Chapter 13Chapter 13

13.1 Ionic Compounds in 13.1 Ionic Compounds in Aqueous SolutionAqueous Solution

Aqueous solution = A solution in which water Aqueous solution = A solution in which water is the solvent (aq).is the solvent (aq). Example: A solution of water (the solvent) and NaCl Example: A solution of water (the solvent) and NaCl

(the solute)(the solute) Aqueous solutions can be electrolytes or non Aqueous solutions can be electrolytes or non

electrolytes.electrolytes. Electrolytes and non-electrolytes are solutes of ionic Electrolytes and non-electrolytes are solutes of ionic

salt solutions.salt solutions. Do electrolytes conduct electricity? YesDo electrolytes conduct electricity? Yes Are non-electrolytes conductors? NoAre non-electrolytes conductors? No Are all electrolytes conductors? YesAre all electrolytes conductors? Yes Are all conductors electrolytes? NoAre all conductors electrolytes? No

Conductors vs. NonconductorsConductors vs. NonconductorsPure Substance Mixtures

Elements Compound Alloys Electrolytic Sol’n

All metals:Cu, Ag, Fe

All ionicCompound

in liquidstate:

NaBr (I),KNO3 (I)

Stainlesssteel,

Sterlingsilver

Water solutions of NaBr, HCl, NH3

(aqueous solution)

Pure Substance Mixtures

Elements Compounds Non-electrolytic Sol’n

Allnonmetals:

I2, P4

All covalentcompounds in liquid state:

HBr (I), Al2Cl6 (I),

All solid compounds:Sucrose, NaBr (s), AlBr (s)

Water solutions of sucrose, isopropylalcohol, ethyl alcohol, glycerin

(aqueous solutions)

Theory of Ionization Theory of Ionization

Theory of Ionization - Some water solutions Theory of Ionization - Some water solutions conduct electricity. These solutions are conduct electricity. These solutions are called called electrolytes.electrolytes. Strong electrolytes:Strong electrolytes:

• NaCl(s) + HNaCl(s) + H22O(l) O(l) Na Na+1+1(aq) + Cl(aq) + Cl-1-1(aq)(aq)

• HCl(g) + HHCl(g) + H22O(l) O(l) H H+1+1(aq) + Cl(aq) + Cl-1-1(aq)(aq)

Weak electrolyte:Weak electrolyte:• HCHC22HH33OO22(l) + H(l) + H22O(l) O(l) H H+1+1(aq) + C(aq) + C22HH33OO22

-1-1(aq)(aq)

Theory of IonizationTheory of Ionization

In 1887, Svante Arrhenius (Sweden) proposed In 1887, Svante Arrhenius (Sweden) proposed the theory of the theory of ionizationionization..

Some substances break into smaller substances Some substances break into smaller substances with charges. He based his ideas on with charges. He based his ideas on observations of changes in observations of changes in freezing and boiling points with different molal concentrationspoints with different molal concentrations NoteNote: Some reactions get a single arrow (: Some reactions get a single arrow () and ) and

some get a double arrow (some get a double arrow () - equilibrium. ) - equilibrium. Arrhenius proposed that when some chemicals are Arrhenius proposed that when some chemicals are

dissolved in water, they produce particles with dissolved in water, they produce particles with charges.charges.

IonizationIonization

ionic compoundsionic compounds dissociatedissociate NaClNaCl(s)(s) + H + H22O O -----> -----> NaNa++ (aq)(aq) + Cl+ Cl-- (aq)(aq)

MgClMgCl2 (s)2 (s) + H + H22O O ----->-----> Mg Mg+2+2(aq)(aq)+ 2C l+ 2C l-- (aq)(aq)

acidsacids ionizeionize (covalent dissociation) (covalent dissociation) HClHCl(g)(g) + H + H22O O -----> H-----> H++

(aq)(aq) + Cl + Cl--(aq)(aq)

HH22SOSO4(g)4(g) + H + H22O O -----> 2H-----> 2H++(aq)(aq) + SO + SO44

2-2-(aq)(aq)

Substances that are not ionic, electrolytes, or acids do not dissociate/ionize.

