Upload
kristina-leonard
View
227
Download
0
Tags:
Embed Size (px)
Citation preview
Chapter 15Solutions
Like Dissolves Like
Definitions
Solution - Solution - homogeneous mixture
Solvent Solvent - present in greater amount
Solute Solute - substance being dissolved
Solvation SolvationSolvation
– the process of dissolving
solute particles are separated and pulled into solution
solute particles are surrounded by solvent particles
Solvation DissociationDissociation
– separation of an ionic solid into aqueous ions
– movie
NaCl(s) Na+(aq) + Cl–(aq)
Solvation
Molecular Molecular SolvationSolvation– molecules
stay intact
C6H12O6(s) C6H12O6(aq)
Solvation
IonizationIonization– breaking apart of
some polar molecules into aqueous ions
HNO3(aq) + H2O(l) H3O+(aq) + NO3–(aq)
Solvation
StrongElectrolyte
Non-Electrolyte
solute exists asions only
- +
salt
- +
sugar
solute exists asmolecules
only
- +
acetic acid
WeakElectrolyte
solute exists asions and
molecules
Making solutionsIn order to dissolve, the solvent
molecules must come in contact with the solute.
Stirring moves fresh solvent next to the solute.
The solvent touches the surface of the solute.
Smaller pieces increase the amount of surface area of the solute.
Solution FormationNature of the solute and the solvent
–Whether a substance will dissolve–How much will dissolve
Factors determining rate of solution...–stirred or shaken (agitation)–particles are made smaller– temperature is increased
Why?
Temperature and Solutions
Higher temperature makes the molecules of the solvent move around faster and contact the solute harder and more often.
– Speeds up dissolving.Usually increases the
amount that will dissolve (exception is gases)
How Much?Solubility- The maximum amount of
substance that will dissolve at a specific temperature (g solute/100 g solvent)
Saturated solution- Contains the maximum amount of solute dissolved. movie
Unsaturated solution- Can still dissolve more solute
Supersaturated- solution that is holding more than it theoretically can; seed crystal will make it come out. movie
Solubility
SATURATED SOLUTION
no more solute dissolves
UNSATURATED SOLUTIONmore solute dissolves
SUPERSATURATED SOLUTION
becomes unstable, crystals form
concentration
Cloud SeedingEver heard of seeding the clouds
to make them produce rain?Clouds- mass of air
supersaturated with water vaporSilver Iodide (AgI) crystals are
dusted into the cloudThe AgI attracts the water,
forming droplets to attract others
LiquidsMiscible means that two liquids can
dissolve in each other–water and antifreeze, water and ethanol
Partially miscible-
slightly dissolveImmiscible can’t mix
–oil and vinegar
Solvation
NONPOLAR
NONPOLAR
POLAR
POLAR
DetergentsDetergents– polar “head” with long nonpolar “tail”– can dissolve nonpolar grease in polar water
““Like Dissolves Like”Like Dissolves Like”““Like Dissolves Like”Like Dissolves Like”
Solubility?For solids in liquids, as the
temperature goes up-the solubility usually goes up.
For gases in a liquid, as the temperature goes up-the solubility goes down.
For gases in a liquid, as the pressure goes up-the solubility goes up.
Mentos and Diet CokeSolubility can also be affected by other
factors such as a reduction in water surface tension and the formation of nucleation sites. Nucleation sites are places where gas molecules gather to form a bubble. Mentos contains compounds that reduce surface tension and at the same time the rough surface of the dissolving tablet provides nucleation sites. The combination in rather dramatic as evidenced in the video clip.
Solubility SolubilitySolubility
– maximum grams of solute that will dissolve in 100 g of solvent at a given temperature
– varies with temp– based on a saturated solution
Solubility Solubility Solubility
CurveCurve– shows the
dependence of solubility on temperature
Solubility
Solids are more soluble at...Solids are more soluble at...– high temperatures.
Gases are more soluble at...Gases are more soluble at...– low temperatures.– high pressures (Henry’s Law).– EX: nitrogen narcosis, the “bends,” soda
The Bends
Shallow Water Blackout
DVD Movie
Gases in liquids...Henry’s Law - says the solubility of a
gas in a liquid is directly proportional to the pressure of the gas above the liquid
– think of a bottle of soda pop, removing the lid releases pres.
