Overview of Ch 11-13

Properties of Properties of SolutionsSolutions

Chapter 11Chapter 11

Solution CompositionSolution Composition

1.Molarity (M) =

2.Mole fraction (A) =

3.Molality (m) =

moles of soluteliters of solution

molestotal moles in solution

A

moles of solutekilograms of solvent

Henry’s LawHenry’s Law

P = kCP = partial pressure of gaseous

solute above the solutionC = concentration of dissolved gask = the Henry’s Law constant

The amount of a gas dissolved in a solution is The amount of a gas dissolved in a solution is directly proportional to the pressure of the gas directly proportional to the pressure of the gas above the solution.above the solution.

Temperature EffectsTemperature Effects

Solubility of gases generally decreases with temperature.

Solubility of solids generally increases with temperature.

Colligative PropertiesColligative Properties

Depend only on the number, not on the identity, of the solute particles in an ideal solution.

Vapor pressure depression Boiling point elevation Freezing point depression Osmotic pressure increase

Raoult’s LawRaoult’s Law

Psoln = solvent Psolvent

Psoln = vapor pressure of the solution

solvent = mole fraction of the solvent

Psolvent = vapor pressure of the pure solvent

The presence of a nonvolatile solute The presence of a nonvolatile solute lowerslowers the vapor pressure of a solvent.the vapor pressure of a solvent.

Boiling Point ElevationBoiling Point Elevation

A nonvolatile solute elevates the boiling point of the solvent.

T = Kbmsolute

Kb = molal boiling point elevation constant

m = molality of the solute

Freezing Point DepressionFreezing Point Depression

A nonvolatile solute depresses the freezing point of the solvent.

T = Kfmsolute

Kf = molal freezing point depression constant

m = molality of the solute

Osmotic PressureOsmotic Pressure

Osmosis: The flow of solvent into the solution through a semipermeable membrane.

Osmotic Pressure: A nonvolatile solute increases the osmotic pressure of the solvent.

Chemical EquilibriumChemical EquilibriumChapter 13Chapter 13

The state where the concentrations of all reactants and products remain constant with time.

Equilibrium ConstantEquilibrium ConstantjA + kB lC + mD

The equilibrium expression:

Kl m

j k C DA B

4NH3(g) + 7O2(g) 4NO2(g) + 6H2O(g)

K NO H O

NH O2

2

24 6

34 7

Manipulations of KManipulations of K

The equilibrium constant for a reaction is the reciprocal of that for the reaction written in reverse.

When the equation for a reaction is multiplied by n, Knew = (Koriginal)n

KK v. v. KKpp

For

jA + kB lC + mDKp = K(RT)n

n = sum of coefficients of gaseous products minus sum of coefficients of gaseous reactants.

Heterogeneous EquilibriaHeterogeneous Equilibria

. . . are equilibria that involve more than one phase.

CaCO3(s) CaO(s) + CO2(g)

K = [CO2]The position of a heterogeneous equilibrium does not depend on the amounts of pure solids or liquids present.

Reaction QuotientReaction Quotient

. . . helps to determine the direction of the move toward equilibrium.

The law of mass action is applied with initial concentrations.

H2(g) + F2(g) 2HF(g)

QHF

H F2 2

02

0 0

•Q < K, shift right

•Q > K, shift left

Solving Equilibrium Solving Equilibrium ProblemsProblems

1. Write the equilibrium expression.2. Set up an “ICE” box with

relevant concentrations.3. Use the stoichiometry of the

reaction to determine changes in products and reactants, solving for unknowns.

Le Châtelier’s Le Châtelier’s PrinciplePrinciple

. . . if a change is imposed on a system at equilibrium, the position of the equilibrium will shift in a direction that tends to reduce that change.

Effects of Changes on the Effects of Changes on the SystemSystem

1. Concentration: The system will shift away from the added component.

2. Temperature: treat the energy change as a reactant (endothermic) or product exothermic).

Effects of Changes on the Effects of Changes on the System System (continued)(continued)

3. Pressure: a. Addition of inert gas does

not affect the equilibrium position.

b. Decreasing the volume shifts the equilibrium toward the side with fewer moles.

Chemical KineticsChemical KineticsChapter 12Chapter 12

The area of chemistry that concerns reaction rates.

Reaction RateReaction Rate

Change in concentration (conc) of a reactant or product per unit time.

Rate = conc of A at time conc of A at time 2 1

2 1

t tt t

At

Reaction rates are positive by convention.

(Differential) Rate (Differential) Rate LawsLaws

Rate = k[NO2]n

k = rate constant n = rate order

Types of Rate LawsTypes of Rate Laws

Differential Rate Law: expresses how rate depends on concentration.

Integrated Rate Law: expresses how concentration depends on time.

Method of Initial RatesMethod of Initial Rates

Initial Rate: the “instantaneous rate” just after the reaction begins.

The initial rate is determined in several experiments using different initial concentrations.

Overall Reaction OrderOverall Reaction Order

Sum of the order of each component in the rate law.

rate = k[H2SeO3][H+]2[I]3

The overall reaction order is 1 + 2 + 3 = 6.

First-Order Rate LawFirst-Order Rate Law

Integrated first-order rate law is

ln[A] = kt + ln[A]o

Rate = A

A

t

k

For For aaA A Products in a 1st-order reaction, Products in a 1st-order reaction,

Half-Life of a 1st-Order RxnHalf-Life of a 1st-Order Rxn

t1/2 = half-life of the reaction k = rate constant

For a first-order reaction, the half-life does not depend on concentration.

tk1/2

0 693.

Second-Order Rate LawSecond-Order Rate Law

For aA products in a second-order reaction,

Rate = A

A

t

k 2

1A

+ 1

A o

kt

Integrated rate law is:

Half-Life of a 2nd-Order RxnHalf-Life of a 2nd-Order Rxn

t1/2 = half-life of the reactionk = rate constantAo = initial concentration of A

The half-life is dependent upon the initial concentration.

tk1/2

oA

1

Zero-Order Rate LawZero-Order Rate Law

For aA products in a zero-order reaction,

Rate= k

[A] = -kt + [A]o

Integrated rate law is

Half-Life of a Zero-Order RxnHalf-Life of a Zero-Order Rxn

t1/2 = half-life of the reactionk = rate constant[A]o = initial concentration of A

The half-life is dependent upon the initial concentration.

t1/2 = [A]o

2k

Reaction MechanismReaction Mechanism

The series of steps by which a reaction occurs.

A chemical equation does not tell us how reactants become products - it is a summary of the overall process.

Reaction Mechanism Reaction Mechanism (continued)(continued)

The reaction

has many steps in the reaction mechanism.

6CO 6H O C H O O2 2light

6 12 6 2 6

Intermediate: formed in one step and used up in a subsequent step and so is never seen as a product.

Molecularity: the number of species that must collide to produce the reaction indicated by that step.

Elementary Step: A reaction for which a rate law can be written from its molecularity.

uni, bi and termolecular

Rate-Determining Rate-Determining StepStep

In a multistep reaction, it is the slowest step. It therefore determines the rate of reaction.

Arrhenius EquationArrhenius Equation

Collisions must have enough energy to produce the reaction (must equal or exceed the activation energy).

Orientation of reactants must allow formation of new bonds.

k = rate constant A = frequency factor Ea = activation energy T = temperature (in K) R = gas constant

k Ae E RT a /

lnk= -Elnk= -Eaa 1 + 1 + ln Aln A R T R T

slope = -Ea /R

•catalysts decrease Ea.