent 3 times as much as nonelectrolytic solution of same molality
193
Sample Problem
What is the expected change in the freezing point of water in a solution of 625 g of barium nitrate Ba(NO3)2 in 100 kg of water
194
1 AnalyzeGiven solute mass and formula = 625 g Ba(NO3)2 solvent mass and identity = 100 kg water Δtf = Kfm
Unknown expected freezing-point depression
195
2 PlanThe molality can be calculated by converting the solute mass to moles and then dividing by the number of kilograms of solvent That molality is in terms of formula units of Ba(NO3)2 and must be converted to molality in terms of dissociated ions in solution It must be multiplied by the number of moles of ions produced per mole of formula unit This adjusted molality can then be used to calculate the freezing-point depression
196
197
198
3 Compute
Practice Problem
What is the expected freezing-point depression for a solution that contains 20 mol of magnesium sulfate dissolved in 10 kg of water
minus74degC199
What is the expected boiling-point elevation of water for a solution that contains 150 g of sodium chloride dissolved in 10 kg of water 27degC The freezing point of an aqueous sodium chloride solution is minus200degC What is the molality of the solution 54 m NaCl
200
Actual Values for Electrolyte SolutionsRemember values calculated are only expected values Actual values of colligative properties for all strong electrolytes are almost what would be expected Difference between expected and actual values caused by attractive forces that exist between dissociated ions in aqueous solution
201
Attraction between hydrated ions in solution is small compared with those in crystalline solid However forces of attraction donrsquot interfere with movements of aqueous ions More concentrated solution means ions closer together and attraction is greater Also depends on charge of ions ndash higher charge greater attraction
202
When cap removed pressure reduced to 1 atm and some CO2 escapes as gas bubbles Effervescence - rapid escape of a gas from a liquid in which it is dissolved
68
Effects of Temperature on Solubility
Letrsquos consider gas solubility Increasing temperature usually decreases gas solubility As temp increases average kinetic energy of molecules in solution increases Greater number of solute molecules escape from attraction of solvent and return to gas phase 69
At higher temps eq is reached with fewer gas molecules in solution Gases generally less soluble
70
Effect of temp on solubility of solids in liquids more difficult to predict
Often increasing temp increases solubility
However the same temp increase can result in large increase in solubility in one case and only slight increase in the next
71
72
enthalpies of Solution
Formation of solution comes with energy change Dissolve KI in water container feels cold Dissolve LiCl in water feels hot Formation of solid-liquid solution can absorb or release heat
73
During formation of solution solvent and solute particles experience changes in forces attracting them to other particles Before dissolving begins solvent molecules held by intermolecular forces (solvent-solvent attraction) In solute molecules held by Ifs (solute-solute attraction)
74
Energy is required to separate solute molecules and solvent molecules from their neighbors
A solute particle that is surrounded by solvent molecules is said to be solvated
75
Step 1 solute particles become separated from solid (energy absorbed) Step 2 solvent particles move apart to allow solute particles to enter liquid (energy absorbed)
76
Step 3 solvent particles attracted to and solvate solute particles (energy released)
77
Heat of solution - net amount of heat energy absorbed or released when a specific amount of solute dissolves in a solvent
Heat of solution negative (heat is released) if sums of energy in steps 1 and 2 is less than step 3
If 1+2 gt 3 HOS is positive (heat absorbed)78
In gaseous state molecules are so far apart there are nearly no forces between them Solute-solute interaction has little effect on heat of solution of a gas Energy released when gas dissolved in liquid bc attraction between solute gas and solvent molecules greater than energy needed to separate solvent molecules
79
Concentration of Solutions80
Concentration - measure of amount of solute in given amount of solvent or solution ldquodiluterdquo just means small amount of solute ldquoconcentratedrdquo just means relatively large amount of solute Unrelated to degree to which solution is saturated Ex Saturated solution of substance not very soluble might be dilute
81
Molarity
Molarity - number of moles of solute in one liter of solution To find molarity must know molar mass A ldquoone molarrdquo solution of NaOH is 1 mole NaOH per liter of solution (1 M)
82
1 mol NaOH = 400 g If this quantity dissolve in enough water to make EXACTLY 100 L solution solution is 1 M If 200 g NaOH dissolve in enough to make 100 L solution what is molarity 0500 M
83
1 M solution is not made by adding 1 mol solute to 1 L solvent That would be more than 1 L Instead 1 mol solute dissolved in less than 1 L Then diluted with more solvent to bring TOTAL VOLUME to 1 L
84
Practice ProblemYou have 340 L of solution that contains 900 g sodium chloride What is the molarity of that solution 0440 M NaCl You have 08 L of a 05 M HCl solution How many moles of HCl does this solution contain 04 mol HCl
85
What is the molarity of a solution composed of 585 g of potassium iodide dissolved in enough water to make a 0125 L of solution 0282 M KI
86
How many moles of H2SO4 are present in 0500 L of a 0150 M H2SO4 solution 00750 mol What volume of 300 M NaCl is needed for reaction that requires 1463 g of NaCl 0834 L
87
Molality
Molality - concentration of solution expressed in moles of solute per kilogram of solvent Solution that contains 1 mol solute (NaOH) dissolved in exactly 1 kg of solvent is ldquoone molalrdquo solution (1 m NaOH)
88
1 mol NaOH = 400 g 400 g NaOH dissolved in 1 kg water is 1 m NaOH
200 g NaOH in 1 kg water = 0500 m NaOH
89
Practice Problem
A solution was prepared by dissolving 171 g sucrose (C12H22O11) in 125 g of water Find the molal concentration of this solution 0400 m C12H22O11
90
A solution of iodine in carbon tetrachloride is used when iodine is needed for certain chemical tests How much iodine must be added to prepare a 0480 m solution of iodine in CCl4 if 1000 g of CCl4 is used 122 g I2
91
What is the molality of a solution composed of 255 g acetone (CH3)2CO dissolved in 200 g of water 22 m acetone What quantity in grams of methanol CH3OH is required to prepare a 0244 m solution in 400 g water 312 g methanol
92
How many grams of AgNO3 are needed to prepare a 0125 m solution in 250 mL of water 531 g AgNO3
What is the molality of a solution containing 182 g HCl and 250 g water 200 m
93
Ions in Aqueous Solutions and Colligative Properties
94
95
compounds in aqueous solutions
DissociationWhen compound made from ions dissolves in water ions separate Dissociation - separation of ions that occurs when an ionic compound dissolves
96
Notice number of ions made per formula unit in equations 1 formula unit of NaCl gives 2 units of ions in solution 1 formula unit of CaCl2 gives 3 units of ions
97
Assuming 100 dissociation solution that contains 1 mol NaCl contains 1 mol Na+ and 1 mol Cl- Can assume 100 dissociation of all soluble ionic compounds
98
Practice Problem
Write the equation for the dissolution of aluminum sulfate in water How many moles of aluminum ions are made by dissolving 1 mol aluminum sulfate What is the total number of moles of ions made by dissolving 1 mol of aluminum sulfate
99
1 AnalyzeGiven Amount of solute = 1 mol Al2(SO4)3 Solvent identity = water Unknown a moles of aluminum ions and sulfate ions Total number of moles of solute ions produced
100
2 Plan
The coefficients in the balanced dissociation equation will tell mole relationships You can use the equation to find out the number of moles of solute ions produced
101
3 Compute
a
b102
Write the equation for the dissolution of each of the following in water and then determine the number of moles of each ions made as well as the total number of ions made a 1 mol ammonium chloride b 1 mol sodium sulfide c 05 mol barium nitrate
103
a 1 mol ammonium chloride
1 mol NH4+ 1 mol Cl- 2 mol total ions
104
b 1 mol sodium sulfide
2 mol Na+ 1 mol S-2 3 mol total ions
105
c 05 mol barium nitrate
05 mol Ba+2 1 mol NO3- 15 mol total ions
106
Precipitation Reactions
Even though no compound is completely insoluble compounds of very low solubility can be considered insoluble for practical purposes There are general guidelines to help predict whether a compound made of certain combination of ions is soluble
107
General Solubility Guidelines
Sodium potassium and ammonium compounds are soluble in water
108
Nitrates acetates and chlorates are soluble
109
110
Most chlorides are soluble except those of silver mercury (I) and lead Lead (II) chloride is soluble in hot water
Most sulfates are soluble except those of barium strontium lead calcium and mercury
111
Most carbonates phosphates and silicates are insoluble except those of sodium potassium and ammonium
112
Most sulfides are insoluble except those of calcium strontium sodium potassium and ammonium
113
Is calcium phosphate Ca3(PO4)2 soluble or insoluble
Not one of the exceptions so it is insoluble
Dissociation equations are not written for insoluble compounds
114
Guidelines also useful in predicting what will happen if 2 solutions of 2 different soluble compounds are mixed
If mixing results in combination of ions that forms insoluble compound a double-replacement and precipitation reaction will happen
115
Will precipitate form when solutions of ammonium sulfide and cadmium nitrate are combined You can tell calcium nitrate is soluble from the guidelines (Most nitrates acetates and chlorates are soluble) You can also tell ammonium sulfide is soluble (Most sulfides are insoluble except those of calcium strontium sodium potassium and ammonium)
116
117
To decide whether precipitate can form must know solubilities of two possible products
Ammonium nitrate ndash soluble (Most nitrates acetates and chlorates are soluble) Cadmium sulfide ndash insoluble (Most sulfides are insoluble except those of calcium strontium sodium potassium and ammonium)
118
bc one product is insoluble double-replacement and precipitation reaction will happen (NH4)2S (aq) + Cd(NO3)2 (aq) rarr NH4NO3 (aq) + CdS (s)
119
Net Ionic Equations
Reactions of ions in aqueous solution usually represented by net ionic equations instead of formula equations Net ionic equation - includes only those compounds and ions that undergo a chemical change in a reaction in an aqueous solution
120
121
To write NIE (net ionic equation) first convert chemical equation into overall ionic equation All soluble ionic compounds shown as dissociated ions in solution Precipitates shown as solids
122
Notice ammonium and nitrate ions appear on both sides They havenrsquot gone through any chemical change Spectator ions - ions that do not take part in a chemical reaction and are found in solution both before and after the reaction
123
To change ionic equation to NIE eliminate spectator ions Cd+2(aq) + S-2(aq) rarr CdS(s)
This NIE applies to any reaction in which precipitate of CdS forms
124
Practice Problem
Identify the precipitate that forms when aqueous solutions of zinc nitrate and ammonium sulfide are combined Write the equation for the possible double-replacement reaction Then write the formula equation overall ionic equation and the net ionic equation
125
1 AnalyzeGiven Identity of reactants zinc nitrate and ammonium sulfide Reaction medium aqueous solution Unknown a equation for the possible double-replacement reaction b identity of the precipitate c formula equation d overall ionic equation e net ionic equation
126
2 PlanWrite the possible double-replacement reaction between Zn(NO3)2 and (NH4)2S Use the rules to determine if any of the products will precipitate Write a formula equation and overall net ionic equation Then cancel spectator ions to make net ionic equation
127
3 Computea possible double-replacement reaction
b rules show that zinc sulfide is not soluble so will precipitate (ammonium nitrate is soluble)
c formula equation128
d overall ionic equation
e net ionic equation129
Will a precipitate form if solutions of potassium sulfate and barium nitrate are combined If so write the net ionic equation for the reaction
Yes Ba+2(aq) + SO4-2(aq) rarr BaSO4(s)
130
Will a precipitate form if solutions of potassium nitrate and magnesium sulfate are combined If so write the net ionic equation for the reaction
No 131
Will a precipitate form if solutions of barium chloride and sodium sulfate are combined If so identify the spectator ions and write the net ionic equation for the reaction
Yes Na+ and Cl- Ba+2(aq) + SO4-2(aq) rarr BaSO4(s)
132
Write the net ionic equation for the precipitation of nickel(II) sulfide
Ni+2(aq) + S-2(aq) rarr NiS(s)
133
IonizationSome molecular compounds can also form ions in solution Usually polar Ionization - ions formed from solute molecules by action of the solvent (creation of ions where there were none) Ionization different from dissociation
134
Dissociation ionic compounds dissolve and ions already present separate Ionization ions formed where non existed before
Ions formed are hydrated Heat released during hydration of ions gives enough energy to break covalent bonds
135
Extent to which solute ionizes depends on strength of bonds within molecules of solute and strength of attraction between solute and solvent molecules
136
HCl ndash molecular compound that ionizes in aqueous solution Contains highly polar bond Attraction between polar HCl molecule and polar water strong enough to break HCl bond
137
The Hydronium Ion
H+ ions from HCl attracts other molecules or ions so strongly that it doesnrsquot normally exist alone Ionization of HCl better described as direct transfer of proton from HCl to H2O forming H3O+
138
139
Hydration of H+ to form hydronium is highly exothermic
Energy released gives a lot of energy needed to ionize a molecular solute
Many molecular compounds that ionize in aqueous solution contain hydrogen and form hydronium ion
140
Strong and Weak Electrolytes
Substances that make ions and conduct current in solution are electrolytes HCl is one of series of compounds that has hydrogen and halogen Hydrogen halides are all molecular compounds with single polar-covalent bonds
141
All are gases Very soluble in water All are electrolytes HCl HBr HI strongly conduct current in solution HF only weakly conducts current at same concentration Strength of conduction related to ability to ionize
142
143
Strong Electrolytes
HCl HBr HI are 100 ionized in solution
Strong electrolyte - any compound whose dilute aqueous solutions conduct electricity well this is due to the presence of all or almost all of the dissolve compound in the form of ions
144
Unique characteristic of strong electrolyte is that they yield only ions
Ex Ionic compound may be highly soluble in water and dissociate into ions (NaCl)
Others may not dissolve much but amount that does dissolve exists only as ions
145
Weak ElectrolytesHF dissolves in water to give acid solution called hydrofluoric acid HF bond stronger than bonds between hydrogen and other halogens When HF dissolves some molecule ionize Reverse reaction also happens HF(aq) + H2O(l) H3O+(aq) + F-(aq)
146
HF(aq) + H2O(l) H3O+
(aq) + F-(aq)Concentration of dissolved unionized HF stays high and concentration of ions stays low
Weak electrolyte - any compound whose aqueous solutions conduct electricity poorly this is due to the presence of small amount of the dissolved compound in the form of ions
147
Different from nonelectrolyte ndash NO ions at all Description of electrolyte as strong or weak not related to concentration of solution Electrolytes differ in degree of ionization not amount of solute dissolved
148
149
Colligative properties of solutions
Presence of solute affects properties of solutions Some properties not dependent on nature of dissolved substance but on how may dissolve particles are present
Colligative properties - properties that depend on the concentration of solute particles but not on their identity
150
Vapor-Pressure Lowering
Boiling and freezing point of solution different from pure solvent Nonvolatile solute raises boiling point and lowers freezing point Nonvolatile substance - one that has little tendency to become gas under existing conditions
151
152
To understand why nonvolatile solute changes boiling and freezing point consider equilibrium vapor pressure
Vapor pressure pressure caused by molecules that have escape liquid phase to gas phase
Can be thought of a measure of tendency of molecules to escape from a liquid
153
Addition of sucrose (nonvolatile solute) lowers concentration of water molecules at surface of liquid This lowers tendency of water molecules to leave solution and enter gas phase Vapor pressure of solution is lower than vapor pressure of pure water
154
Aqueous13 solution13 of13 nonvolatile13 solutePure13 water
Nonelectrolyte solutions of same molality have same concentration of particles
Dilute solutions of same solvent and equal molality of any nonelectrolyte solute lower vapor pressure equally
155
Example
1 m aqueous solution of nonelectrolyte glucose lowers vapor pressure of water 55 x 10-4 atm at 25 1 m aqueous solution of sucrose also lowers water vapor pressure to 55 x 10-4 atm at 25
156
157
bc vapor-pressure lowering depends on concentration of nonelectrolyte solute and doesnrsquot depend on the type of solute it is a colligative proprerty
bc vapor pressure has been lowered solution remains liquid over larger temperature range
Lowers freezing point and raises boiling point
158
Can assume changes in boiling and freezing point also depend on concentration of solute
They are colligative properties
159
Freezing-Point Depression
Freezing point of 1-molal solution of any nonelectrolyte solute in water is found (by experiment) to be 186 lower than freezing point of water When 1 mol of nonelectrolyte solute dissolved in 1 kg water freezing point is -186 not 000 160
When 2 mol nonelectrolyte solute dissolved in 1 kg water freezing point is -372
For any concentration of nonelectrolyte solute in water decrease in freezing point can be determined using value of -186m
161
This value called the molal freezing-point contstant (Kf) is the freezing-point depression of the solvent in a 1-molal solution of a nonvolatile nonelectrolyte solute
162
Each solvent has own characteristic molal freezing-point constant Freezing-point depression (Δtf) the difference between the freezing points of the pure solvent and a solute of nonelectrolyte in that solvent and it is directly proportional to the molal concentration of the solution Δtf = Kfm
163
Δtf = Kfm
Kf expressed as m
m expressed in mol solutekg solvent (molality)
164
Sample Problem
What is the freezing point depression of water in a solution of 171 g of sucrose C12H22O11 and 200 g of water What is the actual freezing point of the solution
165
1 AnalyzeGiven Solute mass and chemical formula 171 g C12H22O11 Solvent mass and identity 2000 g water Unknown a freezing-point depression b freezing point of the solution 166
2 Plan
167
168
Calculate
Unknown a freezing-point depression b freezing point of the solution
169
Practice Problem
A water solution containing an unknown quantity of a nonelectrolyte solute is found to have a freezing point of ndash 023degCWhat is the molal concentration of the solution
012 m170
A solution consists of 103 g of the nonelectrolyte glucose C6H12O6 dissolved in 250 g of water What is the freezing-point depression of the solution
minus0426degC171
In a laboratory experiment the freezing point of an aqueous solution of glucose is found to be minus0325degCWhat is the molal concentration of this solution
0175 m172
If 0500 mol of a nonelectrolyte solute are dissolved in 5000 g of ether what is the freezing point of the solution
minus1181degC173
The freezing point of an aqueous solution that contains a nonelectrolyte is minus90degC aWhat is the freezing-point depression of the solution bWhat is the molal concentration of the solution a minus90degC b 48 m
174
Boiling-Point Elevation
Boiling point is temp at which vapor pressure is equal to atmospheric pressure Change in vapor pressure will cause corresponding change in boiling point
175
176
Vapor pressure of nonvolatile solution is lower than vapor pressure of pure solvent More heat will be required to raise vapor pressure of solution Boiling point of solution is higher than boiling point of pure solvent
Molal boiling-point constant (Kb) - boiling-point elevation of the solvent in a 1-molal solution of a nonvolatile nonelectrolyte solute b-p elevation of any nonelectrolyte in water found by experiment to be 051 So molal b-p constant for water is 051m
177
For different solvents b-p elevations of 1-molal solutions have different values (Table 14-2)
Like freezing-point constants values most accurate for dilute solutions
178
Boiling-point elevation Δtb difference between the boiling points of the pure solvent and a nonelectrolyte solution of that solvent and it is directly proportional to the molal concentration of the solution B-p elevation can be calculated using Δtb = Kbm Δtb (boiling point elevation) expressed in m and m expressed in mol solutekg solvent
Sample Problem
What is the boiling-point elevation of a solution made from 200 g of a nonelectrolyte solute and 4000 g of water The molar mass of the solute is 620 gmol
180
1 AnalyzeGiven solute mass = 200 g solute molar mass = 620 gmol solvent mass and identity = 4000 g of water
Unknown boiling-point elevation
181
2 Plan
182
183
3 Compute
Practice Problem
A solution contains 500 g of sucrose C12H22O11 a nonelectrolyte dissolved in 5000 g of water What is the boiling-point elevation
015degC184
A solution contains 4500 g of sucrose C12H22O11 a nonelectrolyte dissolved in 250 g of waterWhat is the boiling point of the solution
1027degC185
If the boiling point elevation of an aqueous solution containing a nonvolatile electrolyte is 102degC what is the molality of the solution
20 m186
The boiling point of an aqueous solution containing a nonvolatile electrolyte is 10075degC aWhat is the boiling-point elevation bWhat is the molality of the solution a 075degC b 15 m
187
Electrolytes and Colligative PropertiesEarly investigators confused by experiments where certain substances depressed freezing point or elevated boiling point more than expected
188
189
Ex 01 m solution of NaCl lowers freezing point of solvent almost twice as much as 01 m solution of sucrose 01 m CaCl2 lowers fp almost 3 times as much as 01 m solution of sucrose
To understand why this happens contrast behavior or sucrose with NaCl in aqueous solution Each sucrose dissolves to make only 1 particle in solution So 1 mol sucrose dissolves to 1 mol particles 1 mol NaCl dissolves to make 2 moles of particles (1 mol Na+ and 1 mol Cl-)
190
191
Calculated Values for Electrolyte SolutionsColligative properties depend on total concentration of solute particles regardless of their identity Electrolytes cause changes in colligative properties proportional to total molality in terms of all dissolves particles instead of formula units
192
What about barium nitrate Ba(NO3)2 Each mole of barium nitrate yields 3 mol of ions in solution Ba(NO3)2(s) rarr Ba2+(aq) + 2NO3-(aq) You would expect solution of given molality to lower fp of solvent 3 times as much as nonelectrolytic solution of same molality
193
Sample Problem
What is the expected change in the freezing point of water in a solution of 625 g of barium nitrate Ba(NO3)2 in 100 kg of water
194
1 AnalyzeGiven solute mass and formula = 625 g Ba(NO3)2 solvent mass and identity = 100 kg water Δtf = Kfm
Unknown expected freezing-point depression
195
2 PlanThe molality can be calculated by converting the solute mass to moles and then dividing by the number of kilograms of solvent That molality is in terms of formula units of Ba(NO3)2 and must be converted to molality in terms of dissociated ions in solution It must be multiplied by the number of moles of ions produced per mole of formula unit This adjusted molality can then be used to calculate the freezing-point depression
196
197
198
3 Compute
Practice Problem
What is the expected freezing-point depression for a solution that contains 20 mol of magnesium sulfate dissolved in 10 kg of water
minus74degC199
What is the expected boiling-point elevation of water for a solution that contains 150 g of sodium chloride dissolved in 10 kg of water 27degC The freezing point of an aqueous sodium chloride solution is minus200degC What is the molality of the solution 54 m NaCl
200
Actual Values for Electrolyte SolutionsRemember values calculated are only expected values Actual values of colligative properties for all strong electrolytes are almost what would be expected Difference between expected and actual values caused by attractive forces that exist between dissociated ions in aqueous solution
201
Attraction between hydrated ions in solution is small compared with those in crystalline solid However forces of attraction donrsquot interfere with movements of aqueous ions More concentrated solution means ions closer together and attraction is greater Also depends on charge of ions ndash higher charge greater attraction
202
Effects of Temperature on Solubility
Letrsquos consider gas solubility Increasing temperature usually decreases gas solubility As temp increases average kinetic energy of molecules in solution increases Greater number of solute molecules escape from attraction of solvent and return to gas phase 69
At higher temps eq is reached with fewer gas molecules in solution Gases generally less soluble
70
Effect of temp on solubility of solids in liquids more difficult to predict
Often increasing temp increases solubility
However the same temp increase can result in large increase in solubility in one case and only slight increase in the next
71
72
enthalpies of Solution
Formation of solution comes with energy change Dissolve KI in water container feels cold Dissolve LiCl in water feels hot Formation of solid-liquid solution can absorb or release heat
73
During formation of solution solvent and solute particles experience changes in forces attracting them to other particles Before dissolving begins solvent molecules held by intermolecular forces (solvent-solvent attraction) In solute molecules held by Ifs (solute-solute attraction)
74
Energy is required to separate solute molecules and solvent molecules from their neighbors
A solute particle that is surrounded by solvent molecules is said to be solvated
75
Step 1 solute particles become separated from solid (energy absorbed) Step 2 solvent particles move apart to allow solute particles to enter liquid (energy absorbed)
76
Step 3 solvent particles attracted to and solvate solute particles (energy released)
77
Heat of solution - net amount of heat energy absorbed or released when a specific amount of solute dissolves in a solvent
Heat of solution negative (heat is released) if sums of energy in steps 1 and 2 is less than step 3
If 1+2 gt 3 HOS is positive (heat absorbed)78
In gaseous state molecules are so far apart there are nearly no forces between them Solute-solute interaction has little effect on heat of solution of a gas Energy released when gas dissolved in liquid bc attraction between solute gas and solvent molecules greater than energy needed to separate solvent molecules
79
Concentration of Solutions80
Concentration - measure of amount of solute in given amount of solvent or solution ldquodiluterdquo just means small amount of solute ldquoconcentratedrdquo just means relatively large amount of solute Unrelated to degree to which solution is saturated Ex Saturated solution of substance not very soluble might be dilute
81
Molarity
Molarity - number of moles of solute in one liter of solution To find molarity must know molar mass A ldquoone molarrdquo solution of NaOH is 1 mole NaOH per liter of solution (1 M)
82
1 mol NaOH = 400 g If this quantity dissolve in enough water to make EXACTLY 100 L solution solution is 1 M If 200 g NaOH dissolve in enough to make 100 L solution what is molarity 0500 M
83
1 M solution is not made by adding 1 mol solute to 1 L solvent That would be more than 1 L Instead 1 mol solute dissolved in less than 1 L Then diluted with more solvent to bring TOTAL VOLUME to 1 L
84
Practice ProblemYou have 340 L of solution that contains 900 g sodium chloride What is the molarity of that solution 0440 M NaCl You have 08 L of a 05 M HCl solution How many moles of HCl does this solution contain 04 mol HCl
85
What is the molarity of a solution composed of 585 g of potassium iodide dissolved in enough water to make a 0125 L of solution 0282 M KI
86
How many moles of H2SO4 are present in 0500 L of a 0150 M H2SO4 solution 00750 mol What volume of 300 M NaCl is needed for reaction that requires 1463 g of NaCl 0834 L
87
Molality
Molality - concentration of solution expressed in moles of solute per kilogram of solvent Solution that contains 1 mol solute (NaOH) dissolved in exactly 1 kg of solvent is ldquoone molalrdquo solution (1 m NaOH)
88
1 mol NaOH = 400 g 400 g NaOH dissolved in 1 kg water is 1 m NaOH
200 g NaOH in 1 kg water = 0500 m NaOH
89
Practice Problem
A solution was prepared by dissolving 171 g sucrose (C12H22O11) in 125 g of water Find the molal concentration of this solution 0400 m C12H22O11
90
A solution of iodine in carbon tetrachloride is used when iodine is needed for certain chemical tests How much iodine must be added to prepare a 0480 m solution of iodine in CCl4 if 1000 g of CCl4 is used 122 g I2
91
What is the molality of a solution composed of 255 g acetone (CH3)2CO dissolved in 200 g of water 22 m acetone What quantity in grams of methanol CH3OH is required to prepare a 0244 m solution in 400 g water 312 g methanol
92
How many grams of AgNO3 are needed to prepare a 0125 m solution in 250 mL of water 531 g AgNO3
What is the molality of a solution containing 182 g HCl and 250 g water 200 m
93
Ions in Aqueous Solutions and Colligative Properties
94
95
compounds in aqueous solutions
DissociationWhen compound made from ions dissolves in water ions separate Dissociation - separation of ions that occurs when an ionic compound dissolves
96
Notice number of ions made per formula unit in equations 1 formula unit of NaCl gives 2 units of ions in solution 1 formula unit of CaCl2 gives 3 units of ions
97
Assuming 100 dissociation solution that contains 1 mol NaCl contains 1 mol Na+ and 1 mol Cl- Can assume 100 dissociation of all soluble ionic compounds
98
Practice Problem
Write the equation for the dissolution of aluminum sulfate in water How many moles of aluminum ions are made by dissolving 1 mol aluminum sulfate What is the total number of moles of ions made by dissolving 1 mol of aluminum sulfate
99
1 AnalyzeGiven Amount of solute = 1 mol Al2(SO4)3 Solvent identity = water Unknown a moles of aluminum ions and sulfate ions Total number of moles of solute ions produced
100
2 Plan
The coefficients in the balanced dissociation equation will tell mole relationships You can use the equation to find out the number of moles of solute ions produced
101
3 Compute
a
b102
Write the equation for the dissolution of each of the following in water and then determine the number of moles of each ions made as well as the total number of ions made a 1 mol ammonium chloride b 1 mol sodium sulfide c 05 mol barium nitrate
103
a 1 mol ammonium chloride
1 mol NH4+ 1 mol Cl- 2 mol total ions
104
b 1 mol sodium sulfide
2 mol Na+ 1 mol S-2 3 mol total ions
105
c 05 mol barium nitrate
05 mol Ba+2 1 mol NO3- 15 mol total ions
106
Precipitation Reactions
Even though no compound is completely insoluble compounds of very low solubility can be considered insoluble for practical purposes There are general guidelines to help predict whether a compound made of certain combination of ions is soluble
107
General Solubility Guidelines
Sodium potassium and ammonium compounds are soluble in water
108
Nitrates acetates and chlorates are soluble
109
110
Most chlorides are soluble except those of silver mercury (I) and lead Lead (II) chloride is soluble in hot water
Most sulfates are soluble except those of barium strontium lead calcium and mercury
111
Most carbonates phosphates and silicates are insoluble except those of sodium potassium and ammonium
112
Most sulfides are insoluble except those of calcium strontium sodium potassium and ammonium
113
Is calcium phosphate Ca3(PO4)2 soluble or insoluble
Not one of the exceptions so it is insoluble
Dissociation equations are not written for insoluble compounds
114
Guidelines also useful in predicting what will happen if 2 solutions of 2 different soluble compounds are mixed
If mixing results in combination of ions that forms insoluble compound a double-replacement and precipitation reaction will happen
115
Will precipitate form when solutions of ammonium sulfide and cadmium nitrate are combined You can tell calcium nitrate is soluble from the guidelines (Most nitrates acetates and chlorates are soluble) You can also tell ammonium sulfide is soluble (Most sulfides are insoluble except those of calcium strontium sodium potassium and ammonium)
116
117
To decide whether precipitate can form must know solubilities of two possible products
Ammonium nitrate ndash soluble (Most nitrates acetates and chlorates are soluble) Cadmium sulfide ndash insoluble (Most sulfides are insoluble except those of calcium strontium sodium potassium and ammonium)
118
bc one product is insoluble double-replacement and precipitation reaction will happen (NH4)2S (aq) + Cd(NO3)2 (aq) rarr NH4NO3 (aq) + CdS (s)
119
Net Ionic Equations
Reactions of ions in aqueous solution usually represented by net ionic equations instead of formula equations Net ionic equation - includes only those compounds and ions that undergo a chemical change in a reaction in an aqueous solution
120
121
To write NIE (net ionic equation) first convert chemical equation into overall ionic equation All soluble ionic compounds shown as dissociated ions in solution Precipitates shown as solids
122
Notice ammonium and nitrate ions appear on both sides They havenrsquot gone through any chemical change Spectator ions - ions that do not take part in a chemical reaction and are found in solution both before and after the reaction
123
To change ionic equation to NIE eliminate spectator ions Cd+2(aq) + S-2(aq) rarr CdS(s)
This NIE applies to any reaction in which precipitate of CdS forms
124
Practice Problem
Identify the precipitate that forms when aqueous solutions of zinc nitrate and ammonium sulfide are combined Write the equation for the possible double-replacement reaction Then write the formula equation overall ionic equation and the net ionic equation
125
1 AnalyzeGiven Identity of reactants zinc nitrate and ammonium sulfide Reaction medium aqueous solution Unknown a equation for the possible double-replacement reaction b identity of the precipitate c formula equation d overall ionic equation e net ionic equation
126
2 PlanWrite the possible double-replacement reaction between Zn(NO3)2 and (NH4)2S Use the rules to determine if any of the products will precipitate Write a formula equation and overall net ionic equation Then cancel spectator ions to make net ionic equation
127
3 Computea possible double-replacement reaction
b rules show that zinc sulfide is not soluble so will precipitate (ammonium nitrate is soluble)
c formula equation128
d overall ionic equation
e net ionic equation129
Will a precipitate form if solutions of potassium sulfate and barium nitrate are combined If so write the net ionic equation for the reaction
Yes Ba+2(aq) + SO4-2(aq) rarr BaSO4(s)
130
Will a precipitate form if solutions of potassium nitrate and magnesium sulfate are combined If so write the net ionic equation for the reaction
No 131
Will a precipitate form if solutions of barium chloride and sodium sulfate are combined If so identify the spectator ions and write the net ionic equation for the reaction
Yes Na+ and Cl- Ba+2(aq) + SO4-2(aq) rarr BaSO4(s)
132
Write the net ionic equation for the precipitation of nickel(II) sulfide
Ni+2(aq) + S-2(aq) rarr NiS(s)
133
IonizationSome molecular compounds can also form ions in solution Usually polar Ionization - ions formed from solute molecules by action of the solvent (creation of ions where there were none) Ionization different from dissociation
134
Dissociation ionic compounds dissolve and ions already present separate Ionization ions formed where non existed before
Ions formed are hydrated Heat released during hydration of ions gives enough energy to break covalent bonds
135
Extent to which solute ionizes depends on strength of bonds within molecules of solute and strength of attraction between solute and solvent molecules
136
HCl ndash molecular compound that ionizes in aqueous solution Contains highly polar bond Attraction between polar HCl molecule and polar water strong enough to break HCl bond
137
The Hydronium Ion
H+ ions from HCl attracts other molecules or ions so strongly that it doesnrsquot normally exist alone Ionization of HCl better described as direct transfer of proton from HCl to H2O forming H3O+
138
139
Hydration of H+ to form hydronium is highly exothermic
Energy released gives a lot of energy needed to ionize a molecular solute
Many molecular compounds that ionize in aqueous solution contain hydrogen and form hydronium ion
140
Strong and Weak Electrolytes
Substances that make ions and conduct current in solution are electrolytes HCl is one of series of compounds that has hydrogen and halogen Hydrogen halides are all molecular compounds with single polar-covalent bonds
141
All are gases Very soluble in water All are electrolytes HCl HBr HI strongly conduct current in solution HF only weakly conducts current at same concentration Strength of conduction related to ability to ionize
142
143
Strong Electrolytes
HCl HBr HI are 100 ionized in solution
Strong electrolyte - any compound whose dilute aqueous solutions conduct electricity well this is due to the presence of all or almost all of the dissolve compound in the form of ions
144
Unique characteristic of strong electrolyte is that they yield only ions
Ex Ionic compound may be highly soluble in water and dissociate into ions (NaCl)
Others may not dissolve much but amount that does dissolve exists only as ions
145
Weak ElectrolytesHF dissolves in water to give acid solution called hydrofluoric acid HF bond stronger than bonds between hydrogen and other halogens When HF dissolves some molecule ionize Reverse reaction also happens HF(aq) + H2O(l) H3O+(aq) + F-(aq)
146
HF(aq) + H2O(l) H3O+
(aq) + F-(aq)Concentration of dissolved unionized HF stays high and concentration of ions stays low
Weak electrolyte - any compound whose aqueous solutions conduct electricity poorly this is due to the presence of small amount of the dissolved compound in the form of ions
147
Different from nonelectrolyte ndash NO ions at all Description of electrolyte as strong or weak not related to concentration of solution Electrolytes differ in degree of ionization not amount of solute dissolved
148
149
Colligative properties of solutions
Presence of solute affects properties of solutions Some properties not dependent on nature of dissolved substance but on how may dissolve particles are present
Colligative properties - properties that depend on the concentration of solute particles but not on their identity
150
Vapor-Pressure Lowering
Boiling and freezing point of solution different from pure solvent Nonvolatile solute raises boiling point and lowers freezing point Nonvolatile substance - one that has little tendency to become gas under existing conditions
151
152
To understand why nonvolatile solute changes boiling and freezing point consider equilibrium vapor pressure
Vapor pressure pressure caused by molecules that have escape liquid phase to gas phase
Can be thought of a measure of tendency of molecules to escape from a liquid
153
Addition of sucrose (nonvolatile solute) lowers concentration of water molecules at surface of liquid This lowers tendency of water molecules to leave solution and enter gas phase Vapor pressure of solution is lower than vapor pressure of pure water
154
Aqueous13 solution13 of13 nonvolatile13 solutePure13 water
Nonelectrolyte solutions of same molality have same concentration of particles
Dilute solutions of same solvent and equal molality of any nonelectrolyte solute lower vapor pressure equally
155
Example
1 m aqueous solution of nonelectrolyte glucose lowers vapor pressure of water 55 x 10-4 atm at 25 1 m aqueous solution of sucrose also lowers water vapor pressure to 55 x 10-4 atm at 25
156
157
bc vapor-pressure lowering depends on concentration of nonelectrolyte solute and doesnrsquot depend on the type of solute it is a colligative proprerty
bc vapor pressure has been lowered solution remains liquid over larger temperature range
Lowers freezing point and raises boiling point
158
Can assume changes in boiling and freezing point also depend on concentration of solute
They are colligative properties
159
Freezing-Point Depression
Freezing point of 1-molal solution of any nonelectrolyte solute in water is found (by experiment) to be 186 lower than freezing point of water When 1 mol of nonelectrolyte solute dissolved in 1 kg water freezing point is -186 not 000 160
When 2 mol nonelectrolyte solute dissolved in 1 kg water freezing point is -372
For any concentration of nonelectrolyte solute in water decrease in freezing point can be determined using value of -186m
161
This value called the molal freezing-point contstant (Kf) is the freezing-point depression of the solvent in a 1-molal solution of a nonvolatile nonelectrolyte solute
162
Each solvent has own characteristic molal freezing-point constant Freezing-point depression (Δtf) the difference between the freezing points of the pure solvent and a solute of nonelectrolyte in that solvent and it is directly proportional to the molal concentration of the solution Δtf = Kfm
163
Δtf = Kfm
Kf expressed as m
m expressed in mol solutekg solvent (molality)
164
Sample Problem
What is the freezing point depression of water in a solution of 171 g of sucrose C12H22O11 and 200 g of water What is the actual freezing point of the solution
165
1 AnalyzeGiven Solute mass and chemical formula 171 g C12H22O11 Solvent mass and identity 2000 g water Unknown a freezing-point depression b freezing point of the solution 166
2 Plan
167
168
Calculate
Unknown a freezing-point depression b freezing point of the solution
169
Practice Problem
A water solution containing an unknown quantity of a nonelectrolyte solute is found to have a freezing point of ndash 023degCWhat is the molal concentration of the solution
012 m170
A solution consists of 103 g of the nonelectrolyte glucose C6H12O6 dissolved in 250 g of water What is the freezing-point depression of the solution
minus0426degC171
In a laboratory experiment the freezing point of an aqueous solution of glucose is found to be minus0325degCWhat is the molal concentration of this solution
0175 m172
If 0500 mol of a nonelectrolyte solute are dissolved in 5000 g of ether what is the freezing point of the solution
minus1181degC173
The freezing point of an aqueous solution that contains a nonelectrolyte is minus90degC aWhat is the freezing-point depression of the solution bWhat is the molal concentration of the solution a minus90degC b 48 m
174
Boiling-Point Elevation
Boiling point is temp at which vapor pressure is equal to atmospheric pressure Change in vapor pressure will cause corresponding change in boiling point
175
176
Vapor pressure of nonvolatile solution is lower than vapor pressure of pure solvent More heat will be required to raise vapor pressure of solution Boiling point of solution is higher than boiling point of pure solvent
Molal boiling-point constant (Kb) - boiling-point elevation of the solvent in a 1-molal solution of a nonvolatile nonelectrolyte solute b-p elevation of any nonelectrolyte in water found by experiment to be 051 So molal b-p constant for water is 051m
177
For different solvents b-p elevations of 1-molal solutions have different values (Table 14-2)
Like freezing-point constants values most accurate for dilute solutions
178
Boiling-point elevation Δtb difference between the boiling points of the pure solvent and a nonelectrolyte solution of that solvent and it is directly proportional to the molal concentration of the solution B-p elevation can be calculated using Δtb = Kbm Δtb (boiling point elevation) expressed in m and m expressed in mol solutekg solvent
Sample Problem
What is the boiling-point elevation of a solution made from 200 g of a nonelectrolyte solute and 4000 g of water The molar mass of the solute is 620 gmol
180
1 AnalyzeGiven solute mass = 200 g solute molar mass = 620 gmol solvent mass and identity = 4000 g of water
Unknown boiling-point elevation
181
2 Plan
182
183
3 Compute
Practice Problem
A solution contains 500 g of sucrose C12H22O11 a nonelectrolyte dissolved in 5000 g of water What is the boiling-point elevation
015degC184
A solution contains 4500 g of sucrose C12H22O11 a nonelectrolyte dissolved in 250 g of waterWhat is the boiling point of the solution
1027degC185
If the boiling point elevation of an aqueous solution containing a nonvolatile electrolyte is 102degC what is the molality of the solution
20 m186
The boiling point of an aqueous solution containing a nonvolatile electrolyte is 10075degC aWhat is the boiling-point elevation bWhat is the molality of the solution a 075degC b 15 m
187
Electrolytes and Colligative PropertiesEarly investigators confused by experiments where certain substances depressed freezing point or elevated boiling point more than expected
188
189
Ex 01 m solution of NaCl lowers freezing point of solvent almost twice as much as 01 m solution of sucrose 01 m CaCl2 lowers fp almost 3 times as much as 01 m solution of sucrose
To understand why this happens contrast behavior or sucrose with NaCl in aqueous solution Each sucrose dissolves to make only 1 particle in solution So 1 mol sucrose dissolves to 1 mol particles 1 mol NaCl dissolves to make 2 moles of particles (1 mol Na+ and 1 mol Cl-)
190
191
Calculated Values for Electrolyte SolutionsColligative properties depend on total concentration of solute particles regardless of their identity Electrolytes cause changes in colligative properties proportional to total molality in terms of all dissolves particles instead of formula units
192
What about barium nitrate Ba(NO3)2 Each mole of barium nitrate yields 3 mol of ions in solution Ba(NO3)2(s) rarr Ba2+(aq) + 2NO3-(aq) You would expect solution of given molality to lower fp of solvent 3 times as much as nonelectrolytic solution of same molality
193
Sample Problem
What is the expected change in the freezing point of water in a solution of 625 g of barium nitrate Ba(NO3)2 in 100 kg of water
194
1 AnalyzeGiven solute mass and formula = 625 g Ba(NO3)2 solvent mass and identity = 100 kg water Δtf = Kfm
Unknown expected freezing-point depression
195
2 PlanThe molality can be calculated by converting the solute mass to moles and then dividing by the number of kilograms of solvent That molality is in terms of formula units of Ba(NO3)2 and must be converted to molality in terms of dissociated ions in solution It must be multiplied by the number of moles of ions produced per mole of formula unit This adjusted molality can then be used to calculate the freezing-point depression
196
197
198
3 Compute
Practice Problem
What is the expected freezing-point depression for a solution that contains 20 mol of magnesium sulfate dissolved in 10 kg of water
minus74degC199
What is the expected boiling-point elevation of water for a solution that contains 150 g of sodium chloride dissolved in 10 kg of water 27degC The freezing point of an aqueous sodium chloride solution is minus200degC What is the molality of the solution 54 m NaCl
200
Actual Values for Electrolyte SolutionsRemember values calculated are only expected values Actual values of colligative properties for all strong electrolytes are almost what would be expected Difference between expected and actual values caused by attractive forces that exist between dissociated ions in aqueous solution
201
Attraction between hydrated ions in solution is small compared with those in crystalline solid However forces of attraction donrsquot interfere with movements of aqueous ions More concentrated solution means ions closer together and attraction is greater Also depends on charge of ions ndash higher charge greater attraction
202
At higher temps eq is reached with fewer gas molecules in solution Gases generally less soluble
70
Effect of temp on solubility of solids in liquids more difficult to predict
Often increasing temp increases solubility
However the same temp increase can result in large increase in solubility in one case and only slight increase in the next
71
72
enthalpies of Solution
Formation of solution comes with energy change Dissolve KI in water container feels cold Dissolve LiCl in water feels hot Formation of solid-liquid solution can absorb or release heat
73
During formation of solution solvent and solute particles experience changes in forces attracting them to other particles Before dissolving begins solvent molecules held by intermolecular forces (solvent-solvent attraction) In solute molecules held by Ifs (solute-solute attraction)
74
Energy is required to separate solute molecules and solvent molecules from their neighbors
A solute particle that is surrounded by solvent molecules is said to be solvated
75
Step 1 solute particles become separated from solid (energy absorbed) Step 2 solvent particles move apart to allow solute particles to enter liquid (energy absorbed)
76
Step 3 solvent particles attracted to and solvate solute particles (energy released)
77
Heat of solution - net amount of heat energy absorbed or released when a specific amount of solute dissolves in a solvent
Heat of solution negative (heat is released) if sums of energy in steps 1 and 2 is less than step 3
If 1+2 gt 3 HOS is positive (heat absorbed)78
In gaseous state molecules are so far apart there are nearly no forces between them Solute-solute interaction has little effect on heat of solution of a gas Energy released when gas dissolved in liquid bc attraction between solute gas and solvent molecules greater than energy needed to separate solvent molecules
79
Concentration of Solutions80
Concentration - measure of amount of solute in given amount of solvent or solution ldquodiluterdquo just means small amount of solute ldquoconcentratedrdquo just means relatively large amount of solute Unrelated to degree to which solution is saturated Ex Saturated solution of substance not very soluble might be dilute
81
Molarity
Molarity - number of moles of solute in one liter of solution To find molarity must know molar mass A ldquoone molarrdquo solution of NaOH is 1 mole NaOH per liter of solution (1 M)
82
1 mol NaOH = 400 g If this quantity dissolve in enough water to make EXACTLY 100 L solution solution is 1 M If 200 g NaOH dissolve in enough to make 100 L solution what is molarity 0500 M
83
1 M solution is not made by adding 1 mol solute to 1 L solvent That would be more than 1 L Instead 1 mol solute dissolved in less than 1 L Then diluted with more solvent to bring TOTAL VOLUME to 1 L
84
Practice ProblemYou have 340 L of solution that contains 900 g sodium chloride What is the molarity of that solution 0440 M NaCl You have 08 L of a 05 M HCl solution How many moles of HCl does this solution contain 04 mol HCl
85
What is the molarity of a solution composed of 585 g of potassium iodide dissolved in enough water to make a 0125 L of solution 0282 M KI
86
How many moles of H2SO4 are present in 0500 L of a 0150 M H2SO4 solution 00750 mol What volume of 300 M NaCl is needed for reaction that requires 1463 g of NaCl 0834 L
87
Molality
Molality - concentration of solution expressed in moles of solute per kilogram of solvent Solution that contains 1 mol solute (NaOH) dissolved in exactly 1 kg of solvent is ldquoone molalrdquo solution (1 m NaOH)
88
1 mol NaOH = 400 g 400 g NaOH dissolved in 1 kg water is 1 m NaOH
200 g NaOH in 1 kg water = 0500 m NaOH
89
Practice Problem
A solution was prepared by dissolving 171 g sucrose (C12H22O11) in 125 g of water Find the molal concentration of this solution 0400 m C12H22O11
90
A solution of iodine in carbon tetrachloride is used when iodine is needed for certain chemical tests How much iodine must be added to prepare a 0480 m solution of iodine in CCl4 if 1000 g of CCl4 is used 122 g I2
91
What is the molality of a solution composed of 255 g acetone (CH3)2CO dissolved in 200 g of water 22 m acetone What quantity in grams of methanol CH3OH is required to prepare a 0244 m solution in 400 g water 312 g methanol
92
How many grams of AgNO3 are needed to prepare a 0125 m solution in 250 mL of water 531 g AgNO3
What is the molality of a solution containing 182 g HCl and 250 g water 200 m
93
Ions in Aqueous Solutions and Colligative Properties
94
95
compounds in aqueous solutions
DissociationWhen compound made from ions dissolves in water ions separate Dissociation - separation of ions that occurs when an ionic compound dissolves
96
Notice number of ions made per formula unit in equations 1 formula unit of NaCl gives 2 units of ions in solution 1 formula unit of CaCl2 gives 3 units of ions
97
Assuming 100 dissociation solution that contains 1 mol NaCl contains 1 mol Na+ and 1 mol Cl- Can assume 100 dissociation of all soluble ionic compounds
98
Practice Problem
Write the equation for the dissolution of aluminum sulfate in water How many moles of aluminum ions are made by dissolving 1 mol aluminum sulfate What is the total number of moles of ions made by dissolving 1 mol of aluminum sulfate
99
1 AnalyzeGiven Amount of solute = 1 mol Al2(SO4)3 Solvent identity = water Unknown a moles of aluminum ions and sulfate ions Total number of moles of solute ions produced
100
2 Plan
The coefficients in the balanced dissociation equation will tell mole relationships You can use the equation to find out the number of moles of solute ions produced
101
3 Compute
a
b102
Write the equation for the dissolution of each of the following in water and then determine the number of moles of each ions made as well as the total number of ions made a 1 mol ammonium chloride b 1 mol sodium sulfide c 05 mol barium nitrate
103
a 1 mol ammonium chloride
1 mol NH4+ 1 mol Cl- 2 mol total ions
104
b 1 mol sodium sulfide
2 mol Na+ 1 mol S-2 3 mol total ions
105
c 05 mol barium nitrate
05 mol Ba+2 1 mol NO3- 15 mol total ions
106
Precipitation Reactions
Even though no compound is completely insoluble compounds of very low solubility can be considered insoluble for practical purposes There are general guidelines to help predict whether a compound made of certain combination of ions is soluble
107
General Solubility Guidelines
Sodium potassium and ammonium compounds are soluble in water
108
Nitrates acetates and chlorates are soluble
109
110
Most chlorides are soluble except those of silver mercury (I) and lead Lead (II) chloride is soluble in hot water
Most sulfates are soluble except those of barium strontium lead calcium and mercury
111
Most carbonates phosphates and silicates are insoluble except those of sodium potassium and ammonium
112
Most sulfides are insoluble except those of calcium strontium sodium potassium and ammonium
113
Is calcium phosphate Ca3(PO4)2 soluble or insoluble
Not one of the exceptions so it is insoluble
Dissociation equations are not written for insoluble compounds
114
Guidelines also useful in predicting what will happen if 2 solutions of 2 different soluble compounds are mixed
If mixing results in combination of ions that forms insoluble compound a double-replacement and precipitation reaction will happen
115
Will precipitate form when solutions of ammonium sulfide and cadmium nitrate are combined You can tell calcium nitrate is soluble from the guidelines (Most nitrates acetates and chlorates are soluble) You can also tell ammonium sulfide is soluble (Most sulfides are insoluble except those of calcium strontium sodium potassium and ammonium)
116
117
To decide whether precipitate can form must know solubilities of two possible products
Ammonium nitrate ndash soluble (Most nitrates acetates and chlorates are soluble) Cadmium sulfide ndash insoluble (Most sulfides are insoluble except those of calcium strontium sodium potassium and ammonium)
118
bc one product is insoluble double-replacement and precipitation reaction will happen (NH4)2S (aq) + Cd(NO3)2 (aq) rarr NH4NO3 (aq) + CdS (s)
119
Net Ionic Equations
Reactions of ions in aqueous solution usually represented by net ionic equations instead of formula equations Net ionic equation - includes only those compounds and ions that undergo a chemical change in a reaction in an aqueous solution
120
121
To write NIE (net ionic equation) first convert chemical equation into overall ionic equation All soluble ionic compounds shown as dissociated ions in solution Precipitates shown as solids
122
Notice ammonium and nitrate ions appear on both sides They havenrsquot gone through any chemical change Spectator ions - ions that do not take part in a chemical reaction and are found in solution both before and after the reaction
123
To change ionic equation to NIE eliminate spectator ions Cd+2(aq) + S-2(aq) rarr CdS(s)
This NIE applies to any reaction in which precipitate of CdS forms
124
Practice Problem
Identify the precipitate that forms when aqueous solutions of zinc nitrate and ammonium sulfide are combined Write the equation for the possible double-replacement reaction Then write the formula equation overall ionic equation and the net ionic equation
125
1 AnalyzeGiven Identity of reactants zinc nitrate and ammonium sulfide Reaction medium aqueous solution Unknown a equation for the possible double-replacement reaction b identity of the precipitate c formula equation d overall ionic equation e net ionic equation
126
2 PlanWrite the possible double-replacement reaction between Zn(NO3)2 and (NH4)2S Use the rules to determine if any of the products will precipitate Write a formula equation and overall net ionic equation Then cancel spectator ions to make net ionic equation
127
3 Computea possible double-replacement reaction
b rules show that zinc sulfide is not soluble so will precipitate (ammonium nitrate is soluble)
c formula equation128
d overall ionic equation
e net ionic equation129
Will a precipitate form if solutions of potassium sulfate and barium nitrate are combined If so write the net ionic equation for the reaction
Yes Ba+2(aq) + SO4-2(aq) rarr BaSO4(s)
130
Will a precipitate form if solutions of potassium nitrate and magnesium sulfate are combined If so write the net ionic equation for the reaction
No 131
Will a precipitate form if solutions of barium chloride and sodium sulfate are combined If so identify the spectator ions and write the net ionic equation for the reaction
Yes Na+ and Cl- Ba+2(aq) + SO4-2(aq) rarr BaSO4(s)
132
Write the net ionic equation for the precipitation of nickel(II) sulfide
Ni+2(aq) + S-2(aq) rarr NiS(s)
133
IonizationSome molecular compounds can also form ions in solution Usually polar Ionization - ions formed from solute molecules by action of the solvent (creation of ions where there were none) Ionization different from dissociation
134
Dissociation ionic compounds dissolve and ions already present separate Ionization ions formed where non existed before
Ions formed are hydrated Heat released during hydration of ions gives enough energy to break covalent bonds
135
Extent to which solute ionizes depends on strength of bonds within molecules of solute and strength of attraction between solute and solvent molecules
136
HCl ndash molecular compound that ionizes in aqueous solution Contains highly polar bond Attraction between polar HCl molecule and polar water strong enough to break HCl bond
137
The Hydronium Ion
H+ ions from HCl attracts other molecules or ions so strongly that it doesnrsquot normally exist alone Ionization of HCl better described as direct transfer of proton from HCl to H2O forming H3O+
138
139
Hydration of H+ to form hydronium is highly exothermic
Energy released gives a lot of energy needed to ionize a molecular solute
Many molecular compounds that ionize in aqueous solution contain hydrogen and form hydronium ion
140
Strong and Weak Electrolytes
Substances that make ions and conduct current in solution are electrolytes HCl is one of series of compounds that has hydrogen and halogen Hydrogen halides are all molecular compounds with single polar-covalent bonds
141
All are gases Very soluble in water All are electrolytes HCl HBr HI strongly conduct current in solution HF only weakly conducts current at same concentration Strength of conduction related to ability to ionize
142
143
Strong Electrolytes
HCl HBr HI are 100 ionized in solution
Strong electrolyte - any compound whose dilute aqueous solutions conduct electricity well this is due to the presence of all or almost all of the dissolve compound in the form of ions
144
Unique characteristic of strong electrolyte is that they yield only ions
Ex Ionic compound may be highly soluble in water and dissociate into ions (NaCl)
Others may not dissolve much but amount that does dissolve exists only as ions
145
Weak ElectrolytesHF dissolves in water to give acid solution called hydrofluoric acid HF bond stronger than bonds between hydrogen and other halogens When HF dissolves some molecule ionize Reverse reaction also happens HF(aq) + H2O(l) H3O+(aq) + F-(aq)
146
HF(aq) + H2O(l) H3O+
(aq) + F-(aq)Concentration of dissolved unionized HF stays high and concentration of ions stays low
Weak electrolyte - any compound whose aqueous solutions conduct electricity poorly this is due to the presence of small amount of the dissolved compound in the form of ions
147
Different from nonelectrolyte ndash NO ions at all Description of electrolyte as strong or weak not related to concentration of solution Electrolytes differ in degree of ionization not amount of solute dissolved
148
149
Colligative properties of solutions
Presence of solute affects properties of solutions Some properties not dependent on nature of dissolved substance but on how may dissolve particles are present
Colligative properties - properties that depend on the concentration of solute particles but not on their identity
150
Vapor-Pressure Lowering
Boiling and freezing point of solution different from pure solvent Nonvolatile solute raises boiling point and lowers freezing point Nonvolatile substance - one that has little tendency to become gas under existing conditions
151
152
To understand why nonvolatile solute changes boiling and freezing point consider equilibrium vapor pressure
Vapor pressure pressure caused by molecules that have escape liquid phase to gas phase
Can be thought of a measure of tendency of molecules to escape from a liquid
153
Addition of sucrose (nonvolatile solute) lowers concentration of water molecules at surface of liquid This lowers tendency of water molecules to leave solution and enter gas phase Vapor pressure of solution is lower than vapor pressure of pure water
154
Aqueous13 solution13 of13 nonvolatile13 solutePure13 water
Nonelectrolyte solutions of same molality have same concentration of particles
Dilute solutions of same solvent and equal molality of any nonelectrolyte solute lower vapor pressure equally
155
Example
1 m aqueous solution of nonelectrolyte glucose lowers vapor pressure of water 55 x 10-4 atm at 25 1 m aqueous solution of sucrose also lowers water vapor pressure to 55 x 10-4 atm at 25
156
157
bc vapor-pressure lowering depends on concentration of nonelectrolyte solute and doesnrsquot depend on the type of solute it is a colligative proprerty
bc vapor pressure has been lowered solution remains liquid over larger temperature range
Lowers freezing point and raises boiling point
158
Can assume changes in boiling and freezing point also depend on concentration of solute
They are colligative properties
159
Freezing-Point Depression
Freezing point of 1-molal solution of any nonelectrolyte solute in water is found (by experiment) to be 186 lower than freezing point of water When 1 mol of nonelectrolyte solute dissolved in 1 kg water freezing point is -186 not 000 160
When 2 mol nonelectrolyte solute dissolved in 1 kg water freezing point is -372
For any concentration of nonelectrolyte solute in water decrease in freezing point can be determined using value of -186m
161
This value called the molal freezing-point contstant (Kf) is the freezing-point depression of the solvent in a 1-molal solution of a nonvolatile nonelectrolyte solute
162
Each solvent has own characteristic molal freezing-point constant Freezing-point depression (Δtf) the difference between the freezing points of the pure solvent and a solute of nonelectrolyte in that solvent and it is directly proportional to the molal concentration of the solution Δtf = Kfm
163
Δtf = Kfm
Kf expressed as m
m expressed in mol solutekg solvent (molality)
164
Sample Problem
What is the freezing point depression of water in a solution of 171 g of sucrose C12H22O11 and 200 g of water What is the actual freezing point of the solution
165
1 AnalyzeGiven Solute mass and chemical formula 171 g C12H22O11 Solvent mass and identity 2000 g water Unknown a freezing-point depression b freezing point of the solution 166
2 Plan
167
168
Calculate
Unknown a freezing-point depression b freezing point of the solution
169
Practice Problem
A water solution containing an unknown quantity of a nonelectrolyte solute is found to have a freezing point of ndash 023degCWhat is the molal concentration of the solution
012 m170
A solution consists of 103 g of the nonelectrolyte glucose C6H12O6 dissolved in 250 g of water What is the freezing-point depression of the solution
minus0426degC171
In a laboratory experiment the freezing point of an aqueous solution of glucose is found to be minus0325degCWhat is the molal concentration of this solution
0175 m172
If 0500 mol of a nonelectrolyte solute are dissolved in 5000 g of ether what is the freezing point of the solution
minus1181degC173
The freezing point of an aqueous solution that contains a nonelectrolyte is minus90degC aWhat is the freezing-point depression of the solution bWhat is the molal concentration of the solution a minus90degC b 48 m
174
Boiling-Point Elevation
Boiling point is temp at which vapor pressure is equal to atmospheric pressure Change in vapor pressure will cause corresponding change in boiling point
175
176
Vapor pressure of nonvolatile solution is lower than vapor pressure of pure solvent More heat will be required to raise vapor pressure of solution Boiling point of solution is higher than boiling point of pure solvent
Molal boiling-point constant (Kb) - boiling-point elevation of the solvent in a 1-molal solution of a nonvolatile nonelectrolyte solute b-p elevation of any nonelectrolyte in water found by experiment to be 051 So molal b-p constant for water is 051m
177
For different solvents b-p elevations of 1-molal solutions have different values (Table 14-2)
Like freezing-point constants values most accurate for dilute solutions
178
Boiling-point elevation Δtb difference between the boiling points of the pure solvent and a nonelectrolyte solution of that solvent and it is directly proportional to the molal concentration of the solution B-p elevation can be calculated using Δtb = Kbm Δtb (boiling point elevation) expressed in m and m expressed in mol solutekg solvent
Sample Problem
What is the boiling-point elevation of a solution made from 200 g of a nonelectrolyte solute and 4000 g of water The molar mass of the solute is 620 gmol
180
1 AnalyzeGiven solute mass = 200 g solute molar mass = 620 gmol solvent mass and identity = 4000 g of water
Unknown boiling-point elevation
181
2 Plan
182
183
3 Compute
Practice Problem
A solution contains 500 g of sucrose C12H22O11 a nonelectrolyte dissolved in 5000 g of water What is the boiling-point elevation
015degC184
A solution contains 4500 g of sucrose C12H22O11 a nonelectrolyte dissolved in 250 g of waterWhat is the boiling point of the solution
1027degC185
If the boiling point elevation of an aqueous solution containing a nonvolatile electrolyte is 102degC what is the molality of the solution
20 m186
The boiling point of an aqueous solution containing a nonvolatile electrolyte is 10075degC aWhat is the boiling-point elevation bWhat is the molality of the solution a 075degC b 15 m
187
Electrolytes and Colligative PropertiesEarly investigators confused by experiments where certain substances depressed freezing point or elevated boiling point more than expected
188
189
Ex 01 m solution of NaCl lowers freezing point of solvent almost twice as much as 01 m solution of sucrose 01 m CaCl2 lowers fp almost 3 times as much as 01 m solution of sucrose
To understand why this happens contrast behavior or sucrose with NaCl in aqueous solution Each sucrose dissolves to make only 1 particle in solution So 1 mol sucrose dissolves to 1 mol particles 1 mol NaCl dissolves to make 2 moles of particles (1 mol Na+ and 1 mol Cl-)
190
191
Calculated Values for Electrolyte SolutionsColligative properties depend on total concentration of solute particles regardless of their identity Electrolytes cause changes in colligative properties proportional to total molality in terms of all dissolves particles instead of formula units
192
What about barium nitrate Ba(NO3)2 Each mole of barium nitrate yields 3 mol of ions in solution Ba(NO3)2(s) rarr Ba2+(aq) + 2NO3-(aq) You would expect solution of given molality to lower fp of solvent 3 times as much as nonelectrolytic solution of same molality
193
Sample Problem
What is the expected change in the freezing point of water in a solution of 625 g of barium nitrate Ba(NO3)2 in 100 kg of water
194
1 AnalyzeGiven solute mass and formula = 625 g Ba(NO3)2 solvent mass and identity = 100 kg water Δtf = Kfm
Unknown expected freezing-point depression
195
2 PlanThe molality can be calculated by converting the solute mass to moles and then dividing by the number of kilograms of solvent That molality is in terms of formula units of Ba(NO3)2 and must be converted to molality in terms of dissociated ions in solution It must be multiplied by the number of moles of ions produced per mole of formula unit This adjusted molality can then be used to calculate the freezing-point depression
196
197
198
3 Compute
Practice Problem
What is the expected freezing-point depression for a solution that contains 20 mol of magnesium sulfate dissolved in 10 kg of water
minus74degC199
What is the expected boiling-point elevation of water for a solution that contains 150 g of sodium chloride dissolved in 10 kg of water 27degC The freezing point of an aqueous sodium chloride solution is minus200degC What is the molality of the solution 54 m NaCl
200
Actual Values for Electrolyte SolutionsRemember values calculated are only expected values Actual values of colligative properties for all strong electrolytes are almost what would be expected Difference between expected and actual values caused by attractive forces that exist between dissociated ions in aqueous solution
201
Attraction between hydrated ions in solution is small compared with those in crystalline solid However forces of attraction donrsquot interfere with movements of aqueous ions More concentrated solution means ions closer together and attraction is greater Also depends on charge of ions ndash higher charge greater attraction
202
Effect of temp on solubility of solids in liquids more difficult to predict
Often increasing temp increases solubility
However the same temp increase can result in large increase in solubility in one case and only slight increase in the next
71
72
enthalpies of Solution
Formation of solution comes with energy change Dissolve KI in water container feels cold Dissolve LiCl in water feels hot Formation of solid-liquid solution can absorb or release heat
73
During formation of solution solvent and solute particles experience changes in forces attracting them to other particles Before dissolving begins solvent molecules held by intermolecular forces (solvent-solvent attraction) In solute molecules held by Ifs (solute-solute attraction)
74
Energy is required to separate solute molecules and solvent molecules from their neighbors
A solute particle that is surrounded by solvent molecules is said to be solvated
75
Step 1 solute particles become separated from solid (energy absorbed) Step 2 solvent particles move apart to allow solute particles to enter liquid (energy absorbed)
76
Step 3 solvent particles attracted to and solvate solute particles (energy released)
77
Heat of solution - net amount of heat energy absorbed or released when a specific amount of solute dissolves in a solvent
Heat of solution negative (heat is released) if sums of energy in steps 1 and 2 is less than step 3
If 1+2 gt 3 HOS is positive (heat absorbed)78
In gaseous state molecules are so far apart there are nearly no forces between them Solute-solute interaction has little effect on heat of solution of a gas Energy released when gas dissolved in liquid bc attraction between solute gas and solvent molecules greater than energy needed to separate solvent molecules
79
Concentration of Solutions80
Concentration - measure of amount of solute in given amount of solvent or solution ldquodiluterdquo just means small amount of solute ldquoconcentratedrdquo just means relatively large amount of solute Unrelated to degree to which solution is saturated Ex Saturated solution of substance not very soluble might be dilute
81
Molarity
Molarity - number of moles of solute in one liter of solution To find molarity must know molar mass A ldquoone molarrdquo solution of NaOH is 1 mole NaOH per liter of solution (1 M)
82
1 mol NaOH = 400 g If this quantity dissolve in enough water to make EXACTLY 100 L solution solution is 1 M If 200 g NaOH dissolve in enough to make 100 L solution what is molarity 0500 M
83
1 M solution is not made by adding 1 mol solute to 1 L solvent That would be more than 1 L Instead 1 mol solute dissolved in less than 1 L Then diluted with more solvent to bring TOTAL VOLUME to 1 L
84
Practice ProblemYou have 340 L of solution that contains 900 g sodium chloride What is the molarity of that solution 0440 M NaCl You have 08 L of a 05 M HCl solution How many moles of HCl does this solution contain 04 mol HCl
85
What is the molarity of a solution composed of 585 g of potassium iodide dissolved in enough water to make a 0125 L of solution 0282 M KI
86
How many moles of H2SO4 are present in 0500 L of a 0150 M H2SO4 solution 00750 mol What volume of 300 M NaCl is needed for reaction that requires 1463 g of NaCl 0834 L
87
Molality
Molality - concentration of solution expressed in moles of solute per kilogram of solvent Solution that contains 1 mol solute (NaOH) dissolved in exactly 1 kg of solvent is ldquoone molalrdquo solution (1 m NaOH)
88
1 mol NaOH = 400 g 400 g NaOH dissolved in 1 kg water is 1 m NaOH
200 g NaOH in 1 kg water = 0500 m NaOH
89
Practice Problem
A solution was prepared by dissolving 171 g sucrose (C12H22O11) in 125 g of water Find the molal concentration of this solution 0400 m C12H22O11
90
A solution of iodine in carbon tetrachloride is used when iodine is needed for certain chemical tests How much iodine must be added to prepare a 0480 m solution of iodine in CCl4 if 1000 g of CCl4 is used 122 g I2
91
What is the molality of a solution composed of 255 g acetone (CH3)2CO dissolved in 200 g of water 22 m acetone What quantity in grams of methanol CH3OH is required to prepare a 0244 m solution in 400 g water 312 g methanol
92
How many grams of AgNO3 are needed to prepare a 0125 m solution in 250 mL of water 531 g AgNO3
What is the molality of a solution containing 182 g HCl and 250 g water 200 m
93
Ions in Aqueous Solutions and Colligative Properties
94
95
compounds in aqueous solutions
DissociationWhen compound made from ions dissolves in water ions separate Dissociation - separation of ions that occurs when an ionic compound dissolves
96
Notice number of ions made per formula unit in equations 1 formula unit of NaCl gives 2 units of ions in solution 1 formula unit of CaCl2 gives 3 units of ions
97
Assuming 100 dissociation solution that contains 1 mol NaCl contains 1 mol Na+ and 1 mol Cl- Can assume 100 dissociation of all soluble ionic compounds
98
Practice Problem
Write the equation for the dissolution of aluminum sulfate in water How many moles of aluminum ions are made by dissolving 1 mol aluminum sulfate What is the total number of moles of ions made by dissolving 1 mol of aluminum sulfate
99
1 AnalyzeGiven Amount of solute = 1 mol Al2(SO4)3 Solvent identity = water Unknown a moles of aluminum ions and sulfate ions Total number of moles of solute ions produced
100
2 Plan
The coefficients in the balanced dissociation equation will tell mole relationships You can use the equation to find out the number of moles of solute ions produced
101
3 Compute
a
b102
Write the equation for the dissolution of each of the following in water and then determine the number of moles of each ions made as well as the total number of ions made a 1 mol ammonium chloride b 1 mol sodium sulfide c 05 mol barium nitrate
103
a 1 mol ammonium chloride
1 mol NH4+ 1 mol Cl- 2 mol total ions
104
b 1 mol sodium sulfide
2 mol Na+ 1 mol S-2 3 mol total ions
105
c 05 mol barium nitrate
05 mol Ba+2 1 mol NO3- 15 mol total ions
106
Precipitation Reactions
Even though no compound is completely insoluble compounds of very low solubility can be considered insoluble for practical purposes There are general guidelines to help predict whether a compound made of certain combination of ions is soluble
107
General Solubility Guidelines
Sodium potassium and ammonium compounds are soluble in water
108
Nitrates acetates and chlorates are soluble
109
110
Most chlorides are soluble except those of silver mercury (I) and lead Lead (II) chloride is soluble in hot water
Most sulfates are soluble except those of barium strontium lead calcium and mercury
111
Most carbonates phosphates and silicates are insoluble except those of sodium potassium and ammonium
112
Most sulfides are insoluble except those of calcium strontium sodium potassium and ammonium
113
Is calcium phosphate Ca3(PO4)2 soluble or insoluble
Not one of the exceptions so it is insoluble
Dissociation equations are not written for insoluble compounds
114
Guidelines also useful in predicting what will happen if 2 solutions of 2 different soluble compounds are mixed
If mixing results in combination of ions that forms insoluble compound a double-replacement and precipitation reaction will happen
115
Will precipitate form when solutions of ammonium sulfide and cadmium nitrate are combined You can tell calcium nitrate is soluble from the guidelines (Most nitrates acetates and chlorates are soluble) You can also tell ammonium sulfide is soluble (Most sulfides are insoluble except those of calcium strontium sodium potassium and ammonium)
116
117
To decide whether precipitate can form must know solubilities of two possible products
Ammonium nitrate ndash soluble (Most nitrates acetates and chlorates are soluble) Cadmium sulfide ndash insoluble (Most sulfides are insoluble except those of calcium strontium sodium potassium and ammonium)
118
bc one product is insoluble double-replacement and precipitation reaction will happen (NH4)2S (aq) + Cd(NO3)2 (aq) rarr NH4NO3 (aq) + CdS (s)
119
Net Ionic Equations
Reactions of ions in aqueous solution usually represented by net ionic equations instead of formula equations Net ionic equation - includes only those compounds and ions that undergo a chemical change in a reaction in an aqueous solution
120
121
To write NIE (net ionic equation) first convert chemical equation into overall ionic equation All soluble ionic compounds shown as dissociated ions in solution Precipitates shown as solids
122
Notice ammonium and nitrate ions appear on both sides They havenrsquot gone through any chemical change Spectator ions - ions that do not take part in a chemical reaction and are found in solution both before and after the reaction
123
To change ionic equation to NIE eliminate spectator ions Cd+2(aq) + S-2(aq) rarr CdS(s)
This NIE applies to any reaction in which precipitate of CdS forms
124
Practice Problem
Identify the precipitate that forms when aqueous solutions of zinc nitrate and ammonium sulfide are combined Write the equation for the possible double-replacement reaction Then write the formula equation overall ionic equation and the net ionic equation
125
1 AnalyzeGiven Identity of reactants zinc nitrate and ammonium sulfide Reaction medium aqueous solution Unknown a equation for the possible double-replacement reaction b identity of the precipitate c formula equation d overall ionic equation e net ionic equation
126
2 PlanWrite the possible double-replacement reaction between Zn(NO3)2 and (NH4)2S Use the rules to determine if any of the products will precipitate Write a formula equation and overall net ionic equation Then cancel spectator ions to make net ionic equation
127
3 Computea possible double-replacement reaction
b rules show that zinc sulfide is not soluble so will precipitate (ammonium nitrate is soluble)
c formula equation128
d overall ionic equation
e net ionic equation129
Will a precipitate form if solutions of potassium sulfate and barium nitrate are combined If so write the net ionic equation for the reaction
Yes Ba+2(aq) + SO4-2(aq) rarr BaSO4(s)
130
Will a precipitate form if solutions of potassium nitrate and magnesium sulfate are combined If so write the net ionic equation for the reaction
No 131
Will a precipitate form if solutions of barium chloride and sodium sulfate are combined If so identify the spectator ions and write the net ionic equation for the reaction
Yes Na+ and Cl- Ba+2(aq) + SO4-2(aq) rarr BaSO4(s)
132
Write the net ionic equation for the precipitation of nickel(II) sulfide
Ni+2(aq) + S-2(aq) rarr NiS(s)
133
IonizationSome molecular compounds can also form ions in solution Usually polar Ionization - ions formed from solute molecules by action of the solvent (creation of ions where there were none) Ionization different from dissociation
134
Dissociation ionic compounds dissolve and ions already present separate Ionization ions formed where non existed before
Ions formed are hydrated Heat released during hydration of ions gives enough energy to break covalent bonds
135
Extent to which solute ionizes depends on strength of bonds within molecules of solute and strength of attraction between solute and solvent molecules
136
HCl ndash molecular compound that ionizes in aqueous solution Contains highly polar bond Attraction between polar HCl molecule and polar water strong enough to break HCl bond
137
The Hydronium Ion
H+ ions from HCl attracts other molecules or ions so strongly that it doesnrsquot normally exist alone Ionization of HCl better described as direct transfer of proton from HCl to H2O forming H3O+
138
139
Hydration of H+ to form hydronium is highly exothermic
Energy released gives a lot of energy needed to ionize a molecular solute
Many molecular compounds that ionize in aqueous solution contain hydrogen and form hydronium ion
140
Strong and Weak Electrolytes
Substances that make ions and conduct current in solution are electrolytes HCl is one of series of compounds that has hydrogen and halogen Hydrogen halides are all molecular compounds with single polar-covalent bonds
141
All are gases Very soluble in water All are electrolytes HCl HBr HI strongly conduct current in solution HF only weakly conducts current at same concentration Strength of conduction related to ability to ionize
142
143
Strong Electrolytes
HCl HBr HI are 100 ionized in solution
Strong electrolyte - any compound whose dilute aqueous solutions conduct electricity well this is due to the presence of all or almost all of the dissolve compound in the form of ions
144
Unique characteristic of strong electrolyte is that they yield only ions
Ex Ionic compound may be highly soluble in water and dissociate into ions (NaCl)
Others may not dissolve much but amount that does dissolve exists only as ions
145
Weak ElectrolytesHF dissolves in water to give acid solution called hydrofluoric acid HF bond stronger than bonds between hydrogen and other halogens When HF dissolves some molecule ionize Reverse reaction also happens HF(aq) + H2O(l) H3O+(aq) + F-(aq)
146
HF(aq) + H2O(l) H3O+
(aq) + F-(aq)Concentration of dissolved unionized HF stays high and concentration of ions stays low
Weak electrolyte - any compound whose aqueous solutions conduct electricity poorly this is due to the presence of small amount of the dissolved compound in the form of ions
147
Different from nonelectrolyte ndash NO ions at all Description of electrolyte as strong or weak not related to concentration of solution Electrolytes differ in degree of ionization not amount of solute dissolved
148
149
Colligative properties of solutions
Presence of solute affects properties of solutions Some properties not dependent on nature of dissolved substance but on how may dissolve particles are present
Colligative properties - properties that depend on the concentration of solute particles but not on their identity
150
Vapor-Pressure Lowering
Boiling and freezing point of solution different from pure solvent Nonvolatile solute raises boiling point and lowers freezing point Nonvolatile substance - one that has little tendency to become gas under existing conditions
151
152
To understand why nonvolatile solute changes boiling and freezing point consider equilibrium vapor pressure
Vapor pressure pressure caused by molecules that have escape liquid phase to gas phase
Can be thought of a measure of tendency of molecules to escape from a liquid
153
Addition of sucrose (nonvolatile solute) lowers concentration of water molecules at surface of liquid This lowers tendency of water molecules to leave solution and enter gas phase Vapor pressure of solution is lower than vapor pressure of pure water
154
Aqueous13 solution13 of13 nonvolatile13 solutePure13 water
Nonelectrolyte solutions of same molality have same concentration of particles
Dilute solutions of same solvent and equal molality of any nonelectrolyte solute lower vapor pressure equally
155
Example
1 m aqueous solution of nonelectrolyte glucose lowers vapor pressure of water 55 x 10-4 atm at 25 1 m aqueous solution of sucrose also lowers water vapor pressure to 55 x 10-4 atm at 25
156
157
bc vapor-pressure lowering depends on concentration of nonelectrolyte solute and doesnrsquot depend on the type of solute it is a colligative proprerty
bc vapor pressure has been lowered solution remains liquid over larger temperature range
Lowers freezing point and raises boiling point
158
Can assume changes in boiling and freezing point also depend on concentration of solute
They are colligative properties
159
Freezing-Point Depression
Freezing point of 1-molal solution of any nonelectrolyte solute in water is found (by experiment) to be 186 lower than freezing point of water When 1 mol of nonelectrolyte solute dissolved in 1 kg water freezing point is -186 not 000 160
When 2 mol nonelectrolyte solute dissolved in 1 kg water freezing point is -372
For any concentration of nonelectrolyte solute in water decrease in freezing point can be determined using value of -186m
161
This value called the molal freezing-point contstant (Kf) is the freezing-point depression of the solvent in a 1-molal solution of a nonvolatile nonelectrolyte solute
162
Each solvent has own characteristic molal freezing-point constant Freezing-point depression (Δtf) the difference between the freezing points of the pure solvent and a solute of nonelectrolyte in that solvent and it is directly proportional to the molal concentration of the solution Δtf = Kfm
163
Δtf = Kfm
Kf expressed as m
m expressed in mol solutekg solvent (molality)
164
Sample Problem
What is the freezing point depression of water in a solution of 171 g of sucrose C12H22O11 and 200 g of water What is the actual freezing point of the solution
165
1 AnalyzeGiven Solute mass and chemical formula 171 g C12H22O11 Solvent mass and identity 2000 g water Unknown a freezing-point depression b freezing point of the solution 166
2 Plan
167
168
Calculate
Unknown a freezing-point depression b freezing point of the solution
169
Practice Problem
A water solution containing an unknown quantity of a nonelectrolyte solute is found to have a freezing point of ndash 023degCWhat is the molal concentration of the solution
012 m170
A solution consists of 103 g of the nonelectrolyte glucose C6H12O6 dissolved in 250 g of water What is the freezing-point depression of the solution
minus0426degC171
In a laboratory experiment the freezing point of an aqueous solution of glucose is found to be minus0325degCWhat is the molal concentration of this solution
0175 m172
If 0500 mol of a nonelectrolyte solute are dissolved in 5000 g of ether what is the freezing point of the solution
minus1181degC173
The freezing point of an aqueous solution that contains a nonelectrolyte is minus90degC aWhat is the freezing-point depression of the solution bWhat is the molal concentration of the solution a minus90degC b 48 m
174
Boiling-Point Elevation
Boiling point is temp at which vapor pressure is equal to atmospheric pressure Change in vapor pressure will cause corresponding change in boiling point
175
176
Vapor pressure of nonvolatile solution is lower than vapor pressure of pure solvent More heat will be required to raise vapor pressure of solution Boiling point of solution is higher than boiling point of pure solvent
Molal boiling-point constant (Kb) - boiling-point elevation of the solvent in a 1-molal solution of a nonvolatile nonelectrolyte solute b-p elevation of any nonelectrolyte in water found by experiment to be 051 So molal b-p constant for water is 051m
177
For different solvents b-p elevations of 1-molal solutions have different values (Table 14-2)
Like freezing-point constants values most accurate for dilute solutions
178
Boiling-point elevation Δtb difference between the boiling points of the pure solvent and a nonelectrolyte solution of that solvent and it is directly proportional to the molal concentration of the solution B-p elevation can be calculated using Δtb = Kbm Δtb (boiling point elevation) expressed in m and m expressed in mol solutekg solvent
Sample Problem
What is the boiling-point elevation of a solution made from 200 g of a nonelectrolyte solute and 4000 g of water The molar mass of the solute is 620 gmol
180
1 AnalyzeGiven solute mass = 200 g solute molar mass = 620 gmol solvent mass and identity = 4000 g of water
Unknown boiling-point elevation
181
2 Plan
182
183
3 Compute
Practice Problem
A solution contains 500 g of sucrose C12H22O11 a nonelectrolyte dissolved in 5000 g of water What is the boiling-point elevation
015degC184
A solution contains 4500 g of sucrose C12H22O11 a nonelectrolyte dissolved in 250 g of waterWhat is the boiling point of the solution
1027degC185
If the boiling point elevation of an aqueous solution containing a nonvolatile electrolyte is 102degC what is the molality of the solution
20 m186
The boiling point of an aqueous solution containing a nonvolatile electrolyte is 10075degC aWhat is the boiling-point elevation bWhat is the molality of the solution a 075degC b 15 m
187
Electrolytes and Colligative PropertiesEarly investigators confused by experiments where certain substances depressed freezing point or elevated boiling point more than expected
188
189
Ex 01 m solution of NaCl lowers freezing point of solvent almost twice as much as 01 m solution of sucrose 01 m CaCl2 lowers fp almost 3 times as much as 01 m solution of sucrose
To understand why this happens contrast behavior or sucrose with NaCl in aqueous solution Each sucrose dissolves to make only 1 particle in solution So 1 mol sucrose dissolves to 1 mol particles 1 mol NaCl dissolves to make 2 moles of particles (1 mol Na+ and 1 mol Cl-)
190
191
Calculated Values for Electrolyte SolutionsColligative properties depend on total concentration of solute particles regardless of their identity Electrolytes cause changes in colligative properties proportional to total molality in terms of all dissolves particles instead of formula units
192
What about barium nitrate Ba(NO3)2 Each mole of barium nitrate yields 3 mol of ions in solution Ba(NO3)2(s) rarr Ba2+(aq) + 2NO3-(aq) You would expect solution of given molality to lower fp of solvent 3 times as much as nonelectrolytic solution of same molality
193
Sample Problem
What is the expected change in the freezing point of water in a solution of 625 g of barium nitrate Ba(NO3)2 in 100 kg of water
194
1 AnalyzeGiven solute mass and formula = 625 g Ba(NO3)2 solvent mass and identity = 100 kg water Δtf = Kfm
Unknown expected freezing-point depression
195
2 PlanThe molality can be calculated by converting the solute mass to moles and then dividing by the number of kilograms of solvent That molality is in terms of formula units of Ba(NO3)2 and must be converted to molality in terms of dissociated ions in solution It must be multiplied by the number of moles of ions produced per mole of formula unit This adjusted molality can then be used to calculate the freezing-point depression
196
197
198
3 Compute
Practice Problem
What is the expected freezing-point depression for a solution that contains 20 mol of magnesium sulfate dissolved in 10 kg of water
minus74degC199
What is the expected boiling-point elevation of water for a solution that contains 150 g of sodium chloride dissolved in 10 kg of water 27degC The freezing point of an aqueous sodium chloride solution is minus200degC What is the molality of the solution 54 m NaCl
200
Actual Values for Electrolyte SolutionsRemember values calculated are only expected values Actual values of colligative properties for all strong electrolytes are almost what would be expected Difference between expected and actual values caused by attractive forces that exist between dissociated ions in aqueous solution
201
Attraction between hydrated ions in solution is small compared with those in crystalline solid However forces of attraction donrsquot interfere with movements of aqueous ions More concentrated solution means ions closer together and attraction is greater Also depends on charge of ions ndash higher charge greater attraction
202
72
enthalpies of Solution
Formation of solution comes with energy change Dissolve KI in water container feels cold Dissolve LiCl in water feels hot Formation of solid-liquid solution can absorb or release heat
73
During formation of solution solvent and solute particles experience changes in forces attracting them to other particles Before dissolving begins solvent molecules held by intermolecular forces (solvent-solvent attraction) In solute molecules held by Ifs (solute-solute attraction)
74
Energy is required to separate solute molecules and solvent molecules from their neighbors
A solute particle that is surrounded by solvent molecules is said to be solvated
75
Step 1 solute particles become separated from solid (energy absorbed) Step 2 solvent particles move apart to allow solute particles to enter liquid (energy absorbed)
76
Step 3 solvent particles attracted to and solvate solute particles (energy released)
77
Heat of solution - net amount of heat energy absorbed or released when a specific amount of solute dissolves in a solvent
Heat of solution negative (heat is released) if sums of energy in steps 1 and 2 is less than step 3
If 1+2 gt 3 HOS is positive (heat absorbed)78
In gaseous state molecules are so far apart there are nearly no forces between them Solute-solute interaction has little effect on heat of solution of a gas Energy released when gas dissolved in liquid bc attraction between solute gas and solvent molecules greater than energy needed to separate solvent molecules
79
Concentration of Solutions80
Concentration - measure of amount of solute in given amount of solvent or solution ldquodiluterdquo just means small amount of solute ldquoconcentratedrdquo just means relatively large amount of solute Unrelated to degree to which solution is saturated Ex Saturated solution of substance not very soluble might be dilute
81
Molarity
Molarity - number of moles of solute in one liter of solution To find molarity must know molar mass A ldquoone molarrdquo solution of NaOH is 1 mole NaOH per liter of solution (1 M)
82
1 mol NaOH = 400 g If this quantity dissolve in enough water to make EXACTLY 100 L solution solution is 1 M If 200 g NaOH dissolve in enough to make 100 L solution what is molarity 0500 M
83
1 M solution is not made by adding 1 mol solute to 1 L solvent That would be more than 1 L Instead 1 mol solute dissolved in less than 1 L Then diluted with more solvent to bring TOTAL VOLUME to 1 L
84
Practice ProblemYou have 340 L of solution that contains 900 g sodium chloride What is the molarity of that solution 0440 M NaCl You have 08 L of a 05 M HCl solution How many moles of HCl does this solution contain 04 mol HCl
85
What is the molarity of a solution composed of 585 g of potassium iodide dissolved in enough water to make a 0125 L of solution 0282 M KI
86
How many moles of H2SO4 are present in 0500 L of a 0150 M H2SO4 solution 00750 mol What volume of 300 M NaCl is needed for reaction that requires 1463 g of NaCl 0834 L
87
Molality
Molality - concentration of solution expressed in moles of solute per kilogram of solvent Solution that contains 1 mol solute (NaOH) dissolved in exactly 1 kg of solvent is ldquoone molalrdquo solution (1 m NaOH)
88
1 mol NaOH = 400 g 400 g NaOH dissolved in 1 kg water is 1 m NaOH
200 g NaOH in 1 kg water = 0500 m NaOH
89
Practice Problem
A solution was prepared by dissolving 171 g sucrose (C12H22O11) in 125 g of water Find the molal concentration of this solution 0400 m C12H22O11
90
A solution of iodine in carbon tetrachloride is used when iodine is needed for certain chemical tests How much iodine must be added to prepare a 0480 m solution of iodine in CCl4 if 1000 g of CCl4 is used 122 g I2
91
What is the molality of a solution composed of 255 g acetone (CH3)2CO dissolved in 200 g of water 22 m acetone What quantity in grams of methanol CH3OH is required to prepare a 0244 m solution in 400 g water 312 g methanol
92
How many grams of AgNO3 are needed to prepare a 0125 m solution in 250 mL of water 531 g AgNO3
What is the molality of a solution containing 182 g HCl and 250 g water 200 m
93
Ions in Aqueous Solutions and Colligative Properties
94
95
compounds in aqueous solutions
DissociationWhen compound made from ions dissolves in water ions separate Dissociation - separation of ions that occurs when an ionic compound dissolves
96
Notice number of ions made per formula unit in equations 1 formula unit of NaCl gives 2 units of ions in solution 1 formula unit of CaCl2 gives 3 units of ions
97
Assuming 100 dissociation solution that contains 1 mol NaCl contains 1 mol Na+ and 1 mol Cl- Can assume 100 dissociation of all soluble ionic compounds
98
Practice Problem
Write the equation for the dissolution of aluminum sulfate in water How many moles of aluminum ions are made by dissolving 1 mol aluminum sulfate What is the total number of moles of ions made by dissolving 1 mol of aluminum sulfate
99
1 AnalyzeGiven Amount of solute = 1 mol Al2(SO4)3 Solvent identity = water Unknown a moles of aluminum ions and sulfate ions Total number of moles of solute ions produced
100
2 Plan
The coefficients in the balanced dissociation equation will tell mole relationships You can use the equation to find out the number of moles of solute ions produced
101
3 Compute
a
b102
Write the equation for the dissolution of each of the following in water and then determine the number of moles of each ions made as well as the total number of ions made a 1 mol ammonium chloride b 1 mol sodium sulfide c 05 mol barium nitrate
103
a 1 mol ammonium chloride
1 mol NH4+ 1 mol Cl- 2 mol total ions
104
b 1 mol sodium sulfide
2 mol Na+ 1 mol S-2 3 mol total ions
105
c 05 mol barium nitrate
05 mol Ba+2 1 mol NO3- 15 mol total ions
106
Precipitation Reactions
Even though no compound is completely insoluble compounds of very low solubility can be considered insoluble for practical purposes There are general guidelines to help predict whether a compound made of certain combination of ions is soluble
107
General Solubility Guidelines
Sodium potassium and ammonium compounds are soluble in water
108
Nitrates acetates and chlorates are soluble
109
110
Most chlorides are soluble except those of silver mercury (I) and lead Lead (II) chloride is soluble in hot water
Most sulfates are soluble except those of barium strontium lead calcium and mercury
111
Most carbonates phosphates and silicates are insoluble except those of sodium potassium and ammonium
112
Most sulfides are insoluble except those of calcium strontium sodium potassium and ammonium
113
Is calcium phosphate Ca3(PO4)2 soluble or insoluble
Not one of the exceptions so it is insoluble
Dissociation equations are not written for insoluble compounds
114
Guidelines also useful in predicting what will happen if 2 solutions of 2 different soluble compounds are mixed
If mixing results in combination of ions that forms insoluble compound a double-replacement and precipitation reaction will happen
115
Will precipitate form when solutions of ammonium sulfide and cadmium nitrate are combined You can tell calcium nitrate is soluble from the guidelines (Most nitrates acetates and chlorates are soluble) You can also tell ammonium sulfide is soluble (Most sulfides are insoluble except those of calcium strontium sodium potassium and ammonium)
116
117
To decide whether precipitate can form must know solubilities of two possible products
Ammonium nitrate ndash soluble (Most nitrates acetates and chlorates are soluble) Cadmium sulfide ndash insoluble (Most sulfides are insoluble except those of calcium strontium sodium potassium and ammonium)
118
bc one product is insoluble double-replacement and precipitation reaction will happen (NH4)2S (aq) + Cd(NO3)2 (aq) rarr NH4NO3 (aq) + CdS (s)
119
Net Ionic Equations
Reactions of ions in aqueous solution usually represented by net ionic equations instead of formula equations Net ionic equation - includes only those compounds and ions that undergo a chemical change in a reaction in an aqueous solution
120
121
To write NIE (net ionic equation) first convert chemical equation into overall ionic equation All soluble ionic compounds shown as dissociated ions in solution Precipitates shown as solids
122
Notice ammonium and nitrate ions appear on both sides They havenrsquot gone through any chemical change Spectator ions - ions that do not take part in a chemical reaction and are found in solution both before and after the reaction
123
To change ionic equation to NIE eliminate spectator ions Cd+2(aq) + S-2(aq) rarr CdS(s)
This NIE applies to any reaction in which precipitate of CdS forms
124
Practice Problem
Identify the precipitate that forms when aqueous solutions of zinc nitrate and ammonium sulfide are combined Write the equation for the possible double-replacement reaction Then write the formula equation overall ionic equation and the net ionic equation
125
1 AnalyzeGiven Identity of reactants zinc nitrate and ammonium sulfide Reaction medium aqueous solution Unknown a equation for the possible double-replacement reaction b identity of the precipitate c formula equation d overall ionic equation e net ionic equation
126
2 PlanWrite the possible double-replacement reaction between Zn(NO3)2 and (NH4)2S Use the rules to determine if any of the products will precipitate Write a formula equation and overall net ionic equation Then cancel spectator ions to make net ionic equation
127
3 Computea possible double-replacement reaction
b rules show that zinc sulfide is not soluble so will precipitate (ammonium nitrate is soluble)
c formula equation128
d overall ionic equation
e net ionic equation129
Will a precipitate form if solutions of potassium sulfate and barium nitrate are combined If so write the net ionic equation for the reaction
Yes Ba+2(aq) + SO4-2(aq) rarr BaSO4(s)
130
Will a precipitate form if solutions of potassium nitrate and magnesium sulfate are combined If so write the net ionic equation for the reaction
No 131
Will a precipitate form if solutions of barium chloride and sodium sulfate are combined If so identify the spectator ions and write the net ionic equation for the reaction
Yes Na+ and Cl- Ba+2(aq) + SO4-2(aq) rarr BaSO4(s)
132
Write the net ionic equation for the precipitation of nickel(II) sulfide
Ni+2(aq) + S-2(aq) rarr NiS(s)
133
IonizationSome molecular compounds can also form ions in solution Usually polar Ionization - ions formed from solute molecules by action of the solvent (creation of ions where there were none) Ionization different from dissociation
134
Dissociation ionic compounds dissolve and ions already present separate Ionization ions formed where non existed before
Ions formed are hydrated Heat released during hydration of ions gives enough energy to break covalent bonds
135
Extent to which solute ionizes depends on strength of bonds within molecules of solute and strength of attraction between solute and solvent molecules
136
HCl ndash molecular compound that ionizes in aqueous solution Contains highly polar bond Attraction between polar HCl molecule and polar water strong enough to break HCl bond
137
The Hydronium Ion
H+ ions from HCl attracts other molecules or ions so strongly that it doesnrsquot normally exist alone Ionization of HCl better described as direct transfer of proton from HCl to H2O forming H3O+
138
139
Hydration of H+ to form hydronium is highly exothermic
Energy released gives a lot of energy needed to ionize a molecular solute
Many molecular compounds that ionize in aqueous solution contain hydrogen and form hydronium ion
140
Strong and Weak Electrolytes
Substances that make ions and conduct current in solution are electrolytes HCl is one of series of compounds that has hydrogen and halogen Hydrogen halides are all molecular compounds with single polar-covalent bonds
141
All are gases Very soluble in water All are electrolytes HCl HBr HI strongly conduct current in solution HF only weakly conducts current at same concentration Strength of conduction related to ability to ionize
142
143
Strong Electrolytes
HCl HBr HI are 100 ionized in solution
Strong electrolyte - any compound whose dilute aqueous solutions conduct electricity well this is due to the presence of all or almost all of the dissolve compound in the form of ions
144
Unique characteristic of strong electrolyte is that they yield only ions
Ex Ionic compound may be highly soluble in water and dissociate into ions (NaCl)
Others may not dissolve much but amount that does dissolve exists only as ions
145
Weak ElectrolytesHF dissolves in water to give acid solution called hydrofluoric acid HF bond stronger than bonds between hydrogen and other halogens When HF dissolves some molecule ionize Reverse reaction also happens HF(aq) + H2O(l) H3O+(aq) + F-(aq)
146
HF(aq) + H2O(l) H3O+
(aq) + F-(aq)Concentration of dissolved unionized HF stays high and concentration of ions stays low
Weak electrolyte - any compound whose aqueous solutions conduct electricity poorly this is due to the presence of small amount of the dissolved compound in the form of ions
147
Different from nonelectrolyte ndash NO ions at all Description of electrolyte as strong or weak not related to concentration of solution Electrolytes differ in degree of ionization not amount of solute dissolved
148
149
Colligative properties of solutions
Presence of solute affects properties of solutions Some properties not dependent on nature of dissolved substance but on how may dissolve particles are present
Colligative properties - properties that depend on the concentration of solute particles but not on their identity
150
Vapor-Pressure Lowering
Boiling and freezing point of solution different from pure solvent Nonvolatile solute raises boiling point and lowers freezing point Nonvolatile substance - one that has little tendency to become gas under existing conditions
151
152
To understand why nonvolatile solute changes boiling and freezing point consider equilibrium vapor pressure
Vapor pressure pressure caused by molecules that have escape liquid phase to gas phase
Can be thought of a measure of tendency of molecules to escape from a liquid
153
Addition of sucrose (nonvolatile solute) lowers concentration of water molecules at surface of liquid This lowers tendency of water molecules to leave solution and enter gas phase Vapor pressure of solution is lower than vapor pressure of pure water
154
Aqueous13 solution13 of13 nonvolatile13 solutePure13 water
Nonelectrolyte solutions of same molality have same concentration of particles
Dilute solutions of same solvent and equal molality of any nonelectrolyte solute lower vapor pressure equally
155
Example
1 m aqueous solution of nonelectrolyte glucose lowers vapor pressure of water 55 x 10-4 atm at 25 1 m aqueous solution of sucrose also lowers water vapor pressure to 55 x 10-4 atm at 25
156
157
bc vapor-pressure lowering depends on concentration of nonelectrolyte solute and doesnrsquot depend on the type of solute it is a colligative proprerty
bc vapor pressure has been lowered solution remains liquid over larger temperature range
Lowers freezing point and raises boiling point
158
Can assume changes in boiling and freezing point also depend on concentration of solute
They are colligative properties
159
Freezing-Point Depression
Freezing point of 1-molal solution of any nonelectrolyte solute in water is found (by experiment) to be 186 lower than freezing point of water When 1 mol of nonelectrolyte solute dissolved in 1 kg water freezing point is -186 not 000 160
When 2 mol nonelectrolyte solute dissolved in 1 kg water freezing point is -372
For any concentration of nonelectrolyte solute in water decrease in freezing point can be determined using value of -186m
161
This value called the molal freezing-point contstant (Kf) is the freezing-point depression of the solvent in a 1-molal solution of a nonvolatile nonelectrolyte solute
162
Each solvent has own characteristic molal freezing-point constant Freezing-point depression (Δtf) the difference between the freezing points of the pure solvent and a solute of nonelectrolyte in that solvent and it is directly proportional to the molal concentration of the solution Δtf = Kfm
163
Δtf = Kfm
Kf expressed as m
m expressed in mol solutekg solvent (molality)
164
Sample Problem
What is the freezing point depression of water in a solution of 171 g of sucrose C12H22O11 and 200 g of water What is the actual freezing point of the solution
165
1 AnalyzeGiven Solute mass and chemical formula 171 g C12H22O11 Solvent mass and identity 2000 g water Unknown a freezing-point depression b freezing point of the solution 166
2 Plan
167
168
Calculate
Unknown a freezing-point depression b freezing point of the solution
169
Practice Problem
A water solution containing an unknown quantity of a nonelectrolyte solute is found to have a freezing point of ndash 023degCWhat is the molal concentration of the solution
012 m170
A solution consists of 103 g of the nonelectrolyte glucose C6H12O6 dissolved in 250 g of water What is the freezing-point depression of the solution
minus0426degC171
In a laboratory experiment the freezing point of an aqueous solution of glucose is found to be minus0325degCWhat is the molal concentration of this solution
0175 m172
If 0500 mol of a nonelectrolyte solute are dissolved in 5000 g of ether what is the freezing point of the solution
minus1181degC173
The freezing point of an aqueous solution that contains a nonelectrolyte is minus90degC aWhat is the freezing-point depression of the solution bWhat is the molal concentration of the solution a minus90degC b 48 m
174
Boiling-Point Elevation
Boiling point is temp at which vapor pressure is equal to atmospheric pressure Change in vapor pressure will cause corresponding change in boiling point
175
176
Vapor pressure of nonvolatile solution is lower than vapor pressure of pure solvent More heat will be required to raise vapor pressure of solution Boiling point of solution is higher than boiling point of pure solvent
Molal boiling-point constant (Kb) - boiling-point elevation of the solvent in a 1-molal solution of a nonvolatile nonelectrolyte solute b-p elevation of any nonelectrolyte in water found by experiment to be 051 So molal b-p constant for water is 051m
177
For different solvents b-p elevations of 1-molal solutions have different values (Table 14-2)
Like freezing-point constants values most accurate for dilute solutions
178
Boiling-point elevation Δtb difference between the boiling points of the pure solvent and a nonelectrolyte solution of that solvent and it is directly proportional to the molal concentration of the solution B-p elevation can be calculated using Δtb = Kbm Δtb (boiling point elevation) expressed in m and m expressed in mol solutekg solvent
Sample Problem
What is the boiling-point elevation of a solution made from 200 g of a nonelectrolyte solute and 4000 g of water The molar mass of the solute is 620 gmol
180
1 AnalyzeGiven solute mass = 200 g solute molar mass = 620 gmol solvent mass and identity = 4000 g of water
Unknown boiling-point elevation
181
2 Plan
182
183
3 Compute
Practice Problem
A solution contains 500 g of sucrose C12H22O11 a nonelectrolyte dissolved in 5000 g of water What is the boiling-point elevation
015degC184
A solution contains 4500 g of sucrose C12H22O11 a nonelectrolyte dissolved in 250 g of waterWhat is the boiling point of the solution
1027degC185
If the boiling point elevation of an aqueous solution containing a nonvolatile electrolyte is 102degC what is the molality of the solution
20 m186
The boiling point of an aqueous solution containing a nonvolatile electrolyte is 10075degC aWhat is the boiling-point elevation bWhat is the molality of the solution a 075degC b 15 m
187
Electrolytes and Colligative PropertiesEarly investigators confused by experiments where certain substances depressed freezing point or elevated boiling point more than expected
188
189
Ex 01 m solution of NaCl lowers freezing point of solvent almost twice as much as 01 m solution of sucrose 01 m CaCl2 lowers fp almost 3 times as much as 01 m solution of sucrose
To understand why this happens contrast behavior or sucrose with NaCl in aqueous solution Each sucrose dissolves to make only 1 particle in solution So 1 mol sucrose dissolves to 1 mol particles 1 mol NaCl dissolves to make 2 moles of particles (1 mol Na+ and 1 mol Cl-)
190
191
Calculated Values for Electrolyte SolutionsColligative properties depend on total concentration of solute particles regardless of their identity Electrolytes cause changes in colligative properties proportional to total molality in terms of all dissolves particles instead of formula units
192
What about barium nitrate Ba(NO3)2 Each mole of barium nitrate yields 3 mol of ions in solution Ba(NO3)2(s) rarr Ba2+(aq) + 2NO3-(aq) You would expect solution of given molality to lower fp of solvent 3 times as much as nonelectrolytic solution of same molality
193
Sample Problem
What is the expected change in the freezing point of water in a solution of 625 g of barium nitrate Ba(NO3)2 in 100 kg of water
194
1 AnalyzeGiven solute mass and formula = 625 g Ba(NO3)2 solvent mass and identity = 100 kg water Δtf = Kfm
Unknown expected freezing-point depression
195
2 PlanThe molality can be calculated by converting the solute mass to moles and then dividing by the number of kilograms of solvent That molality is in terms of formula units of Ba(NO3)2 and must be converted to molality in terms of dissociated ions in solution It must be multiplied by the number of moles of ions produced per mole of formula unit This adjusted molality can then be used to calculate the freezing-point depression
196
197
198
3 Compute
Practice Problem
What is the expected freezing-point depression for a solution that contains 20 mol of magnesium sulfate dissolved in 10 kg of water
minus74degC199
What is the expected boiling-point elevation of water for a solution that contains 150 g of sodium chloride dissolved in 10 kg of water 27degC The freezing point of an aqueous sodium chloride solution is minus200degC What is the molality of the solution 54 m NaCl
200
Actual Values for Electrolyte SolutionsRemember values calculated are only expected values Actual values of colligative properties for all strong electrolytes are almost what would be expected Difference between expected and actual values caused by attractive forces that exist between dissociated ions in aqueous solution
201
Attraction between hydrated ions in solution is small compared with those in crystalline solid However forces of attraction donrsquot interfere with movements of aqueous ions More concentrated solution means ions closer together and attraction is greater Also depends on charge of ions ndash higher charge greater attraction
202
enthalpies of Solution
Formation of solution comes with energy change Dissolve KI in water container feels cold Dissolve LiCl in water feels hot Formation of solid-liquid solution can absorb or release heat
73
During formation of solution solvent and solute particles experience changes in forces attracting them to other particles Before dissolving begins solvent molecules held by intermolecular forces (solvent-solvent attraction) In solute molecules held by Ifs (solute-solute attraction)
74
Energy is required to separate solute molecules and solvent molecules from their neighbors
A solute particle that is surrounded by solvent molecules is said to be solvated
75
Step 1 solute particles become separated from solid (energy absorbed) Step 2 solvent particles move apart to allow solute particles to enter liquid (energy absorbed)
76
Step 3 solvent particles attracted to and solvate solute particles (energy released)
77
Heat of solution - net amount of heat energy absorbed or released when a specific amount of solute dissolves in a solvent
Heat of solution negative (heat is released) if sums of energy in steps 1 and 2 is less than step 3
If 1+2 gt 3 HOS is positive (heat absorbed)78
In gaseous state molecules are so far apart there are nearly no forces between them Solute-solute interaction has little effect on heat of solution of a gas Energy released when gas dissolved in liquid bc attraction between solute gas and solvent molecules greater than energy needed to separate solvent molecules
79
Concentration of Solutions80
Concentration - measure of amount of solute in given amount of solvent or solution ldquodiluterdquo just means small amount of solute ldquoconcentratedrdquo just means relatively large amount of solute Unrelated to degree to which solution is saturated Ex Saturated solution of substance not very soluble might be dilute
81
Molarity
Molarity - number of moles of solute in one liter of solution To find molarity must know molar mass A ldquoone molarrdquo solution of NaOH is 1 mole NaOH per liter of solution (1 M)
82
1 mol NaOH = 400 g If this quantity dissolve in enough water to make EXACTLY 100 L solution solution is 1 M If 200 g NaOH dissolve in enough to make 100 L solution what is molarity 0500 M
83
1 M solution is not made by adding 1 mol solute to 1 L solvent That would be more than 1 L Instead 1 mol solute dissolved in less than 1 L Then diluted with more solvent to bring TOTAL VOLUME to 1 L
84
Practice ProblemYou have 340 L of solution that contains 900 g sodium chloride What is the molarity of that solution 0440 M NaCl You have 08 L of a 05 M HCl solution How many moles of HCl does this solution contain 04 mol HCl
85
What is the molarity of a solution composed of 585 g of potassium iodide dissolved in enough water to make a 0125 L of solution 0282 M KI
86
How many moles of H2SO4 are present in 0500 L of a 0150 M H2SO4 solution 00750 mol What volume of 300 M NaCl is needed for reaction that requires 1463 g of NaCl 0834 L
87
Molality
Molality - concentration of solution expressed in moles of solute per kilogram of solvent Solution that contains 1 mol solute (NaOH) dissolved in exactly 1 kg of solvent is ldquoone molalrdquo solution (1 m NaOH)
88
1 mol NaOH = 400 g 400 g NaOH dissolved in 1 kg water is 1 m NaOH
200 g NaOH in 1 kg water = 0500 m NaOH
89
Practice Problem
A solution was prepared by dissolving 171 g sucrose (C12H22O11) in 125 g of water Find the molal concentration of this solution 0400 m C12H22O11
90
A solution of iodine in carbon tetrachloride is used when iodine is needed for certain chemical tests How much iodine must be added to prepare a 0480 m solution of iodine in CCl4 if 1000 g of CCl4 is used 122 g I2
91
What is the molality of a solution composed of 255 g acetone (CH3)2CO dissolved in 200 g of water 22 m acetone What quantity in grams of methanol CH3OH is required to prepare a 0244 m solution in 400 g water 312 g methanol
92
How many grams of AgNO3 are needed to prepare a 0125 m solution in 250 mL of water 531 g AgNO3
What is the molality of a solution containing 182 g HCl and 250 g water 200 m
93
Ions in Aqueous Solutions and Colligative Properties
94
95
compounds in aqueous solutions
DissociationWhen compound made from ions dissolves in water ions separate Dissociation - separation of ions that occurs when an ionic compound dissolves
96
Notice number of ions made per formula unit in equations 1 formula unit of NaCl gives 2 units of ions in solution 1 formula unit of CaCl2 gives 3 units of ions
97
Assuming 100 dissociation solution that contains 1 mol NaCl contains 1 mol Na+ and 1 mol Cl- Can assume 100 dissociation of all soluble ionic compounds
98
Practice Problem
Write the equation for the dissolution of aluminum sulfate in water How many moles of aluminum ions are made by dissolving 1 mol aluminum sulfate What is the total number of moles of ions made by dissolving 1 mol of aluminum sulfate
99
1 AnalyzeGiven Amount of solute = 1 mol Al2(SO4)3 Solvent identity = water Unknown a moles of aluminum ions and sulfate ions Total number of moles of solute ions produced
100
2 Plan
The coefficients in the balanced dissociation equation will tell mole relationships You can use the equation to find out the number of moles of solute ions produced
101
3 Compute
a
b102
Write the equation for the dissolution of each of the following in water and then determine the number of moles of each ions made as well as the total number of ions made a 1 mol ammonium chloride b 1 mol sodium sulfide c 05 mol barium nitrate
103
a 1 mol ammonium chloride
1 mol NH4+ 1 mol Cl- 2 mol total ions
104
b 1 mol sodium sulfide
2 mol Na+ 1 mol S-2 3 mol total ions
105
c 05 mol barium nitrate
05 mol Ba+2 1 mol NO3- 15 mol total ions
106
Precipitation Reactions
Even though no compound is completely insoluble compounds of very low solubility can be considered insoluble for practical purposes There are general guidelines to help predict whether a compound made of certain combination of ions is soluble
107
General Solubility Guidelines
Sodium potassium and ammonium compounds are soluble in water
108
Nitrates acetates and chlorates are soluble
109
110
Most chlorides are soluble except those of silver mercury (I) and lead Lead (II) chloride is soluble in hot water
Most sulfates are soluble except those of barium strontium lead calcium and mercury
111
Most carbonates phosphates and silicates are insoluble except those of sodium potassium and ammonium
112
Most sulfides are insoluble except those of calcium strontium sodium potassium and ammonium
113
Is calcium phosphate Ca3(PO4)2 soluble or insoluble
Not one of the exceptions so it is insoluble
Dissociation equations are not written for insoluble compounds
114
Guidelines also useful in predicting what will happen if 2 solutions of 2 different soluble compounds are mixed
If mixing results in combination of ions that forms insoluble compound a double-replacement and precipitation reaction will happen
115
Will precipitate form when solutions of ammonium sulfide and cadmium nitrate are combined You can tell calcium nitrate is soluble from the guidelines (Most nitrates acetates and chlorates are soluble) You can also tell ammonium sulfide is soluble (Most sulfides are insoluble except those of calcium strontium sodium potassium and ammonium)
116
117
To decide whether precipitate can form must know solubilities of two possible products
Ammonium nitrate ndash soluble (Most nitrates acetates and chlorates are soluble) Cadmium sulfide ndash insoluble (Most sulfides are insoluble except those of calcium strontium sodium potassium and ammonium)
118
bc one product is insoluble double-replacement and precipitation reaction will happen (NH4)2S (aq) + Cd(NO3)2 (aq) rarr NH4NO3 (aq) + CdS (s)
119
Net Ionic Equations
Reactions of ions in aqueous solution usually represented by net ionic equations instead of formula equations Net ionic equation - includes only those compounds and ions that undergo a chemical change in a reaction in an aqueous solution
120
121
To write NIE (net ionic equation) first convert chemical equation into overall ionic equation All soluble ionic compounds shown as dissociated ions in solution Precipitates shown as solids
122
Notice ammonium and nitrate ions appear on both sides They havenrsquot gone through any chemical change Spectator ions - ions that do not take part in a chemical reaction and are found in solution both before and after the reaction
123
To change ionic equation to NIE eliminate spectator ions Cd+2(aq) + S-2(aq) rarr CdS(s)
This NIE applies to any reaction in which precipitate of CdS forms
124
Practice Problem
Identify the precipitate that forms when aqueous solutions of zinc nitrate and ammonium sulfide are combined Write the equation for the possible double-replacement reaction Then write the formula equation overall ionic equation and the net ionic equation
125
1 AnalyzeGiven Identity of reactants zinc nitrate and ammonium sulfide Reaction medium aqueous solution Unknown a equation for the possible double-replacement reaction b identity of the precipitate c formula equation d overall ionic equation e net ionic equation
126
2 PlanWrite the possible double-replacement reaction between Zn(NO3)2 and (NH4)2S Use the rules to determine if any of the products will precipitate Write a formula equation and overall net ionic equation Then cancel spectator ions to make net ionic equation
127
3 Computea possible double-replacement reaction
b rules show that zinc sulfide is not soluble so will precipitate (ammonium nitrate is soluble)
c formula equation128
d overall ionic equation
e net ionic equation129
Will a precipitate form if solutions of potassium sulfate and barium nitrate are combined If so write the net ionic equation for the reaction
Yes Ba+2(aq) + SO4-2(aq) rarr BaSO4(s)
130
Will a precipitate form if solutions of potassium nitrate and magnesium sulfate are combined If so write the net ionic equation for the reaction
No 131
Will a precipitate form if solutions of barium chloride and sodium sulfate are combined If so identify the spectator ions and write the net ionic equation for the reaction
Yes Na+ and Cl- Ba+2(aq) + SO4-2(aq) rarr BaSO4(s)
132
Write the net ionic equation for the precipitation of nickel(II) sulfide
Ni+2(aq) + S-2(aq) rarr NiS(s)
133
IonizationSome molecular compounds can also form ions in solution Usually polar Ionization - ions formed from solute molecules by action of the solvent (creation of ions where there were none) Ionization different from dissociation
134
Dissociation ionic compounds dissolve and ions already present separate Ionization ions formed where non existed before
Ions formed are hydrated Heat released during hydration of ions gives enough energy to break covalent bonds
135
Extent to which solute ionizes depends on strength of bonds within molecules of solute and strength of attraction between solute and solvent molecules
136
HCl ndash molecular compound that ionizes in aqueous solution Contains highly polar bond Attraction between polar HCl molecule and polar water strong enough to break HCl bond
137
The Hydronium Ion
H+ ions from HCl attracts other molecules or ions so strongly that it doesnrsquot normally exist alone Ionization of HCl better described as direct transfer of proton from HCl to H2O forming H3O+
138
139
Hydration of H+ to form hydronium is highly exothermic
Energy released gives a lot of energy needed to ionize a molecular solute
Many molecular compounds that ionize in aqueous solution contain hydrogen and form hydronium ion
140
Strong and Weak Electrolytes
Substances that make ions and conduct current in solution are electrolytes HCl is one of series of compounds that has hydrogen and halogen Hydrogen halides are all molecular compounds with single polar-covalent bonds
141
All are gases Very soluble in water All are electrolytes HCl HBr HI strongly conduct current in solution HF only weakly conducts current at same concentration Strength of conduction related to ability to ionize
142
143
Strong Electrolytes
HCl HBr HI are 100 ionized in solution
Strong electrolyte - any compound whose dilute aqueous solutions conduct electricity well this is due to the presence of all or almost all of the dissolve compound in the form of ions
144
Unique characteristic of strong electrolyte is that they yield only ions
Ex Ionic compound may be highly soluble in water and dissociate into ions (NaCl)
Others may not dissolve much but amount that does dissolve exists only as ions
145
Weak ElectrolytesHF dissolves in water to give acid solution called hydrofluoric acid HF bond stronger than bonds between hydrogen and other halogens When HF dissolves some molecule ionize Reverse reaction also happens HF(aq) + H2O(l) H3O+(aq) + F-(aq)
146
HF(aq) + H2O(l) H3O+
(aq) + F-(aq)Concentration of dissolved unionized HF stays high and concentration of ions stays low
Weak electrolyte - any compound whose aqueous solutions conduct electricity poorly this is due to the presence of small amount of the dissolved compound in the form of ions
147
Different from nonelectrolyte ndash NO ions at all Description of electrolyte as strong or weak not related to concentration of solution Electrolytes differ in degree of ionization not amount of solute dissolved
148
149
Colligative properties of solutions
Presence of solute affects properties of solutions Some properties not dependent on nature of dissolved substance but on how may dissolve particles are present
Colligative properties - properties that depend on the concentration of solute particles but not on their identity
150
Vapor-Pressure Lowering
Boiling and freezing point of solution different from pure solvent Nonvolatile solute raises boiling point and lowers freezing point Nonvolatile substance - one that has little tendency to become gas under existing conditions
151
152
To understand why nonvolatile solute changes boiling and freezing point consider equilibrium vapor pressure
Vapor pressure pressure caused by molecules that have escape liquid phase to gas phase
Can be thought of a measure of tendency of molecules to escape from a liquid
153
Addition of sucrose (nonvolatile solute) lowers concentration of water molecules at surface of liquid This lowers tendency of water molecules to leave solution and enter gas phase Vapor pressure of solution is lower than vapor pressure of pure water
154
Aqueous13 solution13 of13 nonvolatile13 solutePure13 water
Nonelectrolyte solutions of same molality have same concentration of particles
Dilute solutions of same solvent and equal molality of any nonelectrolyte solute lower vapor pressure equally
155
Example
1 m aqueous solution of nonelectrolyte glucose lowers vapor pressure of water 55 x 10-4 atm at 25 1 m aqueous solution of sucrose also lowers water vapor pressure to 55 x 10-4 atm at 25
156
157
bc vapor-pressure lowering depends on concentration of nonelectrolyte solute and doesnrsquot depend on the type of solute it is a colligative proprerty
bc vapor pressure has been lowered solution remains liquid over larger temperature range
Lowers freezing point and raises boiling point
158
Can assume changes in boiling and freezing point also depend on concentration of solute
They are colligative properties
159
Freezing-Point Depression
Freezing point of 1-molal solution of any nonelectrolyte solute in water is found (by experiment) to be 186 lower than freezing point of water When 1 mol of nonelectrolyte solute dissolved in 1 kg water freezing point is -186 not 000 160
When 2 mol nonelectrolyte solute dissolved in 1 kg water freezing point is -372
For any concentration of nonelectrolyte solute in water decrease in freezing point can be determined using value of -186m
161
This value called the molal freezing-point contstant (Kf) is the freezing-point depression of the solvent in a 1-molal solution of a nonvolatile nonelectrolyte solute
162
Each solvent has own characteristic molal freezing-point constant Freezing-point depression (Δtf) the difference between the freezing points of the pure solvent and a solute of nonelectrolyte in that solvent and it is directly proportional to the molal concentration of the solution Δtf = Kfm
163
Δtf = Kfm
Kf expressed as m
m expressed in mol solutekg solvent (molality)
164
Sample Problem
What is the freezing point depression of water in a solution of 171 g of sucrose C12H22O11 and 200 g of water What is the actual freezing point of the solution
165
1 AnalyzeGiven Solute mass and chemical formula 171 g C12H22O11 Solvent mass and identity 2000 g water Unknown a freezing-point depression b freezing point of the solution 166
2 Plan
167
168
Calculate
Unknown a freezing-point depression b freezing point of the solution
169
Practice Problem
A water solution containing an unknown quantity of a nonelectrolyte solute is found to have a freezing point of ndash 023degCWhat is the molal concentration of the solution
012 m170
A solution consists of 103 g of the nonelectrolyte glucose C6H12O6 dissolved in 250 g of water What is the freezing-point depression of the solution
minus0426degC171
In a laboratory experiment the freezing point of an aqueous solution of glucose is found to be minus0325degCWhat is the molal concentration of this solution
0175 m172
If 0500 mol of a nonelectrolyte solute are dissolved in 5000 g of ether what is the freezing point of the solution
minus1181degC173
The freezing point of an aqueous solution that contains a nonelectrolyte is minus90degC aWhat is the freezing-point depression of the solution bWhat is the molal concentration of the solution a minus90degC b 48 m
174
Boiling-Point Elevation
Boiling point is temp at which vapor pressure is equal to atmospheric pressure Change in vapor pressure will cause corresponding change in boiling point
175
176
Vapor pressure of nonvolatile solution is lower than vapor pressure of pure solvent More heat will be required to raise vapor pressure of solution Boiling point of solution is higher than boiling point of pure solvent
Molal boiling-point constant (Kb) - boiling-point elevation of the solvent in a 1-molal solution of a nonvolatile nonelectrolyte solute b-p elevation of any nonelectrolyte in water found by experiment to be 051 So molal b-p constant for water is 051m
177
For different solvents b-p elevations of 1-molal solutions have different values (Table 14-2)
Like freezing-point constants values most accurate for dilute solutions
178
Boiling-point elevation Δtb difference between the boiling points of the pure solvent and a nonelectrolyte solution of that solvent and it is directly proportional to the molal concentration of the solution B-p elevation can be calculated using Δtb = Kbm Δtb (boiling point elevation) expressed in m and m expressed in mol solutekg solvent
Sample Problem
What is the boiling-point elevation of a solution made from 200 g of a nonelectrolyte solute and 4000 g of water The molar mass of the solute is 620 gmol
180
1 AnalyzeGiven solute mass = 200 g solute molar mass = 620 gmol solvent mass and identity = 4000 g of water
Unknown boiling-point elevation
181
2 Plan
182
183
3 Compute
Practice Problem
A solution contains 500 g of sucrose C12H22O11 a nonelectrolyte dissolved in 5000 g of water What is the boiling-point elevation
015degC184
A solution contains 4500 g of sucrose C12H22O11 a nonelectrolyte dissolved in 250 g of waterWhat is the boiling point of the solution
1027degC185
If the boiling point elevation of an aqueous solution containing a nonvolatile electrolyte is 102degC what is the molality of the solution
20 m186
The boiling point of an aqueous solution containing a nonvolatile electrolyte is 10075degC aWhat is the boiling-point elevation bWhat is the molality of the solution a 075degC b 15 m
187
Electrolytes and Colligative PropertiesEarly investigators confused by experiments where certain substances depressed freezing point or elevated boiling point more than expected
188
189
Ex 01 m solution of NaCl lowers freezing point of solvent almost twice as much as 01 m solution of sucrose 01 m CaCl2 lowers fp almost 3 times as much as 01 m solution of sucrose
To understand why this happens contrast behavior or sucrose with NaCl in aqueous solution Each sucrose dissolves to make only 1 particle in solution So 1 mol sucrose dissolves to 1 mol particles 1 mol NaCl dissolves to make 2 moles of particles (1 mol Na+ and 1 mol Cl-)
190
191
Calculated Values for Electrolyte SolutionsColligative properties depend on total concentration of solute particles regardless of their identity Electrolytes cause changes in colligative properties proportional to total molality in terms of all dissolves particles instead of formula units
192
What about barium nitrate Ba(NO3)2 Each mole of barium nitrate yields 3 mol of ions in solution Ba(NO3)2(s) rarr Ba2+(aq) + 2NO3-(aq) You would expect solution of given molality to lower fp of solvent 3 times as much as nonelectrolytic solution of same molality
193
Sample Problem
What is the expected change in the freezing point of water in a solution of 625 g of barium nitrate Ba(NO3)2 in 100 kg of water
194
1 AnalyzeGiven solute mass and formula = 625 g Ba(NO3)2 solvent mass and identity = 100 kg water Δtf = Kfm
Unknown expected freezing-point depression
195
2 PlanThe molality can be calculated by converting the solute mass to moles and then dividing by the number of kilograms of solvent That molality is in terms of formula units of Ba(NO3)2 and must be converted to molality in terms of dissociated ions in solution It must be multiplied by the number of moles of ions produced per mole of formula unit This adjusted molality can then be used to calculate the freezing-point depression
196
197
198
3 Compute
Practice Problem
What is the expected freezing-point depression for a solution that contains 20 mol of magnesium sulfate dissolved in 10 kg of water
minus74degC199
What is the expected boiling-point elevation of water for a solution that contains 150 g of sodium chloride dissolved in 10 kg of water 27degC The freezing point of an aqueous sodium chloride solution is minus200degC What is the molality of the solution 54 m NaCl
200
Actual Values for Electrolyte SolutionsRemember values calculated are only expected values Actual values of colligative properties for all strong electrolytes are almost what would be expected Difference between expected and actual values caused by attractive forces that exist between dissociated ions in aqueous solution
201
Attraction between hydrated ions in solution is small compared with those in crystalline solid However forces of attraction donrsquot interfere with movements of aqueous ions More concentrated solution means ions closer together and attraction is greater Also depends on charge of ions ndash higher charge greater attraction
202
During formation of solution solvent and solute particles experience changes in forces attracting them to other particles Before dissolving begins solvent molecules held by intermolecular forces (solvent-solvent attraction) In solute molecules held by Ifs (solute-solute attraction)
74
Energy is required to separate solute molecules and solvent molecules from their neighbors
A solute particle that is surrounded by solvent molecules is said to be solvated
75
Step 1 solute particles become separated from solid (energy absorbed) Step 2 solvent particles move apart to allow solute particles to enter liquid (energy absorbed)
76
Step 3 solvent particles attracted to and solvate solute particles (energy released)
77
Heat of solution - net amount of heat energy absorbed or released when a specific amount of solute dissolves in a solvent
Heat of solution negative (heat is released) if sums of energy in steps 1 and 2 is less than step 3
If 1+2 gt 3 HOS is positive (heat absorbed)78
In gaseous state molecules are so far apart there are nearly no forces between them Solute-solute interaction has little effect on heat of solution of a gas Energy released when gas dissolved in liquid bc attraction between solute gas and solvent molecules greater than energy needed to separate solvent molecules
79
Concentration of Solutions80
Concentration - measure of amount of solute in given amount of solvent or solution ldquodiluterdquo just means small amount of solute ldquoconcentratedrdquo just means relatively large amount of solute Unrelated to degree to which solution is saturated Ex Saturated solution of substance not very soluble might be dilute
81
Molarity
Molarity - number of moles of solute in one liter of solution To find molarity must know molar mass A ldquoone molarrdquo solution of NaOH is 1 mole NaOH per liter of solution (1 M)
82
1 mol NaOH = 400 g If this quantity dissolve in enough water to make EXACTLY 100 L solution solution is 1 M If 200 g NaOH dissolve in enough to make 100 L solution what is molarity 0500 M
83
1 M solution is not made by adding 1 mol solute to 1 L solvent That would be more than 1 L Instead 1 mol solute dissolved in less than 1 L Then diluted with more solvent to bring TOTAL VOLUME to 1 L
84
Practice ProblemYou have 340 L of solution that contains 900 g sodium chloride What is the molarity of that solution 0440 M NaCl You have 08 L of a 05 M HCl solution How many moles of HCl does this solution contain 04 mol HCl
85
What is the molarity of a solution composed of 585 g of potassium iodide dissolved in enough water to make a 0125 L of solution 0282 M KI
86
How many moles of H2SO4 are present in 0500 L of a 0150 M H2SO4 solution 00750 mol What volume of 300 M NaCl is needed for reaction that requires 1463 g of NaCl 0834 L
87
Molality
Molality - concentration of solution expressed in moles of solute per kilogram of solvent Solution that contains 1 mol solute (NaOH) dissolved in exactly 1 kg of solvent is ldquoone molalrdquo solution (1 m NaOH)
88
1 mol NaOH = 400 g 400 g NaOH dissolved in 1 kg water is 1 m NaOH
200 g NaOH in 1 kg water = 0500 m NaOH
89
Practice Problem
A solution was prepared by dissolving 171 g sucrose (C12H22O11) in 125 g of water Find the molal concentration of this solution 0400 m C12H22O11
90
A solution of iodine in carbon tetrachloride is used when iodine is needed for certain chemical tests How much iodine must be added to prepare a 0480 m solution of iodine in CCl4 if 1000 g of CCl4 is used 122 g I2
91
What is the molality of a solution composed of 255 g acetone (CH3)2CO dissolved in 200 g of water 22 m acetone What quantity in grams of methanol CH3OH is required to prepare a 0244 m solution in 400 g water 312 g methanol
92
How many grams of AgNO3 are needed to prepare a 0125 m solution in 250 mL of water 531 g AgNO3
What is the molality of a solution containing 182 g HCl and 250 g water 200 m
93
Ions in Aqueous Solutions and Colligative Properties
94
95
compounds in aqueous solutions
DissociationWhen compound made from ions dissolves in water ions separate Dissociation - separation of ions that occurs when an ionic compound dissolves
96
Notice number of ions made per formula unit in equations 1 formula unit of NaCl gives 2 units of ions in solution 1 formula unit of CaCl2 gives 3 units of ions
97
Assuming 100 dissociation solution that contains 1 mol NaCl contains 1 mol Na+ and 1 mol Cl- Can assume 100 dissociation of all soluble ionic compounds
98
Practice Problem
Write the equation for the dissolution of aluminum sulfate in water How many moles of aluminum ions are made by dissolving 1 mol aluminum sulfate What is the total number of moles of ions made by dissolving 1 mol of aluminum sulfate
99
1 AnalyzeGiven Amount of solute = 1 mol Al2(SO4)3 Solvent identity = water Unknown a moles of aluminum ions and sulfate ions Total number of moles of solute ions produced
100
2 Plan
The coefficients in the balanced dissociation equation will tell mole relationships You can use the equation to find out the number of moles of solute ions produced
101
3 Compute
a
b102
Write the equation for the dissolution of each of the following in water and then determine the number of moles of each ions made as well as the total number of ions made a 1 mol ammonium chloride b 1 mol sodium sulfide c 05 mol barium nitrate
103
a 1 mol ammonium chloride
1 mol NH4+ 1 mol Cl- 2 mol total ions
104
b 1 mol sodium sulfide
2 mol Na+ 1 mol S-2 3 mol total ions
105
c 05 mol barium nitrate
05 mol Ba+2 1 mol NO3- 15 mol total ions
106
Precipitation Reactions
Even though no compound is completely insoluble compounds of very low solubility can be considered insoluble for practical purposes There are general guidelines to help predict whether a compound made of certain combination of ions is soluble
107
General Solubility Guidelines
Sodium potassium and ammonium compounds are soluble in water
108
Nitrates acetates and chlorates are soluble
109
110
Most chlorides are soluble except those of silver mercury (I) and lead Lead (II) chloride is soluble in hot water
Most sulfates are soluble except those of barium strontium lead calcium and mercury
111
Most carbonates phosphates and silicates are insoluble except those of sodium potassium and ammonium
112
Most sulfides are insoluble except those of calcium strontium sodium potassium and ammonium
113
Is calcium phosphate Ca3(PO4)2 soluble or insoluble
Not one of the exceptions so it is insoluble
Dissociation equations are not written for insoluble compounds
114
Guidelines also useful in predicting what will happen if 2 solutions of 2 different soluble compounds are mixed
If mixing results in combination of ions that forms insoluble compound a double-replacement and precipitation reaction will happen
115
Will precipitate form when solutions of ammonium sulfide and cadmium nitrate are combined You can tell calcium nitrate is soluble from the guidelines (Most nitrates acetates and chlorates are soluble) You can also tell ammonium sulfide is soluble (Most sulfides are insoluble except those of calcium strontium sodium potassium and ammonium)
116
117
To decide whether precipitate can form must know solubilities of two possible products
Ammonium nitrate ndash soluble (Most nitrates acetates and chlorates are soluble) Cadmium sulfide ndash insoluble (Most sulfides are insoluble except those of calcium strontium sodium potassium and ammonium)
118
bc one product is insoluble double-replacement and precipitation reaction will happen (NH4)2S (aq) + Cd(NO3)2 (aq) rarr NH4NO3 (aq) + CdS (s)
119
Net Ionic Equations
Reactions of ions in aqueous solution usually represented by net ionic equations instead of formula equations Net ionic equation - includes only those compounds and ions that undergo a chemical change in a reaction in an aqueous solution
120
121
To write NIE (net ionic equation) first convert chemical equation into overall ionic equation All soluble ionic compounds shown as dissociated ions in solution Precipitates shown as solids
122
Notice ammonium and nitrate ions appear on both sides They havenrsquot gone through any chemical change Spectator ions - ions that do not take part in a chemical reaction and are found in solution both before and after the reaction
123
To change ionic equation to NIE eliminate spectator ions Cd+2(aq) + S-2(aq) rarr CdS(s)
This NIE applies to any reaction in which precipitate of CdS forms
124
Practice Problem
Identify the precipitate that forms when aqueous solutions of zinc nitrate and ammonium sulfide are combined Write the equation for the possible double-replacement reaction Then write the formula equation overall ionic equation and the net ionic equation
125
1 AnalyzeGiven Identity of reactants zinc nitrate and ammonium sulfide Reaction medium aqueous solution Unknown a equation for the possible double-replacement reaction b identity of the precipitate c formula equation d overall ionic equation e net ionic equation
126
2 PlanWrite the possible double-replacement reaction between Zn(NO3)2 and (NH4)2S Use the rules to determine if any of the products will precipitate Write a formula equation and overall net ionic equation Then cancel spectator ions to make net ionic equation
127
3 Computea possible double-replacement reaction
b rules show that zinc sulfide is not soluble so will precipitate (ammonium nitrate is soluble)
c formula equation128
d overall ionic equation
e net ionic equation129
Will a precipitate form if solutions of potassium sulfate and barium nitrate are combined If so write the net ionic equation for the reaction
Yes Ba+2(aq) + SO4-2(aq) rarr BaSO4(s)
130
Will a precipitate form if solutions of potassium nitrate and magnesium sulfate are combined If so write the net ionic equation for the reaction
No 131
Will a precipitate form if solutions of barium chloride and sodium sulfate are combined If so identify the spectator ions and write the net ionic equation for the reaction
Yes Na+ and Cl- Ba+2(aq) + SO4-2(aq) rarr BaSO4(s)
132
Write the net ionic equation for the precipitation of nickel(II) sulfide
Ni+2(aq) + S-2(aq) rarr NiS(s)
133
IonizationSome molecular compounds can also form ions in solution Usually polar Ionization - ions formed from solute molecules by action of the solvent (creation of ions where there were none) Ionization different from dissociation
134
Dissociation ionic compounds dissolve and ions already present separate Ionization ions formed where non existed before
Ions formed are hydrated Heat released during hydration of ions gives enough energy to break covalent bonds
135
Extent to which solute ionizes depends on strength of bonds within molecules of solute and strength of attraction between solute and solvent molecules
136
HCl ndash molecular compound that ionizes in aqueous solution Contains highly polar bond Attraction between polar HCl molecule and polar water strong enough to break HCl bond
137
The Hydronium Ion
H+ ions from HCl attracts other molecules or ions so strongly that it doesnrsquot normally exist alone Ionization of HCl better described as direct transfer of proton from HCl to H2O forming H3O+
138
139
Hydration of H+ to form hydronium is highly exothermic
Energy released gives a lot of energy needed to ionize a molecular solute
Many molecular compounds that ionize in aqueous solution contain hydrogen and form hydronium ion
140
Strong and Weak Electrolytes
Substances that make ions and conduct current in solution are electrolytes HCl is one of series of compounds that has hydrogen and halogen Hydrogen halides are all molecular compounds with single polar-covalent bonds
141
All are gases Very soluble in water All are electrolytes HCl HBr HI strongly conduct current in solution HF only weakly conducts current at same concentration Strength of conduction related to ability to ionize
142
143
Strong Electrolytes
HCl HBr HI are 100 ionized in solution
Strong electrolyte - any compound whose dilute aqueous solutions conduct electricity well this is due to the presence of all or almost all of the dissolve compound in the form of ions
144
Unique characteristic of strong electrolyte is that they yield only ions
Ex Ionic compound may be highly soluble in water and dissociate into ions (NaCl)
Others may not dissolve much but amount that does dissolve exists only as ions
145
Weak ElectrolytesHF dissolves in water to give acid solution called hydrofluoric acid HF bond stronger than bonds between hydrogen and other halogens When HF dissolves some molecule ionize Reverse reaction also happens HF(aq) + H2O(l) H3O+(aq) + F-(aq)
146
HF(aq) + H2O(l) H3O+
(aq) + F-(aq)Concentration of dissolved unionized HF stays high and concentration of ions stays low
Weak electrolyte - any compound whose aqueous solutions conduct electricity poorly this is due to the presence of small amount of the dissolved compound in the form of ions
147
Different from nonelectrolyte ndash NO ions at all Description of electrolyte as strong or weak not related to concentration of solution Electrolytes differ in degree of ionization not amount of solute dissolved
148
149
Colligative properties of solutions
Presence of solute affects properties of solutions Some properties not dependent on nature of dissolved substance but on how may dissolve particles are present
Colligative properties - properties that depend on the concentration of solute particles but not on their identity
150
Vapor-Pressure Lowering
Boiling and freezing point of solution different from pure solvent Nonvolatile solute raises boiling point and lowers freezing point Nonvolatile substance - one that has little tendency to become gas under existing conditions
151
152
To understand why nonvolatile solute changes boiling and freezing point consider equilibrium vapor pressure
Vapor pressure pressure caused by molecules that have escape liquid phase to gas phase
Can be thought of a measure of tendency of molecules to escape from a liquid
153
Addition of sucrose (nonvolatile solute) lowers concentration of water molecules at surface of liquid This lowers tendency of water molecules to leave solution and enter gas phase Vapor pressure of solution is lower than vapor pressure of pure water
154
Aqueous13 solution13 of13 nonvolatile13 solutePure13 water
Nonelectrolyte solutions of same molality have same concentration of particles
Dilute solutions of same solvent and equal molality of any nonelectrolyte solute lower vapor pressure equally
155
Example
1 m aqueous solution of nonelectrolyte glucose lowers vapor pressure of water 55 x 10-4 atm at 25 1 m aqueous solution of sucrose also lowers water vapor pressure to 55 x 10-4 atm at 25
156
157
bc vapor-pressure lowering depends on concentration of nonelectrolyte solute and doesnrsquot depend on the type of solute it is a colligative proprerty
bc vapor pressure has been lowered solution remains liquid over larger temperature range
Lowers freezing point and raises boiling point
158
Can assume changes in boiling and freezing point also depend on concentration of solute
They are colligative properties
159
Freezing-Point Depression
Freezing point of 1-molal solution of any nonelectrolyte solute in water is found (by experiment) to be 186 lower than freezing point of water When 1 mol of nonelectrolyte solute dissolved in 1 kg water freezing point is -186 not 000 160
When 2 mol nonelectrolyte solute dissolved in 1 kg water freezing point is -372
For any concentration of nonelectrolyte solute in water decrease in freezing point can be determined using value of -186m
161
This value called the molal freezing-point contstant (Kf) is the freezing-point depression of the solvent in a 1-molal solution of a nonvolatile nonelectrolyte solute
162
Each solvent has own characteristic molal freezing-point constant Freezing-point depression (Δtf) the difference between the freezing points of the pure solvent and a solute of nonelectrolyte in that solvent and it is directly proportional to the molal concentration of the solution Δtf = Kfm
163
Δtf = Kfm
Kf expressed as m
m expressed in mol solutekg solvent (molality)
164
Sample Problem
What is the freezing point depression of water in a solution of 171 g of sucrose C12H22O11 and 200 g of water What is the actual freezing point of the solution
165
1 AnalyzeGiven Solute mass and chemical formula 171 g C12H22O11 Solvent mass and identity 2000 g water Unknown a freezing-point depression b freezing point of the solution 166
2 Plan
167
168
Calculate
Unknown a freezing-point depression b freezing point of the solution
169
Practice Problem
A water solution containing an unknown quantity of a nonelectrolyte solute is found to have a freezing point of ndash 023degCWhat is the molal concentration of the solution
012 m170
A solution consists of 103 g of the nonelectrolyte glucose C6H12O6 dissolved in 250 g of water What is the freezing-point depression of the solution
minus0426degC171
In a laboratory experiment the freezing point of an aqueous solution of glucose is found to be minus0325degCWhat is the molal concentration of this solution
0175 m172
If 0500 mol of a nonelectrolyte solute are dissolved in 5000 g of ether what is the freezing point of the solution
minus1181degC173
The freezing point of an aqueous solution that contains a nonelectrolyte is minus90degC aWhat is the freezing-point depression of the solution bWhat is the molal concentration of the solution a minus90degC b 48 m
174
Boiling-Point Elevation
Boiling point is temp at which vapor pressure is equal to atmospheric pressure Change in vapor pressure will cause corresponding change in boiling point
175
176
Vapor pressure of nonvolatile solution is lower than vapor pressure of pure solvent More heat will be required to raise vapor pressure of solution Boiling point of solution is higher than boiling point of pure solvent
Molal boiling-point constant (Kb) - boiling-point elevation of the solvent in a 1-molal solution of a nonvolatile nonelectrolyte solute b-p elevation of any nonelectrolyte in water found by experiment to be 051 So molal b-p constant for water is 051m
177
For different solvents b-p elevations of 1-molal solutions have different values (Table 14-2)
Like freezing-point constants values most accurate for dilute solutions
178
Boiling-point elevation Δtb difference between the boiling points of the pure solvent and a nonelectrolyte solution of that solvent and it is directly proportional to the molal concentration of the solution B-p elevation can be calculated using Δtb = Kbm Δtb (boiling point elevation) expressed in m and m expressed in mol solutekg solvent
Sample Problem
What is the boiling-point elevation of a solution made from 200 g of a nonelectrolyte solute and 4000 g of water The molar mass of the solute is 620 gmol
180
1 AnalyzeGiven solute mass = 200 g solute molar mass = 620 gmol solvent mass and identity = 4000 g of water
Unknown boiling-point elevation
181
2 Plan
182
183
3 Compute
Practice Problem
A solution contains 500 g of sucrose C12H22O11 a nonelectrolyte dissolved in 5000 g of water What is the boiling-point elevation
015degC184
A solution contains 4500 g of sucrose C12H22O11 a nonelectrolyte dissolved in 250 g of waterWhat is the boiling point of the solution
1027degC185
If the boiling point elevation of an aqueous solution containing a nonvolatile electrolyte is 102degC what is the molality of the solution
20 m186
The boiling point of an aqueous solution containing a nonvolatile electrolyte is 10075degC aWhat is the boiling-point elevation bWhat is the molality of the solution a 075degC b 15 m
187
Electrolytes and Colligative PropertiesEarly investigators confused by experiments where certain substances depressed freezing point or elevated boiling point more than expected
188
189
Ex 01 m solution of NaCl lowers freezing point of solvent almost twice as much as 01 m solution of sucrose 01 m CaCl2 lowers fp almost 3 times as much as 01 m solution of sucrose
To understand why this happens contrast behavior or sucrose with NaCl in aqueous solution Each sucrose dissolves to make only 1 particle in solution So 1 mol sucrose dissolves to 1 mol particles 1 mol NaCl dissolves to make 2 moles of particles (1 mol Na+ and 1 mol Cl-)
190
191
Calculated Values for Electrolyte SolutionsColligative properties depend on total concentration of solute particles regardless of their identity Electrolytes cause changes in colligative properties proportional to total molality in terms of all dissolves particles instead of formula units
192
What about barium nitrate Ba(NO3)2 Each mole of barium nitrate yields 3 mol of ions in solution Ba(NO3)2(s) rarr Ba2+(aq) + 2NO3-(aq) You would expect solution of given molality to lower fp of solvent 3 times as much as nonelectrolytic solution of same molality
193
Sample Problem
What is the expected change in the freezing point of water in a solution of 625 g of barium nitrate Ba(NO3)2 in 100 kg of water
194
1 AnalyzeGiven solute mass and formula = 625 g Ba(NO3)2 solvent mass and identity = 100 kg water Δtf = Kfm
Unknown expected freezing-point depression
195
2 PlanThe molality can be calculated by converting the solute mass to moles and then dividing by the number of kilograms of solvent That molality is in terms of formula units of Ba(NO3)2 and must be converted to molality in terms of dissociated ions in solution It must be multiplied by the number of moles of ions produced per mole of formula unit This adjusted molality can then be used to calculate the freezing-point depression
196
197
198
3 Compute
Practice Problem
What is the expected freezing-point depression for a solution that contains 20 mol of magnesium sulfate dissolved in 10 kg of water
minus74degC199
What is the expected boiling-point elevation of water for a solution that contains 150 g of sodium chloride dissolved in 10 kg of water 27degC The freezing point of an aqueous sodium chloride solution is minus200degC What is the molality of the solution 54 m NaCl
200
Actual Values for Electrolyte SolutionsRemember values calculated are only expected values Actual values of colligative properties for all strong electrolytes are almost what would be expected Difference between expected and actual values caused by attractive forces that exist between dissociated ions in aqueous solution
201
Attraction between hydrated ions in solution is small compared with those in crystalline solid However forces of attraction donrsquot interfere with movements of aqueous ions More concentrated solution means ions closer together and attraction is greater Also depends on charge of ions ndash higher charge greater attraction
202
Energy is required to separate solute molecules and solvent molecules from their neighbors
A solute particle that is surrounded by solvent molecules is said to be solvated
75
Step 1 solute particles become separated from solid (energy absorbed) Step 2 solvent particles move apart to allow solute particles to enter liquid (energy absorbed)
76
Step 3 solvent particles attracted to and solvate solute particles (energy released)
77
Heat of solution - net amount of heat energy absorbed or released when a specific amount of solute dissolves in a solvent
Heat of solution negative (heat is released) if sums of energy in steps 1 and 2 is less than step 3
If 1+2 gt 3 HOS is positive (heat absorbed)78
In gaseous state molecules are so far apart there are nearly no forces between them Solute-solute interaction has little effect on heat of solution of a gas Energy released when gas dissolved in liquid bc attraction between solute gas and solvent molecules greater than energy needed to separate solvent molecules
79
Concentration of Solutions80
Concentration - measure of amount of solute in given amount of solvent or solution ldquodiluterdquo just means small amount of solute ldquoconcentratedrdquo just means relatively large amount of solute Unrelated to degree to which solution is saturated Ex Saturated solution of substance not very soluble might be dilute
81
Molarity
Molarity - number of moles of solute in one liter of solution To find molarity must know molar mass A ldquoone molarrdquo solution of NaOH is 1 mole NaOH per liter of solution (1 M)
82
1 mol NaOH = 400 g If this quantity dissolve in enough water to make EXACTLY 100 L solution solution is 1 M If 200 g NaOH dissolve in enough to make 100 L solution what is molarity 0500 M
83
1 M solution is not made by adding 1 mol solute to 1 L solvent That would be more than 1 L Instead 1 mol solute dissolved in less than 1 L Then diluted with more solvent to bring TOTAL VOLUME to 1 L
84
Practice ProblemYou have 340 L of solution that contains 900 g sodium chloride What is the molarity of that solution 0440 M NaCl You have 08 L of a 05 M HCl solution How many moles of HCl does this solution contain 04 mol HCl
85
What is the molarity of a solution composed of 585 g of potassium iodide dissolved in enough water to make a 0125 L of solution 0282 M KI
86
How many moles of H2SO4 are present in 0500 L of a 0150 M H2SO4 solution 00750 mol What volume of 300 M NaCl is needed for reaction that requires 1463 g of NaCl 0834 L
87
Molality
Molality - concentration of solution expressed in moles of solute per kilogram of solvent Solution that contains 1 mol solute (NaOH) dissolved in exactly 1 kg of solvent is ldquoone molalrdquo solution (1 m NaOH)
88
1 mol NaOH = 400 g 400 g NaOH dissolved in 1 kg water is 1 m NaOH
200 g NaOH in 1 kg water = 0500 m NaOH
89
Practice Problem
A solution was prepared by dissolving 171 g sucrose (C12H22O11) in 125 g of water Find the molal concentration of this solution 0400 m C12H22O11
90
A solution of iodine in carbon tetrachloride is used when iodine is needed for certain chemical tests How much iodine must be added to prepare a 0480 m solution of iodine in CCl4 if 1000 g of CCl4 is used 122 g I2
91
What is the molality of a solution composed of 255 g acetone (CH3)2CO dissolved in 200 g of water 22 m acetone What quantity in grams of methanol CH3OH is required to prepare a 0244 m solution in 400 g water 312 g methanol
92
How many grams of AgNO3 are needed to prepare a 0125 m solution in 250 mL of water 531 g AgNO3
What is the molality of a solution containing 182 g HCl and 250 g water 200 m
93
Ions in Aqueous Solutions and Colligative Properties
94
95
compounds in aqueous solutions
DissociationWhen compound made from ions dissolves in water ions separate Dissociation - separation of ions that occurs when an ionic compound dissolves
96
Notice number of ions made per formula unit in equations 1 formula unit of NaCl gives 2 units of ions in solution 1 formula unit of CaCl2 gives 3 units of ions
97
Assuming 100 dissociation solution that contains 1 mol NaCl contains 1 mol Na+ and 1 mol Cl- Can assume 100 dissociation of all soluble ionic compounds
98
Practice Problem
Write the equation for the dissolution of aluminum sulfate in water How many moles of aluminum ions are made by dissolving 1 mol aluminum sulfate What is the total number of moles of ions made by dissolving 1 mol of aluminum sulfate
99
1 AnalyzeGiven Amount of solute = 1 mol Al2(SO4)3 Solvent identity = water Unknown a moles of aluminum ions and sulfate ions Total number of moles of solute ions produced
100
2 Plan
The coefficients in the balanced dissociation equation will tell mole relationships You can use the equation to find out the number of moles of solute ions produced
101
3 Compute
a
b102
Write the equation for the dissolution of each of the following in water and then determine the number of moles of each ions made as well as the total number of ions made a 1 mol ammonium chloride b 1 mol sodium sulfide c 05 mol barium nitrate
103
a 1 mol ammonium chloride
1 mol NH4+ 1 mol Cl- 2 mol total ions
104
b 1 mol sodium sulfide
2 mol Na+ 1 mol S-2 3 mol total ions
105
c 05 mol barium nitrate
05 mol Ba+2 1 mol NO3- 15 mol total ions
106
Precipitation Reactions
Even though no compound is completely insoluble compounds of very low solubility can be considered insoluble for practical purposes There are general guidelines to help predict whether a compound made of certain combination of ions is soluble
107
General Solubility Guidelines
Sodium potassium and ammonium compounds are soluble in water
108
Nitrates acetates and chlorates are soluble
109
110
Most chlorides are soluble except those of silver mercury (I) and lead Lead (II) chloride is soluble in hot water
Most sulfates are soluble except those of barium strontium lead calcium and mercury
111
Most carbonates phosphates and silicates are insoluble except those of sodium potassium and ammonium
112
Most sulfides are insoluble except those of calcium strontium sodium potassium and ammonium
113
Is calcium phosphate Ca3(PO4)2 soluble or insoluble
Not one of the exceptions so it is insoluble
Dissociation equations are not written for insoluble compounds
114
Guidelines also useful in predicting what will happen if 2 solutions of 2 different soluble compounds are mixed
If mixing results in combination of ions that forms insoluble compound a double-replacement and precipitation reaction will happen
115
Will precipitate form when solutions of ammonium sulfide and cadmium nitrate are combined You can tell calcium nitrate is soluble from the guidelines (Most nitrates acetates and chlorates are soluble) You can also tell ammonium sulfide is soluble (Most sulfides are insoluble except those of calcium strontium sodium potassium and ammonium)
116
117
To decide whether precipitate can form must know solubilities of two possible products
Ammonium nitrate ndash soluble (Most nitrates acetates and chlorates are soluble) Cadmium sulfide ndash insoluble (Most sulfides are insoluble except those of calcium strontium sodium potassium and ammonium)
118
bc one product is insoluble double-replacement and precipitation reaction will happen (NH4)2S (aq) + Cd(NO3)2 (aq) rarr NH4NO3 (aq) + CdS (s)
119
Net Ionic Equations
Reactions of ions in aqueous solution usually represented by net ionic equations instead of formula equations Net ionic equation - includes only those compounds and ions that undergo a chemical change in a reaction in an aqueous solution
120
121
To write NIE (net ionic equation) first convert chemical equation into overall ionic equation All soluble ionic compounds shown as dissociated ions in solution Precipitates shown as solids
122
Notice ammonium and nitrate ions appear on both sides They havenrsquot gone through any chemical change Spectator ions - ions that do not take part in a chemical reaction and are found in solution both before and after the reaction
123
To change ionic equation to NIE eliminate spectator ions Cd+2(aq) + S-2(aq) rarr CdS(s)
This NIE applies to any reaction in which precipitate of CdS forms
124
Practice Problem
Identify the precipitate that forms when aqueous solutions of zinc nitrate and ammonium sulfide are combined Write the equation for the possible double-replacement reaction Then write the formula equation overall ionic equation and the net ionic equation
125
1 AnalyzeGiven Identity of reactants zinc nitrate and ammonium sulfide Reaction medium aqueous solution Unknown a equation for the possible double-replacement reaction b identity of the precipitate c formula equation d overall ionic equation e net ionic equation
126
2 PlanWrite the possible double-replacement reaction between Zn(NO3)2 and (NH4)2S Use the rules to determine if any of the products will precipitate Write a formula equation and overall net ionic equation Then cancel spectator ions to make net ionic equation
127
3 Computea possible double-replacement reaction
b rules show that zinc sulfide is not soluble so will precipitate (ammonium nitrate is soluble)
c formula equation128
d overall ionic equation
e net ionic equation129
Will a precipitate form if solutions of potassium sulfate and barium nitrate are combined If so write the net ionic equation for the reaction
Yes Ba+2(aq) + SO4-2(aq) rarr BaSO4(s)
130
Will a precipitate form if solutions of potassium nitrate and magnesium sulfate are combined If so write the net ionic equation for the reaction
No 131
Will a precipitate form if solutions of barium chloride and sodium sulfate are combined If so identify the spectator ions and write the net ionic equation for the reaction
Yes Na+ and Cl- Ba+2(aq) + SO4-2(aq) rarr BaSO4(s)
132
Write the net ionic equation for the precipitation of nickel(II) sulfide
Ni+2(aq) + S-2(aq) rarr NiS(s)
133
IonizationSome molecular compounds can also form ions in solution Usually polar Ionization - ions formed from solute molecules by action of the solvent (creation of ions where there were none) Ionization different from dissociation
134
Dissociation ionic compounds dissolve and ions already present separate Ionization ions formed where non existed before
Ions formed are hydrated Heat released during hydration of ions gives enough energy to break covalent bonds
135
Extent to which solute ionizes depends on strength of bonds within molecules of solute and strength of attraction between solute and solvent molecules
136
HCl ndash molecular compound that ionizes in aqueous solution Contains highly polar bond Attraction between polar HCl molecule and polar water strong enough to break HCl bond
137
The Hydronium Ion
H+ ions from HCl attracts other molecules or ions so strongly that it doesnrsquot normally exist alone Ionization of HCl better described as direct transfer of proton from HCl to H2O forming H3O+
138
139
Hydration of H+ to form hydronium is highly exothermic
Energy released gives a lot of energy needed to ionize a molecular solute
Many molecular compounds that ionize in aqueous solution contain hydrogen and form hydronium ion
140
Strong and Weak Electrolytes
Substances that make ions and conduct current in solution are electrolytes HCl is one of series of compounds that has hydrogen and halogen Hydrogen halides are all molecular compounds with single polar-covalent bonds
141
All are gases Very soluble in water All are electrolytes HCl HBr HI strongly conduct current in solution HF only weakly conducts current at same concentration Strength of conduction related to ability to ionize
142
143
Strong Electrolytes
HCl HBr HI are 100 ionized in solution
Strong electrolyte - any compound whose dilute aqueous solutions conduct electricity well this is due to the presence of all or almost all of the dissolve compound in the form of ions
144
Unique characteristic of strong electrolyte is that they yield only ions
Ex Ionic compound may be highly soluble in water and dissociate into ions (NaCl)
Others may not dissolve much but amount that does dissolve exists only as ions
145
Weak ElectrolytesHF dissolves in water to give acid solution called hydrofluoric acid HF bond stronger than bonds between hydrogen and other halogens When HF dissolves some molecule ionize Reverse reaction also happens HF(aq) + H2O(l) H3O+(aq) + F-(aq)
146
HF(aq) + H2O(l) H3O+
(aq) + F-(aq)Concentration of dissolved unionized HF stays high and concentration of ions stays low
Weak electrolyte - any compound whose aqueous solutions conduct electricity poorly this is due to the presence of small amount of the dissolved compound in the form of ions
147
Different from nonelectrolyte ndash NO ions at all Description of electrolyte as strong or weak not related to concentration of solution Electrolytes differ in degree of ionization not amount of solute dissolved
148
149
Colligative properties of solutions
Presence of solute affects properties of solutions Some properties not dependent on nature of dissolved substance but on how may dissolve particles are present
Colligative properties - properties that depend on the concentration of solute particles but not on their identity
150
Vapor-Pressure Lowering
Boiling and freezing point of solution different from pure solvent Nonvolatile solute raises boiling point and lowers freezing point Nonvolatile substance - one that has little tendency to become gas under existing conditions
151
152
To understand why nonvolatile solute changes boiling and freezing point consider equilibrium vapor pressure
Vapor pressure pressure caused by molecules that have escape liquid phase to gas phase
Can be thought of a measure of tendency of molecules to escape from a liquid
153
Addition of sucrose (nonvolatile solute) lowers concentration of water molecules at surface of liquid This lowers tendency of water molecules to leave solution and enter gas phase Vapor pressure of solution is lower than vapor pressure of pure water
154
Aqueous13 solution13 of13 nonvolatile13 solutePure13 water
Nonelectrolyte solutions of same molality have same concentration of particles
Dilute solutions of same solvent and equal molality of any nonelectrolyte solute lower vapor pressure equally
155
Example
1 m aqueous solution of nonelectrolyte glucose lowers vapor pressure of water 55 x 10-4 atm at 25 1 m aqueous solution of sucrose also lowers water vapor pressure to 55 x 10-4 atm at 25
156
157
bc vapor-pressure lowering depends on concentration of nonelectrolyte solute and doesnrsquot depend on the type of solute it is a colligative proprerty
bc vapor pressure has been lowered solution remains liquid over larger temperature range
Lowers freezing point and raises boiling point
158
Can assume changes in boiling and freezing point also depend on concentration of solute
They are colligative properties
159
Freezing-Point Depression
Freezing point of 1-molal solution of any nonelectrolyte solute in water is found (by experiment) to be 186 lower than freezing point of water When 1 mol of nonelectrolyte solute dissolved in 1 kg water freezing point is -186 not 000 160
When 2 mol nonelectrolyte solute dissolved in 1 kg water freezing point is -372
For any concentration of nonelectrolyte solute in water decrease in freezing point can be determined using value of -186m
161
This value called the molal freezing-point contstant (Kf) is the freezing-point depression of the solvent in a 1-molal solution of a nonvolatile nonelectrolyte solute
162
Each solvent has own characteristic molal freezing-point constant Freezing-point depression (Δtf) the difference between the freezing points of the pure solvent and a solute of nonelectrolyte in that solvent and it is directly proportional to the molal concentration of the solution Δtf = Kfm
163
Δtf = Kfm
Kf expressed as m
m expressed in mol solutekg solvent (molality)
164
Sample Problem
What is the freezing point depression of water in a solution of 171 g of sucrose C12H22O11 and 200 g of water What is the actual freezing point of the solution
165
1 AnalyzeGiven Solute mass and chemical formula 171 g C12H22O11 Solvent mass and identity 2000 g water Unknown a freezing-point depression b freezing point of the solution 166
2 Plan
167
168
Calculate
Unknown a freezing-point depression b freezing point of the solution
169
Practice Problem
A water solution containing an unknown quantity of a nonelectrolyte solute is found to have a freezing point of ndash 023degCWhat is the molal concentration of the solution
012 m170
A solution consists of 103 g of the nonelectrolyte glucose C6H12O6 dissolved in 250 g of water What is the freezing-point depression of the solution
minus0426degC171
In a laboratory experiment the freezing point of an aqueous solution of glucose is found to be minus0325degCWhat is the molal concentration of this solution
0175 m172
If 0500 mol of a nonelectrolyte solute are dissolved in 5000 g of ether what is the freezing point of the solution
minus1181degC173
The freezing point of an aqueous solution that contains a nonelectrolyte is minus90degC aWhat is the freezing-point depression of the solution bWhat is the molal concentration of the solution a minus90degC b 48 m
174
Boiling-Point Elevation
Boiling point is temp at which vapor pressure is equal to atmospheric pressure Change in vapor pressure will cause corresponding change in boiling point
175
176
Vapor pressure of nonvolatile solution is lower than vapor pressure of pure solvent More heat will be required to raise vapor pressure of solution Boiling point of solution is higher than boiling point of pure solvent
Molal boiling-point constant (Kb) - boiling-point elevation of the solvent in a 1-molal solution of a nonvolatile nonelectrolyte solute b-p elevation of any nonelectrolyte in water found by experiment to be 051 So molal b-p constant for water is 051m
177
For different solvents b-p elevations of 1-molal solutions have different values (Table 14-2)
Like freezing-point constants values most accurate for dilute solutions
178
Boiling-point elevation Δtb difference between the boiling points of the pure solvent and a nonelectrolyte solution of that solvent and it is directly proportional to the molal concentration of the solution B-p elevation can be calculated using Δtb = Kbm Δtb (boiling point elevation) expressed in m and m expressed in mol solutekg solvent
Sample Problem
What is the boiling-point elevation of a solution made from 200 g of a nonelectrolyte solute and 4000 g of water The molar mass of the solute is 620 gmol
180
1 AnalyzeGiven solute mass = 200 g solute molar mass = 620 gmol solvent mass and identity = 4000 g of water
Unknown boiling-point elevation
181
2 Plan
182
183
3 Compute
Practice Problem
A solution contains 500 g of sucrose C12H22O11 a nonelectrolyte dissolved in 5000 g of water What is the boiling-point elevation
015degC184
A solution contains 4500 g of sucrose C12H22O11 a nonelectrolyte dissolved in 250 g of waterWhat is the boiling point of the solution
1027degC185
If the boiling point elevation of an aqueous solution containing a nonvolatile electrolyte is 102degC what is the molality of the solution
20 m186
The boiling point of an aqueous solution containing a nonvolatile electrolyte is 10075degC aWhat is the boiling-point elevation bWhat is the molality of the solution a 075degC b 15 m
187
Electrolytes and Colligative PropertiesEarly investigators confused by experiments where certain substances depressed freezing point or elevated boiling point more than expected
188
189
Ex 01 m solution of NaCl lowers freezing point of solvent almost twice as much as 01 m solution of sucrose 01 m CaCl2 lowers fp almost 3 times as much as 01 m solution of sucrose
To understand why this happens contrast behavior or sucrose with NaCl in aqueous solution Each sucrose dissolves to make only 1 particle in solution So 1 mol sucrose dissolves to 1 mol particles 1 mol NaCl dissolves to make 2 moles of particles (1 mol Na+ and 1 mol Cl-)
190
191
Calculated Values for Electrolyte SolutionsColligative properties depend on total concentration of solute particles regardless of their identity Electrolytes cause changes in colligative properties proportional to total molality in terms of all dissolves particles instead of formula units
192
What about barium nitrate Ba(NO3)2 Each mole of barium nitrate yields 3 mol of ions in solution Ba(NO3)2(s) rarr Ba2+(aq) + 2NO3-(aq) You would expect solution of given molality to lower fp of solvent 3 times as much as nonelectrolytic solution of same molality
193
Sample Problem
What is the expected change in the freezing point of water in a solution of 625 g of barium nitrate Ba(NO3)2 in 100 kg of water
194
1 AnalyzeGiven solute mass and formula = 625 g Ba(NO3)2 solvent mass and identity = 100 kg water Δtf = Kfm
Unknown expected freezing-point depression
195
2 PlanThe molality can be calculated by converting the solute mass to moles and then dividing by the number of kilograms of solvent That molality is in terms of formula units of Ba(NO3)2 and must be converted to molality in terms of dissociated ions in solution It must be multiplied by the number of moles of ions produced per mole of formula unit This adjusted molality can then be used to calculate the freezing-point depression
196
197
198
3 Compute
Practice Problem
What is the expected freezing-point depression for a solution that contains 20 mol of magnesium sulfate dissolved in 10 kg of water
minus74degC199
What is the expected boiling-point elevation of water for a solution that contains 150 g of sodium chloride dissolved in 10 kg of water 27degC The freezing point of an aqueous sodium chloride solution is minus200degC What is the molality of the solution 54 m NaCl
200
Actual Values for Electrolyte SolutionsRemember values calculated are only expected values Actual values of colligative properties for all strong electrolytes are almost what would be expected Difference between expected and actual values caused by attractive forces that exist between dissociated ions in aqueous solution
201
Attraction between hydrated ions in solution is small compared with those in crystalline solid However forces of attraction donrsquot interfere with movements of aqueous ions More concentrated solution means ions closer together and attraction is greater Also depends on charge of ions ndash higher charge greater attraction
202
Step 1 solute particles become separated from solid (energy absorbed) Step 2 solvent particles move apart to allow solute particles to enter liquid (energy absorbed)
76
Step 3 solvent particles attracted to and solvate solute particles (energy released)
77
Heat of solution - net amount of heat energy absorbed or released when a specific amount of solute dissolves in a solvent
Heat of solution negative (heat is released) if sums of energy in steps 1 and 2 is less than step 3
If 1+2 gt 3 HOS is positive (heat absorbed)78
In gaseous state molecules are so far apart there are nearly no forces between them Solute-solute interaction has little effect on heat of solution of a gas Energy released when gas dissolved in liquid bc attraction between solute gas and solvent molecules greater than energy needed to separate solvent molecules
79
Concentration of Solutions80
Concentration - measure of amount of solute in given amount of solvent or solution ldquodiluterdquo just means small amount of solute ldquoconcentratedrdquo just means relatively large amount of solute Unrelated to degree to which solution is saturated Ex Saturated solution of substance not very soluble might be dilute
81
Molarity
Molarity - number of moles of solute in one liter of solution To find molarity must know molar mass A ldquoone molarrdquo solution of NaOH is 1 mole NaOH per liter of solution (1 M)
82
1 mol NaOH = 400 g If this quantity dissolve in enough water to make EXACTLY 100 L solution solution is 1 M If 200 g NaOH dissolve in enough to make 100 L solution what is molarity 0500 M
83
1 M solution is not made by adding 1 mol solute to 1 L solvent That would be more than 1 L Instead 1 mol solute dissolved in less than 1 L Then diluted with more solvent to bring TOTAL VOLUME to 1 L
84
Practice ProblemYou have 340 L of solution that contains 900 g sodium chloride What is the molarity of that solution 0440 M NaCl You have 08 L of a 05 M HCl solution How many moles of HCl does this solution contain 04 mol HCl
85
What is the molarity of a solution composed of 585 g of potassium iodide dissolved in enough water to make a 0125 L of solution 0282 M KI
86
How many moles of H2SO4 are present in 0500 L of a 0150 M H2SO4 solution 00750 mol What volume of 300 M NaCl is needed for reaction that requires 1463 g of NaCl 0834 L
87
Molality
Molality - concentration of solution expressed in moles of solute per kilogram of solvent Solution that contains 1 mol solute (NaOH) dissolved in exactly 1 kg of solvent is ldquoone molalrdquo solution (1 m NaOH)
88
1 mol NaOH = 400 g 400 g NaOH dissolved in 1 kg water is 1 m NaOH
200 g NaOH in 1 kg water = 0500 m NaOH
89
Practice Problem
A solution was prepared by dissolving 171 g sucrose (C12H22O11) in 125 g of water Find the molal concentration of this solution 0400 m C12H22O11
90
A solution of iodine in carbon tetrachloride is used when iodine is needed for certain chemical tests How much iodine must be added to prepare a 0480 m solution of iodine in CCl4 if 1000 g of CCl4 is used 122 g I2
91
What is the molality of a solution composed of 255 g acetone (CH3)2CO dissolved in 200 g of water 22 m acetone What quantity in grams of methanol CH3OH is required to prepare a 0244 m solution in 400 g water 312 g methanol
92
How many grams of AgNO3 are needed to prepare a 0125 m solution in 250 mL of water 531 g AgNO3
What is the molality of a solution containing 182 g HCl and 250 g water 200 m
93
Ions in Aqueous Solutions and Colligative Properties
94
95
compounds in aqueous solutions
DissociationWhen compound made from ions dissolves in water ions separate Dissociation - separation of ions that occurs when an ionic compound dissolves
96
Notice number of ions made per formula unit in equations 1 formula unit of NaCl gives 2 units of ions in solution 1 formula unit of CaCl2 gives 3 units of ions
97
Assuming 100 dissociation solution that contains 1 mol NaCl contains 1 mol Na+ and 1 mol Cl- Can assume 100 dissociation of all soluble ionic compounds
98
Practice Problem
Write the equation for the dissolution of aluminum sulfate in water How many moles of aluminum ions are made by dissolving 1 mol aluminum sulfate What is the total number of moles of ions made by dissolving 1 mol of aluminum sulfate
99
1 AnalyzeGiven Amount of solute = 1 mol Al2(SO4)3 Solvent identity = water Unknown a moles of aluminum ions and sulfate ions Total number of moles of solute ions produced
100
2 Plan
The coefficients in the balanced dissociation equation will tell mole relationships You can use the equation to find out the number of moles of solute ions produced
101
3 Compute
a
b102
Write the equation for the dissolution of each of the following in water and then determine the number of moles of each ions made as well as the total number of ions made a 1 mol ammonium chloride b 1 mol sodium sulfide c 05 mol barium nitrate
103
a 1 mol ammonium chloride
1 mol NH4+ 1 mol Cl- 2 mol total ions
104
b 1 mol sodium sulfide
2 mol Na+ 1 mol S-2 3 mol total ions
105
c 05 mol barium nitrate
05 mol Ba+2 1 mol NO3- 15 mol total ions
106
Precipitation Reactions
Even though no compound is completely insoluble compounds of very low solubility can be considered insoluble for practical purposes There are general guidelines to help predict whether a compound made of certain combination of ions is soluble
107
General Solubility Guidelines
Sodium potassium and ammonium compounds are soluble in water
108
Nitrates acetates and chlorates are soluble
109
110
Most chlorides are soluble except those of silver mercury (I) and lead Lead (II) chloride is soluble in hot water
Most sulfates are soluble except those of barium strontium lead calcium and mercury
111
Most carbonates phosphates and silicates are insoluble except those of sodium potassium and ammonium
112
Most sulfides are insoluble except those of calcium strontium sodium potassium and ammonium
113
Is calcium phosphate Ca3(PO4)2 soluble or insoluble
Not one of the exceptions so it is insoluble
Dissociation equations are not written for insoluble compounds
114
Guidelines also useful in predicting what will happen if 2 solutions of 2 different soluble compounds are mixed
If mixing results in combination of ions that forms insoluble compound a double-replacement and precipitation reaction will happen
115
Will precipitate form when solutions of ammonium sulfide and cadmium nitrate are combined You can tell calcium nitrate is soluble from the guidelines (Most nitrates acetates and chlorates are soluble) You can also tell ammonium sulfide is soluble (Most sulfides are insoluble except those of calcium strontium sodium potassium and ammonium)
116
117
To decide whether precipitate can form must know solubilities of two possible products
Ammonium nitrate ndash soluble (Most nitrates acetates and chlorates are soluble) Cadmium sulfide ndash insoluble (Most sulfides are insoluble except those of calcium strontium sodium potassium and ammonium)
118
bc one product is insoluble double-replacement and precipitation reaction will happen (NH4)2S (aq) + Cd(NO3)2 (aq) rarr NH4NO3 (aq) + CdS (s)
119
Net Ionic Equations
Reactions of ions in aqueous solution usually represented by net ionic equations instead of formula equations Net ionic equation - includes only those compounds and ions that undergo a chemical change in a reaction in an aqueous solution
120
121
To write NIE (net ionic equation) first convert chemical equation into overall ionic equation All soluble ionic compounds shown as dissociated ions in solution Precipitates shown as solids
122
Notice ammonium and nitrate ions appear on both sides They havenrsquot gone through any chemical change Spectator ions - ions that do not take part in a chemical reaction and are found in solution both before and after the reaction
123
To change ionic equation to NIE eliminate spectator ions Cd+2(aq) + S-2(aq) rarr CdS(s)
This NIE applies to any reaction in which precipitate of CdS forms
124
Practice Problem
Identify the precipitate that forms when aqueous solutions of zinc nitrate and ammonium sulfide are combined Write the equation for the possible double-replacement reaction Then write the formula equation overall ionic equation and the net ionic equation
125
1 AnalyzeGiven Identity of reactants zinc nitrate and ammonium sulfide Reaction medium aqueous solution Unknown a equation for the possible double-replacement reaction b identity of the precipitate c formula equation d overall ionic equation e net ionic equation
126
2 PlanWrite the possible double-replacement reaction between Zn(NO3)2 and (NH4)2S Use the rules to determine if any of the products will precipitate Write a formula equation and overall net ionic equation Then cancel spectator ions to make net ionic equation
127
3 Computea possible double-replacement reaction
b rules show that zinc sulfide is not soluble so will precipitate (ammonium nitrate is soluble)
c formula equation128
d overall ionic equation
e net ionic equation129
Will a precipitate form if solutions of potassium sulfate and barium nitrate are combined If so write the net ionic equation for the reaction
Yes Ba+2(aq) + SO4-2(aq) rarr BaSO4(s)
130
Will a precipitate form if solutions of potassium nitrate and magnesium sulfate are combined If so write the net ionic equation for the reaction
No 131
Will a precipitate form if solutions of barium chloride and sodium sulfate are combined If so identify the spectator ions and write the net ionic equation for the reaction
Yes Na+ and Cl- Ba+2(aq) + SO4-2(aq) rarr BaSO4(s)
132
Write the net ionic equation for the precipitation of nickel(II) sulfide
Ni+2(aq) + S-2(aq) rarr NiS(s)
133
IonizationSome molecular compounds can also form ions in solution Usually polar Ionization - ions formed from solute molecules by action of the solvent (creation of ions where there were none) Ionization different from dissociation
134
Dissociation ionic compounds dissolve and ions already present separate Ionization ions formed where non existed before
Ions formed are hydrated Heat released during hydration of ions gives enough energy to break covalent bonds
135
Extent to which solute ionizes depends on strength of bonds within molecules of solute and strength of attraction between solute and solvent molecules
136
HCl ndash molecular compound that ionizes in aqueous solution Contains highly polar bond Attraction between polar HCl molecule and polar water strong enough to break HCl bond
137
The Hydronium Ion
H+ ions from HCl attracts other molecules or ions so strongly that it doesnrsquot normally exist alone Ionization of HCl better described as direct transfer of proton from HCl to H2O forming H3O+
138
139
Hydration of H+ to form hydronium is highly exothermic
Energy released gives a lot of energy needed to ionize a molecular solute
Many molecular compounds that ionize in aqueous solution contain hydrogen and form hydronium ion
140
Strong and Weak Electrolytes
Substances that make ions and conduct current in solution are electrolytes HCl is one of series of compounds that has hydrogen and halogen Hydrogen halides are all molecular compounds with single polar-covalent bonds
141
All are gases Very soluble in water All are electrolytes HCl HBr HI strongly conduct current in solution HF only weakly conducts current at same concentration Strength of conduction related to ability to ionize
142
143
Strong Electrolytes
HCl HBr HI are 100 ionized in solution
Strong electrolyte - any compound whose dilute aqueous solutions conduct electricity well this is due to the presence of all or almost all of the dissolve compound in the form of ions
144
Unique characteristic of strong electrolyte is that they yield only ions
Ex Ionic compound may be highly soluble in water and dissociate into ions (NaCl)
Others may not dissolve much but amount that does dissolve exists only as ions
145
Weak ElectrolytesHF dissolves in water to give acid solution called hydrofluoric acid HF bond stronger than bonds between hydrogen and other halogens When HF dissolves some molecule ionize Reverse reaction also happens HF(aq) + H2O(l) H3O+(aq) + F-(aq)
146
HF(aq) + H2O(l) H3O+
(aq) + F-(aq)Concentration of dissolved unionized HF stays high and concentration of ions stays low
Weak electrolyte - any compound whose aqueous solutions conduct electricity poorly this is due to the presence of small amount of the dissolved compound in the form of ions
147
Different from nonelectrolyte ndash NO ions at all Description of electrolyte as strong or weak not related to concentration of solution Electrolytes differ in degree of ionization not amount of solute dissolved
148
149
Colligative properties of solutions
Presence of solute affects properties of solutions Some properties not dependent on nature of dissolved substance but on how may dissolve particles are present
Colligative properties - properties that depend on the concentration of solute particles but not on their identity
150
Vapor-Pressure Lowering
Boiling and freezing point of solution different from pure solvent Nonvolatile solute raises boiling point and lowers freezing point Nonvolatile substance - one that has little tendency to become gas under existing conditions
151
152
To understand why nonvolatile solute changes boiling and freezing point consider equilibrium vapor pressure
Vapor pressure pressure caused by molecules that have escape liquid phase to gas phase
Can be thought of a measure of tendency of molecules to escape from a liquid
153
Addition of sucrose (nonvolatile solute) lowers concentration of water molecules at surface of liquid This lowers tendency of water molecules to leave solution and enter gas phase Vapor pressure of solution is lower than vapor pressure of pure water
154
Aqueous13 solution13 of13 nonvolatile13 solutePure13 water
Nonelectrolyte solutions of same molality have same concentration of particles
Dilute solutions of same solvent and equal molality of any nonelectrolyte solute lower vapor pressure equally
155
Example
1 m aqueous solution of nonelectrolyte glucose lowers vapor pressure of water 55 x 10-4 atm at 25 1 m aqueous solution of sucrose also lowers water vapor pressure to 55 x 10-4 atm at 25
156
157
bc vapor-pressure lowering depends on concentration of nonelectrolyte solute and doesnrsquot depend on the type of solute it is a colligative proprerty
bc vapor pressure has been lowered solution remains liquid over larger temperature range
Lowers freezing point and raises boiling point
158
Can assume changes in boiling and freezing point also depend on concentration of solute
They are colligative properties
159
Freezing-Point Depression
Freezing point of 1-molal solution of any nonelectrolyte solute in water is found (by experiment) to be 186 lower than freezing point of water When 1 mol of nonelectrolyte solute dissolved in 1 kg water freezing point is -186 not 000 160
When 2 mol nonelectrolyte solute dissolved in 1 kg water freezing point is -372
For any concentration of nonelectrolyte solute in water decrease in freezing point can be determined using value of -186m
161
This value called the molal freezing-point contstant (Kf) is the freezing-point depression of the solvent in a 1-molal solution of a nonvolatile nonelectrolyte solute
162
Each solvent has own characteristic molal freezing-point constant Freezing-point depression (Δtf) the difference between the freezing points of the pure solvent and a solute of nonelectrolyte in that solvent and it is directly proportional to the molal concentration of the solution Δtf = Kfm
163
Δtf = Kfm
Kf expressed as m
m expressed in mol solutekg solvent (molality)
164
Sample Problem
What is the freezing point depression of water in a solution of 171 g of sucrose C12H22O11 and 200 g of water What is the actual freezing point of the solution
165
1 AnalyzeGiven Solute mass and chemical formula 171 g C12H22O11 Solvent mass and identity 2000 g water Unknown a freezing-point depression b freezing point of the solution 166
2 Plan
167
168
Calculate
Unknown a freezing-point depression b freezing point of the solution
169
Practice Problem
A water solution containing an unknown quantity of a nonelectrolyte solute is found to have a freezing point of ndash 023degCWhat is the molal concentration of the solution
012 m170
A solution consists of 103 g of the nonelectrolyte glucose C6H12O6 dissolved in 250 g of water What is the freezing-point depression of the solution
minus0426degC171
In a laboratory experiment the freezing point of an aqueous solution of glucose is found to be minus0325degCWhat is the molal concentration of this solution
0175 m172
If 0500 mol of a nonelectrolyte solute are dissolved in 5000 g of ether what is the freezing point of the solution
minus1181degC173
The freezing point of an aqueous solution that contains a nonelectrolyte is minus90degC aWhat is the freezing-point depression of the solution bWhat is the molal concentration of the solution a minus90degC b 48 m
174
Boiling-Point Elevation
Boiling point is temp at which vapor pressure is equal to atmospheric pressure Change in vapor pressure will cause corresponding change in boiling point
175
176
Vapor pressure of nonvolatile solution is lower than vapor pressure of pure solvent More heat will be required to raise vapor pressure of solution Boiling point of solution is higher than boiling point of pure solvent
Molal boiling-point constant (Kb) - boiling-point elevation of the solvent in a 1-molal solution of a nonvolatile nonelectrolyte solute b-p elevation of any nonelectrolyte in water found by experiment to be 051 So molal b-p constant for water is 051m
177
For different solvents b-p elevations of 1-molal solutions have different values (Table 14-2)
Like freezing-point constants values most accurate for dilute solutions
178
