Chemical EquilibriumChemical Equilibrium

Equilibrium ConstantEquilibrium ConstantEquilibrium constant, K for the reaction

when K >1, the reaction is favored

D C B A dcba

ba

dc

KBADC

Solutions – M or Moles per liter Gases - bars Solids, liquids, solvents are omitted because ~ 1

Conjugate PairsConjugate Pairs Reaction between an acid and base forms another pair of

acid and base as products.

A conjugate acid-base pair are related by the gain or loss of H+.

AutoprotolysisAutoprotolysis

Water is unique because it can behave as either an acid or a base

Reaction with itself is called autoprotolysis

– As an acid

– As a base

Protic solvents undergo some autoprotolysis

14-2

-322 101 OH H Oor H OH O H O H OH wK

OHCH OH OCH OH 3-

32

-32 Br OH HBr OH

pH ScalepH Scale

pH= - log [H+]

pOH=-log [OH-] pH+pOH=-log

Kw=14.00

Strengths of Acids and BasesStrengths of Acids and Bases

“strong” acid or base – completely dissociated in water

– Equilibrium dissociation constant for K>>1

– Hydroxides with metal or organo-ammonium cations

– “strong” doesn’t refer to concentrations, only to

– extent of dissociation

Weak AcidsWeak AcidsOnly partially dissociated in water, hence the Ka for a weak acid is small

HA, react with water by donating a proton to H2O

Equilibrium constant, Ka, is the acid dissociation constant

-32 AHHAor AOH OHHA

aK

HA

A H -

aK

Weak BasesWeak BasesOnly partially dissociated in water, hence the Kb for a weak base is small

Denoted B, react with water by taking a proton from H2O

Equilibrium constant, Kb, is called the base hydrolysis constant

OHBH OHB 2bK

BOH BH -

bK

Examples of weak acids and Examples of weak acids and basesbases

Carboxylic acids

Amines

CH3

O

O H CH3

O

O-

Acetic Acid (HA)

Acetate (A-)

Ka = 1.75 x10-

5H++

Kb = 4.4 x10-4N

CH3

H

H

N+CH3

H

H

H

OH2 OH-+ +

Methylamine (B)

Methylammonium (BH+)

Predict whether basic or acidic a solution of methylammonium chloride in water

)(Cl NHCH ClNHCH -3333 aq(aq)(s)

Polyprotic acids and basesPolyprotic acids and basesPolyprotic acids and bases can donate or accept more than one proton

Constants for polyprotic compounds follow the notation:– K1, K2, K3

– Kb1, Kb2

K1 refers to the acid with the most protons and Kb1 to the base with the fewest protons

15.1220.715.2

3344

22442

14243

pKPOHHPOpKHPOHPOHpKPOHHPOH

Relationship Between Relationship Between KKaa and and KKbb

Ka and Kb of a conjugate acid-base pair

ba

b

a

KKKK

OHHOH OHHAOHA

A H HA

2

2

-

HA

A H -

aK

-

-

AOH HA bK

wba KKK

-

--

AOH HA

HAA H

KaKb=Kw for any conjugate acid-base pair in aqueous solution

Relationship Between Relationship Between KKaa and and KKbb

Calculate Kb for acetate ion if the Ka for acetic acid is 1.75 x 10-5

105

14107.51075.1

100.1

a

wb K

KK

wba KKK

Polyprotic acidsPolyprotic acids

Derive the following relationship for a – Diprotic acid

– Triprotic acid

wba KKK 21

wba KKK 12

wba KKK 31 wba KKK 22

wba KKK 13