Equation:
S1 S2
P1 P2
=
Henry’s Law Example
Concentration is...a measure of the amount of solute
dissolved in a given quantity of solventA concentrated solution has a large
amount of soluteA dilute solution has a small amount of
solute– thus, only qualitative descriptions
But, there are ways to express solution concentration quantitatively
Concentration The amount of solute in a solution. Describing Concentration
– % by mass - medicated creams– % by volume - rubbing alcohol– ppm, ppb - water contaminants– molarity - used by chemists– molality - used by chemists
Molarity - Most ImportantThe number of moles of solute in 1
Liter of the solution. Note: not solvent.
M = moles/Liter; such as 6.0 molarWhat is the molarity of a solution with
2.0 moles of NaCl in 250 mL of solution?
Dilution
Adding water to a solution
2211 VMVM
DilutionPreparation of a desired solution by
adding water to a concentrate.Moles of solute remain the same.
DilutionThe number of moles of solute doesn’t
change if you add more solvent!The # moles before = the # moles afterSince M x L = moles thenM1 x V1 = M2 x V2
M1 and V1 are the starting concentration and volume.
M2 and V2 are the final concentration and volume.
Stock solutions are pre-made to a known Molarity
Dilution What volume of 15.8M HNO3 is required to
make 250 mL of a 6.0M solution?
GIVEN:
M1 = 15.8M
V1 = ?
M2 = 6.0M
V2 = 250 mL
WORK:
M1 V1 = M2 V2
(15.8M) V1 = (6.0M)(250mL)
V1 = 95 mL of 15.8M HNO3
Preparing Solutions
250 mL of 6.0M HNO3 by dilution
– measure 95 mL of 15.8M HNO3
95 mL of15.8M HNO3
water for
safety
250 mL mark
– combine with water until total volume is 250 mL
– Safety: “Do as you oughtta, add the acid to the watta!”
Making SolutionsPour in a small amount of solventThen add the solute (to dissolve it)Carefully fill to final volume.Also remember: M x L = moles of
soluteHow many moles of NaCl are
needed to make 8.0 L of a 0.75 M NaCl solution?
Practice2.0 L of a 0.88 M solution are diluted
to 3.8 L. What is the new molarity?You have 150 mL of 6.0 M HCl. What
volume of 1.3 M HCl can you make?You need 450 mL of 0.15 M NaOH.
All you have available is a 2.0 M stock solution of NaOH. How do you make the required solution?
Making Solutions10.3 g of NaCl are dissolved in a
small amount of water, then diluted to 250 mL. What is the concentration?
How many grams of sugar are needed to make 125 mL of a 0.50 M C6H12O6 solution?
Percent Solutions...Percent means parts per 100, soPercent by volume:
= Volume of solute x 100 Volume of solution
indicated %(v/v)What is the percent solution if 25 mL of CH3OH is diluted to 150 mL with water?
Percent SolutionsPercent by mass:
= Mass of solute(g) x 100 Mass of solution(g)
Indicated %(m/m)More commonly used4.8 g of NaCl are dissolved in 82 g of
solution. What is the percent of the solution?
Colligative Properties
Depend only on the number of dissolved particles
Not on what kind of particle
Colligative ApplicationsCommon Applications
– salting icy roads– making ice cream– antifreeze
• cars (-64°C to 136°C)• fish
Solute particles weaken IMF in the solvent.
Boiling Point Elevation
Boiling Point ElevationThe vapor pressure determines
the boiling point.Lower vapor pressure = higher
boiling point.Salt water boils above 100ºCThe number of dissolved
particles determines how much, as well as the solvent itself.
Depends on # of PiecesElectrolytes form ions when they
are dissolved = more pieces.NaCl Na+ + Cl- (= 2 pieces) More pieces = bigger effect
Vapor Pressure DecreasedThe bonds between molecules keep
molecules from escaping.In a solution, some of the solvent is
busy keeping the solute dissolved.Lowers the vapor pressure
Freezing Point DepressionSolids form when molecules make
an orderly pattern.The solute molecules break up the
orderly pattern. Makes the freezing point lower.Salt water freezes below 0ºCHow much depends on the number
of solute particles dissolved.