Boiling-point elevation Δtb difference between the boiling points of the pure solvent and a nonelectrolyte solution of that solvent and it is directly proportional to the molal concentration of the solution B-p elevation can be calculated using Δtb = Kbm Δtb (boiling point elevation) expressed in m and m expressed in mol solutekg solvent
Sample Problem
What is the boiling-point elevation of a solution made from 200 g of a nonelectrolyte solute and 4000 g of water The molar mass of the solute is 620 gmol
180
1 AnalyzeGiven solute mass = 200 g solute molar mass = 620 gmol solvent mass and identity = 4000 g of water
Unknown boiling-point elevation
181
2 Plan
182
183
3 Compute
Practice Problem
A solution contains 500 g of sucrose C12H22O11 a nonelectrolyte dissolved in 5000 g of water What is the boiling-point elevation
015degC184
A solution contains 4500 g of sucrose C12H22O11 a nonelectrolyte dissolved in 250 g of waterWhat is the boiling point of the solution
1027degC185
If the boiling point elevation of an aqueous solution containing a nonvolatile electrolyte is 102degC what is the molality of the solution
20 m186
The boiling point of an aqueous solution containing a nonvolatile electrolyte is 10075degC aWhat is the boiling-point elevation bWhat is the molality of the solution a 075degC b 15 m
187
Electrolytes and Colligative PropertiesEarly investigators confused by experiments where certain substances depressed freezing point or elevated boiling point more than expected
188
189
Ex 01 m solution of NaCl lowers freezing point of solvent almost twice as much as 01 m solution of sucrose 01 m CaCl2 lowers fp almost 3 times as much as 01 m solution of sucrose
To understand why this happens contrast behavior or sucrose with NaCl in aqueous solution Each sucrose dissolves to make only 1 particle in solution So 1 mol sucrose dissolves to 1 mol particles 1 mol NaCl dissolves to make 2 moles of particles (1 mol Na+ and 1 mol Cl-)
190
191
Calculated Values for Electrolyte SolutionsColligative properties depend on total concentration of solute particles regardless of their identity Electrolytes cause changes in colligative properties proportional to total molality in terms of all dissolves particles instead of formula units
192
What about barium nitrate Ba(NO3)2 Each mole of barium nitrate yields 3 mol of ions in solution Ba(NO3)2(s) rarr Ba2+(aq) + 2NO3-(aq) You would expect solution of given molality to lower fp of solvent 3 times as much as nonelectrolytic solution of same molality
193
Sample Problem
What is the expected change in the freezing point of water in a solution of 625 g of barium nitrate Ba(NO3)2 in 100 kg of water
194
1 AnalyzeGiven solute mass and formula = 625 g Ba(NO3)2 solvent mass and identity = 100 kg water Δtf = Kfm
Unknown expected freezing-point depression
195
2 PlanThe molality can be calculated by converting the solute mass to moles and then dividing by the number of kilograms of solvent That molality is in terms of formula units of Ba(NO3)2 and must be converted to molality in terms of dissociated ions in solution It must be multiplied by the number of moles of ions produced per mole of formula unit This adjusted molality can then be used to calculate the freezing-point depression
196
197
198
3 Compute
Practice Problem
What is the expected freezing-point depression for a solution that contains 20 mol of magnesium sulfate dissolved in 10 kg of water
minus74degC199
What is the expected boiling-point elevation of water for a solution that contains 150 g of sodium chloride dissolved in 10 kg of water 27degC The freezing point of an aqueous sodium chloride solution is minus200degC What is the molality of the solution 54 m NaCl
200
Actual Values for Electrolyte SolutionsRemember values calculated are only expected values Actual values of colligative properties for all strong electrolytes are almost what would be expected Difference between expected and actual values caused by attractive forces that exist between dissociated ions in aqueous solution
201
Attraction between hydrated ions in solution is small compared with those in crystalline solid However forces of attraction donrsquot interfere with movements of aqueous ions More concentrated solution means ions closer together and attraction is greater Also depends on charge of ions ndash higher charge greater attraction
202
Step 3 solvent particles attracted to and solvate solute particles (energy released)
77
Heat of solution - net amount of heat energy absorbed or released when a specific amount of solute dissolves in a solvent
Heat of solution negative (heat is released) if sums of energy in steps 1 and 2 is less than step 3
If 1+2 gt 3 HOS is positive (heat absorbed)78
In gaseous state molecules are so far apart there are nearly no forces between them Solute-solute interaction has little effect on heat of solution of a gas Energy released when gas dissolved in liquid bc attraction between solute gas and solvent molecules greater than energy needed to separate solvent molecules
79
Concentration of Solutions80
Concentration - measure of amount of solute in given amount of solvent or solution ldquodiluterdquo just means small amount of solute ldquoconcentratedrdquo just means relatively large amount of solute Unrelated to degree to which solution is saturated Ex Saturated solution of substance not very soluble might be dilute
81
Molarity
Molarity - number of moles of solute in one liter of solution To find molarity must know molar mass A ldquoone molarrdquo solution of NaOH is 1 mole NaOH per liter of solution (1 M)
82
1 mol NaOH = 400 g If this quantity dissolve in enough water to make EXACTLY 100 L solution solution is 1 M If 200 g NaOH dissolve in enough to make 100 L solution what is molarity 0500 M
83
1 M solution is not made by adding 1 mol solute to 1 L solvent That would be more than 1 L Instead 1 mol solute dissolved in less than 1 L Then diluted with more solvent to bring TOTAL VOLUME to 1 L
84
Practice ProblemYou have 340 L of solution that contains 900 g sodium chloride What is the molarity of that solution 0440 M NaCl You have 08 L of a 05 M HCl solution How many moles of HCl does this solution contain 04 mol HCl
85
What is the molarity of a solution composed of 585 g of potassium iodide dissolved in enough water to make a 0125 L of solution 0282 M KI
86
How many moles of H2SO4 are present in 0500 L of a 0150 M H2SO4 solution 00750 mol What volume of 300 M NaCl is needed for reaction that requires 1463 g of NaCl 0834 L
87
Molality
Molality - concentration of solution expressed in moles of solute per kilogram of solvent Solution that contains 1 mol solute (NaOH) dissolved in exactly 1 kg of solvent is ldquoone molalrdquo solution (1 m NaOH)
88
1 mol NaOH = 400 g 400 g NaOH dissolved in 1 kg water is 1 m NaOH
200 g NaOH in 1 kg water = 0500 m NaOH
89
Practice Problem
A solution was prepared by dissolving 171 g sucrose (C12H22O11) in 125 g of water Find the molal concentration of this solution 0400 m C12H22O11
90
A solution of iodine in carbon tetrachloride is used when iodine is needed for certain chemical tests How much iodine must be added to prepare a 0480 m solution of iodine in CCl4 if 1000 g of CCl4 is used 122 g I2
91
What is the molality of a solution composed of 255 g acetone (CH3)2CO dissolved in 200 g of water 22 m acetone What quantity in grams of methanol CH3OH is required to prepare a 0244 m solution in 400 g water 312 g methanol
92
How many grams of AgNO3 are needed to prepare a 0125 m solution in 250 mL of water 531 g AgNO3
What is the molality of a solution containing 182 g HCl and 250 g water 200 m
93
Ions in Aqueous Solutions and Colligative Properties
94
95
compounds in aqueous solutions
DissociationWhen compound made from ions dissolves in water ions separate Dissociation - separation of ions that occurs when an ionic compound dissolves
96
Notice number of ions made per formula unit in equations 1 formula unit of NaCl gives 2 units of ions in solution 1 formula unit of CaCl2 gives 3 units of ions
97
Assuming 100 dissociation solution that contains 1 mol NaCl contains 1 mol Na+ and 1 mol Cl- Can assume 100 dissociation of all soluble ionic compounds
98
Practice Problem
Write the equation for the dissolution of aluminum sulfate in water How many moles of aluminum ions are made by dissolving 1 mol aluminum sulfate What is the total number of moles of ions made by dissolving 1 mol of aluminum sulfate
99
1 AnalyzeGiven Amount of solute = 1 mol Al2(SO4)3 Solvent identity = water Unknown a moles of aluminum ions and sulfate ions Total number of moles of solute ions produced
100
2 Plan
The coefficients in the balanced dissociation equation will tell mole relationships You can use the equation to find out the number of moles of solute ions produced
101
3 Compute
a
b102
Write the equation for the dissolution of each of the following in water and then determine the number of moles of each ions made as well as the total number of ions made a 1 mol ammonium chloride b 1 mol sodium sulfide c 05 mol barium nitrate
103
a 1 mol ammonium chloride
1 mol NH4+ 1 mol Cl- 2 mol total ions
104
b 1 mol sodium sulfide
2 mol Na+ 1 mol S-2 3 mol total ions
105
c 05 mol barium nitrate
05 mol Ba+2 1 mol NO3- 15 mol total ions
106
Precipitation Reactions
Even though no compound is completely insoluble compounds of very low solubility can be considered insoluble for practical purposes There are general guidelines to help predict whether a compound made of certain combination of ions is soluble
107
General Solubility Guidelines
Sodium potassium and ammonium compounds are soluble in water
108
Nitrates acetates and chlorates are soluble
109
110
Most chlorides are soluble except those of silver mercury (I) and lead Lead (II) chloride is soluble in hot water
Most sulfates are soluble except those of barium strontium lead calcium and mercury
111
Most carbonates phosphates and silicates are insoluble except those of sodium potassium and ammonium
112
Most sulfides are insoluble except those of calcium strontium sodium potassium and ammonium
113
Is calcium phosphate Ca3(PO4)2 soluble or insoluble
Not one of the exceptions so it is insoluble
Dissociation equations are not written for insoluble compounds
114
Guidelines also useful in predicting what will happen if 2 solutions of 2 different soluble compounds are mixed
If mixing results in combination of ions that forms insoluble compound a double-replacement and precipitation reaction will happen
115
Will precipitate form when solutions of ammonium sulfide and cadmium nitrate are combined You can tell calcium nitrate is soluble from the guidelines (Most nitrates acetates and chlorates are soluble) You can also tell ammonium sulfide is soluble (Most sulfides are insoluble except those of calcium strontium sodium potassium and ammonium)
116
117
To decide whether precipitate can form must know solubilities of two possible products
Ammonium nitrate ndash soluble (Most nitrates acetates and chlorates are soluble) Cadmium sulfide ndash insoluble (Most sulfides are insoluble except those of calcium strontium sodium potassium and ammonium)
118
bc one product is insoluble double-replacement and precipitation reaction will happen (NH4)2S (aq) + Cd(NO3)2 (aq) rarr NH4NO3 (aq) + CdS (s)
119
Net Ionic Equations
Reactions of ions in aqueous solution usually represented by net ionic equations instead of formula equations Net ionic equation - includes only those compounds and ions that undergo a chemical change in a reaction in an aqueous solution
120
121
To write NIE (net ionic equation) first convert chemical equation into overall ionic equation All soluble ionic compounds shown as dissociated ions in solution Precipitates shown as solids
122
Notice ammonium and nitrate ions appear on both sides They havenrsquot gone through any chemical change Spectator ions - ions that do not take part in a chemical reaction and are found in solution both before and after the reaction
123
To change ionic equation to NIE eliminate spectator ions Cd+2(aq) + S-2(aq) rarr CdS(s)
This NIE applies to any reaction in which precipitate of CdS forms
124
Practice Problem
Identify the precipitate that forms when aqueous solutions of zinc nitrate and ammonium sulfide are combined Write the equation for the possible double-replacement reaction Then write the formula equation overall ionic equation and the net ionic equation
125
1 AnalyzeGiven Identity of reactants zinc nitrate and ammonium sulfide Reaction medium aqueous solution Unknown a equation for the possible double-replacement reaction b identity of the precipitate c formula equation d overall ionic equation e net ionic equation
126
2 PlanWrite the possible double-replacement reaction between Zn(NO3)2 and (NH4)2S Use the rules to determine if any of the products will precipitate Write a formula equation and overall net ionic equation Then cancel spectator ions to make net ionic equation
127
3 Computea possible double-replacement reaction
b rules show that zinc sulfide is not soluble so will precipitate (ammonium nitrate is soluble)
c formula equation128
d overall ionic equation
e net ionic equation129
Will a precipitate form if solutions of potassium sulfate and barium nitrate are combined If so write the net ionic equation for the reaction
Yes Ba+2(aq) + SO4-2(aq) rarr BaSO4(s)
130
Will a precipitate form if solutions of potassium nitrate and magnesium sulfate are combined If so write the net ionic equation for the reaction
No 131
Will a precipitate form if solutions of barium chloride and sodium sulfate are combined If so identify the spectator ions and write the net ionic equation for the reaction
Yes Na+ and Cl- Ba+2(aq) + SO4-2(aq) rarr BaSO4(s)
132
Write the net ionic equation for the precipitation of nickel(II) sulfide
Ni+2(aq) + S-2(aq) rarr NiS(s)
133
IonizationSome molecular compounds can also form ions in solution Usually polar Ionization - ions formed from solute molecules by action of the solvent (creation of ions where there were none) Ionization different from dissociation
134
Dissociation ionic compounds dissolve and ions already present separate Ionization ions formed where non existed before
Ions formed are hydrated Heat released during hydration of ions gives enough energy to break covalent bonds
135
Extent to which solute ionizes depends on strength of bonds within molecules of solute and strength of attraction between solute and solvent molecules
136
HCl ndash molecular compound that ionizes in aqueous solution Contains highly polar bond Attraction between polar HCl molecule and polar water strong enough to break HCl bond
137
The Hydronium Ion
H+ ions from HCl attracts other molecules or ions so strongly that it doesnrsquot normally exist alone Ionization of HCl better described as direct transfer of proton from HCl to H2O forming H3O+
138
139
Hydration of H+ to form hydronium is highly exothermic
Energy released gives a lot of energy needed to ionize a molecular solute
Many molecular compounds that ionize in aqueous solution contain hydrogen and form hydronium ion
140
Strong and Weak Electrolytes
Substances that make ions and conduct current in solution are electrolytes HCl is one of series of compounds that has hydrogen and halogen Hydrogen halides are all molecular compounds with single polar-covalent bonds
141
All are gases Very soluble in water All are electrolytes HCl HBr HI strongly conduct current in solution HF only weakly conducts current at same concentration Strength of conduction related to ability to ionize
142
143
Strong Electrolytes
HCl HBr HI are 100 ionized in solution
Strong electrolyte - any compound whose dilute aqueous solutions conduct electricity well this is due to the presence of all or almost all of the dissolve compound in the form of ions
144
Unique characteristic of strong electrolyte is that they yield only ions
Ex Ionic compound may be highly soluble in water and dissociate into ions (NaCl)
Others may not dissolve much but amount that does dissolve exists only as ions
145
Weak ElectrolytesHF dissolves in water to give acid solution called hydrofluoric acid HF bond stronger than bonds between hydrogen and other halogens When HF dissolves some molecule ionize Reverse reaction also happens HF(aq) + H2O(l) H3O+(aq) + F-(aq)
146
HF(aq) + H2O(l) H3O+
(aq) + F-(aq)Concentration of dissolved unionized HF stays high and concentration of ions stays low
Weak electrolyte - any compound whose aqueous solutions conduct electricity poorly this is due to the presence of small amount of the dissolved compound in the form of ions
147
Different from nonelectrolyte ndash NO ions at all Description of electrolyte as strong or weak not related to concentration of solution Electrolytes differ in degree of ionization not amount of solute dissolved
148
149
Colligative properties of solutions
Presence of solute affects properties of solutions Some properties not dependent on nature of dissolved substance but on how may dissolve particles are present
Colligative properties - properties that depend on the concentration of solute particles but not on their identity
150
Vapor-Pressure Lowering
Boiling and freezing point of solution different from pure solvent Nonvolatile solute raises boiling point and lowers freezing point Nonvolatile substance - one that has little tendency to become gas under existing conditions
151
152
To understand why nonvolatile solute changes boiling and freezing point consider equilibrium vapor pressure
Vapor pressure pressure caused by molecules that have escape liquid phase to gas phase
Can be thought of a measure of tendency of molecules to escape from a liquid
153
Addition of sucrose (nonvolatile solute) lowers concentration of water molecules at surface of liquid This lowers tendency of water molecules to leave solution and enter gas phase Vapor pressure of solution is lower than vapor pressure of pure water
154
Aqueous13 solution13 of13 nonvolatile13 solutePure13 water
Nonelectrolyte solutions of same molality have same concentration of particles
Dilute solutions of same solvent and equal molality of any nonelectrolyte solute lower vapor pressure equally
155
Example
1 m aqueous solution of nonelectrolyte glucose lowers vapor pressure of water 55 x 10-4 atm at 25 1 m aqueous solution of sucrose also lowers water vapor pressure to 55 x 10-4 atm at 25
156
157
bc vapor-pressure lowering depends on concentration of nonelectrolyte solute and doesnrsquot depend on the type of solute it is a colligative proprerty
bc vapor pressure has been lowered solution remains liquid over larger temperature range
Lowers freezing point and raises boiling point
158
Can assume changes in boiling and freezing point also depend on concentration of solute
They are colligative properties
159
Freezing-Point Depression
Freezing point of 1-molal solution of any nonelectrolyte solute in water is found (by experiment) to be 186 lower than freezing point of water When 1 mol of nonelectrolyte solute dissolved in 1 kg water freezing point is -186 not 000 160
When 2 mol nonelectrolyte solute dissolved in 1 kg water freezing point is -372
For any concentration of nonelectrolyte solute in water decrease in freezing point can be determined using value of -186m
161
This value called the molal freezing-point contstant (Kf) is the freezing-point depression of the solvent in a 1-molal solution of a nonvolatile nonelectrolyte solute
162
Each solvent has own characteristic molal freezing-point constant Freezing-point depression (Δtf) the difference between the freezing points of the pure solvent and a solute of nonelectrolyte in that solvent and it is directly proportional to the molal concentration of the solution Δtf = Kfm
163
Δtf = Kfm
Kf expressed as m
m expressed in mol solutekg solvent (molality)
164
Sample Problem
What is the freezing point depression of water in a solution of 171 g of sucrose C12H22O11 and 200 g of water What is the actual freezing point of the solution
165
1 AnalyzeGiven Solute mass and chemical formula 171 g C12H22O11 Solvent mass and identity 2000 g water Unknown a freezing-point depression b freezing point of the solution 166
2 Plan
167
168
Calculate
Unknown a freezing-point depression b freezing point of the solution
169
Practice Problem
A water solution containing an unknown quantity of a nonelectrolyte solute is found to have a freezing point of ndash 023degCWhat is the molal concentration of the solution
012 m170
A solution consists of 103 g of the nonelectrolyte glucose C6H12O6 dissolved in 250 g of water What is the freezing-point depression of the solution
minus0426degC171
In a laboratory experiment the freezing point of an aqueous solution of glucose is found to be minus0325degCWhat is the molal concentration of this solution
0175 m172
If 0500 mol of a nonelectrolyte solute are dissolved in 5000 g of ether what is the freezing point of the solution
minus1181degC173
The freezing point of an aqueous solution that contains a nonelectrolyte is minus90degC aWhat is the freezing-point depression of the solution bWhat is the molal concentration of the solution a minus90degC b 48 m
174
Boiling-Point Elevation
Boiling point is temp at which vapor pressure is equal to atmospheric pressure Change in vapor pressure will cause corresponding change in boiling point
175
176
Vapor pressure of nonvolatile solution is lower than vapor pressure of pure solvent More heat will be required to raise vapor pressure of solution Boiling point of solution is higher than boiling point of pure solvent
Molal boiling-point constant (Kb) - boiling-point elevation of the solvent in a 1-molal solution of a nonvolatile nonelectrolyte solute b-p elevation of any nonelectrolyte in water found by experiment to be 051 So molal b-p constant for water is 051m
177
For different solvents b-p elevations of 1-molal solutions have different values (Table 14-2)
Like freezing-point constants values most accurate for dilute solutions
178
Boiling-point elevation Δtb difference between the boiling points of the pure solvent and a nonelectrolyte solution of that solvent and it is directly proportional to the molal concentration of the solution B-p elevation can be calculated using Δtb = Kbm Δtb (boiling point elevation) expressed in m and m expressed in mol solutekg solvent
Sample Problem
What is the boiling-point elevation of a solution made from 200 g of a nonelectrolyte solute and 4000 g of water The molar mass of the solute is 620 gmol
180
1 AnalyzeGiven solute mass = 200 g solute molar mass = 620 gmol solvent mass and identity = 4000 g of water
Unknown boiling-point elevation
181
2 Plan
182
183
3 Compute
Practice Problem
A solution contains 500 g of sucrose C12H22O11 a nonelectrolyte dissolved in 5000 g of water What is the boiling-point elevation
015degC184
A solution contains 4500 g of sucrose C12H22O11 a nonelectrolyte dissolved in 250 g of waterWhat is the boiling point of the solution
1027degC185
If the boiling point elevation of an aqueous solution containing a nonvolatile electrolyte is 102degC what is the molality of the solution
20 m186
The boiling point of an aqueous solution containing a nonvolatile electrolyte is 10075degC aWhat is the boiling-point elevation bWhat is the molality of the solution a 075degC b 15 m
187
Electrolytes and Colligative PropertiesEarly investigators confused by experiments where certain substances depressed freezing point or elevated boiling point more than expected
188
189
Ex 01 m solution of NaCl lowers freezing point of solvent almost twice as much as 01 m solution of sucrose 01 m CaCl2 lowers fp almost 3 times as much as 01 m solution of sucrose
To understand why this happens contrast behavior or sucrose with NaCl in aqueous solution Each sucrose dissolves to make only 1 particle in solution So 1 mol sucrose dissolves to 1 mol particles 1 mol NaCl dissolves to make 2 moles of particles (1 mol Na+ and 1 mol Cl-)
190
191
Calculated Values for Electrolyte SolutionsColligative properties depend on total concentration of solute particles regardless of their identity Electrolytes cause changes in colligative properties proportional to total molality in terms of all dissolves particles instead of formula units
192
What about barium nitrate Ba(NO3)2 Each mole of barium nitrate yields 3 mol of ions in solution Ba(NO3)2(s) rarr Ba2+(aq) + 2NO3-(aq) You would expect solution of given molality to lower fp of solvent 3 times as much as nonelectrolytic solution of same molality
193
Sample Problem
What is the expected change in the freezing point of water in a solution of 625 g of barium nitrate Ba(NO3)2 in 100 kg of water
194
1 AnalyzeGiven solute mass and formula = 625 g Ba(NO3)2 solvent mass and identity = 100 kg water Δtf = Kfm
Unknown expected freezing-point depression
195
2 PlanThe molality can be calculated by converting the solute mass to moles and then dividing by the number of kilograms of solvent That molality is in terms of formula units of Ba(NO3)2 and must be converted to molality in terms of dissociated ions in solution It must be multiplied by the number of moles of ions produced per mole of formula unit This adjusted molality can then be used to calculate the freezing-point depression
196
197
198
3 Compute
Practice Problem
What is the expected freezing-point depression for a solution that contains 20 mol of magnesium sulfate dissolved in 10 kg of water
minus74degC199
What is the expected boiling-point elevation of water for a solution that contains 150 g of sodium chloride dissolved in 10 kg of water 27degC The freezing point of an aqueous sodium chloride solution is minus200degC What is the molality of the solution 54 m NaCl
200
Actual Values for Electrolyte SolutionsRemember values calculated are only expected values Actual values of colligative properties for all strong electrolytes are almost what would be expected Difference between expected and actual values caused by attractive forces that exist between dissociated ions in aqueous solution
201
Attraction between hydrated ions in solution is small compared with those in crystalline solid However forces of attraction donrsquot interfere with movements of aqueous ions More concentrated solution means ions closer together and attraction is greater Also depends on charge of ions ndash higher charge greater attraction
202
Heat of solution - net amount of heat energy absorbed or released when a specific amount of solute dissolves in a solvent
Heat of solution negative (heat is released) if sums of energy in steps 1 and 2 is less than step 3
If 1+2 gt 3 HOS is positive (heat absorbed)78
In gaseous state molecules are so far apart there are nearly no forces between them Solute-solute interaction has little effect on heat of solution of a gas Energy released when gas dissolved in liquid bc attraction between solute gas and solvent molecules greater than energy needed to separate solvent molecules
79
Concentration of Solutions80
Concentration - measure of amount of solute in given amount of solvent or solution ldquodiluterdquo just means small amount of solute ldquoconcentratedrdquo just means relatively large amount of solute Unrelated to degree to which solution is saturated Ex Saturated solution of substance not very soluble might be dilute
81
Molarity
Molarity - number of moles of solute in one liter of solution To find molarity must know molar mass A ldquoone molarrdquo solution of NaOH is 1 mole NaOH per liter of solution (1 M)
82
1 mol NaOH = 400 g If this quantity dissolve in enough water to make EXACTLY 100 L solution solution is 1 M If 200 g NaOH dissolve in enough to make 100 L solution what is molarity 0500 M
83
1 M solution is not made by adding 1 mol solute to 1 L solvent That would be more than 1 L Instead 1 mol solute dissolved in less than 1 L Then diluted with more solvent to bring TOTAL VOLUME to 1 L
84
Practice ProblemYou have 340 L of solution that contains 900 g sodium chloride What is the molarity of that solution 0440 M NaCl You have 08 L of a 05 M HCl solution How many moles of HCl does this solution contain 04 mol HCl
85
What is the molarity of a solution composed of 585 g of potassium iodide dissolved in enough water to make a 0125 L of solution 0282 M KI
86
How many moles of H2SO4 are present in 0500 L of a 0150 M H2SO4 solution 00750 mol What volume of 300 M NaCl is needed for reaction that requires 1463 g of NaCl 0834 L
87
Molality
Molality - concentration of solution expressed in moles of solute per kilogram of solvent Solution that contains 1 mol solute (NaOH) dissolved in exactly 1 kg of solvent is ldquoone molalrdquo solution (1 m NaOH)
88
1 mol NaOH = 400 g 400 g NaOH dissolved in 1 kg water is 1 m NaOH
200 g NaOH in 1 kg water = 0500 m NaOH
89
Practice Problem
A solution was prepared by dissolving 171 g sucrose (C12H22O11) in 125 g of water Find the molal concentration of this solution 0400 m C12H22O11
90
A solution of iodine in carbon tetrachloride is used when iodine is needed for certain chemical tests How much iodine must be added to prepare a 0480 m solution of iodine in CCl4 if 1000 g of CCl4 is used 122 g I2
91
What is the molality of a solution composed of 255 g acetone (CH3)2CO dissolved in 200 g of water 22 m acetone What quantity in grams of methanol CH3OH is required to prepare a 0244 m solution in 400 g water 312 g methanol
92
How many grams of AgNO3 are needed to prepare a 0125 m solution in 250 mL of water 531 g AgNO3
What is the molality of a solution containing 182 g HCl and 250 g water 200 m
93
Ions in Aqueous Solutions and Colligative Properties
94
95
compounds in aqueous solutions
DissociationWhen compound made from ions dissolves in water ions separate Dissociation - separation of ions that occurs when an ionic compound dissolves
96
Notice number of ions made per formula unit in equations 1 formula unit of NaCl gives 2 units of ions in solution 1 formula unit of CaCl2 gives 3 units of ions
97
Assuming 100 dissociation solution that contains 1 mol NaCl contains 1 mol Na+ and 1 mol Cl- Can assume 100 dissociation of all soluble ionic compounds
98
Practice Problem
Write the equation for the dissolution of aluminum sulfate in water How many moles of aluminum ions are made by dissolving 1 mol aluminum sulfate What is the total number of moles of ions made by dissolving 1 mol of aluminum sulfate
99
1 AnalyzeGiven Amount of solute = 1 mol Al2(SO4)3 Solvent identity = water Unknown a moles of aluminum ions and sulfate ions Total number of moles of solute ions produced
100
2 Plan
The coefficients in the balanced dissociation equation will tell mole relationships You can use the equation to find out the number of moles of solute ions produced
101
3 Compute
a
b102
Write the equation for the dissolution of each of the following in water and then determine the number of moles of each ions made as well as the total number of ions made a 1 mol ammonium chloride b 1 mol sodium sulfide c 05 mol barium nitrate
103
a 1 mol ammonium chloride
1 mol NH4+ 1 mol Cl- 2 mol total ions
104
b 1 mol sodium sulfide
2 mol Na+ 1 mol S-2 3 mol total ions
105
c 05 mol barium nitrate
05 mol Ba+2 1 mol NO3- 15 mol total ions
106
Precipitation Reactions
Even though no compound is completely insoluble compounds of very low solubility can be considered insoluble for practical purposes There are general guidelines to help predict whether a compound made of certain combination of ions is soluble
107
General Solubility Guidelines
Sodium potassium and ammonium compounds are soluble in water
108
Nitrates acetates and chlorates are soluble
109
110
Most chlorides are soluble except those of silver mercury (I) and lead Lead (II) chloride is soluble in hot water
Most sulfates are soluble except those of barium strontium lead calcium and mercury
111
Most carbonates phosphates and silicates are insoluble except those of sodium potassium and ammonium
112
Most sulfides are insoluble except those of calcium strontium sodium potassium and ammonium
113
Is calcium phosphate Ca3(PO4)2 soluble or insoluble
Not one of the exceptions so it is insoluble
Dissociation equations are not written for insoluble compounds
114
Guidelines also useful in predicting what will happen if 2 solutions of 2 different soluble compounds are mixed
If mixing results in combination of ions that forms insoluble compound a double-replacement and precipitation reaction will happen
115
Will precipitate form when solutions of ammonium sulfide and cadmium nitrate are combined You can tell calcium nitrate is soluble from the guidelines (Most nitrates acetates and chlorates are soluble) You can also tell ammonium sulfide is soluble (Most sulfides are insoluble except those of calcium strontium sodium potassium and ammonium)
116
117
To decide whether precipitate can form must know solubilities of two possible products
Ammonium nitrate ndash soluble (Most nitrates acetates and chlorates are soluble) Cadmium sulfide ndash insoluble (Most sulfides are insoluble except those of calcium strontium sodium potassium and ammonium)
118
bc one product is insoluble double-replacement and precipitation reaction will happen (NH4)2S (aq) + Cd(NO3)2 (aq) rarr NH4NO3 (aq) + CdS (s)
119
Net Ionic Equations
Reactions of ions in aqueous solution usually represented by net ionic equations instead of formula equations Net ionic equation - includes only those compounds and ions that undergo a chemical change in a reaction in an aqueous solution
120
121
To write NIE (net ionic equation) first convert chemical equation into overall ionic equation All soluble ionic compounds shown as dissociated ions in solution Precipitates shown as solids
122
Notice ammonium and nitrate ions appear on both sides They havenrsquot gone through any chemical change Spectator ions - ions that do not take part in a chemical reaction and are found in solution both before and after the reaction
123
To change ionic equation to NIE eliminate spectator ions Cd+2(aq) + S-2(aq) rarr CdS(s)
This NIE applies to any reaction in which precipitate of CdS forms
124
Practice Problem
Identify the precipitate that forms when aqueous solutions of zinc nitrate and ammonium sulfide are combined Write the equation for the possible double-replacement reaction Then write the formula equation overall ionic equation and the net ionic equation
125
1 AnalyzeGiven Identity of reactants zinc nitrate and ammonium sulfide Reaction medium aqueous solution Unknown a equation for the possible double-replacement reaction b identity of the precipitate c formula equation d overall ionic equation e net ionic equation
126
2 PlanWrite the possible double-replacement reaction between Zn(NO3)2 and (NH4)2S Use the rules to determine if any of the products will precipitate Write a formula equation and overall net ionic equation Then cancel spectator ions to make net ionic equation
127
3 Computea possible double-replacement reaction
b rules show that zinc sulfide is not soluble so will precipitate (ammonium nitrate is soluble)
c formula equation128
d overall ionic equation
e net ionic equation129
Will a precipitate form if solutions of potassium sulfate and barium nitrate are combined If so write the net ionic equation for the reaction
Yes Ba+2(aq) + SO4-2(aq) rarr BaSO4(s)
130
Will a precipitate form if solutions of potassium nitrate and magnesium sulfate are combined If so write the net ionic equation for the reaction
No 131
Will a precipitate form if solutions of barium chloride and sodium sulfate are combined If so identify the spectator ions and write the net ionic equation for the reaction
Yes Na+ and Cl- Ba+2(aq) + SO4-2(aq) rarr BaSO4(s)
132
Write the net ionic equation for the precipitation of nickel(II) sulfide
Ni+2(aq) + S-2(aq) rarr NiS(s)
133
IonizationSome molecular compounds can also form ions in solution Usually polar Ionization - ions formed from solute molecules by action of the solvent (creation of ions where there were none) Ionization different from dissociation
134
Dissociation ionic compounds dissolve and ions already present separate Ionization ions formed where non existed before
Ions formed are hydrated Heat released during hydration of ions gives enough energy to break covalent bonds
135
Extent to which solute ionizes depends on strength of bonds within molecules of solute and strength of attraction between solute and solvent molecules
136
HCl ndash molecular compound that ionizes in aqueous solution Contains highly polar bond Attraction between polar HCl molecule and polar water strong enough to break HCl bond
137
The Hydronium Ion
H+ ions from HCl attracts other molecules or ions so strongly that it doesnrsquot normally exist alone Ionization of HCl better described as direct transfer of proton from HCl to H2O forming H3O+
138
139
Hydration of H+ to form hydronium is highly exothermic
Energy released gives a lot of energy needed to ionize a molecular solute
Many molecular compounds that ionize in aqueous solution contain hydrogen and form hydronium ion
140
Strong and Weak Electrolytes
Substances that make ions and conduct current in solution are electrolytes HCl is one of series of compounds that has hydrogen and halogen Hydrogen halides are all molecular compounds with single polar-covalent bonds
141
All are gases Very soluble in water All are electrolytes HCl HBr HI strongly conduct current in solution HF only weakly conducts current at same concentration Strength of conduction related to ability to ionize
142
143
Strong Electrolytes
HCl HBr HI are 100 ionized in solution
Strong electrolyte - any compound whose dilute aqueous solutions conduct electricity well this is due to the presence of all or almost all of the dissolve compound in the form of ions
144
Unique characteristic of strong electrolyte is that they yield only ions
Ex Ionic compound may be highly soluble in water and dissociate into ions (NaCl)
Others may not dissolve much but amount that does dissolve exists only as ions
145
Weak ElectrolytesHF dissolves in water to give acid solution called hydrofluoric acid HF bond stronger than bonds between hydrogen and other halogens When HF dissolves some molecule ionize Reverse reaction also happens HF(aq) + H2O(l) H3O+(aq) + F-(aq)
146
HF(aq) + H2O(l) H3O+
(aq) + F-(aq)Concentration of dissolved unionized HF stays high and concentration of ions stays low
Weak electrolyte - any compound whose aqueous solutions conduct electricity poorly this is due to the presence of small amount of the dissolved compound in the form of ions
147
Different from nonelectrolyte ndash NO ions at all Description of electrolyte as strong or weak not related to concentration of solution Electrolytes differ in degree of ionization not amount of solute dissolved
148
149
Colligative properties of solutions
Presence of solute affects properties of solutions Some properties not dependent on nature of dissolved substance but on how may dissolve particles are present
Colligative properties - properties that depend on the concentration of solute particles but not on their identity
150
Vapor-Pressure Lowering
Boiling and freezing point of solution different from pure solvent Nonvolatile solute raises boiling point and lowers freezing point Nonvolatile substance - one that has little tendency to become gas under existing conditions
151
152
To understand why nonvolatile solute changes boiling and freezing point consider equilibrium vapor pressure
Vapor pressure pressure caused by molecules that have escape liquid phase to gas phase
Can be thought of a measure of tendency of molecules to escape from a liquid
153
Addition of sucrose (nonvolatile solute) lowers concentration of water molecules at surface of liquid This lowers tendency of water molecules to leave solution and enter gas phase Vapor pressure of solution is lower than vapor pressure of pure water
154
Aqueous13 solution13 of13 nonvolatile13 solutePure13 water
Nonelectrolyte solutions of same molality have same concentration of particles
Dilute solutions of same solvent and equal molality of any nonelectrolyte solute lower vapor pressure equally
155
Example
1 m aqueous solution of nonelectrolyte glucose lowers vapor pressure of water 55 x 10-4 atm at 25 1 m aqueous solution of sucrose also lowers water vapor pressure to 55 x 10-4 atm at 25
156
157
bc vapor-pressure lowering depends on concentration of nonelectrolyte solute and doesnrsquot depend on the type of solute it is a colligative proprerty
bc vapor pressure has been lowered solution remains liquid over larger temperature range
Lowers freezing point and raises boiling point
158
Can assume changes in boiling and freezing point also depend on concentration of solute
They are colligative properties
159
Freezing-Point Depression
Freezing point of 1-molal solution of any nonelectrolyte solute in water is found (by experiment) to be 186 lower than freezing point of water When 1 mol of nonelectrolyte solute dissolved in 1 kg water freezing point is -186 not 000 160
When 2 mol nonelectrolyte solute dissolved in 1 kg water freezing point is -372
For any concentration of nonelectrolyte solute in water decrease in freezing point can be determined using value of -186m
161
This value called the molal freezing-point contstant (Kf) is the freezing-point depression of the solvent in a 1-molal solution of a nonvolatile nonelectrolyte solute
162
Each solvent has own characteristic molal freezing-point constant Freezing-point depression (Δtf) the difference between the freezing points of the pure solvent and a solute of nonelectrolyte in that solvent and it is directly proportional to the molal concentration of the solution Δtf = Kfm
163
Δtf = Kfm
Kf expressed as m
m expressed in mol solutekg solvent (molality)
164
Sample Problem
What is the freezing point depression of water in a solution of 171 g of sucrose C12H22O11 and 200 g of water What is the actual freezing point of the solution
165
1 AnalyzeGiven Solute mass and chemical formula 171 g C12H22O11 Solvent mass and identity 2000 g water Unknown a freezing-point depression b freezing point of the solution 166
2 Plan
167
168
Calculate
Unknown a freezing-point depression b freezing point of the solution
169
Practice Problem
A water solution containing an unknown quantity of a nonelectrolyte solute is found to have a freezing point of ndash 023degCWhat is the molal concentration of the solution
012 m170
A solution consists of 103 g of the nonelectrolyte glucose C6H12O6 dissolved in 250 g of water What is the freezing-point depression of the solution
minus0426degC171
In a laboratory experiment the freezing point of an aqueous solution of glucose is found to be minus0325degCWhat is the molal concentration of this solution
0175 m172
If 0500 mol of a nonelectrolyte solute are dissolved in 5000 g of ether what is the freezing point of the solution
minus1181degC173
The freezing point of an aqueous solution that contains a nonelectrolyte is minus90degC aWhat is the freezing-point depression of the solution bWhat is the molal concentration of the solution a minus90degC b 48 m
174
Boiling-Point Elevation
Boiling point is temp at which vapor pressure is equal to atmospheric pressure Change in vapor pressure will cause corresponding change in boiling point
175
176
Vapor pressure of nonvolatile solution is lower than vapor pressure of pure solvent More heat will be required to raise vapor pressure of solution Boiling point of solution is higher than boiling point of pure solvent
Molal boiling-point constant (Kb) - boiling-point elevation of the solvent in a 1-molal solution of a nonvolatile nonelectrolyte solute b-p elevation of any nonelectrolyte in water found by experiment to be 051 So molal b-p constant for water is 051m
177
For different solvents b-p elevations of 1-molal solutions have different values (Table 14-2)
Like freezing-point constants values most accurate for dilute solutions
178
Boiling-point elevation Δtb difference between the boiling points of the pure solvent and a nonelectrolyte solution of that solvent and it is directly proportional to the molal concentration of the solution B-p elevation can be calculated using Δtb = Kbm Δtb (boiling point elevation) expressed in m and m expressed in mol solutekg solvent
Sample Problem
What is the boiling-point elevation of a solution made from 200 g of a nonelectrolyte solute and 4000 g of water The molar mass of the solute is 620 gmol
180
1 AnalyzeGiven solute mass = 200 g solute molar mass = 620 gmol solvent mass and identity = 4000 g of water
Unknown boiling-point elevation
181
2 Plan
182
183
3 Compute
Practice Problem
A solution contains 500 g of sucrose C12H22O11 a nonelectrolyte dissolved in 5000 g of water What is the boiling-point elevation
015degC184
A solution contains 4500 g of sucrose C12H22O11 a nonelectrolyte dissolved in 250 g of waterWhat is the boiling point of the solution
1027degC185
If the boiling point elevation of an aqueous solution containing a nonvolatile electrolyte is 102degC what is the molality of the solution
20 m186
The boiling point of an aqueous solution containing a nonvolatile electrolyte is 10075degC aWhat is the boiling-point elevation bWhat is the molality of the solution a 075degC b 15 m
187
Electrolytes and Colligative PropertiesEarly investigators confused by experiments where certain substances depressed freezing point or elevated boiling point more than expected
188
189
Ex 01 m solution of NaCl lowers freezing point of solvent almost twice as much as 01 m solution of sucrose 01 m CaCl2 lowers fp almost 3 times as much as 01 m solution of sucrose
To understand why this happens contrast behavior or sucrose with NaCl in aqueous solution Each sucrose dissolves to make only 1 particle in solution So 1 mol sucrose dissolves to 1 mol particles 1 mol NaCl dissolves to make 2 moles of particles (1 mol Na+ and 1 mol Cl-)
190
191
Calculated Values for Electrolyte SolutionsColligative properties depend on total concentration of solute particles regardless of their identity Electrolytes cause changes in colligative properties proportional to total molality in terms of all dissolves particles instead of formula units
192
What about barium nitrate Ba(NO3)2 Each mole of barium nitrate yields 3 mol of ions in solution Ba(NO3)2(s) rarr Ba2+(aq) + 2NO3-(aq) You would expect solution of given molality to lower fp of solvent 3 times as much as nonelectrolytic solution of same molality
193
Sample Problem
What is the expected change in the freezing point of water in a solution of 625 g of barium nitrate Ba(NO3)2 in 100 kg of water
194
1 AnalyzeGiven solute mass and formula = 625 g Ba(NO3)2 solvent mass and identity = 100 kg water Δtf = Kfm
Unknown expected freezing-point depression
195
2 PlanThe molality can be calculated by converting the solute mass to moles and then dividing by the number of kilograms of solvent That molality is in terms of formula units of Ba(NO3)2 and must be converted to molality in terms of dissociated ions in solution It must be multiplied by the number of moles of ions produced per mole of formula unit This adjusted molality can then be used to calculate the freezing-point depression
196
197
198
3 Compute
Practice Problem
What is the expected freezing-point depression for a solution that contains 20 mol of magnesium sulfate dissolved in 10 kg of water
minus74degC199
What is the expected boiling-point elevation of water for a solution that contains 150 g of sodium chloride dissolved in 10 kg of water 27degC The freezing point of an aqueous sodium chloride solution is minus200degC What is the molality of the solution 54 m NaCl
200
Actual Values for Electrolyte SolutionsRemember values calculated are only expected values Actual values of colligative properties for all strong electrolytes are almost what would be expected Difference between expected and actual values caused by attractive forces that exist between dissociated ions in aqueous solution
201
Attraction between hydrated ions in solution is small compared with those in crystalline solid However forces of attraction donrsquot interfere with movements of aqueous ions More concentrated solution means ions closer together and attraction is greater Also depends on charge of ions ndash higher charge greater attraction
202
In gaseous state molecules are so far apart there are nearly no forces between them Solute-solute interaction has little effect on heat of solution of a gas Energy released when gas dissolved in liquid bc attraction between solute gas and solvent molecules greater than energy needed to separate solvent molecules
79
Concentration of Solutions80
Concentration - measure of amount of solute in given amount of solvent or solution ldquodiluterdquo just means small amount of solute ldquoconcentratedrdquo just means relatively large amount of solute Unrelated to degree to which solution is saturated Ex Saturated solution of substance not very soluble might be dilute
81
Molarity
Molarity - number of moles of solute in one liter of solution To find molarity must know molar mass A ldquoone molarrdquo solution of NaOH is 1 mole NaOH per liter of solution (1 M)
82
1 mol NaOH = 400 g If this quantity dissolve in enough water to make EXACTLY 100 L solution solution is 1 M If 200 g NaOH dissolve in enough to make 100 L solution what is molarity 0500 M
83
1 M solution is not made by adding 1 mol solute to 1 L solvent That would be more than 1 L Instead 1 mol solute dissolved in less than 1 L Then diluted with more solvent to bring TOTAL VOLUME to 1 L
84
Practice ProblemYou have 340 L of solution that contains 900 g sodium chloride What is the molarity of that solution 0440 M NaCl You have 08 L of a 05 M HCl solution How many moles of HCl does this solution contain 04 mol HCl
85
What is the molarity of a solution composed of 585 g of potassium iodide dissolved in enough water to make a 0125 L of solution 0282 M KI
86
How many moles of H2SO4 are present in 0500 L of a 0150 M H2SO4 solution 00750 mol What volume of 300 M NaCl is needed for reaction that requires 1463 g of NaCl 0834 L
87
Molality
Molality - concentration of solution expressed in moles of solute per kilogram of solvent Solution that contains 1 mol solute (NaOH) dissolved in exactly 1 kg of solvent is ldquoone molalrdquo solution (1 m NaOH)
88
1 mol NaOH = 400 g 400 g NaOH dissolved in 1 kg water is 1 m NaOH
200 g NaOH in 1 kg water = 0500 m NaOH
89
Practice Problem
A solution was prepared by dissolving 171 g sucrose (C12H22O11) in 125 g of water Find the molal concentration of this solution 0400 m C12H22O11
90
A solution of iodine in carbon tetrachloride is used when iodine is needed for certain chemical tests How much iodine must be added to prepare a 0480 m solution of iodine in CCl4 if 1000 g of CCl4 is used 122 g I2
91
What is the molality of a solution composed of 255 g acetone (CH3)2CO dissolved in 200 g of water 22 m acetone What quantity in grams of methanol CH3OH is required to prepare a 0244 m solution in 400 g water 312 g methanol
92
How many grams of AgNO3 are needed to prepare a 0125 m solution in 250 mL of water 531 g AgNO3
What is the molality of a solution containing 182 g HCl and 250 g water 200 m
93
Ions in Aqueous Solutions and Colligative Properties
94
95
compounds in aqueous solutions
DissociationWhen compound made from ions dissolves in water ions separate Dissociation - separation of ions that occurs when an ionic compound dissolves
96
Notice number of ions made per formula unit in equations 1 formula unit of NaCl gives 2 units of ions in solution 1 formula unit of CaCl2 gives 3 units of ions
97
Assuming 100 dissociation solution that contains 1 mol NaCl contains 1 mol Na+ and 1 mol Cl- Can assume 100 dissociation of all soluble ionic compounds
98
Practice Problem
Write the equation for the dissolution of aluminum sulfate in water How many moles of aluminum ions are made by dissolving 1 mol aluminum sulfate What is the total number of moles of ions made by dissolving 1 mol of aluminum sulfate
99
1 AnalyzeGiven Amount of solute = 1 mol Al2(SO4)3 Solvent identity = water Unknown a moles of aluminum ions and sulfate ions Total number of moles of solute ions produced
100
2 Plan
The coefficients in the balanced dissociation equation will tell mole relationships You can use the equation to find out the number of moles of solute ions produced
101
3 Compute
a
b102
Write the equation for the dissolution of each of the following in water and then determine the number of moles of each ions made as well as the total number of ions made a 1 mol ammonium chloride b 1 mol sodium sulfide c 05 mol barium nitrate
103
a 1 mol ammonium chloride
1 mol NH4+ 1 mol Cl- 2 mol total ions
104
b 1 mol sodium sulfide
2 mol Na+ 1 mol S-2 3 mol total ions
105
c 05 mol barium nitrate
05 mol Ba+2 1 mol NO3- 15 mol total ions
106
Precipitation Reactions
Even though no compound is completely insoluble compounds of very low solubility can be considered insoluble for practical purposes There are general guidelines to help predict whether a compound made of certain combination of ions is soluble
107
General Solubility Guidelines
Sodium potassium and ammonium compounds are soluble in water
108
Nitrates acetates and chlorates are soluble
109
110
Most chlorides are soluble except those of silver mercury (I) and lead Lead (II) chloride is soluble in hot water
Most sulfates are soluble except those of barium strontium lead calcium and mercury
111
Most carbonates phosphates and silicates are insoluble except those of sodium potassium and ammonium
112
Most sulfides are insoluble except those of calcium strontium sodium potassium and ammonium
113
Is calcium phosphate Ca3(PO4)2 soluble or insoluble
Not one of the exceptions so it is insoluble
Dissociation equations are not written for insoluble compounds
114
Guidelines also useful in predicting what will happen if 2 solutions of 2 different soluble compounds are mixed
If mixing results in combination of ions that forms insoluble compound a double-replacement and precipitation reaction will happen
115
Will precipitate form when solutions of ammonium sulfide and cadmium nitrate are combined You can tell calcium nitrate is soluble from the guidelines (Most nitrates acetates and chlorates are soluble) You can also tell ammonium sulfide is soluble (Most sulfides are insoluble except those of calcium strontium sodium potassium and ammonium)
116
117
To decide whether precipitate can form must know solubilities of two possible products
Ammonium nitrate ndash soluble (Most nitrates acetates and chlorates are soluble) Cadmium sulfide ndash insoluble (Most sulfides are insoluble except those of calcium strontium sodium potassium and ammonium)
118
bc one product is insoluble double-replacement and precipitation reaction will happen (NH4)2S (aq) + Cd(NO3)2 (aq) rarr NH4NO3 (aq) + CdS (s)
119
Net Ionic Equations
Reactions of ions in aqueous solution usually represented by net ionic equations instead of formula equations Net ionic equation - includes only those compounds and ions that undergo a chemical change in a reaction in an aqueous solution
120
121
To write NIE (net ionic equation) first convert chemical equation into overall ionic equation All soluble ionic compounds shown as dissociated ions in solution Precipitates shown as solids
122
Notice ammonium and nitrate ions appear on both sides They havenrsquot gone through any chemical change Spectator ions - ions that do not take part in a chemical reaction and are found in solution both before and after the reaction
123
To change ionic equation to NIE eliminate spectator ions Cd+2(aq) + S-2(aq) rarr CdS(s)
This NIE applies to any reaction in which precipitate of CdS forms
124
Practice Problem
Identify the precipitate that forms when aqueous solutions of zinc nitrate and ammonium sulfide are combined Write the equation for the possible double-replacement reaction Then write the formula equation overall ionic equation and the net ionic equation
125
1 AnalyzeGiven Identity of reactants zinc nitrate and ammonium sulfide Reaction medium aqueous solution Unknown a equation for the possible double-replacement reaction b identity of the precipitate c formula equation d overall ionic equation e net ionic equation
126
2 PlanWrite the possible double-replacement reaction between Zn(NO3)2 and (NH4)2S Use the rules to determine if any of the products will precipitate Write a formula equation and overall net ionic equation Then cancel spectator ions to make net ionic equation
127
3 Computea possible double-replacement reaction
b rules show that zinc sulfide is not soluble so will precipitate (ammonium nitrate is soluble)
c formula equation128
d overall ionic equation
e net ionic equation129
Will a precipitate form if solutions of potassium sulfate and barium nitrate are combined If so write the net ionic equation for the reaction
Yes Ba+2(aq) + SO4-2(aq) rarr BaSO4(s)
130
Will a precipitate form if solutions of potassium nitrate and magnesium sulfate are combined If so write the net ionic equation for the reaction
No 131
Will a precipitate form if solutions of barium chloride and sodium sulfate are combined If so identify the spectator ions and write the net ionic equation for the reaction
Yes Na+ and Cl- Ba+2(aq) + SO4-2(aq) rarr BaSO4(s)
132
Write the net ionic equation for the precipitation of nickel(II) sulfide
Ni+2(aq) + S-2(aq) rarr NiS(s)
133
IonizationSome molecular compounds can also form ions in solution Usually polar Ionization - ions formed from solute molecules by action of the solvent (creation of ions where there were none) Ionization different from dissociation
134
Dissociation ionic compounds dissolve and ions already present separate Ionization ions formed where non existed before
Ions formed are hydrated Heat released during hydration of ions gives enough energy to break covalent bonds
135
Extent to which solute ionizes depends on strength of bonds within molecules of solute and strength of attraction between solute and solvent molecules
136
HCl ndash molecular compound that ionizes in aqueous solution Contains highly polar bond Attraction between polar HCl molecule and polar water strong enough to break HCl bond
137
The Hydronium Ion
H+ ions from HCl attracts other molecules or ions so strongly that it doesnrsquot normally exist alone Ionization of HCl better described as direct transfer of proton from HCl to H2O forming H3O+
138
139
Hydration of H+ to form hydronium is highly exothermic
Energy released gives a lot of energy needed to ionize a molecular solute
Many molecular compounds that ionize in aqueous solution contain hydrogen and form hydronium ion
140
Strong and Weak Electrolytes
Substances that make ions and conduct current in solution are electrolytes HCl is one of series of compounds that has hydrogen and halogen Hydrogen halides are all molecular compounds with single polar-covalent bonds
141
All are gases Very soluble in water All are electrolytes HCl HBr HI strongly conduct current in solution HF only weakly conducts current at same concentration Strength of conduction related to ability to ionize
142
143
Strong Electrolytes
HCl HBr HI are 100 ionized in solution
Strong electrolyte - any compound whose dilute aqueous solutions conduct electricity well this is due to the presence of all or almost all of the dissolve compound in the form of ions
144
Unique characteristic of strong electrolyte is that they yield only ions
Ex Ionic compound may be highly soluble in water and dissociate into ions (NaCl)
Others may not dissolve much but amount that does dissolve exists only as ions
145
Weak ElectrolytesHF dissolves in water to give acid solution called hydrofluoric acid HF bond stronger than bonds between hydrogen and other halogens When HF dissolves some molecule ionize Reverse reaction also happens HF(aq) + H2O(l) H3O+(aq) + F-(aq)
146
HF(aq) + H2O(l) H3O+
(aq) + F-(aq)Concentration of dissolved unionized HF stays high and concentration of ions stays low
Weak electrolyte - any compound whose aqueous solutions conduct electricity poorly this is due to the presence of small amount of the dissolved compound in the form of ions
147
Different from nonelectrolyte ndash NO ions at all Description of electrolyte as strong or weak not related to concentration of solution Electrolytes differ in degree of ionization not amount of solute dissolved
148
149
Colligative properties of solutions
Presence of solute affects properties of solutions Some properties not dependent on nature of dissolved substance but on how may dissolve particles are present
Colligative properties - properties that depend on the concentration of solute particles but not on their identity
150
Vapor-Pressure Lowering
Boiling and freezing point of solution different from pure solvent Nonvolatile solute raises boiling point and lowers freezing point Nonvolatile substance - one that has little tendency to become gas under existing conditions
151
152
To understand why nonvolatile solute changes boiling and freezing point consider equilibrium vapor pressure
Vapor pressure pressure caused by molecules that have escape liquid phase to gas phase
Can be thought of a measure of tendency of molecules to escape from a liquid
153
Addition of sucrose (nonvolatile solute) lowers concentration of water molecules at surface of liquid This lowers tendency of water molecules to leave solution and enter gas phase Vapor pressure of solution is lower than vapor pressure of pure water
154
Aqueous13 solution13 of13 nonvolatile13 solutePure13 water
Nonelectrolyte solutions of same molality have same concentration of particles
Dilute solutions of same solvent and equal molality of any nonelectrolyte solute lower vapor pressure equally
155
Example
1 m aqueous solution of nonelectrolyte glucose lowers vapor pressure of water 55 x 10-4 atm at 25 1 m aqueous solution of sucrose also lowers water vapor pressure to 55 x 10-4 atm at 25
156
157
bc vapor-pressure lowering depends on concentration of nonelectrolyte solute and doesnrsquot depend on the type of solute it is a colligative proprerty
bc vapor pressure has been lowered solution remains liquid over larger temperature range
Lowers freezing point and raises boiling point
158
Can assume changes in boiling and freezing point also depend on concentration of solute
They are colligative properties
159
Freezing-Point Depression
Freezing point of 1-molal solution of any nonelectrolyte solute in water is found (by experiment) to be 186 lower than freezing point of water When 1 mol of nonelectrolyte solute dissolved in 1 kg water freezing point is -186 not 000 160
When 2 mol nonelectrolyte solute dissolved in 1 kg water freezing point is -372
For any concentration of nonelectrolyte solute in water decrease in freezing point can be determined using value of -186m
161
This value called the molal freezing-point contstant (Kf) is the freezing-point depression of the solvent in a 1-molal solution of a nonvolatile nonelectrolyte solute
162
Each solvent has own characteristic molal freezing-point constant Freezing-point depression (Δtf) the difference between the freezing points of the pure solvent and a solute of nonelectrolyte in that solvent and it is directly proportional to the molal concentration of the solution Δtf = Kfm
163
Δtf = Kfm
Kf expressed as m
m expressed in mol solutekg solvent (molality)
164
Sample Problem
What is the freezing point depression of water in a solution of 171 g of sucrose C12H22O11 and 200 g of water What is the actual freezing point of the solution
165
1 AnalyzeGiven Solute mass and chemical formula 171 g C12H22O11 Solvent mass and identity 2000 g water Unknown a freezing-point depression b freezing point of the solution 166
2 Plan
167
168
Calculate
Unknown a freezing-point depression b freezing point of the solution
169
Practice Problem
A water solution containing an unknown quantity of a nonelectrolyte solute is found to have a freezing point of ndash 023degCWhat is the molal concentration of the solution
012 m170
A solution consists of 103 g of the nonelectrolyte glucose C6H12O6 dissolved in 250 g of water What is the freezing-point depression of the solution
minus0426degC171
In a laboratory experiment the freezing point of an aqueous solution of glucose is found to be minus0325degCWhat is the molal concentration of this solution
0175 m172
If 0500 mol of a nonelectrolyte solute are dissolved in 5000 g of ether what is the freezing point of the solution
minus1181degC173
The freezing point of an aqueous solution that contains a nonelectrolyte is minus90degC aWhat is the freezing-point depression of the solution bWhat is the molal concentration of the solution a minus90degC b 48 m
174
Boiling-Point Elevation
Boiling point is temp at which vapor pressure is equal to atmospheric pressure Change in vapor pressure will cause corresponding change in boiling point
175
176
Vapor pressure of nonvolatile solution is lower than vapor pressure of pure solvent More heat will be required to raise vapor pressure of solution Boiling point of solution is higher than boiling point of pure solvent
Molal boiling-point constant (Kb) - boiling-point elevation of the solvent in a 1-molal solution of a nonvolatile nonelectrolyte solute b-p elevation of any nonelectrolyte in water found by experiment to be 051 So molal b-p constant for water is 051m
177
For different solvents b-p elevations of 1-molal solutions have different values (Table 14-2)
Like freezing-point constants values most accurate for dilute solutions
178
Boiling-point elevation Δtb difference between the boiling points of the pure solvent and a nonelectrolyte solution of that solvent and it is directly proportional to the molal concentration of the solution B-p elevation can be calculated using Δtb = Kbm Δtb (boiling point elevation) expressed in m and m expressed in mol solutekg solvent
Sample Problem
What is the boiling-point elevation of a solution made from 200 g of a nonelectrolyte solute and 4000 g of water The molar mass of the solute is 620 gmol
180
1 AnalyzeGiven solute mass = 200 g solute molar mass = 620 gmol solvent mass and identity = 4000 g of water
Unknown boiling-point elevation
181
2 Plan
182
183
3 Compute
Practice Problem
A solution contains 500 g of sucrose C12H22O11 a nonelectrolyte dissolved in 5000 g of water What is the boiling-point elevation
015degC184
A solution contains 4500 g of sucrose C12H22O11 a nonelectrolyte dissolved in 250 g of waterWhat is the boiling point of the solution
1027degC185
If the boiling point elevation of an aqueous solution containing a nonvolatile electrolyte is 102degC what is the molality of the solution
20 m186
The boiling point of an aqueous solution containing a nonvolatile electrolyte is 10075degC aWhat is the boiling-point elevation bWhat is the molality of the solution a 075degC b 15 m
187
Electrolytes and Colligative PropertiesEarly investigators confused by experiments where certain substances depressed freezing point or elevated boiling point more than expected
188
189
Ex 01 m solution of NaCl lowers freezing point of solvent almost twice as much as 01 m solution of sucrose 01 m CaCl2 lowers fp almost 3 times as much as 01 m solution of sucrose
To understand why this happens contrast behavior or sucrose with NaCl in aqueous solution Each sucrose dissolves to make only 1 particle in solution So 1 mol sucrose dissolves to 1 mol particles 1 mol NaCl dissolves to make 2 moles of particles (1 mol Na+ and 1 mol Cl-)
190
191
Calculated Values for Electrolyte SolutionsColligative properties depend on total concentration of solute particles regardless of their identity Electrolytes cause changes in colligative properties proportional to total molality in terms of all dissolves particles instead of formula units
192
What about barium nitrate Ba(NO3)2 Each mole of barium nitrate yields 3 mol of ions in solution Ba(NO3)2(s) rarr Ba2+(aq) + 2NO3-(aq) You would expect solution of given molality to lower fp of solvent 3 times as much as nonelectrolytic solution of same molality
193
Sample Problem
What is the expected change in the freezing point of water in a solution of 625 g of barium nitrate Ba(NO3)2 in 100 kg of water
194
1 AnalyzeGiven solute mass and formula = 625 g Ba(NO3)2 solvent mass and identity = 100 kg water Δtf = Kfm
Unknown expected freezing-point depression
195
2 PlanThe molality can be calculated by converting the solute mass to moles and then dividing by the number of kilograms of solvent That molality is in terms of formula units of Ba(NO3)2 and must be converted to molality in terms of dissociated ions in solution It must be multiplied by the number of moles of ions produced per mole of formula unit This adjusted molality can then be used to calculate the freezing-point depression
196
197
198
3 Compute
Practice Problem
What is the expected freezing-point depression for a solution that contains 20 mol of magnesium sulfate dissolved in 10 kg of water
minus74degC199
What is the expected boiling-point elevation of water for a solution that contains 150 g of sodium chloride dissolved in 10 kg of water 27degC The freezing point of an aqueous sodium chloride solution is minus200degC What is the molality of the solution 54 m NaCl
200
Actual Values for Electrolyte SolutionsRemember values calculated are only expected values Actual values of colligative properties for all strong electrolytes are almost what would be expected Difference between expected and actual values caused by attractive forces that exist between dissociated ions in aqueous solution
201
Attraction between hydrated ions in solution is small compared with those in crystalline solid However forces of attraction donrsquot interfere with movements of aqueous ions More concentrated solution means ions closer together and attraction is greater Also depends on charge of ions ndash higher charge greater attraction
202
Concentration of Solutions80
Concentration - measure of amount of solute in given amount of solvent or solution ldquodiluterdquo just means small amount of solute ldquoconcentratedrdquo just means relatively large amount of solute Unrelated to degree to which solution is saturated Ex Saturated solution of substance not very soluble might be dilute
81
Molarity
Molarity - number of moles of solute in one liter of solution To find molarity must know molar mass A ldquoone molarrdquo solution of NaOH is 1 mole NaOH per liter of solution (1 M)
82
1 mol NaOH = 400 g If this quantity dissolve in enough water to make EXACTLY 100 L solution solution is 1 M If 200 g NaOH dissolve in enough to make 100 L solution what is molarity 0500 M
83
1 M solution is not made by adding 1 mol solute to 1 L solvent That would be more than 1 L Instead 1 mol solute dissolved in less than 1 L Then diluted with more solvent to bring TOTAL VOLUME to 1 L
84
Practice ProblemYou have 340 L of solution that contains 900 g sodium chloride What is the molarity of that solution 0440 M NaCl You have 08 L of a 05 M HCl solution How many moles of HCl does this solution contain 04 mol HCl
85
What is the molarity of a solution composed of 585 g of potassium iodide dissolved in enough water to make a 0125 L of solution 0282 M KI
86
How many moles of H2SO4 are present in 0500 L of a 0150 M H2SO4 solution 00750 mol What volume of 300 M NaCl is needed for reaction that requires 1463 g of NaCl 0834 L
87
Molality
Molality - concentration of solution expressed in moles of solute per kilogram of solvent Solution that contains 1 mol solute (NaOH) dissolved in exactly 1 kg of solvent is ldquoone molalrdquo solution (1 m NaOH)
88
1 mol NaOH = 400 g 400 g NaOH dissolved in 1 kg water is 1 m NaOH
200 g NaOH in 1 kg water = 0500 m NaOH
89
Practice Problem
A solution was prepared by dissolving 171 g sucrose (C12H22O11) in 125 g of water Find the molal concentration of this solution 0400 m C12H22O11
90
A solution of iodine in carbon tetrachloride is used when iodine is needed for certain chemical tests How much iodine must be added to prepare a 0480 m solution of iodine in CCl4 if 1000 g of CCl4 is used 122 g I2
91
What is the molality of a solution composed of 255 g acetone (CH3)2CO dissolved in 200 g of water 22 m acetone What quantity in grams of methanol CH3OH is required to prepare a 0244 m solution in 400 g water 312 g methanol
92
How many grams of AgNO3 are needed to prepare a 0125 m solution in 250 mL of water 531 g AgNO3
What is the molality of a solution containing 182 g HCl and 250 g water 200 m
93
Ions in Aqueous Solutions and Colligative Properties
94
95
compounds in aqueous solutions
DissociationWhen compound made from ions dissolves in water ions separate Dissociation - separation of ions that occurs when an ionic compound dissolves
96
Notice number of ions made per formula unit in equations 1 formula unit of NaCl gives 2 units of ions in solution 1 formula unit of CaCl2 gives 3 units of ions
97
Assuming 100 dissociation solution that contains 1 mol NaCl contains 1 mol Na+ and 1 mol Cl- Can assume 100 dissociation of all soluble ionic compounds
98
Practice Problem
Write the equation for the dissolution of aluminum sulfate in water How many moles of aluminum ions are made by dissolving 1 mol aluminum sulfate What is the total number of moles of ions made by dissolving 1 mol of aluminum sulfate
99
1 AnalyzeGiven Amount of solute = 1 mol Al2(SO4)3 Solvent identity = water Unknown a moles of aluminum ions and sulfate ions Total number of moles of solute ions produced
100
2 Plan
The coefficients in the balanced dissociation equation will tell mole relationships You can use the equation to find out the number of moles of solute ions produced
101
3 Compute
a
b102
Write the equation for the dissolution of each of the following in water and then determine the number of moles of each ions made as well as the total number of ions made a 1 mol ammonium chloride b 1 mol sodium sulfide c 05 mol barium nitrate
103
a 1 mol ammonium chloride
1 mol NH4+ 1 mol Cl- 2 mol total ions
104
b 1 mol sodium sulfide
2 mol Na+ 1 mol S-2 3 mol total ions
105
c 05 mol barium nitrate
05 mol Ba+2 1 mol NO3- 15 mol total ions
106
Precipitation Reactions
Even though no compound is completely insoluble compounds of very low solubility can be considered insoluble for practical purposes There are general guidelines to help predict whether a compound made of certain combination of ions is soluble
107
General Solubility Guidelines
Sodium potassium and ammonium compounds are soluble in water
108
Nitrates acetates and chlorates are soluble
109
110
Most chlorides are soluble except those of silver mercury (I) and lead Lead (II) chloride is soluble in hot water
Most sulfates are soluble except those of barium strontium lead calcium and mercury
111
Most carbonates phosphates and silicates are insoluble except those of sodium potassium and ammonium
112
Most sulfides are insoluble except those of calcium strontium sodium potassium and ammonium
113
Is calcium phosphate Ca3(PO4)2 soluble or insoluble
Not one of the exceptions so it is insoluble
Dissociation equations are not written for insoluble compounds
114
Guidelines also useful in predicting what will happen if 2 solutions of 2 different soluble compounds are mixed
If mixing results in combination of ions that forms insoluble compound a double-replacement and precipitation reaction will happen
115
Will precipitate form when solutions of ammonium sulfide and cadmium nitrate are combined You can tell calcium nitrate is soluble from the guidelines (Most nitrates acetates and chlorates are soluble) You can also tell ammonium sulfide is soluble (Most sulfides are insoluble except those of calcium strontium sodium potassium and ammonium)
116
117
To decide whether precipitate can form must know solubilities of two possible products
Ammonium nitrate ndash soluble (Most nitrates acetates and chlorates are soluble) Cadmium sulfide ndash insoluble (Most sulfides are insoluble except those of calcium strontium sodium potassium and ammonium)
118
bc one product is insoluble double-replacement and precipitation reaction will happen (NH4)2S (aq) + Cd(NO3)2 (aq) rarr NH4NO3 (aq) + CdS (s)
119
Net Ionic Equations
Reactions of ions in aqueous solution usually represented by net ionic equations instead of formula equations Net ionic equation - includes only those compounds and ions that undergo a chemical change in a reaction in an aqueous solution
120
121
To write NIE (net ionic equation) first convert chemical equation into overall ionic equation All soluble ionic compounds shown as dissociated ions in solution Precipitates shown as solids
122
Notice ammonium and nitrate ions appear on both sides They havenrsquot gone through any chemical change Spectator ions - ions that do not take part in a chemical reaction and are found in solution both before and after the reaction
123
To change ionic equation to NIE eliminate spectator ions Cd+2(aq) + S-2(aq) rarr CdS(s)
This NIE applies to any reaction in which precipitate of CdS forms
124
Practice Problem
Identify the precipitate that forms when aqueous solutions of zinc nitrate and ammonium sulfide are combined Write the equation for the possible double-replacement reaction Then write the formula equation overall ionic equation and the net ionic equation
125
1 AnalyzeGiven Identity of reactants zinc nitrate and ammonium sulfide Reaction medium aqueous solution Unknown a equation for the possible double-replacement reaction b identity of the precipitate c formula equation d overall ionic equation e net ionic equation
126
2 PlanWrite the possible double-replacement reaction between Zn(NO3)2 and (NH4)2S Use the rules to determine if any of the products will precipitate Write a formula equation and overall net ionic equation Then cancel spectator ions to make net ionic equation
127
3 Computea possible double-replacement reaction
b rules show that zinc sulfide is not soluble so will precipitate (ammonium nitrate is soluble)
c formula equation128
d overall ionic equation
e net ionic equation129
Will a precipitate form if solutions of potassium sulfate and barium nitrate are combined If so write the net ionic equation for the reaction
Yes Ba+2(aq) + SO4-2(aq) rarr BaSO4(s)
130
Will a precipitate form if solutions of potassium nitrate and magnesium sulfate are combined If so write the net ionic equation for the reaction
No 131
Will a precipitate form if solutions of barium chloride and sodium sulfate are combined If so identify the spectator ions and write the net ionic equation for the reaction
Yes Na+ and Cl- Ba+2(aq) + SO4-2(aq) rarr BaSO4(s)
132
Write the net ionic equation for the precipitation of nickel(II) sulfide
Ni+2(aq) + S-2(aq) rarr NiS(s)
133
IonizationSome molecular compounds can also form ions in solution Usually polar Ionization - ions formed from solute molecules by action of the solvent (creation of ions where there were none) Ionization different from dissociation
134
Dissociation ionic compounds dissolve and ions already present separate Ionization ions formed where non existed before
Ions formed are hydrated Heat released during hydration of ions gives enough energy to break covalent bonds
135
Extent to which solute ionizes depends on strength of bonds within molecules of solute and strength of attraction between solute and solvent molecules
136
HCl ndash molecular compound that ionizes in aqueous solution Contains highly polar bond Attraction between polar HCl molecule and polar water strong enough to break HCl bond
137
The Hydronium Ion
H+ ions from HCl attracts other molecules or ions so strongly that it doesnrsquot normally exist alone Ionization of HCl better described as direct transfer of proton from HCl to H2O forming H3O+
138
139
Hydration of H+ to form hydronium is highly exothermic
Energy released gives a lot of energy needed to ionize a molecular solute
Many molecular compounds that ionize in aqueous solution contain hydrogen and form hydronium ion
140
Strong and Weak Electrolytes
Substances that make ions and conduct current in solution are electrolytes HCl is one of series of compounds that has hydrogen and halogen Hydrogen halides are all molecular compounds with single polar-covalent bonds
141
All are gases Very soluble in water All are electrolytes HCl HBr HI strongly conduct current in solution HF only weakly conducts current at same concentration Strength of conduction related to ability to ionize
142
143
Strong Electrolytes
HCl HBr HI are 100 ionized in solution
Strong electrolyte - any compound whose dilute aqueous solutions conduct electricity well this is due to the presence of all or almost all of the dissolve compound in the form of ions
144
Unique characteristic of strong electrolyte is that they yield only ions
Ex Ionic compound may be highly soluble in water and dissociate into ions (NaCl)
Others may not dissolve much but amount that does dissolve exists only as ions
145
Weak ElectrolytesHF dissolves in water to give acid solution called hydrofluoric acid HF bond stronger than bonds between hydrogen and other halogens When HF dissolves some molecule ionize Reverse reaction also happens HF(aq) + H2O(l) H3O+(aq) + F-(aq)
146
HF(aq) + H2O(l) H3O+
(aq) + F-(aq)Concentration of dissolved unionized HF stays high and concentration of ions stays low
Weak electrolyte - any compound whose aqueous solutions conduct electricity poorly this is due to the presence of small amount of the dissolved compound in the form of ions
147
Different from nonelectrolyte ndash NO ions at all Description of electrolyte as strong or weak not related to concentration of solution Electrolytes differ in degree of ionization not amount of solute dissolved
148
149
Colligative properties of solutions
Presence of solute affects properties of solutions Some properties not dependent on nature of dissolved substance but on how may dissolve particles are present
Colligative properties - properties that depend on the concentration of solute particles but not on their identity
150
Vapor-Pressure Lowering
Boiling and freezing point of solution different from pure solvent Nonvolatile solute raises boiling point and lowers freezing point Nonvolatile substance - one that has little tendency to become gas under existing conditions
151
152
To understand why nonvolatile solute changes boiling and freezing point consider equilibrium vapor pressure
Vapor pressure pressure caused by molecules that have escape liquid phase to gas phase
Can be thought of a measure of tendency of molecules to escape from a liquid
153
Addition of sucrose (nonvolatile solute) lowers concentration of water molecules at surface of liquid This lowers tendency of water molecules to leave solution and enter gas phase Vapor pressure of solution is lower than vapor pressure of pure water
154
Aqueous13 solution13 of13 nonvolatile13 solutePure13 water
Nonelectrolyte solutions of same molality have same concentration of particles
Dilute solutions of same solvent and equal molality of any nonelectrolyte solute lower vapor pressure equally
155
Example
1 m aqueous solution of nonelectrolyte glucose lowers vapor pressure of water 55 x 10-4 atm at 25 1 m aqueous solution of sucrose also lowers water vapor pressure to 55 x 10-4 atm at 25
156
157
bc vapor-pressure lowering depends on concentration of nonelectrolyte solute and doesnrsquot depend on the type of solute it is a colligative proprerty
bc vapor pressure has been lowered solution remains liquid over larger temperature range
Lowers freezing point and raises boiling point
158
Can assume changes in boiling and freezing point also depend on concentration of solute
They are colligative properties
159
Freezing-Point Depression
Freezing point of 1-molal solution of any nonelectrolyte solute in water is found (by experiment) to be 186 lower than freezing point of water When 1 mol of nonelectrolyte solute dissolved in 1 kg water freezing point is -186 not 000 160
When 2 mol nonelectrolyte solute dissolved in 1 kg water freezing point is -372
For any concentration of nonelectrolyte solute in water decrease in freezing point can be determined using value of -186m
161
This value called the molal freezing-point contstant (Kf) is the freezing-point depression of the solvent in a 1-molal solution of a nonvolatile nonelectrolyte solute
162
Each solvent has own characteristic molal freezing-point constant Freezing-point depression (Δtf) the difference between the freezing points of the pure solvent and a solute of nonelectrolyte in that solvent and it is directly proportional to the molal concentration of the solution Δtf = Kfm
163
Δtf = Kfm
Kf expressed as m
m expressed in mol solutekg solvent (molality)
164
Sample Problem
What is the freezing point depression of water in a solution of 171 g of sucrose C12H22O11 and 200 g of water What is the actual freezing point of the solution
165
1 AnalyzeGiven Solute mass and chemical formula 171 g C12H22O11 Solvent mass and identity 2000 g water Unknown a freezing-point depression b freezing point of the solution 166
2 Plan
167
168
Calculate
Unknown a freezing-point depression b freezing point of the solution
169
Practice Problem
A water solution containing an unknown quantity of a nonelectrolyte solute is found to have a freezing point of ndash 023degCWhat is the molal concentration of the solution
012 m170
A solution consists of 103 g of the nonelectrolyte glucose C6H12O6 dissolved in 250 g of water What is the freezing-point depression of the solution
minus0426degC171
In a laboratory experiment the freezing point of an aqueous solution of glucose is found to be minus0325degCWhat is the molal concentration of this solution
0175 m172
If 0500 mol of a nonelectrolyte solute are dissolved in 5000 g of ether what is the freezing point of the solution
minus1181degC173
The freezing point of an aqueous solution that contains a nonelectrolyte is minus90degC aWhat is the freezing-point depression of the solution bWhat is the molal concentration of the solution a minus90degC b 48 m
174
Boiling-Point Elevation
Boiling point is temp at which vapor pressure is equal to atmospheric pressure Change in vapor pressure will cause corresponding change in boiling point
175
176
Vapor pressure of nonvolatile solution is lower than vapor pressure of pure solvent More heat will be required to raise vapor pressure of solution Boiling point of solution is higher than boiling point of pure solvent
Molal boiling-point constant (Kb) - boiling-point elevation of the solvent in a 1-molal solution of a nonvolatile nonelectrolyte solute b-p elevation of any nonelectrolyte in water found by experiment to be 051 So molal b-p constant for water is 051m
177
For different solvents b-p elevations of 1-molal solutions have different values (Table 14-2)
Like freezing-point constants values most accurate for dilute solutions
178
Boiling-point elevation Δtb difference between the boiling points of the pure solvent and a nonelectrolyte solution of that solvent and it is directly proportional to the molal concentration of the solution B-p elevation can be calculated using Δtb = Kbm Δtb (boiling point elevation) expressed in m and m expressed in mol solutekg solvent
Sample Problem
What is the boiling-point elevation of a solution made from 200 g of a nonelectrolyte solute and 4000 g of water The molar mass of the solute is 620 gmol
180
1 AnalyzeGiven solute mass = 200 g solute molar mass = 620 gmol solvent mass and identity = 4000 g of water
Unknown boiling-point elevation
181
2 Plan
182
183
3 Compute
Practice Problem
A solution contains 500 g of sucrose C12H22O11 a nonelectrolyte dissolved in 5000 g of water What is the boiling-point elevation
015degC184
A solution contains 4500 g of sucrose C12H22O11 a nonelectrolyte dissolved in 250 g of waterWhat is the boiling point of the solution
1027degC185
If the boiling point elevation of an aqueous solution containing a nonvolatile electrolyte is 102degC what is the molality of the solution
20 m186
The boiling point of an aqueous solution containing a nonvolatile electrolyte is 10075degC aWhat is the boiling-point elevation bWhat is the molality of the solution a 075degC b 15 m
187
Electrolytes and Colligative PropertiesEarly investigators confused by experiments where certain substances depressed freezing point or elevated boiling point more than expected
188
189
Ex 01 m solution of NaCl lowers freezing point of solvent almost twice as much as 01 m solution of sucrose 01 m CaCl2 lowers fp almost 3 times as much as 01 m solution of sucrose
To understand why this happens contrast behavior or sucrose with NaCl in aqueous solution Each sucrose dissolves to make only 1 particle in solution So 1 mol sucrose dissolves to 1 mol particles 1 mol NaCl dissolves to make 2 moles of particles (1 mol Na+ and 1 mol Cl-)
190
191
Calculated Values for Electrolyte SolutionsColligative properties depend on total concentration of solute particles regardless of their identity Electrolytes cause changes in colligative properties proportional to total molality in terms of all dissolves particles instead of formula units
192
What about barium nitrate Ba(NO3)2 Each mole of barium nitrate yields 3 mol of ions in solution Ba(NO3)2(s) rarr Ba2+(aq) + 2NO3-(aq) You would expect solution of given molality to lower fp of solvent 3 times as much as nonelectrolytic solution of same molality
193
Sample Problem
What is the expected change in the freezing point of water in a solution of 625 g of barium nitrate Ba(NO3)2 in 100 kg of water
194
1 AnalyzeGiven solute mass and formula = 625 g Ba(NO3)2 solvent mass and identity = 100 kg water Δtf = Kfm
Unknown expected freezing-point depression
195
2 PlanThe molality can be calculated by converting the solute mass to moles and then dividing by the number of kilograms of solvent That molality is in terms of formula units of Ba(NO3)2 and must be converted to molality in terms of dissociated ions in solution It must be multiplied by the number of moles of ions produced per mole of formula unit This adjusted molality can then be used to calculate the freezing-point depression
196
197
198
3 Compute
Practice Problem
What is the expected freezing-point depression for a solution that contains 20 mol of magnesium sulfate dissolved in 10 kg of water
minus74degC199
What is the expected boiling-point elevation of water for a solution that contains 150 g of sodium chloride dissolved in 10 kg of water 27degC The freezing point of an aqueous sodium chloride solution is minus200degC What is the molality of the solution 54 m NaCl
200
Actual Values for Electrolyte SolutionsRemember values calculated are only expected values Actual values of colligative properties for all strong electrolytes are almost what would be expected Difference between expected and actual values caused by attractive forces that exist between dissociated ions in aqueous solution
201
Attraction between hydrated ions in solution is small compared with those in crystalline solid However forces of attraction donrsquot interfere with movements of aqueous ions More concentrated solution means ions closer together and attraction is greater Also depends on charge of ions ndash higher charge greater attraction
202
Concentration - measure of amount of solute in given amount of solvent or solution ldquodiluterdquo just means small amount of solute ldquoconcentratedrdquo just means relatively large amount of solute Unrelated to degree to which solution is saturated Ex Saturated solution of substance not very soluble might be dilute
81
Molarity
Molarity - number of moles of solute in one liter of solution To find molarity must know molar mass A ldquoone molarrdquo solution of NaOH is 1 mole NaOH per liter of solution (1 M)
82
1 mol NaOH = 400 g If this quantity dissolve in enough water to make EXACTLY 100 L solution solution is 1 M If 200 g NaOH dissolve in enough to make 100 L solution what is molarity 0500 M
83
1 M solution is not made by adding 1 mol solute to 1 L solvent That would be more than 1 L Instead 1 mol solute dissolved in less than 1 L Then diluted with more solvent to bring TOTAL VOLUME to 1 L
84
Practice ProblemYou have 340 L of solution that contains 900 g sodium chloride What is the molarity of that solution 0440 M NaCl You have 08 L of a 05 M HCl solution How many moles of HCl does this solution contain 04 mol HCl
85
What is the molarity of a solution composed of 585 g of potassium iodide dissolved in enough water to make a 0125 L of solution 0282 M KI
86
How many moles of H2SO4 are present in 0500 L of a 0150 M H2SO4 solution 00750 mol What volume of 300 M NaCl is needed for reaction that requires 1463 g of NaCl 0834 L
87
Molality
Molality - concentration of solution expressed in moles of solute per kilogram of solvent Solution that contains 1 mol solute (NaOH) dissolved in exactly 1 kg of solvent is ldquoone molalrdquo solution (1 m NaOH)
88
1 mol NaOH = 400 g 400 g NaOH dissolved in 1 kg water is 1 m NaOH
200 g NaOH in 1 kg water = 0500 m NaOH
89
Practice Problem
A solution was prepared by dissolving 171 g sucrose (C12H22O11) in 125 g of water Find the molal concentration of this solution 0400 m C12H22O11
90
A solution of iodine in carbon tetrachloride is used when iodine is needed for certain chemical tests How much iodine must be added to prepare a 0480 m solution of iodine in CCl4 if 1000 g of CCl4 is used 122 g I2
91
What is the molality of a solution composed of 255 g acetone (CH3)2CO dissolved in 200 g of water 22 m acetone What quantity in grams of methanol CH3OH is required to prepare a 0244 m solution in 400 g water 312 g methanol
92
How many grams of AgNO3 are needed to prepare a 0125 m solution in 250 mL of water 531 g AgNO3
What is the molality of a solution containing 182 g HCl and 250 g water 200 m
93
Ions in Aqueous Solutions and Colligative Properties
94
95
compounds in aqueous solutions
DissociationWhen compound made from ions dissolves in water ions separate Dissociation - separation of ions that occurs when an ionic compound dissolves
96
Notice number of ions made per formula unit in equations 1 formula unit of NaCl gives 2 units of ions in solution 1 formula unit of CaCl2 gives 3 units of ions
97
Assuming 100 dissociation solution that contains 1 mol NaCl contains 1 mol Na+ and 1 mol Cl- Can assume 100 dissociation of all soluble ionic compounds
98
Practice Problem
Write the equation for the dissolution of aluminum sulfate in water How many moles of aluminum ions are made by dissolving 1 mol aluminum sulfate What is the total number of moles of ions made by dissolving 1 mol of aluminum sulfate
99
1 AnalyzeGiven Amount of solute = 1 mol Al2(SO4)3 Solvent identity = water Unknown a moles of aluminum ions and sulfate ions Total number of moles of solute ions produced
100
2 Plan
The coefficients in the balanced dissociation equation will tell mole relationships You can use the equation to find out the number of moles of solute ions produced
101
3 Compute
a
b102
Write the equation for the dissolution of each of the following in water and then determine the number of moles of each ions made as well as the total number of ions made a 1 mol ammonium chloride b 1 mol sodium sulfide c 05 mol barium nitrate
103
a 1 mol ammonium chloride
1 mol NH4+ 1 mol Cl- 2 mol total ions
104
b 1 mol sodium sulfide
2 mol Na+ 1 mol S-2 3 mol total ions
105
c 05 mol barium nitrate
05 mol Ba+2 1 mol NO3- 15 mol total ions
106
Precipitation Reactions
Even though no compound is completely insoluble compounds of very low solubility can be considered insoluble for practical purposes There are general guidelines to help predict whether a compound made of certain combination of ions is soluble
107
General Solubility Guidelines
Sodium potassium and ammonium compounds are soluble in water
108
Nitrates acetates and chlorates are soluble
109
110
Most chlorides are soluble except those of silver mercury (I) and lead Lead (II) chloride is soluble in hot water
Most sulfates are soluble except those of barium strontium lead calcium and mercury
111
Most carbonates phosphates and silicates are insoluble except those of sodium potassium and ammonium
112
Most sulfides are insoluble except those of calcium strontium sodium potassium and ammonium
113
Is calcium phosphate Ca3(PO4)2 soluble or insoluble
Not one of the exceptions so it is insoluble
Dissociation equations are not written for insoluble compounds
114
Guidelines also useful in predicting what will happen if 2 solutions of 2 different soluble compounds are mixed
If mixing results in combination of ions that forms insoluble compound a double-replacement and precipitation reaction will happen
115
Will precipitate form when solutions of ammonium sulfide and cadmium nitrate are combined You can tell calcium nitrate is soluble from the guidelines (Most nitrates acetates and chlorates are soluble) You can also tell ammonium sulfide is soluble (Most sulfides are insoluble except those of calcium strontium sodium potassium and ammonium)
116
117
To decide whether precipitate can form must know solubilities of two possible products
Ammonium nitrate ndash soluble (Most nitrates acetates and chlorates are soluble) Cadmium sulfide ndash insoluble (Most sulfides are insoluble except those of calcium strontium sodium potassium and ammonium)
118
bc one product is insoluble double-replacement and precipitation reaction will happen (NH4)2S (aq) + Cd(NO3)2 (aq) rarr NH4NO3 (aq) + CdS (s)
119
Net Ionic Equations
Reactions of ions in aqueous solution usually represented by net ionic equations instead of formula equations Net ionic equation - includes only those compounds and ions that undergo a chemical change in a reaction in an aqueous solution
120
121
To write NIE (net ionic equation) first convert chemical equation into overall ionic equation All soluble ionic compounds shown as dissociated ions in solution Precipitates shown as solids
122
Notice ammonium and nitrate ions appear on both sides They havenrsquot gone through any chemical change Spectator ions - ions that do not take part in a chemical reaction and are found in solution both before and after the reaction
123
To change ionic equation to NIE eliminate spectator ions Cd+2(aq) + S-2(aq) rarr CdS(s)
This NIE applies to any reaction in which precipitate of CdS forms
124
Practice Problem
Identify the precipitate that forms when aqueous solutions of zinc nitrate and ammonium sulfide are combined Write the equation for the possible double-replacement reaction Then write the formula equation overall ionic equation and the net ionic equation
125
1 AnalyzeGiven Identity of reactants zinc nitrate and ammonium sulfide Reaction medium aqueous solution Unknown a equation for the possible double-replacement reaction b identity of the precipitate c formula equation d overall ionic equation e net ionic equation
126
2 PlanWrite the possible double-replacement reaction between Zn(NO3)2 and (NH4)2S Use the rules to determine if any of the products will precipitate Write a formula equation and overall net ionic equation Then cancel spectator ions to make net ionic equation
127
3 Computea possible double-replacement reaction
b rules show that zinc sulfide is not soluble so will precipitate (ammonium nitrate is soluble)
c formula equation128
d overall ionic equation
e net ionic equation129
Will a precipitate form if solutions of potassium sulfate and barium nitrate are combined If so write the net ionic equation for the reaction
Yes Ba+2(aq) + SO4-2(aq) rarr BaSO4(s)
130
Will a precipitate form if solutions of potassium nitrate and magnesium sulfate are combined If so write the net ionic equation for the reaction
No 131
Will a precipitate form if solutions of barium chloride and sodium sulfate are combined If so identify the spectator ions and write the net ionic equation for the reaction
Yes Na+ and Cl- Ba+2(aq) + SO4-2(aq) rarr BaSO4(s)
132
Write the net ionic equation for the precipitation of nickel(II) sulfide
Ni+2(aq) + S-2(aq) rarr NiS(s)
133
IonizationSome molecular compounds can also form ions in solution Usually polar Ionization - ions formed from solute molecules by action of the solvent (creation of ions where there were none) Ionization different from dissociation
134
Dissociation ionic compounds dissolve and ions already present separate Ionization ions formed where non existed before
Ions formed are hydrated Heat released during hydration of ions gives enough energy to break covalent bonds
135
Extent to which solute ionizes depends on strength of bonds within molecules of solute and strength of attraction between solute and solvent molecules
136
HCl ndash molecular compound that ionizes in aqueous solution Contains highly polar bond Attraction between polar HCl molecule and polar water strong enough to break HCl bond
137
The Hydronium Ion
H+ ions from HCl attracts other molecules or ions so strongly that it doesnrsquot normally exist alone Ionization of HCl better described as direct transfer of proton from HCl to H2O forming H3O+
138
139
Hydration of H+ to form hydronium is highly exothermic
Energy released gives a lot of energy needed to ionize a molecular solute
Many molecular compounds that ionize in aqueous solution contain hydrogen and form hydronium ion
140
Strong and Weak Electrolytes
Substances that make ions and conduct current in solution are electrolytes HCl is one of series of compounds that has hydrogen and halogen Hydrogen halides are all molecular compounds with single polar-covalent bonds
141
All are gases Very soluble in water All are electrolytes HCl HBr HI strongly conduct current in solution HF only weakly conducts current at same concentration Strength of conduction related to ability to ionize
142
143
Strong Electrolytes
HCl HBr HI are 100 ionized in solution
Strong electrolyte - any compound whose dilute aqueous solutions conduct electricity well this is due to the presence of all or almost all of the dissolve compound in the form of ions
144
Unique characteristic of strong electrolyte is that they yield only ions
Ex Ionic compound may be highly soluble in water and dissociate into ions (NaCl)
Others may not dissolve much but amount that does dissolve exists only as ions
145
Weak ElectrolytesHF dissolves in water to give acid solution called hydrofluoric acid HF bond stronger than bonds between hydrogen and other halogens When HF dissolves some molecule ionize Reverse reaction also happens HF(aq) + H2O(l) H3O+(aq) + F-(aq)
146
HF(aq) + H2O(l) H3O+
(aq) + F-(aq)Concentration of dissolved unionized HF stays high and concentration of ions stays low
Weak electrolyte - any compound whose aqueous solutions conduct electricity poorly this is due to the presence of small amount of the dissolved compound in the form of ions
147
Different from nonelectrolyte ndash NO ions at all Description of electrolyte as strong or weak not related to concentration of solution Electrolytes differ in degree of ionization not amount of solute dissolved
148
149
Colligative properties of solutions
Presence of solute affects properties of solutions Some properties not dependent on nature of dissolved substance but on how may dissolve particles are present
Colligative properties - properties that depend on the concentration of solute particles but not on their identity
150
Vapor-Pressure Lowering
Boiling and freezing point of solution different from pure solvent Nonvolatile solute raises boiling point and lowers freezing point Nonvolatile substance - one that has little tendency to become gas under existing conditions
151
152
To understand why nonvolatile solute changes boiling and freezing point consider equilibrium vapor pressure
Vapor pressure pressure caused by molecules that have escape liquid phase to gas phase
Can be thought of a measure of tendency of molecules to escape from a liquid
153
Addition of sucrose (nonvolatile solute) lowers concentration of water molecules at surface of liquid This lowers tendency of water molecules to leave solution and enter gas phase Vapor pressure of solution is lower than vapor pressure of pure water
154
Aqueous13 solution13 of13 nonvolatile13 solutePure13 water
Nonelectrolyte solutions of same molality have same concentration of particles
Dilute solutions of same solvent and equal molality of any nonelectrolyte solute lower vapor pressure equally
155
Example
1 m aqueous solution of nonelectrolyte glucose lowers vapor pressure of water 55 x 10-4 atm at 25 1 m aqueous solution of sucrose also lowers water vapor pressure to 55 x 10-4 atm at 25
156
157
bc vapor-pressure lowering depends on concentration of nonelectrolyte solute and doesnrsquot depend on the type of solute it is a colligative proprerty
bc vapor pressure has been lowered solution remains liquid over larger temperature range
Lowers freezing point and raises boiling point
158
Can assume changes in boiling and freezing point also depend on concentration of solute
They are colligative properties
159
Freezing-Point Depression
Freezing point of 1-molal solution of any nonelectrolyte solute in water is found (by experiment) to be 186 lower than freezing point of water When 1 mol of nonelectrolyte solute dissolved in 1 kg water freezing point is -186 not 000 160
When 2 mol nonelectrolyte solute dissolved in 1 kg water freezing point is -372
For any concentration of nonelectrolyte solute in water decrease in freezing point can be determined using value of -186m
161
This value called the molal freezing-point contstant (Kf) is the freezing-point depression of the solvent in a 1-molal solution of a nonvolatile nonelectrolyte solute
162
Each solvent has own characteristic molal freezing-point constant Freezing-point depression (Δtf) the difference between the freezing points of the pure solvent and a solute of nonelectrolyte in that solvent and it is directly proportional to the molal concentration of the solution Δtf = Kfm
163
Δtf = Kfm
Kf expressed as m
m expressed in mol solutekg solvent (molality)
164
Sample Problem
What is the freezing point depression of water in a solution of 171 g of sucrose C12H22O11 and 200 g of water What is the actual freezing point of the solution
165
1 AnalyzeGiven Solute mass and chemical formula 171 g C12H22O11 Solvent mass and identity 2000 g water Unknown a freezing-point depression b freezing point of the solution 166
2 Plan
167
168
Calculate
Unknown a freezing-point depression b freezing point of the solution
169
Practice Problem
A water solution containing an unknown quantity of a nonelectrolyte solute is found to have a freezing point of ndash 023degCWhat is the molal concentration of the solution
012 m170
A solution consists of 103 g of the nonelectrolyte glucose C6H12O6 dissolved in 250 g of water What is the freezing-point depression of the solution
minus0426degC171
In a laboratory experiment the freezing point of an aqueous solution of glucose is found to be minus0325degCWhat is the molal concentration of this solution
0175 m172
If 0500 mol of a nonelectrolyte solute are dissolved in 5000 g of ether what is the freezing point of the solution
minus1181degC173
The freezing point of an aqueous solution that contains a nonelectrolyte is minus90degC aWhat is the freezing-point depression of the solution bWhat is the molal concentration of the solution a minus90degC b 48 m
174
Boiling-Point Elevation
Boiling point is temp at which vapor pressure is equal to atmospheric pressure Change in vapor pressure will cause corresponding change in boiling point
175
176
Vapor pressure of nonvolatile solution is lower than vapor pressure of pure solvent More heat will be required to raise vapor pressure of solution Boiling point of solution is higher than boiling point of pure solvent
Molal boiling-point constant (Kb) - boiling-point elevation of the solvent in a 1-molal solution of a nonvolatile nonelectrolyte solute b-p elevation of any nonelectrolyte in water found by experiment to be 051 So molal b-p constant for water is 051m
177
For different solvents b-p elevations of 1-molal solutions have different values (Table 14-2)
Like freezing-point constants values most accurate for dilute solutions
178
Boiling-point elevation Δtb difference between the boiling points of the pure solvent and a nonelectrolyte solution of that solvent and it is directly proportional to the molal concentration of the solution B-p elevation can be calculated using Δtb = Kbm Δtb (boiling point elevation) expressed in m and m expressed in mol solutekg solvent
Sample Problem
What is the boiling-point elevation of a solution made from 200 g of a nonelectrolyte solute and 4000 g of water The molar mass of the solute is 620 gmol
180
1 AnalyzeGiven solute mass = 200 g solute molar mass = 620 gmol solvent mass and identity = 4000 g of water
Unknown boiling-point elevation
181
2 Plan
182
183
3 Compute
Practice Problem
A solution contains 500 g of sucrose C12H22O11 a nonelectrolyte dissolved in 5000 g of water What is the boiling-point elevation
015degC184
A solution contains 4500 g of sucrose C12H22O11 a nonelectrolyte dissolved in 250 g of waterWhat is the boiling point of the solution
1027degC185
If the boiling point elevation of an aqueous solution containing a nonvolatile electrolyte is 102degC what is the molality of the solution
20 m186
The boiling point of an aqueous solution containing a nonvolatile electrolyte is 10075degC aWhat is the boiling-point elevation bWhat is the molality of the solution a 075degC b 15 m
187
Electrolytes and Colligative PropertiesEarly investigators confused by experiments where certain substances depressed freezing point or elevated boiling point more than expected
188
189
Ex 01 m solution of NaCl lowers freezing point of solvent almost twice as much as 01 m solution of sucrose 01 m CaCl2 lowers fp almost 3 times as much as 01 m solution of sucrose
To understand why this happens contrast behavior or sucrose with NaCl in aqueous solution Each sucrose dissolves to make only 1 particle in solution So 1 mol sucrose dissolves to 1 mol particles 1 mol NaCl dissolves to make 2 moles of particles (1 mol Na+ and 1 mol Cl-)
190
191
Calculated Values for Electrolyte SolutionsColligative properties depend on total concentration of solute particles regardless of their identity Electrolytes cause changes in colligative properties proportional to total molality in terms of all dissolves particles instead of formula units
192
What about barium nitrate Ba(NO3)2 Each mole of barium nitrate yields 3 mol of ions in solution Ba(NO3)2(s) rarr Ba2+(aq) + 2NO3-(aq) You would expect solution of given molality to lower fp of solvent 3 times as much as nonelectrolytic solution of same molality
193
Sample Problem
What is the expected change in the freezing point of water in a solution of 625 g of barium nitrate Ba(NO3)2 in 100 kg of water
194
1 AnalyzeGiven solute mass and formula = 625 g Ba(NO3)2 solvent mass and identity = 100 kg water Δtf = Kfm
Unknown expected freezing-point depression
195
2 PlanThe molality can be calculated by converting the solute mass to moles and then dividing by the number of kilograms of solvent That molality is in terms of formula units of Ba(NO3)2 and must be converted to molality in terms of dissociated ions in solution It must be multiplied by the number of moles of ions produced per mole of formula unit This adjusted molality can then be used to calculate the freezing-point depression
196
197
198
3 Compute
Practice Problem
What is the expected freezing-point depression for a solution that contains 20 mol of magnesium sulfate dissolved in 10 kg of water
minus74degC199
What is the expected boiling-point elevation of water for a solution that contains 150 g of sodium chloride dissolved in 10 kg of water 27degC The freezing point of an aqueous sodium chloride solution is minus200degC What is the molality of the solution 54 m NaCl
200
Actual Values for Electrolyte SolutionsRemember values calculated are only expected values Actual values of colligative properties for all strong electrolytes are almost what would be expected Difference between expected and actual values caused by attractive forces that exist between dissociated ions in aqueous solution
201
Attraction between hydrated ions in solution is small compared with those in crystalline solid However forces of attraction donrsquot interfere with movements of aqueous ions More concentrated solution means ions closer together and attraction is greater Also depends on charge of ions ndash higher charge greater attraction
202
Molarity
Molarity - number of moles of solute in one liter of solution To find molarity must know molar mass A ldquoone molarrdquo solution of NaOH is 1 mole NaOH per liter of solution (1 M)
82
1 mol NaOH = 400 g If this quantity dissolve in enough water to make EXACTLY 100 L solution solution is 1 M If 200 g NaOH dissolve in enough to make 100 L solution what is molarity 0500 M
83
1 M solution is not made by adding 1 mol solute to 1 L solvent That would be more than 1 L Instead 1 mol solute dissolved in less than 1 L Then diluted with more solvent to bring TOTAL VOLUME to 1 L
84
Practice ProblemYou have 340 L of solution that contains 900 g sodium chloride What is the molarity of that solution 0440 M NaCl You have 08 L of a 05 M HCl solution How many moles of HCl does this solution contain 04 mol HCl
85
What is the molarity of a solution composed of 585 g of potassium iodide dissolved in enough water to make a 0125 L of solution 0282 M KI
86
How many moles of H2SO4 are present in 0500 L of a 0150 M H2SO4 solution 00750 mol What volume of 300 M NaCl is needed for reaction that requires 1463 g of NaCl 0834 L
87
Molality
Molality - concentration of solution expressed in moles of solute per kilogram of solvent Solution that contains 1 mol solute (NaOH) dissolved in exactly 1 kg of solvent is ldquoone molalrdquo solution (1 m NaOH)
88
1 mol NaOH = 400 g 400 g NaOH dissolved in 1 kg water is 1 m NaOH
200 g NaOH in 1 kg water = 0500 m NaOH
89
Practice Problem
A solution was prepared by dissolving 171 g sucrose (C12H22O11) in 125 g of water Find the molal concentration of this solution 0400 m C12H22O11
90
A solution of iodine in carbon tetrachloride is used when iodine is needed for certain chemical tests How much iodine must be added to prepare a 0480 m solution of iodine in CCl4 if 1000 g of CCl4 is used 122 g I2
91
What is the molality of a solution composed of 255 g acetone (CH3)2CO dissolved in 200 g of water 22 m acetone What quantity in grams of methanol CH3OH is required to prepare a 0244 m solution in 400 g water 312 g methanol
92
How many grams of AgNO3 are needed to prepare a 0125 m solution in 250 mL of water 531 g AgNO3
What is the molality of a solution containing 182 g HCl and 250 g water 200 m
93
Ions in Aqueous Solutions and Colligative Properties
94
95
compounds in aqueous solutions
DissociationWhen compound made from ions dissolves in water ions separate Dissociation - separation of ions that occurs when an ionic compound dissolves
96
Notice number of ions made per formula unit in equations 1 formula unit of NaCl gives 2 units of ions in solution 1 formula unit of CaCl2 gives 3 units of ions
97
Assuming 100 dissociation solution that contains 1 mol NaCl contains 1 mol Na+ and 1 mol Cl- Can assume 100 dissociation of all soluble ionic compounds
98
Practice Problem
Write the equation for the dissolution of aluminum sulfate in water How many moles of aluminum ions are made by dissolving 1 mol aluminum sulfate What is the total number of moles of ions made by dissolving 1 mol of aluminum sulfate
99
1 AnalyzeGiven Amount of solute = 1 mol Al2(SO4)3 Solvent identity = water Unknown a moles of aluminum ions and sulfate ions Total number of moles of solute ions produced
100
2 Plan
The coefficients in the balanced dissociation equation will tell mole relationships You can use the equation to find out the number of moles of solute ions produced
101
3 Compute
a
b102
Write the equation for the dissolution of each of the following in water and then determine the number of moles of each ions made as well as the total number of ions made a 1 mol ammonium chloride b 1 mol sodium sulfide c 05 mol barium nitrate
103
a 1 mol ammonium chloride
1 mol NH4+ 1 mol Cl- 2 mol total ions
104
b 1 mol sodium sulfide
2 mol Na+ 1 mol S-2 3 mol total ions
105
c 05 mol barium nitrate
05 mol Ba+2 1 mol NO3- 15 mol total ions
106
Precipitation Reactions
Even though no compound is completely insoluble compounds of very low solubility can be considered insoluble for practical purposes There are general guidelines to help predict whether a compound made of certain combination of ions is soluble
107
General Solubility Guidelines
Sodium potassium and ammonium compounds are soluble in water
108
Nitrates acetates and chlorates are soluble
109
110
Most chlorides are soluble except those of silver mercury (I) and lead Lead (II) chloride is soluble in hot water
Most sulfates are soluble except those of barium strontium lead calcium and mercury
111
Most carbonates phosphates and silicates are insoluble except those of sodium potassium and ammonium
112
Most sulfides are insoluble except those of calcium strontium sodium potassium and ammonium
113
Is calcium phosphate Ca3(PO4)2 soluble or insoluble
Not one of the exceptions so it is insoluble
Dissociation equations are not written for insoluble compounds
114
Guidelines also useful in predicting what will happen if 2 solutions of 2 different soluble compounds are mixed
If mixing results in combination of ions that forms insoluble compound a double-replacement and precipitation reaction will happen
115
Will precipitate form when solutions of ammonium sulfide and cadmium nitrate are combined You can tell calcium nitrate is soluble from the guidelines (Most nitrates acetates and chlorates are soluble) You can also tell ammonium sulfide is soluble (Most sulfides are insoluble except those of calcium strontium sodium potassium and ammonium)
116
117
To decide whether precipitate can form must know solubilities of two possible products
Ammonium nitrate ndash soluble (Most nitrates acetates and chlorates are soluble) Cadmium sulfide ndash insoluble (Most sulfides are insoluble except those of calcium strontium sodium potassium and ammonium)
118
bc one product is insoluble double-replacement and precipitation reaction will happen (NH4)2S (aq) + Cd(NO3)2 (aq) rarr NH4NO3 (aq) + CdS (s)
119
Net Ionic Equations
Reactions of ions in aqueous solution usually represented by net ionic equations instead of formula equations Net ionic equation - includes only those compounds and ions that undergo a chemical change in a reaction in an aqueous solution
120
121
To write NIE (net ionic equation) first convert chemical equation into overall ionic equation All soluble ionic compounds shown as dissociated ions in solution Precipitates shown as solids
122
Notice ammonium and nitrate ions appear on both sides They havenrsquot gone through any chemical change Spectator ions - ions that do not take part in a chemical reaction and are found in solution both before and after the reaction
123
To change ionic equation to NIE eliminate spectator ions Cd+2(aq) + S-2(aq) rarr CdS(s)
This NIE applies to any reaction in which precipitate of CdS forms
124
Practice Problem
Identify the precipitate that forms when aqueous solutions of zinc nitrate and ammonium sulfide are combined Write the equation for the possible double-replacement reaction Then write the formula equation overall ionic equation and the net ionic equation
125
1 AnalyzeGiven Identity of reactants zinc nitrate and ammonium sulfide Reaction medium aqueous solution Unknown a equation for the possible double-replacement reaction b identity of the precipitate c formula equation d overall ionic equation e net ionic equation
126
2 PlanWrite the possible double-replacement reaction between Zn(NO3)2 and (NH4)2S Use the rules to determine if any of the products will precipitate Write a formula equation and overall net ionic equation Then cancel spectator ions to make net ionic equation
127
3 Computea possible double-replacement reaction
b rules show that zinc sulfide is not soluble so will precipitate (ammonium nitrate is soluble)
c formula equation128
d overall ionic equation
e net ionic equation129
Will a precipitate form if solutions of potassium sulfate and barium nitrate are combined If so write the net ionic equation for the reaction
Yes Ba+2(aq) + SO4-2(aq) rarr BaSO4(s)
130
Will a precipitate form if solutions of potassium nitrate and magnesium sulfate are combined If so write the net ionic equation for the reaction
No 131
Will a precipitate form if solutions of barium chloride and sodium sulfate are combined If so identify the spectator ions and write the net ionic equation for the reaction
Yes Na+ and Cl- Ba+2(aq) + SO4-2(aq) rarr BaSO4(s)
132
Write the net ionic equation for the precipitation of nickel(II) sulfide
Ni+2(aq) + S-2(aq) rarr NiS(s)
133
IonizationSome molecular compounds can also form ions in solution Usually polar Ionization - ions formed from solute molecules by action of the solvent (creation of ions where there were none) Ionization different from dissociation
134
Dissociation ionic compounds dissolve and ions already present separate Ionization ions formed where non existed before
Ions formed are hydrated Heat released during hydration of ions gives enough energy to break covalent bonds
135
Extent to which solute ionizes depends on strength of bonds within molecules of solute and strength of attraction between solute and solvent molecules
136
HCl ndash molecular compound that ionizes in aqueous solution Contains highly polar bond Attraction between polar HCl molecule and polar water strong enough to break HCl bond
137
The Hydronium Ion
H+ ions from HCl attracts other molecules or ions so strongly that it doesnrsquot normally exist alone Ionization of HCl better described as direct transfer of proton from HCl to H2O forming H3O+
138
139
Hydration of H+ to form hydronium is highly exothermic
Energy released gives a lot of energy needed to ionize a molecular solute
Many molecular compounds that ionize in aqueous solution contain hydrogen and form hydronium ion
140
Strong and Weak Electrolytes
Substances that make ions and conduct current in solution are electrolytes HCl is one of series of compounds that has hydrogen and halogen Hydrogen halides are all molecular compounds with single polar-covalent bonds
141
All are gases Very soluble in water All are electrolytes HCl HBr HI strongly conduct current in solution HF only weakly conducts current at same concentration Strength of conduction related to ability to ionize
142
143
Strong Electrolytes
HCl HBr HI are 100 ionized in solution
Strong electrolyte - any compound whose dilute aqueous solutions conduct electricity well this is due to the presence of all or almost all of the dissolve compound in the form of ions
144
Unique characteristic of strong electrolyte is that they yield only ions
Ex Ionic compound may be highly soluble in water and dissociate into ions (NaCl)
Others may not dissolve much but amount that does dissolve exists only as ions
145
Weak ElectrolytesHF dissolves in water to give acid solution called hydrofluoric acid HF bond stronger than bonds between hydrogen and other halogens When HF dissolves some molecule ionize Reverse reaction also happens HF(aq) + H2O(l) H3O+(aq) + F-(aq)
146
HF(aq) + H2O(l) H3O+
(aq) + F-(aq)Concentration of dissolved unionized HF stays high and concentration of ions stays low
Weak electrolyte - any compound whose aqueous solutions conduct electricity poorly this is due to the presence of small amount of the dissolved compound in the form of ions
147
Different from nonelectrolyte ndash NO ions at all Description of electrolyte as strong or weak not related to concentration of solution Electrolytes differ in degree of ionization not amount of solute dissolved
148
149
Colligative properties of solutions
Presence of solute affects properties of solutions Some properties not dependent on nature of dissolved substance but on how may dissolve particles are present
Colligative properties - properties that depend on the concentration of solute particles but not on their identity
150
Vapor-Pressure Lowering
Boiling and freezing point of solution different from pure solvent Nonvolatile solute raises boiling point and lowers freezing point Nonvolatile substance - one that has little tendency to become gas under existing conditions
151
152
To understand why nonvolatile solute changes boiling and freezing point consider equilibrium vapor pressure
Vapor pressure pressure caused by molecules that have escape liquid phase to gas phase
Can be thought of a measure of tendency of molecules to escape from a liquid
153
Addition of sucrose (nonvolatile solute) lowers concentration of water molecules at surface of liquid This lowers tendency of water molecules to leave solution and enter gas phase Vapor pressure of solution is lower than vapor pressure of pure water
154
Aqueous13 solution13 of13 nonvolatile13 solutePure13 water
Nonelectrolyte solutions of same molality have same concentration of particles
Dilute solutions of same solvent and equal molality of any nonelectrolyte solute lower vapor pressure equally
155
Example
1 m aqueous solution of nonelectrolyte glucose lowers vapor pressure of water 55 x 10-4 atm at 25 1 m aqueous solution of sucrose also lowers water vapor pressure to 55 x 10-4 atm at 25
156
157
bc vapor-pressure lowering depends on concentration of nonelectrolyte solute and doesnrsquot depend on the type of solute it is a colligative proprerty
bc vapor pressure has been lowered solution remains liquid over larger temperature range
Lowers freezing point and raises boiling point
158
Can assume changes in boiling and freezing point also depend on concentration of solute
They are colligative properties
159
Freezing-Point Depression
Freezing point of 1-molal solution of any nonelectrolyte solute in water is found (by experiment) to be 186 lower than freezing point of water When 1 mol of nonelectrolyte solute dissolved in 1 kg water freezing point is -186 not 000 160
When 2 mol nonelectrolyte solute dissolved in 1 kg water freezing point is -372
For any concentration of nonelectrolyte solute in water decrease in freezing point can be determined using value of -186m
161
This value called the molal freezing-point contstant (Kf) is the freezing-point depression of the solvent in a 1-molal solution of a nonvolatile nonelectrolyte solute
162
Each solvent has own characteristic molal freezing-point constant Freezing-point depression (Δtf) the difference between the freezing points of the pure solvent and a solute of nonelectrolyte in that solvent and it is directly proportional to the molal concentration of the solution Δtf = Kfm
163
Δtf = Kfm
Kf expressed as m
m expressed in mol solutekg solvent (molality)
164
Sample Problem
What is the freezing point depression of water in a solution of 171 g of sucrose C12H22O11 and 200 g of water What is the actual freezing point of the solution
165
1 AnalyzeGiven Solute mass and chemical formula 171 g C12H22O11 Solvent mass and identity 2000 g water Unknown a freezing-point depression b freezing point of the solution 166
2 Plan
167
168
Calculate
Unknown a freezing-point depression b freezing point of the solution
169
Practice Problem
A water solution containing an unknown quantity of a nonelectrolyte solute is found to have a freezing point of ndash 023degCWhat is the molal concentration of the solution
012 m170
A solution consists of 103 g of the nonelectrolyte glucose C6H12O6 dissolved in 250 g of water What is the freezing-point depression of the solution
minus0426degC171
In a laboratory experiment the freezing point of an aqueous solution of glucose is found to be minus0325degCWhat is the molal concentration of this solution
0175 m172
If 0500 mol of a nonelectrolyte solute are dissolved in 5000 g of ether what is the freezing point of the solution
minus1181degC173
The freezing point of an aqueous solution that contains a nonelectrolyte is minus90degC aWhat is the freezing-point depression of the solution bWhat is the molal concentration of the solution a minus90degC b 48 m
174
Boiling-Point Elevation
Boiling point is temp at which vapor pressure is equal to atmospheric pressure Change in vapor pressure will cause corresponding change in boiling point
175
176
Vapor pressure of nonvolatile solution is lower than vapor pressure of pure solvent More heat will be required to raise vapor pressure of solution Boiling point of solution is higher than boiling point of pure solvent
Molal boiling-point constant (Kb) - boiling-point elevation of the solvent in a 1-molal solution of a nonvolatile nonelectrolyte solute b-p elevation of any nonelectrolyte in water found by experiment to be 051 So molal b-p constant for water is 051m
177
For different solvents b-p elevations of 1-molal solutions have different values (Table 14-2)
Like freezing-point constants values most accurate for dilute solutions
178
Boiling-point elevation Δtb difference between the boiling points of the pure solvent and a nonelectrolyte solution of that solvent and it is directly proportional to the molal concentration of the solution B-p elevation can be calculated using Δtb = Kbm Δtb (boiling point elevation) expressed in m and m expressed in mol solutekg solvent
Sample Problem
What is the boiling-point elevation of a solution made from 200 g of a nonelectrolyte solute and 4000 g of water The molar mass of the solute is 620 gmol
180
1 AnalyzeGiven solute mass = 200 g solute molar mass = 620 gmol solvent mass and identity = 4000 g of water
Unknown boiling-point elevation
181
2 Plan
182
183
3 Compute
Practice Problem
A solution contains 500 g of sucrose C12H22O11 a nonelectrolyte dissolved in 5000 g of water What is the boiling-point elevation
015degC184
A solution contains 4500 g of sucrose C12H22O11 a nonelectrolyte dissolved in 250 g of waterWhat is the boiling point of the solution
1027degC185
If the boiling point elevation of an aqueous solution containing a nonvolatile electrolyte is 102degC what is the molality of the solution
20 m186
The boiling point of an aqueous solution containing a nonvolatile electrolyte is 10075degC aWhat is the boiling-point elevation bWhat is the molality of the solution a 075degC b 15 m
187
Electrolytes and Colligative PropertiesEarly investigators confused by experiments where certain substances depressed freezing point or elevated boiling point more than expected
188
189
Ex 01 m solution of NaCl lowers freezing point of solvent almost twice as much as 01 m solution of sucrose 01 m CaCl2 lowers fp almost 3 times as much as 01 m solution of sucrose
To understand why this happens contrast behavior or sucrose with NaCl in aqueous solution Each sucrose dissolves to make only 1 particle in solution So 1 mol sucrose dissolves to 1 mol particles 1 mol NaCl dissolves to make 2 moles of particles (1 mol Na+ and 1 mol Cl-)
190
191
Calculated Values for Electrolyte SolutionsColligative properties depend on total concentration of solute particles regardless of their identity Electrolytes cause changes in colligative properties proportional to total molality in terms of all dissolves particles instead of formula units
192
What about barium nitrate Ba(NO3)2 Each mole of barium nitrate yields 3 mol of ions in solution Ba(NO3)2(s) rarr Ba2+(aq) + 2NO3-(aq) You would expect solution of given molality to lower fp of solvent 3 times as much as nonelectrolytic solution of same molality
193
Sample Problem
What is the expected change in the freezing point of water in a solution of 625 g of barium nitrate Ba(NO3)2 in 100 kg of water
194
1 AnalyzeGiven solute mass and formula = 625 g Ba(NO3)2 solvent mass and identity = 100 kg water Δtf = Kfm
Unknown expected freezing-point depression
195
2 PlanThe molality can be calculated by converting the solute mass to moles and then dividing by the number of kilograms of solvent That molality is in terms of formula units of Ba(NO3)2 and must be converted to molality in terms of dissociated ions in solution It must be multiplied by the number of moles of ions produced per mole of formula unit This adjusted molality can then be used to calculate the freezing-point depression
196
197
198
3 Compute
Practice Problem
What is the expected freezing-point depression for a solution that contains 20 mol of magnesium sulfate dissolved in 10 kg of water
minus74degC199
What is the expected boiling-point elevation of water for a solution that contains 150 g of sodium chloride dissolved in 10 kg of water 27degC The freezing point of an aqueous sodium chloride solution is minus200degC What is the molality of the solution 54 m NaCl
200
Actual Values for Electrolyte SolutionsRemember values calculated are only expected values Actual values of colligative properties for all strong electrolytes are almost what would be expected Difference between expected and actual values caused by attractive forces that exist between dissociated ions in aqueous solution
201
Attraction between hydrated ions in solution is small compared with those in crystalline solid However forces of attraction donrsquot interfere with movements of aqueous ions More concentrated solution means ions closer together and attraction is greater Also depends on charge of ions ndash higher charge greater attraction
202
1 mol NaOH = 400 g If this quantity dissolve in enough water to make EXACTLY 100 L solution solution is 1 M If 200 g NaOH dissolve in enough to make 100 L solution what is molarity 0500 M
83
1 M solution is not made by adding 1 mol solute to 1 L solvent That would be more than 1 L Instead 1 mol solute dissolved in less than 1 L Then diluted with more solvent to bring TOTAL VOLUME to 1 L
84
Practice ProblemYou have 340 L of solution that contains 900 g sodium chloride What is the molarity of that solution 0440 M NaCl You have 08 L of a 05 M HCl solution How many moles of HCl does this solution contain 04 mol HCl
85
What is the molarity of a solution composed of 585 g of potassium iodide dissolved in enough water to make a 0125 L of solution 0282 M KI
86
How many moles of H2SO4 are present in 0500 L of a 0150 M H2SO4 solution 00750 mol What volume of 300 M NaCl is needed for reaction that requires 1463 g of NaCl 0834 L
87
Molality
Molality - concentration of solution expressed in moles of solute per kilogram of solvent Solution that contains 1 mol solute (NaOH) dissolved in exactly 1 kg of solvent is ldquoone molalrdquo solution (1 m NaOH)
88
1 mol NaOH = 400 g 400 g NaOH dissolved in 1 kg water is 1 m NaOH
200 g NaOH in 1 kg water = 0500 m NaOH
89
Practice Problem
A solution was prepared by dissolving 171 g sucrose (C12H22O11) in 125 g of water Find the molal concentration of this solution 0400 m C12H22O11
90
A solution of iodine in carbon tetrachloride is used when iodine is needed for certain chemical tests How much iodine must be added to prepare a 0480 m solution of iodine in CCl4 if 1000 g of CCl4 is used 122 g I2
91
What is the molality of a solution composed of 255 g acetone (CH3)2CO dissolved in 200 g of water 22 m acetone What quantity in grams of methanol CH3OH is required to prepare a 0244 m solution in 400 g water 312 g methanol
92
How many grams of AgNO3 are needed to prepare a 0125 m solution in 250 mL of water 531 g AgNO3
What is the molality of a solution containing 182 g HCl and 250 g water 200 m
93
Ions in Aqueous Solutions and Colligative Properties
94
95
compounds in aqueous solutions
DissociationWhen compound made from ions dissolves in water ions separate Dissociation - separation of ions that occurs when an ionic compound dissolves
96
Notice number of ions made per formula unit in equations 1 formula unit of NaCl gives 2 units of ions in solution 1 formula unit of CaCl2 gives 3 units of ions
97
Assuming 100 dissociation solution that contains 1 mol NaCl contains 1 mol Na+ and 1 mol Cl- Can assume 100 dissociation of all soluble ionic compounds
98
Practice Problem
Write the equation for the dissolution of aluminum sulfate in water How many moles of aluminum ions are made by dissolving 1 mol aluminum sulfate What is the total number of moles of ions made by dissolving 1 mol of aluminum sulfate
99
1 AnalyzeGiven Amount of solute = 1 mol Al2(SO4)3 Solvent identity = water Unknown a moles of aluminum ions and sulfate ions Total number of moles of solute ions produced
100
2 Plan
The coefficients in the balanced dissociation equation will tell mole relationships You can use the equation to find out the number of moles of solute ions produced
101
3 Compute
a
b102
Write the equation for the dissolution of each of the following in water and then determine the number of moles of each ions made as well as the total number of ions made a 1 mol ammonium chloride b 1 mol sodium sulfide c 05 mol barium nitrate
103
a 1 mol ammonium chloride
1 mol NH4+ 1 mol Cl- 2 mol total ions
104
b 1 mol sodium sulfide
2 mol Na+ 1 mol S-2 3 mol total ions
105
c 05 mol barium nitrate
05 mol Ba+2 1 mol NO3- 15 mol total ions
106
Precipitation Reactions
Even though no compound is completely insoluble compounds of very low solubility can be considered insoluble for practical purposes There are general guidelines to help predict whether a compound made of certain combination of ions is soluble
107
General Solubility Guidelines
Sodium potassium and ammonium compounds are soluble in water
108
Nitrates acetates and chlorates are soluble
109
110
Most chlorides are soluble except those of silver mercury (I) and lead Lead (II) chloride is soluble in hot water
Most sulfates are soluble except those of barium strontium lead calcium and mercury
111
Most carbonates phosphates and silicates are insoluble except those of sodium potassium and ammonium
112
Most sulfides are insoluble except those of calcium strontium sodium potassium and ammonium
113
Is calcium phosphate Ca3(PO4)2 soluble or insoluble
Not one of the exceptions so it is insoluble
Dissociation equations are not written for insoluble compounds
114
Guidelines also useful in predicting what will happen if 2 solutions of 2 different soluble compounds are mixed
If mixing results in combination of ions that forms insoluble compound a double-replacement and precipitation reaction will happen
115
Will precipitate form when solutions of ammonium sulfide and cadmium nitrate are combined You can tell calcium nitrate is soluble from the guidelines (Most nitrates acetates and chlorates are soluble) You can also tell ammonium sulfide is soluble (Most sulfides are insoluble except those of calcium strontium sodium potassium and ammonium)
116
117
To decide whether precipitate can form must know solubilities of two possible products
Ammonium nitrate ndash soluble (Most nitrates acetates and chlorates are soluble) Cadmium sulfide ndash insoluble (Most sulfides are insoluble except those of calcium strontium sodium potassium and ammonium)
118
bc one product is insoluble double-replacement and precipitation reaction will happen (NH4)2S (aq) + Cd(NO3)2 (aq) rarr NH4NO3 (aq) + CdS (s)
119
Net Ionic Equations
Reactions of ions in aqueous solution usually represented by net ionic equations instead of formula equations Net ionic equation - includes only those compounds and ions that undergo a chemical change in a reaction in an aqueous solution
120
121
To write NIE (net ionic equation) first convert chemical equation into overall ionic equation All soluble ionic compounds shown as dissociated ions in solution Precipitates shown as solids
122
Notice ammonium and nitrate ions appear on both sides They havenrsquot gone through any chemical change Spectator ions - ions that do not take part in a chemical reaction and are found in solution both before and after the reaction
123
To change ionic equation to NIE eliminate spectator ions Cd+2(aq) + S-2(aq) rarr CdS(s)
This NIE applies to any reaction in which precipitate of CdS forms
124
Practice Problem
Identify the precipitate that forms when aqueous solutions of zinc nitrate and ammonium sulfide are combined Write the equation for the possible double-replacement reaction Then write the formula equation overall ionic equation and the net ionic equation
125
1 AnalyzeGiven Identity of reactants zinc nitrate and ammonium sulfide Reaction medium aqueous solution Unknown a equation for the possible double-replacement reaction b identity of the precipitate c formula equation d overall ionic equation e net ionic equation
126
2 PlanWrite the possible double-replacement reaction between Zn(NO3)2 and (NH4)2S Use the rules to determine if any of the products will precipitate Write a formula equation and overall net ionic equation Then cancel spectator ions to make net ionic equation
127
3 Computea possible double-replacement reaction
b rules show that zinc sulfide is not soluble so will precipitate (ammonium nitrate is soluble)
c formula equation128
d overall ionic equation
e net ionic equation129
Will a precipitate form if solutions of potassium sulfate and barium nitrate are combined If so write the net ionic equation for the reaction
Yes Ba+2(aq) + SO4-2(aq) rarr BaSO4(s)
130
Will a precipitate form if solutions of potassium nitrate and magnesium sulfate are combined If so write the net ionic equation for the reaction
No 131
Will a precipitate form if solutions of barium chloride and sodium sulfate are combined If so identify the spectator ions and write the net ionic equation for the reaction
Yes Na+ and Cl- Ba+2(aq) + SO4-2(aq) rarr BaSO4(s)
132
Write the net ionic equation for the precipitation of nickel(II) sulfide
Ni+2(aq) + S-2(aq) rarr NiS(s)
133
IonizationSome molecular compounds can also form ions in solution Usually polar Ionization - ions formed from solute molecules by action of the solvent (creation of ions where there were none) Ionization different from dissociation
134
Dissociation ionic compounds dissolve and ions already present separate Ionization ions formed where non existed before
Ions formed are hydrated Heat released during hydration of ions gives enough energy to break covalent bonds
135
Extent to which solute ionizes depends on strength of bonds within molecules of solute and strength of attraction between solute and solvent molecules
136
HCl ndash molecular compound that ionizes in aqueous solution Contains highly polar bond Attraction between polar HCl molecule and polar water strong enough to break HCl bond
137
The Hydronium Ion
H+ ions from HCl attracts other molecules or ions so strongly that it doesnrsquot normally exist alone Ionization of HCl better described as direct transfer of proton from HCl to H2O forming H3O+
138
139
Hydration of H+ to form hydronium is highly exothermic
Energy released gives a lot of energy needed to ionize a molecular solute
Many molecular compounds that ionize in aqueous solution contain hydrogen and form hydronium ion
140
Strong and Weak Electrolytes
Substances that make ions and conduct current in solution are electrolytes HCl is one of series of compounds that has hydrogen and halogen Hydrogen halides are all molecular compounds with single polar-covalent bonds
141
All are gases Very soluble in water All are electrolytes HCl HBr HI strongly conduct current in solution HF only weakly conducts current at same concentration Strength of conduction related to ability to ionize
142
143
Strong Electrolytes
HCl HBr HI are 100 ionized in solution
Strong electrolyte - any compound whose dilute aqueous solutions conduct electricity well this is due to the presence of all or almost all of the dissolve compound in the form of ions
144
Unique characteristic of strong electrolyte is that they yield only ions
Ex Ionic compound may be highly soluble in water and dissociate into ions (NaCl)
Others may not dissolve much but amount that does dissolve exists only as ions
145
Weak ElectrolytesHF dissolves in water to give acid solution called hydrofluoric acid HF bond stronger than bonds between hydrogen and other halogens When HF dissolves some molecule ionize Reverse reaction also happens HF(aq) + H2O(l) H3O+(aq) + F-(aq)
146
HF(aq) + H2O(l) H3O+
(aq) + F-(aq)Concentration of dissolved unionized HF stays high and concentration of ions stays low
Weak electrolyte - any compound whose aqueous solutions conduct electricity poorly this is due to the presence of small amount of the dissolved compound in the form of ions
147
Different from nonelectrolyte ndash NO ions at all Description of electrolyte as strong or weak not related to concentration of solution Electrolytes differ in degree of ionization not amount of solute dissolved
148
149
Colligative properties of solutions
Presence of solute affects properties of solutions Some properties not dependent on nature of dissolved substance but on how may dissolve particles are present
Colligative properties - properties that depend on the concentration of solute particles but not on their identity
150
Vapor-Pressure Lowering
Boiling and freezing point of solution different from pure solvent Nonvolatile solute raises boiling point and lowers freezing point Nonvolatile substance - one that has little tendency to become gas under existing conditions
151
152
To understand why nonvolatile solute changes boiling and freezing point consider equilibrium vapor pressure
Vapor pressure pressure caused by molecules that have escape liquid phase to gas phase
Can be thought of a measure of tendency of molecules to escape from a liquid
153
Addition of sucrose (nonvolatile solute) lowers concentration of water molecules at surface of liquid This lowers tendency of water molecules to leave solution and enter gas phase Vapor pressure of solution is lower than vapor pressure of pure water
154
Aqueous13 solution13 of13 nonvolatile13 solutePure13 water
Nonelectrolyte solutions of same molality have same concentration of particles
Dilute solutions of same solvent and equal molality of any nonelectrolyte solute lower vapor pressure equally
155
Example
1 m aqueous solution of nonelectrolyte glucose lowers vapor pressure of water 55 x 10-4 atm at 25 1 m aqueous solution of sucrose also lowers water vapor pressure to 55 x 10-4 atm at 25
156
157
bc vapor-pressure lowering depends on concentration of nonelectrolyte solute and doesnrsquot depend on the type of solute it is a colligative proprerty
bc vapor pressure has been lowered solution remains liquid over larger temperature range
Lowers freezing point and raises boiling point
158
Can assume changes in boiling and freezing point also depend on concentration of solute
They are colligative properties
159
Freezing-Point Depression
Freezing point of 1-molal solution of any nonelectrolyte solute in water is found (by experiment) to be 186 lower than freezing point of water When 1 mol of nonelectrolyte solute dissolved in 1 kg water freezing point is -186 not 000 160
When 2 mol nonelectrolyte solute dissolved in 1 kg water freezing point is -372
For any concentration of nonelectrolyte solute in water decrease in freezing point can be determined using value of -186m
161
This value called the molal freezing-point contstant (Kf) is the freezing-point depression of the solvent in a 1-molal solution of a nonvolatile nonelectrolyte solute
162
Each solvent has own characteristic molal freezing-point constant Freezing-point depression (Δtf) the difference between the freezing points of the pure solvent and a solute of nonelectrolyte in that solvent and it is directly proportional to the molal concentration of the solution Δtf = Kfm
163
Δtf = Kfm
Kf expressed as m
m expressed in mol solutekg solvent (molality)
164
Sample Problem
What is the freezing point depression of water in a solution of 171 g of sucrose C12H22O11 and 200 g of water What is the actual freezing point of the solution
165
1 AnalyzeGiven Solute mass and chemical formula 171 g C12H22O11 Solvent mass and identity 2000 g water Unknown a freezing-point depression b freezing point of the solution 166
2 Plan
167
168
Calculate
Unknown a freezing-point depression b freezing point of the solution
169
Practice Problem
A water solution containing an unknown quantity of a nonelectrolyte solute is found to have a freezing point of ndash 023degCWhat is the molal concentration of the solution
012 m170
A solution consists of 103 g of the nonelectrolyte glucose C6H12O6 dissolved in 250 g of water What is the freezing-point depression of the solution
minus0426degC171
In a laboratory experiment the freezing point of an aqueous solution of glucose is found to be minus0325degCWhat is the molal concentration of this solution
0175 m172
If 0500 mol of a nonelectrolyte solute are dissolved in 5000 g of ether what is the freezing point of the solution
minus1181degC173
The freezing point of an aqueous solution that contains a nonelectrolyte is minus90degC aWhat is the freezing-point depression of the solution bWhat is the molal concentration of the solution a minus90degC b 48 m
174
Boiling-Point Elevation
Boiling point is temp at which vapor pressure is equal to atmospheric pressure Change in vapor pressure will cause corresponding change in boiling point
175
176
Vapor pressure of nonvolatile solution is lower than vapor pressure of pure solvent More heat will be required to raise vapor pressure of solution Boiling point of solution is higher than boiling point of pure solvent
Molal boiling-point constant (Kb) - boiling-point elevation of the solvent in a 1-molal solution of a nonvolatile nonelectrolyte solute b-p elevation of any nonelectrolyte in water found by experiment to be 051 So molal b-p constant for water is 051m
177
For different solvents b-p elevations of 1-molal solutions have different values (Table 14-2)
Like freezing-point constants values most accurate for dilute solutions
178
Boiling-point elevation Δtb difference between the boiling points of the pure solvent and a nonelectrolyte solution of that solvent and it is directly proportional to the molal concentration of the solution B-p elevation can be calculated using Δtb = Kbm Δtb (boiling point elevation) expressed in m and m expressed in mol solutekg solvent
Sample Problem
What is the boiling-point elevation of a solution made from 200 g of a nonelectrolyte solute and 4000 g of water The molar mass of the solute is 620 gmol
180
1 AnalyzeGiven solute mass = 200 g solute molar mass = 620 gmol solvent mass and identity = 4000 g of water
Unknown boiling-point elevation
181
2 Plan
182
183
3 Compute
Practice Problem
A solution contains 500 g of sucrose C12H22O11 a nonelectrolyte dissolved in 5000 g of water What is the boiling-point elevation
015degC184
A solution contains 4500 g of sucrose C12H22O11 a nonelectrolyte dissolved in 250 g of waterWhat is the boiling point of the solution
1027degC185
If the boiling point elevation of an aqueous solution containing a nonvolatile electrolyte is 102degC what is the molality of the solution
20 m186
The boiling point of an aqueous solution containing a nonvolatile electrolyte is 10075degC aWhat is the boiling-point elevation bWhat is the molality of the solution a 075degC b 15 m
187
Electrolytes and Colligative PropertiesEarly investigators confused by experiments where certain substances depressed freezing point or elevated boiling point more than expected
188
189
Ex 01 m solution of NaCl lowers freezing point of solvent almost twice as much as 01 m solution of sucrose 01 m CaCl2 lowers fp almost 3 times as much as 01 m solution of sucrose
To understand why this happens contrast behavior or sucrose with NaCl in aqueous solution Each sucrose dissolves to make only 1 particle in solution So 1 mol sucrose dissolves to 1 mol particles 1 mol NaCl dissolves to make 2 moles of particles (1 mol Na+ and 1 mol Cl-)
190
191
Calculated Values for Electrolyte SolutionsColligative properties depend on total concentration of solute particles regardless of their identity Electrolytes cause changes in colligative properties proportional to total molality in terms of all dissolves particles instead of formula units
192
What about barium nitrate Ba(NO3)2 Each mole of barium nitrate yields 3 mol of ions in solution Ba(NO3)2(s) rarr Ba2+(aq) + 2NO3-(aq) You would expect solution of given molality to lower fp of solvent 3 times as much as nonelectrolytic solution of same molality
193
Sample Problem
What is the expected change in the freezing point of water in a solution of 625 g of barium nitrate Ba(NO3)2 in 100 kg of water
194
1 AnalyzeGiven solute mass and formula = 625 g Ba(NO3)2 solvent mass and identity = 100 kg water Δtf = Kfm
Unknown expected freezing-point depression
195
2 PlanThe molality can be calculated by converting the solute mass to moles and then dividing by the number of kilograms of solvent That molality is in terms of formula units of Ba(NO3)2 and must be converted to molality in terms of dissociated ions in solution It must be multiplied by the number of moles of ions produced per mole of formula unit This adjusted molality can then be used to calculate the freezing-point depression
196
197
198
3 Compute
Practice Problem
What is the expected freezing-point depression for a solution that contains 20 mol of magnesium sulfate dissolved in 10 kg of water
minus74degC199
What is the expected boiling-point elevation of water for a solution that contains 150 g of sodium chloride dissolved in 10 kg of water 27degC The freezing point of an aqueous sodium chloride solution is minus200degC What is the molality of the solution 54 m NaCl
200
Actual Values for Electrolyte SolutionsRemember values calculated are only expected values Actual values of colligative properties for all strong electrolytes are almost what would be expected Difference between expected and actual values caused by attractive forces that exist between dissociated ions in aqueous solution
201
Attraction between hydrated ions in solution is small compared with those in crystalline solid However forces of attraction donrsquot interfere with movements of aqueous ions More concentrated solution means ions closer together and attraction is greater Also depends on charge of ions ndash higher charge greater attraction
202
1 M solution is not made by adding 1 mol solute to 1 L solvent That would be more than 1 L Instead 1 mol solute dissolved in less than 1 L Then diluted with more solvent to bring TOTAL VOLUME to 1 L
84
Practice ProblemYou have 340 L of solution that contains 900 g sodium chloride What is the molarity of that solution 0440 M NaCl You have 08 L of a 05 M HCl solution How many moles of HCl does this solution contain 04 mol HCl
85
What is the molarity of a solution composed of 585 g of potassium iodide dissolved in enough water to make a 0125 L of solution 0282 M KI
86
How many moles of H2SO4 are present in 0500 L of a 0150 M H2SO4 solution 00750 mol What volume of 300 M NaCl is needed for reaction that requires 1463 g of NaCl 0834 L
87
Molality
Molality - concentration of solution expressed in moles of solute per kilogram of solvent Solution that contains 1 mol solute (NaOH) dissolved in exactly 1 kg of solvent is ldquoone molalrdquo solution (1 m NaOH)
88
1 mol NaOH = 400 g 400 g NaOH dissolved in 1 kg water is 1 m NaOH
200 g NaOH in 1 kg water = 0500 m NaOH
89
Practice Problem
A solution was prepared by dissolving 171 g sucrose (C12H22O11) in 125 g of water Find the molal concentration of this solution 0400 m C12H22O11
90
A solution of iodine in carbon tetrachloride is used when iodine is needed for certain chemical tests How much iodine must be added to prepare a 0480 m solution of iodine in CCl4 if 1000 g of CCl4 is used 122 g I2
91
What is the molality of a solution composed of 255 g acetone (CH3)2CO dissolved in 200 g of water 22 m acetone What quantity in grams of methanol CH3OH is required to prepare a 0244 m solution in 400 g water 312 g methanol
92
How many grams of AgNO3 are needed to prepare a 0125 m solution in 250 mL of water 531 g AgNO3
What is the molality of a solution containing 182 g HCl and 250 g water 200 m
93
Ions in Aqueous Solutions and Colligative Properties
94
95
compounds in aqueous solutions
DissociationWhen compound made from ions dissolves in water ions separate Dissociation - separation of ions that occurs when an ionic compound dissolves
96
Notice number of ions made per formula unit in equations 1 formula unit of NaCl gives 2 units of ions in solution 1 formula unit of CaCl2 gives 3 units of ions
97
Assuming 100 dissociation solution that contains 1 mol NaCl contains 1 mol Na+ and 1 mol Cl- Can assume 100 dissociation of all soluble ionic compounds
98
Practice Problem
Write the equation for the dissolution of aluminum sulfate in water How many moles of aluminum ions are made by dissolving 1 mol aluminum sulfate What is the total number of moles of ions made by dissolving 1 mol of aluminum sulfate
99
1 AnalyzeGiven Amount of solute = 1 mol Al2(SO4)3 Solvent identity = water Unknown a moles of aluminum ions and sulfate ions Total number of moles of solute ions produced
100
2 Plan
The coefficients in the balanced dissociation equation will tell mole relationships You can use the equation to find out the number of moles of solute ions produced
101
3 Compute
a
b102
Write the equation for the dissolution of each of the following in water and then determine the number of moles of each ions made as well as the total number of ions made a 1 mol ammonium chloride b 1 mol sodium sulfide c 05 mol barium nitrate
103
a 1 mol ammonium chloride
1 mol NH4+ 1 mol Cl- 2 mol total ions
104
b 1 mol sodium sulfide
2 mol Na+ 1 mol S-2 3 mol total ions
105
c 05 mol barium nitrate
05 mol Ba+2 1 mol NO3- 15 mol total ions
106
Precipitation Reactions
Even though no compound is completely insoluble compounds of very low solubility can be considered insoluble for practical purposes There are general guidelines to help predict whether a compound made of certain combination of ions is soluble
107
General Solubility Guidelines
Sodium potassium and ammonium compounds are soluble in water
108
Nitrates acetates and chlorates are soluble
109
110
Most chlorides are soluble except those of silver mercury (I) and lead Lead (II) chloride is soluble in hot water
Most sulfates are soluble except those of barium strontium lead calcium and mercury
111
Most carbonates phosphates and silicates are insoluble except those of sodium potassium and ammonium
112
Most sulfides are insoluble except those of calcium strontium sodium potassium and ammonium
113
Is calcium phosphate Ca3(PO4)2 soluble or insoluble
Not one of the exceptions so it is insoluble
Dissociation equations are not written for insoluble compounds
114
Guidelines also useful in predicting what will happen if 2 solutions of 2 different soluble compounds are mixed
If mixing results in combination of ions that forms insoluble compound a double-replacement and precipitation reaction will happen
115
Will precipitate form when solutions of ammonium sulfide and cadmium nitrate are combined You can tell calcium nitrate is soluble from the guidelines (Most nitrates acetates and chlorates are soluble) You can also tell ammonium sulfide is soluble (Most sulfides are insoluble except those of calcium strontium sodium potassium and ammonium)
116
117
To decide whether precipitate can form must know solubilities of two possible products
Ammonium nitrate ndash soluble (Most nitrates acetates and chlorates are soluble) Cadmium sulfide ndash insoluble (Most sulfides are insoluble except those of calcium strontium sodium potassium and ammonium)
118
bc one product is insoluble double-replacement and precipitation reaction will happen (NH4)2S (aq) + Cd(NO3)2 (aq) rarr NH4NO3 (aq) + CdS (s)
119
Net Ionic Equations
Reactions of ions in aqueous solution usually represented by net ionic equations instead of formula equations Net ionic equation - includes only those compounds and ions that undergo a chemical change in a reaction in an aqueous solution
120
121
To write NIE (net ionic equation) first convert chemical equation into overall ionic equation All soluble ionic compounds shown as dissociated ions in solution Precipitates shown as solids
122
Notice ammonium and nitrate ions appear on both sides They havenrsquot gone through any chemical change Spectator ions - ions that do not take part in a chemical reaction and are found in solution both before and after the reaction
123
To change ionic equation to NIE eliminate spectator ions Cd+2(aq) + S-2(aq) rarr CdS(s)
This NIE applies to any reaction in which precipitate of CdS forms
124
Practice Problem
Identify the precipitate that forms when aqueous solutions of zinc nitrate and ammonium sulfide are combined Write the equation for the possible double-replacement reaction Then write the formula equation overall ionic equation and the net ionic equation
125
1 AnalyzeGiven Identity of reactants zinc nitrate and ammonium sulfide Reaction medium aqueous solution Unknown a equation for the possible double-replacement reaction b identity of the precipitate c formula equation d overall ionic equation e net ionic equation
126
2 PlanWrite the possible double-replacement reaction between Zn(NO3)2 and (NH4)2S Use the rules to determine if any of the products will precipitate Write a formula equation and overall net ionic equation Then cancel spectator ions to make net ionic equation
127
3 Computea possible double-replacement reaction
b rules show that zinc sulfide is not soluble so will precipitate (ammonium nitrate is soluble)
c formula equation128
d overall ionic equation
e net ionic equation129
Will a precipitate form if solutions of potassium sulfate and barium nitrate are combined If so write the net ionic equation for the reaction
Yes Ba+2(aq) + SO4-2(aq) rarr BaSO4(s)
130
Will a precipitate form if solutions of potassium nitrate and magnesium sulfate are combined If so write the net ionic equation for the reaction
No 131
Will a precipitate form if solutions of barium chloride and sodium sulfate are combined If so identify the spectator ions and write the net ionic equation for the reaction
Yes Na+ and Cl- Ba+2(aq) + SO4-2(aq) rarr BaSO4(s)
132
Write the net ionic equation for the precipitation of nickel(II) sulfide
Ni+2(aq) + S-2(aq) rarr NiS(s)
133
IonizationSome molecular compounds can also form ions in solution Usually polar Ionization - ions formed from solute molecules by action of the solvent (creation of ions where there were none) Ionization different from dissociation
134
Dissociation ionic compounds dissolve and ions already present separate Ionization ions formed where non existed before
Ions formed are hydrated Heat released during hydration of ions gives enough energy to break covalent bonds
135
Extent to which solute ionizes depends on strength of bonds within molecules of solute and strength of attraction between solute and solvent molecules
136
HCl ndash molecular compound that ionizes in aqueous solution Contains highly polar bond Attraction between polar HCl molecule and polar water strong enough to break HCl bond
137
The Hydronium Ion
H+ ions from HCl attracts other molecules or ions so strongly that it doesnrsquot normally exist alone Ionization of HCl better described as direct transfer of proton from HCl to H2O forming H3O+
138
139
Hydration of H+ to form hydronium is highly exothermic
Energy released gives a lot of energy needed to ionize a molecular solute
Many molecular compounds that ionize in aqueous solution contain hydrogen and form hydronium ion
140
Strong and Weak Electrolytes
Substances that make ions and conduct current in solution are electrolytes HCl is one of series of compounds that has hydrogen and halogen Hydrogen halides are all molecular compounds with single polar-covalent bonds
141
All are gases Very soluble in water All are electrolytes HCl HBr HI strongly conduct current in solution HF only weakly conducts current at same concentration Strength of conduction related to ability to ionize
142
143
Strong Electrolytes
HCl HBr HI are 100 ionized in solution
Strong electrolyte - any compound whose dilute aqueous solutions conduct electricity well this is due to the presence of all or almost all of the dissolve compound in the form of ions
144
Unique characteristic of strong electrolyte is that they yield only ions
Ex Ionic compound may be highly soluble in water and dissociate into ions (NaCl)
Others may not dissolve much but amount that does dissolve exists only as ions
145
Weak ElectrolytesHF dissolves in water to give acid solution called hydrofluoric acid HF bond stronger than bonds between hydrogen and other halogens When HF dissolves some molecule ionize Reverse reaction also happens HF(aq) + H2O(l) H3O+(aq) + F-(aq)
146
HF(aq) + H2O(l) H3O+
(aq) + F-(aq)Concentration of dissolved unionized HF stays high and concentration of ions stays low
Weak electrolyte - any compound whose aqueous solutions conduct electricity poorly this is due to the presence of small amount of the dissolved compound in the form of ions
147
Different from nonelectrolyte ndash NO ions at all Description of electrolyte as strong or weak not related to concentration of solution Electrolytes differ in degree of ionization not amount of solute dissolved
148
149
Colligative properties of solutions
Presence of solute affects properties of solutions Some properties not dependent on nature of dissolved substance but on how may dissolve particles are present
Colligative properties - properties that depend on the concentration of solute particles but not on their identity
150
Vapor-Pressure Lowering
Boiling and freezing point of solution different from pure solvent Nonvolatile solute raises boiling point and lowers freezing point Nonvolatile substance - one that has little tendency to become gas under existing conditions
151
152
To understand why nonvolatile solute changes boiling and freezing point consider equilibrium vapor pressure
Vapor pressure pressure caused by molecules that have escape liquid phase to gas phase
Can be thought of a measure of tendency of molecules to escape from a liquid
153
Addition of sucrose (nonvolatile solute) lowers concentration of water molecules at surface of liquid This lowers tendency of water molecules to leave solution and enter gas phase Vapor pressure of solution is lower than vapor pressure of pure water
154
Aqueous13 solution13 of13 nonvolatile13 solutePure13 water
Nonelectrolyte solutions of same molality have same concentration of particles
Dilute solutions of same solvent and equal molality of any nonelectrolyte solute lower vapor pressure equally
155
Example
1 m aqueous solution of nonelectrolyte glucose lowers vapor pressure of water 55 x 10-4 atm at 25 1 m aqueous solution of sucrose also lowers water vapor pressure to 55 x 10-4 atm at 25
156
157
bc vapor-pressure lowering depends on concentration of nonelectrolyte solute and doesnrsquot depend on the type of solute it is a colligative proprerty
bc vapor pressure has been lowered solution remains liquid over larger temperature range
Lowers freezing point and raises boiling point
158
Can assume changes in boiling and freezing point also depend on concentration of solute
They are colligative properties
159
Freezing-Point Depression
Freezing point of 1-molal solution of any nonelectrolyte solute in water is found (by experiment) to be 186 lower than freezing point of water When 1 mol of nonelectrolyte solute dissolved in 1 kg water freezing point is -186 not 000 160
When 2 mol nonelectrolyte solute dissolved in 1 kg water freezing point is -372
For any concentration of nonelectrolyte solute in water decrease in freezing point can be determined using value of -186m
161
This value called the molal freezing-point contstant (Kf) is the freezing-point depression of the solvent in a 1-molal solution of a nonvolatile nonelectrolyte solute
162
Each solvent has own characteristic molal freezing-point constant Freezing-point depression (Δtf) the difference between the freezing points of the pure solvent and a solute of nonelectrolyte in that solvent and it is directly proportional to the molal concentration of the solution Δtf = Kfm
163
Δtf = Kfm
Kf expressed as m
m expressed in mol solutekg solvent (molality)
164
Sample Problem
What is the freezing point depression of water in a solution of 171 g of sucrose C12H22O11 and 200 g of water What is the actual freezing point of the solution
165
1 AnalyzeGiven Solute mass and chemical formula 171 g C12H22O11 Solvent mass and identity 2000 g water Unknown a freezing-point depression b freezing point of the solution 166
2 Plan
167
168
Calculate
Unknown a freezing-point depression b freezing point of the solution
169
Practice Problem
A water solution containing an unknown quantity of a nonelectrolyte solute is found to have a freezing point of ndash 023degCWhat is the molal concentration of the solution
012 m170
A solution consists of 103 g of the nonelectrolyte glucose C6H12O6 dissolved in 250 g of water What is the freezing-point depression of the solution
minus0426degC171
In a laboratory experiment the freezing point of an aqueous solution of glucose is found to be minus0325degCWhat is the molal concentration of this solution
0175 m172
If 0500 mol of a nonelectrolyte solute are dissolved in 5000 g of ether what is the freezing point of the solution
minus1181degC173
The freezing point of an aqueous solution that contains a nonelectrolyte is minus90degC aWhat is the freezing-point depression of the solution bWhat is the molal concentration of the solution a minus90degC b 48 m
174
Boiling-Point Elevation
Boiling point is temp at which vapor pressure is equal to atmospheric pressure Change in vapor pressure will cause corresponding change in boiling point
175
176
Vapor pressure of nonvolatile solution is lower than vapor pressure of pure solvent More heat will be required to raise vapor pressure of solution Boiling point of solution is higher than boiling point of pure solvent
Molal boiling-point constant (Kb) - boiling-point elevation of the solvent in a 1-molal solution of a nonvolatile nonelectrolyte solute b-p elevation of any nonelectrolyte in water found by experiment to be 051 So molal b-p constant for water is 051m
177
For different solvents b-p elevations of 1-molal solutions have different values (Table 14-2)
Like freezing-point constants values most accurate for dilute solutions
178
Boiling-point elevation Δtb difference between the boiling points of the pure solvent and a nonelectrolyte solution of that solvent and it is directly proportional to the molal concentration of the solution B-p elevation can be calculated using Δtb = Kbm Δtb (boiling point elevation) expressed in m and m expressed in mol solutekg solvent
Sample Problem
What is the boiling-point elevation of a solution made from 200 g of a nonelectrolyte solute and 4000 g of water The molar mass of the solute is 620 gmol
180
1 AnalyzeGiven solute mass = 200 g solute molar mass = 620 gmol solvent mass and identity = 4000 g of water
Unknown boiling-point elevation
181
2 Plan
182
183
3 Compute
Practice Problem
A solution contains 500 g of sucrose C12H22O11 a nonelectrolyte dissolved in 5000 g of water What is the boiling-point elevation
015degC184
A solution contains 4500 g of sucrose C12H22O11 a nonelectrolyte dissolved in 250 g of waterWhat is the boiling point of the solution
1027degC185
If the boiling point elevation of an aqueous solution containing a nonvolatile electrolyte is 102degC what is the molality of the solution
20 m186
The boiling point of an aqueous solution containing a nonvolatile electrolyte is 10075degC aWhat is the boiling-point elevation bWhat is the molality of the solution a 075degC b 15 m
187
Electrolytes and Colligative PropertiesEarly investigators confused by experiments where certain substances depressed freezing point or elevated boiling point more than expected
188
189
Ex 01 m solution of NaCl lowers freezing point of solvent almost twice as much as 01 m solution of sucrose 01 m CaCl2 lowers fp almost 3 times as much as 01 m solution of sucrose
To understand why this happens contrast behavior or sucrose with NaCl in aqueous solution Each sucrose dissolves to make only 1 particle in solution So 1 mol sucrose dissolves to 1 mol particles 1 mol NaCl dissolves to make 2 moles of particles (1 mol Na+ and 1 mol Cl-)
190
191
Calculated Values for Electrolyte SolutionsColligative properties depend on total concentration of solute particles regardless of their identity Electrolytes cause changes in colligative properties proportional to total molality in terms of all dissolves particles instead of formula units
192
What about barium nitrate Ba(NO3)2 Each mole of barium nitrate yields 3 mol of ions in solution Ba(NO3)2(s) rarr Ba2+(aq) + 2NO3-(aq) You would expect solution of given molality to lower fp of solvent 3 times as much as nonelectrolytic solution of same molality
193
Sample Problem
What is the expected change in the freezing point of water in a solution of 625 g of barium nitrate Ba(NO3)2 in 100 kg of water
194
1 AnalyzeGiven solute mass and formula = 625 g Ba(NO3)2 solvent mass and identity = 100 kg water Δtf = Kfm
Unknown expected freezing-point depression
195
2 PlanThe molality can be calculated by converting the solute mass to moles and then dividing by the number of kilograms of solvent That molality is in terms of formula units of Ba(NO3)2 and must be converted to molality in terms of dissociated ions in solution It must be multiplied by the number of moles of ions produced per mole of formula unit This adjusted molality can then be used to calculate the freezing-point depression
196
197
198
3 Compute
Practice Problem
What is the expected freezing-point depression for a solution that contains 20 mol of magnesium sulfate dissolved in 10 kg of water
minus74degC199
What is the expected boiling-point elevation of water for a solution that contains 150 g of sodium chloride dissolved in 10 kg of water 27degC The freezing point of an aqueous sodium chloride solution is minus200degC What is the molality of the solution 54 m NaCl
200
Actual Values for Electrolyte SolutionsRemember values calculated are only expected values Actual values of colligative properties for all strong electrolytes are almost what would be expected Difference between expected and actual values caused by attractive forces that exist between dissociated ions in aqueous solution
201
Attraction between hydrated ions in solution is small compared with those in crystalline solid However forces of attraction donrsquot interfere with movements of aqueous ions More concentrated solution means ions closer together and attraction is greater Also depends on charge of ions ndash higher charge greater attraction
202
Practice ProblemYou have 340 L of solution that contains 900 g sodium chloride What is the molarity of that solution 0440 M NaCl You have 08 L of a 05 M HCl solution How many moles of HCl does this solution contain 04 mol HCl
85
What is the molarity of a solution composed of 585 g of potassium iodide dissolved in enough water to make a 0125 L of solution 0282 M KI
86
How many moles of H2SO4 are present in 0500 L of a 0150 M H2SO4 solution 00750 mol What volume of 300 M NaCl is needed for reaction that requires 1463 g of NaCl 0834 L
87
Molality
Molality - concentration of solution expressed in moles of solute per kilogram of solvent Solution that contains 1 mol solute (NaOH) dissolved in exactly 1 kg of solvent is ldquoone molalrdquo solution (1 m NaOH)
88
1 mol NaOH = 400 g 400 g NaOH dissolved in 1 kg water is 1 m NaOH
200 g NaOH in 1 kg water = 0500 m NaOH
89
Practice Problem
A solution was prepared by dissolving 171 g sucrose (C12H22O11) in 125 g of water Find the molal concentration of this solution 0400 m C12H22O11
90
A solution of iodine in carbon tetrachloride is used when iodine is needed for certain chemical tests How much iodine must be added to prepare a 0480 m solution of iodine in CCl4 if 1000 g of CCl4 is used 122 g I2
91
What is the molality of a solution composed of 255 g acetone (CH3)2CO dissolved in 200 g of water 22 m acetone What quantity in grams of methanol CH3OH is required to prepare a 0244 m solution in 400 g water 312 g methanol
92
How many grams of AgNO3 are needed to prepare a 0125 m solution in 250 mL of water 531 g AgNO3
What is the molality of a solution containing 182 g HCl and 250 g water 200 m
93
Ions in Aqueous Solutions and Colligative Properties
94
95
compounds in aqueous solutions
DissociationWhen compound made from ions dissolves in water ions separate Dissociation - separation of ions that occurs when an ionic compound dissolves
96
Notice number of ions made per formula unit in equations 1 formula unit of NaCl gives 2 units of ions in solution 1 formula unit of CaCl2 gives 3 units of ions
97
Assuming 100 dissociation solution that contains 1 mol NaCl contains 1 mol Na+ and 1 mol Cl- Can assume 100 dissociation of all soluble ionic compounds
98
Practice Problem
Write the equation for the dissolution of aluminum sulfate in water How many moles of aluminum ions are made by dissolving 1 mol aluminum sulfate What is the total number of moles of ions made by dissolving 1 mol of aluminum sulfate
99
1 AnalyzeGiven Amount of solute = 1 mol Al2(SO4)3 Solvent identity = water Unknown a moles of aluminum ions and sulfate ions Total number of moles of solute ions produced
100
2 Plan
The coefficients in the balanced dissociation equation will tell mole relationships You can use the equation to find out the number of moles of solute ions produced
101
3 Compute
a
b102
Write the equation for the dissolution of each of the following in water and then determine the number of moles of each ions made as well as the total number of ions made a 1 mol ammonium chloride b 1 mol sodium sulfide c 05 mol barium nitrate
103
a 1 mol ammonium chloride
1 mol NH4+ 1 mol Cl- 2 mol total ions
104
b 1 mol sodium sulfide
2 mol Na+ 1 mol S-2 3 mol total ions
105
c 05 mol barium nitrate
05 mol Ba+2 1 mol NO3- 15 mol total ions
106
Precipitation Reactions
Even though no compound is completely insoluble compounds of very low solubility can be considered insoluble for practical purposes There are general guidelines to help predict whether a compound made of certain combination of ions is soluble
107
General Solubility Guidelines
Sodium potassium and ammonium compounds are soluble in water
108
Nitrates acetates and chlorates are soluble
109
110
Most chlorides are soluble except those of silver mercury (I) and lead Lead (II) chloride is soluble in hot water
Most sulfates are soluble except those of barium strontium lead calcium and mercury
111
Most carbonates phosphates and silicates are insoluble except those of sodium potassium and ammonium
112
Most sulfides are insoluble except those of calcium strontium sodium potassium and ammonium
113
Is calcium phosphate Ca3(PO4)2 soluble or insoluble
Not one of the exceptions so it is insoluble
Dissociation equations are not written for insoluble compounds
114
Guidelines also useful in predicting what will happen if 2 solutions of 2 different soluble compounds are mixed
If mixing results in combination of ions that forms insoluble compound a double-replacement and precipitation reaction will happen
115
Will precipitate form when solutions of ammonium sulfide and cadmium nitrate are combined You can tell calcium nitrate is soluble from the guidelines (Most nitrates acetates and chlorates are soluble) You can also tell ammonium sulfide is soluble (Most sulfides are insoluble except those of calcium strontium sodium potassium and ammonium)
116
117
To decide whether precipitate can form must know solubilities of two possible products
Ammonium nitrate ndash soluble (Most nitrates acetates and chlorates are soluble) Cadmium sulfide ndash insoluble (Most sulfides are insoluble except those of calcium strontium sodium potassium and ammonium)
118
bc one product is insoluble double-replacement and precipitation reaction will happen (NH4)2S (aq) + Cd(NO3)2 (aq) rarr NH4NO3 (aq) + CdS (s)
119
Net Ionic Equations
Reactions of ions in aqueous solution usually represented by net ionic equations instead of formula equations Net ionic equation - includes only those compounds and ions that undergo a chemical change in a reaction in an aqueous solution
120
121
To write NIE (net ionic equation) first convert chemical equation into overall ionic equation All soluble ionic compounds shown as dissociated ions in solution Precipitates shown as solids
122
Notice ammonium and nitrate ions appear on both sides They havenrsquot gone through any chemical change Spectator ions - ions that do not take part in a chemical reaction and are found in solution both before and after the reaction
123
To change ionic equation to NIE eliminate spectator ions Cd+2(aq) + S-2(aq) rarr CdS(s)
This NIE applies to any reaction in which precipitate of CdS forms
124
Practice Problem
Identify the precipitate that forms when aqueous solutions of zinc nitrate and ammonium sulfide are combined Write the equation for the possible double-replacement reaction Then write the formula equation overall ionic equation and the net ionic equation
125
1 AnalyzeGiven Identity of reactants zinc nitrate and ammonium sulfide Reaction medium aqueous solution Unknown a equation for the possible double-replacement reaction b identity of the precipitate c formula equation d overall ionic equation e net ionic equation
126
2 PlanWrite the possible double-replacement reaction between Zn(NO3)2 and (NH4)2S Use the rules to determine if any of the products will precipitate Write a formula equation and overall net ionic equation Then cancel spectator ions to make net ionic equation
127
3 Computea possible double-replacement reaction
b rules show that zinc sulfide is not soluble so will precipitate (ammonium nitrate is soluble)
c formula equation128
d overall ionic equation
e net ionic equation129
Will a precipitate form if solutions of potassium sulfate and barium nitrate are combined If so write the net ionic equation for the reaction
Yes Ba+2(aq) + SO4-2(aq) rarr BaSO4(s)
130
Will a precipitate form if solutions of potassium nitrate and magnesium sulfate are combined If so write the net ionic equation for the reaction
No 131
Will a precipitate form if solutions of barium chloride and sodium sulfate are combined If so identify the spectator ions and write the net ionic equation for the reaction
Yes Na+ and Cl- Ba+2(aq) + SO4-2(aq) rarr BaSO4(s)
132
Write the net ionic equation for the precipitation of nickel(II) sulfide
Ni+2(aq) + S-2(aq) rarr NiS(s)
133
IonizationSome molecular compounds can also form ions in solution Usually polar Ionization - ions formed from solute molecules by action of the solvent (creation of ions where there were none) Ionization different from dissociation
134
Dissociation ionic compounds dissolve and ions already present separate Ionization ions formed where non existed before
Ions formed are hydrated Heat released during hydration of ions gives enough energy to break covalent bonds
135
Extent to which solute ionizes depends on strength of bonds within molecules of solute and strength of attraction between solute and solvent molecules
136
HCl ndash molecular compound that ionizes in aqueous solution Contains highly polar bond Attraction between polar HCl molecule and polar water strong enough to break HCl bond
137
The Hydronium Ion
H+ ions from HCl attracts other molecules or ions so strongly that it doesnrsquot normally exist alone Ionization of HCl better described as direct transfer of proton from HCl to H2O forming H3O+
138
139
Hydration of H+ to form hydronium is highly exothermic
Energy released gives a lot of energy needed to ionize a molecular solute
Many molecular compounds that ionize in aqueous solution contain hydrogen and form hydronium ion
140
Strong and Weak Electrolytes
Substances that make ions and conduct current in solution are electrolytes HCl is one of series of compounds that has hydrogen and halogen Hydrogen halides are all molecular compounds with single polar-covalent bonds
141
All are gases Very soluble in water All are electrolytes HCl HBr HI strongly conduct current in solution HF only weakly conducts current at same concentration Strength of conduction related to ability to ionize
142
143
Strong Electrolytes
HCl HBr HI are 100 ionized in solution
Strong electrolyte - any compound whose dilute aqueous solutions conduct electricity well this is due to the presence of all or almost all of the dissolve compound in the form of ions
144
Unique characteristic of strong electrolyte is that they yield only ions
Ex Ionic compound may be highly soluble in water and dissociate into ions (NaCl)
Others may not dissolve much but amount that does dissolve exists only as ions
145
Weak ElectrolytesHF dissolves in water to give acid solution called hydrofluoric acid HF bond stronger than bonds between hydrogen and other halogens When HF dissolves some molecule ionize Reverse reaction also happens HF(aq) + H2O(l) H3O+(aq) + F-(aq)
146
HF(aq) + H2O(l) H3O+
(aq) + F-(aq)Concentration of dissolved unionized HF stays high and concentration of ions stays low
Weak electrolyte - any compound whose aqueous solutions conduct electricity poorly this is due to the presence of small amount of the dissolved compound in the form of ions
147
Different from nonelectrolyte ndash NO ions at all Description of electrolyte as strong or weak not related to concentration of solution Electrolytes differ in degree of ionization not amount of solute dissolved
148
149
Colligative properties of solutions
Presence of solute affects properties of solutions Some properties not dependent on nature of dissolved substance but on how may dissolve particles are present
Colligative properties - properties that depend on the concentration of solute particles but not on their identity
150
Vapor-Pressure Lowering
Boiling and freezing point of solution different from pure solvent Nonvolatile solute raises boiling point and lowers freezing point Nonvolatile substance - one that has little tendency to become gas under existing conditions
151
152
To understand why nonvolatile solute changes boiling and freezing point consider equilibrium vapor pressure
Vapor pressure pressure caused by molecules that have escape liquid phase to gas phase
Can be thought of a measure of tendency of molecules to escape from a liquid
153
Addition of sucrose (nonvolatile solute) lowers concentration of water molecules at surface of liquid This lowers tendency of water molecules to leave solution and enter gas phase Vapor pressure of solution is lower than vapor pressure of pure water
154
Aqueous13 solution13 of13 nonvolatile13 solutePure13 water
Nonelectrolyte solutions of same molality have same concentration of particles
Dilute solutions of same solvent and equal molality of any nonelectrolyte solute lower vapor pressure equally
155
Example
1 m aqueous solution of nonelectrolyte glucose lowers vapor pressure of water 55 x 10-4 atm at 25 1 m aqueous solution of sucrose also lowers water vapor pressure to 55 x 10-4 atm at 25
156
157
bc vapor-pressure lowering depends on concentration of nonelectrolyte solute and doesnrsquot depend on the type of solute it is a colligative proprerty
bc vapor pressure has been lowered solution remains liquid over larger temperature range
Lowers freezing point and raises boiling point
158
Can assume changes in boiling and freezing point also depend on concentration of solute
They are colligative properties
159
Freezing-Point Depression
Freezing point of 1-molal solution of any nonelectrolyte solute in water is found (by experiment) to be 186 lower than freezing point of water When 1 mol of nonelectrolyte solute dissolved in 1 kg water freezing point is -186 not 000 160
When 2 mol nonelectrolyte solute dissolved in 1 kg water freezing point is -372
For any concentration of nonelectrolyte solute in water decrease in freezing point can be determined using value of -186m
161
This value called the molal freezing-point contstant (Kf) is the freezing-point depression of the solvent in a 1-molal solution of a nonvolatile nonelectrolyte solute
162
Each solvent has own characteristic molal freezing-point constant Freezing-point depression (Δtf) the difference between the freezing points of the pure solvent and a solute of nonelectrolyte in that solvent and it is directly proportional to the molal concentration of the solution Δtf = Kfm
163
Δtf = Kfm
Kf expressed as m
m expressed in mol solutekg solvent (molality)
164
Sample Problem
What is the freezing point depression of water in a solution of 171 g of sucrose C12H22O11 and 200 g of water What is the actual freezing point of the solution
165
1 AnalyzeGiven Solute mass and chemical formula 171 g C12H22O11 Solvent mass and identity 2000 g water Unknown a freezing-point depression b freezing point of the solution 166
2 Plan
167
168
Calculate
Unknown a freezing-point depression b freezing point of the solution
169
Practice Problem
A water solution containing an unknown quantity of a nonelectrolyte solute is found to have a freezing point of ndash 023degCWhat is the molal concentration of the solution
012 m170
A solution consists of 103 g of the nonelectrolyte glucose C6H12O6 dissolved in 250 g of water What is the freezing-point depression of the solution
minus0426degC171
In a laboratory experiment the freezing point of an aqueous solution of glucose is found to be minus0325degCWhat is the molal concentration of this solution
0175 m172
If 0500 mol of a nonelectrolyte solute are dissolved in 5000 g of ether what is the freezing point of the solution
minus1181degC173
The freezing point of an aqueous solution that contains a nonelectrolyte is minus90degC aWhat is the freezing-point depression of the solution bWhat is the molal concentration of the solution a minus90degC b 48 m
174
Boiling-Point Elevation
Boiling point is temp at which vapor pressure is equal to atmospheric pressure Change in vapor pressure will cause corresponding change in boiling point
175
176
Vapor pressure of nonvolatile solution is lower than vapor pressure of pure solvent More heat will be required to raise vapor pressure of solution Boiling point of solution is higher than boiling point of pure solvent
Molal boiling-point constant (Kb) - boiling-point elevation of the solvent in a 1-molal solution of a nonvolatile nonelectrolyte solute b-p elevation of any nonelectrolyte in water found by experiment to be 051 So molal b-p constant for water is 051m
177
For different solvents b-p elevations of 1-molal solutions have different values (Table 14-2)
Like freezing-point constants values most accurate for dilute solutions
178
Boiling-point elevation Δtb difference between the boiling points of the pure solvent and a nonelectrolyte solution of that solvent and it is directly proportional to the molal concentration of the solution B-p elevation can be calculated using Δtb = Kbm Δtb (boiling point elevation) expressed in m and m expressed in mol solutekg solvent
Sample Problem
What is the boiling-point elevation of a solution made from 200 g of a nonelectrolyte solute and 4000 g of water The molar mass of the solute is 620 gmol
180
1 AnalyzeGiven solute mass = 200 g solute molar mass = 620 gmol solvent mass and identity = 4000 g of water
Unknown boiling-point elevation
181
2 Plan
182
183
3 Compute
Practice Problem
A solution contains 500 g of sucrose C12H22O11 a nonelectrolyte dissolved in 5000 g of water What is the boiling-point elevation
015degC184
A solution contains 4500 g of sucrose C12H22O11 a nonelectrolyte dissolved in 250 g of waterWhat is the boiling point of the solution
1027degC185
If the boiling point elevation of an aqueous solution containing a nonvolatile electrolyte is 102degC what is the molality of the solution
20 m186
The boiling point of an aqueous solution containing a nonvolatile electrolyte is 10075degC aWhat is the boiling-point elevation bWhat is the molality of the solution a 075degC b 15 m
187
Electrolytes and Colligative PropertiesEarly investigators confused by experiments where certain substances depressed freezing point or elevated boiling point more than expected
188
189
Ex 01 m solution of NaCl lowers freezing point of solvent almost twice as much as 01 m solution of sucrose 01 m CaCl2 lowers fp almost 3 times as much as 01 m solution of sucrose
To understand why this happens contrast behavior or sucrose with NaCl in aqueous solution Each sucrose dissolves to make only 1 particle in solution So 1 mol sucrose dissolves to 1 mol particles 1 mol NaCl dissolves to make 2 moles of particles (1 mol Na+ and 1 mol Cl-)
190
191
Calculated Values for Electrolyte SolutionsColligative properties depend on total concentration of solute particles regardless of their identity Electrolytes cause changes in colligative properties proportional to total molality in terms of all dissolves particles instead of formula units
192
What about barium nitrate Ba(NO3)2 Each mole of barium nitrate yields 3 mol of ions in solution Ba(NO3)2(s) rarr Ba2+(aq) + 2NO3-(aq) You would expect solution of given molality to lower fp of solvent 3 times as much as nonelectrolytic solution of same molality
193
Sample Problem
What is the expected change in the freezing point of water in a solution of 625 g of barium nitrate Ba(NO3)2 in 100 kg of water
194
1 AnalyzeGiven solute mass and formula = 625 g Ba(NO3)2 solvent mass and identity = 100 kg water Δtf = Kfm
Unknown expected freezing-point depression
195
2 PlanThe molality can be calculated by converting the solute mass to moles and then dividing by the number of kilograms of solvent That molality is in terms of formula units of Ba(NO3)2 and must be converted to molality in terms of dissociated ions in solution It must be multiplied by the number of moles of ions produced per mole of formula unit This adjusted molality can then be used to calculate the freezing-point depression
196
197
198
3 Compute
Practice Problem
What is the expected freezing-point depression for a solution that contains 20 mol of magnesium sulfate dissolved in 10 kg of water
minus74degC199
What is the expected boiling-point elevation of water for a solution that contains 150 g of sodium chloride dissolved in 10 kg of water 27degC The freezing point of an aqueous sodium chloride solution is minus200degC What is the molality of the solution 54 m NaCl
200
Actual Values for Electrolyte SolutionsRemember values calculated are only expected values Actual values of colligative properties for all strong electrolytes are almost what would be expected Difference between expected and actual values caused by attractive forces that exist between dissociated ions in aqueous solution
201
Attraction between hydrated ions in solution is small compared with those in crystalline solid However forces of attraction donrsquot interfere with movements of aqueous ions More concentrated solution means ions closer together and attraction is greater Also depends on charge of ions ndash higher charge greater attraction
202
What is the molarity of a solution composed of 585 g of potassium iodide dissolved in enough water to make a 0125 L of solution 0282 M KI
86
How many moles of H2SO4 are present in 0500 L of a 0150 M H2SO4 solution 00750 mol What volume of 300 M NaCl is needed for reaction that requires 1463 g of NaCl 0834 L
87
Molality
Molality - concentration of solution expressed in moles of solute per kilogram of solvent Solution that contains 1 mol solute (NaOH) dissolved in exactly 1 kg of solvent is ldquoone molalrdquo solution (1 m NaOH)
88
1 mol NaOH = 400 g 400 g NaOH dissolved in 1 kg water is 1 m NaOH
200 g NaOH in 1 kg water = 0500 m NaOH
89
Practice Problem
A solution was prepared by dissolving 171 g sucrose (C12H22O11) in 125 g of water Find the molal concentration of this solution 0400 m C12H22O11
90
A solution of iodine in carbon tetrachloride is used when iodine is needed for certain chemical tests How much iodine must be added to prepare a 0480 m solution of iodine in CCl4 if 1000 g of CCl4 is used 122 g I2
91
What is the molality of a solution composed of 255 g acetone (CH3)2CO dissolved in 200 g of water 22 m acetone What quantity in grams of methanol CH3OH is required to prepare a 0244 m solution in 400 g water 312 g methanol
92
How many grams of AgNO3 are needed to prepare a 0125 m solution in 250 mL of water 531 g AgNO3
What is the molality of a solution containing 182 g HCl and 250 g water 200 m
93
Ions in Aqueous Solutions and Colligative Properties
94
95
compounds in aqueous solutions
DissociationWhen compound made from ions dissolves in water ions separate Dissociation - separation of ions that occurs when an ionic compound dissolves
96
Notice number of ions made per formula unit in equations 1 formula unit of NaCl gives 2 units of ions in solution 1 formula unit of CaCl2 gives 3 units of ions
97
Assuming 100 dissociation solution that contains 1 mol NaCl contains 1 mol Na+ and 1 mol Cl- Can assume 100 dissociation of all soluble ionic compounds
98
Practice Problem
Write the equation for the dissolution of aluminum sulfate in water How many moles of aluminum ions are made by dissolving 1 mol aluminum sulfate What is the total number of moles of ions made by dissolving 1 mol of aluminum sulfate
99
1 AnalyzeGiven Amount of solute = 1 mol Al2(SO4)3 Solvent identity = water Unknown a moles of aluminum ions and sulfate ions Total number of moles of solute ions produced
100
2 Plan
The coefficients in the balanced dissociation equation will tell mole relationships You can use the equation to find out the number of moles of solute ions produced
101
3 Compute
a
b102
Write the equation for the dissolution of each of the following in water and then determine the number of moles of each ions made as well as the total number of ions made a 1 mol ammonium chloride b 1 mol sodium sulfide c 05 mol barium nitrate
103
a 1 mol ammonium chloride
1 mol NH4+ 1 mol Cl- 2 mol total ions
104
b 1 mol sodium sulfide
2 mol Na+ 1 mol S-2 3 mol total ions
105
c 05 mol barium nitrate
05 mol Ba+2 1 mol NO3- 15 mol total ions
106
Precipitation Reactions
Even though no compound is completely insoluble compounds of very low solubility can be considered insoluble for practical purposes There are general guidelines to help predict whether a compound made of certain combination of ions is soluble
107
General Solubility Guidelines
Sodium potassium and ammonium compounds are soluble in water
108
Nitrates acetates and chlorates are soluble
109
110
Most chlorides are soluble except those of silver mercury (I) and lead Lead (II) chloride is soluble in hot water
Most sulfates are soluble except those of barium strontium lead calcium and mercury
111
Most carbonates phosphates and silicates are insoluble except those of sodium potassium and ammonium
112
Most sulfides are insoluble except those of calcium strontium sodium potassium and ammonium
113
Is calcium phosphate Ca3(PO4)2 soluble or insoluble
Not one of the exceptions so it is insoluble
Dissociation equations are not written for insoluble compounds
114
Guidelines also useful in predicting what will happen if 2 solutions of 2 different soluble compounds are mixed
If mixing results in combination of ions that forms insoluble compound a double-replacement and precipitation reaction will happen
115
Will precipitate form when solutions of ammonium sulfide and cadmium nitrate are combined You can tell calcium nitrate is soluble from the guidelines (Most nitrates acetates and chlorates are soluble) You can also tell ammonium sulfide is soluble (Most sulfides are insoluble except those of calcium strontium sodium potassium and ammonium)
116
117
To decide whether precipitate can form must know solubilities of two possible products
Ammonium nitrate ndash soluble (Most nitrates acetates and chlorates are soluble) Cadmium sulfide ndash insoluble (Most sulfides are insoluble except those of calcium strontium sodium potassium and ammonium)
118
bc one product is insoluble double-replacement and precipitation reaction will happen (NH4)2S (aq) + Cd(NO3)2 (aq) rarr NH4NO3 (aq) + CdS (s)
119
Net Ionic Equations
Reactions of ions in aqueous solution usually represented by net ionic equations instead of formula equations Net ionic equation - includes only those compounds and ions that undergo a chemical change in a reaction in an aqueous solution
120
121
To write NIE (net ionic equation) first convert chemical equation into overall ionic equation All soluble ionic compounds shown as dissociated ions in solution Precipitates shown as solids
122
Notice ammonium and nitrate ions appear on both sides They havenrsquot gone through any chemical change Spectator ions - ions that do not take part in a chemical reaction and are found in solution both before and after the reaction
123
To change ionic equation to NIE eliminate spectator ions Cd+2(aq) + S-2(aq) rarr CdS(s)
This NIE applies to any reaction in which precipitate of CdS forms
124
Practice Problem
Identify the precipitate that forms when aqueous solutions of zinc nitrate and ammonium sulfide are combined Write the equation for the possible double-replacement reaction Then write the formula equation overall ionic equation and the net ionic equation
125
1 AnalyzeGiven Identity of reactants zinc nitrate and ammonium sulfide Reaction medium aqueous solution Unknown a equation for the possible double-replacement reaction b identity of the precipitate c formula equation d overall ionic equation e net ionic equation
126
2 PlanWrite the possible double-replacement reaction between Zn(NO3)2 and (NH4)2S Use the rules to determine if any of the products will precipitate Write a formula equation and overall net ionic equation Then cancel spectator ions to make net ionic equation
127
3 Computea possible double-replacement reaction
b rules show that zinc sulfide is not soluble so will precipitate (ammonium nitrate is soluble)
c formula equation128
d overall ionic equation
e net ionic equation129
Will a precipitate form if solutions of potassium sulfate and barium nitrate are combined If so write the net ionic equation for the reaction
Yes Ba+2(aq) + SO4-2(aq) rarr BaSO4(s)
130
Will a precipitate form if solutions of potassium nitrate and magnesium sulfate are combined If so write the net ionic equation for the reaction
No 131
Will a precipitate form if solutions of barium chloride and sodium sulfate are combined If so identify the spectator ions and write the net ionic equation for the reaction
Yes Na+ and Cl- Ba+2(aq) + SO4-2(aq) rarr BaSO4(s)
132
Write the net ionic equation for the precipitation of nickel(II) sulfide
Ni+2(aq) + S-2(aq) rarr NiS(s)
133
IonizationSome molecular compounds can also form ions in solution Usually polar Ionization - ions formed from solute molecules by action of the solvent (creation of ions where there were none) Ionization different from dissociation
134
Dissociation ionic compounds dissolve and ions already present separate Ionization ions formed where non existed before
Ions formed are hydrated Heat released during hydration of ions gives enough energy to break covalent bonds
135
Extent to which solute ionizes depends on strength of bonds within molecules of solute and strength of attraction between solute and solvent molecules
136
HCl ndash molecular compound that ionizes in aqueous solution Contains highly polar bond Attraction between polar HCl molecule and polar water strong enough to break HCl bond
137
The Hydronium Ion
H+ ions from HCl attracts other molecules or ions so strongly that it doesnrsquot normally exist alone Ionization of HCl better described as direct transfer of proton from HCl to H2O forming H3O+
138
139