Dissolving Ionic Compounds Dissolving Ionic Compounds

Hydration: Water molecules surround each Hydration: Water molecules surround each ion in solution; the entire ions from the ion in solution; the entire ions from the crystal dissolves and hydrated ions crystal dissolves and hydrated ions become uniformly dissolved.become uniformly dissolved.

Heat of Hydration Heat of Hydration energy released when ions are surrounded by water molecules.

The # of water molecules used depends on the size and charge of the ion.

↑ Heat released (more negative) as the size of the ion ↓

Li+1 -523 kJ/mole vs Na+1 -418 kJ/mole ↑ Heat released (more negative) as the charge of the

ion ↑ Na+1 -418 kJ/mole vs Mg+2 -1949 kJ/mole

Li and Mg are close to the same size, so charge is more important

Heat of HydrationHeat of Hydration

Three interactions contribute to the heats of Three interactions contribute to the heats of solution (by forming a solution)solution (by forming a solution) Step 1Step 1 dissociationdissociation - separation of ions (they already - separation of ions (they already

exist, Hexist, H22O separates them) — O separates them) — solute-solutesolute-solute

connections break apart = energy absorbedconnections break apart = energy absorbed Step 2 Step 2 solventsolvent — — solvent-solventsolvent-solvent connections connections

break apart = energy absorbedbreak apart = energy absorbed Step 3Step 3 hydrationhydration – – Solute-solventSolute-solvent - particles are - particles are

surrounded by water = energy releasedsurrounded by water = energy released

Heat of HydrationHeat of Hydration

Endothermic Energy Level DiagramEndothermic Energy Level Diagram

E

Time

Solute

Hydrate

Solution

Heat of HydrationHeat of Hydration

Exothermic Energy Level DiagramExothermic Energy Level Diagram

E

23

1

Time

Solute

Hydrate

Solution

If step #1 + step #2 are less than step #3, than the overall reaction is exothermic

Dissociation Dissociation

The separation of ions that occurs when The separation of ions that occurs when an ionic compound dissolves.an ionic compound dissolves. Ex. A 1.0 M solution of sodium chloride Ex. A 1.0 M solution of sodium chloride

contains:contains:• 1 mol of Na+ ions and 1 mol of Cl- ions.1 mol of Na+ ions and 1 mol of Cl- ions.

• NaCl(s) ---------> NaNaCl(s) ---------> Na++(aq)(aq) + Cl + Cl--(aq)(aq)

1 mol 1 mol 1 mol1 mol 1 mol 1 mol• Total of 2 moles of ionsTotal of 2 moles of ions

DissociationDissociation

A 1.0 M solution of calcium chloride A 1.0 M solution of calcium chloride contains:contains:

CaClCaCl22(s) ----> Ca(s) ----> Ca2+2+(aq)(aq) + 2Cl + 2Cl--(aq)(aq)

1 mol of Ca1 mol of Ca22+ ions and 2 mol of Cl- ions + ions and 2 mol of Cl- ions a total of 3 mole of ions.a total of 3 mole of ions.

DissociationDissociation

(a) Dissolve Al(a) Dissolve Al22(SO(SO44))33 in water. (b) How many in water. (b) How many

moles of aluminum ions and sulfate ions are moles of aluminum ions and sulfate ions are produced by dissolving 1 mol of Alproduced by dissolving 1 mol of Al22(SO(SO44))33. (c) . (c)

What is the total number of moles of ions What is the total number of moles of ions produced by dissolving 1 mol of Alproduced by dissolving 1 mol of Al22(SO(SO44))33??

(a) Al(a) Al22(SO(SO44))33 -----> 2Al -----> 2Al3+3+(aq) + 3SO(aq) + 3SO44-2-2(aq).(aq).

