I II III IV V

Ch. 19 – Strengths of Acids & Bases

19.3pp. 602 –

611

A. Strong & Weak Acids

Strong acids are completely ionized in aqueous solution

0

20

40

60

80

100

Relative Number of Moles

HA H3O+ A-

Strong Acid

HA(aq) + H2O(l) → H3O+(aq) + A-(aq)

Equilibrium Amounts

A. Strong & Weak Acids

Weak acids ionize only slightly in aqueous solution CH3COOH(aq) + H2O(l) H3O+(aq) + CH3COO–(aq)

~99% ~1%

Initial Amounts

B. Acid Dissociation Constant

You can write the equilibrium-constant expression from balanced chemical equation CH3COOH(aq) + H2O(l) H3O+(aq) + CH3COO–(aq)

][

]][[

3

33

COOHCH

COOCHOHKeq

The acid dissociation constant, Ka, is ratio of concentrations of dissociated and undissociated form

aeq KCOOHCH

COOCHOHK

][

]][[

3

33

B. Acid Dissociation Constant

Reflects the fraction of the acid in the ionized form If Ka is small, then degree of dissociation is small

Weak acids have small Ka values

The stronger an acid is, the larger its Ka value Often written in generic form:

][

]][[

HA

AHKa

HA H+ + A–

C. Strong & Weak Bases

Strong bases completely dissociate into metal ions and hydroxide ions in aqueous solution

Weak bases react with water to form the hydroxide ion and the conjugate acid of the base

NH3(aq) + H2O(l) NH4+(aq) + OH-(aq)

D. Base Dissociation Constant

You can write the equilibrium-constant expression from balanced chemical equation

][

]][[

3

33

COOHCH

COOCHOHKeq

The base dissociation constant, Kb, is ratio of concentrations of dissociated and undissociated form

beq KNH

OHNHK

][

]][[

3

4

NH3(aq) + H2O(l) NH4+(aq) + OH-(aq)

D. Base Dissociation Constant

Size of Kb indicates ability of weak base to compete with very strong base OH– for hydrogen ions

Weak bases have small Kb values

The stronger a base is, the larger its Kb value Often written in generic form:

][

]][[

B

OHBHKb

B + H2O BH+ + OH–

E. Calculating Dissociation Constants A 0.100M solution of acetic acid is only

partially ionized. From the pH, [H+] is determined to be 1.34 x 10-3M. What is Ka of acetic acid?

Brackets indicate concentrations at equilibrium – you need to find equilibrium values!

][

]][[

3

3

COOHCH

COOCHHKa

E. Calculating Dissociation Constants

Concentration [CH3COOH] [H+] [CH3COO-]

Initial 0.100 0 0

Change -1.34 x 10-3 +1.34 x 10-3 +1.34 x 10-3

Equilibrium 0.0987 1.34 x 10-3 1.34 x 10-3

][

]][[

3

3

COOHCH

COOCHHKa

533

3

3 1082.10987.0

)1034.1)(1034.1(

][

]][[

xxx

COOHCH

COOCHHKa

F. Percent Dissociation Often useful to specify amount of weak acid

or base that has dissociated in achieving equilibrium

%100)(

)(x

Mionconcentrat initial

Mddissociate amountonDissociati Percent

Ex) 1.00M solution of HF, [H+] = 2.7x10-2 M

%7.2%10000.1

107.2%

2

xM

MxDissoc

G. Weak Acids and Bases WS1) Find the pH and pOH of a 0.325M

acetic acid solution. Ka = 1.8 x 10-5

Concentration [CH3COOH] [H+] [CH3COO-]

Initial 0.325 0 0

Change -x +x +x

Equilibrium 0.325 - x x x

HA H+ + A–

325.0325.0325.0

))((

][

]][[108.1

22

3

35 x

x

x

x

xx

COOHCH

COOCHHxKa

G. Weak Acids and Bases WS1) Find the pH and pOH of a 0.325M

acetic acid solution. Ka = 1.8 x 10-5

][1042.2

325.0108.1

3

25

HMxx

xxKa

38.11

62.2

pOH

pH

%74.0%100325.0

1042.2%

3

xM

MxDissoc