Hydration of H+ to form hydronium is highly exothermic
Energy released gives a lot of energy needed to ionize a molecular solute
Many molecular compounds that ionize in aqueous solution contain hydrogen and form hydronium ion
140
Strong and Weak Electrolytes
Substances that make ions and conduct current in solution are electrolytes HCl is one of series of compounds that has hydrogen and halogen Hydrogen halides are all molecular compounds with single polar-covalent bonds
141
All are gases Very soluble in water All are electrolytes HCl HBr HI strongly conduct current in solution HF only weakly conducts current at same concentration Strength of conduction related to ability to ionize
142
143
Strong Electrolytes
HCl HBr HI are 100 ionized in solution
Strong electrolyte - any compound whose dilute aqueous solutions conduct electricity well this is due to the presence of all or almost all of the dissolve compound in the form of ions
144
Unique characteristic of strong electrolyte is that they yield only ions
Ex Ionic compound may be highly soluble in water and dissociate into ions (NaCl)
Others may not dissolve much but amount that does dissolve exists only as ions
145
Weak ElectrolytesHF dissolves in water to give acid solution called hydrofluoric acid HF bond stronger than bonds between hydrogen and other halogens When HF dissolves some molecule ionize Reverse reaction also happens HF(aq) + H2O(l) H3O+(aq) + F-(aq)
146
HF(aq) + H2O(l) H3O+
(aq) + F-(aq)Concentration of dissolved unionized HF stays high and concentration of ions stays low
Weak electrolyte - any compound whose aqueous solutions conduct electricity poorly this is due to the presence of small amount of the dissolved compound in the form of ions
147
Different from nonelectrolyte ndash NO ions at all Description of electrolyte as strong or weak not related to concentration of solution Electrolytes differ in degree of ionization not amount of solute dissolved
148
149
Colligative properties of solutions
Presence of solute affects properties of solutions Some properties not dependent on nature of dissolved substance but on how may dissolve particles are present
Colligative properties - properties that depend on the concentration of solute particles but not on their identity
150
Vapor-Pressure Lowering
Boiling and freezing point of solution different from pure solvent Nonvolatile solute raises boiling point and lowers freezing point Nonvolatile substance - one that has little tendency to become gas under existing conditions
151
152
To understand why nonvolatile solute changes boiling and freezing point consider equilibrium vapor pressure
Vapor pressure pressure caused by molecules that have escape liquid phase to gas phase
Can be thought of a measure of tendency of molecules to escape from a liquid
153
Addition of sucrose (nonvolatile solute) lowers concentration of water molecules at surface of liquid This lowers tendency of water molecules to leave solution and enter gas phase Vapor pressure of solution is lower than vapor pressure of pure water
154
Aqueous13 solution13 of13 nonvolatile13 solutePure13 water
Nonelectrolyte solutions of same molality have same concentration of particles
Dilute solutions of same solvent and equal molality of any nonelectrolyte solute lower vapor pressure equally
155
Example
1 m aqueous solution of nonelectrolyte glucose lowers vapor pressure of water 55 x 10-4 atm at 25 1 m aqueous solution of sucrose also lowers water vapor pressure to 55 x 10-4 atm at 25
156
157
bc vapor-pressure lowering depends on concentration of nonelectrolyte solute and doesnrsquot depend on the type of solute it is a colligative proprerty
bc vapor pressure has been lowered solution remains liquid over larger temperature range
Lowers freezing point and raises boiling point
158
Can assume changes in boiling and freezing point also depend on concentration of solute
They are colligative properties
159
Freezing-Point Depression
Freezing point of 1-molal solution of any nonelectrolyte solute in water is found (by experiment) to be 186 lower than freezing point of water When 1 mol of nonelectrolyte solute dissolved in 1 kg water freezing point is -186 not 000 160
When 2 mol nonelectrolyte solute dissolved in 1 kg water freezing point is -372
For any concentration of nonelectrolyte solute in water decrease in freezing point can be determined using value of -186m
161
This value called the molal freezing-point contstant (Kf) is the freezing-point depression of the solvent in a 1-molal solution of a nonvolatile nonelectrolyte solute
162
Each solvent has own characteristic molal freezing-point constant Freezing-point depression (Δtf) the difference between the freezing points of the pure solvent and a solute of nonelectrolyte in that solvent and it is directly proportional to the molal concentration of the solution Δtf = Kfm
163
Δtf = Kfm
Kf expressed as m
m expressed in mol solutekg solvent (molality)
164
Sample Problem
What is the freezing point depression of water in a solution of 171 g of sucrose C12H22O11 and 200 g of water What is the actual freezing point of the solution
165
1 AnalyzeGiven Solute mass and chemical formula 171 g C12H22O11 Solvent mass and identity 2000 g water Unknown a freezing-point depression b freezing point of the solution 166
2 Plan
167
168
Calculate
Unknown a freezing-point depression b freezing point of the solution
169
Practice Problem
A water solution containing an unknown quantity of a nonelectrolyte solute is found to have a freezing point of ndash 023degCWhat is the molal concentration of the solution
012 m170
A solution consists of 103 g of the nonelectrolyte glucose C6H12O6 dissolved in 250 g of water What is the freezing-point depression of the solution
minus0426degC171
In a laboratory experiment the freezing point of an aqueous solution of glucose is found to be minus0325degCWhat is the molal concentration of this solution
0175 m172
If 0500 mol of a nonelectrolyte solute are dissolved in 5000 g of ether what is the freezing point of the solution
minus1181degC173
The freezing point of an aqueous solution that contains a nonelectrolyte is minus90degC aWhat is the freezing-point depression of the solution bWhat is the molal concentration of the solution a minus90degC b 48 m
174
Boiling-Point Elevation
Boiling point is temp at which vapor pressure is equal to atmospheric pressure Change in vapor pressure will cause corresponding change in boiling point
175
176
Vapor pressure of nonvolatile solution is lower than vapor pressure of pure solvent More heat will be required to raise vapor pressure of solution Boiling point of solution is higher than boiling point of pure solvent
Molal boiling-point constant (Kb) - boiling-point elevation of the solvent in a 1-molal solution of a nonvolatile nonelectrolyte solute b-p elevation of any nonelectrolyte in water found by experiment to be 051 So molal b-p constant for water is 051m
177
For different solvents b-p elevations of 1-molal solutions have different values (Table 14-2)
Like freezing-point constants values most accurate for dilute solutions
178
Boiling-point elevation Δtb difference between the boiling points of the pure solvent and a nonelectrolyte solution of that solvent and it is directly proportional to the molal concentration of the solution B-p elevation can be calculated using Δtb = Kbm Δtb (boiling point elevation) expressed in m and m expressed in mol solutekg solvent
Sample Problem
What is the boiling-point elevation of a solution made from 200 g of a nonelectrolyte solute and 4000 g of water The molar mass of the solute is 620 gmol
180
1 AnalyzeGiven solute mass = 200 g solute molar mass = 620 gmol solvent mass and identity = 4000 g of water
Unknown boiling-point elevation
181
2 Plan
182
183
3 Compute
Practice Problem
A solution contains 500 g of sucrose C12H22O11 a nonelectrolyte dissolved in 5000 g of water What is the boiling-point elevation
015degC184
A solution contains 4500 g of sucrose C12H22O11 a nonelectrolyte dissolved in 250 g of waterWhat is the boiling point of the solution
1027degC185
If the boiling point elevation of an aqueous solution containing a nonvolatile electrolyte is 102degC what is the molality of the solution
20 m186
The boiling point of an aqueous solution containing a nonvolatile electrolyte is 10075degC aWhat is the boiling-point elevation bWhat is the molality of the solution a 075degC b 15 m
187
Electrolytes and Colligative PropertiesEarly investigators confused by experiments where certain substances depressed freezing point or elevated boiling point more than expected
188
189
Ex 01 m solution of NaCl lowers freezing point of solvent almost twice as much as 01 m solution of sucrose 01 m CaCl2 lowers fp almost 3 times as much as 01 m solution of sucrose
To understand why this happens contrast behavior or sucrose with NaCl in aqueous solution Each sucrose dissolves to make only 1 particle in solution So 1 mol sucrose dissolves to 1 mol particles 1 mol NaCl dissolves to make 2 moles of particles (1 mol Na+ and 1 mol Cl-)
190
191
Calculated Values for Electrolyte SolutionsColligative properties depend on total concentration of solute particles regardless of their identity Electrolytes cause changes in colligative properties proportional to total molality in terms of all dissolves particles instead of formula units
192
What about barium nitrate Ba(NO3)2 Each mole of barium nitrate yields 3 mol of ions in solution Ba(NO3)2(s) rarr Ba2+(aq) + 2NO3-(aq) You would expect solution of given molality to lower fp of solvent 3 times as much as nonelectrolytic solution of same molality
193
Sample Problem
What is the expected change in the freezing point of water in a solution of 625 g of barium nitrate Ba(NO3)2 in 100 kg of water
194
1 AnalyzeGiven solute mass and formula = 625 g Ba(NO3)2 solvent mass and identity = 100 kg water Δtf = Kfm
Unknown expected freezing-point depression
195
2 PlanThe molality can be calculated by converting the solute mass to moles and then dividing by the number of kilograms of solvent That molality is in terms of formula units of Ba(NO3)2 and must be converted to molality in terms of dissociated ions in solution It must be multiplied by the number of moles of ions produced per mole of formula unit This adjusted molality can then be used to calculate the freezing-point depression
196
197
198
3 Compute
Practice Problem
What is the expected freezing-point depression for a solution that contains 20 mol of magnesium sulfate dissolved in 10 kg of water
minus74degC199
What is the expected boiling-point elevation of water for a solution that contains 150 g of sodium chloride dissolved in 10 kg of water 27degC The freezing point of an aqueous sodium chloride solution is minus200degC What is the molality of the solution 54 m NaCl
200
Actual Values for Electrolyte SolutionsRemember values calculated are only expected values Actual values of colligative properties for all strong electrolytes are almost what would be expected Difference between expected and actual values caused by attractive forces that exist between dissociated ions in aqueous solution
201
Attraction between hydrated ions in solution is small compared with those in crystalline solid However forces of attraction donrsquot interfere with movements of aqueous ions More concentrated solution means ions closer together and attraction is greater Also depends on charge of ions ndash higher charge greater attraction
202
How many moles of H2SO4 are present in 0500 L of a 0150 M H2SO4 solution 00750 mol What volume of 300 M NaCl is needed for reaction that requires 1463 g of NaCl 0834 L
87
Molality
Molality - concentration of solution expressed in moles of solute per kilogram of solvent Solution that contains 1 mol solute (NaOH) dissolved in exactly 1 kg of solvent is ldquoone molalrdquo solution (1 m NaOH)
88
1 mol NaOH = 400 g 400 g NaOH dissolved in 1 kg water is 1 m NaOH
200 g NaOH in 1 kg water = 0500 m NaOH
89
Practice Problem
A solution was prepared by dissolving 171 g sucrose (C12H22O11) in 125 g of water Find the molal concentration of this solution 0400 m C12H22O11
90
A solution of iodine in carbon tetrachloride is used when iodine is needed for certain chemical tests How much iodine must be added to prepare a 0480 m solution of iodine in CCl4 if 1000 g of CCl4 is used 122 g I2
91
What is the molality of a solution composed of 255 g acetone (CH3)2CO dissolved in 200 g of water 22 m acetone What quantity in grams of methanol CH3OH is required to prepare a 0244 m solution in 400 g water 312 g methanol
92
How many grams of AgNO3 are needed to prepare a 0125 m solution in 250 mL of water 531 g AgNO3
What is the molality of a solution containing 182 g HCl and 250 g water 200 m
93
Ions in Aqueous Solutions and Colligative Properties
94
95
compounds in aqueous solutions
DissociationWhen compound made from ions dissolves in water ions separate Dissociation - separation of ions that occurs when an ionic compound dissolves
96
Notice number of ions made per formula unit in equations 1 formula unit of NaCl gives 2 units of ions in solution 1 formula unit of CaCl2 gives 3 units of ions
97
Assuming 100 dissociation solution that contains 1 mol NaCl contains 1 mol Na+ and 1 mol Cl- Can assume 100 dissociation of all soluble ionic compounds
98
Practice Problem
Write the equation for the dissolution of aluminum sulfate in water How many moles of aluminum ions are made by dissolving 1 mol aluminum sulfate What is the total number of moles of ions made by dissolving 1 mol of aluminum sulfate
99
1 AnalyzeGiven Amount of solute = 1 mol Al2(SO4)3 Solvent identity = water Unknown a moles of aluminum ions and sulfate ions Total number of moles of solute ions produced
100
2 Plan
The coefficients in the balanced dissociation equation will tell mole relationships You can use the equation to find out the number of moles of solute ions produced
101
3 Compute
a
b102
Write the equation for the dissolution of each of the following in water and then determine the number of moles of each ions made as well as the total number of ions made a 1 mol ammonium chloride b 1 mol sodium sulfide c 05 mol barium nitrate
103
a 1 mol ammonium chloride
1 mol NH4+ 1 mol Cl- 2 mol total ions
104
b 1 mol sodium sulfide
2 mol Na+ 1 mol S-2 3 mol total ions
105
c 05 mol barium nitrate
05 mol Ba+2 1 mol NO3- 15 mol total ions
106
Precipitation Reactions
Even though no compound is completely insoluble compounds of very low solubility can be considered insoluble for practical purposes There are general guidelines to help predict whether a compound made of certain combination of ions is soluble
107
General Solubility Guidelines
Sodium potassium and ammonium compounds are soluble in water
108
Nitrates acetates and chlorates are soluble
109
110
Most chlorides are soluble except those of silver mercury (I) and lead Lead (II) chloride is soluble in hot water
Most sulfates are soluble except those of barium strontium lead calcium and mercury
111
Most carbonates phosphates and silicates are insoluble except those of sodium potassium and ammonium
112
Most sulfides are insoluble except those of calcium strontium sodium potassium and ammonium
113
Is calcium phosphate Ca3(PO4)2 soluble or insoluble
Not one of the exceptions so it is insoluble
Dissociation equations are not written for insoluble compounds
114
Guidelines also useful in predicting what will happen if 2 solutions of 2 different soluble compounds are mixed
If mixing results in combination of ions that forms insoluble compound a double-replacement and precipitation reaction will happen
115
Will precipitate form when solutions of ammonium sulfide and cadmium nitrate are combined You can tell calcium nitrate is soluble from the guidelines (Most nitrates acetates and chlorates are soluble) You can also tell ammonium sulfide is soluble (Most sulfides are insoluble except those of calcium strontium sodium potassium and ammonium)
116
117
To decide whether precipitate can form must know solubilities of two possible products
Ammonium nitrate ndash soluble (Most nitrates acetates and chlorates are soluble) Cadmium sulfide ndash insoluble (Most sulfides are insoluble except those of calcium strontium sodium potassium and ammonium)
118
bc one product is insoluble double-replacement and precipitation reaction will happen (NH4)2S (aq) + Cd(NO3)2 (aq) rarr NH4NO3 (aq) + CdS (s)
119
Net Ionic Equations
Reactions of ions in aqueous solution usually represented by net ionic equations instead of formula equations Net ionic equation - includes only those compounds and ions that undergo a chemical change in a reaction in an aqueous solution
120
121
To write NIE (net ionic equation) first convert chemical equation into overall ionic equation All soluble ionic compounds shown as dissociated ions in solution Precipitates shown as solids
122
Notice ammonium and nitrate ions appear on both sides They havenrsquot gone through any chemical change Spectator ions - ions that do not take part in a chemical reaction and are found in solution both before and after the reaction
123
To change ionic equation to NIE eliminate spectator ions Cd+2(aq) + S-2(aq) rarr CdS(s)
This NIE applies to any reaction in which precipitate of CdS forms
124
Practice Problem
Identify the precipitate that forms when aqueous solutions of zinc nitrate and ammonium sulfide are combined Write the equation for the possible double-replacement reaction Then write the formula equation overall ionic equation and the net ionic equation
125
1 AnalyzeGiven Identity of reactants zinc nitrate and ammonium sulfide Reaction medium aqueous solution Unknown a equation for the possible double-replacement reaction b identity of the precipitate c formula equation d overall ionic equation e net ionic equation
126
2 PlanWrite the possible double-replacement reaction between Zn(NO3)2 and (NH4)2S Use the rules to determine if any of the products will precipitate Write a formula equation and overall net ionic equation Then cancel spectator ions to make net ionic equation
127
3 Computea possible double-replacement reaction
b rules show that zinc sulfide is not soluble so will precipitate (ammonium nitrate is soluble)
c formula equation128
d overall ionic equation
e net ionic equation129
Will a precipitate form if solutions of potassium sulfate and barium nitrate are combined If so write the net ionic equation for the reaction
Yes Ba+2(aq) + SO4-2(aq) rarr BaSO4(s)
130
Will a precipitate form if solutions of potassium nitrate and magnesium sulfate are combined If so write the net ionic equation for the reaction
No 131
Will a precipitate form if solutions of barium chloride and sodium sulfate are combined If so identify the spectator ions and write the net ionic equation for the reaction
Yes Na+ and Cl- Ba+2(aq) + SO4-2(aq) rarr BaSO4(s)
132
Write the net ionic equation for the precipitation of nickel(II) sulfide
Ni+2(aq) + S-2(aq) rarr NiS(s)
133
IonizationSome molecular compounds can also form ions in solution Usually polar Ionization - ions formed from solute molecules by action of the solvent (creation of ions where there were none) Ionization different from dissociation
134
Dissociation ionic compounds dissolve and ions already present separate Ionization ions formed where non existed before
Ions formed are hydrated Heat released during hydration of ions gives enough energy to break covalent bonds
135
Extent to which solute ionizes depends on strength of bonds within molecules of solute and strength of attraction between solute and solvent molecules
136
HCl ndash molecular compound that ionizes in aqueous solution Contains highly polar bond Attraction between polar HCl molecule and polar water strong enough to break HCl bond
137
The Hydronium Ion
H+ ions from HCl attracts other molecules or ions so strongly that it doesnrsquot normally exist alone Ionization of HCl better described as direct transfer of proton from HCl to H2O forming H3O+
138
139
Hydration of H+ to form hydronium is highly exothermic
Energy released gives a lot of energy needed to ionize a molecular solute
Many molecular compounds that ionize in aqueous solution contain hydrogen and form hydronium ion
140
Strong and Weak Electrolytes
Substances that make ions and conduct current in solution are electrolytes HCl is one of series of compounds that has hydrogen and halogen Hydrogen halides are all molecular compounds with single polar-covalent bonds
141
All are gases Very soluble in water All are electrolytes HCl HBr HI strongly conduct current in solution HF only weakly conducts current at same concentration Strength of conduction related to ability to ionize
142
143
Strong Electrolytes
HCl HBr HI are 100 ionized in solution
Strong electrolyte - any compound whose dilute aqueous solutions conduct electricity well this is due to the presence of all or almost all of the dissolve compound in the form of ions
144
Unique characteristic of strong electrolyte is that they yield only ions
Ex Ionic compound may be highly soluble in water and dissociate into ions (NaCl)
Others may not dissolve much but amount that does dissolve exists only as ions
145
Weak ElectrolytesHF dissolves in water to give acid solution called hydrofluoric acid HF bond stronger than bonds between hydrogen and other halogens When HF dissolves some molecule ionize Reverse reaction also happens HF(aq) + H2O(l) H3O+(aq) + F-(aq)
146
HF(aq) + H2O(l) H3O+
(aq) + F-(aq)Concentration of dissolved unionized HF stays high and concentration of ions stays low
Weak electrolyte - any compound whose aqueous solutions conduct electricity poorly this is due to the presence of small amount of the dissolved compound in the form of ions
147
Different from nonelectrolyte ndash NO ions at all Description of electrolyte as strong or weak not related to concentration of solution Electrolytes differ in degree of ionization not amount of solute dissolved
148
149
Colligative properties of solutions
Presence of solute affects properties of solutions Some properties not dependent on nature of dissolved substance but on how may dissolve particles are present
Colligative properties - properties that depend on the concentration of solute particles but not on their identity
150
Vapor-Pressure Lowering
Boiling and freezing point of solution different from pure solvent Nonvolatile solute raises boiling point and lowers freezing point Nonvolatile substance - one that has little tendency to become gas under existing conditions
151
152
To understand why nonvolatile solute changes boiling and freezing point consider equilibrium vapor pressure
Vapor pressure pressure caused by molecules that have escape liquid phase to gas phase
Can be thought of a measure of tendency of molecules to escape from a liquid
153
Addition of sucrose (nonvolatile solute) lowers concentration of water molecules at surface of liquid This lowers tendency of water molecules to leave solution and enter gas phase Vapor pressure of solution is lower than vapor pressure of pure water
154
Aqueous13 solution13 of13 nonvolatile13 solutePure13 water
Nonelectrolyte solutions of same molality have same concentration of particles
Dilute solutions of same solvent and equal molality of any nonelectrolyte solute lower vapor pressure equally
155
Example
1 m aqueous solution of nonelectrolyte glucose lowers vapor pressure of water 55 x 10-4 atm at 25 1 m aqueous solution of sucrose also lowers water vapor pressure to 55 x 10-4 atm at 25
156
157
bc vapor-pressure lowering depends on concentration of nonelectrolyte solute and doesnrsquot depend on the type of solute it is a colligative proprerty
bc vapor pressure has been lowered solution remains liquid over larger temperature range
Lowers freezing point and raises boiling point
158
Can assume changes in boiling and freezing point also depend on concentration of solute
They are colligative properties
159
Freezing-Point Depression
Freezing point of 1-molal solution of any nonelectrolyte solute in water is found (by experiment) to be 186 lower than freezing point of water When 1 mol of nonelectrolyte solute dissolved in 1 kg water freezing point is -186 not 000 160
When 2 mol nonelectrolyte solute dissolved in 1 kg water freezing point is -372
For any concentration of nonelectrolyte solute in water decrease in freezing point can be determined using value of -186m
161
This value called the molal freezing-point contstant (Kf) is the freezing-point depression of the solvent in a 1-molal solution of a nonvolatile nonelectrolyte solute
162
Each solvent has own characteristic molal freezing-point constant Freezing-point depression (Δtf) the difference between the freezing points of the pure solvent and a solute of nonelectrolyte in that solvent and it is directly proportional to the molal concentration of the solution Δtf = Kfm
163
Δtf = Kfm
Kf expressed as m
m expressed in mol solutekg solvent (molality)
164
Sample Problem
What is the freezing point depression of water in a solution of 171 g of sucrose C12H22O11 and 200 g of water What is the actual freezing point of the solution
165
1 AnalyzeGiven Solute mass and chemical formula 171 g C12H22O11 Solvent mass and identity 2000 g water Unknown a freezing-point depression b freezing point of the solution 166
2 Plan
167
168
Calculate
Unknown a freezing-point depression b freezing point of the solution
169
Practice Problem
A water solution containing an unknown quantity of a nonelectrolyte solute is found to have a freezing point of ndash 023degCWhat is the molal concentration of the solution
012 m170
A solution consists of 103 g of the nonelectrolyte glucose C6H12O6 dissolved in 250 g of water What is the freezing-point depression of the solution
minus0426degC171
In a laboratory experiment the freezing point of an aqueous solution of glucose is found to be minus0325degCWhat is the molal concentration of this solution
0175 m172
If 0500 mol of a nonelectrolyte solute are dissolved in 5000 g of ether what is the freezing point of the solution
minus1181degC173
The freezing point of an aqueous solution that contains a nonelectrolyte is minus90degC aWhat is the freezing-point depression of the solution bWhat is the molal concentration of the solution a minus90degC b 48 m
174
Boiling-Point Elevation
Boiling point is temp at which vapor pressure is equal to atmospheric pressure Change in vapor pressure will cause corresponding change in boiling point
175
176
Vapor pressure of nonvolatile solution is lower than vapor pressure of pure solvent More heat will be required to raise vapor pressure of solution Boiling point of solution is higher than boiling point of pure solvent
Molal boiling-point constant (Kb) - boiling-point elevation of the solvent in a 1-molal solution of a nonvolatile nonelectrolyte solute b-p elevation of any nonelectrolyte in water found by experiment to be 051 So molal b-p constant for water is 051m
177
For different solvents b-p elevations of 1-molal solutions have different values (Table 14-2)
Like freezing-point constants values most accurate for dilute solutions
178
Boiling-point elevation Δtb difference between the boiling points of the pure solvent and a nonelectrolyte solution of that solvent and it is directly proportional to the molal concentration of the solution B-p elevation can be calculated using Δtb = Kbm Δtb (boiling point elevation) expressed in m and m expressed in mol solutekg solvent
Sample Problem
What is the boiling-point elevation of a solution made from 200 g of a nonelectrolyte solute and 4000 g of water The molar mass of the solute is 620 gmol
180
1 AnalyzeGiven solute mass = 200 g solute molar mass = 620 gmol solvent mass and identity = 4000 g of water
Unknown boiling-point elevation
181
2 Plan
182
183
3 Compute
Practice Problem
A solution contains 500 g of sucrose C12H22O11 a nonelectrolyte dissolved in 5000 g of water What is the boiling-point elevation
015degC184
A solution contains 4500 g of sucrose C12H22O11 a nonelectrolyte dissolved in 250 g of waterWhat is the boiling point of the solution
1027degC185
If the boiling point elevation of an aqueous solution containing a nonvolatile electrolyte is 102degC what is the molality of the solution
20 m186
The boiling point of an aqueous solution containing a nonvolatile electrolyte is 10075degC aWhat is the boiling-point elevation bWhat is the molality of the solution a 075degC b 15 m
187
Electrolytes and Colligative PropertiesEarly investigators confused by experiments where certain substances depressed freezing point or elevated boiling point more than expected
188
189
Ex 01 m solution of NaCl lowers freezing point of solvent almost twice as much as 01 m solution of sucrose 01 m CaCl2 lowers fp almost 3 times as much as 01 m solution of sucrose
To understand why this happens contrast behavior or sucrose with NaCl in aqueous solution Each sucrose dissolves to make only 1 particle in solution So 1 mol sucrose dissolves to 1 mol particles 1 mol NaCl dissolves to make 2 moles of particles (1 mol Na+ and 1 mol Cl-)
190
191
Calculated Values for Electrolyte SolutionsColligative properties depend on total concentration of solute particles regardless of their identity Electrolytes cause changes in colligative properties proportional to total molality in terms of all dissolves particles instead of formula units
192
What about barium nitrate Ba(NO3)2 Each mole of barium nitrate yields 3 mol of ions in solution Ba(NO3)2(s) rarr Ba2+(aq) + 2NO3-(aq) You would expect solution of given molality to lower fp of solvent 3 times as much as nonelectrolytic solution of same molality
193
Sample Problem
What is the expected change in the freezing point of water in a solution of 625 g of barium nitrate Ba(NO3)2 in 100 kg of water
194
1 AnalyzeGiven solute mass and formula = 625 g Ba(NO3)2 solvent mass and identity = 100 kg water Δtf = Kfm
Unknown expected freezing-point depression
195
2 PlanThe molality can be calculated by converting the solute mass to moles and then dividing by the number of kilograms of solvent That molality is in terms of formula units of Ba(NO3)2 and must be converted to molality in terms of dissociated ions in solution It must be multiplied by the number of moles of ions produced per mole of formula unit This adjusted molality can then be used to calculate the freezing-point depression
196
197
198
3 Compute
Practice Problem
What is the expected freezing-point depression for a solution that contains 20 mol of magnesium sulfate dissolved in 10 kg of water
minus74degC199
What is the expected boiling-point elevation of water for a solution that contains 150 g of sodium chloride dissolved in 10 kg of water 27degC The freezing point of an aqueous sodium chloride solution is minus200degC What is the molality of the solution 54 m NaCl
200
Actual Values for Electrolyte SolutionsRemember values calculated are only expected values Actual values of colligative properties for all strong electrolytes are almost what would be expected Difference between expected and actual values caused by attractive forces that exist between dissociated ions in aqueous solution
201
Attraction between hydrated ions in solution is small compared with those in crystalline solid However forces of attraction donrsquot interfere with movements of aqueous ions More concentrated solution means ions closer together and attraction is greater Also depends on charge of ions ndash higher charge greater attraction
202
Molality
Molality - concentration of solution expressed in moles of solute per kilogram of solvent Solution that contains 1 mol solute (NaOH) dissolved in exactly 1 kg of solvent is ldquoone molalrdquo solution (1 m NaOH)
88
1 mol NaOH = 400 g 400 g NaOH dissolved in 1 kg water is 1 m NaOH
200 g NaOH in 1 kg water = 0500 m NaOH
89
Practice Problem
A solution was prepared by dissolving 171 g sucrose (C12H22O11) in 125 g of water Find the molal concentration of this solution 0400 m C12H22O11
90
A solution of iodine in carbon tetrachloride is used when iodine is needed for certain chemical tests How much iodine must be added to prepare a 0480 m solution of iodine in CCl4 if 1000 g of CCl4 is used 122 g I2
91
What is the molality of a solution composed of 255 g acetone (CH3)2CO dissolved in 200 g of water 22 m acetone What quantity in grams of methanol CH3OH is required to prepare a 0244 m solution in 400 g water 312 g methanol
92
How many grams of AgNO3 are needed to prepare a 0125 m solution in 250 mL of water 531 g AgNO3
What is the molality of a solution containing 182 g HCl and 250 g water 200 m
93
Ions in Aqueous Solutions and Colligative Properties
94
95
compounds in aqueous solutions
DissociationWhen compound made from ions dissolves in water ions separate Dissociation - separation of ions that occurs when an ionic compound dissolves
96
Notice number of ions made per formula unit in equations 1 formula unit of NaCl gives 2 units of ions in solution 1 formula unit of CaCl2 gives 3 units of ions
97
Assuming 100 dissociation solution that contains 1 mol NaCl contains 1 mol Na+ and 1 mol Cl- Can assume 100 dissociation of all soluble ionic compounds
98
Practice Problem
Write the equation for the dissolution of aluminum sulfate in water How many moles of aluminum ions are made by dissolving 1 mol aluminum sulfate What is the total number of moles of ions made by dissolving 1 mol of aluminum sulfate
99
1 AnalyzeGiven Amount of solute = 1 mol Al2(SO4)3 Solvent identity = water Unknown a moles of aluminum ions and sulfate ions Total number of moles of solute ions produced
100
2 Plan
The coefficients in the balanced dissociation equation will tell mole relationships You can use the equation to find out the number of moles of solute ions produced
101
3 Compute
a
b102
Write the equation for the dissolution of each of the following in water and then determine the number of moles of each ions made as well as the total number of ions made a 1 mol ammonium chloride b 1 mol sodium sulfide c 05 mol barium nitrate
103
a 1 mol ammonium chloride
1 mol NH4+ 1 mol Cl- 2 mol total ions
104
b 1 mol sodium sulfide
2 mol Na+ 1 mol S-2 3 mol total ions
105
c 05 mol barium nitrate
05 mol Ba+2 1 mol NO3- 15 mol total ions
106
Precipitation Reactions
Even though no compound is completely insoluble compounds of very low solubility can be considered insoluble for practical purposes There are general guidelines to help predict whether a compound made of certain combination of ions is soluble
107
General Solubility Guidelines
Sodium potassium and ammonium compounds are soluble in water
108
Nitrates acetates and chlorates are soluble
109
110
Most chlorides are soluble except those of silver mercury (I) and lead Lead (II) chloride is soluble in hot water
Most sulfates are soluble except those of barium strontium lead calcium and mercury
111
Most carbonates phosphates and silicates are insoluble except those of sodium potassium and ammonium
112
Most sulfides are insoluble except those of calcium strontium sodium potassium and ammonium
113
Is calcium phosphate Ca3(PO4)2 soluble or insoluble
Not one of the exceptions so it is insoluble
Dissociation equations are not written for insoluble compounds
114
Guidelines also useful in predicting what will happen if 2 solutions of 2 different soluble compounds are mixed
If mixing results in combination of ions that forms insoluble compound a double-replacement and precipitation reaction will happen
115
Will precipitate form when solutions of ammonium sulfide and cadmium nitrate are combined You can tell calcium nitrate is soluble from the guidelines (Most nitrates acetates and chlorates are soluble) You can also tell ammonium sulfide is soluble (Most sulfides are insoluble except those of calcium strontium sodium potassium and ammonium)
116
117
To decide whether precipitate can form must know solubilities of two possible products
Ammonium nitrate ndash soluble (Most nitrates acetates and chlorates are soluble) Cadmium sulfide ndash insoluble (Most sulfides are insoluble except those of calcium strontium sodium potassium and ammonium)
118
bc one product is insoluble double-replacement and precipitation reaction will happen (NH4)2S (aq) + Cd(NO3)2 (aq) rarr NH4NO3 (aq) + CdS (s)
119
Net Ionic Equations
Reactions of ions in aqueous solution usually represented by net ionic equations instead of formula equations Net ionic equation - includes only those compounds and ions that undergo a chemical change in a reaction in an aqueous solution
120
121
To write NIE (net ionic equation) first convert chemical equation into overall ionic equation All soluble ionic compounds shown as dissociated ions in solution Precipitates shown as solids
122
Notice ammonium and nitrate ions appear on both sides They havenrsquot gone through any chemical change Spectator ions - ions that do not take part in a chemical reaction and are found in solution both before and after the reaction
123
To change ionic equation to NIE eliminate spectator ions Cd+2(aq) + S-2(aq) rarr CdS(s)
This NIE applies to any reaction in which precipitate of CdS forms
124
Practice Problem
Identify the precipitate that forms when aqueous solutions of zinc nitrate and ammonium sulfide are combined Write the equation for the possible double-replacement reaction Then write the formula equation overall ionic equation and the net ionic equation
125
1 AnalyzeGiven Identity of reactants zinc nitrate and ammonium sulfide Reaction medium aqueous solution Unknown a equation for the possible double-replacement reaction b identity of the precipitate c formula equation d overall ionic equation e net ionic equation
126
2 PlanWrite the possible double-replacement reaction between Zn(NO3)2 and (NH4)2S Use the rules to determine if any of the products will precipitate Write a formula equation and overall net ionic equation Then cancel spectator ions to make net ionic equation
127
3 Computea possible double-replacement reaction
b rules show that zinc sulfide is not soluble so will precipitate (ammonium nitrate is soluble)
c formula equation128
d overall ionic equation
e net ionic equation129
Will a precipitate form if solutions of potassium sulfate and barium nitrate are combined If so write the net ionic equation for the reaction
Yes Ba+2(aq) + SO4-2(aq) rarr BaSO4(s)
130
Will a precipitate form if solutions of potassium nitrate and magnesium sulfate are combined If so write the net ionic equation for the reaction
No 131
Will a precipitate form if solutions of barium chloride and sodium sulfate are combined If so identify the spectator ions and write the net ionic equation for the reaction
Yes Na+ and Cl- Ba+2(aq) + SO4-2(aq) rarr BaSO4(s)
132
Write the net ionic equation for the precipitation of nickel(II) sulfide
Ni+2(aq) + S-2(aq) rarr NiS(s)
133
IonizationSome molecular compounds can also form ions in solution Usually polar Ionization - ions formed from solute molecules by action of the solvent (creation of ions where there were none) Ionization different from dissociation
134
Dissociation ionic compounds dissolve and ions already present separate Ionization ions formed where non existed before
Ions formed are hydrated Heat released during hydration of ions gives enough energy to break covalent bonds
135
Extent to which solute ionizes depends on strength of bonds within molecules of solute and strength of attraction between solute and solvent molecules
136
HCl ndash molecular compound that ionizes in aqueous solution Contains highly polar bond Attraction between polar HCl molecule and polar water strong enough to break HCl bond
137
The Hydronium Ion
H+ ions from HCl attracts other molecules or ions so strongly that it doesnrsquot normally exist alone Ionization of HCl better described as direct transfer of proton from HCl to H2O forming H3O+
138
139
Hydration of H+ to form hydronium is highly exothermic
Energy released gives a lot of energy needed to ionize a molecular solute
Many molecular compounds that ionize in aqueous solution contain hydrogen and form hydronium ion
140
Strong and Weak Electrolytes
Substances that make ions and conduct current in solution are electrolytes HCl is one of series of compounds that has hydrogen and halogen Hydrogen halides are all molecular compounds with single polar-covalent bonds
141
All are gases Very soluble in water All are electrolytes HCl HBr HI strongly conduct current in solution HF only weakly conducts current at same concentration Strength of conduction related to ability to ionize
142
143
Strong Electrolytes
HCl HBr HI are 100 ionized in solution
Strong electrolyte - any compound whose dilute aqueous solutions conduct electricity well this is due to the presence of all or almost all of the dissolve compound in the form of ions
144
Unique characteristic of strong electrolyte is that they yield only ions
Ex Ionic compound may be highly soluble in water and dissociate into ions (NaCl)
Others may not dissolve much but amount that does dissolve exists only as ions
145
Weak ElectrolytesHF dissolves in water to give acid solution called hydrofluoric acid HF bond stronger than bonds between hydrogen and other halogens When HF dissolves some molecule ionize Reverse reaction also happens HF(aq) + H2O(l) H3O+(aq) + F-(aq)
146
HF(aq) + H2O(l) H3O+
(aq) + F-(aq)Concentration of dissolved unionized HF stays high and concentration of ions stays low
Weak electrolyte - any compound whose aqueous solutions conduct electricity poorly this is due to the presence of small amount of the dissolved compound in the form of ions
147
Different from nonelectrolyte ndash NO ions at all Description of electrolyte as strong or weak not related to concentration of solution Electrolytes differ in degree of ionization not amount of solute dissolved
148
149
Colligative properties of solutions
Presence of solute affects properties of solutions Some properties not dependent on nature of dissolved substance but on how may dissolve particles are present
Colligative properties - properties that depend on the concentration of solute particles but not on their identity
150
Vapor-Pressure Lowering
Boiling and freezing point of solution different from pure solvent Nonvolatile solute raises boiling point and lowers freezing point Nonvolatile substance - one that has little tendency to become gas under existing conditions
151
152
To understand why nonvolatile solute changes boiling and freezing point consider equilibrium vapor pressure
Vapor pressure pressure caused by molecules that have escape liquid phase to gas phase
Can be thought of a measure of tendency of molecules to escape from a liquid
153
Addition of sucrose (nonvolatile solute) lowers concentration of water molecules at surface of liquid This lowers tendency of water molecules to leave solution and enter gas phase Vapor pressure of solution is lower than vapor pressure of pure water
154
Aqueous13 solution13 of13 nonvolatile13 solutePure13 water
Nonelectrolyte solutions of same molality have same concentration of particles
Dilute solutions of same solvent and equal molality of any nonelectrolyte solute lower vapor pressure equally
155
Example
1 m aqueous solution of nonelectrolyte glucose lowers vapor pressure of water 55 x 10-4 atm at 25 1 m aqueous solution of sucrose also lowers water vapor pressure to 55 x 10-4 atm at 25
156
157
bc vapor-pressure lowering depends on concentration of nonelectrolyte solute and doesnrsquot depend on the type of solute it is a colligative proprerty
bc vapor pressure has been lowered solution remains liquid over larger temperature range
Lowers freezing point and raises boiling point
158
Can assume changes in boiling and freezing point also depend on concentration of solute
They are colligative properties
159
Freezing-Point Depression
Freezing point of 1-molal solution of any nonelectrolyte solute in water is found (by experiment) to be 186 lower than freezing point of water When 1 mol of nonelectrolyte solute dissolved in 1 kg water freezing point is -186 not 000 160
When 2 mol nonelectrolyte solute dissolved in 1 kg water freezing point is -372
For any concentration of nonelectrolyte solute in water decrease in freezing point can be determined using value of -186m
161
This value called the molal freezing-point contstant (Kf) is the freezing-point depression of the solvent in a 1-molal solution of a nonvolatile nonelectrolyte solute
162
Each solvent has own characteristic molal freezing-point constant Freezing-point depression (Δtf) the difference between the freezing points of the pure solvent and a solute of nonelectrolyte in that solvent and it is directly proportional to the molal concentration of the solution Δtf = Kfm
163
Δtf = Kfm
Kf expressed as m
m expressed in mol solutekg solvent (molality)
164
Sample Problem
What is the freezing point depression of water in a solution of 171 g of sucrose C12H22O11 and 200 g of water What is the actual freezing point of the solution
165
1 AnalyzeGiven Solute mass and chemical formula 171 g C12H22O11 Solvent mass and identity 2000 g water Unknown a freezing-point depression b freezing point of the solution 166
2 Plan
167
168
Calculate
Unknown a freezing-point depression b freezing point of the solution
169
Practice Problem
A water solution containing an unknown quantity of a nonelectrolyte solute is found to have a freezing point of ndash 023degCWhat is the molal concentration of the solution
012 m170
A solution consists of 103 g of the nonelectrolyte glucose C6H12O6 dissolved in 250 g of water What is the freezing-point depression of the solution
minus0426degC171
In a laboratory experiment the freezing point of an aqueous solution of glucose is found to be minus0325degCWhat is the molal concentration of this solution
0175 m172
If 0500 mol of a nonelectrolyte solute are dissolved in 5000 g of ether what is the freezing point of the solution
minus1181degC173
The freezing point of an aqueous solution that contains a nonelectrolyte is minus90degC aWhat is the freezing-point depression of the solution bWhat is the molal concentration of the solution a minus90degC b 48 m
174
Boiling-Point Elevation
Boiling point is temp at which vapor pressure is equal to atmospheric pressure Change in vapor pressure will cause corresponding change in boiling point
175
176
Vapor pressure of nonvolatile solution is lower than vapor pressure of pure solvent More heat will be required to raise vapor pressure of solution Boiling point of solution is higher than boiling point of pure solvent
Molal boiling-point constant (Kb) - boiling-point elevation of the solvent in a 1-molal solution of a nonvolatile nonelectrolyte solute b-p elevation of any nonelectrolyte in water found by experiment to be 051 So molal b-p constant for water is 051m
177
For different solvents b-p elevations of 1-molal solutions have different values (Table 14-2)
Like freezing-point constants values most accurate for dilute solutions
178
Boiling-point elevation Δtb difference between the boiling points of the pure solvent and a nonelectrolyte solution of that solvent and it is directly proportional to the molal concentration of the solution B-p elevation can be calculated using Δtb = Kbm Δtb (boiling point elevation) expressed in m and m expressed in mol solutekg solvent
Sample Problem
What is the boiling-point elevation of a solution made from 200 g of a nonelectrolyte solute and 4000 g of water The molar mass of the solute is 620 gmol
180
1 AnalyzeGiven solute mass = 200 g solute molar mass = 620 gmol solvent mass and identity = 4000 g of water
Unknown boiling-point elevation
181
2 Plan
182
183
3 Compute
Practice Problem
A solution contains 500 g of sucrose C12H22O11 a nonelectrolyte dissolved in 5000 g of water What is the boiling-point elevation
015degC184
A solution contains 4500 g of sucrose C12H22O11 a nonelectrolyte dissolved in 250 g of waterWhat is the boiling point of the solution
1027degC185
If the boiling point elevation of an aqueous solution containing a nonvolatile electrolyte is 102degC what is the molality of the solution
20 m186
The boiling point of an aqueous solution containing a nonvolatile electrolyte is 10075degC aWhat is the boiling-point elevation bWhat is the molality of the solution a 075degC b 15 m
187
Electrolytes and Colligative PropertiesEarly investigators confused by experiments where certain substances depressed freezing point or elevated boiling point more than expected
188
189
Ex 01 m solution of NaCl lowers freezing point of solvent almost twice as much as 01 m solution of sucrose 01 m CaCl2 lowers fp almost 3 times as much as 01 m solution of sucrose
To understand why this happens contrast behavior or sucrose with NaCl in aqueous solution Each sucrose dissolves to make only 1 particle in solution So 1 mol sucrose dissolves to 1 mol particles 1 mol NaCl dissolves to make 2 moles of particles (1 mol Na+ and 1 mol Cl-)
190
191
Calculated Values for Electrolyte SolutionsColligative properties depend on total concentration of solute particles regardless of their identity Electrolytes cause changes in colligative properties proportional to total molality in terms of all dissolves particles instead of formula units
192
What about barium nitrate Ba(NO3)2 Each mole of barium nitrate yields 3 mol of ions in solution Ba(NO3)2(s) rarr Ba2+(aq) + 2NO3-(aq) You would expect solution of given molality to lower fp of solvent 3 times as much as nonelectrolytic solution of same molality
193
Sample Problem
What is the expected change in the freezing point of water in a solution of 625 g of barium nitrate Ba(NO3)2 in 100 kg of water
194
1 AnalyzeGiven solute mass and formula = 625 g Ba(NO3)2 solvent mass and identity = 100 kg water Δtf = Kfm
Unknown expected freezing-point depression
195
2 PlanThe molality can be calculated by converting the solute mass to moles and then dividing by the number of kilograms of solvent That molality is in terms of formula units of Ba(NO3)2 and must be converted to molality in terms of dissociated ions in solution It must be multiplied by the number of moles of ions produced per mole of formula unit This adjusted molality can then be used to calculate the freezing-point depression
196
197
198
3 Compute
Practice Problem
What is the expected freezing-point depression for a solution that contains 20 mol of magnesium sulfate dissolved in 10 kg of water
minus74degC199
What is the expected boiling-point elevation of water for a solution that contains 150 g of sodium chloride dissolved in 10 kg of water 27degC The freezing point of an aqueous sodium chloride solution is minus200degC What is the molality of the solution 54 m NaCl
200
Actual Values for Electrolyte SolutionsRemember values calculated are only expected values Actual values of colligative properties for all strong electrolytes are almost what would be expected Difference between expected and actual values caused by attractive forces that exist between dissociated ions in aqueous solution
201
Attraction between hydrated ions in solution is small compared with those in crystalline solid However forces of attraction donrsquot interfere with movements of aqueous ions More concentrated solution means ions closer together and attraction is greater Also depends on charge of ions ndash higher charge greater attraction
202
1 mol NaOH = 400 g 400 g NaOH dissolved in 1 kg water is 1 m NaOH
200 g NaOH in 1 kg water = 0500 m NaOH
89
Practice Problem
A solution was prepared by dissolving 171 g sucrose (C12H22O11) in 125 g of water Find the molal concentration of this solution 0400 m C12H22O11
90
A solution of iodine in carbon tetrachloride is used when iodine is needed for certain chemical tests How much iodine must be added to prepare a 0480 m solution of iodine in CCl4 if 1000 g of CCl4 is used 122 g I2
91
What is the molality of a solution composed of 255 g acetone (CH3)2CO dissolved in 200 g of water 22 m acetone What quantity in grams of methanol CH3OH is required to prepare a 0244 m solution in 400 g water 312 g methanol
92
How many grams of AgNO3 are needed to prepare a 0125 m solution in 250 mL of water 531 g AgNO3
What is the molality of a solution containing 182 g HCl and 250 g water 200 m
93
Ions in Aqueous Solutions and Colligative Properties
94
95
compounds in aqueous solutions
DissociationWhen compound made from ions dissolves in water ions separate Dissociation - separation of ions that occurs when an ionic compound dissolves
96
Notice number of ions made per formula unit in equations 1 formula unit of NaCl gives 2 units of ions in solution 1 formula unit of CaCl2 gives 3 units of ions
97
Assuming 100 dissociation solution that contains 1 mol NaCl contains 1 mol Na+ and 1 mol Cl- Can assume 100 dissociation of all soluble ionic compounds
98
Practice Problem
Write the equation for the dissolution of aluminum sulfate in water How many moles of aluminum ions are made by dissolving 1 mol aluminum sulfate What is the total number of moles of ions made by dissolving 1 mol of aluminum sulfate
99
1 AnalyzeGiven Amount of solute = 1 mol Al2(SO4)3 Solvent identity = water Unknown a moles of aluminum ions and sulfate ions Total number of moles of solute ions produced
100
2 Plan
The coefficients in the balanced dissociation equation will tell mole relationships You can use the equation to find out the number of moles of solute ions produced
101
3 Compute
a
b102
Write the equation for the dissolution of each of the following in water and then determine the number of moles of each ions made as well as the total number of ions made a 1 mol ammonium chloride b 1 mol sodium sulfide c 05 mol barium nitrate
103
a 1 mol ammonium chloride
1 mol NH4+ 1 mol Cl- 2 mol total ions
104
b 1 mol sodium sulfide
2 mol Na+ 1 mol S-2 3 mol total ions
105
c 05 mol barium nitrate
05 mol Ba+2 1 mol NO3- 15 mol total ions
106
Precipitation Reactions
Even though no compound is completely insoluble compounds of very low solubility can be considered insoluble for practical purposes There are general guidelines to help predict whether a compound made of certain combination of ions is soluble
107
General Solubility Guidelines
Sodium potassium and ammonium compounds are soluble in water
108
Nitrates acetates and chlorates are soluble
109
110
Most chlorides are soluble except those of silver mercury (I) and lead Lead (II) chloride is soluble in hot water
Most sulfates are soluble except those of barium strontium lead calcium and mercury
111
Most carbonates phosphates and silicates are insoluble except those of sodium potassium and ammonium
112
Most sulfides are insoluble except those of calcium strontium sodium potassium and ammonium
113
Is calcium phosphate Ca3(PO4)2 soluble or insoluble
Not one of the exceptions so it is insoluble
Dissociation equations are not written for insoluble compounds
114
Guidelines also useful in predicting what will happen if 2 solutions of 2 different soluble compounds are mixed
If mixing results in combination of ions that forms insoluble compound a double-replacement and precipitation reaction will happen
115
Will precipitate form when solutions of ammonium sulfide and cadmium nitrate are combined You can tell calcium nitrate is soluble from the guidelines (Most nitrates acetates and chlorates are soluble) You can also tell ammonium sulfide is soluble (Most sulfides are insoluble except those of calcium strontium sodium potassium and ammonium)
116
117
To decide whether precipitate can form must know solubilities of two possible products
Ammonium nitrate ndash soluble (Most nitrates acetates and chlorates are soluble) Cadmium sulfide ndash insoluble (Most sulfides are insoluble except those of calcium strontium sodium potassium and ammonium)
118
bc one product is insoluble double-replacement and precipitation reaction will happen (NH4)2S (aq) + Cd(NO3)2 (aq) rarr NH4NO3 (aq) + CdS (s)
119
Net Ionic Equations
Reactions of ions in aqueous solution usually represented by net ionic equations instead of formula equations Net ionic equation - includes only those compounds and ions that undergo a chemical change in a reaction in an aqueous solution
120
121
To write NIE (net ionic equation) first convert chemical equation into overall ionic equation All soluble ionic compounds shown as dissociated ions in solution Precipitates shown as solids
122
Notice ammonium and nitrate ions appear on both sides They havenrsquot gone through any chemical change Spectator ions - ions that do not take part in a chemical reaction and are found in solution both before and after the reaction
123
To change ionic equation to NIE eliminate spectator ions Cd+2(aq) + S-2(aq) rarr CdS(s)
This NIE applies to any reaction in which precipitate of CdS forms
124
Practice Problem
Identify the precipitate that forms when aqueous solutions of zinc nitrate and ammonium sulfide are combined Write the equation for the possible double-replacement reaction Then write the formula equation overall ionic equation and the net ionic equation
125
1 AnalyzeGiven Identity of reactants zinc nitrate and ammonium sulfide Reaction medium aqueous solution Unknown a equation for the possible double-replacement reaction b identity of the precipitate c formula equation d overall ionic equation e net ionic equation
126
2 PlanWrite the possible double-replacement reaction between Zn(NO3)2 and (NH4)2S Use the rules to determine if any of the products will precipitate Write a formula equation and overall net ionic equation Then cancel spectator ions to make net ionic equation
127
3 Computea possible double-replacement reaction
b rules show that zinc sulfide is not soluble so will precipitate (ammonium nitrate is soluble)
c formula equation128
d overall ionic equation
e net ionic equation129
Will a precipitate form if solutions of potassium sulfate and barium nitrate are combined If so write the net ionic equation for the reaction
Yes Ba+2(aq) + SO4-2(aq) rarr BaSO4(s)
130
Will a precipitate form if solutions of potassium nitrate and magnesium sulfate are combined If so write the net ionic equation for the reaction
No 131
Will a precipitate form if solutions of barium chloride and sodium sulfate are combined If so identify the spectator ions and write the net ionic equation for the reaction
Yes Na+ and Cl- Ba+2(aq) + SO4-2(aq) rarr BaSO4(s)
132
Write the net ionic equation for the precipitation of nickel(II) sulfide
Ni+2(aq) + S-2(aq) rarr NiS(s)
133
IonizationSome molecular compounds can also form ions in solution Usually polar Ionization - ions formed from solute molecules by action of the solvent (creation of ions where there were none) Ionization different from dissociation
134
Dissociation ionic compounds dissolve and ions already present separate Ionization ions formed where non existed before
Ions formed are hydrated Heat released during hydration of ions gives enough energy to break covalent bonds
135
Extent to which solute ionizes depends on strength of bonds within molecules of solute and strength of attraction between solute and solvent molecules
136
HCl ndash molecular compound that ionizes in aqueous solution Contains highly polar bond Attraction between polar HCl molecule and polar water strong enough to break HCl bond
137
The Hydronium Ion
H+ ions from HCl attracts other molecules or ions so strongly that it doesnrsquot normally exist alone Ionization of HCl better described as direct transfer of proton from HCl to H2O forming H3O+
138
139
Hydration of H+ to form hydronium is highly exothermic
Energy released gives a lot of energy needed to ionize a molecular solute
Many molecular compounds that ionize in aqueous solution contain hydrogen and form hydronium ion
140
Strong and Weak Electrolytes
Substances that make ions and conduct current in solution are electrolytes HCl is one of series of compounds that has hydrogen and halogen Hydrogen halides are all molecular compounds with single polar-covalent bonds
141
All are gases Very soluble in water All are electrolytes HCl HBr HI strongly conduct current in solution HF only weakly conducts current at same concentration Strength of conduction related to ability to ionize
142
143
Strong Electrolytes
HCl HBr HI are 100 ionized in solution
Strong electrolyte - any compound whose dilute aqueous solutions conduct electricity well this is due to the presence of all or almost all of the dissolve compound in the form of ions
144
Unique characteristic of strong electrolyte is that they yield only ions
Ex Ionic compound may be highly soluble in water and dissociate into ions (NaCl)
Others may not dissolve much but amount that does dissolve exists only as ions
145
Weak ElectrolytesHF dissolves in water to give acid solution called hydrofluoric acid HF bond stronger than bonds between hydrogen and other halogens When HF dissolves some molecule ionize Reverse reaction also happens HF(aq) + H2O(l) H3O+(aq) + F-(aq)
146
HF(aq) + H2O(l) H3O+
(aq) + F-(aq)Concentration of dissolved unionized HF stays high and concentration of ions stays low
Weak electrolyte - any compound whose aqueous solutions conduct electricity poorly this is due to the presence of small amount of the dissolved compound in the form of ions
147
Different from nonelectrolyte ndash NO ions at all Description of electrolyte as strong or weak not related to concentration of solution Electrolytes differ in degree of ionization not amount of solute dissolved
148
149
Colligative properties of solutions
Presence of solute affects properties of solutions Some properties not dependent on nature of dissolved substance but on how may dissolve particles are present
Colligative properties - properties that depend on the concentration of solute particles but not on their identity
150
Vapor-Pressure Lowering
Boiling and freezing point of solution different from pure solvent Nonvolatile solute raises boiling point and lowers freezing point Nonvolatile substance - one that has little tendency to become gas under existing conditions
151
152
To understand why nonvolatile solute changes boiling and freezing point consider equilibrium vapor pressure
Vapor pressure pressure caused by molecules that have escape liquid phase to gas phase
Can be thought of a measure of tendency of molecules to escape from a liquid
153
Addition of sucrose (nonvolatile solute) lowers concentration of water molecules at surface of liquid This lowers tendency of water molecules to leave solution and enter gas phase Vapor pressure of solution is lower than vapor pressure of pure water
154
Aqueous13 solution13 of13 nonvolatile13 solutePure13 water
Nonelectrolyte solutions of same molality have same concentration of particles
Dilute solutions of same solvent and equal molality of any nonelectrolyte solute lower vapor pressure equally
155
Example
1 m aqueous solution of nonelectrolyte glucose lowers vapor pressure of water 55 x 10-4 atm at 25 1 m aqueous solution of sucrose also lowers water vapor pressure to 55 x 10-4 atm at 25
156
157
bc vapor-pressure lowering depends on concentration of nonelectrolyte solute and doesnrsquot depend on the type of solute it is a colligative proprerty
bc vapor pressure has been lowered solution remains liquid over larger temperature range
Lowers freezing point and raises boiling point
158
Can assume changes in boiling and freezing point also depend on concentration of solute
They are colligative properties
159
Freezing-Point Depression
Freezing point of 1-molal solution of any nonelectrolyte solute in water is found (by experiment) to be 186 lower than freezing point of water When 1 mol of nonelectrolyte solute dissolved in 1 kg water freezing point is -186 not 000 160
When 2 mol nonelectrolyte solute dissolved in 1 kg water freezing point is -372
For any concentration of nonelectrolyte solute in water decrease in freezing point can be determined using value of -186m
161
This value called the molal freezing-point contstant (Kf) is the freezing-point depression of the solvent in a 1-molal solution of a nonvolatile nonelectrolyte solute
162
Each solvent has own characteristic molal freezing-point constant Freezing-point depression (Δtf) the difference between the freezing points of the pure solvent and a solute of nonelectrolyte in that solvent and it is directly proportional to the molal concentration of the solution Δtf = Kfm
163
Δtf = Kfm
Kf expressed as m
m expressed in mol solutekg solvent (molality)
164
Sample Problem
What is the freezing point depression of water in a solution of 171 g of sucrose C12H22O11 and 200 g of water What is the actual freezing point of the solution
165
1 AnalyzeGiven Solute mass and chemical formula 171 g C12H22O11 Solvent mass and identity 2000 g water Unknown a freezing-point depression b freezing point of the solution 166
2 Plan
167
168
Calculate
Unknown a freezing-point depression b freezing point of the solution
169
Practice Problem
A water solution containing an unknown quantity of a nonelectrolyte solute is found to have a freezing point of ndash 023degCWhat is the molal concentration of the solution
012 m170
A solution consists of 103 g of the nonelectrolyte glucose C6H12O6 dissolved in 250 g of water What is the freezing-point depression of the solution
minus0426degC171
In a laboratory experiment the freezing point of an aqueous solution of glucose is found to be minus0325degCWhat is the molal concentration of this solution
0175 m172
If 0500 mol of a nonelectrolyte solute are dissolved in 5000 g of ether what is the freezing point of the solution
minus1181degC173
The freezing point of an aqueous solution that contains a nonelectrolyte is minus90degC aWhat is the freezing-point depression of the solution bWhat is the molal concentration of the solution a minus90degC b 48 m
174
Boiling-Point Elevation
Boiling point is temp at which vapor pressure is equal to atmospheric pressure Change in vapor pressure will cause corresponding change in boiling point
175
176
Vapor pressure of nonvolatile solution is lower than vapor pressure of pure solvent More heat will be required to raise vapor pressure of solution Boiling point of solution is higher than boiling point of pure solvent
Molal boiling-point constant (Kb) - boiling-point elevation of the solvent in a 1-molal solution of a nonvolatile nonelectrolyte solute b-p elevation of any nonelectrolyte in water found by experiment to be 051 So molal b-p constant for water is 051m
177
For different solvents b-p elevations of 1-molal solutions have different values (Table 14-2)
Like freezing-point constants values most accurate for dilute solutions
178
Boiling-point elevation Δtb difference between the boiling points of the pure solvent and a nonelectrolyte solution of that solvent and it is directly proportional to the molal concentration of the solution B-p elevation can be calculated using Δtb = Kbm Δtb (boiling point elevation) expressed in m and m expressed in mol solutekg solvent
Sample Problem
What is the boiling-point elevation of a solution made from 200 g of a nonelectrolyte solute and 4000 g of water The molar mass of the solute is 620 gmol
180
1 AnalyzeGiven solute mass = 200 g solute molar mass = 620 gmol solvent mass and identity = 4000 g of water
Unknown boiling-point elevation
181
2 Plan
182
183
3 Compute
Practice Problem
A solution contains 500 g of sucrose C12H22O11 a nonelectrolyte dissolved in 5000 g of water What is the boiling-point elevation
015degC184
A solution contains 4500 g of sucrose C12H22O11 a nonelectrolyte dissolved in 250 g of waterWhat is the boiling point of the solution
1027degC185
If the boiling point elevation of an aqueous solution containing a nonvolatile electrolyte is 102degC what is the molality of the solution
20 m186
The boiling point of an aqueous solution containing a nonvolatile electrolyte is 10075degC aWhat is the boiling-point elevation bWhat is the molality of the solution a 075degC b 15 m
187
Electrolytes and Colligative PropertiesEarly investigators confused by experiments where certain substances depressed freezing point or elevated boiling point more than expected
188
189
Ex 01 m solution of NaCl lowers freezing point of solvent almost twice as much as 01 m solution of sucrose 01 m CaCl2 lowers fp almost 3 times as much as 01 m solution of sucrose
To understand why this happens contrast behavior or sucrose with NaCl in aqueous solution Each sucrose dissolves to make only 1 particle in solution So 1 mol sucrose dissolves to 1 mol particles 1 mol NaCl dissolves to make 2 moles of particles (1 mol Na+ and 1 mol Cl-)
190
191
Calculated Values for Electrolyte SolutionsColligative properties depend on total concentration of solute particles regardless of their identity Electrolytes cause changes in colligative properties proportional to total molality in terms of all dissolves particles instead of formula units
192
What about barium nitrate Ba(NO3)2 Each mole of barium nitrate yields 3 mol of ions in solution Ba(NO3)2(s) rarr Ba2+(aq) + 2NO3-(aq) You would expect solution of given molality to lower fp of solvent 3 times as much as nonelectrolytic solution of same molality
193
Sample Problem
What is the expected change in the freezing point of water in a solution of 625 g of barium nitrate Ba(NO3)2 in 100 kg of water
194
1 AnalyzeGiven solute mass and formula = 625 g Ba(NO3)2 solvent mass and identity = 100 kg water Δtf = Kfm
Unknown expected freezing-point depression
195
2 PlanThe molality can be calculated by converting the solute mass to moles and then dividing by the number of kilograms of solvent That molality is in terms of formula units of Ba(NO3)2 and must be converted to molality in terms of dissociated ions in solution It must be multiplied by the number of moles of ions produced per mole of formula unit This adjusted molality can then be used to calculate the freezing-point depression
196
197
198
3 Compute
Practice Problem
What is the expected freezing-point depression for a solution that contains 20 mol of magnesium sulfate dissolved in 10 kg of water
minus74degC199
What is the expected boiling-point elevation of water for a solution that contains 150 g of sodium chloride dissolved in 10 kg of water 27degC The freezing point of an aqueous sodium chloride solution is minus200degC What is the molality of the solution 54 m NaCl
200
Actual Values for Electrolyte SolutionsRemember values calculated are only expected values Actual values of colligative properties for all strong electrolytes are almost what would be expected Difference between expected and actual values caused by attractive forces that exist between dissociated ions in aqueous solution
201
Attraction between hydrated ions in solution is small compared with those in crystalline solid However forces of attraction donrsquot interfere with movements of aqueous ions More concentrated solution means ions closer together and attraction is greater Also depends on charge of ions ndash higher charge greater attraction
202
Practice Problem
A solution was prepared by dissolving 171 g sucrose (C12H22O11) in 125 g of water Find the molal concentration of this solution 0400 m C12H22O11
90
A solution of iodine in carbon tetrachloride is used when iodine is needed for certain chemical tests How much iodine must be added to prepare a 0480 m solution of iodine in CCl4 if 1000 g of CCl4 is used 122 g I2
91
What is the molality of a solution composed of 255 g acetone (CH3)2CO dissolved in 200 g of water 22 m acetone What quantity in grams of methanol CH3OH is required to prepare a 0244 m solution in 400 g water 312 g methanol
92
How many grams of AgNO3 are needed to prepare a 0125 m solution in 250 mL of water 531 g AgNO3
What is the molality of a solution containing 182 g HCl and 250 g water 200 m
93
Ions in Aqueous Solutions and Colligative Properties
94
95
compounds in aqueous solutions
DissociationWhen compound made from ions dissolves in water ions separate Dissociation - separation of ions that occurs when an ionic compound dissolves
96
Notice number of ions made per formula unit in equations 1 formula unit of NaCl gives 2 units of ions in solution 1 formula unit of CaCl2 gives 3 units of ions
97
Assuming 100 dissociation solution that contains 1 mol NaCl contains 1 mol Na+ and 1 mol Cl- Can assume 100 dissociation of all soluble ionic compounds
98
Practice Problem
Write the equation for the dissolution of aluminum sulfate in water How many moles of aluminum ions are made by dissolving 1 mol aluminum sulfate What is the total number of moles of ions made by dissolving 1 mol of aluminum sulfate
99
1 AnalyzeGiven Amount of solute = 1 mol Al2(SO4)3 Solvent identity = water Unknown a moles of aluminum ions and sulfate ions Total number of moles of solute ions produced
100
2 Plan
The coefficients in the balanced dissociation equation will tell mole relationships You can use the equation to find out the number of moles of solute ions produced
101
3 Compute
a
b102
Write the equation for the dissolution of each of the following in water and then determine the number of moles of each ions made as well as the total number of ions made a 1 mol ammonium chloride b 1 mol sodium sulfide c 05 mol barium nitrate
103
a 1 mol ammonium chloride
1 mol NH4+ 1 mol Cl- 2 mol total ions
104
b 1 mol sodium sulfide
2 mol Na+ 1 mol S-2 3 mol total ions
105
c 05 mol barium nitrate
05 mol Ba+2 1 mol NO3- 15 mol total ions
106
Precipitation Reactions
Even though no compound is completely insoluble compounds of very low solubility can be considered insoluble for practical purposes There are general guidelines to help predict whether a compound made of certain combination of ions is soluble
107
General Solubility Guidelines
Sodium potassium and ammonium compounds are soluble in water
108
Nitrates acetates and chlorates are soluble
109
110
Most chlorides are soluble except those of silver mercury (I) and lead Lead (II) chloride is soluble in hot water
Most sulfates are soluble except those of barium strontium lead calcium and mercury
111
Most carbonates phosphates and silicates are insoluble except those of sodium potassium and ammonium
112
Most sulfides are insoluble except those of calcium strontium sodium potassium and ammonium
113
Is calcium phosphate Ca3(PO4)2 soluble or insoluble
Not one of the exceptions so it is insoluble
Dissociation equations are not written for insoluble compounds
114
Guidelines also useful in predicting what will happen if 2 solutions of 2 different soluble compounds are mixed
If mixing results in combination of ions that forms insoluble compound a double-replacement and precipitation reaction will happen
115
Will precipitate form when solutions of ammonium sulfide and cadmium nitrate are combined You can tell calcium nitrate is soluble from the guidelines (Most nitrates acetates and chlorates are soluble) You can also tell ammonium sulfide is soluble (Most sulfides are insoluble except those of calcium strontium sodium potassium and ammonium)
116
117
To decide whether precipitate can form must know solubilities of two possible products
Ammonium nitrate ndash soluble (Most nitrates acetates and chlorates are soluble) Cadmium sulfide ndash insoluble (Most sulfides are insoluble except those of calcium strontium sodium potassium and ammonium)
118
bc one product is insoluble double-replacement and precipitation reaction will happen (NH4)2S (aq) + Cd(NO3)2 (aq) rarr NH4NO3 (aq) + CdS (s)
119
Net Ionic Equations
Reactions of ions in aqueous solution usually represented by net ionic equations instead of formula equations Net ionic equation - includes only those compounds and ions that undergo a chemical change in a reaction in an aqueous solution
120
121
To write NIE (net ionic equation) first convert chemical equation into overall ionic equation All soluble ionic compounds shown as dissociated ions in solution Precipitates shown as solids
122
Notice ammonium and nitrate ions appear on both sides They havenrsquot gone through any chemical change Spectator ions - ions that do not take part in a chemical reaction and are found in solution both before and after the reaction
123
To change ionic equation to NIE eliminate spectator ions Cd+2(aq) + S-2(aq) rarr CdS(s)
This NIE applies to any reaction in which precipitate of CdS forms
124
Practice Problem
Identify the precipitate that forms when aqueous solutions of zinc nitrate and ammonium sulfide are combined Write the equation for the possible double-replacement reaction Then write the formula equation overall ionic equation and the net ionic equation
125
1 AnalyzeGiven Identity of reactants zinc nitrate and ammonium sulfide Reaction medium aqueous solution Unknown a equation for the possible double-replacement reaction b identity of the precipitate c formula equation d overall ionic equation e net ionic equation
126
2 PlanWrite the possible double-replacement reaction between Zn(NO3)2 and (NH4)2S Use the rules to determine if any of the products will precipitate Write a formula equation and overall net ionic equation Then cancel spectator ions to make net ionic equation
127
3 Computea possible double-replacement reaction
b rules show that zinc sulfide is not soluble so will precipitate (ammonium nitrate is soluble)
c formula equation128
d overall ionic equation
e net ionic equation129
Will a precipitate form if solutions of potassium sulfate and barium nitrate are combined If so write the net ionic equation for the reaction
Yes Ba+2(aq) + SO4-2(aq) rarr BaSO4(s)
130
Will a precipitate form if solutions of potassium nitrate and magnesium sulfate are combined If so write the net ionic equation for the reaction
No 131
Will a precipitate form if solutions of barium chloride and sodium sulfate are combined If so identify the spectator ions and write the net ionic equation for the reaction
Yes Na+ and Cl- Ba+2(aq) + SO4-2(aq) rarr BaSO4(s)
132
Write the net ionic equation for the precipitation of nickel(II) sulfide
Ni+2(aq) + S-2(aq) rarr NiS(s)
133
IonizationSome molecular compounds can also form ions in solution Usually polar Ionization - ions formed from solute molecules by action of the solvent (creation of ions where there were none) Ionization different from dissociation
134
Dissociation ionic compounds dissolve and ions already present separate Ionization ions formed where non existed before
Ions formed are hydrated Heat released during hydration of ions gives enough energy to break covalent bonds
135
Extent to which solute ionizes depends on strength of bonds within molecules of solute and strength of attraction between solute and solvent molecules
136
HCl ndash molecular compound that ionizes in aqueous solution Contains highly polar bond Attraction between polar HCl molecule and polar water strong enough to break HCl bond
137
The Hydronium Ion
H+ ions from HCl attracts other molecules or ions so strongly that it doesnrsquot normally exist alone Ionization of HCl better described as direct transfer of proton from HCl to H2O forming H3O+
138
139
Hydration of H+ to form hydronium is highly exothermic
Energy released gives a lot of energy needed to ionize a molecular solute
Many molecular compounds that ionize in aqueous solution contain hydrogen and form hydronium ion
140
Strong and Weak Electrolytes
Substances that make ions and conduct current in solution are electrolytes HCl is one of series of compounds that has hydrogen and halogen Hydrogen halides are all molecular compounds with single polar-covalent bonds
141
All are gases Very soluble in water All are electrolytes HCl HBr HI strongly conduct current in solution HF only weakly conducts current at same concentration Strength of conduction related to ability to ionize
142
143
Strong Electrolytes
HCl HBr HI are 100 ionized in solution
Strong electrolyte - any compound whose dilute aqueous solutions conduct electricity well this is due to the presence of all or almost all of the dissolve compound in the form of ions
144
Unique characteristic of strong electrolyte is that they yield only ions
Ex Ionic compound may be highly soluble in water and dissociate into ions (NaCl)
Others may not dissolve much but amount that does dissolve exists only as ions
145
Weak ElectrolytesHF dissolves in water to give acid solution called hydrofluoric acid HF bond stronger than bonds between hydrogen and other halogens When HF dissolves some molecule ionize Reverse reaction also happens HF(aq) + H2O(l) H3O+(aq) + F-(aq)
146
HF(aq) + H2O(l) H3O+
(aq) + F-(aq)Concentration of dissolved unionized HF stays high and concentration of ions stays low
Weak electrolyte - any compound whose aqueous solutions conduct electricity poorly this is due to the presence of small amount of the dissolved compound in the form of ions
147
Different from nonelectrolyte ndash NO ions at all Description of electrolyte as strong or weak not related to concentration of solution Electrolytes differ in degree of ionization not amount of solute dissolved
148
149
Colligative properties of solutions
Presence of solute affects properties of solutions Some properties not dependent on nature of dissolved substance but on how may dissolve particles are present
Colligative properties - properties that depend on the concentration of solute particles but not on their identity
150
Vapor-Pressure Lowering
Boiling and freezing point of solution different from pure solvent Nonvolatile solute raises boiling point and lowers freezing point Nonvolatile substance - one that has little tendency to become gas under existing conditions
151
152
To understand why nonvolatile solute changes boiling and freezing point consider equilibrium vapor pressure
Vapor pressure pressure caused by molecules that have escape liquid phase to gas phase
Can be thought of a measure of tendency of molecules to escape from a liquid
153
Addition of sucrose (nonvolatile solute) lowers concentration of water molecules at surface of liquid This lowers tendency of water molecules to leave solution and enter gas phase Vapor pressure of solution is lower than vapor pressure of pure water
154
Aqueous13 solution13 of13 nonvolatile13 solutePure13 water
Nonelectrolyte solutions of same molality have same concentration of particles
Dilute solutions of same solvent and equal molality of any nonelectrolyte solute lower vapor pressure equally
155
Example
1 m aqueous solution of nonelectrolyte glucose lowers vapor pressure of water 55 x 10-4 atm at 25 1 m aqueous solution of sucrose also lowers water vapor pressure to 55 x 10-4 atm at 25
156
157
bc vapor-pressure lowering depends on concentration of nonelectrolyte solute and doesnrsquot depend on the type of solute it is a colligative proprerty
bc vapor pressure has been lowered solution remains liquid over larger temperature range
Lowers freezing point and raises boiling point
158
Can assume changes in boiling and freezing point also depend on concentration of solute
They are colligative properties
159
Freezing-Point Depression
Freezing point of 1-molal solution of any nonelectrolyte solute in water is found (by experiment) to be 186 lower than freezing point of water When 1 mol of nonelectrolyte solute dissolved in 1 kg water freezing point is -186 not 000 160
When 2 mol nonelectrolyte solute dissolved in 1 kg water freezing point is -372
For any concentration of nonelectrolyte solute in water decrease in freezing point can be determined using value of -186m
161
This value called the molal freezing-point contstant (Kf) is the freezing-point depression of the solvent in a 1-molal solution of a nonvolatile nonelectrolyte solute
162
Each solvent has own characteristic molal freezing-point constant Freezing-point depression (Δtf) the difference between the freezing points of the pure solvent and a solute of nonelectrolyte in that solvent and it is directly proportional to the molal concentration of the solution Δtf = Kfm
163
Δtf = Kfm
Kf expressed as m
m expressed in mol solutekg solvent (molality)
164
Sample Problem
What is the freezing point depression of water in a solution of 171 g of sucrose C12H22O11 and 200 g of water What is the actual freezing point of the solution
165
1 AnalyzeGiven Solute mass and chemical formula 171 g C12H22O11 Solvent mass and identity 2000 g water Unknown a freezing-point depression b freezing point of the solution 166
2 Plan
167
168
Calculate
Unknown a freezing-point depression b freezing point of the solution
169
Practice Problem
A water solution containing an unknown quantity of a nonelectrolyte solute is found to have a freezing point of ndash 023degCWhat is the molal concentration of the solution
012 m170
A solution consists of 103 g of the nonelectrolyte glucose C6H12O6 dissolved in 250 g of water What is the freezing-point depression of the solution
minus0426degC171
In a laboratory experiment the freezing point of an aqueous solution of glucose is found to be minus0325degCWhat is the molal concentration of this solution
0175 m172
If 0500 mol of a nonelectrolyte solute are dissolved in 5000 g of ether what is the freezing point of the solution
minus1181degC173
The freezing point of an aqueous solution that contains a nonelectrolyte is minus90degC aWhat is the freezing-point depression of the solution bWhat is the molal concentration of the solution a minus90degC b 48 m
174
Boiling-Point Elevation
Boiling point is temp at which vapor pressure is equal to atmospheric pressure Change in vapor pressure will cause corresponding change in boiling point
175
176
Vapor pressure of nonvolatile solution is lower than vapor pressure of pure solvent More heat will be required to raise vapor pressure of solution Boiling point of solution is higher than boiling point of pure solvent
Molal boiling-point constant (Kb) - boiling-point elevation of the solvent in a 1-molal solution of a nonvolatile nonelectrolyte solute b-p elevation of any nonelectrolyte in water found by experiment to be 051 So molal b-p constant for water is 051m
177
For different solvents b-p elevations of 1-molal solutions have different values (Table 14-2)
Like freezing-point constants values most accurate for dilute solutions
178
Boiling-point elevation Δtb difference between the boiling points of the pure solvent and a nonelectrolyte solution of that solvent and it is directly proportional to the molal concentration of the solution B-p elevation can be calculated using Δtb = Kbm Δtb (boiling point elevation) expressed in m and m expressed in mol solutekg solvent
Sample Problem
What is the boiling-point elevation of a solution made from 200 g of a nonelectrolyte solute and 4000 g of water The molar mass of the solute is 620 gmol
180
1 AnalyzeGiven solute mass = 200 g solute molar mass = 620 gmol solvent mass and identity = 4000 g of water
Unknown boiling-point elevation
181
2 Plan
182
183
3 Compute
Practice Problem
A solution contains 500 g of sucrose C12H22O11 a nonelectrolyte dissolved in 5000 g of water What is the boiling-point elevation
015degC184
A solution contains 4500 g of sucrose C12H22O11 a nonelectrolyte dissolved in 250 g of waterWhat is the boiling point of the solution
1027degC185
If the boiling point elevation of an aqueous solution containing a nonvolatile electrolyte is 102degC what is the molality of the solution
20 m186
The boiling point of an aqueous solution containing a nonvolatile electrolyte is 10075degC aWhat is the boiling-point elevation bWhat is the molality of the solution a 075degC b 15 m
187
Electrolytes and Colligative PropertiesEarly investigators confused by experiments where certain substances depressed freezing point or elevated boiling point more than expected
188
189
Ex 01 m solution of NaCl lowers freezing point of solvent almost twice as much as 01 m solution of sucrose 01 m CaCl2 lowers fp almost 3 times as much as 01 m solution of sucrose
To understand why this happens contrast behavior or sucrose with NaCl in aqueous solution Each sucrose dissolves to make only 1 particle in solution So 1 mol sucrose dissolves to 1 mol particles 1 mol NaCl dissolves to make 2 moles of particles (1 mol Na+ and 1 mol Cl-)
190
191
Calculated Values for Electrolyte SolutionsColligative properties depend on total concentration of solute particles regardless of their identity Electrolytes cause changes in colligative properties proportional to total molality in terms of all dissolves particles instead of formula units
192
What about barium nitrate Ba(NO3)2 Each mole of barium nitrate yields 3 mol of ions in solution Ba(NO3)2(s) rarr Ba2+(aq) + 2NO3-(aq) You would expect solution of given molality to lower fp of solvent 3 times as much as nonelectrolytic solution of same molality
193
Sample Problem
What is the expected change in the freezing point of water in a solution of 625 g of barium nitrate Ba(NO3)2 in 100 kg of water
194
1 AnalyzeGiven solute mass and formula = 625 g Ba(NO3)2 solvent mass and identity = 100 kg water Δtf = Kfm
Unknown expected freezing-point depression
195
2 PlanThe molality can be calculated by converting the solute mass to moles and then dividing by the number of kilograms of solvent That molality is in terms of formula units of Ba(NO3)2 and must be converted to molality in terms of dissociated ions in solution It must be multiplied by the number of moles of ions produced per mole of formula unit This adjusted molality can then be used to calculate the freezing-point depression
196
197
198
3 Compute
Practice Problem
What is the expected freezing-point depression for a solution that contains 20 mol of magnesium sulfate dissolved in 10 kg of water
minus74degC199
What is the expected boiling-point elevation of water for a solution that contains 150 g of sodium chloride dissolved in 10 kg of water 27degC The freezing point of an aqueous sodium chloride solution is minus200degC What is the molality of the solution 54 m NaCl
200
Actual Values for Electrolyte SolutionsRemember values calculated are only expected values Actual values of colligative properties for all strong electrolytes are almost what would be expected Difference between expected and actual values caused by attractive forces that exist between dissociated ions in aqueous solution
201
Attraction between hydrated ions in solution is small compared with those in crystalline solid However forces of attraction donrsquot interfere with movements of aqueous ions More concentrated solution means ions closer together and attraction is greater Also depends on charge of ions ndash higher charge greater attraction
202
A solution of iodine in carbon tetrachloride is used when iodine is needed for certain chemical tests How much iodine must be added to prepare a 0480 m solution of iodine in CCl4 if 1000 g of CCl4 is used 122 g I2
91
What is the molality of a solution composed of 255 g acetone (CH3)2CO dissolved in 200 g of water 22 m acetone What quantity in grams of methanol CH3OH is required to prepare a 0244 m solution in 400 g water 312 g methanol
92
How many grams of AgNO3 are needed to prepare a 0125 m solution in 250 mL of water 531 g AgNO3
What is the molality of a solution containing 182 g HCl and 250 g water 200 m
93
Ions in Aqueous Solutions and Colligative Properties
94
95
compounds in aqueous solutions
DissociationWhen compound made from ions dissolves in water ions separate Dissociation - separation of ions that occurs when an ionic compound dissolves
96
Notice number of ions made per formula unit in equations 1 formula unit of NaCl gives 2 units of ions in solution 1 formula unit of CaCl2 gives 3 units of ions
97
Assuming 100 dissociation solution that contains 1 mol NaCl contains 1 mol Na+ and 1 mol Cl- Can assume 100 dissociation of all soluble ionic compounds
98
Practice Problem
Write the equation for the dissolution of aluminum sulfate in water How many moles of aluminum ions are made by dissolving 1 mol aluminum sulfate What is the total number of moles of ions made by dissolving 1 mol of aluminum sulfate
99
1 AnalyzeGiven Amount of solute = 1 mol Al2(SO4)3 Solvent identity = water Unknown a moles of aluminum ions and sulfate ions Total number of moles of solute ions produced
100
2 Plan
The coefficients in the balanced dissociation equation will tell mole relationships You can use the equation to find out the number of moles of solute ions produced
101
3 Compute
a
b102
Write the equation for the dissolution of each of the following in water and then determine the number of moles of each ions made as well as the total number of ions made a 1 mol ammonium chloride b 1 mol sodium sulfide c 05 mol barium nitrate
103
a 1 mol ammonium chloride
1 mol NH4+ 1 mol Cl- 2 mol total ions
104
b 1 mol sodium sulfide
2 mol Na+ 1 mol S-2 3 mol total ions
105
c 05 mol barium nitrate
05 mol Ba+2 1 mol NO3- 15 mol total ions
106
Precipitation Reactions
Even though no compound is completely insoluble compounds of very low solubility can be considered insoluble for practical purposes There are general guidelines to help predict whether a compound made of certain combination of ions is soluble
107
General Solubility Guidelines
Sodium potassium and ammonium compounds are soluble in water
108
Nitrates acetates and chlorates are soluble
109
110
Most chlorides are soluble except those of silver mercury (I) and lead Lead (II) chloride is soluble in hot water
Most sulfates are soluble except those of barium strontium lead calcium and mercury
111
Most carbonates phosphates and silicates are insoluble except those of sodium potassium and ammonium
112
Most sulfides are insoluble except those of calcium strontium sodium potassium and ammonium
113
Is calcium phosphate Ca3(PO4)2 soluble or insoluble
Not one of the exceptions so it is insoluble
Dissociation equations are not written for insoluble compounds
114
Guidelines also useful in predicting what will happen if 2 solutions of 2 different soluble compounds are mixed
If mixing results in combination of ions that forms insoluble compound a double-replacement and precipitation reaction will happen
115
Will precipitate form when solutions of ammonium sulfide and cadmium nitrate are combined You can tell calcium nitrate is soluble from the guidelines (Most nitrates acetates and chlorates are soluble) You can also tell ammonium sulfide is soluble (Most sulfides are insoluble except those of calcium strontium sodium potassium and ammonium)
116
117
To decide whether precipitate can form must know solubilities of two possible products
Ammonium nitrate ndash soluble (Most nitrates acetates and chlorates are soluble) Cadmium sulfide ndash insoluble (Most sulfides are insoluble except those of calcium strontium sodium potassium and ammonium)
118
bc one product is insoluble double-replacement and precipitation reaction will happen (NH4)2S (aq) + Cd(NO3)2 (aq) rarr NH4NO3 (aq) + CdS (s)
119
Net Ionic Equations
Reactions of ions in aqueous solution usually represented by net ionic equations instead of formula equations Net ionic equation - includes only those compounds and ions that undergo a chemical change in a reaction in an aqueous solution
120
121
To write NIE (net ionic equation) first convert chemical equation into overall ionic equation All soluble ionic compounds shown as dissociated ions in solution Precipitates shown as solids
122
Notice ammonium and nitrate ions appear on both sides They havenrsquot gone through any chemical change Spectator ions - ions that do not take part in a chemical reaction and are found in solution both before and after the reaction
123
To change ionic equation to NIE eliminate spectator ions Cd+2(aq) + S-2(aq) rarr CdS(s)
This NIE applies to any reaction in which precipitate of CdS forms
124
Practice Problem
Identify the precipitate that forms when aqueous solutions of zinc nitrate and ammonium sulfide are combined Write the equation for the possible double-replacement reaction Then write the formula equation overall ionic equation and the net ionic equation
125
1 AnalyzeGiven Identity of reactants zinc nitrate and ammonium sulfide Reaction medium aqueous solution Unknown a equation for the possible double-replacement reaction b identity of the precipitate c formula equation d overall ionic equation e net ionic equation
126
2 PlanWrite the possible double-replacement reaction between Zn(NO3)2 and (NH4)2S Use the rules to determine if any of the products will precipitate Write a formula equation and overall net ionic equation Then cancel spectator ions to make net ionic equation
127
3 Computea possible double-replacement reaction
b rules show that zinc sulfide is not soluble so will precipitate (ammonium nitrate is soluble)
c formula equation128
d overall ionic equation
e net ionic equation129
Will a precipitate form if solutions of potassium sulfate and barium nitrate are combined If so write the net ionic equation for the reaction
Yes Ba+2(aq) + SO4-2(aq) rarr BaSO4(s)
130
Will a precipitate form if solutions of potassium nitrate and magnesium sulfate are combined If so write the net ionic equation for the reaction
No 131
Will a precipitate form if solutions of barium chloride and sodium sulfate are combined If so identify the spectator ions and write the net ionic equation for the reaction
Yes Na+ and Cl- Ba+2(aq) + SO4-2(aq) rarr BaSO4(s)
132
Write the net ionic equation for the precipitation of nickel(II) sulfide
Ni+2(aq) + S-2(aq) rarr NiS(s)
133
IonizationSome molecular compounds can also form ions in solution Usually polar Ionization - ions formed from solute molecules by action of the solvent (creation of ions where there were none) Ionization different from dissociation
134
Dissociation ionic compounds dissolve and ions already present separate Ionization ions formed where non existed before
Ions formed are hydrated Heat released during hydration of ions gives enough energy to break covalent bonds
135
Extent to which solute ionizes depends on strength of bonds within molecules of solute and strength of attraction between solute and solvent molecules
136
HCl ndash molecular compound that ionizes in aqueous solution Contains highly polar bond Attraction between polar HCl molecule and polar water strong enough to break HCl bond
137
The Hydronium Ion
H+ ions from HCl attracts other molecules or ions so strongly that it doesnrsquot normally exist alone Ionization of HCl better described as direct transfer of proton from HCl to H2O forming H3O+
138
139
Hydration of H+ to form hydronium is highly exothermic
Energy released gives a lot of energy needed to ionize a molecular solute
Many molecular compounds that ionize in aqueous solution contain hydrogen and form hydronium ion
140
Strong and Weak Electrolytes
Substances that make ions and conduct current in solution are electrolytes HCl is one of series of compounds that has hydrogen and halogen Hydrogen halides are all molecular compounds with single polar-covalent bonds
141
All are gases Very soluble in water All are electrolytes HCl HBr HI strongly conduct current in solution HF only weakly conducts current at same concentration Strength of conduction related to ability to ionize
142
143
Strong Electrolytes
HCl HBr HI are 100 ionized in solution
Strong electrolyte - any compound whose dilute aqueous solutions conduct electricity well this is due to the presence of all or almost all of the dissolve compound in the form of ions
144
Unique characteristic of strong electrolyte is that they yield only ions
Ex Ionic compound may be highly soluble in water and dissociate into ions (NaCl)
Others may not dissolve much but amount that does dissolve exists only as ions
145
Weak ElectrolytesHF dissolves in water to give acid solution called hydrofluoric acid HF bond stronger than bonds between hydrogen and other halogens When HF dissolves some molecule ionize Reverse reaction also happens HF(aq) + H2O(l) H3O+(aq) + F-(aq)
146
HF(aq) + H2O(l) H3O+
(aq) + F-(aq)Concentration of dissolved unionized HF stays high and concentration of ions stays low
Weak electrolyte - any compound whose aqueous solutions conduct electricity poorly this is due to the presence of small amount of the dissolved compound in the form of ions
147
Different from nonelectrolyte ndash NO ions at all Description of electrolyte as strong or weak not related to concentration of solution Electrolytes differ in degree of ionization not amount of solute dissolved
148
149
Colligative properties of solutions
Presence of solute affects properties of solutions Some properties not dependent on nature of dissolved substance but on how may dissolve particles are present
Colligative properties - properties that depend on the concentration of solute particles but not on their identity
150
Vapor-Pressure Lowering
Boiling and freezing point of solution different from pure solvent Nonvolatile solute raises boiling point and lowers freezing point Nonvolatile substance - one that has little tendency to become gas under existing conditions
151
152
To understand why nonvolatile solute changes boiling and freezing point consider equilibrium vapor pressure
Vapor pressure pressure caused by molecules that have escape liquid phase to gas phase
Can be thought of a measure of tendency of molecules to escape from a liquid
153
Addition of sucrose (nonvolatile solute) lowers concentration of water molecules at surface of liquid This lowers tendency of water molecules to leave solution and enter gas phase Vapor pressure of solution is lower than vapor pressure of pure water
154
Aqueous13 solution13 of13 nonvolatile13 solutePure13 water
Nonelectrolyte solutions of same molality have same concentration of particles
Dilute solutions of same solvent and equal molality of any nonelectrolyte solute lower vapor pressure equally
155
Example
1 m aqueous solution of nonelectrolyte glucose lowers vapor pressure of water 55 x 10-4 atm at 25 1 m aqueous solution of sucrose also lowers water vapor pressure to 55 x 10-4 atm at 25
156
157
bc vapor-pressure lowering depends on concentration of nonelectrolyte solute and doesnrsquot depend on the type of solute it is a colligative proprerty
bc vapor pressure has been lowered solution remains liquid over larger temperature range
Lowers freezing point and raises boiling point
158
Can assume changes in boiling and freezing point also depend on concentration of solute
They are colligative properties
159
Freezing-Point Depression
Freezing point of 1-molal solution of any nonelectrolyte solute in water is found (by experiment) to be 186 lower than freezing point of water When 1 mol of nonelectrolyte solute dissolved in 1 kg water freezing point is -186 not 000 160
When 2 mol nonelectrolyte solute dissolved in 1 kg water freezing point is -372
For any concentration of nonelectrolyte solute in water decrease in freezing point can be determined using value of -186m
161
This value called the molal freezing-point contstant (Kf) is the freezing-point depression of the solvent in a 1-molal solution of a nonvolatile nonelectrolyte solute
162
Each solvent has own characteristic molal freezing-point constant Freezing-point depression (Δtf) the difference between the freezing points of the pure solvent and a solute of nonelectrolyte in that solvent and it is directly proportional to the molal concentration of the solution Δtf = Kfm
163
Δtf = Kfm
Kf expressed as m
m expressed in mol solutekg solvent (molality)
164
Sample Problem
What is the freezing point depression of water in a solution of 171 g of sucrose C12H22O11 and 200 g of water What is the actual freezing point of the solution
165
1 AnalyzeGiven Solute mass and chemical formula 171 g C12H22O11 Solvent mass and identity 2000 g water Unknown a freezing-point depression b freezing point of the solution 166
2 Plan
167
168
Calculate
Unknown a freezing-point depression b freezing point of the solution
169
Practice Problem
A water solution containing an unknown quantity of a nonelectrolyte solute is found to have a freezing point of ndash 023degCWhat is the molal concentration of the solution
012 m170
A solution consists of 103 g of the nonelectrolyte glucose C6H12O6 dissolved in 250 g of water What is the freezing-point depression of the solution
minus0426degC171
In a laboratory experiment the freezing point of an aqueous solution of glucose is found to be minus0325degCWhat is the molal concentration of this solution
0175 m172
If 0500 mol of a nonelectrolyte solute are dissolved in 5000 g of ether what is the freezing point of the solution
minus1181degC173
The freezing point of an aqueous solution that contains a nonelectrolyte is minus90degC aWhat is the freezing-point depression of the solution bWhat is the molal concentration of the solution a minus90degC b 48 m
174
Boiling-Point Elevation
Boiling point is temp at which vapor pressure is equal to atmospheric pressure Change in vapor pressure will cause corresponding change in boiling point
175
176
Vapor pressure of nonvolatile solution is lower than vapor pressure of pure solvent More heat will be required to raise vapor pressure of solution Boiling point of solution is higher than boiling point of pure solvent
Molal boiling-point constant (Kb) - boiling-point elevation of the solvent in a 1-molal solution of a nonvolatile nonelectrolyte solute b-p elevation of any nonelectrolyte in water found by experiment to be 051 So molal b-p constant for water is 051m
177
For different solvents b-p elevations of 1-molal solutions have different values (Table 14-2)
Like freezing-point constants values most accurate for dilute solutions
178
Boiling-point elevation Δtb difference between the boiling points of the pure solvent and a nonelectrolyte solution of that solvent and it is directly proportional to the molal concentration of the solution B-p elevation can be calculated using Δtb = Kbm Δtb (boiling point elevation) expressed in m and m expressed in mol solutekg solvent
Sample Problem
What is the boiling-point elevation of a solution made from 200 g of a nonelectrolyte solute and 4000 g of water The molar mass of the solute is 620 gmol
180
1 AnalyzeGiven solute mass = 200 g solute molar mass = 620 gmol solvent mass and identity = 4000 g of water
Unknown boiling-point elevation
181
2 Plan
182
183
3 Compute
Practice Problem
A solution contains 500 g of sucrose C12H22O11 a nonelectrolyte dissolved in 5000 g of water What is the boiling-point elevation
015degC184
A solution contains 4500 g of sucrose C12H22O11 a nonelectrolyte dissolved in 250 g of waterWhat is the boiling point of the solution
1027degC185
If the boiling point elevation of an aqueous solution containing a nonvolatile electrolyte is 102degC what is the molality of the solution
20 m186
The boiling point of an aqueous solution containing a nonvolatile electrolyte is 10075degC aWhat is the boiling-point elevation bWhat is the molality of the solution a 075degC b 15 m
187
Electrolytes and Colligative PropertiesEarly investigators confused by experiments where certain substances depressed freezing point or elevated boiling point more than expected
188
189
Ex 01 m solution of NaCl lowers freezing point of solvent almost twice as much as 01 m solution of sucrose 01 m CaCl2 lowers fp almost 3 times as much as 01 m solution of sucrose
To understand why this happens contrast behavior or sucrose with NaCl in aqueous solution Each sucrose dissolves to make only 1 particle in solution So 1 mol sucrose dissolves to 1 mol particles 1 mol NaCl dissolves to make 2 moles of particles (1 mol Na+ and 1 mol Cl-)
190
191
Calculated Values for Electrolyte SolutionsColligative properties depend on total concentration of solute particles regardless of their identity Electrolytes cause changes in colligative properties proportional to total molality in terms of all dissolves particles instead of formula units
192
What about barium nitrate Ba(NO3)2 Each mole of barium nitrate yields 3 mol of ions in solution Ba(NO3)2(s) rarr Ba2+(aq) + 2NO3-(aq) You would expect solution of given molality to lower fp of solvent 3 times as much as nonelectrolytic solution of same molality
193
Sample Problem
What is the expected change in the freezing point of water in a solution of 625 g of barium nitrate Ba(NO3)2 in 100 kg of water
194
1 AnalyzeGiven solute mass and formula = 625 g Ba(NO3)2 solvent mass and identity = 100 kg water Δtf = Kfm
Unknown expected freezing-point depression
195
2 PlanThe molality can be calculated by converting the solute mass to moles and then dividing by the number of kilograms of solvent That molality is in terms of formula units of Ba(NO3)2 and must be converted to molality in terms of dissociated ions in solution It must be multiplied by the number of moles of ions produced per mole of formula unit This adjusted molality can then be used to calculate the freezing-point depression
196
197
198
3 Compute
Practice Problem
What is the expected freezing-point depression for a solution that contains 20 mol of magnesium sulfate dissolved in 10 kg of water
minus74degC199
What is the expected boiling-point elevation of water for a solution that contains 150 g of sodium chloride dissolved in 10 kg of water 27degC The freezing point of an aqueous sodium chloride solution is minus200degC What is the molality of the solution 54 m NaCl
200
Actual Values for Electrolyte SolutionsRemember values calculated are only expected values Actual values of colligative properties for all strong electrolytes are almost what would be expected Difference between expected and actual values caused by attractive forces that exist between dissociated ions in aqueous solution
201
Attraction between hydrated ions in solution is small compared with those in crystalline solid However forces of attraction donrsquot interfere with movements of aqueous ions More concentrated solution means ions closer together and attraction is greater Also depends on charge of ions ndash higher charge greater attraction
202
What is the molality of a solution composed of 255 g acetone (CH3)2CO dissolved in 200 g of water 22 m acetone What quantity in grams of methanol CH3OH is required to prepare a 0244 m solution in 400 g water 312 g methanol
92
How many grams of AgNO3 are needed to prepare a 0125 m solution in 250 mL of water 531 g AgNO3
What is the molality of a solution containing 182 g HCl and 250 g water 200 m
93
Ions in Aqueous Solutions and Colligative Properties
94
95
compounds in aqueous solutions
DissociationWhen compound made from ions dissolves in water ions separate Dissociation - separation of ions that occurs when an ionic compound dissolves
96
Notice number of ions made per formula unit in equations 1 formula unit of NaCl gives 2 units of ions in solution 1 formula unit of CaCl2 gives 3 units of ions
97
Assuming 100 dissociation solution that contains 1 mol NaCl contains 1 mol Na+ and 1 mol Cl- Can assume 100 dissociation of all soluble ionic compounds
98
Practice Problem
Write the equation for the dissolution of aluminum sulfate in water How many moles of aluminum ions are made by dissolving 1 mol aluminum sulfate What is the total number of moles of ions made by dissolving 1 mol of aluminum sulfate
99
1 AnalyzeGiven Amount of solute = 1 mol Al2(SO4)3 Solvent identity = water Unknown a moles of aluminum ions and sulfate ions Total number of moles of solute ions produced
100
2 Plan
The coefficients in the balanced dissociation equation will tell mole relationships You can use the equation to find out the number of moles of solute ions produced
101
3 Compute
a
b102
Write the equation for the dissolution of each of the following in water and then determine the number of moles of each ions made as well as the total number of ions made a 1 mol ammonium chloride b 1 mol sodium sulfide c 05 mol barium nitrate
103
a 1 mol ammonium chloride
1 mol NH4+ 1 mol Cl- 2 mol total ions
104
b 1 mol sodium sulfide
2 mol Na+ 1 mol S-2 3 mol total ions
105
c 05 mol barium nitrate
05 mol Ba+2 1 mol NO3- 15 mol total ions
106
Precipitation Reactions
Even though no compound is completely insoluble compounds of very low solubility can be considered insoluble for practical purposes There are general guidelines to help predict whether a compound made of certain combination of ions is soluble
107
General Solubility Guidelines
Sodium potassium and ammonium compounds are soluble in water
108
Nitrates acetates and chlorates are soluble
109
110
Most chlorides are soluble except those of silver mercury (I) and lead Lead (II) chloride is soluble in hot water
Most sulfates are soluble except those of barium strontium lead calcium and mercury
111
Most carbonates phosphates and silicates are insoluble except those of sodium potassium and ammonium
112
Most sulfides are insoluble except those of calcium strontium sodium potassium and ammonium
113
Is calcium phosphate Ca3(PO4)2 soluble or insoluble
Not one of the exceptions so it is insoluble
Dissociation equations are not written for insoluble compounds
114
Guidelines also useful in predicting what will happen if 2 solutions of 2 different soluble compounds are mixed
If mixing results in combination of ions that forms insoluble compound a double-replacement and precipitation reaction will happen
115
Will precipitate form when solutions of ammonium sulfide and cadmium nitrate are combined You can tell calcium nitrate is soluble from the guidelines (Most nitrates acetates and chlorates are soluble) You can also tell ammonium sulfide is soluble (Most sulfides are insoluble except those of calcium strontium sodium potassium and ammonium)
116
117
To decide whether precipitate can form must know solubilities of two possible products
Ammonium nitrate ndash soluble (Most nitrates acetates and chlorates are soluble) Cadmium sulfide ndash insoluble (Most sulfides are insoluble except those of calcium strontium sodium potassium and ammonium)
118
bc one product is insoluble double-replacement and precipitation reaction will happen (NH4)2S (aq) + Cd(NO3)2 (aq) rarr NH4NO3 (aq) + CdS (s)
119
Net Ionic Equations
Reactions of ions in aqueous solution usually represented by net ionic equations instead of formula equations Net ionic equation - includes only those compounds and ions that undergo a chemical change in a reaction in an aqueous solution
120
121
To write NIE (net ionic equation) first convert chemical equation into overall ionic equation All soluble ionic compounds shown as dissociated ions in solution Precipitates shown as solids
122
Notice ammonium and nitrate ions appear on both sides They havenrsquot gone through any chemical change Spectator ions - ions that do not take part in a chemical reaction and are found in solution both before and after the reaction
123
To change ionic equation to NIE eliminate spectator ions Cd+2(aq) + S-2(aq) rarr CdS(s)
This NIE applies to any reaction in which precipitate of CdS forms
124
Practice Problem
Identify the precipitate that forms when aqueous solutions of zinc nitrate and ammonium sulfide are combined Write the equation for the possible double-replacement reaction Then write the formula equation overall ionic equation and the net ionic equation
125
1 AnalyzeGiven Identity of reactants zinc nitrate and ammonium sulfide Reaction medium aqueous solution Unknown a equation for the possible double-replacement reaction b identity of the precipitate c formula equation d overall ionic equation e net ionic equation
126
2 PlanWrite the possible double-replacement reaction between Zn(NO3)2 and (NH4)2S Use the rules to determine if any of the products will precipitate Write a formula equation and overall net ionic equation Then cancel spectator ions to make net ionic equation
127
3 Computea possible double-replacement reaction
b rules show that zinc sulfide is not soluble so will precipitate (ammonium nitrate is soluble)
c formula equation128
d overall ionic equation
e net ionic equation129
Will a precipitate form if solutions of potassium sulfate and barium nitrate are combined If so write the net ionic equation for the reaction
Yes Ba+2(aq) + SO4-2(aq) rarr BaSO4(s)
130
Will a precipitate form if solutions of potassium nitrate and magnesium sulfate are combined If so write the net ionic equation for the reaction
No 131
Will a precipitate form if solutions of barium chloride and sodium sulfate are combined If so identify the spectator ions and write the net ionic equation for the reaction
Yes Na+ and Cl- Ba+2(aq) + SO4-2(aq) rarr BaSO4(s)
132
Write the net ionic equation for the precipitation of nickel(II) sulfide
Ni+2(aq) + S-2(aq) rarr NiS(s)
133
IonizationSome molecular compounds can also form ions in solution Usually polar Ionization - ions formed from solute molecules by action of the solvent (creation of ions where there were none) Ionization different from dissociation
134
Dissociation ionic compounds dissolve and ions already present separate Ionization ions formed where non existed before
Ions formed are hydrated Heat released during hydration of ions gives enough energy to break covalent bonds
135
Extent to which solute ionizes depends on strength of bonds within molecules of solute and strength of attraction between solute and solvent molecules
136
HCl ndash molecular compound that ionizes in aqueous solution Contains highly polar bond Attraction between polar HCl molecule and polar water strong enough to break HCl bond
137
The Hydronium Ion
H+ ions from HCl attracts other molecules or ions so strongly that it doesnrsquot normally exist alone Ionization of HCl better described as direct transfer of proton from HCl to H2O forming H3O+
138
139
Hydration of H+ to form hydronium is highly exothermic
Energy released gives a lot of energy needed to ionize a molecular solute
Many molecular compounds that ionize in aqueous solution contain hydrogen and form hydronium ion
140
Strong and Weak Electrolytes
Substances that make ions and conduct current in solution are electrolytes HCl is one of series of compounds that has hydrogen and halogen Hydrogen halides are all molecular compounds with single polar-covalent bonds
141
All are gases Very soluble in water All are electrolytes HCl HBr HI strongly conduct current in solution HF only weakly conducts current at same concentration Strength of conduction related to ability to ionize
142
143
Strong Electrolytes
HCl HBr HI are 100 ionized in solution
Strong electrolyte - any compound whose dilute aqueous solutions conduct electricity well this is due to the presence of all or almost all of the dissolve compound in the form of ions
144
Unique characteristic of strong electrolyte is that they yield only ions
Ex Ionic compound may be highly soluble in water and dissociate into ions (NaCl)
Others may not dissolve much but amount that does dissolve exists only as ions
145
Weak ElectrolytesHF dissolves in water to give acid solution called hydrofluoric acid HF bond stronger than bonds between hydrogen and other halogens When HF dissolves some molecule ionize Reverse reaction also happens HF(aq) + H2O(l) H3O+(aq) + F-(aq)
146
HF(aq) + H2O(l) H3O+
(aq) + F-(aq)Concentration of dissolved unionized HF stays high and concentration of ions stays low
Weak electrolyte - any compound whose aqueous solutions conduct electricity poorly this is due to the presence of small amount of the dissolved compound in the form of ions
147
Different from nonelectrolyte ndash NO ions at all Description of electrolyte as strong or weak not related to concentration of solution Electrolytes differ in degree of ionization not amount of solute dissolved
148
149
Colligative properties of solutions
Presence of solute affects properties of solutions Some properties not dependent on nature of dissolved substance but on how may dissolve particles are present
Colligative properties - properties that depend on the concentration of solute particles but not on their identity
150
Vapor-Pressure Lowering
Boiling and freezing point of solution different from pure solvent Nonvolatile solute raises boiling point and lowers freezing point Nonvolatile substance - one that has little tendency to become gas under existing conditions
151
152
To understand why nonvolatile solute changes boiling and freezing point consider equilibrium vapor pressure
Vapor pressure pressure caused by molecules that have escape liquid phase to gas phase
Can be thought of a measure of tendency of molecules to escape from a liquid
153
Addition of sucrose (nonvolatile solute) lowers concentration of water molecules at surface of liquid This lowers tendency of water molecules to leave solution and enter gas phase Vapor pressure of solution is lower than vapor pressure of pure water
154
Aqueous13 solution13 of13 nonvolatile13 solutePure13 water
Nonelectrolyte solutions of same molality have same concentration of particles
Dilute solutions of same solvent and equal molality of any nonelectrolyte solute lower vapor pressure equally
155
Example
1 m aqueous solution of nonelectrolyte glucose lowers vapor pressure of water 55 x 10-4 atm at 25 1 m aqueous solution of sucrose also lowers water vapor pressure to 55 x 10-4 atm at 25
156
157
bc vapor-pressure lowering depends on concentration of nonelectrolyte solute and doesnrsquot depend on the type of solute it is a colligative proprerty
bc vapor pressure has been lowered solution remains liquid over larger temperature range
Lowers freezing point and raises boiling point
158
Can assume changes in boiling and freezing point also depend on concentration of solute
They are colligative properties
159
Freezing-Point Depression
Freezing point of 1-molal solution of any nonelectrolyte solute in water is found (by experiment) to be 186 lower than freezing point of water When 1 mol of nonelectrolyte solute dissolved in 1 kg water freezing point is -186 not 000 160
When 2 mol nonelectrolyte solute dissolved in 1 kg water freezing point is -372
For any concentration of nonelectrolyte solute in water decrease in freezing point can be determined using value of -186m
161
This value called the molal freezing-point contstant (Kf) is the freezing-point depression of the solvent in a 1-molal solution of a nonvolatile nonelectrolyte solute
162
Each solvent has own characteristic molal freezing-point constant Freezing-point depression (Δtf) the difference between the freezing points of the pure solvent and a solute of nonelectrolyte in that solvent and it is directly proportional to the molal concentration of the solution Δtf = Kfm
163
Δtf = Kfm
Kf expressed as m
m expressed in mol solutekg solvent (molality)
164
Sample Problem
What is the freezing point depression of water in a solution of 171 g of sucrose C12H22O11 and 200 g of water What is the actual freezing point of the solution
165
1 AnalyzeGiven Solute mass and chemical formula 171 g C12H22O11 Solvent mass and identity 2000 g water Unknown a freezing-point depression b freezing point of the solution 166
2 Plan
167
168
Calculate
Unknown a freezing-point depression b freezing point of the solution
169
Practice Problem
A water solution containing an unknown quantity of a nonelectrolyte solute is found to have a freezing point of ndash 023degCWhat is the molal concentration of the solution
012 m170
A solution consists of 103 g of the nonelectrolyte glucose C6H12O6 dissolved in 250 g of water What is the freezing-point depression of the solution
minus0426degC171
In a laboratory experiment the freezing point of an aqueous solution of glucose is found to be minus0325degCWhat is the molal concentration of this solution
0175 m172
If 0500 mol of a nonelectrolyte solute are dissolved in 5000 g of ether what is the freezing point of the solution
minus1181degC173
The freezing point of an aqueous solution that contains a nonelectrolyte is minus90degC aWhat is the freezing-point depression of the solution bWhat is the molal concentration of the solution a minus90degC b 48 m
174
Boiling-Point Elevation
Boiling point is temp at which vapor pressure is equal to atmospheric pressure Change in vapor pressure will cause corresponding change in boiling point
175
176
Vapor pressure of nonvolatile solution is lower than vapor pressure of pure solvent More heat will be required to raise vapor pressure of solution Boiling point of solution is higher than boiling point of pure solvent
Molal boiling-point constant (Kb) - boiling-point elevation of the solvent in a 1-molal solution of a nonvolatile nonelectrolyte solute b-p elevation of any nonelectrolyte in water found by experiment to be 051 So molal b-p constant for water is 051m
177
For different solvents b-p elevations of 1-molal solutions have different values (Table 14-2)
Like freezing-point constants values most accurate for dilute solutions
178
Boiling-point elevation Δtb difference between the boiling points of the pure solvent and a nonelectrolyte solution of that solvent and it is directly proportional to the molal concentration of the solution B-p elevation can be calculated using Δtb = Kbm Δtb (boiling point elevation) expressed in m and m expressed in mol solutekg solvent
Sample Problem
What is the boiling-point elevation of a solution made from 200 g of a nonelectrolyte solute and 4000 g of water The molar mass of the solute is 620 gmol
180
1 AnalyzeGiven solute mass = 200 g solute molar mass = 620 gmol solvent mass and identity = 4000 g of water
Unknown boiling-point elevation
181
2 Plan
182
183
3 Compute
Practice Problem
A solution contains 500 g of sucrose C12H22O11 a nonelectrolyte dissolved in 5000 g of water What is the boiling-point elevation
015degC184
A solution contains 4500 g of sucrose C12H22O11 a nonelectrolyte dissolved in 250 g of waterWhat is the boiling point of the solution
1027degC185
If the boiling point elevation of an aqueous solution containing a nonvolatile electrolyte is 102degC what is the molality of the solution
20 m186
The boiling point of an aqueous solution containing a nonvolatile electrolyte is 10075degC aWhat is the boiling-point elevation bWhat is the molality of the solution a 075degC b 15 m
187
Electrolytes and Colligative PropertiesEarly investigators confused by experiments where certain substances depressed freezing point or elevated boiling point more than expected
188
189
Ex 01 m solution of NaCl lowers freezing point of solvent almost twice as much as 01 m solution of sucrose 01 m CaCl2 lowers fp almost 3 times as much as 01 m solution of sucrose
To understand why this happens contrast behavior or sucrose with NaCl in aqueous solution Each sucrose dissolves to make only 1 particle in solution So 1 mol sucrose dissolves to 1 mol particles 1 mol NaCl dissolves to make 2 moles of particles (1 mol Na+ and 1 mol Cl-)
190
191
Calculated Values for Electrolyte SolutionsColligative properties depend on total concentration of solute particles regardless of their identity Electrolytes cause changes in colligative properties proportional to total molality in terms of all dissolves particles instead of formula units
192
What about barium nitrate Ba(NO3)2 Each mole of barium nitrate yields 3 mol of ions in solution Ba(NO3)2(s) rarr Ba2+(aq) + 2NO3-(aq) You would expect solution of given molality to lower fp of solvent 3 times as much as nonelectrolytic solution of same molality
193
Sample Problem
What is the expected change in the freezing point of water in a solution of 625 g of barium nitrate Ba(NO3)2 in 100 kg of water
194
1 AnalyzeGiven solute mass and formula = 625 g Ba(NO3)2 solvent mass and identity = 100 kg water Δtf = Kfm
Unknown expected freezing-point depression
195
2 PlanThe molality can be calculated by converting the solute mass to moles and then dividing by the number of kilograms of solvent That molality is in terms of formula units of Ba(NO3)2 and must be converted to molality in terms of dissociated ions in solution It must be multiplied by the number of moles of ions produced per mole of formula unit This adjusted molality can then be used to calculate the freezing-point depression
196
197
198
3 Compute
Practice Problem
What is the expected freezing-point depression for a solution that contains 20 mol of magnesium sulfate dissolved in 10 kg of water
minus74degC199
What is the expected boiling-point elevation of water for a solution that contains 150 g of sodium chloride dissolved in 10 kg of water 27degC The freezing point of an aqueous sodium chloride solution is minus200degC What is the molality of the solution 54 m NaCl
200
Actual Values for Electrolyte SolutionsRemember values calculated are only expected values Actual values of colligative properties for all strong electrolytes are almost what would be expected Difference between expected and actual values caused by attractive forces that exist between dissociated ions in aqueous solution
201
Attraction between hydrated ions in solution is small compared with those in crystalline solid However forces of attraction donrsquot interfere with movements of aqueous ions More concentrated solution means ions closer together and attraction is greater Also depends on charge of ions ndash higher charge greater attraction
202
How many grams of AgNO3 are needed to prepare a 0125 m solution in 250 mL of water 531 g AgNO3
What is the molality of a solution containing 182 g HCl and 250 g water 200 m
93
Ions in Aqueous Solutions and Colligative Properties
94
95
compounds in aqueous solutions
DissociationWhen compound made from ions dissolves in water ions separate Dissociation - separation of ions that occurs when an ionic compound dissolves
96
Notice number of ions made per formula unit in equations 1 formula unit of NaCl gives 2 units of ions in solution 1 formula unit of CaCl2 gives 3 units of ions
97
Assuming 100 dissociation solution that contains 1 mol NaCl contains 1 mol Na+ and 1 mol Cl- Can assume 100 dissociation of all soluble ionic compounds
98
Practice Problem
Write the equation for the dissolution of aluminum sulfate in water How many moles of aluminum ions are made by dissolving 1 mol aluminum sulfate What is the total number of moles of ions made by dissolving 1 mol of aluminum sulfate
99
1 AnalyzeGiven Amount of solute = 1 mol Al2(SO4)3 Solvent identity = water Unknown a moles of aluminum ions and sulfate ions Total number of moles of solute ions produced
100
2 Plan
The coefficients in the balanced dissociation equation will tell mole relationships You can use the equation to find out the number of moles of solute ions produced
101
3 Compute
a
b102
Write the equation for the dissolution of each of the following in water and then determine the number of moles of each ions made as well as the total number of ions made a 1 mol ammonium chloride b 1 mol sodium sulfide c 05 mol barium nitrate
103
a 1 mol ammonium chloride
1 mol NH4+ 1 mol Cl- 2 mol total ions
104
b 1 mol sodium sulfide
2 mol Na+ 1 mol S-2 3 mol total ions
105
c 05 mol barium nitrate
05 mol Ba+2 1 mol NO3- 15 mol total ions
106
Precipitation Reactions
Even though no compound is completely insoluble compounds of very low solubility can be considered insoluble for practical purposes There are general guidelines to help predict whether a compound made of certain combination of ions is soluble
107
General Solubility Guidelines
Sodium potassium and ammonium compounds are soluble in water
108
Nitrates acetates and chlorates are soluble
109
110
Most chlorides are soluble except those of silver mercury (I) and lead Lead (II) chloride is soluble in hot water
Most sulfates are soluble except those of barium strontium lead calcium and mercury
111
Most carbonates phosphates and silicates are insoluble except those of sodium potassium and ammonium
112
Most sulfides are insoluble except those of calcium strontium sodium potassium and ammonium
113
Is calcium phosphate Ca3(PO4)2 soluble or insoluble
Not one of the exceptions so it is insoluble
Dissociation equations are not written for insoluble compounds
114
Guidelines also useful in predicting what will happen if 2 solutions of 2 different soluble compounds are mixed
If mixing results in combination of ions that forms insoluble compound a double-replacement and precipitation reaction will happen
115
Will precipitate form when solutions of ammonium sulfide and cadmium nitrate are combined You can tell calcium nitrate is soluble from the guidelines (Most nitrates acetates and chlorates are soluble) You can also tell ammonium sulfide is soluble (Most sulfides are insoluble except those of calcium strontium sodium potassium and ammonium)
116
117
To decide whether precipitate can form must know solubilities of two possible products
Ammonium nitrate ndash soluble (Most nitrates acetates and chlorates are soluble) Cadmium sulfide ndash insoluble (Most sulfides are insoluble except those of calcium strontium sodium potassium and ammonium)
118
bc one product is insoluble double-replacement and precipitation reaction will happen (NH4)2S (aq) + Cd(NO3)2 (aq) rarr NH4NO3 (aq) + CdS (s)
119
Net Ionic Equations
Reactions of ions in aqueous solution usually represented by net ionic equations instead of formula equations Net ionic equation - includes only those compounds and ions that undergo a chemical change in a reaction in an aqueous solution
120
121
To write NIE (net ionic equation) first convert chemical equation into overall ionic equation All soluble ionic compounds shown as dissociated ions in solution Precipitates shown as solids
122
Notice ammonium and nitrate ions appear on both sides They havenrsquot gone through any chemical change Spectator ions - ions that do not take part in a chemical reaction and are found in solution both before and after the reaction
123
To change ionic equation to NIE eliminate spectator ions Cd+2(aq) + S-2(aq) rarr CdS(s)
This NIE applies to any reaction in which precipitate of CdS forms
124
Practice Problem
Identify the precipitate that forms when aqueous solutions of zinc nitrate and ammonium sulfide are combined Write the equation for the possible double-replacement reaction Then write the formula equation overall ionic equation and the net ionic equation
125
1 AnalyzeGiven Identity of reactants zinc nitrate and ammonium sulfide Reaction medium aqueous solution Unknown a equation for the possible double-replacement reaction b identity of the precipitate c formula equation d overall ionic equation e net ionic equation
126
2 PlanWrite the possible double-replacement reaction between Zn(NO3)2 and (NH4)2S Use the rules to determine if any of the products will precipitate Write a formula equation and overall net ionic equation Then cancel spectator ions to make net ionic equation
127
3 Computea possible double-replacement reaction
b rules show that zinc sulfide is not soluble so will precipitate (ammonium nitrate is soluble)
c formula equation128
d overall ionic equation
e net ionic equation129
Will a precipitate form if solutions of potassium sulfate and barium nitrate are combined If so write the net ionic equation for the reaction
Yes Ba+2(aq) + SO4-2(aq) rarr BaSO4(s)
130
Will a precipitate form if solutions of potassium nitrate and magnesium sulfate are combined If so write the net ionic equation for the reaction
No 131
Will a precipitate form if solutions of barium chloride and sodium sulfate are combined If so identify the spectator ions and write the net ionic equation for the reaction
Yes Na+ and Cl- Ba+2(aq) + SO4-2(aq) rarr BaSO4(s)
132
Write the net ionic equation for the precipitation of nickel(II) sulfide
Ni+2(aq) + S-2(aq) rarr NiS(s)
133
IonizationSome molecular compounds can also form ions in solution Usually polar Ionization - ions formed from solute molecules by action of the solvent (creation of ions where there were none) Ionization different from dissociation
134
Dissociation ionic compounds dissolve and ions already present separate Ionization ions formed where non existed before
Ions formed are hydrated Heat released during hydration of ions gives enough energy to break covalent bonds
135
Extent to which solute ionizes depends on strength of bonds within molecules of solute and strength of attraction between solute and solvent molecules
136
HCl ndash molecular compound that ionizes in aqueous solution Contains highly polar bond Attraction between polar HCl molecule and polar water strong enough to break HCl bond
137
The Hydronium Ion
H+ ions from HCl attracts other molecules or ions so strongly that it doesnrsquot normally exist alone Ionization of HCl better described as direct transfer of proton from HCl to H2O forming H3O+
138
139
Hydration of H+ to form hydronium is highly exothermic
Energy released gives a lot of energy needed to ionize a molecular solute
Many molecular compounds that ionize in aqueous solution contain hydrogen and form hydronium ion
140
Strong and Weak Electrolytes
Substances that make ions and conduct current in solution are electrolytes HCl is one of series of compounds that has hydrogen and halogen Hydrogen halides are all molecular compounds with single polar-covalent bonds
141
All are gases Very soluble in water All are electrolytes HCl HBr HI strongly conduct current in solution HF only weakly conducts current at same concentration Strength of conduction related to ability to ionize
142
143
Strong Electrolytes
HCl HBr HI are 100 ionized in solution
Strong electrolyte - any compound whose dilute aqueous solutions conduct electricity well this is due to the presence of all or almost all of the dissolve compound in the form of ions
144
Unique characteristic of strong electrolyte is that they yield only ions
Ex Ionic compound may be highly soluble in water and dissociate into ions (NaCl)
Others may not dissolve much but amount that does dissolve exists only as ions
145
Weak ElectrolytesHF dissolves in water to give acid solution called hydrofluoric acid HF bond stronger than bonds between hydrogen and other halogens When HF dissolves some molecule ionize Reverse reaction also happens HF(aq) + H2O(l) H3O+(aq) + F-(aq)
146
HF(aq) + H2O(l) H3O+
(aq) + F-(aq)Concentration of dissolved unionized HF stays high and concentration of ions stays low
Weak electrolyte - any compound whose aqueous solutions conduct electricity poorly this is due to the presence of small amount of the dissolved compound in the form of ions
147
Different from nonelectrolyte ndash NO ions at all Description of electrolyte as strong or weak not related to concentration of solution Electrolytes differ in degree of ionization not amount of solute dissolved
148
149
Colligative properties of solutions
Presence of solute affects properties of solutions Some properties not dependent on nature of dissolved substance but on how may dissolve particles are present
Colligative properties - properties that depend on the concentration of solute particles but not on their identity
150
Vapor-Pressure Lowering
Boiling and freezing point of solution different from pure solvent Nonvolatile solute raises boiling point and lowers freezing point Nonvolatile substance - one that has little tendency to become gas under existing conditions
151
152
To understand why nonvolatile solute changes boiling and freezing point consider equilibrium vapor pressure
Vapor pressure pressure caused by molecules that have escape liquid phase to gas phase
Can be thought of a measure of tendency of molecules to escape from a liquid
153
Addition of sucrose (nonvolatile solute) lowers concentration of water molecules at surface of liquid This lowers tendency of water molecules to leave solution and enter gas phase Vapor pressure of solution is lower than vapor pressure of pure water
154
Aqueous13 solution13 of13 nonvolatile13 solutePure13 water
Nonelectrolyte solutions of same molality have same concentration of particles
Dilute solutions of same solvent and equal molality of any nonelectrolyte solute lower vapor pressure equally
155
Example
1 m aqueous solution of nonelectrolyte glucose lowers vapor pressure of water 55 x 10-4 atm at 25 1 m aqueous solution of sucrose also lowers water vapor pressure to 55 x 10-4 atm at 25
156
157
bc vapor-pressure lowering depends on concentration of nonelectrolyte solute and doesnrsquot depend on the type of solute it is a colligative proprerty
bc vapor pressure has been lowered solution remains liquid over larger temperature range
Lowers freezing point and raises boiling point
158
Can assume changes in boiling and freezing point also depend on concentration of solute
They are colligative properties
159
Freezing-Point Depression
Freezing point of 1-molal solution of any nonelectrolyte solute in water is found (by experiment) to be 186 lower than freezing point of water When 1 mol of nonelectrolyte solute dissolved in 1 kg water freezing point is -186 not 000 160
When 2 mol nonelectrolyte solute dissolved in 1 kg water freezing point is -372
For any concentration of nonelectrolyte solute in water decrease in freezing point can be determined using value of -186m
161
This value called the molal freezing-point contstant (Kf) is the freezing-point depression of the solvent in a 1-molal solution of a nonvolatile nonelectrolyte solute
162
Each solvent has own characteristic molal freezing-point constant Freezing-point depression (Δtf) the difference between the freezing points of the pure solvent and a solute of nonelectrolyte in that solvent and it is directly proportional to the molal concentration of the solution Δtf = Kfm
163
Δtf = Kfm
Kf expressed as m
m expressed in mol solutekg solvent (molality)
164
Sample Problem
What is the freezing point depression of water in a solution of 171 g of sucrose C12H22O11 and 200 g of water What is the actual freezing point of the solution
165
1 AnalyzeGiven Solute mass and chemical formula 171 g C12H22O11 Solvent mass and identity 2000 g water Unknown a freezing-point depression b freezing point of the solution 166
2 Plan
167
168
Calculate
Unknown a freezing-point depression b freezing point of the solution
169
Practice Problem
A water solution containing an unknown quantity of a nonelectrolyte solute is found to have a freezing point of ndash 023degCWhat is the molal concentration of the solution
012 m170
A solution consists of 103 g of the nonelectrolyte glucose C6H12O6 dissolved in 250 g of water What is the freezing-point depression of the solution
minus0426degC171
In a laboratory experiment the freezing point of an aqueous solution of glucose is found to be minus0325degCWhat is the molal concentration of this solution
0175 m172
If 0500 mol of a nonelectrolyte solute are dissolved in 5000 g of ether what is the freezing point of the solution
minus1181degC173
The freezing point of an aqueous solution that contains a nonelectrolyte is minus90degC aWhat is the freezing-point depression of the solution bWhat is the molal concentration of the solution a minus90degC b 48 m
174
Boiling-Point Elevation
Boiling point is temp at which vapor pressure is equal to atmospheric pressure Change in vapor pressure will cause corresponding change in boiling point
175
176
Vapor pressure of nonvolatile solution is lower than vapor pressure of pure solvent More heat will be required to raise vapor pressure of solution Boiling point of solution is higher than boiling point of pure solvent
Molal boiling-point constant (Kb) - boiling-point elevation of the solvent in a 1-molal solution of a nonvolatile nonelectrolyte solute b-p elevation of any nonelectrolyte in water found by experiment to be 051 So molal b-p constant for water is 051m
177
For different solvents b-p elevations of 1-molal solutions have different values (Table 14-2)
Like freezing-point constants values most accurate for dilute solutions
178
Boiling-point elevation Δtb difference between the boiling points of the pure solvent and a nonelectrolyte solution of that solvent and it is directly proportional to the molal concentration of the solution B-p elevation can be calculated using Δtb = Kbm Δtb (boiling point elevation) expressed in m and m expressed in mol solutekg solvent
Sample Problem
What is the boiling-point elevation of a solution made from 200 g of a nonelectrolyte solute and 4000 g of water The molar mass of the solute is 620 gmol
180
1 AnalyzeGiven solute mass = 200 g solute molar mass = 620 gmol solvent mass and identity = 4000 g of water
Unknown boiling-point elevation
181
2 Plan
182
183
3 Compute
Practice Problem
A solution contains 500 g of sucrose C12H22O11 a nonelectrolyte dissolved in 5000 g of water What is the boiling-point elevation
015degC184
A solution contains 4500 g of sucrose C12H22O11 a nonelectrolyte dissolved in 250 g of waterWhat is the boiling point of the solution
1027degC185
If the boiling point elevation of an aqueous solution containing a nonvolatile electrolyte is 102degC what is the molality of the solution
20 m186
The boiling point of an aqueous solution containing a nonvolatile electrolyte is 10075degC aWhat is the boiling-point elevation bWhat is the molality of the solution a 075degC b 15 m
187
Electrolytes and Colligative PropertiesEarly investigators confused by experiments where certain substances depressed freezing point or elevated boiling point more than expected
188
189
Ex 01 m solution of NaCl lowers freezing point of solvent almost twice as much as 01 m solution of sucrose 01 m CaCl2 lowers fp almost 3 times as much as 01 m solution of sucrose
To understand why this happens contrast behavior or sucrose with NaCl in aqueous solution Each sucrose dissolves to make only 1 particle in solution So 1 mol sucrose dissolves to 1 mol particles 1 mol NaCl dissolves to make 2 moles of particles (1 mol Na+ and 1 mol Cl-)
190
191
Calculated Values for Electrolyte SolutionsColligative properties depend on total concentration of solute particles regardless of their identity Electrolytes cause changes in colligative properties proportional to total molality in terms of all dissolves particles instead of formula units
192
What about barium nitrate Ba(NO3)2 Each mole of barium nitrate yields 3 mol of ions in solution Ba(NO3)2(s) rarr Ba2+(aq) + 2NO3-(aq) You would expect solution of given molality to lower fp of solvent 3 times as much as nonelectrolytic solution of same molality
193
Sample Problem
What is the expected change in the freezing point of water in a solution of 625 g of barium nitrate Ba(NO3)2 in 100 kg of water
194
1 AnalyzeGiven solute mass and formula = 625 g Ba(NO3)2 solvent mass and identity = 100 kg water Δtf = Kfm
Unknown expected freezing-point depression
195
2 PlanThe molality can be calculated by converting the solute mass to moles and then dividing by the number of kilograms of solvent That molality is in terms of formula units of Ba(NO3)2 and must be converted to molality in terms of dissociated ions in solution It must be multiplied by the number of moles of ions produced per mole of formula unit This adjusted molality can then be used to calculate the freezing-point depression
196
197
198
3 Compute
Practice Problem
What is the expected freezing-point depression for a solution that contains 20 mol of magnesium sulfate dissolved in 10 kg of water
minus74degC199
What is the expected boiling-point elevation of water for a solution that contains 150 g of sodium chloride dissolved in 10 kg of water 27degC The freezing point of an aqueous sodium chloride solution is minus200degC What is the molality of the solution 54 m NaCl
200
Actual Values for Electrolyte SolutionsRemember values calculated are only expected values Actual values of colligative properties for all strong electrolytes are almost what would be expected Difference between expected and actual values caused by attractive forces that exist between dissociated ions in aqueous solution
201
Attraction between hydrated ions in solution is small compared with those in crystalline solid However forces of attraction donrsquot interfere with movements of aqueous ions More concentrated solution means ions closer together and attraction is greater Also depends on charge of ions ndash higher charge greater attraction
202
Ions in Aqueous Solutions and Colligative Properties
94
95
compounds in aqueous solutions
DissociationWhen compound made from ions dissolves in water ions separate Dissociation - separation of ions that occurs when an ionic compound dissolves
96
Notice number of ions made per formula unit in equations 1 formula unit of NaCl gives 2 units of ions in solution 1 formula unit of CaCl2 gives 3 units of ions
97
Assuming 100 dissociation solution that contains 1 mol NaCl contains 1 mol Na+ and 1 mol Cl- Can assume 100 dissociation of all soluble ionic compounds
98
Practice Problem
Write the equation for the dissolution of aluminum sulfate in water How many moles of aluminum ions are made by dissolving 1 mol aluminum sulfate What is the total number of moles of ions made by dissolving 1 mol of aluminum sulfate
99
1 AnalyzeGiven Amount of solute = 1 mol Al2(SO4)3 Solvent identity = water Unknown a moles of aluminum ions and sulfate ions Total number of moles of solute ions produced
100
2 Plan
The coefficients in the balanced dissociation equation will tell mole relationships You can use the equation to find out the number of moles of solute ions produced
101
3 Compute
a
b102
Write the equation for the dissolution of each of the following in water and then determine the number of moles of each ions made as well as the total number of ions made a 1 mol ammonium chloride b 1 mol sodium sulfide c 05 mol barium nitrate
103
a 1 mol ammonium chloride
1 mol NH4+ 1 mol Cl- 2 mol total ions
104
b 1 mol sodium sulfide
2 mol Na+ 1 mol S-2 3 mol total ions
105
c 05 mol barium nitrate
05 mol Ba+2 1 mol NO3- 15 mol total ions
106
Precipitation Reactions
Even though no compound is completely insoluble compounds of very low solubility can be considered insoluble for practical purposes There are general guidelines to help predict whether a compound made of certain combination of ions is soluble
107
General Solubility Guidelines
Sodium potassium and ammonium compounds are soluble in water
108
Nitrates acetates and chlorates are soluble
109
110
Most chlorides are soluble except those of silver mercury (I) and lead Lead (II) chloride is soluble in hot water
Most sulfates are soluble except those of barium strontium lead calcium and mercury
111
Most carbonates phosphates and silicates are insoluble except those of sodium potassium and ammonium
112
Most sulfides are insoluble except those of calcium strontium sodium potassium and ammonium
113
Is calcium phosphate Ca3(PO4)2 soluble or insoluble
Not one of the exceptions so it is insoluble
Dissociation equations are not written for insoluble compounds
114
Guidelines also useful in predicting what will happen if 2 solutions of 2 different soluble compounds are mixed
If mixing results in combination of ions that forms insoluble compound a double-replacement and precipitation reaction will happen
115
Will precipitate form when solutions of ammonium sulfide and cadmium nitrate are combined You can tell calcium nitrate is soluble from the guidelines (Most nitrates acetates and chlorates are soluble) You can also tell ammonium sulfide is soluble (Most sulfides are insoluble except those of calcium strontium sodium potassium and ammonium)
116
117
To decide whether precipitate can form must know solubilities of two possible products
Ammonium nitrate ndash soluble (Most nitrates acetates and chlorates are soluble) Cadmium sulfide ndash insoluble (Most sulfides are insoluble except those of calcium strontium sodium potassium and ammonium)
118
bc one product is insoluble double-replacement and precipitation reaction will happen (NH4)2S (aq) + Cd(NO3)2 (aq) rarr NH4NO3 (aq) + CdS (s)
119
Net Ionic Equations
Reactions of ions in aqueous solution usually represented by net ionic equations instead of formula equations Net ionic equation - includes only those compounds and ions that undergo a chemical change in a reaction in an aqueous solution
120
121
To write NIE (net ionic equation) first convert chemical equation into overall ionic equation All soluble ionic compounds shown as dissociated ions in solution Precipitates shown as solids
122
Notice ammonium and nitrate ions appear on both sides They havenrsquot gone through any chemical change Spectator ions - ions that do not take part in a chemical reaction and are found in solution both before and after the reaction
123
To change ionic equation to NIE eliminate spectator ions Cd+2(aq) + S-2(aq) rarr CdS(s)
This NIE applies to any reaction in which precipitate of CdS forms
124
Practice Problem
Identify the precipitate that forms when aqueous solutions of zinc nitrate and ammonium sulfide are combined Write the equation for the possible double-replacement reaction Then write the formula equation overall ionic equation and the net ionic equation
125
1 AnalyzeGiven Identity of reactants zinc nitrate and ammonium sulfide Reaction medium aqueous solution Unknown a equation for the possible double-replacement reaction b identity of the precipitate c formula equation d overall ionic equation e net ionic equation
126
2 PlanWrite the possible double-replacement reaction between Zn(NO3)2 and (NH4)2S Use the rules to determine if any of the products will precipitate Write a formula equation and overall net ionic equation Then cancel spectator ions to make net ionic equation
127
3 Computea possible double-replacement reaction
b rules show that zinc sulfide is not soluble so will precipitate (ammonium nitrate is soluble)
c formula equation128
d overall ionic equation
e net ionic equation129
Will a precipitate form if solutions of potassium sulfate and barium nitrate are combined If so write the net ionic equation for the reaction
Yes Ba+2(aq) + SO4-2(aq) rarr BaSO4(s)
130
Will a precipitate form if solutions of potassium nitrate and magnesium sulfate are combined If so write the net ionic equation for the reaction
No 131
Will a precipitate form if solutions of barium chloride and sodium sulfate are combined If so identify the spectator ions and write the net ionic equation for the reaction
Yes Na+ and Cl- Ba+2(aq) + SO4-2(aq) rarr BaSO4(s)
132
Write the net ionic equation for the precipitation of nickel(II) sulfide
Ni+2(aq) + S-2(aq) rarr NiS(s)
133
IonizationSome molecular compounds can also form ions in solution Usually polar Ionization - ions formed from solute molecules by action of the solvent (creation of ions where there were none) Ionization different from dissociation
134
Dissociation ionic compounds dissolve and ions already present separate Ionization ions formed where non existed before
Ions formed are hydrated Heat released during hydration of ions gives enough energy to break covalent bonds
135
Extent to which solute ionizes depends on strength of bonds within molecules of solute and strength of attraction between solute and solvent molecules
136
HCl ndash molecular compound that ionizes in aqueous solution Contains highly polar bond Attraction between polar HCl molecule and polar water strong enough to break HCl bond
137
The Hydronium Ion
H+ ions from HCl attracts other molecules or ions so strongly that it doesnrsquot normally exist alone Ionization of HCl better described as direct transfer of proton from HCl to H2O forming H3O+
138
139
Hydration of H+ to form hydronium is highly exothermic
Energy released gives a lot of energy needed to ionize a molecular solute
Many molecular compounds that ionize in aqueous solution contain hydrogen and form hydronium ion
140
Strong and Weak Electrolytes
Substances that make ions and conduct current in solution are electrolytes HCl is one of series of compounds that has hydrogen and halogen Hydrogen halides are all molecular compounds with single polar-covalent bonds
141
All are gases Very soluble in water All are electrolytes HCl HBr HI strongly conduct current in solution HF only weakly conducts current at same concentration Strength of conduction related to ability to ionize
142
143
Strong Electrolytes
HCl HBr HI are 100 ionized in solution
Strong electrolyte - any compound whose dilute aqueous solutions conduct electricity well this is due to the presence of all or almost all of the dissolve compound in the form of ions
144
Unique characteristic of strong electrolyte is that they yield only ions
Ex Ionic compound may be highly soluble in water and dissociate into ions (NaCl)
Others may not dissolve much but amount that does dissolve exists only as ions
145
Weak ElectrolytesHF dissolves in water to give acid solution called hydrofluoric acid HF bond stronger than bonds between hydrogen and other halogens When HF dissolves some molecule ionize Reverse reaction also happens HF(aq) + H2O(l) H3O+(aq) + F-(aq)
146
HF(aq) + H2O(l) H3O+
(aq) + F-(aq)Concentration of dissolved unionized HF stays high and concentration of ions stays low
Weak electrolyte - any compound whose aqueous solutions conduct electricity poorly this is due to the presence of small amount of the dissolved compound in the form of ions
147
Different from nonelectrolyte ndash NO ions at all Description of electrolyte as strong or weak not related to concentration of solution Electrolytes differ in degree of ionization not amount of solute dissolved
148
149
Colligative properties of solutions
Presence of solute affects properties of solutions Some properties not dependent on nature of dissolved substance but on how may dissolve particles are present
Colligative properties - properties that depend on the concentration of solute particles but not on their identity
150
Vapor-Pressure Lowering
Boiling and freezing point of solution different from pure solvent Nonvolatile solute raises boiling point and lowers freezing point Nonvolatile substance - one that has little tendency to become gas under existing conditions
151
152
To understand why nonvolatile solute changes boiling and freezing point consider equilibrium vapor pressure
Vapor pressure pressure caused by molecules that have escape liquid phase to gas phase
Can be thought of a measure of tendency of molecules to escape from a liquid
153
Addition of sucrose (nonvolatile solute) lowers concentration of water molecules at surface of liquid This lowers tendency of water molecules to leave solution and enter gas phase Vapor pressure of solution is lower than vapor pressure of pure water
154
Aqueous13 solution13 of13 nonvolatile13 solutePure13 water
Nonelectrolyte solutions of same molality have same concentration of particles
Dilute solutions of same solvent and equal molality of any nonelectrolyte solute lower vapor pressure equally
155
Example
1 m aqueous solution of nonelectrolyte glucose lowers vapor pressure of water 55 x 10-4 atm at 25 1 m aqueous solution of sucrose also lowers water vapor pressure to 55 x 10-4 atm at 25
156
157
bc vapor-pressure lowering depends on concentration of nonelectrolyte solute and doesnrsquot depend on the type of solute it is a colligative proprerty
bc vapor pressure has been lowered solution remains liquid over larger temperature range
Lowers freezing point and raises boiling point
158
Can assume changes in boiling and freezing point also depend on concentration of solute
They are colligative properties
159
Freezing-Point Depression
Freezing point of 1-molal solution of any nonelectrolyte solute in water is found (by experiment) to be 186 lower than freezing point of water When 1 mol of nonelectrolyte solute dissolved in 1 kg water freezing point is -186 not 000 160
When 2 mol nonelectrolyte solute dissolved in 1 kg water freezing point is -372
For any concentration of nonelectrolyte solute in water decrease in freezing point can be determined using value of -186m
161
This value called the molal freezing-point contstant (Kf) is the freezing-point depression of the solvent in a 1-molal solution of a nonvolatile nonelectrolyte solute
162
Each solvent has own characteristic molal freezing-point constant Freezing-point depression (Δtf) the difference between the freezing points of the pure solvent and a solute of nonelectrolyte in that solvent and it is directly proportional to the molal concentration of the solution Δtf = Kfm
163
Δtf = Kfm
Kf expressed as m
m expressed in mol solutekg solvent (molality)
164
Sample Problem
What is the freezing point depression of water in a solution of 171 g of sucrose C12H22O11 and 200 g of water What is the actual freezing point of the solution
165
1 AnalyzeGiven Solute mass and chemical formula 171 g C12H22O11 Solvent mass and identity 2000 g water Unknown a freezing-point depression b freezing point of the solution 166
2 Plan
167
168
Calculate
Unknown a freezing-point depression b freezing point of the solution
169
Practice Problem
A water solution containing an unknown quantity of a nonelectrolyte solute is found to have a freezing point of ndash 023degCWhat is the molal concentration of the solution
012 m170
A solution consists of 103 g of the nonelectrolyte glucose C6H12O6 dissolved in 250 g of water What is the freezing-point depression of the solution
minus0426degC171
In a laboratory experiment the freezing point of an aqueous solution of glucose is found to be minus0325degCWhat is the molal concentration of this solution
0175 m172
If 0500 mol of a nonelectrolyte solute are dissolved in 5000 g of ether what is the freezing point of the solution
minus1181degC173
The freezing point of an aqueous solution that contains a nonelectrolyte is minus90degC aWhat is the freezing-point depression of the solution bWhat is the molal concentration of the solution a minus90degC b 48 m
174
Boiling-Point Elevation
Boiling point is temp at which vapor pressure is equal to atmospheric pressure Change in vapor pressure will cause corresponding change in boiling point
175
176
Vapor pressure of nonvolatile solution is lower than vapor pressure of pure solvent More heat will be required to raise vapor pressure of solution Boiling point of solution is higher than boiling point of pure solvent
Molal boiling-point constant (Kb) - boiling-point elevation of the solvent in a 1-molal solution of a nonvolatile nonelectrolyte solute b-p elevation of any nonelectrolyte in water found by experiment to be 051 So molal b-p constant for water is 051m
177
For different solvents b-p elevations of 1-molal solutions have different values (Table 14-2)
Like freezing-point constants values most accurate for dilute solutions
178
Boiling-point elevation Δtb difference between the boiling points of the pure solvent and a nonelectrolyte solution of that solvent and it is directly proportional to the molal concentration of the solution B-p elevation can be calculated using Δtb = Kbm Δtb (boiling point elevation) expressed in m and m expressed in mol solutekg solvent
Sample Problem
What is the boiling-point elevation of a solution made from 200 g of a nonelectrolyte solute and 4000 g of water The molar mass of the solute is 620 gmol
180
1 AnalyzeGiven solute mass = 200 g solute molar mass = 620 gmol solvent mass and identity = 4000 g of water
Unknown boiling-point elevation
181
2 Plan
182
183
3 Compute
Practice Problem
A solution contains 500 g of sucrose C12H22O11 a nonelectrolyte dissolved in 5000 g of water What is the boiling-point elevation
015degC184
A solution contains 4500 g of sucrose C12H22O11 a nonelectrolyte dissolved in 250 g of waterWhat is the boiling point of the solution
1027degC185
If the boiling point elevation of an aqueous solution containing a nonvolatile electrolyte is 102degC what is the molality of the solution
20 m186
The boiling point of an aqueous solution containing a nonvolatile electrolyte is 10075degC aWhat is the boiling-point elevation bWhat is the molality of the solution a 075degC b 15 m
187
Electrolytes and Colligative PropertiesEarly investigators confused by experiments where certain substances depressed freezing point or elevated boiling point more than expected
188
189
Ex 01 m solution of NaCl lowers freezing point of solvent almost twice as much as 01 m solution of sucrose 01 m CaCl2 lowers fp almost 3 times as much as 01 m solution of sucrose
To understand why this happens contrast behavior or sucrose with NaCl in aqueous solution Each sucrose dissolves to make only 1 particle in solution So 1 mol sucrose dissolves to 1 mol particles 1 mol NaCl dissolves to make 2 moles of particles (1 mol Na+ and 1 mol Cl-)
190
191
Calculated Values for Electrolyte SolutionsColligative properties depend on total concentration of solute particles regardless of their identity Electrolytes cause changes in colligative properties proportional to total molality in terms of all dissolves particles instead of formula units
192
What about barium nitrate Ba(NO3)2 Each mole of barium nitrate yields 3 mol of ions in solution Ba(NO3)2(s) rarr Ba2+(aq) + 2NO3-(aq) You would expect solution of given molality to lower fp of solvent 3 times as much as nonelectrolytic solution of same molality
193
Sample Problem
What is the expected change in the freezing point of water in a solution of 625 g of barium nitrate Ba(NO3)2 in 100 kg of water
194
1 AnalyzeGiven solute mass and formula = 625 g Ba(NO3)2 solvent mass and identity = 100 kg water Δtf = Kfm
Unknown expected freezing-point depression
195
2 PlanThe molality can be calculated by converting the solute mass to moles and then dividing by the number of kilograms of solvent That molality is in terms of formula units of Ba(NO3)2 and must be converted to molality in terms of dissociated ions in solution It must be multiplied by the number of moles of ions produced per mole of formula unit This adjusted molality can then be used to calculate the freezing-point depression
196
197
198
3 Compute
Practice Problem
What is the expected freezing-point depression for a solution that contains 20 mol of magnesium sulfate dissolved in 10 kg of water
minus74degC199
What is the expected boiling-point elevation of water for a solution that contains 150 g of sodium chloride dissolved in 10 kg of water 27degC The freezing point of an aqueous sodium chloride solution is minus200degC What is the molality of the solution 54 m NaCl
200
Actual Values for Electrolyte SolutionsRemember values calculated are only expected values Actual values of colligative properties for all strong electrolytes are almost what would be expected Difference between expected and actual values caused by attractive forces that exist between dissociated ions in aqueous solution
201
Attraction between hydrated ions in solution is small compared with those in crystalline solid However forces of attraction donrsquot interfere with movements of aqueous ions More concentrated solution means ions closer together and attraction is greater Also depends on charge of ions ndash higher charge greater attraction
202
95
compounds in aqueous solutions
DissociationWhen compound made from ions dissolves in water ions separate Dissociation - separation of ions that occurs when an ionic compound dissolves
96
Notice number of ions made per formula unit in equations 1 formula unit of NaCl gives 2 units of ions in solution 1 formula unit of CaCl2 gives 3 units of ions
97
Assuming 100 dissociation solution that contains 1 mol NaCl contains 1 mol Na+ and 1 mol Cl- Can assume 100 dissociation of all soluble ionic compounds
98
Practice Problem
Write the equation for the dissolution of aluminum sulfate in water How many moles of aluminum ions are made by dissolving 1 mol aluminum sulfate What is the total number of moles of ions made by dissolving 1 mol of aluminum sulfate
99
1 AnalyzeGiven Amount of solute = 1 mol Al2(SO4)3 Solvent identity = water Unknown a moles of aluminum ions and sulfate ions Total number of moles of solute ions produced
100
2 Plan
The coefficients in the balanced dissociation equation will tell mole relationships You can use the equation to find out the number of moles of solute ions produced
101
3 Compute
a
b102
Write the equation for the dissolution of each of the following in water and then determine the number of moles of each ions made as well as the total number of ions made a 1 mol ammonium chloride b 1 mol sodium sulfide c 05 mol barium nitrate
103
a 1 mol ammonium chloride
1 mol NH4+ 1 mol Cl- 2 mol total ions
104
b 1 mol sodium sulfide
2 mol Na+ 1 mol S-2 3 mol total ions
105
c 05 mol barium nitrate
05 mol Ba+2 1 mol NO3- 15 mol total ions
106
Precipitation Reactions
Even though no compound is completely insoluble compounds of very low solubility can be considered insoluble for practical purposes There are general guidelines to help predict whether a compound made of certain combination of ions is soluble
107
General Solubility Guidelines
Sodium potassium and ammonium compounds are soluble in water
108
Nitrates acetates and chlorates are soluble
109
110
Most chlorides are soluble except those of silver mercury (I) and lead Lead (II) chloride is soluble in hot water
Most sulfates are soluble except those of barium strontium lead calcium and mercury
111
Most carbonates phosphates and silicates are insoluble except those of sodium potassium and ammonium
112
Most sulfides are insoluble except those of calcium strontium sodium potassium and ammonium
113
Is calcium phosphate Ca3(PO4)2 soluble or insoluble
Not one of the exceptions so it is insoluble
Dissociation equations are not written for insoluble compounds
114
Guidelines also useful in predicting what will happen if 2 solutions of 2 different soluble compounds are mixed
If mixing results in combination of ions that forms insoluble compound a double-replacement and precipitation reaction will happen
115
Will precipitate form when solutions of ammonium sulfide and cadmium nitrate are combined You can tell calcium nitrate is soluble from the guidelines (Most nitrates acetates and chlorates are soluble) You can also tell ammonium sulfide is soluble (Most sulfides are insoluble except those of calcium strontium sodium potassium and ammonium)
116
117
To decide whether precipitate can form must know solubilities of two possible products
Ammonium nitrate ndash soluble (Most nitrates acetates and chlorates are soluble) Cadmium sulfide ndash insoluble (Most sulfides are insoluble except those of calcium strontium sodium potassium and ammonium)
118
bc one product is insoluble double-replacement and precipitation reaction will happen (NH4)2S (aq) + Cd(NO3)2 (aq) rarr NH4NO3 (aq) + CdS (s)
119
Net Ionic Equations
Reactions of ions in aqueous solution usually represented by net ionic equations instead of formula equations Net ionic equation - includes only those compounds and ions that undergo a chemical change in a reaction in an aqueous solution
120
121
To write NIE (net ionic equation) first convert chemical equation into overall ionic equation All soluble ionic compounds shown as dissociated ions in solution Precipitates shown as solids
122
Notice ammonium and nitrate ions appear on both sides They havenrsquot gone through any chemical change Spectator ions - ions that do not take part in a chemical reaction and are found in solution both before and after the reaction
123
To change ionic equation to NIE eliminate spectator ions Cd+2(aq) + S-2(aq) rarr CdS(s)
This NIE applies to any reaction in which precipitate of CdS forms
124
Practice Problem
Identify the precipitate that forms when aqueous solutions of zinc nitrate and ammonium sulfide are combined Write the equation for the possible double-replacement reaction Then write the formula equation overall ionic equation and the net ionic equation
125
1 AnalyzeGiven Identity of reactants zinc nitrate and ammonium sulfide Reaction medium aqueous solution Unknown a equation for the possible double-replacement reaction b identity of the precipitate c formula equation d overall ionic equation e net ionic equation
126
2 PlanWrite the possible double-replacement reaction between Zn(NO3)2 and (NH4)2S Use the rules to determine if any of the products will precipitate Write a formula equation and overall net ionic equation Then cancel spectator ions to make net ionic equation
127
3 Computea possible double-replacement reaction
b rules show that zinc sulfide is not soluble so will precipitate (ammonium nitrate is soluble)
c formula equation128
d overall ionic equation
e net ionic equation129
Will a precipitate form if solutions of potassium sulfate and barium nitrate are combined If so write the net ionic equation for the reaction
Yes Ba+2(aq) + SO4-2(aq) rarr BaSO4(s)
130
Will a precipitate form if solutions of potassium nitrate and magnesium sulfate are combined If so write the net ionic equation for the reaction
No 131
Will a precipitate form if solutions of barium chloride and sodium sulfate are combined If so identify the spectator ions and write the net ionic equation for the reaction
Yes Na+ and Cl- Ba+2(aq) + SO4-2(aq) rarr BaSO4(s)
132
Write the net ionic equation for the precipitation of nickel(II) sulfide
Ni+2(aq) + S-2(aq) rarr NiS(s)
133
IonizationSome molecular compounds can also form ions in solution Usually polar Ionization - ions formed from solute molecules by action of the solvent (creation of ions where there were none) Ionization different from dissociation
134
Dissociation ionic compounds dissolve and ions already present separate Ionization ions formed where non existed before
Ions formed are hydrated Heat released during hydration of ions gives enough energy to break covalent bonds
135
Extent to which solute ionizes depends on strength of bonds within molecules of solute and strength of attraction between solute and solvent molecules
136
HCl ndash molecular compound that ionizes in aqueous solution Contains highly polar bond Attraction between polar HCl molecule and polar water strong enough to break HCl bond
137
The Hydronium Ion
H+ ions from HCl attracts other molecules or ions so strongly that it doesnrsquot normally exist alone Ionization of HCl better described as direct transfer of proton from HCl to H2O forming H3O+
138
139
Hydration of H+ to form hydronium is highly exothermic
Energy released gives a lot of energy needed to ionize a molecular solute
Many molecular compounds that ionize in aqueous solution contain hydrogen and form hydronium ion
140
Strong and Weak Electrolytes
Substances that make ions and conduct current in solution are electrolytes HCl is one of series of compounds that has hydrogen and halogen Hydrogen halides are all molecular compounds with single polar-covalent bonds
141
All are gases Very soluble in water All are electrolytes HCl HBr HI strongly conduct current in solution HF only weakly conducts current at same concentration Strength of conduction related to ability to ionize
142
143
Strong Electrolytes
HCl HBr HI are 100 ionized in solution
Strong electrolyte - any compound whose dilute aqueous solutions conduct electricity well this is due to the presence of all or almost all of the dissolve compound in the form of ions
144
Unique characteristic of strong electrolyte is that they yield only ions
Ex Ionic compound may be highly soluble in water and dissociate into ions (NaCl)
Others may not dissolve much but amount that does dissolve exists only as ions
145
Weak ElectrolytesHF dissolves in water to give acid solution called hydrofluoric acid HF bond stronger than bonds between hydrogen and other halogens When HF dissolves some molecule ionize Reverse reaction also happens HF(aq) + H2O(l) H3O+(aq) + F-(aq)
146
HF(aq) + H2O(l) H3O+
(aq) + F-(aq)Concentration of dissolved unionized HF stays high and concentration of ions stays low
Weak electrolyte - any compound whose aqueous solutions conduct electricity poorly this is due to the presence of small amount of the dissolved compound in the form of ions
147
Different from nonelectrolyte ndash NO ions at all Description of electrolyte as strong or weak not related to concentration of solution Electrolytes differ in degree of ionization not amount of solute dissolved
148
149
Colligative properties of solutions
Presence of solute affects properties of solutions Some properties not dependent on nature of dissolved substance but on how may dissolve particles are present
Colligative properties - properties that depend on the concentration of solute particles but not on their identity
150
Vapor-Pressure Lowering
Boiling and freezing point of solution different from pure solvent Nonvolatile solute raises boiling point and lowers freezing point Nonvolatile substance - one that has little tendency to become gas under existing conditions
151
152
To understand why nonvolatile solute changes boiling and freezing point consider equilibrium vapor pressure
Vapor pressure pressure caused by molecules that have escape liquid phase to gas phase
Can be thought of a measure of tendency of molecules to escape from a liquid
153
Addition of sucrose (nonvolatile solute) lowers concentration of water molecules at surface of liquid This lowers tendency of water molecules to leave solution and enter gas phase Vapor pressure of solution is lower than vapor pressure of pure water
154
Aqueous13 solution13 of13 nonvolatile13 solutePure13 water
Nonelectrolyte solutions of same molality have same concentration of particles
Dilute solutions of same solvent and equal molality of any nonelectrolyte solute lower vapor pressure equally
155
Example
1 m aqueous solution of nonelectrolyte glucose lowers vapor pressure of water 55 x 10-4 atm at 25 1 m aqueous solution of sucrose also lowers water vapor pressure to 55 x 10-4 atm at 25
156
157
bc vapor-pressure lowering depends on concentration of nonelectrolyte solute and doesnrsquot depend on the type of solute it is a colligative proprerty
bc vapor pressure has been lowered solution remains liquid over larger temperature range
Lowers freezing point and raises boiling point
158
Can assume changes in boiling and freezing point also depend on concentration of solute
They are colligative properties
159
Freezing-Point Depression
Freezing point of 1-molal solution of any nonelectrolyte solute in water is found (by experiment) to be 186 lower than freezing point of water When 1 mol of nonelectrolyte solute dissolved in 1 kg water freezing point is -186 not 000 160
When 2 mol nonelectrolyte solute dissolved in 1 kg water freezing point is -372
For any concentration of nonelectrolyte solute in water decrease in freezing point can be determined using value of -186m
161
This value called the molal freezing-point contstant (Kf) is the freezing-point depression of the solvent in a 1-molal solution of a nonvolatile nonelectrolyte solute
162
Each solvent has own characteristic molal freezing-point constant Freezing-point depression (Δtf) the difference between the freezing points of the pure solvent and a solute of nonelectrolyte in that solvent and it is directly proportional to the molal concentration of the solution Δtf = Kfm
163
Δtf = Kfm
Kf expressed as m
m expressed in mol solutekg solvent (molality)
164
Sample Problem
What is the freezing point depression of water in a solution of 171 g of sucrose C12H22O11 and 200 g of water What is the actual freezing point of the solution
165
1 AnalyzeGiven Solute mass and chemical formula 171 g C12H22O11 Solvent mass and identity 2000 g water Unknown a freezing-point depression b freezing point of the solution 166
2 Plan
167
168
Calculate
Unknown a freezing-point depression b freezing point of the solution
169
Practice Problem
A water solution containing an unknown quantity of a nonelectrolyte solute is found to have a freezing point of ndash 023degCWhat is the molal concentration of the solution
012 m170
A solution consists of 103 g of the nonelectrolyte glucose C6H12O6 dissolved in 250 g of water What is the freezing-point depression of the solution
minus0426degC171
In a laboratory experiment the freezing point of an aqueous solution of glucose is found to be minus0325degCWhat is the molal concentration of this solution
0175 m172
If 0500 mol of a nonelectrolyte solute are dissolved in 5000 g of ether what is the freezing point of the solution
minus1181degC173
The freezing point of an aqueous solution that contains a nonelectrolyte is minus90degC aWhat is the freezing-point depression of the solution bWhat is the molal concentration of the solution a minus90degC b 48 m
174
Boiling-Point Elevation
Boiling point is temp at which vapor pressure is equal to atmospheric pressure Change in vapor pressure will cause corresponding change in boiling point
175
176
Vapor pressure of nonvolatile solution is lower than vapor pressure of pure solvent More heat will be required to raise vapor pressure of solution Boiling point of solution is higher than boiling point of pure solvent
Molal boiling-point constant (Kb) - boiling-point elevation of the solvent in a 1-molal solution of a nonvolatile nonelectrolyte solute b-p elevation of any nonelectrolyte in water found by experiment to be 051 So molal b-p constant for water is 051m
177
For different solvents b-p elevations of 1-molal solutions have different values (Table 14-2)
Like freezing-point constants values most accurate for dilute solutions
178
Boiling-point elevation Δtb difference between the boiling points of the pure solvent and a nonelectrolyte solution of that solvent and it is directly proportional to the molal concentration of the solution B-p elevation can be calculated using Δtb = Kbm Δtb (boiling point elevation) expressed in m and m expressed in mol solutekg solvent
Sample Problem
What is the boiling-point elevation of a solution made from 200 g of a nonelectrolyte solute and 4000 g of water The molar mass of the solute is 620 gmol
180
1 AnalyzeGiven solute mass = 200 g solute molar mass = 620 gmol solvent mass and identity = 4000 g of water
Unknown boiling-point elevation
181
2 Plan
182
183
3 Compute
Practice Problem
A solution contains 500 g of sucrose C12H22O11 a nonelectrolyte dissolved in 5000 g of water What is the boiling-point elevation
015degC184
A solution contains 4500 g of sucrose C12H22O11 a nonelectrolyte dissolved in 250 g of waterWhat is the boiling point of the solution
1027degC185
If the boiling point elevation of an aqueous solution containing a nonvolatile electrolyte is 102degC what is the molality of the solution
20 m186
The boiling point of an aqueous solution containing a nonvolatile electrolyte is 10075degC aWhat is the boiling-point elevation bWhat is the molality of the solution a 075degC b 15 m
187
Electrolytes and Colligative PropertiesEarly investigators confused by experiments where certain substances depressed freezing point or elevated boiling point more than expected
188
189
Ex 01 m solution of NaCl lowers freezing point of solvent almost twice as much as 01 m solution of sucrose 01 m CaCl2 lowers fp almost 3 times as much as 01 m solution of sucrose
To understand why this happens contrast behavior or sucrose with NaCl in aqueous solution Each sucrose dissolves to make only 1 particle in solution So 1 mol sucrose dissolves to 1 mol particles 1 mol NaCl dissolves to make 2 moles of particles (1 mol Na+ and 1 mol Cl-)
190
191
Calculated Values for Electrolyte SolutionsColligative properties depend on total concentration of solute particles regardless of their identity Electrolytes cause changes in colligative properties proportional to total molality in terms of all dissolves particles instead of formula units
192
What about barium nitrate Ba(NO3)2 Each mole of barium nitrate yields 3 mol of ions in solution Ba(NO3)2(s) rarr Ba2+(aq) + 2NO3-(aq) You would expect solution of given molality to lower fp of solvent 3 times as much as nonelectrolytic solution of same molality
193
Sample Problem
What is the expected change in the freezing point of water in a solution of 625 g of barium nitrate Ba(NO3)2 in 100 kg of water
194
1 AnalyzeGiven solute mass and formula = 625 g Ba(NO3)2 solvent mass and identity = 100 kg water Δtf = Kfm
Unknown expected freezing-point depression
195
2 PlanThe molality can be calculated by converting the solute mass to moles and then dividing by the number of kilograms of solvent That molality is in terms of formula units of Ba(NO3)2 and must be converted to molality in terms of dissociated ions in solution It must be multiplied by the number of moles of ions produced per mole of formula unit This adjusted molality can then be used to calculate the freezing-point depression
196
197
198
3 Compute
Practice Problem
What is the expected freezing-point depression for a solution that contains 20 mol of magnesium sulfate dissolved in 10 kg of water
minus74degC199
What is the expected boiling-point elevation of water for a solution that contains 150 g of sodium chloride dissolved in 10 kg of water 27degC The freezing point of an aqueous sodium chloride solution is minus200degC What is the molality of the solution 54 m NaCl
200
Actual Values for Electrolyte SolutionsRemember values calculated are only expected values Actual values of colligative properties for all strong electrolytes are almost what would be expected Difference between expected and actual values caused by attractive forces that exist between dissociated ions in aqueous solution
201
Attraction between hydrated ions in solution is small compared with those in crystalline solid However forces of attraction donrsquot interfere with movements of aqueous ions More concentrated solution means ions closer together and attraction is greater Also depends on charge of ions ndash higher charge greater attraction
202
DissociationWhen compound made from ions dissolves in water ions separate Dissociation - separation of ions that occurs when an ionic compound dissolves
96
Notice number of ions made per formula unit in equations 1 formula unit of NaCl gives 2 units of ions in solution 1 formula unit of CaCl2 gives 3 units of ions
97
Assuming 100 dissociation solution that contains 1 mol NaCl contains 1 mol Na+ and 1 mol Cl- Can assume 100 dissociation of all soluble ionic compounds
98
Practice Problem
Write the equation for the dissolution of aluminum sulfate in water How many moles of aluminum ions are made by dissolving 1 mol aluminum sulfate What is the total number of moles of ions made by dissolving 1 mol of aluminum sulfate
99
1 AnalyzeGiven Amount of solute = 1 mol Al2(SO4)3 Solvent identity = water Unknown a moles of aluminum ions and sulfate ions Total number of moles of solute ions produced
100
2 Plan
The coefficients in the balanced dissociation equation will tell mole relationships You can use the equation to find out the number of moles of solute ions produced
101
3 Compute
a
b102
Write the equation for the dissolution of each of the following in water and then determine the number of moles of each ions made as well as the total number of ions made a 1 mol ammonium chloride b 1 mol sodium sulfide c 05 mol barium nitrate
103
a 1 mol ammonium chloride
1 mol NH4+ 1 mol Cl- 2 mol total ions
104
b 1 mol sodium sulfide
2 mol Na+ 1 mol S-2 3 mol total ions
105
c 05 mol barium nitrate
05 mol Ba+2 1 mol NO3- 15 mol total ions
106
Precipitation Reactions
Even though no compound is completely insoluble compounds of very low solubility can be considered insoluble for practical purposes There are general guidelines to help predict whether a compound made of certain combination of ions is soluble
107
General Solubility Guidelines
Sodium potassium and ammonium compounds are soluble in water
108
Nitrates acetates and chlorates are soluble
109
110
Most chlorides are soluble except those of silver mercury (I) and lead Lead (II) chloride is soluble in hot water
Most sulfates are soluble except those of barium strontium lead calcium and mercury
111
Most carbonates phosphates and silicates are insoluble except those of sodium potassium and ammonium
112
Most sulfides are insoluble except those of calcium strontium sodium potassium and ammonium
113
Is calcium phosphate Ca3(PO4)2 soluble or insoluble
Not one of the exceptions so it is insoluble
Dissociation equations are not written for insoluble compounds
114
Guidelines also useful in predicting what will happen if 2 solutions of 2 different soluble compounds are mixed
If mixing results in combination of ions that forms insoluble compound a double-replacement and precipitation reaction will happen
115
Will precipitate form when solutions of ammonium sulfide and cadmium nitrate are combined You can tell calcium nitrate is soluble from the guidelines (Most nitrates acetates and chlorates are soluble) You can also tell ammonium sulfide is soluble (Most sulfides are insoluble except those of calcium strontium sodium potassium and ammonium)
116
117
To decide whether precipitate can form must know solubilities of two possible products
Ammonium nitrate ndash soluble (Most nitrates acetates and chlorates are soluble) Cadmium sulfide ndash insoluble (Most sulfides are insoluble except those of calcium strontium sodium potassium and ammonium)
118
bc one product is insoluble double-replacement and precipitation reaction will happen (NH4)2S (aq) + Cd(NO3)2 (aq) rarr NH4NO3 (aq) + CdS (s)
119
Net Ionic Equations
Reactions of ions in aqueous solution usually represented by net ionic equations instead of formula equations Net ionic equation - includes only those compounds and ions that undergo a chemical change in a reaction in an aqueous solution
120
121
To write NIE (net ionic equation) first convert chemical equation into overall ionic equation All soluble ionic compounds shown as dissociated ions in solution Precipitates shown as solids
122
Notice ammonium and nitrate ions appear on both sides They havenrsquot gone through any chemical change Spectator ions - ions that do not take part in a chemical reaction and are found in solution both before and after the reaction
123
To change ionic equation to NIE eliminate spectator ions Cd+2(aq) + S-2(aq) rarr CdS(s)
This NIE applies to any reaction in which precipitate of CdS forms
124
Practice Problem
Identify the precipitate that forms when aqueous solutions of zinc nitrate and ammonium sulfide are combined Write the equation for the possible double-replacement reaction Then write the formula equation overall ionic equation and the net ionic equation
125
1 AnalyzeGiven Identity of reactants zinc nitrate and ammonium sulfide Reaction medium aqueous solution Unknown a equation for the possible double-replacement reaction b identity of the precipitate c formula equation d overall ionic equation e net ionic equation
126
2 PlanWrite the possible double-replacement reaction between Zn(NO3)2 and (NH4)2S Use the rules to determine if any of the products will precipitate Write a formula equation and overall net ionic equation Then cancel spectator ions to make net ionic equation
127
3 Computea possible double-replacement reaction
b rules show that zinc sulfide is not soluble so will precipitate (ammonium nitrate is soluble)
c formula equation128
d overall ionic equation
e net ionic equation129
Will a precipitate form if solutions of potassium sulfate and barium nitrate are combined If so write the net ionic equation for the reaction
Yes Ba+2(aq) + SO4-2(aq) rarr BaSO4(s)
130
Will a precipitate form if solutions of potassium nitrate and magnesium sulfate are combined If so write the net ionic equation for the reaction
No 131
Will a precipitate form if solutions of barium chloride and sodium sulfate are combined If so identify the spectator ions and write the net ionic equation for the reaction
Yes Na+ and Cl- Ba+2(aq) + SO4-2(aq) rarr BaSO4(s)
132
Write the net ionic equation for the precipitation of nickel(II) sulfide
Ni+2(aq) + S-2(aq) rarr NiS(s)
133
IonizationSome molecular compounds can also form ions in solution Usually polar Ionization - ions formed from solute molecules by action of the solvent (creation of ions where there were none) Ionization different from dissociation
134
Dissociation ionic compounds dissolve and ions already present separate Ionization ions formed where non existed before
Ions formed are hydrated Heat released during hydration of ions gives enough energy to break covalent bonds
135
Extent to which solute ionizes depends on strength of bonds within molecules of solute and strength of attraction between solute and solvent molecules
136
HCl ndash molecular compound that ionizes in aqueous solution Contains highly polar bond Attraction between polar HCl molecule and polar water strong enough to break HCl bond
137
The Hydronium Ion
H+ ions from HCl attracts other molecules or ions so strongly that it doesnrsquot normally exist alone Ionization of HCl better described as direct transfer of proton from HCl to H2O forming H3O+
138
139
Hydration of H+ to form hydronium is highly exothermic
Energy released gives a lot of energy needed to ionize a molecular solute
Many molecular compounds that ionize in aqueous solution contain hydrogen and form hydronium ion
140
Strong and Weak Electrolytes
Substances that make ions and conduct current in solution are electrolytes HCl is one of series of compounds that has hydrogen and halogen Hydrogen halides are all molecular compounds with single polar-covalent bonds
141
All are gases Very soluble in water All are electrolytes HCl HBr HI strongly conduct current in solution HF only weakly conducts current at same concentration Strength of conduction related to ability to ionize
142
143
Strong Electrolytes
HCl HBr HI are 100 ionized in solution
Strong electrolyte - any compound whose dilute aqueous solutions conduct electricity well this is due to the presence of all or almost all of the dissolve compound in the form of ions
144
Unique characteristic of strong electrolyte is that they yield only ions
Ex Ionic compound may be highly soluble in water and dissociate into ions (NaCl)
Others may not dissolve much but amount that does dissolve exists only as ions
145
Weak ElectrolytesHF dissolves in water to give acid solution called hydrofluoric acid HF bond stronger than bonds between hydrogen and other halogens When HF dissolves some molecule ionize Reverse reaction also happens HF(aq) + H2O(l) H3O+(aq) + F-(aq)
146
HF(aq) + H2O(l) H3O+
(aq) + F-(aq)Concentration of dissolved unionized HF stays high and concentration of ions stays low
Weak electrolyte - any compound whose aqueous solutions conduct electricity poorly this is due to the presence of small amount of the dissolved compound in the form of ions
147
Different from nonelectrolyte ndash NO ions at all Description of electrolyte as strong or weak not related to concentration of solution Electrolytes differ in degree of ionization not amount of solute dissolved
148
149
Colligative properties of solutions
Presence of solute affects properties of solutions Some properties not dependent on nature of dissolved substance but on how may dissolve particles are present
Colligative properties - properties that depend on the concentration of solute particles but not on their identity
150
Vapor-Pressure Lowering
Boiling and freezing point of solution different from pure solvent Nonvolatile solute raises boiling point and lowers freezing point Nonvolatile substance - one that has little tendency to become gas under existing conditions
151
152
To understand why nonvolatile solute changes boiling and freezing point consider equilibrium vapor pressure
Vapor pressure pressure caused by molecules that have escape liquid phase to gas phase
Can be thought of a measure of tendency of molecules to escape from a liquid
153
Addition of sucrose (nonvolatile solute) lowers concentration of water molecules at surface of liquid This lowers tendency of water molecules to leave solution and enter gas phase Vapor pressure of solution is lower than vapor pressure of pure water
154
Aqueous13 solution13 of13 nonvolatile13 solutePure13 water
Nonelectrolyte solutions of same molality have same concentration of particles
Dilute solutions of same solvent and equal molality of any nonelectrolyte solute lower vapor pressure equally
155
Example
1 m aqueous solution of nonelectrolyte glucose lowers vapor pressure of water 55 x 10-4 atm at 25 1 m aqueous solution of sucrose also lowers water vapor pressure to 55 x 10-4 atm at 25
156
157
bc vapor-pressure lowering depends on concentration of nonelectrolyte solute and doesnrsquot depend on the type of solute it is a colligative proprerty
bc vapor pressure has been lowered solution remains liquid over larger temperature range
Lowers freezing point and raises boiling point
158
Can assume changes in boiling and freezing point also depend on concentration of solute
They are colligative properties
159
Freezing-Point Depression
Freezing point of 1-molal solution of any nonelectrolyte solute in water is found (by experiment) to be 186 lower than freezing point of water When 1 mol of nonelectrolyte solute dissolved in 1 kg water freezing point is -186 not 000 160
When 2 mol nonelectrolyte solute dissolved in 1 kg water freezing point is -372
For any concentration of nonelectrolyte solute in water decrease in freezing point can be determined using value of -186m
161
This value called the molal freezing-point contstant (Kf) is the freezing-point depression of the solvent in a 1-molal solution of a nonvolatile nonelectrolyte solute
162
Each solvent has own characteristic molal freezing-point constant Freezing-point depression (Δtf) the difference between the freezing points of the pure solvent and a solute of nonelectrolyte in that solvent and it is directly proportional to the molal concentration of the solution Δtf = Kfm
163
Δtf = Kfm
Kf expressed as m
m expressed in mol solutekg solvent (molality)
164
Sample Problem
What is the freezing point depression of water in a solution of 171 g of sucrose C12H22O11 and 200 g of water What is the actual freezing point of the solution
165
1 AnalyzeGiven Solute mass and chemical formula 171 g C12H22O11 Solvent mass and identity 2000 g water Unknown a freezing-point depression b freezing point of the solution 166
2 Plan
167
168
Calculate
Unknown a freezing-point depression b freezing point of the solution
169
Practice Problem
A water solution containing an unknown quantity of a nonelectrolyte solute is found to have a freezing point of ndash 023degCWhat is the molal concentration of the solution
012 m170
A solution consists of 103 g of the nonelectrolyte glucose C6H12O6 dissolved in 250 g of water What is the freezing-point depression of the solution
minus0426degC171
In a laboratory experiment the freezing point of an aqueous solution of glucose is found to be minus0325degCWhat is the molal concentration of this solution
0175 m172
If 0500 mol of a nonelectrolyte solute are dissolved in 5000 g of ether what is the freezing point of the solution
minus1181degC173
The freezing point of an aqueous solution that contains a nonelectrolyte is minus90degC aWhat is the freezing-point depression of the solution bWhat is the molal concentration of the solution a minus90degC b 48 m
174
Boiling-Point Elevation
Boiling point is temp at which vapor pressure is equal to atmospheric pressure Change in vapor pressure will cause corresponding change in boiling point
175
176
Vapor pressure of nonvolatile solution is lower than vapor pressure of pure solvent More heat will be required to raise vapor pressure of solution Boiling point of solution is higher than boiling point of pure solvent
Molal boiling-point constant (Kb) - boiling-point elevation of the solvent in a 1-molal solution of a nonvolatile nonelectrolyte solute b-p elevation of any nonelectrolyte in water found by experiment to be 051 So molal b-p constant for water is 051m
177
For different solvents b-p elevations of 1-molal solutions have different values (Table 14-2)
Like freezing-point constants values most accurate for dilute solutions
178
Boiling-point elevation Δtb difference between the boiling points of the pure solvent and a nonelectrolyte solution of that solvent and it is directly proportional to the molal concentration of the solution B-p elevation can be calculated using Δtb = Kbm Δtb (boiling point elevation) expressed in m and m expressed in mol solutekg solvent
Sample Problem
What is the boiling-point elevation of a solution made from 200 g of a nonelectrolyte solute and 4000 g of water The molar mass of the solute is 620 gmol
180
1 AnalyzeGiven solute mass = 200 g solute molar mass = 620 gmol solvent mass and identity = 4000 g of water
Unknown boiling-point elevation
181
2 Plan
182
183
3 Compute
Practice Problem
A solution contains 500 g of sucrose C12H22O11 a nonelectrolyte dissolved in 5000 g of water What is the boiling-point elevation
015degC184
A solution contains 4500 g of sucrose C12H22O11 a nonelectrolyte dissolved in 250 g of waterWhat is the boiling point of the solution
1027degC185
If the boiling point elevation of an aqueous solution containing a nonvolatile electrolyte is 102degC what is the molality of the solution
20 m186
The boiling point of an aqueous solution containing a nonvolatile electrolyte is 10075degC aWhat is the boiling-point elevation bWhat is the molality of the solution a 075degC b 15 m
187
Electrolytes and Colligative PropertiesEarly investigators confused by experiments where certain substances depressed freezing point or elevated boiling point more than expected
188
189
Ex 01 m solution of NaCl lowers freezing point of solvent almost twice as much as 01 m solution of sucrose 01 m CaCl2 lowers fp almost 3 times as much as 01 m solution of sucrose
To understand why this happens contrast behavior or sucrose with NaCl in aqueous solution Each sucrose dissolves to make only 1 particle in solution So 1 mol sucrose dissolves to 1 mol particles 1 mol NaCl dissolves to make 2 moles of particles (1 mol Na+ and 1 mol Cl-)
190
191
Calculated Values for Electrolyte SolutionsColligative properties depend on total concentration of solute particles regardless of their identity Electrolytes cause changes in colligative properties proportional to total molality in terms of all dissolves particles instead of formula units
192
What about barium nitrate Ba(NO3)2 Each mole of barium nitrate yields 3 mol of ions in solution Ba(NO3)2(s) rarr Ba2+(aq) + 2NO3-(aq) You would expect solution of given molality to lower fp of solvent 3 times as much as nonelectrolytic solution of same molality
193
Sample Problem
What is the expected change in the freezing point of water in a solution of 625 g of barium nitrate Ba(NO3)2 in 100 kg of water
194
1 AnalyzeGiven solute mass and formula = 625 g Ba(NO3)2 solvent mass and identity = 100 kg water Δtf = Kfm
Unknown expected freezing-point depression
195
2 PlanThe molality can be calculated by converting the solute mass to moles and then dividing by the number of kilograms of solvent That molality is in terms of formula units of Ba(NO3)2 and must be converted to molality in terms of dissociated ions in solution It must be multiplied by the number of moles of ions produced per mole of formula unit This adjusted molality can then be used to calculate the freezing-point depression
196
197
198
3 Compute
Practice Problem
What is the expected freezing-point depression for a solution that contains 20 mol of magnesium sulfate dissolved in 10 kg of water
minus74degC199
What is the expected boiling-point elevation of water for a solution that contains 150 g of sodium chloride dissolved in 10 kg of water 27degC The freezing point of an aqueous sodium chloride solution is minus200degC What is the molality of the solution 54 m NaCl
200
Actual Values for Electrolyte SolutionsRemember values calculated are only expected values Actual values of colligative properties for all strong electrolytes are almost what would be expected Difference between expected and actual values caused by attractive forces that exist between dissociated ions in aqueous solution
201
Attraction between hydrated ions in solution is small compared with those in crystalline solid However forces of attraction donrsquot interfere with movements of aqueous ions More concentrated solution means ions closer together and attraction is greater Also depends on charge of ions ndash higher charge greater attraction
