Chapter 13 Chapter 13 Ions in Aqueous Solutions and Ions in Aqueous Solutions and

Colligative Properties Colligative Properties

13-1 Compounds in Aqueous 13-1 Compounds in Aqueous SolutionsSolutions

DissociationDissociation

• One formula unit of NaCl produces two ions:One formula unit of NaCl produces two ions:

NaCl (s) → NaNaCl (s) → Na++ (aq) + Cl (aq) + Cl--(aq)(aq)• One mole of NaCl produces two moles of ions One mole of NaCl produces two moles of ions

• One formula unit of CaClOne formula unit of CaCl22 produces three ions: produces three ions:

CaClCaCl22 (s) → Ca (s) → Ca+2+2 (aq) + 2Cl (aq) + 2Cl-- (aq) (aq)

• One mole of CaClOne mole of CaCl22 produces three moles of ions produces three moles of ions

The separation of ions that occurs when The separation of ions that occurs when an ionic compound dissolvesan ionic compound dissolves

Dissociation EquationsDissociation Equations

NaCl(s) NaCl(s)

AgNOAgNO33(s) (s)

MgClMgCl22(s) (s)

NaNa22SOSO44(s) (s)

AlClAlCl33(s) (s)

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

AgAg++(aq) + NO(aq) + NO33--(aq)(aq)

MgMg2+2+(aq) + 2 Cl(aq) + 2 Cl--(aq)(aq)

2 Na2 Na++(aq) + SO(aq) + SO442-2-(aq)(aq)

AlAl3+3+(aq) + 3 Cl(aq) + 3 Cl--(aq)(aq)

Precipitation ReactionsPrecipitation Reactions

Solubility RulesSolubility Rules • No compound is completely insoluble No compound is completely insoluble • Compounds of very low solubility can be Compounds of very low solubility can be

considered insoluble considered insoluble • Dissociation equations cannot be written Dissociation equations cannot be written

for insoluble compounds for insoluble compounds

More Solubility GuidelinesMore Solubility Guidelines

• Oxalates are generally insoluble except for Oxalates are generally insoluble except for those of alkali metals. (alkaline earth those of alkali metals. (alkaline earth oxalates are insoluble.)oxalates are insoluble.)

• Chromates of silver, lead & mercury are Chromates of silver, lead & mercury are insoluble. Those of alkali metals, calcium, insoluble. Those of alkali metals, calcium, magnesium & ammonium are soluble. magnesium & ammonium are soluble.

Double replacement forming a precipitate…Double replacement forming a precipitate…

Pb(NOPb(NO33))2 2 (aq) + 2KI (aq) (aq) + 2KI (aq) PbI PbI2 2 (s) + 2KNO(s) + 2KNO3 3 (aq)(aq)

PbPb2+ 2+ (aq) + 2 NO(aq) + 2 NO33- - (aq) + 2 K(aq) + 2 K+ + (aq) +2 I(aq) +2 I- - (aq) (aq)

PbIPbI2 2 (s) + 2 K(s) + 2 K+ + (aq) + 2 NO(aq) + 2 NO33- - (aq)(aq)

PbPb2+ 2+ (aq) + 2 I(aq) + 2 I- - (aq) (aq) PbI PbI2 2 (s)(s)

Double replacement (ionic) equationDouble replacement (ionic) equation

Complete ionic equation shows compounds as Complete ionic equation shows compounds as aqueous ionsaqueous ions

Net ionic equationNet ionic equation eliminates the eliminates the spectator ionsspectator ions

Includes only those compounds and ions Includes only those compounds and ions that undergo a chemical changethat undergo a chemical change

Spectator ions are those ions that do not take Spectator ions are those ions that do not take part in a chemical rxn and are found in part in a chemical rxn and are found in solution both before and after the rxn- solution both before and after the rxn-

unchanged!unchanged!