(b) 1 mol ----> 2 mol + 3 mol.(b) 1 mol ----> 2 mol + 3 mol. (c) 2 mol Al(c) 2 mol Al33+ + 3mol SO+ + 3mol SO44

2- 2- = 5 mol of = 5 mol of

solute ionssolute ions

Solubility Equilibria Solubility Equilibria

No ionic compound has No ionic compound has infiniteinfinite solubility. solubility. No ionic compound has No ionic compound has zerozero solubililty. solubililty. Rough rules of solubililty (using the Rough rules of solubililty (using the

solubility tables):solubility tables): If more than 1 gram per 100g HIf more than 1 gram per 100g H220 before 0 before

saturation = saturation = solublesoluble If .1 gram to 1 gram per 100g HIf .1 gram to 1 gram per 100g H220 = 0 = slightlyslightly

solublesoluble If less than .1 gram per 100g HIf less than .1 gram per 100g H220 = 0 = insolubleinsoluble

Solubility EquilibriaSolubility Equilibria

Very slightly soluble ionic compoundsVery slightly soluble ionic compounds – when placed in water, an equilibrium is – when placed in water, an equilibrium is established between the solid compound established between the solid compound and its ions in solution:and its ions in solution:

Example: Example: AgClAgCl(s)(s) Ag Ag++

(aq)(aq) + + ClCl--(aq)(aq)

Fe(OH)Fe(OH)3(s)3(s) Fe Fe3+3+(aq)(aq) + 3OH + 3OH--

(aq)(aq)

AgAg22SS(s)(s) 2Ag 2Ag++(aq (aq ++ SS2-2-

(aq)(aq)

Solubility EquilibriaSolubility Equilibria

Precipitation ReactionsPrecipitation Reactions = Soluble = Soluble compounds form compounds form insolubleinsoluble products. products.

Type of rxn: Type of rxn: double replacementdouble replacement - remember - remember – reactants are soluble in water– reactants are soluble in water

Ex1: Silver nitrate + magnesium chlorideEx1: Silver nitrate + magnesium chloride KCl(aq) +KCl(aq) + AgNOAgNO33(aq)(aq) ---> KNO---> KNO33(aq)(aq) + AgCl(s)+ AgCl(s)

Solubility EquilibriaSolubility Equilibria Net ionic equations Net ionic equations – double replacement – double replacement

reactions and other reactions of ions in reactions and other reactions of ions in aqueous solutions are represented as ‘net aqueous solutions are represented as ‘net ionic equations.’ionic equations.’

Steps:Steps: 1. write an equation & show soluble compounds as 1. write an equation & show soluble compounds as

dissociated ionsdissociated ions 2. write a net ionic equation – only those 2. write a net ionic equation – only those

compounds and ions that undergo a chemical compounds and ions that undergo a chemical change in a reaction in an aqueous sol’n and does change in a reaction in an aqueous sol’n and does not include spectator ions (ions found on the not include spectator ions (ions found on the reactants and products side).reactants and products side).

Net ionic equationsNet ionic equations

Ex1:Ex1: MolecularMolecular

• KCl(aq) + AgNOKCl(aq) + AgNO33(aq) ---> KNO(aq) ---> KNO33(aq) + AgCl(s)(aq) + AgCl(s)

Total IonicTotal Ionic

net ionic equationnet ionic equation

Spectator IoncsSpectator Ioncs

Net Ionic EquationsNet Ionic Equations

Calcium chloride + aluminum carbonateCalcium chloride + aluminum carbonate MolecularMolecular

Total IonicTotal Ionic

Net IonicNet Ionic

Spectator IonsSpectator Ions

13.2 Molecular Electrolytes 13.2 Molecular Electrolytes

Molecular solutes can form electrolytic Molecular solutes can form electrolytic solutions if they are highly solutions if they are highly polarpolar..