202
Notice number of ions made per formula unit in equations 1 formula unit of NaCl gives 2 units of ions in solution 1 formula unit of CaCl2 gives 3 units of ions
97
Assuming 100 dissociation solution that contains 1 mol NaCl contains 1 mol Na+ and 1 mol Cl- Can assume 100 dissociation of all soluble ionic compounds
98
Practice Problem
Write the equation for the dissolution of aluminum sulfate in water How many moles of aluminum ions are made by dissolving 1 mol aluminum sulfate What is the total number of moles of ions made by dissolving 1 mol of aluminum sulfate
99
1 AnalyzeGiven Amount of solute = 1 mol Al2(SO4)3 Solvent identity = water Unknown a moles of aluminum ions and sulfate ions Total number of moles of solute ions produced
100
2 Plan
The coefficients in the balanced dissociation equation will tell mole relationships You can use the equation to find out the number of moles of solute ions produced
101
3 Compute
a
b102
Write the equation for the dissolution of each of the following in water and then determine the number of moles of each ions made as well as the total number of ions made a 1 mol ammonium chloride b 1 mol sodium sulfide c 05 mol barium nitrate
103
a 1 mol ammonium chloride
1 mol NH4+ 1 mol Cl- 2 mol total ions
104
b 1 mol sodium sulfide
2 mol Na+ 1 mol S-2 3 mol total ions
105
c 05 mol barium nitrate
05 mol Ba+2 1 mol NO3- 15 mol total ions
106
Precipitation Reactions
Even though no compound is completely insoluble compounds of very low solubility can be considered insoluble for practical purposes There are general guidelines to help predict whether a compound made of certain combination of ions is soluble
107
General Solubility Guidelines
Sodium potassium and ammonium compounds are soluble in water
108
Nitrates acetates and chlorates are soluble
109
110
Most chlorides are soluble except those of silver mercury (I) and lead Lead (II) chloride is soluble in hot water
Most sulfates are soluble except those of barium strontium lead calcium and mercury
111
Most carbonates phosphates and silicates are insoluble except those of sodium potassium and ammonium
112
Most sulfides are insoluble except those of calcium strontium sodium potassium and ammonium
113
Is calcium phosphate Ca3(PO4)2 soluble or insoluble
Not one of the exceptions so it is insoluble
Dissociation equations are not written for insoluble compounds
114
Guidelines also useful in predicting what will happen if 2 solutions of 2 different soluble compounds are mixed
If mixing results in combination of ions that forms insoluble compound a double-replacement and precipitation reaction will happen
115
Will precipitate form when solutions of ammonium sulfide and cadmium nitrate are combined You can tell calcium nitrate is soluble from the guidelines (Most nitrates acetates and chlorates are soluble) You can also tell ammonium sulfide is soluble (Most sulfides are insoluble except those of calcium strontium sodium potassium and ammonium)
116
117
To decide whether precipitate can form must know solubilities of two possible products
Ammonium nitrate ndash soluble (Most nitrates acetates and chlorates are soluble) Cadmium sulfide ndash insoluble (Most sulfides are insoluble except those of calcium strontium sodium potassium and ammonium)
118
bc one product is insoluble double-replacement and precipitation reaction will happen (NH4)2S (aq) + Cd(NO3)2 (aq) rarr NH4NO3 (aq) + CdS (s)
119
Net Ionic Equations
Reactions of ions in aqueous solution usually represented by net ionic equations instead of formula equations Net ionic equation - includes only those compounds and ions that undergo a chemical change in a reaction in an aqueous solution
120
121
To write NIE (net ionic equation) first convert chemical equation into overall ionic equation All soluble ionic compounds shown as dissociated ions in solution Precipitates shown as solids
122
Notice ammonium and nitrate ions appear on both sides They havenrsquot gone through any chemical change Spectator ions - ions that do not take part in a chemical reaction and are found in solution both before and after the reaction
123
To change ionic equation to NIE eliminate spectator ions Cd+2(aq) + S-2(aq) rarr CdS(s)
This NIE applies to any reaction in which precipitate of CdS forms
124
Practice Problem
Identify the precipitate that forms when aqueous solutions of zinc nitrate and ammonium sulfide are combined Write the equation for the possible double-replacement reaction Then write the formula equation overall ionic equation and the net ionic equation
125
1 AnalyzeGiven Identity of reactants zinc nitrate and ammonium sulfide Reaction medium aqueous solution Unknown a equation for the possible double-replacement reaction b identity of the precipitate c formula equation d overall ionic equation e net ionic equation
126
2 PlanWrite the possible double-replacement reaction between Zn(NO3)2 and (NH4)2S Use the rules to determine if any of the products will precipitate Write a formula equation and overall net ionic equation Then cancel spectator ions to make net ionic equation
127
3 Computea possible double-replacement reaction
b rules show that zinc sulfide is not soluble so will precipitate (ammonium nitrate is soluble)
c formula equation128
d overall ionic equation
e net ionic equation129
Will a precipitate form if solutions of potassium sulfate and barium nitrate are combined If so write the net ionic equation for the reaction
Yes Ba+2(aq) + SO4-2(aq) rarr BaSO4(s)
130
Will a precipitate form if solutions of potassium nitrate and magnesium sulfate are combined If so write the net ionic equation for the reaction
No 131
Will a precipitate form if solutions of barium chloride and sodium sulfate are combined If so identify the spectator ions and write the net ionic equation for the reaction
Yes Na+ and Cl- Ba+2(aq) + SO4-2(aq) rarr BaSO4(s)
132
Write the net ionic equation for the precipitation of nickel(II) sulfide
Ni+2(aq) + S-2(aq) rarr NiS(s)
133
IonizationSome molecular compounds can also form ions in solution Usually polar Ionization - ions formed from solute molecules by action of the solvent (creation of ions where there were none) Ionization different from dissociation
134
Dissociation ionic compounds dissolve and ions already present separate Ionization ions formed where non existed before
Ions formed are hydrated Heat released during hydration of ions gives enough energy to break covalent bonds
135
Extent to which solute ionizes depends on strength of bonds within molecules of solute and strength of attraction between solute and solvent molecules
136
HCl ndash molecular compound that ionizes in aqueous solution Contains highly polar bond Attraction between polar HCl molecule and polar water strong enough to break HCl bond
137
The Hydronium Ion
H+ ions from HCl attracts other molecules or ions so strongly that it doesnrsquot normally exist alone Ionization of HCl better described as direct transfer of proton from HCl to H2O forming H3O+
138
139
Hydration of H+ to form hydronium is highly exothermic
Energy released gives a lot of energy needed to ionize a molecular solute
Many molecular compounds that ionize in aqueous solution contain hydrogen and form hydronium ion
140
Strong and Weak Electrolytes
Substances that make ions and conduct current in solution are electrolytes HCl is one of series of compounds that has hydrogen and halogen Hydrogen halides are all molecular compounds with single polar-covalent bonds
141
All are gases Very soluble in water All are electrolytes HCl HBr HI strongly conduct current in solution HF only weakly conducts current at same concentration Strength of conduction related to ability to ionize
142
143
Strong Electrolytes
HCl HBr HI are 100 ionized in solution
Strong electrolyte - any compound whose dilute aqueous solutions conduct electricity well this is due to the presence of all or almost all of the dissolve compound in the form of ions
144
Unique characteristic of strong electrolyte is that they yield only ions
Ex Ionic compound may be highly soluble in water and dissociate into ions (NaCl)
Others may not dissolve much but amount that does dissolve exists only as ions
145
Weak ElectrolytesHF dissolves in water to give acid solution called hydrofluoric acid HF bond stronger than bonds between hydrogen and other halogens When HF dissolves some molecule ionize Reverse reaction also happens HF(aq) + H2O(l) H3O+(aq) + F-(aq)
146
HF(aq) + H2O(l) H3O+
(aq) + F-(aq)Concentration of dissolved unionized HF stays high and concentration of ions stays low
Weak electrolyte - any compound whose aqueous solutions conduct electricity poorly this is due to the presence of small amount of the dissolved compound in the form of ions
147
Different from nonelectrolyte ndash NO ions at all Description of electrolyte as strong or weak not related to concentration of solution Electrolytes differ in degree of ionization not amount of solute dissolved
148
149
Colligative properties of solutions
Presence of solute affects properties of solutions Some properties not dependent on nature of dissolved substance but on how may dissolve particles are present
Colligative properties - properties that depend on the concentration of solute particles but not on their identity
150
Vapor-Pressure Lowering
Boiling and freezing point of solution different from pure solvent Nonvolatile solute raises boiling point and lowers freezing point Nonvolatile substance - one that has little tendency to become gas under existing conditions
151
152
To understand why nonvolatile solute changes boiling and freezing point consider equilibrium vapor pressure
Vapor pressure pressure caused by molecules that have escape liquid phase to gas phase
Can be thought of a measure of tendency of molecules to escape from a liquid
153
Addition of sucrose (nonvolatile solute) lowers concentration of water molecules at surface of liquid This lowers tendency of water molecules to leave solution and enter gas phase Vapor pressure of solution is lower than vapor pressure of pure water
154
Aqueous13 solution13 of13 nonvolatile13 solutePure13 water
Nonelectrolyte solutions of same molality have same concentration of particles
Dilute solutions of same solvent and equal molality of any nonelectrolyte solute lower vapor pressure equally
155
Example
1 m aqueous solution of nonelectrolyte glucose lowers vapor pressure of water 55 x 10-4 atm at 25 1 m aqueous solution of sucrose also lowers water vapor pressure to 55 x 10-4 atm at 25
156
157
bc vapor-pressure lowering depends on concentration of nonelectrolyte solute and doesnrsquot depend on the type of solute it is a colligative proprerty
bc vapor pressure has been lowered solution remains liquid over larger temperature range
Lowers freezing point and raises boiling point
158
Can assume changes in boiling and freezing point also depend on concentration of solute
They are colligative properties
159
Freezing-Point Depression
Freezing point of 1-molal solution of any nonelectrolyte solute in water is found (by experiment) to be 186 lower than freezing point of water When 1 mol of nonelectrolyte solute dissolved in 1 kg water freezing point is -186 not 000 160
When 2 mol nonelectrolyte solute dissolved in 1 kg water freezing point is -372
For any concentration of nonelectrolyte solute in water decrease in freezing point can be determined using value of -186m
161
This value called the molal freezing-point contstant (Kf) is the freezing-point depression of the solvent in a 1-molal solution of a nonvolatile nonelectrolyte solute
162
Each solvent has own characteristic molal freezing-point constant Freezing-point depression (Δtf) the difference between the freezing points of the pure solvent and a solute of nonelectrolyte in that solvent and it is directly proportional to the molal concentration of the solution Δtf = Kfm
163
Δtf = Kfm
Kf expressed as m
m expressed in mol solutekg solvent (molality)
164
Sample Problem
What is the freezing point depression of water in a solution of 171 g of sucrose C12H22O11 and 200 g of water What is the actual freezing point of the solution
165
1 AnalyzeGiven Solute mass and chemical formula 171 g C12H22O11 Solvent mass and identity 2000 g water Unknown a freezing-point depression b freezing point of the solution 166
2 Plan
167
168
Calculate
Unknown a freezing-point depression b freezing point of the solution
169
Practice Problem
A water solution containing an unknown quantity of a nonelectrolyte solute is found to have a freezing point of ndash 023degCWhat is the molal concentration of the solution
012 m170
A solution consists of 103 g of the nonelectrolyte glucose C6H12O6 dissolved in 250 g of water What is the freezing-point depression of the solution
minus0426degC171
In a laboratory experiment the freezing point of an aqueous solution of glucose is found to be minus0325degCWhat is the molal concentration of this solution
0175 m172
If 0500 mol of a nonelectrolyte solute are dissolved in 5000 g of ether what is the freezing point of the solution
minus1181degC173
The freezing point of an aqueous solution that contains a nonelectrolyte is minus90degC aWhat is the freezing-point depression of the solution bWhat is the molal concentration of the solution a minus90degC b 48 m
174
Boiling-Point Elevation
Boiling point is temp at which vapor pressure is equal to atmospheric pressure Change in vapor pressure will cause corresponding change in boiling point
175
176
Vapor pressure of nonvolatile solution is lower than vapor pressure of pure solvent More heat will be required to raise vapor pressure of solution Boiling point of solution is higher than boiling point of pure solvent
Molal boiling-point constant (Kb) - boiling-point elevation of the solvent in a 1-molal solution of a nonvolatile nonelectrolyte solute b-p elevation of any nonelectrolyte in water found by experiment to be 051 So molal b-p constant for water is 051m
177
For different solvents b-p elevations of 1-molal solutions have different values (Table 14-2)
Like freezing-point constants values most accurate for dilute solutions
178
Boiling-point elevation Δtb difference between the boiling points of the pure solvent and a nonelectrolyte solution of that solvent and it is directly proportional to the molal concentration of the solution B-p elevation can be calculated using Δtb = Kbm Δtb (boiling point elevation) expressed in m and m expressed in mol solutekg solvent
Sample Problem
What is the boiling-point elevation of a solution made from 200 g of a nonelectrolyte solute and 4000 g of water The molar mass of the solute is 620 gmol
180
1 AnalyzeGiven solute mass = 200 g solute molar mass = 620 gmol solvent mass and identity = 4000 g of water
Unknown boiling-point elevation
181
2 Plan
182
183
3 Compute
Practice Problem
A solution contains 500 g of sucrose C12H22O11 a nonelectrolyte dissolved in 5000 g of water What is the boiling-point elevation
015degC184
A solution contains 4500 g of sucrose C12H22O11 a nonelectrolyte dissolved in 250 g of waterWhat is the boiling point of the solution
1027degC185
If the boiling point elevation of an aqueous solution containing a nonvolatile electrolyte is 102degC what is the molality of the solution
20 m186
The boiling point of an aqueous solution containing a nonvolatile electrolyte is 10075degC aWhat is the boiling-point elevation bWhat is the molality of the solution a 075degC b 15 m
187
Electrolytes and Colligative PropertiesEarly investigators confused by experiments where certain substances depressed freezing point or elevated boiling point more than expected
188
189
Ex 01 m solution of NaCl lowers freezing point of solvent almost twice as much as 01 m solution of sucrose 01 m CaCl2 lowers fp almost 3 times as much as 01 m solution of sucrose
To understand why this happens contrast behavior or sucrose with NaCl in aqueous solution Each sucrose dissolves to make only 1 particle in solution So 1 mol sucrose dissolves to 1 mol particles 1 mol NaCl dissolves to make 2 moles of particles (1 mol Na+ and 1 mol Cl-)
190
191
Calculated Values for Electrolyte SolutionsColligative properties depend on total concentration of solute particles regardless of their identity Electrolytes cause changes in colligative properties proportional to total molality in terms of all dissolves particles instead of formula units
192
What about barium nitrate Ba(NO3)2 Each mole of barium nitrate yields 3 mol of ions in solution Ba(NO3)2(s) rarr Ba2+(aq) + 2NO3-(aq) You would expect solution of given molality to lower fp of solvent 3 times as much as nonelectrolytic solution of same molality
193
Sample Problem
What is the expected change in the freezing point of water in a solution of 625 g of barium nitrate Ba(NO3)2 in 100 kg of water
194
1 AnalyzeGiven solute mass and formula = 625 g Ba(NO3)2 solvent mass and identity = 100 kg water Δtf = Kfm
Unknown expected freezing-point depression
195
2 PlanThe molality can be calculated by converting the solute mass to moles and then dividing by the number of kilograms of solvent That molality is in terms of formula units of Ba(NO3)2 and must be converted to molality in terms of dissociated ions in solution It must be multiplied by the number of moles of ions produced per mole of formula unit This adjusted molality can then be used to calculate the freezing-point depression
196
197
198
3 Compute
Practice Problem
What is the expected freezing-point depression for a solution that contains 20 mol of magnesium sulfate dissolved in 10 kg of water
minus74degC199
What is the expected boiling-point elevation of water for a solution that contains 150 g of sodium chloride dissolved in 10 kg of water 27degC The freezing point of an aqueous sodium chloride solution is minus200degC What is the molality of the solution 54 m NaCl
200
Actual Values for Electrolyte SolutionsRemember values calculated are only expected values Actual values of colligative properties for all strong electrolytes are almost what would be expected Difference between expected and actual values caused by attractive forces that exist between dissociated ions in aqueous solution
201
Attraction between hydrated ions in solution is small compared with those in crystalline solid However forces of attraction donrsquot interfere with movements of aqueous ions More concentrated solution means ions closer together and attraction is greater Also depends on charge of ions ndash higher charge greater attraction
202
Assuming 100 dissociation solution that contains 1 mol NaCl contains 1 mol Na+ and 1 mol Cl- Can assume 100 dissociation of all soluble ionic compounds
98
Practice Problem
Write the equation for the dissolution of aluminum sulfate in water How many moles of aluminum ions are made by dissolving 1 mol aluminum sulfate What is the total number of moles of ions made by dissolving 1 mol of aluminum sulfate
99
1 AnalyzeGiven Amount of solute = 1 mol Al2(SO4)3 Solvent identity = water Unknown a moles of aluminum ions and sulfate ions Total number of moles of solute ions produced
100
2 Plan
The coefficients in the balanced dissociation equation will tell mole relationships You can use the equation to find out the number of moles of solute ions produced
101
3 Compute
a
b102
Write the equation for the dissolution of each of the following in water and then determine the number of moles of each ions made as well as the total number of ions made a 1 mol ammonium chloride b 1 mol sodium sulfide c 05 mol barium nitrate
103
a 1 mol ammonium chloride
1 mol NH4+ 1 mol Cl- 2 mol total ions
104
b 1 mol sodium sulfide
2 mol Na+ 1 mol S-2 3 mol total ions
105
c 05 mol barium nitrate
05 mol Ba+2 1 mol NO3- 15 mol total ions
106
Precipitation Reactions
Even though no compound is completely insoluble compounds of very low solubility can be considered insoluble for practical purposes There are general guidelines to help predict whether a compound made of certain combination of ions is soluble
107
General Solubility Guidelines
Sodium potassium and ammonium compounds are soluble in water
108
Nitrates acetates and chlorates are soluble
109
110
Most chlorides are soluble except those of silver mercury (I) and lead Lead (II) chloride is soluble in hot water
Most sulfates are soluble except those of barium strontium lead calcium and mercury
111
Most carbonates phosphates and silicates are insoluble except those of sodium potassium and ammonium
112
Most sulfides are insoluble except those of calcium strontium sodium potassium and ammonium
113
Is calcium phosphate Ca3(PO4)2 soluble or insoluble
Not one of the exceptions so it is insoluble
Dissociation equations are not written for insoluble compounds
114
Guidelines also useful in predicting what will happen if 2 solutions of 2 different soluble compounds are mixed
If mixing results in combination of ions that forms insoluble compound a double-replacement and precipitation reaction will happen
115
Will precipitate form when solutions of ammonium sulfide and cadmium nitrate are combined You can tell calcium nitrate is soluble from the guidelines (Most nitrates acetates and chlorates are soluble) You can also tell ammonium sulfide is soluble (Most sulfides are insoluble except those of calcium strontium sodium potassium and ammonium)
116
117
To decide whether precipitate can form must know solubilities of two possible products
Ammonium nitrate ndash soluble (Most nitrates acetates and chlorates are soluble) Cadmium sulfide ndash insoluble (Most sulfides are insoluble except those of calcium strontium sodium potassium and ammonium)
118
bc one product is insoluble double-replacement and precipitation reaction will happen (NH4)2S (aq) + Cd(NO3)2 (aq) rarr NH4NO3 (aq) + CdS (s)
119
Net Ionic Equations
Reactions of ions in aqueous solution usually represented by net ionic equations instead of formula equations Net ionic equation - includes only those compounds and ions that undergo a chemical change in a reaction in an aqueous solution
120
121
To write NIE (net ionic equation) first convert chemical equation into overall ionic equation All soluble ionic compounds shown as dissociated ions in solution Precipitates shown as solids
122
Notice ammonium and nitrate ions appear on both sides They havenrsquot gone through any chemical change Spectator ions - ions that do not take part in a chemical reaction and are found in solution both before and after the reaction
123
To change ionic equation to NIE eliminate spectator ions Cd+2(aq) + S-2(aq) rarr CdS(s)
This NIE applies to any reaction in which precipitate of CdS forms
124
Practice Problem
Identify the precipitate that forms when aqueous solutions of zinc nitrate and ammonium sulfide are combined Write the equation for the possible double-replacement reaction Then write the formula equation overall ionic equation and the net ionic equation
125
1 AnalyzeGiven Identity of reactants zinc nitrate and ammonium sulfide Reaction medium aqueous solution Unknown a equation for the possible double-replacement reaction b identity of the precipitate c formula equation d overall ionic equation e net ionic equation
126
2 PlanWrite the possible double-replacement reaction between Zn(NO3)2 and (NH4)2S Use the rules to determine if any of the products will precipitate Write a formula equation and overall net ionic equation Then cancel spectator ions to make net ionic equation
127
3 Computea possible double-replacement reaction
b rules show that zinc sulfide is not soluble so will precipitate (ammonium nitrate is soluble)
c formula equation128
d overall ionic equation
e net ionic equation129
Will a precipitate form if solutions of potassium sulfate and barium nitrate are combined If so write the net ionic equation for the reaction
Yes Ba+2(aq) + SO4-2(aq) rarr BaSO4(s)
130
Will a precipitate form if solutions of potassium nitrate and magnesium sulfate are combined If so write the net ionic equation for the reaction
No 131
Will a precipitate form if solutions of barium chloride and sodium sulfate are combined If so identify the spectator ions and write the net ionic equation for the reaction
Yes Na+ and Cl- Ba+2(aq) + SO4-2(aq) rarr BaSO4(s)
132
Write the net ionic equation for the precipitation of nickel(II) sulfide
Ni+2(aq) + S-2(aq) rarr NiS(s)
133
IonizationSome molecular compounds can also form ions in solution Usually polar Ionization - ions formed from solute molecules by action of the solvent (creation of ions where there were none) Ionization different from dissociation
134
Dissociation ionic compounds dissolve and ions already present separate Ionization ions formed where non existed before
Ions formed are hydrated Heat released during hydration of ions gives enough energy to break covalent bonds
135
Extent to which solute ionizes depends on strength of bonds within molecules of solute and strength of attraction between solute and solvent molecules
136
HCl ndash molecular compound that ionizes in aqueous solution Contains highly polar bond Attraction between polar HCl molecule and polar water strong enough to break HCl bond
137
The Hydronium Ion
H+ ions from HCl attracts other molecules or ions so strongly that it doesnrsquot normally exist alone Ionization of HCl better described as direct transfer of proton from HCl to H2O forming H3O+
138
139
Hydration of H+ to form hydronium is highly exothermic
Energy released gives a lot of energy needed to ionize a molecular solute
Many molecular compounds that ionize in aqueous solution contain hydrogen and form hydronium ion
140
Strong and Weak Electrolytes
Substances that make ions and conduct current in solution are electrolytes HCl is one of series of compounds that has hydrogen and halogen Hydrogen halides are all molecular compounds with single polar-covalent bonds
141
All are gases Very soluble in water All are electrolytes HCl HBr HI strongly conduct current in solution HF only weakly conducts current at same concentration Strength of conduction related to ability to ionize
142
143
Strong Electrolytes
HCl HBr HI are 100 ionized in solution
Strong electrolyte - any compound whose dilute aqueous solutions conduct electricity well this is due to the presence of all or almost all of the dissolve compound in the form of ions
144
Unique characteristic of strong electrolyte is that they yield only ions
Ex Ionic compound may be highly soluble in water and dissociate into ions (NaCl)
Others may not dissolve much but amount that does dissolve exists only as ions
145
Weak ElectrolytesHF dissolves in water to give acid solution called hydrofluoric acid HF bond stronger than bonds between hydrogen and other halogens When HF dissolves some molecule ionize Reverse reaction also happens HF(aq) + H2O(l) H3O+(aq) + F-(aq)
146
HF(aq) + H2O(l) H3O+
(aq) + F-(aq)Concentration of dissolved unionized HF stays high and concentration of ions stays low
Weak electrolyte - any compound whose aqueous solutions conduct electricity poorly this is due to the presence of small amount of the dissolved compound in the form of ions
147
Different from nonelectrolyte ndash NO ions at all Description of electrolyte as strong or weak not related to concentration of solution Electrolytes differ in degree of ionization not amount of solute dissolved
148
149
Colligative properties of solutions
Presence of solute affects properties of solutions Some properties not dependent on nature of dissolved substance but on how may dissolve particles are present
Colligative properties - properties that depend on the concentration of solute particles but not on their identity
150
Vapor-Pressure Lowering
Boiling and freezing point of solution different from pure solvent Nonvolatile solute raises boiling point and lowers freezing point Nonvolatile substance - one that has little tendency to become gas under existing conditions
151
152
To understand why nonvolatile solute changes boiling and freezing point consider equilibrium vapor pressure
Vapor pressure pressure caused by molecules that have escape liquid phase to gas phase
Can be thought of a measure of tendency of molecules to escape from a liquid
153
Addition of sucrose (nonvolatile solute) lowers concentration of water molecules at surface of liquid This lowers tendency of water molecules to leave solution and enter gas phase Vapor pressure of solution is lower than vapor pressure of pure water
154
Aqueous13 solution13 of13 nonvolatile13 solutePure13 water
Nonelectrolyte solutions of same molality have same concentration of particles
Dilute solutions of same solvent and equal molality of any nonelectrolyte solute lower vapor pressure equally
155
Example
1 m aqueous solution of nonelectrolyte glucose lowers vapor pressure of water 55 x 10-4 atm at 25 1 m aqueous solution of sucrose also lowers water vapor pressure to 55 x 10-4 atm at 25
156
157
bc vapor-pressure lowering depends on concentration of nonelectrolyte solute and doesnrsquot depend on the type of solute it is a colligative proprerty
bc vapor pressure has been lowered solution remains liquid over larger temperature range
Lowers freezing point and raises boiling point
158
Can assume changes in boiling and freezing point also depend on concentration of solute
They are colligative properties
159
Freezing-Point Depression
Freezing point of 1-molal solution of any nonelectrolyte solute in water is found (by experiment) to be 186 lower than freezing point of water When 1 mol of nonelectrolyte solute dissolved in 1 kg water freezing point is -186 not 000 160
When 2 mol nonelectrolyte solute dissolved in 1 kg water freezing point is -372
For any concentration of nonelectrolyte solute in water decrease in freezing point can be determined using value of -186m
161
This value called the molal freezing-point contstant (Kf) is the freezing-point depression of the solvent in a 1-molal solution of a nonvolatile nonelectrolyte solute
162
Each solvent has own characteristic molal freezing-point constant Freezing-point depression (Δtf) the difference between the freezing points of the pure solvent and a solute of nonelectrolyte in that solvent and it is directly proportional to the molal concentration of the solution Δtf = Kfm
163
Δtf = Kfm
Kf expressed as m
m expressed in mol solutekg solvent (molality)
164
Sample Problem
What is the freezing point depression of water in a solution of 171 g of sucrose C12H22O11 and 200 g of water What is the actual freezing point of the solution
165
1 AnalyzeGiven Solute mass and chemical formula 171 g C12H22O11 Solvent mass and identity 2000 g water Unknown a freezing-point depression b freezing point of the solution 166
2 Plan
167
168
Calculate
Unknown a freezing-point depression b freezing point of the solution
169
Practice Problem
A water solution containing an unknown quantity of a nonelectrolyte solute is found to have a freezing point of ndash 023degCWhat is the molal concentration of the solution
012 m170
A solution consists of 103 g of the nonelectrolyte glucose C6H12O6 dissolved in 250 g of water What is the freezing-point depression of the solution
minus0426degC171
In a laboratory experiment the freezing point of an aqueous solution of glucose is found to be minus0325degCWhat is the molal concentration of this solution
0175 m172
If 0500 mol of a nonelectrolyte solute are dissolved in 5000 g of ether what is the freezing point of the solution
minus1181degC173
The freezing point of an aqueous solution that contains a nonelectrolyte is minus90degC aWhat is the freezing-point depression of the solution bWhat is the molal concentration of the solution a minus90degC b 48 m
174
Boiling-Point Elevation
Boiling point is temp at which vapor pressure is equal to atmospheric pressure Change in vapor pressure will cause corresponding change in boiling point
175
176
Vapor pressure of nonvolatile solution is lower than vapor pressure of pure solvent More heat will be required to raise vapor pressure of solution Boiling point of solution is higher than boiling point of pure solvent
Molal boiling-point constant (Kb) - boiling-point elevation of the solvent in a 1-molal solution of a nonvolatile nonelectrolyte solute b-p elevation of any nonelectrolyte in water found by experiment to be 051 So molal b-p constant for water is 051m
177
For different solvents b-p elevations of 1-molal solutions have different values (Table 14-2)
Like freezing-point constants values most accurate for dilute solutions
178
Boiling-point elevation Δtb difference between the boiling points of the pure solvent and a nonelectrolyte solution of that solvent and it is directly proportional to the molal concentration of the solution B-p elevation can be calculated using Δtb = Kbm Δtb (boiling point elevation) expressed in m and m expressed in mol solutekg solvent
Sample Problem
What is the boiling-point elevation of a solution made from 200 g of a nonelectrolyte solute and 4000 g of water The molar mass of the solute is 620 gmol
180
1 AnalyzeGiven solute mass = 200 g solute molar mass = 620 gmol solvent mass and identity = 4000 g of water
Unknown boiling-point elevation
181
2 Plan
182
183
3 Compute
Practice Problem
A solution contains 500 g of sucrose C12H22O11 a nonelectrolyte dissolved in 5000 g of water What is the boiling-point elevation
015degC184
A solution contains 4500 g of sucrose C12H22O11 a nonelectrolyte dissolved in 250 g of waterWhat is the boiling point of the solution
1027degC185
If the boiling point elevation of an aqueous solution containing a nonvolatile electrolyte is 102degC what is the molality of the solution
20 m186
The boiling point of an aqueous solution containing a nonvolatile electrolyte is 10075degC aWhat is the boiling-point elevation bWhat is the molality of the solution a 075degC b 15 m
187
Electrolytes and Colligative PropertiesEarly investigators confused by experiments where certain substances depressed freezing point or elevated boiling point more than expected
188
189
Ex 01 m solution of NaCl lowers freezing point of solvent almost twice as much as 01 m solution of sucrose 01 m CaCl2 lowers fp almost 3 times as much as 01 m solution of sucrose
To understand why this happens contrast behavior or sucrose with NaCl in aqueous solution Each sucrose dissolves to make only 1 particle in solution So 1 mol sucrose dissolves to 1 mol particles 1 mol NaCl dissolves to make 2 moles of particles (1 mol Na+ and 1 mol Cl-)
190
191
Calculated Values for Electrolyte SolutionsColligative properties depend on total concentration of solute particles regardless of their identity Electrolytes cause changes in colligative properties proportional to total molality in terms of all dissolves particles instead of formula units
192
What about barium nitrate Ba(NO3)2 Each mole of barium nitrate yields 3 mol of ions in solution Ba(NO3)2(s) rarr Ba2+(aq) + 2NO3-(aq) You would expect solution of given molality to lower fp of solvent 3 times as much as nonelectrolytic solution of same molality
193
Sample Problem
What is the expected change in the freezing point of water in a solution of 625 g of barium nitrate Ba(NO3)2 in 100 kg of water
194
1 AnalyzeGiven solute mass and formula = 625 g Ba(NO3)2 solvent mass and identity = 100 kg water Δtf = Kfm
Unknown expected freezing-point depression
195
2 PlanThe molality can be calculated by converting the solute mass to moles and then dividing by the number of kilograms of solvent That molality is in terms of formula units of Ba(NO3)2 and must be converted to molality in terms of dissociated ions in solution It must be multiplied by the number of moles of ions produced per mole of formula unit This adjusted molality can then be used to calculate the freezing-point depression
196
197
198
3 Compute
Practice Problem
What is the expected freezing-point depression for a solution that contains 20 mol of magnesium sulfate dissolved in 10 kg of water
minus74degC199
What is the expected boiling-point elevation of water for a solution that contains 150 g of sodium chloride dissolved in 10 kg of water 27degC The freezing point of an aqueous sodium chloride solution is minus200degC What is the molality of the solution 54 m NaCl
200
Actual Values for Electrolyte SolutionsRemember values calculated are only expected values Actual values of colligative properties for all strong electrolytes are almost what would be expected Difference between expected and actual values caused by attractive forces that exist between dissociated ions in aqueous solution
201
Attraction between hydrated ions in solution is small compared with those in crystalline solid However forces of attraction donrsquot interfere with movements of aqueous ions More concentrated solution means ions closer together and attraction is greater Also depends on charge of ions ndash higher charge greater attraction
202
Practice Problem
Write the equation for the dissolution of aluminum sulfate in water How many moles of aluminum ions are made by dissolving 1 mol aluminum sulfate What is the total number of moles of ions made by dissolving 1 mol of aluminum sulfate
99
1 AnalyzeGiven Amount of solute = 1 mol Al2(SO4)3 Solvent identity = water Unknown a moles of aluminum ions and sulfate ions Total number of moles of solute ions produced
100
2 Plan
The coefficients in the balanced dissociation equation will tell mole relationships You can use the equation to find out the number of moles of solute ions produced
101
3 Compute
a
b102
Write the equation for the dissolution of each of the following in water and then determine the number of moles of each ions made as well as the total number of ions made a 1 mol ammonium chloride b 1 mol sodium sulfide c 05 mol barium nitrate
103
a 1 mol ammonium chloride
1 mol NH4+ 1 mol Cl- 2 mol total ions
104
b 1 mol sodium sulfide
2 mol Na+ 1 mol S-2 3 mol total ions
105
c 05 mol barium nitrate
05 mol Ba+2 1 mol NO3- 15 mol total ions
106
Precipitation Reactions
Even though no compound is completely insoluble compounds of very low solubility can be considered insoluble for practical purposes There are general guidelines to help predict whether a compound made of certain combination of ions is soluble
107
General Solubility Guidelines
Sodium potassium and ammonium compounds are soluble in water
108
Nitrates acetates and chlorates are soluble
109
110
Most chlorides are soluble except those of silver mercury (I) and lead Lead (II) chloride is soluble in hot water
Most sulfates are soluble except those of barium strontium lead calcium and mercury
111
Most carbonates phosphates and silicates are insoluble except those of sodium potassium and ammonium
112
Most sulfides are insoluble except those of calcium strontium sodium potassium and ammonium
113
Is calcium phosphate Ca3(PO4)2 soluble or insoluble
Not one of the exceptions so it is insoluble
Dissociation equations are not written for insoluble compounds
114
Guidelines also useful in predicting what will happen if 2 solutions of 2 different soluble compounds are mixed
If mixing results in combination of ions that forms insoluble compound a double-replacement and precipitation reaction will happen
115
Will precipitate form when solutions of ammonium sulfide and cadmium nitrate are combined You can tell calcium nitrate is soluble from the guidelines (Most nitrates acetates and chlorates are soluble) You can also tell ammonium sulfide is soluble (Most sulfides are insoluble except those of calcium strontium sodium potassium and ammonium)
116
117
To decide whether precipitate can form must know solubilities of two possible products
Ammonium nitrate ndash soluble (Most nitrates acetates and chlorates are soluble) Cadmium sulfide ndash insoluble (Most sulfides are insoluble except those of calcium strontium sodium potassium and ammonium)
118
bc one product is insoluble double-replacement and precipitation reaction will happen (NH4)2S (aq) + Cd(NO3)2 (aq) rarr NH4NO3 (aq) + CdS (s)
119
Net Ionic Equations
Reactions of ions in aqueous solution usually represented by net ionic equations instead of formula equations Net ionic equation - includes only those compounds and ions that undergo a chemical change in a reaction in an aqueous solution
120
121
To write NIE (net ionic equation) first convert chemical equation into overall ionic equation All soluble ionic compounds shown as dissociated ions in solution Precipitates shown as solids
122
Notice ammonium and nitrate ions appear on both sides They havenrsquot gone through any chemical change Spectator ions - ions that do not take part in a chemical reaction and are found in solution both before and after the reaction
123
To change ionic equation to NIE eliminate spectator ions Cd+2(aq) + S-2(aq) rarr CdS(s)
This NIE applies to any reaction in which precipitate of CdS forms
124
Practice Problem
Identify the precipitate that forms when aqueous solutions of zinc nitrate and ammonium sulfide are combined Write the equation for the possible double-replacement reaction Then write the formula equation overall ionic equation and the net ionic equation
125
1 AnalyzeGiven Identity of reactants zinc nitrate and ammonium sulfide Reaction medium aqueous solution Unknown a equation for the possible double-replacement reaction b identity of the precipitate c formula equation d overall ionic equation e net ionic equation
126
2 PlanWrite the possible double-replacement reaction between Zn(NO3)2 and (NH4)2S Use the rules to determine if any of the products will precipitate Write a formula equation and overall net ionic equation Then cancel spectator ions to make net ionic equation
127
3 Computea possible double-replacement reaction
b rules show that zinc sulfide is not soluble so will precipitate (ammonium nitrate is soluble)
c formula equation128
d overall ionic equation
e net ionic equation129
Will a precipitate form if solutions of potassium sulfate and barium nitrate are combined If so write the net ionic equation for the reaction
Yes Ba+2(aq) + SO4-2(aq) rarr BaSO4(s)
130
Will a precipitate form if solutions of potassium nitrate and magnesium sulfate are combined If so write the net ionic equation for the reaction
No 131
Will a precipitate form if solutions of barium chloride and sodium sulfate are combined If so identify the spectator ions and write the net ionic equation for the reaction
Yes Na+ and Cl- Ba+2(aq) + SO4-2(aq) rarr BaSO4(s)
132
Write the net ionic equation for the precipitation of nickel(II) sulfide
Ni+2(aq) + S-2(aq) rarr NiS(s)
133
IonizationSome molecular compounds can also form ions in solution Usually polar Ionization - ions formed from solute molecules by action of the solvent (creation of ions where there were none) Ionization different from dissociation
134
Dissociation ionic compounds dissolve and ions already present separate Ionization ions formed where non existed before
Ions formed are hydrated Heat released during hydration of ions gives enough energy to break covalent bonds
135
Extent to which solute ionizes depends on strength of bonds within molecules of solute and strength of attraction between solute and solvent molecules
136
HCl ndash molecular compound that ionizes in aqueous solution Contains highly polar bond Attraction between polar HCl molecule and polar water strong enough to break HCl bond
137
The Hydronium Ion
H+ ions from HCl attracts other molecules or ions so strongly that it doesnrsquot normally exist alone Ionization of HCl better described as direct transfer of proton from HCl to H2O forming H3O+
138
139
Hydration of H+ to form hydronium is highly exothermic
Energy released gives a lot of energy needed to ionize a molecular solute
Many molecular compounds that ionize in aqueous solution contain hydrogen and form hydronium ion
140
Strong and Weak Electrolytes
Substances that make ions and conduct current in solution are electrolytes HCl is one of series of compounds that has hydrogen and halogen Hydrogen halides are all molecular compounds with single polar-covalent bonds
141
All are gases Very soluble in water All are electrolytes HCl HBr HI strongly conduct current in solution HF only weakly conducts current at same concentration Strength of conduction related to ability to ionize
142
143
Strong Electrolytes
HCl HBr HI are 100 ionized in solution
Strong electrolyte - any compound whose dilute aqueous solutions conduct electricity well this is due to the presence of all or almost all of the dissolve compound in the form of ions
144
Unique characteristic of strong electrolyte is that they yield only ions
Ex Ionic compound may be highly soluble in water and dissociate into ions (NaCl)
Others may not dissolve much but amount that does dissolve exists only as ions
145
Weak ElectrolytesHF dissolves in water to give acid solution called hydrofluoric acid HF bond stronger than bonds between hydrogen and other halogens When HF dissolves some molecule ionize Reverse reaction also happens HF(aq) + H2O(l) H3O+(aq) + F-(aq)
146
HF(aq) + H2O(l) H3O+
(aq) + F-(aq)Concentration of dissolved unionized HF stays high and concentration of ions stays low
Weak electrolyte - any compound whose aqueous solutions conduct electricity poorly this is due to the presence of small amount of the dissolved compound in the form of ions
147
Different from nonelectrolyte ndash NO ions at all Description of electrolyte as strong or weak not related to concentration of solution Electrolytes differ in degree of ionization not amount of solute dissolved
148
149
Colligative properties of solutions
Presence of solute affects properties of solutions Some properties not dependent on nature of dissolved substance but on how may dissolve particles are present
Colligative properties - properties that depend on the concentration of solute particles but not on their identity
150
Vapor-Pressure Lowering
Boiling and freezing point of solution different from pure solvent Nonvolatile solute raises boiling point and lowers freezing point Nonvolatile substance - one that has little tendency to become gas under existing conditions
151
152
To understand why nonvolatile solute changes boiling and freezing point consider equilibrium vapor pressure
Vapor pressure pressure caused by molecules that have escape liquid phase to gas phase
Can be thought of a measure of tendency of molecules to escape from a liquid
153
Addition of sucrose (nonvolatile solute) lowers concentration of water molecules at surface of liquid This lowers tendency of water molecules to leave solution and enter gas phase Vapor pressure of solution is lower than vapor pressure of pure water
154
Aqueous13 solution13 of13 nonvolatile13 solutePure13 water
Nonelectrolyte solutions of same molality have same concentration of particles
Dilute solutions of same solvent and equal molality of any nonelectrolyte solute lower vapor pressure equally
155
Example
1 m aqueous solution of nonelectrolyte glucose lowers vapor pressure of water 55 x 10-4 atm at 25 1 m aqueous solution of sucrose also lowers water vapor pressure to 55 x 10-4 atm at 25
156
157
bc vapor-pressure lowering depends on concentration of nonelectrolyte solute and doesnrsquot depend on the type of solute it is a colligative proprerty
bc vapor pressure has been lowered solution remains liquid over larger temperature range
Lowers freezing point and raises boiling point
158
Can assume changes in boiling and freezing point also depend on concentration of solute
They are colligative properties
159
Freezing-Point Depression
Freezing point of 1-molal solution of any nonelectrolyte solute in water is found (by experiment) to be 186 lower than freezing point of water When 1 mol of nonelectrolyte solute dissolved in 1 kg water freezing point is -186 not 000 160
When 2 mol nonelectrolyte solute dissolved in 1 kg water freezing point is -372
For any concentration of nonelectrolyte solute in water decrease in freezing point can be determined using value of -186m
161
This value called the molal freezing-point contstant (Kf) is the freezing-point depression of the solvent in a 1-molal solution of a nonvolatile nonelectrolyte solute
162
Each solvent has own characteristic molal freezing-point constant Freezing-point depression (Δtf) the difference between the freezing points of the pure solvent and a solute of nonelectrolyte in that solvent and it is directly proportional to the molal concentration of the solution Δtf = Kfm
163
Δtf = Kfm
Kf expressed as m
m expressed in mol solutekg solvent (molality)
164
Sample Problem
What is the freezing point depression of water in a solution of 171 g of sucrose C12H22O11 and 200 g of water What is the actual freezing point of the solution
165
1 AnalyzeGiven Solute mass and chemical formula 171 g C12H22O11 Solvent mass and identity 2000 g water Unknown a freezing-point depression b freezing point of the solution 166
2 Plan
167
168
Calculate
Unknown a freezing-point depression b freezing point of the solution
169
Practice Problem
A water solution containing an unknown quantity of a nonelectrolyte solute is found to have a freezing point of ndash 023degCWhat is the molal concentration of the solution
012 m170
A solution consists of 103 g of the nonelectrolyte glucose C6H12O6 dissolved in 250 g of water What is the freezing-point depression of the solution
minus0426degC171
In a laboratory experiment the freezing point of an aqueous solution of glucose is found to be minus0325degCWhat is the molal concentration of this solution
0175 m172
If 0500 mol of a nonelectrolyte solute are dissolved in 5000 g of ether what is the freezing point of the solution
minus1181degC173
The freezing point of an aqueous solution that contains a nonelectrolyte is minus90degC aWhat is the freezing-point depression of the solution bWhat is the molal concentration of the solution a minus90degC b 48 m
174
Boiling-Point Elevation
Boiling point is temp at which vapor pressure is equal to atmospheric pressure Change in vapor pressure will cause corresponding change in boiling point
175
176
Vapor pressure of nonvolatile solution is lower than vapor pressure of pure solvent More heat will be required to raise vapor pressure of solution Boiling point of solution is higher than boiling point of pure solvent
Molal boiling-point constant (Kb) - boiling-point elevation of the solvent in a 1-molal solution of a nonvolatile nonelectrolyte solute b-p elevation of any nonelectrolyte in water found by experiment to be 051 So molal b-p constant for water is 051m
177
For different solvents b-p elevations of 1-molal solutions have different values (Table 14-2)
Like freezing-point constants values most accurate for dilute solutions
178
Boiling-point elevation Δtb difference between the boiling points of the pure solvent and a nonelectrolyte solution of that solvent and it is directly proportional to the molal concentration of the solution B-p elevation can be calculated using Δtb = Kbm Δtb (boiling point elevation) expressed in m and m expressed in mol solutekg solvent
Sample Problem
What is the boiling-point elevation of a solution made from 200 g of a nonelectrolyte solute and 4000 g of water The molar mass of the solute is 620 gmol
180
1 AnalyzeGiven solute mass = 200 g solute molar mass = 620 gmol solvent mass and identity = 4000 g of water
Unknown boiling-point elevation
181
2 Plan
182
183
3 Compute
Practice Problem
A solution contains 500 g of sucrose C12H22O11 a nonelectrolyte dissolved in 5000 g of water What is the boiling-point elevation
015degC184
A solution contains 4500 g of sucrose C12H22O11 a nonelectrolyte dissolved in 250 g of waterWhat is the boiling point of the solution
1027degC185
If the boiling point elevation of an aqueous solution containing a nonvolatile electrolyte is 102degC what is the molality of the solution
20 m186
The boiling point of an aqueous solution containing a nonvolatile electrolyte is 10075degC aWhat is the boiling-point elevation bWhat is the molality of the solution a 075degC b 15 m
187
Electrolytes and Colligative PropertiesEarly investigators confused by experiments where certain substances depressed freezing point or elevated boiling point more than expected
188
189
Ex 01 m solution of NaCl lowers freezing point of solvent almost twice as much as 01 m solution of sucrose 01 m CaCl2 lowers fp almost 3 times as much as 01 m solution of sucrose
To understand why this happens contrast behavior or sucrose with NaCl in aqueous solution Each sucrose dissolves to make only 1 particle in solution So 1 mol sucrose dissolves to 1 mol particles 1 mol NaCl dissolves to make 2 moles of particles (1 mol Na+ and 1 mol Cl-)
190
191
Calculated Values for Electrolyte SolutionsColligative properties depend on total concentration of solute particles regardless of their identity Electrolytes cause changes in colligative properties proportional to total molality in terms of all dissolves particles instead of formula units
192
What about barium nitrate Ba(NO3)2 Each mole of barium nitrate yields 3 mol of ions in solution Ba(NO3)2(s) rarr Ba2+(aq) + 2NO3-(aq) You would expect solution of given molality to lower fp of solvent 3 times as much as nonelectrolytic solution of same molality
193
Sample Problem
What is the expected change in the freezing point of water in a solution of 625 g of barium nitrate Ba(NO3)2 in 100 kg of water
194
1 AnalyzeGiven solute mass and formula = 625 g Ba(NO3)2 solvent mass and identity = 100 kg water Δtf = Kfm
Unknown expected freezing-point depression
195
2 PlanThe molality can be calculated by converting the solute mass to moles and then dividing by the number of kilograms of solvent That molality is in terms of formula units of Ba(NO3)2 and must be converted to molality in terms of dissociated ions in solution It must be multiplied by the number of moles of ions produced per mole of formula unit This adjusted molality can then be used to calculate the freezing-point depression
196
197
198
3 Compute
Practice Problem
What is the expected freezing-point depression for a solution that contains 20 mol of magnesium sulfate dissolved in 10 kg of water
minus74degC199
What is the expected boiling-point elevation of water for a solution that contains 150 g of sodium chloride dissolved in 10 kg of water 27degC The freezing point of an aqueous sodium chloride solution is minus200degC What is the molality of the solution 54 m NaCl
200
Actual Values for Electrolyte SolutionsRemember values calculated are only expected values Actual values of colligative properties for all strong electrolytes are almost what would be expected Difference between expected and actual values caused by attractive forces that exist between dissociated ions in aqueous solution
201
Attraction between hydrated ions in solution is small compared with those in crystalline solid However forces of attraction donrsquot interfere with movements of aqueous ions More concentrated solution means ions closer together and attraction is greater Also depends on charge of ions ndash higher charge greater attraction
202
1 AnalyzeGiven Amount of solute = 1 mol Al2(SO4)3 Solvent identity = water Unknown a moles of aluminum ions and sulfate ions Total number of moles of solute ions produced
100
2 Plan
The coefficients in the balanced dissociation equation will tell mole relationships You can use the equation to find out the number of moles of solute ions produced
101
3 Compute
a
b102
Write the equation for the dissolution of each of the following in water and then determine the number of moles of each ions made as well as the total number of ions made a 1 mol ammonium chloride b 1 mol sodium sulfide c 05 mol barium nitrate
103
a 1 mol ammonium chloride
1 mol NH4+ 1 mol Cl- 2 mol total ions
104
b 1 mol sodium sulfide
2 mol Na+ 1 mol S-2 3 mol total ions
105
c 05 mol barium nitrate
05 mol Ba+2 1 mol NO3- 15 mol total ions
106
Precipitation Reactions
Even though no compound is completely insoluble compounds of very low solubility can be considered insoluble for practical purposes There are general guidelines to help predict whether a compound made of certain combination of ions is soluble
107
General Solubility Guidelines
Sodium potassium and ammonium compounds are soluble in water
108
Nitrates acetates and chlorates are soluble
109
110
Most chlorides are soluble except those of silver mercury (I) and lead Lead (II) chloride is soluble in hot water
Most sulfates are soluble except those of barium strontium lead calcium and mercury
111
Most carbonates phosphates and silicates are insoluble except those of sodium potassium and ammonium
112
Most sulfides are insoluble except those of calcium strontium sodium potassium and ammonium
113
Is calcium phosphate Ca3(PO4)2 soluble or insoluble
Not one of the exceptions so it is insoluble
Dissociation equations are not written for insoluble compounds
114
Guidelines also useful in predicting what will happen if 2 solutions of 2 different soluble compounds are mixed
If mixing results in combination of ions that forms insoluble compound a double-replacement and precipitation reaction will happen
115
Will precipitate form when solutions of ammonium sulfide and cadmium nitrate are combined You can tell calcium nitrate is soluble from the guidelines (Most nitrates acetates and chlorates are soluble) You can also tell ammonium sulfide is soluble (Most sulfides are insoluble except those of calcium strontium sodium potassium and ammonium)
116
117
To decide whether precipitate can form must know solubilities of two possible products
Ammonium nitrate ndash soluble (Most nitrates acetates and chlorates are soluble) Cadmium sulfide ndash insoluble (Most sulfides are insoluble except those of calcium strontium sodium potassium and ammonium)
118
bc one product is insoluble double-replacement and precipitation reaction will happen (NH4)2S (aq) + Cd(NO3)2 (aq) rarr NH4NO3 (aq) + CdS (s)
119
Net Ionic Equations
Reactions of ions in aqueous solution usually represented by net ionic equations instead of formula equations Net ionic equation - includes only those compounds and ions that undergo a chemical change in a reaction in an aqueous solution
120
121
To write NIE (net ionic equation) first convert chemical equation into overall ionic equation All soluble ionic compounds shown as dissociated ions in solution Precipitates shown as solids
122
Notice ammonium and nitrate ions appear on both sides They havenrsquot gone through any chemical change Spectator ions - ions that do not take part in a chemical reaction and are found in solution both before and after the reaction
123
To change ionic equation to NIE eliminate spectator ions Cd+2(aq) + S-2(aq) rarr CdS(s)
This NIE applies to any reaction in which precipitate of CdS forms
124
Practice Problem
Identify the precipitate that forms when aqueous solutions of zinc nitrate and ammonium sulfide are combined Write the equation for the possible double-replacement reaction Then write the formula equation overall ionic equation and the net ionic equation
125
1 AnalyzeGiven Identity of reactants zinc nitrate and ammonium sulfide Reaction medium aqueous solution Unknown a equation for the possible double-replacement reaction b identity of the precipitate c formula equation d overall ionic equation e net ionic equation
126
2 PlanWrite the possible double-replacement reaction between Zn(NO3)2 and (NH4)2S Use the rules to determine if any of the products will precipitate Write a formula equation and overall net ionic equation Then cancel spectator ions to make net ionic equation
127
3 Computea possible double-replacement reaction
b rules show that zinc sulfide is not soluble so will precipitate (ammonium nitrate is soluble)
c formula equation128
d overall ionic equation
e net ionic equation129
Will a precipitate form if solutions of potassium sulfate and barium nitrate are combined If so write the net ionic equation for the reaction
Yes Ba+2(aq) + SO4-2(aq) rarr BaSO4(s)
130
Will a precipitate form if solutions of potassium nitrate and magnesium sulfate are combined If so write the net ionic equation for the reaction
No 131
Will a precipitate form if solutions of barium chloride and sodium sulfate are combined If so identify the spectator ions and write the net ionic equation for the reaction
Yes Na+ and Cl- Ba+2(aq) + SO4-2(aq) rarr BaSO4(s)
132
Write the net ionic equation for the precipitation of nickel(II) sulfide
Ni+2(aq) + S-2(aq) rarr NiS(s)
133
IonizationSome molecular compounds can also form ions in solution Usually polar Ionization - ions formed from solute molecules by action of the solvent (creation of ions where there were none) Ionization different from dissociation
134
Dissociation ionic compounds dissolve and ions already present separate Ionization ions formed where non existed before
Ions formed are hydrated Heat released during hydration of ions gives enough energy to break covalent bonds
135
Extent to which solute ionizes depends on strength of bonds within molecules of solute and strength of attraction between solute and solvent molecules
136
HCl ndash molecular compound that ionizes in aqueous solution Contains highly polar bond Attraction between polar HCl molecule and polar water strong enough to break HCl bond
137
The Hydronium Ion
H+ ions from HCl attracts other molecules or ions so strongly that it doesnrsquot normally exist alone Ionization of HCl better described as direct transfer of proton from HCl to H2O forming H3O+
138
139
Hydration of H+ to form hydronium is highly exothermic
Energy released gives a lot of energy needed to ionize a molecular solute
Many molecular compounds that ionize in aqueous solution contain hydrogen and form hydronium ion
140
Strong and Weak Electrolytes
Substances that make ions and conduct current in solution are electrolytes HCl is one of series of compounds that has hydrogen and halogen Hydrogen halides are all molecular compounds with single polar-covalent bonds
141
All are gases Very soluble in water All are electrolytes HCl HBr HI strongly conduct current in solution HF only weakly conducts current at same concentration Strength of conduction related to ability to ionize
142
143
Strong Electrolytes
HCl HBr HI are 100 ionized in solution
Strong electrolyte - any compound whose dilute aqueous solutions conduct electricity well this is due to the presence of all or almost all of the dissolve compound in the form of ions
144
Unique characteristic of strong electrolyte is that they yield only ions
Ex Ionic compound may be highly soluble in water and dissociate into ions (NaCl)
Others may not dissolve much but amount that does dissolve exists only as ions
145
Weak ElectrolytesHF dissolves in water to give acid solution called hydrofluoric acid HF bond stronger than bonds between hydrogen and other halogens When HF dissolves some molecule ionize Reverse reaction also happens HF(aq) + H2O(l) H3O+(aq) + F-(aq)
146
HF(aq) + H2O(l) H3O+
(aq) + F-(aq)Concentration of dissolved unionized HF stays high and concentration of ions stays low
Weak electrolyte - any compound whose aqueous solutions conduct electricity poorly this is due to the presence of small amount of the dissolved compound in the form of ions
147
Different from nonelectrolyte ndash NO ions at all Description of electrolyte as strong or weak not related to concentration of solution Electrolytes differ in degree of ionization not amount of solute dissolved
148
149
Colligative properties of solutions
Presence of solute affects properties of solutions Some properties not dependent on nature of dissolved substance but on how may dissolve particles are present
Colligative properties - properties that depend on the concentration of solute particles but not on their identity
150
Vapor-Pressure Lowering
Boiling and freezing point of solution different from pure solvent Nonvolatile solute raises boiling point and lowers freezing point Nonvolatile substance - one that has little tendency to become gas under existing conditions
151
152
To understand why nonvolatile solute changes boiling and freezing point consider equilibrium vapor pressure
Vapor pressure pressure caused by molecules that have escape liquid phase to gas phase
Can be thought of a measure of tendency of molecules to escape from a liquid
153
Addition of sucrose (nonvolatile solute) lowers concentration of water molecules at surface of liquid This lowers tendency of water molecules to leave solution and enter gas phase Vapor pressure of solution is lower than vapor pressure of pure water
154
Aqueous13 solution13 of13 nonvolatile13 solutePure13 water
Nonelectrolyte solutions of same molality have same concentration of particles
Dilute solutions of same solvent and equal molality of any nonelectrolyte solute lower vapor pressure equally
155
Example
1 m aqueous solution of nonelectrolyte glucose lowers vapor pressure of water 55 x 10-4 atm at 25 1 m aqueous solution of sucrose also lowers water vapor pressure to 55 x 10-4 atm at 25
156
157
bc vapor-pressure lowering depends on concentration of nonelectrolyte solute and doesnrsquot depend on the type of solute it is a colligative proprerty
bc vapor pressure has been lowered solution remains liquid over larger temperature range
Lowers freezing point and raises boiling point
158
Can assume changes in boiling and freezing point also depend on concentration of solute
They are colligative properties
159
Freezing-Point Depression
Freezing point of 1-molal solution of any nonelectrolyte solute in water is found (by experiment) to be 186 lower than freezing point of water When 1 mol of nonelectrolyte solute dissolved in 1 kg water freezing point is -186 not 000 160
When 2 mol nonelectrolyte solute dissolved in 1 kg water freezing point is -372
For any concentration of nonelectrolyte solute in water decrease in freezing point can be determined using value of -186m
161
This value called the molal freezing-point contstant (Kf) is the freezing-point depression of the solvent in a 1-molal solution of a nonvolatile nonelectrolyte solute
162
Each solvent has own characteristic molal freezing-point constant Freezing-point depression (Δtf) the difference between the freezing points of the pure solvent and a solute of nonelectrolyte in that solvent and it is directly proportional to the molal concentration of the solution Δtf = Kfm
163
Δtf = Kfm
Kf expressed as m
m expressed in mol solutekg solvent (molality)
164
Sample Problem
What is the freezing point depression of water in a solution of 171 g of sucrose C12H22O11 and 200 g of water What is the actual freezing point of the solution
165
1 AnalyzeGiven Solute mass and chemical formula 171 g C12H22O11 Solvent mass and identity 2000 g water Unknown a freezing-point depression b freezing point of the solution 166
2 Plan
167
168
Calculate
Unknown a freezing-point depression b freezing point of the solution
169
Practice Problem
A water solution containing an unknown quantity of a nonelectrolyte solute is found to have a freezing point of ndash 023degCWhat is the molal concentration of the solution
012 m170
A solution consists of 103 g of the nonelectrolyte glucose C6H12O6 dissolved in 250 g of water What is the freezing-point depression of the solution
minus0426degC171
In a laboratory experiment the freezing point of an aqueous solution of glucose is found to be minus0325degCWhat is the molal concentration of this solution
0175 m172
If 0500 mol of a nonelectrolyte solute are dissolved in 5000 g of ether what is the freezing point of the solution
minus1181degC173
The freezing point of an aqueous solution that contains a nonelectrolyte is minus90degC aWhat is the freezing-point depression of the solution bWhat is the molal concentration of the solution a minus90degC b 48 m
174
Boiling-Point Elevation
Boiling point is temp at which vapor pressure is equal to atmospheric pressure Change in vapor pressure will cause corresponding change in boiling point
175
176
Vapor pressure of nonvolatile solution is lower than vapor pressure of pure solvent More heat will be required to raise vapor pressure of solution Boiling point of solution is higher than boiling point of pure solvent
Molal boiling-point constant (Kb) - boiling-point elevation of the solvent in a 1-molal solution of a nonvolatile nonelectrolyte solute b-p elevation of any nonelectrolyte in water found by experiment to be 051 So molal b-p constant for water is 051m
177
For different solvents b-p elevations of 1-molal solutions have different values (Table 14-2)
Like freezing-point constants values most accurate for dilute solutions
178
Boiling-point elevation Δtb difference between the boiling points of the pure solvent and a nonelectrolyte solution of that solvent and it is directly proportional to the molal concentration of the solution B-p elevation can be calculated using Δtb = Kbm Δtb (boiling point elevation) expressed in m and m expressed in mol solutekg solvent
Sample Problem
What is the boiling-point elevation of a solution made from 200 g of a nonelectrolyte solute and 4000 g of water The molar mass of the solute is 620 gmol
180
1 AnalyzeGiven solute mass = 200 g solute molar mass = 620 gmol solvent mass and identity = 4000 g of water
Unknown boiling-point elevation
181
2 Plan
182
183
3 Compute
Practice Problem
A solution contains 500 g of sucrose C12H22O11 a nonelectrolyte dissolved in 5000 g of water What is the boiling-point elevation
015degC184
A solution contains 4500 g of sucrose C12H22O11 a nonelectrolyte dissolved in 250 g of waterWhat is the boiling point of the solution
1027degC185
If the boiling point elevation of an aqueous solution containing a nonvolatile electrolyte is 102degC what is the molality of the solution
20 m186
The boiling point of an aqueous solution containing a nonvolatile electrolyte is 10075degC aWhat is the boiling-point elevation bWhat is the molality of the solution a 075degC b 15 m
187
Electrolytes and Colligative PropertiesEarly investigators confused by experiments where certain substances depressed freezing point or elevated boiling point more than expected
188
189
Ex 01 m solution of NaCl lowers freezing point of solvent almost twice as much as 01 m solution of sucrose 01 m CaCl2 lowers fp almost 3 times as much as 01 m solution of sucrose
To understand why this happens contrast behavior or sucrose with NaCl in aqueous solution Each sucrose dissolves to make only 1 particle in solution So 1 mol sucrose dissolves to 1 mol particles 1 mol NaCl dissolves to make 2 moles of particles (1 mol Na+ and 1 mol Cl-)
190
191
Calculated Values for Electrolyte SolutionsColligative properties depend on total concentration of solute particles regardless of their identity Electrolytes cause changes in colligative properties proportional to total molality in terms of all dissolves particles instead of formula units
192
What about barium nitrate Ba(NO3)2 Each mole of barium nitrate yields 3 mol of ions in solution Ba(NO3)2(s) rarr Ba2+(aq) + 2NO3-(aq) You would expect solution of given molality to lower fp of solvent 3 times as much as nonelectrolytic solution of same molality
193
Sample Problem
What is the expected change in the freezing point of water in a solution of 625 g of barium nitrate Ba(NO3)2 in 100 kg of water
194
1 AnalyzeGiven solute mass and formula = 625 g Ba(NO3)2 solvent mass and identity = 100 kg water Δtf = Kfm
Unknown expected freezing-point depression
195
2 PlanThe molality can be calculated by converting the solute mass to moles and then dividing by the number of kilograms of solvent That molality is in terms of formula units of Ba(NO3)2 and must be converted to molality in terms of dissociated ions in solution It must be multiplied by the number of moles of ions produced per mole of formula unit This adjusted molality can then be used to calculate the freezing-point depression
196
197
198
3 Compute
Practice Problem
What is the expected freezing-point depression for a solution that contains 20 mol of magnesium sulfate dissolved in 10 kg of water
minus74degC199
What is the expected boiling-point elevation of water for a solution that contains 150 g of sodium chloride dissolved in 10 kg of water 27degC The freezing point of an aqueous sodium chloride solution is minus200degC What is the molality of the solution 54 m NaCl
200
Actual Values for Electrolyte SolutionsRemember values calculated are only expected values Actual values of colligative properties for all strong electrolytes are almost what would be expected Difference between expected and actual values caused by attractive forces that exist between dissociated ions in aqueous solution
201
Attraction between hydrated ions in solution is small compared with those in crystalline solid However forces of attraction donrsquot interfere with movements of aqueous ions More concentrated solution means ions closer together and attraction is greater Also depends on charge of ions ndash higher charge greater attraction
202
2 Plan
The coefficients in the balanced dissociation equation will tell mole relationships You can use the equation to find out the number of moles of solute ions produced
101
3 Compute
a
b102
Write the equation for the dissolution of each of the following in water and then determine the number of moles of each ions made as well as the total number of ions made a 1 mol ammonium chloride b 1 mol sodium sulfide c 05 mol barium nitrate
103
a 1 mol ammonium chloride
1 mol NH4+ 1 mol Cl- 2 mol total ions
104
b 1 mol sodium sulfide
2 mol Na+ 1 mol S-2 3 mol total ions
105
c 05 mol barium nitrate
05 mol Ba+2 1 mol NO3- 15 mol total ions
106
Precipitation Reactions
Even though no compound is completely insoluble compounds of very low solubility can be considered insoluble for practical purposes There are general guidelines to help predict whether a compound made of certain combination of ions is soluble
107
General Solubility Guidelines
Sodium potassium and ammonium compounds are soluble in water
108
Nitrates acetates and chlorates are soluble
109
110
Most chlorides are soluble except those of silver mercury (I) and lead Lead (II) chloride is soluble in hot water
Most sulfates are soluble except those of barium strontium lead calcium and mercury
111
Most carbonates phosphates and silicates are insoluble except those of sodium potassium and ammonium
112
Most sulfides are insoluble except those of calcium strontium sodium potassium and ammonium
113
Is calcium phosphate Ca3(PO4)2 soluble or insoluble
Not one of the exceptions so it is insoluble
Dissociation equations are not written for insoluble compounds
114
Guidelines also useful in predicting what will happen if 2 solutions of 2 different soluble compounds are mixed
If mixing results in combination of ions that forms insoluble compound a double-replacement and precipitation reaction will happen
115
Will precipitate form when solutions of ammonium sulfide and cadmium nitrate are combined You can tell calcium nitrate is soluble from the guidelines (Most nitrates acetates and chlorates are soluble) You can also tell ammonium sulfide is soluble (Most sulfides are insoluble except those of calcium strontium sodium potassium and ammonium)
116
117
To decide whether precipitate can form must know solubilities of two possible products
Ammonium nitrate ndash soluble (Most nitrates acetates and chlorates are soluble) Cadmium sulfide ndash insoluble (Most sulfides are insoluble except those of calcium strontium sodium potassium and ammonium)
118
bc one product is insoluble double-replacement and precipitation reaction will happen (NH4)2S (aq) + Cd(NO3)2 (aq) rarr NH4NO3 (aq) + CdS (s)
119
Net Ionic Equations
Reactions of ions in aqueous solution usually represented by net ionic equations instead of formula equations Net ionic equation - includes only those compounds and ions that undergo a chemical change in a reaction in an aqueous solution
120
121
To write NIE (net ionic equation) first convert chemical equation into overall ionic equation All soluble ionic compounds shown as dissociated ions in solution Precipitates shown as solids
122
Notice ammonium and nitrate ions appear on both sides They havenrsquot gone through any chemical change Spectator ions - ions that do not take part in a chemical reaction and are found in solution both before and after the reaction
123
To change ionic equation to NIE eliminate spectator ions Cd+2(aq) + S-2(aq) rarr CdS(s)
This NIE applies to any reaction in which precipitate of CdS forms
124
Practice Problem
Identify the precipitate that forms when aqueous solutions of zinc nitrate and ammonium sulfide are combined Write the equation for the possible double-replacement reaction Then write the formula equation overall ionic equation and the net ionic equation
125
1 AnalyzeGiven Identity of reactants zinc nitrate and ammonium sulfide Reaction medium aqueous solution Unknown a equation for the possible double-replacement reaction b identity of the precipitate c formula equation d overall ionic equation e net ionic equation
126
2 PlanWrite the possible double-replacement reaction between Zn(NO3)2 and (NH4)2S Use the rules to determine if any of the products will precipitate Write a formula equation and overall net ionic equation Then cancel spectator ions to make net ionic equation
127
3 Computea possible double-replacement reaction
b rules show that zinc sulfide is not soluble so will precipitate (ammonium nitrate is soluble)
c formula equation128
d overall ionic equation
e net ionic equation129
Will a precipitate form if solutions of potassium sulfate and barium nitrate are combined If so write the net ionic equation for the reaction
Yes Ba+2(aq) + SO4-2(aq) rarr BaSO4(s)
130
Will a precipitate form if solutions of potassium nitrate and magnesium sulfate are combined If so write the net ionic equation for the reaction
No 131
Will a precipitate form if solutions of barium chloride and sodium sulfate are combined If so identify the spectator ions and write the net ionic equation for the reaction
Yes Na+ and Cl- Ba+2(aq) + SO4-2(aq) rarr BaSO4(s)
132
Write the net ionic equation for the precipitation of nickel(II) sulfide
Ni+2(aq) + S-2(aq) rarr NiS(s)
133
IonizationSome molecular compounds can also form ions in solution Usually polar Ionization - ions formed from solute molecules by action of the solvent (creation of ions where there were none) Ionization different from dissociation
134
Dissociation ionic compounds dissolve and ions already present separate Ionization ions formed where non existed before
Ions formed are hydrated Heat released during hydration of ions gives enough energy to break covalent bonds
135
Extent to which solute ionizes depends on strength of bonds within molecules of solute and strength of attraction between solute and solvent molecules
136
HCl ndash molecular compound that ionizes in aqueous solution Contains highly polar bond Attraction between polar HCl molecule and polar water strong enough to break HCl bond
137
The Hydronium Ion
H+ ions from HCl attracts other molecules or ions so strongly that it doesnrsquot normally exist alone Ionization of HCl better described as direct transfer of proton from HCl to H2O forming H3O+
138
139
Hydration of H+ to form hydronium is highly exothermic
Energy released gives a lot of energy needed to ionize a molecular solute
Many molecular compounds that ionize in aqueous solution contain hydrogen and form hydronium ion
140
Strong and Weak Electrolytes
Substances that make ions and conduct current in solution are electrolytes HCl is one of series of compounds that has hydrogen and halogen Hydrogen halides are all molecular compounds with single polar-covalent bonds
141
All are gases Very soluble in water All are electrolytes HCl HBr HI strongly conduct current in solution HF only weakly conducts current at same concentration Strength of conduction related to ability to ionize
142
143
Strong Electrolytes
HCl HBr HI are 100 ionized in solution
Strong electrolyte - any compound whose dilute aqueous solutions conduct electricity well this is due to the presence of all or almost all of the dissolve compound in the form of ions
144
Unique characteristic of strong electrolyte is that they yield only ions
Ex Ionic compound may be highly soluble in water and dissociate into ions (NaCl)
Others may not dissolve much but amount that does dissolve exists only as ions
145
Weak ElectrolytesHF dissolves in water to give acid solution called hydrofluoric acid HF bond stronger than bonds between hydrogen and other halogens When HF dissolves some molecule ionize Reverse reaction also happens HF(aq) + H2O(l) H3O+(aq) + F-(aq)
146
HF(aq) + H2O(l) H3O+
(aq) + F-(aq)Concentration of dissolved unionized HF stays high and concentration of ions stays low
Weak electrolyte - any compound whose aqueous solutions conduct electricity poorly this is due to the presence of small amount of the dissolved compound in the form of ions
147
Different from nonelectrolyte ndash NO ions at all Description of electrolyte as strong or weak not related to concentration of solution Electrolytes differ in degree of ionization not amount of solute dissolved
148
149
Colligative properties of solutions
Presence of solute affects properties of solutions Some properties not dependent on nature of dissolved substance but on how may dissolve particles are present
Colligative properties - properties that depend on the concentration of solute particles but not on their identity
150
Vapor-Pressure Lowering
Boiling and freezing point of solution different from pure solvent Nonvolatile solute raises boiling point and lowers freezing point Nonvolatile substance - one that has little tendency to become gas under existing conditions
151
152
To understand why nonvolatile solute changes boiling and freezing point consider equilibrium vapor pressure
Vapor pressure pressure caused by molecules that have escape liquid phase to gas phase
Can be thought of a measure of tendency of molecules to escape from a liquid
153
Addition of sucrose (nonvolatile solute) lowers concentration of water molecules at surface of liquid This lowers tendency of water molecules to leave solution and enter gas phase Vapor pressure of solution is lower than vapor pressure of pure water
154
Aqueous13 solution13 of13 nonvolatile13 solutePure13 water
Nonelectrolyte solutions of same molality have same concentration of particles
Dilute solutions of same solvent and equal molality of any nonelectrolyte solute lower vapor pressure equally
155
Example
1 m aqueous solution of nonelectrolyte glucose lowers vapor pressure of water 55 x 10-4 atm at 25 1 m aqueous solution of sucrose also lowers water vapor pressure to 55 x 10-4 atm at 25
156
157
bc vapor-pressure lowering depends on concentration of nonelectrolyte solute and doesnrsquot depend on the type of solute it is a colligative proprerty
bc vapor pressure has been lowered solution remains liquid over larger temperature range
Lowers freezing point and raises boiling point
158
Can assume changes in boiling and freezing point also depend on concentration of solute
They are colligative properties
159
Freezing-Point Depression
Freezing point of 1-molal solution of any nonelectrolyte solute in water is found (by experiment) to be 186 lower than freezing point of water When 1 mol of nonelectrolyte solute dissolved in 1 kg water freezing point is -186 not 000 160
When 2 mol nonelectrolyte solute dissolved in 1 kg water freezing point is -372
For any concentration of nonelectrolyte solute in water decrease in freezing point can be determined using value of -186m
161
This value called the molal freezing-point contstant (Kf) is the freezing-point depression of the solvent in a 1-molal solution of a nonvolatile nonelectrolyte solute
162
Each solvent has own characteristic molal freezing-point constant Freezing-point depression (Δtf) the difference between the freezing points of the pure solvent and a solute of nonelectrolyte in that solvent and it is directly proportional to the molal concentration of the solution Δtf = Kfm
163
Δtf = Kfm
Kf expressed as m
m expressed in mol solutekg solvent (molality)
164
Sample Problem
What is the freezing point depression of water in a solution of 171 g of sucrose C12H22O11 and 200 g of water What is the actual freezing point of the solution
165
1 AnalyzeGiven Solute mass and chemical formula 171 g C12H22O11 Solvent mass and identity 2000 g water Unknown a freezing-point depression b freezing point of the solution 166
2 Plan
167
168
Calculate
Unknown a freezing-point depression b freezing point of the solution
169
Practice Problem
A water solution containing an unknown quantity of a nonelectrolyte solute is found to have a freezing point of ndash 023degCWhat is the molal concentration of the solution
012 m170
A solution consists of 103 g of the nonelectrolyte glucose C6H12O6 dissolved in 250 g of water What is the freezing-point depression of the solution
minus0426degC171
In a laboratory experiment the freezing point of an aqueous solution of glucose is found to be minus0325degCWhat is the molal concentration of this solution
0175 m172
If 0500 mol of a nonelectrolyte solute are dissolved in 5000 g of ether what is the freezing point of the solution
minus1181degC173
The freezing point of an aqueous solution that contains a nonelectrolyte is minus90degC aWhat is the freezing-point depression of the solution bWhat is the molal concentration of the solution a minus90degC b 48 m
174
Boiling-Point Elevation
Boiling point is temp at which vapor pressure is equal to atmospheric pressure Change in vapor pressure will cause corresponding change in boiling point
175
176
Vapor pressure of nonvolatile solution is lower than vapor pressure of pure solvent More heat will be required to raise vapor pressure of solution Boiling point of solution is higher than boiling point of pure solvent
Molal boiling-point constant (Kb) - boiling-point elevation of the solvent in a 1-molal solution of a nonvolatile nonelectrolyte solute b-p elevation of any nonelectrolyte in water found by experiment to be 051 So molal b-p constant for water is 051m
177
For different solvents b-p elevations of 1-molal solutions have different values (Table 14-2)
Like freezing-point constants values most accurate for dilute solutions
178
Boiling-point elevation Δtb difference between the boiling points of the pure solvent and a nonelectrolyte solution of that solvent and it is directly proportional to the molal concentration of the solution B-p elevation can be calculated using Δtb = Kbm Δtb (boiling point elevation) expressed in m and m expressed in mol solutekg solvent
Sample Problem
What is the boiling-point elevation of a solution made from 200 g of a nonelectrolyte solute and 4000 g of water The molar mass of the solute is 620 gmol
180
1 AnalyzeGiven solute mass = 200 g solute molar mass = 620 gmol solvent mass and identity = 4000 g of water
Unknown boiling-point elevation
181
2 Plan
182
183
3 Compute
Practice Problem
A solution contains 500 g of sucrose C12H22O11 a nonelectrolyte dissolved in 5000 g of water What is the boiling-point elevation
015degC184
A solution contains 4500 g of sucrose C12H22O11 a nonelectrolyte dissolved in 250 g of waterWhat is the boiling point of the solution
1027degC185
If the boiling point elevation of an aqueous solution containing a nonvolatile electrolyte is 102degC what is the molality of the solution
20 m186
The boiling point of an aqueous solution containing a nonvolatile electrolyte is 10075degC aWhat is the boiling-point elevation bWhat is the molality of the solution a 075degC b 15 m
187
Electrolytes and Colligative PropertiesEarly investigators confused by experiments where certain substances depressed freezing point or elevated boiling point more than expected
188
189
Ex 01 m solution of NaCl lowers freezing point of solvent almost twice as much as 01 m solution of sucrose 01 m CaCl2 lowers fp almost 3 times as much as 01 m solution of sucrose
To understand why this happens contrast behavior or sucrose with NaCl in aqueous solution Each sucrose dissolves to make only 1 particle in solution So 1 mol sucrose dissolves to 1 mol particles 1 mol NaCl dissolves to make 2 moles of particles (1 mol Na+ and 1 mol Cl-)
190
191
Calculated Values for Electrolyte SolutionsColligative properties depend on total concentration of solute particles regardless of their identity Electrolytes cause changes in colligative properties proportional to total molality in terms of all dissolves particles instead of formula units
192
What about barium nitrate Ba(NO3)2 Each mole of barium nitrate yields 3 mol of ions in solution Ba(NO3)2(s) rarr Ba2+(aq) + 2NO3-(aq) You would expect solution of given molality to lower fp of solvent 3 times as much as nonelectrolytic solution of same molality
193
Sample Problem
What is the expected change in the freezing point of water in a solution of 625 g of barium nitrate Ba(NO3)2 in 100 kg of water
194
1 AnalyzeGiven solute mass and formula = 625 g Ba(NO3)2 solvent mass and identity = 100 kg water Δtf = Kfm
Unknown expected freezing-point depression
195
2 PlanThe molality can be calculated by converting the solute mass to moles and then dividing by the number of kilograms of solvent That molality is in terms of formula units of Ba(NO3)2 and must be converted to molality in terms of dissociated ions in solution It must be multiplied by the number of moles of ions produced per mole of formula unit This adjusted molality can then be used to calculate the freezing-point depression
196
197
198
3 Compute
Practice Problem
What is the expected freezing-point depression for a solution that contains 20 mol of magnesium sulfate dissolved in 10 kg of water
minus74degC199
What is the expected boiling-point elevation of water for a solution that contains 150 g of sodium chloride dissolved in 10 kg of water 27degC The freezing point of an aqueous sodium chloride solution is minus200degC What is the molality of the solution 54 m NaCl
200
Actual Values for Electrolyte SolutionsRemember values calculated are only expected values Actual values of colligative properties for all strong electrolytes are almost what would be expected Difference between expected and actual values caused by attractive forces that exist between dissociated ions in aqueous solution
201
Attraction between hydrated ions in solution is small compared with those in crystalline solid However forces of attraction donrsquot interfere with movements of aqueous ions More concentrated solution means ions closer together and attraction is greater Also depends on charge of ions ndash higher charge greater attraction
202
3 Compute
a
b102
Write the equation for the dissolution of each of the following in water and then determine the number of moles of each ions made as well as the total number of ions made a 1 mol ammonium chloride b 1 mol sodium sulfide c 05 mol barium nitrate
103
a 1 mol ammonium chloride
1 mol NH4+ 1 mol Cl- 2 mol total ions
104
b 1 mol sodium sulfide
2 mol Na+ 1 mol S-2 3 mol total ions
105
c 05 mol barium nitrate
05 mol Ba+2 1 mol NO3- 15 mol total ions
106
Precipitation Reactions
Even though no compound is completely insoluble compounds of very low solubility can be considered insoluble for practical purposes There are general guidelines to help predict whether a compound made of certain combination of ions is soluble
107
General Solubility Guidelines
Sodium potassium and ammonium compounds are soluble in water
108
Nitrates acetates and chlorates are soluble
109
110
Most chlorides are soluble except those of silver mercury (I) and lead Lead (II) chloride is soluble in hot water
Most sulfates are soluble except those of barium strontium lead calcium and mercury
111
Most carbonates phosphates and silicates are insoluble except those of sodium potassium and ammonium
112
Most sulfides are insoluble except those of calcium strontium sodium potassium and ammonium
113
Is calcium phosphate Ca3(PO4)2 soluble or insoluble
Not one of the exceptions so it is insoluble
Dissociation equations are not written for insoluble compounds
114
Guidelines also useful in predicting what will happen if 2 solutions of 2 different soluble compounds are mixed
If mixing results in combination of ions that forms insoluble compound a double-replacement and precipitation reaction will happen
115
Will precipitate form when solutions of ammonium sulfide and cadmium nitrate are combined You can tell calcium nitrate is soluble from the guidelines (Most nitrates acetates and chlorates are soluble) You can also tell ammonium sulfide is soluble (Most sulfides are insoluble except those of calcium strontium sodium potassium and ammonium)
116
117
To decide whether precipitate can form must know solubilities of two possible products
Ammonium nitrate ndash soluble (Most nitrates acetates and chlorates are soluble) Cadmium sulfide ndash insoluble (Most sulfides are insoluble except those of calcium strontium sodium potassium and ammonium)
118
bc one product is insoluble double-replacement and precipitation reaction will happen (NH4)2S (aq) + Cd(NO3)2 (aq) rarr NH4NO3 (aq) + CdS (s)
119
Net Ionic Equations
Reactions of ions in aqueous solution usually represented by net ionic equations instead of formula equations Net ionic equation - includes only those compounds and ions that undergo a chemical change in a reaction in an aqueous solution
120
121
To write NIE (net ionic equation) first convert chemical equation into overall ionic equation All soluble ionic compounds shown as dissociated ions in solution Precipitates shown as solids
122
Notice ammonium and nitrate ions appear on both sides They havenrsquot gone through any chemical change Spectator ions - ions that do not take part in a chemical reaction and are found in solution both before and after the reaction
123
To change ionic equation to NIE eliminate spectator ions Cd+2(aq) + S-2(aq) rarr CdS(s)
This NIE applies to any reaction in which precipitate of CdS forms
124
Practice Problem
Identify the precipitate that forms when aqueous solutions of zinc nitrate and ammonium sulfide are combined Write the equation for the possible double-replacement reaction Then write the formula equation overall ionic equation and the net ionic equation
125
1 AnalyzeGiven Identity of reactants zinc nitrate and ammonium sulfide Reaction medium aqueous solution Unknown a equation for the possible double-replacement reaction b identity of the precipitate c formula equation d overall ionic equation e net ionic equation
126
2 PlanWrite the possible double-replacement reaction between Zn(NO3)2 and (NH4)2S Use the rules to determine if any of the products will precipitate Write a formula equation and overall net ionic equation Then cancel spectator ions to make net ionic equation
127
3 Computea possible double-replacement reaction
b rules show that zinc sulfide is not soluble so will precipitate (ammonium nitrate is soluble)
c formula equation128
d overall ionic equation
e net ionic equation129
Will a precipitate form if solutions of potassium sulfate and barium nitrate are combined If so write the net ionic equation for the reaction
Yes Ba+2(aq) + SO4-2(aq) rarr BaSO4(s)
130
Will a precipitate form if solutions of potassium nitrate and magnesium sulfate are combined If so write the net ionic equation for the reaction
No 131
Will a precipitate form if solutions of barium chloride and sodium sulfate are combined If so identify the spectator ions and write the net ionic equation for the reaction
Yes Na+ and Cl- Ba+2(aq) + SO4-2(aq) rarr BaSO4(s)
132
Write the net ionic equation for the precipitation of nickel(II) sulfide
Ni+2(aq) + S-2(aq) rarr NiS(s)
133
IonizationSome molecular compounds can also form ions in solution Usually polar Ionization - ions formed from solute molecules by action of the solvent (creation of ions where there were none) Ionization different from dissociation
134
Dissociation ionic compounds dissolve and ions already present separate Ionization ions formed where non existed before
Ions formed are hydrated Heat released during hydration of ions gives enough energy to break covalent bonds
135
Extent to which solute ionizes depends on strength of bonds within molecules of solute and strength of attraction between solute and solvent molecules
136
HCl ndash molecular compound that ionizes in aqueous solution Contains highly polar bond Attraction between polar HCl molecule and polar water strong enough to break HCl bond
137
The Hydronium Ion
H+ ions from HCl attracts other molecules or ions so strongly that it doesnrsquot normally exist alone Ionization of HCl better described as direct transfer of proton from HCl to H2O forming H3O+
138
139
Hydration of H+ to form hydronium is highly exothermic
Energy released gives a lot of energy needed to ionize a molecular solute
Many molecular compounds that ionize in aqueous solution contain hydrogen and form hydronium ion
140
Strong and Weak Electrolytes
Substances that make ions and conduct current in solution are electrolytes HCl is one of series of compounds that has hydrogen and halogen Hydrogen halides are all molecular compounds with single polar-covalent bonds
141
All are gases Very soluble in water All are electrolytes HCl HBr HI strongly conduct current in solution HF only weakly conducts current at same concentration Strength of conduction related to ability to ionize
142
143
Strong Electrolytes
HCl HBr HI are 100 ionized in solution
Strong electrolyte - any compound whose dilute aqueous solutions conduct electricity well this is due to the presence of all or almost all of the dissolve compound in the form of ions
144
Unique characteristic of strong electrolyte is that they yield only ions
Ex Ionic compound may be highly soluble in water and dissociate into ions (NaCl)
Others may not dissolve much but amount that does dissolve exists only as ions
145
Weak ElectrolytesHF dissolves in water to give acid solution called hydrofluoric acid HF bond stronger than bonds between hydrogen and other halogens When HF dissolves some molecule ionize Reverse reaction also happens HF(aq) + H2O(l) H3O+(aq) + F-(aq)
146
HF(aq) + H2O(l) H3O+
(aq) + F-(aq)Concentration of dissolved unionized HF stays high and concentration of ions stays low
Weak electrolyte - any compound whose aqueous solutions conduct electricity poorly this is due to the presence of small amount of the dissolved compound in the form of ions
147
Different from nonelectrolyte ndash NO ions at all Description of electrolyte as strong or weak not related to concentration of solution Electrolytes differ in degree of ionization not amount of solute dissolved
148
149
Colligative properties of solutions
Presence of solute affects properties of solutions Some properties not dependent on nature of dissolved substance but on how may dissolve particles are present
Colligative properties - properties that depend on the concentration of solute particles but not on their identity
150
Vapor-Pressure Lowering
Boiling and freezing point of solution different from pure solvent Nonvolatile solute raises boiling point and lowers freezing point Nonvolatile substance - one that has little tendency to become gas under existing conditions
151
152
To understand why nonvolatile solute changes boiling and freezing point consider equilibrium vapor pressure
Vapor pressure pressure caused by molecules that have escape liquid phase to gas phase
Can be thought of a measure of tendency of molecules to escape from a liquid
153
Addition of sucrose (nonvolatile solute) lowers concentration of water molecules at surface of liquid This lowers tendency of water molecules to leave solution and enter gas phase Vapor pressure of solution is lower than vapor pressure of pure water
154
Aqueous13 solution13 of13 nonvolatile13 solutePure13 water
Nonelectrolyte solutions of same molality have same concentration of particles
Dilute solutions of same solvent and equal molality of any nonelectrolyte solute lower vapor pressure equally
155
Example
1 m aqueous solution of nonelectrolyte glucose lowers vapor pressure of water 55 x 10-4 atm at 25 1 m aqueous solution of sucrose also lowers water vapor pressure to 55 x 10-4 atm at 25
156
157
bc vapor-pressure lowering depends on concentration of nonelectrolyte solute and doesnrsquot depend on the type of solute it is a colligative proprerty
bc vapor pressure has been lowered solution remains liquid over larger temperature range
Lowers freezing point and raises boiling point
158
Can assume changes in boiling and freezing point also depend on concentration of solute
They are colligative properties
159
Freezing-Point Depression
Freezing point of 1-molal solution of any nonelectrolyte solute in water is found (by experiment) to be 186 lower than freezing point of water When 1 mol of nonelectrolyte solute dissolved in 1 kg water freezing point is -186 not 000 160
When 2 mol nonelectrolyte solute dissolved in 1 kg water freezing point is -372
For any concentration of nonelectrolyte solute in water decrease in freezing point can be determined using value of -186m
161
This value called the molal freezing-point contstant (Kf) is the freezing-point depression of the solvent in a 1-molal solution of a nonvolatile nonelectrolyte solute
162
Each solvent has own characteristic molal freezing-point constant Freezing-point depression (Δtf) the difference between the freezing points of the pure solvent and a solute of nonelectrolyte in that solvent and it is directly proportional to the molal concentration of the solution Δtf = Kfm
163
Δtf = Kfm
Kf expressed as m
m expressed in mol solutekg solvent (molality)
164
Sample Problem
What is the freezing point depression of water in a solution of 171 g of sucrose C12H22O11 and 200 g of water What is the actual freezing point of the solution
165
1 AnalyzeGiven Solute mass and chemical formula 171 g C12H22O11 Solvent mass and identity 2000 g water Unknown a freezing-point depression b freezing point of the solution 166
2 Plan
167
168
Calculate
Unknown a freezing-point depression b freezing point of the solution
169
Practice Problem
A water solution containing an unknown quantity of a nonelectrolyte solute is found to have a freezing point of ndash 023degCWhat is the molal concentration of the solution
012 m170
A solution consists of 103 g of the nonelectrolyte glucose C6H12O6 dissolved in 250 g of water What is the freezing-point depression of the solution
minus0426degC171
In a laboratory experiment the freezing point of an aqueous solution of glucose is found to be minus0325degCWhat is the molal concentration of this solution
0175 m172
If 0500 mol of a nonelectrolyte solute are dissolved in 5000 g of ether what is the freezing point of the solution
minus1181degC173
The freezing point of an aqueous solution that contains a nonelectrolyte is minus90degC aWhat is the freezing-point depression of the solution bWhat is the molal concentration of the solution a minus90degC b 48 m
174
Boiling-Point Elevation
Boiling point is temp at which vapor pressure is equal to atmospheric pressure Change in vapor pressure will cause corresponding change in boiling point
175
176
Vapor pressure of nonvolatile solution is lower than vapor pressure of pure solvent More heat will be required to raise vapor pressure of solution Boiling point of solution is higher than boiling point of pure solvent
Molal boiling-point constant (Kb) - boiling-point elevation of the solvent in a 1-molal solution of a nonvolatile nonelectrolyte solute b-p elevation of any nonelectrolyte in water found by experiment to be 051 So molal b-p constant for water is 051m
177
For different solvents b-p elevations of 1-molal solutions have different values (Table 14-2)
Like freezing-point constants values most accurate for dilute solutions
178
Boiling-point elevation Δtb difference between the boiling points of the pure solvent and a nonelectrolyte solution of that solvent and it is directly proportional to the molal concentration of the solution B-p elevation can be calculated using Δtb = Kbm Δtb (boiling point elevation) expressed in m and m expressed in mol solutekg solvent
Sample Problem
What is the boiling-point elevation of a solution made from 200 g of a nonelectrolyte solute and 4000 g of water The molar mass of the solute is 620 gmol
180
1 AnalyzeGiven solute mass = 200 g solute molar mass = 620 gmol solvent mass and identity = 4000 g of water
Unknown boiling-point elevation
181
2 Plan
182
183
3 Compute
Practice Problem
A solution contains 500 g of sucrose C12H22O11 a nonelectrolyte dissolved in 5000 g of water What is the boiling-point elevation
015degC184
A solution contains 4500 g of sucrose C12H22O11 a nonelectrolyte dissolved in 250 g of waterWhat is the boiling point of the solution
1027degC185
If the boiling point elevation of an aqueous solution containing a nonvolatile electrolyte is 102degC what is the molality of the solution
20 m186
The boiling point of an aqueous solution containing a nonvolatile electrolyte is 10075degC aWhat is the boiling-point elevation bWhat is the molality of the solution a 075degC b 15 m
187
Electrolytes and Colligative PropertiesEarly investigators confused by experiments where certain substances depressed freezing point or elevated boiling point more than expected
188
189
Ex 01 m solution of NaCl lowers freezing point of solvent almost twice as much as 01 m solution of sucrose 01 m CaCl2 lowers fp almost 3 times as much as 01 m solution of sucrose
To understand why this happens contrast behavior or sucrose with NaCl in aqueous solution Each sucrose dissolves to make only 1 particle in solution So 1 mol sucrose dissolves to 1 mol particles 1 mol NaCl dissolves to make 2 moles of particles (1 mol Na+ and 1 mol Cl-)
190
191
Calculated Values for Electrolyte SolutionsColligative properties depend on total concentration of solute particles regardless of their identity Electrolytes cause changes in colligative properties proportional to total molality in terms of all dissolves particles instead of formula units
192
What about barium nitrate Ba(NO3)2 Each mole of barium nitrate yields 3 mol of ions in solution Ba(NO3)2(s) rarr Ba2+(aq) + 2NO3-(aq) You would expect solution of given molality to lower fp of solvent 3 times as much as nonelectrolytic solution of same molality
193
Sample Problem
What is the expected change in the freezing point of water in a solution of 625 g of barium nitrate Ba(NO3)2 in 100 kg of water
194
1 AnalyzeGiven solute mass and formula = 625 g Ba(NO3)2 solvent mass and identity = 100 kg water Δtf = Kfm
Unknown expected freezing-point depression
195
2 PlanThe molality can be calculated by converting the solute mass to moles and then dividing by the number of kilograms of solvent That molality is in terms of formula units of Ba(NO3)2 and must be converted to molality in terms of dissociated ions in solution It must be multiplied by the number of moles of ions produced per mole of formula unit This adjusted molality can then be used to calculate the freezing-point depression
196
197
198
3 Compute
Practice Problem
What is the expected freezing-point depression for a solution that contains 20 mol of magnesium sulfate dissolved in 10 kg of water
minus74degC199
What is the expected boiling-point elevation of water for a solution that contains 150 g of sodium chloride dissolved in 10 kg of water 27degC The freezing point of an aqueous sodium chloride solution is minus200degC What is the molality of the solution 54 m NaCl
200
Actual Values for Electrolyte SolutionsRemember values calculated are only expected values Actual values of colligative properties for all strong electrolytes are almost what would be expected Difference between expected and actual values caused by attractive forces that exist between dissociated ions in aqueous solution
201
Attraction between hydrated ions in solution is small compared with those in crystalline solid However forces of attraction donrsquot interfere with movements of aqueous ions More concentrated solution means ions closer together and attraction is greater Also depends on charge of ions ndash higher charge greater attraction
202
Write the equation for the dissolution of each of the following in water and then determine the number of moles of each ions made as well as the total number of ions made a 1 mol ammonium chloride b 1 mol sodium sulfide c 05 mol barium nitrate
103
a 1 mol ammonium chloride
1 mol NH4+ 1 mol Cl- 2 mol total ions
104
b 1 mol sodium sulfide
2 mol Na+ 1 mol S-2 3 mol total ions
105
c 05 mol barium nitrate
05 mol Ba+2 1 mol NO3- 15 mol total ions
106
Precipitation Reactions
Even though no compound is completely insoluble compounds of very low solubility can be considered insoluble for practical purposes There are general guidelines to help predict whether a compound made of certain combination of ions is soluble
107
General Solubility Guidelines
Sodium potassium and ammonium compounds are soluble in water
108
Nitrates acetates and chlorates are soluble
109
110
Most chlorides are soluble except those of silver mercury (I) and lead Lead (II) chloride is soluble in hot water
Most sulfates are soluble except those of barium strontium lead calcium and mercury
111
Most carbonates phosphates and silicates are insoluble except those of sodium potassium and ammonium
112
Most sulfides are insoluble except those of calcium strontium sodium potassium and ammonium
113
Is calcium phosphate Ca3(PO4)2 soluble or insoluble
Not one of the exceptions so it is insoluble
Dissociation equations are not written for insoluble compounds
114
Guidelines also useful in predicting what will happen if 2 solutions of 2 different soluble compounds are mixed
If mixing results in combination of ions that forms insoluble compound a double-replacement and precipitation reaction will happen
115
Will precipitate form when solutions of ammonium sulfide and cadmium nitrate are combined You can tell calcium nitrate is soluble from the guidelines (Most nitrates acetates and chlorates are soluble) You can also tell ammonium sulfide is soluble (Most sulfides are insoluble except those of calcium strontium sodium potassium and ammonium)
116
117
To decide whether precipitate can form must know solubilities of two possible products
Ammonium nitrate ndash soluble (Most nitrates acetates and chlorates are soluble) Cadmium sulfide ndash insoluble (Most sulfides are insoluble except those of calcium strontium sodium potassium and ammonium)
118
bc one product is insoluble double-replacement and precipitation reaction will happen (NH4)2S (aq) + Cd(NO3)2 (aq) rarr NH4NO3 (aq) + CdS (s)
119
Net Ionic Equations
Reactions of ions in aqueous solution usually represented by net ionic equations instead of formula equations Net ionic equation - includes only those compounds and ions that undergo a chemical change in a reaction in an aqueous solution
120
121
To write NIE (net ionic equation) first convert chemical equation into overall ionic equation All soluble ionic compounds shown as dissociated ions in solution Precipitates shown as solids
122
Notice ammonium and nitrate ions appear on both sides They havenrsquot gone through any chemical change Spectator ions - ions that do not take part in a chemical reaction and are found in solution both before and after the reaction
123
To change ionic equation to NIE eliminate spectator ions Cd+2(aq) + S-2(aq) rarr CdS(s)
This NIE applies to any reaction in which precipitate of CdS forms
124
Practice Problem
Identify the precipitate that forms when aqueous solutions of zinc nitrate and ammonium sulfide are combined Write the equation for the possible double-replacement reaction Then write the formula equation overall ionic equation and the net ionic equation
125
1 AnalyzeGiven Identity of reactants zinc nitrate and ammonium sulfide Reaction medium aqueous solution Unknown a equation for the possible double-replacement reaction b identity of the precipitate c formula equation d overall ionic equation e net ionic equation
126
2 PlanWrite the possible double-replacement reaction between Zn(NO3)2 and (NH4)2S Use the rules to determine if any of the products will precipitate Write a formula equation and overall net ionic equation Then cancel spectator ions to make net ionic equation
127
3 Computea possible double-replacement reaction
b rules show that zinc sulfide is not soluble so will precipitate (ammonium nitrate is soluble)
c formula equation128
d overall ionic equation
e net ionic equation129
Will a precipitate form if solutions of potassium sulfate and barium nitrate are combined If so write the net ionic equation for the reaction
Yes Ba+2(aq) + SO4-2(aq) rarr BaSO4(s)
130
Will a precipitate form if solutions of potassium nitrate and magnesium sulfate are combined If so write the net ionic equation for the reaction
No 131
Will a precipitate form if solutions of barium chloride and sodium sulfate are combined If so identify the spectator ions and write the net ionic equation for the reaction
Yes Na+ and Cl- Ba+2(aq) + SO4-2(aq) rarr BaSO4(s)
132
Write the net ionic equation for the precipitation of nickel(II) sulfide
Ni+2(aq) + S-2(aq) rarr NiS(s)
133
IonizationSome molecular compounds can also form ions in solution Usually polar Ionization - ions formed from solute molecules by action of the solvent (creation of ions where there were none) Ionization different from dissociation
134
Dissociation ionic compounds dissolve and ions already present separate Ionization ions formed where non existed before
Ions formed are hydrated Heat released during hydration of ions gives enough energy to break covalent bonds
135
Extent to which solute ionizes depends on strength of bonds within molecules of solute and strength of attraction between solute and solvent molecules
136
HCl ndash molecular compound that ionizes in aqueous solution Contains highly polar bond Attraction between polar HCl molecule and polar water strong enough to break HCl bond
137
The Hydronium Ion
H+ ions from HCl attracts other molecules or ions so strongly that it doesnrsquot normally exist alone Ionization of HCl better described as direct transfer of proton from HCl to H2O forming H3O+
138
139
Hydration of H+ to form hydronium is highly exothermic
Energy released gives a lot of energy needed to ionize a molecular solute
Many molecular compounds that ionize in aqueous solution contain hydrogen and form hydronium ion
140
Strong and Weak Electrolytes
Substances that make ions and conduct current in solution are electrolytes HCl is one of series of compounds that has hydrogen and halogen Hydrogen halides are all molecular compounds with single polar-covalent bonds
141
All are gases Very soluble in water All are electrolytes HCl HBr HI strongly conduct current in solution HF only weakly conducts current at same concentration Strength of conduction related to ability to ionize
142
143
Strong Electrolytes
HCl HBr HI are 100 ionized in solution
Strong electrolyte - any compound whose dilute aqueous solutions conduct electricity well this is due to the presence of all or almost all of the dissolve compound in the form of ions
144
Unique characteristic of strong electrolyte is that they yield only ions
Ex Ionic compound may be highly soluble in water and dissociate into ions (NaCl)
Others may not dissolve much but amount that does dissolve exists only as ions
145
Weak ElectrolytesHF dissolves in water to give acid solution called hydrofluoric acid HF bond stronger than bonds between hydrogen and other halogens When HF dissolves some molecule ionize Reverse reaction also happens HF(aq) + H2O(l) H3O+(aq) + F-(aq)
146
HF(aq) + H2O(l) H3O+
(aq) + F-(aq)Concentration of dissolved unionized HF stays high and concentration of ions stays low
Weak electrolyte - any compound whose aqueous solutions conduct electricity poorly this is due to the presence of small amount of the dissolved compound in the form of ions
147
Different from nonelectrolyte ndash NO ions at all Description of electrolyte as strong or weak not related to concentration of solution Electrolytes differ in degree of ionization not amount of solute dissolved
148
149
Colligative properties of solutions
Presence of solute affects properties of solutions Some properties not dependent on nature of dissolved substance but on how may dissolve particles are present
Colligative properties - properties that depend on the concentration of solute particles but not on their identity
150
Vapor-Pressure Lowering
Boiling and freezing point of solution different from pure solvent Nonvolatile solute raises boiling point and lowers freezing point Nonvolatile substance - one that has little tendency to become gas under existing conditions
151
152
To understand why nonvolatile solute changes boiling and freezing point consider equilibrium vapor pressure
Vapor pressure pressure caused by molecules that have escape liquid phase to gas phase
Can be thought of a measure of tendency of molecules to escape from a liquid
153
Addition of sucrose (nonvolatile solute) lowers concentration of water molecules at surface of liquid This lowers tendency of water molecules to leave solution and enter gas phase Vapor pressure of solution is lower than vapor pressure of pure water
154
Aqueous13 solution13 of13 nonvolatile13 solutePure13 water
Nonelectrolyte solutions of same molality have same concentration of particles
Dilute solutions of same solvent and equal molality of any nonelectrolyte solute lower vapor pressure equally
155
Example
1 m aqueous solution of nonelectrolyte glucose lowers vapor pressure of water 55 x 10-4 atm at 25 1 m aqueous solution of sucrose also lowers water vapor pressure to 55 x 10-4 atm at 25
156
157
bc vapor-pressure lowering depends on concentration of nonelectrolyte solute and doesnrsquot depend on the type of solute it is a colligative proprerty
bc vapor pressure has been lowered solution remains liquid over larger temperature range
Lowers freezing point and raises boiling point
158
Can assume changes in boiling and freezing point also depend on concentration of solute
They are colligative properties
159
Freezing-Point Depression
Freezing point of 1-molal solution of any nonelectrolyte solute in water is found (by experiment) to be 186 lower than freezing point of water When 1 mol of nonelectrolyte solute dissolved in 1 kg water freezing point is -186 not 000 160
When 2 mol nonelectrolyte solute dissolved in 1 kg water freezing point is -372
For any concentration of nonelectrolyte solute in water decrease in freezing point can be determined using value of -186m
161
This value called the molal freezing-point contstant (Kf) is the freezing-point depression of the solvent in a 1-molal solution of a nonvolatile nonelectrolyte solute
162
Each solvent has own characteristic molal freezing-point constant Freezing-point depression (Δtf) the difference between the freezing points of the pure solvent and a solute of nonelectrolyte in that solvent and it is directly proportional to the molal concentration of the solution Δtf = Kfm
163
Δtf = Kfm
Kf expressed as m
m expressed in mol solutekg solvent (molality)
164
Sample Problem
What is the freezing point depression of water in a solution of 171 g of sucrose C12H22O11 and 200 g of water What is the actual freezing point of the solution
165
1 AnalyzeGiven Solute mass and chemical formula 171 g C12H22O11 Solvent mass and identity 2000 g water Unknown a freezing-point depression b freezing point of the solution 166
2 Plan
167
168
Calculate
Unknown a freezing-point depression b freezing point of the solution
169
Practice Problem
A water solution containing an unknown quantity of a nonelectrolyte solute is found to have a freezing point of ndash 023degCWhat is the molal concentration of the solution
012 m170
A solution consists of 103 g of the nonelectrolyte glucose C6H12O6 dissolved in 250 g of water What is the freezing-point depression of the solution
minus0426degC171
In a laboratory experiment the freezing point of an aqueous solution of glucose is found to be minus0325degCWhat is the molal concentration of this solution
0175 m172
If 0500 mol of a nonelectrolyte solute are dissolved in 5000 g of ether what is the freezing point of the solution
minus1181degC173
The freezing point of an aqueous solution that contains a nonelectrolyte is minus90degC aWhat is the freezing-point depression of the solution bWhat is the molal concentration of the solution a minus90degC b 48 m
174
Boiling-Point Elevation
Boiling point is temp at which vapor pressure is equal to atmospheric pressure Change in vapor pressure will cause corresponding change in boiling point
175
176
Vapor pressure of nonvolatile solution is lower than vapor pressure of pure solvent More heat will be required to raise vapor pressure of solution Boiling point of solution is higher than boiling point of pure solvent
Molal boiling-point constant (Kb) - boiling-point elevation of the solvent in a 1-molal solution of a nonvolatile nonelectrolyte solute b-p elevation of any nonelectrolyte in water found by experiment to be 051 So molal b-p constant for water is 051m
177
For different solvents b-p elevations of 1-molal solutions have different values (Table 14-2)
Like freezing-point constants values most accurate for dilute solutions
178
Boiling-point elevation Δtb difference between the boiling points of the pure solvent and a nonelectrolyte solution of that solvent and it is directly proportional to the molal concentration of the solution B-p elevation can be calculated using Δtb = Kbm Δtb (boiling point elevation) expressed in m and m expressed in mol solutekg solvent
Sample Problem
What is the boiling-point elevation of a solution made from 200 g of a nonelectrolyte solute and 4000 g of water The molar mass of the solute is 620 gmol
180
1 AnalyzeGiven solute mass = 200 g solute molar mass = 620 gmol solvent mass and identity = 4000 g of water
Unknown boiling-point elevation
181
2 Plan
182
183
3 Compute
Practice Problem
A solution contains 500 g of sucrose C12H22O11 a nonelectrolyte dissolved in 5000 g of water What is the boiling-point elevation
015degC184
A solution contains 4500 g of sucrose C12H22O11 a nonelectrolyte dissolved in 250 g of waterWhat is the boiling point of the solution
1027degC185
If the boiling point elevation of an aqueous solution containing a nonvolatile electrolyte is 102degC what is the molality of the solution
20 m186
The boiling point of an aqueous solution containing a nonvolatile electrolyte is 10075degC aWhat is the boiling-point elevation bWhat is the molality of the solution a 075degC b 15 m
187
Electrolytes and Colligative PropertiesEarly investigators confused by experiments where certain substances depressed freezing point or elevated boiling point more than expected
188
189
Ex 01 m solution of NaCl lowers freezing point of solvent almost twice as much as 01 m solution of sucrose 01 m CaCl2 lowers fp almost 3 times as much as 01 m solution of sucrose
To understand why this happens contrast behavior or sucrose with NaCl in aqueous solution Each sucrose dissolves to make only 1 particle in solution So 1 mol sucrose dissolves to 1 mol particles 1 mol NaCl dissolves to make 2 moles of particles (1 mol Na+ and 1 mol Cl-)
190
191
Calculated Values for Electrolyte SolutionsColligative properties depend on total concentration of solute particles regardless of their identity Electrolytes cause changes in colligative properties proportional to total molality in terms of all dissolves particles instead of formula units
192
What about barium nitrate Ba(NO3)2 Each mole of barium nitrate yields 3 mol of ions in solution Ba(NO3)2(s) rarr Ba2+(aq) + 2NO3-(aq) You would expect solution of given molality to lower fp of solvent 3 times as much as nonelectrolytic solution of same molality
193
Sample Problem
What is the expected change in the freezing point of water in a solution of 625 g of barium nitrate Ba(NO3)2 in 100 kg of water
194
1 AnalyzeGiven solute mass and formula = 625 g Ba(NO3)2 solvent mass and identity = 100 kg water Δtf = Kfm
Unknown expected freezing-point depression
195
2 PlanThe molality can be calculated by converting the solute mass to moles and then dividing by the number of kilograms of solvent That molality is in terms of formula units of Ba(NO3)2 and must be converted to molality in terms of dissociated ions in solution It must be multiplied by the number of moles of ions produced per mole of formula unit This adjusted molality can then be used to calculate the freezing-point depression
196
197
198
3 Compute
Practice Problem
What is the expected freezing-point depression for a solution that contains 20 mol of magnesium sulfate dissolved in 10 kg of water
minus74degC199
What is the expected boiling-point elevation of water for a solution that contains 150 g of sodium chloride dissolved in 10 kg of water 27degC The freezing point of an aqueous sodium chloride solution is minus200degC What is the molality of the solution 54 m NaCl
200
Actual Values for Electrolyte SolutionsRemember values calculated are only expected values Actual values of colligative properties for all strong electrolytes are almost what would be expected Difference between expected and actual values caused by attractive forces that exist between dissociated ions in aqueous solution
201
Attraction between hydrated ions in solution is small compared with those in crystalline solid However forces of attraction donrsquot interfere with movements of aqueous ions More concentrated solution means ions closer together and attraction is greater Also depends on charge of ions ndash higher charge greater attraction
202
a 1 mol ammonium chloride
1 mol NH4+ 1 mol Cl- 2 mol total ions
104
b 1 mol sodium sulfide
2 mol Na+ 1 mol S-2 3 mol total ions
105
c 05 mol barium nitrate
05 mol Ba+2 1 mol NO3- 15 mol total ions
106
Precipitation Reactions
Even though no compound is completely insoluble compounds of very low solubility can be considered insoluble for practical purposes There are general guidelines to help predict whether a compound made of certain combination of ions is soluble
107
General Solubility Guidelines
Sodium potassium and ammonium compounds are soluble in water
108
Nitrates acetates and chlorates are soluble
109
110
Most chlorides are soluble except those of silver mercury (I) and lead Lead (II) chloride is soluble in hot water
Most sulfates are soluble except those of barium strontium lead calcium and mercury
111
Most carbonates phosphates and silicates are insoluble except those of sodium potassium and ammonium
112
Most sulfides are insoluble except those of calcium strontium sodium potassium and ammonium
113
Is calcium phosphate Ca3(PO4)2 soluble or insoluble
Not one of the exceptions so it is insoluble
Dissociation equations are not written for insoluble compounds
114
Guidelines also useful in predicting what will happen if 2 solutions of 2 different soluble compounds are mixed
If mixing results in combination of ions that forms insoluble compound a double-replacement and precipitation reaction will happen
115
Will precipitate form when solutions of ammonium sulfide and cadmium nitrate are combined You can tell calcium nitrate is soluble from the guidelines (Most nitrates acetates and chlorates are soluble) You can also tell ammonium sulfide is soluble (Most sulfides are insoluble except those of calcium strontium sodium potassium and ammonium)
116
117
To decide whether precipitate can form must know solubilities of two possible products
Ammonium nitrate ndash soluble (Most nitrates acetates and chlorates are soluble) Cadmium sulfide ndash insoluble (Most sulfides are insoluble except those of calcium strontium sodium potassium and ammonium)
118
bc one product is insoluble double-replacement and precipitation reaction will happen (NH4)2S (aq) + Cd(NO3)2 (aq) rarr NH4NO3 (aq) + CdS (s)
119
Net Ionic Equations
Reactions of ions in aqueous solution usually represented by net ionic equations instead of formula equations Net ionic equation - includes only those compounds and ions that undergo a chemical change in a reaction in an aqueous solution
120
121
To write NIE (net ionic equation) first convert chemical equation into overall ionic equation All soluble ionic compounds shown as dissociated ions in solution Precipitates shown as solids
122
Notice ammonium and nitrate ions appear on both sides They havenrsquot gone through any chemical change Spectator ions - ions that do not take part in a chemical reaction and are found in solution both before and after the reaction
123
To change ionic equation to NIE eliminate spectator ions Cd+2(aq) + S-2(aq) rarr CdS(s)
This NIE applies to any reaction in which precipitate of CdS forms
124
Practice Problem
Identify the precipitate that forms when aqueous solutions of zinc nitrate and ammonium sulfide are combined Write the equation for the possible double-replacement reaction Then write the formula equation overall ionic equation and the net ionic equation
125
1 AnalyzeGiven Identity of reactants zinc nitrate and ammonium sulfide Reaction medium aqueous solution Unknown a equation for the possible double-replacement reaction b identity of the precipitate c formula equation d overall ionic equation e net ionic equation
126
2 PlanWrite the possible double-replacement reaction between Zn(NO3)2 and (NH4)2S Use the rules to determine if any of the products will precipitate Write a formula equation and overall net ionic equation Then cancel spectator ions to make net ionic equation
127
3 Computea possible double-replacement reaction
b rules show that zinc sulfide is not soluble so will precipitate (ammonium nitrate is soluble)
c formula equation128
d overall ionic equation
e net ionic equation129
Will a precipitate form if solutions of potassium sulfate and barium nitrate are combined If so write the net ionic equation for the reaction
Yes Ba+2(aq) + SO4-2(aq) rarr BaSO4(s)
130
Will a precipitate form if solutions of potassium nitrate and magnesium sulfate are combined If so write the net ionic equation for the reaction
No 131
Will a precipitate form if solutions of barium chloride and sodium sulfate are combined If so identify the spectator ions and write the net ionic equation for the reaction
Yes Na+ and Cl- Ba+2(aq) + SO4-2(aq) rarr BaSO4(s)
132
Write the net ionic equation for the precipitation of nickel(II) sulfide
Ni+2(aq) + S-2(aq) rarr NiS(s)
133
IonizationSome molecular compounds can also form ions in solution Usually polar Ionization - ions formed from solute molecules by action of the solvent (creation of ions where there were none) Ionization different from dissociation
134
Dissociation ionic compounds dissolve and ions already present separate Ionization ions formed where non existed before
Ions formed are hydrated Heat released during hydration of ions gives enough energy to break covalent bonds
135
Extent to which solute ionizes depends on strength of bonds within molecules of solute and strength of attraction between solute and solvent molecules
136
HCl ndash molecular compound that ionizes in aqueous solution Contains highly polar bond Attraction between polar HCl molecule and polar water strong enough to break HCl bond
137
The Hydronium Ion
H+ ions from HCl attracts other molecules or ions so strongly that it doesnrsquot normally exist alone Ionization of HCl better described as direct transfer of proton from HCl to H2O forming H3O+
138
139
Hydration of H+ to form hydronium is highly exothermic
Energy released gives a lot of energy needed to ionize a molecular solute
Many molecular compounds that ionize in aqueous solution contain hydrogen and form hydronium ion
140
Strong and Weak Electrolytes
Substances that make ions and conduct current in solution are electrolytes HCl is one of series of compounds that has hydrogen and halogen Hydrogen halides are all molecular compounds with single polar-covalent bonds
141
All are gases Very soluble in water All are electrolytes HCl HBr HI strongly conduct current in solution HF only weakly conducts current at same concentration Strength of conduction related to ability to ionize
142
143
Strong Electrolytes
HCl HBr HI are 100 ionized in solution
Strong electrolyte - any compound whose dilute aqueous solutions conduct electricity well this is due to the presence of all or almost all of the dissolve compound in the form of ions
144
Unique characteristic of strong electrolyte is that they yield only ions
Ex Ionic compound may be highly soluble in water and dissociate into ions (NaCl)
Others may not dissolve much but amount that does dissolve exists only as ions
145
Weak ElectrolytesHF dissolves in water to give acid solution called hydrofluoric acid HF bond stronger than bonds between hydrogen and other halogens When HF dissolves some molecule ionize Reverse reaction also happens HF(aq) + H2O(l) H3O+(aq) + F-(aq)
146
HF(aq) + H2O(l) H3O+
(aq) + F-(aq)Concentration of dissolved unionized HF stays high and concentration of ions stays low
Weak electrolyte - any compound whose aqueous solutions conduct electricity poorly this is due to the presence of small amount of the dissolved compound in the form of ions
147
Different from nonelectrolyte ndash NO ions at all Description of electrolyte as strong or weak not related to concentration of solution Electrolytes differ in degree of ionization not amount of solute dissolved
148
149
Colligative properties of solutions
Presence of solute affects properties of solutions Some properties not dependent on nature of dissolved substance but on how may dissolve particles are present
Colligative properties - properties that depend on the concentration of solute particles but not on their identity
150
Vapor-Pressure Lowering
Boiling and freezing point of solution different from pure solvent Nonvolatile solute raises boiling point and lowers freezing point Nonvolatile substance - one that has little tendency to become gas under existing conditions
151
152
To understand why nonvolatile solute changes boiling and freezing point consider equilibrium vapor pressure
Vapor pressure pressure caused by molecules that have escape liquid phase to gas phase
Can be thought of a measure of tendency of molecules to escape from a liquid
153
Addition of sucrose (nonvolatile solute) lowers concentration of water molecules at surface of liquid This lowers tendency of water molecules to leave solution and enter gas phase Vapor pressure of solution is lower than vapor pressure of pure water
154
Aqueous13 solution13 of13 nonvolatile13 solutePure13 water
Nonelectrolyte solutions of same molality have same concentration of particles
Dilute solutions of same solvent and equal molality of any nonelectrolyte solute lower vapor pressure equally
155
Example
1 m aqueous solution of nonelectrolyte glucose lowers vapor pressure of water 55 x 10-4 atm at 25 1 m aqueous solution of sucrose also lowers water vapor pressure to 55 x 10-4 atm at 25
156
157
bc vapor-pressure lowering depends on concentration of nonelectrolyte solute and doesnrsquot depend on the type of solute it is a colligative proprerty
bc vapor pressure has been lowered solution remains liquid over larger temperature range
Lowers freezing point and raises boiling point
158
Can assume changes in boiling and freezing point also depend on concentration of solute
They are colligative properties
159
Freezing-Point Depression
Freezing point of 1-molal solution of any nonelectrolyte solute in water is found (by experiment) to be 186 lower than freezing point of water When 1 mol of nonelectrolyte solute dissolved in 1 kg water freezing point is -186 not 000 160
When 2 mol nonelectrolyte solute dissolved in 1 kg water freezing point is -372
For any concentration of nonelectrolyte solute in water decrease in freezing point can be determined using value of -186m
161
This value called the molal freezing-point contstant (Kf) is the freezing-point depression of the solvent in a 1-molal solution of a nonvolatile nonelectrolyte solute
162
Each solvent has own characteristic molal freezing-point constant Freezing-point depression (Δtf) the difference between the freezing points of the pure solvent and a solute of nonelectrolyte in that solvent and it is directly proportional to the molal concentration of the solution Δtf = Kfm
163
Δtf = Kfm
Kf expressed as m
m expressed in mol solutekg solvent (molality)
164
Sample Problem
What is the freezing point depression of water in a solution of 171 g of sucrose C12H22O11 and 200 g of water What is the actual freezing point of the solution
165
1 AnalyzeGiven Solute mass and chemical formula 171 g C12H22O11 Solvent mass and identity 2000 g water Unknown a freezing-point depression b freezing point of the solution 166
2 Plan
167
168
Calculate
Unknown a freezing-point depression b freezing point of the solution
169
Practice Problem
A water solution containing an unknown quantity of a nonelectrolyte solute is found to have a freezing point of ndash 023degCWhat is the molal concentration of the solution
012 m170
A solution consists of 103 g of the nonelectrolyte glucose C6H12O6 dissolved in 250 g of water What is the freezing-point depression of the solution
minus0426degC171
In a laboratory experiment the freezing point of an aqueous solution of glucose is found to be minus0325degCWhat is the molal concentration of this solution
0175 m172
If 0500 mol of a nonelectrolyte solute are dissolved in 5000 g of ether what is the freezing point of the solution
minus1181degC173
The freezing point of an aqueous solution that contains a nonelectrolyte is minus90degC aWhat is the freezing-point depression of the solution bWhat is the molal concentration of the solution a minus90degC b 48 m
174
Boiling-Point Elevation
Boiling point is temp at which vapor pressure is equal to atmospheric pressure Change in vapor pressure will cause corresponding change in boiling point
175
176
Vapor pressure of nonvolatile solution is lower than vapor pressure of pure solvent More heat will be required to raise vapor pressure of solution Boiling point of solution is higher than boiling point of pure solvent
Molal boiling-point constant (Kb) - boiling-point elevation of the solvent in a 1-molal solution of a nonvolatile nonelectrolyte solute b-p elevation of any nonelectrolyte in water found by experiment to be 051 So molal b-p constant for water is 051m
177
For different solvents b-p elevations of 1-molal solutions have different values (Table 14-2)
Like freezing-point constants values most accurate for dilute solutions
178
Boiling-point elevation Δtb difference between the boiling points of the pure solvent and a nonelectrolyte solution of that solvent and it is directly proportional to the molal concentration of the solution B-p elevation can be calculated using Δtb = Kbm Δtb (boiling point elevation) expressed in m and m expressed in mol solutekg solvent
Sample Problem
What is the boiling-point elevation of a solution made from 200 g of a nonelectrolyte solute and 4000 g of water The molar mass of the solute is 620 gmol
180
1 AnalyzeGiven solute mass = 200 g solute molar mass = 620 gmol solvent mass and identity = 4000 g of water
Unknown boiling-point elevation
181
2 Plan
182
183
3 Compute
Practice Problem
A solution contains 500 g of sucrose C12H22O11 a nonelectrolyte dissolved in 5000 g of water What is the boiling-point elevation
015degC184
A solution contains 4500 g of sucrose C12H22O11 a nonelectrolyte dissolved in 250 g of waterWhat is the boiling point of the solution
1027degC185
If the boiling point elevation of an aqueous solution containing a nonvolatile electrolyte is 102degC what is the molality of the solution
20 m186
The boiling point of an aqueous solution containing a nonvolatile electrolyte is 10075degC aWhat is the boiling-point elevation bWhat is the molality of the solution a 075degC b 15 m
187
Electrolytes and Colligative PropertiesEarly investigators confused by experiments where certain substances depressed freezing point or elevated boiling point more than expected
188
189
Ex 01 m solution of NaCl lowers freezing point of solvent almost twice as much as 01 m solution of sucrose 01 m CaCl2 lowers fp almost 3 times as much as 01 m solution of sucrose
To understand why this happens contrast behavior or sucrose with NaCl in aqueous solution Each sucrose dissolves to make only 1 particle in solution So 1 mol sucrose dissolves to 1 mol particles 1 mol NaCl dissolves to make 2 moles of particles (1 mol Na+ and 1 mol Cl-)
190
191
Calculated Values for Electrolyte SolutionsColligative properties depend on total concentration of solute particles regardless of their identity Electrolytes cause changes in colligative properties proportional to total molality in terms of all dissolves particles instead of formula units
192
What about barium nitrate Ba(NO3)2 Each mole of barium nitrate yields 3 mol of ions in solution Ba(NO3)2(s) rarr Ba2+(aq) + 2NO3-(aq) You would expect solution of given molality to lower fp of solvent 3 times as much as nonelectrolytic solution of same molality
193
Sample Problem
What is the expected change in the freezing point of water in a solution of 625 g of barium nitrate Ba(NO3)2 in 100 kg of water
194
1 AnalyzeGiven solute mass and formula = 625 g Ba(NO3)2 solvent mass and identity = 100 kg water Δtf = Kfm
Unknown expected freezing-point depression
195
2 PlanThe molality can be calculated by converting the solute mass to moles and then dividing by the number of kilograms of solvent That molality is in terms of formula units of Ba(NO3)2 and must be converted to molality in terms of dissociated ions in solution It must be multiplied by the number of moles of ions produced per mole of formula unit This adjusted molality can then be used to calculate the freezing-point depression
196
197
198
3 Compute
Practice Problem
What is the expected freezing-point depression for a solution that contains 20 mol of magnesium sulfate dissolved in 10 kg of water
minus74degC199
What is the expected boiling-point elevation of water for a solution that contains 150 g of sodium chloride dissolved in 10 kg of water 27degC The freezing point of an aqueous sodium chloride solution is minus200degC What is the molality of the solution 54 m NaCl
200
Actual Values for Electrolyte SolutionsRemember values calculated are only expected values Actual values of colligative properties for all strong electrolytes are almost what would be expected Difference between expected and actual values caused by attractive forces that exist between dissociated ions in aqueous solution
201
Attraction between hydrated ions in solution is small compared with those in crystalline solid However forces of attraction donrsquot interfere with movements of aqueous ions More concentrated solution means ions closer together and attraction is greater Also depends on charge of ions ndash higher charge greater attraction
202
b 1 mol sodium sulfide
2 mol Na+ 1 mol S-2 3 mol total ions
105
c 05 mol barium nitrate
05 mol Ba+2 1 mol NO3- 15 mol total ions
106
Precipitation Reactions
Even though no compound is completely insoluble compounds of very low solubility can be considered insoluble for practical purposes There are general guidelines to help predict whether a compound made of certain combination of ions is soluble
107
General Solubility Guidelines
Sodium potassium and ammonium compounds are soluble in water
108
Nitrates acetates and chlorates are soluble
109
110
Most chlorides are soluble except those of silver mercury (I) and lead Lead (II) chloride is soluble in hot water
Most sulfates are soluble except those of barium strontium lead calcium and mercury
111
Most carbonates phosphates and silicates are insoluble except those of sodium potassium and ammonium
112
Most sulfides are insoluble except those of calcium strontium sodium potassium and ammonium
113
Is calcium phosphate Ca3(PO4)2 soluble or insoluble
Not one of the exceptions so it is insoluble
Dissociation equations are not written for insoluble compounds
114
Guidelines also useful in predicting what will happen if 2 solutions of 2 different soluble compounds are mixed
If mixing results in combination of ions that forms insoluble compound a double-replacement and precipitation reaction will happen
115
Will precipitate form when solutions of ammonium sulfide and cadmium nitrate are combined You can tell calcium nitrate is soluble from the guidelines (Most nitrates acetates and chlorates are soluble) You can also tell ammonium sulfide is soluble (Most sulfides are insoluble except those of calcium strontium sodium potassium and ammonium)
116
117
To decide whether precipitate can form must know solubilities of two possible products
Ammonium nitrate ndash soluble (Most nitrates acetates and chlorates are soluble) Cadmium sulfide ndash insoluble (Most sulfides are insoluble except those of calcium strontium sodium potassium and ammonium)
118
bc one product is insoluble double-replacement and precipitation reaction will happen (NH4)2S (aq) + Cd(NO3)2 (aq) rarr NH4NO3 (aq) + CdS (s)
119
Net Ionic Equations
Reactions of ions in aqueous solution usually represented by net ionic equations instead of formula equations Net ionic equation - includes only those compounds and ions that undergo a chemical change in a reaction in an aqueous solution
120
121
To write NIE (net ionic equation) first convert chemical equation into overall ionic equation All soluble ionic compounds shown as dissociated ions in solution Precipitates shown as solids
122
Notice ammonium and nitrate ions appear on both sides They havenrsquot gone through any chemical change Spectator ions - ions that do not take part in a chemical reaction and are found in solution both before and after the reaction
123
To change ionic equation to NIE eliminate spectator ions Cd+2(aq) + S-2(aq) rarr CdS(s)
This NIE applies to any reaction in which precipitate of CdS forms
124
Practice Problem
Identify the precipitate that forms when aqueous solutions of zinc nitrate and ammonium sulfide are combined Write the equation for the possible double-replacement reaction Then write the formula equation overall ionic equation and the net ionic equation
125
1 AnalyzeGiven Identity of reactants zinc nitrate and ammonium sulfide Reaction medium aqueous solution Unknown a equation for the possible double-replacement reaction b identity of the precipitate c formula equation d overall ionic equation e net ionic equation
126
2 PlanWrite the possible double-replacement reaction between Zn(NO3)2 and (NH4)2S Use the rules to determine if any of the products will precipitate Write a formula equation and overall net ionic equation Then cancel spectator ions to make net ionic equation
127
3 Computea possible double-replacement reaction
b rules show that zinc sulfide is not soluble so will precipitate (ammonium nitrate is soluble)
c formula equation128
d overall ionic equation
e net ionic equation129
Will a precipitate form if solutions of potassium sulfate and barium nitrate are combined If so write the net ionic equation for the reaction
Yes Ba+2(aq) + SO4-2(aq) rarr BaSO4(s)
130
Will a precipitate form if solutions of potassium nitrate and magnesium sulfate are combined If so write the net ionic equation for the reaction
No 131
Will a precipitate form if solutions of barium chloride and sodium sulfate are combined If so identify the spectator ions and write the net ionic equation for the reaction
Yes Na+ and Cl- Ba+2(aq) + SO4-2(aq) rarr BaSO4(s)
132
Write the net ionic equation for the precipitation of nickel(II) sulfide
Ni+2(aq) + S-2(aq) rarr NiS(s)
133
IonizationSome molecular compounds can also form ions in solution Usually polar Ionization - ions formed from solute molecules by action of the solvent (creation of ions where there were none) Ionization different from dissociation
134
Dissociation ionic compounds dissolve and ions already present separate Ionization ions formed where non existed before
Ions formed are hydrated Heat released during hydration of ions gives enough energy to break covalent bonds
135
Extent to which solute ionizes depends on strength of bonds within molecules of solute and strength of attraction between solute and solvent molecules
136
HCl ndash molecular compound that ionizes in aqueous solution Contains highly polar bond Attraction between polar HCl molecule and polar water strong enough to break HCl bond
137
The Hydronium Ion
H+ ions from HCl attracts other molecules or ions so strongly that it doesnrsquot normally exist alone Ionization of HCl better described as direct transfer of proton from HCl to H2O forming H3O+
138
139
Hydration of H+ to form hydronium is highly exothermic
Energy released gives a lot of energy needed to ionize a molecular solute
Many molecular compounds that ionize in aqueous solution contain hydrogen and form hydronium ion
140
Strong and Weak Electrolytes
Substances that make ions and conduct current in solution are electrolytes HCl is one of series of compounds that has hydrogen and halogen Hydrogen halides are all molecular compounds with single polar-covalent bonds
141
All are gases Very soluble in water All are electrolytes HCl HBr HI strongly conduct current in solution HF only weakly conducts current at same concentration Strength of conduction related to ability to ionize
142
143
Strong Electrolytes
HCl HBr HI are 100 ionized in solution
Strong electrolyte - any compound whose dilute aqueous solutions conduct electricity well this is due to the presence of all or almost all of the dissolve compound in the form of ions
144
Unique characteristic of strong electrolyte is that they yield only ions
Ex Ionic compound may be highly soluble in water and dissociate into ions (NaCl)
Others may not dissolve much but amount that does dissolve exists only as ions
145
Weak ElectrolytesHF dissolves in water to give acid solution called hydrofluoric acid HF bond stronger than bonds between hydrogen and other halogens When HF dissolves some molecule ionize Reverse reaction also happens HF(aq) + H2O(l) H3O+(aq) + F-(aq)
146
HF(aq) + H2O(l) H3O+
(aq) + F-(aq)Concentration of dissolved unionized HF stays high and concentration of ions stays low
Weak electrolyte - any compound whose aqueous solutions conduct electricity poorly this is due to the presence of small amount of the dissolved compound in the form of ions
147
Different from nonelectrolyte ndash NO ions at all Description of electrolyte as strong or weak not related to concentration of solution Electrolytes differ in degree of ionization not amount of solute dissolved
148
149
Colligative properties of solutions
Presence of solute affects properties of solutions Some properties not dependent on nature of dissolved substance but on how may dissolve particles are present
Colligative properties - properties that depend on the concentration of solute particles but not on their identity
150
Vapor-Pressure Lowering
Boiling and freezing point of solution different from pure solvent Nonvolatile solute raises boiling point and lowers freezing point Nonvolatile substance - one that has little tendency to become gas under existing conditions
151
152
To understand why nonvolatile solute changes boiling and freezing point consider equilibrium vapor pressure
Vapor pressure pressure caused by molecules that have escape liquid phase to gas phase
Can be thought of a measure of tendency of molecules to escape from a liquid
153
Addition of sucrose (nonvolatile solute) lowers concentration of water molecules at surface of liquid This lowers tendency of water molecules to leave solution and enter gas phase Vapor pressure of solution is lower than vapor pressure of pure water
154
Aqueous13 solution13 of13 nonvolatile13 solutePure13 water
Nonelectrolyte solutions of same molality have same concentration of particles
Dilute solutions of same solvent and equal molality of any nonelectrolyte solute lower vapor pressure equally
155
Example
1 m aqueous solution of nonelectrolyte glucose lowers vapor pressure of water 55 x 10-4 atm at 25 1 m aqueous solution of sucrose also lowers water vapor pressure to 55 x 10-4 atm at 25
156
157
bc vapor-pressure lowering depends on concentration of nonelectrolyte solute and doesnrsquot depend on the type of solute it is a colligative proprerty
bc vapor pressure has been lowered solution remains liquid over larger temperature range
Lowers freezing point and raises boiling point
158
Can assume changes in boiling and freezing point also depend on concentration of solute
They are colligative properties
159
Freezing-Point Depression
Freezing point of 1-molal solution of any nonelectrolyte solute in water is found (by experiment) to be 186 lower than freezing point of water When 1 mol of nonelectrolyte solute dissolved in 1 kg water freezing point is -186 not 000 160
When 2 mol nonelectrolyte solute dissolved in 1 kg water freezing point is -372
For any concentration of nonelectrolyte solute in water decrease in freezing point can be determined using value of -186m
161
This value called the molal freezing-point contstant (Kf) is the freezing-point depression of the solvent in a 1-molal solution of a nonvolatile nonelectrolyte solute
162
Each solvent has own characteristic molal freezing-point constant Freezing-point depression (Δtf) the difference between the freezing points of the pure solvent and a solute of nonelectrolyte in that solvent and it is directly proportional to the molal concentration of the solution Δtf = Kfm
163
Δtf = Kfm
Kf expressed as m
m expressed in mol solutekg solvent (molality)
164
Sample Problem
What is the freezing point depression of water in a solution of 171 g of sucrose C12H22O11 and 200 g of water What is the actual freezing point of the solution
165
1 AnalyzeGiven Solute mass and chemical formula 171 g C12H22O11 Solvent mass and identity 2000 g water Unknown a freezing-point depression b freezing point of the solution 166
2 Plan
167
168
Calculate
Unknown a freezing-point depression b freezing point of the solution
169
Practice Problem
A water solution containing an unknown quantity of a nonelectrolyte solute is found to have a freezing point of ndash 023degCWhat is the molal concentration of the solution
012 m170
A solution consists of 103 g of the nonelectrolyte glucose C6H12O6 dissolved in 250 g of water What is the freezing-point depression of the solution
minus0426degC171
In a laboratory experiment the freezing point of an aqueous solution of glucose is found to be minus0325degCWhat is the molal concentration of this solution
0175 m172
If 0500 mol of a nonelectrolyte solute are dissolved in 5000 g of ether what is the freezing point of the solution
minus1181degC173
The freezing point of an aqueous solution that contains a nonelectrolyte is minus90degC aWhat is the freezing-point depression of the solution bWhat is the molal concentration of the solution a minus90degC b 48 m
174
Boiling-Point Elevation
Boiling point is temp at which vapor pressure is equal to atmospheric pressure Change in vapor pressure will cause corresponding change in boiling point
175
176
Vapor pressure of nonvolatile solution is lower than vapor pressure of pure solvent More heat will be required to raise vapor pressure of solution Boiling point of solution is higher than boiling point of pure solvent
Molal boiling-point constant (Kb) - boiling-point elevation of the solvent in a 1-molal solution of a nonvolatile nonelectrolyte solute b-p elevation of any nonelectrolyte in water found by experiment to be 051 So molal b-p constant for water is 051m
177
For different solvents b-p elevations of 1-molal solutions have different values (Table 14-2)
Like freezing-point constants values most accurate for dilute solutions
178
Boiling-point elevation Δtb difference between the boiling points of the pure solvent and a nonelectrolyte solution of that solvent and it is directly proportional to the molal concentration of the solution B-p elevation can be calculated using Δtb = Kbm Δtb (boiling point elevation) expressed in m and m expressed in mol solutekg solvent
Sample Problem
What is the boiling-point elevation of a solution made from 200 g of a nonelectrolyte solute and 4000 g of water The molar mass of the solute is 620 gmol
180
1 AnalyzeGiven solute mass = 200 g solute molar mass = 620 gmol solvent mass and identity = 4000 g of water
Unknown boiling-point elevation
181
2 Plan
182
183
3 Compute
Practice Problem
A solution contains 500 g of sucrose C12H22O11 a nonelectrolyte dissolved in 5000 g of water What is the boiling-point elevation
015degC184
A solution contains 4500 g of sucrose C12H22O11 a nonelectrolyte dissolved in 250 g of waterWhat is the boiling point of the solution
1027degC185
If the boiling point elevation of an aqueous solution containing a nonvolatile electrolyte is 102degC what is the molality of the solution
20 m186
The boiling point of an aqueous solution containing a nonvolatile electrolyte is 10075degC aWhat is the boiling-point elevation bWhat is the molality of the solution a 075degC b 15 m
187
Electrolytes and Colligative PropertiesEarly investigators confused by experiments where certain substances depressed freezing point or elevated boiling point more than expected
188
189
Ex 01 m solution of NaCl lowers freezing point of solvent almost twice as much as 01 m solution of sucrose 01 m CaCl2 lowers fp almost 3 times as much as 01 m solution of sucrose
To understand why this happens contrast behavior or sucrose with NaCl in aqueous solution Each sucrose dissolves to make only 1 particle in solution So 1 mol sucrose dissolves to 1 mol particles 1 mol NaCl dissolves to make 2 moles of particles (1 mol Na+ and 1 mol Cl-)
190
191
Calculated Values for Electrolyte SolutionsColligative properties depend on total concentration of solute particles regardless of their identity Electrolytes cause changes in colligative properties proportional to total molality in terms of all dissolves particles instead of formula units
192
What about barium nitrate Ba(NO3)2 Each mole of barium nitrate yields 3 mol of ions in solution Ba(NO3)2(s) rarr Ba2+(aq) + 2NO3-(aq) You would expect solution of given molality to lower fp of solvent 3 times as much as nonelectrolytic solution of same molality
193
Sample Problem
What is the expected change in the freezing point of water in a solution of 625 g of barium nitrate Ba(NO3)2 in 100 kg of water
194
1 AnalyzeGiven solute mass and formula = 625 g Ba(NO3)2 solvent mass and identity = 100 kg water Δtf = Kfm
Unknown expected freezing-point depression
195
2 PlanThe molality can be calculated by converting the solute mass to moles and then dividing by the number of kilograms of solvent That molality is in terms of formula units of Ba(NO3)2 and must be converted to molality in terms of dissociated ions in solution It must be multiplied by the number of moles of ions produced per mole of formula unit This adjusted molality can then be used to calculate the freezing-point depression
196
197
198
3 Compute
Practice Problem
What is the expected freezing-point depression for a solution that contains 20 mol of magnesium sulfate dissolved in 10 kg of water
minus74degC199
What is the expected boiling-point elevation of water for a solution that contains 150 g of sodium chloride dissolved in 10 kg of water 27degC The freezing point of an aqueous sodium chloride solution is minus200degC What is the molality of the solution 54 m NaCl
200
Actual Values for Electrolyte SolutionsRemember values calculated are only expected values Actual values of colligative properties for all strong electrolytes are almost what would be expected Difference between expected and actual values caused by attractive forces that exist between dissociated ions in aqueous solution
201
Attraction between hydrated ions in solution is small compared with those in crystalline solid However forces of attraction donrsquot interfere with movements of aqueous ions More concentrated solution means ions closer together and attraction is greater Also depends on charge of ions ndash higher charge greater attraction
202
c 05 mol barium nitrate
05 mol Ba+2 1 mol NO3- 15 mol total ions
106
Precipitation Reactions
Even though no compound is completely insoluble compounds of very low solubility can be considered insoluble for practical purposes There are general guidelines to help predict whether a compound made of certain combination of ions is soluble
107
General Solubility Guidelines
Sodium potassium and ammonium compounds are soluble in water
108
Nitrates acetates and chlorates are soluble
109
110
Most chlorides are soluble except those of silver mercury (I) and lead Lead (II) chloride is soluble in hot water
Most sulfates are soluble except those of barium strontium lead calcium and mercury
111
Most carbonates phosphates and silicates are insoluble except those of sodium potassium and ammonium
112
Most sulfides are insoluble except those of calcium strontium sodium potassium and ammonium
113
Is calcium phosphate Ca3(PO4)2 soluble or insoluble
Not one of the exceptions so it is insoluble
Dissociation equations are not written for insoluble compounds
114
Guidelines also useful in predicting what will happen if 2 solutions of 2 different soluble compounds are mixed
If mixing results in combination of ions that forms insoluble compound a double-replacement and precipitation reaction will happen
115
Will precipitate form when solutions of ammonium sulfide and cadmium nitrate are combined You can tell calcium nitrate is soluble from the guidelines (Most nitrates acetates and chlorates are soluble) You can also tell ammonium sulfide is soluble (Most sulfides are insoluble except those of calcium strontium sodium potassium and ammonium)
116
117
To decide whether precipitate can form must know solubilities of two possible products
Ammonium nitrate ndash soluble (Most nitrates acetates and chlorates are soluble) Cadmium sulfide ndash insoluble (Most sulfides are insoluble except those of calcium strontium sodium potassium and ammonium)
118
bc one product is insoluble double-replacement and precipitation reaction will happen (NH4)2S (aq) + Cd(NO3)2 (aq) rarr NH4NO3 (aq) + CdS (s)
119
Net Ionic Equations
Reactions of ions in aqueous solution usually represented by net ionic equations instead of formula equations Net ionic equation - includes only those compounds and ions that undergo a chemical change in a reaction in an aqueous solution
120
121
To write NIE (net ionic equation) first convert chemical equation into overall ionic equation All soluble ionic compounds shown as dissociated ions in solution Precipitates shown as solids
122
Notice ammonium and nitrate ions appear on both sides They havenrsquot gone through any chemical change Spectator ions - ions that do not take part in a chemical reaction and are found in solution both before and after the reaction
123
To change ionic equation to NIE eliminate spectator ions Cd+2(aq) + S-2(aq) rarr CdS(s)
This NIE applies to any reaction in which precipitate of CdS forms
124
Practice Problem
Identify the precipitate that forms when aqueous solutions of zinc nitrate and ammonium sulfide are combined Write the equation for the possible double-replacement reaction Then write the formula equation overall ionic equation and the net ionic equation
125
1 AnalyzeGiven Identity of reactants zinc nitrate and ammonium sulfide Reaction medium aqueous solution Unknown a equation for the possible double-replacement reaction b identity of the precipitate c formula equation d overall ionic equation e net ionic equation
126
2 PlanWrite the possible double-replacement reaction between Zn(NO3)2 and (NH4)2S Use the rules to determine if any of the products will precipitate Write a formula equation and overall net ionic equation Then cancel spectator ions to make net ionic equation
127
3 Computea possible double-replacement reaction
b rules show that zinc sulfide is not soluble so will precipitate (ammonium nitrate is soluble)
c formula equation128
d overall ionic equation
e net ionic equation129
Will a precipitate form if solutions of potassium sulfate and barium nitrate are combined If so write the net ionic equation for the reaction
Yes Ba+2(aq) + SO4-2(aq) rarr BaSO4(s)
130
Will a precipitate form if solutions of potassium nitrate and magnesium sulfate are combined If so write the net ionic equation for the reaction
No 131
Will a precipitate form if solutions of barium chloride and sodium sulfate are combined If so identify the spectator ions and write the net ionic equation for the reaction
Yes Na+ and Cl- Ba+2(aq) + SO4-2(aq) rarr BaSO4(s)
132
Write the net ionic equation for the precipitation of nickel(II) sulfide
Ni+2(aq) + S-2(aq) rarr NiS(s)
133
IonizationSome molecular compounds can also form ions in solution Usually polar Ionization - ions formed from solute molecules by action of the solvent (creation of ions where there were none) Ionization different from dissociation
134
Dissociation ionic compounds dissolve and ions already present separate Ionization ions formed where non existed before
Ions formed are hydrated Heat released during hydration of ions gives enough energy to break covalent bonds
135
Extent to which solute ionizes depends on strength of bonds within molecules of solute and strength of attraction between solute and solvent molecules
136
HCl ndash molecular compound that ionizes in aqueous solution Contains highly polar bond Attraction between polar HCl molecule and polar water strong enough to break HCl bond
137
The Hydronium Ion
H+ ions from HCl attracts other molecules or ions so strongly that it doesnrsquot normally exist alone Ionization of HCl better described as direct transfer of proton from HCl to H2O forming H3O+
138
139
Hydration of H+ to form hydronium is highly exothermic
Energy released gives a lot of energy needed to ionize a molecular solute
Many molecular compounds that ionize in aqueous solution contain hydrogen and form hydronium ion
140
Strong and Weak Electrolytes
Substances that make ions and conduct current in solution are electrolytes HCl is one of series of compounds that has hydrogen and halogen Hydrogen halides are all molecular compounds with single polar-covalent bonds
141
All are gases Very soluble in water All are electrolytes HCl HBr HI strongly conduct current in solution HF only weakly conducts current at same concentration Strength of conduction related to ability to ionize
142
143
Strong Electrolytes
HCl HBr HI are 100 ionized in solution
Strong electrolyte - any compound whose dilute aqueous solutions conduct electricity well this is due to the presence of all or almost all of the dissolve compound in the form of ions
144
Unique characteristic of strong electrolyte is that they yield only ions
Ex Ionic compound may be highly soluble in water and dissociate into ions (NaCl)
Others may not dissolve much but amount that does dissolve exists only as ions
145
Weak ElectrolytesHF dissolves in water to give acid solution called hydrofluoric acid HF bond stronger than bonds between hydrogen and other halogens When HF dissolves some molecule ionize Reverse reaction also happens HF(aq) + H2O(l) H3O+(aq) + F-(aq)
146
HF(aq) + H2O(l) H3O+
(aq) + F-(aq)Concentration of dissolved unionized HF stays high and concentration of ions stays low
Weak electrolyte - any compound whose aqueous solutions conduct electricity poorly this is due to the presence of small amount of the dissolved compound in the form of ions
147
Different from nonelectrolyte ndash NO ions at all Description of electrolyte as strong or weak not related to concentration of solution Electrolytes differ in degree of ionization not amount of solute dissolved
148
149
Colligative properties of solutions
Presence of solute affects properties of solutions Some properties not dependent on nature of dissolved substance but on how may dissolve particles are present
Colligative properties - properties that depend on the concentration of solute particles but not on their identity
150
Vapor-Pressure Lowering
Boiling and freezing point of solution different from pure solvent Nonvolatile solute raises boiling point and lowers freezing point Nonvolatile substance - one that has little tendency to become gas under existing conditions
151
152
To understand why nonvolatile solute changes boiling and freezing point consider equilibrium vapor pressure
Vapor pressure pressure caused by molecules that have escape liquid phase to gas phase
Can be thought of a measure of tendency of molecules to escape from a liquid
153
Addition of sucrose (nonvolatile solute) lowers concentration of water molecules at surface of liquid This lowers tendency of water molecules to leave solution and enter gas phase Vapor pressure of solution is lower than vapor pressure of pure water
154
Aqueous13 solution13 of13 nonvolatile13 solutePure13 water
Nonelectrolyte solutions of same molality have same concentration of particles
Dilute solutions of same solvent and equal molality of any nonelectrolyte solute lower vapor pressure equally
155
Example
1 m aqueous solution of nonelectrolyte glucose lowers vapor pressure of water 55 x 10-4 atm at 25 1 m aqueous solution of sucrose also lowers water vapor pressure to 55 x 10-4 atm at 25
156
157
bc vapor-pressure lowering depends on concentration of nonelectrolyte solute and doesnrsquot depend on the type of solute it is a colligative proprerty
bc vapor pressure has been lowered solution remains liquid over larger temperature range
Lowers freezing point and raises boiling point
158
Can assume changes in boiling and freezing point also depend on concentration of solute
They are colligative properties
159
Freezing-Point Depression
Freezing point of 1-molal solution of any nonelectrolyte solute in water is found (by experiment) to be 186 lower than freezing point of water When 1 mol of nonelectrolyte solute dissolved in 1 kg water freezing point is -186 not 000 160
When 2 mol nonelectrolyte solute dissolved in 1 kg water freezing point is -372
For any concentration of nonelectrolyte solute in water decrease in freezing point can be determined using value of -186m
161
This value called the molal freezing-point contstant (Kf) is the freezing-point depression of the solvent in a 1-molal solution of a nonvolatile nonelectrolyte solute
162
Each solvent has own characteristic molal freezing-point constant Freezing-point depression (Δtf) the difference between the freezing points of the pure solvent and a solute of nonelectrolyte in that solvent and it is directly proportional to the molal concentration of the solution Δtf = Kfm
163
Δtf = Kfm
Kf expressed as m
m expressed in mol solutekg solvent (molality)
164
Sample Problem
What is the freezing point depression of water in a solution of 171 g of sucrose C12H22O11 and 200 g of water What is the actual freezing point of the solution
165
1 AnalyzeGiven Solute mass and chemical formula 171 g C12H22O11 Solvent mass and identity 2000 g water Unknown a freezing-point depression b freezing point of the solution 166
2 Plan
167
168
Calculate
Unknown a freezing-point depression b freezing point of the solution
169
Practice Problem
A water solution containing an unknown quantity of a nonelectrolyte solute is found to have a freezing point of ndash 023degCWhat is the molal concentration of the solution
012 m170
A solution consists of 103 g of the nonelectrolyte glucose C6H12O6 dissolved in 250 g of water What is the freezing-point depression of the solution
minus0426degC171
In a laboratory experiment the freezing point of an aqueous solution of glucose is found to be minus0325degCWhat is the molal concentration of this solution
0175 m172
If 0500 mol of a nonelectrolyte solute are dissolved in 5000 g of ether what is the freezing point of the solution
minus1181degC173
The freezing point of an aqueous solution that contains a nonelectrolyte is minus90degC aWhat is the freezing-point depression of the solution bWhat is the molal concentration of the solution a minus90degC b 48 m
174
Boiling-Point Elevation
Boiling point is temp at which vapor pressure is equal to atmospheric pressure Change in vapor pressure will cause corresponding change in boiling point
175
176
Vapor pressure of nonvolatile solution is lower than vapor pressure of pure solvent More heat will be required to raise vapor pressure of solution Boiling point of solution is higher than boiling point of pure solvent
Molal boiling-point constant (Kb) - boiling-point elevation of the solvent in a 1-molal solution of a nonvolatile nonelectrolyte solute b-p elevation of any nonelectrolyte in water found by experiment to be 051 So molal b-p constant for water is 051m
177
For different solvents b-p elevations of 1-molal solutions have different values (Table 14-2)
Like freezing-point constants values most accurate for dilute solutions
178
Boiling-point elevation Δtb difference between the boiling points of the pure solvent and a nonelectrolyte solution of that solvent and it is directly proportional to the molal concentration of the solution B-p elevation can be calculated using Δtb = Kbm Δtb (boiling point elevation) expressed in m and m expressed in mol solutekg solvent
Sample Problem
What is the boiling-point elevation of a solution made from 200 g of a nonelectrolyte solute and 4000 g of water The molar mass of the solute is 620 gmol
180
1 AnalyzeGiven solute mass = 200 g solute molar mass = 620 gmol solvent mass and identity = 4000 g of water
Unknown boiling-point elevation
181
2 Plan
182
183
3 Compute
Practice Problem
A solution contains 500 g of sucrose C12H22O11 a nonelectrolyte dissolved in 5000 g of water What is the boiling-point elevation
015degC184
A solution contains 4500 g of sucrose C12H22O11 a nonelectrolyte dissolved in 250 g of waterWhat is the boiling point of the solution
1027degC185
If the boiling point elevation of an aqueous solution containing a nonvolatile electrolyte is 102degC what is the molality of the solution
20 m186
The boiling point of an aqueous solution containing a nonvolatile electrolyte is 10075degC aWhat is the boiling-point elevation bWhat is the molality of the solution a 075degC b 15 m
187
Electrolytes and Colligative PropertiesEarly investigators confused by experiments where certain substances depressed freezing point or elevated boiling point more than expected
188
189
Ex 01 m solution of NaCl lowers freezing point of solvent almost twice as much as 01 m solution of sucrose 01 m CaCl2 lowers fp almost 3 times as much as 01 m solution of sucrose
To understand why this happens contrast behavior or sucrose with NaCl in aqueous solution Each sucrose dissolves to make only 1 particle in solution So 1 mol sucrose dissolves to 1 mol particles 1 mol NaCl dissolves to make 2 moles of particles (1 mol Na+ and 1 mol Cl-)
190
191
Calculated Values for Electrolyte SolutionsColligative properties depend on total concentration of solute particles regardless of their identity Electrolytes cause changes in colligative properties proportional to total molality in terms of all dissolves particles instead of formula units
192
What about barium nitrate Ba(NO3)2 Each mole of barium nitrate yields 3 mol of ions in solution Ba(NO3)2(s) rarr Ba2+(aq) + 2NO3-(aq) You would expect solution of given molality to lower fp of solvent 3 times as much as nonelectrolytic solution of same molality
193
Sample Problem
What is the expected change in the freezing point of water in a solution of 625 g of barium nitrate Ba(NO3)2 in 100 kg of water
194
1 AnalyzeGiven solute mass and formula = 625 g Ba(NO3)2 solvent mass and identity = 100 kg water Δtf = Kfm
Unknown expected freezing-point depression
195
2 PlanThe molality can be calculated by converting the solute mass to moles and then dividing by the number of kilograms of solvent That molality is in terms of formula units of Ba(NO3)2 and must be converted to molality in terms of dissociated ions in solution It must be multiplied by the number of moles of ions produced per mole of formula unit This adjusted molality can then be used to calculate the freezing-point depression
196
197
198
3 Compute
Practice Problem
What is the expected freezing-point depression for a solution that contains 20 mol of magnesium sulfate dissolved in 10 kg of water
minus74degC199
What is the expected boiling-point elevation of water for a solution that contains 150 g of sodium chloride dissolved in 10 kg of water 27degC The freezing point of an aqueous sodium chloride solution is minus200degC What is the molality of the solution 54 m NaCl
200
Actual Values for Electrolyte SolutionsRemember values calculated are only expected values Actual values of colligative properties for all strong electrolytes are almost what would be expected Difference between expected and actual values caused by attractive forces that exist between dissociated ions in aqueous solution
201
Attraction between hydrated ions in solution is small compared with those in crystalline solid However forces of attraction donrsquot interfere with movements of aqueous ions More concentrated solution means ions closer together and attraction is greater Also depends on charge of ions ndash higher charge greater attraction
202
Precipitation Reactions
Even though no compound is completely insoluble compounds of very low solubility can be considered insoluble for practical purposes There are general guidelines to help predict whether a compound made of certain combination of ions is soluble
107
General Solubility Guidelines
Sodium potassium and ammonium compounds are soluble in water
108
Nitrates acetates and chlorates are soluble
109
110
Most chlorides are soluble except those of silver mercury (I) and lead Lead (II) chloride is soluble in hot water
Most sulfates are soluble except those of barium strontium lead calcium and mercury
111
Most carbonates phosphates and silicates are insoluble except those of sodium potassium and ammonium
112
Most sulfides are insoluble except those of calcium strontium sodium potassium and ammonium
113
Is calcium phosphate Ca3(PO4)2 soluble or insoluble
Not one of the exceptions so it is insoluble
Dissociation equations are not written for insoluble compounds
114
Guidelines also useful in predicting what will happen if 2 solutions of 2 different soluble compounds are mixed
If mixing results in combination of ions that forms insoluble compound a double-replacement and precipitation reaction will happen
115
Will precipitate form when solutions of ammonium sulfide and cadmium nitrate are combined You can tell calcium nitrate is soluble from the guidelines (Most nitrates acetates and chlorates are soluble) You can also tell ammonium sulfide is soluble (Most sulfides are insoluble except those of calcium strontium sodium potassium and ammonium)
116
117
To decide whether precipitate can form must know solubilities of two possible products
Ammonium nitrate ndash soluble (Most nitrates acetates and chlorates are soluble) Cadmium sulfide ndash insoluble (Most sulfides are insoluble except those of calcium strontium sodium potassium and ammonium)
118
bc one product is insoluble double-replacement and precipitation reaction will happen (NH4)2S (aq) + Cd(NO3)2 (aq) rarr NH4NO3 (aq) + CdS (s)
119
Net Ionic Equations
Reactions of ions in aqueous solution usually represented by net ionic equations instead of formula equations Net ionic equation - includes only those compounds and ions that undergo a chemical change in a reaction in an aqueous solution
120
121
To write NIE (net ionic equation) first convert chemical equation into overall ionic equation All soluble ionic compounds shown as dissociated ions in solution Precipitates shown as solids
122
Notice ammonium and nitrate ions appear on both sides They havenrsquot gone through any chemical change Spectator ions - ions that do not take part in a chemical reaction and are found in solution both before and after the reaction
123
To change ionic equation to NIE eliminate spectator ions Cd+2(aq) + S-2(aq) rarr CdS(s)
This NIE applies to any reaction in which precipitate of CdS forms
124
Practice Problem
Identify the precipitate that forms when aqueous solutions of zinc nitrate and ammonium sulfide are combined Write the equation for the possible double-replacement reaction Then write the formula equation overall ionic equation and the net ionic equation
125
1 AnalyzeGiven Identity of reactants zinc nitrate and ammonium sulfide Reaction medium aqueous solution Unknown a equation for the possible double-replacement reaction b identity of the precipitate c formula equation d overall ionic equation e net ionic equation
126
2 PlanWrite the possible double-replacement reaction between Zn(NO3)2 and (NH4)2S Use the rules to determine if any of the products will precipitate Write a formula equation and overall net ionic equation Then cancel spectator ions to make net ionic equation
127
3 Computea possible double-replacement reaction
b rules show that zinc sulfide is not soluble so will precipitate (ammonium nitrate is soluble)
c formula equation128
d overall ionic equation
e net ionic equation129
Will a precipitate form if solutions of potassium sulfate and barium nitrate are combined If so write the net ionic equation for the reaction
Yes Ba+2(aq) + SO4-2(aq) rarr BaSO4(s)
130
Will a precipitate form if solutions of potassium nitrate and magnesium sulfate are combined If so write the net ionic equation for the reaction
No 131
Will a precipitate form if solutions of barium chloride and sodium sulfate are combined If so identify the spectator ions and write the net ionic equation for the reaction
Yes Na+ and Cl- Ba+2(aq) + SO4-2(aq) rarr BaSO4(s)
132
Write the net ionic equation for the precipitation of nickel(II) sulfide
Ni+2(aq) + S-2(aq) rarr NiS(s)
133
IonizationSome molecular compounds can also form ions in solution Usually polar Ionization - ions formed from solute molecules by action of the solvent (creation of ions where there were none) Ionization different from dissociation
134
Dissociation ionic compounds dissolve and ions already present separate Ionization ions formed where non existed before
Ions formed are hydrated Heat released during hydration of ions gives enough energy to break covalent bonds
135
Extent to which solute ionizes depends on strength of bonds within molecules of solute and strength of attraction between solute and solvent molecules
136
HCl ndash molecular compound that ionizes in aqueous solution Contains highly polar bond Attraction between polar HCl molecule and polar water strong enough to break HCl bond
137
The Hydronium Ion
H+ ions from HCl attracts other molecules or ions so strongly that it doesnrsquot normally exist alone Ionization of HCl better described as direct transfer of proton from HCl to H2O forming H3O+
138
139
Hydration of H+ to form hydronium is highly exothermic
Energy released gives a lot of energy needed to ionize a molecular solute
Many molecular compounds that ionize in aqueous solution contain hydrogen and form hydronium ion
140
Strong and Weak Electrolytes
Substances that make ions and conduct current in solution are electrolytes HCl is one of series of compounds that has hydrogen and halogen Hydrogen halides are all molecular compounds with single polar-covalent bonds
141
All are gases Very soluble in water All are electrolytes HCl HBr HI strongly conduct current in solution HF only weakly conducts current at same concentration Strength of conduction related to ability to ionize
142
143
Strong Electrolytes
HCl HBr HI are 100 ionized in solution
Strong electrolyte - any compound whose dilute aqueous solutions conduct electricity well this is due to the presence of all or almost all of the dissolve compound in the form of ions
144
Unique characteristic of strong electrolyte is that they yield only ions
Ex Ionic compound may be highly soluble in water and dissociate into ions (NaCl)
Others may not dissolve much but amount that does dissolve exists only as ions
145
Weak ElectrolytesHF dissolves in water to give acid solution called hydrofluoric acid HF bond stronger than bonds between hydrogen and other halogens When HF dissolves some molecule ionize Reverse reaction also happens HF(aq) + H2O(l) H3O+(aq) + F-(aq)
146
HF(aq) + H2O(l) H3O+
(aq) + F-(aq)Concentration of dissolved unionized HF stays high and concentration of ions stays low
Weak electrolyte - any compound whose aqueous solutions conduct electricity poorly this is due to the presence of small amount of the dissolved compound in the form of ions
147
Different from nonelectrolyte ndash NO ions at all Description of electrolyte as strong or weak not related to concentration of solution Electrolytes differ in degree of ionization not amount of solute dissolved
148
149
Colligative properties of solutions
Presence of solute affects properties of solutions Some properties not dependent on nature of dissolved substance but on how may dissolve particles are present
Colligative properties - properties that depend on the concentration of solute particles but not on their identity
150
Vapor-Pressure Lowering
Boiling and freezing point of solution different from pure solvent Nonvolatile solute raises boiling point and lowers freezing point Nonvolatile substance - one that has little tendency to become gas under existing conditions
151
152
To understand why nonvolatile solute changes boiling and freezing point consider equilibrium vapor pressure
Vapor pressure pressure caused by molecules that have escape liquid phase to gas phase
Can be thought of a measure of tendency of molecules to escape from a liquid
153
Addition of sucrose (nonvolatile solute) lowers concentration of water molecules at surface of liquid This lowers tendency of water molecules to leave solution and enter gas phase Vapor pressure of solution is lower than vapor pressure of pure water
154
Aqueous13 solution13 of13 nonvolatile13 solutePure13 water
Nonelectrolyte solutions of same molality have same concentration of particles
Dilute solutions of same solvent and equal molality of any nonelectrolyte solute lower vapor pressure equally
155
Example
1 m aqueous solution of nonelectrolyte glucose lowers vapor pressure of water 55 x 10-4 atm at 25 1 m aqueous solution of sucrose also lowers water vapor pressure to 55 x 10-4 atm at 25
156
157
bc vapor-pressure lowering depends on concentration of nonelectrolyte solute and doesnrsquot depend on the type of solute it is a colligative proprerty
bc vapor pressure has been lowered solution remains liquid over larger temperature range
Lowers freezing point and raises boiling point
158
Can assume changes in boiling and freezing point also depend on concentration of solute
They are colligative properties
159
Freezing-Point Depression
Freezing point of 1-molal solution of any nonelectrolyte solute in water is found (by experiment) to be 186 lower than freezing point of water When 1 mol of nonelectrolyte solute dissolved in 1 kg water freezing point is -186 not 000 160
When 2 mol nonelectrolyte solute dissolved in 1 kg water freezing point is -372
For any concentration of nonelectrolyte solute in water decrease in freezing point can be determined using value of -186m
161
This value called the molal freezing-point contstant (Kf) is the freezing-point depression of the solvent in a 1-molal solution of a nonvolatile nonelectrolyte solute
162
Each solvent has own characteristic molal freezing-point constant Freezing-point depression (Δtf) the difference between the freezing points of the pure solvent and a solute of nonelectrolyte in that solvent and it is directly proportional to the molal concentration of the solution Δtf = Kfm
163
Δtf = Kfm
Kf expressed as m
m expressed in mol solutekg solvent (molality)
164
Sample Problem
What is the freezing point depression of water in a solution of 171 g of sucrose C12H22O11 and 200 g of water What is the actual freezing point of the solution
165
1 AnalyzeGiven Solute mass and chemical formula 171 g C12H22O11 Solvent mass and identity 2000 g water Unknown a freezing-point depression b freezing point of the solution 166
2 Plan
167
168
Calculate
Unknown a freezing-point depression b freezing point of the solution
169
Practice Problem
A water solution containing an unknown quantity of a nonelectrolyte solute is found to have a freezing point of ndash 023degCWhat is the molal concentration of the solution
012 m170
A solution consists of 103 g of the nonelectrolyte glucose C6H12O6 dissolved in 250 g of water What is the freezing-point depression of the solution
minus0426degC171
In a laboratory experiment the freezing point of an aqueous solution of glucose is found to be minus0325degCWhat is the molal concentration of this solution
0175 m172
If 0500 mol of a nonelectrolyte solute are dissolved in 5000 g of ether what is the freezing point of the solution
minus1181degC173
The freezing point of an aqueous solution that contains a nonelectrolyte is minus90degC aWhat is the freezing-point depression of the solution bWhat is the molal concentration of the solution a minus90degC b 48 m
174
Boiling-Point Elevation
Boiling point is temp at which vapor pressure is equal to atmospheric pressure Change in vapor pressure will cause corresponding change in boiling point
175
176
Vapor pressure of nonvolatile solution is lower than vapor pressure of pure solvent More heat will be required to raise vapor pressure of solution Boiling point of solution is higher than boiling point of pure solvent
Molal boiling-point constant (Kb) - boiling-point elevation of the solvent in a 1-molal solution of a nonvolatile nonelectrolyte solute b-p elevation of any nonelectrolyte in water found by experiment to be 051 So molal b-p constant for water is 051m
177
For different solvents b-p elevations of 1-molal solutions have different values (Table 14-2)
Like freezing-point constants values most accurate for dilute solutions
178
Boiling-point elevation Δtb difference between the boiling points of the pure solvent and a nonelectrolyte solution of that solvent and it is directly proportional to the molal concentration of the solution B-p elevation can be calculated using Δtb = Kbm Δtb (boiling point elevation) expressed in m and m expressed in mol solutekg solvent
Sample Problem
What is the boiling-point elevation of a solution made from 200 g of a nonelectrolyte solute and 4000 g of water The molar mass of the solute is 620 gmol
180
1 AnalyzeGiven solute mass = 200 g solute molar mass = 620 gmol solvent mass and identity = 4000 g of water
Unknown boiling-point elevation
181
2 Plan
182
183
3 Compute
Practice Problem
A solution contains 500 g of sucrose C12H22O11 a nonelectrolyte dissolved in 5000 g of water What is the boiling-point elevation
015degC184
A solution contains 4500 g of sucrose C12H22O11 a nonelectrolyte dissolved in 250 g of waterWhat is the boiling point of the solution
1027degC185
If the boiling point elevation of an aqueous solution containing a nonvolatile electrolyte is 102degC what is the molality of the solution
20 m186
The boiling point of an aqueous solution containing a nonvolatile electrolyte is 10075degC aWhat is the boiling-point elevation bWhat is the molality of the solution a 075degC b 15 m
187
Electrolytes and Colligative PropertiesEarly investigators confused by experiments where certain substances depressed freezing point or elevated boiling point more than expected
188
189
Ex 01 m solution of NaCl lowers freezing point of solvent almost twice as much as 01 m solution of sucrose 01 m CaCl2 lowers fp almost 3 times as much as 01 m solution of sucrose
To understand why this happens contrast behavior or sucrose with NaCl in aqueous solution Each sucrose dissolves to make only 1 particle in solution So 1 mol sucrose dissolves to 1 mol particles 1 mol NaCl dissolves to make 2 moles of particles (1 mol Na+ and 1 mol Cl-)
190
191
Calculated Values for Electrolyte SolutionsColligative properties depend on total concentration of solute particles regardless of their identity Electrolytes cause changes in colligative properties proportional to total molality in terms of all dissolves particles instead of formula units
192
What about barium nitrate Ba(NO3)2 Each mole of barium nitrate yields 3 mol of ions in solution Ba(NO3)2(s) rarr Ba2+(aq) + 2NO3-(aq) You would expect solution of given molality to lower fp of solvent 3 times as much as nonelectrolytic solution of same molality
193
Sample Problem
What is the expected change in the freezing point of water in a solution of 625 g of barium nitrate Ba(NO3)2 in 100 kg of water
194
1 AnalyzeGiven solute mass and formula = 625 g Ba(NO3)2 solvent mass and identity = 100 kg water Δtf = Kfm
Unknown expected freezing-point depression
195
2 PlanThe molality can be calculated by converting the solute mass to moles and then dividing by the number of kilograms of solvent That molality is in terms of formula units of Ba(NO3)2 and must be converted to molality in terms of dissociated ions in solution It must be multiplied by the number of moles of ions produced per mole of formula unit This adjusted molality can then be used to calculate the freezing-point depression
196
197
198
3 Compute
Practice Problem
What is the expected freezing-point depression for a solution that contains 20 mol of magnesium sulfate dissolved in 10 kg of water
minus74degC199
What is the expected boiling-point elevation of water for a solution that contains 150 g of sodium chloride dissolved in 10 kg of water 27degC The freezing point of an aqueous sodium chloride solution is minus200degC What is the molality of the solution 54 m NaCl
200
Actual Values for Electrolyte SolutionsRemember values calculated are only expected values Actual values of colligative properties for all strong electrolytes are almost what would be expected Difference between expected and actual values caused by attractive forces that exist between dissociated ions in aqueous solution
201
Attraction between hydrated ions in solution is small compared with those in crystalline solid However forces of attraction donrsquot interfere with movements of aqueous ions More concentrated solution means ions closer together and attraction is greater Also depends on charge of ions ndash higher charge greater attraction
202
General Solubility Guidelines
Sodium potassium and ammonium compounds are soluble in water
108
Nitrates acetates and chlorates are soluble
109
110
Most chlorides are soluble except those of silver mercury (I) and lead Lead (II) chloride is soluble in hot water
Most sulfates are soluble except those of barium strontium lead calcium and mercury
111
Most carbonates phosphates and silicates are insoluble except those of sodium potassium and ammonium
112
Most sulfides are insoluble except those of calcium strontium sodium potassium and ammonium
113
Is calcium phosphate Ca3(PO4)2 soluble or insoluble
Not one of the exceptions so it is insoluble
Dissociation equations are not written for insoluble compounds
114
Guidelines also useful in predicting what will happen if 2 solutions of 2 different soluble compounds are mixed
If mixing results in combination of ions that forms insoluble compound a double-replacement and precipitation reaction will happen
115
Will precipitate form when solutions of ammonium sulfide and cadmium nitrate are combined You can tell calcium nitrate is soluble from the guidelines (Most nitrates acetates and chlorates are soluble) You can also tell ammonium sulfide is soluble (Most sulfides are insoluble except those of calcium strontium sodium potassium and ammonium)
116
117
To decide whether precipitate can form must know solubilities of two possible products
Ammonium nitrate ndash soluble (Most nitrates acetates and chlorates are soluble) Cadmium sulfide ndash insoluble (Most sulfides are insoluble except those of calcium strontium sodium potassium and ammonium)
118
bc one product is insoluble double-replacement and precipitation reaction will happen (NH4)2S (aq) + Cd(NO3)2 (aq) rarr NH4NO3 (aq) + CdS (s)
119
Net Ionic Equations
Reactions of ions in aqueous solution usually represented by net ionic equations instead of formula equations Net ionic equation - includes only those compounds and ions that undergo a chemical change in a reaction in an aqueous solution
120
121
To write NIE (net ionic equation) first convert chemical equation into overall ionic equation All soluble ionic compounds shown as dissociated ions in solution Precipitates shown as solids
122
Notice ammonium and nitrate ions appear on both sides They havenrsquot gone through any chemical change Spectator ions - ions that do not take part in a chemical reaction and are found in solution both before and after the reaction
123
To change ionic equation to NIE eliminate spectator ions Cd+2(aq) + S-2(aq) rarr CdS(s)
This NIE applies to any reaction in which precipitate of CdS forms
124
Practice Problem
Identify the precipitate that forms when aqueous solutions of zinc nitrate and ammonium sulfide are combined Write the equation for the possible double-replacement reaction Then write the formula equation overall ionic equation and the net ionic equation
125
1 AnalyzeGiven Identity of reactants zinc nitrate and ammonium sulfide Reaction medium aqueous solution Unknown a equation for the possible double-replacement reaction b identity of the precipitate c formula equation d overall ionic equation e net ionic equation
126
2 PlanWrite the possible double-replacement reaction between Zn(NO3)2 and (NH4)2S Use the rules to determine if any of the products will precipitate Write a formula equation and overall net ionic equation Then cancel spectator ions to make net ionic equation
127
3 Computea possible double-replacement reaction
b rules show that zinc sulfide is not soluble so will precipitate (ammonium nitrate is soluble)
c formula equation128
d overall ionic equation
e net ionic equation129
Will a precipitate form if solutions of potassium sulfate and barium nitrate are combined If so write the net ionic equation for the reaction
Yes Ba+2(aq) + SO4-2(aq) rarr BaSO4(s)
130
Will a precipitate form if solutions of potassium nitrate and magnesium sulfate are combined If so write the net ionic equation for the reaction
No 131
Will a precipitate form if solutions of barium chloride and sodium sulfate are combined If so identify the spectator ions and write the net ionic equation for the reaction
Yes Na+ and Cl- Ba+2(aq) + SO4-2(aq) rarr BaSO4(s)
132
Write the net ionic equation for the precipitation of nickel(II) sulfide
Ni+2(aq) + S-2(aq) rarr NiS(s)
133
IonizationSome molecular compounds can also form ions in solution Usually polar Ionization - ions formed from solute molecules by action of the solvent (creation of ions where there were none) Ionization different from dissociation
134
Dissociation ionic compounds dissolve and ions already present separate Ionization ions formed where non existed before
Ions formed are hydrated Heat released during hydration of ions gives enough energy to break covalent bonds
135
Extent to which solute ionizes depends on strength of bonds within molecules of solute and strength of attraction between solute and solvent molecules
136
HCl ndash molecular compound that ionizes in aqueous solution Contains highly polar bond Attraction between polar HCl molecule and polar water strong enough to break HCl bond
137
The Hydronium Ion
H+ ions from HCl attracts other molecules or ions so strongly that it doesnrsquot normally exist alone Ionization of HCl better described as direct transfer of proton from HCl to H2O forming H3O+
138
139
Hydration of H+ to form hydronium is highly exothermic
Energy released gives a lot of energy needed to ionize a molecular solute
Many molecular compounds that ionize in aqueous solution contain hydrogen and form hydronium ion
140
Strong and Weak Electrolytes
Substances that make ions and conduct current in solution are electrolytes HCl is one of series of compounds that has hydrogen and halogen Hydrogen halides are all molecular compounds with single polar-covalent bonds
141
All are gases Very soluble in water All are electrolytes HCl HBr HI strongly conduct current in solution HF only weakly conducts current at same concentration Strength of conduction related to ability to ionize
142
143
Strong Electrolytes
HCl HBr HI are 100 ionized in solution
Strong electrolyte - any compound whose dilute aqueous solutions conduct electricity well this is due to the presence of all or almost all of the dissolve compound in the form of ions
144
Unique characteristic of strong electrolyte is that they yield only ions
Ex Ionic compound may be highly soluble in water and dissociate into ions (NaCl)
Others may not dissolve much but amount that does dissolve exists only as ions
145
Weak ElectrolytesHF dissolves in water to give acid solution called hydrofluoric acid HF bond stronger than bonds between hydrogen and other halogens When HF dissolves some molecule ionize Reverse reaction also happens HF(aq) + H2O(l) H3O+(aq) + F-(aq)
146
HF(aq) + H2O(l) H3O+
(aq) + F-(aq)Concentration of dissolved unionized HF stays high and concentration of ions stays low
Weak electrolyte - any compound whose aqueous solutions conduct electricity poorly this is due to the presence of small amount of the dissolved compound in the form of ions
147
Different from nonelectrolyte ndash NO ions at all Description of electrolyte as strong or weak not related to concentration of solution Electrolytes differ in degree of ionization not amount of solute dissolved
148
149
Colligative properties of solutions
Presence of solute affects properties of solutions Some properties not dependent on nature of dissolved substance but on how may dissolve particles are present
Colligative properties - properties that depend on the concentration of solute particles but not on their identity
150
Vapor-Pressure Lowering
Boiling and freezing point of solution different from pure solvent Nonvolatile solute raises boiling point and lowers freezing point Nonvolatile substance - one that has little tendency to become gas under existing conditions
151
152
To understand why nonvolatile solute changes boiling and freezing point consider equilibrium vapor pressure
Vapor pressure pressure caused by molecules that have escape liquid phase to gas phase
Can be thought of a measure of tendency of molecules to escape from a liquid
153
Addition of sucrose (nonvolatile solute) lowers concentration of water molecules at surface of liquid This lowers tendency of water molecules to leave solution and enter gas phase Vapor pressure of solution is lower than vapor pressure of pure water
154
Aqueous13 solution13 of13 nonvolatile13 solutePure13 water
Nonelectrolyte solutions of same molality have same concentration of particles
Dilute solutions of same solvent and equal molality of any nonelectrolyte solute lower vapor pressure equally
155
Example
1 m aqueous solution of nonelectrolyte glucose lowers vapor pressure of water 55 x 10-4 atm at 25 1 m aqueous solution of sucrose also lowers water vapor pressure to 55 x 10-4 atm at 25
156
157
bc vapor-pressure lowering depends on concentration of nonelectrolyte solute and doesnrsquot depend on the type of solute it is a colligative proprerty
bc vapor pressure has been lowered solution remains liquid over larger temperature range
Lowers freezing point and raises boiling point
158
Can assume changes in boiling and freezing point also depend on concentration of solute
They are colligative properties
159
Freezing-Point Depression