IonizationIonization• Ions are formed from solute Ions are formed from solute

compounds by the action of the compounds by the action of the solvent solvent

• Polar water molecules are attracted Polar water molecules are attracted to polar or charged solute particles to polar or charged solute particles

• Electronegative oxygen of water is Electronegative oxygen of water is attracted to electropositive portion of attracted to electropositive portion of a solute compound a solute compound

• Electropositive hydrogen of water is Electropositive hydrogen of water is attracted to the electronegative attracted to the electronegative portion of a solute compoundportion of a solute compound

The Hydronium Ion The Hydronium Ion • HH33OO++ is called the "hydronium" ion is called the "hydronium" ion

HH22O (l) + HCl (g) → HO (l) + HCl (g) → H33OO++ (aq) + Cl (aq) + Cl-- (aq) (aq)

Strong ElectrolytesStrong Electrolytes• Any compound of which all or almost all of Any compound of which all or almost all of

the dissolved compound exists as ions in the dissolved compound exists as ions in an aqueous solution an aqueous solution

• All soluble ionic compounds are strong All soluble ionic compounds are strong electrolytes electrolytes

• Hydrogen halides :Hydrogen halides : HCl, HBr, HI HCl, HBr, HI

Weak Electrolytes Weak Electrolytes

• A compound of which a relatively small A compound of which a relatively small amount of the dissolved compound exists amount of the dissolved compound exists as ions in an aqueous solution as ions in an aqueous solution

• Examples:Examples: HF, organic acids HF, organic acids

Chapter 13 Chapter 13 Ions in Aqueous Solutions and Ions in Aqueous Solutions and

Colligative Properties Colligative Properties

13-2 Colligative Properties13-2 Colligative Properties

Colligative PropertiesColligative Properties

• These are properties that are dependent These are properties that are dependent onlyonly on the number & concentration of on the number & concentration of solute particles.solute particles.

Vapor Pressure LoweringVapor Pressure Lowering

• Effect of Solutes on Vapor-Pressure Effect of Solutes on Vapor-Pressure

Any nonvolatile solute will lower the vapor Any nonvolatile solute will lower the vapor pressure of a solution, having two pressure of a solution, having two

noticeable effects:noticeable effects:

• Raising the boiling point of the solution Raising the boiling point of the solution

• Lowering the freezing point of the solution Lowering the freezing point of the solution

So vapor pressure is lowered, boiling occurs at a So vapor pressure is lowered, boiling occurs at a higher temp b/c more energy is required for the vapor higher temp b/c more energy is required for the vapor

pressure to equal the atmospheric pressure.pressure to equal the atmospheric pressure.

Solute particles take up space @ the liquid-air surface.Solute particles take up space @ the liquid-air surface.

Freezing Point DepressionFreezing Point Depression

• Solutions have lower freezing points than pure Solutions have lower freezing points than pure solvents. solvents.

• If the solution is aqueous, its freezing point will If the solution is aqueous, its freezing point will always be lower than 0always be lower than 0C.C.

• How much lower?How much lower?• Depends on the # & concentration of solute Depends on the # & concentration of solute

particles.particles.• Ex: Salt on ice in the winterEx: Salt on ice in the winter

Colligative PropertiesColligative Properties

• Colligative properties depend on:Colligative properties depend on:

number & concentration of solute particles number & concentration of solute particles

• Since ionic substances dissolve into Since ionic substances dissolve into multiple particles, their colligative effects multiple particles, their colligative effects are greater than those of covalent are greater than those of covalent substances.substances.

Freezing Point DepressionFreezing Point Depression

• Ionic solutes depress the freezing point more than Ionic solutes depress the freezing point more than covalent solutes. Look at their solubility rxns, note covalent solutes. Look at their solubility rxns, note the number of particles formed:the number of particles formed:

• IonicIonic: NaCl (s) + H: NaCl (s) + H22O (l) O (l) Na Na++ (aq) + Cl (aq) + Cl-- (aq) (aq)

• CovalentCovalent: C: C66HH1212OO66 (s) + H (s) + H22O (l) O (l) C C66HH1212OO66 (aq) (aq)

2 particles formed

1 particle formed

Molal Freezing-Point Constant for WaterMolal Freezing-Point Constant for Water

• The freezing-point depression of the solvent in a 1-The freezing-point depression of the solvent in a 1-molal solution of a nonvolatile, nonelectrolyte solute molal solution of a nonvolatile, nonelectrolyte solute

KKff = -1.86 °C/m = -1.86 °C/m • Freezing-Point DepressionFreezing-Point Depression • The difference between the freezing points of the pure The difference between the freezing points of the pure

solvent and a solution of a nonelectrolyte in that solvent and a solution of a nonelectrolyte in that solvent solvent

ΔΔttff = K = Kff mmWhere: m = molalityWhere: m = molality

ΔΔttff = change in freezing point = change in freezing point

Boiling Point ElevationBoiling Point Elevation• Solutions have higher boiling points than pure Solutions have higher boiling points than pure

solvents. solvents.