Freezing Point Depression
Properties of Water Ice is less dense than water
movie
Molality
solvent ofkg
solute of moles(m)molality
mass of solvent only
1 kg water = 1 L waterkg 1
mol0.25 0.25m
Molalitya new unit for concentrationm = Moles of solute
kilogram of solventm = Moles of solute
1000 g of solvent What is the molality of a solution
with 9.3 mole of NaCl in 450 g of water?
MolalityFind the molality of a solution containing 75 g
of MgCl2 in 250 mL of water.
75 g MgCl2 1 mol MgCl2
95.21 g MgCl2
= 3.2m MgCl2
0.250 kg water
kg
molm
MolalityHow many grams of NaCl are req’d to make a
1.54m solution using 0.500 kg of water?
0.500 kg water 1.54 mol NaCl
1 kg water
= 45.0 g NaCl
58.44 g NaCl
1 mol NaCl
kg 1
mol1.5 1.5m
Preparing Solutions 500 mL of 1.54M NaCl
500 mLwater
45.0 gNaCl
– mass 45.0 g of NaCl– add water until total
volume is 500 mL– mass 45.0 g of NaCl– add 0.500 kg of water
500 mLmark
500 mLvolumetric
flask
1.54m NaCl in 0.500 kg of water
Types of Property Change Boiling Point ElevationBoiling Point Elevation (tb)
– b.p. of a solution is higher than b.p. of the pure solvent
Freezing Point DepressionFreezing Point Depression (tf)
– f.p. of a solution is lower than f.p. of the pure solvent
Boiling Point Change?The size of the change in boiling
point is determined by the molality.Tb = Kb x m x nTb is the change in the boiling pointKb is a constant determined by the
solvent.m is the molality of the solution.n is the number of pieces it falls into
when it dissolves.
What About Freezing?The size of the change in freezing
point is also determined by molality.Tf = Kf x m x nTf is the change in freezing pointKf is a constant determined by the
solvent.m is the molality of the solution.n is the number of pieces it falls into
when it dissolves.
Calculations n = the # of Particlesn = the # of Particles
– Nonelectrolytes (covalent)• remain intact when dissolved • 1 particle n=1
– Electrolytes (ionic)• dissociate into ions when dissolved• 2 or more particles n=2 or more
Boiling and Freezing Points and Constants
Boiling–Point Elevation and Freezing–Point Depression
Calculations
t: change in temperature (°C)
k: constant based on the solvent (°C·kg/mol)
m: molality (m)
n: # of particles
t = k · m · n
Calculations At what temperature will a solution that is composed
of 0.73 moles of glucose in 225 g of phenol boil?
m = 3.2mn = 1
WORK:
m = 0.73mol ÷ 0.225kg
GIVEN:b.p. = ? tb = ?
tb = kb · m · n
kb = 3.60°C·kg/mol
tb = (3.60°C·kg/mol)(3.2m)(1)
tb = 12°C
b.p. = 181.8°C + 12°C
b.p. = 194°C
Calculations Find the freezing point of a saturated solution of
NaCl containing 28 g NaCl in 100. mL water.
m = 4.8m
n = 2
WORK:
m = 0.48mol ÷ 0.100kg
GIVEN:
f.p. = ?
tf = ?
tf = kf · m · n
kf = 1.86°C·kg/mol
tf = (1.86°C·kg/mol)(4.8m)(2)
tf = 18°C
f.p. = 0.00°C - 18°C
f.p. = -18°C
ProblemsWhat is the boiling point of a
solution made by dissolving 1.20 moles of NaCl in 7.50e2 g of water?
What is the freezing point?What is the boiling point of a
solution made by dissolving 1.20 moles of CaCl2 in 750. g of water?
What is the freezing point?
Mole FractionThis is another way to express
concentrationIt is the ratio of moles of solute to
total number of moles of solute + solvent
nsolute
nsolute + nsolvent
X =
Mole Fraction ProblemWhat is the mole fraction of solute in
a 35.5 percent by mass aqueous solution of formic acid (HCOOH)?
Molar MassWe can use changes in boiling
and freezing to calculate the molar mass of a substance
Find: 1) molality 2) moles, and then 3) molar mass
MM Example ProblemAn aqueous solution made using
500. mL of solvent has a freezing point of -5.00°C. If 245 g of covalent solid was used, what is the molar mass of the solute?
If the empirical formula of the solute above is CH2O, what is the molecular formula?