Freezing point of 1-molal solution of any nonelectrolyte solute in water is found (by experiment) to be 186 lower than freezing point of water When 1 mol of nonelectrolyte solute dissolved in 1 kg water freezing point is -186 not 000 160
When 2 mol nonelectrolyte solute dissolved in 1 kg water freezing point is -372
For any concentration of nonelectrolyte solute in water decrease in freezing point can be determined using value of -186m
161
This value called the molal freezing-point contstant (Kf) is the freezing-point depression of the solvent in a 1-molal solution of a nonvolatile nonelectrolyte solute
162
Each solvent has own characteristic molal freezing-point constant Freezing-point depression (Δtf) the difference between the freezing points of the pure solvent and a solute of nonelectrolyte in that solvent and it is directly proportional to the molal concentration of the solution Δtf = Kfm
163
Δtf = Kfm
Kf expressed as m
m expressed in mol solutekg solvent (molality)
164
Sample Problem
What is the freezing point depression of water in a solution of 171 g of sucrose C12H22O11 and 200 g of water What is the actual freezing point of the solution
165
1 AnalyzeGiven Solute mass and chemical formula 171 g C12H22O11 Solvent mass and identity 2000 g water Unknown a freezing-point depression b freezing point of the solution 166
2 Plan
167
168
Calculate
Unknown a freezing-point depression b freezing point of the solution
169
Practice Problem
A water solution containing an unknown quantity of a nonelectrolyte solute is found to have a freezing point of ndash 023degCWhat is the molal concentration of the solution
012 m170
A solution consists of 103 g of the nonelectrolyte glucose C6H12O6 dissolved in 250 g of water What is the freezing-point depression of the solution
minus0426degC171
In a laboratory experiment the freezing point of an aqueous solution of glucose is found to be minus0325degCWhat is the molal concentration of this solution
0175 m172
If 0500 mol of a nonelectrolyte solute are dissolved in 5000 g of ether what is the freezing point of the solution
minus1181degC173
The freezing point of an aqueous solution that contains a nonelectrolyte is minus90degC aWhat is the freezing-point depression of the solution bWhat is the molal concentration of the solution a minus90degC b 48 m
174
Boiling-Point Elevation
Boiling point is temp at which vapor pressure is equal to atmospheric pressure Change in vapor pressure will cause corresponding change in boiling point
175
176
Vapor pressure of nonvolatile solution is lower than vapor pressure of pure solvent More heat will be required to raise vapor pressure of solution Boiling point of solution is higher than boiling point of pure solvent
Molal boiling-point constant (Kb) - boiling-point elevation of the solvent in a 1-molal solution of a nonvolatile nonelectrolyte solute b-p elevation of any nonelectrolyte in water found by experiment to be 051 So molal b-p constant for water is 051m
177
For different solvents b-p elevations of 1-molal solutions have different values (Table 14-2)
Like freezing-point constants values most accurate for dilute solutions
178
Boiling-point elevation Δtb difference between the boiling points of the pure solvent and a nonelectrolyte solution of that solvent and it is directly proportional to the molal concentration of the solution B-p elevation can be calculated using Δtb = Kbm Δtb (boiling point elevation) expressed in m and m expressed in mol solutekg solvent
Sample Problem
What is the boiling-point elevation of a solution made from 200 g of a nonelectrolyte solute and 4000 g of water The molar mass of the solute is 620 gmol
180
1 AnalyzeGiven solute mass = 200 g solute molar mass = 620 gmol solvent mass and identity = 4000 g of water
Unknown boiling-point elevation
181
2 Plan
182
183
3 Compute
Practice Problem
A solution contains 500 g of sucrose C12H22O11 a nonelectrolyte dissolved in 5000 g of water What is the boiling-point elevation
015degC184
A solution contains 4500 g of sucrose C12H22O11 a nonelectrolyte dissolved in 250 g of waterWhat is the boiling point of the solution
1027degC185
If the boiling point elevation of an aqueous solution containing a nonvolatile electrolyte is 102degC what is the molality of the solution
20 m186
The boiling point of an aqueous solution containing a nonvolatile electrolyte is 10075degC aWhat is the boiling-point elevation bWhat is the molality of the solution a 075degC b 15 m
187
Electrolytes and Colligative PropertiesEarly investigators confused by experiments where certain substances depressed freezing point or elevated boiling point more than expected
188
189
Ex 01 m solution of NaCl lowers freezing point of solvent almost twice as much as 01 m solution of sucrose 01 m CaCl2 lowers fp almost 3 times as much as 01 m solution of sucrose
To understand why this happens contrast behavior or sucrose with NaCl in aqueous solution Each sucrose dissolves to make only 1 particle in solution So 1 mol sucrose dissolves to 1 mol particles 1 mol NaCl dissolves to make 2 moles of particles (1 mol Na+ and 1 mol Cl-)
190
191
Calculated Values for Electrolyte SolutionsColligative properties depend on total concentration of solute particles regardless of their identity Electrolytes cause changes in colligative properties proportional to total molality in terms of all dissolves particles instead of formula units
192
What about barium nitrate Ba(NO3)2 Each mole of barium nitrate yields 3 mol of ions in solution Ba(NO3)2(s) rarr Ba2+(aq) + 2NO3-(aq) You would expect solution of given molality to lower fp of solvent 3 times as much as nonelectrolytic solution of same molality
193
Sample Problem
What is the expected change in the freezing point of water in a solution of 625 g of barium nitrate Ba(NO3)2 in 100 kg of water
194
1 AnalyzeGiven solute mass and formula = 625 g Ba(NO3)2 solvent mass and identity = 100 kg water Δtf = Kfm
Unknown expected freezing-point depression
195
2 PlanThe molality can be calculated by converting the solute mass to moles and then dividing by the number of kilograms of solvent That molality is in terms of formula units of Ba(NO3)2 and must be converted to molality in terms of dissociated ions in solution It must be multiplied by the number of moles of ions produced per mole of formula unit This adjusted molality can then be used to calculate the freezing-point depression
196
197
198
3 Compute
Practice Problem
What is the expected freezing-point depression for a solution that contains 20 mol of magnesium sulfate dissolved in 10 kg of water
minus74degC199
What is the expected boiling-point elevation of water for a solution that contains 150 g of sodium chloride dissolved in 10 kg of water 27degC The freezing point of an aqueous sodium chloride solution is minus200degC What is the molality of the solution 54 m NaCl
200
Actual Values for Electrolyte SolutionsRemember values calculated are only expected values Actual values of colligative properties for all strong electrolytes are almost what would be expected Difference between expected and actual values caused by attractive forces that exist between dissociated ions in aqueous solution
201
Attraction between hydrated ions in solution is small compared with those in crystalline solid However forces of attraction donrsquot interfere with movements of aqueous ions More concentrated solution means ions closer together and attraction is greater Also depends on charge of ions ndash higher charge greater attraction
202
Nitrates acetates and chlorates are soluble
109
110
Most chlorides are soluble except those of silver mercury (I) and lead Lead (II) chloride is soluble in hot water
Most sulfates are soluble except those of barium strontium lead calcium and mercury
111
Most carbonates phosphates and silicates are insoluble except those of sodium potassium and ammonium
112
Most sulfides are insoluble except those of calcium strontium sodium potassium and ammonium
113
Is calcium phosphate Ca3(PO4)2 soluble or insoluble
Not one of the exceptions so it is insoluble
Dissociation equations are not written for insoluble compounds
114
Guidelines also useful in predicting what will happen if 2 solutions of 2 different soluble compounds are mixed
If mixing results in combination of ions that forms insoluble compound a double-replacement and precipitation reaction will happen
115
Will precipitate form when solutions of ammonium sulfide and cadmium nitrate are combined You can tell calcium nitrate is soluble from the guidelines (Most nitrates acetates and chlorates are soluble) You can also tell ammonium sulfide is soluble (Most sulfides are insoluble except those of calcium strontium sodium potassium and ammonium)
116
117
To decide whether precipitate can form must know solubilities of two possible products
Ammonium nitrate ndash soluble (Most nitrates acetates and chlorates are soluble) Cadmium sulfide ndash insoluble (Most sulfides are insoluble except those of calcium strontium sodium potassium and ammonium)
118
bc one product is insoluble double-replacement and precipitation reaction will happen (NH4)2S (aq) + Cd(NO3)2 (aq) rarr NH4NO3 (aq) + CdS (s)
119
Net Ionic Equations
Reactions of ions in aqueous solution usually represented by net ionic equations instead of formula equations Net ionic equation - includes only those compounds and ions that undergo a chemical change in a reaction in an aqueous solution
120
121
To write NIE (net ionic equation) first convert chemical equation into overall ionic equation All soluble ionic compounds shown as dissociated ions in solution Precipitates shown as solids
122
Notice ammonium and nitrate ions appear on both sides They havenrsquot gone through any chemical change Spectator ions - ions that do not take part in a chemical reaction and are found in solution both before and after the reaction
123
To change ionic equation to NIE eliminate spectator ions Cd+2(aq) + S-2(aq) rarr CdS(s)
This NIE applies to any reaction in which precipitate of CdS forms
124
Practice Problem
Identify the precipitate that forms when aqueous solutions of zinc nitrate and ammonium sulfide are combined Write the equation for the possible double-replacement reaction Then write the formula equation overall ionic equation and the net ionic equation
125
1 AnalyzeGiven Identity of reactants zinc nitrate and ammonium sulfide Reaction medium aqueous solution Unknown a equation for the possible double-replacement reaction b identity of the precipitate c formula equation d overall ionic equation e net ionic equation
126
2 PlanWrite the possible double-replacement reaction between Zn(NO3)2 and (NH4)2S Use the rules to determine if any of the products will precipitate Write a formula equation and overall net ionic equation Then cancel spectator ions to make net ionic equation
127
3 Computea possible double-replacement reaction
b rules show that zinc sulfide is not soluble so will precipitate (ammonium nitrate is soluble)
c formula equation128
d overall ionic equation
e net ionic equation129
Will a precipitate form if solutions of potassium sulfate and barium nitrate are combined If so write the net ionic equation for the reaction
Yes Ba+2(aq) + SO4-2(aq) rarr BaSO4(s)
130
Will a precipitate form if solutions of potassium nitrate and magnesium sulfate are combined If so write the net ionic equation for the reaction
No 131
Will a precipitate form if solutions of barium chloride and sodium sulfate are combined If so identify the spectator ions and write the net ionic equation for the reaction
Yes Na+ and Cl- Ba+2(aq) + SO4-2(aq) rarr BaSO4(s)
132
Write the net ionic equation for the precipitation of nickel(II) sulfide
Ni+2(aq) + S-2(aq) rarr NiS(s)
133
IonizationSome molecular compounds can also form ions in solution Usually polar Ionization - ions formed from solute molecules by action of the solvent (creation of ions where there were none) Ionization different from dissociation
134
Dissociation ionic compounds dissolve and ions already present separate Ionization ions formed where non existed before
Ions formed are hydrated Heat released during hydration of ions gives enough energy to break covalent bonds
135
Extent to which solute ionizes depends on strength of bonds within molecules of solute and strength of attraction between solute and solvent molecules
136
HCl ndash molecular compound that ionizes in aqueous solution Contains highly polar bond Attraction between polar HCl molecule and polar water strong enough to break HCl bond
137
The Hydronium Ion
H+ ions from HCl attracts other molecules or ions so strongly that it doesnrsquot normally exist alone Ionization of HCl better described as direct transfer of proton from HCl to H2O forming H3O+
138
139
Hydration of H+ to form hydronium is highly exothermic
Energy released gives a lot of energy needed to ionize a molecular solute
Many molecular compounds that ionize in aqueous solution contain hydrogen and form hydronium ion
140
Strong and Weak Electrolytes
Substances that make ions and conduct current in solution are electrolytes HCl is one of series of compounds that has hydrogen and halogen Hydrogen halides are all molecular compounds with single polar-covalent bonds
141
All are gases Very soluble in water All are electrolytes HCl HBr HI strongly conduct current in solution HF only weakly conducts current at same concentration Strength of conduction related to ability to ionize
142
143
Strong Electrolytes
HCl HBr HI are 100 ionized in solution
Strong electrolyte - any compound whose dilute aqueous solutions conduct electricity well this is due to the presence of all or almost all of the dissolve compound in the form of ions
144
Unique characteristic of strong electrolyte is that they yield only ions
Ex Ionic compound may be highly soluble in water and dissociate into ions (NaCl)
Others may not dissolve much but amount that does dissolve exists only as ions
145
Weak ElectrolytesHF dissolves in water to give acid solution called hydrofluoric acid HF bond stronger than bonds between hydrogen and other halogens When HF dissolves some molecule ionize Reverse reaction also happens HF(aq) + H2O(l) H3O+(aq) + F-(aq)
146
HF(aq) + H2O(l) H3O+
(aq) + F-(aq)Concentration of dissolved unionized HF stays high and concentration of ions stays low
Weak electrolyte - any compound whose aqueous solutions conduct electricity poorly this is due to the presence of small amount of the dissolved compound in the form of ions
147
Different from nonelectrolyte ndash NO ions at all Description of electrolyte as strong or weak not related to concentration of solution Electrolytes differ in degree of ionization not amount of solute dissolved
148
149
Colligative properties of solutions
Presence of solute affects properties of solutions Some properties not dependent on nature of dissolved substance but on how may dissolve particles are present
Colligative properties - properties that depend on the concentration of solute particles but not on their identity
150
Vapor-Pressure Lowering
Boiling and freezing point of solution different from pure solvent Nonvolatile solute raises boiling point and lowers freezing point Nonvolatile substance - one that has little tendency to become gas under existing conditions
151
152
To understand why nonvolatile solute changes boiling and freezing point consider equilibrium vapor pressure
Vapor pressure pressure caused by molecules that have escape liquid phase to gas phase
Can be thought of a measure of tendency of molecules to escape from a liquid
153
Addition of sucrose (nonvolatile solute) lowers concentration of water molecules at surface of liquid This lowers tendency of water molecules to leave solution and enter gas phase Vapor pressure of solution is lower than vapor pressure of pure water
154
Aqueous13 solution13 of13 nonvolatile13 solutePure13 water
Nonelectrolyte solutions of same molality have same concentration of particles
Dilute solutions of same solvent and equal molality of any nonelectrolyte solute lower vapor pressure equally
155
Example
1 m aqueous solution of nonelectrolyte glucose lowers vapor pressure of water 55 x 10-4 atm at 25 1 m aqueous solution of sucrose also lowers water vapor pressure to 55 x 10-4 atm at 25
156
157
bc vapor-pressure lowering depends on concentration of nonelectrolyte solute and doesnrsquot depend on the type of solute it is a colligative proprerty
bc vapor pressure has been lowered solution remains liquid over larger temperature range
Lowers freezing point and raises boiling point
158
Can assume changes in boiling and freezing point also depend on concentration of solute
They are colligative properties
159
Freezing-Point Depression
Freezing point of 1-molal solution of any nonelectrolyte solute in water is found (by experiment) to be 186 lower than freezing point of water When 1 mol of nonelectrolyte solute dissolved in 1 kg water freezing point is -186 not 000 160
When 2 mol nonelectrolyte solute dissolved in 1 kg water freezing point is -372
For any concentration of nonelectrolyte solute in water decrease in freezing point can be determined using value of -186m
161
This value called the molal freezing-point contstant (Kf) is the freezing-point depression of the solvent in a 1-molal solution of a nonvolatile nonelectrolyte solute
162
Each solvent has own characteristic molal freezing-point constant Freezing-point depression (Δtf) the difference between the freezing points of the pure solvent and a solute of nonelectrolyte in that solvent and it is directly proportional to the molal concentration of the solution Δtf = Kfm
163
Δtf = Kfm
Kf expressed as m
m expressed in mol solutekg solvent (molality)
164
Sample Problem
What is the freezing point depression of water in a solution of 171 g of sucrose C12H22O11 and 200 g of water What is the actual freezing point of the solution
165
1 AnalyzeGiven Solute mass and chemical formula 171 g C12H22O11 Solvent mass and identity 2000 g water Unknown a freezing-point depression b freezing point of the solution 166
2 Plan
167
168
Calculate
Unknown a freezing-point depression b freezing point of the solution
169
Practice Problem
A water solution containing an unknown quantity of a nonelectrolyte solute is found to have a freezing point of ndash 023degCWhat is the molal concentration of the solution
012 m170
A solution consists of 103 g of the nonelectrolyte glucose C6H12O6 dissolved in 250 g of water What is the freezing-point depression of the solution
minus0426degC171
In a laboratory experiment the freezing point of an aqueous solution of glucose is found to be minus0325degCWhat is the molal concentration of this solution
0175 m172
If 0500 mol of a nonelectrolyte solute are dissolved in 5000 g of ether what is the freezing point of the solution
minus1181degC173
The freezing point of an aqueous solution that contains a nonelectrolyte is minus90degC aWhat is the freezing-point depression of the solution bWhat is the molal concentration of the solution a minus90degC b 48 m
174
Boiling-Point Elevation
Boiling point is temp at which vapor pressure is equal to atmospheric pressure Change in vapor pressure will cause corresponding change in boiling point
175
176
Vapor pressure of nonvolatile solution is lower than vapor pressure of pure solvent More heat will be required to raise vapor pressure of solution Boiling point of solution is higher than boiling point of pure solvent
Molal boiling-point constant (Kb) - boiling-point elevation of the solvent in a 1-molal solution of a nonvolatile nonelectrolyte solute b-p elevation of any nonelectrolyte in water found by experiment to be 051 So molal b-p constant for water is 051m
177
For different solvents b-p elevations of 1-molal solutions have different values (Table 14-2)
Like freezing-point constants values most accurate for dilute solutions
178
Boiling-point elevation Δtb difference between the boiling points of the pure solvent and a nonelectrolyte solution of that solvent and it is directly proportional to the molal concentration of the solution B-p elevation can be calculated using Δtb = Kbm Δtb (boiling point elevation) expressed in m and m expressed in mol solutekg solvent
Sample Problem
What is the boiling-point elevation of a solution made from 200 g of a nonelectrolyte solute and 4000 g of water The molar mass of the solute is 620 gmol
180
1 AnalyzeGiven solute mass = 200 g solute molar mass = 620 gmol solvent mass and identity = 4000 g of water
Unknown boiling-point elevation
181
2 Plan
182
183
3 Compute
Practice Problem
A solution contains 500 g of sucrose C12H22O11 a nonelectrolyte dissolved in 5000 g of water What is the boiling-point elevation
015degC184
A solution contains 4500 g of sucrose C12H22O11 a nonelectrolyte dissolved in 250 g of waterWhat is the boiling point of the solution
1027degC185
If the boiling point elevation of an aqueous solution containing a nonvolatile electrolyte is 102degC what is the molality of the solution
20 m186
The boiling point of an aqueous solution containing a nonvolatile electrolyte is 10075degC aWhat is the boiling-point elevation bWhat is the molality of the solution a 075degC b 15 m
187
Electrolytes and Colligative PropertiesEarly investigators confused by experiments where certain substances depressed freezing point or elevated boiling point more than expected
188
189
Ex 01 m solution of NaCl lowers freezing point of solvent almost twice as much as 01 m solution of sucrose 01 m CaCl2 lowers fp almost 3 times as much as 01 m solution of sucrose
To understand why this happens contrast behavior or sucrose with NaCl in aqueous solution Each sucrose dissolves to make only 1 particle in solution So 1 mol sucrose dissolves to 1 mol particles 1 mol NaCl dissolves to make 2 moles of particles (1 mol Na+ and 1 mol Cl-)
190
191
Calculated Values for Electrolyte SolutionsColligative properties depend on total concentration of solute particles regardless of their identity Electrolytes cause changes in colligative properties proportional to total molality in terms of all dissolves particles instead of formula units
192
What about barium nitrate Ba(NO3)2 Each mole of barium nitrate yields 3 mol of ions in solution Ba(NO3)2(s) rarr Ba2+(aq) + 2NO3-(aq) You would expect solution of given molality to lower fp of solvent 3 times as much as nonelectrolytic solution of same molality
193
Sample Problem
What is the expected change in the freezing point of water in a solution of 625 g of barium nitrate Ba(NO3)2 in 100 kg of water
194
1 AnalyzeGiven solute mass and formula = 625 g Ba(NO3)2 solvent mass and identity = 100 kg water Δtf = Kfm
Unknown expected freezing-point depression
195
2 PlanThe molality can be calculated by converting the solute mass to moles and then dividing by the number of kilograms of solvent That molality is in terms of formula units of Ba(NO3)2 and must be converted to molality in terms of dissociated ions in solution It must be multiplied by the number of moles of ions produced per mole of formula unit This adjusted molality can then be used to calculate the freezing-point depression
196
197
198
3 Compute
Practice Problem
What is the expected freezing-point depression for a solution that contains 20 mol of magnesium sulfate dissolved in 10 kg of water
minus74degC199
What is the expected boiling-point elevation of water for a solution that contains 150 g of sodium chloride dissolved in 10 kg of water 27degC The freezing point of an aqueous sodium chloride solution is minus200degC What is the molality of the solution 54 m NaCl
200
Actual Values for Electrolyte SolutionsRemember values calculated are only expected values Actual values of colligative properties for all strong electrolytes are almost what would be expected Difference between expected and actual values caused by attractive forces that exist between dissociated ions in aqueous solution
201
Attraction between hydrated ions in solution is small compared with those in crystalline solid However forces of attraction donrsquot interfere with movements of aqueous ions More concentrated solution means ions closer together and attraction is greater Also depends on charge of ions ndash higher charge greater attraction
202
110
Most chlorides are soluble except those of silver mercury (I) and lead Lead (II) chloride is soluble in hot water
Most sulfates are soluble except those of barium strontium lead calcium and mercury
111
Most carbonates phosphates and silicates are insoluble except those of sodium potassium and ammonium
112
Most sulfides are insoluble except those of calcium strontium sodium potassium and ammonium
113
Is calcium phosphate Ca3(PO4)2 soluble or insoluble
Not one of the exceptions so it is insoluble
Dissociation equations are not written for insoluble compounds
114
Guidelines also useful in predicting what will happen if 2 solutions of 2 different soluble compounds are mixed
If mixing results in combination of ions that forms insoluble compound a double-replacement and precipitation reaction will happen
115
Will precipitate form when solutions of ammonium sulfide and cadmium nitrate are combined You can tell calcium nitrate is soluble from the guidelines (Most nitrates acetates and chlorates are soluble) You can also tell ammonium sulfide is soluble (Most sulfides are insoluble except those of calcium strontium sodium potassium and ammonium)
116
117
To decide whether precipitate can form must know solubilities of two possible products
Ammonium nitrate ndash soluble (Most nitrates acetates and chlorates are soluble) Cadmium sulfide ndash insoluble (Most sulfides are insoluble except those of calcium strontium sodium potassium and ammonium)
118
bc one product is insoluble double-replacement and precipitation reaction will happen (NH4)2S (aq) + Cd(NO3)2 (aq) rarr NH4NO3 (aq) + CdS (s)
119
Net Ionic Equations
Reactions of ions in aqueous solution usually represented by net ionic equations instead of formula equations Net ionic equation - includes only those compounds and ions that undergo a chemical change in a reaction in an aqueous solution
120
121
To write NIE (net ionic equation) first convert chemical equation into overall ionic equation All soluble ionic compounds shown as dissociated ions in solution Precipitates shown as solids
122
Notice ammonium and nitrate ions appear on both sides They havenrsquot gone through any chemical change Spectator ions - ions that do not take part in a chemical reaction and are found in solution both before and after the reaction
123
To change ionic equation to NIE eliminate spectator ions Cd+2(aq) + S-2(aq) rarr CdS(s)
This NIE applies to any reaction in which precipitate of CdS forms
124
Practice Problem
Identify the precipitate that forms when aqueous solutions of zinc nitrate and ammonium sulfide are combined Write the equation for the possible double-replacement reaction Then write the formula equation overall ionic equation and the net ionic equation
125
1 AnalyzeGiven Identity of reactants zinc nitrate and ammonium sulfide Reaction medium aqueous solution Unknown a equation for the possible double-replacement reaction b identity of the precipitate c formula equation d overall ionic equation e net ionic equation
126
2 PlanWrite the possible double-replacement reaction between Zn(NO3)2 and (NH4)2S Use the rules to determine if any of the products will precipitate Write a formula equation and overall net ionic equation Then cancel spectator ions to make net ionic equation
127
3 Computea possible double-replacement reaction
b rules show that zinc sulfide is not soluble so will precipitate (ammonium nitrate is soluble)
c formula equation128
d overall ionic equation
e net ionic equation129
Will a precipitate form if solutions of potassium sulfate and barium nitrate are combined If so write the net ionic equation for the reaction
Yes Ba+2(aq) + SO4-2(aq) rarr BaSO4(s)
130
Will a precipitate form if solutions of potassium nitrate and magnesium sulfate are combined If so write the net ionic equation for the reaction
No 131
Will a precipitate form if solutions of barium chloride and sodium sulfate are combined If so identify the spectator ions and write the net ionic equation for the reaction
Yes Na+ and Cl- Ba+2(aq) + SO4-2(aq) rarr BaSO4(s)
132
Write the net ionic equation for the precipitation of nickel(II) sulfide
Ni+2(aq) + S-2(aq) rarr NiS(s)
133
IonizationSome molecular compounds can also form ions in solution Usually polar Ionization - ions formed from solute molecules by action of the solvent (creation of ions where there were none) Ionization different from dissociation
134
Dissociation ionic compounds dissolve and ions already present separate Ionization ions formed where non existed before
Ions formed are hydrated Heat released during hydration of ions gives enough energy to break covalent bonds
135
Extent to which solute ionizes depends on strength of bonds within molecules of solute and strength of attraction between solute and solvent molecules
136
HCl ndash molecular compound that ionizes in aqueous solution Contains highly polar bond Attraction between polar HCl molecule and polar water strong enough to break HCl bond
137
The Hydronium Ion
H+ ions from HCl attracts other molecules or ions so strongly that it doesnrsquot normally exist alone Ionization of HCl better described as direct transfer of proton from HCl to H2O forming H3O+
138
139
Hydration of H+ to form hydronium is highly exothermic
Energy released gives a lot of energy needed to ionize a molecular solute
Many molecular compounds that ionize in aqueous solution contain hydrogen and form hydronium ion
140
Strong and Weak Electrolytes
Substances that make ions and conduct current in solution are electrolytes HCl is one of series of compounds that has hydrogen and halogen Hydrogen halides are all molecular compounds with single polar-covalent bonds
141
All are gases Very soluble in water All are electrolytes HCl HBr HI strongly conduct current in solution HF only weakly conducts current at same concentration Strength of conduction related to ability to ionize
142
143
Strong Electrolytes
HCl HBr HI are 100 ionized in solution
Strong electrolyte - any compound whose dilute aqueous solutions conduct electricity well this is due to the presence of all or almost all of the dissolve compound in the form of ions
144
Unique characteristic of strong electrolyte is that they yield only ions
Ex Ionic compound may be highly soluble in water and dissociate into ions (NaCl)
Others may not dissolve much but amount that does dissolve exists only as ions
145
Weak ElectrolytesHF dissolves in water to give acid solution called hydrofluoric acid HF bond stronger than bonds between hydrogen and other halogens When HF dissolves some molecule ionize Reverse reaction also happens HF(aq) + H2O(l) H3O+(aq) + F-(aq)
146
HF(aq) + H2O(l) H3O+
(aq) + F-(aq)Concentration of dissolved unionized HF stays high and concentration of ions stays low
Weak electrolyte - any compound whose aqueous solutions conduct electricity poorly this is due to the presence of small amount of the dissolved compound in the form of ions
147
Different from nonelectrolyte ndash NO ions at all Description of electrolyte as strong or weak not related to concentration of solution Electrolytes differ in degree of ionization not amount of solute dissolved
148
149
Colligative properties of solutions
Presence of solute affects properties of solutions Some properties not dependent on nature of dissolved substance but on how may dissolve particles are present
Colligative properties - properties that depend on the concentration of solute particles but not on their identity
150
Vapor-Pressure Lowering
Boiling and freezing point of solution different from pure solvent Nonvolatile solute raises boiling point and lowers freezing point Nonvolatile substance - one that has little tendency to become gas under existing conditions
151
152
To understand why nonvolatile solute changes boiling and freezing point consider equilibrium vapor pressure
Vapor pressure pressure caused by molecules that have escape liquid phase to gas phase
Can be thought of a measure of tendency of molecules to escape from a liquid
153
Addition of sucrose (nonvolatile solute) lowers concentration of water molecules at surface of liquid This lowers tendency of water molecules to leave solution and enter gas phase Vapor pressure of solution is lower than vapor pressure of pure water
154
Aqueous13 solution13 of13 nonvolatile13 solutePure13 water
Nonelectrolyte solutions of same molality have same concentration of particles
Dilute solutions of same solvent and equal molality of any nonelectrolyte solute lower vapor pressure equally
155
Example
1 m aqueous solution of nonelectrolyte glucose lowers vapor pressure of water 55 x 10-4 atm at 25 1 m aqueous solution of sucrose also lowers water vapor pressure to 55 x 10-4 atm at 25
156
157
bc vapor-pressure lowering depends on concentration of nonelectrolyte solute and doesnrsquot depend on the type of solute it is a colligative proprerty
bc vapor pressure has been lowered solution remains liquid over larger temperature range
Lowers freezing point and raises boiling point
158
Can assume changes in boiling and freezing point also depend on concentration of solute
They are colligative properties
159
Freezing-Point Depression
Freezing point of 1-molal solution of any nonelectrolyte solute in water is found (by experiment) to be 186 lower than freezing point of water When 1 mol of nonelectrolyte solute dissolved in 1 kg water freezing point is -186 not 000 160
When 2 mol nonelectrolyte solute dissolved in 1 kg water freezing point is -372
For any concentration of nonelectrolyte solute in water decrease in freezing point can be determined using value of -186m
161
This value called the molal freezing-point contstant (Kf) is the freezing-point depression of the solvent in a 1-molal solution of a nonvolatile nonelectrolyte solute
162
Each solvent has own characteristic molal freezing-point constant Freezing-point depression (Δtf) the difference between the freezing points of the pure solvent and a solute of nonelectrolyte in that solvent and it is directly proportional to the molal concentration of the solution Δtf = Kfm
163
Δtf = Kfm
Kf expressed as m
m expressed in mol solutekg solvent (molality)
164
Sample Problem
What is the freezing point depression of water in a solution of 171 g of sucrose C12H22O11 and 200 g of water What is the actual freezing point of the solution
165
1 AnalyzeGiven Solute mass and chemical formula 171 g C12H22O11 Solvent mass and identity 2000 g water Unknown a freezing-point depression b freezing point of the solution 166
2 Plan
167
168
Calculate
Unknown a freezing-point depression b freezing point of the solution
169
Practice Problem
A water solution containing an unknown quantity of a nonelectrolyte solute is found to have a freezing point of ndash 023degCWhat is the molal concentration of the solution
012 m170
A solution consists of 103 g of the nonelectrolyte glucose C6H12O6 dissolved in 250 g of water What is the freezing-point depression of the solution
minus0426degC171
In a laboratory experiment the freezing point of an aqueous solution of glucose is found to be minus0325degCWhat is the molal concentration of this solution
0175 m172
If 0500 mol of a nonelectrolyte solute are dissolved in 5000 g of ether what is the freezing point of the solution
minus1181degC173
The freezing point of an aqueous solution that contains a nonelectrolyte is minus90degC aWhat is the freezing-point depression of the solution bWhat is the molal concentration of the solution a minus90degC b 48 m
174
Boiling-Point Elevation
Boiling point is temp at which vapor pressure is equal to atmospheric pressure Change in vapor pressure will cause corresponding change in boiling point
175
176
Vapor pressure of nonvolatile solution is lower than vapor pressure of pure solvent More heat will be required to raise vapor pressure of solution Boiling point of solution is higher than boiling point of pure solvent
Molal boiling-point constant (Kb) - boiling-point elevation of the solvent in a 1-molal solution of a nonvolatile nonelectrolyte solute b-p elevation of any nonelectrolyte in water found by experiment to be 051 So molal b-p constant for water is 051m
177
For different solvents b-p elevations of 1-molal solutions have different values (Table 14-2)
Like freezing-point constants values most accurate for dilute solutions
178
Boiling-point elevation Δtb difference between the boiling points of the pure solvent and a nonelectrolyte solution of that solvent and it is directly proportional to the molal concentration of the solution B-p elevation can be calculated using Δtb = Kbm Δtb (boiling point elevation) expressed in m and m expressed in mol solutekg solvent
Sample Problem
What is the boiling-point elevation of a solution made from 200 g of a nonelectrolyte solute and 4000 g of water The molar mass of the solute is 620 gmol
180
1 AnalyzeGiven solute mass = 200 g solute molar mass = 620 gmol solvent mass and identity = 4000 g of water
Unknown boiling-point elevation
181
2 Plan
182
183
3 Compute
Practice Problem
A solution contains 500 g of sucrose C12H22O11 a nonelectrolyte dissolved in 5000 g of water What is the boiling-point elevation
015degC184
A solution contains 4500 g of sucrose C12H22O11 a nonelectrolyte dissolved in 250 g of waterWhat is the boiling point of the solution
1027degC185
If the boiling point elevation of an aqueous solution containing a nonvolatile electrolyte is 102degC what is the molality of the solution
20 m186
The boiling point of an aqueous solution containing a nonvolatile electrolyte is 10075degC aWhat is the boiling-point elevation bWhat is the molality of the solution a 075degC b 15 m
187
Electrolytes and Colligative PropertiesEarly investigators confused by experiments where certain substances depressed freezing point or elevated boiling point more than expected
188
189
Ex 01 m solution of NaCl lowers freezing point of solvent almost twice as much as 01 m solution of sucrose 01 m CaCl2 lowers fp almost 3 times as much as 01 m solution of sucrose
To understand why this happens contrast behavior or sucrose with NaCl in aqueous solution Each sucrose dissolves to make only 1 particle in solution So 1 mol sucrose dissolves to 1 mol particles 1 mol NaCl dissolves to make 2 moles of particles (1 mol Na+ and 1 mol Cl-)
190
191
Calculated Values for Electrolyte SolutionsColligative properties depend on total concentration of solute particles regardless of their identity Electrolytes cause changes in colligative properties proportional to total molality in terms of all dissolves particles instead of formula units
192
What about barium nitrate Ba(NO3)2 Each mole of barium nitrate yields 3 mol of ions in solution Ba(NO3)2(s) rarr Ba2+(aq) + 2NO3-(aq) You would expect solution of given molality to lower fp of solvent 3 times as much as nonelectrolytic solution of same molality
193
Sample Problem
What is the expected change in the freezing point of water in a solution of 625 g of barium nitrate Ba(NO3)2 in 100 kg of water
194
1 AnalyzeGiven solute mass and formula = 625 g Ba(NO3)2 solvent mass and identity = 100 kg water Δtf = Kfm
Unknown expected freezing-point depression
195
2 PlanThe molality can be calculated by converting the solute mass to moles and then dividing by the number of kilograms of solvent That molality is in terms of formula units of Ba(NO3)2 and must be converted to molality in terms of dissociated ions in solution It must be multiplied by the number of moles of ions produced per mole of formula unit This adjusted molality can then be used to calculate the freezing-point depression
196
197
198
3 Compute
Practice Problem
What is the expected freezing-point depression for a solution that contains 20 mol of magnesium sulfate dissolved in 10 kg of water
minus74degC199
What is the expected boiling-point elevation of water for a solution that contains 150 g of sodium chloride dissolved in 10 kg of water 27degC The freezing point of an aqueous sodium chloride solution is minus200degC What is the molality of the solution 54 m NaCl
200
Actual Values for Electrolyte SolutionsRemember values calculated are only expected values Actual values of colligative properties for all strong electrolytes are almost what would be expected Difference between expected and actual values caused by attractive forces that exist between dissociated ions in aqueous solution
201
Attraction between hydrated ions in solution is small compared with those in crystalline solid However forces of attraction donrsquot interfere with movements of aqueous ions More concentrated solution means ions closer together and attraction is greater Also depends on charge of ions ndash higher charge greater attraction
202
Most sulfates are soluble except those of barium strontium lead calcium and mercury
111
Most carbonates phosphates and silicates are insoluble except those of sodium potassium and ammonium
112
Most sulfides are insoluble except those of calcium strontium sodium potassium and ammonium
113
Is calcium phosphate Ca3(PO4)2 soluble or insoluble
Not one of the exceptions so it is insoluble
Dissociation equations are not written for insoluble compounds
114
Guidelines also useful in predicting what will happen if 2 solutions of 2 different soluble compounds are mixed
If mixing results in combination of ions that forms insoluble compound a double-replacement and precipitation reaction will happen
115
Will precipitate form when solutions of ammonium sulfide and cadmium nitrate are combined You can tell calcium nitrate is soluble from the guidelines (Most nitrates acetates and chlorates are soluble) You can also tell ammonium sulfide is soluble (Most sulfides are insoluble except those of calcium strontium sodium potassium and ammonium)
116
117
To decide whether precipitate can form must know solubilities of two possible products
Ammonium nitrate ndash soluble (Most nitrates acetates and chlorates are soluble) Cadmium sulfide ndash insoluble (Most sulfides are insoluble except those of calcium strontium sodium potassium and ammonium)
118
bc one product is insoluble double-replacement and precipitation reaction will happen (NH4)2S (aq) + Cd(NO3)2 (aq) rarr NH4NO3 (aq) + CdS (s)
119
Net Ionic Equations
Reactions of ions in aqueous solution usually represented by net ionic equations instead of formula equations Net ionic equation - includes only those compounds and ions that undergo a chemical change in a reaction in an aqueous solution
120
121
To write NIE (net ionic equation) first convert chemical equation into overall ionic equation All soluble ionic compounds shown as dissociated ions in solution Precipitates shown as solids
122
Notice ammonium and nitrate ions appear on both sides They havenrsquot gone through any chemical change Spectator ions - ions that do not take part in a chemical reaction and are found in solution both before and after the reaction
123
To change ionic equation to NIE eliminate spectator ions Cd+2(aq) + S-2(aq) rarr CdS(s)
This NIE applies to any reaction in which precipitate of CdS forms
124
Practice Problem
Identify the precipitate that forms when aqueous solutions of zinc nitrate and ammonium sulfide are combined Write the equation for the possible double-replacement reaction Then write the formula equation overall ionic equation and the net ionic equation
125
1 AnalyzeGiven Identity of reactants zinc nitrate and ammonium sulfide Reaction medium aqueous solution Unknown a equation for the possible double-replacement reaction b identity of the precipitate c formula equation d overall ionic equation e net ionic equation
126
2 PlanWrite the possible double-replacement reaction between Zn(NO3)2 and (NH4)2S Use the rules to determine if any of the products will precipitate Write a formula equation and overall net ionic equation Then cancel spectator ions to make net ionic equation
127
3 Computea possible double-replacement reaction
b rules show that zinc sulfide is not soluble so will precipitate (ammonium nitrate is soluble)
c formula equation128
d overall ionic equation
e net ionic equation129
Will a precipitate form if solutions of potassium sulfate and barium nitrate are combined If so write the net ionic equation for the reaction
Yes Ba+2(aq) + SO4-2(aq) rarr BaSO4(s)
130
Will a precipitate form if solutions of potassium nitrate and magnesium sulfate are combined If so write the net ionic equation for the reaction
No 131
Will a precipitate form if solutions of barium chloride and sodium sulfate are combined If so identify the spectator ions and write the net ionic equation for the reaction
Yes Na+ and Cl- Ba+2(aq) + SO4-2(aq) rarr BaSO4(s)
132
Write the net ionic equation for the precipitation of nickel(II) sulfide
Ni+2(aq) + S-2(aq) rarr NiS(s)
133
IonizationSome molecular compounds can also form ions in solution Usually polar Ionization - ions formed from solute molecules by action of the solvent (creation of ions where there were none) Ionization different from dissociation
134
Dissociation ionic compounds dissolve and ions already present separate Ionization ions formed where non existed before
Ions formed are hydrated Heat released during hydration of ions gives enough energy to break covalent bonds
135
Extent to which solute ionizes depends on strength of bonds within molecules of solute and strength of attraction between solute and solvent molecules
136
HCl ndash molecular compound that ionizes in aqueous solution Contains highly polar bond Attraction between polar HCl molecule and polar water strong enough to break HCl bond
137
The Hydronium Ion
H+ ions from HCl attracts other molecules or ions so strongly that it doesnrsquot normally exist alone Ionization of HCl better described as direct transfer of proton from HCl to H2O forming H3O+
138
139
Hydration of H+ to form hydronium is highly exothermic
Energy released gives a lot of energy needed to ionize a molecular solute
Many molecular compounds that ionize in aqueous solution contain hydrogen and form hydronium ion
140
Strong and Weak Electrolytes
Substances that make ions and conduct current in solution are electrolytes HCl is one of series of compounds that has hydrogen and halogen Hydrogen halides are all molecular compounds with single polar-covalent bonds
141
All are gases Very soluble in water All are electrolytes HCl HBr HI strongly conduct current in solution HF only weakly conducts current at same concentration Strength of conduction related to ability to ionize
142
143
Strong Electrolytes
HCl HBr HI are 100 ionized in solution
Strong electrolyte - any compound whose dilute aqueous solutions conduct electricity well this is due to the presence of all or almost all of the dissolve compound in the form of ions
144
Unique characteristic of strong electrolyte is that they yield only ions
Ex Ionic compound may be highly soluble in water and dissociate into ions (NaCl)
Others may not dissolve much but amount that does dissolve exists only as ions
145
Weak ElectrolytesHF dissolves in water to give acid solution called hydrofluoric acid HF bond stronger than bonds between hydrogen and other halogens When HF dissolves some molecule ionize Reverse reaction also happens HF(aq) + H2O(l) H3O+(aq) + F-(aq)
146
HF(aq) + H2O(l) H3O+
(aq) + F-(aq)Concentration of dissolved unionized HF stays high and concentration of ions stays low
Weak electrolyte - any compound whose aqueous solutions conduct electricity poorly this is due to the presence of small amount of the dissolved compound in the form of ions
147
Different from nonelectrolyte ndash NO ions at all Description of electrolyte as strong or weak not related to concentration of solution Electrolytes differ in degree of ionization not amount of solute dissolved
148
149
Colligative properties of solutions
Presence of solute affects properties of solutions Some properties not dependent on nature of dissolved substance but on how may dissolve particles are present
Colligative properties - properties that depend on the concentration of solute particles but not on their identity
150
Vapor-Pressure Lowering
Boiling and freezing point of solution different from pure solvent Nonvolatile solute raises boiling point and lowers freezing point Nonvolatile substance - one that has little tendency to become gas under existing conditions
151
152
To understand why nonvolatile solute changes boiling and freezing point consider equilibrium vapor pressure
Vapor pressure pressure caused by molecules that have escape liquid phase to gas phase
Can be thought of a measure of tendency of molecules to escape from a liquid
153
Addition of sucrose (nonvolatile solute) lowers concentration of water molecules at surface of liquid This lowers tendency of water molecules to leave solution and enter gas phase Vapor pressure of solution is lower than vapor pressure of pure water
154
Aqueous13 solution13 of13 nonvolatile13 solutePure13 water
Nonelectrolyte solutions of same molality have same concentration of particles
Dilute solutions of same solvent and equal molality of any nonelectrolyte solute lower vapor pressure equally
155
Example
1 m aqueous solution of nonelectrolyte glucose lowers vapor pressure of water 55 x 10-4 atm at 25 1 m aqueous solution of sucrose also lowers water vapor pressure to 55 x 10-4 atm at 25
156
157
bc vapor-pressure lowering depends on concentration of nonelectrolyte solute and doesnrsquot depend on the type of solute it is a colligative proprerty
bc vapor pressure has been lowered solution remains liquid over larger temperature range
Lowers freezing point and raises boiling point
158
Can assume changes in boiling and freezing point also depend on concentration of solute
They are colligative properties
159
Freezing-Point Depression
Freezing point of 1-molal solution of any nonelectrolyte solute in water is found (by experiment) to be 186 lower than freezing point of water When 1 mol of nonelectrolyte solute dissolved in 1 kg water freezing point is -186 not 000 160
When 2 mol nonelectrolyte solute dissolved in 1 kg water freezing point is -372
For any concentration of nonelectrolyte solute in water decrease in freezing point can be determined using value of -186m
161
This value called the molal freezing-point contstant (Kf) is the freezing-point depression of the solvent in a 1-molal solution of a nonvolatile nonelectrolyte solute
162
Each solvent has own characteristic molal freezing-point constant Freezing-point depression (Δtf) the difference between the freezing points of the pure solvent and a solute of nonelectrolyte in that solvent and it is directly proportional to the molal concentration of the solution Δtf = Kfm
163
Δtf = Kfm
Kf expressed as m
m expressed in mol solutekg solvent (molality)
164
Sample Problem
What is the freezing point depression of water in a solution of 171 g of sucrose C12H22O11 and 200 g of water What is the actual freezing point of the solution
165
1 AnalyzeGiven Solute mass and chemical formula 171 g C12H22O11 Solvent mass and identity 2000 g water Unknown a freezing-point depression b freezing point of the solution 166
2 Plan
167
168
Calculate
Unknown a freezing-point depression b freezing point of the solution
169
Practice Problem
A water solution containing an unknown quantity of a nonelectrolyte solute is found to have a freezing point of ndash 023degCWhat is the molal concentration of the solution
012 m170
A solution consists of 103 g of the nonelectrolyte glucose C6H12O6 dissolved in 250 g of water What is the freezing-point depression of the solution
minus0426degC171
In a laboratory experiment the freezing point of an aqueous solution of glucose is found to be minus0325degCWhat is the molal concentration of this solution
0175 m172
If 0500 mol of a nonelectrolyte solute are dissolved in 5000 g of ether what is the freezing point of the solution
minus1181degC173
The freezing point of an aqueous solution that contains a nonelectrolyte is minus90degC aWhat is the freezing-point depression of the solution bWhat is the molal concentration of the solution a minus90degC b 48 m
174
Boiling-Point Elevation
Boiling point is temp at which vapor pressure is equal to atmospheric pressure Change in vapor pressure will cause corresponding change in boiling point
175
176
Vapor pressure of nonvolatile solution is lower than vapor pressure of pure solvent More heat will be required to raise vapor pressure of solution Boiling point of solution is higher than boiling point of pure solvent
Molal boiling-point constant (Kb) - boiling-point elevation of the solvent in a 1-molal solution of a nonvolatile nonelectrolyte solute b-p elevation of any nonelectrolyte in water found by experiment to be 051 So molal b-p constant for water is 051m
177
For different solvents b-p elevations of 1-molal solutions have different values (Table 14-2)
Like freezing-point constants values most accurate for dilute solutions
178
Boiling-point elevation Δtb difference between the boiling points of the pure solvent and a nonelectrolyte solution of that solvent and it is directly proportional to the molal concentration of the solution B-p elevation can be calculated using Δtb = Kbm Δtb (boiling point elevation) expressed in m and m expressed in mol solutekg solvent
Sample Problem
What is the boiling-point elevation of a solution made from 200 g of a nonelectrolyte solute and 4000 g of water The molar mass of the solute is 620 gmol
180
1 AnalyzeGiven solute mass = 200 g solute molar mass = 620 gmol solvent mass and identity = 4000 g of water
Unknown boiling-point elevation
181
2 Plan
182
183
3 Compute
Practice Problem
A solution contains 500 g of sucrose C12H22O11 a nonelectrolyte dissolved in 5000 g of water What is the boiling-point elevation
015degC184
A solution contains 4500 g of sucrose C12H22O11 a nonelectrolyte dissolved in 250 g of waterWhat is the boiling point of the solution
1027degC185
If the boiling point elevation of an aqueous solution containing a nonvolatile electrolyte is 102degC what is the molality of the solution
20 m186
The boiling point of an aqueous solution containing a nonvolatile electrolyte is 10075degC aWhat is the boiling-point elevation bWhat is the molality of the solution a 075degC b 15 m
187
Electrolytes and Colligative PropertiesEarly investigators confused by experiments where certain substances depressed freezing point or elevated boiling point more than expected
188
189
Ex 01 m solution of NaCl lowers freezing point of solvent almost twice as much as 01 m solution of sucrose 01 m CaCl2 lowers fp almost 3 times as much as 01 m solution of sucrose
To understand why this happens contrast behavior or sucrose with NaCl in aqueous solution Each sucrose dissolves to make only 1 particle in solution So 1 mol sucrose dissolves to 1 mol particles 1 mol NaCl dissolves to make 2 moles of particles (1 mol Na+ and 1 mol Cl-)
190
191
Calculated Values for Electrolyte SolutionsColligative properties depend on total concentration of solute particles regardless of their identity Electrolytes cause changes in colligative properties proportional to total molality in terms of all dissolves particles instead of formula units
192
What about barium nitrate Ba(NO3)2 Each mole of barium nitrate yields 3 mol of ions in solution Ba(NO3)2(s) rarr Ba2+(aq) + 2NO3-(aq) You would expect solution of given molality to lower fp of solvent 3 times as much as nonelectrolytic solution of same molality
193
Sample Problem
What is the expected change in the freezing point of water in a solution of 625 g of barium nitrate Ba(NO3)2 in 100 kg of water
194
1 AnalyzeGiven solute mass and formula = 625 g Ba(NO3)2 solvent mass and identity = 100 kg water Δtf = Kfm
Unknown expected freezing-point depression
195
2 PlanThe molality can be calculated by converting the solute mass to moles and then dividing by the number of kilograms of solvent That molality is in terms of formula units of Ba(NO3)2 and must be converted to molality in terms of dissociated ions in solution It must be multiplied by the number of moles of ions produced per mole of formula unit This adjusted molality can then be used to calculate the freezing-point depression
196
197
198
3 Compute
Practice Problem
What is the expected freezing-point depression for a solution that contains 20 mol of magnesium sulfate dissolved in 10 kg of water
minus74degC199
What is the expected boiling-point elevation of water for a solution that contains 150 g of sodium chloride dissolved in 10 kg of water 27degC The freezing point of an aqueous sodium chloride solution is minus200degC What is the molality of the solution 54 m NaCl
200
Actual Values for Electrolyte SolutionsRemember values calculated are only expected values Actual values of colligative properties for all strong electrolytes are almost what would be expected Difference between expected and actual values caused by attractive forces that exist between dissociated ions in aqueous solution
201
Attraction between hydrated ions in solution is small compared with those in crystalline solid However forces of attraction donrsquot interfere with movements of aqueous ions More concentrated solution means ions closer together and attraction is greater Also depends on charge of ions ndash higher charge greater attraction
202
Most carbonates phosphates and silicates are insoluble except those of sodium potassium and ammonium
112
Most sulfides are insoluble except those of calcium strontium sodium potassium and ammonium
113
Is calcium phosphate Ca3(PO4)2 soluble or insoluble
Not one of the exceptions so it is insoluble
Dissociation equations are not written for insoluble compounds
114
Guidelines also useful in predicting what will happen if 2 solutions of 2 different soluble compounds are mixed
If mixing results in combination of ions that forms insoluble compound a double-replacement and precipitation reaction will happen
115
Will precipitate form when solutions of ammonium sulfide and cadmium nitrate are combined You can tell calcium nitrate is soluble from the guidelines (Most nitrates acetates and chlorates are soluble) You can also tell ammonium sulfide is soluble (Most sulfides are insoluble except those of calcium strontium sodium potassium and ammonium)
116
117
To decide whether precipitate can form must know solubilities of two possible products
Ammonium nitrate ndash soluble (Most nitrates acetates and chlorates are soluble) Cadmium sulfide ndash insoluble (Most sulfides are insoluble except those of calcium strontium sodium potassium and ammonium)
118
bc one product is insoluble double-replacement and precipitation reaction will happen (NH4)2S (aq) + Cd(NO3)2 (aq) rarr NH4NO3 (aq) + CdS (s)
119
Net Ionic Equations
Reactions of ions in aqueous solution usually represented by net ionic equations instead of formula equations Net ionic equation - includes only those compounds and ions that undergo a chemical change in a reaction in an aqueous solution
120
121
To write NIE (net ionic equation) first convert chemical equation into overall ionic equation All soluble ionic compounds shown as dissociated ions in solution Precipitates shown as solids
122
Notice ammonium and nitrate ions appear on both sides They havenrsquot gone through any chemical change Spectator ions - ions that do not take part in a chemical reaction and are found in solution both before and after the reaction
123
To change ionic equation to NIE eliminate spectator ions Cd+2(aq) + S-2(aq) rarr CdS(s)
This NIE applies to any reaction in which precipitate of CdS forms
124
Practice Problem
Identify the precipitate that forms when aqueous solutions of zinc nitrate and ammonium sulfide are combined Write the equation for the possible double-replacement reaction Then write the formula equation overall ionic equation and the net ionic equation
125
1 AnalyzeGiven Identity of reactants zinc nitrate and ammonium sulfide Reaction medium aqueous solution Unknown a equation for the possible double-replacement reaction b identity of the precipitate c formula equation d overall ionic equation e net ionic equation
126
2 PlanWrite the possible double-replacement reaction between Zn(NO3)2 and (NH4)2S Use the rules to determine if any of the products will precipitate Write a formula equation and overall net ionic equation Then cancel spectator ions to make net ionic equation
127
3 Computea possible double-replacement reaction
b rules show that zinc sulfide is not soluble so will precipitate (ammonium nitrate is soluble)
c formula equation128
d overall ionic equation
e net ionic equation129
Will a precipitate form if solutions of potassium sulfate and barium nitrate are combined If so write the net ionic equation for the reaction
Yes Ba+2(aq) + SO4-2(aq) rarr BaSO4(s)
130
Will a precipitate form if solutions of potassium nitrate and magnesium sulfate are combined If so write the net ionic equation for the reaction
No 131
Will a precipitate form if solutions of barium chloride and sodium sulfate are combined If so identify the spectator ions and write the net ionic equation for the reaction
Yes Na+ and Cl- Ba+2(aq) + SO4-2(aq) rarr BaSO4(s)
132
Write the net ionic equation for the precipitation of nickel(II) sulfide
Ni+2(aq) + S-2(aq) rarr NiS(s)
133
IonizationSome molecular compounds can also form ions in solution Usually polar Ionization - ions formed from solute molecules by action of the solvent (creation of ions where there were none) Ionization different from dissociation
134
Dissociation ionic compounds dissolve and ions already present separate Ionization ions formed where non existed before
Ions formed are hydrated Heat released during hydration of ions gives enough energy to break covalent bonds
135
Extent to which solute ionizes depends on strength of bonds within molecules of solute and strength of attraction between solute and solvent molecules
136
HCl ndash molecular compound that ionizes in aqueous solution Contains highly polar bond Attraction between polar HCl molecule and polar water strong enough to break HCl bond
137
The Hydronium Ion
H+ ions from HCl attracts other molecules or ions so strongly that it doesnrsquot normally exist alone Ionization of HCl better described as direct transfer of proton from HCl to H2O forming H3O+
138
139
Hydration of H+ to form hydronium is highly exothermic
Energy released gives a lot of energy needed to ionize a molecular solute
Many molecular compounds that ionize in aqueous solution contain hydrogen and form hydronium ion
140
Strong and Weak Electrolytes
Substances that make ions and conduct current in solution are electrolytes HCl is one of series of compounds that has hydrogen and halogen Hydrogen halides are all molecular compounds with single polar-covalent bonds
141
All are gases Very soluble in water All are electrolytes HCl HBr HI strongly conduct current in solution HF only weakly conducts current at same concentration Strength of conduction related to ability to ionize
142
143
Strong Electrolytes
HCl HBr HI are 100 ionized in solution
Strong electrolyte - any compound whose dilute aqueous solutions conduct electricity well this is due to the presence of all or almost all of the dissolve compound in the form of ions
144
Unique characteristic of strong electrolyte is that they yield only ions
Ex Ionic compound may be highly soluble in water and dissociate into ions (NaCl)
Others may not dissolve much but amount that does dissolve exists only as ions
145
Weak ElectrolytesHF dissolves in water to give acid solution called hydrofluoric acid HF bond stronger than bonds between hydrogen and other halogens When HF dissolves some molecule ionize Reverse reaction also happens HF(aq) + H2O(l) H3O+(aq) + F-(aq)
146
HF(aq) + H2O(l) H3O+
(aq) + F-(aq)Concentration of dissolved unionized HF stays high and concentration of ions stays low
Weak electrolyte - any compound whose aqueous solutions conduct electricity poorly this is due to the presence of small amount of the dissolved compound in the form of ions
147
Different from nonelectrolyte ndash NO ions at all Description of electrolyte as strong or weak not related to concentration of solution Electrolytes differ in degree of ionization not amount of solute dissolved
148
149
Colligative properties of solutions
Presence of solute affects properties of solutions Some properties not dependent on nature of dissolved substance but on how may dissolve particles are present
Colligative properties - properties that depend on the concentration of solute particles but not on their identity
150
Vapor-Pressure Lowering
Boiling and freezing point of solution different from pure solvent Nonvolatile solute raises boiling point and lowers freezing point Nonvolatile substance - one that has little tendency to become gas under existing conditions
151
152
To understand why nonvolatile solute changes boiling and freezing point consider equilibrium vapor pressure
Vapor pressure pressure caused by molecules that have escape liquid phase to gas phase
Can be thought of a measure of tendency of molecules to escape from a liquid
153
Addition of sucrose (nonvolatile solute) lowers concentration of water molecules at surface of liquid This lowers tendency of water molecules to leave solution and enter gas phase Vapor pressure of solution is lower than vapor pressure of pure water
154
Aqueous13 solution13 of13 nonvolatile13 solutePure13 water
Nonelectrolyte solutions of same molality have same concentration of particles
Dilute solutions of same solvent and equal molality of any nonelectrolyte solute lower vapor pressure equally
155
Example
1 m aqueous solution of nonelectrolyte glucose lowers vapor pressure of water 55 x 10-4 atm at 25 1 m aqueous solution of sucrose also lowers water vapor pressure to 55 x 10-4 atm at 25
156
157
bc vapor-pressure lowering depends on concentration of nonelectrolyte solute and doesnrsquot depend on the type of solute it is a colligative proprerty
bc vapor pressure has been lowered solution remains liquid over larger temperature range
Lowers freezing point and raises boiling point
158
Can assume changes in boiling and freezing point also depend on concentration of solute
They are colligative properties
159
Freezing-Point Depression
Freezing point of 1-molal solution of any nonelectrolyte solute in water is found (by experiment) to be 186 lower than freezing point of water When 1 mol of nonelectrolyte solute dissolved in 1 kg water freezing point is -186 not 000 160
When 2 mol nonelectrolyte solute dissolved in 1 kg water freezing point is -372
For any concentration of nonelectrolyte solute in water decrease in freezing point can be determined using value of -186m
161
This value called the molal freezing-point contstant (Kf) is the freezing-point depression of the solvent in a 1-molal solution of a nonvolatile nonelectrolyte solute
162
Each solvent has own characteristic molal freezing-point constant Freezing-point depression (Δtf) the difference between the freezing points of the pure solvent and a solute of nonelectrolyte in that solvent and it is directly proportional to the molal concentration of the solution Δtf = Kfm
163
Δtf = Kfm
Kf expressed as m
m expressed in mol solutekg solvent (molality)
164
Sample Problem
What is the freezing point depression of water in a solution of 171 g of sucrose C12H22O11 and 200 g of water What is the actual freezing point of the solution
165
1 AnalyzeGiven Solute mass and chemical formula 171 g C12H22O11 Solvent mass and identity 2000 g water Unknown a freezing-point depression b freezing point of the solution 166
2 Plan
167
168
Calculate
Unknown a freezing-point depression b freezing point of the solution
169
Practice Problem
A water solution containing an unknown quantity of a nonelectrolyte solute is found to have a freezing point of ndash 023degCWhat is the molal concentration of the solution
012 m170
A solution consists of 103 g of the nonelectrolyte glucose C6H12O6 dissolved in 250 g of water What is the freezing-point depression of the solution
minus0426degC171
In a laboratory experiment the freezing point of an aqueous solution of glucose is found to be minus0325degCWhat is the molal concentration of this solution
0175 m172
If 0500 mol of a nonelectrolyte solute are dissolved in 5000 g of ether what is the freezing point of the solution
minus1181degC173
The freezing point of an aqueous solution that contains a nonelectrolyte is minus90degC aWhat is the freezing-point depression of the solution bWhat is the molal concentration of the solution a minus90degC b 48 m
174
Boiling-Point Elevation
Boiling point is temp at which vapor pressure is equal to atmospheric pressure Change in vapor pressure will cause corresponding change in boiling point
175
176
Vapor pressure of nonvolatile solution is lower than vapor pressure of pure solvent More heat will be required to raise vapor pressure of solution Boiling point of solution is higher than boiling point of pure solvent
Molal boiling-point constant (Kb) - boiling-point elevation of the solvent in a 1-molal solution of a nonvolatile nonelectrolyte solute b-p elevation of any nonelectrolyte in water found by experiment to be 051 So molal b-p constant for water is 051m
177
For different solvents b-p elevations of 1-molal solutions have different values (Table 14-2)
Like freezing-point constants values most accurate for dilute solutions
178
Boiling-point elevation Δtb difference between the boiling points of the pure solvent and a nonelectrolyte solution of that solvent and it is directly proportional to the molal concentration of the solution B-p elevation can be calculated using Δtb = Kbm Δtb (boiling point elevation) expressed in m and m expressed in mol solutekg solvent
Sample Problem
What is the boiling-point elevation of a solution made from 200 g of a nonelectrolyte solute and 4000 g of water The molar mass of the solute is 620 gmol
180
1 AnalyzeGiven solute mass = 200 g solute molar mass = 620 gmol solvent mass and identity = 4000 g of water
Unknown boiling-point elevation
181
2 Plan
182
183
3 Compute
Practice Problem
A solution contains 500 g of sucrose C12H22O11 a nonelectrolyte dissolved in 5000 g of water What is the boiling-point elevation
015degC184
A solution contains 4500 g of sucrose C12H22O11 a nonelectrolyte dissolved in 250 g of waterWhat is the boiling point of the solution
1027degC185
If the boiling point elevation of an aqueous solution containing a nonvolatile electrolyte is 102degC what is the molality of the solution
20 m186
The boiling point of an aqueous solution containing a nonvolatile electrolyte is 10075degC aWhat is the boiling-point elevation bWhat is the molality of the solution a 075degC b 15 m
187
Electrolytes and Colligative PropertiesEarly investigators confused by experiments where certain substances depressed freezing point or elevated boiling point more than expected
188
189
Ex 01 m solution of NaCl lowers freezing point of solvent almost twice as much as 01 m solution of sucrose 01 m CaCl2 lowers fp almost 3 times as much as 01 m solution of sucrose
To understand why this happens contrast behavior or sucrose with NaCl in aqueous solution Each sucrose dissolves to make only 1 particle in solution So 1 mol sucrose dissolves to 1 mol particles 1 mol NaCl dissolves to make 2 moles of particles (1 mol Na+ and 1 mol Cl-)
190
191
Calculated Values for Electrolyte SolutionsColligative properties depend on total concentration of solute particles regardless of their identity Electrolytes cause changes in colligative properties proportional to total molality in terms of all dissolves particles instead of formula units
192
What about barium nitrate Ba(NO3)2 Each mole of barium nitrate yields 3 mol of ions in solution Ba(NO3)2(s) rarr Ba2+(aq) + 2NO3-(aq) You would expect solution of given molality to lower fp of solvent 3 times as much as nonelectrolytic solution of same molality
193
Sample Problem
What is the expected change in the freezing point of water in a solution of 625 g of barium nitrate Ba(NO3)2 in 100 kg of water
194
1 AnalyzeGiven solute mass and formula = 625 g Ba(NO3)2 solvent mass and identity = 100 kg water Δtf = Kfm
Unknown expected freezing-point depression
195
2 PlanThe molality can be calculated by converting the solute mass to moles and then dividing by the number of kilograms of solvent That molality is in terms of formula units of Ba(NO3)2 and must be converted to molality in terms of dissociated ions in solution It must be multiplied by the number of moles of ions produced per mole of formula unit This adjusted molality can then be used to calculate the freezing-point depression
196
197
198
3 Compute
Practice Problem
What is the expected freezing-point depression for a solution that contains 20 mol of magnesium sulfate dissolved in 10 kg of water
minus74degC199
What is the expected boiling-point elevation of water for a solution that contains 150 g of sodium chloride dissolved in 10 kg of water 27degC The freezing point of an aqueous sodium chloride solution is minus200degC What is the molality of the solution 54 m NaCl
200
Actual Values for Electrolyte SolutionsRemember values calculated are only expected values Actual values of colligative properties for all strong electrolytes are almost what would be expected Difference between expected and actual values caused by attractive forces that exist between dissociated ions in aqueous solution
201
Attraction between hydrated ions in solution is small compared with those in crystalline solid However forces of attraction donrsquot interfere with movements of aqueous ions More concentrated solution means ions closer together and attraction is greater Also depends on charge of ions ndash higher charge greater attraction
202
Most sulfides are insoluble except those of calcium strontium sodium potassium and ammonium
113
Is calcium phosphate Ca3(PO4)2 soluble or insoluble
Not one of the exceptions so it is insoluble
Dissociation equations are not written for insoluble compounds
114
Guidelines also useful in predicting what will happen if 2 solutions of 2 different soluble compounds are mixed
If mixing results in combination of ions that forms insoluble compound a double-replacement and precipitation reaction will happen
115
Will precipitate form when solutions of ammonium sulfide and cadmium nitrate are combined You can tell calcium nitrate is soluble from the guidelines (Most nitrates acetates and chlorates are soluble) You can also tell ammonium sulfide is soluble (Most sulfides are insoluble except those of calcium strontium sodium potassium and ammonium)
116
117
To decide whether precipitate can form must know solubilities of two possible products
Ammonium nitrate ndash soluble (Most nitrates acetates and chlorates are soluble) Cadmium sulfide ndash insoluble (Most sulfides are insoluble except those of calcium strontium sodium potassium and ammonium)
118
bc one product is insoluble double-replacement and precipitation reaction will happen (NH4)2S (aq) + Cd(NO3)2 (aq) rarr NH4NO3 (aq) + CdS (s)
119
Net Ionic Equations
Reactions of ions in aqueous solution usually represented by net ionic equations instead of formula equations Net ionic equation - includes only those compounds and ions that undergo a chemical change in a reaction in an aqueous solution
120
121
To write NIE (net ionic equation) first convert chemical equation into overall ionic equation All soluble ionic compounds shown as dissociated ions in solution Precipitates shown as solids
122
Notice ammonium and nitrate ions appear on both sides They havenrsquot gone through any chemical change Spectator ions - ions that do not take part in a chemical reaction and are found in solution both before and after the reaction
123
To change ionic equation to NIE eliminate spectator ions Cd+2(aq) + S-2(aq) rarr CdS(s)
This NIE applies to any reaction in which precipitate of CdS forms
124
Practice Problem
Identify the precipitate that forms when aqueous solutions of zinc nitrate and ammonium sulfide are combined Write the equation for the possible double-replacement reaction Then write the formula equation overall ionic equation and the net ionic equation
125
1 AnalyzeGiven Identity of reactants zinc nitrate and ammonium sulfide Reaction medium aqueous solution Unknown a equation for the possible double-replacement reaction b identity of the precipitate c formula equation d overall ionic equation e net ionic equation
126
2 PlanWrite the possible double-replacement reaction between Zn(NO3)2 and (NH4)2S Use the rules to determine if any of the products will precipitate Write a formula equation and overall net ionic equation Then cancel spectator ions to make net ionic equation
127
3 Computea possible double-replacement reaction
b rules show that zinc sulfide is not soluble so will precipitate (ammonium nitrate is soluble)
c formula equation128
d overall ionic equation
e net ionic equation129
Will a precipitate form if solutions of potassium sulfate and barium nitrate are combined If so write the net ionic equation for the reaction
Yes Ba+2(aq) + SO4-2(aq) rarr BaSO4(s)
130
Will a precipitate form if solutions of potassium nitrate and magnesium sulfate are combined If so write the net ionic equation for the reaction
No 131
Will a precipitate form if solutions of barium chloride and sodium sulfate are combined If so identify the spectator ions and write the net ionic equation for the reaction
Yes Na+ and Cl- Ba+2(aq) + SO4-2(aq) rarr BaSO4(s)
132
Write the net ionic equation for the precipitation of nickel(II) sulfide
Ni+2(aq) + S-2(aq) rarr NiS(s)
133
IonizationSome molecular compounds can also form ions in solution Usually polar Ionization - ions formed from solute molecules by action of the solvent (creation of ions where there were none) Ionization different from dissociation
134
Dissociation ionic compounds dissolve and ions already present separate Ionization ions formed where non existed before
Ions formed are hydrated Heat released during hydration of ions gives enough energy to break covalent bonds
135
Extent to which solute ionizes depends on strength of bonds within molecules of solute and strength of attraction between solute and solvent molecules
136
HCl ndash molecular compound that ionizes in aqueous solution Contains highly polar bond Attraction between polar HCl molecule and polar water strong enough to break HCl bond
137
The Hydronium Ion
H+ ions from HCl attracts other molecules or ions so strongly that it doesnrsquot normally exist alone Ionization of HCl better described as direct transfer of proton from HCl to H2O forming H3O+
138
139
Hydration of H+ to form hydronium is highly exothermic
Energy released gives a lot of energy needed to ionize a molecular solute
Many molecular compounds that ionize in aqueous solution contain hydrogen and form hydronium ion
140
Strong and Weak Electrolytes
Substances that make ions and conduct current in solution are electrolytes HCl is one of series of compounds that has hydrogen and halogen Hydrogen halides are all molecular compounds with single polar-covalent bonds
141
All are gases Very soluble in water All are electrolytes HCl HBr HI strongly conduct current in solution HF only weakly conducts current at same concentration Strength of conduction related to ability to ionize
142
143
Strong Electrolytes
HCl HBr HI are 100 ionized in solution
Strong electrolyte - any compound whose dilute aqueous solutions conduct electricity well this is due to the presence of all or almost all of the dissolve compound in the form of ions
144
Unique characteristic of strong electrolyte is that they yield only ions
Ex Ionic compound may be highly soluble in water and dissociate into ions (NaCl)
Others may not dissolve much but amount that does dissolve exists only as ions
145
Weak ElectrolytesHF dissolves in water to give acid solution called hydrofluoric acid HF bond stronger than bonds between hydrogen and other halogens When HF dissolves some molecule ionize Reverse reaction also happens HF(aq) + H2O(l) H3O+(aq) + F-(aq)
146
HF(aq) + H2O(l) H3O+
(aq) + F-(aq)Concentration of dissolved unionized HF stays high and concentration of ions stays low
Weak electrolyte - any compound whose aqueous solutions conduct electricity poorly this is due to the presence of small amount of the dissolved compound in the form of ions
147
Different from nonelectrolyte ndash NO ions at all Description of electrolyte as strong or weak not related to concentration of solution Electrolytes differ in degree of ionization not amount of solute dissolved
148
149
Colligative properties of solutions
Presence of solute affects properties of solutions Some properties not dependent on nature of dissolved substance but on how may dissolve particles are present
Colligative properties - properties that depend on the concentration of solute particles but not on their identity
150
Vapor-Pressure Lowering
Boiling and freezing point of solution different from pure solvent Nonvolatile solute raises boiling point and lowers freezing point Nonvolatile substance - one that has little tendency to become gas under existing conditions
151
152
To understand why nonvolatile solute changes boiling and freezing point consider equilibrium vapor pressure
Vapor pressure pressure caused by molecules that have escape liquid phase to gas phase
Can be thought of a measure of tendency of molecules to escape from a liquid
153
Addition of sucrose (nonvolatile solute) lowers concentration of water molecules at surface of liquid This lowers tendency of water molecules to leave solution and enter gas phase Vapor pressure of solution is lower than vapor pressure of pure water
154
Aqueous13 solution13 of13 nonvolatile13 solutePure13 water
Nonelectrolyte solutions of same molality have same concentration of particles
Dilute solutions of same solvent and equal molality of any nonelectrolyte solute lower vapor pressure equally
155
Example
1 m aqueous solution of nonelectrolyte glucose lowers vapor pressure of water 55 x 10-4 atm at 25 1 m aqueous solution of sucrose also lowers water vapor pressure to 55 x 10-4 atm at 25
156
157
bc vapor-pressure lowering depends on concentration of nonelectrolyte solute and doesnrsquot depend on the type of solute it is a colligative proprerty
bc vapor pressure has been lowered solution remains liquid over larger temperature range
Lowers freezing point and raises boiling point
158
Can assume changes in boiling and freezing point also depend on concentration of solute
They are colligative properties
159
Freezing-Point Depression
Freezing point of 1-molal solution of any nonelectrolyte solute in water is found (by experiment) to be 186 lower than freezing point of water When 1 mol of nonelectrolyte solute dissolved in 1 kg water freezing point is -186 not 000 160
When 2 mol nonelectrolyte solute dissolved in 1 kg water freezing point is -372
For any concentration of nonelectrolyte solute in water decrease in freezing point can be determined using value of -186m
161
This value called the molal freezing-point contstant (Kf) is the freezing-point depression of the solvent in a 1-molal solution of a nonvolatile nonelectrolyte solute
162
Each solvent has own characteristic molal freezing-point constant Freezing-point depression (Δtf) the difference between the freezing points of the pure solvent and a solute of nonelectrolyte in that solvent and it is directly proportional to the molal concentration of the solution Δtf = Kfm
163
Δtf = Kfm
Kf expressed as m
m expressed in mol solutekg solvent (molality)
164
Sample Problem
What is the freezing point depression of water in a solution of 171 g of sucrose C12H22O11 and 200 g of water What is the actual freezing point of the solution
165
1 AnalyzeGiven Solute mass and chemical formula 171 g C12H22O11 Solvent mass and identity 2000 g water Unknown a freezing-point depression b freezing point of the solution 166
2 Plan
167
168
Calculate
Unknown a freezing-point depression b freezing point of the solution
169
Practice Problem
A water solution containing an unknown quantity of a nonelectrolyte solute is found to have a freezing point of ndash 023degCWhat is the molal concentration of the solution
012 m170
A solution consists of 103 g of the nonelectrolyte glucose C6H12O6 dissolved in 250 g of water What is the freezing-point depression of the solution
minus0426degC171
In a laboratory experiment the freezing point of an aqueous solution of glucose is found to be minus0325degCWhat is the molal concentration of this solution
0175 m172
If 0500 mol of a nonelectrolyte solute are dissolved in 5000 g of ether what is the freezing point of the solution
minus1181degC173
The freezing point of an aqueous solution that contains a nonelectrolyte is minus90degC aWhat is the freezing-point depression of the solution bWhat is the molal concentration of the solution a minus90degC b 48 m
174
Boiling-Point Elevation
Boiling point is temp at which vapor pressure is equal to atmospheric pressure Change in vapor pressure will cause corresponding change in boiling point
175
176
Vapor pressure of nonvolatile solution is lower than vapor pressure of pure solvent More heat will be required to raise vapor pressure of solution Boiling point of solution is higher than boiling point of pure solvent
Molal boiling-point constant (Kb) - boiling-point elevation of the solvent in a 1-molal solution of a nonvolatile nonelectrolyte solute b-p elevation of any nonelectrolyte in water found by experiment to be 051 So molal b-p constant for water is 051m
177
For different solvents b-p elevations of 1-molal solutions have different values (Table 14-2)
Like freezing-point constants values most accurate for dilute solutions
178
Boiling-point elevation Δtb difference between the boiling points of the pure solvent and a nonelectrolyte solution of that solvent and it is directly proportional to the molal concentration of the solution B-p elevation can be calculated using Δtb = Kbm Δtb (boiling point elevation) expressed in m and m expressed in mol solutekg solvent
Sample Problem
What is the boiling-point elevation of a solution made from 200 g of a nonelectrolyte solute and 4000 g of water The molar mass of the solute is 620 gmol
180
1 AnalyzeGiven solute mass = 200 g solute molar mass = 620 gmol solvent mass and identity = 4000 g of water
Unknown boiling-point elevation
181
2 Plan
182
183
3 Compute
Practice Problem
A solution contains 500 g of sucrose C12H22O11 a nonelectrolyte dissolved in 5000 g of water What is the boiling-point elevation
015degC184
A solution contains 4500 g of sucrose C12H22O11 a nonelectrolyte dissolved in 250 g of waterWhat is the boiling point of the solution
1027degC185
If the boiling point elevation of an aqueous solution containing a nonvolatile electrolyte is 102degC what is the molality of the solution
20 m186
The boiling point of an aqueous solution containing a nonvolatile electrolyte is 10075degC aWhat is the boiling-point elevation bWhat is the molality of the solution a 075degC b 15 m
187
Electrolytes and Colligative PropertiesEarly investigators confused by experiments where certain substances depressed freezing point or elevated boiling point more than expected
188
189
Ex 01 m solution of NaCl lowers freezing point of solvent almost twice as much as 01 m solution of sucrose 01 m CaCl2 lowers fp almost 3 times as much as 01 m solution of sucrose
To understand why this happens contrast behavior or sucrose with NaCl in aqueous solution Each sucrose dissolves to make only 1 particle in solution So 1 mol sucrose dissolves to 1 mol particles 1 mol NaCl dissolves to make 2 moles of particles (1 mol Na+ and 1 mol Cl-)
190
191
Calculated Values for Electrolyte SolutionsColligative properties depend on total concentration of solute particles regardless of their identity Electrolytes cause changes in colligative properties proportional to total molality in terms of all dissolves particles instead of formula units
192
What about barium nitrate Ba(NO3)2 Each mole of barium nitrate yields 3 mol of ions in solution Ba(NO3)2(s) rarr Ba2+(aq) + 2NO3-(aq) You would expect solution of given molality to lower fp of solvent 3 times as much as nonelectrolytic solution of same molality
193
Sample Problem
What is the expected change in the freezing point of water in a solution of 625 g of barium nitrate Ba(NO3)2 in 100 kg of water
194
1 AnalyzeGiven solute mass and formula = 625 g Ba(NO3)2 solvent mass and identity = 100 kg water Δtf = Kfm
Unknown expected freezing-point depression
195
2 PlanThe molality can be calculated by converting the solute mass to moles and then dividing by the number of kilograms of solvent That molality is in terms of formula units of Ba(NO3)2 and must be converted to molality in terms of dissociated ions in solution It must be multiplied by the number of moles of ions produced per mole of formula unit This adjusted molality can then be used to calculate the freezing-point depression
196
197
198
3 Compute
Practice Problem
What is the expected freezing-point depression for a solution that contains 20 mol of magnesium sulfate dissolved in 10 kg of water
minus74degC199
What is the expected boiling-point elevation of water for a solution that contains 150 g of sodium chloride dissolved in 10 kg of water 27degC The freezing point of an aqueous sodium chloride solution is minus200degC What is the molality of the solution 54 m NaCl
200
Actual Values for Electrolyte SolutionsRemember values calculated are only expected values Actual values of colligative properties for all strong electrolytes are almost what would be expected Difference between expected and actual values caused by attractive forces that exist between dissociated ions in aqueous solution
201
Attraction between hydrated ions in solution is small compared with those in crystalline solid However forces of attraction donrsquot interfere with movements of aqueous ions More concentrated solution means ions closer together and attraction is greater Also depends on charge of ions ndash higher charge greater attraction
202
Is calcium phosphate Ca3(PO4)2 soluble or insoluble
Not one of the exceptions so it is insoluble
Dissociation equations are not written for insoluble compounds
114
Guidelines also useful in predicting what will happen if 2 solutions of 2 different soluble compounds are mixed
If mixing results in combination of ions that forms insoluble compound a double-replacement and precipitation reaction will happen
115
Will precipitate form when solutions of ammonium sulfide and cadmium nitrate are combined You can tell calcium nitrate is soluble from the guidelines (Most nitrates acetates and chlorates are soluble) You can also tell ammonium sulfide is soluble (Most sulfides are insoluble except those of calcium strontium sodium potassium and ammonium)
116
117
To decide whether precipitate can form must know solubilities of two possible products
Ammonium nitrate ndash soluble (Most nitrates acetates and chlorates are soluble) Cadmium sulfide ndash insoluble (Most sulfides are insoluble except those of calcium strontium sodium potassium and ammonium)
118
bc one product is insoluble double-replacement and precipitation reaction will happen (NH4)2S (aq) + Cd(NO3)2 (aq) rarr NH4NO3 (aq) + CdS (s)
119
Net Ionic Equations
Reactions of ions in aqueous solution usually represented by net ionic equations instead of formula equations Net ionic equation - includes only those compounds and ions that undergo a chemical change in a reaction in an aqueous solution
120
121
To write NIE (net ionic equation) first convert chemical equation into overall ionic equation All soluble ionic compounds shown as dissociated ions in solution Precipitates shown as solids
122
Notice ammonium and nitrate ions appear on both sides They havenrsquot gone through any chemical change Spectator ions - ions that do not take part in a chemical reaction and are found in solution both before and after the reaction
123
To change ionic equation to NIE eliminate spectator ions Cd+2(aq) + S-2(aq) rarr CdS(s)
This NIE applies to any reaction in which precipitate of CdS forms
124
Practice Problem
Identify the precipitate that forms when aqueous solutions of zinc nitrate and ammonium sulfide are combined Write the equation for the possible double-replacement reaction Then write the formula equation overall ionic equation and the net ionic equation
125
1 AnalyzeGiven Identity of reactants zinc nitrate and ammonium sulfide Reaction medium aqueous solution Unknown a equation for the possible double-replacement reaction b identity of the precipitate c formula equation d overall ionic equation e net ionic equation
126
2 PlanWrite the possible double-replacement reaction between Zn(NO3)2 and (NH4)2S Use the rules to determine if any of the products will precipitate Write a formula equation and overall net ionic equation Then cancel spectator ions to make net ionic equation
127
3 Computea possible double-replacement reaction
b rules show that zinc sulfide is not soluble so will precipitate (ammonium nitrate is soluble)
c formula equation128
d overall ionic equation
e net ionic equation129
Will a precipitate form if solutions of potassium sulfate and barium nitrate are combined If so write the net ionic equation for the reaction
Yes Ba+2(aq) + SO4-2(aq) rarr BaSO4(s)
130
Will a precipitate form if solutions of potassium nitrate and magnesium sulfate are combined If so write the net ionic equation for the reaction
No 131
Will a precipitate form if solutions of barium chloride and sodium sulfate are combined If so identify the spectator ions and write the net ionic equation for the reaction
Yes Na+ and Cl- Ba+2(aq) + SO4-2(aq) rarr BaSO4(s)
132
Write the net ionic equation for the precipitation of nickel(II) sulfide
Ni+2(aq) + S-2(aq) rarr NiS(s)
133
IonizationSome molecular compounds can also form ions in solution Usually polar Ionization - ions formed from solute molecules by action of the solvent (creation of ions where there were none) Ionization different from dissociation
134
Dissociation ionic compounds dissolve and ions already present separate Ionization ions formed where non existed before
Ions formed are hydrated Heat released during hydration of ions gives enough energy to break covalent bonds
135
Extent to which solute ionizes depends on strength of bonds within molecules of solute and strength of attraction between solute and solvent molecules
136
HCl ndash molecular compound that ionizes in aqueous solution Contains highly polar bond Attraction between polar HCl molecule and polar water strong enough to break HCl bond
137
The Hydronium Ion
H+ ions from HCl attracts other molecules or ions so strongly that it doesnrsquot normally exist alone Ionization of HCl better described as direct transfer of proton from HCl to H2O forming H3O+
138
139
Hydration of H+ to form hydronium is highly exothermic
Energy released gives a lot of energy needed to ionize a molecular solute
Many molecular compounds that ionize in aqueous solution contain hydrogen and form hydronium ion
140
Strong and Weak Electrolytes
Substances that make ions and conduct current in solution are electrolytes HCl is one of series of compounds that has hydrogen and halogen Hydrogen halides are all molecular compounds with single polar-covalent bonds
141
All are gases Very soluble in water All are electrolytes HCl HBr HI strongly conduct current in solution HF only weakly conducts current at same concentration Strength of conduction related to ability to ionize
142
143
Strong Electrolytes
HCl HBr HI are 100 ionized in solution
Strong electrolyte - any compound whose dilute aqueous solutions conduct electricity well this is due to the presence of all or almost all of the dissolve compound in the form of ions
144
Unique characteristic of strong electrolyte is that they yield only ions
Ex Ionic compound may be highly soluble in water and dissociate into ions (NaCl)
Others may not dissolve much but amount that does dissolve exists only as ions
145
Weak ElectrolytesHF dissolves in water to give acid solution called hydrofluoric acid HF bond stronger than bonds between hydrogen and other halogens When HF dissolves some molecule ionize Reverse reaction also happens HF(aq) + H2O(l) H3O+(aq) + F-(aq)
146
HF(aq) + H2O(l) H3O+
(aq) + F-(aq)Concentration of dissolved unionized HF stays high and concentration of ions stays low
Weak electrolyte - any compound whose aqueous solutions conduct electricity poorly this is due to the presence of small amount of the dissolved compound in the form of ions
147
Different from nonelectrolyte ndash NO ions at all Description of electrolyte as strong or weak not related to concentration of solution Electrolytes differ in degree of ionization not amount of solute dissolved
148
149
Colligative properties of solutions
Presence of solute affects properties of solutions Some properties not dependent on nature of dissolved substance but on how may dissolve particles are present
Colligative properties - properties that depend on the concentration of solute particles but not on their identity
150
Vapor-Pressure Lowering
Boiling and freezing point of solution different from pure solvent Nonvolatile solute raises boiling point and lowers freezing point Nonvolatile substance - one that has little tendency to become gas under existing conditions
151
152
To understand why nonvolatile solute changes boiling and freezing point consider equilibrium vapor pressure
Vapor pressure pressure caused by molecules that have escape liquid phase to gas phase
Can be thought of a measure of tendency of molecules to escape from a liquid
153
Addition of sucrose (nonvolatile solute) lowers concentration of water molecules at surface of liquid This lowers tendency of water molecules to leave solution and enter gas phase Vapor pressure of solution is lower than vapor pressure of pure water
154
Aqueous13 solution13 of13 nonvolatile13 solutePure13 water
Nonelectrolyte solutions of same molality have same concentration of particles
Dilute solutions of same solvent and equal molality of any nonelectrolyte solute lower vapor pressure equally
155
Example
1 m aqueous solution of nonelectrolyte glucose lowers vapor pressure of water 55 x 10-4 atm at 25 1 m aqueous solution of sucrose also lowers water vapor pressure to 55 x 10-4 atm at 25
156
157
bc vapor-pressure lowering depends on concentration of nonelectrolyte solute and doesnrsquot depend on the type of solute it is a colligative proprerty
bc vapor pressure has been lowered solution remains liquid over larger temperature range
Lowers freezing point and raises boiling point
158
Can assume changes in boiling and freezing point also depend on concentration of solute
They are colligative properties
159
Freezing-Point Depression
Freezing point of 1-molal solution of any nonelectrolyte solute in water is found (by experiment) to be 186 lower than freezing point of water When 1 mol of nonelectrolyte solute dissolved in 1 kg water freezing point is -186 not 000 160
When 2 mol nonelectrolyte solute dissolved in 1 kg water freezing point is -372
For any concentration of nonelectrolyte solute in water decrease in freezing point can be determined using value of -186m
161
This value called the molal freezing-point contstant (Kf) is the freezing-point depression of the solvent in a 1-molal solution of a nonvolatile nonelectrolyte solute
162
Each solvent has own characteristic molal freezing-point constant Freezing-point depression (Δtf) the difference between the freezing points of the pure solvent and a solute of nonelectrolyte in that solvent and it is directly proportional to the molal concentration of the solution Δtf = Kfm
163
Δtf = Kfm
Kf expressed as m
m expressed in mol solutekg solvent (molality)
164
Sample Problem
What is the freezing point depression of water in a solution of 171 g of sucrose C12H22O11 and 200 g of water What is the actual freezing point of the solution
165
1 AnalyzeGiven Solute mass and chemical formula 171 g C12H22O11 Solvent mass and identity 2000 g water Unknown a freezing-point depression b freezing point of the solution 166
2 Plan
167
168
Calculate
Unknown a freezing-point depression b freezing point of the solution
169
Practice Problem
A water solution containing an unknown quantity of a nonelectrolyte solute is found to have a freezing point of ndash 023degCWhat is the molal concentration of the solution
012 m170
A solution consists of 103 g of the nonelectrolyte glucose C6H12O6 dissolved in 250 g of water What is the freezing-point depression of the solution
minus0426degC171
In a laboratory experiment the freezing point of an aqueous solution of glucose is found to be minus0325degCWhat is the molal concentration of this solution
0175 m172
If 0500 mol of a nonelectrolyte solute are dissolved in 5000 g of ether what is the freezing point of the solution
minus1181degC173
The freezing point of an aqueous solution that contains a nonelectrolyte is minus90degC aWhat is the freezing-point depression of the solution bWhat is the molal concentration of the solution a minus90degC b 48 m
174
Boiling-Point Elevation
Boiling point is temp at which vapor pressure is equal to atmospheric pressure Change in vapor pressure will cause corresponding change in boiling point
175
176
Vapor pressure of nonvolatile solution is lower than vapor pressure of pure solvent More heat will be required to raise vapor pressure of solution Boiling point of solution is higher than boiling point of pure solvent
Molal boiling-point constant (Kb) - boiling-point elevation of the solvent in a 1-molal solution of a nonvolatile nonelectrolyte solute b-p elevation of any nonelectrolyte in water found by experiment to be 051 So molal b-p constant for water is 051m
177
For different solvents b-p elevations of 1-molal solutions have different values (Table 14-2)
Like freezing-point constants values most accurate for dilute solutions
178
Boiling-point elevation Δtb difference between the boiling points of the pure solvent and a nonelectrolyte solution of that solvent and it is directly proportional to the molal concentration of the solution B-p elevation can be calculated using Δtb = Kbm Δtb (boiling point elevation) expressed in m and m expressed in mol solutekg solvent
Sample Problem
What is the boiling-point elevation of a solution made from 200 g of a nonelectrolyte solute and 4000 g of water The molar mass of the solute is 620 gmol
180
1 AnalyzeGiven solute mass = 200 g solute molar mass = 620 gmol solvent mass and identity = 4000 g of water
Unknown boiling-point elevation
181
2 Plan
182
183
3 Compute
Practice Problem
A solution contains 500 g of sucrose C12H22O11 a nonelectrolyte dissolved in 5000 g of water What is the boiling-point elevation
015degC184
A solution contains 4500 g of sucrose C12H22O11 a nonelectrolyte dissolved in 250 g of waterWhat is the boiling point of the solution
1027degC185
If the boiling point elevation of an aqueous solution containing a nonvolatile electrolyte is 102degC what is the molality of the solution
20 m186
The boiling point of an aqueous solution containing a nonvolatile electrolyte is 10075degC aWhat is the boiling-point elevation bWhat is the molality of the solution a 075degC b 15 m
187
Electrolytes and Colligative PropertiesEarly investigators confused by experiments where certain substances depressed freezing point or elevated boiling point more than expected
188
189
Ex 01 m solution of NaCl lowers freezing point of solvent almost twice as much as 01 m solution of sucrose 01 m CaCl2 lowers fp almost 3 times as much as 01 m solution of sucrose
To understand why this happens contrast behavior or sucrose with NaCl in aqueous solution Each sucrose dissolves to make only 1 particle in solution So 1 mol sucrose dissolves to 1 mol particles 1 mol NaCl dissolves to make 2 moles of particles (1 mol Na+ and 1 mol Cl-)
190
191
Calculated Values for Electrolyte SolutionsColligative properties depend on total concentration of solute particles regardless of their identity Electrolytes cause changes in colligative properties proportional to total molality in terms of all dissolves particles instead of formula units
192
What about barium nitrate Ba(NO3)2 Each mole of barium nitrate yields 3 mol of ions in solution Ba(NO3)2(s) rarr Ba2+(aq) + 2NO3-(aq) You would expect solution of given molality to lower fp of solvent 3 times as much as nonelectrolytic solution of same molality
193
Sample Problem
What is the expected change in the freezing point of water in a solution of 625 g of barium nitrate Ba(NO3)2 in 100 kg of water
194
1 AnalyzeGiven solute mass and formula = 625 g Ba(NO3)2 solvent mass and identity = 100 kg water Δtf = Kfm
Unknown expected freezing-point depression
195
2 PlanThe molality can be calculated by converting the solute mass to moles and then dividing by the number of kilograms of solvent That molality is in terms of formula units of Ba(NO3)2 and must be converted to molality in terms of dissociated ions in solution It must be multiplied by the number of moles of ions produced per mole of formula unit This adjusted molality can then be used to calculate the freezing-point depression
196
197
198
3 Compute
Practice Problem
What is the expected freezing-point depression for a solution that contains 20 mol of magnesium sulfate dissolved in 10 kg of water
minus74degC199
What is the expected boiling-point elevation of water for a solution that contains 150 g of sodium chloride dissolved in 10 kg of water 27degC The freezing point of an aqueous sodium chloride solution is minus200degC What is the molality of the solution 54 m NaCl
200
Actual Values for Electrolyte SolutionsRemember values calculated are only expected values Actual values of colligative properties for all strong electrolytes are almost what would be expected Difference between expected and actual values caused by attractive forces that exist between dissociated ions in aqueous solution
201
Attraction between hydrated ions in solution is small compared with those in crystalline solid However forces of attraction donrsquot interfere with movements of aqueous ions More concentrated solution means ions closer together and attraction is greater Also depends on charge of ions ndash higher charge greater attraction
202
Guidelines also useful in predicting what will happen if 2 solutions of 2 different soluble compounds are mixed
If mixing results in combination of ions that forms insoluble compound a double-replacement and precipitation reaction will happen
115
Will precipitate form when solutions of ammonium sulfide and cadmium nitrate are combined You can tell calcium nitrate is soluble from the guidelines (Most nitrates acetates and chlorates are soluble) You can also tell ammonium sulfide is soluble (Most sulfides are insoluble except those of calcium strontium sodium potassium and ammonium)
116
117
To decide whether precipitate can form must know solubilities of two possible products
Ammonium nitrate ndash soluble (Most nitrates acetates and chlorates are soluble) Cadmium sulfide ndash insoluble (Most sulfides are insoluble except those of calcium strontium sodium potassium and ammonium)
118
bc one product is insoluble double-replacement and precipitation reaction will happen (NH4)2S (aq) + Cd(NO3)2 (aq) rarr NH4NO3 (aq) + CdS (s)
119
Net Ionic Equations
Reactions of ions in aqueous solution usually represented by net ionic equations instead of formula equations Net ionic equation - includes only those compounds and ions that undergo a chemical change in a reaction in an aqueous solution
120
121
To write NIE (net ionic equation) first convert chemical equation into overall ionic equation All soluble ionic compounds shown as dissociated ions in solution Precipitates shown as solids
122
Notice ammonium and nitrate ions appear on both sides They havenrsquot gone through any chemical change Spectator ions - ions that do not take part in a chemical reaction and are found in solution both before and after the reaction
123
To change ionic equation to NIE eliminate spectator ions Cd+2(aq) + S-2(aq) rarr CdS(s)
This NIE applies to any reaction in which precipitate of CdS forms
124
Practice Problem
Identify the precipitate that forms when aqueous solutions of zinc nitrate and ammonium sulfide are combined Write the equation for the possible double-replacement reaction Then write the formula equation overall ionic equation and the net ionic equation
125
1 AnalyzeGiven Identity of reactants zinc nitrate and ammonium sulfide Reaction medium aqueous solution Unknown a equation for the possible double-replacement reaction b identity of the precipitate c formula equation d overall ionic equation e net ionic equation
126
2 PlanWrite the possible double-replacement reaction between Zn(NO3)2 and (NH4)2S Use the rules to determine if any of the products will precipitate Write a formula equation and overall net ionic equation Then cancel spectator ions to make net ionic equation
127
3 Computea possible double-replacement reaction
b rules show that zinc sulfide is not soluble so will precipitate (ammonium nitrate is soluble)
c formula equation128
d overall ionic equation
e net ionic equation129
Will a precipitate form if solutions of potassium sulfate and barium nitrate are combined If so write the net ionic equation for the reaction
Yes Ba+2(aq) + SO4-2(aq) rarr BaSO4(s)
130
Will a precipitate form if solutions of potassium nitrate and magnesium sulfate are combined If so write the net ionic equation for the reaction
No 131
Will a precipitate form if solutions of barium chloride and sodium sulfate are combined If so identify the spectator ions and write the net ionic equation for the reaction
Yes Na+ and Cl- Ba+2(aq) + SO4-2(aq) rarr BaSO4(s)
132
Write the net ionic equation for the precipitation of nickel(II) sulfide
Ni+2(aq) + S-2(aq) rarr NiS(s)
133
IonizationSome molecular compounds can also form ions in solution Usually polar Ionization - ions formed from solute molecules by action of the solvent (creation of ions where there were none) Ionization different from dissociation
134
Dissociation ionic compounds dissolve and ions already present separate Ionization ions formed where non existed before
Ions formed are hydrated Heat released during hydration of ions gives enough energy to break covalent bonds
135
Extent to which solute ionizes depends on strength of bonds within molecules of solute and strength of attraction between solute and solvent molecules
136
HCl ndash molecular compound that ionizes in aqueous solution Contains highly polar bond Attraction between polar HCl molecule and polar water strong enough to break HCl bond
137
The Hydronium Ion
H+ ions from HCl attracts other molecules or ions so strongly that it doesnrsquot normally exist alone Ionization of HCl better described as direct transfer of proton from HCl to H2O forming H3O+
138
139
Hydration of H+ to form hydronium is highly exothermic
Energy released gives a lot of energy needed to ionize a molecular solute
Many molecular compounds that ionize in aqueous solution contain hydrogen and form hydronium ion
140
Strong and Weak Electrolytes
Substances that make ions and conduct current in solution are electrolytes HCl is one of series of compounds that has hydrogen and halogen Hydrogen halides are all molecular compounds with single polar-covalent bonds
141
All are gases Very soluble in water All are electrolytes HCl HBr HI strongly conduct current in solution HF only weakly conducts current at same concentration Strength of conduction related to ability to ionize
142
143
Strong Electrolytes
HCl HBr HI are 100 ionized in solution
Strong electrolyte - any compound whose dilute aqueous solutions conduct electricity well this is due to the presence of all or almost all of the dissolve compound in the form of ions
144
Unique characteristic of strong electrolyte is that they yield only ions
Ex Ionic compound may be highly soluble in water and dissociate into ions (NaCl)
Others may not dissolve much but amount that does dissolve exists only as ions
145
Weak ElectrolytesHF dissolves in water to give acid solution called hydrofluoric acid HF bond stronger than bonds between hydrogen and other halogens When HF dissolves some molecule ionize Reverse reaction also happens HF(aq) + H2O(l) H3O+(aq) + F-(aq)
146
HF(aq) + H2O(l) H3O+
(aq) + F-(aq)Concentration of dissolved unionized HF stays high and concentration of ions stays low
Weak electrolyte - any compound whose aqueous solutions conduct electricity poorly this is due to the presence of small amount of the dissolved compound in the form of ions
147
Different from nonelectrolyte ndash NO ions at all Description of electrolyte as strong or weak not related to concentration of solution Electrolytes differ in degree of ionization not amount of solute dissolved
148
149
Colligative properties of solutions
Presence of solute affects properties of solutions Some properties not dependent on nature of dissolved substance but on how may dissolve particles are present
Colligative properties - properties that depend on the concentration of solute particles but not on their identity
150
Vapor-Pressure Lowering
Boiling and freezing point of solution different from pure solvent Nonvolatile solute raises boiling point and lowers freezing point Nonvolatile substance - one that has little tendency to become gas under existing conditions
151
152
To understand why nonvolatile solute changes boiling and freezing point consider equilibrium vapor pressure
Vapor pressure pressure caused by molecules that have escape liquid phase to gas phase
Can be thought of a measure of tendency of molecules to escape from a liquid
153
Addition of sucrose (nonvolatile solute) lowers concentration of water molecules at surface of liquid This lowers tendency of water molecules to leave solution and enter gas phase Vapor pressure of solution is lower than vapor pressure of pure water
154
Aqueous13 solution13 of13 nonvolatile13 solutePure13 water
Nonelectrolyte solutions of same molality have same concentration of particles
Dilute solutions of same solvent and equal molality of any nonelectrolyte solute lower vapor pressure equally
155
Example
1 m aqueous solution of nonelectrolyte glucose lowers vapor pressure of water 55 x 10-4 atm at 25 1 m aqueous solution of sucrose also lowers water vapor pressure to 55 x 10-4 atm at 25
156
157
bc vapor-pressure lowering depends on concentration of nonelectrolyte solute and doesnrsquot depend on the type of solute it is a colligative proprerty
bc vapor pressure has been lowered solution remains liquid over larger temperature range
Lowers freezing point and raises boiling point
158
Can assume changes in boiling and freezing point also depend on concentration of solute
They are colligative properties
159
Freezing-Point Depression
Freezing point of 1-molal solution of any nonelectrolyte solute in water is found (by experiment) to be 186 lower than freezing point of water When 1 mol of nonelectrolyte solute dissolved in 1 kg water freezing point is -186 not 000 160
When 2 mol nonelectrolyte solute dissolved in 1 kg water freezing point is -372
For any concentration of nonelectrolyte solute in water decrease in freezing point can be determined using value of -186m
161
This value called the molal freezing-point contstant (Kf) is the freezing-point depression of the solvent in a 1-molal solution of a nonvolatile nonelectrolyte solute
162
Each solvent has own characteristic molal freezing-point constant Freezing-point depression (Δtf) the difference between the freezing points of the pure solvent and a solute of nonelectrolyte in that solvent and it is directly proportional to the molal concentration of the solution Δtf = Kfm
163
Δtf = Kfm
Kf expressed as m
m expressed in mol solutekg solvent (molality)
164
Sample Problem
What is the freezing point depression of water in a solution of 171 g of sucrose C12H22O11 and 200 g of water What is the actual freezing point of the solution
165
1 AnalyzeGiven Solute mass and chemical formula 171 g C12H22O11 Solvent mass and identity 2000 g water Unknown a freezing-point depression b freezing point of the solution 166
2 Plan
167
168
Calculate
Unknown a freezing-point depression b freezing point of the solution
169
Practice Problem
A water solution containing an unknown quantity of a nonelectrolyte solute is found to have a freezing point of ndash 023degCWhat is the molal concentration of the solution
012 m170
A solution consists of 103 g of the nonelectrolyte glucose C6H12O6 dissolved in 250 g of water What is the freezing-point depression of the solution
minus0426degC171
In a laboratory experiment the freezing point of an aqueous solution of glucose is found to be minus0325degCWhat is the molal concentration of this solution
0175 m172
If 0500 mol of a nonelectrolyte solute are dissolved in 5000 g of ether what is the freezing point of the solution
minus1181degC173
The freezing point of an aqueous solution that contains a nonelectrolyte is minus90degC aWhat is the freezing-point depression of the solution bWhat is the molal concentration of the solution a minus90degC b 48 m
174
Boiling-Point Elevation
Boiling point is temp at which vapor pressure is equal to atmospheric pressure Change in vapor pressure will cause corresponding change in boiling point
175
176
Vapor pressure of nonvolatile solution is lower than vapor pressure of pure solvent More heat will be required to raise vapor pressure of solution Boiling point of solution is higher than boiling point of pure solvent
Molal boiling-point constant (Kb) - boiling-point elevation of the solvent in a 1-molal solution of a nonvolatile nonelectrolyte solute b-p elevation of any nonelectrolyte in water found by experiment to be 051 So molal b-p constant for water is 051m
177
For different solvents b-p elevations of 1-molal solutions have different values (Table 14-2)
Like freezing-point constants values most accurate for dilute solutions
178
Boiling-point elevation Δtb difference between the boiling points of the pure solvent and a nonelectrolyte solution of that solvent and it is directly proportional to the molal concentration of the solution B-p elevation can be calculated using Δtb = Kbm Δtb (boiling point elevation) expressed in m and m expressed in mol solutekg solvent
Sample Problem
What is the boiling-point elevation of a solution made from 200 g of a nonelectrolyte solute and 4000 g of water The molar mass of the solute is 620 gmol
180
1 AnalyzeGiven solute mass = 200 g solute molar mass = 620 gmol solvent mass and identity = 4000 g of water
Unknown boiling-point elevation
181
2 Plan
182
183
3 Compute
Practice Problem
A solution contains 500 g of sucrose C12H22O11 a nonelectrolyte dissolved in 5000 g of water What is the boiling-point elevation
015degC184
A solution contains 4500 g of sucrose C12H22O11 a nonelectrolyte dissolved in 250 g of waterWhat is the boiling point of the solution
1027degC185
If the boiling point elevation of an aqueous solution containing a nonvolatile electrolyte is 102degC what is the molality of the solution
20 m186
The boiling point of an aqueous solution containing a nonvolatile electrolyte is 10075degC aWhat is the boiling-point elevation bWhat is the molality of the solution a 075degC b 15 m
187
Electrolytes and Colligative PropertiesEarly investigators confused by experiments where certain substances depressed freezing point or elevated boiling point more than expected
188
189
Ex 01 m solution of NaCl lowers freezing point of solvent almost twice as much as 01 m solution of sucrose 01 m CaCl2 lowers fp almost 3 times as much as 01 m solution of sucrose
To understand why this happens contrast behavior or sucrose with NaCl in aqueous solution Each sucrose dissolves to make only 1 particle in solution So 1 mol sucrose dissolves to 1 mol particles 1 mol NaCl dissolves to make 2 moles of particles (1 mol Na+ and 1 mol Cl-)
190
191
Calculated Values for Electrolyte SolutionsColligative properties depend on total concentration of solute particles regardless of their identity Electrolytes cause changes in colligative properties proportional to total molality in terms of all dissolves particles instead of formula units
192
What about barium nitrate Ba(NO3)2 Each mole of barium nitrate yields 3 mol of ions in solution Ba(NO3)2(s) rarr Ba2+(aq) + 2NO3-(aq) You would expect solution of given molality to lower fp of solvent 3 times as much as nonelectrolytic solution of same molality
193
Sample Problem
What is the expected change in the freezing point of water in a solution of 625 g of barium nitrate Ba(NO3)2 in 100 kg of water
194
1 AnalyzeGiven solute mass and formula = 625 g Ba(NO3)2 solvent mass and identity = 100 kg water Δtf = Kfm
Unknown expected freezing-point depression
195
2 PlanThe molality can be calculated by converting the solute mass to moles and then dividing by the number of kilograms of solvent That molality is in terms of formula units of Ba(NO3)2 and must be converted to molality in terms of dissociated ions in solution It must be multiplied by the number of moles of ions produced per mole of formula unit This adjusted molality can then be used to calculate the freezing-point depression
196
197
198
3 Compute
Practice Problem
What is the expected freezing-point depression for a solution that contains 20 mol of magnesium sulfate dissolved in 10 kg of water
minus74degC199
What is the expected boiling-point elevation of water for a solution that contains 150 g of sodium chloride dissolved in 10 kg of water 27degC The freezing point of an aqueous sodium chloride solution is minus200degC What is the molality of the solution 54 m NaCl
200
Actual Values for Electrolyte SolutionsRemember values calculated are only expected values Actual values of colligative properties for all strong electrolytes are almost what would be expected Difference between expected and actual values caused by attractive forces that exist between dissociated ions in aqueous solution
201
Attraction between hydrated ions in solution is small compared with those in crystalline solid However forces of attraction donrsquot interfere with movements of aqueous ions More concentrated solution means ions closer together and attraction is greater Also depends on charge of ions ndash higher charge greater attraction
202
Will precipitate form when solutions of ammonium sulfide and cadmium nitrate are combined You can tell calcium nitrate is soluble from the guidelines (Most nitrates acetates and chlorates are soluble) You can also tell ammonium sulfide is soluble (Most sulfides are insoluble except those of calcium strontium sodium potassium and ammonium)
116
117
To decide whether precipitate can form must know solubilities of two possible products
Ammonium nitrate ndash soluble (Most nitrates acetates and chlorates are soluble) Cadmium sulfide ndash insoluble (Most sulfides are insoluble except those of calcium strontium sodium potassium and ammonium)
118
bc one product is insoluble double-replacement and precipitation reaction will happen (NH4)2S (aq) + Cd(NO3)2 (aq) rarr NH4NO3 (aq) + CdS (s)
119
Net Ionic Equations
Reactions of ions in aqueous solution usually represented by net ionic equations instead of formula equations Net ionic equation - includes only those compounds and ions that undergo a chemical change in a reaction in an aqueous solution
120
121
To write NIE (net ionic equation) first convert chemical equation into overall ionic equation All soluble ionic compounds shown as dissociated ions in solution Precipitates shown as solids
122
Notice ammonium and nitrate ions appear on both sides They havenrsquot gone through any chemical change Spectator ions - ions that do not take part in a chemical reaction and are found in solution both before and after the reaction
123
To change ionic equation to NIE eliminate spectator ions Cd+2(aq) + S-2(aq) rarr CdS(s)
This NIE applies to any reaction in which precipitate of CdS forms
124
Practice Problem
Identify the precipitate that forms when aqueous solutions of zinc nitrate and ammonium sulfide are combined Write the equation for the possible double-replacement reaction Then write the formula equation overall ionic equation and the net ionic equation
125
1 AnalyzeGiven Identity of reactants zinc nitrate and ammonium sulfide Reaction medium aqueous solution Unknown a equation for the possible double-replacement reaction b identity of the precipitate c formula equation d overall ionic equation e net ionic equation
126
2 PlanWrite the possible double-replacement reaction between Zn(NO3)2 and (NH4)2S Use the rules to determine if any of the products will precipitate Write a formula equation and overall net ionic equation Then cancel spectator ions to make net ionic equation
127
3 Computea possible double-replacement reaction
b rules show that zinc sulfide is not soluble so will precipitate (ammonium nitrate is soluble)
c formula equation128
d overall ionic equation
e net ionic equation129
Will a precipitate form if solutions of potassium sulfate and barium nitrate are combined If so write the net ionic equation for the reaction
Yes Ba+2(aq) + SO4-2(aq) rarr BaSO4(s)
130
Will a precipitate form if solutions of potassium nitrate and magnesium sulfate are combined If so write the net ionic equation for the reaction
No 131
Will a precipitate form if solutions of barium chloride and sodium sulfate are combined If so identify the spectator ions and write the net ionic equation for the reaction
Yes Na+ and Cl- Ba+2(aq) + SO4-2(aq) rarr BaSO4(s)
132
Write the net ionic equation for the precipitation of nickel(II) sulfide
Ni+2(aq) + S-2(aq) rarr NiS(s)
133
IonizationSome molecular compounds can also form ions in solution Usually polar Ionization - ions formed from solute molecules by action of the solvent (creation of ions where there were none) Ionization different from dissociation
134
Dissociation ionic compounds dissolve and ions already present separate Ionization ions formed where non existed before
Ions formed are hydrated Heat released during hydration of ions gives enough energy to break covalent bonds
135
Extent to which solute ionizes depends on strength of bonds within molecules of solute and strength of attraction between solute and solvent molecules
136
HCl ndash molecular compound that ionizes in aqueous solution Contains highly polar bond Attraction between polar HCl molecule and polar water strong enough to break HCl bond
137
The Hydronium Ion
H+ ions from HCl attracts other molecules or ions so strongly that it doesnrsquot normally exist alone Ionization of HCl better described as direct transfer of proton from HCl to H2O forming H3O+
138
139
Hydration of H+ to form hydronium is highly exothermic
Energy released gives a lot of energy needed to ionize a molecular solute
Many molecular compounds that ionize in aqueous solution contain hydrogen and form hydronium ion
140
Strong and Weak Electrolytes
Substances that make ions and conduct current in solution are electrolytes HCl is one of series of compounds that has hydrogen and halogen Hydrogen halides are all molecular compounds with single polar-covalent bonds
141
All are gases Very soluble in water All are electrolytes HCl HBr HI strongly conduct current in solution HF only weakly conducts current at same concentration Strength of conduction related to ability to ionize
142
143
Strong Electrolytes
HCl HBr HI are 100 ionized in solution
Strong electrolyte - any compound whose dilute aqueous solutions conduct electricity well this is due to the presence of all or almost all of the dissolve compound in the form of ions
144
Unique characteristic of strong electrolyte is that they yield only ions
Ex Ionic compound may be highly soluble in water and dissociate into ions (NaCl)
Others may not dissolve much but amount that does dissolve exists only as ions
145
Weak ElectrolytesHF dissolves in water to give acid solution called hydrofluoric acid HF bond stronger than bonds between hydrogen and other halogens When HF dissolves some molecule ionize Reverse reaction also happens HF(aq) + H2O(l) H3O+(aq) + F-(aq)
146
HF(aq) + H2O(l) H3O+
(aq) + F-(aq)Concentration of dissolved unionized HF stays high and concentration of ions stays low
Weak electrolyte - any compound whose aqueous solutions conduct electricity poorly this is due to the presence of small amount of the dissolved compound in the form of ions
147
Different from nonelectrolyte ndash NO ions at all Description of electrolyte as strong or weak not related to concentration of solution Electrolytes differ in degree of ionization not amount of solute dissolved
148
149
Colligative properties of solutions
Presence of solute affects properties of solutions Some properties not dependent on nature of dissolved substance but on how may dissolve particles are present
Colligative properties - properties that depend on the concentration of solute particles but not on their identity
150
Vapor-Pressure Lowering
Boiling and freezing point of solution different from pure solvent Nonvolatile solute raises boiling point and lowers freezing point Nonvolatile substance - one that has little tendency to become gas under existing conditions
151
152
To understand why nonvolatile solute changes boiling and freezing point consider equilibrium vapor pressure
Vapor pressure pressure caused by molecules that have escape liquid phase to gas phase
Can be thought of a measure of tendency of molecules to escape from a liquid
153
Addition of sucrose (nonvolatile solute) lowers concentration of water molecules at surface of liquid This lowers tendency of water molecules to leave solution and enter gas phase Vapor pressure of solution is lower than vapor pressure of pure water
154
Aqueous13 solution13 of13 nonvolatile13 solutePure13 water
Nonelectrolyte solutions of same molality have same concentration of particles
Dilute solutions of same solvent and equal molality of any nonelectrolyte solute lower vapor pressure equally
155
Example
1 m aqueous solution of nonelectrolyte glucose lowers vapor pressure of water 55 x 10-4 atm at 25 1 m aqueous solution of sucrose also lowers water vapor pressure to 55 x 10-4 atm at 25
156
157
bc vapor-pressure lowering depends on concentration of nonelectrolyte solute and doesnrsquot depend on the type of solute it is a colligative proprerty
bc vapor pressure has been lowered solution remains liquid over larger temperature range
Lowers freezing point and raises boiling point
158
Can assume changes in boiling and freezing point also depend on concentration of solute
They are colligative properties
159
Freezing-Point Depression
Freezing point of 1-molal solution of any nonelectrolyte solute in water is found (by experiment) to be 186 lower than freezing point of water When 1 mol of nonelectrolyte solute dissolved in 1 kg water freezing point is -186 not 000 160
When 2 mol nonelectrolyte solute dissolved in 1 kg water freezing point is -372
For any concentration of nonelectrolyte solute in water decrease in freezing point can be determined using value of -186m
161
This value called the molal freezing-point contstant (Kf) is the freezing-point depression of the solvent in a 1-molal solution of a nonvolatile nonelectrolyte solute
162
Each solvent has own characteristic molal freezing-point constant Freezing-point depression (Δtf) the difference between the freezing points of the pure solvent and a solute of nonelectrolyte in that solvent and it is directly proportional to the molal concentration of the solution Δtf = Kfm
163
Δtf = Kfm
Kf expressed as m
m expressed in mol solutekg solvent (molality)
164
Sample Problem
What is the freezing point depression of water in a solution of 171 g of sucrose C12H22O11 and 200 g of water What is the actual freezing point of the solution
165
1 AnalyzeGiven Solute mass and chemical formula 171 g C12H22O11 Solvent mass and identity 2000 g water Unknown a freezing-point depression b freezing point of the solution 166
2 Plan
167
168
Calculate
Unknown a freezing-point depression b freezing point of the solution
169
Practice Problem
A water solution containing an unknown quantity of a nonelectrolyte solute is found to have a freezing point of ndash 023degCWhat is the molal concentration of the solution
012 m170
A solution consists of 103 g of the nonelectrolyte glucose C6H12O6 dissolved in 250 g of water What is the freezing-point depression of the solution
minus0426degC171
In a laboratory experiment the freezing point of an aqueous solution of glucose is found to be minus0325degCWhat is the molal concentration of this solution
0175 m172
If 0500 mol of a nonelectrolyte solute are dissolved in 5000 g of ether what is the freezing point of the solution
minus1181degC173
The freezing point of an aqueous solution that contains a nonelectrolyte is minus90degC aWhat is the freezing-point depression of the solution bWhat is the molal concentration of the solution a minus90degC b 48 m
174
Boiling-Point Elevation
Boiling point is temp at which vapor pressure is equal to atmospheric pressure Change in vapor pressure will cause corresponding change in boiling point
175
176
Vapor pressure of nonvolatile solution is lower than vapor pressure of pure solvent More heat will be required to raise vapor pressure of solution Boiling point of solution is higher than boiling point of pure solvent
Molal boiling-point constant (Kb) - boiling-point elevation of the solvent in a 1-molal solution of a nonvolatile nonelectrolyte solute b-p elevation of any nonelectrolyte in water found by experiment to be 051 So molal b-p constant for water is 051m
177
For different solvents b-p elevations of 1-molal solutions have different values (Table 14-2)
Like freezing-point constants values most accurate for dilute solutions
178
Boiling-point elevation Δtb difference between the boiling points of the pure solvent and a nonelectrolyte solution of that solvent and it is directly proportional to the molal concentration of the solution B-p elevation can be calculated using Δtb = Kbm Δtb (boiling point elevation) expressed in m and m expressed in mol solutekg solvent
Sample Problem
What is the boiling-point elevation of a solution made from 200 g of a nonelectrolyte solute and 4000 g of water The molar mass of the solute is 620 gmol
180
1 AnalyzeGiven solute mass = 200 g solute molar mass = 620 gmol solvent mass and identity = 4000 g of water
Unknown boiling-point elevation
181
2 Plan
182
183
3 Compute
Practice Problem
A solution contains 500 g of sucrose C12H22O11 a nonelectrolyte dissolved in 5000 g of water What is the boiling-point elevation
015degC184
A solution contains 4500 g of sucrose C12H22O11 a nonelectrolyte dissolved in 250 g of waterWhat is the boiling point of the solution
1027degC185
If the boiling point elevation of an aqueous solution containing a nonvolatile electrolyte is 102degC what is the molality of the solution
20 m186
The boiling point of an aqueous solution containing a nonvolatile electrolyte is 10075degC aWhat is the boiling-point elevation bWhat is the molality of the solution a 075degC b 15 m
187
Electrolytes and Colligative PropertiesEarly investigators confused by experiments where certain substances depressed freezing point or elevated boiling point more than expected
188
189
Ex 01 m solution of NaCl lowers freezing point of solvent almost twice as much as 01 m solution of sucrose 01 m CaCl2 lowers fp almost 3 times as much as 01 m solution of sucrose
To understand why this happens contrast behavior or sucrose with NaCl in aqueous solution Each sucrose dissolves to make only 1 particle in solution So 1 mol sucrose dissolves to 1 mol particles 1 mol NaCl dissolves to make 2 moles of particles (1 mol Na+ and 1 mol Cl-)
190
191
Calculated Values for Electrolyte SolutionsColligative properties depend on total concentration of solute particles regardless of their identity Electrolytes cause changes in colligative properties proportional to total molality in terms of all dissolves particles instead of formula units
192
What about barium nitrate Ba(NO3)2 Each mole of barium nitrate yields 3 mol of ions in solution Ba(NO3)2(s) rarr Ba2+(aq) + 2NO3-(aq) You would expect solution of given molality to lower fp of solvent 3 times as much as nonelectrolytic solution of same molality
193
Sample Problem
What is the expected change in the freezing point of water in a solution of 625 g of barium nitrate Ba(NO3)2 in 100 kg of water
194
1 AnalyzeGiven solute mass and formula = 625 g Ba(NO3)2 solvent mass and identity = 100 kg water Δtf = Kfm
Unknown expected freezing-point depression
195
2 PlanThe molality can be calculated by converting the solute mass to moles and then dividing by the number of kilograms of solvent That molality is in terms of formula units of Ba(NO3)2 and must be converted to molality in terms of dissociated ions in solution It must be multiplied by the number of moles of ions produced per mole of formula unit This adjusted molality can then be used to calculate the freezing-point depression
196
197
198
3 Compute
Practice Problem
What is the expected freezing-point depression for a solution that contains 20 mol of magnesium sulfate dissolved in 10 kg of water
minus74degC199
What is the expected boiling-point elevation of water for a solution that contains 150 g of sodium chloride dissolved in 10 kg of water 27degC The freezing point of an aqueous sodium chloride solution is minus200degC What is the molality of the solution 54 m NaCl
200
Actual Values for Electrolyte SolutionsRemember values calculated are only expected values Actual values of colligative properties for all strong electrolytes are almost what would be expected Difference between expected and actual values caused by attractive forces that exist between dissociated ions in aqueous solution
201
Attraction between hydrated ions in solution is small compared with those in crystalline solid However forces of attraction donrsquot interfere with movements of aqueous ions More concentrated solution means ions closer together and attraction is greater Also depends on charge of ions ndash higher charge greater attraction
202
117
To decide whether precipitate can form must know solubilities of two possible products
Ammonium nitrate ndash soluble (Most nitrates acetates and chlorates are soluble) Cadmium sulfide ndash insoluble (Most sulfides are insoluble except those of calcium strontium sodium potassium and ammonium)
118
bc one product is insoluble double-replacement and precipitation reaction will happen (NH4)2S (aq) + Cd(NO3)2 (aq) rarr NH4NO3 (aq) + CdS (s)
119
Net Ionic Equations
Reactions of ions in aqueous solution usually represented by net ionic equations instead of formula equations Net ionic equation - includes only those compounds and ions that undergo a chemical change in a reaction in an aqueous solution
120
121
To write NIE (net ionic equation) first convert chemical equation into overall ionic equation All soluble ionic compounds shown as dissociated ions in solution Precipitates shown as solids
122
Notice ammonium and nitrate ions appear on both sides They havenrsquot gone through any chemical change Spectator ions - ions that do not take part in a chemical reaction and are found in solution both before and after the reaction
123
To change ionic equation to NIE eliminate spectator ions Cd+2(aq) + S-2(aq) rarr CdS(s)
This NIE applies to any reaction in which precipitate of CdS forms
124
Practice Problem
Identify the precipitate that forms when aqueous solutions of zinc nitrate and ammonium sulfide are combined Write the equation for the possible double-replacement reaction Then write the formula equation overall ionic equation and the net ionic equation
125
1 AnalyzeGiven Identity of reactants zinc nitrate and ammonium sulfide Reaction medium aqueous solution Unknown a equation for the possible double-replacement reaction b identity of the precipitate c formula equation d overall ionic equation e net ionic equation
126
2 PlanWrite the possible double-replacement reaction between Zn(NO3)2 and (NH4)2S Use the rules to determine if any of the products will precipitate Write a formula equation and overall net ionic equation Then cancel spectator ions to make net ionic equation
127
3 Computea possible double-replacement reaction
b rules show that zinc sulfide is not soluble so will precipitate (ammonium nitrate is soluble)
c formula equation128
d overall ionic equation
e net ionic equation129
Will a precipitate form if solutions of potassium sulfate and barium nitrate are combined If so write the net ionic equation for the reaction
Yes Ba+2(aq) + SO4-2(aq) rarr BaSO4(s)
130
Will a precipitate form if solutions of potassium nitrate and magnesium sulfate are combined If so write the net ionic equation for the reaction
No 131
Will a precipitate form if solutions of barium chloride and sodium sulfate are combined If so identify the spectator ions and write the net ionic equation for the reaction
Yes Na+ and Cl- Ba+2(aq) + SO4-2(aq) rarr BaSO4(s)
132
Write the net ionic equation for the precipitation of nickel(II) sulfide
Ni+2(aq) + S-2(aq) rarr NiS(s)
133
IonizationSome molecular compounds can also form ions in solution Usually polar Ionization - ions formed from solute molecules by action of the solvent (creation of ions where there were none) Ionization different from dissociation
134
Dissociation ionic compounds dissolve and ions already present separate Ionization ions formed where non existed before
Ions formed are hydrated Heat released during hydration of ions gives enough energy to break covalent bonds
135
Extent to which solute ionizes depends on strength of bonds within molecules of solute and strength of attraction between solute and solvent molecules
136
HCl ndash molecular compound that ionizes in aqueous solution Contains highly polar bond Attraction between polar HCl molecule and polar water strong enough to break HCl bond
137
The Hydronium Ion
H+ ions from HCl attracts other molecules or ions so strongly that it doesnrsquot normally exist alone Ionization of HCl better described as direct transfer of proton from HCl to H2O forming H3O+
138
139
Hydration of H+ to form hydronium is highly exothermic
Energy released gives a lot of energy needed to ionize a molecular solute
Many molecular compounds that ionize in aqueous solution contain hydrogen and form hydronium ion
140
Strong and Weak Electrolytes
Substances that make ions and conduct current in solution are electrolytes HCl is one of series of compounds that has hydrogen and halogen Hydrogen halides are all molecular compounds with single polar-covalent bonds
141
All are gases Very soluble in water All are electrolytes HCl HBr HI strongly conduct current in solution HF only weakly conducts current at same concentration Strength of conduction related to ability to ionize
142
143
Strong Electrolytes
HCl HBr HI are 100 ionized in solution
Strong electrolyte - any compound whose dilute aqueous solutions conduct electricity well this is due to the presence of all or almost all of the dissolve compound in the form of ions
144
Unique characteristic of strong electrolyte is that they yield only ions
Ex Ionic compound may be highly soluble in water and dissociate into ions (NaCl)
Others may not dissolve much but amount that does dissolve exists only as ions
145
Weak ElectrolytesHF dissolves in water to give acid solution called hydrofluoric acid HF bond stronger than bonds between hydrogen and other halogens When HF dissolves some molecule ionize Reverse reaction also happens HF(aq) + H2O(l) H3O+(aq) + F-(aq)
146
HF(aq) + H2O(l) H3O+
(aq) + F-(aq)Concentration of dissolved unionized HF stays high and concentration of ions stays low
Weak electrolyte - any compound whose aqueous solutions conduct electricity poorly this is due to the presence of small amount of the dissolved compound in the form of ions
147
Different from nonelectrolyte ndash NO ions at all Description of electrolyte as strong or weak not related to concentration of solution Electrolytes differ in degree of ionization not amount of solute dissolved
148
149
Colligative properties of solutions
Presence of solute affects properties of solutions Some properties not dependent on nature of dissolved substance but on how may dissolve particles are present
Colligative properties - properties that depend on the concentration of solute particles but not on their identity
150
Vapor-Pressure Lowering
Boiling and freezing point of solution different from pure solvent Nonvolatile solute raises boiling point and lowers freezing point Nonvolatile substance - one that has little tendency to become gas under existing conditions
151
152
To understand why nonvolatile solute changes boiling and freezing point consider equilibrium vapor pressure
Vapor pressure pressure caused by molecules that have escape liquid phase to gas phase
Can be thought of a measure of tendency of molecules to escape from a liquid
153
Addition of sucrose (nonvolatile solute) lowers concentration of water molecules at surface of liquid This lowers tendency of water molecules to leave solution and enter gas phase Vapor pressure of solution is lower than vapor pressure of pure water
154
Aqueous13 solution13 of13 nonvolatile13 solutePure13 water
Nonelectrolyte solutions of same molality have same concentration of particles
Dilute solutions of same solvent and equal molality of any nonelectrolyte solute lower vapor pressure equally
155
Example
1 m aqueous solution of nonelectrolyte glucose lowers vapor pressure of water 55 x 10-4 atm at 25 1 m aqueous solution of sucrose also lowers water vapor pressure to 55 x 10-4 atm at 25
156
157
bc vapor-pressure lowering depends on concentration of nonelectrolyte solute and doesnrsquot depend on the type of solute it is a colligative proprerty
bc vapor pressure has been lowered solution remains liquid over larger temperature range
Lowers freezing point and raises boiling point
158
Can assume changes in boiling and freezing point also depend on concentration of solute
They are colligative properties
159
Freezing-Point Depression
Freezing point of 1-molal solution of any nonelectrolyte solute in water is found (by experiment) to be 186 lower than freezing point of water When 1 mol of nonelectrolyte solute dissolved in 1 kg water freezing point is -186 not 000 160
When 2 mol nonelectrolyte solute dissolved in 1 kg water freezing point is -372
For any concentration of nonelectrolyte solute in water decrease in freezing point can be determined using value of -186m
161
This value called the molal freezing-point contstant (Kf) is the freezing-point depression of the solvent in a 1-molal solution of a nonvolatile nonelectrolyte solute
162
Each solvent has own characteristic molal freezing-point constant Freezing-point depression (Δtf) the difference between the freezing points of the pure solvent and a solute of nonelectrolyte in that solvent and it is directly proportional to the molal concentration of the solution Δtf = Kfm
163
Δtf = Kfm
Kf expressed as m
m expressed in mol solutekg solvent (molality)
164
Sample Problem
What is the freezing point depression of water in a solution of 171 g of sucrose C12H22O11 and 200 g of water What is the actual freezing point of the solution
165
1 AnalyzeGiven Solute mass and chemical formula 171 g C12H22O11 Solvent mass and identity 2000 g water Unknown a freezing-point depression b freezing point of the solution 166
2 Plan
167
168
Calculate
Unknown a freezing-point depression b freezing point of the solution
169
Practice Problem
A water solution containing an unknown quantity of a nonelectrolyte solute is found to have a freezing point of ndash 023degCWhat is the molal concentration of the solution
012 m170
A solution consists of 103 g of the nonelectrolyte glucose C6H12O6 dissolved in 250 g of water What is the freezing-point depression of the solution
minus0426degC171
In a laboratory experiment the freezing point of an aqueous solution of glucose is found to be minus0325degCWhat is the molal concentration of this solution
0175 m172
If 0500 mol of a nonelectrolyte solute are dissolved in 5000 g of ether what is the freezing point of the solution
minus1181degC173
The freezing point of an aqueous solution that contains a nonelectrolyte is minus90degC aWhat is the freezing-point depression of the solution bWhat is the molal concentration of the solution a minus90degC b 48 m
174
Boiling-Point Elevation
Boiling point is temp at which vapor pressure is equal to atmospheric pressure Change in vapor pressure will cause corresponding change in boiling point
175
176
Vapor pressure of nonvolatile solution is lower than vapor pressure of pure solvent More heat will be required to raise vapor pressure of solution Boiling point of solution is higher than boiling point of pure solvent
Molal boiling-point constant (Kb) - boiling-point elevation of the solvent in a 1-molal solution of a nonvolatile nonelectrolyte solute b-p elevation of any nonelectrolyte in water found by experiment to be 051 So molal b-p constant for water is 051m
177
For different solvents b-p elevations of 1-molal solutions have different values (Table 14-2)
Like freezing-point constants values most accurate for dilute solutions
178
Boiling-point elevation Δtb difference between the boiling points of the pure solvent and a nonelectrolyte solution of that solvent and it is directly proportional to the molal concentration of the solution B-p elevation can be calculated using Δtb = Kbm Δtb (boiling point elevation) expressed in m and m expressed in mol solutekg solvent
Sample Problem
What is the boiling-point elevation of a solution made from 200 g of a nonelectrolyte solute and 4000 g of water The molar mass of the solute is 620 gmol
180
1 AnalyzeGiven solute mass = 200 g solute molar mass = 620 gmol solvent mass and identity = 4000 g of water
Unknown boiling-point elevation
181
2 Plan
182
183
3 Compute
Practice Problem
A solution contains 500 g of sucrose C12H22O11 a nonelectrolyte dissolved in 5000 g of water What is the boiling-point elevation
015degC184
A solution contains 4500 g of sucrose C12H22O11 a nonelectrolyte dissolved in 250 g of waterWhat is the boiling point of the solution
1027degC185
If the boiling point elevation of an aqueous solution containing a nonvolatile electrolyte is 102degC what is the molality of the solution
20 m186
The boiling point of an aqueous solution containing a nonvolatile electrolyte is 10075degC aWhat is the boiling-point elevation bWhat is the molality of the solution a 075degC b 15 m
187
Electrolytes and Colligative PropertiesEarly investigators confused by experiments where certain substances depressed freezing point or elevated boiling point more than expected
188
189
Ex 01 m solution of NaCl lowers freezing point of solvent almost twice as much as 01 m solution of sucrose 01 m CaCl2 lowers fp almost 3 times as much as 01 m solution of sucrose
To understand why this happens contrast behavior or sucrose with NaCl in aqueous solution Each sucrose dissolves to make only 1 particle in solution So 1 mol sucrose dissolves to 1 mol particles 1 mol NaCl dissolves to make 2 moles of particles (1 mol Na+ and 1 mol Cl-)
190
191
Calculated Values for Electrolyte SolutionsColligative properties depend on total concentration of solute particles regardless of their identity Electrolytes cause changes in colligative properties proportional to total molality in terms of all dissolves particles instead of formula units
192
What about barium nitrate Ba(NO3)2 Each mole of barium nitrate yields 3 mol of ions in solution Ba(NO3)2(s) rarr Ba2+(aq) + 2NO3-(aq) You would expect solution of given molality to lower fp of solvent 3 times as much as nonelectrolytic solution of same molality
193
Sample Problem
What is the expected change in the freezing point of water in a solution of 625 g of barium nitrate Ba(NO3)2 in 100 kg of water
194
1 AnalyzeGiven solute mass and formula = 625 g Ba(NO3)2 solvent mass and identity = 100 kg water Δtf = Kfm
Unknown expected freezing-point depression
195
2 PlanThe molality can be calculated by converting the solute mass to moles and then dividing by the number of kilograms of solvent That molality is in terms of formula units of Ba(NO3)2 and must be converted to molality in terms of dissociated ions in solution It must be multiplied by the number of moles of ions produced per mole of formula unit This adjusted molality can then be used to calculate the freezing-point depression
196
197
198
3 Compute
Practice Problem
What is the expected freezing-point depression for a solution that contains 20 mol of magnesium sulfate dissolved in 10 kg of water
minus74degC199
What is the expected boiling-point elevation of water for a solution that contains 150 g of sodium chloride dissolved in 10 kg of water 27degC The freezing point of an aqueous sodium chloride solution is minus200degC What is the molality of the solution 54 m NaCl
200
Actual Values for Electrolyte SolutionsRemember values calculated are only expected values Actual values of colligative properties for all strong electrolytes are almost what would be expected Difference between expected and actual values caused by attractive forces that exist between dissociated ions in aqueous solution
201
Attraction between hydrated ions in solution is small compared with those in crystalline solid However forces of attraction donrsquot interfere with movements of aqueous ions More concentrated solution means ions closer together and attraction is greater Also depends on charge of ions ndash higher charge greater attraction
202
To decide whether precipitate can form must know solubilities of two possible products
Ammonium nitrate ndash soluble (Most nitrates acetates and chlorates are soluble) Cadmium sulfide ndash insoluble (Most sulfides are insoluble except those of calcium strontium sodium potassium and ammonium)
118
bc one product is insoluble double-replacement and precipitation reaction will happen (NH4)2S (aq) + Cd(NO3)2 (aq) rarr NH4NO3 (aq) + CdS (s)
119
Net Ionic Equations
Reactions of ions in aqueous solution usually represented by net ionic equations instead of formula equations Net ionic equation - includes only those compounds and ions that undergo a chemical change in a reaction in an aqueous solution
120
121
To write NIE (net ionic equation) first convert chemical equation into overall ionic equation All soluble ionic compounds shown as dissociated ions in solution Precipitates shown as solids
122
Notice ammonium and nitrate ions appear on both sides They havenrsquot gone through any chemical change Spectator ions - ions that do not take part in a chemical reaction and are found in solution both before and after the reaction
123
To change ionic equation to NIE eliminate spectator ions Cd+2(aq) + S-2(aq) rarr CdS(s)
This NIE applies to any reaction in which precipitate of CdS forms
124
Practice Problem
Identify the precipitate that forms when aqueous solutions of zinc nitrate and ammonium sulfide are combined Write the equation for the possible double-replacement reaction Then write the formula equation overall ionic equation and the net ionic equation
125
1 AnalyzeGiven Identity of reactants zinc nitrate and ammonium sulfide Reaction medium aqueous solution Unknown a equation for the possible double-replacement reaction b identity of the precipitate c formula equation d overall ionic equation e net ionic equation
126
2 PlanWrite the possible double-replacement reaction between Zn(NO3)2 and (NH4)2S Use the rules to determine if any of the products will precipitate Write a formula equation and overall net ionic equation Then cancel spectator ions to make net ionic equation
127
3 Computea possible double-replacement reaction
b rules show that zinc sulfide is not soluble so will precipitate (ammonium nitrate is soluble)
c formula equation128
d overall ionic equation
e net ionic equation129
Will a precipitate form if solutions of potassium sulfate and barium nitrate are combined If so write the net ionic equation for the reaction
Yes Ba+2(aq) + SO4-2(aq) rarr BaSO4(s)
130
Will a precipitate form if solutions of potassium nitrate and magnesium sulfate are combined If so write the net ionic equation for the reaction
No 131
Will a precipitate form if solutions of barium chloride and sodium sulfate are combined If so identify the spectator ions and write the net ionic equation for the reaction
Yes Na+ and Cl- Ba+2(aq) + SO4-2(aq) rarr BaSO4(s)
132
Write the net ionic equation for the precipitation of nickel(II) sulfide
Ni+2(aq) + S-2(aq) rarr NiS(s)
133
IonizationSome molecular compounds can also form ions in solution Usually polar Ionization - ions formed from solute molecules by action of the solvent (creation of ions where there were none) Ionization different from dissociation
134
Dissociation ionic compounds dissolve and ions already present separate Ionization ions formed where non existed before
Ions formed are hydrated Heat released during hydration of ions gives enough energy to break covalent bonds
135
Extent to which solute ionizes depends on strength of bonds within molecules of solute and strength of attraction between solute and solvent molecules
136
HCl ndash molecular compound that ionizes in aqueous solution Contains highly polar bond Attraction between polar HCl molecule and polar water strong enough to break HCl bond
137
The Hydronium Ion
H+ ions from HCl attracts other molecules or ions so strongly that it doesnrsquot normally exist alone Ionization of HCl better described as direct transfer of proton from HCl to H2O forming H3O+
138
139
Hydration of H+ to form hydronium is highly exothermic
Energy released gives a lot of energy needed to ionize a molecular solute
Many molecular compounds that ionize in aqueous solution contain hydrogen and form hydronium ion
140
Strong and Weak Electrolytes
Substances that make ions and conduct current in solution are electrolytes HCl is one of series of compounds that has hydrogen and halogen Hydrogen halides are all molecular compounds with single polar-covalent bonds
141
All are gases Very soluble in water All are electrolytes HCl HBr HI strongly conduct current in solution HF only weakly conducts current at same concentration Strength of conduction related to ability to ionize
142
143
Strong Electrolytes
HCl HBr HI are 100 ionized in solution
Strong electrolyte - any compound whose dilute aqueous solutions conduct electricity well this is due to the presence of all or almost all of the dissolve compound in the form of ions
144
Unique characteristic of strong electrolyte is that they yield only ions
Ex Ionic compound may be highly soluble in water and dissociate into ions (NaCl)
Others may not dissolve much but amount that does dissolve exists only as ions
145
Weak ElectrolytesHF dissolves in water to give acid solution called hydrofluoric acid HF bond stronger than bonds between hydrogen and other halogens When HF dissolves some molecule ionize Reverse reaction also happens HF(aq) + H2O(l) H3O+(aq) + F-(aq)
146
HF(aq) + H2O(l) H3O+
(aq) + F-(aq)Concentration of dissolved unionized HF stays high and concentration of ions stays low
Weak electrolyte - any compound whose aqueous solutions conduct electricity poorly this is due to the presence of small amount of the dissolved compound in the form of ions
147
Different from nonelectrolyte ndash NO ions at all Description of electrolyte as strong or weak not related to concentration of solution Electrolytes differ in degree of ionization not amount of solute dissolved
148
149
Colligative properties of solutions
Presence of solute affects properties of solutions Some properties not dependent on nature of dissolved substance but on how may dissolve particles are present
Colligative properties - properties that depend on the concentration of solute particles but not on their identity
150
Vapor-Pressure Lowering
Boiling and freezing point of solution different from pure solvent Nonvolatile solute raises boiling point and lowers freezing point Nonvolatile substance - one that has little tendency to become gas under existing conditions
151
152
To understand why nonvolatile solute changes boiling and freezing point consider equilibrium vapor pressure
Vapor pressure pressure caused by molecules that have escape liquid phase to gas phase
Can be thought of a measure of tendency of molecules to escape from a liquid
153
Addition of sucrose (nonvolatile solute) lowers concentration of water molecules at surface of liquid This lowers tendency of water molecules to leave solution and enter gas phase Vapor pressure of solution is lower than vapor pressure of pure water
154
Aqueous13 solution13 of13 nonvolatile13 solutePure13 water
Nonelectrolyte solutions of same molality have same concentration of particles
Dilute solutions of same solvent and equal molality of any nonelectrolyte solute lower vapor pressure equally
155
Example
1 m aqueous solution of nonelectrolyte glucose lowers vapor pressure of water 55 x 10-4 atm at 25 1 m aqueous solution of sucrose also lowers water vapor pressure to 55 x 10-4 atm at 25
156
157
bc vapor-pressure lowering depends on concentration of nonelectrolyte solute and doesnrsquot depend on the type of solute it is a colligative proprerty
bc vapor pressure has been lowered solution remains liquid over larger temperature range
Lowers freezing point and raises boiling point
158
Can assume changes in boiling and freezing point also depend on concentration of solute
They are colligative properties
159
Freezing-Point Depression
Freezing point of 1-molal solution of any nonelectrolyte solute in water is found (by experiment) to be 186 lower than freezing point of water When 1 mol of nonelectrolyte solute dissolved in 1 kg water freezing point is -186 not 000 160
When 2 mol nonelectrolyte solute dissolved in 1 kg water freezing point is -372
For any concentration of nonelectrolyte solute in water decrease in freezing point can be determined using value of -186m
161
This value called the molal freezing-point contstant (Kf) is the freezing-point depression of the solvent in a 1-molal solution of a nonvolatile nonelectrolyte solute
162
Each solvent has own characteristic molal freezing-point constant Freezing-point depression (Δtf) the difference between the freezing points of the pure solvent and a solute of nonelectrolyte in that solvent and it is directly proportional to the molal concentration of the solution Δtf = Kfm
163
Δtf = Kfm
Kf expressed as m
m expressed in mol solutekg solvent (molality)
164
Sample Problem
What is the freezing point depression of water in a solution of 171 g of sucrose C12H22O11 and 200 g of water What is the actual freezing point of the solution
165
1 AnalyzeGiven Solute mass and chemical formula 171 g C12H22O11 Solvent mass and identity 2000 g water Unknown a freezing-point depression b freezing point of the solution 166
2 Plan
167
168
Calculate
Unknown a freezing-point depression b freezing point of the solution
169
Practice Problem
A water solution containing an unknown quantity of a nonelectrolyte solute is found to have a freezing point of ndash 023degCWhat is the molal concentration of the solution
012 m170
A solution consists of 103 g of the nonelectrolyte glucose C6H12O6 dissolved in 250 g of water What is the freezing-point depression of the solution
minus0426degC171
In a laboratory experiment the freezing point of an aqueous solution of glucose is found to be minus0325degCWhat is the molal concentration of this solution
0175 m172
If 0500 mol of a nonelectrolyte solute are dissolved in 5000 g of ether what is the freezing point of the solution
minus1181degC173
The freezing point of an aqueous solution that contains a nonelectrolyte is minus90degC aWhat is the freezing-point depression of the solution bWhat is the molal concentration of the solution a minus90degC b 48 m
174
Boiling-Point Elevation
Boiling point is temp at which vapor pressure is equal to atmospheric pressure Change in vapor pressure will cause corresponding change in boiling point
175
176
Vapor pressure of nonvolatile solution is lower than vapor pressure of pure solvent More heat will be required to raise vapor pressure of solution Boiling point of solution is higher than boiling point of pure solvent
Molal boiling-point constant (Kb) - boiling-point elevation of the solvent in a 1-molal solution of a nonvolatile nonelectrolyte solute b-p elevation of any nonelectrolyte in water found by experiment to be 051 So molal b-p constant for water is 051m
177
For different solvents b-p elevations of 1-molal solutions have different values (Table 14-2)
Like freezing-point constants values most accurate for dilute solutions
178
Boiling-point elevation Δtb difference between the boiling points of the pure solvent and a nonelectrolyte solution of that solvent and it is directly proportional to the molal concentration of the solution B-p elevation can be calculated using Δtb = Kbm Δtb (boiling point elevation) expressed in m and m expressed in mol solutekg solvent
Sample Problem
What is the boiling-point elevation of a solution made from 200 g of a nonelectrolyte solute and 4000 g of water The molar mass of the solute is 620 gmol
180
1 AnalyzeGiven solute mass = 200 g solute molar mass = 620 gmol solvent mass and identity = 4000 g of water
Unknown boiling-point elevation
181
2 Plan
182
183
3 Compute
Practice Problem
A solution contains 500 g of sucrose C12H22O11 a nonelectrolyte dissolved in 5000 g of water What is the boiling-point elevation
015degC184
A solution contains 4500 g of sucrose C12H22O11 a nonelectrolyte dissolved in 250 g of waterWhat is the boiling point of the solution
1027degC185
If the boiling point elevation of an aqueous solution containing a nonvolatile electrolyte is 102degC what is the molality of the solution
20 m186
The boiling point of an aqueous solution containing a nonvolatile electrolyte is 10075degC aWhat is the boiling-point elevation bWhat is the molality of the solution a 075degC b 15 m
187
Electrolytes and Colligative PropertiesEarly investigators confused by experiments where certain substances depressed freezing point or elevated boiling point more than expected
188
189
Ex 01 m solution of NaCl lowers freezing point of solvent almost twice as much as 01 m solution of sucrose 01 m CaCl2 lowers fp almost 3 times as much as 01 m solution of sucrose
To understand why this happens contrast behavior or sucrose with NaCl in aqueous solution Each sucrose dissolves to make only 1 particle in solution So 1 mol sucrose dissolves to 1 mol particles 1 mol NaCl dissolves to make 2 moles of particles (1 mol Na+ and 1 mol Cl-)
190
191
Calculated Values for Electrolyte SolutionsColligative properties depend on total concentration of solute particles regardless of their identity Electrolytes cause changes in colligative properties proportional to total molality in terms of all dissolves particles instead of formula units
192
What about barium nitrate Ba(NO3)2 Each mole of barium nitrate yields 3 mol of ions in solution Ba(NO3)2(s) rarr Ba2+(aq) + 2NO3-(aq) You would expect solution of given molality to lower fp of solvent 3 times as much as nonelectrolytic solution of same molality
193
Sample Problem
What is the expected change in the freezing point of water in a solution of 625 g of barium nitrate Ba(NO3)2 in 100 kg of water
194
1 AnalyzeGiven solute mass and formula = 625 g Ba(NO3)2 solvent mass and identity = 100 kg water Δtf = Kfm
Unknown expected freezing-point depression
195
2 PlanThe molality can be calculated by converting the solute mass to moles and then dividing by the number of kilograms of solvent That molality is in terms of formula units of Ba(NO3)2 and must be converted to molality in terms of dissociated ions in solution It must be multiplied by the number of moles of ions produced per mole of formula unit This adjusted molality can then be used to calculate the freezing-point depression
196
197
198
3 Compute
Practice Problem
What is the expected freezing-point depression for a solution that contains 20 mol of magnesium sulfate dissolved in 10 kg of water
minus74degC199
What is the expected boiling-point elevation of water for a solution that contains 150 g of sodium chloride dissolved in 10 kg of water 27degC The freezing point of an aqueous sodium chloride solution is minus200degC What is the molality of the solution 54 m NaCl
200
Actual Values for Electrolyte SolutionsRemember values calculated are only expected values Actual values of colligative properties for all strong electrolytes are almost what would be expected Difference between expected and actual values caused by attractive forces that exist between dissociated ions in aqueous solution
201
Attraction between hydrated ions in solution is small compared with those in crystalline solid However forces of attraction donrsquot interfere with movements of aqueous ions More concentrated solution means ions closer together and attraction is greater Also depends on charge of ions ndash higher charge greater attraction
202
bc one product is insoluble double-replacement and precipitation reaction will happen (NH4)2S (aq) + Cd(NO3)2 (aq) rarr NH4NO3 (aq) + CdS (s)
119
Net Ionic Equations
Reactions of ions in aqueous solution usually represented by net ionic equations instead of formula equations Net ionic equation - includes only those compounds and ions that undergo a chemical change in a reaction in an aqueous solution
120
121
To write NIE (net ionic equation) first convert chemical equation into overall ionic equation All soluble ionic compounds shown as dissociated ions in solution Precipitates shown as solids
122
Notice ammonium and nitrate ions appear on both sides They havenrsquot gone through any chemical change Spectator ions - ions that do not take part in a chemical reaction and are found in solution both before and after the reaction
123
To change ionic equation to NIE eliminate spectator ions Cd+2(aq) + S-2(aq) rarr CdS(s)
This NIE applies to any reaction in which precipitate of CdS forms
124
Practice Problem
Identify the precipitate that forms when aqueous solutions of zinc nitrate and ammonium sulfide are combined Write the equation for the possible double-replacement reaction Then write the formula equation overall ionic equation and the net ionic equation
125
1 AnalyzeGiven Identity of reactants zinc nitrate and ammonium sulfide Reaction medium aqueous solution Unknown a equation for the possible double-replacement reaction b identity of the precipitate c formula equation d overall ionic equation e net ionic equation
126
2 PlanWrite the possible double-replacement reaction between Zn(NO3)2 and (NH4)2S Use the rules to determine if any of the products will precipitate Write a formula equation and overall net ionic equation Then cancel spectator ions to make net ionic equation
127
3 Computea possible double-replacement reaction
b rules show that zinc sulfide is not soluble so will precipitate (ammonium nitrate is soluble)
c formula equation128
d overall ionic equation
e net ionic equation129
Will a precipitate form if solutions of potassium sulfate and barium nitrate are combined If so write the net ionic equation for the reaction
Yes Ba+2(aq) + SO4-2(aq) rarr BaSO4(s)
130
Will a precipitate form if solutions of potassium nitrate and magnesium sulfate are combined If so write the net ionic equation for the reaction
No 131
Will a precipitate form if solutions of barium chloride and sodium sulfate are combined If so identify the spectator ions and write the net ionic equation for the reaction
Yes Na+ and Cl- Ba+2(aq) + SO4-2(aq) rarr BaSO4(s)
132
Write the net ionic equation for the precipitation of nickel(II) sulfide
Ni+2(aq) + S-2(aq) rarr NiS(s)
133
IonizationSome molecular compounds can also form ions in solution Usually polar Ionization - ions formed from solute molecules by action of the solvent (creation of ions where there were none) Ionization different from dissociation
134
Dissociation ionic compounds dissolve and ions already present separate Ionization ions formed where non existed before
Ions formed are hydrated Heat released during hydration of ions gives enough energy to break covalent bonds
135
Extent to which solute ionizes depends on strength of bonds within molecules of solute and strength of attraction between solute and solvent molecules
136
HCl ndash molecular compound that ionizes in aqueous solution Contains highly polar bond Attraction between polar HCl molecule and polar water strong enough to break HCl bond
137
The Hydronium Ion
H+ ions from HCl attracts other molecules or ions so strongly that it doesnrsquot normally exist alone Ionization of HCl better described as direct transfer of proton from HCl to H2O forming H3O+
138
139
Hydration of H+ to form hydronium is highly exothermic
Energy released gives a lot of energy needed to ionize a molecular solute
Many molecular compounds that ionize in aqueous solution contain hydrogen and form hydronium ion
140
Strong and Weak Electrolytes
Substances that make ions and conduct current in solution are electrolytes HCl is one of series of compounds that has hydrogen and halogen Hydrogen halides are all molecular compounds with single polar-covalent bonds
141
All are gases Very soluble in water All are electrolytes HCl HBr HI strongly conduct current in solution HF only weakly conducts current at same concentration Strength of conduction related to ability to ionize
142
143
Strong Electrolytes
HCl HBr HI are 100 ionized in solution
Strong electrolyte - any compound whose dilute aqueous solutions conduct electricity well this is due to the presence of all or almost all of the dissolve compound in the form of ions
144
Unique characteristic of strong electrolyte is that they yield only ions
Ex Ionic compound may be highly soluble in water and dissociate into ions (NaCl)
Others may not dissolve much but amount that does dissolve exists only as ions
145
Weak ElectrolytesHF dissolves in water to give acid solution called hydrofluoric acid HF bond stronger than bonds between hydrogen and other halogens When HF dissolves some molecule ionize Reverse reaction also happens HF(aq) + H2O(l) H3O+(aq) + F-(aq)
146
HF(aq) + H2O(l) H3O+
(aq) + F-(aq)Concentration of dissolved unionized HF stays high and concentration of ions stays low
Weak electrolyte - any compound whose aqueous solutions conduct electricity poorly this is due to the presence of small amount of the dissolved compound in the form of ions
147
Different from nonelectrolyte ndash NO ions at all Description of electrolyte as strong or weak not related to concentration of solution Electrolytes differ in degree of ionization not amount of solute dissolved
148
149
Colligative properties of solutions
Presence of solute affects properties of solutions Some properties not dependent on nature of dissolved substance but on how may dissolve particles are present
Colligative properties - properties that depend on the concentration of solute particles but not on their identity
150
Vapor-Pressure Lowering
Boiling and freezing point of solution different from pure solvent Nonvolatile solute raises boiling point and lowers freezing point Nonvolatile substance - one that has little tendency to become gas under existing conditions
151
152
To understand why nonvolatile solute changes boiling and freezing point consider equilibrium vapor pressure
Vapor pressure pressure caused by molecules that have escape liquid phase to gas phase
Can be thought of a measure of tendency of molecules to escape from a liquid
153
Addition of sucrose (nonvolatile solute) lowers concentration of water molecules at surface of liquid This lowers tendency of water molecules to leave solution and enter gas phase Vapor pressure of solution is lower than vapor pressure of pure water
154
Aqueous13 solution13 of13 nonvolatile13 solutePure13 water
Nonelectrolyte solutions of same molality have same concentration of particles
Dilute solutions of same solvent and equal molality of any nonelectrolyte solute lower vapor pressure equally
155
Example
1 m aqueous solution of nonelectrolyte glucose lowers vapor pressure of water 55 x 10-4 atm at 25 1 m aqueous solution of sucrose also lowers water vapor pressure to 55 x 10-4 atm at 25
156
157
bc vapor-pressure lowering depends on concentration of nonelectrolyte solute and doesnrsquot depend on the type of solute it is a colligative proprerty
bc vapor pressure has been lowered solution remains liquid over larger temperature range
Lowers freezing point and raises boiling point
158
Can assume changes in boiling and freezing point also depend on concentration of solute
They are colligative properties
159
Freezing-Point Depression
Freezing point of 1-molal solution of any nonelectrolyte solute in water is found (by experiment) to be 186 lower than freezing point of water When 1 mol of nonelectrolyte solute dissolved in 1 kg water freezing point is -186 not 000 160
When 2 mol nonelectrolyte solute dissolved in 1 kg water freezing point is -372
For any concentration of nonelectrolyte solute in water decrease in freezing point can be determined using value of -186m
161
This value called the molal freezing-point contstant (Kf) is the freezing-point depression of the solvent in a 1-molal solution of a nonvolatile nonelectrolyte solute
162
Each solvent has own characteristic molal freezing-point constant Freezing-point depression (Δtf) the difference between the freezing points of the pure solvent and a solute of nonelectrolyte in that solvent and it is directly proportional to the molal concentration of the solution Δtf = Kfm
163
Δtf = Kfm
Kf expressed as m
m expressed in mol solutekg solvent (molality)
164
Sample Problem
What is the freezing point depression of water in a solution of 171 g of sucrose C12H22O11 and 200 g of water What is the actual freezing point of the solution
165
1 AnalyzeGiven Solute mass and chemical formula 171 g C12H22O11 Solvent mass and identity 2000 g water Unknown a freezing-point depression b freezing point of the solution 166
2 Plan
167
168
Calculate
Unknown a freezing-point depression b freezing point of the solution
169
Practice Problem
A water solution containing an unknown quantity of a nonelectrolyte solute is found to have a freezing point of ndash 023degCWhat is the molal concentration of the solution
012 m170
A solution consists of 103 g of the nonelectrolyte glucose C6H12O6 dissolved in 250 g of water What is the freezing-point depression of the solution
minus0426degC171
In a laboratory experiment the freezing point of an aqueous solution of glucose is found to be minus0325degCWhat is the molal concentration of this solution
0175 m172
If 0500 mol of a nonelectrolyte solute are dissolved in 5000 g of ether what is the freezing point of the solution
minus1181degC173
The freezing point of an aqueous solution that contains a nonelectrolyte is minus90degC aWhat is the freezing-point depression of the solution bWhat is the molal concentration of the solution a minus90degC b 48 m
174
Boiling-Point Elevation
Boiling point is temp at which vapor pressure is equal to atmospheric pressure Change in vapor pressure will cause corresponding change in boiling point
175
176
Vapor pressure of nonvolatile solution is lower than vapor pressure of pure solvent More heat will be required to raise vapor pressure of solution Boiling point of solution is higher than boiling point of pure solvent
Molal boiling-point constant (Kb) - boiling-point elevation of the solvent in a 1-molal solution of a nonvolatile nonelectrolyte solute b-p elevation of any nonelectrolyte in water found by experiment to be 051 So molal b-p constant for water is 051m
177
For different solvents b-p elevations of 1-molal solutions have different values (Table 14-2)
Like freezing-point constants values most accurate for dilute solutions
178
Boiling-point elevation Δtb difference between the boiling points of the pure solvent and a nonelectrolyte solution of that solvent and it is directly proportional to the molal concentration of the solution B-p elevation can be calculated using Δtb = Kbm Δtb (boiling point elevation) expressed in m and m expressed in mol solutekg solvent
Sample Problem
What is the boiling-point elevation of a solution made from 200 g of a nonelectrolyte solute and 4000 g of water The molar mass of the solute is 620 gmol
180
1 AnalyzeGiven solute mass = 200 g solute molar mass = 620 gmol solvent mass and identity = 4000 g of water
Unknown boiling-point elevation
181
2 Plan
182
183
3 Compute
Practice Problem
A solution contains 500 g of sucrose C12H22O11 a nonelectrolyte dissolved in 5000 g of water What is the boiling-point elevation
015degC184
A solution contains 4500 g of sucrose C12H22O11 a nonelectrolyte dissolved in 250 g of waterWhat is the boiling point of the solution
1027degC185
If the boiling point elevation of an aqueous solution containing a nonvolatile electrolyte is 102degC what is the molality of the solution
20 m186
The boiling point of an aqueous solution containing a nonvolatile electrolyte is 10075degC aWhat is the boiling-point elevation bWhat is the molality of the solution a 075degC b 15 m
187
Electrolytes and Colligative PropertiesEarly investigators confused by experiments where certain substances depressed freezing point or elevated boiling point more than expected
188
189
Ex 01 m solution of NaCl lowers freezing point of solvent almost twice as much as 01 m solution of sucrose 01 m CaCl2 lowers fp almost 3 times as much as 01 m solution of sucrose
To understand why this happens contrast behavior or sucrose with NaCl in aqueous solution Each sucrose dissolves to make only 1 particle in solution So 1 mol sucrose dissolves to 1 mol particles 1 mol NaCl dissolves to make 2 moles of particles (1 mol Na+ and 1 mol Cl-)
190
191
Calculated Values for Electrolyte SolutionsColligative properties depend on total concentration of solute particles regardless of their identity Electrolytes cause changes in colligative properties proportional to total molality in terms of all dissolves particles instead of formula units
192
What about barium nitrate Ba(NO3)2 Each mole of barium nitrate yields 3 mol of ions in solution Ba(NO3)2(s) rarr Ba2+(aq) + 2NO3-(aq) You would expect solution of given molality to lower fp of solvent 3 times as much as nonelectrolytic solution of same molality
193
Sample Problem
What is the expected change in the freezing point of water in a solution of 625 g of barium nitrate Ba(NO3)2 in 100 kg of water
194
1 AnalyzeGiven solute mass and formula = 625 g Ba(NO3)2 solvent mass and identity = 100 kg water Δtf = Kfm
Unknown expected freezing-point depression
195
2 PlanThe molality can be calculated by converting the solute mass to moles and then dividing by the number of kilograms of solvent That molality is in terms of formula units of Ba(NO3)2 and must be converted to molality in terms of dissociated ions in solution It must be multiplied by the number of moles of ions produced per mole of formula unit This adjusted molality can then be used to calculate the freezing-point depression
196
197
198
3 Compute
Practice Problem
What is the expected freezing-point depression for a solution that contains 20 mol of magnesium sulfate dissolved in 10 kg of water
minus74degC199
What is the expected boiling-point elevation of water for a solution that contains 150 g of sodium chloride dissolved in 10 kg of water 27degC The freezing point of an aqueous sodium chloride solution is minus200degC What is the molality of the solution 54 m NaCl
200
Actual Values for Electrolyte SolutionsRemember values calculated are only expected values Actual values of colligative properties for all strong electrolytes are almost what would be expected Difference between expected and actual values caused by attractive forces that exist between dissociated ions in aqueous solution
201
Attraction between hydrated ions in solution is small compared with those in crystalline solid However forces of attraction donrsquot interfere with movements of aqueous ions More concentrated solution means ions closer together and attraction is greater Also depends on charge of ions ndash higher charge greater attraction
202
Net Ionic Equations
Reactions of ions in aqueous solution usually represented by net ionic equations instead of formula equations Net ionic equation - includes only those compounds and ions that undergo a chemical change in a reaction in an aqueous solution
120
121
To write NIE (net ionic equation) first convert chemical equation into overall ionic equation All soluble ionic compounds shown as dissociated ions in solution Precipitates shown as solids
122
Notice ammonium and nitrate ions appear on both sides They havenrsquot gone through any chemical change Spectator ions - ions that do not take part in a chemical reaction and are found in solution both before and after the reaction
123
To change ionic equation to NIE eliminate spectator ions Cd+2(aq) + S-2(aq) rarr CdS(s)
This NIE applies to any reaction in which precipitate of CdS forms
124
Practice Problem
Identify the precipitate that forms when aqueous solutions of zinc nitrate and ammonium sulfide are combined Write the equation for the possible double-replacement reaction Then write the formula equation overall ionic equation and the net ionic equation
125
1 AnalyzeGiven Identity of reactants zinc nitrate and ammonium sulfide Reaction medium aqueous solution Unknown a equation for the possible double-replacement reaction b identity of the precipitate c formula equation d overall ionic equation e net ionic equation
126
2 PlanWrite the possible double-replacement reaction between Zn(NO3)2 and (NH4)2S Use the rules to determine if any of the products will precipitate Write a formula equation and overall net ionic equation Then cancel spectator ions to make net ionic equation
127
3 Computea possible double-replacement reaction
b rules show that zinc sulfide is not soluble so will precipitate (ammonium nitrate is soluble)
c formula equation128
d overall ionic equation
e net ionic equation129
Will a precipitate form if solutions of potassium sulfate and barium nitrate are combined If so write the net ionic equation for the reaction
Yes Ba+2(aq) + SO4-2(aq) rarr BaSO4(s)
130
Will a precipitate form if solutions of potassium nitrate and magnesium sulfate are combined If so write the net ionic equation for the reaction
No 131
Will a precipitate form if solutions of barium chloride and sodium sulfate are combined If so identify the spectator ions and write the net ionic equation for the reaction
Yes Na+ and Cl- Ba+2(aq) + SO4-2(aq) rarr BaSO4(s)
132
Write the net ionic equation for the precipitation of nickel(II) sulfide
Ni+2(aq) + S-2(aq) rarr NiS(s)
133
IonizationSome molecular compounds can also form ions in solution Usually polar Ionization - ions formed from solute molecules by action of the solvent (creation of ions where there were none) Ionization different from dissociation
134
Dissociation ionic compounds dissolve and ions already present separate Ionization ions formed where non existed before
Ions formed are hydrated Heat released during hydration of ions gives enough energy to break covalent bonds
135
Extent to which solute ionizes depends on strength of bonds within molecules of solute and strength of attraction between solute and solvent molecules
136
HCl ndash molecular compound that ionizes in aqueous solution Contains highly polar bond Attraction between polar HCl molecule and polar water strong enough to break HCl bond
137
The Hydronium Ion
H+ ions from HCl attracts other molecules or ions so strongly that it doesnrsquot normally exist alone Ionization of HCl better described as direct transfer of proton from HCl to H2O forming H3O+
138
139
Hydration of H+ to form hydronium is highly exothermic
Energy released gives a lot of energy needed to ionize a molecular solute
Many molecular compounds that ionize in aqueous solution contain hydrogen and form hydronium ion
140
Strong and Weak Electrolytes
Substances that make ions and conduct current in solution are electrolytes HCl is one of series of compounds that has hydrogen and halogen Hydrogen halides are all molecular compounds with single polar-covalent bonds
141
All are gases Very soluble in water All are electrolytes HCl HBr HI strongly conduct current in solution HF only weakly conducts current at same concentration Strength of conduction related to ability to ionize
142
143
Strong Electrolytes
HCl HBr HI are 100 ionized in solution
Strong electrolyte - any compound whose dilute aqueous solutions conduct electricity well this is due to the presence of all or almost all of the dissolve compound in the form of ions
144
Unique characteristic of strong electrolyte is that they yield only ions
Ex Ionic compound may be highly soluble in water and dissociate into ions (NaCl)
Others may not dissolve much but amount that does dissolve exists only as ions
145
Weak ElectrolytesHF dissolves in water to give acid solution called hydrofluoric acid HF bond stronger than bonds between hydrogen and other halogens When HF dissolves some molecule ionize Reverse reaction also happens HF(aq) + H2O(l) H3O+(aq) + F-(aq)
146
HF(aq) + H2O(l) H3O+
(aq) + F-(aq)Concentration of dissolved unionized HF stays high and concentration of ions stays low
Weak electrolyte - any compound whose aqueous solutions conduct electricity poorly this is due to the presence of small amount of the dissolved compound in the form of ions
147
Different from nonelectrolyte ndash NO ions at all Description of electrolyte as strong or weak not related to concentration of solution Electrolytes differ in degree of ionization not amount of solute dissolved
148
149
Colligative properties of solutions
Presence of solute affects properties of solutions Some properties not dependent on nature of dissolved substance but on how may dissolve particles are present
Colligative properties - properties that depend on the concentration of solute particles but not on their identity
150
Vapor-Pressure Lowering
Boiling and freezing point of solution different from pure solvent Nonvolatile solute raises boiling point and lowers freezing point Nonvolatile substance - one that has little tendency to become gas under existing conditions
151
152
To understand why nonvolatile solute changes boiling and freezing point consider equilibrium vapor pressure
Vapor pressure pressure caused by molecules that have escape liquid phase to gas phase
Can be thought of a measure of tendency of molecules to escape from a liquid
153
Addition of sucrose (nonvolatile solute) lowers concentration of water molecules at surface of liquid This lowers tendency of water molecules to leave solution and enter gas phase Vapor pressure of solution is lower than vapor pressure of pure water
154
Aqueous13 solution13 of13 nonvolatile13 solutePure13 water
Nonelectrolyte solutions of same molality have same concentration of particles
Dilute solutions of same solvent and equal molality of any nonelectrolyte solute lower vapor pressure equally
155
Example
1 m aqueous solution of nonelectrolyte glucose lowers vapor pressure of water 55 x 10-4 atm at 25 1 m aqueous solution of sucrose also lowers water vapor pressure to 55 x 10-4 atm at 25
156
157
bc vapor-pressure lowering depends on concentration of nonelectrolyte solute and doesnrsquot depend on the type of solute it is a colligative proprerty
bc vapor pressure has been lowered solution remains liquid over larger temperature range
Lowers freezing point and raises boiling point
158
Can assume changes in boiling and freezing point also depend on concentration of solute
They are colligative properties
159
Freezing-Point Depression
Freezing point of 1-molal solution of any nonelectrolyte solute in water is found (by experiment) to be 186 lower than freezing point of water When 1 mol of nonelectrolyte solute dissolved in 1 kg water freezing point is -186 not 000 160
When 2 mol nonelectrolyte solute dissolved in 1 kg water freezing point is -372
For any concentration of nonelectrolyte solute in water decrease in freezing point can be determined using value of -186m
161
This value called the molal freezing-point contstant (Kf) is the freezing-point depression of the solvent in a 1-molal solution of a nonvolatile nonelectrolyte solute
162
Each solvent has own characteristic molal freezing-point constant Freezing-point depression (Δtf) the difference between the freezing points of the pure solvent and a solute of nonelectrolyte in that solvent and it is directly proportional to the molal concentration of the solution Δtf = Kfm
163
Δtf = Kfm
Kf expressed as m
m expressed in mol solutekg solvent (molality)
164
Sample Problem
What is the freezing point depression of water in a solution of 171 g of sucrose C12H22O11 and 200 g of water What is the actual freezing point of the solution
165
1 AnalyzeGiven Solute mass and chemical formula 171 g C12H22O11 Solvent mass and identity 2000 g water Unknown a freezing-point depression b freezing point of the solution 166
2 Plan
167
168
Calculate
Unknown a freezing-point depression b freezing point of the solution
169
Practice Problem
A water solution containing an unknown quantity of a nonelectrolyte solute is found to have a freezing point of ndash 023degCWhat is the molal concentration of the solution
012 m170
A solution consists of 103 g of the nonelectrolyte glucose C6H12O6 dissolved in 250 g of water What is the freezing-point depression of the solution
minus0426degC171
In a laboratory experiment the freezing point of an aqueous solution of glucose is found to be minus0325degCWhat is the molal concentration of this solution
0175 m172
If 0500 mol of a nonelectrolyte solute are dissolved in 5000 g of ether what is the freezing point of the solution
minus1181degC173
The freezing point of an aqueous solution that contains a nonelectrolyte is minus90degC aWhat is the freezing-point depression of the solution bWhat is the molal concentration of the solution a minus90degC b 48 m
174
Boiling-Point Elevation
Boiling point is temp at which vapor pressure is equal to atmospheric pressure Change in vapor pressure will cause corresponding change in boiling point
175
176
Vapor pressure of nonvolatile solution is lower than vapor pressure of pure solvent More heat will be required to raise vapor pressure of solution Boiling point of solution is higher than boiling point of pure solvent
Molal boiling-point constant (Kb) - boiling-point elevation of the solvent in a 1-molal solution of a nonvolatile nonelectrolyte solute b-p elevation of any nonelectrolyte in water found by experiment to be 051 So molal b-p constant for water is 051m
177
For different solvents b-p elevations of 1-molal solutions have different values (Table 14-2)
Like freezing-point constants values most accurate for dilute solutions
178
Boiling-point elevation Δtb difference between the boiling points of the pure solvent and a nonelectrolyte solution of that solvent and it is directly proportional to the molal concentration of the solution B-p elevation can be calculated using Δtb = Kbm Δtb (boiling point elevation) expressed in m and m expressed in mol solutekg solvent
Sample Problem
What is the boiling-point elevation of a solution made from 200 g of a nonelectrolyte solute and 4000 g of water The molar mass of the solute is 620 gmol
180
1 AnalyzeGiven solute mass = 200 g solute molar mass = 620 gmol solvent mass and identity = 4000 g of water
Unknown boiling-point elevation
181
2 Plan
182
183
3 Compute
Practice Problem
A solution contains 500 g of sucrose C12H22O11 a nonelectrolyte dissolved in 5000 g of water What is the boiling-point elevation
015degC184
A solution contains 4500 g of sucrose C12H22O11 a nonelectrolyte dissolved in 250 g of waterWhat is the boiling point of the solution
1027degC185
If the boiling point elevation of an aqueous solution containing a nonvolatile electrolyte is 102degC what is the molality of the solution
20 m186
The boiling point of an aqueous solution containing a nonvolatile electrolyte is 10075degC aWhat is the boiling-point elevation bWhat is the molality of the solution a 075degC b 15 m
187
Electrolytes and Colligative PropertiesEarly investigators confused by experiments where certain substances depressed freezing point or elevated boiling point more than expected
188
189
Ex 01 m solution of NaCl lowers freezing point of solvent almost twice as much as 01 m solution of sucrose 01 m CaCl2 lowers fp almost 3 times as much as 01 m solution of sucrose
To understand why this happens contrast behavior or sucrose with NaCl in aqueous solution Each sucrose dissolves to make only 1 particle in solution So 1 mol sucrose dissolves to 1 mol particles 1 mol NaCl dissolves to make 2 moles of particles (1 mol Na+ and 1 mol Cl-)
190
191
Calculated Values for Electrolyte SolutionsColligative properties depend on total concentration of solute particles regardless of their identity Electrolytes cause changes in colligative properties proportional to total molality in terms of all dissolves particles instead of formula units
192
What about barium nitrate Ba(NO3)2 Each mole of barium nitrate yields 3 mol of ions in solution Ba(NO3)2(s) rarr Ba2+(aq) + 2NO3-(aq) You would expect solution of given molality to lower fp of solvent 3 times as much as nonelectrolytic solution of same molality
193
Sample Problem
What is the expected change in the freezing point of water in a solution of 625 g of barium nitrate Ba(NO3)2 in 100 kg of water
194
1 AnalyzeGiven solute mass and formula = 625 g Ba(NO3)2 solvent mass and identity = 100 kg water Δtf = Kfm
Unknown expected freezing-point depression
195
2 PlanThe molality can be calculated by converting the solute mass to moles and then dividing by the number of kilograms of solvent That molality is in terms of formula units of Ba(NO3)2 and must be converted to molality in terms of dissociated ions in solution It must be multiplied by the number of moles of ions produced per mole of formula unit This adjusted molality can then be used to calculate the freezing-point depression
196
197
198
3 Compute
Practice Problem
What is the expected freezing-point depression for a solution that contains 20 mol of magnesium sulfate dissolved in 10 kg of water
minus74degC199
What is the expected boiling-point elevation of water for a solution that contains 150 g of sodium chloride dissolved in 10 kg of water 27degC The freezing point of an aqueous sodium chloride solution is minus200degC What is the molality of the solution 54 m NaCl
200
Actual Values for Electrolyte SolutionsRemember values calculated are only expected values Actual values of colligative properties for all strong electrolytes are almost what would be expected Difference between expected and actual values caused by attractive forces that exist between dissociated ions in aqueous solution
201
Attraction between hydrated ions in solution is small compared with those in crystalline solid However forces of attraction donrsquot interfere with movements of aqueous ions More concentrated solution means ions closer together and attraction is greater Also depends on charge of ions ndash higher charge greater attraction
202
121
To write NIE (net ionic equation) first convert chemical equation into overall ionic equation All soluble ionic compounds shown as dissociated ions in solution Precipitates shown as solids
122
Notice ammonium and nitrate ions appear on both sides They havenrsquot gone through any chemical change Spectator ions - ions that do not take part in a chemical reaction and are found in solution both before and after the reaction
123
To change ionic equation to NIE eliminate spectator ions Cd+2(aq) + S-2(aq) rarr CdS(s)
This NIE applies to any reaction in which precipitate of CdS forms
124
Practice Problem
Identify the precipitate that forms when aqueous solutions of zinc nitrate and ammonium sulfide are combined Write the equation for the possible double-replacement reaction Then write the formula equation overall ionic equation and the net ionic equation
125
1 AnalyzeGiven Identity of reactants zinc nitrate and ammonium sulfide Reaction medium aqueous solution Unknown a equation for the possible double-replacement reaction b identity of the precipitate c formula equation d overall ionic equation e net ionic equation
126
2 PlanWrite the possible double-replacement reaction between Zn(NO3)2 and (NH4)2S Use the rules to determine if any of the products will precipitate Write a formula equation and overall net ionic equation Then cancel spectator ions to make net ionic equation
127
3 Computea possible double-replacement reaction
b rules show that zinc sulfide is not soluble so will precipitate (ammonium nitrate is soluble)
c formula equation128
d overall ionic equation
e net ionic equation129
Will a precipitate form if solutions of potassium sulfate and barium nitrate are combined If so write the net ionic equation for the reaction
Yes Ba+2(aq) + SO4-2(aq) rarr BaSO4(s)
130
Will a precipitate form if solutions of potassium nitrate and magnesium sulfate are combined If so write the net ionic equation for the reaction
No 131
Will a precipitate form if solutions of barium chloride and sodium sulfate are combined If so identify the spectator ions and write the net ionic equation for the reaction
Yes Na+ and Cl- Ba+2(aq) + SO4-2(aq) rarr BaSO4(s)
132
Write the net ionic equation for the precipitation of nickel(II) sulfide
Ni+2(aq) + S-2(aq) rarr NiS(s)
133
IonizationSome molecular compounds can also form ions in solution Usually polar Ionization - ions formed from solute molecules by action of the solvent (creation of ions where there were none) Ionization different from dissociation
134
Dissociation ionic compounds dissolve and ions already present separate Ionization ions formed where non existed before
Ions formed are hydrated Heat released during hydration of ions gives enough energy to break covalent bonds
135
Extent to which solute ionizes depends on strength of bonds within molecules of solute and strength of attraction between solute and solvent molecules
136
HCl ndash molecular compound that ionizes in aqueous solution Contains highly polar bond Attraction between polar HCl molecule and polar water strong enough to break HCl bond
137
The Hydronium Ion
H+ ions from HCl attracts other molecules or ions so strongly that it doesnrsquot normally exist alone Ionization of HCl better described as direct transfer of proton from HCl to H2O forming H3O+
138
139
Hydration of H+ to form hydronium is highly exothermic
Energy released gives a lot of energy needed to ionize a molecular solute
Many molecular compounds that ionize in aqueous solution contain hydrogen and form hydronium ion
140
Strong and Weak Electrolytes
Substances that make ions and conduct current in solution are electrolytes HCl is one of series of compounds that has hydrogen and halogen Hydrogen halides are all molecular compounds with single polar-covalent bonds
141
All are gases Very soluble in water All are electrolytes HCl HBr HI strongly conduct current in solution HF only weakly conducts current at same concentration Strength of conduction related to ability to ionize
142
143
Strong Electrolytes
HCl HBr HI are 100 ionized in solution