• If the solution is aqueous, its boiling point will If the solution is aqueous, its boiling point will always be higher than 100always be higher than 100C. C.

• BoilingBoiling: Temperature at which vapor pressure : Temperature at which vapor pressure equals atmospheric pressure. equals atmospheric pressure.

Molal Boiling-Point Constant for WaterMolal Boiling-Point Constant for Water

• The boiling point elevation of the solvent in a 1-molal The boiling point elevation of the solvent in a 1-molal solution of a nonvolatile, nonelectrolyte solute solution of a nonvolatile, nonelectrolyte solute

KKbb = 0.51 °C/m = 0.51 °C/m • Boiling-Point Elevation Boiling-Point Elevation • The difference between the boiling points of the pure The difference between the boiling points of the pure

solvent and a solution of a nonelectrolyte in that solvent and a solution of a nonelectrolyte in that solvent solvent

ΔΔttbb = K = Kbb mmWhere: m = molalityWhere: m = molality

ΔΔttbb = change in boiling point = change in boiling point

The van’t Hoff Factor, The van’t Hoff Factor, ii

Electrolytes may have two, three or more Electrolytes may have two, three or more times the effect on boiling point and freezing times the effect on boiling point and freezing point, depending on its dissociation.point, depending on its dissociation.

T = i T = i K K mm

Freezing Point Depression and Boiling Freezing Point Depression and Boiling Point Elevation ConstantsPoint Elevation Constants

Dissociation EquationsDissociation Equations

NaCl(s) NaCl(s)

AgNOAgNO33(s) (s)

MgClMgCl22(s) (s)

NaNa22SOSO44(s) (s)

AlClAlCl33(s) (s)

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

AgAg++(aq) + NO(aq) + NO33--(aq)(aq)

MgMg2+2+(aq) + 2 Cl(aq) + 2 Cl--(aq)(aq)

2 Na2 Na++(aq) + SO(aq) + SO442-2-(aq)(aq)

AlAl3+3+(aq) + 3 Cl(aq) + 3 Cl--(aq)(aq)

i = 2

i = 2

i = 3

i = 3

i = 4

Ideal vs. Real van’t Hoff FactorIdeal vs. Real van’t Hoff Factor• Attractive forces between ions cause clustering Attractive forces between ions cause clustering

• Clustering is greatest in concentrated solutions Clustering is greatest in concentrated solutions

• Ideal and real results are closest in very dilute solutions Ideal and real results are closest in very dilute solutions

• The Debye-Huckel TheoryThe Debye-Huckel Theory

• Clustering hinders the movements of ions, so fewer ions Clustering hinders the movements of ions, so fewer ions appear to be present appear to be present

Ideal vs. Real van’t Hoff FactorIdeal vs. Real van’t Hoff Factor

The ideal van’t Hoff Factor is only achieved in The ideal van’t Hoff Factor is only achieved in VERY DILUTEVERY DILUTE solution. solution.

Preventing Preventing icing of roads icing of roads using CaClusing CaCl22

Osmotic PressureOsmotic Pressure• Semipermeable membranes Semipermeable membranes • Membranes that allow the movement of some Membranes that allow the movement of some

particles while blocking the movement of others particles while blocking the movement of others • Osmosis Osmosis • The movement of solvent through a semipermeable The movement of solvent through a semipermeable

membrane from the side of lower solute concentration membrane from the side of lower solute concentration to the side of higher solute concentration to the side of higher solute concentration

• Osmosis occurs when two solutions of different Osmosis occurs when two solutions of different concentration are separated by a semipermeable concentration are separated by a semipermeable membrane membrane

Osmotic PressureOsmotic Pressure• The external pressure that must be applied to The external pressure that must be applied to

stop osmosis stop osmosis

• Osmotic pressure increases with the Osmotic pressure increases with the concentration of solute particles concentration of solute particles

• Osmotic pressure is not dependent on the Osmotic pressure is not dependent on the TYPE of solute particles TYPE of solute particles