Ionization versus dissociationIonization versus dissociation Dissociation = The separation of ions that Dissociation = The separation of ions that

occurs occurs when the ionic compound dissolveswhen the ionic compound dissolves Ionization = The formation of ions that occurs Ionization = The formation of ions that occurs

when a when a polar covalent compoundpolar covalent compound dissolves in dissolves in water (water rips apart molecules and turns water (water rips apart molecules and turns them into ionsthem into ions

Molecular ElectrolytesMolecular Electrolytes

Ionization example = HCl in water Ionization example = HCl in water HH22O + HCl -----> HO + HCl -----> H33OO++ + Cl + Cl--

When a hydrogen chloride molecule ionizes in When a hydrogen chloride molecule ionizes in water, its hydrogen ion bonds covalently to a water, its hydrogen ion bonds covalently to a water molecule. A water molecule. A hydroniumhydronium ion and a ion and a chloridechloride ion are formed ion are formed

Hydronium Ion: HHydronium Ion: H33OO++

The HThe H++ ion attracts other molecules or ions ion attracts other molecules or ions so strongly that it does not normally exist, so strongly that it does not normally exist, so the Hso the H++ ion becomes covalently bonded ion becomes covalently bonded to oxygen.to oxygen.

Substances which form Substances which form electrolytic solutions electrolytic solutions

1. Acids1. Acids HX HX ex: HCl, HNO ex: HCl, HNO33

polarpolar 2. Bases2. Bases MOH ex: NaOH MOH ex: NaOH

ionicionic 3. Salts MX3. Salts MX ex: NaCl, KBr, CaCO ex: NaCl, KBr, CaCO33 ionicionic

H = HydrogenM = MetalOH = HydroxideX = NM or P-ion

•Why? The solute pulls ions into solutions

Which of the following form Which of the following form electrolytic solutions?electrolytic solutions?

MgBrMgBr22

CC88HH1818

KOH KOH CC1212HH2222OO1111

HNOHNO33

Strong vs. weak electrolytesStrong vs. weak electrolytes

Some compounds ionize / dissociate Some compounds ionize / dissociate completely, while others don’t. completely, while others don’t.

Strong electrolyteStrong electrolyte – a compound that – a compound that when dissolved/ionized, yields 100% ions. when dissolved/ionized, yields 100% ions. Distinguishing factor of strong electrolytes – to Distinguishing factor of strong electrolytes – to

whatever extent they dissolve in water, they whatever extent they dissolve in water, they yield only ions: HCl, HBr and HI are 100% yield only ions: HCl, HBr and HI are 100% ionized in dilute aqueous solutions. ionized in dilute aqueous solutions.

Strong vs. weak electrolytesStrong vs. weak electrolytes

Weak electrolyteWeak electrolyte – a solute that yields a – a solute that yields a relatively low concentration of ions in an relatively low concentration of ions in an aqueous solution.aqueous solution. HF(aq) + HHF(aq) + H22O(l) O(l) H H33O+ (aq) + F-(aq) O+ (aq) + F-(aq)

In an aqueous solution, the majority of HF In an aqueous solution, the majority of HF molecules are present as dissolved HF molecules are present as dissolved HF molecules.molecules.

Strong vs. weak electrolytesStrong vs. weak electrolytes

In general, the extent to which a solute In general, the extent to which a solute ionizes in solution depends on the bonds ionizes in solution depends on the bonds within the molecules of the solute and the within the molecules of the solute and the strength of attraction to solvent molecules.strength of attraction to solvent molecules.

Note: If the strength of bonds in solute Note: If the strength of bonds in solute molecules < the attractive forces of the molecules < the attractive forces of the water dipoles, then the covalent bonds water dipoles, then the covalent bonds break and the molecule separates into break and the molecule separates into ions.ions.

13.3 Properties of Electrolyte 13.3 Properties of Electrolyte SolutionsSolutions

Conductivity of SolutionsConductivity of Solutions To compare the conductivities of strong and weak To compare the conductivities of strong and weak

electrolytes, the conductivities of solutions of equal electrolytes, the conductivities of solutions of equal concentration must be compared.concentration must be compared.

Ionization of pure waterIonization of pure water

HH22O(l) + HO(l) + H22O(l) O(l) H H33OO++ (aq) + OH- (aq) + OH-

(aq) (aq) So why does water that comes out of the tap conduct So why does water that comes out of the tap conduct

electricity?electricity?• It contains a high enough concentration of dissolved ions to It contains a high enough concentration of dissolved ions to

make it a better conductor than pure water.make it a better conductor than pure water.