Strong electrolyte - any compound whose dilute aqueous solutions conduct electricity well this is due to the presence of all or almost all of the dissolve compound in the form of ions
144
Unique characteristic of strong electrolyte is that they yield only ions
Ex Ionic compound may be highly soluble in water and dissociate into ions (NaCl)
Others may not dissolve much but amount that does dissolve exists only as ions
145
Weak ElectrolytesHF dissolves in water to give acid solution called hydrofluoric acid HF bond stronger than bonds between hydrogen and other halogens When HF dissolves some molecule ionize Reverse reaction also happens HF(aq) + H2O(l) H3O+(aq) + F-(aq)
146
HF(aq) + H2O(l) H3O+
(aq) + F-(aq)Concentration of dissolved unionized HF stays high and concentration of ions stays low
Weak electrolyte - any compound whose aqueous solutions conduct electricity poorly this is due to the presence of small amount of the dissolved compound in the form of ions
147
Different from nonelectrolyte ndash NO ions at all Description of electrolyte as strong or weak not related to concentration of solution Electrolytes differ in degree of ionization not amount of solute dissolved
148
149
Colligative properties of solutions
Presence of solute affects properties of solutions Some properties not dependent on nature of dissolved substance but on how may dissolve particles are present
Colligative properties - properties that depend on the concentration of solute particles but not on their identity
150
Vapor-Pressure Lowering
Boiling and freezing point of solution different from pure solvent Nonvolatile solute raises boiling point and lowers freezing point Nonvolatile substance - one that has little tendency to become gas under existing conditions
151
152
To understand why nonvolatile solute changes boiling and freezing point consider equilibrium vapor pressure
Vapor pressure pressure caused by molecules that have escape liquid phase to gas phase
Can be thought of a measure of tendency of molecules to escape from a liquid
153
Addition of sucrose (nonvolatile solute) lowers concentration of water molecules at surface of liquid This lowers tendency of water molecules to leave solution and enter gas phase Vapor pressure of solution is lower than vapor pressure of pure water
154
Aqueous13 solution13 of13 nonvolatile13 solutePure13 water
Nonelectrolyte solutions of same molality have same concentration of particles
Dilute solutions of same solvent and equal molality of any nonelectrolyte solute lower vapor pressure equally
155
Example
1 m aqueous solution of nonelectrolyte glucose lowers vapor pressure of water 55 x 10-4 atm at 25 1 m aqueous solution of sucrose also lowers water vapor pressure to 55 x 10-4 atm at 25
156
157
bc vapor-pressure lowering depends on concentration of nonelectrolyte solute and doesnrsquot depend on the type of solute it is a colligative proprerty
bc vapor pressure has been lowered solution remains liquid over larger temperature range
Lowers freezing point and raises boiling point
158
Can assume changes in boiling and freezing point also depend on concentration of solute
They are colligative properties
159
Freezing-Point Depression
Freezing point of 1-molal solution of any nonelectrolyte solute in water is found (by experiment) to be 186 lower than freezing point of water When 1 mol of nonelectrolyte solute dissolved in 1 kg water freezing point is -186 not 000 160
When 2 mol nonelectrolyte solute dissolved in 1 kg water freezing point is -372
For any concentration of nonelectrolyte solute in water decrease in freezing point can be determined using value of -186m
161
This value called the molal freezing-point contstant (Kf) is the freezing-point depression of the solvent in a 1-molal solution of a nonvolatile nonelectrolyte solute
162
Each solvent has own characteristic molal freezing-point constant Freezing-point depression (Δtf) the difference between the freezing points of the pure solvent and a solute of nonelectrolyte in that solvent and it is directly proportional to the molal concentration of the solution Δtf = Kfm
163
Δtf = Kfm
Kf expressed as m
m expressed in mol solutekg solvent (molality)
164
Sample Problem
What is the freezing point depression of water in a solution of 171 g of sucrose C12H22O11 and 200 g of water What is the actual freezing point of the solution
165
1 AnalyzeGiven Solute mass and chemical formula 171 g C12H22O11 Solvent mass and identity 2000 g water Unknown a freezing-point depression b freezing point of the solution 166
2 Plan
167
168
Calculate
Unknown a freezing-point depression b freezing point of the solution
169
Practice Problem
A water solution containing an unknown quantity of a nonelectrolyte solute is found to have a freezing point of ndash 023degCWhat is the molal concentration of the solution
012 m170
A solution consists of 103 g of the nonelectrolyte glucose C6H12O6 dissolved in 250 g of water What is the freezing-point depression of the solution
minus0426degC171
In a laboratory experiment the freezing point of an aqueous solution of glucose is found to be minus0325degCWhat is the molal concentration of this solution
0175 m172
If 0500 mol of a nonelectrolyte solute are dissolved in 5000 g of ether what is the freezing point of the solution
minus1181degC173
The freezing point of an aqueous solution that contains a nonelectrolyte is minus90degC aWhat is the freezing-point depression of the solution bWhat is the molal concentration of the solution a minus90degC b 48 m
174
Boiling-Point Elevation
Boiling point is temp at which vapor pressure is equal to atmospheric pressure Change in vapor pressure will cause corresponding change in boiling point
175
176
Vapor pressure of nonvolatile solution is lower than vapor pressure of pure solvent More heat will be required to raise vapor pressure of solution Boiling point of solution is higher than boiling point of pure solvent
Molal boiling-point constant (Kb) - boiling-point elevation of the solvent in a 1-molal solution of a nonvolatile nonelectrolyte solute b-p elevation of any nonelectrolyte in water found by experiment to be 051 So molal b-p constant for water is 051m
177
For different solvents b-p elevations of 1-molal solutions have different values (Table 14-2)
Like freezing-point constants values most accurate for dilute solutions
178
Boiling-point elevation Δtb difference between the boiling points of the pure solvent and a nonelectrolyte solution of that solvent and it is directly proportional to the molal concentration of the solution B-p elevation can be calculated using Δtb = Kbm Δtb (boiling point elevation) expressed in m and m expressed in mol solutekg solvent
Sample Problem
What is the boiling-point elevation of a solution made from 200 g of a nonelectrolyte solute and 4000 g of water The molar mass of the solute is 620 gmol
180
1 AnalyzeGiven solute mass = 200 g solute molar mass = 620 gmol solvent mass and identity = 4000 g of water
Unknown boiling-point elevation
181
2 Plan
182
183
3 Compute
Practice Problem
A solution contains 500 g of sucrose C12H22O11 a nonelectrolyte dissolved in 5000 g of water What is the boiling-point elevation
015degC184
A solution contains 4500 g of sucrose C12H22O11 a nonelectrolyte dissolved in 250 g of waterWhat is the boiling point of the solution
1027degC185
If the boiling point elevation of an aqueous solution containing a nonvolatile electrolyte is 102degC what is the molality of the solution
20 m186
The boiling point of an aqueous solution containing a nonvolatile electrolyte is 10075degC aWhat is the boiling-point elevation bWhat is the molality of the solution a 075degC b 15 m
187
Electrolytes and Colligative PropertiesEarly investigators confused by experiments where certain substances depressed freezing point or elevated boiling point more than expected
188
189
Ex 01 m solution of NaCl lowers freezing point of solvent almost twice as much as 01 m solution of sucrose 01 m CaCl2 lowers fp almost 3 times as much as 01 m solution of sucrose
To understand why this happens contrast behavior or sucrose with NaCl in aqueous solution Each sucrose dissolves to make only 1 particle in solution So 1 mol sucrose dissolves to 1 mol particles 1 mol NaCl dissolves to make 2 moles of particles (1 mol Na+ and 1 mol Cl-)
190
191
Calculated Values for Electrolyte SolutionsColligative properties depend on total concentration of solute particles regardless of their identity Electrolytes cause changes in colligative properties proportional to total molality in terms of all dissolves particles instead of formula units
192
What about barium nitrate Ba(NO3)2 Each mole of barium nitrate yields 3 mol of ions in solution Ba(NO3)2(s) rarr Ba2+(aq) + 2NO3-(aq) You would expect solution of given molality to lower fp of solvent 3 times as much as nonelectrolytic solution of same molality
193
Sample Problem
What is the expected change in the freezing point of water in a solution of 625 g of barium nitrate Ba(NO3)2 in 100 kg of water
194
1 AnalyzeGiven solute mass and formula = 625 g Ba(NO3)2 solvent mass and identity = 100 kg water Δtf = Kfm
Unknown expected freezing-point depression
195
2 PlanThe molality can be calculated by converting the solute mass to moles and then dividing by the number of kilograms of solvent That molality is in terms of formula units of Ba(NO3)2 and must be converted to molality in terms of dissociated ions in solution It must be multiplied by the number of moles of ions produced per mole of formula unit This adjusted molality can then be used to calculate the freezing-point depression
196
197
198
3 Compute
Practice Problem
What is the expected freezing-point depression for a solution that contains 20 mol of magnesium sulfate dissolved in 10 kg of water
minus74degC199
What is the expected boiling-point elevation of water for a solution that contains 150 g of sodium chloride dissolved in 10 kg of water 27degC The freezing point of an aqueous sodium chloride solution is minus200degC What is the molality of the solution 54 m NaCl
200
Actual Values for Electrolyte SolutionsRemember values calculated are only expected values Actual values of colligative properties for all strong electrolytes are almost what would be expected Difference between expected and actual values caused by attractive forces that exist between dissociated ions in aqueous solution
201
Attraction between hydrated ions in solution is small compared with those in crystalline solid However forces of attraction donrsquot interfere with movements of aqueous ions More concentrated solution means ions closer together and attraction is greater Also depends on charge of ions ndash higher charge greater attraction
202
To write NIE (net ionic equation) first convert chemical equation into overall ionic equation All soluble ionic compounds shown as dissociated ions in solution Precipitates shown as solids
122
Notice ammonium and nitrate ions appear on both sides They havenrsquot gone through any chemical change Spectator ions - ions that do not take part in a chemical reaction and are found in solution both before and after the reaction
123
To change ionic equation to NIE eliminate spectator ions Cd+2(aq) + S-2(aq) rarr CdS(s)
This NIE applies to any reaction in which precipitate of CdS forms
124
Practice Problem
Identify the precipitate that forms when aqueous solutions of zinc nitrate and ammonium sulfide are combined Write the equation for the possible double-replacement reaction Then write the formula equation overall ionic equation and the net ionic equation
125
1 AnalyzeGiven Identity of reactants zinc nitrate and ammonium sulfide Reaction medium aqueous solution Unknown a equation for the possible double-replacement reaction b identity of the precipitate c formula equation d overall ionic equation e net ionic equation
126
2 PlanWrite the possible double-replacement reaction between Zn(NO3)2 and (NH4)2S Use the rules to determine if any of the products will precipitate Write a formula equation and overall net ionic equation Then cancel spectator ions to make net ionic equation
127
3 Computea possible double-replacement reaction
b rules show that zinc sulfide is not soluble so will precipitate (ammonium nitrate is soluble)
c formula equation128
d overall ionic equation
e net ionic equation129
Will a precipitate form if solutions of potassium sulfate and barium nitrate are combined If so write the net ionic equation for the reaction
Yes Ba+2(aq) + SO4-2(aq) rarr BaSO4(s)
130
Will a precipitate form if solutions of potassium nitrate and magnesium sulfate are combined If so write the net ionic equation for the reaction
No 131
Will a precipitate form if solutions of barium chloride and sodium sulfate are combined If so identify the spectator ions and write the net ionic equation for the reaction
Yes Na+ and Cl- Ba+2(aq) + SO4-2(aq) rarr BaSO4(s)
132
Write the net ionic equation for the precipitation of nickel(II) sulfide
Ni+2(aq) + S-2(aq) rarr NiS(s)
133
IonizationSome molecular compounds can also form ions in solution Usually polar Ionization - ions formed from solute molecules by action of the solvent (creation of ions where there were none) Ionization different from dissociation
134
Dissociation ionic compounds dissolve and ions already present separate Ionization ions formed where non existed before
Ions formed are hydrated Heat released during hydration of ions gives enough energy to break covalent bonds
135
Extent to which solute ionizes depends on strength of bonds within molecules of solute and strength of attraction between solute and solvent molecules
136
HCl ndash molecular compound that ionizes in aqueous solution Contains highly polar bond Attraction between polar HCl molecule and polar water strong enough to break HCl bond
137
The Hydronium Ion
H+ ions from HCl attracts other molecules or ions so strongly that it doesnrsquot normally exist alone Ionization of HCl better described as direct transfer of proton from HCl to H2O forming H3O+
138
139
Hydration of H+ to form hydronium is highly exothermic
Energy released gives a lot of energy needed to ionize a molecular solute
Many molecular compounds that ionize in aqueous solution contain hydrogen and form hydronium ion
140
Strong and Weak Electrolytes
Substances that make ions and conduct current in solution are electrolytes HCl is one of series of compounds that has hydrogen and halogen Hydrogen halides are all molecular compounds with single polar-covalent bonds
141
All are gases Very soluble in water All are electrolytes HCl HBr HI strongly conduct current in solution HF only weakly conducts current at same concentration Strength of conduction related to ability to ionize
142
143
Strong Electrolytes
HCl HBr HI are 100 ionized in solution
Strong electrolyte - any compound whose dilute aqueous solutions conduct electricity well this is due to the presence of all or almost all of the dissolve compound in the form of ions
144
Unique characteristic of strong electrolyte is that they yield only ions
Ex Ionic compound may be highly soluble in water and dissociate into ions (NaCl)
Others may not dissolve much but amount that does dissolve exists only as ions
145
Weak ElectrolytesHF dissolves in water to give acid solution called hydrofluoric acid HF bond stronger than bonds between hydrogen and other halogens When HF dissolves some molecule ionize Reverse reaction also happens HF(aq) + H2O(l) H3O+(aq) + F-(aq)
146
HF(aq) + H2O(l) H3O+
(aq) + F-(aq)Concentration of dissolved unionized HF stays high and concentration of ions stays low
Weak electrolyte - any compound whose aqueous solutions conduct electricity poorly this is due to the presence of small amount of the dissolved compound in the form of ions
147
Different from nonelectrolyte ndash NO ions at all Description of electrolyte as strong or weak not related to concentration of solution Electrolytes differ in degree of ionization not amount of solute dissolved
148
149
Colligative properties of solutions
Presence of solute affects properties of solutions Some properties not dependent on nature of dissolved substance but on how may dissolve particles are present
Colligative properties - properties that depend on the concentration of solute particles but not on their identity
150
Vapor-Pressure Lowering
Boiling and freezing point of solution different from pure solvent Nonvolatile solute raises boiling point and lowers freezing point Nonvolatile substance - one that has little tendency to become gas under existing conditions
151
152
To understand why nonvolatile solute changes boiling and freezing point consider equilibrium vapor pressure
Vapor pressure pressure caused by molecules that have escape liquid phase to gas phase
Can be thought of a measure of tendency of molecules to escape from a liquid
153
Addition of sucrose (nonvolatile solute) lowers concentration of water molecules at surface of liquid This lowers tendency of water molecules to leave solution and enter gas phase Vapor pressure of solution is lower than vapor pressure of pure water
154
Aqueous13 solution13 of13 nonvolatile13 solutePure13 water
Nonelectrolyte solutions of same molality have same concentration of particles
Dilute solutions of same solvent and equal molality of any nonelectrolyte solute lower vapor pressure equally
155
Example
1 m aqueous solution of nonelectrolyte glucose lowers vapor pressure of water 55 x 10-4 atm at 25 1 m aqueous solution of sucrose also lowers water vapor pressure to 55 x 10-4 atm at 25
156
157
bc vapor-pressure lowering depends on concentration of nonelectrolyte solute and doesnrsquot depend on the type of solute it is a colligative proprerty
bc vapor pressure has been lowered solution remains liquid over larger temperature range
Lowers freezing point and raises boiling point
158
Can assume changes in boiling and freezing point also depend on concentration of solute
They are colligative properties
159
Freezing-Point Depression
Freezing point of 1-molal solution of any nonelectrolyte solute in water is found (by experiment) to be 186 lower than freezing point of water When 1 mol of nonelectrolyte solute dissolved in 1 kg water freezing point is -186 not 000 160
When 2 mol nonelectrolyte solute dissolved in 1 kg water freezing point is -372
For any concentration of nonelectrolyte solute in water decrease in freezing point can be determined using value of -186m
161
This value called the molal freezing-point contstant (Kf) is the freezing-point depression of the solvent in a 1-molal solution of a nonvolatile nonelectrolyte solute
162
Each solvent has own characteristic molal freezing-point constant Freezing-point depression (Δtf) the difference between the freezing points of the pure solvent and a solute of nonelectrolyte in that solvent and it is directly proportional to the molal concentration of the solution Δtf = Kfm
163
Δtf = Kfm
Kf expressed as m
m expressed in mol solutekg solvent (molality)
164
Sample Problem
What is the freezing point depression of water in a solution of 171 g of sucrose C12H22O11 and 200 g of water What is the actual freezing point of the solution
165
1 AnalyzeGiven Solute mass and chemical formula 171 g C12H22O11 Solvent mass and identity 2000 g water Unknown a freezing-point depression b freezing point of the solution 166
2 Plan
167
168
Calculate
Unknown a freezing-point depression b freezing point of the solution
169
Practice Problem
A water solution containing an unknown quantity of a nonelectrolyte solute is found to have a freezing point of ndash 023degCWhat is the molal concentration of the solution
012 m170
A solution consists of 103 g of the nonelectrolyte glucose C6H12O6 dissolved in 250 g of water What is the freezing-point depression of the solution
minus0426degC171
In a laboratory experiment the freezing point of an aqueous solution of glucose is found to be minus0325degCWhat is the molal concentration of this solution
0175 m172
If 0500 mol of a nonelectrolyte solute are dissolved in 5000 g of ether what is the freezing point of the solution
minus1181degC173
The freezing point of an aqueous solution that contains a nonelectrolyte is minus90degC aWhat is the freezing-point depression of the solution bWhat is the molal concentration of the solution a minus90degC b 48 m
174
Boiling-Point Elevation
Boiling point is temp at which vapor pressure is equal to atmospheric pressure Change in vapor pressure will cause corresponding change in boiling point
175
176
Vapor pressure of nonvolatile solution is lower than vapor pressure of pure solvent More heat will be required to raise vapor pressure of solution Boiling point of solution is higher than boiling point of pure solvent
Molal boiling-point constant (Kb) - boiling-point elevation of the solvent in a 1-molal solution of a nonvolatile nonelectrolyte solute b-p elevation of any nonelectrolyte in water found by experiment to be 051 So molal b-p constant for water is 051m
177
For different solvents b-p elevations of 1-molal solutions have different values (Table 14-2)
Like freezing-point constants values most accurate for dilute solutions
178
Boiling-point elevation Δtb difference between the boiling points of the pure solvent and a nonelectrolyte solution of that solvent and it is directly proportional to the molal concentration of the solution B-p elevation can be calculated using Δtb = Kbm Δtb (boiling point elevation) expressed in m and m expressed in mol solutekg solvent
Sample Problem
What is the boiling-point elevation of a solution made from 200 g of a nonelectrolyte solute and 4000 g of water The molar mass of the solute is 620 gmol
180
1 AnalyzeGiven solute mass = 200 g solute molar mass = 620 gmol solvent mass and identity = 4000 g of water
Unknown boiling-point elevation
181
2 Plan
182
183
3 Compute
Practice Problem
A solution contains 500 g of sucrose C12H22O11 a nonelectrolyte dissolved in 5000 g of water What is the boiling-point elevation
015degC184
A solution contains 4500 g of sucrose C12H22O11 a nonelectrolyte dissolved in 250 g of waterWhat is the boiling point of the solution
1027degC185
If the boiling point elevation of an aqueous solution containing a nonvolatile electrolyte is 102degC what is the molality of the solution
20 m186
The boiling point of an aqueous solution containing a nonvolatile electrolyte is 10075degC aWhat is the boiling-point elevation bWhat is the molality of the solution a 075degC b 15 m
187
Electrolytes and Colligative PropertiesEarly investigators confused by experiments where certain substances depressed freezing point or elevated boiling point more than expected
188
189
Ex 01 m solution of NaCl lowers freezing point of solvent almost twice as much as 01 m solution of sucrose 01 m CaCl2 lowers fp almost 3 times as much as 01 m solution of sucrose
To understand why this happens contrast behavior or sucrose with NaCl in aqueous solution Each sucrose dissolves to make only 1 particle in solution So 1 mol sucrose dissolves to 1 mol particles 1 mol NaCl dissolves to make 2 moles of particles (1 mol Na+ and 1 mol Cl-)
190
191
Calculated Values for Electrolyte SolutionsColligative properties depend on total concentration of solute particles regardless of their identity Electrolytes cause changes in colligative properties proportional to total molality in terms of all dissolves particles instead of formula units
192
What about barium nitrate Ba(NO3)2 Each mole of barium nitrate yields 3 mol of ions in solution Ba(NO3)2(s) rarr Ba2+(aq) + 2NO3-(aq) You would expect solution of given molality to lower fp of solvent 3 times as much as nonelectrolytic solution of same molality
193
Sample Problem
What is the expected change in the freezing point of water in a solution of 625 g of barium nitrate Ba(NO3)2 in 100 kg of water
194
1 AnalyzeGiven solute mass and formula = 625 g Ba(NO3)2 solvent mass and identity = 100 kg water Δtf = Kfm
Unknown expected freezing-point depression
195
2 PlanThe molality can be calculated by converting the solute mass to moles and then dividing by the number of kilograms of solvent That molality is in terms of formula units of Ba(NO3)2 and must be converted to molality in terms of dissociated ions in solution It must be multiplied by the number of moles of ions produced per mole of formula unit This adjusted molality can then be used to calculate the freezing-point depression
196
197
198
3 Compute
Practice Problem
What is the expected freezing-point depression for a solution that contains 20 mol of magnesium sulfate dissolved in 10 kg of water
minus74degC199
What is the expected boiling-point elevation of water for a solution that contains 150 g of sodium chloride dissolved in 10 kg of water 27degC The freezing point of an aqueous sodium chloride solution is minus200degC What is the molality of the solution 54 m NaCl
200
Actual Values for Electrolyte SolutionsRemember values calculated are only expected values Actual values of colligative properties for all strong electrolytes are almost what would be expected Difference between expected and actual values caused by attractive forces that exist between dissociated ions in aqueous solution
201
Attraction between hydrated ions in solution is small compared with those in crystalline solid However forces of attraction donrsquot interfere with movements of aqueous ions More concentrated solution means ions closer together and attraction is greater Also depends on charge of ions ndash higher charge greater attraction
202
Notice ammonium and nitrate ions appear on both sides They havenrsquot gone through any chemical change Spectator ions - ions that do not take part in a chemical reaction and are found in solution both before and after the reaction
123
To change ionic equation to NIE eliminate spectator ions Cd+2(aq) + S-2(aq) rarr CdS(s)
This NIE applies to any reaction in which precipitate of CdS forms
124
Practice Problem
Identify the precipitate that forms when aqueous solutions of zinc nitrate and ammonium sulfide are combined Write the equation for the possible double-replacement reaction Then write the formula equation overall ionic equation and the net ionic equation
125
1 AnalyzeGiven Identity of reactants zinc nitrate and ammonium sulfide Reaction medium aqueous solution Unknown a equation for the possible double-replacement reaction b identity of the precipitate c formula equation d overall ionic equation e net ionic equation
126
2 PlanWrite the possible double-replacement reaction between Zn(NO3)2 and (NH4)2S Use the rules to determine if any of the products will precipitate Write a formula equation and overall net ionic equation Then cancel spectator ions to make net ionic equation
127
3 Computea possible double-replacement reaction
b rules show that zinc sulfide is not soluble so will precipitate (ammonium nitrate is soluble)
c formula equation128
d overall ionic equation
e net ionic equation129
Will a precipitate form if solutions of potassium sulfate and barium nitrate are combined If so write the net ionic equation for the reaction
Yes Ba+2(aq) + SO4-2(aq) rarr BaSO4(s)
130
Will a precipitate form if solutions of potassium nitrate and magnesium sulfate are combined If so write the net ionic equation for the reaction
No 131
Will a precipitate form if solutions of barium chloride and sodium sulfate are combined If so identify the spectator ions and write the net ionic equation for the reaction
Yes Na+ and Cl- Ba+2(aq) + SO4-2(aq) rarr BaSO4(s)
132
Write the net ionic equation for the precipitation of nickel(II) sulfide
Ni+2(aq) + S-2(aq) rarr NiS(s)
133
IonizationSome molecular compounds can also form ions in solution Usually polar Ionization - ions formed from solute molecules by action of the solvent (creation of ions where there were none) Ionization different from dissociation
134
Dissociation ionic compounds dissolve and ions already present separate Ionization ions formed where non existed before
Ions formed are hydrated Heat released during hydration of ions gives enough energy to break covalent bonds
135
Extent to which solute ionizes depends on strength of bonds within molecules of solute and strength of attraction between solute and solvent molecules
136
HCl ndash molecular compound that ionizes in aqueous solution Contains highly polar bond Attraction between polar HCl molecule and polar water strong enough to break HCl bond
137
The Hydronium Ion
H+ ions from HCl attracts other molecules or ions so strongly that it doesnrsquot normally exist alone Ionization of HCl better described as direct transfer of proton from HCl to H2O forming H3O+
138
139
Hydration of H+ to form hydronium is highly exothermic
Energy released gives a lot of energy needed to ionize a molecular solute
Many molecular compounds that ionize in aqueous solution contain hydrogen and form hydronium ion
140
Strong and Weak Electrolytes
Substances that make ions and conduct current in solution are electrolytes HCl is one of series of compounds that has hydrogen and halogen Hydrogen halides are all molecular compounds with single polar-covalent bonds
141
All are gases Very soluble in water All are electrolytes HCl HBr HI strongly conduct current in solution HF only weakly conducts current at same concentration Strength of conduction related to ability to ionize
142
143
Strong Electrolytes
HCl HBr HI are 100 ionized in solution
Strong electrolyte - any compound whose dilute aqueous solutions conduct electricity well this is due to the presence of all or almost all of the dissolve compound in the form of ions
144
Unique characteristic of strong electrolyte is that they yield only ions
Ex Ionic compound may be highly soluble in water and dissociate into ions (NaCl)
Others may not dissolve much but amount that does dissolve exists only as ions
145
Weak ElectrolytesHF dissolves in water to give acid solution called hydrofluoric acid HF bond stronger than bonds between hydrogen and other halogens When HF dissolves some molecule ionize Reverse reaction also happens HF(aq) + H2O(l) H3O+(aq) + F-(aq)
146
HF(aq) + H2O(l) H3O+
(aq) + F-(aq)Concentration of dissolved unionized HF stays high and concentration of ions stays low
Weak electrolyte - any compound whose aqueous solutions conduct electricity poorly this is due to the presence of small amount of the dissolved compound in the form of ions
147
Different from nonelectrolyte ndash NO ions at all Description of electrolyte as strong or weak not related to concentration of solution Electrolytes differ in degree of ionization not amount of solute dissolved
148
149
Colligative properties of solutions
Presence of solute affects properties of solutions Some properties not dependent on nature of dissolved substance but on how may dissolve particles are present
Colligative properties - properties that depend on the concentration of solute particles but not on their identity
150
Vapor-Pressure Lowering
Boiling and freezing point of solution different from pure solvent Nonvolatile solute raises boiling point and lowers freezing point Nonvolatile substance - one that has little tendency to become gas under existing conditions
151
152
To understand why nonvolatile solute changes boiling and freezing point consider equilibrium vapor pressure
Vapor pressure pressure caused by molecules that have escape liquid phase to gas phase
Can be thought of a measure of tendency of molecules to escape from a liquid
153
Addition of sucrose (nonvolatile solute) lowers concentration of water molecules at surface of liquid This lowers tendency of water molecules to leave solution and enter gas phase Vapor pressure of solution is lower than vapor pressure of pure water
154
Aqueous13 solution13 of13 nonvolatile13 solutePure13 water
Nonelectrolyte solutions of same molality have same concentration of particles
Dilute solutions of same solvent and equal molality of any nonelectrolyte solute lower vapor pressure equally
155
Example
1 m aqueous solution of nonelectrolyte glucose lowers vapor pressure of water 55 x 10-4 atm at 25 1 m aqueous solution of sucrose also lowers water vapor pressure to 55 x 10-4 atm at 25
156
157
bc vapor-pressure lowering depends on concentration of nonelectrolyte solute and doesnrsquot depend on the type of solute it is a colligative proprerty
bc vapor pressure has been lowered solution remains liquid over larger temperature range
Lowers freezing point and raises boiling point
158
Can assume changes in boiling and freezing point also depend on concentration of solute
They are colligative properties
159
Freezing-Point Depression
Freezing point of 1-molal solution of any nonelectrolyte solute in water is found (by experiment) to be 186 lower than freezing point of water When 1 mol of nonelectrolyte solute dissolved in 1 kg water freezing point is -186 not 000 160
When 2 mol nonelectrolyte solute dissolved in 1 kg water freezing point is -372
For any concentration of nonelectrolyte solute in water decrease in freezing point can be determined using value of -186m
161
This value called the molal freezing-point contstant (Kf) is the freezing-point depression of the solvent in a 1-molal solution of a nonvolatile nonelectrolyte solute
162
Each solvent has own characteristic molal freezing-point constant Freezing-point depression (Δtf) the difference between the freezing points of the pure solvent and a solute of nonelectrolyte in that solvent and it is directly proportional to the molal concentration of the solution Δtf = Kfm
163
Δtf = Kfm
Kf expressed as m
m expressed in mol solutekg solvent (molality)
164
Sample Problem
What is the freezing point depression of water in a solution of 171 g of sucrose C12H22O11 and 200 g of water What is the actual freezing point of the solution
165
1 AnalyzeGiven Solute mass and chemical formula 171 g C12H22O11 Solvent mass and identity 2000 g water Unknown a freezing-point depression b freezing point of the solution 166
2 Plan
167
168
Calculate
Unknown a freezing-point depression b freezing point of the solution
169
Practice Problem
A water solution containing an unknown quantity of a nonelectrolyte solute is found to have a freezing point of ndash 023degCWhat is the molal concentration of the solution
012 m170
A solution consists of 103 g of the nonelectrolyte glucose C6H12O6 dissolved in 250 g of water What is the freezing-point depression of the solution
minus0426degC171
In a laboratory experiment the freezing point of an aqueous solution of glucose is found to be minus0325degCWhat is the molal concentration of this solution
0175 m172
If 0500 mol of a nonelectrolyte solute are dissolved in 5000 g of ether what is the freezing point of the solution
minus1181degC173
The freezing point of an aqueous solution that contains a nonelectrolyte is minus90degC aWhat is the freezing-point depression of the solution bWhat is the molal concentration of the solution a minus90degC b 48 m
174
Boiling-Point Elevation
Boiling point is temp at which vapor pressure is equal to atmospheric pressure Change in vapor pressure will cause corresponding change in boiling point
175
176
Vapor pressure of nonvolatile solution is lower than vapor pressure of pure solvent More heat will be required to raise vapor pressure of solution Boiling point of solution is higher than boiling point of pure solvent
Molal boiling-point constant (Kb) - boiling-point elevation of the solvent in a 1-molal solution of a nonvolatile nonelectrolyte solute b-p elevation of any nonelectrolyte in water found by experiment to be 051 So molal b-p constant for water is 051m
177
For different solvents b-p elevations of 1-molal solutions have different values (Table 14-2)
Like freezing-point constants values most accurate for dilute solutions
178
Boiling-point elevation Δtb difference between the boiling points of the pure solvent and a nonelectrolyte solution of that solvent and it is directly proportional to the molal concentration of the solution B-p elevation can be calculated using Δtb = Kbm Δtb (boiling point elevation) expressed in m and m expressed in mol solutekg solvent
Sample Problem
What is the boiling-point elevation of a solution made from 200 g of a nonelectrolyte solute and 4000 g of water The molar mass of the solute is 620 gmol
180
1 AnalyzeGiven solute mass = 200 g solute molar mass = 620 gmol solvent mass and identity = 4000 g of water
Unknown boiling-point elevation
181
2 Plan
182
183
3 Compute
Practice Problem
A solution contains 500 g of sucrose C12H22O11 a nonelectrolyte dissolved in 5000 g of water What is the boiling-point elevation
015degC184
A solution contains 4500 g of sucrose C12H22O11 a nonelectrolyte dissolved in 250 g of waterWhat is the boiling point of the solution
1027degC185
If the boiling point elevation of an aqueous solution containing a nonvolatile electrolyte is 102degC what is the molality of the solution
20 m186
The boiling point of an aqueous solution containing a nonvolatile electrolyte is 10075degC aWhat is the boiling-point elevation bWhat is the molality of the solution a 075degC b 15 m
187
Electrolytes and Colligative PropertiesEarly investigators confused by experiments where certain substances depressed freezing point or elevated boiling point more than expected
188
189
Ex 01 m solution of NaCl lowers freezing point of solvent almost twice as much as 01 m solution of sucrose 01 m CaCl2 lowers fp almost 3 times as much as 01 m solution of sucrose
To understand why this happens contrast behavior or sucrose with NaCl in aqueous solution Each sucrose dissolves to make only 1 particle in solution So 1 mol sucrose dissolves to 1 mol particles 1 mol NaCl dissolves to make 2 moles of particles (1 mol Na+ and 1 mol Cl-)
190
191
Calculated Values for Electrolyte SolutionsColligative properties depend on total concentration of solute particles regardless of their identity Electrolytes cause changes in colligative properties proportional to total molality in terms of all dissolves particles instead of formula units
192
What about barium nitrate Ba(NO3)2 Each mole of barium nitrate yields 3 mol of ions in solution Ba(NO3)2(s) rarr Ba2+(aq) + 2NO3-(aq) You would expect solution of given molality to lower fp of solvent 3 times as much as nonelectrolytic solution of same molality
193
Sample Problem
What is the expected change in the freezing point of water in a solution of 625 g of barium nitrate Ba(NO3)2 in 100 kg of water
194
1 AnalyzeGiven solute mass and formula = 625 g Ba(NO3)2 solvent mass and identity = 100 kg water Δtf = Kfm
Unknown expected freezing-point depression
195
2 PlanThe molality can be calculated by converting the solute mass to moles and then dividing by the number of kilograms of solvent That molality is in terms of formula units of Ba(NO3)2 and must be converted to molality in terms of dissociated ions in solution It must be multiplied by the number of moles of ions produced per mole of formula unit This adjusted molality can then be used to calculate the freezing-point depression
196
197
198
3 Compute
Practice Problem
What is the expected freezing-point depression for a solution that contains 20 mol of magnesium sulfate dissolved in 10 kg of water
minus74degC199
What is the expected boiling-point elevation of water for a solution that contains 150 g of sodium chloride dissolved in 10 kg of water 27degC The freezing point of an aqueous sodium chloride solution is minus200degC What is the molality of the solution 54 m NaCl
200
Actual Values for Electrolyte SolutionsRemember values calculated are only expected values Actual values of colligative properties for all strong electrolytes are almost what would be expected Difference between expected and actual values caused by attractive forces that exist between dissociated ions in aqueous solution
201
Attraction between hydrated ions in solution is small compared with those in crystalline solid However forces of attraction donrsquot interfere with movements of aqueous ions More concentrated solution means ions closer together and attraction is greater Also depends on charge of ions ndash higher charge greater attraction
202
To change ionic equation to NIE eliminate spectator ions Cd+2(aq) + S-2(aq) rarr CdS(s)
This NIE applies to any reaction in which precipitate of CdS forms
124
Practice Problem
Identify the precipitate that forms when aqueous solutions of zinc nitrate and ammonium sulfide are combined Write the equation for the possible double-replacement reaction Then write the formula equation overall ionic equation and the net ionic equation
125
1 AnalyzeGiven Identity of reactants zinc nitrate and ammonium sulfide Reaction medium aqueous solution Unknown a equation for the possible double-replacement reaction b identity of the precipitate c formula equation d overall ionic equation e net ionic equation
126
2 PlanWrite the possible double-replacement reaction between Zn(NO3)2 and (NH4)2S Use the rules to determine if any of the products will precipitate Write a formula equation and overall net ionic equation Then cancel spectator ions to make net ionic equation
127
3 Computea possible double-replacement reaction
b rules show that zinc sulfide is not soluble so will precipitate (ammonium nitrate is soluble)
c formula equation128
d overall ionic equation
e net ionic equation129
Will a precipitate form if solutions of potassium sulfate and barium nitrate are combined If so write the net ionic equation for the reaction
Yes Ba+2(aq) + SO4-2(aq) rarr BaSO4(s)
130
Will a precipitate form if solutions of potassium nitrate and magnesium sulfate are combined If so write the net ionic equation for the reaction
No 131
Will a precipitate form if solutions of barium chloride and sodium sulfate are combined If so identify the spectator ions and write the net ionic equation for the reaction
Yes Na+ and Cl- Ba+2(aq) + SO4-2(aq) rarr BaSO4(s)
132
Write the net ionic equation for the precipitation of nickel(II) sulfide
Ni+2(aq) + S-2(aq) rarr NiS(s)
133
IonizationSome molecular compounds can also form ions in solution Usually polar Ionization - ions formed from solute molecules by action of the solvent (creation of ions where there were none) Ionization different from dissociation
134
Dissociation ionic compounds dissolve and ions already present separate Ionization ions formed where non existed before
Ions formed are hydrated Heat released during hydration of ions gives enough energy to break covalent bonds
135
Extent to which solute ionizes depends on strength of bonds within molecules of solute and strength of attraction between solute and solvent molecules
136
HCl ndash molecular compound that ionizes in aqueous solution Contains highly polar bond Attraction between polar HCl molecule and polar water strong enough to break HCl bond
137
The Hydronium Ion
H+ ions from HCl attracts other molecules or ions so strongly that it doesnrsquot normally exist alone Ionization of HCl better described as direct transfer of proton from HCl to H2O forming H3O+
138
139
Hydration of H+ to form hydronium is highly exothermic
Energy released gives a lot of energy needed to ionize a molecular solute
Many molecular compounds that ionize in aqueous solution contain hydrogen and form hydronium ion
140
Strong and Weak Electrolytes
Substances that make ions and conduct current in solution are electrolytes HCl is one of series of compounds that has hydrogen and halogen Hydrogen halides are all molecular compounds with single polar-covalent bonds
141
All are gases Very soluble in water All are electrolytes HCl HBr HI strongly conduct current in solution HF only weakly conducts current at same concentration Strength of conduction related to ability to ionize
142
143
Strong Electrolytes
HCl HBr HI are 100 ionized in solution
Strong electrolyte - any compound whose dilute aqueous solutions conduct electricity well this is due to the presence of all or almost all of the dissolve compound in the form of ions
144
Unique characteristic of strong electrolyte is that they yield only ions
Ex Ionic compound may be highly soluble in water and dissociate into ions (NaCl)
Others may not dissolve much but amount that does dissolve exists only as ions
145
Weak ElectrolytesHF dissolves in water to give acid solution called hydrofluoric acid HF bond stronger than bonds between hydrogen and other halogens When HF dissolves some molecule ionize Reverse reaction also happens HF(aq) + H2O(l) H3O+(aq) + F-(aq)
146
HF(aq) + H2O(l) H3O+
(aq) + F-(aq)Concentration of dissolved unionized HF stays high and concentration of ions stays low
Weak electrolyte - any compound whose aqueous solutions conduct electricity poorly this is due to the presence of small amount of the dissolved compound in the form of ions
147
Different from nonelectrolyte ndash NO ions at all Description of electrolyte as strong or weak not related to concentration of solution Electrolytes differ in degree of ionization not amount of solute dissolved
148
149
Colligative properties of solutions
Presence of solute affects properties of solutions Some properties not dependent on nature of dissolved substance but on how may dissolve particles are present
Colligative properties - properties that depend on the concentration of solute particles but not on their identity
150
Vapor-Pressure Lowering
Boiling and freezing point of solution different from pure solvent Nonvolatile solute raises boiling point and lowers freezing point Nonvolatile substance - one that has little tendency to become gas under existing conditions
151
152
To understand why nonvolatile solute changes boiling and freezing point consider equilibrium vapor pressure
Vapor pressure pressure caused by molecules that have escape liquid phase to gas phase
Can be thought of a measure of tendency of molecules to escape from a liquid
153
Addition of sucrose (nonvolatile solute) lowers concentration of water molecules at surface of liquid This lowers tendency of water molecules to leave solution and enter gas phase Vapor pressure of solution is lower than vapor pressure of pure water
154
Aqueous13 solution13 of13 nonvolatile13 solutePure13 water
Nonelectrolyte solutions of same molality have same concentration of particles
Dilute solutions of same solvent and equal molality of any nonelectrolyte solute lower vapor pressure equally
155
Example
1 m aqueous solution of nonelectrolyte glucose lowers vapor pressure of water 55 x 10-4 atm at 25 1 m aqueous solution of sucrose also lowers water vapor pressure to 55 x 10-4 atm at 25
156
157
bc vapor-pressure lowering depends on concentration of nonelectrolyte solute and doesnrsquot depend on the type of solute it is a colligative proprerty
bc vapor pressure has been lowered solution remains liquid over larger temperature range
Lowers freezing point and raises boiling point
158
Can assume changes in boiling and freezing point also depend on concentration of solute
They are colligative properties
159
Freezing-Point Depression
Freezing point of 1-molal solution of any nonelectrolyte solute in water is found (by experiment) to be 186 lower than freezing point of water When 1 mol of nonelectrolyte solute dissolved in 1 kg water freezing point is -186 not 000 160
When 2 mol nonelectrolyte solute dissolved in 1 kg water freezing point is -372
For any concentration of nonelectrolyte solute in water decrease in freezing point can be determined using value of -186m
161
This value called the molal freezing-point contstant (Kf) is the freezing-point depression of the solvent in a 1-molal solution of a nonvolatile nonelectrolyte solute
162
Each solvent has own characteristic molal freezing-point constant Freezing-point depression (Δtf) the difference between the freezing points of the pure solvent and a solute of nonelectrolyte in that solvent and it is directly proportional to the molal concentration of the solution Δtf = Kfm
163
Δtf = Kfm
Kf expressed as m
m expressed in mol solutekg solvent (molality)
164
Sample Problem
What is the freezing point depression of water in a solution of 171 g of sucrose C12H22O11 and 200 g of water What is the actual freezing point of the solution
165
1 AnalyzeGiven Solute mass and chemical formula 171 g C12H22O11 Solvent mass and identity 2000 g water Unknown a freezing-point depression b freezing point of the solution 166
2 Plan
167
168
Calculate
Unknown a freezing-point depression b freezing point of the solution
169
Practice Problem
A water solution containing an unknown quantity of a nonelectrolyte solute is found to have a freezing point of ndash 023degCWhat is the molal concentration of the solution
012 m170
A solution consists of 103 g of the nonelectrolyte glucose C6H12O6 dissolved in 250 g of water What is the freezing-point depression of the solution
minus0426degC171
In a laboratory experiment the freezing point of an aqueous solution of glucose is found to be minus0325degCWhat is the molal concentration of this solution
0175 m172
If 0500 mol of a nonelectrolyte solute are dissolved in 5000 g of ether what is the freezing point of the solution
minus1181degC173
The freezing point of an aqueous solution that contains a nonelectrolyte is minus90degC aWhat is the freezing-point depression of the solution bWhat is the molal concentration of the solution a minus90degC b 48 m
174
Boiling-Point Elevation
Boiling point is temp at which vapor pressure is equal to atmospheric pressure Change in vapor pressure will cause corresponding change in boiling point
175
176
Vapor pressure of nonvolatile solution is lower than vapor pressure of pure solvent More heat will be required to raise vapor pressure of solution Boiling point of solution is higher than boiling point of pure solvent
Molal boiling-point constant (Kb) - boiling-point elevation of the solvent in a 1-molal solution of a nonvolatile nonelectrolyte solute b-p elevation of any nonelectrolyte in water found by experiment to be 051 So molal b-p constant for water is 051m
177
For different solvents b-p elevations of 1-molal solutions have different values (Table 14-2)
Like freezing-point constants values most accurate for dilute solutions
178
Boiling-point elevation Δtb difference between the boiling points of the pure solvent and a nonelectrolyte solution of that solvent and it is directly proportional to the molal concentration of the solution B-p elevation can be calculated using Δtb = Kbm Δtb (boiling point elevation) expressed in m and m expressed in mol solutekg solvent
Sample Problem
What is the boiling-point elevation of a solution made from 200 g of a nonelectrolyte solute and 4000 g of water The molar mass of the solute is 620 gmol
180
1 AnalyzeGiven solute mass = 200 g solute molar mass = 620 gmol solvent mass and identity = 4000 g of water
Unknown boiling-point elevation
181
2 Plan
182
183
3 Compute
Practice Problem
A solution contains 500 g of sucrose C12H22O11 a nonelectrolyte dissolved in 5000 g of water What is the boiling-point elevation
015degC184
A solution contains 4500 g of sucrose C12H22O11 a nonelectrolyte dissolved in 250 g of waterWhat is the boiling point of the solution
1027degC185
If the boiling point elevation of an aqueous solution containing a nonvolatile electrolyte is 102degC what is the molality of the solution
20 m186
The boiling point of an aqueous solution containing a nonvolatile electrolyte is 10075degC aWhat is the boiling-point elevation bWhat is the molality of the solution a 075degC b 15 m
187
Electrolytes and Colligative PropertiesEarly investigators confused by experiments where certain substances depressed freezing point or elevated boiling point more than expected
188
189
Ex 01 m solution of NaCl lowers freezing point of solvent almost twice as much as 01 m solution of sucrose 01 m CaCl2 lowers fp almost 3 times as much as 01 m solution of sucrose
To understand why this happens contrast behavior or sucrose with NaCl in aqueous solution Each sucrose dissolves to make only 1 particle in solution So 1 mol sucrose dissolves to 1 mol particles 1 mol NaCl dissolves to make 2 moles of particles (1 mol Na+ and 1 mol Cl-)
190
191
Calculated Values for Electrolyte SolutionsColligative properties depend on total concentration of solute particles regardless of their identity Electrolytes cause changes in colligative properties proportional to total molality in terms of all dissolves particles instead of formula units
192
What about barium nitrate Ba(NO3)2 Each mole of barium nitrate yields 3 mol of ions in solution Ba(NO3)2(s) rarr Ba2+(aq) + 2NO3-(aq) You would expect solution of given molality to lower fp of solvent 3 times as much as nonelectrolytic solution of same molality
193
Sample Problem
What is the expected change in the freezing point of water in a solution of 625 g of barium nitrate Ba(NO3)2 in 100 kg of water
194
1 AnalyzeGiven solute mass and formula = 625 g Ba(NO3)2 solvent mass and identity = 100 kg water Δtf = Kfm
Unknown expected freezing-point depression
195
2 PlanThe molality can be calculated by converting the solute mass to moles and then dividing by the number of kilograms of solvent That molality is in terms of formula units of Ba(NO3)2 and must be converted to molality in terms of dissociated ions in solution It must be multiplied by the number of moles of ions produced per mole of formula unit This adjusted molality can then be used to calculate the freezing-point depression
196
197
198
3 Compute
Practice Problem
What is the expected freezing-point depression for a solution that contains 20 mol of magnesium sulfate dissolved in 10 kg of water
minus74degC199
What is the expected boiling-point elevation of water for a solution that contains 150 g of sodium chloride dissolved in 10 kg of water 27degC The freezing point of an aqueous sodium chloride solution is minus200degC What is the molality of the solution 54 m NaCl
200
Actual Values for Electrolyte SolutionsRemember values calculated are only expected values Actual values of colligative properties for all strong electrolytes are almost what would be expected Difference between expected and actual values caused by attractive forces that exist between dissociated ions in aqueous solution
201
Attraction between hydrated ions in solution is small compared with those in crystalline solid However forces of attraction donrsquot interfere with movements of aqueous ions More concentrated solution means ions closer together and attraction is greater Also depends on charge of ions ndash higher charge greater attraction
202
Practice Problem
Identify the precipitate that forms when aqueous solutions of zinc nitrate and ammonium sulfide are combined Write the equation for the possible double-replacement reaction Then write the formula equation overall ionic equation and the net ionic equation
125
1 AnalyzeGiven Identity of reactants zinc nitrate and ammonium sulfide Reaction medium aqueous solution Unknown a equation for the possible double-replacement reaction b identity of the precipitate c formula equation d overall ionic equation e net ionic equation
126
2 PlanWrite the possible double-replacement reaction between Zn(NO3)2 and (NH4)2S Use the rules to determine if any of the products will precipitate Write a formula equation and overall net ionic equation Then cancel spectator ions to make net ionic equation
127
3 Computea possible double-replacement reaction
b rules show that zinc sulfide is not soluble so will precipitate (ammonium nitrate is soluble)
c formula equation128
d overall ionic equation
e net ionic equation129
Will a precipitate form if solutions of potassium sulfate and barium nitrate are combined If so write the net ionic equation for the reaction
Yes Ba+2(aq) + SO4-2(aq) rarr BaSO4(s)
130
Will a precipitate form if solutions of potassium nitrate and magnesium sulfate are combined If so write the net ionic equation for the reaction
No 131
Will a precipitate form if solutions of barium chloride and sodium sulfate are combined If so identify the spectator ions and write the net ionic equation for the reaction
Yes Na+ and Cl- Ba+2(aq) + SO4-2(aq) rarr BaSO4(s)
132
Write the net ionic equation for the precipitation of nickel(II) sulfide
Ni+2(aq) + S-2(aq) rarr NiS(s)
133
IonizationSome molecular compounds can also form ions in solution Usually polar Ionization - ions formed from solute molecules by action of the solvent (creation of ions where there were none) Ionization different from dissociation
134
Dissociation ionic compounds dissolve and ions already present separate Ionization ions formed where non existed before
Ions formed are hydrated Heat released during hydration of ions gives enough energy to break covalent bonds
135
Extent to which solute ionizes depends on strength of bonds within molecules of solute and strength of attraction between solute and solvent molecules
136
HCl ndash molecular compound that ionizes in aqueous solution Contains highly polar bond Attraction between polar HCl molecule and polar water strong enough to break HCl bond
137
The Hydronium Ion
H+ ions from HCl attracts other molecules or ions so strongly that it doesnrsquot normally exist alone Ionization of HCl better described as direct transfer of proton from HCl to H2O forming H3O+
138
139
Hydration of H+ to form hydronium is highly exothermic
Energy released gives a lot of energy needed to ionize a molecular solute
Many molecular compounds that ionize in aqueous solution contain hydrogen and form hydronium ion
140
Strong and Weak Electrolytes
Substances that make ions and conduct current in solution are electrolytes HCl is one of series of compounds that has hydrogen and halogen Hydrogen halides are all molecular compounds with single polar-covalent bonds
141
All are gases Very soluble in water All are electrolytes HCl HBr HI strongly conduct current in solution HF only weakly conducts current at same concentration Strength of conduction related to ability to ionize
142
143
Strong Electrolytes
HCl HBr HI are 100 ionized in solution
Strong electrolyte - any compound whose dilute aqueous solutions conduct electricity well this is due to the presence of all or almost all of the dissolve compound in the form of ions
144
Unique characteristic of strong electrolyte is that they yield only ions
Ex Ionic compound may be highly soluble in water and dissociate into ions (NaCl)
Others may not dissolve much but amount that does dissolve exists only as ions
145
Weak ElectrolytesHF dissolves in water to give acid solution called hydrofluoric acid HF bond stronger than bonds between hydrogen and other halogens When HF dissolves some molecule ionize Reverse reaction also happens HF(aq) + H2O(l) H3O+(aq) + F-(aq)
146
HF(aq) + H2O(l) H3O+
(aq) + F-(aq)Concentration of dissolved unionized HF stays high and concentration of ions stays low
Weak electrolyte - any compound whose aqueous solutions conduct electricity poorly this is due to the presence of small amount of the dissolved compound in the form of ions
147
Different from nonelectrolyte ndash NO ions at all Description of electrolyte as strong or weak not related to concentration of solution Electrolytes differ in degree of ionization not amount of solute dissolved
148
149
Colligative properties of solutions
Presence of solute affects properties of solutions Some properties not dependent on nature of dissolved substance but on how may dissolve particles are present
Colligative properties - properties that depend on the concentration of solute particles but not on their identity
150
Vapor-Pressure Lowering
Boiling and freezing point of solution different from pure solvent Nonvolatile solute raises boiling point and lowers freezing point Nonvolatile substance - one that has little tendency to become gas under existing conditions
151
152
To understand why nonvolatile solute changes boiling and freezing point consider equilibrium vapor pressure
Vapor pressure pressure caused by molecules that have escape liquid phase to gas phase
Can be thought of a measure of tendency of molecules to escape from a liquid
153
Addition of sucrose (nonvolatile solute) lowers concentration of water molecules at surface of liquid This lowers tendency of water molecules to leave solution and enter gas phase Vapor pressure of solution is lower than vapor pressure of pure water
154
Aqueous13 solution13 of13 nonvolatile13 solutePure13 water
Nonelectrolyte solutions of same molality have same concentration of particles
Dilute solutions of same solvent and equal molality of any nonelectrolyte solute lower vapor pressure equally
155
Example
1 m aqueous solution of nonelectrolyte glucose lowers vapor pressure of water 55 x 10-4 atm at 25 1 m aqueous solution of sucrose also lowers water vapor pressure to 55 x 10-4 atm at 25
156
157
bc vapor-pressure lowering depends on concentration of nonelectrolyte solute and doesnrsquot depend on the type of solute it is a colligative proprerty
bc vapor pressure has been lowered solution remains liquid over larger temperature range
Lowers freezing point and raises boiling point
158
Can assume changes in boiling and freezing point also depend on concentration of solute
They are colligative properties
159
Freezing-Point Depression
Freezing point of 1-molal solution of any nonelectrolyte solute in water is found (by experiment) to be 186 lower than freezing point of water When 1 mol of nonelectrolyte solute dissolved in 1 kg water freezing point is -186 not 000 160
When 2 mol nonelectrolyte solute dissolved in 1 kg water freezing point is -372
For any concentration of nonelectrolyte solute in water decrease in freezing point can be determined using value of -186m
161
This value called the molal freezing-point contstant (Kf) is the freezing-point depression of the solvent in a 1-molal solution of a nonvolatile nonelectrolyte solute
162
Each solvent has own characteristic molal freezing-point constant Freezing-point depression (Δtf) the difference between the freezing points of the pure solvent and a solute of nonelectrolyte in that solvent and it is directly proportional to the molal concentration of the solution Δtf = Kfm
163
Δtf = Kfm
Kf expressed as m
m expressed in mol solutekg solvent (molality)
164
Sample Problem
What is the freezing point depression of water in a solution of 171 g of sucrose C12H22O11 and 200 g of water What is the actual freezing point of the solution
165
1 AnalyzeGiven Solute mass and chemical formula 171 g C12H22O11 Solvent mass and identity 2000 g water Unknown a freezing-point depression b freezing point of the solution 166
2 Plan
167
168
Calculate
Unknown a freezing-point depression b freezing point of the solution
169
Practice Problem
A water solution containing an unknown quantity of a nonelectrolyte solute is found to have a freezing point of ndash 023degCWhat is the molal concentration of the solution
012 m170
A solution consists of 103 g of the nonelectrolyte glucose C6H12O6 dissolved in 250 g of water What is the freezing-point depression of the solution
minus0426degC171
In a laboratory experiment the freezing point of an aqueous solution of glucose is found to be minus0325degCWhat is the molal concentration of this solution
0175 m172
If 0500 mol of a nonelectrolyte solute are dissolved in 5000 g of ether what is the freezing point of the solution
minus1181degC173
The freezing point of an aqueous solution that contains a nonelectrolyte is minus90degC aWhat is the freezing-point depression of the solution bWhat is the molal concentration of the solution a minus90degC b 48 m
174
Boiling-Point Elevation
Boiling point is temp at which vapor pressure is equal to atmospheric pressure Change in vapor pressure will cause corresponding change in boiling point
175
176
Vapor pressure of nonvolatile solution is lower than vapor pressure of pure solvent More heat will be required to raise vapor pressure of solution Boiling point of solution is higher than boiling point of pure solvent
Molal boiling-point constant (Kb) - boiling-point elevation of the solvent in a 1-molal solution of a nonvolatile nonelectrolyte solute b-p elevation of any nonelectrolyte in water found by experiment to be 051 So molal b-p constant for water is 051m
177
For different solvents b-p elevations of 1-molal solutions have different values (Table 14-2)
Like freezing-point constants values most accurate for dilute solutions
178
Boiling-point elevation Δtb difference between the boiling points of the pure solvent and a nonelectrolyte solution of that solvent and it is directly proportional to the molal concentration of the solution B-p elevation can be calculated using Δtb = Kbm Δtb (boiling point elevation) expressed in m and m expressed in mol solutekg solvent
Sample Problem
What is the boiling-point elevation of a solution made from 200 g of a nonelectrolyte solute and 4000 g of water The molar mass of the solute is 620 gmol
180
1 AnalyzeGiven solute mass = 200 g solute molar mass = 620 gmol solvent mass and identity = 4000 g of water
Unknown boiling-point elevation
181
2 Plan
182
183
3 Compute
Practice Problem
A solution contains 500 g of sucrose C12H22O11 a nonelectrolyte dissolved in 5000 g of water What is the boiling-point elevation
015degC184
A solution contains 4500 g of sucrose C12H22O11 a nonelectrolyte dissolved in 250 g of waterWhat is the boiling point of the solution
1027degC185
If the boiling point elevation of an aqueous solution containing a nonvolatile electrolyte is 102degC what is the molality of the solution
20 m186
The boiling point of an aqueous solution containing a nonvolatile electrolyte is 10075degC aWhat is the boiling-point elevation bWhat is the molality of the solution a 075degC b 15 m
187
Electrolytes and Colligative PropertiesEarly investigators confused by experiments where certain substances depressed freezing point or elevated boiling point more than expected
188
189
Ex 01 m solution of NaCl lowers freezing point of solvent almost twice as much as 01 m solution of sucrose 01 m CaCl2 lowers fp almost 3 times as much as 01 m solution of sucrose
To understand why this happens contrast behavior or sucrose with NaCl in aqueous solution Each sucrose dissolves to make only 1 particle in solution So 1 mol sucrose dissolves to 1 mol particles 1 mol NaCl dissolves to make 2 moles of particles (1 mol Na+ and 1 mol Cl-)
190
191
Calculated Values for Electrolyte SolutionsColligative properties depend on total concentration of solute particles regardless of their identity Electrolytes cause changes in colligative properties proportional to total molality in terms of all dissolves particles instead of formula units
192
What about barium nitrate Ba(NO3)2 Each mole of barium nitrate yields 3 mol of ions in solution Ba(NO3)2(s) rarr Ba2+(aq) + 2NO3-(aq) You would expect solution of given molality to lower fp of solvent 3 times as much as nonelectrolytic solution of same molality
193
Sample Problem
What is the expected change in the freezing point of water in a solution of 625 g of barium nitrate Ba(NO3)2 in 100 kg of water
194
1 AnalyzeGiven solute mass and formula = 625 g Ba(NO3)2 solvent mass and identity = 100 kg water Δtf = Kfm
Unknown expected freezing-point depression
195
2 PlanThe molality can be calculated by converting the solute mass to moles and then dividing by the number of kilograms of solvent That molality is in terms of formula units of Ba(NO3)2 and must be converted to molality in terms of dissociated ions in solution It must be multiplied by the number of moles of ions produced per mole of formula unit This adjusted molality can then be used to calculate the freezing-point depression
196
197
198
3 Compute
Practice Problem
What is the expected freezing-point depression for a solution that contains 20 mol of magnesium sulfate dissolved in 10 kg of water
minus74degC199
What is the expected boiling-point elevation of water for a solution that contains 150 g of sodium chloride dissolved in 10 kg of water 27degC The freezing point of an aqueous sodium chloride solution is minus200degC What is the molality of the solution 54 m NaCl
200
Actual Values for Electrolyte SolutionsRemember values calculated are only expected values Actual values of colligative properties for all strong electrolytes are almost what would be expected Difference between expected and actual values caused by attractive forces that exist between dissociated ions in aqueous solution
201
Attraction between hydrated ions in solution is small compared with those in crystalline solid However forces of attraction donrsquot interfere with movements of aqueous ions More concentrated solution means ions closer together and attraction is greater Also depends on charge of ions ndash higher charge greater attraction
202
1 AnalyzeGiven Identity of reactants zinc nitrate and ammonium sulfide Reaction medium aqueous solution Unknown a equation for the possible double-replacement reaction b identity of the precipitate c formula equation d overall ionic equation e net ionic equation
126
2 PlanWrite the possible double-replacement reaction between Zn(NO3)2 and (NH4)2S Use the rules to determine if any of the products will precipitate Write a formula equation and overall net ionic equation Then cancel spectator ions to make net ionic equation
127
3 Computea possible double-replacement reaction
b rules show that zinc sulfide is not soluble so will precipitate (ammonium nitrate is soluble)
c formula equation128
d overall ionic equation
e net ionic equation129
Will a precipitate form if solutions of potassium sulfate and barium nitrate are combined If so write the net ionic equation for the reaction
Yes Ba+2(aq) + SO4-2(aq) rarr BaSO4(s)
130
Will a precipitate form if solutions of potassium nitrate and magnesium sulfate are combined If so write the net ionic equation for the reaction
No 131
Will a precipitate form if solutions of barium chloride and sodium sulfate are combined If so identify the spectator ions and write the net ionic equation for the reaction
Yes Na+ and Cl- Ba+2(aq) + SO4-2(aq) rarr BaSO4(s)
132
Write the net ionic equation for the precipitation of nickel(II) sulfide
Ni+2(aq) + S-2(aq) rarr NiS(s)
133
IonizationSome molecular compounds can also form ions in solution Usually polar Ionization - ions formed from solute molecules by action of the solvent (creation of ions where there were none) Ionization different from dissociation
134
Dissociation ionic compounds dissolve and ions already present separate Ionization ions formed where non existed before
Ions formed are hydrated Heat released during hydration of ions gives enough energy to break covalent bonds
135
Extent to which solute ionizes depends on strength of bonds within molecules of solute and strength of attraction between solute and solvent molecules
136
HCl ndash molecular compound that ionizes in aqueous solution Contains highly polar bond Attraction between polar HCl molecule and polar water strong enough to break HCl bond
137
The Hydronium Ion
H+ ions from HCl attracts other molecules or ions so strongly that it doesnrsquot normally exist alone Ionization of HCl better described as direct transfer of proton from HCl to H2O forming H3O+
138
139
Hydration of H+ to form hydronium is highly exothermic
Energy released gives a lot of energy needed to ionize a molecular solute
Many molecular compounds that ionize in aqueous solution contain hydrogen and form hydronium ion
140
Strong and Weak Electrolytes
Substances that make ions and conduct current in solution are electrolytes HCl is one of series of compounds that has hydrogen and halogen Hydrogen halides are all molecular compounds with single polar-covalent bonds
141
All are gases Very soluble in water All are electrolytes HCl HBr HI strongly conduct current in solution HF only weakly conducts current at same concentration Strength of conduction related to ability to ionize
142
143
Strong Electrolytes
HCl HBr HI are 100 ionized in solution
Strong electrolyte - any compound whose dilute aqueous solutions conduct electricity well this is due to the presence of all or almost all of the dissolve compound in the form of ions
144
Unique characteristic of strong electrolyte is that they yield only ions
Ex Ionic compound may be highly soluble in water and dissociate into ions (NaCl)
Others may not dissolve much but amount that does dissolve exists only as ions
145
Weak ElectrolytesHF dissolves in water to give acid solution called hydrofluoric acid HF bond stronger than bonds between hydrogen and other halogens When HF dissolves some molecule ionize Reverse reaction also happens HF(aq) + H2O(l) H3O+(aq) + F-(aq)
146
HF(aq) + H2O(l) H3O+
(aq) + F-(aq)Concentration of dissolved unionized HF stays high and concentration of ions stays low
Weak electrolyte - any compound whose aqueous solutions conduct electricity poorly this is due to the presence of small amount of the dissolved compound in the form of ions
147
Different from nonelectrolyte ndash NO ions at all Description of electrolyte as strong or weak not related to concentration of solution Electrolytes differ in degree of ionization not amount of solute dissolved
148
149
Colligative properties of solutions
Presence of solute affects properties of solutions Some properties not dependent on nature of dissolved substance but on how may dissolve particles are present
Colligative properties - properties that depend on the concentration of solute particles but not on their identity
150
Vapor-Pressure Lowering
Boiling and freezing point of solution different from pure solvent Nonvolatile solute raises boiling point and lowers freezing point Nonvolatile substance - one that has little tendency to become gas under existing conditions
151
152
To understand why nonvolatile solute changes boiling and freezing point consider equilibrium vapor pressure
Vapor pressure pressure caused by molecules that have escape liquid phase to gas phase
Can be thought of a measure of tendency of molecules to escape from a liquid
153
Addition of sucrose (nonvolatile solute) lowers concentration of water molecules at surface of liquid This lowers tendency of water molecules to leave solution and enter gas phase Vapor pressure of solution is lower than vapor pressure of pure water
154
Aqueous13 solution13 of13 nonvolatile13 solutePure13 water
Nonelectrolyte solutions of same molality have same concentration of particles
Dilute solutions of same solvent and equal molality of any nonelectrolyte solute lower vapor pressure equally
155
Example
1 m aqueous solution of nonelectrolyte glucose lowers vapor pressure of water 55 x 10-4 atm at 25 1 m aqueous solution of sucrose also lowers water vapor pressure to 55 x 10-4 atm at 25
156
157
bc vapor-pressure lowering depends on concentration of nonelectrolyte solute and doesnrsquot depend on the type of solute it is a colligative proprerty
bc vapor pressure has been lowered solution remains liquid over larger temperature range
Lowers freezing point and raises boiling point
158
Can assume changes in boiling and freezing point also depend on concentration of solute
They are colligative properties
159
Freezing-Point Depression
Freezing point of 1-molal solution of any nonelectrolyte solute in water is found (by experiment) to be 186 lower than freezing point of water When 1 mol of nonelectrolyte solute dissolved in 1 kg water freezing point is -186 not 000 160
When 2 mol nonelectrolyte solute dissolved in 1 kg water freezing point is -372
For any concentration of nonelectrolyte solute in water decrease in freezing point can be determined using value of -186m
161
This value called the molal freezing-point contstant (Kf) is the freezing-point depression of the solvent in a 1-molal solution of a nonvolatile nonelectrolyte solute
162
Each solvent has own characteristic molal freezing-point constant Freezing-point depression (Δtf) the difference between the freezing points of the pure solvent and a solute of nonelectrolyte in that solvent and it is directly proportional to the molal concentration of the solution Δtf = Kfm
163
Δtf = Kfm
Kf expressed as m
m expressed in mol solutekg solvent (molality)
164
Sample Problem
What is the freezing point depression of water in a solution of 171 g of sucrose C12H22O11 and 200 g of water What is the actual freezing point of the solution
165
1 AnalyzeGiven Solute mass and chemical formula 171 g C12H22O11 Solvent mass and identity 2000 g water Unknown a freezing-point depression b freezing point of the solution 166
2 Plan
167
168
Calculate
Unknown a freezing-point depression b freezing point of the solution
169
Practice Problem
A water solution containing an unknown quantity of a nonelectrolyte solute is found to have a freezing point of ndash 023degCWhat is the molal concentration of the solution
012 m170
A solution consists of 103 g of the nonelectrolyte glucose C6H12O6 dissolved in 250 g of water What is the freezing-point depression of the solution
minus0426degC171
In a laboratory experiment the freezing point of an aqueous solution of glucose is found to be minus0325degCWhat is the molal concentration of this solution
0175 m172
If 0500 mol of a nonelectrolyte solute are dissolved in 5000 g of ether what is the freezing point of the solution
minus1181degC173
The freezing point of an aqueous solution that contains a nonelectrolyte is minus90degC aWhat is the freezing-point depression of the solution bWhat is the molal concentration of the solution a minus90degC b 48 m
174
Boiling-Point Elevation
Boiling point is temp at which vapor pressure is equal to atmospheric pressure Change in vapor pressure will cause corresponding change in boiling point
175
176
Vapor pressure of nonvolatile solution is lower than vapor pressure of pure solvent More heat will be required to raise vapor pressure of solution Boiling point of solution is higher than boiling point of pure solvent
Molal boiling-point constant (Kb) - boiling-point elevation of the solvent in a 1-molal solution of a nonvolatile nonelectrolyte solute b-p elevation of any nonelectrolyte in water found by experiment to be 051 So molal b-p constant for water is 051m
177
For different solvents b-p elevations of 1-molal solutions have different values (Table 14-2)
Like freezing-point constants values most accurate for dilute solutions
178
Boiling-point elevation Δtb difference between the boiling points of the pure solvent and a nonelectrolyte solution of that solvent and it is directly proportional to the molal concentration of the solution B-p elevation can be calculated using Δtb = Kbm Δtb (boiling point elevation) expressed in m and m expressed in mol solutekg solvent
Sample Problem
What is the boiling-point elevation of a solution made from 200 g of a nonelectrolyte solute and 4000 g of water The molar mass of the solute is 620 gmol
180
1 AnalyzeGiven solute mass = 200 g solute molar mass = 620 gmol solvent mass and identity = 4000 g of water
Unknown boiling-point elevation
181
2 Plan
182
183
3 Compute
Practice Problem
A solution contains 500 g of sucrose C12H22O11 a nonelectrolyte dissolved in 5000 g of water What is the boiling-point elevation
015degC184
A solution contains 4500 g of sucrose C12H22O11 a nonelectrolyte dissolved in 250 g of waterWhat is the boiling point of the solution
1027degC185
If the boiling point elevation of an aqueous solution containing a nonvolatile electrolyte is 102degC what is the molality of the solution
20 m186
The boiling point of an aqueous solution containing a nonvolatile electrolyte is 10075degC aWhat is the boiling-point elevation bWhat is the molality of the solution a 075degC b 15 m
187
Electrolytes and Colligative PropertiesEarly investigators confused by experiments where certain substances depressed freezing point or elevated boiling point more than expected
188
189
Ex 01 m solution of NaCl lowers freezing point of solvent almost twice as much as 01 m solution of sucrose 01 m CaCl2 lowers fp almost 3 times as much as 01 m solution of sucrose
To understand why this happens contrast behavior or sucrose with NaCl in aqueous solution Each sucrose dissolves to make only 1 particle in solution So 1 mol sucrose dissolves to 1 mol particles 1 mol NaCl dissolves to make 2 moles of particles (1 mol Na+ and 1 mol Cl-)
190
191
Calculated Values for Electrolyte SolutionsColligative properties depend on total concentration of solute particles regardless of their identity Electrolytes cause changes in colligative properties proportional to total molality in terms of all dissolves particles instead of formula units
192
What about barium nitrate Ba(NO3)2 Each mole of barium nitrate yields 3 mol of ions in solution Ba(NO3)2(s) rarr Ba2+(aq) + 2NO3-(aq) You would expect solution of given molality to lower fp of solvent 3 times as much as nonelectrolytic solution of same molality
193
Sample Problem
What is the expected change in the freezing point of water in a solution of 625 g of barium nitrate Ba(NO3)2 in 100 kg of water
194
1 AnalyzeGiven solute mass and formula = 625 g Ba(NO3)2 solvent mass and identity = 100 kg water Δtf = Kfm
Unknown expected freezing-point depression
195
2 PlanThe molality can be calculated by converting the solute mass to moles and then dividing by the number of kilograms of solvent That molality is in terms of formula units of Ba(NO3)2 and must be converted to molality in terms of dissociated ions in solution It must be multiplied by the number of moles of ions produced per mole of formula unit This adjusted molality can then be used to calculate the freezing-point depression
196
197
198
3 Compute
Practice Problem
What is the expected freezing-point depression for a solution that contains 20 mol of magnesium sulfate dissolved in 10 kg of water
minus74degC199
What is the expected boiling-point elevation of water for a solution that contains 150 g of sodium chloride dissolved in 10 kg of water 27degC The freezing point of an aqueous sodium chloride solution is minus200degC What is the molality of the solution 54 m NaCl
200
Actual Values for Electrolyte SolutionsRemember values calculated are only expected values Actual values of colligative properties for all strong electrolytes are almost what would be expected Difference between expected and actual values caused by attractive forces that exist between dissociated ions in aqueous solution
201
Attraction between hydrated ions in solution is small compared with those in crystalline solid However forces of attraction donrsquot interfere with movements of aqueous ions More concentrated solution means ions closer together and attraction is greater Also depends on charge of ions ndash higher charge greater attraction
202
2 PlanWrite the possible double-replacement reaction between Zn(NO3)2 and (NH4)2S Use the rules to determine if any of the products will precipitate Write a formula equation and overall net ionic equation Then cancel spectator ions to make net ionic equation
127
3 Computea possible double-replacement reaction
b rules show that zinc sulfide is not soluble so will precipitate (ammonium nitrate is soluble)
c formula equation128
d overall ionic equation
e net ionic equation129
Will a precipitate form if solutions of potassium sulfate and barium nitrate are combined If so write the net ionic equation for the reaction
Yes Ba+2(aq) + SO4-2(aq) rarr BaSO4(s)
130
Will a precipitate form if solutions of potassium nitrate and magnesium sulfate are combined If so write the net ionic equation for the reaction
No 131
Will a precipitate form if solutions of barium chloride and sodium sulfate are combined If so identify the spectator ions and write the net ionic equation for the reaction
Yes Na+ and Cl- Ba+2(aq) + SO4-2(aq) rarr BaSO4(s)
132
Write the net ionic equation for the precipitation of nickel(II) sulfide
Ni+2(aq) + S-2(aq) rarr NiS(s)
133
IonizationSome molecular compounds can also form ions in solution Usually polar Ionization - ions formed from solute molecules by action of the solvent (creation of ions where there were none) Ionization different from dissociation
134
Dissociation ionic compounds dissolve and ions already present separate Ionization ions formed where non existed before
Ions formed are hydrated Heat released during hydration of ions gives enough energy to break covalent bonds
135
Extent to which solute ionizes depends on strength of bonds within molecules of solute and strength of attraction between solute and solvent molecules
136
HCl ndash molecular compound that ionizes in aqueous solution Contains highly polar bond Attraction between polar HCl molecule and polar water strong enough to break HCl bond
137
The Hydronium Ion
H+ ions from HCl attracts other molecules or ions so strongly that it doesnrsquot normally exist alone Ionization of HCl better described as direct transfer of proton from HCl to H2O forming H3O+
138
139
Hydration of H+ to form hydronium is highly exothermic
Energy released gives a lot of energy needed to ionize a molecular solute
Many molecular compounds that ionize in aqueous solution contain hydrogen and form hydronium ion
140
Strong and Weak Electrolytes
Substances that make ions and conduct current in solution are electrolytes HCl is one of series of compounds that has hydrogen and halogen Hydrogen halides are all molecular compounds with single polar-covalent bonds
141
All are gases Very soluble in water All are electrolytes HCl HBr HI strongly conduct current in solution HF only weakly conducts current at same concentration Strength of conduction related to ability to ionize
142
143
Strong Electrolytes
HCl HBr HI are 100 ionized in solution
Strong electrolyte - any compound whose dilute aqueous solutions conduct electricity well this is due to the presence of all or almost all of the dissolve compound in the form of ions
144
Unique characteristic of strong electrolyte is that they yield only ions
Ex Ionic compound may be highly soluble in water and dissociate into ions (NaCl)
Others may not dissolve much but amount that does dissolve exists only as ions
145
Weak ElectrolytesHF dissolves in water to give acid solution called hydrofluoric acid HF bond stronger than bonds between hydrogen and other halogens When HF dissolves some molecule ionize Reverse reaction also happens HF(aq) + H2O(l) H3O+(aq) + F-(aq)
146
HF(aq) + H2O(l) H3O+
(aq) + F-(aq)Concentration of dissolved unionized HF stays high and concentration of ions stays low
Weak electrolyte - any compound whose aqueous solutions conduct electricity poorly this is due to the presence of small amount of the dissolved compound in the form of ions
147
Different from nonelectrolyte ndash NO ions at all Description of electrolyte as strong or weak not related to concentration of solution Electrolytes differ in degree of ionization not amount of solute dissolved
148
149
Colligative properties of solutions
Presence of solute affects properties of solutions Some properties not dependent on nature of dissolved substance but on how may dissolve particles are present
Colligative properties - properties that depend on the concentration of solute particles but not on their identity
150
Vapor-Pressure Lowering
Boiling and freezing point of solution different from pure solvent Nonvolatile solute raises boiling point and lowers freezing point Nonvolatile substance - one that has little tendency to become gas under existing conditions
151
152
To understand why nonvolatile solute changes boiling and freezing point consider equilibrium vapor pressure
Vapor pressure pressure caused by molecules that have escape liquid phase to gas phase
Can be thought of a measure of tendency of molecules to escape from a liquid
153
Addition of sucrose (nonvolatile solute) lowers concentration of water molecules at surface of liquid This lowers tendency of water molecules to leave solution and enter gas phase Vapor pressure of solution is lower than vapor pressure of pure water
154
Aqueous13 solution13 of13 nonvolatile13 solutePure13 water
Nonelectrolyte solutions of same molality have same concentration of particles
Dilute solutions of same solvent and equal molality of any nonelectrolyte solute lower vapor pressure equally
155
Example
1 m aqueous solution of nonelectrolyte glucose lowers vapor pressure of water 55 x 10-4 atm at 25 1 m aqueous solution of sucrose also lowers water vapor pressure to 55 x 10-4 atm at 25
156
157
bc vapor-pressure lowering depends on concentration of nonelectrolyte solute and doesnrsquot depend on the type of solute it is a colligative proprerty
bc vapor pressure has been lowered solution remains liquid over larger temperature range
Lowers freezing point and raises boiling point
158
Can assume changes in boiling and freezing point also depend on concentration of solute
They are colligative properties
159
Freezing-Point Depression
Freezing point of 1-molal solution of any nonelectrolyte solute in water is found (by experiment) to be 186 lower than freezing point of water When 1 mol of nonelectrolyte solute dissolved in 1 kg water freezing point is -186 not 000 160
When 2 mol nonelectrolyte solute dissolved in 1 kg water freezing point is -372
For any concentration of nonelectrolyte solute in water decrease in freezing point can be determined using value of -186m
161
This value called the molal freezing-point contstant (Kf) is the freezing-point depression of the solvent in a 1-molal solution of a nonvolatile nonelectrolyte solute
162
Each solvent has own characteristic molal freezing-point constant Freezing-point depression (Δtf) the difference between the freezing points of the pure solvent and a solute of nonelectrolyte in that solvent and it is directly proportional to the molal concentration of the solution Δtf = Kfm
163
Δtf = Kfm
Kf expressed as m
m expressed in mol solutekg solvent (molality)
164
Sample Problem
What is the freezing point depression of water in a solution of 171 g of sucrose C12H22O11 and 200 g of water What is the actual freezing point of the solution
165
1 AnalyzeGiven Solute mass and chemical formula 171 g C12H22O11 Solvent mass and identity 2000 g water Unknown a freezing-point depression b freezing point of the solution 166
2 Plan
167
168
Calculate
Unknown a freezing-point depression b freezing point of the solution
169
Practice Problem
A water solution containing an unknown quantity of a nonelectrolyte solute is found to have a freezing point of ndash 023degCWhat is the molal concentration of the solution
012 m170
A solution consists of 103 g of the nonelectrolyte glucose C6H12O6 dissolved in 250 g of water What is the freezing-point depression of the solution
minus0426degC171
In a laboratory experiment the freezing point of an aqueous solution of glucose is found to be minus0325degCWhat is the molal concentration of this solution
0175 m172
If 0500 mol of a nonelectrolyte solute are dissolved in 5000 g of ether what is the freezing point of the solution
minus1181degC173
The freezing point of an aqueous solution that contains a nonelectrolyte is minus90degC aWhat is the freezing-point depression of the solution bWhat is the molal concentration of the solution a minus90degC b 48 m
174
Boiling-Point Elevation
Boiling point is temp at which vapor pressure is equal to atmospheric pressure Change in vapor pressure will cause corresponding change in boiling point
175
176
Vapor pressure of nonvolatile solution is lower than vapor pressure of pure solvent More heat will be required to raise vapor pressure of solution Boiling point of solution is higher than boiling point of pure solvent
Molal boiling-point constant (Kb) - boiling-point elevation of the solvent in a 1-molal solution of a nonvolatile nonelectrolyte solute b-p elevation of any nonelectrolyte in water found by experiment to be 051 So molal b-p constant for water is 051m
177
For different solvents b-p elevations of 1-molal solutions have different values (Table 14-2)
Like freezing-point constants values most accurate for dilute solutions
178
Boiling-point elevation Δtb difference between the boiling points of the pure solvent and a nonelectrolyte solution of that solvent and it is directly proportional to the molal concentration of the solution B-p elevation can be calculated using Δtb = Kbm Δtb (boiling point elevation) expressed in m and m expressed in mol solutekg solvent
Sample Problem
What is the boiling-point elevation of a solution made from 200 g of a nonelectrolyte solute and 4000 g of water The molar mass of the solute is 620 gmol
180
1 AnalyzeGiven solute mass = 200 g solute molar mass = 620 gmol solvent mass and identity = 4000 g of water
Unknown boiling-point elevation
181
2 Plan
182
183
3 Compute
Practice Problem
A solution contains 500 g of sucrose C12H22O11 a nonelectrolyte dissolved in 5000 g of water What is the boiling-point elevation
015degC184
A solution contains 4500 g of sucrose C12H22O11 a nonelectrolyte dissolved in 250 g of waterWhat is the boiling point of the solution
1027degC185
If the boiling point elevation of an aqueous solution containing a nonvolatile electrolyte is 102degC what is the molality of the solution
20 m186
The boiling point of an aqueous solution containing a nonvolatile electrolyte is 10075degC aWhat is the boiling-point elevation bWhat is the molality of the solution a 075degC b 15 m
187
Electrolytes and Colligative PropertiesEarly investigators confused by experiments where certain substances depressed freezing point or elevated boiling point more than expected
188
189
Ex 01 m solution of NaCl lowers freezing point of solvent almost twice as much as 01 m solution of sucrose 01 m CaCl2 lowers fp almost 3 times as much as 01 m solution of sucrose
To understand why this happens contrast behavior or sucrose with NaCl in aqueous solution Each sucrose dissolves to make only 1 particle in solution So 1 mol sucrose dissolves to 1 mol particles 1 mol NaCl dissolves to make 2 moles of particles (1 mol Na+ and 1 mol Cl-)
190
191
Calculated Values for Electrolyte SolutionsColligative properties depend on total concentration of solute particles regardless of their identity Electrolytes cause changes in colligative properties proportional to total molality in terms of all dissolves particles instead of formula units
192
What about barium nitrate Ba(NO3)2 Each mole of barium nitrate yields 3 mol of ions in solution Ba(NO3)2(s) rarr Ba2+(aq) + 2NO3-(aq) You would expect solution of given molality to lower fp of solvent 3 times as much as nonelectrolytic solution of same molality
193
Sample Problem
What is the expected change in the freezing point of water in a solution of 625 g of barium nitrate Ba(NO3)2 in 100 kg of water
194
1 AnalyzeGiven solute mass and formula = 625 g Ba(NO3)2 solvent mass and identity = 100 kg water Δtf = Kfm
Unknown expected freezing-point depression
195
2 PlanThe molality can be calculated by converting the solute mass to moles and then dividing by the number of kilograms of solvent That molality is in terms of formula units of Ba(NO3)2 and must be converted to molality in terms of dissociated ions in solution It must be multiplied by the number of moles of ions produced per mole of formula unit This adjusted molality can then be used to calculate the freezing-point depression
196
197
198
3 Compute
Practice Problem
What is the expected freezing-point depression for a solution that contains 20 mol of magnesium sulfate dissolved in 10 kg of water
minus74degC199
What is the expected boiling-point elevation of water for a solution that contains 150 g of sodium chloride dissolved in 10 kg of water 27degC The freezing point of an aqueous sodium chloride solution is minus200degC What is the molality of the solution 54 m NaCl
200
Actual Values for Electrolyte SolutionsRemember values calculated are only expected values Actual values of colligative properties for all strong electrolytes are almost what would be expected Difference between expected and actual values caused by attractive forces that exist between dissociated ions in aqueous solution
201
Attraction between hydrated ions in solution is small compared with those in crystalline solid However forces of attraction donrsquot interfere with movements of aqueous ions More concentrated solution means ions closer together and attraction is greater Also depends on charge of ions ndash higher charge greater attraction
202
3 Computea possible double-replacement reaction
b rules show that zinc sulfide is not soluble so will precipitate (ammonium nitrate is soluble)
c formula equation128
d overall ionic equation
e net ionic equation129
Will a precipitate form if solutions of potassium sulfate and barium nitrate are combined If so write the net ionic equation for the reaction
Yes Ba+2(aq) + SO4-2(aq) rarr BaSO4(s)
130
Will a precipitate form if solutions of potassium nitrate and magnesium sulfate are combined If so write the net ionic equation for the reaction
No 131
Will a precipitate form if solutions of barium chloride and sodium sulfate are combined If so identify the spectator ions and write the net ionic equation for the reaction
Yes Na+ and Cl- Ba+2(aq) + SO4-2(aq) rarr BaSO4(s)
132
Write the net ionic equation for the precipitation of nickel(II) sulfide
Ni+2(aq) + S-2(aq) rarr NiS(s)
133
IonizationSome molecular compounds can also form ions in solution Usually polar Ionization - ions formed from solute molecules by action of the solvent (creation of ions where there were none) Ionization different from dissociation
134
Dissociation ionic compounds dissolve and ions already present separate Ionization ions formed where non existed before
Ions formed are hydrated Heat released during hydration of ions gives enough energy to break covalent bonds
135
Extent to which solute ionizes depends on strength of bonds within molecules of solute and strength of attraction between solute and solvent molecules
136
HCl ndash molecular compound that ionizes in aqueous solution Contains highly polar bond Attraction between polar HCl molecule and polar water strong enough to break HCl bond
137
The Hydronium Ion
H+ ions from HCl attracts other molecules or ions so strongly that it doesnrsquot normally exist alone Ionization of HCl better described as direct transfer of proton from HCl to H2O forming H3O+
138
139
Hydration of H+ to form hydronium is highly exothermic
Energy released gives a lot of energy needed to ionize a molecular solute
Many molecular compounds that ionize in aqueous solution contain hydrogen and form hydronium ion
140
Strong and Weak Electrolytes
Substances that make ions and conduct current in solution are electrolytes HCl is one of series of compounds that has hydrogen and halogen Hydrogen halides are all molecular compounds with single polar-covalent bonds
141
All are gases Very soluble in water All are electrolytes HCl HBr HI strongly conduct current in solution HF only weakly conducts current at same concentration Strength of conduction related to ability to ionize
142
143
Strong Electrolytes
HCl HBr HI are 100 ionized in solution
Strong electrolyte - any compound whose dilute aqueous solutions conduct electricity well this is due to the presence of all or almost all of the dissolve compound in the form of ions
144
Unique characteristic of strong electrolyte is that they yield only ions
Ex Ionic compound may be highly soluble in water and dissociate into ions (NaCl)
Others may not dissolve much but amount that does dissolve exists only as ions
145
Weak ElectrolytesHF dissolves in water to give acid solution called hydrofluoric acid HF bond stronger than bonds between hydrogen and other halogens When HF dissolves some molecule ionize Reverse reaction also happens HF(aq) + H2O(l) H3O+(aq) + F-(aq)
146
HF(aq) + H2O(l) H3O+
(aq) + F-(aq)Concentration of dissolved unionized HF stays high and concentration of ions stays low
Weak electrolyte - any compound whose aqueous solutions conduct electricity poorly this is due to the presence of small amount of the dissolved compound in the form of ions
147
Different from nonelectrolyte ndash NO ions at all Description of electrolyte as strong or weak not related to concentration of solution Electrolytes differ in degree of ionization not amount of solute dissolved
148
149
Colligative properties of solutions
Presence of solute affects properties of solutions Some properties not dependent on nature of dissolved substance but on how may dissolve particles are present
Colligative properties - properties that depend on the concentration of solute particles but not on their identity
150
Vapor-Pressure Lowering
Boiling and freezing point of solution different from pure solvent Nonvolatile solute raises boiling point and lowers freezing point Nonvolatile substance - one that has little tendency to become gas under existing conditions
151
152
To understand why nonvolatile solute changes boiling and freezing point consider equilibrium vapor pressure
Vapor pressure pressure caused by molecules that have escape liquid phase to gas phase
Can be thought of a measure of tendency of molecules to escape from a liquid
153
Addition of sucrose (nonvolatile solute) lowers concentration of water molecules at surface of liquid This lowers tendency of water molecules to leave solution and enter gas phase Vapor pressure of solution is lower than vapor pressure of pure water
154
Aqueous13 solution13 of13 nonvolatile13 solutePure13 water
Nonelectrolyte solutions of same molality have same concentration of particles
Dilute solutions of same solvent and equal molality of any nonelectrolyte solute lower vapor pressure equally
155
Example
1 m aqueous solution of nonelectrolyte glucose lowers vapor pressure of water 55 x 10-4 atm at 25 1 m aqueous solution of sucrose also lowers water vapor pressure to 55 x 10-4 atm at 25
156
157
bc vapor-pressure lowering depends on concentration of nonelectrolyte solute and doesnrsquot depend on the type of solute it is a colligative proprerty
bc vapor pressure has been lowered solution remains liquid over larger temperature range
Lowers freezing point and raises boiling point
158
Can assume changes in boiling and freezing point also depend on concentration of solute
They are colligative properties
159
Freezing-Point Depression
Freezing point of 1-molal solution of any nonelectrolyte solute in water is found (by experiment) to be 186 lower than freezing point of water When 1 mol of nonelectrolyte solute dissolved in 1 kg water freezing point is -186 not 000 160
When 2 mol nonelectrolyte solute dissolved in 1 kg water freezing point is -372
For any concentration of nonelectrolyte solute in water decrease in freezing point can be determined using value of -186m
161
This value called the molal freezing-point contstant (Kf) is the freezing-point depression of the solvent in a 1-molal solution of a nonvolatile nonelectrolyte solute
162
Each solvent has own characteristic molal freezing-point constant Freezing-point depression (Δtf) the difference between the freezing points of the pure solvent and a solute of nonelectrolyte in that solvent and it is directly proportional to the molal concentration of the solution Δtf = Kfm
163
Δtf = Kfm
Kf expressed as m
m expressed in mol solutekg solvent (molality)
164
Sample Problem
What is the freezing point depression of water in a solution of 171 g of sucrose C12H22O11 and 200 g of water What is the actual freezing point of the solution
165
1 AnalyzeGiven Solute mass and chemical formula 171 g C12H22O11 Solvent mass and identity 2000 g water Unknown a freezing-point depression b freezing point of the solution 166
2 Plan
167
168
Calculate
Unknown a freezing-point depression b freezing point of the solution
169
Practice Problem
A water solution containing an unknown quantity of a nonelectrolyte solute is found to have a freezing point of ndash 023degCWhat is the molal concentration of the solution
012 m170
A solution consists of 103 g of the nonelectrolyte glucose C6H12O6 dissolved in 250 g of water What is the freezing-point depression of the solution
minus0426degC171
In a laboratory experiment the freezing point of an aqueous solution of glucose is found to be minus0325degCWhat is the molal concentration of this solution
0175 m172
If 0500 mol of a nonelectrolyte solute are dissolved in 5000 g of ether what is the freezing point of the solution
minus1181degC173
The freezing point of an aqueous solution that contains a nonelectrolyte is minus90degC aWhat is the freezing-point depression of the solution bWhat is the molal concentration of the solution a minus90degC b 48 m
174
Boiling-Point Elevation
Boiling point is temp at which vapor pressure is equal to atmospheric pressure Change in vapor pressure will cause corresponding change in boiling point
175
176
Vapor pressure of nonvolatile solution is lower than vapor pressure of pure solvent More heat will be required to raise vapor pressure of solution Boiling point of solution is higher than boiling point of pure solvent
Molal boiling-point constant (Kb) - boiling-point elevation of the solvent in a 1-molal solution of a nonvolatile nonelectrolyte solute b-p elevation of any nonelectrolyte in water found by experiment to be 051 So molal b-p constant for water is 051m
177
For different solvents b-p elevations of 1-molal solutions have different values (Table 14-2)
Like freezing-point constants values most accurate for dilute solutions
178
Boiling-point elevation Δtb difference between the boiling points of the pure solvent and a nonelectrolyte solution of that solvent and it is directly proportional to the molal concentration of the solution B-p elevation can be calculated using Δtb = Kbm Δtb (boiling point elevation) expressed in m and m expressed in mol solutekg solvent
Sample Problem
What is the boiling-point elevation of a solution made from 200 g of a nonelectrolyte solute and 4000 g of water The molar mass of the solute is 620 gmol
180
1 AnalyzeGiven solute mass = 200 g solute molar mass = 620 gmol solvent mass and identity = 4000 g of water
Unknown boiling-point elevation
181
2 Plan
182
183
3 Compute
Practice Problem
A solution contains 500 g of sucrose C12H22O11 a nonelectrolyte dissolved in 5000 g of water What is the boiling-point elevation
015degC184
A solution contains 4500 g of sucrose C12H22O11 a nonelectrolyte dissolved in 250 g of waterWhat is the boiling point of the solution
1027degC185
If the boiling point elevation of an aqueous solution containing a nonvolatile electrolyte is 102degC what is the molality of the solution
20 m186
The boiling point of an aqueous solution containing a nonvolatile electrolyte is 10075degC aWhat is the boiling-point elevation bWhat is the molality of the solution a 075degC b 15 m
187
Electrolytes and Colligative PropertiesEarly investigators confused by experiments where certain substances depressed freezing point or elevated boiling point more than expected
188
189
Ex 01 m solution of NaCl lowers freezing point of solvent almost twice as much as 01 m solution of sucrose 01 m CaCl2 lowers fp almost 3 times as much as 01 m solution of sucrose
To understand why this happens contrast behavior or sucrose with NaCl in aqueous solution Each sucrose dissolves to make only 1 particle in solution So 1 mol sucrose dissolves to 1 mol particles 1 mol NaCl dissolves to make 2 moles of particles (1 mol Na+ and 1 mol Cl-)
190
191
Calculated Values for Electrolyte SolutionsColligative properties depend on total concentration of solute particles regardless of their identity Electrolytes cause changes in colligative properties proportional to total molality in terms of all dissolves particles instead of formula units
192
What about barium nitrate Ba(NO3)2 Each mole of barium nitrate yields 3 mol of ions in solution Ba(NO3)2(s) rarr Ba2+(aq) + 2NO3-(aq) You would expect solution of given molality to lower fp of solvent 3 times as much as nonelectrolytic solution of same molality
193
Sample Problem
What is the expected change in the freezing point of water in a solution of 625 g of barium nitrate Ba(NO3)2 in 100 kg of water
194
1 AnalyzeGiven solute mass and formula = 625 g Ba(NO3)2 solvent mass and identity = 100 kg water Δtf = Kfm
Unknown expected freezing-point depression
195
2 PlanThe molality can be calculated by converting the solute mass to moles and then dividing by the number of kilograms of solvent That molality is in terms of formula units of Ba(NO3)2 and must be converted to molality in terms of dissociated ions in solution It must be multiplied by the number of moles of ions produced per mole of formula unit This adjusted molality can then be used to calculate the freezing-point depression
196
197
198
3 Compute
Practice Problem
What is the expected freezing-point depression for a solution that contains 20 mol of magnesium sulfate dissolved in 10 kg of water
minus74degC199
What is the expected boiling-point elevation of water for a solution that contains 150 g of sodium chloride dissolved in 10 kg of water 27degC The freezing point of an aqueous sodium chloride solution is minus200degC What is the molality of the solution 54 m NaCl
200
Actual Values for Electrolyte SolutionsRemember values calculated are only expected values Actual values of colligative properties for all strong electrolytes are almost what would be expected Difference between expected and actual values caused by attractive forces that exist between dissociated ions in aqueous solution
201
Attraction between hydrated ions in solution is small compared with those in crystalline solid However forces of attraction donrsquot interfere with movements of aqueous ions More concentrated solution means ions closer together and attraction is greater Also depends on charge of ions ndash higher charge greater attraction
202
d overall ionic equation
e net ionic equation129
Will a precipitate form if solutions of potassium sulfate and barium nitrate are combined If so write the net ionic equation for the reaction
Yes Ba+2(aq) + SO4-2(aq) rarr BaSO4(s)
130
Will a precipitate form if solutions of potassium nitrate and magnesium sulfate are combined If so write the net ionic equation for the reaction
No 131
Will a precipitate form if solutions of barium chloride and sodium sulfate are combined If so identify the spectator ions and write the net ionic equation for the reaction
Yes Na+ and Cl- Ba+2(aq) + SO4-2(aq) rarr BaSO4(s)
132
Write the net ionic equation for the precipitation of nickel(II) sulfide
Ni+2(aq) + S-2(aq) rarr NiS(s)
133
IonizationSome molecular compounds can also form ions in solution Usually polar Ionization - ions formed from solute molecules by action of the solvent (creation of ions where there were none) Ionization different from dissociation
134
Dissociation ionic compounds dissolve and ions already present separate Ionization ions formed where non existed before
Ions formed are hydrated Heat released during hydration of ions gives enough energy to break covalent bonds
135
Extent to which solute ionizes depends on strength of bonds within molecules of solute and strength of attraction between solute and solvent molecules
136
HCl ndash molecular compound that ionizes in aqueous solution Contains highly polar bond Attraction between polar HCl molecule and polar water strong enough to break HCl bond
137
The Hydronium Ion
H+ ions from HCl attracts other molecules or ions so strongly that it doesnrsquot normally exist alone Ionization of HCl better described as direct transfer of proton from HCl to H2O forming H3O+
138
139
Hydration of H+ to form hydronium is highly exothermic
Energy released gives a lot of energy needed to ionize a molecular solute
Many molecular compounds that ionize in aqueous solution contain hydrogen and form hydronium ion
140
Strong and Weak Electrolytes
Substances that make ions and conduct current in solution are electrolytes HCl is one of series of compounds that has hydrogen and halogen Hydrogen halides are all molecular compounds with single polar-covalent bonds
141
All are gases Very soluble in water All are electrolytes HCl HBr HI strongly conduct current in solution HF only weakly conducts current at same concentration Strength of conduction related to ability to ionize
142
143
Strong Electrolytes
HCl HBr HI are 100 ionized in solution
Strong electrolyte - any compound whose dilute aqueous solutions conduct electricity well this is due to the presence of all or almost all of the dissolve compound in the form of ions
144
Unique characteristic of strong electrolyte is that they yield only ions
Ex Ionic compound may be highly soluble in water and dissociate into ions (NaCl)
Others may not dissolve much but amount that does dissolve exists only as ions
145
Weak ElectrolytesHF dissolves in water to give acid solution called hydrofluoric acid HF bond stronger than bonds between hydrogen and other halogens When HF dissolves some molecule ionize Reverse reaction also happens HF(aq) + H2O(l) H3O+(aq) + F-(aq)
146
HF(aq) + H2O(l) H3O+
(aq) + F-(aq)Concentration of dissolved unionized HF stays high and concentration of ions stays low
Weak electrolyte - any compound whose aqueous solutions conduct electricity poorly this is due to the presence of small amount of the dissolved compound in the form of ions
147
Different from nonelectrolyte ndash NO ions at all Description of electrolyte as strong or weak not related to concentration of solution Electrolytes differ in degree of ionization not amount of solute dissolved
148
149
Colligative properties of solutions
Presence of solute affects properties of solutions Some properties not dependent on nature of dissolved substance but on how may dissolve particles are present
Colligative properties - properties that depend on the concentration of solute particles but not on their identity
150
Vapor-Pressure Lowering
Boiling and freezing point of solution different from pure solvent Nonvolatile solute raises boiling point and lowers freezing point Nonvolatile substance - one that has little tendency to become gas under existing conditions
151
152
To understand why nonvolatile solute changes boiling and freezing point consider equilibrium vapor pressure
Vapor pressure pressure caused by molecules that have escape liquid phase to gas phase
Can be thought of a measure of tendency of molecules to escape from a liquid
153
Addition of sucrose (nonvolatile solute) lowers concentration of water molecules at surface of liquid This lowers tendency of water molecules to leave solution and enter gas phase Vapor pressure of solution is lower than vapor pressure of pure water
154
Aqueous13 solution13 of13 nonvolatile13 solutePure13 water
Nonelectrolyte solutions of same molality have same concentration of particles
Dilute solutions of same solvent and equal molality of any nonelectrolyte solute lower vapor pressure equally
155
Example
1 m aqueous solution of nonelectrolyte glucose lowers vapor pressure of water 55 x 10-4 atm at 25 1 m aqueous solution of sucrose also lowers water vapor pressure to 55 x 10-4 atm at 25
156
157
bc vapor-pressure lowering depends on concentration of nonelectrolyte solute and doesnrsquot depend on the type of solute it is a colligative proprerty
bc vapor pressure has been lowered solution remains liquid over larger temperature range
Lowers freezing point and raises boiling point
158
Can assume changes in boiling and freezing point also depend on concentration of solute
They are colligative properties
159
Freezing-Point Depression
Freezing point of 1-molal solution of any nonelectrolyte solute in water is found (by experiment) to be 186 lower than freezing point of water When 1 mol of nonelectrolyte solute dissolved in 1 kg water freezing point is -186 not 000 160
When 2 mol nonelectrolyte solute dissolved in 1 kg water freezing point is -372
For any concentration of nonelectrolyte solute in water decrease in freezing point can be determined using value of -186m
161
This value called the molal freezing-point contstant (Kf) is the freezing-point depression of the solvent in a 1-molal solution of a nonvolatile nonelectrolyte solute
162
Each solvent has own characteristic molal freezing-point constant Freezing-point depression (Δtf) the difference between the freezing points of the pure solvent and a solute of nonelectrolyte in that solvent and it is directly proportional to the molal concentration of the solution Δtf = Kfm
163
Δtf = Kfm
Kf expressed as m
m expressed in mol solutekg solvent (molality)
164
Sample Problem
What is the freezing point depression of water in a solution of 171 g of sucrose C12H22O11 and 200 g of water What is the actual freezing point of the solution
165
1 AnalyzeGiven Solute mass and chemical formula 171 g C12H22O11 Solvent mass and identity 2000 g water Unknown a freezing-point depression b freezing point of the solution 166
2 Plan
167
168
Calculate
Unknown a freezing-point depression b freezing point of the solution
169
Practice Problem
A water solution containing an unknown quantity of a nonelectrolyte solute is found to have a freezing point of ndash 023degCWhat is the molal concentration of the solution
012 m170
A solution consists of 103 g of the nonelectrolyte glucose C6H12O6 dissolved in 250 g of water What is the freezing-point depression of the solution
minus0426degC171
In a laboratory experiment the freezing point of an aqueous solution of glucose is found to be minus0325degCWhat is the molal concentration of this solution
0175 m172
If 0500 mol of a nonelectrolyte solute are dissolved in 5000 g of ether what is the freezing point of the solution
minus1181degC173
The freezing point of an aqueous solution that contains a nonelectrolyte is minus90degC aWhat is the freezing-point depression of the solution bWhat is the molal concentration of the solution a minus90degC b 48 m
174
Boiling-Point Elevation
Boiling point is temp at which vapor pressure is equal to atmospheric pressure Change in vapor pressure will cause corresponding change in boiling point
175
176
Vapor pressure of nonvolatile solution is lower than vapor pressure of pure solvent More heat will be required to raise vapor pressure of solution Boiling point of solution is higher than boiling point of pure solvent
Molal boiling-point constant (Kb) - boiling-point elevation of the solvent in a 1-molal solution of a nonvolatile nonelectrolyte solute b-p elevation of any nonelectrolyte in water found by experiment to be 051 So molal b-p constant for water is 051m
177
For different solvents b-p elevations of 1-molal solutions have different values (Table 14-2)
Like freezing-point constants values most accurate for dilute solutions
178
Boiling-point elevation Δtb difference between the boiling points of the pure solvent and a nonelectrolyte solution of that solvent and it is directly proportional to the molal concentration of the solution B-p elevation can be calculated using Δtb = Kbm Δtb (boiling point elevation) expressed in m and m expressed in mol solutekg solvent
Sample Problem
What is the boiling-point elevation of a solution made from 200 g of a nonelectrolyte solute and 4000 g of water The molar mass of the solute is 620 gmol
180
1 AnalyzeGiven solute mass = 200 g solute molar mass = 620 gmol solvent mass and identity = 4000 g of water
Unknown boiling-point elevation
181
2 Plan
182
183
3 Compute
Practice Problem
A solution contains 500 g of sucrose C12H22O11 a nonelectrolyte dissolved in 5000 g of water What is the boiling-point elevation
015degC184
A solution contains 4500 g of sucrose C12H22O11 a nonelectrolyte dissolved in 250 g of waterWhat is the boiling point of the solution
1027degC185
If the boiling point elevation of an aqueous solution containing a nonvolatile electrolyte is 102degC what is the molality of the solution
20 m186
The boiling point of an aqueous solution containing a nonvolatile electrolyte is 10075degC aWhat is the boiling-point elevation bWhat is the molality of the solution a 075degC b 15 m
187
Electrolytes and Colligative PropertiesEarly investigators confused by experiments where certain substances depressed freezing point or elevated boiling point more than expected
188
189
Ex 01 m solution of NaCl lowers freezing point of solvent almost twice as much as 01 m solution of sucrose 01 m CaCl2 lowers fp almost 3 times as much as 01 m solution of sucrose
To understand why this happens contrast behavior or sucrose with NaCl in aqueous solution Each sucrose dissolves to make only 1 particle in solution So 1 mol sucrose dissolves to 1 mol particles 1 mol NaCl dissolves to make 2 moles of particles (1 mol Na+ and 1 mol Cl-)
190
191
Calculated Values for Electrolyte SolutionsColligative properties depend on total concentration of solute particles regardless of their identity Electrolytes cause changes in colligative properties proportional to total molality in terms of all dissolves particles instead of formula units
192
What about barium nitrate Ba(NO3)2 Each mole of barium nitrate yields 3 mol of ions in solution Ba(NO3)2(s) rarr Ba2+(aq) + 2NO3-(aq) You would expect solution of given molality to lower fp of solvent 3 times as much as nonelectrolytic solution of same molality
193
Sample Problem
What is the expected change in the freezing point of water in a solution of 625 g of barium nitrate Ba(NO3)2 in 100 kg of water
194
1 AnalyzeGiven solute mass and formula = 625 g Ba(NO3)2 solvent mass and identity = 100 kg water Δtf = Kfm
Unknown expected freezing-point depression
195
2 PlanThe molality can be calculated by converting the solute mass to moles and then dividing by the number of kilograms of solvent That molality is in terms of formula units of Ba(NO3)2 and must be converted to molality in terms of dissociated ions in solution It must be multiplied by the number of moles of ions produced per mole of formula unit This adjusted molality can then be used to calculate the freezing-point depression
196
197
198
3 Compute
Practice Problem
What is the expected freezing-point depression for a solution that contains 20 mol of magnesium sulfate dissolved in 10 kg of water
minus74degC199
What is the expected boiling-point elevation of water for a solution that contains 150 g of sodium chloride dissolved in 10 kg of water 27degC The freezing point of an aqueous sodium chloride solution is minus200degC What is the molality of the solution 54 m NaCl
200
Actual Values for Electrolyte SolutionsRemember values calculated are only expected values Actual values of colligative properties for all strong electrolytes are almost what would be expected Difference between expected and actual values caused by attractive forces that exist between dissociated ions in aqueous solution
201
Attraction between hydrated ions in solution is small compared with those in crystalline solid However forces of attraction donrsquot interfere with movements of aqueous ions More concentrated solution means ions closer together and attraction is greater Also depends on charge of ions ndash higher charge greater attraction
202
Will a precipitate form if solutions of potassium sulfate and barium nitrate are combined If so write the net ionic equation for the reaction
Yes Ba+2(aq) + SO4-2(aq) rarr BaSO4(s)
130
Will a precipitate form if solutions of potassium nitrate and magnesium sulfate are combined If so write the net ionic equation for the reaction
No 131
Will a precipitate form if solutions of barium chloride and sodium sulfate are combined If so identify the spectator ions and write the net ionic equation for the reaction
Yes Na+ and Cl- Ba+2(aq) + SO4-2(aq) rarr BaSO4(s)
132
Write the net ionic equation for the precipitation of nickel(II) sulfide
Ni+2(aq) + S-2(aq) rarr NiS(s)
133
IonizationSome molecular compounds can also form ions in solution Usually polar Ionization - ions formed from solute molecules by action of the solvent (creation of ions where there were none) Ionization different from dissociation
134
Dissociation ionic compounds dissolve and ions already present separate Ionization ions formed where non existed before
Ions formed are hydrated Heat released during hydration of ions gives enough energy to break covalent bonds
135
Extent to which solute ionizes depends on strength of bonds within molecules of solute and strength of attraction between solute and solvent molecules
136
HCl ndash molecular compound that ionizes in aqueous solution Contains highly polar bond Attraction between polar HCl molecule and polar water strong enough to break HCl bond
137
The Hydronium Ion
H+ ions from HCl attracts other molecules or ions so strongly that it doesnrsquot normally exist alone Ionization of HCl better described as direct transfer of proton from HCl to H2O forming H3O+
138
139
Hydration of H+ to form hydronium is highly exothermic
Energy released gives a lot of energy needed to ionize a molecular solute
Many molecular compounds that ionize in aqueous solution contain hydrogen and form hydronium ion
140
Strong and Weak Electrolytes
Substances that make ions and conduct current in solution are electrolytes HCl is one of series of compounds that has hydrogen and halogen Hydrogen halides are all molecular compounds with single polar-covalent bonds
141
All are gases Very soluble in water All are electrolytes HCl HBr HI strongly conduct current in solution HF only weakly conducts current at same concentration Strength of conduction related to ability to ionize
142
143
Strong Electrolytes
HCl HBr HI are 100 ionized in solution
Strong electrolyte - any compound whose dilute aqueous solutions conduct electricity well this is due to the presence of all or almost all of the dissolve compound in the form of ions
144
Unique characteristic of strong electrolyte is that they yield only ions
Ex Ionic compound may be highly soluble in water and dissociate into ions (NaCl)
Others may not dissolve much but amount that does dissolve exists only as ions
145
Weak ElectrolytesHF dissolves in water to give acid solution called hydrofluoric acid HF bond stronger than bonds between hydrogen and other halogens When HF dissolves some molecule ionize Reverse reaction also happens HF(aq) + H2O(l) H3O+(aq) + F-(aq)
146
HF(aq) + H2O(l) H3O+
(aq) + F-(aq)Concentration of dissolved unionized HF stays high and concentration of ions stays low
Weak electrolyte - any compound whose aqueous solutions conduct electricity poorly this is due to the presence of small amount of the dissolved compound in the form of ions
147
Different from nonelectrolyte ndash NO ions at all Description of electrolyte as strong or weak not related to concentration of solution Electrolytes differ in degree of ionization not amount of solute dissolved
148
149
Colligative properties of solutions
Presence of solute affects properties of solutions Some properties not dependent on nature of dissolved substance but on how may dissolve particles are present
Colligative properties - properties that depend on the concentration of solute particles but not on their identity
150
Vapor-Pressure Lowering
Boiling and freezing point of solution different from pure solvent Nonvolatile solute raises boiling point and lowers freezing point Nonvolatile substance - one that has little tendency to become gas under existing conditions
151
152
To understand why nonvolatile solute changes boiling and freezing point consider equilibrium vapor pressure
Vapor pressure pressure caused by molecules that have escape liquid phase to gas phase
Can be thought of a measure of tendency of molecules to escape from a liquid
153
Addition of sucrose (nonvolatile solute) lowers concentration of water molecules at surface of liquid This lowers tendency of water molecules to leave solution and enter gas phase Vapor pressure of solution is lower than vapor pressure of pure water
154
Aqueous13 solution13 of13 nonvolatile13 solutePure13 water
Nonelectrolyte solutions of same molality have same concentration of particles
Dilute solutions of same solvent and equal molality of any nonelectrolyte solute lower vapor pressure equally
155
Example
1 m aqueous solution of nonelectrolyte glucose lowers vapor pressure of water 55 x 10-4 atm at 25 1 m aqueous solution of sucrose also lowers water vapor pressure to 55 x 10-4 atm at 25
156
157
bc vapor-pressure lowering depends on concentration of nonelectrolyte solute and doesnrsquot depend on the type of solute it is a colligative proprerty
bc vapor pressure has been lowered solution remains liquid over larger temperature range
Lowers freezing point and raises boiling point
158
Can assume changes in boiling and freezing point also depend on concentration of solute
They are colligative properties
159
Freezing-Point Depression
Freezing point of 1-molal solution of any nonelectrolyte solute in water is found (by experiment) to be 186 lower than freezing point of water When 1 mol of nonelectrolyte solute dissolved in 1 kg water freezing point is -186 not 000 160
When 2 mol nonelectrolyte solute dissolved in 1 kg water freezing point is -372
For any concentration of nonelectrolyte solute in water decrease in freezing point can be determined using value of -186m
161
This value called the molal freezing-point contstant (Kf) is the freezing-point depression of the solvent in a 1-molal solution of a nonvolatile nonelectrolyte solute
162
Each solvent has own characteristic molal freezing-point constant Freezing-point depression (Δtf) the difference between the freezing points of the pure solvent and a solute of nonelectrolyte in that solvent and it is directly proportional to the molal concentration of the solution Δtf = Kfm
163
Δtf = Kfm
Kf expressed as m
m expressed in mol solutekg solvent (molality)
164
Sample Problem
What is the freezing point depression of water in a solution of 171 g of sucrose C12H22O11 and 200 g of water What is the actual freezing point of the solution
165
1 AnalyzeGiven Solute mass and chemical formula 171 g C12H22O11 Solvent mass and identity 2000 g water Unknown a freezing-point depression b freezing point of the solution 166
2 Plan
167
168
Calculate
Unknown a freezing-point depression b freezing point of the solution
169
Practice Problem
A water solution containing an unknown quantity of a nonelectrolyte solute is found to have a freezing point of ndash 023degCWhat is the molal concentration of the solution
012 m170
A solution consists of 103 g of the nonelectrolyte glucose C6H12O6 dissolved in 250 g of water What is the freezing-point depression of the solution
minus0426degC171
In a laboratory experiment the freezing point of an aqueous solution of glucose is found to be minus0325degCWhat is the molal concentration of this solution
0175 m172
If 0500 mol of a nonelectrolyte solute are dissolved in 5000 g of ether what is the freezing point of the solution
minus1181degC173
The freezing point of an aqueous solution that contains a nonelectrolyte is minus90degC aWhat is the freezing-point depression of the solution bWhat is the molal concentration of the solution a minus90degC b 48 m
174
Boiling-Point Elevation
Boiling point is temp at which vapor pressure is equal to atmospheric pressure Change in vapor pressure will cause corresponding change in boiling point
175
176
Vapor pressure of nonvolatile solution is lower than vapor pressure of pure solvent More heat will be required to raise vapor pressure of solution Boiling point of solution is higher than boiling point of pure solvent
Molal boiling-point constant (Kb) - boiling-point elevation of the solvent in a 1-molal solution of a nonvolatile nonelectrolyte solute b-p elevation of any nonelectrolyte in water found by experiment to be 051 So molal b-p constant for water is 051m
177
For different solvents b-p elevations of 1-molal solutions have different values (Table 14-2)
Like freezing-point constants values most accurate for dilute solutions
178
Boiling-point elevation Δtb difference between the boiling points of the pure solvent and a nonelectrolyte solution of that solvent and it is directly proportional to the molal concentration of the solution B-p elevation can be calculated using Δtb = Kbm Δtb (boiling point elevation) expressed in m and m expressed in mol solutekg solvent
Sample Problem
What is the boiling-point elevation of a solution made from 200 g of a nonelectrolyte solute and 4000 g of water The molar mass of the solute is 620 gmol
180
1 AnalyzeGiven solute mass = 200 g solute molar mass = 620 gmol solvent mass and identity = 4000 g of water
Unknown boiling-point elevation
181
2 Plan
182
183
3 Compute
Practice Problem
A solution contains 500 g of sucrose C12H22O11 a nonelectrolyte dissolved in 5000 g of water What is the boiling-point elevation
015degC184
A solution contains 4500 g of sucrose C12H22O11 a nonelectrolyte dissolved in 250 g of waterWhat is the boiling point of the solution
1027degC185
If the boiling point elevation of an aqueous solution containing a nonvolatile electrolyte is 102degC what is the molality of the solution
20 m186
The boiling point of an aqueous solution containing a nonvolatile electrolyte is 10075degC aWhat is the boiling-point elevation bWhat is the molality of the solution a 075degC b 15 m
187
Electrolytes and Colligative PropertiesEarly investigators confused by experiments where certain substances depressed freezing point or elevated boiling point more than expected
188
189
Ex 01 m solution of NaCl lowers freezing point of solvent almost twice as much as 01 m solution of sucrose 01 m CaCl2 lowers fp almost 3 times as much as 01 m solution of sucrose
To understand why this happens contrast behavior or sucrose with NaCl in aqueous solution Each sucrose dissolves to make only 1 particle in solution So 1 mol sucrose dissolves to 1 mol particles 1 mol NaCl dissolves to make 2 moles of particles (1 mol Na+ and 1 mol Cl-)
190
191
Calculated Values for Electrolyte SolutionsColligative properties depend on total concentration of solute particles regardless of their identity Electrolytes cause changes in colligative properties proportional to total molality in terms of all dissolves particles instead of formula units
192
What about barium nitrate Ba(NO3)2 Each mole of barium nitrate yields 3 mol of ions in solution Ba(NO3)2(s) rarr Ba2+(aq) + 2NO3-(aq) You would expect solution of given molality to lower fp of solvent 3 times as much as nonelectrolytic solution of same molality
193
Sample Problem
What is the expected change in the freezing point of water in a solution of 625 g of barium nitrate Ba(NO3)2 in 100 kg of water
194
1 AnalyzeGiven solute mass and formula = 625 g Ba(NO3)2 solvent mass and identity = 100 kg water Δtf = Kfm
Unknown expected freezing-point depression
195
2 PlanThe molality can be calculated by converting the solute mass to moles and then dividing by the number of kilograms of solvent That molality is in terms of formula units of Ba(NO3)2 and must be converted to molality in terms of dissociated ions in solution It must be multiplied by the number of moles of ions produced per mole of formula unit This adjusted molality can then be used to calculate the freezing-point depression
196
197
198
3 Compute
Practice Problem
What is the expected freezing-point depression for a solution that contains 20 mol of magnesium sulfate dissolved in 10 kg of water
minus74degC199
What is the expected boiling-point elevation of water for a solution that contains 150 g of sodium chloride dissolved in 10 kg of water 27degC The freezing point of an aqueous sodium chloride solution is minus200degC What is the molality of the solution 54 m NaCl
200
Actual Values for Electrolyte SolutionsRemember values calculated are only expected values Actual values of colligative properties for all strong electrolytes are almost what would be expected Difference between expected and actual values caused by attractive forces that exist between dissociated ions in aqueous solution
201
Attraction between hydrated ions in solution is small compared with those in crystalline solid However forces of attraction donrsquot interfere with movements of aqueous ions More concentrated solution means ions closer together and attraction is greater Also depends on charge of ions ndash higher charge greater attraction
202
Will a precipitate form if solutions of potassium nitrate and magnesium sulfate are combined If so write the net ionic equation for the reaction
No 131
Will a precipitate form if solutions of barium chloride and sodium sulfate are combined If so identify the spectator ions and write the net ionic equation for the reaction
Yes Na+ and Cl- Ba+2(aq) + SO4-2(aq) rarr BaSO4(s)
132
Write the net ionic equation for the precipitation of nickel(II) sulfide
Ni+2(aq) + S-2(aq) rarr NiS(s)
133
IonizationSome molecular compounds can also form ions in solution Usually polar Ionization - ions formed from solute molecules by action of the solvent (creation of ions where there were none) Ionization different from dissociation
134
Dissociation ionic compounds dissolve and ions already present separate Ionization ions formed where non existed before
Ions formed are hydrated Heat released during hydration of ions gives enough energy to break covalent bonds
135
Extent to which solute ionizes depends on strength of bonds within molecules of solute and strength of attraction between solute and solvent molecules
136
HCl ndash molecular compound that ionizes in aqueous solution Contains highly polar bond Attraction between polar HCl molecule and polar water strong enough to break HCl bond
137
The Hydronium Ion
H+ ions from HCl attracts other molecules or ions so strongly that it doesnrsquot normally exist alone Ionization of HCl better described as direct transfer of proton from HCl to H2O forming H3O+
138
139
Hydration of H+ to form hydronium is highly exothermic
Energy released gives a lot of energy needed to ionize a molecular solute
Many molecular compounds that ionize in aqueous solution contain hydrogen and form hydronium ion
140
Strong and Weak Electrolytes
Substances that make ions and conduct current in solution are electrolytes HCl is one of series of compounds that has hydrogen and halogen Hydrogen halides are all molecular compounds with single polar-covalent bonds
141
All are gases Very soluble in water All are electrolytes HCl HBr HI strongly conduct current in solution HF only weakly conducts current at same concentration Strength of conduction related to ability to ionize
142
143
Strong Electrolytes
HCl HBr HI are 100 ionized in solution
Strong electrolyte - any compound whose dilute aqueous solutions conduct electricity well this is due to the presence of all or almost all of the dissolve compound in the form of ions
144
Unique characteristic of strong electrolyte is that they yield only ions
Ex Ionic compound may be highly soluble in water and dissociate into ions (NaCl)
Others may not dissolve much but amount that does dissolve exists only as ions
145
Weak ElectrolytesHF dissolves in water to give acid solution called hydrofluoric acid HF bond stronger than bonds between hydrogen and other halogens When HF dissolves some molecule ionize Reverse reaction also happens HF(aq) + H2O(l) H3O+(aq) + F-(aq)
146
HF(aq) + H2O(l) H3O+
(aq) + F-(aq)Concentration of dissolved unionized HF stays high and concentration of ions stays low
Weak electrolyte - any compound whose aqueous solutions conduct electricity poorly this is due to the presence of small amount of the dissolved compound in the form of ions
147
Different from nonelectrolyte ndash NO ions at all Description of electrolyte as strong or weak not related to concentration of solution Electrolytes differ in degree of ionization not amount of solute dissolved
148
149
Colligative properties of solutions
Presence of solute affects properties of solutions Some properties not dependent on nature of dissolved substance but on how may dissolve particles are present
Colligative properties - properties that depend on the concentration of solute particles but not on their identity
150
Vapor-Pressure Lowering
Boiling and freezing point of solution different from pure solvent Nonvolatile solute raises boiling point and lowers freezing point Nonvolatile substance - one that has little tendency to become gas under existing conditions
151
152
To understand why nonvolatile solute changes boiling and freezing point consider equilibrium vapor pressure
Vapor pressure pressure caused by molecules that have escape liquid phase to gas phase
Can be thought of a measure of tendency of molecules to escape from a liquid
153
Addition of sucrose (nonvolatile solute) lowers concentration of water molecules at surface of liquid This lowers tendency of water molecules to leave solution and enter gas phase Vapor pressure of solution is lower than vapor pressure of pure water
154
Aqueous13 solution13 of13 nonvolatile13 solutePure13 water
Nonelectrolyte solutions of same molality have same concentration of particles
Dilute solutions of same solvent and equal molality of any nonelectrolyte solute lower vapor pressure equally
155
Example
1 m aqueous solution of nonelectrolyte glucose lowers vapor pressure of water 55 x 10-4 atm at 25 1 m aqueous solution of sucrose also lowers water vapor pressure to 55 x 10-4 atm at 25
156
157
bc vapor-pressure lowering depends on concentration of nonelectrolyte solute and doesnrsquot depend on the type of solute it is a colligative proprerty
bc vapor pressure has been lowered solution remains liquid over larger temperature range
Lowers freezing point and raises boiling point
158
Can assume changes in boiling and freezing point also depend on concentration of solute
They are colligative properties
159
Freezing-Point Depression
Freezing point of 1-molal solution of any nonelectrolyte solute in water is found (by experiment) to be 186 lower than freezing point of water When 1 mol of nonelectrolyte solute dissolved in 1 kg water freezing point is -186 not 000 160
When 2 mol nonelectrolyte solute dissolved in 1 kg water freezing point is -372
For any concentration of nonelectrolyte solute in water decrease in freezing point can be determined using value of -186m
161
This value called the molal freezing-point contstant (Kf) is the freezing-point depression of the solvent in a 1-molal solution of a nonvolatile nonelectrolyte solute
162
Each solvent has own characteristic molal freezing-point constant Freezing-point depression (Δtf) the difference between the freezing points of the pure solvent and a solute of nonelectrolyte in that solvent and it is directly proportional to the molal concentration of the solution Δtf = Kfm
163
Δtf = Kfm
Kf expressed as m
m expressed in mol solutekg solvent (molality)
164
Sample Problem
What is the freezing point depression of water in a solution of 171 g of sucrose C12H22O11 and 200 g of water What is the actual freezing point of the solution
165
1 AnalyzeGiven Solute mass and chemical formula 171 g C12H22O11 Solvent mass and identity 2000 g water Unknown a freezing-point depression b freezing point of the solution 166
2 Plan
167
168
Calculate
Unknown a freezing-point depression b freezing point of the solution
169
Practice Problem
A water solution containing an unknown quantity of a nonelectrolyte solute is found to have a freezing point of ndash 023degCWhat is the molal concentration of the solution
012 m170
A solution consists of 103 g of the nonelectrolyte glucose C6H12O6 dissolved in 250 g of water What is the freezing-point depression of the solution
minus0426degC171
In a laboratory experiment the freezing point of an aqueous solution of glucose is found to be minus0325degCWhat is the molal concentration of this solution
0175 m172
If 0500 mol of a nonelectrolyte solute are dissolved in 5000 g of ether what is the freezing point of the solution
minus1181degC173
The freezing point of an aqueous solution that contains a nonelectrolyte is minus90degC aWhat is the freezing-point depression of the solution bWhat is the molal concentration of the solution a minus90degC b 48 m
174
Boiling-Point Elevation
Boiling point is temp at which vapor pressure is equal to atmospheric pressure Change in vapor pressure will cause corresponding change in boiling point
175
176
Vapor pressure of nonvolatile solution is lower than vapor pressure of pure solvent More heat will be required to raise vapor pressure of solution Boiling point of solution is higher than boiling point of pure solvent
Molal boiling-point constant (Kb) - boiling-point elevation of the solvent in a 1-molal solution of a nonvolatile nonelectrolyte solute b-p elevation of any nonelectrolyte in water found by experiment to be 051 So molal b-p constant for water is 051m
177
For different solvents b-p elevations of 1-molal solutions have different values (Table 14-2)
Like freezing-point constants values most accurate for dilute solutions
178
Boiling-point elevation Δtb difference between the boiling points of the pure solvent and a nonelectrolyte solution of that solvent and it is directly proportional to the molal concentration of the solution B-p elevation can be calculated using Δtb = Kbm Δtb (boiling point elevation) expressed in m and m expressed in mol solutekg solvent
Sample Problem
What is the boiling-point elevation of a solution made from 200 g of a nonelectrolyte solute and 4000 g of water The molar mass of the solute is 620 gmol
180
1 AnalyzeGiven solute mass = 200 g solute molar mass = 620 gmol solvent mass and identity = 4000 g of water
Unknown boiling-point elevation
181
2 Plan
182
183
3 Compute
Practice Problem
A solution contains 500 g of sucrose C12H22O11 a nonelectrolyte dissolved in 5000 g of water What is the boiling-point elevation
015degC184
A solution contains 4500 g of sucrose C12H22O11 a nonelectrolyte dissolved in 250 g of waterWhat is the boiling point of the solution
1027degC185
If the boiling point elevation of an aqueous solution containing a nonvolatile electrolyte is 102degC what is the molality of the solution
20 m186
The boiling point of an aqueous solution containing a nonvolatile electrolyte is 10075degC aWhat is the boiling-point elevation bWhat is the molality of the solution a 075degC b 15 m
187
Electrolytes and Colligative PropertiesEarly investigators confused by experiments where certain substances depressed freezing point or elevated boiling point more than expected
188
189
Ex 01 m solution of NaCl lowers freezing point of solvent almost twice as much as 01 m solution of sucrose 01 m CaCl2 lowers fp almost 3 times as much as 01 m solution of sucrose
To understand why this happens contrast behavior or sucrose with NaCl in aqueous solution Each sucrose dissolves to make only 1 particle in solution So 1 mol sucrose dissolves to 1 mol particles 1 mol NaCl dissolves to make 2 moles of particles (1 mol Na+ and 1 mol Cl-)
190
191
Calculated Values for Electrolyte SolutionsColligative properties depend on total concentration of solute particles regardless of their identity Electrolytes cause changes in colligative properties proportional to total molality in terms of all dissolves particles instead of formula units
192
What about barium nitrate Ba(NO3)2 Each mole of barium nitrate yields 3 mol of ions in solution Ba(NO3)2(s) rarr Ba2+(aq) + 2NO3-(aq) You would expect solution of given molality to lower fp of solvent 3 times as much as nonelectrolytic solution of same molality
193
Sample Problem
What is the expected change in the freezing point of water in a solution of 625 g of barium nitrate Ba(NO3)2 in 100 kg of water
194
1 AnalyzeGiven solute mass and formula = 625 g Ba(NO3)2 solvent mass and identity = 100 kg water Δtf = Kfm
Unknown expected freezing-point depression
195
2 PlanThe molality can be calculated by converting the solute mass to moles and then dividing by the number of kilograms of solvent That molality is in terms of formula units of Ba(NO3)2 and must be converted to molality in terms of dissociated ions in solution It must be multiplied by the number of moles of ions produced per mole of formula unit This adjusted molality can then be used to calculate the freezing-point depression
196
197
198
3 Compute
Practice Problem
What is the expected freezing-point depression for a solution that contains 20 mol of magnesium sulfate dissolved in 10 kg of water
minus74degC199
What is the expected boiling-point elevation of water for a solution that contains 150 g of sodium chloride dissolved in 10 kg of water 27degC The freezing point of an aqueous sodium chloride solution is minus200degC What is the molality of the solution 54 m NaCl
200
Actual Values for Electrolyte SolutionsRemember values calculated are only expected values Actual values of colligative properties for all strong electrolytes are almost what would be expected Difference between expected and actual values caused by attractive forces that exist between dissociated ions in aqueous solution
201
Attraction between hydrated ions in solution is small compared with those in crystalline solid However forces of attraction donrsquot interfere with movements of aqueous ions More concentrated solution means ions closer together and attraction is greater Also depends on charge of ions ndash higher charge greater attraction
202
Will a precipitate form if solutions of barium chloride and sodium sulfate are combined If so identify the spectator ions and write the net ionic equation for the reaction
Yes Na+ and Cl- Ba+2(aq) + SO4-2(aq) rarr BaSO4(s)
132
Write the net ionic equation for the precipitation of nickel(II) sulfide
Ni+2(aq) + S-2(aq) rarr NiS(s)
133
IonizationSome molecular compounds can also form ions in solution Usually polar Ionization - ions formed from solute molecules by action of the solvent (creation of ions where there were none) Ionization different from dissociation
134
Dissociation ionic compounds dissolve and ions already present separate Ionization ions formed where non existed before
Ions formed are hydrated Heat released during hydration of ions gives enough energy to break covalent bonds
135
Extent to which solute ionizes depends on strength of bonds within molecules of solute and strength of attraction between solute and solvent molecules
136
HCl ndash molecular compound that ionizes in aqueous solution Contains highly polar bond Attraction between polar HCl molecule and polar water strong enough to break HCl bond
137
The Hydronium Ion
H+ ions from HCl attracts other molecules or ions so strongly that it doesnrsquot normally exist alone Ionization of HCl better described as direct transfer of proton from HCl to H2O forming H3O+
138
139
Hydration of H+ to form hydronium is highly exothermic
Energy released gives a lot of energy needed to ionize a molecular solute
Many molecular compounds that ionize in aqueous solution contain hydrogen and form hydronium ion
140
Strong and Weak Electrolytes
Substances that make ions and conduct current in solution are electrolytes HCl is one of series of compounds that has hydrogen and halogen Hydrogen halides are all molecular compounds with single polar-covalent bonds
141
All are gases Very soluble in water All are electrolytes HCl HBr HI strongly conduct current in solution HF only weakly conducts current at same concentration Strength of conduction related to ability to ionize
142
143
Strong Electrolytes
HCl HBr HI are 100 ionized in solution
Strong electrolyte - any compound whose dilute aqueous solutions conduct electricity well this is due to the presence of all or almost all of the dissolve compound in the form of ions
144
Unique characteristic of strong electrolyte is that they yield only ions
Ex Ionic compound may be highly soluble in water and dissociate into ions (NaCl)
Others may not dissolve much but amount that does dissolve exists only as ions
145
Weak ElectrolytesHF dissolves in water to give acid solution called hydrofluoric acid HF bond stronger than bonds between hydrogen and other halogens When HF dissolves some molecule ionize Reverse reaction also happens HF(aq) + H2O(l) H3O+(aq) + F-(aq)
146
HF(aq) + H2O(l) H3O+
(aq) + F-(aq)Concentration of dissolved unionized HF stays high and concentration of ions stays low
Weak electrolyte - any compound whose aqueous solutions conduct electricity poorly this is due to the presence of small amount of the dissolved compound in the form of ions
147
Different from nonelectrolyte ndash NO ions at all Description of electrolyte as strong or weak not related to concentration of solution Electrolytes differ in degree of ionization not amount of solute dissolved
148
149
Colligative properties of solutions
Presence of solute affects properties of solutions Some properties not dependent on nature of dissolved substance but on how may dissolve particles are present
Colligative properties - properties that depend on the concentration of solute particles but not on their identity
150
Vapor-Pressure Lowering
Boiling and freezing point of solution different from pure solvent Nonvolatile solute raises boiling point and lowers freezing point Nonvolatile substance - one that has little tendency to become gas under existing conditions
151
152
To understand why nonvolatile solute changes boiling and freezing point consider equilibrium vapor pressure
Vapor pressure pressure caused by molecules that have escape liquid phase to gas phase
Can be thought of a measure of tendency of molecules to escape from a liquid
153
Addition of sucrose (nonvolatile solute) lowers concentration of water molecules at surface of liquid This lowers tendency of water molecules to leave solution and enter gas phase Vapor pressure of solution is lower than vapor pressure of pure water
154
Aqueous13 solution13 of13 nonvolatile13 solutePure13 water
Nonelectrolyte solutions of same molality have same concentration of particles
Dilute solutions of same solvent and equal molality of any nonelectrolyte solute lower vapor pressure equally
155
Example
1 m aqueous solution of nonelectrolyte glucose lowers vapor pressure of water 55 x 10-4 atm at 25 1 m aqueous solution of sucrose also lowers water vapor pressure to 55 x 10-4 atm at 25
156
157
bc vapor-pressure lowering depends on concentration of nonelectrolyte solute and doesnrsquot depend on the type of solute it is a colligative proprerty
bc vapor pressure has been lowered solution remains liquid over larger temperature range
Lowers freezing point and raises boiling point
158
Can assume changes in boiling and freezing point also depend on concentration of solute
They are colligative properties
159
Freezing-Point Depression
Freezing point of 1-molal solution of any nonelectrolyte solute in water is found (by experiment) to be 186 lower than freezing point of water When 1 mol of nonelectrolyte solute dissolved in 1 kg water freezing point is -186 not 000 160
When 2 mol nonelectrolyte solute dissolved in 1 kg water freezing point is -372
For any concentration of nonelectrolyte solute in water decrease in freezing point can be determined using value of -186m
161
This value called the molal freezing-point contstant (Kf) is the freezing-point depression of the solvent in a 1-molal solution of a nonvolatile nonelectrolyte solute
162
Each solvent has own characteristic molal freezing-point constant Freezing-point depression (Δtf) the difference between the freezing points of the pure solvent and a solute of nonelectrolyte in that solvent and it is directly proportional to the molal concentration of the solution Δtf = Kfm
163
Δtf = Kfm
Kf expressed as m
m expressed in mol solutekg solvent (molality)
164
Sample Problem
What is the freezing point depression of water in a solution of 171 g of sucrose C12H22O11 and 200 g of water What is the actual freezing point of the solution
165
1 AnalyzeGiven Solute mass and chemical formula 171 g C12H22O11 Solvent mass and identity 2000 g water Unknown a freezing-point depression b freezing point of the solution 166
2 Plan
167
168
Calculate
Unknown a freezing-point depression b freezing point of the solution
169
Practice Problem
A water solution containing an unknown quantity of a nonelectrolyte solute is found to have a freezing point of ndash 023degCWhat is the molal concentration of the solution
012 m170
A solution consists of 103 g of the nonelectrolyte glucose C6H12O6 dissolved in 250 g of water What is the freezing-point depression of the solution
minus0426degC171
In a laboratory experiment the freezing point of an aqueous solution of glucose is found to be minus0325degCWhat is the molal concentration of this solution
0175 m172
If 0500 mol of a nonelectrolyte solute are dissolved in 5000 g of ether what is the freezing point of the solution
minus1181degC173
The freezing point of an aqueous solution that contains a nonelectrolyte is minus90degC aWhat is the freezing-point depression of the solution bWhat is the molal concentration of the solution a minus90degC b 48 m
174
Boiling-Point Elevation
Boiling point is temp at which vapor pressure is equal to atmospheric pressure Change in vapor pressure will cause corresponding change in boiling point
175
176
Vapor pressure of nonvolatile solution is lower than vapor pressure of pure solvent More heat will be required to raise vapor pressure of solution Boiling point of solution is higher than boiling point of pure solvent
Molal boiling-point constant (Kb) - boiling-point elevation of the solvent in a 1-molal solution of a nonvolatile nonelectrolyte solute b-p elevation of any nonelectrolyte in water found by experiment to be 051 So molal b-p constant for water is 051m
177
For different solvents b-p elevations of 1-molal solutions have different values (Table 14-2)
Like freezing-point constants values most accurate for dilute solutions
178
Boiling-point elevation Δtb difference between the boiling points of the pure solvent and a nonelectrolyte solution of that solvent and it is directly proportional to the molal concentration of the solution B-p elevation can be calculated using Δtb = Kbm Δtb (boiling point elevation) expressed in m and m expressed in mol solutekg solvent
Sample Problem
What is the boiling-point elevation of a solution made from 200 g of a nonelectrolyte solute and 4000 g of water The molar mass of the solute is 620 gmol
180
1 AnalyzeGiven solute mass = 200 g solute molar mass = 620 gmol solvent mass and identity = 4000 g of water
Unknown boiling-point elevation
181
2 Plan
182
183
3 Compute
Practice Problem
A solution contains 500 g of sucrose C12H22O11 a nonelectrolyte dissolved in 5000 g of water What is the boiling-point elevation
015degC184
A solution contains 4500 g of sucrose C12H22O11 a nonelectrolyte dissolved in 250 g of waterWhat is the boiling point of the solution
1027degC185
If the boiling point elevation of an aqueous solution containing a nonvolatile electrolyte is 102degC what is the molality of the solution
20 m186
The boiling point of an aqueous solution containing a nonvolatile electrolyte is 10075degC aWhat is the boiling-point elevation bWhat is the molality of the solution a 075degC b 15 m
187
Electrolytes and Colligative PropertiesEarly investigators confused by experiments where certain substances depressed freezing point or elevated boiling point more than expected
188
189
Ex 01 m solution of NaCl lowers freezing point of solvent almost twice as much as 01 m solution of sucrose 01 m CaCl2 lowers fp almost 3 times as much as 01 m solution of sucrose
To understand why this happens contrast behavior or sucrose with NaCl in aqueous solution Each sucrose dissolves to make only 1 particle in solution So 1 mol sucrose dissolves to 1 mol particles 1 mol NaCl dissolves to make 2 moles of particles (1 mol Na+ and 1 mol Cl-)
190
191
Calculated Values for Electrolyte SolutionsColligative properties depend on total concentration of solute particles regardless of their identity Electrolytes cause changes in colligative properties proportional to total molality in terms of all dissolves particles instead of formula units
192
What about barium nitrate Ba(NO3)2 Each mole of barium nitrate yields 3 mol of ions in solution Ba(NO3)2(s) rarr Ba2+(aq) + 2NO3-(aq) You would expect solution of given molality to lower fp of solvent 3 times as much as nonelectrolytic solution of same molality
193
Sample Problem
What is the expected change in the freezing point of water in a solution of 625 g of barium nitrate Ba(NO3)2 in 100 kg of water
194
1 AnalyzeGiven solute mass and formula = 625 g Ba(NO3)2 solvent mass and identity = 100 kg water Δtf = Kfm
Unknown expected freezing-point depression
195
2 PlanThe molality can be calculated by converting the solute mass to moles and then dividing by the number of kilograms of solvent That molality is in terms of formula units of Ba(NO3)2 and must be converted to molality in terms of dissociated ions in solution It must be multiplied by the number of moles of ions produced per mole of formula unit This adjusted molality can then be used to calculate the freezing-point depression
196
197
198
3 Compute
Practice Problem
What is the expected freezing-point depression for a solution that contains 20 mol of magnesium sulfate dissolved in 10 kg of water
minus74degC199
What is the expected boiling-point elevation of water for a solution that contains 150 g of sodium chloride dissolved in 10 kg of water 27degC The freezing point of an aqueous sodium chloride solution is minus200degC What is the molality of the solution 54 m NaCl
200
Actual Values for Electrolyte SolutionsRemember values calculated are only expected values Actual values of colligative properties for all strong electrolytes are almost what would be expected Difference between expected and actual values caused by attractive forces that exist between dissociated ions in aqueous solution
201
Attraction between hydrated ions in solution is small compared with those in crystalline solid However forces of attraction donrsquot interfere with movements of aqueous ions More concentrated solution means ions closer together and attraction is greater Also depends on charge of ions ndash higher charge greater attraction
202
Write the net ionic equation for the precipitation of nickel(II) sulfide
Ni+2(aq) + S-2(aq) rarr NiS(s)
133
IonizationSome molecular compounds can also form ions in solution Usually polar Ionization - ions formed from solute molecules by action of the solvent (creation of ions where there were none) Ionization different from dissociation
134
Dissociation ionic compounds dissolve and ions already present separate Ionization ions formed where non existed before
Ions formed are hydrated Heat released during hydration of ions gives enough energy to break covalent bonds
135
Extent to which solute ionizes depends on strength of bonds within molecules of solute and strength of attraction between solute and solvent molecules
136
HCl ndash molecular compound that ionizes in aqueous solution Contains highly polar bond Attraction between polar HCl molecule and polar water strong enough to break HCl bond
137
The Hydronium Ion
H+ ions from HCl attracts other molecules or ions so strongly that it doesnrsquot normally exist alone Ionization of HCl better described as direct transfer of proton from HCl to H2O forming H3O+
138
139
Hydration of H+ to form hydronium is highly exothermic
Energy released gives a lot of energy needed to ionize a molecular solute
Many molecular compounds that ionize in aqueous solution contain hydrogen and form hydronium ion
140
Strong and Weak Electrolytes
Substances that make ions and conduct current in solution are electrolytes HCl is one of series of compounds that has hydrogen and halogen Hydrogen halides are all molecular compounds with single polar-covalent bonds
141
All are gases Very soluble in water All are electrolytes HCl HBr HI strongly conduct current in solution HF only weakly conducts current at same concentration Strength of conduction related to ability to ionize
142
143
Strong Electrolytes
HCl HBr HI are 100 ionized in solution
Strong electrolyte - any compound whose dilute aqueous solutions conduct electricity well this is due to the presence of all or almost all of the dissolve compound in the form of ions
144
Unique characteristic of strong electrolyte is that they yield only ions
Ex Ionic compound may be highly soluble in water and dissociate into ions (NaCl)
Others may not dissolve much but amount that does dissolve exists only as ions
145
Weak ElectrolytesHF dissolves in water to give acid solution called hydrofluoric acid HF bond stronger than bonds between hydrogen and other halogens When HF dissolves some molecule ionize Reverse reaction also happens HF(aq) + H2O(l) H3O+(aq) + F-(aq)
146
HF(aq) + H2O(l) H3O+
(aq) + F-(aq)Concentration of dissolved unionized HF stays high and concentration of ions stays low
Weak electrolyte - any compound whose aqueous solutions conduct electricity poorly this is due to the presence of small amount of the dissolved compound in the form of ions
147
Different from nonelectrolyte ndash NO ions at all Description of electrolyte as strong or weak not related to concentration of solution Electrolytes differ in degree of ionization not amount of solute dissolved
148
149
Colligative properties of solutions
Presence of solute affects properties of solutions Some properties not dependent on nature of dissolved substance but on how may dissolve particles are present
Colligative properties - properties that depend on the concentration of solute particles but not on their identity
150
Vapor-Pressure Lowering
Boiling and freezing point of solution different from pure solvent Nonvolatile solute raises boiling point and lowers freezing point Nonvolatile substance - one that has little tendency to become gas under existing conditions
151
152
To understand why nonvolatile solute changes boiling and freezing point consider equilibrium vapor pressure
Vapor pressure pressure caused by molecules that have escape liquid phase to gas phase
Can be thought of a measure of tendency of molecules to escape from a liquid
153
Addition of sucrose (nonvolatile solute) lowers concentration of water molecules at surface of liquid This lowers tendency of water molecules to leave solution and enter gas phase Vapor pressure of solution is lower than vapor pressure of pure water
154
Aqueous13 solution13 of13 nonvolatile13 solutePure13 water
Nonelectrolyte solutions of same molality have same concentration of particles
Dilute solutions of same solvent and equal molality of any nonelectrolyte solute lower vapor pressure equally
155
Example
1 m aqueous solution of nonelectrolyte glucose lowers vapor pressure of water 55 x 10-4 atm at 25 1 m aqueous solution of sucrose also lowers water vapor pressure to 55 x 10-4 atm at 25
156
157
bc vapor-pressure lowering depends on concentration of nonelectrolyte solute and doesnrsquot depend on the type of solute it is a colligative proprerty
bc vapor pressure has been lowered solution remains liquid over larger temperature range
Lowers freezing point and raises boiling point
158
Can assume changes in boiling and freezing point also depend on concentration of solute
They are colligative properties
159
Freezing-Point Depression
Freezing point of 1-molal solution of any nonelectrolyte solute in water is found (by experiment) to be 186 lower than freezing point of water When 1 mol of nonelectrolyte solute dissolved in 1 kg water freezing point is -186 not 000 160
When 2 mol nonelectrolyte solute dissolved in 1 kg water freezing point is -372
For any concentration of nonelectrolyte solute in water decrease in freezing point can be determined using value of -186m
161
This value called the molal freezing-point contstant (Kf) is the freezing-point depression of the solvent in a 1-molal solution of a nonvolatile nonelectrolyte solute
162
Each solvent has own characteristic molal freezing-point constant Freezing-point depression (Δtf) the difference between the freezing points of the pure solvent and a solute of nonelectrolyte in that solvent and it is directly proportional to the molal concentration of the solution Δtf = Kfm
163
Δtf = Kfm
Kf expressed as m
m expressed in mol solutekg solvent (molality)
164
Sample Problem
What is the freezing point depression of water in a solution of 171 g of sucrose C12H22O11 and 200 g of water What is the actual freezing point of the solution
165
1 AnalyzeGiven Solute mass and chemical formula 171 g C12H22O11 Solvent mass and identity 2000 g water Unknown a freezing-point depression b freezing point of the solution 166
2 Plan
167
168
Calculate
Unknown a freezing-point depression b freezing point of the solution
169
Practice Problem
A water solution containing an unknown quantity of a nonelectrolyte solute is found to have a freezing point of ndash 023degCWhat is the molal concentration of the solution
012 m170
A solution consists of 103 g of the nonelectrolyte glucose C6H12O6 dissolved in 250 g of water What is the freezing-point depression of the solution
minus0426degC171
In a laboratory experiment the freezing point of an aqueous solution of glucose is found to be minus0325degCWhat is the molal concentration of this solution
0175 m172
If 0500 mol of a nonelectrolyte solute are dissolved in 5000 g of ether what is the freezing point of the solution
minus1181degC173
The freezing point of an aqueous solution that contains a nonelectrolyte is minus90degC aWhat is the freezing-point depression of the solution bWhat is the molal concentration of the solution a minus90degC b 48 m
174
Boiling-Point Elevation
Boiling point is temp at which vapor pressure is equal to atmospheric pressure Change in vapor pressure will cause corresponding change in boiling point
175
176
Vapor pressure of nonvolatile solution is lower than vapor pressure of pure solvent More heat will be required to raise vapor pressure of solution Boiling point of solution is higher than boiling point of pure solvent
Molal boiling-point constant (Kb) - boiling-point elevation of the solvent in a 1-molal solution of a nonvolatile nonelectrolyte solute b-p elevation of any nonelectrolyte in water found by experiment to be 051 So molal b-p constant for water is 051m
177
For different solvents b-p elevations of 1-molal solutions have different values (Table 14-2)
Like freezing-point constants values most accurate for dilute solutions
178
Boiling-point elevation Δtb difference between the boiling points of the pure solvent and a nonelectrolyte solution of that solvent and it is directly proportional to the molal concentration of the solution B-p elevation can be calculated using Δtb = Kbm Δtb (boiling point elevation) expressed in m and m expressed in mol solutekg solvent
Sample Problem
What is the boiling-point elevation of a solution made from 200 g of a nonelectrolyte solute and 4000 g of water The molar mass of the solute is 620 gmol
180
1 AnalyzeGiven solute mass = 200 g solute molar mass = 620 gmol solvent mass and identity = 4000 g of water
Unknown boiling-point elevation
181
2 Plan
182
183
3 Compute
Practice Problem
A solution contains 500 g of sucrose C12H22O11 a nonelectrolyte dissolved in 5000 g of water What is the boiling-point elevation
015degC184
A solution contains 4500 g of sucrose C12H22O11 a nonelectrolyte dissolved in 250 g of waterWhat is the boiling point of the solution
1027degC185
If the boiling point elevation of an aqueous solution containing a nonvolatile electrolyte is 102degC what is the molality of the solution
20 m186
The boiling point of an aqueous solution containing a nonvolatile electrolyte is 10075degC aWhat is the boiling-point elevation bWhat is the molality of the solution a 075degC b 15 m
187
Electrolytes and Colligative PropertiesEarly investigators confused by experiments where certain substances depressed freezing point or elevated boiling point more than expected
188
189
Ex 01 m solution of NaCl lowers freezing point of solvent almost twice as much as 01 m solution of sucrose 01 m CaCl2 lowers fp almost 3 times as much as 01 m solution of sucrose
To understand why this happens contrast behavior or sucrose with NaCl in aqueous solution Each sucrose dissolves to make only 1 particle in solution So 1 mol sucrose dissolves to 1 mol particles 1 mol NaCl dissolves to make 2 moles of particles (1 mol Na+ and 1 mol Cl-)
190
191
Calculated Values for Electrolyte SolutionsColligative properties depend on total concentration of solute particles regardless of their identity Electrolytes cause changes in colligative properties proportional to total molality in terms of all dissolves particles instead of formula units
192
What about barium nitrate Ba(NO3)2 Each mole of barium nitrate yields 3 mol of ions in solution Ba(NO3)2(s) rarr Ba2+(aq) + 2NO3-(aq) You would expect solution of given molality to lower fp of solvent 3 times as much as nonelectrolytic solution of same molality
193
Sample Problem
What is the expected change in the freezing point of water in a solution of 625 g of barium nitrate Ba(NO3)2 in 100 kg of water
194
1 AnalyzeGiven solute mass and formula = 625 g Ba(NO3)2 solvent mass and identity = 100 kg water Δtf = Kfm
Unknown expected freezing-point depression
195
2 PlanThe molality can be calculated by converting the solute mass to moles and then dividing by the number of kilograms of solvent That molality is in terms of formula units of Ba(NO3)2 and must be converted to molality in terms of dissociated ions in solution It must be multiplied by the number of moles of ions produced per mole of formula unit This adjusted molality can then be used to calculate the freezing-point depression
196
197
198
3 Compute
Practice Problem
What is the expected freezing-point depression for a solution that contains 20 mol of magnesium sulfate dissolved in 10 kg of water
minus74degC199
What is the expected boiling-point elevation of water for a solution that contains 150 g of sodium chloride dissolved in 10 kg of water 27degC The freezing point of an aqueous sodium chloride solution is minus200degC What is the molality of the solution 54 m NaCl
200
Actual Values for Electrolyte SolutionsRemember values calculated are only expected values Actual values of colligative properties for all strong electrolytes are almost what would be expected Difference between expected and actual values caused by attractive forces that exist between dissociated ions in aqueous solution
201
Attraction between hydrated ions in solution is small compared with those in crystalline solid However forces of attraction donrsquot interfere with movements of aqueous ions More concentrated solution means ions closer together and attraction is greater Also depends on charge of ions ndash higher charge greater attraction
202
IonizationSome molecular compounds can also form ions in solution Usually polar Ionization - ions formed from solute molecules by action of the solvent (creation of ions where there were none) Ionization different from dissociation
134
Dissociation ionic compounds dissolve and ions already present separate Ionization ions formed where non existed before
Ions formed are hydrated Heat released during hydration of ions gives enough energy to break covalent bonds
135
Extent to which solute ionizes depends on strength of bonds within molecules of solute and strength of attraction between solute and solvent molecules
136
HCl ndash molecular compound that ionizes in aqueous solution Contains highly polar bond Attraction between polar HCl molecule and polar water strong enough to break HCl bond
137
The Hydronium Ion
H+ ions from HCl attracts other molecules or ions so strongly that it doesnrsquot normally exist alone Ionization of HCl better described as direct transfer of proton from HCl to H2O forming H3O+
138
139
Hydration of H+ to form hydronium is highly exothermic
Energy released gives a lot of energy needed to ionize a molecular solute
Many molecular compounds that ionize in aqueous solution contain hydrogen and form hydronium ion
140
Strong and Weak Electrolytes
Substances that make ions and conduct current in solution are electrolytes HCl is one of series of compounds that has hydrogen and halogen Hydrogen halides are all molecular compounds with single polar-covalent bonds
141
All are gases Very soluble in water All are electrolytes HCl HBr HI strongly conduct current in solution HF only weakly conducts current at same concentration Strength of conduction related to ability to ionize
142
143
Strong Electrolytes
HCl HBr HI are 100 ionized in solution
Strong electrolyte - any compound whose dilute aqueous solutions conduct electricity well this is due to the presence of all or almost all of the dissolve compound in the form of ions
144
Unique characteristic of strong electrolyte is that they yield only ions
Ex Ionic compound may be highly soluble in water and dissociate into ions (NaCl)
Others may not dissolve much but amount that does dissolve exists only as ions
145
Weak ElectrolytesHF dissolves in water to give acid solution called hydrofluoric acid HF bond stronger than bonds between hydrogen and other halogens When HF dissolves some molecule ionize Reverse reaction also happens HF(aq) + H2O(l) H3O+(aq) + F-(aq)
146
HF(aq) + H2O(l) H3O+
(aq) + F-(aq)Concentration of dissolved unionized HF stays high and concentration of ions stays low
Weak electrolyte - any compound whose aqueous solutions conduct electricity poorly this is due to the presence of small amount of the dissolved compound in the form of ions
147
Different from nonelectrolyte ndash NO ions at all Description of electrolyte as strong or weak not related to concentration of solution Electrolytes differ in degree of ionization not amount of solute dissolved
148
149
Colligative properties of solutions
Presence of solute affects properties of solutions Some properties not dependent on nature of dissolved substance but on how may dissolve particles are present
Colligative properties - properties that depend on the concentration of solute particles but not on their identity
150
Vapor-Pressure Lowering
Boiling and freezing point of solution different from pure solvent Nonvolatile solute raises boiling point and lowers freezing point Nonvolatile substance - one that has little tendency to become gas under existing conditions
151
152
To understand why nonvolatile solute changes boiling and freezing point consider equilibrium vapor pressure
Vapor pressure pressure caused by molecules that have escape liquid phase to gas phase
Can be thought of a measure of tendency of molecules to escape from a liquid
153
Addition of sucrose (nonvolatile solute) lowers concentration of water molecules at surface of liquid This lowers tendency of water molecules to leave solution and enter gas phase Vapor pressure of solution is lower than vapor pressure of pure water
154
Aqueous13 solution13 of13 nonvolatile13 solutePure13 water
Nonelectrolyte solutions of same molality have same concentration of particles
Dilute solutions of same solvent and equal molality of any nonelectrolyte solute lower vapor pressure equally
155
Example
1 m aqueous solution of nonelectrolyte glucose lowers vapor pressure of water 55 x 10-4 atm at 25 1 m aqueous solution of sucrose also lowers water vapor pressure to 55 x 10-4 atm at 25
156
157
bc vapor-pressure lowering depends on concentration of nonelectrolyte solute and doesnrsquot depend on the type of solute it is a colligative proprerty
bc vapor pressure has been lowered solution remains liquid over larger temperature range
Lowers freezing point and raises boiling point
158
Can assume changes in boiling and freezing point also depend on concentration of solute
They are colligative properties
159
Freezing-Point Depression
Freezing point of 1-molal solution of any nonelectrolyte solute in water is found (by experiment) to be 186 lower than freezing point of water When 1 mol of nonelectrolyte solute dissolved in 1 kg water freezing point is -186 not 000 160
When 2 mol nonelectrolyte solute dissolved in 1 kg water freezing point is -372
For any concentration of nonelectrolyte solute in water decrease in freezing point can be determined using value of -186m
161
This value called the molal freezing-point contstant (Kf) is the freezing-point depression of the solvent in a 1-molal solution of a nonvolatile nonelectrolyte solute
162
Each solvent has own characteristic molal freezing-point constant Freezing-point depression (Δtf) the difference between the freezing points of the pure solvent and a solute of nonelectrolyte in that solvent and it is directly proportional to the molal concentration of the solution Δtf = Kfm
163
Δtf = Kfm
Kf expressed as m
m expressed in mol solutekg solvent (molality)
164
Sample Problem
What is the freezing point depression of water in a solution of 171 g of sucrose C12H22O11 and 200 g of water What is the actual freezing point of the solution
165
1 AnalyzeGiven Solute mass and chemical formula 171 g C12H22O11 Solvent mass and identity 2000 g water Unknown a freezing-point depression b freezing point of the solution 166
2 Plan
167
168
Calculate
Unknown a freezing-point depression b freezing point of the solution
169
Practice Problem
A water solution containing an unknown quantity of a nonelectrolyte solute is found to have a freezing point of ndash 023degCWhat is the molal concentration of the solution
012 m170
A solution consists of 103 g of the nonelectrolyte glucose C6H12O6 dissolved in 250 g of water What is the freezing-point depression of the solution
minus0426degC171
In a laboratory experiment the freezing point of an aqueous solution of glucose is found to be minus0325degCWhat is the molal concentration of this solution
0175 m172
If 0500 mol of a nonelectrolyte solute are dissolved in 5000 g of ether what is the freezing point of the solution
minus1181degC173
The freezing point of an aqueous solution that contains a nonelectrolyte is minus90degC aWhat is the freezing-point depression of the solution bWhat is the molal concentration of the solution a minus90degC b 48 m
174
Boiling-Point Elevation
Boiling point is temp at which vapor pressure is equal to atmospheric pressure Change in vapor pressure will cause corresponding change in boiling point
175
176
Vapor pressure of nonvolatile solution is lower than vapor pressure of pure solvent More heat will be required to raise vapor pressure of solution Boiling point of solution is higher than boiling point of pure solvent
Molal boiling-point constant (Kb) - boiling-point elevation of the solvent in a 1-molal solution of a nonvolatile nonelectrolyte solute b-p elevation of any nonelectrolyte in water found by experiment to be 051 So molal b-p constant for water is 051m
177
For different solvents b-p elevations of 1-molal solutions have different values (Table 14-2)
Like freezing-point constants values most accurate for dilute solutions
178
Boiling-point elevation Δtb difference between the boiling points of the pure solvent and a nonelectrolyte solution of that solvent and it is directly proportional to the molal concentration of the solution B-p elevation can be calculated using Δtb = Kbm Δtb (boiling point elevation) expressed in m and m expressed in mol solutekg solvent
Sample Problem
What is the boiling-point elevation of a solution made from 200 g of a nonelectrolyte solute and 4000 g of water The molar mass of the solute is 620 gmol
180
1 AnalyzeGiven solute mass = 200 g solute molar mass = 620 gmol solvent mass and identity = 4000 g of water
Unknown boiling-point elevation
181
2 Plan
182
183
3 Compute
Practice Problem
A solution contains 500 g of sucrose C12H22O11 a nonelectrolyte dissolved in 5000 g of water What is the boiling-point elevation
015degC184
A solution contains 4500 g of sucrose C12H22O11 a nonelectrolyte dissolved in 250 g of waterWhat is the boiling point of the solution
1027degC185
If the boiling point elevation of an aqueous solution containing a nonvolatile electrolyte is 102degC what is the molality of the solution
20 m186
The boiling point of an aqueous solution containing a nonvolatile electrolyte is 10075degC aWhat is the boiling-point elevation bWhat is the molality of the solution a 075degC b 15 m
187
Electrolytes and Colligative PropertiesEarly investigators confused by experiments where certain substances depressed freezing point or elevated boiling point more than expected
188
189
Ex 01 m solution of NaCl lowers freezing point of solvent almost twice as much as 01 m solution of sucrose 01 m CaCl2 lowers fp almost 3 times as much as 01 m solution of sucrose
To understand why this happens contrast behavior or sucrose with NaCl in aqueous solution Each sucrose dissolves to make only 1 particle in solution So 1 mol sucrose dissolves to 1 mol particles 1 mol NaCl dissolves to make 2 moles of particles (1 mol Na+ and 1 mol Cl-)
190
191
Calculated Values for Electrolyte SolutionsColligative properties depend on total concentration of solute particles regardless of their identity Electrolytes cause changes in colligative properties proportional to total molality in terms of all dissolves particles instead of formula units
192
What about barium nitrate Ba(NO3)2 Each mole of barium nitrate yields 3 mol of ions in solution Ba(NO3)2(s) rarr Ba2+(aq) + 2NO3-(aq) You would expect solution of given molality to lower fp of solvent 3 times as much as nonelectrolytic solution of same molality
193
Sample Problem
What is the expected change in the freezing point of water in a solution of 625 g of barium nitrate Ba(NO3)2 in 100 kg of water
194
1 AnalyzeGiven solute mass and formula = 625 g Ba(NO3)2 solvent mass and identity = 100 kg water Δtf = Kfm
Unknown expected freezing-point depression
195
2 PlanThe molality can be calculated by converting the solute mass to moles and then dividing by the number of kilograms of solvent That molality is in terms of formula units of Ba(NO3)2 and must be converted to molality in terms of dissociated ions in solution It must be multiplied by the number of moles of ions produced per mole of formula unit This adjusted molality can then be used to calculate the freezing-point depression
196
197
198
3 Compute
Practice Problem
What is the expected freezing-point depression for a solution that contains 20 mol of magnesium sulfate dissolved in 10 kg of water
minus74degC199
What is the expected boiling-point elevation of water for a solution that contains 150 g of sodium chloride dissolved in 10 kg of water 27degC The freezing point of an aqueous sodium chloride solution is minus200degC What is the molality of the solution 54 m NaCl
200
Actual Values for Electrolyte SolutionsRemember values calculated are only expected values Actual values of colligative properties for all strong electrolytes are almost what would be expected Difference between expected and actual values caused by attractive forces that exist between dissociated ions in aqueous solution
201
Attraction between hydrated ions in solution is small compared with those in crystalline solid However forces of attraction donrsquot interfere with movements of aqueous ions More concentrated solution means ions closer together and attraction is greater Also depends on charge of ions ndash higher charge greater attraction
202
Dissociation ionic compounds dissolve and ions already present separate Ionization ions formed where non existed before
Ions formed are hydrated Heat released during hydration of ions gives enough energy to break covalent bonds
135
Extent to which solute ionizes depends on strength of bonds within molecules of solute and strength of attraction between solute and solvent molecules
136
HCl ndash molecular compound that ionizes in aqueous solution Contains highly polar bond Attraction between polar HCl molecule and polar water strong enough to break HCl bond
137
The Hydronium Ion
H+ ions from HCl attracts other molecules or ions so strongly that it doesnrsquot normally exist alone Ionization of HCl better described as direct transfer of proton from HCl to H2O forming H3O+
138
139
Hydration of H+ to form hydronium is highly exothermic
Energy released gives a lot of energy needed to ionize a molecular solute
Many molecular compounds that ionize in aqueous solution contain hydrogen and form hydronium ion
140
Strong and Weak Electrolytes
Substances that make ions and conduct current in solution are electrolytes HCl is one of series of compounds that has hydrogen and halogen Hydrogen halides are all molecular compounds with single polar-covalent bonds
141
All are gases Very soluble in water All are electrolytes HCl HBr HI strongly conduct current in solution HF only weakly conducts current at same concentration Strength of conduction related to ability to ionize
142
143
Strong Electrolytes
HCl HBr HI are 100 ionized in solution
Strong electrolyte - any compound whose dilute aqueous solutions conduct electricity well this is due to the presence of all or almost all of the dissolve compound in the form of ions
144
Unique characteristic of strong electrolyte is that they yield only ions
Ex Ionic compound may be highly soluble in water and dissociate into ions (NaCl)
Others may not dissolve much but amount that does dissolve exists only as ions
145
Weak ElectrolytesHF dissolves in water to give acid solution called hydrofluoric acid HF bond stronger than bonds between hydrogen and other halogens When HF dissolves some molecule ionize Reverse reaction also happens HF(aq) + H2O(l) H3O+(aq) + F-(aq)
146
HF(aq) + H2O(l) H3O+
(aq) + F-(aq)Concentration of dissolved unionized HF stays high and concentration of ions stays low
Weak electrolyte - any compound whose aqueous solutions conduct electricity poorly this is due to the presence of small amount of the dissolved compound in the form of ions
147
Different from nonelectrolyte ndash NO ions at all Description of electrolyte as strong or weak not related to concentration of solution Electrolytes differ in degree of ionization not amount of solute dissolved
148
149
Colligative properties of solutions
Presence of solute affects properties of solutions Some properties not dependent on nature of dissolved substance but on how may dissolve particles are present
Colligative properties - properties that depend on the concentration of solute particles but not on their identity
150
Vapor-Pressure Lowering
Boiling and freezing point of solution different from pure solvent Nonvolatile solute raises boiling point and lowers freezing point Nonvolatile substance - one that has little tendency to become gas under existing conditions
151
152
To understand why nonvolatile solute changes boiling and freezing point consider equilibrium vapor pressure
Vapor pressure pressure caused by molecules that have escape liquid phase to gas phase
Can be thought of a measure of tendency of molecules to escape from a liquid
153
Addition of sucrose (nonvolatile solute) lowers concentration of water molecules at surface of liquid This lowers tendency of water molecules to leave solution and enter gas phase Vapor pressure of solution is lower than vapor pressure of pure water
154
Aqueous13 solution13 of13 nonvolatile13 solutePure13 water
Nonelectrolyte solutions of same molality have same concentration of particles
Dilute solutions of same solvent and equal molality of any nonelectrolyte solute lower vapor pressure equally
155
Example
1 m aqueous solution of nonelectrolyte glucose lowers vapor pressure of water 55 x 10-4 atm at 25 1 m aqueous solution of sucrose also lowers water vapor pressure to 55 x 10-4 atm at 25
156
157
bc vapor-pressure lowering depends on concentration of nonelectrolyte solute and doesnrsquot depend on the type of solute it is a colligative proprerty
bc vapor pressure has been lowered solution remains liquid over larger temperature range
Lowers freezing point and raises boiling point
158
Can assume changes in boiling and freezing point also depend on concentration of solute
They are colligative properties
159
Freezing-Point Depression
Freezing point of 1-molal solution of any nonelectrolyte solute in water is found (by experiment) to be 186 lower than freezing point of water When 1 mol of nonelectrolyte solute dissolved in 1 kg water freezing point is -186 not 000 160
When 2 mol nonelectrolyte solute dissolved in 1 kg water freezing point is -372
For any concentration of nonelectrolyte solute in water decrease in freezing point can be determined using value of -186m
161
This value called the molal freezing-point contstant (Kf) is the freezing-point depression of the solvent in a 1-molal solution of a nonvolatile nonelectrolyte solute
162
Each solvent has own characteristic molal freezing-point constant Freezing-point depression (Δtf) the difference between the freezing points of the pure solvent and a solute of nonelectrolyte in that solvent and it is directly proportional to the molal concentration of the solution Δtf = Kfm
163
Δtf = Kfm
Kf expressed as m
m expressed in mol solutekg solvent (molality)
164
Sample Problem
What is the freezing point depression of water in a solution of 171 g of sucrose C12H22O11 and 200 g of water What is the actual freezing point of the solution
165
1 AnalyzeGiven Solute mass and chemical formula 171 g C12H22O11 Solvent mass and identity 2000 g water Unknown a freezing-point depression b freezing point of the solution 166
2 Plan
167
168
Calculate
Unknown a freezing-point depression b freezing point of the solution
169
Practice Problem
A water solution containing an unknown quantity of a nonelectrolyte solute is found to have a freezing point of ndash 023degCWhat is the molal concentration of the solution
012 m170
A solution consists of 103 g of the nonelectrolyte glucose C6H12O6 dissolved in 250 g of water What is the freezing-point depression of the solution
minus0426degC171
In a laboratory experiment the freezing point of an aqueous solution of glucose is found to be minus0325degCWhat is the molal concentration of this solution
0175 m172
If 0500 mol of a nonelectrolyte solute are dissolved in 5000 g of ether what is the freezing point of the solution
minus1181degC173
The freezing point of an aqueous solution that contains a nonelectrolyte is minus90degC aWhat is the freezing-point depression of the solution bWhat is the molal concentration of the solution a minus90degC b 48 m
174
Boiling-Point Elevation
Boiling point is temp at which vapor pressure is equal to atmospheric pressure Change in vapor pressure will cause corresponding change in boiling point
175
176
Vapor pressure of nonvolatile solution is lower than vapor pressure of pure solvent More heat will be required to raise vapor pressure of solution Boiling point of solution is higher than boiling point of pure solvent
Molal boiling-point constant (Kb) - boiling-point elevation of the solvent in a 1-molal solution of a nonvolatile nonelectrolyte solute b-p elevation of any nonelectrolyte in water found by experiment to be 051 So molal b-p constant for water is 051m
177
For different solvents b-p elevations of 1-molal solutions have different values (Table 14-2)
Like freezing-point constants values most accurate for dilute solutions
178
Boiling-point elevation Δtb difference between the boiling points of the pure solvent and a nonelectrolyte solution of that solvent and it is directly proportional to the molal concentration of the solution B-p elevation can be calculated using Δtb = Kbm Δtb (boiling point elevation) expressed in m and m expressed in mol solutekg solvent
Sample Problem
What is the boiling-point elevation of a solution made from 200 g of a nonelectrolyte solute and 4000 g of water The molar mass of the solute is 620 gmol
180
1 AnalyzeGiven solute mass = 200 g solute molar mass = 620 gmol solvent mass and identity = 4000 g of water
Unknown boiling-point elevation
181
2 Plan
182
183
3 Compute
Practice Problem
A solution contains 500 g of sucrose C12H22O11 a nonelectrolyte dissolved in 5000 g of water What is the boiling-point elevation
015degC184
A solution contains 4500 g of sucrose C12H22O11 a nonelectrolyte dissolved in 250 g of waterWhat is the boiling point of the solution
1027degC185
If the boiling point elevation of an aqueous solution containing a nonvolatile electrolyte is 102degC what is the molality of the solution
20 m186
The boiling point of an aqueous solution containing a nonvolatile electrolyte is 10075degC aWhat is the boiling-point elevation bWhat is the molality of the solution a 075degC b 15 m
187
Electrolytes and Colligative PropertiesEarly investigators confused by experiments where certain substances depressed freezing point or elevated boiling point more than expected
188
189
Ex 01 m solution of NaCl lowers freezing point of solvent almost twice as much as 01 m solution of sucrose 01 m CaCl2 lowers fp almost 3 times as much as 01 m solution of sucrose
To understand why this happens contrast behavior or sucrose with NaCl in aqueous solution Each sucrose dissolves to make only 1 particle in solution So 1 mol sucrose dissolves to 1 mol particles 1 mol NaCl dissolves to make 2 moles of particles (1 mol Na+ and 1 mol Cl-)
190
191
Calculated Values for Electrolyte SolutionsColligative properties depend on total concentration of solute particles regardless of their identity Electrolytes cause changes in colligative properties proportional to total molality in terms of all dissolves particles instead of formula units
192
What about barium nitrate Ba(NO3)2 Each mole of barium nitrate yields 3 mol of ions in solution Ba(NO3)2(s) rarr Ba2+(aq) + 2NO3-(aq) You would expect solution of given molality to lower fp of solvent 3 times as much as nonelectrolytic solution of same molality
193
Sample Problem
What is the expected change in the freezing point of water in a solution of 625 g of barium nitrate Ba(NO3)2 in 100 kg of water
194
1 AnalyzeGiven solute mass and formula = 625 g Ba(NO3)2 solvent mass and identity = 100 kg water Δtf = Kfm
Unknown expected freezing-point depression
195
2 PlanThe molality can be calculated by converting the solute mass to moles and then dividing by the number of kilograms of solvent That molality is in terms of formula units of Ba(NO3)2 and must be converted to molality in terms of dissociated ions in solution It must be multiplied by the number of moles of ions produced per mole of formula unit This adjusted molality can then be used to calculate the freezing-point depression
196
197
198
3 Compute
Practice Problem
What is the expected freezing-point depression for a solution that contains 20 mol of magnesium sulfate dissolved in 10 kg of water
minus74degC199
What is the expected boiling-point elevation of water for a solution that contains 150 g of sodium chloride dissolved in 10 kg of water 27degC The freezing point of an aqueous sodium chloride solution is minus200degC What is the molality of the solution 54 m NaCl
200
Actual Values for Electrolyte SolutionsRemember values calculated are only expected values Actual values of colligative properties for all strong electrolytes are almost what would be expected Difference between expected and actual values caused by attractive forces that exist between dissociated ions in aqueous solution
201
Attraction between hydrated ions in solution is small compared with those in crystalline solid However forces of attraction donrsquot interfere with movements of aqueous ions More concentrated solution means ions closer together and attraction is greater Also depends on charge of ions ndash higher charge greater attraction
202
Extent to which solute ionizes depends on strength of bonds within molecules of solute and strength of attraction between solute and solvent molecules
136
HCl ndash molecular compound that ionizes in aqueous solution Contains highly polar bond Attraction between polar HCl molecule and polar water strong enough to break HCl bond
137
The Hydronium Ion
H+ ions from HCl attracts other molecules or ions so strongly that it doesnrsquot normally exist alone Ionization of HCl better described as direct transfer of proton from HCl to H2O forming H3O+
138
139
Hydration of H+ to form hydronium is highly exothermic
Energy released gives a lot of energy needed to ionize a molecular solute
Many molecular compounds that ionize in aqueous solution contain hydrogen and form hydronium ion
140
Strong and Weak Electrolytes
Substances that make ions and conduct current in solution are electrolytes HCl is one of series of compounds that has hydrogen and halogen Hydrogen halides are all molecular compounds with single polar-covalent bonds
141
All are gases Very soluble in water All are electrolytes HCl HBr HI strongly conduct current in solution HF only weakly conducts current at same concentration Strength of conduction related to ability to ionize
142
143
Strong Electrolytes
HCl HBr HI are 100 ionized in solution
Strong electrolyte - any compound whose dilute aqueous solutions conduct electricity well this is due to the presence of all or almost all of the dissolve compound in the form of ions
144
Unique characteristic of strong electrolyte is that they yield only ions
Ex Ionic compound may be highly soluble in water and dissociate into ions (NaCl)
Others may not dissolve much but amount that does dissolve exists only as ions
145
Weak ElectrolytesHF dissolves in water to give acid solution called hydrofluoric acid HF bond stronger than bonds between hydrogen and other halogens When HF dissolves some molecule ionize Reverse reaction also happens HF(aq) + H2O(l) H3O+(aq) + F-(aq)
146
HF(aq) + H2O(l) H3O+
(aq) + F-(aq)Concentration of dissolved unionized HF stays high and concentration of ions stays low
Weak electrolyte - any compound whose aqueous solutions conduct electricity poorly this is due to the presence of small amount of the dissolved compound in the form of ions
147
Different from nonelectrolyte ndash NO ions at all Description of electrolyte as strong or weak not related to concentration of solution Electrolytes differ in degree of ionization not amount of solute dissolved
148
149
Colligative properties of solutions
Presence of solute affects properties of solutions Some properties not dependent on nature of dissolved substance but on how may dissolve particles are present
Colligative properties - properties that depend on the concentration of solute particles but not on their identity
150
Vapor-Pressure Lowering
Boiling and freezing point of solution different from pure solvent Nonvolatile solute raises boiling point and lowers freezing point Nonvolatile substance - one that has little tendency to become gas under existing conditions
151
152
To understand why nonvolatile solute changes boiling and freezing point consider equilibrium vapor pressure
Vapor pressure pressure caused by molecules that have escape liquid phase to gas phase
Can be thought of a measure of tendency of molecules to escape from a liquid
153
Addition of sucrose (nonvolatile solute) lowers concentration of water molecules at surface of liquid This lowers tendency of water molecules to leave solution and enter gas phase Vapor pressure of solution is lower than vapor pressure of pure water
154
Aqueous13 solution13 of13 nonvolatile13 solutePure13 water
Nonelectrolyte solutions of same molality have same concentration of particles
Dilute solutions of same solvent and equal molality of any nonelectrolyte solute lower vapor pressure equally
155
Example
1 m aqueous solution of nonelectrolyte glucose lowers vapor pressure of water 55 x 10-4 atm at 25 1 m aqueous solution of sucrose also lowers water vapor pressure to 55 x 10-4 atm at 25
156
157
bc vapor-pressure lowering depends on concentration of nonelectrolyte solute and doesnrsquot depend on the type of solute it is a colligative proprerty
bc vapor pressure has been lowered solution remains liquid over larger temperature range
Lowers freezing point and raises boiling point
158
Can assume changes in boiling and freezing point also depend on concentration of solute
They are colligative properties
159
Freezing-Point Depression
Freezing point of 1-molal solution of any nonelectrolyte solute in water is found (by experiment) to be 186 lower than freezing point of water When 1 mol of nonelectrolyte solute dissolved in 1 kg water freezing point is -186 not 000 160
When 2 mol nonelectrolyte solute dissolved in 1 kg water freezing point is -372
For any concentration of nonelectrolyte solute in water decrease in freezing point can be determined using value of -186m
161
This value called the molal freezing-point contstant (Kf) is the freezing-point depression of the solvent in a 1-molal solution of a nonvolatile nonelectrolyte solute
162
Each solvent has own characteristic molal freezing-point constant Freezing-point depression (Δtf) the difference between the freezing points of the pure solvent and a solute of nonelectrolyte in that solvent and it is directly proportional to the molal concentration of the solution Δtf = Kfm
163
Δtf = Kfm
Kf expressed as m
m expressed in mol solutekg solvent (molality)
164
Sample Problem
What is the freezing point depression of water in a solution of 171 g of sucrose C12H22O11 and 200 g of water What is the actual freezing point of the solution
165
1 AnalyzeGiven Solute mass and chemical formula 171 g C12H22O11 Solvent mass and identity 2000 g water Unknown a freezing-point depression b freezing point of the solution 166
2 Plan
167
168
Calculate
Unknown a freezing-point depression b freezing point of the solution
169
Practice Problem
A water solution containing an unknown quantity of a nonelectrolyte solute is found to have a freezing point of ndash 023degCWhat is the molal concentration of the solution
012 m170
A solution consists of 103 g of the nonelectrolyte glucose C6H12O6 dissolved in 250 g of water What is the freezing-point depression of the solution
minus0426degC171
In a laboratory experiment the freezing point of an aqueous solution of glucose is found to be minus0325degCWhat is the molal concentration of this solution
0175 m172
If 0500 mol of a nonelectrolyte solute are dissolved in 5000 g of ether what is the freezing point of the solution
minus1181degC173
The freezing point of an aqueous solution that contains a nonelectrolyte is minus90degC aWhat is the freezing-point depression of the solution bWhat is the molal concentration of the solution a minus90degC b 48 m
174
Boiling-Point Elevation
Boiling point is temp at which vapor pressure is equal to atmospheric pressure Change in vapor pressure will cause corresponding change in boiling point
175
176
Vapor pressure of nonvolatile solution is lower than vapor pressure of pure solvent More heat will be required to raise vapor pressure of solution Boiling point of solution is higher than boiling point of pure solvent
Molal boiling-point constant (Kb) - boiling-point elevation of the solvent in a 1-molal solution of a nonvolatile nonelectrolyte solute b-p elevation of any nonelectrolyte in water found by experiment to be 051 So molal b-p constant for water is 051m
177
For different solvents b-p elevations of 1-molal solutions have different values (Table 14-2)
Like freezing-point constants values most accurate for dilute solutions
178
Boiling-point elevation Δtb difference between the boiling points of the pure solvent and a nonelectrolyte solution of that solvent and it is directly proportional to the molal concentration of the solution B-p elevation can be calculated using Δtb = Kbm Δtb (boiling point elevation) expressed in m and m expressed in mol solutekg solvent
Sample Problem
What is the boiling-point elevation of a solution made from 200 g of a nonelectrolyte solute and 4000 g of water The molar mass of the solute is 620 gmol
180
1 AnalyzeGiven solute mass = 200 g solute molar mass = 620 gmol solvent mass and identity = 4000 g of water
Unknown boiling-point elevation
181
2 Plan
182
183
3 Compute
Practice Problem
A solution contains 500 g of sucrose C12H22O11 a nonelectrolyte dissolved in 5000 g of water What is the boiling-point elevation
015degC184
A solution contains 4500 g of sucrose C12H22O11 a nonelectrolyte dissolved in 250 g of waterWhat is the boiling point of the solution
1027degC185
If the boiling point elevation of an aqueous solution containing a nonvolatile electrolyte is 102degC what is the molality of the solution
20 m186
The boiling point of an aqueous solution containing a nonvolatile electrolyte is 10075degC aWhat is the boiling-point elevation bWhat is the molality of the solution a 075degC b 15 m
187
Electrolytes and Colligative PropertiesEarly investigators confused by experiments where certain substances depressed freezing point or elevated boiling point more than expected
188
189
Ex 01 m solution of NaCl lowers freezing point of solvent almost twice as much as 01 m solution of sucrose 01 m CaCl2 lowers fp almost 3 times as much as 01 m solution of sucrose
To understand why this happens contrast behavior or sucrose with NaCl in aqueous solution Each sucrose dissolves to make only 1 particle in solution So 1 mol sucrose dissolves to 1 mol particles 1 mol NaCl dissolves to make 2 moles of particles (1 mol Na+ and 1 mol Cl-)
190
191
Calculated Values for Electrolyte SolutionsColligative properties depend on total concentration of solute particles regardless of their identity Electrolytes cause changes in colligative properties proportional to total molality in terms of all dissolves particles instead of formula units
192
What about barium nitrate Ba(NO3)2 Each mole of barium nitrate yields 3 mol of ions in solution Ba(NO3)2(s) rarr Ba2+(aq) + 2NO3-(aq) You would expect solution of given molality to lower fp of solvent 3 times as much as nonelectrolytic solution of same molality
193
Sample Problem
What is the expected change in the freezing point of water in a solution of 625 g of barium nitrate Ba(NO3)2 in 100 kg of water
194
1 AnalyzeGiven solute mass and formula = 625 g Ba(NO3)2 solvent mass and identity = 100 kg water Δtf = Kfm
Unknown expected freezing-point depression
195
2 PlanThe molality can be calculated by converting the solute mass to moles and then dividing by the number of kilograms of solvent That molality is in terms of formula units of Ba(NO3)2 and must be converted to molality in terms of dissociated ions in solution It must be multiplied by the number of moles of ions produced per mole of formula unit This adjusted molality can then be used to calculate the freezing-point depression
196
197
198
3 Compute
Practice Problem
What is the expected freezing-point depression for a solution that contains 20 mol of magnesium sulfate dissolved in 10 kg of water
minus74degC199
What is the expected boiling-point elevation of water for a solution that contains 150 g of sodium chloride dissolved in 10 kg of water 27degC The freezing point of an aqueous sodium chloride solution is minus200degC What is the molality of the solution 54 m NaCl
200
Actual Values for Electrolyte SolutionsRemember values calculated are only expected values Actual values of colligative properties for all strong electrolytes are almost what would be expected Difference between expected and actual values caused by attractive forces that exist between dissociated ions in aqueous solution
201
Attraction between hydrated ions in solution is small compared with those in crystalline solid However forces of attraction donrsquot interfere with movements of aqueous ions More concentrated solution means ions closer together and attraction is greater Also depends on charge of ions ndash higher charge greater attraction
202
HCl ndash molecular compound that ionizes in aqueous solution Contains highly polar bond Attraction between polar HCl molecule and polar water strong enough to break HCl bond
137
The Hydronium Ion
H+ ions from HCl attracts other molecules or ions so strongly that it doesnrsquot normally exist alone Ionization of HCl better described as direct transfer of proton from HCl to H2O forming H3O+
138
139
Hydration of H+ to form hydronium is highly exothermic
Energy released gives a lot of energy needed to ionize a molecular solute
Many molecular compounds that ionize in aqueous solution contain hydrogen and form hydronium ion
140
Strong and Weak Electrolytes
Substances that make ions and conduct current in solution are electrolytes HCl is one of series of compounds that has hydrogen and halogen Hydrogen halides are all molecular compounds with single polar-covalent bonds
141
All are gases Very soluble in water All are electrolytes HCl HBr HI strongly conduct current in solution HF only weakly conducts current at same concentration Strength of conduction related to ability to ionize
142
143
Strong Electrolytes
HCl HBr HI are 100 ionized in solution
Strong electrolyte - any compound whose dilute aqueous solutions conduct electricity well this is due to the presence of all or almost all of the dissolve compound in the form of ions
144
Unique characteristic of strong electrolyte is that they yield only ions
Ex Ionic compound may be highly soluble in water and dissociate into ions (NaCl)
Others may not dissolve much but amount that does dissolve exists only as ions
145
Weak ElectrolytesHF dissolves in water to give acid solution called hydrofluoric acid HF bond stronger than bonds between hydrogen and other halogens When HF dissolves some molecule ionize Reverse reaction also happens HF(aq) + H2O(l) H3O+(aq) + F-(aq)
146
HF(aq) + H2O(l) H3O+
(aq) + F-(aq)Concentration of dissolved unionized HF stays high and concentration of ions stays low
Weak electrolyte - any compound whose aqueous solutions conduct electricity poorly this is due to the presence of small amount of the dissolved compound in the form of ions
147
Different from nonelectrolyte ndash NO ions at all Description of electrolyte as strong or weak not related to concentration of solution Electrolytes differ in degree of ionization not amount of solute dissolved
148
149
Colligative properties of solutions
Presence of solute affects properties of solutions Some properties not dependent on nature of dissolved substance but on how may dissolve particles are present
Colligative properties - properties that depend on the concentration of solute particles but not on their identity
150
Vapor-Pressure Lowering
Boiling and freezing point of solution different from pure solvent Nonvolatile solute raises boiling point and lowers freezing point Nonvolatile substance - one that has little tendency to become gas under existing conditions
151
152
To understand why nonvolatile solute changes boiling and freezing point consider equilibrium vapor pressure
Vapor pressure pressure caused by molecules that have escape liquid phase to gas phase
Can be thought of a measure of tendency of molecules to escape from a liquid
153
Addition of sucrose (nonvolatile solute) lowers concentration of water molecules at surface of liquid This lowers tendency of water molecules to leave solution and enter gas phase Vapor pressure of solution is lower than vapor pressure of pure water
154
Aqueous13 solution13 of13 nonvolatile13 solutePure13 water
Nonelectrolyte solutions of same molality have same concentration of particles
Dilute solutions of same solvent and equal molality of any nonelectrolyte solute lower vapor pressure equally
155
Example
1 m aqueous solution of nonelectrolyte glucose lowers vapor pressure of water 55 x 10-4 atm at 25 1 m aqueous solution of sucrose also lowers water vapor pressure to 55 x 10-4 atm at 25
156
157
bc vapor-pressure lowering depends on concentration of nonelectrolyte solute and doesnrsquot depend on the type of solute it is a colligative proprerty
bc vapor pressure has been lowered solution remains liquid over larger temperature range
Lowers freezing point and raises boiling point
158
Can assume changes in boiling and freezing point also depend on concentration of solute
They are colligative properties
159
Freezing-Point Depression
Freezing point of 1-molal solution of any nonelectrolyte solute in water is found (by experiment) to be 186 lower than freezing point of water When 1 mol of nonelectrolyte solute dissolved in 1 kg water freezing point is -186 not 000 160
When 2 mol nonelectrolyte solute dissolved in 1 kg water freezing point is -372
For any concentration of nonelectrolyte solute in water decrease in freezing point can be determined using value of -186m
161
This value called the molal freezing-point contstant (Kf) is the freezing-point depression of the solvent in a 1-molal solution of a nonvolatile nonelectrolyte solute
162
Each solvent has own characteristic molal freezing-point constant Freezing-point depression (Δtf) the difference between the freezing points of the pure solvent and a solute of nonelectrolyte in that solvent and it is directly proportional to the molal concentration of the solution Δtf = Kfm
163
Δtf = Kfm
Kf expressed as m
m expressed in mol solutekg solvent (molality)
164
Sample Problem
What is the freezing point depression of water in a solution of 171 g of sucrose C12H22O11 and 200 g of water What is the actual freezing point of the solution
165
1 AnalyzeGiven Solute mass and chemical formula 171 g C12H22O11 Solvent mass and identity 2000 g water Unknown a freezing-point depression b freezing point of the solution 166
2 Plan
167
168
Calculate
Unknown a freezing-point depression b freezing point of the solution
169
Practice Problem
A water solution containing an unknown quantity of a nonelectrolyte solute is found to have a freezing point of ndash 023degCWhat is the molal concentration of the solution
012 m170
A solution consists of 103 g of the nonelectrolyte glucose C6H12O6 dissolved in 250 g of water What is the freezing-point depression of the solution
minus0426degC171
In a laboratory experiment the freezing point of an aqueous solution of glucose is found to be minus0325degCWhat is the molal concentration of this solution
0175 m172
If 0500 mol of a nonelectrolyte solute are dissolved in 5000 g of ether what is the freezing point of the solution
minus1181degC173
The freezing point of an aqueous solution that contains a nonelectrolyte is minus90degC aWhat is the freezing-point depression of the solution bWhat is the molal concentration of the solution a minus90degC b 48 m
174
Boiling-Point Elevation
Boiling point is temp at which vapor pressure is equal to atmospheric pressure Change in vapor pressure will cause corresponding change in boiling point
175
176
Vapor pressure of nonvolatile solution is lower than vapor pressure of pure solvent More heat will be required to raise vapor pressure of solution Boiling point of solution is higher than boiling point of pure solvent
Molal boiling-point constant (Kb) - boiling-point elevation of the solvent in a 1-molal solution of a nonvolatile nonelectrolyte solute b-p elevation of any nonelectrolyte in water found by experiment to be 051 So molal b-p constant for water is 051m
177
For different solvents b-p elevations of 1-molal solutions have different values (Table 14-2)
Like freezing-point constants values most accurate for dilute solutions
178
Boiling-point elevation Δtb difference between the boiling points of the pure solvent and a nonelectrolyte solution of that solvent and it is directly proportional to the molal concentration of the solution B-p elevation can be calculated using Δtb = Kbm Δtb (boiling point elevation) expressed in m and m expressed in mol solutekg solvent
Sample Problem
What is the boiling-point elevation of a solution made from 200 g of a nonelectrolyte solute and 4000 g of water The molar mass of the solute is 620 gmol
180
1 AnalyzeGiven solute mass = 200 g solute molar mass = 620 gmol solvent mass and identity = 4000 g of water
Unknown boiling-point elevation
181
2 Plan
182
183
3 Compute
Practice Problem
A solution contains 500 g of sucrose C12H22O11 a nonelectrolyte dissolved in 5000 g of water What is the boiling-point elevation
015degC184
A solution contains 4500 g of sucrose C12H22O11 a nonelectrolyte dissolved in 250 g of waterWhat is the boiling point of the solution
1027degC185
If the boiling point elevation of an aqueous solution containing a nonvolatile electrolyte is 102degC what is the molality of the solution
20 m186
The boiling point of an aqueous solution containing a nonvolatile electrolyte is 10075degC aWhat is the boiling-point elevation bWhat is the molality of the solution a 075degC b 15 m
187
Electrolytes and Colligative PropertiesEarly investigators confused by experiments where certain substances depressed freezing point or elevated boiling point more than expected
188
189
Ex 01 m solution of NaCl lowers freezing point of solvent almost twice as much as 01 m solution of sucrose 01 m CaCl2 lowers fp almost 3 times as much as 01 m solution of sucrose
To understand why this happens contrast behavior or sucrose with NaCl in aqueous solution Each sucrose dissolves to make only 1 particle in solution So 1 mol sucrose dissolves to 1 mol particles 1 mol NaCl dissolves to make 2 moles of particles (1 mol Na+ and 1 mol Cl-)
190
191
Calculated Values for Electrolyte SolutionsColligative properties depend on total concentration of solute particles regardless of their identity Electrolytes cause changes in colligative properties proportional to total molality in terms of all dissolves particles instead of formula units
192
What about barium nitrate Ba(NO3)2 Each mole of barium nitrate yields 3 mol of ions in solution Ba(NO3)2(s) rarr Ba2+(aq) + 2NO3-(aq) You would expect solution of given molality to lower fp of solvent 3 times as much as nonelectrolytic solution of same molality
193
Sample Problem
What is the expected change in the freezing point of water in a solution of 625 g of barium nitrate Ba(NO3)2 in 100 kg of water
194
1 AnalyzeGiven solute mass and formula = 625 g Ba(NO3)2 solvent mass and identity = 100 kg water Δtf = Kfm
Unknown expected freezing-point depression
195
2 PlanThe molality can be calculated by converting the solute mass to moles and then dividing by the number of kilograms of solvent That molality is in terms of formula units of Ba(NO3)2 and must be converted to molality in terms of dissociated ions in solution It must be multiplied by the number of moles of ions produced per mole of formula unit This adjusted molality can then be used to calculate the freezing-point depression
196
197
198
3 Compute
Practice Problem
What is the expected freezing-point depression for a solution that contains 20 mol of magnesium sulfate dissolved in 10 kg of water
minus74degC199
What is the expected boiling-point elevation of water for a solution that contains 150 g of sodium chloride dissolved in 10 kg of water 27degC The freezing point of an aqueous sodium chloride solution is minus200degC What is the molality of the solution 54 m NaCl
200
Actual Values for Electrolyte SolutionsRemember values calculated are only expected values Actual values of colligative properties for all strong electrolytes are almost what would be expected Difference between expected and actual values caused by attractive forces that exist between dissociated ions in aqueous solution
201
Attraction between hydrated ions in solution is small compared with those in crystalline solid However forces of attraction donrsquot interfere with movements of aqueous ions More concentrated solution means ions closer together and attraction is greater Also depends on charge of ions ndash higher charge greater attraction
202
The Hydronium Ion
H+ ions from HCl attracts other molecules or ions so strongly that it doesnrsquot normally exist alone Ionization of HCl better described as direct transfer of proton from HCl to H2O forming H3O+
138
139
Hydration of H+ to form hydronium is highly exothermic
Energy released gives a lot of energy needed to ionize a molecular solute
Many molecular compounds that ionize in aqueous solution contain hydrogen and form hydronium ion
140
Strong and Weak Electrolytes
Substances that make ions and conduct current in solution are electrolytes HCl is one of series of compounds that has hydrogen and halogen Hydrogen halides are all molecular compounds with single polar-covalent bonds
141
All are gases Very soluble in water All are electrolytes HCl HBr HI strongly conduct current in solution HF only weakly conducts current at same concentration Strength of conduction related to ability to ionize
142
143
Strong Electrolytes
HCl HBr HI are 100 ionized in solution
Strong electrolyte - any compound whose dilute aqueous solutions conduct electricity well this is due to the presence of all or almost all of the dissolve compound in the form of ions
144
Unique characteristic of strong electrolyte is that they yield only ions
Ex Ionic compound may be highly soluble in water and dissociate into ions (NaCl)
Others may not dissolve much but amount that does dissolve exists only as ions
145
Weak ElectrolytesHF dissolves in water to give acid solution called hydrofluoric acid HF bond stronger than bonds between hydrogen and other halogens When HF dissolves some molecule ionize Reverse reaction also happens HF(aq) + H2O(l) H3O+(aq) + F-(aq)
146
HF(aq) + H2O(l) H3O+
(aq) + F-(aq)Concentration of dissolved unionized HF stays high and concentration of ions stays low
Weak electrolyte - any compound whose aqueous solutions conduct electricity poorly this is due to the presence of small amount of the dissolved compound in the form of ions
147
Different from nonelectrolyte ndash NO ions at all Description of electrolyte as strong or weak not related to concentration of solution Electrolytes differ in degree of ionization not amount of solute dissolved
148
149
Colligative properties of solutions
Presence of solute affects properties of solutions Some properties not dependent on nature of dissolved substance but on how may dissolve particles are present
Colligative properties - properties that depend on the concentration of solute particles but not on their identity
150
Vapor-Pressure Lowering
Boiling and freezing point of solution different from pure solvent Nonvolatile solute raises boiling point and lowers freezing point Nonvolatile substance - one that has little tendency to become gas under existing conditions
151
152
To understand why nonvolatile solute changes boiling and freezing point consider equilibrium vapor pressure
Vapor pressure pressure caused by molecules that have escape liquid phase to gas phase
Can be thought of a measure of tendency of molecules to escape from a liquid
153
Addition of sucrose (nonvolatile solute) lowers concentration of water molecules at surface of liquid This lowers tendency of water molecules to leave solution and enter gas phase Vapor pressure of solution is lower than vapor pressure of pure water
154
Aqueous13 solution13 of13 nonvolatile13 solutePure13 water
Nonelectrolyte solutions of same molality have same concentration of particles
Dilute solutions of same solvent and equal molality of any nonelectrolyte solute lower vapor pressure equally
155
Example
1 m aqueous solution of nonelectrolyte glucose lowers vapor pressure of water 55 x 10-4 atm at 25 1 m aqueous solution of sucrose also lowers water vapor pressure to 55 x 10-4 atm at 25
156
157
bc vapor-pressure lowering depends on concentration of nonelectrolyte solute and doesnrsquot depend on the type of solute it is a colligative proprerty
bc vapor pressure has been lowered solution remains liquid over larger temperature range
Lowers freezing point and raises boiling point
158
Can assume changes in boiling and freezing point also depend on concentration of solute
They are colligative properties
159
Freezing-Point Depression
Freezing point of 1-molal solution of any nonelectrolyte solute in water is found (by experiment) to be 186 lower than freezing point of water When 1 mol of nonelectrolyte solute dissolved in 1 kg water freezing point is -186 not 000 160
When 2 mol nonelectrolyte solute dissolved in 1 kg water freezing point is -372
For any concentration of nonelectrolyte solute in water decrease in freezing point can be determined using value of -186m
161
This value called the molal freezing-point contstant (Kf) is the freezing-point depression of the solvent in a 1-molal solution of a nonvolatile nonelectrolyte solute
162
Each solvent has own characteristic molal freezing-point constant Freezing-point depression (Δtf) the difference between the freezing points of the pure solvent and a solute of nonelectrolyte in that solvent and it is directly proportional to the molal concentration of the solution Δtf = Kfm
163
Δtf = Kfm
Kf expressed as m
m expressed in mol solutekg solvent (molality)
164
Sample Problem
What is the freezing point depression of water in a solution of 171 g of sucrose C12H22O11 and 200 g of water What is the actual freezing point of the solution
165
1 AnalyzeGiven Solute mass and chemical formula 171 g C12H22O11 Solvent mass and identity 2000 g water Unknown a freezing-point depression b freezing point of the solution 166
2 Plan
167
168
Calculate
Unknown a freezing-point depression b freezing point of the solution
169
Practice Problem
A water solution containing an unknown quantity of a nonelectrolyte solute is found to have a freezing point of ndash 023degCWhat is the molal concentration of the solution
012 m170
A solution consists of 103 g of the nonelectrolyte glucose C6H12O6 dissolved in 250 g of water What is the freezing-point depression of the solution
minus0426degC171
In a laboratory experiment the freezing point of an aqueous solution of glucose is found to be minus0325degCWhat is the molal concentration of this solution
0175 m172
If 0500 mol of a nonelectrolyte solute are dissolved in 5000 g of ether what is the freezing point of the solution
minus1181degC173
The freezing point of an aqueous solution that contains a nonelectrolyte is minus90degC aWhat is the freezing-point depression of the solution bWhat is the molal concentration of the solution a minus90degC b 48 m
174
Boiling-Point Elevation
Boiling point is temp at which vapor pressure is equal to atmospheric pressure Change in vapor pressure will cause corresponding change in boiling point
175
176
Vapor pressure of nonvolatile solution is lower than vapor pressure of pure solvent More heat will be required to raise vapor pressure of solution Boiling point of solution is higher than boiling point of pure solvent
Molal boiling-point constant (Kb) - boiling-point elevation of the solvent in a 1-molal solution of a nonvolatile nonelectrolyte solute b-p elevation of any nonelectrolyte in water found by experiment to be 051 So molal b-p constant for water is 051m
177
For different solvents b-p elevations of 1-molal solutions have different values (Table 14-2)
Like freezing-point constants values most accurate for dilute solutions
178
Boiling-point elevation Δtb difference between the boiling points of the pure solvent and a nonelectrolyte solution of that solvent and it is directly proportional to the molal concentration of the solution B-p elevation can be calculated using Δtb = Kbm Δtb (boiling point elevation) expressed in m and m expressed in mol solutekg solvent
Sample Problem
What is the boiling-point elevation of a solution made from 200 g of a nonelectrolyte solute and 4000 g of water The molar mass of the solute is 620 gmol
180
1 AnalyzeGiven solute mass = 200 g solute molar mass = 620 gmol solvent mass and identity = 4000 g of water
Unknown boiling-point elevation
181
2 Plan
182
183
3 Compute
Practice Problem
A solution contains 500 g of sucrose C12H22O11 a nonelectrolyte dissolved in 5000 g of water What is the boiling-point elevation
015degC184
A solution contains 4500 g of sucrose C12H22O11 a nonelectrolyte dissolved in 250 g of waterWhat is the boiling point of the solution
1027degC185
If the boiling point elevation of an aqueous solution containing a nonvolatile electrolyte is 102degC what is the molality of the solution
20 m186
The boiling point of an aqueous solution containing a nonvolatile electrolyte is 10075degC aWhat is the boiling-point elevation bWhat is the molality of the solution a 075degC b 15 m
187
Electrolytes and Colligative PropertiesEarly investigators confused by experiments where certain substances depressed freezing point or elevated boiling point more than expected
188
189
Ex 01 m solution of NaCl lowers freezing point of solvent almost twice as much as 01 m solution of sucrose 01 m CaCl2 lowers fp almost 3 times as much as 01 m solution of sucrose
To understand why this happens contrast behavior or sucrose with NaCl in aqueous solution Each sucrose dissolves to make only 1 particle in solution So 1 mol sucrose dissolves to 1 mol particles 1 mol NaCl dissolves to make 2 moles of particles (1 mol Na+ and 1 mol Cl-)
190
191
Calculated Values for Electrolyte SolutionsColligative properties depend on total concentration of solute particles regardless of their identity Electrolytes cause changes in colligative properties proportional to total molality in terms of all dissolves particles instead of formula units
192
What about barium nitrate Ba(NO3)2 Each mole of barium nitrate yields 3 mol of ions in solution Ba(NO3)2(s) rarr Ba2+(aq) + 2NO3-(aq) You would expect solution of given molality to lower fp of solvent 3 times as much as nonelectrolytic solution of same molality
193
Sample Problem
What is the expected change in the freezing point of water in a solution of 625 g of barium nitrate Ba(NO3)2 in 100 kg of water
194
1 AnalyzeGiven solute mass and formula = 625 g Ba(NO3)2 solvent mass and identity = 100 kg water Δtf = Kfm
Unknown expected freezing-point depression
195
2 PlanThe molality can be calculated by converting the solute mass to moles and then dividing by the number of kilograms of solvent That molality is in terms of formula units of Ba(NO3)2 and must be converted to molality in terms of dissociated ions in solution It must be multiplied by the number of moles of ions produced per mole of formula unit This adjusted molality can then be used to calculate the freezing-point depression
196
197
198
3 Compute
Practice Problem
What is the expected freezing-point depression for a solution that contains 20 mol of magnesium sulfate dissolved in 10 kg of water
minus74degC199
What is the expected boiling-point elevation of water for a solution that contains 150 g of sodium chloride dissolved in 10 kg of water 27degC The freezing point of an aqueous sodium chloride solution is minus200degC What is the molality of the solution 54 m NaCl
200
Actual Values for Electrolyte SolutionsRemember values calculated are only expected values Actual values of colligative properties for all strong electrolytes are almost what would be expected Difference between expected and actual values caused by attractive forces that exist between dissociated ions in aqueous solution
201
Attraction between hydrated ions in solution is small compared with those in crystalline solid However forces of attraction donrsquot interfere with movements of aqueous ions More concentrated solution means ions closer together and attraction is greater Also depends on charge of ions ndash higher charge greater attraction
202
139
Hydration of H+ to form hydronium is highly exothermic
Energy released gives a lot of energy needed to ionize a molecular solute
Many molecular compounds that ionize in aqueous solution contain hydrogen and form hydronium ion
140
Strong and Weak Electrolytes
Substances that make ions and conduct current in solution are electrolytes HCl is one of series of compounds that has hydrogen and halogen Hydrogen halides are all molecular compounds with single polar-covalent bonds
141
All are gases Very soluble in water All are electrolytes HCl HBr HI strongly conduct current in solution HF only weakly conducts current at same concentration Strength of conduction related to ability to ionize
142
143
Strong Electrolytes
HCl HBr HI are 100 ionized in solution
Strong electrolyte - any compound whose dilute aqueous solutions conduct electricity well this is due to the presence of all or almost all of the dissolve compound in the form of ions
144
Unique characteristic of strong electrolyte is that they yield only ions
Ex Ionic compound may be highly soluble in water and dissociate into ions (NaCl)
Others may not dissolve much but amount that does dissolve exists only as ions
145
Weak ElectrolytesHF dissolves in water to give acid solution called hydrofluoric acid HF bond stronger than bonds between hydrogen and other halogens When HF dissolves some molecule ionize Reverse reaction also happens HF(aq) + H2O(l) H3O+(aq) + F-(aq)
146
HF(aq) + H2O(l) H3O+
(aq) + F-(aq)Concentration of dissolved unionized HF stays high and concentration of ions stays low
Weak electrolyte - any compound whose aqueous solutions conduct electricity poorly this is due to the presence of small amount of the dissolved compound in the form of ions
147
Different from nonelectrolyte ndash NO ions at all Description of electrolyte as strong or weak not related to concentration of solution Electrolytes differ in degree of ionization not amount of solute dissolved
148
149
Colligative properties of solutions
Presence of solute affects properties of solutions Some properties not dependent on nature of dissolved substance but on how may dissolve particles are present
Colligative properties - properties that depend on the concentration of solute particles but not on their identity
150
Vapor-Pressure Lowering
Boiling and freezing point of solution different from pure solvent Nonvolatile solute raises boiling point and lowers freezing point Nonvolatile substance - one that has little tendency to become gas under existing conditions
151
152
To understand why nonvolatile solute changes boiling and freezing point consider equilibrium vapor pressure
Vapor pressure pressure caused by molecules that have escape liquid phase to gas phase
Can be thought of a measure of tendency of molecules to escape from a liquid
153
Addition of sucrose (nonvolatile solute) lowers concentration of water molecules at surface of liquid This lowers tendency of water molecules to leave solution and enter gas phase Vapor pressure of solution is lower than vapor pressure of pure water
154
Aqueous13 solution13 of13 nonvolatile13 solutePure13 water
Nonelectrolyte solutions of same molality have same concentration of particles
Dilute solutions of same solvent and equal molality of any nonelectrolyte solute lower vapor pressure equally
155
Example
1 m aqueous solution of nonelectrolyte glucose lowers vapor pressure of water 55 x 10-4 atm at 25 1 m aqueous solution of sucrose also lowers water vapor pressure to 55 x 10-4 atm at 25
156
157
bc vapor-pressure lowering depends on concentration of nonelectrolyte solute and doesnrsquot depend on the type of solute it is a colligative proprerty
bc vapor pressure has been lowered solution remains liquid over larger temperature range
Lowers freezing point and raises boiling point
158
Can assume changes in boiling and freezing point also depend on concentration of solute
They are colligative properties
159
Freezing-Point Depression
Freezing point of 1-molal solution of any nonelectrolyte solute in water is found (by experiment) to be 186 lower than freezing point of water When 1 mol of nonelectrolyte solute dissolved in 1 kg water freezing point is -186 not 000 160
When 2 mol nonelectrolyte solute dissolved in 1 kg water freezing point is -372
For any concentration of nonelectrolyte solute in water decrease in freezing point can be determined using value of -186m
161
This value called the molal freezing-point contstant (Kf) is the freezing-point depression of the solvent in a 1-molal solution of a nonvolatile nonelectrolyte solute
162
Each solvent has own characteristic molal freezing-point constant Freezing-point depression (Δtf) the difference between the freezing points of the pure solvent and a solute of nonelectrolyte in that solvent and it is directly proportional to the molal concentration of the solution Δtf = Kfm
163
Δtf = Kfm
Kf expressed as m
m expressed in mol solutekg solvent (molality)
164
Sample Problem
What is the freezing point depression of water in a solution of 171 g of sucrose C12H22O11 and 200 g of water What is the actual freezing point of the solution
165
1 AnalyzeGiven Solute mass and chemical formula 171 g C12H22O11 Solvent mass and identity 2000 g water Unknown a freezing-point depression b freezing point of the solution 166
2 Plan
167
168
Calculate
Unknown a freezing-point depression b freezing point of the solution
169
Practice Problem
A water solution containing an unknown quantity of a nonelectrolyte solute is found to have a freezing point of ndash 023degCWhat is the molal concentration of the solution
012 m170
A solution consists of 103 g of the nonelectrolyte glucose C6H12O6 dissolved in 250 g of water What is the freezing-point depression of the solution
minus0426degC171
In a laboratory experiment the freezing point of an aqueous solution of glucose is found to be minus0325degCWhat is the molal concentration of this solution
0175 m172
If 0500 mol of a nonelectrolyte solute are dissolved in 5000 g of ether what is the freezing point of the solution
minus1181degC173
The freezing point of an aqueous solution that contains a nonelectrolyte is minus90degC aWhat is the freezing-point depression of the solution bWhat is the molal concentration of the solution a minus90degC b 48 m
174
Boiling-Point Elevation
Boiling point is temp at which vapor pressure is equal to atmospheric pressure Change in vapor pressure will cause corresponding change in boiling point
175
176
Vapor pressure of nonvolatile solution is lower than vapor pressure of pure solvent More heat will be required to raise vapor pressure of solution Boiling point of solution is higher than boiling point of pure solvent
Molal boiling-point constant (Kb) - boiling-point elevation of the solvent in a 1-molal solution of a nonvolatile nonelectrolyte solute b-p elevation of any nonelectrolyte in water found by experiment to be 051 So molal b-p constant for water is 051m
177
For different solvents b-p elevations of 1-molal solutions have different values (Table 14-2)
Like freezing-point constants values most accurate for dilute solutions
178
Boiling-point elevation Δtb difference between the boiling points of the pure solvent and a nonelectrolyte solution of that solvent and it is directly proportional to the molal concentration of the solution B-p elevation can be calculated using Δtb = Kbm Δtb (boiling point elevation) expressed in m and m expressed in mol solutekg solvent
Sample Problem
What is the boiling-point elevation of a solution made from 200 g of a nonelectrolyte solute and 4000 g of water The molar mass of the solute is 620 gmol
180
1 AnalyzeGiven solute mass = 200 g solute molar mass = 620 gmol solvent mass and identity = 4000 g of water
Unknown boiling-point elevation
181
2 Plan
182
183
3 Compute
Practice Problem
A solution contains 500 g of sucrose C12H22O11 a nonelectrolyte dissolved in 5000 g of water What is the boiling-point elevation
015degC184
A solution contains 4500 g of sucrose C12H22O11 a nonelectrolyte dissolved in 250 g of waterWhat is the boiling point of the solution
1027degC185
If the boiling point elevation of an aqueous solution containing a nonvolatile electrolyte is 102degC what is the molality of the solution
20 m186
The boiling point of an aqueous solution containing a nonvolatile electrolyte is 10075degC aWhat is the boiling-point elevation bWhat is the molality of the solution a 075degC b 15 m
187
Electrolytes and Colligative PropertiesEarly investigators confused by experiments where certain substances depressed freezing point or elevated boiling point more than expected
188
189
Ex 01 m solution of NaCl lowers freezing point of solvent almost twice as much as 01 m solution of sucrose 01 m CaCl2 lowers fp almost 3 times as much as 01 m solution of sucrose
To understand why this happens contrast behavior or sucrose with NaCl in aqueous solution Each sucrose dissolves to make only 1 particle in solution So 1 mol sucrose dissolves to 1 mol particles 1 mol NaCl dissolves to make 2 moles of particles (1 mol Na+ and 1 mol Cl-)
190
191
Calculated Values for Electrolyte SolutionsColligative properties depend on total concentration of solute particles regardless of their identity Electrolytes cause changes in colligative properties proportional to total molality in terms of all dissolves particles instead of formula units
192
What about barium nitrate Ba(NO3)2 Each mole of barium nitrate yields 3 mol of ions in solution Ba(NO3)2(s) rarr Ba2+(aq) + 2NO3-(aq) You would expect solution of given molality to lower fp of solvent 3 times as much as nonelectrolytic solution of same molality
193
Sample Problem
What is the expected change in the freezing point of water in a solution of 625 g of barium nitrate Ba(NO3)2 in 100 kg of water
194
1 AnalyzeGiven solute mass and formula = 625 g Ba(NO3)2 solvent mass and identity = 100 kg water Δtf = Kfm
Unknown expected freezing-point depression
195
2 PlanThe molality can be calculated by converting the solute mass to moles and then dividing by the number of kilograms of solvent That molality is in terms of formula units of Ba(NO3)2 and must be converted to molality in terms of dissociated ions in solution It must be multiplied by the number of moles of ions produced per mole of formula unit This adjusted molality can then be used to calculate the freezing-point depression
196
197
198
3 Compute
Practice Problem
What is the expected freezing-point depression for a solution that contains 20 mol of magnesium sulfate dissolved in 10 kg of water
minus74degC199
What is the expected boiling-point elevation of water for a solution that contains 150 g of sodium chloride dissolved in 10 kg of water 27degC The freezing point of an aqueous sodium chloride solution is minus200degC What is the molality of the solution 54 m NaCl
200
Actual Values for Electrolyte SolutionsRemember values calculated are only expected values Actual values of colligative properties for all strong electrolytes are almost what would be expected Difference between expected and actual values caused by attractive forces that exist between dissociated ions in aqueous solution
201
Attraction between hydrated ions in solution is small compared with those in crystalline solid However forces of attraction donrsquot interfere with movements of aqueous ions More concentrated solution means ions closer together and attraction is greater Also depends on charge of ions ndash higher charge greater attraction
202
Hydration of H+ to form hydronium is highly exothermic
Energy released gives a lot of energy needed to ionize a molecular solute
Many molecular compounds that ionize in aqueous solution contain hydrogen and form hydronium ion
140
Strong and Weak Electrolytes
Substances that make ions and conduct current in solution are electrolytes HCl is one of series of compounds that has hydrogen and halogen Hydrogen halides are all molecular compounds with single polar-covalent bonds
141
All are gases Very soluble in water All are electrolytes HCl HBr HI strongly conduct current in solution HF only weakly conducts current at same concentration Strength of conduction related to ability to ionize
142
143
Strong Electrolytes
HCl HBr HI are 100 ionized in solution
Strong electrolyte - any compound whose dilute aqueous solutions conduct electricity well this is due to the presence of all or almost all of the dissolve compound in the form of ions
144
Unique characteristic of strong electrolyte is that they yield only ions
Ex Ionic compound may be highly soluble in water and dissociate into ions (NaCl)
Others may not dissolve much but amount that does dissolve exists only as ions
145
Weak ElectrolytesHF dissolves in water to give acid solution called hydrofluoric acid HF bond stronger than bonds between hydrogen and other halogens When HF dissolves some molecule ionize Reverse reaction also happens HF(aq) + H2O(l) H3O+(aq) + F-(aq)
146
HF(aq) + H2O(l) H3O+
(aq) + F-(aq)Concentration of dissolved unionized HF stays high and concentration of ions stays low
Weak electrolyte - any compound whose aqueous solutions conduct electricity poorly this is due to the presence of small amount of the dissolved compound in the form of ions
147
Different from nonelectrolyte ndash NO ions at all Description of electrolyte as strong or weak not related to concentration of solution Electrolytes differ in degree of ionization not amount of solute dissolved
148
149
Colligative properties of solutions
Presence of solute affects properties of solutions Some properties not dependent on nature of dissolved substance but on how may dissolve particles are present
Colligative properties - properties that depend on the concentration of solute particles but not on their identity
150
Vapor-Pressure Lowering
Boiling and freezing point of solution different from pure solvent Nonvolatile solute raises boiling point and lowers freezing point Nonvolatile substance - one that has little tendency to become gas under existing conditions
151
152
To understand why nonvolatile solute changes boiling and freezing point consider equilibrium vapor pressure
Vapor pressure pressure caused by molecules that have escape liquid phase to gas phase
Can be thought of a measure of tendency of molecules to escape from a liquid
153
Addition of sucrose (nonvolatile solute) lowers concentration of water molecules at surface of liquid This lowers tendency of water molecules to leave solution and enter gas phase Vapor pressure of solution is lower than vapor pressure of pure water
154
Aqueous13 solution13 of13 nonvolatile13 solutePure13 water
Nonelectrolyte solutions of same molality have same concentration of particles
Dilute solutions of same solvent and equal molality of any nonelectrolyte solute lower vapor pressure equally
155
Example
1 m aqueous solution of nonelectrolyte glucose lowers vapor pressure of water 55 x 10-4 atm at 25 1 m aqueous solution of sucrose also lowers water vapor pressure to 55 x 10-4 atm at 25
156
157
bc vapor-pressure lowering depends on concentration of nonelectrolyte solute and doesnrsquot depend on the type of solute it is a colligative proprerty
bc vapor pressure has been lowered solution remains liquid over larger temperature range
Lowers freezing point and raises boiling point
158
Can assume changes in boiling and freezing point also depend on concentration of solute
They are colligative properties
159
Freezing-Point Depression
Freezing point of 1-molal solution of any nonelectrolyte solute in water is found (by experiment) to be 186 lower than freezing point of water When 1 mol of nonelectrolyte solute dissolved in 1 kg water freezing point is -186 not 000 160
When 2 mol nonelectrolyte solute dissolved in 1 kg water freezing point is -372
For any concentration of nonelectrolyte solute in water decrease in freezing point can be determined using value of -186m
161
This value called the molal freezing-point contstant (Kf) is the freezing-point depression of the solvent in a 1-molal solution of a nonvolatile nonelectrolyte solute
162
Each solvent has own characteristic molal freezing-point constant Freezing-point depression (Δtf) the difference between the freezing points of the pure solvent and a solute of nonelectrolyte in that solvent and it is directly proportional to the molal concentration of the solution Δtf = Kfm
163
Δtf = Kfm
Kf expressed as m
m expressed in mol solutekg solvent (molality)
164
Sample Problem
What is the freezing point depression of water in a solution of 171 g of sucrose C12H22O11 and 200 g of water What is the actual freezing point of the solution
165
1 AnalyzeGiven Solute mass and chemical formula 171 g C12H22O11 Solvent mass and identity 2000 g water Unknown a freezing-point depression b freezing point of the solution 166
2 Plan
167
168
Calculate
Unknown a freezing-point depression b freezing point of the solution
169
Practice Problem
A water solution containing an unknown quantity of a nonelectrolyte solute is found to have a freezing point of ndash 023degCWhat is the molal concentration of the solution
012 m170
A solution consists of 103 g of the nonelectrolyte glucose C6H12O6 dissolved in 250 g of water What is the freezing-point depression of the solution
minus0426degC171
In a laboratory experiment the freezing point of an aqueous solution of glucose is found to be minus0325degCWhat is the molal concentration of this solution
0175 m172
If 0500 mol of a nonelectrolyte solute are dissolved in 5000 g of ether what is the freezing point of the solution
minus1181degC173
The freezing point of an aqueous solution that contains a nonelectrolyte is minus90degC aWhat is the freezing-point depression of the solution bWhat is the molal concentration of the solution a minus90degC b 48 m
174
Boiling-Point Elevation
Boiling point is temp at which vapor pressure is equal to atmospheric pressure Change in vapor pressure will cause corresponding change in boiling point
175
176
Vapor pressure of nonvolatile solution is lower than vapor pressure of pure solvent More heat will be required to raise vapor pressure of solution Boiling point of solution is higher than boiling point of pure solvent
Molal boiling-point constant (Kb) - boiling-point elevation of the solvent in a 1-molal solution of a nonvolatile nonelectrolyte solute b-p elevation of any nonelectrolyte in water found by experiment to be 051 So molal b-p constant for water is 051m
177
For different solvents b-p elevations of 1-molal solutions have different values (Table 14-2)
Like freezing-point constants values most accurate for dilute solutions
178
Boiling-point elevation Δtb difference between the boiling points of the pure solvent and a nonelectrolyte solution of that solvent and it is directly proportional to the molal concentration of the solution B-p elevation can be calculated using Δtb = Kbm Δtb (boiling point elevation) expressed in m and m expressed in mol solutekg solvent
Sample Problem
What is the boiling-point elevation of a solution made from 200 g of a nonelectrolyte solute and 4000 g of water The molar mass of the solute is 620 gmol
180
1 AnalyzeGiven solute mass = 200 g solute molar mass = 620 gmol solvent mass and identity = 4000 g of water
Unknown boiling-point elevation
181
2 Plan
182
183
3 Compute
Practice Problem
A solution contains 500 g of sucrose C12H22O11 a nonelectrolyte dissolved in 5000 g of water What is the boiling-point elevation
015degC184
A solution contains 4500 g of sucrose C12H22O11 a nonelectrolyte dissolved in 250 g of waterWhat is the boiling point of the solution
1027degC185
If the boiling point elevation of an aqueous solution containing a nonvolatile electrolyte is 102degC what is the molality of the solution
20 m186
The boiling point of an aqueous solution containing a nonvolatile electrolyte is 10075degC aWhat is the boiling-point elevation bWhat is the molality of the solution a 075degC b 15 m
187
Electrolytes and Colligative PropertiesEarly investigators confused by experiments where certain substances depressed freezing point or elevated boiling point more than expected
188
189
Ex 01 m solution of NaCl lowers freezing point of solvent almost twice as much as 01 m solution of sucrose 01 m CaCl2 lowers fp almost 3 times as much as 01 m solution of sucrose
To understand why this happens contrast behavior or sucrose with NaCl in aqueous solution Each sucrose dissolves to make only 1 particle in solution So 1 mol sucrose dissolves to 1 mol particles 1 mol NaCl dissolves to make 2 moles of particles (1 mol Na+ and 1 mol Cl-)
190
191
Calculated Values for Electrolyte SolutionsColligative properties depend on total concentration of solute particles regardless of their identity Electrolytes cause changes in colligative properties proportional to total molality in terms of all dissolves particles instead of formula units
192
What about barium nitrate Ba(NO3)2 Each mole of barium nitrate yields 3 mol of ions in solution Ba(NO3)2(s) rarr Ba2+(aq) + 2NO3-(aq) You would expect solution of given molality to lower fp of solvent 3 times as much as nonelectrolytic solution of same molality
193
Sample Problem
What is the expected change in the freezing point of water in a solution of 625 g of barium nitrate Ba(NO3)2 in 100 kg of water
194
1 AnalyzeGiven solute mass and formula = 625 g Ba(NO3)2 solvent mass and identity = 100 kg water Δtf = Kfm
Unknown expected freezing-point depression
195
2 PlanThe molality can be calculated by converting the solute mass to moles and then dividing by the number of kilograms of solvent That molality is in terms of formula units of Ba(NO3)2 and must be converted to molality in terms of dissociated ions in solution It must be multiplied by the number of moles of ions produced per mole of formula unit This adjusted molality can then be used to calculate the freezing-point depression
196
197
198
3 Compute
Practice Problem
What is the expected freezing-point depression for a solution that contains 20 mol of magnesium sulfate dissolved in 10 kg of water
minus74degC199
What is the expected boiling-point elevation of water for a solution that contains 150 g of sodium chloride dissolved in 10 kg of water 27degC The freezing point of an aqueous sodium chloride solution is minus200degC What is the molality of the solution 54 m NaCl
200
Actual Values for Electrolyte SolutionsRemember values calculated are only expected values Actual values of colligative properties for all strong electrolytes are almost what would be expected Difference between expected and actual values caused by attractive forces that exist between dissociated ions in aqueous solution
201
Attraction between hydrated ions in solution is small compared with those in crystalline solid However forces of attraction donrsquot interfere with movements of aqueous ions More concentrated solution means ions closer together and attraction is greater Also depends on charge of ions ndash higher charge greater attraction
202
Strong and Weak Electrolytes
Substances that make ions and conduct current in solution are electrolytes HCl is one of series of compounds that has hydrogen and halogen Hydrogen halides are all molecular compounds with single polar-covalent bonds
141
All are gases Very soluble in water All are electrolytes HCl HBr HI strongly conduct current in solution HF only weakly conducts current at same concentration Strength of conduction related to ability to ionize
142
143
Strong Electrolytes
HCl HBr HI are 100 ionized in solution
Strong electrolyte - any compound whose dilute aqueous solutions conduct electricity well this is due to the presence of all or almost all of the dissolve compound in the form of ions
144
Unique characteristic of strong electrolyte is that they yield only ions
Ex Ionic compound may be highly soluble in water and dissociate into ions (NaCl)
Others may not dissolve much but amount that does dissolve exists only as ions
145
Weak ElectrolytesHF dissolves in water to give acid solution called hydrofluoric acid HF bond stronger than bonds between hydrogen and other halogens When HF dissolves some molecule ionize Reverse reaction also happens HF(aq) + H2O(l) H3O+(aq) + F-(aq)
146
HF(aq) + H2O(l) H3O+
(aq) + F-(aq)Concentration of dissolved unionized HF stays high and concentration of ions stays low
Weak electrolyte - any compound whose aqueous solutions conduct electricity poorly this is due to the presence of small amount of the dissolved compound in the form of ions
147
Different from nonelectrolyte ndash NO ions at all Description of electrolyte as strong or weak not related to concentration of solution Electrolytes differ in degree of ionization not amount of solute dissolved
148
149
Colligative properties of solutions
Presence of solute affects properties of solutions Some properties not dependent on nature of dissolved substance but on how may dissolve particles are present
Colligative properties - properties that depend on the concentration of solute particles but not on their identity
150
Vapor-Pressure Lowering
Boiling and freezing point of solution different from pure solvent Nonvolatile solute raises boiling point and lowers freezing point Nonvolatile substance - one that has little tendency to become gas under existing conditions
151
152
To understand why nonvolatile solute changes boiling and freezing point consider equilibrium vapor pressure
Vapor pressure pressure caused by molecules that have escape liquid phase to gas phase
Can be thought of a measure of tendency of molecules to escape from a liquid
153
Addition of sucrose (nonvolatile solute) lowers concentration of water molecules at surface of liquid This lowers tendency of water molecules to leave solution and enter gas phase Vapor pressure of solution is lower than vapor pressure of pure water
154
Aqueous13 solution13 of13 nonvolatile13 solutePure13 water
Nonelectrolyte solutions of same molality have same concentration of particles
Dilute solutions of same solvent and equal molality of any nonelectrolyte solute lower vapor pressure equally
155
Example
1 m aqueous solution of nonelectrolyte glucose lowers vapor pressure of water 55 x 10-4 atm at 25 1 m aqueous solution of sucrose also lowers water vapor pressure to 55 x 10-4 atm at 25
156
157
bc vapor-pressure lowering depends on concentration of nonelectrolyte solute and doesnrsquot depend on the type of solute it is a colligative proprerty
bc vapor pressure has been lowered solution remains liquid over larger temperature range
Lowers freezing point and raises boiling point
158
Can assume changes in boiling and freezing point also depend on concentration of solute
They are colligative properties
159
Freezing-Point Depression
Freezing point of 1-molal solution of any nonelectrolyte solute in water is found (by experiment) to be 186 lower than freezing point of water When 1 mol of nonelectrolyte solute dissolved in 1 kg water freezing point is -186 not 000 160
When 2 mol nonelectrolyte solute dissolved in 1 kg water freezing point is -372
For any concentration of nonelectrolyte solute in water decrease in freezing point can be determined using value of -186m
161
This value called the molal freezing-point contstant (Kf) is the freezing-point depression of the solvent in a 1-molal solution of a nonvolatile nonelectrolyte solute
162
Each solvent has own characteristic molal freezing-point constant Freezing-point depression (Δtf) the difference between the freezing points of the pure solvent and a solute of nonelectrolyte in that solvent and it is directly proportional to the molal concentration of the solution Δtf = Kfm
163
Δtf = Kfm
Kf expressed as m
m expressed in mol solutekg solvent (molality)
164
Sample Problem
What is the freezing point depression of water in a solution of 171 g of sucrose C12H22O11 and 200 g of water What is the actual freezing point of the solution
165
1 AnalyzeGiven Solute mass and chemical formula 171 g C12H22O11 Solvent mass and identity 2000 g water Unknown a freezing-point depression b freezing point of the solution 166
2 Plan
167
168
Calculate
Unknown a freezing-point depression b freezing point of the solution
169
Practice Problem
A water solution containing an unknown quantity of a nonelectrolyte solute is found to have a freezing point of ndash 023degCWhat is the molal concentration of the solution
012 m170
A solution consists of 103 g of the nonelectrolyte glucose C6H12O6 dissolved in 250 g of water What is the freezing-point depression of the solution
minus0426degC171
In a laboratory experiment the freezing point of an aqueous solution of glucose is found to be minus0325degCWhat is the molal concentration of this solution
0175 m172
If 0500 mol of a nonelectrolyte solute are dissolved in 5000 g of ether what is the freezing point of the solution
minus1181degC173
The freezing point of an aqueous solution that contains a nonelectrolyte is minus90degC aWhat is the freezing-point depression of the solution bWhat is the molal concentration of the solution a minus90degC b 48 m
174
Boiling-Point Elevation
Boiling point is temp at which vapor pressure is equal to atmospheric pressure Change in vapor pressure will cause corresponding change in boiling point
175
176
Vapor pressure of nonvolatile solution is lower than vapor pressure of pure solvent More heat will be required to raise vapor pressure of solution Boiling point of solution is higher than boiling point of pure solvent
Molal boiling-point constant (Kb) - boiling-point elevation of the solvent in a 1-molal solution of a nonvolatile nonelectrolyte solute b-p elevation of any nonelectrolyte in water found by experiment to be 051 So molal b-p constant for water is 051m
177
For different solvents b-p elevations of 1-molal solutions have different values (Table 14-2)
Like freezing-point constants values most accurate for dilute solutions
178
Boiling-point elevation Δtb difference between the boiling points of the pure solvent and a nonelectrolyte solution of that solvent and it is directly proportional to the molal concentration of the solution B-p elevation can be calculated using Δtb = Kbm Δtb (boiling point elevation) expressed in m and m expressed in mol solutekg solvent
Sample Problem
What is the boiling-point elevation of a solution made from 200 g of a nonelectrolyte solute and 4000 g of water The molar mass of the solute is 620 gmol
180
1 AnalyzeGiven solute mass = 200 g solute molar mass = 620 gmol solvent mass and identity = 4000 g of water
Unknown boiling-point elevation
181
2 Plan
182
183
3 Compute
Practice Problem
A solution contains 500 g of sucrose C12H22O11 a nonelectrolyte dissolved in 5000 g of water What is the boiling-point elevation
015degC184
A solution contains 4500 g of sucrose C12H22O11 a nonelectrolyte dissolved in 250 g of waterWhat is the boiling point of the solution
1027degC185
If the boiling point elevation of an aqueous solution containing a nonvolatile electrolyte is 102degC what is the molality of the solution
20 m186
The boiling point of an aqueous solution containing a nonvolatile electrolyte is 10075degC aWhat is the boiling-point elevation bWhat is the molality of the solution a 075degC b 15 m
187
Electrolytes and Colligative PropertiesEarly investigators confused by experiments where certain substances depressed freezing point or elevated boiling point more than expected
188
189
Ex 01 m solution of NaCl lowers freezing point of solvent almost twice as much as 01 m solution of sucrose 01 m CaCl2 lowers fp almost 3 times as much as 01 m solution of sucrose
To understand why this happens contrast behavior or sucrose with NaCl in aqueous solution Each sucrose dissolves to make only 1 particle in solution So 1 mol sucrose dissolves to 1 mol particles 1 mol NaCl dissolves to make 2 moles of particles (1 mol Na+ and 1 mol Cl-)
190
191
Calculated Values for Electrolyte SolutionsColligative properties depend on total concentration of solute particles regardless of their identity Electrolytes cause changes in colligative properties proportional to total molality in terms of all dissolves particles instead of formula units
192
What about barium nitrate Ba(NO3)2 Each mole of barium nitrate yields 3 mol of ions in solution Ba(NO3)2(s) rarr Ba2+(aq) + 2NO3-(aq) You would expect solution of given molality to lower fp of solvent 3 times as much as nonelectrolytic solution of same molality
193
Sample Problem
What is the expected change in the freezing point of water in a solution of 625 g of barium nitrate Ba(NO3)2 in 100 kg of water
194
1 AnalyzeGiven solute mass and formula = 625 g Ba(NO3)2 solvent mass and identity = 100 kg water Δtf = Kfm
Unknown expected freezing-point depression
195
2 PlanThe molality can be calculated by converting the solute mass to moles and then dividing by the number of kilograms of solvent That molality is in terms of formula units of Ba(NO3)2 and must be converted to molality in terms of dissociated ions in solution It must be multiplied by the number of moles of ions produced per mole of formula unit This adjusted molality can then be used to calculate the freezing-point depression
196
197
198
3 Compute
Practice Problem
What is the expected freezing-point depression for a solution that contains 20 mol of magnesium sulfate dissolved in 10 kg of water
minus74degC199
What is the expected boiling-point elevation of water for a solution that contains 150 g of sodium chloride dissolved in 10 kg of water 27degC The freezing point of an aqueous sodium chloride solution is minus200degC What is the molality of the solution 54 m NaCl
200
Actual Values for Electrolyte SolutionsRemember values calculated are only expected values Actual values of colligative properties for all strong electrolytes are almost what would be expected Difference between expected and actual values caused by attractive forces that exist between dissociated ions in aqueous solution
201
Attraction between hydrated ions in solution is small compared with those in crystalline solid However forces of attraction donrsquot interfere with movements of aqueous ions More concentrated solution means ions closer together and attraction is greater Also depends on charge of ions ndash higher charge greater attraction
202
All are gases Very soluble in water All are electrolytes HCl HBr HI strongly conduct current in solution HF only weakly conducts current at same concentration Strength of conduction related to ability to ionize
142
143
Strong Electrolytes
HCl HBr HI are 100 ionized in solution
Strong electrolyte - any compound whose dilute aqueous solutions conduct electricity well this is due to the presence of all or almost all of the dissolve compound in the form of ions
144
Unique characteristic of strong electrolyte is that they yield only ions
Ex Ionic compound may be highly soluble in water and dissociate into ions (NaCl)
Others may not dissolve much but amount that does dissolve exists only as ions
145
Weak ElectrolytesHF dissolves in water to give acid solution called hydrofluoric acid HF bond stronger than bonds between hydrogen and other halogens When HF dissolves some molecule ionize Reverse reaction also happens HF(aq) + H2O(l) H3O+(aq) + F-(aq)
146
HF(aq) + H2O(l) H3O+
(aq) + F-(aq)Concentration of dissolved unionized HF stays high and concentration of ions stays low
Weak electrolyte - any compound whose aqueous solutions conduct electricity poorly this is due to the presence of small amount of the dissolved compound in the form of ions
147
Different from nonelectrolyte ndash NO ions at all Description of electrolyte as strong or weak not related to concentration of solution Electrolytes differ in degree of ionization not amount of solute dissolved
148
149
Colligative properties of solutions
Presence of solute affects properties of solutions Some properties not dependent on nature of dissolved substance but on how may dissolve particles are present
Colligative properties - properties that depend on the concentration of solute particles but not on their identity
150
Vapor-Pressure Lowering
Boiling and freezing point of solution different from pure solvent Nonvolatile solute raises boiling point and lowers freezing point Nonvolatile substance - one that has little tendency to become gas under existing conditions
151
152
To understand why nonvolatile solute changes boiling and freezing point consider equilibrium vapor pressure
Vapor pressure pressure caused by molecules that have escape liquid phase to gas phase
Can be thought of a measure of tendency of molecules to escape from a liquid
153
Addition of sucrose (nonvolatile solute) lowers concentration of water molecules at surface of liquid This lowers tendency of water molecules to leave solution and enter gas phase Vapor pressure of solution is lower than vapor pressure of pure water
154
Aqueous13 solution13 of13 nonvolatile13 solutePure13 water
Nonelectrolyte solutions of same molality have same concentration of particles
Dilute solutions of same solvent and equal molality of any nonelectrolyte solute lower vapor pressure equally
155
Example
1 m aqueous solution of nonelectrolyte glucose lowers vapor pressure of water 55 x 10-4 atm at 25 1 m aqueous solution of sucrose also lowers water vapor pressure to 55 x 10-4 atm at 25
156
157
bc vapor-pressure lowering depends on concentration of nonelectrolyte solute and doesnrsquot depend on the type of solute it is a colligative proprerty
bc vapor pressure has been lowered solution remains liquid over larger temperature range
Lowers freezing point and raises boiling point
158
Can assume changes in boiling and freezing point also depend on concentration of solute
They are colligative properties
159
Freezing-Point Depression
Freezing point of 1-molal solution of any nonelectrolyte solute in water is found (by experiment) to be 186 lower than freezing point of water When 1 mol of nonelectrolyte solute dissolved in 1 kg water freezing point is -186 not 000 160
When 2 mol nonelectrolyte solute dissolved in 1 kg water freezing point is -372
For any concentration of nonelectrolyte solute in water decrease in freezing point can be determined using value of -186m
161
This value called the molal freezing-point contstant (Kf) is the freezing-point depression of the solvent in a 1-molal solution of a nonvolatile nonelectrolyte solute
162
Each solvent has own characteristic molal freezing-point constant Freezing-point depression (Δtf) the difference between the freezing points of the pure solvent and a solute of nonelectrolyte in that solvent and it is directly proportional to the molal concentration of the solution Δtf = Kfm
163
Δtf = Kfm
Kf expressed as m
m expressed in mol solutekg solvent (molality)
164
Sample Problem
What is the freezing point depression of water in a solution of 171 g of sucrose C12H22O11 and 200 g of water What is the actual freezing point of the solution
165
1 AnalyzeGiven Solute mass and chemical formula 171 g C12H22O11 Solvent mass and identity 2000 g water Unknown a freezing-point depression b freezing point of the solution 166
2 Plan
167
168
Calculate
Unknown a freezing-point depression b freezing point of the solution
169
Practice Problem
A water solution containing an unknown quantity of a nonelectrolyte solute is found to have a freezing point of ndash 023degCWhat is the molal concentration of the solution
012 m170
A solution consists of 103 g of the nonelectrolyte glucose C6H12O6 dissolved in 250 g of water What is the freezing-point depression of the solution
minus0426degC171
In a laboratory experiment the freezing point of an aqueous solution of glucose is found to be minus0325degCWhat is the molal concentration of this solution
0175 m172
If 0500 mol of a nonelectrolyte solute are dissolved in 5000 g of ether what is the freezing point of the solution
minus1181degC173
The freezing point of an aqueous solution that contains a nonelectrolyte is minus90degC aWhat is the freezing-point depression of the solution bWhat is the molal concentration of the solution a minus90degC b 48 m
174
Boiling-Point Elevation
Boiling point is temp at which vapor pressure is equal to atmospheric pressure Change in vapor pressure will cause corresponding change in boiling point
175
176
Vapor pressure of nonvolatile solution is lower than vapor pressure of pure solvent More heat will be required to raise vapor pressure of solution Boiling point of solution is higher than boiling point of pure solvent
Molal boiling-point constant (Kb) - boiling-point elevation of the solvent in a 1-molal solution of a nonvolatile nonelectrolyte solute b-p elevation of any nonelectrolyte in water found by experiment to be 051 So molal b-p constant for water is 051m
177
For different solvents b-p elevations of 1-molal solutions have different values (Table 14-2)
Like freezing-point constants values most accurate for dilute solutions
178
Boiling-point elevation Δtb difference between the boiling points of the pure solvent and a nonelectrolyte solution of that solvent and it is directly proportional to the molal concentration of the solution B-p elevation can be calculated using Δtb = Kbm Δtb (boiling point elevation) expressed in m and m expressed in mol solutekg solvent
Sample Problem
What is the boiling-point elevation of a solution made from 200 g of a nonelectrolyte solute and 4000 g of water The molar mass of the solute is 620 gmol
180
1 AnalyzeGiven solute mass = 200 g solute molar mass = 620 gmol solvent mass and identity = 4000 g of water
Unknown boiling-point elevation
181
2 Plan
182
183
3 Compute
Practice Problem
A solution contains 500 g of sucrose C12H22O11 a nonelectrolyte dissolved in 5000 g of water What is the boiling-point elevation
015degC184
A solution contains 4500 g of sucrose C12H22O11 a nonelectrolyte dissolved in 250 g of waterWhat is the boiling point of the solution
1027degC185
If the boiling point elevation of an aqueous solution containing a nonvolatile electrolyte is 102degC what is the molality of the solution
20 m186
The boiling point of an aqueous solution containing a nonvolatile electrolyte is 10075degC aWhat is the boiling-point elevation bWhat is the molality of the solution a 075degC b 15 m
187
Electrolytes and Colligative PropertiesEarly investigators confused by experiments where certain substances depressed freezing point or elevated boiling point more than expected
188
189
Ex 01 m solution of NaCl lowers freezing point of solvent almost twice as much as 01 m solution of sucrose 01 m CaCl2 lowers fp almost 3 times as much as 01 m solution of sucrose
To understand why this happens contrast behavior or sucrose with NaCl in aqueous solution Each sucrose dissolves to make only 1 particle in solution So 1 mol sucrose dissolves to 1 mol particles 1 mol NaCl dissolves to make 2 moles of particles (1 mol Na+ and 1 mol Cl-)
190
191
Calculated Values for Electrolyte SolutionsColligative properties depend on total concentration of solute particles regardless of their identity Electrolytes cause changes in colligative properties proportional to total molality in terms of all dissolves particles instead of formula units
192
What about barium nitrate Ba(NO3)2 Each mole of barium nitrate yields 3 mol of ions in solution Ba(NO3)2(s) rarr Ba2+(aq) + 2NO3-(aq) You would expect solution of given molality to lower fp of solvent 3 times as much as nonelectrolytic solution of same molality
193
Sample Problem
What is the expected change in the freezing point of water in a solution of 625 g of barium nitrate Ba(NO3)2 in 100 kg of water
194
1 AnalyzeGiven solute mass and formula = 625 g Ba(NO3)2 solvent mass and identity = 100 kg water Δtf = Kfm
Unknown expected freezing-point depression
195
2 PlanThe molality can be calculated by converting the solute mass to moles and then dividing by the number of kilograms of solvent That molality is in terms of formula units of Ba(NO3)2 and must be converted to molality in terms of dissociated ions in solution It must be multiplied by the number of moles of ions produced per mole of formula unit This adjusted molality can then be used to calculate the freezing-point depression
196
197
198
3 Compute
Practice Problem
What is the expected freezing-point depression for a solution that contains 20 mol of magnesium sulfate dissolved in 10 kg of water
minus74degC199
What is the expected boiling-point elevation of water for a solution that contains 150 g of sodium chloride dissolved in 10 kg of water 27degC The freezing point of an aqueous sodium chloride solution is minus200degC What is the molality of the solution 54 m NaCl
200
Actual Values for Electrolyte SolutionsRemember values calculated are only expected values Actual values of colligative properties for all strong electrolytes are almost what would be expected Difference between expected and actual values caused by attractive forces that exist between dissociated ions in aqueous solution
201
Attraction between hydrated ions in solution is small compared with those in crystalline solid However forces of attraction donrsquot interfere with movements of aqueous ions More concentrated solution means ions closer together and attraction is greater Also depends on charge of ions ndash higher charge greater attraction
202
143
Strong Electrolytes
HCl HBr HI are 100 ionized in solution
Strong electrolyte - any compound whose dilute aqueous solutions conduct electricity well this is due to the presence of all or almost all of the dissolve compound in the form of ions
144
Unique characteristic of strong electrolyte is that they yield only ions
Ex Ionic compound may be highly soluble in water and dissociate into ions (NaCl)
Others may not dissolve much but amount that does dissolve exists only as ions
145
Weak ElectrolytesHF dissolves in water to give acid solution called hydrofluoric acid HF bond stronger than bonds between hydrogen and other halogens When HF dissolves some molecule ionize Reverse reaction also happens HF(aq) + H2O(l) H3O+(aq) + F-(aq)
146
HF(aq) + H2O(l) H3O+
(aq) + F-(aq)Concentration of dissolved unionized HF stays high and concentration of ions stays low
Weak electrolyte - any compound whose aqueous solutions conduct electricity poorly this is due to the presence of small amount of the dissolved compound in the form of ions
147
Different from nonelectrolyte ndash NO ions at all Description of electrolyte as strong or weak not related to concentration of solution Electrolytes differ in degree of ionization not amount of solute dissolved
148
149
Colligative properties of solutions
Presence of solute affects properties of solutions Some properties not dependent on nature of dissolved substance but on how may dissolve particles are present
Colligative properties - properties that depend on the concentration of solute particles but not on their identity
150
Vapor-Pressure Lowering
Boiling and freezing point of solution different from pure solvent Nonvolatile solute raises boiling point and lowers freezing point Nonvolatile substance - one that has little tendency to become gas under existing conditions
151
152
To understand why nonvolatile solute changes boiling and freezing point consider equilibrium vapor pressure
Vapor pressure pressure caused by molecules that have escape liquid phase to gas phase
Can be thought of a measure of tendency of molecules to escape from a liquid
153
Addition of sucrose (nonvolatile solute) lowers concentration of water molecules at surface of liquid This lowers tendency of water molecules to leave solution and enter gas phase Vapor pressure of solution is lower than vapor pressure of pure water
154
Aqueous13 solution13 of13 nonvolatile13 solutePure13 water
Nonelectrolyte solutions of same molality have same concentration of particles
Dilute solutions of same solvent and equal molality of any nonelectrolyte solute lower vapor pressure equally
155
Example
1 m aqueous solution of nonelectrolyte glucose lowers vapor pressure of water 55 x 10-4 atm at 25 1 m aqueous solution of sucrose also lowers water vapor pressure to 55 x 10-4 atm at 25
156
157
bc vapor-pressure lowering depends on concentration of nonelectrolyte solute and doesnrsquot depend on the type of solute it is a colligative proprerty
bc vapor pressure has been lowered solution remains liquid over larger temperature range
Lowers freezing point and raises boiling point
158
Can assume changes in boiling and freezing point also depend on concentration of solute
They are colligative properties
159
Freezing-Point Depression
Freezing point of 1-molal solution of any nonelectrolyte solute in water is found (by experiment) to be 186 lower than freezing point of water When 1 mol of nonelectrolyte solute dissolved in 1 kg water freezing point is -186 not 000 160
When 2 mol nonelectrolyte solute dissolved in 1 kg water freezing point is -372
For any concentration of nonelectrolyte solute in water decrease in freezing point can be determined using value of -186m
161
This value called the molal freezing-point contstant (Kf) is the freezing-point depression of the solvent in a 1-molal solution of a nonvolatile nonelectrolyte solute
162
Each solvent has own characteristic molal freezing-point constant Freezing-point depression (Δtf) the difference between the freezing points of the pure solvent and a solute of nonelectrolyte in that solvent and it is directly proportional to the molal concentration of the solution Δtf = Kfm
163
Δtf = Kfm
Kf expressed as m
m expressed in mol solutekg solvent (molality)
164
Sample Problem
What is the freezing point depression of water in a solution of 171 g of sucrose C12H22O11 and 200 g of water What is the actual freezing point of the solution
165
1 AnalyzeGiven Solute mass and chemical formula 171 g C12H22O11 Solvent mass and identity 2000 g water Unknown a freezing-point depression b freezing point of the solution 166
2 Plan
167
168
Calculate
Unknown a freezing-point depression b freezing point of the solution
169
Practice Problem
A water solution containing an unknown quantity of a nonelectrolyte solute is found to have a freezing point of ndash 023degCWhat is the molal concentration of the solution
012 m170
A solution consists of 103 g of the nonelectrolyte glucose C6H12O6 dissolved in 250 g of water What is the freezing-point depression of the solution
minus0426degC171
In a laboratory experiment the freezing point of an aqueous solution of glucose is found to be minus0325degCWhat is the molal concentration of this solution
0175 m172
If 0500 mol of a nonelectrolyte solute are dissolved in 5000 g of ether what is the freezing point of the solution
minus1181degC173
The freezing point of an aqueous solution that contains a nonelectrolyte is minus90degC aWhat is the freezing-point depression of the solution bWhat is the molal concentration of the solution a minus90degC b 48 m
174
Boiling-Point Elevation
Boiling point is temp at which vapor pressure is equal to atmospheric pressure Change in vapor pressure will cause corresponding change in boiling point
175
176
Vapor pressure of nonvolatile solution is lower than vapor pressure of pure solvent More heat will be required to raise vapor pressure of solution Boiling point of solution is higher than boiling point of pure solvent
Molal boiling-point constant (Kb) - boiling-point elevation of the solvent in a 1-molal solution of a nonvolatile nonelectrolyte solute b-p elevation of any nonelectrolyte in water found by experiment to be 051 So molal b-p constant for water is 051m
177
For different solvents b-p elevations of 1-molal solutions have different values (Table 14-2)
Like freezing-point constants values most accurate for dilute solutions
178
Boiling-point elevation Δtb difference between the boiling points of the pure solvent and a nonelectrolyte solution of that solvent and it is directly proportional to the molal concentration of the solution B-p elevation can be calculated using Δtb = Kbm Δtb (boiling point elevation) expressed in m and m expressed in mol solutekg solvent
Sample Problem
What is the boiling-point elevation of a solution made from 200 g of a nonelectrolyte solute and 4000 g of water The molar mass of the solute is 620 gmol
180
1 AnalyzeGiven solute mass = 200 g solute molar mass = 620 gmol solvent mass and identity = 4000 g of water
Unknown boiling-point elevation
181
2 Plan
182
183
3 Compute
Practice Problem
A solution contains 500 g of sucrose C12H22O11 a nonelectrolyte dissolved in 5000 g of water What is the boiling-point elevation
015degC184
A solution contains 4500 g of sucrose C12H22O11 a nonelectrolyte dissolved in 250 g of waterWhat is the boiling point of the solution
1027degC185
If the boiling point elevation of an aqueous solution containing a nonvolatile electrolyte is 102degC what is the molality of the solution
20 m186
The boiling point of an aqueous solution containing a nonvolatile electrolyte is 10075degC aWhat is the boiling-point elevation bWhat is the molality of the solution a 075degC b 15 m
187
Electrolytes and Colligative PropertiesEarly investigators confused by experiments where certain substances depressed freezing point or elevated boiling point more than expected
188
189
Ex 01 m solution of NaCl lowers freezing point of solvent almost twice as much as 01 m solution of sucrose 01 m CaCl2 lowers fp almost 3 times as much as 01 m solution of sucrose
To understand why this happens contrast behavior or sucrose with NaCl in aqueous solution Each sucrose dissolves to make only 1 particle in solution So 1 mol sucrose dissolves to 1 mol particles 1 mol NaCl dissolves to make 2 moles of particles (1 mol Na+ and 1 mol Cl-)
190
191
Calculated Values for Electrolyte SolutionsColligative properties depend on total concentration of solute particles regardless of their identity Electrolytes cause changes in colligative properties proportional to total molality in terms of all dissolves particles instead of formula units
192
What about barium nitrate Ba(NO3)2 Each mole of barium nitrate yields 3 mol of ions in solution Ba(NO3)2(s) rarr Ba2+(aq) + 2NO3-(aq) You would expect solution of given molality to lower fp of solvent 3 times as much as nonelectrolytic solution of same molality
193
Sample Problem
What is the expected change in the freezing point of water in a solution of 625 g of barium nitrate Ba(NO3)2 in 100 kg of water
194
1 AnalyzeGiven solute mass and formula = 625 g Ba(NO3)2 solvent mass and identity = 100 kg water Δtf = Kfm
Unknown expected freezing-point depression
195
2 PlanThe molality can be calculated by converting the solute mass to moles and then dividing by the number of kilograms of solvent That molality is in terms of formula units of Ba(NO3)2 and must be converted to molality in terms of dissociated ions in solution It must be multiplied by the number of moles of ions produced per mole of formula unit This adjusted molality can then be used to calculate the freezing-point depression
196
197
198
3 Compute
Practice Problem
What is the expected freezing-point depression for a solution that contains 20 mol of magnesium sulfate dissolved in 10 kg of water
minus74degC199
What is the expected boiling-point elevation of water for a solution that contains 150 g of sodium chloride dissolved in 10 kg of water 27degC The freezing point of an aqueous sodium chloride solution is minus200degC What is the molality of the solution 54 m NaCl
200
Actual Values for Electrolyte SolutionsRemember values calculated are only expected values Actual values of colligative properties for all strong electrolytes are almost what would be expected Difference between expected and actual values caused by attractive forces that exist between dissociated ions in aqueous solution
201
Attraction between hydrated ions in solution is small compared with those in crystalline solid However forces of attraction donrsquot interfere with movements of aqueous ions More concentrated solution means ions closer together and attraction is greater Also depends on charge of ions ndash higher charge greater attraction
202
Strong Electrolytes
HCl HBr HI are 100 ionized in solution
Strong electrolyte - any compound whose dilute aqueous solutions conduct electricity well this is due to the presence of all or almost all of the dissolve compound in the form of ions
144
Unique characteristic of strong electrolyte is that they yield only ions
Ex Ionic compound may be highly soluble in water and dissociate into ions (NaCl)
Others may not dissolve much but amount that does dissolve exists only as ions
145
Weak ElectrolytesHF dissolves in water to give acid solution called hydrofluoric acid HF bond stronger than bonds between hydrogen and other halogens When HF dissolves some molecule ionize Reverse reaction also happens HF(aq) + H2O(l) H3O+(aq) + F-(aq)
146
HF(aq) + H2O(l) H3O+
(aq) + F-(aq)Concentration of dissolved unionized HF stays high and concentration of ions stays low
Weak electrolyte - any compound whose aqueous solutions conduct electricity poorly this is due to the presence of small amount of the dissolved compound in the form of ions
147
Different from nonelectrolyte ndash NO ions at all Description of electrolyte as strong or weak not related to concentration of solution Electrolytes differ in degree of ionization not amount of solute dissolved
148
149
Colligative properties of solutions
Presence of solute affects properties of solutions Some properties not dependent on nature of dissolved substance but on how may dissolve particles are present
Colligative properties - properties that depend on the concentration of solute particles but not on their identity
150
Vapor-Pressure Lowering
Boiling and freezing point of solution different from pure solvent Nonvolatile solute raises boiling point and lowers freezing point Nonvolatile substance - one that has little tendency to become gas under existing conditions
151
152
To understand why nonvolatile solute changes boiling and freezing point consider equilibrium vapor pressure
Vapor pressure pressure caused by molecules that have escape liquid phase to gas phase
Can be thought of a measure of tendency of molecules to escape from a liquid
153
Addition of sucrose (nonvolatile solute) lowers concentration of water molecules at surface of liquid This lowers tendency of water molecules to leave solution and enter gas phase Vapor pressure of solution is lower than vapor pressure of pure water
154
Aqueous13 solution13 of13 nonvolatile13 solutePure13 water
Nonelectrolyte solutions of same molality have same concentration of particles
Dilute solutions of same solvent and equal molality of any nonelectrolyte solute lower vapor pressure equally
155
Example
1 m aqueous solution of nonelectrolyte glucose lowers vapor pressure of water 55 x 10-4 atm at 25 1 m aqueous solution of sucrose also lowers water vapor pressure to 55 x 10-4 atm at 25
156
157
bc vapor-pressure lowering depends on concentration of nonelectrolyte solute and doesnrsquot depend on the type of solute it is a colligative proprerty
bc vapor pressure has been lowered solution remains liquid over larger temperature range
Lowers freezing point and raises boiling point
158
Can assume changes in boiling and freezing point also depend on concentration of solute
They are colligative properties
159
Freezing-Point Depression
Freezing point of 1-molal solution of any nonelectrolyte solute in water is found (by experiment) to be 186 lower than freezing point of water When 1 mol of nonelectrolyte solute dissolved in 1 kg water freezing point is -186 not 000 160
When 2 mol nonelectrolyte solute dissolved in 1 kg water freezing point is -372
For any concentration of nonelectrolyte solute in water decrease in freezing point can be determined using value of -186m
161
This value called the molal freezing-point contstant (Kf) is the freezing-point depression of the solvent in a 1-molal solution of a nonvolatile nonelectrolyte solute
162
Each solvent has own characteristic molal freezing-point constant Freezing-point depression (Δtf) the difference between the freezing points of the pure solvent and a solute of nonelectrolyte in that solvent and it is directly proportional to the molal concentration of the solution Δtf = Kfm
163
Δtf = Kfm
Kf expressed as m
m expressed in mol solutekg solvent (molality)
164
Sample Problem
What is the freezing point depression of water in a solution of 171 g of sucrose C12H22O11 and 200 g of water What is the actual freezing point of the solution
165
1 AnalyzeGiven Solute mass and chemical formula 171 g C12H22O11 Solvent mass and identity 2000 g water Unknown a freezing-point depression b freezing point of the solution 166
2 Plan
167
168
Calculate
Unknown a freezing-point depression b freezing point of the solution
169
Practice Problem
A water solution containing an unknown quantity of a nonelectrolyte solute is found to have a freezing point of ndash 023degCWhat is the molal concentration of the solution
012 m170
A solution consists of 103 g of the nonelectrolyte glucose C6H12O6 dissolved in 250 g of water What is the freezing-point depression of the solution
minus0426degC171
In a laboratory experiment the freezing point of an aqueous solution of glucose is found to be minus0325degCWhat is the molal concentration of this solution
0175 m172
If 0500 mol of a nonelectrolyte solute are dissolved in 5000 g of ether what is the freezing point of the solution
minus1181degC173
The freezing point of an aqueous solution that contains a nonelectrolyte is minus90degC aWhat is the freezing-point depression of the solution bWhat is the molal concentration of the solution a minus90degC b 48 m
174
Boiling-Point Elevation
Boiling point is temp at which vapor pressure is equal to atmospheric pressure Change in vapor pressure will cause corresponding change in boiling point
175
176
Vapor pressure of nonvolatile solution is lower than vapor pressure of pure solvent More heat will be required to raise vapor pressure of solution Boiling point of solution is higher than boiling point of pure solvent
Molal boiling-point constant (Kb) - boiling-point elevation of the solvent in a 1-molal solution of a nonvolatile nonelectrolyte solute b-p elevation of any nonelectrolyte in water found by experiment to be 051 So molal b-p constant for water is 051m
177
For different solvents b-p elevations of 1-molal solutions have different values (Table 14-2)
Like freezing-point constants values most accurate for dilute solutions
178
Boiling-point elevation Δtb difference between the boiling points of the pure solvent and a nonelectrolyte solution of that solvent and it is directly proportional to the molal concentration of the solution B-p elevation can be calculated using Δtb = Kbm Δtb (boiling point elevation) expressed in m and m expressed in mol solutekg solvent
Sample Problem
What is the boiling-point elevation of a solution made from 200 g of a nonelectrolyte solute and 4000 g of water The molar mass of the solute is 620 gmol
180
1 AnalyzeGiven solute mass = 200 g solute molar mass = 620 gmol solvent mass and identity = 4000 g of water
Unknown boiling-point elevation
181
2 Plan
182
183
3 Compute
Practice Problem
A solution contains 500 g of sucrose C12H22O11 a nonelectrolyte dissolved in 5000 g of water What is the boiling-point elevation
015degC184
A solution contains 4500 g of sucrose C12H22O11 a nonelectrolyte dissolved in 250 g of waterWhat is the boiling point of the solution
1027degC185
If the boiling point elevation of an aqueous solution containing a nonvolatile electrolyte is 102degC what is the molality of the solution
20 m186
The boiling point of an aqueous solution containing a nonvolatile electrolyte is 10075degC aWhat is the boiling-point elevation bWhat is the molality of the solution a 075degC b 15 m
187
Electrolytes and Colligative PropertiesEarly investigators confused by experiments where certain substances depressed freezing point or elevated boiling point more than expected
188
189
Ex 01 m solution of NaCl lowers freezing point of solvent almost twice as much as 01 m solution of sucrose 01 m CaCl2 lowers fp almost 3 times as much as 01 m solution of sucrose
To understand why this happens contrast behavior or sucrose with NaCl in aqueous solution Each sucrose dissolves to make only 1 particle in solution So 1 mol sucrose dissolves to 1 mol particles 1 mol NaCl dissolves to make 2 moles of particles (1 mol Na+ and 1 mol Cl-)
190
191
Calculated Values for Electrolyte SolutionsColligative properties depend on total concentration of solute particles regardless of their identity Electrolytes cause changes in colligative properties proportional to total molality in terms of all dissolves particles instead of formula units
192
What about barium nitrate Ba(NO3)2 Each mole of barium nitrate yields 3 mol of ions in solution Ba(NO3)2(s) rarr Ba2+(aq) + 2NO3-(aq) You would expect solution of given molality to lower fp of solvent 3 times as much as nonelectrolytic solution of same molality
193
Sample Problem
What is the expected change in the freezing point of water in a solution of 625 g of barium nitrate Ba(NO3)2 in 100 kg of water
194
1 AnalyzeGiven solute mass and formula = 625 g Ba(NO3)2 solvent mass and identity = 100 kg water Δtf = Kfm
Unknown expected freezing-point depression
195
2 PlanThe molality can be calculated by converting the solute mass to moles and then dividing by the number of kilograms of solvent That molality is in terms of formula units of Ba(NO3)2 and must be converted to molality in terms of dissociated ions in solution It must be multiplied by the number of moles of ions produced per mole of formula unit This adjusted molality can then be used to calculate the freezing-point depression
196
197
198
3 Compute
Practice Problem
What is the expected freezing-point depression for a solution that contains 20 mol of magnesium sulfate dissolved in 10 kg of water
minus74degC199
What is the expected boiling-point elevation of water for a solution that contains 150 g of sodium chloride dissolved in 10 kg of water 27degC The freezing point of an aqueous sodium chloride solution is minus200degC What is the molality of the solution 54 m NaCl
200
Actual Values for Electrolyte SolutionsRemember values calculated are only expected values Actual values of colligative properties for all strong electrolytes are almost what would be expected Difference between expected and actual values caused by attractive forces that exist between dissociated ions in aqueous solution
201
Attraction between hydrated ions in solution is small compared with those in crystalline solid However forces of attraction donrsquot interfere with movements of aqueous ions More concentrated solution means ions closer together and attraction is greater Also depends on charge of ions ndash higher charge greater attraction
202
Unique characteristic of strong electrolyte is that they yield only ions
Ex Ionic compound may be highly soluble in water and dissociate into ions (NaCl)
Others may not dissolve much but amount that does dissolve exists only as ions
145
Weak ElectrolytesHF dissolves in water to give acid solution called hydrofluoric acid HF bond stronger than bonds between hydrogen and other halogens When HF dissolves some molecule ionize Reverse reaction also happens HF(aq) + H2O(l) H3O+(aq) + F-(aq)
146
HF(aq) + H2O(l) H3O+
(aq) + F-(aq)Concentration of dissolved unionized HF stays high and concentration of ions stays low
Weak electrolyte - any compound whose aqueous solutions conduct electricity poorly this is due to the presence of small amount of the dissolved compound in the form of ions
147
Different from nonelectrolyte ndash NO ions at all Description of electrolyte as strong or weak not related to concentration of solution Electrolytes differ in degree of ionization not amount of solute dissolved
148
149
Colligative properties of solutions
Presence of solute affects properties of solutions Some properties not dependent on nature of dissolved substance but on how may dissolve particles are present
Colligative properties - properties that depend on the concentration of solute particles but not on their identity
150
Vapor-Pressure Lowering
Boiling and freezing point of solution different from pure solvent Nonvolatile solute raises boiling point and lowers freezing point Nonvolatile substance - one that has little tendency to become gas under existing conditions
151
152
To understand why nonvolatile solute changes boiling and freezing point consider equilibrium vapor pressure
Vapor pressure pressure caused by molecules that have escape liquid phase to gas phase
Can be thought of a measure of tendency of molecules to escape from a liquid
153
Addition of sucrose (nonvolatile solute) lowers concentration of water molecules at surface of liquid This lowers tendency of water molecules to leave solution and enter gas phase Vapor pressure of solution is lower than vapor pressure of pure water
154
Aqueous13 solution13 of13 nonvolatile13 solutePure13 water
Nonelectrolyte solutions of same molality have same concentration of particles
Dilute solutions of same solvent and equal molality of any nonelectrolyte solute lower vapor pressure equally
155
Example
1 m aqueous solution of nonelectrolyte glucose lowers vapor pressure of water 55 x 10-4 atm at 25 1 m aqueous solution of sucrose also lowers water vapor pressure to 55 x 10-4 atm at 25
156
157
bc vapor-pressure lowering depends on concentration of nonelectrolyte solute and doesnrsquot depend on the type of solute it is a colligative proprerty
bc vapor pressure has been lowered solution remains liquid over larger temperature range
Lowers freezing point and raises boiling point
158
Can assume changes in boiling and freezing point also depend on concentration of solute
They are colligative properties
159
Freezing-Point Depression
Freezing point of 1-molal solution of any nonelectrolyte solute in water is found (by experiment) to be 186 lower than freezing point of water When 1 mol of nonelectrolyte solute dissolved in 1 kg water freezing point is -186 not 000 160
When 2 mol nonelectrolyte solute dissolved in 1 kg water freezing point is -372
For any concentration of nonelectrolyte solute in water decrease in freezing point can be determined using value of -186m
161
This value called the molal freezing-point contstant (Kf) is the freezing-point depression of the solvent in a 1-molal solution of a nonvolatile nonelectrolyte solute
162
Each solvent has own characteristic molal freezing-point constant Freezing-point depression (Δtf) the difference between the freezing points of the pure solvent and a solute of nonelectrolyte in that solvent and it is directly proportional to the molal concentration of the solution Δtf = Kfm
163
Δtf = Kfm
Kf expressed as m
m expressed in mol solutekg solvent (molality)
164
Sample Problem
What is the freezing point depression of water in a solution of 171 g of sucrose C12H22O11 and 200 g of water What is the actual freezing point of the solution
165
1 AnalyzeGiven Solute mass and chemical formula 171 g C12H22O11 Solvent mass and identity 2000 g water Unknown a freezing-point depression b freezing point of the solution 166
2 Plan
167
168
Calculate
Unknown a freezing-point depression b freezing point of the solution
169
Practice Problem
A water solution containing an unknown quantity of a nonelectrolyte solute is found to have a freezing point of ndash 023degCWhat is the molal concentration of the solution
012 m170
A solution consists of 103 g of the nonelectrolyte glucose C6H12O6 dissolved in 250 g of water What is the freezing-point depression of the solution
minus0426degC171
In a laboratory experiment the freezing point of an aqueous solution of glucose is found to be minus0325degCWhat is the molal concentration of this solution
0175 m172
If 0500 mol of a nonelectrolyte solute are dissolved in 5000 g of ether what is the freezing point of the solution
minus1181degC173
The freezing point of an aqueous solution that contains a nonelectrolyte is minus90degC aWhat is the freezing-point depression of the solution bWhat is the molal concentration of the solution a minus90degC b 48 m
174
Boiling-Point Elevation
Boiling point is temp at which vapor pressure is equal to atmospheric pressure Change in vapor pressure will cause corresponding change in boiling point
175
176
Vapor pressure of nonvolatile solution is lower than vapor pressure of pure solvent More heat will be required to raise vapor pressure of solution Boiling point of solution is higher than boiling point of pure solvent
Molal boiling-point constant (Kb) - boiling-point elevation of the solvent in a 1-molal solution of a nonvolatile nonelectrolyte solute b-p elevation of any nonelectrolyte in water found by experiment to be 051 So molal b-p constant for water is 051m
177
For different solvents b-p elevations of 1-molal solutions have different values (Table 14-2)
Like freezing-point constants values most accurate for dilute solutions
178
Boiling-point elevation Δtb difference between the boiling points of the pure solvent and a nonelectrolyte solution of that solvent and it is directly proportional to the molal concentration of the solution B-p elevation can be calculated using Δtb = Kbm Δtb (boiling point elevation) expressed in m and m expressed in mol solutekg solvent
Sample Problem
What is the boiling-point elevation of a solution made from 200 g of a nonelectrolyte solute and 4000 g of water The molar mass of the solute is 620 gmol
180
1 AnalyzeGiven solute mass = 200 g solute molar mass = 620 gmol solvent mass and identity = 4000 g of water
Unknown boiling-point elevation
181
2 Plan
182
183
3 Compute
Practice Problem
A solution contains 500 g of sucrose C12H22O11 a nonelectrolyte dissolved in 5000 g of water What is the boiling-point elevation
015degC184
A solution contains 4500 g of sucrose C12H22O11 a nonelectrolyte dissolved in 250 g of waterWhat is the boiling point of the solution
1027degC185
If the boiling point elevation of an aqueous solution containing a nonvolatile electrolyte is 102degC what is the molality of the solution
20 m186
The boiling point of an aqueous solution containing a nonvolatile electrolyte is 10075degC aWhat is the boiling-point elevation bWhat is the molality of the solution a 075degC b 15 m
187
Electrolytes and Colligative PropertiesEarly investigators confused by experiments where certain substances depressed freezing point or elevated boiling point more than expected
188
189
Ex 01 m solution of NaCl lowers freezing point of solvent almost twice as much as 01 m solution of sucrose 01 m CaCl2 lowers fp almost 3 times as much as 01 m solution of sucrose
To understand why this happens contrast behavior or sucrose with NaCl in aqueous solution Each sucrose dissolves to make only 1 particle in solution So 1 mol sucrose dissolves to 1 mol particles 1 mol NaCl dissolves to make 2 moles of particles (1 mol Na+ and 1 mol Cl-)
190
191
Calculated Values for Electrolyte SolutionsColligative properties depend on total concentration of solute particles regardless of their identity Electrolytes cause changes in colligative properties proportional to total molality in terms of all dissolves particles instead of formula units
192
What about barium nitrate Ba(NO3)2 Each mole of barium nitrate yields 3 mol of ions in solution Ba(NO3)2(s) rarr Ba2+(aq) + 2NO3-(aq) You would expect solution of given molality to lower fp of solvent 3 times as much as nonelectrolytic solution of same molality
193
Sample Problem
What is the expected change in the freezing point of water in a solution of 625 g of barium nitrate Ba(NO3)2 in 100 kg of water
194
1 AnalyzeGiven solute mass and formula = 625 g Ba(NO3)2 solvent mass and identity = 100 kg water Δtf = Kfm
Unknown expected freezing-point depression
195
2 PlanThe molality can be calculated by converting the solute mass to moles and then dividing by the number of kilograms of solvent That molality is in terms of formula units of Ba(NO3)2 and must be converted to molality in terms of dissociated ions in solution It must be multiplied by the number of moles of ions produced per mole of formula unit This adjusted molality can then be used to calculate the freezing-point depression
196
197
198
3 Compute
Practice Problem
What is the expected freezing-point depression for a solution that contains 20 mol of magnesium sulfate dissolved in 10 kg of water
minus74degC199
What is the expected boiling-point elevation of water for a solution that contains 150 g of sodium chloride dissolved in 10 kg of water 27degC The freezing point of an aqueous sodium chloride solution is minus200degC What is the molality of the solution 54 m NaCl
200
Actual Values for Electrolyte SolutionsRemember values calculated are only expected values Actual values of colligative properties for all strong electrolytes are almost what would be expected Difference between expected and actual values caused by attractive forces that exist between dissociated ions in aqueous solution
201
Attraction between hydrated ions in solution is small compared with those in crystalline solid However forces of attraction donrsquot interfere with movements of aqueous ions More concentrated solution means ions closer together and attraction is greater Also depends on charge of ions ndash higher charge greater attraction
202
Weak ElectrolytesHF dissolves in water to give acid solution called hydrofluoric acid HF bond stronger than bonds between hydrogen and other halogens When HF dissolves some molecule ionize Reverse reaction also happens HF(aq) + H2O(l) H3O+(aq) + F-(aq)
146
HF(aq) + H2O(l) H3O+
(aq) + F-(aq)Concentration of dissolved unionized HF stays high and concentration of ions stays low
Weak electrolyte - any compound whose aqueous solutions conduct electricity poorly this is due to the presence of small amount of the dissolved compound in the form of ions
147
Different from nonelectrolyte ndash NO ions at all Description of electrolyte as strong or weak not related to concentration of solution Electrolytes differ in degree of ionization not amount of solute dissolved
148
149
Colligative properties of solutions
Presence of solute affects properties of solutions Some properties not dependent on nature of dissolved substance but on how may dissolve particles are present
Colligative properties - properties that depend on the concentration of solute particles but not on their identity
150
Vapor-Pressure Lowering
Boiling and freezing point of solution different from pure solvent Nonvolatile solute raises boiling point and lowers freezing point Nonvolatile substance - one that has little tendency to become gas under existing conditions
151
152
To understand why nonvolatile solute changes boiling and freezing point consider equilibrium vapor pressure
Vapor pressure pressure caused by molecules that have escape liquid phase to gas phase
Can be thought of a measure of tendency of molecules to escape from a liquid
153
Addition of sucrose (nonvolatile solute) lowers concentration of water molecules at surface of liquid This lowers tendency of water molecules to leave solution and enter gas phase Vapor pressure of solution is lower than vapor pressure of pure water
154
Aqueous13 solution13 of13 nonvolatile13 solutePure13 water
Nonelectrolyte solutions of same molality have same concentration of particles
Dilute solutions of same solvent and equal molality of any nonelectrolyte solute lower vapor pressure equally
155
Example
1 m aqueous solution of nonelectrolyte glucose lowers vapor pressure of water 55 x 10-4 atm at 25 1 m aqueous solution of sucrose also lowers water vapor pressure to 55 x 10-4 atm at 25
156
157
bc vapor-pressure lowering depends on concentration of nonelectrolyte solute and doesnrsquot depend on the type of solute it is a colligative proprerty
bc vapor pressure has been lowered solution remains liquid over larger temperature range
Lowers freezing point and raises boiling point
158
Can assume changes in boiling and freezing point also depend on concentration of solute
They are colligative properties
159
Freezing-Point Depression
Freezing point of 1-molal solution of any nonelectrolyte solute in water is found (by experiment) to be 186 lower than freezing point of water When 1 mol of nonelectrolyte solute dissolved in 1 kg water freezing point is -186 not 000 160
When 2 mol nonelectrolyte solute dissolved in 1 kg water freezing point is -372
For any concentration of nonelectrolyte solute in water decrease in freezing point can be determined using value of -186m
161
This value called the molal freezing-point contstant (Kf) is the freezing-point depression of the solvent in a 1-molal solution of a nonvolatile nonelectrolyte solute
162
Each solvent has own characteristic molal freezing-point constant Freezing-point depression (Δtf) the difference between the freezing points of the pure solvent and a solute of nonelectrolyte in that solvent and it is directly proportional to the molal concentration of the solution Δtf = Kfm
163
Δtf = Kfm
Kf expressed as m
m expressed in mol solutekg solvent (molality)
164
Sample Problem
What is the freezing point depression of water in a solution of 171 g of sucrose C12H22O11 and 200 g of water What is the actual freezing point of the solution
165
1 AnalyzeGiven Solute mass and chemical formula 171 g C12H22O11 Solvent mass and identity 2000 g water Unknown a freezing-point depression b freezing point of the solution 166
2 Plan
167
168
Calculate
Unknown a freezing-point depression b freezing point of the solution
169
Practice Problem
A water solution containing an unknown quantity of a nonelectrolyte solute is found to have a freezing point of ndash 023degCWhat is the molal concentration of the solution
012 m170
A solution consists of 103 g of the nonelectrolyte glucose C6H12O6 dissolved in 250 g of water What is the freezing-point depression of the solution
minus0426degC171
In a laboratory experiment the freezing point of an aqueous solution of glucose is found to be minus0325degCWhat is the molal concentration of this solution
0175 m172
If 0500 mol of a nonelectrolyte solute are dissolved in 5000 g of ether what is the freezing point of the solution
minus1181degC173
The freezing point of an aqueous solution that contains a nonelectrolyte is minus90degC aWhat is the freezing-point depression of the solution bWhat is the molal concentration of the solution a minus90degC b 48 m
174
Boiling-Point Elevation
Boiling point is temp at which vapor pressure is equal to atmospheric pressure Change in vapor pressure will cause corresponding change in boiling point
175
176
Vapor pressure of nonvolatile solution is lower than vapor pressure of pure solvent More heat will be required to raise vapor pressure of solution Boiling point of solution is higher than boiling point of pure solvent
Molal boiling-point constant (Kb) - boiling-point elevation of the solvent in a 1-molal solution of a nonvolatile nonelectrolyte solute b-p elevation of any nonelectrolyte in water found by experiment to be 051 So molal b-p constant for water is 051m
177
For different solvents b-p elevations of 1-molal solutions have different values (Table 14-2)
Like freezing-point constants values most accurate for dilute solutions
178
Boiling-point elevation Δtb difference between the boiling points of the pure solvent and a nonelectrolyte solution of that solvent and it is directly proportional to the molal concentration of the solution B-p elevation can be calculated using Δtb = Kbm Δtb (boiling point elevation) expressed in m and m expressed in mol solutekg solvent
Sample Problem
What is the boiling-point elevation of a solution made from 200 g of a nonelectrolyte solute and 4000 g of water The molar mass of the solute is 620 gmol
180
1 AnalyzeGiven solute mass = 200 g solute molar mass = 620 gmol solvent mass and identity = 4000 g of water
Unknown boiling-point elevation
181
2 Plan
182
183
3 Compute
Practice Problem
A solution contains 500 g of sucrose C12H22O11 a nonelectrolyte dissolved in 5000 g of water What is the boiling-point elevation
015degC184
A solution contains 4500 g of sucrose C12H22O11 a nonelectrolyte dissolved in 250 g of waterWhat is the boiling point of the solution
1027degC185
If the boiling point elevation of an aqueous solution containing a nonvolatile electrolyte is 102degC what is the molality of the solution
20 m186
The boiling point of an aqueous solution containing a nonvolatile electrolyte is 10075degC aWhat is the boiling-point elevation bWhat is the molality of the solution a 075degC b 15 m
187
Electrolytes and Colligative PropertiesEarly investigators confused by experiments where certain substances depressed freezing point or elevated boiling point more than expected
188
189
Ex 01 m solution of NaCl lowers freezing point of solvent almost twice as much as 01 m solution of sucrose 01 m CaCl2 lowers fp almost 3 times as much as 01 m solution of sucrose
To understand why this happens contrast behavior or sucrose with NaCl in aqueous solution Each sucrose dissolves to make only 1 particle in solution So 1 mol sucrose dissolves to 1 mol particles 1 mol NaCl dissolves to make 2 moles of particles (1 mol Na+ and 1 mol Cl-)
190
191
Calculated Values for Electrolyte SolutionsColligative properties depend on total concentration of solute particles regardless of their identity Electrolytes cause changes in colligative properties proportional to total molality in terms of all dissolves particles instead of formula units
192
What about barium nitrate Ba(NO3)2 Each mole of barium nitrate yields 3 mol of ions in solution Ba(NO3)2(s) rarr Ba2+(aq) + 2NO3-(aq) You would expect solution of given molality to lower fp of solvent 3 times as much as nonelectrolytic solution of same molality
193
Sample Problem
What is the expected change in the freezing point of water in a solution of 625 g of barium nitrate Ba(NO3)2 in 100 kg of water
194
1 AnalyzeGiven solute mass and formula = 625 g Ba(NO3)2 solvent mass and identity = 100 kg water Δtf = Kfm
Unknown expected freezing-point depression
195
2 PlanThe molality can be calculated by converting the solute mass to moles and then dividing by the number of kilograms of solvent That molality is in terms of formula units of Ba(NO3)2 and must be converted to molality in terms of dissociated ions in solution It must be multiplied by the number of moles of ions produced per mole of formula unit This adjusted molality can then be used to calculate the freezing-point depression
196
197
198
3 Compute
Practice Problem
What is the expected freezing-point depression for a solution that contains 20 mol of magnesium sulfate dissolved in 10 kg of water
minus74degC199
What is the expected boiling-point elevation of water for a solution that contains 150 g of sodium chloride dissolved in 10 kg of water 27degC The freezing point of an aqueous sodium chloride solution is minus200degC What is the molality of the solution 54 m NaCl
200
Actual Values for Electrolyte SolutionsRemember values calculated are only expected values Actual values of colligative properties for all strong electrolytes are almost what would be expected Difference between expected and actual values caused by attractive forces that exist between dissociated ions in aqueous solution
201
Attraction between hydrated ions in solution is small compared with those in crystalline solid However forces of attraction donrsquot interfere with movements of aqueous ions More concentrated solution means ions closer together and attraction is greater Also depends on charge of ions ndash higher charge greater attraction
202
HF(aq) + H2O(l) H3O+
(aq) + F-(aq)Concentration of dissolved unionized HF stays high and concentration of ions stays low
Weak electrolyte - any compound whose aqueous solutions conduct electricity poorly this is due to the presence of small amount of the dissolved compound in the form of ions
147
Different from nonelectrolyte ndash NO ions at all Description of electrolyte as strong or weak not related to concentration of solution Electrolytes differ in degree of ionization not amount of solute dissolved
148
149
Colligative properties of solutions
Presence of solute affects properties of solutions Some properties not dependent on nature of dissolved substance but on how may dissolve particles are present
Colligative properties - properties that depend on the concentration of solute particles but not on their identity
150
Vapor-Pressure Lowering
Boiling and freezing point of solution different from pure solvent Nonvolatile solute raises boiling point and lowers freezing point Nonvolatile substance - one that has little tendency to become gas under existing conditions
151
152
To understand why nonvolatile solute changes boiling and freezing point consider equilibrium vapor pressure
Vapor pressure pressure caused by molecules that have escape liquid phase to gas phase
Can be thought of a measure of tendency of molecules to escape from a liquid
153
Addition of sucrose (nonvolatile solute) lowers concentration of water molecules at surface of liquid This lowers tendency of water molecules to leave solution and enter gas phase Vapor pressure of solution is lower than vapor pressure of pure water
154
Aqueous13 solution13 of13 nonvolatile13 solutePure13 water
Nonelectrolyte solutions of same molality have same concentration of particles
Dilute solutions of same solvent and equal molality of any nonelectrolyte solute lower vapor pressure equally
155
Example
1 m aqueous solution of nonelectrolyte glucose lowers vapor pressure of water 55 x 10-4 atm at 25 1 m aqueous solution of sucrose also lowers water vapor pressure to 55 x 10-4 atm at 25
156
157
bc vapor-pressure lowering depends on concentration of nonelectrolyte solute and doesnrsquot depend on the type of solute it is a colligative proprerty
bc vapor pressure has been lowered solution remains liquid over larger temperature range
Lowers freezing point and raises boiling point
158
Can assume changes in boiling and freezing point also depend on concentration of solute
They are colligative properties
159
Freezing-Point Depression
Freezing point of 1-molal solution of any nonelectrolyte solute in water is found (by experiment) to be 186 lower than freezing point of water When 1 mol of nonelectrolyte solute dissolved in 1 kg water freezing point is -186 not 000 160
When 2 mol nonelectrolyte solute dissolved in 1 kg water freezing point is -372
For any concentration of nonelectrolyte solute in water decrease in freezing point can be determined using value of -186m
161
This value called the molal freezing-point contstant (Kf) is the freezing-point depression of the solvent in a 1-molal solution of a nonvolatile nonelectrolyte solute
162
Each solvent has own characteristic molal freezing-point constant Freezing-point depression (Δtf) the difference between the freezing points of the pure solvent and a solute of nonelectrolyte in that solvent and it is directly proportional to the molal concentration of the solution Δtf = Kfm
163
Δtf = Kfm
Kf expressed as m
m expressed in mol solutekg solvent (molality)
164
Sample Problem
What is the freezing point depression of water in a solution of 171 g of sucrose C12H22O11 and 200 g of water What is the actual freezing point of the solution
165
1 AnalyzeGiven Solute mass and chemical formula 171 g C12H22O11 Solvent mass and identity 2000 g water Unknown a freezing-point depression b freezing point of the solution 166
2 Plan
167
168
Calculate
Unknown a freezing-point depression b freezing point of the solution
169
Practice Problem
A water solution containing an unknown quantity of a nonelectrolyte solute is found to have a freezing point of ndash 023degCWhat is the molal concentration of the solution
012 m170
A solution consists of 103 g of the nonelectrolyte glucose C6H12O6 dissolved in 250 g of water What is the freezing-point depression of the solution
minus0426degC171
In a laboratory experiment the freezing point of an aqueous solution of glucose is found to be minus0325degCWhat is the molal concentration of this solution
0175 m172
If 0500 mol of a nonelectrolyte solute are dissolved in 5000 g of ether what is the freezing point of the solution
minus1181degC173
The freezing point of an aqueous solution that contains a nonelectrolyte is minus90degC aWhat is the freezing-point depression of the solution bWhat is the molal concentration of the solution a minus90degC b 48 m
174
Boiling-Point Elevation
Boiling point is temp at which vapor pressure is equal to atmospheric pressure Change in vapor pressure will cause corresponding change in boiling point
175
176
Vapor pressure of nonvolatile solution is lower than vapor pressure of pure solvent More heat will be required to raise vapor pressure of solution Boiling point of solution is higher than boiling point of pure solvent
Molal boiling-point constant (Kb) - boiling-point elevation of the solvent in a 1-molal solution of a nonvolatile nonelectrolyte solute b-p elevation of any nonelectrolyte in water found by experiment to be 051 So molal b-p constant for water is 051m
177
For different solvents b-p elevations of 1-molal solutions have different values (Table 14-2)
Like freezing-point constants values most accurate for dilute solutions
178
Boiling-point elevation Δtb difference between the boiling points of the pure solvent and a nonelectrolyte solution of that solvent and it is directly proportional to the molal concentration of the solution B-p elevation can be calculated using Δtb = Kbm Δtb (boiling point elevation) expressed in m and m expressed in mol solutekg solvent
Sample Problem
What is the boiling-point elevation of a solution made from 200 g of a nonelectrolyte solute and 4000 g of water The molar mass of the solute is 620 gmol
180
1 AnalyzeGiven solute mass = 200 g solute molar mass = 620 gmol solvent mass and identity = 4000 g of water
Unknown boiling-point elevation
181
2 Plan
182
183
3 Compute
Practice Problem
A solution contains 500 g of sucrose C12H22O11 a nonelectrolyte dissolved in 5000 g of water What is the boiling-point elevation
015degC184
A solution contains 4500 g of sucrose C12H22O11 a nonelectrolyte dissolved in 250 g of waterWhat is the boiling point of the solution
1027degC185
If the boiling point elevation of an aqueous solution containing a nonvolatile electrolyte is 102degC what is the molality of the solution
20 m186
The boiling point of an aqueous solution containing a nonvolatile electrolyte is 10075degC aWhat is the boiling-point elevation bWhat is the molality of the solution a 075degC b 15 m
187
Electrolytes and Colligative PropertiesEarly investigators confused by experiments where certain substances depressed freezing point or elevated boiling point more than expected
188
189
Ex 01 m solution of NaCl lowers freezing point of solvent almost twice as much as 01 m solution of sucrose 01 m CaCl2 lowers fp almost 3 times as much as 01 m solution of sucrose
To understand why this happens contrast behavior or sucrose with NaCl in aqueous solution Each sucrose dissolves to make only 1 particle in solution So 1 mol sucrose dissolves to 1 mol particles 1 mol NaCl dissolves to make 2 moles of particles (1 mol Na+ and 1 mol Cl-)
190
191
Calculated Values for Electrolyte SolutionsColligative properties depend on total concentration of solute particles regardless of their identity Electrolytes cause changes in colligative properties proportional to total molality in terms of all dissolves particles instead of formula units
192
What about barium nitrate Ba(NO3)2 Each mole of barium nitrate yields 3 mol of ions in solution Ba(NO3)2(s) rarr Ba2+(aq) + 2NO3-(aq) You would expect solution of given molality to lower fp of solvent 3 times as much as nonelectrolytic solution of same molality
193
Sample Problem
What is the expected change in the freezing point of water in a solution of 625 g of barium nitrate Ba(NO3)2 in 100 kg of water
194
1 AnalyzeGiven solute mass and formula = 625 g Ba(NO3)2 solvent mass and identity = 100 kg water Δtf = Kfm
Unknown expected freezing-point depression
195
2 PlanThe molality can be calculated by converting the solute mass to moles and then dividing by the number of kilograms of solvent That molality is in terms of formula units of Ba(NO3)2 and must be converted to molality in terms of dissociated ions in solution It must be multiplied by the number of moles of ions produced per mole of formula unit This adjusted molality can then be used to calculate the freezing-point depression
196
197
198
3 Compute
Practice Problem
What is the expected freezing-point depression for a solution that contains 20 mol of magnesium sulfate dissolved in 10 kg of water
minus74degC199
What is the expected boiling-point elevation of water for a solution that contains 150 g of sodium chloride dissolved in 10 kg of water 27degC The freezing point of an aqueous sodium chloride solution is minus200degC What is the molality of the solution 54 m NaCl
200
Actual Values for Electrolyte SolutionsRemember values calculated are only expected values Actual values of colligative properties for all strong electrolytes are almost what would be expected Difference between expected and actual values caused by attractive forces that exist between dissociated ions in aqueous solution
201
Attraction between hydrated ions in solution is small compared with those in crystalline solid However forces of attraction donrsquot interfere with movements of aqueous ions More concentrated solution means ions closer together and attraction is greater Also depends on charge of ions ndash higher charge greater attraction
202
Different from nonelectrolyte ndash NO ions at all Description of electrolyte as strong or weak not related to concentration of solution Electrolytes differ in degree of ionization not amount of solute dissolved
148
149
Colligative properties of solutions
Presence of solute affects properties of solutions Some properties not dependent on nature of dissolved substance but on how may dissolve particles are present
Colligative properties - properties that depend on the concentration of solute particles but not on their identity
150
Vapor-Pressure Lowering
Boiling and freezing point of solution different from pure solvent Nonvolatile solute raises boiling point and lowers freezing point Nonvolatile substance - one that has little tendency to become gas under existing conditions
151
152
To understand why nonvolatile solute changes boiling and freezing point consider equilibrium vapor pressure
Vapor pressure pressure caused by molecules that have escape liquid phase to gas phase
Can be thought of a measure of tendency of molecules to escape from a liquid
153
Addition of sucrose (nonvolatile solute) lowers concentration of water molecules at surface of liquid This lowers tendency of water molecules to leave solution and enter gas phase Vapor pressure of solution is lower than vapor pressure of pure water
154
Aqueous13 solution13 of13 nonvolatile13 solutePure13 water
Nonelectrolyte solutions of same molality have same concentration of particles
Dilute solutions of same solvent and equal molality of any nonelectrolyte solute lower vapor pressure equally
155
Example
1 m aqueous solution of nonelectrolyte glucose lowers vapor pressure of water 55 x 10-4 atm at 25 1 m aqueous solution of sucrose also lowers water vapor pressure to 55 x 10-4 atm at 25
156
157
bc vapor-pressure lowering depends on concentration of nonelectrolyte solute and doesnrsquot depend on the type of solute it is a colligative proprerty
bc vapor pressure has been lowered solution remains liquid over larger temperature range
Lowers freezing point and raises boiling point
158
Can assume changes in boiling and freezing point also depend on concentration of solute
They are colligative properties
159
Freezing-Point Depression
Freezing point of 1-molal solution of any nonelectrolyte solute in water is found (by experiment) to be 186 lower than freezing point of water When 1 mol of nonelectrolyte solute dissolved in 1 kg water freezing point is -186 not 000 160
When 2 mol nonelectrolyte solute dissolved in 1 kg water freezing point is -372
For any concentration of nonelectrolyte solute in water decrease in freezing point can be determined using value of -186m
161
This value called the molal freezing-point contstant (Kf) is the freezing-point depression of the solvent in a 1-molal solution of a nonvolatile nonelectrolyte solute
162
Each solvent has own characteristic molal freezing-point constant Freezing-point depression (Δtf) the difference between the freezing points of the pure solvent and a solute of nonelectrolyte in that solvent and it is directly proportional to the molal concentration of the solution Δtf = Kfm
163
Δtf = Kfm
Kf expressed as m
m expressed in mol solutekg solvent (molality)
164
Sample Problem
What is the freezing point depression of water in a solution of 171 g of sucrose C12H22O11 and 200 g of water What is the actual freezing point of the solution
165
1 AnalyzeGiven Solute mass and chemical formula 171 g C12H22O11 Solvent mass and identity 2000 g water Unknown a freezing-point depression b freezing point of the solution 166
2 Plan
167
168
Calculate
Unknown a freezing-point depression b freezing point of the solution
169
Practice Problem
A water solution containing an unknown quantity of a nonelectrolyte solute is found to have a freezing point of ndash 023degCWhat is the molal concentration of the solution
012 m170
A solution consists of 103 g of the nonelectrolyte glucose C6H12O6 dissolved in 250 g of water What is the freezing-point depression of the solution
minus0426degC171
In a laboratory experiment the freezing point of an aqueous solution of glucose is found to be minus0325degCWhat is the molal concentration of this solution
0175 m172
If 0500 mol of a nonelectrolyte solute are dissolved in 5000 g of ether what is the freezing point of the solution
minus1181degC173
The freezing point of an aqueous solution that contains a nonelectrolyte is minus90degC aWhat is the freezing-point depression of the solution bWhat is the molal concentration of the solution a minus90degC b 48 m
174
Boiling-Point Elevation
Boiling point is temp at which vapor pressure is equal to atmospheric pressure Change in vapor pressure will cause corresponding change in boiling point
175
176
Vapor pressure of nonvolatile solution is lower than vapor pressure of pure solvent More heat will be required to raise vapor pressure of solution Boiling point of solution is higher than boiling point of pure solvent
Molal boiling-point constant (Kb) - boiling-point elevation of the solvent in a 1-molal solution of a nonvolatile nonelectrolyte solute b-p elevation of any nonelectrolyte in water found by experiment to be 051 So molal b-p constant for water is 051m
177
For different solvents b-p elevations of 1-molal solutions have different values (Table 14-2)
Like freezing-point constants values most accurate for dilute solutions
178
Boiling-point elevation Δtb difference between the boiling points of the pure solvent and a nonelectrolyte solution of that solvent and it is directly proportional to the molal concentration of the solution B-p elevation can be calculated using Δtb = Kbm Δtb (boiling point elevation) expressed in m and m expressed in mol solutekg solvent
Sample Problem
What is the boiling-point elevation of a solution made from 200 g of a nonelectrolyte solute and 4000 g of water The molar mass of the solute is 620 gmol
180
1 AnalyzeGiven solute mass = 200 g solute molar mass = 620 gmol solvent mass and identity = 4000 g of water
Unknown boiling-point elevation
181
2 Plan
182
183
3 Compute
Practice Problem
A solution contains 500 g of sucrose C12H22O11 a nonelectrolyte dissolved in 5000 g of water What is the boiling-point elevation
015degC184
A solution contains 4500 g of sucrose C12H22O11 a nonelectrolyte dissolved in 250 g of waterWhat is the boiling point of the solution
1027degC185
If the boiling point elevation of an aqueous solution containing a nonvolatile electrolyte is 102degC what is the molality of the solution
20 m186
The boiling point of an aqueous solution containing a nonvolatile electrolyte is 10075degC aWhat is the boiling-point elevation bWhat is the molality of the solution a 075degC b 15 m
187
Electrolytes and Colligative PropertiesEarly investigators confused by experiments where certain substances depressed freezing point or elevated boiling point more than expected
188
189
Ex 01 m solution of NaCl lowers freezing point of solvent almost twice as much as 01 m solution of sucrose 01 m CaCl2 lowers fp almost 3 times as much as 01 m solution of sucrose
To understand why this happens contrast behavior or sucrose with NaCl in aqueous solution Each sucrose dissolves to make only 1 particle in solution So 1 mol sucrose dissolves to 1 mol particles 1 mol NaCl dissolves to make 2 moles of particles (1 mol Na+ and 1 mol Cl-)
190
191
Calculated Values for Electrolyte SolutionsColligative properties depend on total concentration of solute particles regardless of their identity Electrolytes cause changes in colligative properties proportional to total molality in terms of all dissolves particles instead of formula units
192
What about barium nitrate Ba(NO3)2 Each mole of barium nitrate yields 3 mol of ions in solution Ba(NO3)2(s) rarr Ba2+(aq) + 2NO3-(aq) You would expect solution of given molality to lower fp of solvent 3 times as much as nonelectrolytic solution of same molality
193
Sample Problem
What is the expected change in the freezing point of water in a solution of 625 g of barium nitrate Ba(NO3)2 in 100 kg of water
194
1 AnalyzeGiven solute mass and formula = 625 g Ba(NO3)2 solvent mass and identity = 100 kg water Δtf = Kfm
Unknown expected freezing-point depression
195
2 PlanThe molality can be calculated by converting the solute mass to moles and then dividing by the number of kilograms of solvent That molality is in terms of formula units of Ba(NO3)2 and must be converted to molality in terms of dissociated ions in solution It must be multiplied by the number of moles of ions produced per mole of formula unit This adjusted molality can then be used to calculate the freezing-point depression
196
197
198
3 Compute
Practice Problem
What is the expected freezing-point depression for a solution that contains 20 mol of magnesium sulfate dissolved in 10 kg of water
minus74degC199
What is the expected boiling-point elevation of water for a solution that contains 150 g of sodium chloride dissolved in 10 kg of water 27degC The freezing point of an aqueous sodium chloride solution is minus200degC What is the molality of the solution 54 m NaCl
200
Actual Values for Electrolyte SolutionsRemember values calculated are only expected values Actual values of colligative properties for all strong electrolytes are almost what would be expected Difference between expected and actual values caused by attractive forces that exist between dissociated ions in aqueous solution
201
Attraction between hydrated ions in solution is small compared with those in crystalline solid However forces of attraction donrsquot interfere with movements of aqueous ions More concentrated solution means ions closer together and attraction is greater Also depends on charge of ions ndash higher charge greater attraction
202
149
Colligative properties of solutions
Presence of solute affects properties of solutions Some properties not dependent on nature of dissolved substance but on how may dissolve particles are present
Colligative properties - properties that depend on the concentration of solute particles but not on their identity
150
Vapor-Pressure Lowering
Boiling and freezing point of solution different from pure solvent Nonvolatile solute raises boiling point and lowers freezing point Nonvolatile substance - one that has little tendency to become gas under existing conditions
151
152
To understand why nonvolatile solute changes boiling and freezing point consider equilibrium vapor pressure
Vapor pressure pressure caused by molecules that have escape liquid phase to gas phase
Can be thought of a measure of tendency of molecules to escape from a liquid
153
Addition of sucrose (nonvolatile solute) lowers concentration of water molecules at surface of liquid This lowers tendency of water molecules to leave solution and enter gas phase Vapor pressure of solution is lower than vapor pressure of pure water
154
Aqueous13 solution13 of13 nonvolatile13 solutePure13 water
Nonelectrolyte solutions of same molality have same concentration of particles
Dilute solutions of same solvent and equal molality of any nonelectrolyte solute lower vapor pressure equally
155
Example
1 m aqueous solution of nonelectrolyte glucose lowers vapor pressure of water 55 x 10-4 atm at 25 1 m aqueous solution of sucrose also lowers water vapor pressure to 55 x 10-4 atm at 25
156
157
bc vapor-pressure lowering depends on concentration of nonelectrolyte solute and doesnrsquot depend on the type of solute it is a colligative proprerty
bc vapor pressure has been lowered solution remains liquid over larger temperature range
Lowers freezing point and raises boiling point
158
Can assume changes in boiling and freezing point also depend on concentration of solute
They are colligative properties
159
Freezing-Point Depression
Freezing point of 1-molal solution of any nonelectrolyte solute in water is found (by experiment) to be 186 lower than freezing point of water When 1 mol of nonelectrolyte solute dissolved in 1 kg water freezing point is -186 not 000 160
When 2 mol nonelectrolyte solute dissolved in 1 kg water freezing point is -372
For any concentration of nonelectrolyte solute in water decrease in freezing point can be determined using value of -186m
161
This value called the molal freezing-point contstant (Kf) is the freezing-point depression of the solvent in a 1-molal solution of a nonvolatile nonelectrolyte solute
162
Each solvent has own characteristic molal freezing-point constant Freezing-point depression (Δtf) the difference between the freezing points of the pure solvent and a solute of nonelectrolyte in that solvent and it is directly proportional to the molal concentration of the solution Δtf = Kfm
163
Δtf = Kfm
Kf expressed as m
m expressed in mol solutekg solvent (molality)
164
Sample Problem
What is the freezing point depression of water in a solution of 171 g of sucrose C12H22O11 and 200 g of water What is the actual freezing point of the solution
165
1 AnalyzeGiven Solute mass and chemical formula 171 g C12H22O11 Solvent mass and identity 2000 g water Unknown a freezing-point depression b freezing point of the solution 166
2 Plan
167
168
Calculate
Unknown a freezing-point depression b freezing point of the solution
169
Practice Problem
A water solution containing an unknown quantity of a nonelectrolyte solute is found to have a freezing point of ndash 023degCWhat is the molal concentration of the solution
012 m170
A solution consists of 103 g of the nonelectrolyte glucose C6H12O6 dissolved in 250 g of water What is the freezing-point depression of the solution
minus0426degC171
In a laboratory experiment the freezing point of an aqueous solution of glucose is found to be minus0325degCWhat is the molal concentration of this solution
0175 m172
If 0500 mol of a nonelectrolyte solute are dissolved in 5000 g of ether what is the freezing point of the solution
minus1181degC173
The freezing point of an aqueous solution that contains a nonelectrolyte is minus90degC aWhat is the freezing-point depression of the solution bWhat is the molal concentration of the solution a minus90degC b 48 m
174
Boiling-Point Elevation
Boiling point is temp at which vapor pressure is equal to atmospheric pressure Change in vapor pressure will cause corresponding change in boiling point
175
176
Vapor pressure of nonvolatile solution is lower than vapor pressure of pure solvent More heat will be required to raise vapor pressure of solution Boiling point of solution is higher than boiling point of pure solvent
Molal boiling-point constant (Kb) - boiling-point elevation of the solvent in a 1-molal solution of a nonvolatile nonelectrolyte solute b-p elevation of any nonelectrolyte in water found by experiment to be 051 So molal b-p constant for water is 051m
177
For different solvents b-p elevations of 1-molal solutions have different values (Table 14-2)
Like freezing-point constants values most accurate for dilute solutions
178
Boiling-point elevation Δtb difference between the boiling points of the pure solvent and a nonelectrolyte solution of that solvent and it is directly proportional to the molal concentration of the solution B-p elevation can be calculated using Δtb = Kbm Δtb (boiling point elevation) expressed in m and m expressed in mol solutekg solvent
Sample Problem
What is the boiling-point elevation of a solution made from 200 g of a nonelectrolyte solute and 4000 g of water The molar mass of the solute is 620 gmol
180
1 AnalyzeGiven solute mass = 200 g solute molar mass = 620 gmol solvent mass and identity = 4000 g of water
Unknown boiling-point elevation
181
2 Plan
182
183
3 Compute
Practice Problem
A solution contains 500 g of sucrose C12H22O11 a nonelectrolyte dissolved in 5000 g of water What is the boiling-point elevation
015degC184
A solution contains 4500 g of sucrose C12H22O11 a nonelectrolyte dissolved in 250 g of waterWhat is the boiling point of the solution
1027degC185
If the boiling point elevation of an aqueous solution containing a nonvolatile electrolyte is 102degC what is the molality of the solution
20 m186
The boiling point of an aqueous solution containing a nonvolatile electrolyte is 10075degC aWhat is the boiling-point elevation bWhat is the molality of the solution a 075degC b 15 m
187
Electrolytes and Colligative PropertiesEarly investigators confused by experiments where certain substances depressed freezing point or elevated boiling point more than expected
188
189
Ex 01 m solution of NaCl lowers freezing point of solvent almost twice as much as 01 m solution of sucrose 01 m CaCl2 lowers fp almost 3 times as much as 01 m solution of sucrose
To understand why this happens contrast behavior or sucrose with NaCl in aqueous solution Each sucrose dissolves to make only 1 particle in solution So 1 mol sucrose dissolves to 1 mol particles 1 mol NaCl dissolves to make 2 moles of particles (1 mol Na+ and 1 mol Cl-)
190
191
Calculated Values for Electrolyte SolutionsColligative properties depend on total concentration of solute particles regardless of their identity Electrolytes cause changes in colligative properties proportional to total molality in terms of all dissolves particles instead of formula units
192
What about barium nitrate Ba(NO3)2 Each mole of barium nitrate yields 3 mol of ions in solution Ba(NO3)2(s) rarr Ba2+(aq) + 2NO3-(aq) You would expect solution of given molality to lower fp of solvent 3 times as much as nonelectrolytic solution of same molality
193
Sample Problem
What is the expected change in the freezing point of water in a solution of 625 g of barium nitrate Ba(NO3)2 in 100 kg of water
194
1 AnalyzeGiven solute mass and formula = 625 g Ba(NO3)2 solvent mass and identity = 100 kg water Δtf = Kfm
Unknown expected freezing-point depression
195
2 PlanThe molality can be calculated by converting the solute mass to moles and then dividing by the number of kilograms of solvent That molality is in terms of formula units of Ba(NO3)2 and must be converted to molality in terms of dissociated ions in solution It must be multiplied by the number of moles of ions produced per mole of formula unit This adjusted molality can then be used to calculate the freezing-point depression
196
197
198
3 Compute
Practice Problem
What is the expected freezing-point depression for a solution that contains 20 mol of magnesium sulfate dissolved in 10 kg of water
minus74degC199
What is the expected boiling-point elevation of water for a solution that contains 150 g of sodium chloride dissolved in 10 kg of water 27degC The freezing point of an aqueous sodium chloride solution is minus200degC What is the molality of the solution 54 m NaCl
200
Actual Values for Electrolyte SolutionsRemember values calculated are only expected values Actual values of colligative properties for all strong electrolytes are almost what would be expected Difference between expected and actual values caused by attractive forces that exist between dissociated ions in aqueous solution
201
Attraction between hydrated ions in solution is small compared with those in crystalline solid However forces of attraction donrsquot interfere with movements of aqueous ions More concentrated solution means ions closer together and attraction is greater Also depends on charge of ions ndash higher charge greater attraction
202
Presence of solute affects properties of solutions Some properties not dependent on nature of dissolved substance but on how may dissolve particles are present
Colligative properties - properties that depend on the concentration of solute particles but not on their identity
150
Vapor-Pressure Lowering
Boiling and freezing point of solution different from pure solvent Nonvolatile solute raises boiling point and lowers freezing point Nonvolatile substance - one that has little tendency to become gas under existing conditions
151
152
To understand why nonvolatile solute changes boiling and freezing point consider equilibrium vapor pressure
Vapor pressure pressure caused by molecules that have escape liquid phase to gas phase
Can be thought of a measure of tendency of molecules to escape from a liquid
153
Addition of sucrose (nonvolatile solute) lowers concentration of water molecules at surface of liquid This lowers tendency of water molecules to leave solution and enter gas phase Vapor pressure of solution is lower than vapor pressure of pure water
154
Aqueous13 solution13 of13 nonvolatile13 solutePure13 water
Nonelectrolyte solutions of same molality have same concentration of particles
Dilute solutions of same solvent and equal molality of any nonelectrolyte solute lower vapor pressure equally
155
Example
1 m aqueous solution of nonelectrolyte glucose lowers vapor pressure of water 55 x 10-4 atm at 25 1 m aqueous solution of sucrose also lowers water vapor pressure to 55 x 10-4 atm at 25
156
157
bc vapor-pressure lowering depends on concentration of nonelectrolyte solute and doesnrsquot depend on the type of solute it is a colligative proprerty
bc vapor pressure has been lowered solution remains liquid over larger temperature range
Lowers freezing point and raises boiling point
158
Can assume changes in boiling and freezing point also depend on concentration of solute
They are colligative properties
159
Freezing-Point Depression
Freezing point of 1-molal solution of any nonelectrolyte solute in water is found (by experiment) to be 186 lower than freezing point of water When 1 mol of nonelectrolyte solute dissolved in 1 kg water freezing point is -186 not 000 160
When 2 mol nonelectrolyte solute dissolved in 1 kg water freezing point is -372
For any concentration of nonelectrolyte solute in water decrease in freezing point can be determined using value of -186m
161
This value called the molal freezing-point contstant (Kf) is the freezing-point depression of the solvent in a 1-molal solution of a nonvolatile nonelectrolyte solute
162
Each solvent has own characteristic molal freezing-point constant Freezing-point depression (Δtf) the difference between the freezing points of the pure solvent and a solute of nonelectrolyte in that solvent and it is directly proportional to the molal concentration of the solution Δtf = Kfm
163
Δtf = Kfm
Kf expressed as m
m expressed in mol solutekg solvent (molality)
164
Sample Problem
What is the freezing point depression of water in a solution of 171 g of sucrose C12H22O11 and 200 g of water What is the actual freezing point of the solution
165
1 AnalyzeGiven Solute mass and chemical formula 171 g C12H22O11 Solvent mass and identity 2000 g water Unknown a freezing-point depression b freezing point of the solution 166
2 Plan
167
168
Calculate
Unknown a freezing-point depression b freezing point of the solution
169
Practice Problem
A water solution containing an unknown quantity of a nonelectrolyte solute is found to have a freezing point of ndash 023degCWhat is the molal concentration of the solution
012 m170
A solution consists of 103 g of the nonelectrolyte glucose C6H12O6 dissolved in 250 g of water What is the freezing-point depression of the solution
minus0426degC171
In a laboratory experiment the freezing point of an aqueous solution of glucose is found to be minus0325degCWhat is the molal concentration of this solution
0175 m172
If 0500 mol of a nonelectrolyte solute are dissolved in 5000 g of ether what is the freezing point of the solution
minus1181degC173
The freezing point of an aqueous solution that contains a nonelectrolyte is minus90degC aWhat is the freezing-point depression of the solution bWhat is the molal concentration of the solution a minus90degC b 48 m
174
Boiling-Point Elevation
Boiling point is temp at which vapor pressure is equal to atmospheric pressure Change in vapor pressure will cause corresponding change in boiling point
175
176
Vapor pressure of nonvolatile solution is lower than vapor pressure of pure solvent More heat will be required to raise vapor pressure of solution Boiling point of solution is higher than boiling point of pure solvent
Molal boiling-point constant (Kb) - boiling-point elevation of the solvent in a 1-molal solution of a nonvolatile nonelectrolyte solute b-p elevation of any nonelectrolyte in water found by experiment to be 051 So molal b-p constant for water is 051m
177
For different solvents b-p elevations of 1-molal solutions have different values (Table 14-2)
Like freezing-point constants values most accurate for dilute solutions
178
Boiling-point elevation Δtb difference between the boiling points of the pure solvent and a nonelectrolyte solution of that solvent and it is directly proportional to the molal concentration of the solution B-p elevation can be calculated using Δtb = Kbm Δtb (boiling point elevation) expressed in m and m expressed in mol solutekg solvent
Sample Problem
What is the boiling-point elevation of a solution made from 200 g of a nonelectrolyte solute and 4000 g of water The molar mass of the solute is 620 gmol
180
1 AnalyzeGiven solute mass = 200 g solute molar mass = 620 gmol solvent mass and identity = 4000 g of water
Unknown boiling-point elevation
181
2 Plan
182
183
3 Compute
Practice Problem
A solution contains 500 g of sucrose C12H22O11 a nonelectrolyte dissolved in 5000 g of water What is the boiling-point elevation
015degC184
A solution contains 4500 g of sucrose C12H22O11 a nonelectrolyte dissolved in 250 g of waterWhat is the boiling point of the solution
1027degC185
If the boiling point elevation of an aqueous solution containing a nonvolatile electrolyte is 102degC what is the molality of the solution
20 m186
The boiling point of an aqueous solution containing a nonvolatile electrolyte is 10075degC aWhat is the boiling-point elevation bWhat is the molality of the solution a 075degC b 15 m
187
Electrolytes and Colligative PropertiesEarly investigators confused by experiments where certain substances depressed freezing point or elevated boiling point more than expected
188
189
Ex 01 m solution of NaCl lowers freezing point of solvent almost twice as much as 01 m solution of sucrose 01 m CaCl2 lowers fp almost 3 times as much as 01 m solution of sucrose
To understand why this happens contrast behavior or sucrose with NaCl in aqueous solution Each sucrose dissolves to make only 1 particle in solution So 1 mol sucrose dissolves to 1 mol particles 1 mol NaCl dissolves to make 2 moles of particles (1 mol Na+ and 1 mol Cl-)
190
191
Calculated Values for Electrolyte SolutionsColligative properties depend on total concentration of solute particles regardless of their identity Electrolytes cause changes in colligative properties proportional to total molality in terms of all dissolves particles instead of formula units
192
What about barium nitrate Ba(NO3)2 Each mole of barium nitrate yields 3 mol of ions in solution Ba(NO3)2(s) rarr Ba2+(aq) + 2NO3-(aq) You would expect solution of given molality to lower fp of solvent 3 times as much as nonelectrolytic solution of same molality
193
Sample Problem
What is the expected change in the freezing point of water in a solution of 625 g of barium nitrate Ba(NO3)2 in 100 kg of water
194
1 AnalyzeGiven solute mass and formula = 625 g Ba(NO3)2 solvent mass and identity = 100 kg water Δtf = Kfm
Unknown expected freezing-point depression
195
2 PlanThe molality can be calculated by converting the solute mass to moles and then dividing by the number of kilograms of solvent That molality is in terms of formula units of Ba(NO3)2 and must be converted to molality in terms of dissociated ions in solution It must be multiplied by the number of moles of ions produced per mole of formula unit This adjusted molality can then be used to calculate the freezing-point depression
196
197
198
3 Compute
Practice Problem
What is the expected freezing-point depression for a solution that contains 20 mol of magnesium sulfate dissolved in 10 kg of water
minus74degC199
What is the expected boiling-point elevation of water for a solution that contains 150 g of sodium chloride dissolved in 10 kg of water 27degC The freezing point of an aqueous sodium chloride solution is minus200degC What is the molality of the solution 54 m NaCl
200
Actual Values for Electrolyte SolutionsRemember values calculated are only expected values Actual values of colligative properties for all strong electrolytes are almost what would be expected Difference between expected and actual values caused by attractive forces that exist between dissociated ions in aqueous solution
201
Attraction between hydrated ions in solution is small compared with those in crystalline solid However forces of attraction donrsquot interfere with movements of aqueous ions More concentrated solution means ions closer together and attraction is greater Also depends on charge of ions ndash higher charge greater attraction
202
Vapor-Pressure Lowering
Boiling and freezing point of solution different from pure solvent Nonvolatile solute raises boiling point and lowers freezing point Nonvolatile substance - one that has little tendency to become gas under existing conditions
151
152
To understand why nonvolatile solute changes boiling and freezing point consider equilibrium vapor pressure
Vapor pressure pressure caused by molecules that have escape liquid phase to gas phase
Can be thought of a measure of tendency of molecules to escape from a liquid
153
Addition of sucrose (nonvolatile solute) lowers concentration of water molecules at surface of liquid This lowers tendency of water molecules to leave solution and enter gas phase Vapor pressure of solution is lower than vapor pressure of pure water
154
Aqueous13 solution13 of13 nonvolatile13 solutePure13 water
Nonelectrolyte solutions of same molality have same concentration of particles
Dilute solutions of same solvent and equal molality of any nonelectrolyte solute lower vapor pressure equally
155
Example
1 m aqueous solution of nonelectrolyte glucose lowers vapor pressure of water 55 x 10-4 atm at 25 1 m aqueous solution of sucrose also lowers water vapor pressure to 55 x 10-4 atm at 25
156
157
bc vapor-pressure lowering depends on concentration of nonelectrolyte solute and doesnrsquot depend on the type of solute it is a colligative proprerty
bc vapor pressure has been lowered solution remains liquid over larger temperature range
Lowers freezing point and raises boiling point
158
Can assume changes in boiling and freezing point also depend on concentration of solute
They are colligative properties
159
Freezing-Point Depression
Freezing point of 1-molal solution of any nonelectrolyte solute in water is found (by experiment) to be 186 lower than freezing point of water When 1 mol of nonelectrolyte solute dissolved in 1 kg water freezing point is -186 not 000 160
When 2 mol nonelectrolyte solute dissolved in 1 kg water freezing point is -372
For any concentration of nonelectrolyte solute in water decrease in freezing point can be determined using value of -186m
161
This value called the molal freezing-point contstant (Kf) is the freezing-point depression of the solvent in a 1-molal solution of a nonvolatile nonelectrolyte solute
162
Each solvent has own characteristic molal freezing-point constant Freezing-point depression (Δtf) the difference between the freezing points of the pure solvent and a solute of nonelectrolyte in that solvent and it is directly proportional to the molal concentration of the solution Δtf = Kfm
163
Δtf = Kfm
Kf expressed as m
m expressed in mol solutekg solvent (molality)
164
Sample Problem
What is the freezing point depression of water in a solution of 171 g of sucrose C12H22O11 and 200 g of water What is the actual freezing point of the solution
165
1 AnalyzeGiven Solute mass and chemical formula 171 g C12H22O11 Solvent mass and identity 2000 g water Unknown a freezing-point depression b freezing point of the solution 166
2 Plan
167
168
Calculate
Unknown a freezing-point depression b freezing point of the solution
169
Practice Problem
A water solution containing an unknown quantity of a nonelectrolyte solute is found to have a freezing point of ndash 023degCWhat is the molal concentration of the solution
012 m170
A solution consists of 103 g of the nonelectrolyte glucose C6H12O6 dissolved in 250 g of water What is the freezing-point depression of the solution
minus0426degC171
In a laboratory experiment the freezing point of an aqueous solution of glucose is found to be minus0325degCWhat is the molal concentration of this solution
0175 m172
If 0500 mol of a nonelectrolyte solute are dissolved in 5000 g of ether what is the freezing point of the solution
minus1181degC173
The freezing point of an aqueous solution that contains a nonelectrolyte is minus90degC aWhat is the freezing-point depression of the solution bWhat is the molal concentration of the solution a minus90degC b 48 m
174
Boiling-Point Elevation
Boiling point is temp at which vapor pressure is equal to atmospheric pressure Change in vapor pressure will cause corresponding change in boiling point
175
176
Vapor pressure of nonvolatile solution is lower than vapor pressure of pure solvent More heat will be required to raise vapor pressure of solution Boiling point of solution is higher than boiling point of pure solvent
Molal boiling-point constant (Kb) - boiling-point elevation of the solvent in a 1-molal solution of a nonvolatile nonelectrolyte solute b-p elevation of any nonelectrolyte in water found by experiment to be 051 So molal b-p constant for water is 051m
177
For different solvents b-p elevations of 1-molal solutions have different values (Table 14-2)
Like freezing-point constants values most accurate for dilute solutions
178
Boiling-point elevation Δtb difference between the boiling points of the pure solvent and a nonelectrolyte solution of that solvent and it is directly proportional to the molal concentration of the solution B-p elevation can be calculated using Δtb = Kbm Δtb (boiling point elevation) expressed in m and m expressed in mol solutekg solvent
Sample Problem
What is the boiling-point elevation of a solution made from 200 g of a nonelectrolyte solute and 4000 g of water The molar mass of the solute is 620 gmol
180
1 AnalyzeGiven solute mass = 200 g solute molar mass = 620 gmol solvent mass and identity = 4000 g of water
Unknown boiling-point elevation
181
2 Plan
182
183
3 Compute
Practice Problem
A solution contains 500 g of sucrose C12H22O11 a nonelectrolyte dissolved in 5000 g of water What is the boiling-point elevation
015degC184
A solution contains 4500 g of sucrose C12H22O11 a nonelectrolyte dissolved in 250 g of waterWhat is the boiling point of the solution
1027degC185
If the boiling point elevation of an aqueous solution containing a nonvolatile electrolyte is 102degC what is the molality of the solution
20 m186
The boiling point of an aqueous solution containing a nonvolatile electrolyte is 10075degC aWhat is the boiling-point elevation bWhat is the molality of the solution a 075degC b 15 m
187
Electrolytes and Colligative PropertiesEarly investigators confused by experiments where certain substances depressed freezing point or elevated boiling point more than expected
188
189
Ex 01 m solution of NaCl lowers freezing point of solvent almost twice as much as 01 m solution of sucrose 01 m CaCl2 lowers fp almost 3 times as much as 01 m solution of sucrose
To understand why this happens contrast behavior or sucrose with NaCl in aqueous solution Each sucrose dissolves to make only 1 particle in solution So 1 mol sucrose dissolves to 1 mol particles 1 mol NaCl dissolves to make 2 moles of particles (1 mol Na+ and 1 mol Cl-)
190
191
Calculated Values for Electrolyte SolutionsColligative properties depend on total concentration of solute particles regardless of their identity Electrolytes cause changes in colligative properties proportional to total molality in terms of all dissolves particles instead of formula units
192
What about barium nitrate Ba(NO3)2 Each mole of barium nitrate yields 3 mol of ions in solution Ba(NO3)2(s) rarr Ba2+(aq) + 2NO3-(aq) You would expect solution of given molality to lower fp of solvent 3 times as much as nonelectrolytic solution of same molality
193
Sample Problem
What is the expected change in the freezing point of water in a solution of 625 g of barium nitrate Ba(NO3)2 in 100 kg of water
194
1 AnalyzeGiven solute mass and formula = 625 g Ba(NO3)2 solvent mass and identity = 100 kg water Δtf = Kfm
Unknown expected freezing-point depression
195
2 PlanThe molality can be calculated by converting the solute mass to moles and then dividing by the number of kilograms of solvent That molality is in terms of formula units of Ba(NO3)2 and must be converted to molality in terms of dissociated ions in solution It must be multiplied by the number of moles of ions produced per mole of formula unit This adjusted molality can then be used to calculate the freezing-point depression
196
197
198
3 Compute
Practice Problem
What is the expected freezing-point depression for a solution that contains 20 mol of magnesium sulfate dissolved in 10 kg of water
minus74degC199
What is the expected boiling-point elevation of water for a solution that contains 150 g of sodium chloride dissolved in 10 kg of water 27degC The freezing point of an aqueous sodium chloride solution is minus200degC What is the molality of the solution 54 m NaCl
200
Actual Values for Electrolyte SolutionsRemember values calculated are only expected values Actual values of colligative properties for all strong electrolytes are almost what would be expected Difference between expected and actual values caused by attractive forces that exist between dissociated ions in aqueous solution
201
Attraction between hydrated ions in solution is small compared with those in crystalline solid However forces of attraction donrsquot interfere with movements of aqueous ions More concentrated solution means ions closer together and attraction is greater Also depends on charge of ions ndash higher charge greater attraction
202
152
To understand why nonvolatile solute changes boiling and freezing point consider equilibrium vapor pressure
Vapor pressure pressure caused by molecules that have escape liquid phase to gas phase
Can be thought of a measure of tendency of molecules to escape from a liquid
153
Addition of sucrose (nonvolatile solute) lowers concentration of water molecules at surface of liquid This lowers tendency of water molecules to leave solution and enter gas phase Vapor pressure of solution is lower than vapor pressure of pure water
154
Aqueous13 solution13 of13 nonvolatile13 solutePure13 water
Nonelectrolyte solutions of same molality have same concentration of particles
Dilute solutions of same solvent and equal molality of any nonelectrolyte solute lower vapor pressure equally
155
Example
1 m aqueous solution of nonelectrolyte glucose lowers vapor pressure of water 55 x 10-4 atm at 25 1 m aqueous solution of sucrose also lowers water vapor pressure to 55 x 10-4 atm at 25
156
157
bc vapor-pressure lowering depends on concentration of nonelectrolyte solute and doesnrsquot depend on the type of solute it is a colligative proprerty
bc vapor pressure has been lowered solution remains liquid over larger temperature range
Lowers freezing point and raises boiling point
158
Can assume changes in boiling and freezing point also depend on concentration of solute
They are colligative properties
159
Freezing-Point Depression
Freezing point of 1-molal solution of any nonelectrolyte solute in water is found (by experiment) to be 186 lower than freezing point of water When 1 mol of nonelectrolyte solute dissolved in 1 kg water freezing point is -186 not 000 160
When 2 mol nonelectrolyte solute dissolved in 1 kg water freezing point is -372
For any concentration of nonelectrolyte solute in water decrease in freezing point can be determined using value of -186m
161
This value called the molal freezing-point contstant (Kf) is the freezing-point depression of the solvent in a 1-molal solution of a nonvolatile nonelectrolyte solute
162
Each solvent has own characteristic molal freezing-point constant Freezing-point depression (Δtf) the difference between the freezing points of the pure solvent and a solute of nonelectrolyte in that solvent and it is directly proportional to the molal concentration of the solution Δtf = Kfm
163
Δtf = Kfm
Kf expressed as m
m expressed in mol solutekg solvent (molality)
164
Sample Problem
What is the freezing point depression of water in a solution of 171 g of sucrose C12H22O11 and 200 g of water What is the actual freezing point of the solution
165
1 AnalyzeGiven Solute mass and chemical formula 171 g C12H22O11 Solvent mass and identity 2000 g water Unknown a freezing-point depression b freezing point of the solution 166
2 Plan
167
168
Calculate
Unknown a freezing-point depression b freezing point of the solution
169
Practice Problem
A water solution containing an unknown quantity of a nonelectrolyte solute is found to have a freezing point of ndash 023degCWhat is the molal concentration of the solution
012 m170
A solution consists of 103 g of the nonelectrolyte glucose C6H12O6 dissolved in 250 g of water What is the freezing-point depression of the solution
minus0426degC171
In a laboratory experiment the freezing point of an aqueous solution of glucose is found to be minus0325degCWhat is the molal concentration of this solution
0175 m172
If 0500 mol of a nonelectrolyte solute are dissolved in 5000 g of ether what is the freezing point of the solution
minus1181degC173
The freezing point of an aqueous solution that contains a nonelectrolyte is minus90degC aWhat is the freezing-point depression of the solution bWhat is the molal concentration of the solution a minus90degC b 48 m
174
Boiling-Point Elevation
Boiling point is temp at which vapor pressure is equal to atmospheric pressure Change in vapor pressure will cause corresponding change in boiling point
175
176
Vapor pressure of nonvolatile solution is lower than vapor pressure of pure solvent More heat will be required to raise vapor pressure of solution Boiling point of solution is higher than boiling point of pure solvent
Molal boiling-point constant (Kb) - boiling-point elevation of the solvent in a 1-molal solution of a nonvolatile nonelectrolyte solute b-p elevation of any nonelectrolyte in water found by experiment to be 051 So molal b-p constant for water is 051m
177
For different solvents b-p elevations of 1-molal solutions have different values (Table 14-2)
Like freezing-point constants values most accurate for dilute solutions
178
Boiling-point elevation Δtb difference between the boiling points of the pure solvent and a nonelectrolyte solution of that solvent and it is directly proportional to the molal concentration of the solution B-p elevation can be calculated using Δtb = Kbm Δtb (boiling point elevation) expressed in m and m expressed in mol solutekg solvent
Sample Problem
What is the boiling-point elevation of a solution made from 200 g of a nonelectrolyte solute and 4000 g of water The molar mass of the solute is 620 gmol
180
1 AnalyzeGiven solute mass = 200 g solute molar mass = 620 gmol solvent mass and identity = 4000 g of water
Unknown boiling-point elevation
181
2 Plan
182
183
3 Compute
Practice Problem
A solution contains 500 g of sucrose C12H22O11 a nonelectrolyte dissolved in 5000 g of water What is the boiling-point elevation
015degC184
A solution contains 4500 g of sucrose C12H22O11 a nonelectrolyte dissolved in 250 g of waterWhat is the boiling point of the solution
1027degC185
If the boiling point elevation of an aqueous solution containing a nonvolatile electrolyte is 102degC what is the molality of the solution
20 m186
The boiling point of an aqueous solution containing a nonvolatile electrolyte is 10075degC aWhat is the boiling-point elevation bWhat is the molality of the solution a 075degC b 15 m
187
Electrolytes and Colligative PropertiesEarly investigators confused by experiments where certain substances depressed freezing point or elevated boiling point more than expected
188
189
Ex 01 m solution of NaCl lowers freezing point of solvent almost twice as much as 01 m solution of sucrose 01 m CaCl2 lowers fp almost 3 times as much as 01 m solution of sucrose
To understand why this happens contrast behavior or sucrose with NaCl in aqueous solution Each sucrose dissolves to make only 1 particle in solution So 1 mol sucrose dissolves to 1 mol particles 1 mol NaCl dissolves to make 2 moles of particles (1 mol Na+ and 1 mol Cl-)
190
191
Calculated Values for Electrolyte SolutionsColligative properties depend on total concentration of solute particles regardless of their identity Electrolytes cause changes in colligative properties proportional to total molality in terms of all dissolves particles instead of formula units
192
What about barium nitrate Ba(NO3)2 Each mole of barium nitrate yields 3 mol of ions in solution Ba(NO3)2(s) rarr Ba2+(aq) + 2NO3-(aq) You would expect solution of given molality to lower fp of solvent 3 times as much as nonelectrolytic solution of same molality
193
Sample Problem
What is the expected change in the freezing point of water in a solution of 625 g of barium nitrate Ba(NO3)2 in 100 kg of water
194
1 AnalyzeGiven solute mass and formula = 625 g Ba(NO3)2 solvent mass and identity = 100 kg water Δtf = Kfm
Unknown expected freezing-point depression
195
2 PlanThe molality can be calculated by converting the solute mass to moles and then dividing by the number of kilograms of solvent That molality is in terms of formula units of Ba(NO3)2 and must be converted to molality in terms of dissociated ions in solution It must be multiplied by the number of moles of ions produced per mole of formula unit This adjusted molality can then be used to calculate the freezing-point depression
196
197
198
3 Compute
Practice Problem
What is the expected freezing-point depression for a solution that contains 20 mol of magnesium sulfate dissolved in 10 kg of water
minus74degC199
What is the expected boiling-point elevation of water for a solution that contains 150 g of sodium chloride dissolved in 10 kg of water 27degC The freezing point of an aqueous sodium chloride solution is minus200degC What is the molality of the solution 54 m NaCl
200
Actual Values for Electrolyte SolutionsRemember values calculated are only expected values Actual values of colligative properties for all strong electrolytes are almost what would be expected Difference between expected and actual values caused by attractive forces that exist between dissociated ions in aqueous solution
201
Attraction between hydrated ions in solution is small compared with those in crystalline solid However forces of attraction donrsquot interfere with movements of aqueous ions More concentrated solution means ions closer together and attraction is greater Also depends on charge of ions ndash higher charge greater attraction
202
To understand why nonvolatile solute changes boiling and freezing point consider equilibrium vapor pressure
Vapor pressure pressure caused by molecules that have escape liquid phase to gas phase
Can be thought of a measure of tendency of molecules to escape from a liquid
153
Addition of sucrose (nonvolatile solute) lowers concentration of water molecules at surface of liquid This lowers tendency of water molecules to leave solution and enter gas phase Vapor pressure of solution is lower than vapor pressure of pure water
154
Aqueous13 solution13 of13 nonvolatile13 solutePure13 water
Nonelectrolyte solutions of same molality have same concentration of particles
Dilute solutions of same solvent and equal molality of any nonelectrolyte solute lower vapor pressure equally
155
Example
1 m aqueous solution of nonelectrolyte glucose lowers vapor pressure of water 55 x 10-4 atm at 25 1 m aqueous solution of sucrose also lowers water vapor pressure to 55 x 10-4 atm at 25
156
157
bc vapor-pressure lowering depends on concentration of nonelectrolyte solute and doesnrsquot depend on the type of solute it is a colligative proprerty
bc vapor pressure has been lowered solution remains liquid over larger temperature range
Lowers freezing point and raises boiling point
158
Can assume changes in boiling and freezing point also depend on concentration of solute
They are colligative properties
159
Freezing-Point Depression
Freezing point of 1-molal solution of any nonelectrolyte solute in water is found (by experiment) to be 186 lower than freezing point of water When 1 mol of nonelectrolyte solute dissolved in 1 kg water freezing point is -186 not 000 160
When 2 mol nonelectrolyte solute dissolved in 1 kg water freezing point is -372
For any concentration of nonelectrolyte solute in water decrease in freezing point can be determined using value of -186m
161
This value called the molal freezing-point contstant (Kf) is the freezing-point depression of the solvent in a 1-molal solution of a nonvolatile nonelectrolyte solute
162
Each solvent has own characteristic molal freezing-point constant Freezing-point depression (Δtf) the difference between the freezing points of the pure solvent and a solute of nonelectrolyte in that solvent and it is directly proportional to the molal concentration of the solution Δtf = Kfm
163
Δtf = Kfm
Kf expressed as m
m expressed in mol solutekg solvent (molality)
164
Sample Problem
What is the freezing point depression of water in a solution of 171 g of sucrose C12H22O11 and 200 g of water What is the actual freezing point of the solution
165
1 AnalyzeGiven Solute mass and chemical formula 171 g C12H22O11 Solvent mass and identity 2000 g water Unknown a freezing-point depression b freezing point of the solution 166
2 Plan
167
168
Calculate
Unknown a freezing-point depression b freezing point of the solution
169
Practice Problem
A water solution containing an unknown quantity of a nonelectrolyte solute is found to have a freezing point of ndash 023degCWhat is the molal concentration of the solution
012 m170
A solution consists of 103 g of the nonelectrolyte glucose C6H12O6 dissolved in 250 g of water What is the freezing-point depression of the solution
minus0426degC171
In a laboratory experiment the freezing point of an aqueous solution of glucose is found to be minus0325degCWhat is the molal concentration of this solution
0175 m172
If 0500 mol of a nonelectrolyte solute are dissolved in 5000 g of ether what is the freezing point of the solution
minus1181degC173
The freezing point of an aqueous solution that contains a nonelectrolyte is minus90degC aWhat is the freezing-point depression of the solution bWhat is the molal concentration of the solution a minus90degC b 48 m
174
Boiling-Point Elevation
Boiling point is temp at which vapor pressure is equal to atmospheric pressure Change in vapor pressure will cause corresponding change in boiling point
175
176
Vapor pressure of nonvolatile solution is lower than vapor pressure of pure solvent More heat will be required to raise vapor pressure of solution Boiling point of solution is higher than boiling point of pure solvent
Molal boiling-point constant (Kb) - boiling-point elevation of the solvent in a 1-molal solution of a nonvolatile nonelectrolyte solute b-p elevation of any nonelectrolyte in water found by experiment to be 051 So molal b-p constant for water is 051m
177
For different solvents b-p elevations of 1-molal solutions have different values (Table 14-2)
Like freezing-point constants values most accurate for dilute solutions
178
Boiling-point elevation Δtb difference between the boiling points of the pure solvent and a nonelectrolyte solution of that solvent and it is directly proportional to the molal concentration of the solution B-p elevation can be calculated using Δtb = Kbm Δtb (boiling point elevation) expressed in m and m expressed in mol solutekg solvent
Sample Problem
What is the boiling-point elevation of a solution made from 200 g of a nonelectrolyte solute and 4000 g of water The molar mass of the solute is 620 gmol
180
1 AnalyzeGiven solute mass = 200 g solute molar mass = 620 gmol solvent mass and identity = 4000 g of water
Unknown boiling-point elevation
181
2 Plan
182
183
3 Compute
Practice Problem
A solution contains 500 g of sucrose C12H22O11 a nonelectrolyte dissolved in 5000 g of water What is the boiling-point elevation
015degC184
A solution contains 4500 g of sucrose C12H22O11 a nonelectrolyte dissolved in 250 g of waterWhat is the boiling point of the solution
1027degC185
If the boiling point elevation of an aqueous solution containing a nonvolatile electrolyte is 102degC what is the molality of the solution
20 m186
The boiling point of an aqueous solution containing a nonvolatile electrolyte is 10075degC aWhat is the boiling-point elevation bWhat is the molality of the solution a 075degC b 15 m
187
Electrolytes and Colligative PropertiesEarly investigators confused by experiments where certain substances depressed freezing point or elevated boiling point more than expected
188
189
Ex 01 m solution of NaCl lowers freezing point of solvent almost twice as much as 01 m solution of sucrose 01 m CaCl2 lowers fp almost 3 times as much as 01 m solution of sucrose
To understand why this happens contrast behavior or sucrose with NaCl in aqueous solution Each sucrose dissolves to make only 1 particle in solution So 1 mol sucrose dissolves to 1 mol particles 1 mol NaCl dissolves to make 2 moles of particles (1 mol Na+ and 1 mol Cl-)
190
191
Calculated Values for Electrolyte SolutionsColligative properties depend on total concentration of solute particles regardless of their identity Electrolytes cause changes in colligative properties proportional to total molality in terms of all dissolves particles instead of formula units
192
What about barium nitrate Ba(NO3)2 Each mole of barium nitrate yields 3 mol of ions in solution Ba(NO3)2(s) rarr Ba2+(aq) + 2NO3-(aq) You would expect solution of given molality to lower fp of solvent 3 times as much as nonelectrolytic solution of same molality
193
Sample Problem
What is the expected change in the freezing point of water in a solution of 625 g of barium nitrate Ba(NO3)2 in 100 kg of water
194
1 AnalyzeGiven solute mass and formula = 625 g Ba(NO3)2 solvent mass and identity = 100 kg water Δtf = Kfm
Unknown expected freezing-point depression
195
2 PlanThe molality can be calculated by converting the solute mass to moles and then dividing by the number of kilograms of solvent That molality is in terms of formula units of Ba(NO3)2 and must be converted to molality in terms of dissociated ions in solution It must be multiplied by the number of moles of ions produced per mole of formula unit This adjusted molality can then be used to calculate the freezing-point depression
196
197
198
3 Compute
Practice Problem
What is the expected freezing-point depression for a solution that contains 20 mol of magnesium sulfate dissolved in 10 kg of water
minus74degC199
What is the expected boiling-point elevation of water for a solution that contains 150 g of sodium chloride dissolved in 10 kg of water 27degC The freezing point of an aqueous sodium chloride solution is minus200degC What is the molality of the solution 54 m NaCl
200
Actual Values for Electrolyte SolutionsRemember values calculated are only expected values Actual values of colligative properties for all strong electrolytes are almost what would be expected Difference between expected and actual values caused by attractive forces that exist between dissociated ions in aqueous solution
201
Attraction between hydrated ions in solution is small compared with those in crystalline solid However forces of attraction donrsquot interfere with movements of aqueous ions More concentrated solution means ions closer together and attraction is greater Also depends on charge of ions ndash higher charge greater attraction
202
Addition of sucrose (nonvolatile solute) lowers concentration of water molecules at surface of liquid This lowers tendency of water molecules to leave solution and enter gas phase Vapor pressure of solution is lower than vapor pressure of pure water
154
Aqueous13 solution13 of13 nonvolatile13 solutePure13 water
Nonelectrolyte solutions of same molality have same concentration of particles
Dilute solutions of same solvent and equal molality of any nonelectrolyte solute lower vapor pressure equally
155
Example
1 m aqueous solution of nonelectrolyte glucose lowers vapor pressure of water 55 x 10-4 atm at 25 1 m aqueous solution of sucrose also lowers water vapor pressure to 55 x 10-4 atm at 25
156
157
bc vapor-pressure lowering depends on concentration of nonelectrolyte solute and doesnrsquot depend on the type of solute it is a colligative proprerty
bc vapor pressure has been lowered solution remains liquid over larger temperature range
Lowers freezing point and raises boiling point
158
Can assume changes in boiling and freezing point also depend on concentration of solute
They are colligative properties
159
Freezing-Point Depression
Freezing point of 1-molal solution of any nonelectrolyte solute in water is found (by experiment) to be 186 lower than freezing point of water When 1 mol of nonelectrolyte solute dissolved in 1 kg water freezing point is -186 not 000 160
When 2 mol nonelectrolyte solute dissolved in 1 kg water freezing point is -372
For any concentration of nonelectrolyte solute in water decrease in freezing point can be determined using value of -186m
161
This value called the molal freezing-point contstant (Kf) is the freezing-point depression of the solvent in a 1-molal solution of a nonvolatile nonelectrolyte solute
162
Each solvent has own characteristic molal freezing-point constant Freezing-point depression (Δtf) the difference between the freezing points of the pure solvent and a solute of nonelectrolyte in that solvent and it is directly proportional to the molal concentration of the solution Δtf = Kfm
163
Δtf = Kfm
Kf expressed as m
m expressed in mol solutekg solvent (molality)
164
Sample Problem
What is the freezing point depression of water in a solution of 171 g of sucrose C12H22O11 and 200 g of water What is the actual freezing point of the solution
165
1 AnalyzeGiven Solute mass and chemical formula 171 g C12H22O11 Solvent mass and identity 2000 g water Unknown a freezing-point depression b freezing point of the solution 166
2 Plan
167
168
Calculate
Unknown a freezing-point depression b freezing point of the solution
169
Practice Problem
A water solution containing an unknown quantity of a nonelectrolyte solute is found to have a freezing point of ndash 023degCWhat is the molal concentration of the solution
012 m170
A solution consists of 103 g of the nonelectrolyte glucose C6H12O6 dissolved in 250 g of water What is the freezing-point depression of the solution
minus0426degC171
In a laboratory experiment the freezing point of an aqueous solution of glucose is found to be minus0325degCWhat is the molal concentration of this solution
0175 m172
If 0500 mol of a nonelectrolyte solute are dissolved in 5000 g of ether what is the freezing point of the solution
minus1181degC173
The freezing point of an aqueous solution that contains a nonelectrolyte is minus90degC aWhat is the freezing-point depression of the solution bWhat is the molal concentration of the solution a minus90degC b 48 m
174
Boiling-Point Elevation
Boiling point is temp at which vapor pressure is equal to atmospheric pressure Change in vapor pressure will cause corresponding change in boiling point
175
176
Vapor pressure of nonvolatile solution is lower than vapor pressure of pure solvent More heat will be required to raise vapor pressure of solution Boiling point of solution is higher than boiling point of pure solvent
Molal boiling-point constant (Kb) - boiling-point elevation of the solvent in a 1-molal solution of a nonvolatile nonelectrolyte solute b-p elevation of any nonelectrolyte in water found by experiment to be 051 So molal b-p constant for water is 051m
177
For different solvents b-p elevations of 1-molal solutions have different values (Table 14-2)
Like freezing-point constants values most accurate for dilute solutions
178
Boiling-point elevation Δtb difference between the boiling points of the pure solvent and a nonelectrolyte solution of that solvent and it is directly proportional to the molal concentration of the solution B-p elevation can be calculated using Δtb = Kbm Δtb (boiling point elevation) expressed in m and m expressed in mol solutekg solvent
Sample Problem
What is the boiling-point elevation of a solution made from 200 g of a nonelectrolyte solute and 4000 g of water The molar mass of the solute is 620 gmol
180
1 AnalyzeGiven solute mass = 200 g solute molar mass = 620 gmol solvent mass and identity = 4000 g of water
Unknown boiling-point elevation
181
2 Plan
182
183
3 Compute
Practice Problem
A solution contains 500 g of sucrose C12H22O11 a nonelectrolyte dissolved in 5000 g of water What is the boiling-point elevation
015degC184
A solution contains 4500 g of sucrose C12H22O11 a nonelectrolyte dissolved in 250 g of waterWhat is the boiling point of the solution
1027degC185
If the boiling point elevation of an aqueous solution containing a nonvolatile electrolyte is 102degC what is the molality of the solution
20 m186
The boiling point of an aqueous solution containing a nonvolatile electrolyte is 10075degC aWhat is the boiling-point elevation bWhat is the molality of the solution a 075degC b 15 m
187
Electrolytes and Colligative PropertiesEarly investigators confused by experiments where certain substances depressed freezing point or elevated boiling point more than expected
188
189
Ex 01 m solution of NaCl lowers freezing point of solvent almost twice as much as 01 m solution of sucrose 01 m CaCl2 lowers fp almost 3 times as much as 01 m solution of sucrose
To understand why this happens contrast behavior or sucrose with NaCl in aqueous solution Each sucrose dissolves to make only 1 particle in solution So 1 mol sucrose dissolves to 1 mol particles 1 mol NaCl dissolves to make 2 moles of particles (1 mol Na+ and 1 mol Cl-)
190
191
Calculated Values for Electrolyte SolutionsColligative properties depend on total concentration of solute particles regardless of their identity Electrolytes cause changes in colligative properties proportional to total molality in terms of all dissolves particles instead of formula units
192
What about barium nitrate Ba(NO3)2 Each mole of barium nitrate yields 3 mol of ions in solution Ba(NO3)2(s) rarr Ba2+(aq) + 2NO3-(aq) You would expect solution of given molality to lower fp of solvent 3 times as much as nonelectrolytic solution of same molality
193
Sample Problem
What is the expected change in the freezing point of water in a solution of 625 g of barium nitrate Ba(NO3)2 in 100 kg of water
194
1 AnalyzeGiven solute mass and formula = 625 g Ba(NO3)2 solvent mass and identity = 100 kg water Δtf = Kfm
Unknown expected freezing-point depression
195
2 PlanThe molality can be calculated by converting the solute mass to moles and then dividing by the number of kilograms of solvent That molality is in terms of formula units of Ba(NO3)2 and must be converted to molality in terms of dissociated ions in solution It must be multiplied by the number of moles of ions produced per mole of formula unit This adjusted molality can then be used to calculate the freezing-point depression
196
197
198
3 Compute
Practice Problem
What is the expected freezing-point depression for a solution that contains 20 mol of magnesium sulfate dissolved in 10 kg of water
minus74degC199
What is the expected boiling-point elevation of water for a solution that contains 150 g of sodium chloride dissolved in 10 kg of water 27degC The freezing point of an aqueous sodium chloride solution is minus200degC What is the molality of the solution 54 m NaCl
200
Actual Values for Electrolyte SolutionsRemember values calculated are only expected values Actual values of colligative properties for all strong electrolytes are almost what would be expected Difference between expected and actual values caused by attractive forces that exist between dissociated ions in aqueous solution
201
Attraction between hydrated ions in solution is small compared with those in crystalline solid However forces of attraction donrsquot interfere with movements of aqueous ions More concentrated solution means ions closer together and attraction is greater Also depends on charge of ions ndash higher charge greater attraction
202
Nonelectrolyte solutions of same molality have same concentration of particles
Dilute solutions of same solvent and equal molality of any nonelectrolyte solute lower vapor pressure equally
155
Example
1 m aqueous solution of nonelectrolyte glucose lowers vapor pressure of water 55 x 10-4 atm at 25 1 m aqueous solution of sucrose also lowers water vapor pressure to 55 x 10-4 atm at 25
156
157
bc vapor-pressure lowering depends on concentration of nonelectrolyte solute and doesnrsquot depend on the type of solute it is a colligative proprerty
bc vapor pressure has been lowered solution remains liquid over larger temperature range
Lowers freezing point and raises boiling point
158
Can assume changes in boiling and freezing point also depend on concentration of solute
They are colligative properties
159
Freezing-Point Depression
Freezing point of 1-molal solution of any nonelectrolyte solute in water is found (by experiment) to be 186 lower than freezing point of water When 1 mol of nonelectrolyte solute dissolved in 1 kg water freezing point is -186 not 000 160
When 2 mol nonelectrolyte solute dissolved in 1 kg water freezing point is -372
For any concentration of nonelectrolyte solute in water decrease in freezing point can be determined using value of -186m
161
This value called the molal freezing-point contstant (Kf) is the freezing-point depression of the solvent in a 1-molal solution of a nonvolatile nonelectrolyte solute
162
Each solvent has own characteristic molal freezing-point constant Freezing-point depression (Δtf) the difference between the freezing points of the pure solvent and a solute of nonelectrolyte in that solvent and it is directly proportional to the molal concentration of the solution Δtf = Kfm
163
Δtf = Kfm
Kf expressed as m
m expressed in mol solutekg solvent (molality)
164
Sample Problem
What is the freezing point depression of water in a solution of 171 g of sucrose C12H22O11 and 200 g of water What is the actual freezing point of the solution
165
1 AnalyzeGiven Solute mass and chemical formula 171 g C12H22O11 Solvent mass and identity 2000 g water Unknown a freezing-point depression b freezing point of the solution 166
2 Plan
167
168
Calculate
Unknown a freezing-point depression b freezing point of the solution
169
Practice Problem
A water solution containing an unknown quantity of a nonelectrolyte solute is found to have a freezing point of ndash 023degCWhat is the molal concentration of the solution
012 m170
A solution consists of 103 g of the nonelectrolyte glucose C6H12O6 dissolved in 250 g of water What is the freezing-point depression of the solution
minus0426degC171
In a laboratory experiment the freezing point of an aqueous solution of glucose is found to be minus0325degCWhat is the molal concentration of this solution
0175 m172
If 0500 mol of a nonelectrolyte solute are dissolved in 5000 g of ether what is the freezing point of the solution
minus1181degC173
The freezing point of an aqueous solution that contains a nonelectrolyte is minus90degC aWhat is the freezing-point depression of the solution bWhat is the molal concentration of the solution a minus90degC b 48 m
174
Boiling-Point Elevation
Boiling point is temp at which vapor pressure is equal to atmospheric pressure Change in vapor pressure will cause corresponding change in boiling point
175
176
Vapor pressure of nonvolatile solution is lower than vapor pressure of pure solvent More heat will be required to raise vapor pressure of solution Boiling point of solution is higher than boiling point of pure solvent
Molal boiling-point constant (Kb) - boiling-point elevation of the solvent in a 1-molal solution of a nonvolatile nonelectrolyte solute b-p elevation of any nonelectrolyte in water found by experiment to be 051 So molal b-p constant for water is 051m
177
For different solvents b-p elevations of 1-molal solutions have different values (Table 14-2)
Like freezing-point constants values most accurate for dilute solutions
178
Boiling-point elevation Δtb difference between the boiling points of the pure solvent and a nonelectrolyte solution of that solvent and it is directly proportional to the molal concentration of the solution B-p elevation can be calculated using Δtb = Kbm Δtb (boiling point elevation) expressed in m and m expressed in mol solutekg solvent
Sample Problem
What is the boiling-point elevation of a solution made from 200 g of a nonelectrolyte solute and 4000 g of water The molar mass of the solute is 620 gmol
180
1 AnalyzeGiven solute mass = 200 g solute molar mass = 620 gmol solvent mass and identity = 4000 g of water
Unknown boiling-point elevation
181
2 Plan
182
183
3 Compute
Practice Problem
A solution contains 500 g of sucrose C12H22O11 a nonelectrolyte dissolved in 5000 g of water What is the boiling-point elevation
015degC184
A solution contains 4500 g of sucrose C12H22O11 a nonelectrolyte dissolved in 250 g of waterWhat is the boiling point of the solution
1027degC185
If the boiling point elevation of an aqueous solution containing a nonvolatile electrolyte is 102degC what is the molality of the solution
20 m186
The boiling point of an aqueous solution containing a nonvolatile electrolyte is 10075degC aWhat is the boiling-point elevation bWhat is the molality of the solution a 075degC b 15 m
187
Electrolytes and Colligative PropertiesEarly investigators confused by experiments where certain substances depressed freezing point or elevated boiling point more than expected
188
189
Ex 01 m solution of NaCl lowers freezing point of solvent almost twice as much as 01 m solution of sucrose 01 m CaCl2 lowers fp almost 3 times as much as 01 m solution of sucrose
To understand why this happens contrast behavior or sucrose with NaCl in aqueous solution Each sucrose dissolves to make only 1 particle in solution So 1 mol sucrose dissolves to 1 mol particles 1 mol NaCl dissolves to make 2 moles of particles (1 mol Na+ and 1 mol Cl-)
190
191
Calculated Values for Electrolyte SolutionsColligative properties depend on total concentration of solute particles regardless of their identity Electrolytes cause changes in colligative properties proportional to total molality in terms of all dissolves particles instead of formula units
192
What about barium nitrate Ba(NO3)2 Each mole of barium nitrate yields 3 mol of ions in solution Ba(NO3)2(s) rarr Ba2+(aq) + 2NO3-(aq) You would expect solution of given molality to lower fp of solvent 3 times as much as nonelectrolytic solution of same molality
193
Sample Problem
What is the expected change in the freezing point of water in a solution of 625 g of barium nitrate Ba(NO3)2 in 100 kg of water
194
1 AnalyzeGiven solute mass and formula = 625 g Ba(NO3)2 solvent mass and identity = 100 kg water Δtf = Kfm
Unknown expected freezing-point depression
195
2 PlanThe molality can be calculated by converting the solute mass to moles and then dividing by the number of kilograms of solvent That molality is in terms of formula units of Ba(NO3)2 and must be converted to molality in terms of dissociated ions in solution It must be multiplied by the number of moles of ions produced per mole of formula unit This adjusted molality can then be used to calculate the freezing-point depression
196
197
198
3 Compute
Practice Problem
What is the expected freezing-point depression for a solution that contains 20 mol of magnesium sulfate dissolved in 10 kg of water
minus74degC199
What is the expected boiling-point elevation of water for a solution that contains 150 g of sodium chloride dissolved in 10 kg of water 27degC The freezing point of an aqueous sodium chloride solution is minus200degC What is the molality of the solution 54 m NaCl
200
Actual Values for Electrolyte SolutionsRemember values calculated are only expected values Actual values of colligative properties for all strong electrolytes are almost what would be expected Difference between expected and actual values caused by attractive forces that exist between dissociated ions in aqueous solution
201
Attraction between hydrated ions in solution is small compared with those in crystalline solid However forces of attraction donrsquot interfere with movements of aqueous ions More concentrated solution means ions closer together and attraction is greater Also depends on charge of ions ndash higher charge greater attraction
202
Example
1 m aqueous solution of nonelectrolyte glucose lowers vapor pressure of water 55 x 10-4 atm at 25 1 m aqueous solution of sucrose also lowers water vapor pressure to 55 x 10-4 atm at 25
156
157
bc vapor-pressure lowering depends on concentration of nonelectrolyte solute and doesnrsquot depend on the type of solute it is a colligative proprerty
bc vapor pressure has been lowered solution remains liquid over larger temperature range
Lowers freezing point and raises boiling point
158
Can assume changes in boiling and freezing point also depend on concentration of solute
They are colligative properties
159
Freezing-Point Depression
Freezing point of 1-molal solution of any nonelectrolyte solute in water is found (by experiment) to be 186 lower than freezing point of water When 1 mol of nonelectrolyte solute dissolved in 1 kg water freezing point is -186 not 000 160
When 2 mol nonelectrolyte solute dissolved in 1 kg water freezing point is -372
For any concentration of nonelectrolyte solute in water decrease in freezing point can be determined using value of -186m
161
This value called the molal freezing-point contstant (Kf) is the freezing-point depression of the solvent in a 1-molal solution of a nonvolatile nonelectrolyte solute
162
Each solvent has own characteristic molal freezing-point constant Freezing-point depression (Δtf) the difference between the freezing points of the pure solvent and a solute of nonelectrolyte in that solvent and it is directly proportional to the molal concentration of the solution Δtf = Kfm
163
Δtf = Kfm
Kf expressed as m
m expressed in mol solutekg solvent (molality)
164
Sample Problem
What is the freezing point depression of water in a solution of 171 g of sucrose C12H22O11 and 200 g of water What is the actual freezing point of the solution
165
1 AnalyzeGiven Solute mass and chemical formula 171 g C12H22O11 Solvent mass and identity 2000 g water Unknown a freezing-point depression b freezing point of the solution 166
2 Plan
167
168
Calculate
Unknown a freezing-point depression b freezing point of the solution
169
Practice Problem