Colligative Properties of Electrolytic Solutions

Properties that depend on the concentration of the solute particles. Freezing point and boiling point are colligative properties. Freezing point depression – the difference

between the freezing points of a pure solvent and a nonelectrolyte solution in it.

• Solutions that conduct electricity contain electrolytes.

Colligative Properties of Electrolytic Solutions

Ionic compounds dissociate: NaCl(s) + H2O(l) yields Na+

(aq) + Cl-(aq)

MgCl2(s) + H2O(l) yields Mg+2(aq) + 2Cl-(aq)

Acids ionize (dissociation of a covalent compound): HCl(g) + H2O(l) yields H+

(aq) + Cl-(aq)

H2SO4(l) + H2O(l) yields 2H+(aq) + SO4

-2(aq)

Substances that are not acids, bases, and salts do not dissociate/ionize.

When solutes dissolve in liquids, they lower the freezing point.

Freezing Point Depression

Two factors affect the degree of change in the temperature: the amount of the solute and the nature of the solvent.

∆tf = kf (m)(x) x = # of ions produced when the solute dissolves

kf = constant

(kf water = -1.86 oC/m

m = molality (moles solute)

Kg solvent

Why does freezing point depression occur?

O Na+ O

H H ……… O …. . H H

 

H H

Cl-

• The solute (NaCl) interferes with crystal formation. (ex: antifreeze) • As the number of solute particles increase, the freezing point

decreases.

Freezing Point DepressionFreezing Point Depression Ex1: Calculate the freezing point of 10.00 grams of NaCl in

200.0 grams of water.

∆tf = kf (m)(x)

Grams moles

10.00 g NaCl x 1 mole NaCl = .1709 moles NaCl

58.5 g NaCl

Molality

m = .1709 moles = m = .8547 m

.2000 kg

Change in temperature

∆tf = (-1.86 oC/m)(.8547 m)(2) = -3.179 oC

New Freezing point

∆tf = tf - ti -3.179 oC = x – 0 oC x = -3.179 oC

Boiling Point ElevationBoiling Point Elevation

Boiling point elevation – when solutes dissolve in liquids, they raise the boiling points.

Same concept as freezing point depression except boiling point increases.

kb water = 0.512 oC/m

Why does boiling point elevation occur? The solute takes up space on the surface of a liquid. This

decreases the ability of the liquid to evaporate. Thus, the vapor pressure decreases. Boiling occurs when the atmospheric pressure equals the vapor pressure. So, an increase in energy is needed to increase the vapor pressure to reach the atmospheric pressure.

Boiling Point ElevationBoiling Point Elevation

= solvent

= solute

“A” “B”

Which would produce more vapor? A Which would have a higher vapor pressure? A Which would take less energy to raise the vapor pressure to

atmospheric pressure? A Which would have a higher boiling point? B

Boiling Point ElevationBoiling Point Elevation Ex1: Calculate the boiling point of a solution of 10.00

grams of NaCl in 200.0 grams of water.

∆tb = kb(m)(x)

Grams moles

10.00 g NaCl x 1 mole NaCl = .1709 moles NaCl

58.5 g NaCl

Molality

m = .1709 moles = .8547 m

.2000 kg

Change in temperature 

∆tb = (.512 oC/m)(.8547 m)(2) = .8752 oC

New Temperature

∆tb = tf - ti .8752 oC = x – 100 oC = 100.8752 oC

Ion Pairing

When experiments are done regarding freezing point depression and boiling point elevation, the actual answers are different than the theoretical answers Example: a solution of NaCl in water:Sodium

Chloride can dissociate at a rate of 100% if the concentration of the solution is very low. With increased concentration, ions may come in contact with each other and rejoin resulting in less than 100% dissociation. Only at low, low concentrations do solutions have their “x” factor approach the theoretical value.

Concentration (molality)

Actual change in the freezing

point

Theoretical change in the freezing point

% dissociation

.1 - 0.346 - 0.372 93 %

.01 - 0.0361 - 0.0372 97 %

.001 - 0.00366 - 0.00372 98 %

.0001 - 0.000372 - 0.000372 100 %