A water solution containing an unknown quantity of a nonelectrolyte solute is found to have a freezing point of ndash 023degCWhat is the molal concentration of the solution
012 m170
A solution consists of 103 g of the nonelectrolyte glucose C6H12O6 dissolved in 250 g of water What is the freezing-point depression of the solution
minus0426degC171
In a laboratory experiment the freezing point of an aqueous solution of glucose is found to be minus0325degCWhat is the molal concentration of this solution
0175 m172
If 0500 mol of a nonelectrolyte solute are dissolved in 5000 g of ether what is the freezing point of the solution
minus1181degC173
The freezing point of an aqueous solution that contains a nonelectrolyte is minus90degC aWhat is the freezing-point depression of the solution bWhat is the molal concentration of the solution a minus90degC b 48 m
174
Boiling-Point Elevation
Boiling point is temp at which vapor pressure is equal to atmospheric pressure Change in vapor pressure will cause corresponding change in boiling point
175
176
Vapor pressure of nonvolatile solution is lower than vapor pressure of pure solvent More heat will be required to raise vapor pressure of solution Boiling point of solution is higher than boiling point of pure solvent
Molal boiling-point constant (Kb) - boiling-point elevation of the solvent in a 1-molal solution of a nonvolatile nonelectrolyte solute b-p elevation of any nonelectrolyte in water found by experiment to be 051 So molal b-p constant for water is 051m
177
For different solvents b-p elevations of 1-molal solutions have different values (Table 14-2)
Like freezing-point constants values most accurate for dilute solutions
178
Boiling-point elevation Δtb difference between the boiling points of the pure solvent and a nonelectrolyte solution of that solvent and it is directly proportional to the molal concentration of the solution B-p elevation can be calculated using Δtb = Kbm Δtb (boiling point elevation) expressed in m and m expressed in mol solutekg solvent
Sample Problem
What is the boiling-point elevation of a solution made from 200 g of a nonelectrolyte solute and 4000 g of water The molar mass of the solute is 620 gmol
180
1 AnalyzeGiven solute mass = 200 g solute molar mass = 620 gmol solvent mass and identity = 4000 g of water
Unknown boiling-point elevation
181
2 Plan
182
183
3 Compute
Practice Problem
A solution contains 500 g of sucrose C12H22O11 a nonelectrolyte dissolved in 5000 g of water What is the boiling-point elevation
015degC184
A solution contains 4500 g of sucrose C12H22O11 a nonelectrolyte dissolved in 250 g of waterWhat is the boiling point of the solution
1027degC185
If the boiling point elevation of an aqueous solution containing a nonvolatile electrolyte is 102degC what is the molality of the solution
20 m186
The boiling point of an aqueous solution containing a nonvolatile electrolyte is 10075degC aWhat is the boiling-point elevation bWhat is the molality of the solution a 075degC b 15 m
187
Electrolytes and Colligative PropertiesEarly investigators confused by experiments where certain substances depressed freezing point or elevated boiling point more than expected
188
189
Ex 01 m solution of NaCl lowers freezing point of solvent almost twice as much as 01 m solution of sucrose 01 m CaCl2 lowers fp almost 3 times as much as 01 m solution of sucrose
To understand why this happens contrast behavior or sucrose with NaCl in aqueous solution Each sucrose dissolves to make only 1 particle in solution So 1 mol sucrose dissolves to 1 mol particles 1 mol NaCl dissolves to make 2 moles of particles (1 mol Na+ and 1 mol Cl-)
190
191
Calculated Values for Electrolyte SolutionsColligative properties depend on total concentration of solute particles regardless of their identity Electrolytes cause changes in colligative properties proportional to total molality in terms of all dissolves particles instead of formula units
192
What about barium nitrate Ba(NO3)2 Each mole of barium nitrate yields 3 mol of ions in solution Ba(NO3)2(s) rarr Ba2+(aq) + 2NO3-(aq) You would expect solution of given molality to lower fp of solvent 3 times as much as nonelectrolytic solution of same molality
193
Sample Problem
What is the expected change in the freezing point of water in a solution of 625 g of barium nitrate Ba(NO3)2 in 100 kg of water
194
1 AnalyzeGiven solute mass and formula = 625 g Ba(NO3)2 solvent mass and identity = 100 kg water Δtf = Kfm
Unknown expected freezing-point depression
195
2 PlanThe molality can be calculated by converting the solute mass to moles and then dividing by the number of kilograms of solvent That molality is in terms of formula units of Ba(NO3)2 and must be converted to molality in terms of dissociated ions in solution It must be multiplied by the number of moles of ions produced per mole of formula unit This adjusted molality can then be used to calculate the freezing-point depression
196
197
198
3 Compute
Practice Problem
What is the expected freezing-point depression for a solution that contains 20 mol of magnesium sulfate dissolved in 10 kg of water
minus74degC199
What is the expected boiling-point elevation of water for a solution that contains 150 g of sodium chloride dissolved in 10 kg of water 27degC The freezing point of an aqueous sodium chloride solution is minus200degC What is the molality of the solution 54 m NaCl
200
Actual Values for Electrolyte SolutionsRemember values calculated are only expected values Actual values of colligative properties for all strong electrolytes are almost what would be expected Difference between expected and actual values caused by attractive forces that exist between dissociated ions in aqueous solution
201
Attraction between hydrated ions in solution is small compared with those in crystalline solid However forces of attraction donrsquot interfere with movements of aqueous ions More concentrated solution means ions closer together and attraction is greater Also depends on charge of ions ndash higher charge greater attraction
202
157
bc vapor-pressure lowering depends on concentration of nonelectrolyte solute and doesnrsquot depend on the type of solute it is a colligative proprerty
bc vapor pressure has been lowered solution remains liquid over larger temperature range
Lowers freezing point and raises boiling point
158
Can assume changes in boiling and freezing point also depend on concentration of solute
They are colligative properties
159
Freezing-Point Depression
Freezing point of 1-molal solution of any nonelectrolyte solute in water is found (by experiment) to be 186 lower than freezing point of water When 1 mol of nonelectrolyte solute dissolved in 1 kg water freezing point is -186 not 000 160
When 2 mol nonelectrolyte solute dissolved in 1 kg water freezing point is -372
For any concentration of nonelectrolyte solute in water decrease in freezing point can be determined using value of -186m
161
This value called the molal freezing-point contstant (Kf) is the freezing-point depression of the solvent in a 1-molal solution of a nonvolatile nonelectrolyte solute
162
Each solvent has own characteristic molal freezing-point constant Freezing-point depression (Δtf) the difference between the freezing points of the pure solvent and a solute of nonelectrolyte in that solvent and it is directly proportional to the molal concentration of the solution Δtf = Kfm
163
Δtf = Kfm
Kf expressed as m
m expressed in mol solutekg solvent (molality)
164
Sample Problem
What is the freezing point depression of water in a solution of 171 g of sucrose C12H22O11 and 200 g of water What is the actual freezing point of the solution
165
1 AnalyzeGiven Solute mass and chemical formula 171 g C12H22O11 Solvent mass and identity 2000 g water Unknown a freezing-point depression b freezing point of the solution 166
2 Plan
167
168
Calculate
Unknown a freezing-point depression b freezing point of the solution
169
Practice Problem
A water solution containing an unknown quantity of a nonelectrolyte solute is found to have a freezing point of ndash 023degCWhat is the molal concentration of the solution
012 m170
A solution consists of 103 g of the nonelectrolyte glucose C6H12O6 dissolved in 250 g of water What is the freezing-point depression of the solution
minus0426degC171
In a laboratory experiment the freezing point of an aqueous solution of glucose is found to be minus0325degCWhat is the molal concentration of this solution
0175 m172
If 0500 mol of a nonelectrolyte solute are dissolved in 5000 g of ether what is the freezing point of the solution
minus1181degC173
The freezing point of an aqueous solution that contains a nonelectrolyte is minus90degC aWhat is the freezing-point depression of the solution bWhat is the molal concentration of the solution a minus90degC b 48 m
174
Boiling-Point Elevation
Boiling point is temp at which vapor pressure is equal to atmospheric pressure Change in vapor pressure will cause corresponding change in boiling point
175
176
Vapor pressure of nonvolatile solution is lower than vapor pressure of pure solvent More heat will be required to raise vapor pressure of solution Boiling point of solution is higher than boiling point of pure solvent
Molal boiling-point constant (Kb) - boiling-point elevation of the solvent in a 1-molal solution of a nonvolatile nonelectrolyte solute b-p elevation of any nonelectrolyte in water found by experiment to be 051 So molal b-p constant for water is 051m
177
For different solvents b-p elevations of 1-molal solutions have different values (Table 14-2)
Like freezing-point constants values most accurate for dilute solutions
178
Boiling-point elevation Δtb difference between the boiling points of the pure solvent and a nonelectrolyte solution of that solvent and it is directly proportional to the molal concentration of the solution B-p elevation can be calculated using Δtb = Kbm Δtb (boiling point elevation) expressed in m and m expressed in mol solutekg solvent
Sample Problem
What is the boiling-point elevation of a solution made from 200 g of a nonelectrolyte solute and 4000 g of water The molar mass of the solute is 620 gmol
180
1 AnalyzeGiven solute mass = 200 g solute molar mass = 620 gmol solvent mass and identity = 4000 g of water
Unknown boiling-point elevation
181
2 Plan
182
183
3 Compute
Practice Problem
A solution contains 500 g of sucrose C12H22O11 a nonelectrolyte dissolved in 5000 g of water What is the boiling-point elevation
015degC184
A solution contains 4500 g of sucrose C12H22O11 a nonelectrolyte dissolved in 250 g of waterWhat is the boiling point of the solution
1027degC185
If the boiling point elevation of an aqueous solution containing a nonvolatile electrolyte is 102degC what is the molality of the solution
20 m186
The boiling point of an aqueous solution containing a nonvolatile electrolyte is 10075degC aWhat is the boiling-point elevation bWhat is the molality of the solution a 075degC b 15 m
187
Electrolytes and Colligative PropertiesEarly investigators confused by experiments where certain substances depressed freezing point or elevated boiling point more than expected
188
189
Ex 01 m solution of NaCl lowers freezing point of solvent almost twice as much as 01 m solution of sucrose 01 m CaCl2 lowers fp almost 3 times as much as 01 m solution of sucrose
To understand why this happens contrast behavior or sucrose with NaCl in aqueous solution Each sucrose dissolves to make only 1 particle in solution So 1 mol sucrose dissolves to 1 mol particles 1 mol NaCl dissolves to make 2 moles of particles (1 mol Na+ and 1 mol Cl-)
190
191
Calculated Values for Electrolyte SolutionsColligative properties depend on total concentration of solute particles regardless of their identity Electrolytes cause changes in colligative properties proportional to total molality in terms of all dissolves particles instead of formula units
192
What about barium nitrate Ba(NO3)2 Each mole of barium nitrate yields 3 mol of ions in solution Ba(NO3)2(s) rarr Ba2+(aq) + 2NO3-(aq) You would expect solution of given molality to lower fp of solvent 3 times as much as nonelectrolytic solution of same molality
193
Sample Problem
What is the expected change in the freezing point of water in a solution of 625 g of barium nitrate Ba(NO3)2 in 100 kg of water
194
1 AnalyzeGiven solute mass and formula = 625 g Ba(NO3)2 solvent mass and identity = 100 kg water Δtf = Kfm
Unknown expected freezing-point depression
195
2 PlanThe molality can be calculated by converting the solute mass to moles and then dividing by the number of kilograms of solvent That molality is in terms of formula units of Ba(NO3)2 and must be converted to molality in terms of dissociated ions in solution It must be multiplied by the number of moles of ions produced per mole of formula unit This adjusted molality can then be used to calculate the freezing-point depression
196
197
198
3 Compute
Practice Problem
What is the expected freezing-point depression for a solution that contains 20 mol of magnesium sulfate dissolved in 10 kg of water
minus74degC199
What is the expected boiling-point elevation of water for a solution that contains 150 g of sodium chloride dissolved in 10 kg of water 27degC The freezing point of an aqueous sodium chloride solution is minus200degC What is the molality of the solution 54 m NaCl
200
Actual Values for Electrolyte SolutionsRemember values calculated are only expected values Actual values of colligative properties for all strong electrolytes are almost what would be expected Difference between expected and actual values caused by attractive forces that exist between dissociated ions in aqueous solution
201
Attraction between hydrated ions in solution is small compared with those in crystalline solid However forces of attraction donrsquot interfere with movements of aqueous ions More concentrated solution means ions closer together and attraction is greater Also depends on charge of ions ndash higher charge greater attraction
202
bc vapor pressure has been lowered solution remains liquid over larger temperature range
Lowers freezing point and raises boiling point
158
Can assume changes in boiling and freezing point also depend on concentration of solute
They are colligative properties
159
Freezing-Point Depression
Freezing point of 1-molal solution of any nonelectrolyte solute in water is found (by experiment) to be 186 lower than freezing point of water When 1 mol of nonelectrolyte solute dissolved in 1 kg water freezing point is -186 not 000 160
When 2 mol nonelectrolyte solute dissolved in 1 kg water freezing point is -372
For any concentration of nonelectrolyte solute in water decrease in freezing point can be determined using value of -186m
161
This value called the molal freezing-point contstant (Kf) is the freezing-point depression of the solvent in a 1-molal solution of a nonvolatile nonelectrolyte solute
162
Each solvent has own characteristic molal freezing-point constant Freezing-point depression (Δtf) the difference between the freezing points of the pure solvent and a solute of nonelectrolyte in that solvent and it is directly proportional to the molal concentration of the solution Δtf = Kfm
163
Δtf = Kfm
Kf expressed as m
m expressed in mol solutekg solvent (molality)
164
Sample Problem
What is the freezing point depression of water in a solution of 171 g of sucrose C12H22O11 and 200 g of water What is the actual freezing point of the solution
165
1 AnalyzeGiven Solute mass and chemical formula 171 g C12H22O11 Solvent mass and identity 2000 g water Unknown a freezing-point depression b freezing point of the solution 166
2 Plan
167
168
Calculate
Unknown a freezing-point depression b freezing point of the solution
169
Practice Problem
A water solution containing an unknown quantity of a nonelectrolyte solute is found to have a freezing point of ndash 023degCWhat is the molal concentration of the solution
012 m170
A solution consists of 103 g of the nonelectrolyte glucose C6H12O6 dissolved in 250 g of water What is the freezing-point depression of the solution
minus0426degC171
In a laboratory experiment the freezing point of an aqueous solution of glucose is found to be minus0325degCWhat is the molal concentration of this solution
0175 m172
If 0500 mol of a nonelectrolyte solute are dissolved in 5000 g of ether what is the freezing point of the solution
minus1181degC173
The freezing point of an aqueous solution that contains a nonelectrolyte is minus90degC aWhat is the freezing-point depression of the solution bWhat is the molal concentration of the solution a minus90degC b 48 m
174
Boiling-Point Elevation
Boiling point is temp at which vapor pressure is equal to atmospheric pressure Change in vapor pressure will cause corresponding change in boiling point
175
176
Vapor pressure of nonvolatile solution is lower than vapor pressure of pure solvent More heat will be required to raise vapor pressure of solution Boiling point of solution is higher than boiling point of pure solvent
Molal boiling-point constant (Kb) - boiling-point elevation of the solvent in a 1-molal solution of a nonvolatile nonelectrolyte solute b-p elevation of any nonelectrolyte in water found by experiment to be 051 So molal b-p constant for water is 051m
177
For different solvents b-p elevations of 1-molal solutions have different values (Table 14-2)
Like freezing-point constants values most accurate for dilute solutions
178
Boiling-point elevation Δtb difference between the boiling points of the pure solvent and a nonelectrolyte solution of that solvent and it is directly proportional to the molal concentration of the solution B-p elevation can be calculated using Δtb = Kbm Δtb (boiling point elevation) expressed in m and m expressed in mol solutekg solvent
Sample Problem
What is the boiling-point elevation of a solution made from 200 g of a nonelectrolyte solute and 4000 g of water The molar mass of the solute is 620 gmol
180
1 AnalyzeGiven solute mass = 200 g solute molar mass = 620 gmol solvent mass and identity = 4000 g of water
Unknown boiling-point elevation
181
2 Plan
182
183
3 Compute
Practice Problem
A solution contains 500 g of sucrose C12H22O11 a nonelectrolyte dissolved in 5000 g of water What is the boiling-point elevation
015degC184
A solution contains 4500 g of sucrose C12H22O11 a nonelectrolyte dissolved in 250 g of waterWhat is the boiling point of the solution
1027degC185
If the boiling point elevation of an aqueous solution containing a nonvolatile electrolyte is 102degC what is the molality of the solution
20 m186
The boiling point of an aqueous solution containing a nonvolatile electrolyte is 10075degC aWhat is the boiling-point elevation bWhat is the molality of the solution a 075degC b 15 m
187
Electrolytes and Colligative PropertiesEarly investigators confused by experiments where certain substances depressed freezing point or elevated boiling point more than expected
188
189
Ex 01 m solution of NaCl lowers freezing point of solvent almost twice as much as 01 m solution of sucrose 01 m CaCl2 lowers fp almost 3 times as much as 01 m solution of sucrose
To understand why this happens contrast behavior or sucrose with NaCl in aqueous solution Each sucrose dissolves to make only 1 particle in solution So 1 mol sucrose dissolves to 1 mol particles 1 mol NaCl dissolves to make 2 moles of particles (1 mol Na+ and 1 mol Cl-)
190
191
Calculated Values for Electrolyte SolutionsColligative properties depend on total concentration of solute particles regardless of their identity Electrolytes cause changes in colligative properties proportional to total molality in terms of all dissolves particles instead of formula units
192
What about barium nitrate Ba(NO3)2 Each mole of barium nitrate yields 3 mol of ions in solution Ba(NO3)2(s) rarr Ba2+(aq) + 2NO3-(aq) You would expect solution of given molality to lower fp of solvent 3 times as much as nonelectrolytic solution of same molality
193
Sample Problem
What is the expected change in the freezing point of water in a solution of 625 g of barium nitrate Ba(NO3)2 in 100 kg of water
194
1 AnalyzeGiven solute mass and formula = 625 g Ba(NO3)2 solvent mass and identity = 100 kg water Δtf = Kfm
Unknown expected freezing-point depression
195
2 PlanThe molality can be calculated by converting the solute mass to moles and then dividing by the number of kilograms of solvent That molality is in terms of formula units of Ba(NO3)2 and must be converted to molality in terms of dissociated ions in solution It must be multiplied by the number of moles of ions produced per mole of formula unit This adjusted molality can then be used to calculate the freezing-point depression
196
197
198
3 Compute
Practice Problem
What is the expected freezing-point depression for a solution that contains 20 mol of magnesium sulfate dissolved in 10 kg of water
minus74degC199
What is the expected boiling-point elevation of water for a solution that contains 150 g of sodium chloride dissolved in 10 kg of water 27degC The freezing point of an aqueous sodium chloride solution is minus200degC What is the molality of the solution 54 m NaCl
200
Actual Values for Electrolyte SolutionsRemember values calculated are only expected values Actual values of colligative properties for all strong electrolytes are almost what would be expected Difference between expected and actual values caused by attractive forces that exist between dissociated ions in aqueous solution
201
Attraction between hydrated ions in solution is small compared with those in crystalline solid However forces of attraction donrsquot interfere with movements of aqueous ions More concentrated solution means ions closer together and attraction is greater Also depends on charge of ions ndash higher charge greater attraction
202
Can assume changes in boiling and freezing point also depend on concentration of solute
They are colligative properties
159
Freezing-Point Depression
Freezing point of 1-molal solution of any nonelectrolyte solute in water is found (by experiment) to be 186 lower than freezing point of water When 1 mol of nonelectrolyte solute dissolved in 1 kg water freezing point is -186 not 000 160
When 2 mol nonelectrolyte solute dissolved in 1 kg water freezing point is -372
For any concentration of nonelectrolyte solute in water decrease in freezing point can be determined using value of -186m
161
This value called the molal freezing-point contstant (Kf) is the freezing-point depression of the solvent in a 1-molal solution of a nonvolatile nonelectrolyte solute
162
Each solvent has own characteristic molal freezing-point constant Freezing-point depression (Δtf) the difference between the freezing points of the pure solvent and a solute of nonelectrolyte in that solvent and it is directly proportional to the molal concentration of the solution Δtf = Kfm
163
Δtf = Kfm
Kf expressed as m
m expressed in mol solutekg solvent (molality)
164
Sample Problem
What is the freezing point depression of water in a solution of 171 g of sucrose C12H22O11 and 200 g of water What is the actual freezing point of the solution
165
1 AnalyzeGiven Solute mass and chemical formula 171 g C12H22O11 Solvent mass and identity 2000 g water Unknown a freezing-point depression b freezing point of the solution 166
2 Plan
167
168
Calculate
Unknown a freezing-point depression b freezing point of the solution
169
Practice Problem
A water solution containing an unknown quantity of a nonelectrolyte solute is found to have a freezing point of ndash 023degCWhat is the molal concentration of the solution
012 m170
A solution consists of 103 g of the nonelectrolyte glucose C6H12O6 dissolved in 250 g of water What is the freezing-point depression of the solution
minus0426degC171
In a laboratory experiment the freezing point of an aqueous solution of glucose is found to be minus0325degCWhat is the molal concentration of this solution
0175 m172
If 0500 mol of a nonelectrolyte solute are dissolved in 5000 g of ether what is the freezing point of the solution
minus1181degC173
The freezing point of an aqueous solution that contains a nonelectrolyte is minus90degC aWhat is the freezing-point depression of the solution bWhat is the molal concentration of the solution a minus90degC b 48 m
174
Boiling-Point Elevation
Boiling point is temp at which vapor pressure is equal to atmospheric pressure Change in vapor pressure will cause corresponding change in boiling point
175
176
Vapor pressure of nonvolatile solution is lower than vapor pressure of pure solvent More heat will be required to raise vapor pressure of solution Boiling point of solution is higher than boiling point of pure solvent
Molal boiling-point constant (Kb) - boiling-point elevation of the solvent in a 1-molal solution of a nonvolatile nonelectrolyte solute b-p elevation of any nonelectrolyte in water found by experiment to be 051 So molal b-p constant for water is 051m
177
For different solvents b-p elevations of 1-molal solutions have different values (Table 14-2)
Like freezing-point constants values most accurate for dilute solutions
178
Boiling-point elevation Δtb difference between the boiling points of the pure solvent and a nonelectrolyte solution of that solvent and it is directly proportional to the molal concentration of the solution B-p elevation can be calculated using Δtb = Kbm Δtb (boiling point elevation) expressed in m and m expressed in mol solutekg solvent
Sample Problem
What is the boiling-point elevation of a solution made from 200 g of a nonelectrolyte solute and 4000 g of water The molar mass of the solute is 620 gmol
180
1 AnalyzeGiven solute mass = 200 g solute molar mass = 620 gmol solvent mass and identity = 4000 g of water
Unknown boiling-point elevation
181
2 Plan
182
183
3 Compute
Practice Problem
A solution contains 500 g of sucrose C12H22O11 a nonelectrolyte dissolved in 5000 g of water What is the boiling-point elevation
015degC184
A solution contains 4500 g of sucrose C12H22O11 a nonelectrolyte dissolved in 250 g of waterWhat is the boiling point of the solution
1027degC185
If the boiling point elevation of an aqueous solution containing a nonvolatile electrolyte is 102degC what is the molality of the solution
20 m186
The boiling point of an aqueous solution containing a nonvolatile electrolyte is 10075degC aWhat is the boiling-point elevation bWhat is the molality of the solution a 075degC b 15 m
187
Electrolytes and Colligative PropertiesEarly investigators confused by experiments where certain substances depressed freezing point or elevated boiling point more than expected
188
189
Ex 01 m solution of NaCl lowers freezing point of solvent almost twice as much as 01 m solution of sucrose 01 m CaCl2 lowers fp almost 3 times as much as 01 m solution of sucrose
To understand why this happens contrast behavior or sucrose with NaCl in aqueous solution Each sucrose dissolves to make only 1 particle in solution So 1 mol sucrose dissolves to 1 mol particles 1 mol NaCl dissolves to make 2 moles of particles (1 mol Na+ and 1 mol Cl-)
190
191
Calculated Values for Electrolyte SolutionsColligative properties depend on total concentration of solute particles regardless of their identity Electrolytes cause changes in colligative properties proportional to total molality in terms of all dissolves particles instead of formula units
192
What about barium nitrate Ba(NO3)2 Each mole of barium nitrate yields 3 mol of ions in solution Ba(NO3)2(s) rarr Ba2+(aq) + 2NO3-(aq) You would expect solution of given molality to lower fp of solvent 3 times as much as nonelectrolytic solution of same molality
193
Sample Problem
What is the expected change in the freezing point of water in a solution of 625 g of barium nitrate Ba(NO3)2 in 100 kg of water
194
1 AnalyzeGiven solute mass and formula = 625 g Ba(NO3)2 solvent mass and identity = 100 kg water Δtf = Kfm
Unknown expected freezing-point depression
195
2 PlanThe molality can be calculated by converting the solute mass to moles and then dividing by the number of kilograms of solvent That molality is in terms of formula units of Ba(NO3)2 and must be converted to molality in terms of dissociated ions in solution It must be multiplied by the number of moles of ions produced per mole of formula unit This adjusted molality can then be used to calculate the freezing-point depression
196
197
198
3 Compute
Practice Problem
What is the expected freezing-point depression for a solution that contains 20 mol of magnesium sulfate dissolved in 10 kg of water
minus74degC199
What is the expected boiling-point elevation of water for a solution that contains 150 g of sodium chloride dissolved in 10 kg of water 27degC The freezing point of an aqueous sodium chloride solution is minus200degC What is the molality of the solution 54 m NaCl
200
Actual Values for Electrolyte SolutionsRemember values calculated are only expected values Actual values of colligative properties for all strong electrolytes are almost what would be expected Difference between expected and actual values caused by attractive forces that exist between dissociated ions in aqueous solution
201
Attraction between hydrated ions in solution is small compared with those in crystalline solid However forces of attraction donrsquot interfere with movements of aqueous ions More concentrated solution means ions closer together and attraction is greater Also depends on charge of ions ndash higher charge greater attraction
202
Freezing-Point Depression
Freezing point of 1-molal solution of any nonelectrolyte solute in water is found (by experiment) to be 186 lower than freezing point of water When 1 mol of nonelectrolyte solute dissolved in 1 kg water freezing point is -186 not 000 160
When 2 mol nonelectrolyte solute dissolved in 1 kg water freezing point is -372
For any concentration of nonelectrolyte solute in water decrease in freezing point can be determined using value of -186m
161
This value called the molal freezing-point contstant (Kf) is the freezing-point depression of the solvent in a 1-molal solution of a nonvolatile nonelectrolyte solute
162
Each solvent has own characteristic molal freezing-point constant Freezing-point depression (Δtf) the difference between the freezing points of the pure solvent and a solute of nonelectrolyte in that solvent and it is directly proportional to the molal concentration of the solution Δtf = Kfm
163
Δtf = Kfm
Kf expressed as m
m expressed in mol solutekg solvent (molality)
164
Sample Problem
What is the freezing point depression of water in a solution of 171 g of sucrose C12H22O11 and 200 g of water What is the actual freezing point of the solution
165
1 AnalyzeGiven Solute mass and chemical formula 171 g C12H22O11 Solvent mass and identity 2000 g water Unknown a freezing-point depression b freezing point of the solution 166
2 Plan
167
168
Calculate
Unknown a freezing-point depression b freezing point of the solution
169
Practice Problem
A water solution containing an unknown quantity of a nonelectrolyte solute is found to have a freezing point of ndash 023degCWhat is the molal concentration of the solution
012 m170
A solution consists of 103 g of the nonelectrolyte glucose C6H12O6 dissolved in 250 g of water What is the freezing-point depression of the solution
minus0426degC171
In a laboratory experiment the freezing point of an aqueous solution of glucose is found to be minus0325degCWhat is the molal concentration of this solution
0175 m172
If 0500 mol of a nonelectrolyte solute are dissolved in 5000 g of ether what is the freezing point of the solution
minus1181degC173
The freezing point of an aqueous solution that contains a nonelectrolyte is minus90degC aWhat is the freezing-point depression of the solution bWhat is the molal concentration of the solution a minus90degC b 48 m
174
Boiling-Point Elevation
Boiling point is temp at which vapor pressure is equal to atmospheric pressure Change in vapor pressure will cause corresponding change in boiling point
175
176
Vapor pressure of nonvolatile solution is lower than vapor pressure of pure solvent More heat will be required to raise vapor pressure of solution Boiling point of solution is higher than boiling point of pure solvent
Molal boiling-point constant (Kb) - boiling-point elevation of the solvent in a 1-molal solution of a nonvolatile nonelectrolyte solute b-p elevation of any nonelectrolyte in water found by experiment to be 051 So molal b-p constant for water is 051m
177
For different solvents b-p elevations of 1-molal solutions have different values (Table 14-2)
Like freezing-point constants values most accurate for dilute solutions
178
Boiling-point elevation Δtb difference between the boiling points of the pure solvent and a nonelectrolyte solution of that solvent and it is directly proportional to the molal concentration of the solution B-p elevation can be calculated using Δtb = Kbm Δtb (boiling point elevation) expressed in m and m expressed in mol solutekg solvent
Sample Problem
What is the boiling-point elevation of a solution made from 200 g of a nonelectrolyte solute and 4000 g of water The molar mass of the solute is 620 gmol
180
1 AnalyzeGiven solute mass = 200 g solute molar mass = 620 gmol solvent mass and identity = 4000 g of water
Unknown boiling-point elevation
181
2 Plan
182
183
3 Compute
Practice Problem
A solution contains 500 g of sucrose C12H22O11 a nonelectrolyte dissolved in 5000 g of water What is the boiling-point elevation
015degC184
A solution contains 4500 g of sucrose C12H22O11 a nonelectrolyte dissolved in 250 g of waterWhat is the boiling point of the solution
1027degC185
If the boiling point elevation of an aqueous solution containing a nonvolatile electrolyte is 102degC what is the molality of the solution
20 m186
The boiling point of an aqueous solution containing a nonvolatile electrolyte is 10075degC aWhat is the boiling-point elevation bWhat is the molality of the solution a 075degC b 15 m
187
Electrolytes and Colligative PropertiesEarly investigators confused by experiments where certain substances depressed freezing point or elevated boiling point more than expected
188
189
Ex 01 m solution of NaCl lowers freezing point of solvent almost twice as much as 01 m solution of sucrose 01 m CaCl2 lowers fp almost 3 times as much as 01 m solution of sucrose
To understand why this happens contrast behavior or sucrose with NaCl in aqueous solution Each sucrose dissolves to make only 1 particle in solution So 1 mol sucrose dissolves to 1 mol particles 1 mol NaCl dissolves to make 2 moles of particles (1 mol Na+ and 1 mol Cl-)
190
191
Calculated Values for Electrolyte SolutionsColligative properties depend on total concentration of solute particles regardless of their identity Electrolytes cause changes in colligative properties proportional to total molality in terms of all dissolves particles instead of formula units
192
What about barium nitrate Ba(NO3)2 Each mole of barium nitrate yields 3 mol of ions in solution Ba(NO3)2(s) rarr Ba2+(aq) + 2NO3-(aq) You would expect solution of given molality to lower fp of solvent 3 times as much as nonelectrolytic solution of same molality
193
Sample Problem
What is the expected change in the freezing point of water in a solution of 625 g of barium nitrate Ba(NO3)2 in 100 kg of water
194
1 AnalyzeGiven solute mass and formula = 625 g Ba(NO3)2 solvent mass and identity = 100 kg water Δtf = Kfm
Unknown expected freezing-point depression
195
2 PlanThe molality can be calculated by converting the solute mass to moles and then dividing by the number of kilograms of solvent That molality is in terms of formula units of Ba(NO3)2 and must be converted to molality in terms of dissociated ions in solution It must be multiplied by the number of moles of ions produced per mole of formula unit This adjusted molality can then be used to calculate the freezing-point depression
196
197
198
3 Compute
Practice Problem
What is the expected freezing-point depression for a solution that contains 20 mol of magnesium sulfate dissolved in 10 kg of water
minus74degC199
What is the expected boiling-point elevation of water for a solution that contains 150 g of sodium chloride dissolved in 10 kg of water 27degC The freezing point of an aqueous sodium chloride solution is minus200degC What is the molality of the solution 54 m NaCl
200
Actual Values for Electrolyte SolutionsRemember values calculated are only expected values Actual values of colligative properties for all strong electrolytes are almost what would be expected Difference between expected and actual values caused by attractive forces that exist between dissociated ions in aqueous solution
201
Attraction between hydrated ions in solution is small compared with those in crystalline solid However forces of attraction donrsquot interfere with movements of aqueous ions More concentrated solution means ions closer together and attraction is greater Also depends on charge of ions ndash higher charge greater attraction
202
When 2 mol nonelectrolyte solute dissolved in 1 kg water freezing point is -372
For any concentration of nonelectrolyte solute in water decrease in freezing point can be determined using value of -186m
161
This value called the molal freezing-point contstant (Kf) is the freezing-point depression of the solvent in a 1-molal solution of a nonvolatile nonelectrolyte solute
162
Each solvent has own characteristic molal freezing-point constant Freezing-point depression (Δtf) the difference between the freezing points of the pure solvent and a solute of nonelectrolyte in that solvent and it is directly proportional to the molal concentration of the solution Δtf = Kfm
163
Δtf = Kfm
Kf expressed as m
m expressed in mol solutekg solvent (molality)
164
Sample Problem
What is the freezing point depression of water in a solution of 171 g of sucrose C12H22O11 and 200 g of water What is the actual freezing point of the solution
165
1 AnalyzeGiven Solute mass and chemical formula 171 g C12H22O11 Solvent mass and identity 2000 g water Unknown a freezing-point depression b freezing point of the solution 166
2 Plan
167
168
Calculate
Unknown a freezing-point depression b freezing point of the solution
169
Practice Problem
A water solution containing an unknown quantity of a nonelectrolyte solute is found to have a freezing point of ndash 023degCWhat is the molal concentration of the solution
012 m170
A solution consists of 103 g of the nonelectrolyte glucose C6H12O6 dissolved in 250 g of water What is the freezing-point depression of the solution
minus0426degC171
In a laboratory experiment the freezing point of an aqueous solution of glucose is found to be minus0325degCWhat is the molal concentration of this solution
0175 m172
If 0500 mol of a nonelectrolyte solute are dissolved in 5000 g of ether what is the freezing point of the solution
minus1181degC173
The freezing point of an aqueous solution that contains a nonelectrolyte is minus90degC aWhat is the freezing-point depression of the solution bWhat is the molal concentration of the solution a minus90degC b 48 m
174
Boiling-Point Elevation
Boiling point is temp at which vapor pressure is equal to atmospheric pressure Change in vapor pressure will cause corresponding change in boiling point
175
176
Vapor pressure of nonvolatile solution is lower than vapor pressure of pure solvent More heat will be required to raise vapor pressure of solution Boiling point of solution is higher than boiling point of pure solvent
Molal boiling-point constant (Kb) - boiling-point elevation of the solvent in a 1-molal solution of a nonvolatile nonelectrolyte solute b-p elevation of any nonelectrolyte in water found by experiment to be 051 So molal b-p constant for water is 051m
177
For different solvents b-p elevations of 1-molal solutions have different values (Table 14-2)
Like freezing-point constants values most accurate for dilute solutions
178
Boiling-point elevation Δtb difference between the boiling points of the pure solvent and a nonelectrolyte solution of that solvent and it is directly proportional to the molal concentration of the solution B-p elevation can be calculated using Δtb = Kbm Δtb (boiling point elevation) expressed in m and m expressed in mol solutekg solvent
Sample Problem
What is the boiling-point elevation of a solution made from 200 g of a nonelectrolyte solute and 4000 g of water The molar mass of the solute is 620 gmol
180
1 AnalyzeGiven solute mass = 200 g solute molar mass = 620 gmol solvent mass and identity = 4000 g of water
Unknown boiling-point elevation
181
2 Plan
182
183
3 Compute
Practice Problem
A solution contains 500 g of sucrose C12H22O11 a nonelectrolyte dissolved in 5000 g of water What is the boiling-point elevation
015degC184
A solution contains 4500 g of sucrose C12H22O11 a nonelectrolyte dissolved in 250 g of waterWhat is the boiling point of the solution
1027degC185
If the boiling point elevation of an aqueous solution containing a nonvolatile electrolyte is 102degC what is the molality of the solution
20 m186
The boiling point of an aqueous solution containing a nonvolatile electrolyte is 10075degC aWhat is the boiling-point elevation bWhat is the molality of the solution a 075degC b 15 m
187
Electrolytes and Colligative PropertiesEarly investigators confused by experiments where certain substances depressed freezing point or elevated boiling point more than expected
188
189
Ex 01 m solution of NaCl lowers freezing point of solvent almost twice as much as 01 m solution of sucrose 01 m CaCl2 lowers fp almost 3 times as much as 01 m solution of sucrose
To understand why this happens contrast behavior or sucrose with NaCl in aqueous solution Each sucrose dissolves to make only 1 particle in solution So 1 mol sucrose dissolves to 1 mol particles 1 mol NaCl dissolves to make 2 moles of particles (1 mol Na+ and 1 mol Cl-)
190
191
Calculated Values for Electrolyte SolutionsColligative properties depend on total concentration of solute particles regardless of their identity Electrolytes cause changes in colligative properties proportional to total molality in terms of all dissolves particles instead of formula units
192
What about barium nitrate Ba(NO3)2 Each mole of barium nitrate yields 3 mol of ions in solution Ba(NO3)2(s) rarr Ba2+(aq) + 2NO3-(aq) You would expect solution of given molality to lower fp of solvent 3 times as much as nonelectrolytic solution of same molality
193
Sample Problem
What is the expected change in the freezing point of water in a solution of 625 g of barium nitrate Ba(NO3)2 in 100 kg of water
194
1 AnalyzeGiven solute mass and formula = 625 g Ba(NO3)2 solvent mass and identity = 100 kg water Δtf = Kfm
Unknown expected freezing-point depression
195
2 PlanThe molality can be calculated by converting the solute mass to moles and then dividing by the number of kilograms of solvent That molality is in terms of formula units of Ba(NO3)2 and must be converted to molality in terms of dissociated ions in solution It must be multiplied by the number of moles of ions produced per mole of formula unit This adjusted molality can then be used to calculate the freezing-point depression
196
197
198
3 Compute
Practice Problem
What is the expected freezing-point depression for a solution that contains 20 mol of magnesium sulfate dissolved in 10 kg of water
minus74degC199
What is the expected boiling-point elevation of water for a solution that contains 150 g of sodium chloride dissolved in 10 kg of water 27degC The freezing point of an aqueous sodium chloride solution is minus200degC What is the molality of the solution 54 m NaCl
200
Actual Values for Electrolyte SolutionsRemember values calculated are only expected values Actual values of colligative properties for all strong electrolytes are almost what would be expected Difference between expected and actual values caused by attractive forces that exist between dissociated ions in aqueous solution
201
Attraction between hydrated ions in solution is small compared with those in crystalline solid However forces of attraction donrsquot interfere with movements of aqueous ions More concentrated solution means ions closer together and attraction is greater Also depends on charge of ions ndash higher charge greater attraction
202
This value called the molal freezing-point contstant (Kf) is the freezing-point depression of the solvent in a 1-molal solution of a nonvolatile nonelectrolyte solute
162
Each solvent has own characteristic molal freezing-point constant Freezing-point depression (Δtf) the difference between the freezing points of the pure solvent and a solute of nonelectrolyte in that solvent and it is directly proportional to the molal concentration of the solution Δtf = Kfm
163
Δtf = Kfm
Kf expressed as m
m expressed in mol solutekg solvent (molality)
164
Sample Problem
What is the freezing point depression of water in a solution of 171 g of sucrose C12H22O11 and 200 g of water What is the actual freezing point of the solution
165
1 AnalyzeGiven Solute mass and chemical formula 171 g C12H22O11 Solvent mass and identity 2000 g water Unknown a freezing-point depression b freezing point of the solution 166
2 Plan
167
168
Calculate
Unknown a freezing-point depression b freezing point of the solution
169
Practice Problem
A water solution containing an unknown quantity of a nonelectrolyte solute is found to have a freezing point of ndash 023degCWhat is the molal concentration of the solution
012 m170
A solution consists of 103 g of the nonelectrolyte glucose C6H12O6 dissolved in 250 g of water What is the freezing-point depression of the solution
minus0426degC171
In a laboratory experiment the freezing point of an aqueous solution of glucose is found to be minus0325degCWhat is the molal concentration of this solution
0175 m172
If 0500 mol of a nonelectrolyte solute are dissolved in 5000 g of ether what is the freezing point of the solution
minus1181degC173
The freezing point of an aqueous solution that contains a nonelectrolyte is minus90degC aWhat is the freezing-point depression of the solution bWhat is the molal concentration of the solution a minus90degC b 48 m
174
Boiling-Point Elevation
Boiling point is temp at which vapor pressure is equal to atmospheric pressure Change in vapor pressure will cause corresponding change in boiling point
175
176
Vapor pressure of nonvolatile solution is lower than vapor pressure of pure solvent More heat will be required to raise vapor pressure of solution Boiling point of solution is higher than boiling point of pure solvent
Molal boiling-point constant (Kb) - boiling-point elevation of the solvent in a 1-molal solution of a nonvolatile nonelectrolyte solute b-p elevation of any nonelectrolyte in water found by experiment to be 051 So molal b-p constant for water is 051m
177
For different solvents b-p elevations of 1-molal solutions have different values (Table 14-2)
Like freezing-point constants values most accurate for dilute solutions
178
Boiling-point elevation Δtb difference between the boiling points of the pure solvent and a nonelectrolyte solution of that solvent and it is directly proportional to the molal concentration of the solution B-p elevation can be calculated using Δtb = Kbm Δtb (boiling point elevation) expressed in m and m expressed in mol solutekg solvent
Sample Problem
What is the boiling-point elevation of a solution made from 200 g of a nonelectrolyte solute and 4000 g of water The molar mass of the solute is 620 gmol
180
1 AnalyzeGiven solute mass = 200 g solute molar mass = 620 gmol solvent mass and identity = 4000 g of water
Unknown boiling-point elevation
181
2 Plan
182
183
3 Compute
Practice Problem
A solution contains 500 g of sucrose C12H22O11 a nonelectrolyte dissolved in 5000 g of water What is the boiling-point elevation
015degC184
A solution contains 4500 g of sucrose C12H22O11 a nonelectrolyte dissolved in 250 g of waterWhat is the boiling point of the solution
1027degC185
If the boiling point elevation of an aqueous solution containing a nonvolatile electrolyte is 102degC what is the molality of the solution
20 m186
The boiling point of an aqueous solution containing a nonvolatile electrolyte is 10075degC aWhat is the boiling-point elevation bWhat is the molality of the solution a 075degC b 15 m
187
Electrolytes and Colligative PropertiesEarly investigators confused by experiments where certain substances depressed freezing point or elevated boiling point more than expected
188
189
Ex 01 m solution of NaCl lowers freezing point of solvent almost twice as much as 01 m solution of sucrose 01 m CaCl2 lowers fp almost 3 times as much as 01 m solution of sucrose
To understand why this happens contrast behavior or sucrose with NaCl in aqueous solution Each sucrose dissolves to make only 1 particle in solution So 1 mol sucrose dissolves to 1 mol particles 1 mol NaCl dissolves to make 2 moles of particles (1 mol Na+ and 1 mol Cl-)
190
191
Calculated Values for Electrolyte SolutionsColligative properties depend on total concentration of solute particles regardless of their identity Electrolytes cause changes in colligative properties proportional to total molality in terms of all dissolves particles instead of formula units
192
What about barium nitrate Ba(NO3)2 Each mole of barium nitrate yields 3 mol of ions in solution Ba(NO3)2(s) rarr Ba2+(aq) + 2NO3-(aq) You would expect solution of given molality to lower fp of solvent 3 times as much as nonelectrolytic solution of same molality
193
Sample Problem
What is the expected change in the freezing point of water in a solution of 625 g of barium nitrate Ba(NO3)2 in 100 kg of water
194
1 AnalyzeGiven solute mass and formula = 625 g Ba(NO3)2 solvent mass and identity = 100 kg water Δtf = Kfm
Unknown expected freezing-point depression
195
2 PlanThe molality can be calculated by converting the solute mass to moles and then dividing by the number of kilograms of solvent That molality is in terms of formula units of Ba(NO3)2 and must be converted to molality in terms of dissociated ions in solution It must be multiplied by the number of moles of ions produced per mole of formula unit This adjusted molality can then be used to calculate the freezing-point depression
196
197
198
3 Compute
Practice Problem
What is the expected freezing-point depression for a solution that contains 20 mol of magnesium sulfate dissolved in 10 kg of water
minus74degC199
What is the expected boiling-point elevation of water for a solution that contains 150 g of sodium chloride dissolved in 10 kg of water 27degC The freezing point of an aqueous sodium chloride solution is minus200degC What is the molality of the solution 54 m NaCl
200
Actual Values for Electrolyte SolutionsRemember values calculated are only expected values Actual values of colligative properties for all strong electrolytes are almost what would be expected Difference between expected and actual values caused by attractive forces that exist between dissociated ions in aqueous solution
201
Attraction between hydrated ions in solution is small compared with those in crystalline solid However forces of attraction donrsquot interfere with movements of aqueous ions More concentrated solution means ions closer together and attraction is greater Also depends on charge of ions ndash higher charge greater attraction
202
Each solvent has own characteristic molal freezing-point constant Freezing-point depression (Δtf) the difference between the freezing points of the pure solvent and a solute of nonelectrolyte in that solvent and it is directly proportional to the molal concentration of the solution Δtf = Kfm
163
Δtf = Kfm
Kf expressed as m
m expressed in mol solutekg solvent (molality)
164
Sample Problem
What is the freezing point depression of water in a solution of 171 g of sucrose C12H22O11 and 200 g of water What is the actual freezing point of the solution
165
1 AnalyzeGiven Solute mass and chemical formula 171 g C12H22O11 Solvent mass and identity 2000 g water Unknown a freezing-point depression b freezing point of the solution 166
2 Plan
167
168
Calculate
Unknown a freezing-point depression b freezing point of the solution
169
Practice Problem
A water solution containing an unknown quantity of a nonelectrolyte solute is found to have a freezing point of ndash 023degCWhat is the molal concentration of the solution
012 m170
A solution consists of 103 g of the nonelectrolyte glucose C6H12O6 dissolved in 250 g of water What is the freezing-point depression of the solution
minus0426degC171
In a laboratory experiment the freezing point of an aqueous solution of glucose is found to be minus0325degCWhat is the molal concentration of this solution
0175 m172
If 0500 mol of a nonelectrolyte solute are dissolved in 5000 g of ether what is the freezing point of the solution
minus1181degC173
The freezing point of an aqueous solution that contains a nonelectrolyte is minus90degC aWhat is the freezing-point depression of the solution bWhat is the molal concentration of the solution a minus90degC b 48 m
174
Boiling-Point Elevation
Boiling point is temp at which vapor pressure is equal to atmospheric pressure Change in vapor pressure will cause corresponding change in boiling point
175
176
Vapor pressure of nonvolatile solution is lower than vapor pressure of pure solvent More heat will be required to raise vapor pressure of solution Boiling point of solution is higher than boiling point of pure solvent
Molal boiling-point constant (Kb) - boiling-point elevation of the solvent in a 1-molal solution of a nonvolatile nonelectrolyte solute b-p elevation of any nonelectrolyte in water found by experiment to be 051 So molal b-p constant for water is 051m
177
For different solvents b-p elevations of 1-molal solutions have different values (Table 14-2)
Like freezing-point constants values most accurate for dilute solutions
178
Boiling-point elevation Δtb difference between the boiling points of the pure solvent and a nonelectrolyte solution of that solvent and it is directly proportional to the molal concentration of the solution B-p elevation can be calculated using Δtb = Kbm Δtb (boiling point elevation) expressed in m and m expressed in mol solutekg solvent
Sample Problem
What is the boiling-point elevation of a solution made from 200 g of a nonelectrolyte solute and 4000 g of water The molar mass of the solute is 620 gmol
180
1 AnalyzeGiven solute mass = 200 g solute molar mass = 620 gmol solvent mass and identity = 4000 g of water
Unknown boiling-point elevation
181
2 Plan
182
183
3 Compute
Practice Problem
A solution contains 500 g of sucrose C12H22O11 a nonelectrolyte dissolved in 5000 g of water What is the boiling-point elevation
015degC184
A solution contains 4500 g of sucrose C12H22O11 a nonelectrolyte dissolved in 250 g of waterWhat is the boiling point of the solution
1027degC185
If the boiling point elevation of an aqueous solution containing a nonvolatile electrolyte is 102degC what is the molality of the solution
20 m186
The boiling point of an aqueous solution containing a nonvolatile electrolyte is 10075degC aWhat is the boiling-point elevation bWhat is the molality of the solution a 075degC b 15 m
187
Electrolytes and Colligative PropertiesEarly investigators confused by experiments where certain substances depressed freezing point or elevated boiling point more than expected
188
189
Ex 01 m solution of NaCl lowers freezing point of solvent almost twice as much as 01 m solution of sucrose 01 m CaCl2 lowers fp almost 3 times as much as 01 m solution of sucrose
To understand why this happens contrast behavior or sucrose with NaCl in aqueous solution Each sucrose dissolves to make only 1 particle in solution So 1 mol sucrose dissolves to 1 mol particles 1 mol NaCl dissolves to make 2 moles of particles (1 mol Na+ and 1 mol Cl-)
190
191
Calculated Values for Electrolyte SolutionsColligative properties depend on total concentration of solute particles regardless of their identity Electrolytes cause changes in colligative properties proportional to total molality in terms of all dissolves particles instead of formula units
192
What about barium nitrate Ba(NO3)2 Each mole of barium nitrate yields 3 mol of ions in solution Ba(NO3)2(s) rarr Ba2+(aq) + 2NO3-(aq) You would expect solution of given molality to lower fp of solvent 3 times as much as nonelectrolytic solution of same molality
193
Sample Problem
What is the expected change in the freezing point of water in a solution of 625 g of barium nitrate Ba(NO3)2 in 100 kg of water
194
1 AnalyzeGiven solute mass and formula = 625 g Ba(NO3)2 solvent mass and identity = 100 kg water Δtf = Kfm
Unknown expected freezing-point depression
195
2 PlanThe molality can be calculated by converting the solute mass to moles and then dividing by the number of kilograms of solvent That molality is in terms of formula units of Ba(NO3)2 and must be converted to molality in terms of dissociated ions in solution It must be multiplied by the number of moles of ions produced per mole of formula unit This adjusted molality can then be used to calculate the freezing-point depression
196
197
198
3 Compute
Practice Problem
What is the expected freezing-point depression for a solution that contains 20 mol of magnesium sulfate dissolved in 10 kg of water
minus74degC199
What is the expected boiling-point elevation of water for a solution that contains 150 g of sodium chloride dissolved in 10 kg of water 27degC The freezing point of an aqueous sodium chloride solution is minus200degC What is the molality of the solution 54 m NaCl
200
Actual Values for Electrolyte SolutionsRemember values calculated are only expected values Actual values of colligative properties for all strong electrolytes are almost what would be expected Difference between expected and actual values caused by attractive forces that exist between dissociated ions in aqueous solution
201
Attraction between hydrated ions in solution is small compared with those in crystalline solid However forces of attraction donrsquot interfere with movements of aqueous ions More concentrated solution means ions closer together and attraction is greater Also depends on charge of ions ndash higher charge greater attraction
202
Δtf = Kfm
Kf expressed as m
m expressed in mol solutekg solvent (molality)
164
Sample Problem
What is the freezing point depression of water in a solution of 171 g of sucrose C12H22O11 and 200 g of water What is the actual freezing point of the solution
165
1 AnalyzeGiven Solute mass and chemical formula 171 g C12H22O11 Solvent mass and identity 2000 g water Unknown a freezing-point depression b freezing point of the solution 166
2 Plan
167
168
Calculate
Unknown a freezing-point depression b freezing point of the solution
169
Practice Problem
A water solution containing an unknown quantity of a nonelectrolyte solute is found to have a freezing point of ndash 023degCWhat is the molal concentration of the solution
012 m170
A solution consists of 103 g of the nonelectrolyte glucose C6H12O6 dissolved in 250 g of water What is the freezing-point depression of the solution
minus0426degC171
In a laboratory experiment the freezing point of an aqueous solution of glucose is found to be minus0325degCWhat is the molal concentration of this solution
0175 m172
If 0500 mol of a nonelectrolyte solute are dissolved in 5000 g of ether what is the freezing point of the solution
minus1181degC173
The freezing point of an aqueous solution that contains a nonelectrolyte is minus90degC aWhat is the freezing-point depression of the solution bWhat is the molal concentration of the solution a minus90degC b 48 m
174
Boiling-Point Elevation
Boiling point is temp at which vapor pressure is equal to atmospheric pressure Change in vapor pressure will cause corresponding change in boiling point
175
176
Vapor pressure of nonvolatile solution is lower than vapor pressure of pure solvent More heat will be required to raise vapor pressure of solution Boiling point of solution is higher than boiling point of pure solvent
Molal boiling-point constant (Kb) - boiling-point elevation of the solvent in a 1-molal solution of a nonvolatile nonelectrolyte solute b-p elevation of any nonelectrolyte in water found by experiment to be 051 So molal b-p constant for water is 051m
177
For different solvents b-p elevations of 1-molal solutions have different values (Table 14-2)
Like freezing-point constants values most accurate for dilute solutions
178
Boiling-point elevation Δtb difference between the boiling points of the pure solvent and a nonelectrolyte solution of that solvent and it is directly proportional to the molal concentration of the solution B-p elevation can be calculated using Δtb = Kbm Δtb (boiling point elevation) expressed in m and m expressed in mol solutekg solvent
Sample Problem
What is the boiling-point elevation of a solution made from 200 g of a nonelectrolyte solute and 4000 g of water The molar mass of the solute is 620 gmol
180
1 AnalyzeGiven solute mass = 200 g solute molar mass = 620 gmol solvent mass and identity = 4000 g of water
Unknown boiling-point elevation
181
2 Plan
182
183
3 Compute
Practice Problem
A solution contains 500 g of sucrose C12H22O11 a nonelectrolyte dissolved in 5000 g of water What is the boiling-point elevation
015degC184
A solution contains 4500 g of sucrose C12H22O11 a nonelectrolyte dissolved in 250 g of waterWhat is the boiling point of the solution
1027degC185
If the boiling point elevation of an aqueous solution containing a nonvolatile electrolyte is 102degC what is the molality of the solution
20 m186
The boiling point of an aqueous solution containing a nonvolatile electrolyte is 10075degC aWhat is the boiling-point elevation bWhat is the molality of the solution a 075degC b 15 m
187
Electrolytes and Colligative PropertiesEarly investigators confused by experiments where certain substances depressed freezing point or elevated boiling point more than expected
188
189
Ex 01 m solution of NaCl lowers freezing point of solvent almost twice as much as 01 m solution of sucrose 01 m CaCl2 lowers fp almost 3 times as much as 01 m solution of sucrose
To understand why this happens contrast behavior or sucrose with NaCl in aqueous solution Each sucrose dissolves to make only 1 particle in solution So 1 mol sucrose dissolves to 1 mol particles 1 mol NaCl dissolves to make 2 moles of particles (1 mol Na+ and 1 mol Cl-)
190
191
Calculated Values for Electrolyte SolutionsColligative properties depend on total concentration of solute particles regardless of their identity Electrolytes cause changes in colligative properties proportional to total molality in terms of all dissolves particles instead of formula units
192
What about barium nitrate Ba(NO3)2 Each mole of barium nitrate yields 3 mol of ions in solution Ba(NO3)2(s) rarr Ba2+(aq) + 2NO3-(aq) You would expect solution of given molality to lower fp of solvent 3 times as much as nonelectrolytic solution of same molality
193
Sample Problem
What is the expected change in the freezing point of water in a solution of 625 g of barium nitrate Ba(NO3)2 in 100 kg of water
194
1 AnalyzeGiven solute mass and formula = 625 g Ba(NO3)2 solvent mass and identity = 100 kg water Δtf = Kfm
Unknown expected freezing-point depression
195
2 PlanThe molality can be calculated by converting the solute mass to moles and then dividing by the number of kilograms of solvent That molality is in terms of formula units of Ba(NO3)2 and must be converted to molality in terms of dissociated ions in solution It must be multiplied by the number of moles of ions produced per mole of formula unit This adjusted molality can then be used to calculate the freezing-point depression
196
197
198
3 Compute
Practice Problem
What is the expected freezing-point depression for a solution that contains 20 mol of magnesium sulfate dissolved in 10 kg of water
minus74degC199
What is the expected boiling-point elevation of water for a solution that contains 150 g of sodium chloride dissolved in 10 kg of water 27degC The freezing point of an aqueous sodium chloride solution is minus200degC What is the molality of the solution 54 m NaCl
200
Actual Values for Electrolyte SolutionsRemember values calculated are only expected values Actual values of colligative properties for all strong electrolytes are almost what would be expected Difference between expected and actual values caused by attractive forces that exist between dissociated ions in aqueous solution
201
Attraction between hydrated ions in solution is small compared with those in crystalline solid However forces of attraction donrsquot interfere with movements of aqueous ions More concentrated solution means ions closer together and attraction is greater Also depends on charge of ions ndash higher charge greater attraction
202
Sample Problem
What is the freezing point depression of water in a solution of 171 g of sucrose C12H22O11 and 200 g of water What is the actual freezing point of the solution
165
1 AnalyzeGiven Solute mass and chemical formula 171 g C12H22O11 Solvent mass and identity 2000 g water Unknown a freezing-point depression b freezing point of the solution 166
2 Plan
167
168
Calculate
Unknown a freezing-point depression b freezing point of the solution
169
Practice Problem
A water solution containing an unknown quantity of a nonelectrolyte solute is found to have a freezing point of ndash 023degCWhat is the molal concentration of the solution
012 m170
A solution consists of 103 g of the nonelectrolyte glucose C6H12O6 dissolved in 250 g of water What is the freezing-point depression of the solution
minus0426degC171
In a laboratory experiment the freezing point of an aqueous solution of glucose is found to be minus0325degCWhat is the molal concentration of this solution
0175 m172
If 0500 mol of a nonelectrolyte solute are dissolved in 5000 g of ether what is the freezing point of the solution
minus1181degC173
The freezing point of an aqueous solution that contains a nonelectrolyte is minus90degC aWhat is the freezing-point depression of the solution bWhat is the molal concentration of the solution a minus90degC b 48 m
174
Boiling-Point Elevation
Boiling point is temp at which vapor pressure is equal to atmospheric pressure Change in vapor pressure will cause corresponding change in boiling point
175
176
Vapor pressure of nonvolatile solution is lower than vapor pressure of pure solvent More heat will be required to raise vapor pressure of solution Boiling point of solution is higher than boiling point of pure solvent
Molal boiling-point constant (Kb) - boiling-point elevation of the solvent in a 1-molal solution of a nonvolatile nonelectrolyte solute b-p elevation of any nonelectrolyte in water found by experiment to be 051 So molal b-p constant for water is 051m
177
For different solvents b-p elevations of 1-molal solutions have different values (Table 14-2)
Like freezing-point constants values most accurate for dilute solutions
178
Boiling-point elevation Δtb difference between the boiling points of the pure solvent and a nonelectrolyte solution of that solvent and it is directly proportional to the molal concentration of the solution B-p elevation can be calculated using Δtb = Kbm Δtb (boiling point elevation) expressed in m and m expressed in mol solutekg solvent
Sample Problem
What is the boiling-point elevation of a solution made from 200 g of a nonelectrolyte solute and 4000 g of water The molar mass of the solute is 620 gmol
180
1 AnalyzeGiven solute mass = 200 g solute molar mass = 620 gmol solvent mass and identity = 4000 g of water
Unknown boiling-point elevation
181
2 Plan
182
183
3 Compute
Practice Problem
A solution contains 500 g of sucrose C12H22O11 a nonelectrolyte dissolved in 5000 g of water What is the boiling-point elevation
015degC184
A solution contains 4500 g of sucrose C12H22O11 a nonelectrolyte dissolved in 250 g of waterWhat is the boiling point of the solution
1027degC185
If the boiling point elevation of an aqueous solution containing a nonvolatile electrolyte is 102degC what is the molality of the solution
20 m186
The boiling point of an aqueous solution containing a nonvolatile electrolyte is 10075degC aWhat is the boiling-point elevation bWhat is the molality of the solution a 075degC b 15 m
187
Electrolytes and Colligative PropertiesEarly investigators confused by experiments where certain substances depressed freezing point or elevated boiling point more than expected
188
189
Ex 01 m solution of NaCl lowers freezing point of solvent almost twice as much as 01 m solution of sucrose 01 m CaCl2 lowers fp almost 3 times as much as 01 m solution of sucrose
To understand why this happens contrast behavior or sucrose with NaCl in aqueous solution Each sucrose dissolves to make only 1 particle in solution So 1 mol sucrose dissolves to 1 mol particles 1 mol NaCl dissolves to make 2 moles of particles (1 mol Na+ and 1 mol Cl-)
190
191
Calculated Values for Electrolyte SolutionsColligative properties depend on total concentration of solute particles regardless of their identity Electrolytes cause changes in colligative properties proportional to total molality in terms of all dissolves particles instead of formula units
192
What about barium nitrate Ba(NO3)2 Each mole of barium nitrate yields 3 mol of ions in solution Ba(NO3)2(s) rarr Ba2+(aq) + 2NO3-(aq) You would expect solution of given molality to lower fp of solvent 3 times as much as nonelectrolytic solution of same molality
193
Sample Problem
What is the expected change in the freezing point of water in a solution of 625 g of barium nitrate Ba(NO3)2 in 100 kg of water
194
1 AnalyzeGiven solute mass and formula = 625 g Ba(NO3)2 solvent mass and identity = 100 kg water Δtf = Kfm
Unknown expected freezing-point depression
195
2 PlanThe molality can be calculated by converting the solute mass to moles and then dividing by the number of kilograms of solvent That molality is in terms of formula units of Ba(NO3)2 and must be converted to molality in terms of dissociated ions in solution It must be multiplied by the number of moles of ions produced per mole of formula unit This adjusted molality can then be used to calculate the freezing-point depression
196
197
198
3 Compute
Practice Problem
What is the expected freezing-point depression for a solution that contains 20 mol of magnesium sulfate dissolved in 10 kg of water
minus74degC199
What is the expected boiling-point elevation of water for a solution that contains 150 g of sodium chloride dissolved in 10 kg of water 27degC The freezing point of an aqueous sodium chloride solution is minus200degC What is the molality of the solution 54 m NaCl
200
Actual Values for Electrolyte SolutionsRemember values calculated are only expected values Actual values of colligative properties for all strong electrolytes are almost what would be expected Difference between expected and actual values caused by attractive forces that exist between dissociated ions in aqueous solution
201
Attraction between hydrated ions in solution is small compared with those in crystalline solid However forces of attraction donrsquot interfere with movements of aqueous ions More concentrated solution means ions closer together and attraction is greater Also depends on charge of ions ndash higher charge greater attraction
202
1 AnalyzeGiven Solute mass and chemical formula 171 g C12H22O11 Solvent mass and identity 2000 g water Unknown a freezing-point depression b freezing point of the solution 166
2 Plan
167
168
Calculate
Unknown a freezing-point depression b freezing point of the solution
169
Practice Problem
A water solution containing an unknown quantity of a nonelectrolyte solute is found to have a freezing point of ndash 023degCWhat is the molal concentration of the solution
012 m170
A solution consists of 103 g of the nonelectrolyte glucose C6H12O6 dissolved in 250 g of water What is the freezing-point depression of the solution
minus0426degC171
In a laboratory experiment the freezing point of an aqueous solution of glucose is found to be minus0325degCWhat is the molal concentration of this solution
0175 m172
If 0500 mol of a nonelectrolyte solute are dissolved in 5000 g of ether what is the freezing point of the solution
minus1181degC173
The freezing point of an aqueous solution that contains a nonelectrolyte is minus90degC aWhat is the freezing-point depression of the solution bWhat is the molal concentration of the solution a minus90degC b 48 m
174
Boiling-Point Elevation
Boiling point is temp at which vapor pressure is equal to atmospheric pressure Change in vapor pressure will cause corresponding change in boiling point
175
176
Vapor pressure of nonvolatile solution is lower than vapor pressure of pure solvent More heat will be required to raise vapor pressure of solution Boiling point of solution is higher than boiling point of pure solvent
Molal boiling-point constant (Kb) - boiling-point elevation of the solvent in a 1-molal solution of a nonvolatile nonelectrolyte solute b-p elevation of any nonelectrolyte in water found by experiment to be 051 So molal b-p constant for water is 051m
177
For different solvents b-p elevations of 1-molal solutions have different values (Table 14-2)
Like freezing-point constants values most accurate for dilute solutions
178
Boiling-point elevation Δtb difference between the boiling points of the pure solvent and a nonelectrolyte solution of that solvent and it is directly proportional to the molal concentration of the solution B-p elevation can be calculated using Δtb = Kbm Δtb (boiling point elevation) expressed in m and m expressed in mol solutekg solvent
Sample Problem
What is the boiling-point elevation of a solution made from 200 g of a nonelectrolyte solute and 4000 g of water The molar mass of the solute is 620 gmol
180
1 AnalyzeGiven solute mass = 200 g solute molar mass = 620 gmol solvent mass and identity = 4000 g of water
Unknown boiling-point elevation
181
2 Plan
182
183
3 Compute
Practice Problem
A solution contains 500 g of sucrose C12H22O11 a nonelectrolyte dissolved in 5000 g of water What is the boiling-point elevation
015degC184
A solution contains 4500 g of sucrose C12H22O11 a nonelectrolyte dissolved in 250 g of waterWhat is the boiling point of the solution
1027degC185
If the boiling point elevation of an aqueous solution containing a nonvolatile electrolyte is 102degC what is the molality of the solution
20 m186
The boiling point of an aqueous solution containing a nonvolatile electrolyte is 10075degC aWhat is the boiling-point elevation bWhat is the molality of the solution a 075degC b 15 m
187
Electrolytes and Colligative PropertiesEarly investigators confused by experiments where certain substances depressed freezing point or elevated boiling point more than expected
188
189
Ex 01 m solution of NaCl lowers freezing point of solvent almost twice as much as 01 m solution of sucrose 01 m CaCl2 lowers fp almost 3 times as much as 01 m solution of sucrose
To understand why this happens contrast behavior or sucrose with NaCl in aqueous solution Each sucrose dissolves to make only 1 particle in solution So 1 mol sucrose dissolves to 1 mol particles 1 mol NaCl dissolves to make 2 moles of particles (1 mol Na+ and 1 mol Cl-)
190
191
Calculated Values for Electrolyte SolutionsColligative properties depend on total concentration of solute particles regardless of their identity Electrolytes cause changes in colligative properties proportional to total molality in terms of all dissolves particles instead of formula units
192
What about barium nitrate Ba(NO3)2 Each mole of barium nitrate yields 3 mol of ions in solution Ba(NO3)2(s) rarr Ba2+(aq) + 2NO3-(aq) You would expect solution of given molality to lower fp of solvent 3 times as much as nonelectrolytic solution of same molality
193
Sample Problem
What is the expected change in the freezing point of water in a solution of 625 g of barium nitrate Ba(NO3)2 in 100 kg of water
194
1 AnalyzeGiven solute mass and formula = 625 g Ba(NO3)2 solvent mass and identity = 100 kg water Δtf = Kfm
Unknown expected freezing-point depression
195
2 PlanThe molality can be calculated by converting the solute mass to moles and then dividing by the number of kilograms of solvent That molality is in terms of formula units of Ba(NO3)2 and must be converted to molality in terms of dissociated ions in solution It must be multiplied by the number of moles of ions produced per mole of formula unit This adjusted molality can then be used to calculate the freezing-point depression
196
197
198
3 Compute
Practice Problem
What is the expected freezing-point depression for a solution that contains 20 mol of magnesium sulfate dissolved in 10 kg of water
minus74degC199
What is the expected boiling-point elevation of water for a solution that contains 150 g of sodium chloride dissolved in 10 kg of water 27degC The freezing point of an aqueous sodium chloride solution is minus200degC What is the molality of the solution 54 m NaCl
200
Actual Values for Electrolyte SolutionsRemember values calculated are only expected values Actual values of colligative properties for all strong electrolytes are almost what would be expected Difference between expected and actual values caused by attractive forces that exist between dissociated ions in aqueous solution
201
Attraction between hydrated ions in solution is small compared with those in crystalline solid However forces of attraction donrsquot interfere with movements of aqueous ions More concentrated solution means ions closer together and attraction is greater Also depends on charge of ions ndash higher charge greater attraction
202
2 Plan
167
168
Calculate
Unknown a freezing-point depression b freezing point of the solution
169
Practice Problem
A water solution containing an unknown quantity of a nonelectrolyte solute is found to have a freezing point of ndash 023degCWhat is the molal concentration of the solution
012 m170
A solution consists of 103 g of the nonelectrolyte glucose C6H12O6 dissolved in 250 g of water What is the freezing-point depression of the solution
minus0426degC171
In a laboratory experiment the freezing point of an aqueous solution of glucose is found to be minus0325degCWhat is the molal concentration of this solution
0175 m172
If 0500 mol of a nonelectrolyte solute are dissolved in 5000 g of ether what is the freezing point of the solution
minus1181degC173
The freezing point of an aqueous solution that contains a nonelectrolyte is minus90degC aWhat is the freezing-point depression of the solution bWhat is the molal concentration of the solution a minus90degC b 48 m
174
Boiling-Point Elevation
Boiling point is temp at which vapor pressure is equal to atmospheric pressure Change in vapor pressure will cause corresponding change in boiling point
175
176
Vapor pressure of nonvolatile solution is lower than vapor pressure of pure solvent More heat will be required to raise vapor pressure of solution Boiling point of solution is higher than boiling point of pure solvent
Molal boiling-point constant (Kb) - boiling-point elevation of the solvent in a 1-molal solution of a nonvolatile nonelectrolyte solute b-p elevation of any nonelectrolyte in water found by experiment to be 051 So molal b-p constant for water is 051m
177
For different solvents b-p elevations of 1-molal solutions have different values (Table 14-2)
Like freezing-point constants values most accurate for dilute solutions
178
Boiling-point elevation Δtb difference between the boiling points of the pure solvent and a nonelectrolyte solution of that solvent and it is directly proportional to the molal concentration of the solution B-p elevation can be calculated using Δtb = Kbm Δtb (boiling point elevation) expressed in m and m expressed in mol solutekg solvent
Sample Problem
What is the boiling-point elevation of a solution made from 200 g of a nonelectrolyte solute and 4000 g of water The molar mass of the solute is 620 gmol
180
1 AnalyzeGiven solute mass = 200 g solute molar mass = 620 gmol solvent mass and identity = 4000 g of water
Unknown boiling-point elevation
181
2 Plan
182
183
3 Compute
Practice Problem
A solution contains 500 g of sucrose C12H22O11 a nonelectrolyte dissolved in 5000 g of water What is the boiling-point elevation
015degC184
A solution contains 4500 g of sucrose C12H22O11 a nonelectrolyte dissolved in 250 g of waterWhat is the boiling point of the solution
1027degC185
If the boiling point elevation of an aqueous solution containing a nonvolatile electrolyte is 102degC what is the molality of the solution
20 m186
The boiling point of an aqueous solution containing a nonvolatile electrolyte is 10075degC aWhat is the boiling-point elevation bWhat is the molality of the solution a 075degC b 15 m
187
Electrolytes and Colligative PropertiesEarly investigators confused by experiments where certain substances depressed freezing point or elevated boiling point more than expected
188
189
Ex 01 m solution of NaCl lowers freezing point of solvent almost twice as much as 01 m solution of sucrose 01 m CaCl2 lowers fp almost 3 times as much as 01 m solution of sucrose
To understand why this happens contrast behavior or sucrose with NaCl in aqueous solution Each sucrose dissolves to make only 1 particle in solution So 1 mol sucrose dissolves to 1 mol particles 1 mol NaCl dissolves to make 2 moles of particles (1 mol Na+ and 1 mol Cl-)
190
191
Calculated Values for Electrolyte SolutionsColligative properties depend on total concentration of solute particles regardless of their identity Electrolytes cause changes in colligative properties proportional to total molality in terms of all dissolves particles instead of formula units
192
What about barium nitrate Ba(NO3)2 Each mole of barium nitrate yields 3 mol of ions in solution Ba(NO3)2(s) rarr Ba2+(aq) + 2NO3-(aq) You would expect solution of given molality to lower fp of solvent 3 times as much as nonelectrolytic solution of same molality
193
Sample Problem
What is the expected change in the freezing point of water in a solution of 625 g of barium nitrate Ba(NO3)2 in 100 kg of water
194
1 AnalyzeGiven solute mass and formula = 625 g Ba(NO3)2 solvent mass and identity = 100 kg water Δtf = Kfm
Unknown expected freezing-point depression
195
2 PlanThe molality can be calculated by converting the solute mass to moles and then dividing by the number of kilograms of solvent That molality is in terms of formula units of Ba(NO3)2 and must be converted to molality in terms of dissociated ions in solution It must be multiplied by the number of moles of ions produced per mole of formula unit This adjusted molality can then be used to calculate the freezing-point depression
196
197
198
3 Compute
Practice Problem
What is the expected freezing-point depression for a solution that contains 20 mol of magnesium sulfate dissolved in 10 kg of water
minus74degC199
What is the expected boiling-point elevation of water for a solution that contains 150 g of sodium chloride dissolved in 10 kg of water 27degC The freezing point of an aqueous sodium chloride solution is minus200degC What is the molality of the solution 54 m NaCl
200
Actual Values for Electrolyte SolutionsRemember values calculated are only expected values Actual values of colligative properties for all strong electrolytes are almost what would be expected Difference between expected and actual values caused by attractive forces that exist between dissociated ions in aqueous solution
201
Attraction between hydrated ions in solution is small compared with those in crystalline solid However forces of attraction donrsquot interfere with movements of aqueous ions More concentrated solution means ions closer together and attraction is greater Also depends on charge of ions ndash higher charge greater attraction
202
168
Calculate
Unknown a freezing-point depression b freezing point of the solution
169
Practice Problem
A water solution containing an unknown quantity of a nonelectrolyte solute is found to have a freezing point of ndash 023degCWhat is the molal concentration of the solution
012 m170
A solution consists of 103 g of the nonelectrolyte glucose C6H12O6 dissolved in 250 g of water What is the freezing-point depression of the solution
minus0426degC171
In a laboratory experiment the freezing point of an aqueous solution of glucose is found to be minus0325degCWhat is the molal concentration of this solution
0175 m172
If 0500 mol of a nonelectrolyte solute are dissolved in 5000 g of ether what is the freezing point of the solution
minus1181degC173
The freezing point of an aqueous solution that contains a nonelectrolyte is minus90degC aWhat is the freezing-point depression of the solution bWhat is the molal concentration of the solution a minus90degC b 48 m
174
Boiling-Point Elevation
Boiling point is temp at which vapor pressure is equal to atmospheric pressure Change in vapor pressure will cause corresponding change in boiling point
175
176
Vapor pressure of nonvolatile solution is lower than vapor pressure of pure solvent More heat will be required to raise vapor pressure of solution Boiling point of solution is higher than boiling point of pure solvent
Molal boiling-point constant (Kb) - boiling-point elevation of the solvent in a 1-molal solution of a nonvolatile nonelectrolyte solute b-p elevation of any nonelectrolyte in water found by experiment to be 051 So molal b-p constant for water is 051m
177
For different solvents b-p elevations of 1-molal solutions have different values (Table 14-2)
Like freezing-point constants values most accurate for dilute solutions
178
Boiling-point elevation Δtb difference between the boiling points of the pure solvent and a nonelectrolyte solution of that solvent and it is directly proportional to the molal concentration of the solution B-p elevation can be calculated using Δtb = Kbm Δtb (boiling point elevation) expressed in m and m expressed in mol solutekg solvent
Sample Problem
What is the boiling-point elevation of a solution made from 200 g of a nonelectrolyte solute and 4000 g of water The molar mass of the solute is 620 gmol
180
1 AnalyzeGiven solute mass = 200 g solute molar mass = 620 gmol solvent mass and identity = 4000 g of water
Unknown boiling-point elevation
181
2 Plan
182
183
3 Compute
Practice Problem
A solution contains 500 g of sucrose C12H22O11 a nonelectrolyte dissolved in 5000 g of water What is the boiling-point elevation
015degC184
A solution contains 4500 g of sucrose C12H22O11 a nonelectrolyte dissolved in 250 g of waterWhat is the boiling point of the solution
1027degC185
If the boiling point elevation of an aqueous solution containing a nonvolatile electrolyte is 102degC what is the molality of the solution
20 m186
The boiling point of an aqueous solution containing a nonvolatile electrolyte is 10075degC aWhat is the boiling-point elevation bWhat is the molality of the solution a 075degC b 15 m
187
Electrolytes and Colligative PropertiesEarly investigators confused by experiments where certain substances depressed freezing point or elevated boiling point more than expected
188
189
Ex 01 m solution of NaCl lowers freezing point of solvent almost twice as much as 01 m solution of sucrose 01 m CaCl2 lowers fp almost 3 times as much as 01 m solution of sucrose
To understand why this happens contrast behavior or sucrose with NaCl in aqueous solution Each sucrose dissolves to make only 1 particle in solution So 1 mol sucrose dissolves to 1 mol particles 1 mol NaCl dissolves to make 2 moles of particles (1 mol Na+ and 1 mol Cl-)
190
191
Calculated Values for Electrolyte SolutionsColligative properties depend on total concentration of solute particles regardless of their identity Electrolytes cause changes in colligative properties proportional to total molality in terms of all dissolves particles instead of formula units
192
What about barium nitrate Ba(NO3)2 Each mole of barium nitrate yields 3 mol of ions in solution Ba(NO3)2(s) rarr Ba2+(aq) + 2NO3-(aq) You would expect solution of given molality to lower fp of solvent 3 times as much as nonelectrolytic solution of same molality
193
Sample Problem
What is the expected change in the freezing point of water in a solution of 625 g of barium nitrate Ba(NO3)2 in 100 kg of water
194
1 AnalyzeGiven solute mass and formula = 625 g Ba(NO3)2 solvent mass and identity = 100 kg water Δtf = Kfm
Unknown expected freezing-point depression
195
2 PlanThe molality can be calculated by converting the solute mass to moles and then dividing by the number of kilograms of solvent That molality is in terms of formula units of Ba(NO3)2 and must be converted to molality in terms of dissociated ions in solution It must be multiplied by the number of moles of ions produced per mole of formula unit This adjusted molality can then be used to calculate the freezing-point depression
196
197
198
3 Compute
Practice Problem
What is the expected freezing-point depression for a solution that contains 20 mol of magnesium sulfate dissolved in 10 kg of water
minus74degC199
What is the expected boiling-point elevation of water for a solution that contains 150 g of sodium chloride dissolved in 10 kg of water 27degC The freezing point of an aqueous sodium chloride solution is minus200degC What is the molality of the solution 54 m NaCl
200
Actual Values for Electrolyte SolutionsRemember values calculated are only expected values Actual values of colligative properties for all strong electrolytes are almost what would be expected Difference between expected and actual values caused by attractive forces that exist between dissociated ions in aqueous solution
201
Attraction between hydrated ions in solution is small compared with those in crystalline solid However forces of attraction donrsquot interfere with movements of aqueous ions More concentrated solution means ions closer together and attraction is greater Also depends on charge of ions ndash higher charge greater attraction
202
Unknown a freezing-point depression b freezing point of the solution
169
Practice Problem
A water solution containing an unknown quantity of a nonelectrolyte solute is found to have a freezing point of ndash 023degCWhat is the molal concentration of the solution
012 m170
A solution consists of 103 g of the nonelectrolyte glucose C6H12O6 dissolved in 250 g of water What is the freezing-point depression of the solution
minus0426degC171
In a laboratory experiment the freezing point of an aqueous solution of glucose is found to be minus0325degCWhat is the molal concentration of this solution
0175 m172
If 0500 mol of a nonelectrolyte solute are dissolved in 5000 g of ether what is the freezing point of the solution
minus1181degC173
The freezing point of an aqueous solution that contains a nonelectrolyte is minus90degC aWhat is the freezing-point depression of the solution bWhat is the molal concentration of the solution a minus90degC b 48 m
174
Boiling-Point Elevation
Boiling point is temp at which vapor pressure is equal to atmospheric pressure Change in vapor pressure will cause corresponding change in boiling point
175
176
Vapor pressure of nonvolatile solution is lower than vapor pressure of pure solvent More heat will be required to raise vapor pressure of solution Boiling point of solution is higher than boiling point of pure solvent
Molal boiling-point constant (Kb) - boiling-point elevation of the solvent in a 1-molal solution of a nonvolatile nonelectrolyte solute b-p elevation of any nonelectrolyte in water found by experiment to be 051 So molal b-p constant for water is 051m
177
For different solvents b-p elevations of 1-molal solutions have different values (Table 14-2)
Like freezing-point constants values most accurate for dilute solutions
178
Boiling-point elevation Δtb difference between the boiling points of the pure solvent and a nonelectrolyte solution of that solvent and it is directly proportional to the molal concentration of the solution B-p elevation can be calculated using Δtb = Kbm Δtb (boiling point elevation) expressed in m and m expressed in mol solutekg solvent
Sample Problem
What is the boiling-point elevation of a solution made from 200 g of a nonelectrolyte solute and 4000 g of water The molar mass of the solute is 620 gmol
180
1 AnalyzeGiven solute mass = 200 g solute molar mass = 620 gmol solvent mass and identity = 4000 g of water
Unknown boiling-point elevation
181
2 Plan
182
183
3 Compute
Practice Problem
A solution contains 500 g of sucrose C12H22O11 a nonelectrolyte dissolved in 5000 g of water What is the boiling-point elevation
015degC184
A solution contains 4500 g of sucrose C12H22O11 a nonelectrolyte dissolved in 250 g of waterWhat is the boiling point of the solution
1027degC185
If the boiling point elevation of an aqueous solution containing a nonvolatile electrolyte is 102degC what is the molality of the solution
20 m186
The boiling point of an aqueous solution containing a nonvolatile electrolyte is 10075degC aWhat is the boiling-point elevation bWhat is the molality of the solution a 075degC b 15 m
187
Electrolytes and Colligative PropertiesEarly investigators confused by experiments where certain substances depressed freezing point or elevated boiling point more than expected
188
189
Ex 01 m solution of NaCl lowers freezing point of solvent almost twice as much as 01 m solution of sucrose 01 m CaCl2 lowers fp almost 3 times as much as 01 m solution of sucrose
To understand why this happens contrast behavior or sucrose with NaCl in aqueous solution Each sucrose dissolves to make only 1 particle in solution So 1 mol sucrose dissolves to 1 mol particles 1 mol NaCl dissolves to make 2 moles of particles (1 mol Na+ and 1 mol Cl-)
190
191
Calculated Values for Electrolyte SolutionsColligative properties depend on total concentration of solute particles regardless of their identity Electrolytes cause changes in colligative properties proportional to total molality in terms of all dissolves particles instead of formula units
192
What about barium nitrate Ba(NO3)2 Each mole of barium nitrate yields 3 mol of ions in solution Ba(NO3)2(s) rarr Ba2+(aq) + 2NO3-(aq) You would expect solution of given molality to lower fp of solvent 3 times as much as nonelectrolytic solution of same molality
193
Sample Problem
What is the expected change in the freezing point of water in a solution of 625 g of barium nitrate Ba(NO3)2 in 100 kg of water
194
1 AnalyzeGiven solute mass and formula = 625 g Ba(NO3)2 solvent mass and identity = 100 kg water Δtf = Kfm
Unknown expected freezing-point depression
195
2 PlanThe molality can be calculated by converting the solute mass to moles and then dividing by the number of kilograms of solvent That molality is in terms of formula units of Ba(NO3)2 and must be converted to molality in terms of dissociated ions in solution It must be multiplied by the number of moles of ions produced per mole of formula unit This adjusted molality can then be used to calculate the freezing-point depression
196
197
198
3 Compute
Practice Problem
What is the expected freezing-point depression for a solution that contains 20 mol of magnesium sulfate dissolved in 10 kg of water
minus74degC199
What is the expected boiling-point elevation of water for a solution that contains 150 g of sodium chloride dissolved in 10 kg of water 27degC The freezing point of an aqueous sodium chloride solution is minus200degC What is the molality of the solution 54 m NaCl
200
Actual Values for Electrolyte SolutionsRemember values calculated are only expected values Actual values of colligative properties for all strong electrolytes are almost what would be expected Difference between expected and actual values caused by attractive forces that exist between dissociated ions in aqueous solution
201
Attraction between hydrated ions in solution is small compared with those in crystalline solid However forces of attraction donrsquot interfere with movements of aqueous ions More concentrated solution means ions closer together and attraction is greater Also depends on charge of ions ndash higher charge greater attraction
202
Practice Problem
A water solution containing an unknown quantity of a nonelectrolyte solute is found to have a freezing point of ndash 023degCWhat is the molal concentration of the solution
012 m170
A solution consists of 103 g of the nonelectrolyte glucose C6H12O6 dissolved in 250 g of water What is the freezing-point depression of the solution
minus0426degC171
In a laboratory experiment the freezing point of an aqueous solution of glucose is found to be minus0325degCWhat is the molal concentration of this solution
0175 m172
If 0500 mol of a nonelectrolyte solute are dissolved in 5000 g of ether what is the freezing point of the solution
minus1181degC173
The freezing point of an aqueous solution that contains a nonelectrolyte is minus90degC aWhat is the freezing-point depression of the solution bWhat is the molal concentration of the solution a minus90degC b 48 m
174
Boiling-Point Elevation
Boiling point is temp at which vapor pressure is equal to atmospheric pressure Change in vapor pressure will cause corresponding change in boiling point
175
176
Vapor pressure of nonvolatile solution is lower than vapor pressure of pure solvent More heat will be required to raise vapor pressure of solution Boiling point of solution is higher than boiling point of pure solvent
Molal boiling-point constant (Kb) - boiling-point elevation of the solvent in a 1-molal solution of a nonvolatile nonelectrolyte solute b-p elevation of any nonelectrolyte in water found by experiment to be 051 So molal b-p constant for water is 051m
177
For different solvents b-p elevations of 1-molal solutions have different values (Table 14-2)
Like freezing-point constants values most accurate for dilute solutions
178
Boiling-point elevation Δtb difference between the boiling points of the pure solvent and a nonelectrolyte solution of that solvent and it is directly proportional to the molal concentration of the solution B-p elevation can be calculated using Δtb = Kbm Δtb (boiling point elevation) expressed in m and m expressed in mol solutekg solvent
Sample Problem
What is the boiling-point elevation of a solution made from 200 g of a nonelectrolyte solute and 4000 g of water The molar mass of the solute is 620 gmol
180
1 AnalyzeGiven solute mass = 200 g solute molar mass = 620 gmol solvent mass and identity = 4000 g of water
Unknown boiling-point elevation
181
2 Plan
182
183
3 Compute
Practice Problem
A solution contains 500 g of sucrose C12H22O11 a nonelectrolyte dissolved in 5000 g of water What is the boiling-point elevation
015degC184
A solution contains 4500 g of sucrose C12H22O11 a nonelectrolyte dissolved in 250 g of waterWhat is the boiling point of the solution
1027degC185
If the boiling point elevation of an aqueous solution containing a nonvolatile electrolyte is 102degC what is the molality of the solution
20 m186
The boiling point of an aqueous solution containing a nonvolatile electrolyte is 10075degC aWhat is the boiling-point elevation bWhat is the molality of the solution a 075degC b 15 m
187
Electrolytes and Colligative PropertiesEarly investigators confused by experiments where certain substances depressed freezing point or elevated boiling point more than expected
188
189
Ex 01 m solution of NaCl lowers freezing point of solvent almost twice as much as 01 m solution of sucrose 01 m CaCl2 lowers fp almost 3 times as much as 01 m solution of sucrose
To understand why this happens contrast behavior or sucrose with NaCl in aqueous solution Each sucrose dissolves to make only 1 particle in solution So 1 mol sucrose dissolves to 1 mol particles 1 mol NaCl dissolves to make 2 moles of particles (1 mol Na+ and 1 mol Cl-)
190
191
Calculated Values for Electrolyte SolutionsColligative properties depend on total concentration of solute particles regardless of their identity Electrolytes cause changes in colligative properties proportional to total molality in terms of all dissolves particles instead of formula units
192
What about barium nitrate Ba(NO3)2 Each mole of barium nitrate yields 3 mol of ions in solution Ba(NO3)2(s) rarr Ba2+(aq) + 2NO3-(aq) You would expect solution of given molality to lower fp of solvent 3 times as much as nonelectrolytic solution of same molality
193
Sample Problem
What is the expected change in the freezing point of water in a solution of 625 g of barium nitrate Ba(NO3)2 in 100 kg of water
194
1 AnalyzeGiven solute mass and formula = 625 g Ba(NO3)2 solvent mass and identity = 100 kg water Δtf = Kfm
Unknown expected freezing-point depression
195
2 PlanThe molality can be calculated by converting the solute mass to moles and then dividing by the number of kilograms of solvent That molality is in terms of formula units of Ba(NO3)2 and must be converted to molality in terms of dissociated ions in solution It must be multiplied by the number of moles of ions produced per mole of formula unit This adjusted molality can then be used to calculate the freezing-point depression
196
197
198
3 Compute
Practice Problem
What is the expected freezing-point depression for a solution that contains 20 mol of magnesium sulfate dissolved in 10 kg of water
minus74degC199
What is the expected boiling-point elevation of water for a solution that contains 150 g of sodium chloride dissolved in 10 kg of water 27degC The freezing point of an aqueous sodium chloride solution is minus200degC What is the molality of the solution 54 m NaCl
200
Actual Values for Electrolyte SolutionsRemember values calculated are only expected values Actual values of colligative properties for all strong electrolytes are almost what would be expected Difference between expected and actual values caused by attractive forces that exist between dissociated ions in aqueous solution
201
Attraction between hydrated ions in solution is small compared with those in crystalline solid However forces of attraction donrsquot interfere with movements of aqueous ions More concentrated solution means ions closer together and attraction is greater Also depends on charge of ions ndash higher charge greater attraction
202
A solution consists of 103 g of the nonelectrolyte glucose C6H12O6 dissolved in 250 g of water What is the freezing-point depression of the solution
minus0426degC171
In a laboratory experiment the freezing point of an aqueous solution of glucose is found to be minus0325degCWhat is the molal concentration of this solution
0175 m172
If 0500 mol of a nonelectrolyte solute are dissolved in 5000 g of ether what is the freezing point of the solution
minus1181degC173
The freezing point of an aqueous solution that contains a nonelectrolyte is minus90degC aWhat is the freezing-point depression of the solution bWhat is the molal concentration of the solution a minus90degC b 48 m
174
Boiling-Point Elevation
Boiling point is temp at which vapor pressure is equal to atmospheric pressure Change in vapor pressure will cause corresponding change in boiling point
175
176
Vapor pressure of nonvolatile solution is lower than vapor pressure of pure solvent More heat will be required to raise vapor pressure of solution Boiling point of solution is higher than boiling point of pure solvent
Molal boiling-point constant (Kb) - boiling-point elevation of the solvent in a 1-molal solution of a nonvolatile nonelectrolyte solute b-p elevation of any nonelectrolyte in water found by experiment to be 051 So molal b-p constant for water is 051m
177
For different solvents b-p elevations of 1-molal solutions have different values (Table 14-2)
Like freezing-point constants values most accurate for dilute solutions
178
Boiling-point elevation Δtb difference between the boiling points of the pure solvent and a nonelectrolyte solution of that solvent and it is directly proportional to the molal concentration of the solution B-p elevation can be calculated using Δtb = Kbm Δtb (boiling point elevation) expressed in m and m expressed in mol solutekg solvent
Sample Problem
What is the boiling-point elevation of a solution made from 200 g of a nonelectrolyte solute and 4000 g of water The molar mass of the solute is 620 gmol
180
1 AnalyzeGiven solute mass = 200 g solute molar mass = 620 gmol solvent mass and identity = 4000 g of water
Unknown boiling-point elevation
181
2 Plan
182
183
3 Compute
Practice Problem
A solution contains 500 g of sucrose C12H22O11 a nonelectrolyte dissolved in 5000 g of water What is the boiling-point elevation
015degC184
A solution contains 4500 g of sucrose C12H22O11 a nonelectrolyte dissolved in 250 g of waterWhat is the boiling point of the solution
1027degC185
If the boiling point elevation of an aqueous solution containing a nonvolatile electrolyte is 102degC what is the molality of the solution
20 m186
The boiling point of an aqueous solution containing a nonvolatile electrolyte is 10075degC aWhat is the boiling-point elevation bWhat is the molality of the solution a 075degC b 15 m
187
Electrolytes and Colligative PropertiesEarly investigators confused by experiments where certain substances depressed freezing point or elevated boiling point more than expected
188
189
Ex 01 m solution of NaCl lowers freezing point of solvent almost twice as much as 01 m solution of sucrose 01 m CaCl2 lowers fp almost 3 times as much as 01 m solution of sucrose
To understand why this happens contrast behavior or sucrose with NaCl in aqueous solution Each sucrose dissolves to make only 1 particle in solution So 1 mol sucrose dissolves to 1 mol particles 1 mol NaCl dissolves to make 2 moles of particles (1 mol Na+ and 1 mol Cl-)
190
191
Calculated Values for Electrolyte SolutionsColligative properties depend on total concentration of solute particles regardless of their identity Electrolytes cause changes in colligative properties proportional to total molality in terms of all dissolves particles instead of formula units
192
What about barium nitrate Ba(NO3)2 Each mole of barium nitrate yields 3 mol of ions in solution Ba(NO3)2(s) rarr Ba2+(aq) + 2NO3-(aq) You would expect solution of given molality to lower fp of solvent 3 times as much as nonelectrolytic solution of same molality
193
Sample Problem
What is the expected change in the freezing point of water in a solution of 625 g of barium nitrate Ba(NO3)2 in 100 kg of water
194
1 AnalyzeGiven solute mass and formula = 625 g Ba(NO3)2 solvent mass and identity = 100 kg water Δtf = Kfm
Unknown expected freezing-point depression
195
2 PlanThe molality can be calculated by converting the solute mass to moles and then dividing by the number of kilograms of solvent That molality is in terms of formula units of Ba(NO3)2 and must be converted to molality in terms of dissociated ions in solution It must be multiplied by the number of moles of ions produced per mole of formula unit This adjusted molality can then be used to calculate the freezing-point depression
196
197
198
3 Compute
Practice Problem
What is the expected freezing-point depression for a solution that contains 20 mol of magnesium sulfate dissolved in 10 kg of water
minus74degC199
What is the expected boiling-point elevation of water for a solution that contains 150 g of sodium chloride dissolved in 10 kg of water 27degC The freezing point of an aqueous sodium chloride solution is minus200degC What is the molality of the solution 54 m NaCl
200
Actual Values for Electrolyte SolutionsRemember values calculated are only expected values Actual values of colligative properties for all strong electrolytes are almost what would be expected Difference between expected and actual values caused by attractive forces that exist between dissociated ions in aqueous solution
201
Attraction between hydrated ions in solution is small compared with those in crystalline solid However forces of attraction donrsquot interfere with movements of aqueous ions More concentrated solution means ions closer together and attraction is greater Also depends on charge of ions ndash higher charge greater attraction
202
In a laboratory experiment the freezing point of an aqueous solution of glucose is found to be minus0325degCWhat is the molal concentration of this solution
0175 m172
If 0500 mol of a nonelectrolyte solute are dissolved in 5000 g of ether what is the freezing point of the solution
minus1181degC173
The freezing point of an aqueous solution that contains a nonelectrolyte is minus90degC aWhat is the freezing-point depression of the solution bWhat is the molal concentration of the solution a minus90degC b 48 m
174
Boiling-Point Elevation
Boiling point is temp at which vapor pressure is equal to atmospheric pressure Change in vapor pressure will cause corresponding change in boiling point
175
176
Vapor pressure of nonvolatile solution is lower than vapor pressure of pure solvent More heat will be required to raise vapor pressure of solution Boiling point of solution is higher than boiling point of pure solvent
Molal boiling-point constant (Kb) - boiling-point elevation of the solvent in a 1-molal solution of a nonvolatile nonelectrolyte solute b-p elevation of any nonelectrolyte in water found by experiment to be 051 So molal b-p constant for water is 051m
177
For different solvents b-p elevations of 1-molal solutions have different values (Table 14-2)
Like freezing-point constants values most accurate for dilute solutions
178
Boiling-point elevation Δtb difference between the boiling points of the pure solvent and a nonelectrolyte solution of that solvent and it is directly proportional to the molal concentration of the solution B-p elevation can be calculated using Δtb = Kbm Δtb (boiling point elevation) expressed in m and m expressed in mol solutekg solvent
Sample Problem
What is the boiling-point elevation of a solution made from 200 g of a nonelectrolyte solute and 4000 g of water The molar mass of the solute is 620 gmol
180
1 AnalyzeGiven solute mass = 200 g solute molar mass = 620 gmol solvent mass and identity = 4000 g of water
Unknown boiling-point elevation
181
2 Plan
182
183
3 Compute
Practice Problem
A solution contains 500 g of sucrose C12H22O11 a nonelectrolyte dissolved in 5000 g of water What is the boiling-point elevation
015degC184
A solution contains 4500 g of sucrose C12H22O11 a nonelectrolyte dissolved in 250 g of waterWhat is the boiling point of the solution
1027degC185
If the boiling point elevation of an aqueous solution containing a nonvolatile electrolyte is 102degC what is the molality of the solution
20 m186
The boiling point of an aqueous solution containing a nonvolatile electrolyte is 10075degC aWhat is the boiling-point elevation bWhat is the molality of the solution a 075degC b 15 m
187
Electrolytes and Colligative PropertiesEarly investigators confused by experiments where certain substances depressed freezing point or elevated boiling point more than expected
188
189
Ex 01 m solution of NaCl lowers freezing point of solvent almost twice as much as 01 m solution of sucrose 01 m CaCl2 lowers fp almost 3 times as much as 01 m solution of sucrose
To understand why this happens contrast behavior or sucrose with NaCl in aqueous solution Each sucrose dissolves to make only 1 particle in solution So 1 mol sucrose dissolves to 1 mol particles 1 mol NaCl dissolves to make 2 moles of particles (1 mol Na+ and 1 mol Cl-)
190
191
Calculated Values for Electrolyte SolutionsColligative properties depend on total concentration of solute particles regardless of their identity Electrolytes cause changes in colligative properties proportional to total molality in terms of all dissolves particles instead of formula units
192
What about barium nitrate Ba(NO3)2 Each mole of barium nitrate yields 3 mol of ions in solution Ba(NO3)2(s) rarr Ba2+(aq) + 2NO3-(aq) You would expect solution of given molality to lower fp of solvent 3 times as much as nonelectrolytic solution of same molality
193
Sample Problem
What is the expected change in the freezing point of water in a solution of 625 g of barium nitrate Ba(NO3)2 in 100 kg of water
194
1 AnalyzeGiven solute mass and formula = 625 g Ba(NO3)2 solvent mass and identity = 100 kg water Δtf = Kfm
Unknown expected freezing-point depression
195
2 PlanThe molality can be calculated by converting the solute mass to moles and then dividing by the number of kilograms of solvent That molality is in terms of formula units of Ba(NO3)2 and must be converted to molality in terms of dissociated ions in solution It must be multiplied by the number of moles of ions produced per mole of formula unit This adjusted molality can then be used to calculate the freezing-point depression
196
197
198
3 Compute
Practice Problem
What is the expected freezing-point depression for a solution that contains 20 mol of magnesium sulfate dissolved in 10 kg of water
minus74degC199
What is the expected boiling-point elevation of water for a solution that contains 150 g of sodium chloride dissolved in 10 kg of water 27degC The freezing point of an aqueous sodium chloride solution is minus200degC What is the molality of the solution 54 m NaCl
200
Actual Values for Electrolyte SolutionsRemember values calculated are only expected values Actual values of colligative properties for all strong electrolytes are almost what would be expected Difference between expected and actual values caused by attractive forces that exist between dissociated ions in aqueous solution
201
Attraction between hydrated ions in solution is small compared with those in crystalline solid However forces of attraction donrsquot interfere with movements of aqueous ions More concentrated solution means ions closer together and attraction is greater Also depends on charge of ions ndash higher charge greater attraction
202
If 0500 mol of a nonelectrolyte solute are dissolved in 5000 g of ether what is the freezing point of the solution
minus1181degC173
The freezing point of an aqueous solution that contains a nonelectrolyte is minus90degC aWhat is the freezing-point depression of the solution bWhat is the molal concentration of the solution a minus90degC b 48 m
174
Boiling-Point Elevation
Boiling point is temp at which vapor pressure is equal to atmospheric pressure Change in vapor pressure will cause corresponding change in boiling point
175
176
Vapor pressure of nonvolatile solution is lower than vapor pressure of pure solvent More heat will be required to raise vapor pressure of solution Boiling point of solution is higher than boiling point of pure solvent
Molal boiling-point constant (Kb) - boiling-point elevation of the solvent in a 1-molal solution of a nonvolatile nonelectrolyte solute b-p elevation of any nonelectrolyte in water found by experiment to be 051 So molal b-p constant for water is 051m
177
For different solvents b-p elevations of 1-molal solutions have different values (Table 14-2)
Like freezing-point constants values most accurate for dilute solutions
178
Boiling-point elevation Δtb difference between the boiling points of the pure solvent and a nonelectrolyte solution of that solvent and it is directly proportional to the molal concentration of the solution B-p elevation can be calculated using Δtb = Kbm Δtb (boiling point elevation) expressed in m and m expressed in mol solutekg solvent
Sample Problem
What is the boiling-point elevation of a solution made from 200 g of a nonelectrolyte solute and 4000 g of water The molar mass of the solute is 620 gmol
180
1 AnalyzeGiven solute mass = 200 g solute molar mass = 620 gmol solvent mass and identity = 4000 g of water
Unknown boiling-point elevation
181
2 Plan
182
183
3 Compute
Practice Problem
A solution contains 500 g of sucrose C12H22O11 a nonelectrolyte dissolved in 5000 g of water What is the boiling-point elevation
015degC184
A solution contains 4500 g of sucrose C12H22O11 a nonelectrolyte dissolved in 250 g of waterWhat is the boiling point of the solution
1027degC185
If the boiling point elevation of an aqueous solution containing a nonvolatile electrolyte is 102degC what is the molality of the solution
20 m186
The boiling point of an aqueous solution containing a nonvolatile electrolyte is 10075degC aWhat is the boiling-point elevation bWhat is the molality of the solution a 075degC b 15 m
187
Electrolytes and Colligative PropertiesEarly investigators confused by experiments where certain substances depressed freezing point or elevated boiling point more than expected
188
189
Ex 01 m solution of NaCl lowers freezing point of solvent almost twice as much as 01 m solution of sucrose 01 m CaCl2 lowers fp almost 3 times as much as 01 m solution of sucrose
To understand why this happens contrast behavior or sucrose with NaCl in aqueous solution Each sucrose dissolves to make only 1 particle in solution So 1 mol sucrose dissolves to 1 mol particles 1 mol NaCl dissolves to make 2 moles of particles (1 mol Na+ and 1 mol Cl-)
190
191
Calculated Values for Electrolyte SolutionsColligative properties depend on total concentration of solute particles regardless of their identity Electrolytes cause changes in colligative properties proportional to total molality in terms of all dissolves particles instead of formula units
192
What about barium nitrate Ba(NO3)2 Each mole of barium nitrate yields 3 mol of ions in solution Ba(NO3)2(s) rarr Ba2+(aq) + 2NO3-(aq) You would expect solution of given molality to lower fp of solvent 3 times as much as nonelectrolytic solution of same molality
193
Sample Problem
What is the expected change in the freezing point of water in a solution of 625 g of barium nitrate Ba(NO3)2 in 100 kg of water
194
1 AnalyzeGiven solute mass and formula = 625 g Ba(NO3)2 solvent mass and identity = 100 kg water Δtf = Kfm
Unknown expected freezing-point depression
195
2 PlanThe molality can be calculated by converting the solute mass to moles and then dividing by the number of kilograms of solvent That molality is in terms of formula units of Ba(NO3)2 and must be converted to molality in terms of dissociated ions in solution It must be multiplied by the number of moles of ions produced per mole of formula unit This adjusted molality can then be used to calculate the freezing-point depression
196
197
198
3 Compute
Practice Problem
What is the expected freezing-point depression for a solution that contains 20 mol of magnesium sulfate dissolved in 10 kg of water
minus74degC199
What is the expected boiling-point elevation of water for a solution that contains 150 g of sodium chloride dissolved in 10 kg of water 27degC The freezing point of an aqueous sodium chloride solution is minus200degC What is the molality of the solution 54 m NaCl
200
Actual Values for Electrolyte SolutionsRemember values calculated are only expected values Actual values of colligative properties for all strong electrolytes are almost what would be expected Difference between expected and actual values caused by attractive forces that exist between dissociated ions in aqueous solution
201
Attraction between hydrated ions in solution is small compared with those in crystalline solid However forces of attraction donrsquot interfere with movements of aqueous ions More concentrated solution means ions closer together and attraction is greater Also depends on charge of ions ndash higher charge greater attraction
202
The freezing point of an aqueous solution that contains a nonelectrolyte is minus90degC aWhat is the freezing-point depression of the solution bWhat is the molal concentration of the solution a minus90degC b 48 m
174
Boiling-Point Elevation
Boiling point is temp at which vapor pressure is equal to atmospheric pressure Change in vapor pressure will cause corresponding change in boiling point
175
176
Vapor pressure of nonvolatile solution is lower than vapor pressure of pure solvent More heat will be required to raise vapor pressure of solution Boiling point of solution is higher than boiling point of pure solvent
Molal boiling-point constant (Kb) - boiling-point elevation of the solvent in a 1-molal solution of a nonvolatile nonelectrolyte solute b-p elevation of any nonelectrolyte in water found by experiment to be 051 So molal b-p constant for water is 051m
177
For different solvents b-p elevations of 1-molal solutions have different values (Table 14-2)
Like freezing-point constants values most accurate for dilute solutions
178
Boiling-point elevation Δtb difference between the boiling points of the pure solvent and a nonelectrolyte solution of that solvent and it is directly proportional to the molal concentration of the solution B-p elevation can be calculated using Δtb = Kbm Δtb (boiling point elevation) expressed in m and m expressed in mol solutekg solvent
Sample Problem
What is the boiling-point elevation of a solution made from 200 g of a nonelectrolyte solute and 4000 g of water The molar mass of the solute is 620 gmol
180
1 AnalyzeGiven solute mass = 200 g solute molar mass = 620 gmol solvent mass and identity = 4000 g of water
Unknown boiling-point elevation
181
2 Plan
182
183
3 Compute
Practice Problem
A solution contains 500 g of sucrose C12H22O11 a nonelectrolyte dissolved in 5000 g of water What is the boiling-point elevation
015degC184
A solution contains 4500 g of sucrose C12H22O11 a nonelectrolyte dissolved in 250 g of waterWhat is the boiling point of the solution
1027degC185
If the boiling point elevation of an aqueous solution containing a nonvolatile electrolyte is 102degC what is the molality of the solution
20 m186
The boiling point of an aqueous solution containing a nonvolatile electrolyte is 10075degC aWhat is the boiling-point elevation bWhat is the molality of the solution a 075degC b 15 m
187
Electrolytes and Colligative PropertiesEarly investigators confused by experiments where certain substances depressed freezing point or elevated boiling point more than expected
188
189
Ex 01 m solution of NaCl lowers freezing point of solvent almost twice as much as 01 m solution of sucrose 01 m CaCl2 lowers fp almost 3 times as much as 01 m solution of sucrose
To understand why this happens contrast behavior or sucrose with NaCl in aqueous solution Each sucrose dissolves to make only 1 particle in solution So 1 mol sucrose dissolves to 1 mol particles 1 mol NaCl dissolves to make 2 moles of particles (1 mol Na+ and 1 mol Cl-)
190
191
Calculated Values for Electrolyte SolutionsColligative properties depend on total concentration of solute particles regardless of their identity Electrolytes cause changes in colligative properties proportional to total molality in terms of all dissolves particles instead of formula units
192
What about barium nitrate Ba(NO3)2 Each mole of barium nitrate yields 3 mol of ions in solution Ba(NO3)2(s) rarr Ba2+(aq) + 2NO3-(aq) You would expect solution of given molality to lower fp of solvent 3 times as much as nonelectrolytic solution of same molality
193
Sample Problem
What is the expected change in the freezing point of water in a solution of 625 g of barium nitrate Ba(NO3)2 in 100 kg of water
194
1 AnalyzeGiven solute mass and formula = 625 g Ba(NO3)2 solvent mass and identity = 100 kg water Δtf = Kfm
Unknown expected freezing-point depression
195
2 PlanThe molality can be calculated by converting the solute mass to moles and then dividing by the number of kilograms of solvent That molality is in terms of formula units of Ba(NO3)2 and must be converted to molality in terms of dissociated ions in solution It must be multiplied by the number of moles of ions produced per mole of formula unit This adjusted molality can then be used to calculate the freezing-point depression
196
197
198
3 Compute
Practice Problem
What is the expected freezing-point depression for a solution that contains 20 mol of magnesium sulfate dissolved in 10 kg of water
minus74degC199
What is the expected boiling-point elevation of water for a solution that contains 150 g of sodium chloride dissolved in 10 kg of water 27degC The freezing point of an aqueous sodium chloride solution is minus200degC What is the molality of the solution 54 m NaCl
200
Actual Values for Electrolyte SolutionsRemember values calculated are only expected values Actual values of colligative properties for all strong electrolytes are almost what would be expected Difference between expected and actual values caused by attractive forces that exist between dissociated ions in aqueous solution
201
Attraction between hydrated ions in solution is small compared with those in crystalline solid However forces of attraction donrsquot interfere with movements of aqueous ions More concentrated solution means ions closer together and attraction is greater Also depends on charge of ions ndash higher charge greater attraction
202
Boiling-Point Elevation
Boiling point is temp at which vapor pressure is equal to atmospheric pressure Change in vapor pressure will cause corresponding change in boiling point
175
176
Vapor pressure of nonvolatile solution is lower than vapor pressure of pure solvent More heat will be required to raise vapor pressure of solution Boiling point of solution is higher than boiling point of pure solvent
Molal boiling-point constant (Kb) - boiling-point elevation of the solvent in a 1-molal solution of a nonvolatile nonelectrolyte solute b-p elevation of any nonelectrolyte in water found by experiment to be 051 So molal b-p constant for water is 051m
177
For different solvents b-p elevations of 1-molal solutions have different values (Table 14-2)
Like freezing-point constants values most accurate for dilute solutions
178
Boiling-point elevation Δtb difference between the boiling points of the pure solvent and a nonelectrolyte solution of that solvent and it is directly proportional to the molal concentration of the solution B-p elevation can be calculated using Δtb = Kbm Δtb (boiling point elevation) expressed in m and m expressed in mol solutekg solvent
Sample Problem
What is the boiling-point elevation of a solution made from 200 g of a nonelectrolyte solute and 4000 g of water The molar mass of the solute is 620 gmol
180
1 AnalyzeGiven solute mass = 200 g solute molar mass = 620 gmol solvent mass and identity = 4000 g of water
Unknown boiling-point elevation
181
2 Plan
182
183
3 Compute
Practice Problem
A solution contains 500 g of sucrose C12H22O11 a nonelectrolyte dissolved in 5000 g of water What is the boiling-point elevation
015degC184
A solution contains 4500 g of sucrose C12H22O11 a nonelectrolyte dissolved in 250 g of waterWhat is the boiling point of the solution
1027degC185
If the boiling point elevation of an aqueous solution containing a nonvolatile electrolyte is 102degC what is the molality of the solution
20 m186
The boiling point of an aqueous solution containing a nonvolatile electrolyte is 10075degC aWhat is the boiling-point elevation bWhat is the molality of the solution a 075degC b 15 m
187
Electrolytes and Colligative PropertiesEarly investigators confused by experiments where certain substances depressed freezing point or elevated boiling point more than expected
188
189
Ex 01 m solution of NaCl lowers freezing point of solvent almost twice as much as 01 m solution of sucrose 01 m CaCl2 lowers fp almost 3 times as much as 01 m solution of sucrose
To understand why this happens contrast behavior or sucrose with NaCl in aqueous solution Each sucrose dissolves to make only 1 particle in solution So 1 mol sucrose dissolves to 1 mol particles 1 mol NaCl dissolves to make 2 moles of particles (1 mol Na+ and 1 mol Cl-)
190
191
Calculated Values for Electrolyte SolutionsColligative properties depend on total concentration of solute particles regardless of their identity Electrolytes cause changes in colligative properties proportional to total molality in terms of all dissolves particles instead of formula units
192
What about barium nitrate Ba(NO3)2 Each mole of barium nitrate yields 3 mol of ions in solution Ba(NO3)2(s) rarr Ba2+(aq) + 2NO3-(aq) You would expect solution of given molality to lower fp of solvent 3 times as much as nonelectrolytic solution of same molality
193
Sample Problem
What is the expected change in the freezing point of water in a solution of 625 g of barium nitrate Ba(NO3)2 in 100 kg of water
194
1 AnalyzeGiven solute mass and formula = 625 g Ba(NO3)2 solvent mass and identity = 100 kg water Δtf = Kfm
Unknown expected freezing-point depression
195
2 PlanThe molality can be calculated by converting the solute mass to moles and then dividing by the number of kilograms of solvent That molality is in terms of formula units of Ba(NO3)2 and must be converted to molality in terms of dissociated ions in solution It must be multiplied by the number of moles of ions produced per mole of formula unit This adjusted molality can then be used to calculate the freezing-point depression
196
197
198
3 Compute
Practice Problem
What is the expected freezing-point depression for a solution that contains 20 mol of magnesium sulfate dissolved in 10 kg of water
minus74degC199
What is the expected boiling-point elevation of water for a solution that contains 150 g of sodium chloride dissolved in 10 kg of water 27degC The freezing point of an aqueous sodium chloride solution is minus200degC What is the molality of the solution 54 m NaCl
200
Actual Values for Electrolyte SolutionsRemember values calculated are only expected values Actual values of colligative properties for all strong electrolytes are almost what would be expected Difference between expected and actual values caused by attractive forces that exist between dissociated ions in aqueous solution
201
Attraction between hydrated ions in solution is small compared with those in crystalline solid However forces of attraction donrsquot interfere with movements of aqueous ions More concentrated solution means ions closer together and attraction is greater Also depends on charge of ions ndash higher charge greater attraction
202
176
Vapor pressure of nonvolatile solution is lower than vapor pressure of pure solvent More heat will be required to raise vapor pressure of solution Boiling point of solution is higher than boiling point of pure solvent
Molal boiling-point constant (Kb) - boiling-point elevation of the solvent in a 1-molal solution of a nonvolatile nonelectrolyte solute b-p elevation of any nonelectrolyte in water found by experiment to be 051 So molal b-p constant for water is 051m
177
For different solvents b-p elevations of 1-molal solutions have different values (Table 14-2)
Like freezing-point constants values most accurate for dilute solutions
178
Boiling-point elevation Δtb difference between the boiling points of the pure solvent and a nonelectrolyte solution of that solvent and it is directly proportional to the molal concentration of the solution B-p elevation can be calculated using Δtb = Kbm Δtb (boiling point elevation) expressed in m and m expressed in mol solutekg solvent
Sample Problem
What is the boiling-point elevation of a solution made from 200 g of a nonelectrolyte solute and 4000 g of water The molar mass of the solute is 620 gmol
180
1 AnalyzeGiven solute mass = 200 g solute molar mass = 620 gmol solvent mass and identity = 4000 g of water
Unknown boiling-point elevation
181
2 Plan
182
183
3 Compute
Practice Problem
A solution contains 500 g of sucrose C12H22O11 a nonelectrolyte dissolved in 5000 g of water What is the boiling-point elevation
015degC184
A solution contains 4500 g of sucrose C12H22O11 a nonelectrolyte dissolved in 250 g of waterWhat is the boiling point of the solution
1027degC185
If the boiling point elevation of an aqueous solution containing a nonvolatile electrolyte is 102degC what is the molality of the solution
20 m186
The boiling point of an aqueous solution containing a nonvolatile electrolyte is 10075degC aWhat is the boiling-point elevation bWhat is the molality of the solution a 075degC b 15 m
187
Electrolytes and Colligative PropertiesEarly investigators confused by experiments where certain substances depressed freezing point or elevated boiling point more than expected
188
189
Ex 01 m solution of NaCl lowers freezing point of solvent almost twice as much as 01 m solution of sucrose 01 m CaCl2 lowers fp almost 3 times as much as 01 m solution of sucrose
To understand why this happens contrast behavior or sucrose with NaCl in aqueous solution Each sucrose dissolves to make only 1 particle in solution So 1 mol sucrose dissolves to 1 mol particles 1 mol NaCl dissolves to make 2 moles of particles (1 mol Na+ and 1 mol Cl-)
190
191
Calculated Values for Electrolyte SolutionsColligative properties depend on total concentration of solute particles regardless of their identity Electrolytes cause changes in colligative properties proportional to total molality in terms of all dissolves particles instead of formula units
192
What about barium nitrate Ba(NO3)2 Each mole of barium nitrate yields 3 mol of ions in solution Ba(NO3)2(s) rarr Ba2+(aq) + 2NO3-(aq) You would expect solution of given molality to lower fp of solvent 3 times as much as nonelectrolytic solution of same molality
193
Sample Problem
What is the expected change in the freezing point of water in a solution of 625 g of barium nitrate Ba(NO3)2 in 100 kg of water
194
1 AnalyzeGiven solute mass and formula = 625 g Ba(NO3)2 solvent mass and identity = 100 kg water Δtf = Kfm
Unknown expected freezing-point depression
195
2 PlanThe molality can be calculated by converting the solute mass to moles and then dividing by the number of kilograms of solvent That molality is in terms of formula units of Ba(NO3)2 and must be converted to molality in terms of dissociated ions in solution It must be multiplied by the number of moles of ions produced per mole of formula unit This adjusted molality can then be used to calculate the freezing-point depression
196
197
198
3 Compute
Practice Problem
What is the expected freezing-point depression for a solution that contains 20 mol of magnesium sulfate dissolved in 10 kg of water
minus74degC199
What is the expected boiling-point elevation of water for a solution that contains 150 g of sodium chloride dissolved in 10 kg of water 27degC The freezing point of an aqueous sodium chloride solution is minus200degC What is the molality of the solution 54 m NaCl
200
Actual Values for Electrolyte SolutionsRemember values calculated are only expected values Actual values of colligative properties for all strong electrolytes are almost what would be expected Difference between expected and actual values caused by attractive forces that exist between dissociated ions in aqueous solution
201
Attraction between hydrated ions in solution is small compared with those in crystalline solid However forces of attraction donrsquot interfere with movements of aqueous ions More concentrated solution means ions closer together and attraction is greater Also depends on charge of ions ndash higher charge greater attraction
202
Molal boiling-point constant (Kb) - boiling-point elevation of the solvent in a 1-molal solution of a nonvolatile nonelectrolyte solute b-p elevation of any nonelectrolyte in water found by experiment to be 051 So molal b-p constant for water is 051m
177
For different solvents b-p elevations of 1-molal solutions have different values (Table 14-2)
Like freezing-point constants values most accurate for dilute solutions
178
Boiling-point elevation Δtb difference between the boiling points of the pure solvent and a nonelectrolyte solution of that solvent and it is directly proportional to the molal concentration of the solution B-p elevation can be calculated using Δtb = Kbm Δtb (boiling point elevation) expressed in m and m expressed in mol solutekg solvent
Sample Problem
What is the boiling-point elevation of a solution made from 200 g of a nonelectrolyte solute and 4000 g of water The molar mass of the solute is 620 gmol
180
1 AnalyzeGiven solute mass = 200 g solute molar mass = 620 gmol solvent mass and identity = 4000 g of water
Unknown boiling-point elevation
181
2 Plan
182
183
3 Compute
Practice Problem
A solution contains 500 g of sucrose C12H22O11 a nonelectrolyte dissolved in 5000 g of water What is the boiling-point elevation
015degC184
A solution contains 4500 g of sucrose C12H22O11 a nonelectrolyte dissolved in 250 g of waterWhat is the boiling point of the solution
1027degC185
If the boiling point elevation of an aqueous solution containing a nonvolatile electrolyte is 102degC what is the molality of the solution
20 m186
The boiling point of an aqueous solution containing a nonvolatile electrolyte is 10075degC aWhat is the boiling-point elevation bWhat is the molality of the solution a 075degC b 15 m
187
Electrolytes and Colligative PropertiesEarly investigators confused by experiments where certain substances depressed freezing point or elevated boiling point more than expected
188
189
Ex 01 m solution of NaCl lowers freezing point of solvent almost twice as much as 01 m solution of sucrose 01 m CaCl2 lowers fp almost 3 times as much as 01 m solution of sucrose
To understand why this happens contrast behavior or sucrose with NaCl in aqueous solution Each sucrose dissolves to make only 1 particle in solution So 1 mol sucrose dissolves to 1 mol particles 1 mol NaCl dissolves to make 2 moles of particles (1 mol Na+ and 1 mol Cl-)
190
191
Calculated Values for Electrolyte SolutionsColligative properties depend on total concentration of solute particles regardless of their identity Electrolytes cause changes in colligative properties proportional to total molality in terms of all dissolves particles instead of formula units
192
What about barium nitrate Ba(NO3)2 Each mole of barium nitrate yields 3 mol of ions in solution Ba(NO3)2(s) rarr Ba2+(aq) + 2NO3-(aq) You would expect solution of given molality to lower fp of solvent 3 times as much as nonelectrolytic solution of same molality
193
Sample Problem
What is the expected change in the freezing point of water in a solution of 625 g of barium nitrate Ba(NO3)2 in 100 kg of water
194
1 AnalyzeGiven solute mass and formula = 625 g Ba(NO3)2 solvent mass and identity = 100 kg water Δtf = Kfm
Unknown expected freezing-point depression
195
2 PlanThe molality can be calculated by converting the solute mass to moles and then dividing by the number of kilograms of solvent That molality is in terms of formula units of Ba(NO3)2 and must be converted to molality in terms of dissociated ions in solution It must be multiplied by the number of moles of ions produced per mole of formula unit This adjusted molality can then be used to calculate the freezing-point depression
196
197
198
3 Compute
Practice Problem
What is the expected freezing-point depression for a solution that contains 20 mol of magnesium sulfate dissolved in 10 kg of water
minus74degC199
What is the expected boiling-point elevation of water for a solution that contains 150 g of sodium chloride dissolved in 10 kg of water 27degC The freezing point of an aqueous sodium chloride solution is minus200degC What is the molality of the solution 54 m NaCl
200
Actual Values for Electrolyte SolutionsRemember values calculated are only expected values Actual values of colligative properties for all strong electrolytes are almost what would be expected Difference between expected and actual values caused by attractive forces that exist between dissociated ions in aqueous solution
201
Attraction between hydrated ions in solution is small compared with those in crystalline solid However forces of attraction donrsquot interfere with movements of aqueous ions More concentrated solution means ions closer together and attraction is greater Also depends on charge of ions ndash higher charge greater attraction
202
For different solvents b-p elevations of 1-molal solutions have different values (Table 14-2)
Like freezing-point constants values most accurate for dilute solutions
178
Boiling-point elevation Δtb difference between the boiling points of the pure solvent and a nonelectrolyte solution of that solvent and it is directly proportional to the molal concentration of the solution B-p elevation can be calculated using Δtb = Kbm Δtb (boiling point elevation) expressed in m and m expressed in mol solutekg solvent
Sample Problem
What is the boiling-point elevation of a solution made from 200 g of a nonelectrolyte solute and 4000 g of water The molar mass of the solute is 620 gmol
180
1 AnalyzeGiven solute mass = 200 g solute molar mass = 620 gmol solvent mass and identity = 4000 g of water
Unknown boiling-point elevation
181
2 Plan
182
183
3 Compute
Practice Problem
A solution contains 500 g of sucrose C12H22O11 a nonelectrolyte dissolved in 5000 g of water What is the boiling-point elevation
015degC184
A solution contains 4500 g of sucrose C12H22O11 a nonelectrolyte dissolved in 250 g of waterWhat is the boiling point of the solution
1027degC185
If the boiling point elevation of an aqueous solution containing a nonvolatile electrolyte is 102degC what is the molality of the solution
20 m186
The boiling point of an aqueous solution containing a nonvolatile electrolyte is 10075degC aWhat is the boiling-point elevation bWhat is the molality of the solution a 075degC b 15 m
187
Electrolytes and Colligative PropertiesEarly investigators confused by experiments where certain substances depressed freezing point or elevated boiling point more than expected
188
189
Ex 01 m solution of NaCl lowers freezing point of solvent almost twice as much as 01 m solution of sucrose 01 m CaCl2 lowers fp almost 3 times as much as 01 m solution of sucrose
To understand why this happens contrast behavior or sucrose with NaCl in aqueous solution Each sucrose dissolves to make only 1 particle in solution So 1 mol sucrose dissolves to 1 mol particles 1 mol NaCl dissolves to make 2 moles of particles (1 mol Na+ and 1 mol Cl-)
190
191
Calculated Values for Electrolyte SolutionsColligative properties depend on total concentration of solute particles regardless of their identity Electrolytes cause changes in colligative properties proportional to total molality in terms of all dissolves particles instead of formula units
192
What about barium nitrate Ba(NO3)2 Each mole of barium nitrate yields 3 mol of ions in solution Ba(NO3)2(s) rarr Ba2+(aq) + 2NO3-(aq) You would expect solution of given molality to lower fp of solvent 3 times as much as nonelectrolytic solution of same molality
193
Sample Problem
What is the expected change in the freezing point of water in a solution of 625 g of barium nitrate Ba(NO3)2 in 100 kg of water
194
1 AnalyzeGiven solute mass and formula = 625 g Ba(NO3)2 solvent mass and identity = 100 kg water Δtf = Kfm
Unknown expected freezing-point depression
195
2 PlanThe molality can be calculated by converting the solute mass to moles and then dividing by the number of kilograms of solvent That molality is in terms of formula units of Ba(NO3)2 and must be converted to molality in terms of dissociated ions in solution It must be multiplied by the number of moles of ions produced per mole of formula unit This adjusted molality can then be used to calculate the freezing-point depression
196
197
198
3 Compute
Practice Problem
What is the expected freezing-point depression for a solution that contains 20 mol of magnesium sulfate dissolved in 10 kg of water
minus74degC199
What is the expected boiling-point elevation of water for a solution that contains 150 g of sodium chloride dissolved in 10 kg of water 27degC The freezing point of an aqueous sodium chloride solution is minus200degC What is the molality of the solution 54 m NaCl
200
Actual Values for Electrolyte SolutionsRemember values calculated are only expected values Actual values of colligative properties for all strong electrolytes are almost what would be expected Difference between expected and actual values caused by attractive forces that exist between dissociated ions in aqueous solution
201
Attraction between hydrated ions in solution is small compared with those in crystalline solid However forces of attraction donrsquot interfere with movements of aqueous ions More concentrated solution means ions closer together and attraction is greater Also depends on charge of ions ndash higher charge greater attraction
202
Boiling-point elevation Δtb difference between the boiling points of the pure solvent and a nonelectrolyte solution of that solvent and it is directly proportional to the molal concentration of the solution B-p elevation can be calculated using Δtb = Kbm Δtb (boiling point elevation) expressed in m and m expressed in mol solutekg solvent
Sample Problem
What is the boiling-point elevation of a solution made from 200 g of a nonelectrolyte solute and 4000 g of water The molar mass of the solute is 620 gmol
180
1 AnalyzeGiven solute mass = 200 g solute molar mass = 620 gmol solvent mass and identity = 4000 g of water
Unknown boiling-point elevation
181
2 Plan
182
183
3 Compute
Practice Problem
A solution contains 500 g of sucrose C12H22O11 a nonelectrolyte dissolved in 5000 g of water What is the boiling-point elevation
015degC184
A solution contains 4500 g of sucrose C12H22O11 a nonelectrolyte dissolved in 250 g of waterWhat is the boiling point of the solution
1027degC185
If the boiling point elevation of an aqueous solution containing a nonvolatile electrolyte is 102degC what is the molality of the solution
20 m186
The boiling point of an aqueous solution containing a nonvolatile electrolyte is 10075degC aWhat is the boiling-point elevation bWhat is the molality of the solution a 075degC b 15 m
187
Electrolytes and Colligative PropertiesEarly investigators confused by experiments where certain substances depressed freezing point or elevated boiling point more than expected
188
189
Ex 01 m solution of NaCl lowers freezing point of solvent almost twice as much as 01 m solution of sucrose 01 m CaCl2 lowers fp almost 3 times as much as 01 m solution of sucrose
To understand why this happens contrast behavior or sucrose with NaCl in aqueous solution Each sucrose dissolves to make only 1 particle in solution So 1 mol sucrose dissolves to 1 mol particles 1 mol NaCl dissolves to make 2 moles of particles (1 mol Na+ and 1 mol Cl-)
190
191
Calculated Values for Electrolyte SolutionsColligative properties depend on total concentration of solute particles regardless of their identity Electrolytes cause changes in colligative properties proportional to total molality in terms of all dissolves particles instead of formula units
192
What about barium nitrate Ba(NO3)2 Each mole of barium nitrate yields 3 mol of ions in solution Ba(NO3)2(s) rarr Ba2+(aq) + 2NO3-(aq) You would expect solution of given molality to lower fp of solvent 3 times as much as nonelectrolytic solution of same molality
193
Sample Problem
What is the expected change in the freezing point of water in a solution of 625 g of barium nitrate Ba(NO3)2 in 100 kg of water
194
1 AnalyzeGiven solute mass and formula = 625 g Ba(NO3)2 solvent mass and identity = 100 kg water Δtf = Kfm
Unknown expected freezing-point depression
195
2 PlanThe molality can be calculated by converting the solute mass to moles and then dividing by the number of kilograms of solvent That molality is in terms of formula units of Ba(NO3)2 and must be converted to molality in terms of dissociated ions in solution It must be multiplied by the number of moles of ions produced per mole of formula unit This adjusted molality can then be used to calculate the freezing-point depression
196
197
198
3 Compute
Practice Problem
What is the expected freezing-point depression for a solution that contains 20 mol of magnesium sulfate dissolved in 10 kg of water
minus74degC199
What is the expected boiling-point elevation of water for a solution that contains 150 g of sodium chloride dissolved in 10 kg of water 27degC The freezing point of an aqueous sodium chloride solution is minus200degC What is the molality of the solution 54 m NaCl
200
Actual Values for Electrolyte SolutionsRemember values calculated are only expected values Actual values of colligative properties for all strong electrolytes are almost what would be expected Difference between expected and actual values caused by attractive forces that exist between dissociated ions in aqueous solution
201
Attraction between hydrated ions in solution is small compared with those in crystalline solid However forces of attraction donrsquot interfere with movements of aqueous ions More concentrated solution means ions closer together and attraction is greater Also depends on charge of ions ndash higher charge greater attraction
202
Sample Problem
What is the boiling-point elevation of a solution made from 200 g of a nonelectrolyte solute and 4000 g of water The molar mass of the solute is 620 gmol
180
1 AnalyzeGiven solute mass = 200 g solute molar mass = 620 gmol solvent mass and identity = 4000 g of water
Unknown boiling-point elevation
181
2 Plan
182
183
3 Compute
Practice Problem
A solution contains 500 g of sucrose C12H22O11 a nonelectrolyte dissolved in 5000 g of water What is the boiling-point elevation
015degC184
A solution contains 4500 g of sucrose C12H22O11 a nonelectrolyte dissolved in 250 g of waterWhat is the boiling point of the solution
1027degC185
If the boiling point elevation of an aqueous solution containing a nonvolatile electrolyte is 102degC what is the molality of the solution
20 m186
The boiling point of an aqueous solution containing a nonvolatile electrolyte is 10075degC aWhat is the boiling-point elevation bWhat is the molality of the solution a 075degC b 15 m
187
Electrolytes and Colligative PropertiesEarly investigators confused by experiments where certain substances depressed freezing point or elevated boiling point more than expected
188
189
Ex 01 m solution of NaCl lowers freezing point of solvent almost twice as much as 01 m solution of sucrose 01 m CaCl2 lowers fp almost 3 times as much as 01 m solution of sucrose
To understand why this happens contrast behavior or sucrose with NaCl in aqueous solution Each sucrose dissolves to make only 1 particle in solution So 1 mol sucrose dissolves to 1 mol particles 1 mol NaCl dissolves to make 2 moles of particles (1 mol Na+ and 1 mol Cl-)
190
191
Calculated Values for Electrolyte SolutionsColligative properties depend on total concentration of solute particles regardless of their identity Electrolytes cause changes in colligative properties proportional to total molality in terms of all dissolves particles instead of formula units
192
What about barium nitrate Ba(NO3)2 Each mole of barium nitrate yields 3 mol of ions in solution Ba(NO3)2(s) rarr Ba2+(aq) + 2NO3-(aq) You would expect solution of given molality to lower fp of solvent 3 times as much as nonelectrolytic solution of same molality
193
Sample Problem
What is the expected change in the freezing point of water in a solution of 625 g of barium nitrate Ba(NO3)2 in 100 kg of water
194
1 AnalyzeGiven solute mass and formula = 625 g Ba(NO3)2 solvent mass and identity = 100 kg water Δtf = Kfm
Unknown expected freezing-point depression
195
2 PlanThe molality can be calculated by converting the solute mass to moles and then dividing by the number of kilograms of solvent That molality is in terms of formula units of Ba(NO3)2 and must be converted to molality in terms of dissociated ions in solution It must be multiplied by the number of moles of ions produced per mole of formula unit This adjusted molality can then be used to calculate the freezing-point depression
196
197
198
3 Compute
Practice Problem
What is the expected freezing-point depression for a solution that contains 20 mol of magnesium sulfate dissolved in 10 kg of water
minus74degC199
What is the expected boiling-point elevation of water for a solution that contains 150 g of sodium chloride dissolved in 10 kg of water 27degC The freezing point of an aqueous sodium chloride solution is minus200degC What is the molality of the solution 54 m NaCl
200
Actual Values for Electrolyte SolutionsRemember values calculated are only expected values Actual values of colligative properties for all strong electrolytes are almost what would be expected Difference between expected and actual values caused by attractive forces that exist between dissociated ions in aqueous solution
201
Attraction between hydrated ions in solution is small compared with those in crystalline solid However forces of attraction donrsquot interfere with movements of aqueous ions More concentrated solution means ions closer together and attraction is greater Also depends on charge of ions ndash higher charge greater attraction
202
1 AnalyzeGiven solute mass = 200 g solute molar mass = 620 gmol solvent mass and identity = 4000 g of water
Unknown boiling-point elevation
181
2 Plan
182
183
3 Compute
Practice Problem
A solution contains 500 g of sucrose C12H22O11 a nonelectrolyte dissolved in 5000 g of water What is the boiling-point elevation
015degC184
A solution contains 4500 g of sucrose C12H22O11 a nonelectrolyte dissolved in 250 g of waterWhat is the boiling point of the solution
1027degC185
If the boiling point elevation of an aqueous solution containing a nonvolatile electrolyte is 102degC what is the molality of the solution
20 m186
The boiling point of an aqueous solution containing a nonvolatile electrolyte is 10075degC aWhat is the boiling-point elevation bWhat is the molality of the solution a 075degC b 15 m
187
Electrolytes and Colligative PropertiesEarly investigators confused by experiments where certain substances depressed freezing point or elevated boiling point more than expected
188
189
Ex 01 m solution of NaCl lowers freezing point of solvent almost twice as much as 01 m solution of sucrose 01 m CaCl2 lowers fp almost 3 times as much as 01 m solution of sucrose
To understand why this happens contrast behavior or sucrose with NaCl in aqueous solution Each sucrose dissolves to make only 1 particle in solution So 1 mol sucrose dissolves to 1 mol particles 1 mol NaCl dissolves to make 2 moles of particles (1 mol Na+ and 1 mol Cl-)
190
191
Calculated Values for Electrolyte SolutionsColligative properties depend on total concentration of solute particles regardless of their identity Electrolytes cause changes in colligative properties proportional to total molality in terms of all dissolves particles instead of formula units
192
What about barium nitrate Ba(NO3)2 Each mole of barium nitrate yields 3 mol of ions in solution Ba(NO3)2(s) rarr Ba2+(aq) + 2NO3-(aq) You would expect solution of given molality to lower fp of solvent 3 times as much as nonelectrolytic solution of same molality
193
Sample Problem
What is the expected change in the freezing point of water in a solution of 625 g of barium nitrate Ba(NO3)2 in 100 kg of water
194
1 AnalyzeGiven solute mass and formula = 625 g Ba(NO3)2 solvent mass and identity = 100 kg water Δtf = Kfm
Unknown expected freezing-point depression
195
2 PlanThe molality can be calculated by converting the solute mass to moles and then dividing by the number of kilograms of solvent That molality is in terms of formula units of Ba(NO3)2 and must be converted to molality in terms of dissociated ions in solution It must be multiplied by the number of moles of ions produced per mole of formula unit This adjusted molality can then be used to calculate the freezing-point depression
196
197
198
3 Compute
Practice Problem
What is the expected freezing-point depression for a solution that contains 20 mol of magnesium sulfate dissolved in 10 kg of water
minus74degC199
What is the expected boiling-point elevation of water for a solution that contains 150 g of sodium chloride dissolved in 10 kg of water 27degC The freezing point of an aqueous sodium chloride solution is minus200degC What is the molality of the solution 54 m NaCl
200
Actual Values for Electrolyte SolutionsRemember values calculated are only expected values Actual values of colligative properties for all strong electrolytes are almost what would be expected Difference between expected and actual values caused by attractive forces that exist between dissociated ions in aqueous solution
201
Attraction between hydrated ions in solution is small compared with those in crystalline solid However forces of attraction donrsquot interfere with movements of aqueous ions More concentrated solution means ions closer together and attraction is greater Also depends on charge of ions ndash higher charge greater attraction
202
2 Plan
182
183
3 Compute
Practice Problem
A solution contains 500 g of sucrose C12H22O11 a nonelectrolyte dissolved in 5000 g of water What is the boiling-point elevation
015degC184
A solution contains 4500 g of sucrose C12H22O11 a nonelectrolyte dissolved in 250 g of waterWhat is the boiling point of the solution
1027degC185
If the boiling point elevation of an aqueous solution containing a nonvolatile electrolyte is 102degC what is the molality of the solution
20 m186
The boiling point of an aqueous solution containing a nonvolatile electrolyte is 10075degC aWhat is the boiling-point elevation bWhat is the molality of the solution a 075degC b 15 m
187
Electrolytes and Colligative PropertiesEarly investigators confused by experiments where certain substances depressed freezing point or elevated boiling point more than expected
188
189
Ex 01 m solution of NaCl lowers freezing point of solvent almost twice as much as 01 m solution of sucrose 01 m CaCl2 lowers fp almost 3 times as much as 01 m solution of sucrose
To understand why this happens contrast behavior or sucrose with NaCl in aqueous solution Each sucrose dissolves to make only 1 particle in solution So 1 mol sucrose dissolves to 1 mol particles 1 mol NaCl dissolves to make 2 moles of particles (1 mol Na+ and 1 mol Cl-)
190
191
Calculated Values for Electrolyte SolutionsColligative properties depend on total concentration of solute particles regardless of their identity Electrolytes cause changes in colligative properties proportional to total molality in terms of all dissolves particles instead of formula units
192
What about barium nitrate Ba(NO3)2 Each mole of barium nitrate yields 3 mol of ions in solution Ba(NO3)2(s) rarr Ba2+(aq) + 2NO3-(aq) You would expect solution of given molality to lower fp of solvent 3 times as much as nonelectrolytic solution of same molality
193
Sample Problem
What is the expected change in the freezing point of water in a solution of 625 g of barium nitrate Ba(NO3)2 in 100 kg of water
194
1 AnalyzeGiven solute mass and formula = 625 g Ba(NO3)2 solvent mass and identity = 100 kg water Δtf = Kfm
Unknown expected freezing-point depression
195
2 PlanThe molality can be calculated by converting the solute mass to moles and then dividing by the number of kilograms of solvent That molality is in terms of formula units of Ba(NO3)2 and must be converted to molality in terms of dissociated ions in solution It must be multiplied by the number of moles of ions produced per mole of formula unit This adjusted molality can then be used to calculate the freezing-point depression
196
197
198
3 Compute
Practice Problem
What is the expected freezing-point depression for a solution that contains 20 mol of magnesium sulfate dissolved in 10 kg of water
minus74degC199
What is the expected boiling-point elevation of water for a solution that contains 150 g of sodium chloride dissolved in 10 kg of water 27degC The freezing point of an aqueous sodium chloride solution is minus200degC What is the molality of the solution 54 m NaCl
200
Actual Values for Electrolyte SolutionsRemember values calculated are only expected values Actual values of colligative properties for all strong electrolytes are almost what would be expected Difference between expected and actual values caused by attractive forces that exist between dissociated ions in aqueous solution
201
Attraction between hydrated ions in solution is small compared with those in crystalline solid However forces of attraction donrsquot interfere with movements of aqueous ions More concentrated solution means ions closer together and attraction is greater Also depends on charge of ions ndash higher charge greater attraction
202
183
3 Compute
Practice Problem
A solution contains 500 g of sucrose C12H22O11 a nonelectrolyte dissolved in 5000 g of water What is the boiling-point elevation
015degC184
A solution contains 4500 g of sucrose C12H22O11 a nonelectrolyte dissolved in 250 g of waterWhat is the boiling point of the solution
1027degC185
If the boiling point elevation of an aqueous solution containing a nonvolatile electrolyte is 102degC what is the molality of the solution
20 m186
The boiling point of an aqueous solution containing a nonvolatile electrolyte is 10075degC aWhat is the boiling-point elevation bWhat is the molality of the solution a 075degC b 15 m
187
Electrolytes and Colligative PropertiesEarly investigators confused by experiments where certain substances depressed freezing point or elevated boiling point more than expected
188
189
Ex 01 m solution of NaCl lowers freezing point of solvent almost twice as much as 01 m solution of sucrose 01 m CaCl2 lowers fp almost 3 times as much as 01 m solution of sucrose
To understand why this happens contrast behavior or sucrose with NaCl in aqueous solution Each sucrose dissolves to make only 1 particle in solution So 1 mol sucrose dissolves to 1 mol particles 1 mol NaCl dissolves to make 2 moles of particles (1 mol Na+ and 1 mol Cl-)
190
191
Calculated Values for Electrolyte SolutionsColligative properties depend on total concentration of solute particles regardless of their identity Electrolytes cause changes in colligative properties proportional to total molality in terms of all dissolves particles instead of formula units
192
What about barium nitrate Ba(NO3)2 Each mole of barium nitrate yields 3 mol of ions in solution Ba(NO3)2(s) rarr Ba2+(aq) + 2NO3-(aq) You would expect solution of given molality to lower fp of solvent 3 times as much as nonelectrolytic solution of same molality
193
Sample Problem
What is the expected change in the freezing point of water in a solution of 625 g of barium nitrate Ba(NO3)2 in 100 kg of water
194
1 AnalyzeGiven solute mass and formula = 625 g Ba(NO3)2 solvent mass and identity = 100 kg water Δtf = Kfm
Unknown expected freezing-point depression
195
2 PlanThe molality can be calculated by converting the solute mass to moles and then dividing by the number of kilograms of solvent That molality is in terms of formula units of Ba(NO3)2 and must be converted to molality in terms of dissociated ions in solution It must be multiplied by the number of moles of ions produced per mole of formula unit This adjusted molality can then be used to calculate the freezing-point depression
196
197
198
3 Compute
Practice Problem
What is the expected freezing-point depression for a solution that contains 20 mol of magnesium sulfate dissolved in 10 kg of water
minus74degC199
What is the expected boiling-point elevation of water for a solution that contains 150 g of sodium chloride dissolved in 10 kg of water 27degC The freezing point of an aqueous sodium chloride solution is minus200degC What is the molality of the solution 54 m NaCl
200
Actual Values for Electrolyte SolutionsRemember values calculated are only expected values Actual values of colligative properties for all strong electrolytes are almost what would be expected Difference between expected and actual values caused by attractive forces that exist between dissociated ions in aqueous solution
201
Attraction between hydrated ions in solution is small compared with those in crystalline solid However forces of attraction donrsquot interfere with movements of aqueous ions More concentrated solution means ions closer together and attraction is greater Also depends on charge of ions ndash higher charge greater attraction
202
Practice Problem
A solution contains 500 g of sucrose C12H22O11 a nonelectrolyte dissolved in 5000 g of water What is the boiling-point elevation
015degC184
A solution contains 4500 g of sucrose C12H22O11 a nonelectrolyte dissolved in 250 g of waterWhat is the boiling point of the solution
1027degC185
If the boiling point elevation of an aqueous solution containing a nonvolatile electrolyte is 102degC what is the molality of the solution
20 m186
The boiling point of an aqueous solution containing a nonvolatile electrolyte is 10075degC aWhat is the boiling-point elevation bWhat is the molality of the solution a 075degC b 15 m
187
Electrolytes and Colligative PropertiesEarly investigators confused by experiments where certain substances depressed freezing point or elevated boiling point more than expected
188
189
Ex 01 m solution of NaCl lowers freezing point of solvent almost twice as much as 01 m solution of sucrose 01 m CaCl2 lowers fp almost 3 times as much as 01 m solution of sucrose
To understand why this happens contrast behavior or sucrose with NaCl in aqueous solution Each sucrose dissolves to make only 1 particle in solution So 1 mol sucrose dissolves to 1 mol particles 1 mol NaCl dissolves to make 2 moles of particles (1 mol Na+ and 1 mol Cl-)
190
191
Calculated Values for Electrolyte SolutionsColligative properties depend on total concentration of solute particles regardless of their identity Electrolytes cause changes in colligative properties proportional to total molality in terms of all dissolves particles instead of formula units
192
What about barium nitrate Ba(NO3)2 Each mole of barium nitrate yields 3 mol of ions in solution Ba(NO3)2(s) rarr Ba2+(aq) + 2NO3-(aq) You would expect solution of given molality to lower fp of solvent 3 times as much as nonelectrolytic solution of same molality
193
Sample Problem
What is the expected change in the freezing point of water in a solution of 625 g of barium nitrate Ba(NO3)2 in 100 kg of water
194
1 AnalyzeGiven solute mass and formula = 625 g Ba(NO3)2 solvent mass and identity = 100 kg water Δtf = Kfm
Unknown expected freezing-point depression
195
2 PlanThe molality can be calculated by converting the solute mass to moles and then dividing by the number of kilograms of solvent That molality is in terms of formula units of Ba(NO3)2 and must be converted to molality in terms of dissociated ions in solution It must be multiplied by the number of moles of ions produced per mole of formula unit This adjusted molality can then be used to calculate the freezing-point depression
196
197
198
3 Compute
Practice Problem
What is the expected freezing-point depression for a solution that contains 20 mol of magnesium sulfate dissolved in 10 kg of water
minus74degC199
What is the expected boiling-point elevation of water for a solution that contains 150 g of sodium chloride dissolved in 10 kg of water 27degC The freezing point of an aqueous sodium chloride solution is minus200degC What is the molality of the solution 54 m NaCl
200
Actual Values for Electrolyte SolutionsRemember values calculated are only expected values Actual values of colligative properties for all strong electrolytes are almost what would be expected Difference between expected and actual values caused by attractive forces that exist between dissociated ions in aqueous solution
201
Attraction between hydrated ions in solution is small compared with those in crystalline solid However forces of attraction donrsquot interfere with movements of aqueous ions More concentrated solution means ions closer together and attraction is greater Also depends on charge of ions ndash higher charge greater attraction
202
A solution contains 4500 g of sucrose C12H22O11 a nonelectrolyte dissolved in 250 g of waterWhat is the boiling point of the solution
1027degC185
If the boiling point elevation of an aqueous solution containing a nonvolatile electrolyte is 102degC what is the molality of the solution
20 m186
The boiling point of an aqueous solution containing a nonvolatile electrolyte is 10075degC aWhat is the boiling-point elevation bWhat is the molality of the solution a 075degC b 15 m
187
Electrolytes and Colligative PropertiesEarly investigators confused by experiments where certain substances depressed freezing point or elevated boiling point more than expected
188
189
Ex 01 m solution of NaCl lowers freezing point of solvent almost twice as much as 01 m solution of sucrose 01 m CaCl2 lowers fp almost 3 times as much as 01 m solution of sucrose
To understand why this happens contrast behavior or sucrose with NaCl in aqueous solution Each sucrose dissolves to make only 1 particle in solution So 1 mol sucrose dissolves to 1 mol particles 1 mol NaCl dissolves to make 2 moles of particles (1 mol Na+ and 1 mol Cl-)
190
191
Calculated Values for Electrolyte SolutionsColligative properties depend on total concentration of solute particles regardless of their identity Electrolytes cause changes in colligative properties proportional to total molality in terms of all dissolves particles instead of formula units
192
What about barium nitrate Ba(NO3)2 Each mole of barium nitrate yields 3 mol of ions in solution Ba(NO3)2(s) rarr Ba2+(aq) + 2NO3-(aq) You would expect solution of given molality to lower fp of solvent 3 times as much as nonelectrolytic solution of same molality
193
Sample Problem
What is the expected change in the freezing point of water in a solution of 625 g of barium nitrate Ba(NO3)2 in 100 kg of water
194
1 AnalyzeGiven solute mass and formula = 625 g Ba(NO3)2 solvent mass and identity = 100 kg water Δtf = Kfm
Unknown expected freezing-point depression
195
2 PlanThe molality can be calculated by converting the solute mass to moles and then dividing by the number of kilograms of solvent That molality is in terms of formula units of Ba(NO3)2 and must be converted to molality in terms of dissociated ions in solution It must be multiplied by the number of moles of ions produced per mole of formula unit This adjusted molality can then be used to calculate the freezing-point depression
196
197
198
3 Compute
Practice Problem
What is the expected freezing-point depression for a solution that contains 20 mol of magnesium sulfate dissolved in 10 kg of water
minus74degC199
What is the expected boiling-point elevation of water for a solution that contains 150 g of sodium chloride dissolved in 10 kg of water 27degC The freezing point of an aqueous sodium chloride solution is minus200degC What is the molality of the solution 54 m NaCl
200
Actual Values for Electrolyte SolutionsRemember values calculated are only expected values Actual values of colligative properties for all strong electrolytes are almost what would be expected Difference between expected and actual values caused by attractive forces that exist between dissociated ions in aqueous solution
201
Attraction between hydrated ions in solution is small compared with those in crystalline solid However forces of attraction donrsquot interfere with movements of aqueous ions More concentrated solution means ions closer together and attraction is greater Also depends on charge of ions ndash higher charge greater attraction
202
If the boiling point elevation of an aqueous solution containing a nonvolatile electrolyte is 102degC what is the molality of the solution
20 m186
The boiling point of an aqueous solution containing a nonvolatile electrolyte is 10075degC aWhat is the boiling-point elevation bWhat is the molality of the solution a 075degC b 15 m
187
Electrolytes and Colligative PropertiesEarly investigators confused by experiments where certain substances depressed freezing point or elevated boiling point more than expected
188
189
Ex 01 m solution of NaCl lowers freezing point of solvent almost twice as much as 01 m solution of sucrose 01 m CaCl2 lowers fp almost 3 times as much as 01 m solution of sucrose
To understand why this happens contrast behavior or sucrose with NaCl in aqueous solution Each sucrose dissolves to make only 1 particle in solution So 1 mol sucrose dissolves to 1 mol particles 1 mol NaCl dissolves to make 2 moles of particles (1 mol Na+ and 1 mol Cl-)
190
191
Calculated Values for Electrolyte SolutionsColligative properties depend on total concentration of solute particles regardless of their identity Electrolytes cause changes in colligative properties proportional to total molality in terms of all dissolves particles instead of formula units
192
What about barium nitrate Ba(NO3)2 Each mole of barium nitrate yields 3 mol of ions in solution Ba(NO3)2(s) rarr Ba2+(aq) + 2NO3-(aq) You would expect solution of given molality to lower fp of solvent 3 times as much as nonelectrolytic solution of same molality
193
Sample Problem
What is the expected change in the freezing point of water in a solution of 625 g of barium nitrate Ba(NO3)2 in 100 kg of water
194
1 AnalyzeGiven solute mass and formula = 625 g Ba(NO3)2 solvent mass and identity = 100 kg water Δtf = Kfm
Unknown expected freezing-point depression
195
2 PlanThe molality can be calculated by converting the solute mass to moles and then dividing by the number of kilograms of solvent That molality is in terms of formula units of Ba(NO3)2 and must be converted to molality in terms of dissociated ions in solution It must be multiplied by the number of moles of ions produced per mole of formula unit This adjusted molality can then be used to calculate the freezing-point depression
196
197
198
3 Compute
Practice Problem
What is the expected freezing-point depression for a solution that contains 20 mol of magnesium sulfate dissolved in 10 kg of water
minus74degC199
What is the expected boiling-point elevation of water for a solution that contains 150 g of sodium chloride dissolved in 10 kg of water 27degC The freezing point of an aqueous sodium chloride solution is minus200degC What is the molality of the solution 54 m NaCl
200
Actual Values for Electrolyte SolutionsRemember values calculated are only expected values Actual values of colligative properties for all strong electrolytes are almost what would be expected Difference between expected and actual values caused by attractive forces that exist between dissociated ions in aqueous solution
201
Attraction between hydrated ions in solution is small compared with those in crystalline solid However forces of attraction donrsquot interfere with movements of aqueous ions More concentrated solution means ions closer together and attraction is greater Also depends on charge of ions ndash higher charge greater attraction
202
The boiling point of an aqueous solution containing a nonvolatile electrolyte is 10075degC aWhat is the boiling-point elevation bWhat is the molality of the solution a 075degC b 15 m
187
Electrolytes and Colligative PropertiesEarly investigators confused by experiments where certain substances depressed freezing point or elevated boiling point more than expected
188
189
Ex 01 m solution of NaCl lowers freezing point of solvent almost twice as much as 01 m solution of sucrose 01 m CaCl2 lowers fp almost 3 times as much as 01 m solution of sucrose
To understand why this happens contrast behavior or sucrose with NaCl in aqueous solution Each sucrose dissolves to make only 1 particle in solution So 1 mol sucrose dissolves to 1 mol particles 1 mol NaCl dissolves to make 2 moles of particles (1 mol Na+ and 1 mol Cl-)
190
191
Calculated Values for Electrolyte SolutionsColligative properties depend on total concentration of solute particles regardless of their identity Electrolytes cause changes in colligative properties proportional to total molality in terms of all dissolves particles instead of formula units
192
What about barium nitrate Ba(NO3)2 Each mole of barium nitrate yields 3 mol of ions in solution Ba(NO3)2(s) rarr Ba2+(aq) + 2NO3-(aq) You would expect solution of given molality to lower fp of solvent 3 times as much as nonelectrolytic solution of same molality
193
Sample Problem
What is the expected change in the freezing point of water in a solution of 625 g of barium nitrate Ba(NO3)2 in 100 kg of water
194
1 AnalyzeGiven solute mass and formula = 625 g Ba(NO3)2 solvent mass and identity = 100 kg water Δtf = Kfm
Unknown expected freezing-point depression
195
2 PlanThe molality can be calculated by converting the solute mass to moles and then dividing by the number of kilograms of solvent That molality is in terms of formula units of Ba(NO3)2 and must be converted to molality in terms of dissociated ions in solution It must be multiplied by the number of moles of ions produced per mole of formula unit This adjusted molality can then be used to calculate the freezing-point depression
196
197
198
3 Compute
Practice Problem
What is the expected freezing-point depression for a solution that contains 20 mol of magnesium sulfate dissolved in 10 kg of water
minus74degC199
What is the expected boiling-point elevation of water for a solution that contains 150 g of sodium chloride dissolved in 10 kg of water 27degC The freezing point of an aqueous sodium chloride solution is minus200degC What is the molality of the solution 54 m NaCl
200
Actual Values for Electrolyte SolutionsRemember values calculated are only expected values Actual values of colligative properties for all strong electrolytes are almost what would be expected Difference between expected and actual values caused by attractive forces that exist between dissociated ions in aqueous solution
201
Attraction between hydrated ions in solution is small compared with those in crystalline solid However forces of attraction donrsquot interfere with movements of aqueous ions More concentrated solution means ions closer together and attraction is greater Also depends on charge of ions ndash higher charge greater attraction
202
Electrolytes and Colligative PropertiesEarly investigators confused by experiments where certain substances depressed freezing point or elevated boiling point more than expected
188
189
Ex 01 m solution of NaCl lowers freezing point of solvent almost twice as much as 01 m solution of sucrose 01 m CaCl2 lowers fp almost 3 times as much as 01 m solution of sucrose
To understand why this happens contrast behavior or sucrose with NaCl in aqueous solution Each sucrose dissolves to make only 1 particle in solution So 1 mol sucrose dissolves to 1 mol particles 1 mol NaCl dissolves to make 2 moles of particles (1 mol Na+ and 1 mol Cl-)
190
191
Calculated Values for Electrolyte SolutionsColligative properties depend on total concentration of solute particles regardless of their identity Electrolytes cause changes in colligative properties proportional to total molality in terms of all dissolves particles instead of formula units
192
What about barium nitrate Ba(NO3)2 Each mole of barium nitrate yields 3 mol of ions in solution Ba(NO3)2(s) rarr Ba2+(aq) + 2NO3-(aq) You would expect solution of given molality to lower fp of solvent 3 times as much as nonelectrolytic solution of same molality
193
Sample Problem
What is the expected change in the freezing point of water in a solution of 625 g of barium nitrate Ba(NO3)2 in 100 kg of water
194
1 AnalyzeGiven solute mass and formula = 625 g Ba(NO3)2 solvent mass and identity = 100 kg water Δtf = Kfm
Unknown expected freezing-point depression
195
2 PlanThe molality can be calculated by converting the solute mass to moles and then dividing by the number of kilograms of solvent That molality is in terms of formula units of Ba(NO3)2 and must be converted to molality in terms of dissociated ions in solution It must be multiplied by the number of moles of ions produced per mole of formula unit This adjusted molality can then be used to calculate the freezing-point depression
196
197
198
3 Compute
Practice Problem
What is the expected freezing-point depression for a solution that contains 20 mol of magnesium sulfate dissolved in 10 kg of water
minus74degC199
What is the expected boiling-point elevation of water for a solution that contains 150 g of sodium chloride dissolved in 10 kg of water 27degC The freezing point of an aqueous sodium chloride solution is minus200degC What is the molality of the solution 54 m NaCl
200
Actual Values for Electrolyte SolutionsRemember values calculated are only expected values Actual values of colligative properties for all strong electrolytes are almost what would be expected Difference between expected and actual values caused by attractive forces that exist between dissociated ions in aqueous solution
201
Attraction between hydrated ions in solution is small compared with those in crystalline solid However forces of attraction donrsquot interfere with movements of aqueous ions More concentrated solution means ions closer together and attraction is greater Also depends on charge of ions ndash higher charge greater attraction
202
189
Ex 01 m solution of NaCl lowers freezing point of solvent almost twice as much as 01 m solution of sucrose 01 m CaCl2 lowers fp almost 3 times as much as 01 m solution of sucrose
To understand why this happens contrast behavior or sucrose with NaCl in aqueous solution Each sucrose dissolves to make only 1 particle in solution So 1 mol sucrose dissolves to 1 mol particles 1 mol NaCl dissolves to make 2 moles of particles (1 mol Na+ and 1 mol Cl-)
190
191
Calculated Values for Electrolyte SolutionsColligative properties depend on total concentration of solute particles regardless of their identity Electrolytes cause changes in colligative properties proportional to total molality in terms of all dissolves particles instead of formula units
192
What about barium nitrate Ba(NO3)2 Each mole of barium nitrate yields 3 mol of ions in solution Ba(NO3)2(s) rarr Ba2+(aq) + 2NO3-(aq) You would expect solution of given molality to lower fp of solvent 3 times as much as nonelectrolytic solution of same molality
193
Sample Problem
What is the expected change in the freezing point of water in a solution of 625 g of barium nitrate Ba(NO3)2 in 100 kg of water
194
1 AnalyzeGiven solute mass and formula = 625 g Ba(NO3)2 solvent mass and identity = 100 kg water Δtf = Kfm
Unknown expected freezing-point depression
195
2 PlanThe molality can be calculated by converting the solute mass to moles and then dividing by the number of kilograms of solvent That molality is in terms of formula units of Ba(NO3)2 and must be converted to molality in terms of dissociated ions in solution It must be multiplied by the number of moles of ions produced per mole of formula unit This adjusted molality can then be used to calculate the freezing-point depression
196
197
198
3 Compute
Practice Problem
What is the expected freezing-point depression for a solution that contains 20 mol of magnesium sulfate dissolved in 10 kg of water
minus74degC199
What is the expected boiling-point elevation of water for a solution that contains 150 g of sodium chloride dissolved in 10 kg of water 27degC The freezing point of an aqueous sodium chloride solution is minus200degC What is the molality of the solution 54 m NaCl
200
Actual Values for Electrolyte SolutionsRemember values calculated are only expected values Actual values of colligative properties for all strong electrolytes are almost what would be expected Difference between expected and actual values caused by attractive forces that exist between dissociated ions in aqueous solution
201
Attraction between hydrated ions in solution is small compared with those in crystalline solid However forces of attraction donrsquot interfere with movements of aqueous ions More concentrated solution means ions closer together and attraction is greater Also depends on charge of ions ndash higher charge greater attraction
202
To understand why this happens contrast behavior or sucrose with NaCl in aqueous solution Each sucrose dissolves to make only 1 particle in solution So 1 mol sucrose dissolves to 1 mol particles 1 mol NaCl dissolves to make 2 moles of particles (1 mol Na+ and 1 mol Cl-)
190
191
Calculated Values for Electrolyte SolutionsColligative properties depend on total concentration of solute particles regardless of their identity Electrolytes cause changes in colligative properties proportional to total molality in terms of all dissolves particles instead of formula units
192
What about barium nitrate Ba(NO3)2 Each mole of barium nitrate yields 3 mol of ions in solution Ba(NO3)2(s) rarr Ba2+(aq) + 2NO3-(aq) You would expect solution of given molality to lower fp of solvent 3 times as much as nonelectrolytic solution of same molality
193
Sample Problem
What is the expected change in the freezing point of water in a solution of 625 g of barium nitrate Ba(NO3)2 in 100 kg of water
194
1 AnalyzeGiven solute mass and formula = 625 g Ba(NO3)2 solvent mass and identity = 100 kg water Δtf = Kfm
Unknown expected freezing-point depression
195
2 PlanThe molality can be calculated by converting the solute mass to moles and then dividing by the number of kilograms of solvent That molality is in terms of formula units of Ba(NO3)2 and must be converted to molality in terms of dissociated ions in solution It must be multiplied by the number of moles of ions produced per mole of formula unit This adjusted molality can then be used to calculate the freezing-point depression
196
197
198
3 Compute
Practice Problem
What is the expected freezing-point depression for a solution that contains 20 mol of magnesium sulfate dissolved in 10 kg of water
minus74degC199
What is the expected boiling-point elevation of water for a solution that contains 150 g of sodium chloride dissolved in 10 kg of water 27degC The freezing point of an aqueous sodium chloride solution is minus200degC What is the molality of the solution 54 m NaCl
200
Actual Values for Electrolyte SolutionsRemember values calculated are only expected values Actual values of colligative properties for all strong electrolytes are almost what would be expected Difference between expected and actual values caused by attractive forces that exist between dissociated ions in aqueous solution
201
Attraction between hydrated ions in solution is small compared with those in crystalline solid However forces of attraction donrsquot interfere with movements of aqueous ions More concentrated solution means ions closer together and attraction is greater Also depends on charge of ions ndash higher charge greater attraction
202
191
Calculated Values for Electrolyte SolutionsColligative properties depend on total concentration of solute particles regardless of their identity Electrolytes cause changes in colligative properties proportional to total molality in terms of all dissolves particles instead of formula units
192
What about barium nitrate Ba(NO3)2 Each mole of barium nitrate yields 3 mol of ions in solution Ba(NO3)2(s) rarr Ba2+(aq) + 2NO3-(aq) You would expect solution of given molality to lower fp of solvent 3 times as much as nonelectrolytic solution of same molality
193
Sample Problem
What is the expected change in the freezing point of water in a solution of 625 g of barium nitrate Ba(NO3)2 in 100 kg of water
194
1 AnalyzeGiven solute mass and formula = 625 g Ba(NO3)2 solvent mass and identity = 100 kg water Δtf = Kfm
Unknown expected freezing-point depression
195
2 PlanThe molality can be calculated by converting the solute mass to moles and then dividing by the number of kilograms of solvent That molality is in terms of formula units of Ba(NO3)2 and must be converted to molality in terms of dissociated ions in solution It must be multiplied by the number of moles of ions produced per mole of formula unit This adjusted molality can then be used to calculate the freezing-point depression
196
197
198
3 Compute
Practice Problem
What is the expected freezing-point depression for a solution that contains 20 mol of magnesium sulfate dissolved in 10 kg of water
minus74degC199
What is the expected boiling-point elevation of water for a solution that contains 150 g of sodium chloride dissolved in 10 kg of water 27degC The freezing point of an aqueous sodium chloride solution is minus200degC What is the molality of the solution 54 m NaCl
200
Actual Values for Electrolyte SolutionsRemember values calculated are only expected values Actual values of colligative properties for all strong electrolytes are almost what would be expected Difference between expected and actual values caused by attractive forces that exist between dissociated ions in aqueous solution
201
Attraction between hydrated ions in solution is small compared with those in crystalline solid However forces of attraction donrsquot interfere with movements of aqueous ions More concentrated solution means ions closer together and attraction is greater Also depends on charge of ions ndash higher charge greater attraction
202
Calculated Values for Electrolyte SolutionsColligative properties depend on total concentration of solute particles regardless of their identity Electrolytes cause changes in colligative properties proportional to total molality in terms of all dissolves particles instead of formula units
192
What about barium nitrate Ba(NO3)2 Each mole of barium nitrate yields 3 mol of ions in solution Ba(NO3)2(s) rarr Ba2+(aq) + 2NO3-(aq) You would expect solution of given molality to lower fp of solvent 3 times as much as nonelectrolytic solution of same molality
193
Sample Problem
What is the expected change in the freezing point of water in a solution of 625 g of barium nitrate Ba(NO3)2 in 100 kg of water
194
1 AnalyzeGiven solute mass and formula = 625 g Ba(NO3)2 solvent mass and identity = 100 kg water Δtf = Kfm
Unknown expected freezing-point depression
195
2 PlanThe molality can be calculated by converting the solute mass to moles and then dividing by the number of kilograms of solvent That molality is in terms of formula units of Ba(NO3)2 and must be converted to molality in terms of dissociated ions in solution It must be multiplied by the number of moles of ions produced per mole of formula unit This adjusted molality can then be used to calculate the freezing-point depression
196
197
198
3 Compute
Practice Problem
What is the expected freezing-point depression for a solution that contains 20 mol of magnesium sulfate dissolved in 10 kg of water
minus74degC199
What is the expected boiling-point elevation of water for a solution that contains 150 g of sodium chloride dissolved in 10 kg of water 27degC The freezing point of an aqueous sodium chloride solution is minus200degC What is the molality of the solution 54 m NaCl
200
Actual Values for Electrolyte SolutionsRemember values calculated are only expected values Actual values of colligative properties for all strong electrolytes are almost what would be expected Difference between expected and actual values caused by attractive forces that exist between dissociated ions in aqueous solution
201
Attraction between hydrated ions in solution is small compared with those in crystalline solid However forces of attraction donrsquot interfere with movements of aqueous ions More concentrated solution means ions closer together and attraction is greater Also depends on charge of ions ndash higher charge greater attraction
202
What about barium nitrate Ba(NO3)2 Each mole of barium nitrate yields 3 mol of ions in solution Ba(NO3)2(s) rarr Ba2+(aq) + 2NO3-(aq) You would expect solution of given molality to lower fp of solvent 3 times as much as nonelectrolytic solution of same molality
193
Sample Problem
What is the expected change in the freezing point of water in a solution of 625 g of barium nitrate Ba(NO3)2 in 100 kg of water
194
1 AnalyzeGiven solute mass and formula = 625 g Ba(NO3)2 solvent mass and identity = 100 kg water Δtf = Kfm
Unknown expected freezing-point depression
195
2 PlanThe molality can be calculated by converting the solute mass to moles and then dividing by the number of kilograms of solvent That molality is in terms of formula units of Ba(NO3)2 and must be converted to molality in terms of dissociated ions in solution It must be multiplied by the number of moles of ions produced per mole of formula unit This adjusted molality can then be used to calculate the freezing-point depression
196
197
198
3 Compute
Practice Problem
What is the expected freezing-point depression for a solution that contains 20 mol of magnesium sulfate dissolved in 10 kg of water
minus74degC199
What is the expected boiling-point elevation of water for a solution that contains 150 g of sodium chloride dissolved in 10 kg of water 27degC The freezing point of an aqueous sodium chloride solution is minus200degC What is the molality of the solution 54 m NaCl
200
Actual Values for Electrolyte SolutionsRemember values calculated are only expected values Actual values of colligative properties for all strong electrolytes are almost what would be expected Difference between expected and actual values caused by attractive forces that exist between dissociated ions in aqueous solution
201
Attraction between hydrated ions in solution is small compared with those in crystalline solid However forces of attraction donrsquot interfere with movements of aqueous ions More concentrated solution means ions closer together and attraction is greater Also depends on charge of ions ndash higher charge greater attraction
202
Sample Problem
What is the expected change in the freezing point of water in a solution of 625 g of barium nitrate Ba(NO3)2 in 100 kg of water
194
1 AnalyzeGiven solute mass and formula = 625 g Ba(NO3)2 solvent mass and identity = 100 kg water Δtf = Kfm
Unknown expected freezing-point depression
195
2 PlanThe molality can be calculated by converting the solute mass to moles and then dividing by the number of kilograms of solvent That molality is in terms of formula units of Ba(NO3)2 and must be converted to molality in terms of dissociated ions in solution It must be multiplied by the number of moles of ions produced per mole of formula unit This adjusted molality can then be used to calculate the freezing-point depression
196
197
198
3 Compute
Practice Problem
What is the expected freezing-point depression for a solution that contains 20 mol of magnesium sulfate dissolved in 10 kg of water
minus74degC199
What is the expected boiling-point elevation of water for a solution that contains 150 g of sodium chloride dissolved in 10 kg of water 27degC The freezing point of an aqueous sodium chloride solution is minus200degC What is the molality of the solution 54 m NaCl
200
Actual Values for Electrolyte SolutionsRemember values calculated are only expected values Actual values of colligative properties for all strong electrolytes are almost what would be expected Difference between expected and actual values caused by attractive forces that exist between dissociated ions in aqueous solution
201
Attraction between hydrated ions in solution is small compared with those in crystalline solid However forces of attraction donrsquot interfere with movements of aqueous ions More concentrated solution means ions closer together and attraction is greater Also depends on charge of ions ndash higher charge greater attraction
202
1 AnalyzeGiven solute mass and formula = 625 g Ba(NO3)2 solvent mass and identity = 100 kg water Δtf = Kfm
Unknown expected freezing-point depression
195
2 PlanThe molality can be calculated by converting the solute mass to moles and then dividing by the number of kilograms of solvent That molality is in terms of formula units of Ba(NO3)2 and must be converted to molality in terms of dissociated ions in solution It must be multiplied by the number of moles of ions produced per mole of formula unit This adjusted molality can then be used to calculate the freezing-point depression
196
197
198
3 Compute
Practice Problem
What is the expected freezing-point depression for a solution that contains 20 mol of magnesium sulfate dissolved in 10 kg of water
minus74degC199
What is the expected boiling-point elevation of water for a solution that contains 150 g of sodium chloride dissolved in 10 kg of water 27degC The freezing point of an aqueous sodium chloride solution is minus200degC What is the molality of the solution 54 m NaCl
200
Actual Values for Electrolyte SolutionsRemember values calculated are only expected values Actual values of colligative properties for all strong electrolytes are almost what would be expected Difference between expected and actual values caused by attractive forces that exist between dissociated ions in aqueous solution
201
Attraction between hydrated ions in solution is small compared with those in crystalline solid However forces of attraction donrsquot interfere with movements of aqueous ions More concentrated solution means ions closer together and attraction is greater Also depends on charge of ions ndash higher charge greater attraction
202
2 PlanThe molality can be calculated by converting the solute mass to moles and then dividing by the number of kilograms of solvent That molality is in terms of formula units of Ba(NO3)2 and must be converted to molality in terms of dissociated ions in solution It must be multiplied by the number of moles of ions produced per mole of formula unit This adjusted molality can then be used to calculate the freezing-point depression
196
197
198
3 Compute
Practice Problem
What is the expected freezing-point depression for a solution that contains 20 mol of magnesium sulfate dissolved in 10 kg of water
minus74degC199
What is the expected boiling-point elevation of water for a solution that contains 150 g of sodium chloride dissolved in 10 kg of water 27degC The freezing point of an aqueous sodium chloride solution is minus200degC What is the molality of the solution 54 m NaCl
200
Actual Values for Electrolyte SolutionsRemember values calculated are only expected values Actual values of colligative properties for all strong electrolytes are almost what would be expected Difference between expected and actual values caused by attractive forces that exist between dissociated ions in aqueous solution
201
Attraction between hydrated ions in solution is small compared with those in crystalline solid However forces of attraction donrsquot interfere with movements of aqueous ions More concentrated solution means ions closer together and attraction is greater Also depends on charge of ions ndash higher charge greater attraction
202
197
198
3 Compute
Practice Problem
What is the expected freezing-point depression for a solution that contains 20 mol of magnesium sulfate dissolved in 10 kg of water
minus74degC199
What is the expected boiling-point elevation of water for a solution that contains 150 g of sodium chloride dissolved in 10 kg of water 27degC The freezing point of an aqueous sodium chloride solution is minus200degC What is the molality of the solution 54 m NaCl
200
Actual Values for Electrolyte SolutionsRemember values calculated are only expected values Actual values of colligative properties for all strong electrolytes are almost what would be expected Difference between expected and actual values caused by attractive forces that exist between dissociated ions in aqueous solution
201
Attraction between hydrated ions in solution is small compared with those in crystalline solid However forces of attraction donrsquot interfere with movements of aqueous ions More concentrated solution means ions closer together and attraction is greater Also depends on charge of ions ndash higher charge greater attraction
202
198
3 Compute
Practice Problem
What is the expected freezing-point depression for a solution that contains 20 mol of magnesium sulfate dissolved in 10 kg of water
minus74degC199
What is the expected boiling-point elevation of water for a solution that contains 150 g of sodium chloride dissolved in 10 kg of water 27degC The freezing point of an aqueous sodium chloride solution is minus200degC What is the molality of the solution 54 m NaCl
200
Actual Values for Electrolyte SolutionsRemember values calculated are only expected values Actual values of colligative properties for all strong electrolytes are almost what would be expected Difference between expected and actual values caused by attractive forces that exist between dissociated ions in aqueous solution
201
Attraction between hydrated ions in solution is small compared with those in crystalline solid However forces of attraction donrsquot interfere with movements of aqueous ions More concentrated solution means ions closer together and attraction is greater Also depends on charge of ions ndash higher charge greater attraction
202
Practice Problem
What is the expected freezing-point depression for a solution that contains 20 mol of magnesium sulfate dissolved in 10 kg of water
minus74degC199
What is the expected boiling-point elevation of water for a solution that contains 150 g of sodium chloride dissolved in 10 kg of water 27degC The freezing point of an aqueous sodium chloride solution is minus200degC What is the molality of the solution 54 m NaCl
200
Actual Values for Electrolyte SolutionsRemember values calculated are only expected values Actual values of colligative properties for all strong electrolytes are almost what would be expected Difference between expected and actual values caused by attractive forces that exist between dissociated ions in aqueous solution
201
Attraction between hydrated ions in solution is small compared with those in crystalline solid However forces of attraction donrsquot interfere with movements of aqueous ions More concentrated solution means ions closer together and attraction is greater Also depends on charge of ions ndash higher charge greater attraction
202
What is the expected boiling-point elevation of water for a solution that contains 150 g of sodium chloride dissolved in 10 kg of water 27degC The freezing point of an aqueous sodium chloride solution is minus200degC What is the molality of the solution 54 m NaCl
200
Actual Values for Electrolyte SolutionsRemember values calculated are only expected values Actual values of colligative properties for all strong electrolytes are almost what would be expected Difference between expected and actual values caused by attractive forces that exist between dissociated ions in aqueous solution
201
Attraction between hydrated ions in solution is small compared with those in crystalline solid However forces of attraction donrsquot interfere with movements of aqueous ions More concentrated solution means ions closer together and attraction is greater Also depends on charge of ions ndash higher charge greater attraction
202
Actual Values for Electrolyte SolutionsRemember values calculated are only expected values Actual values of colligative properties for all strong electrolytes are almost what would be expected Difference between expected and actual values caused by attractive forces that exist between dissociated ions in aqueous solution
201
Attraction between hydrated ions in solution is small compared with those in crystalline solid However forces of attraction donrsquot interfere with movements of aqueous ions More concentrated solution means ions closer together and attraction is greater Also depends on charge of ions ndash higher charge greater attraction
202
Attraction between hydrated ions in solution is small compared with those in crystalline solid However forces of attraction donrsquot interfere with movements of aqueous ions More concentrated solution means ions closer together and attraction is greater Also depends on charge of ions ndash higher charge greater attraction
202
