1

Week 10 CHEM 1310 - Sections L and M 1

Ch 8 - Applications of Equilibrium

8.1 Solutions of Acids or Bases Containing a Common Ion

8.2 Buffered Solutions 8.4 Buffer Capacity 8.5 Titrations and pH Curves 8.6 Acid-Base Indicators 8.8 Solubility Product Sections to Skip: 8.3, 8.7, 8.9, and 8.10

Week 10 CHEM 1310 - Sections L and M 2

Review of an Example

What is the [H+] and the % dissociation of asolution containing 1.0 M HF?HF (aq) ↔ H+ (aq) + F- (aq) Ka = 7.2 x 10-4

Weak acid

Ka = [H+] x [F-]

[HA]7.2 x 10-4 =

X2

1.0

Thus, X = [H+] = √ (7.2 x 10-4) = 0.027 M

% Dissociation = 0.0271.0

x 100 = 2.7%

2

Week 10 CHEM 1310 - Sections L and M 3

Common Ion Effect

What happens when more F- is added froma different source?

HF (aq) ↔ H+ (aq) + F- (aq)

NaF (s) → Na+ (aq) + F– (aq)

More F- added

Le Châtelier’s Principle:Equilibrium shifts to left

Common Ion

What impact doesthe additional F-

have on [H+]?

Week 10 CHEM 1310 - Sections L and M 4

Common Ion Effect

HF (aq) ↔ H+ (aq) + F- (aq)

NaF (s) → Na+ (aq) + F– (aq)

More F- added

Equilibrium Concentrations[HF] = 1.0 - x, where x is negligible

[H+] = X

[F-] = [F-]from NaF + [F-]from HF = 1.0 + X

[HF] = 1.0 M[NaF] = 1.0 M

Ka = [H+] x [F-]

[HA]

3

Week 10 CHEM 1310 - Sections L and M 5

Common Ion Effect

Equilibrium Concentrations[HF] = 1.0 - X, where X is negligible

[H+] = X

[F-] = [F-]from NaF + [F-]from HF = 1.0 + X,where X is negligible

Ka = [H+] x [F-]

[HA]

7.4 x 10-4 = X x (1.0)

1.0[H+] = 0.00074 M

when a common ion at1.0 M is added

Week 10 CHEM 1310 - Sections L and M 6

Common Ion Effect: Impact

Without Common Ion

With Common Ion

[H+] 0.027M decrease 0.00074M

pH 1.57 increase 3.13

% Dissociation 2.7% decrease 0.074%

HF (aq) ↔ H+ (aq) + F- (aq)

Le Châtelier’s Principle:Equilibrium shifts to left in this example

4

Week 10 CHEM 1310 - Sections L and M 7

What are Buffered Solutions?

Most important application of acid-basesolutions is buffering

Buffer is a solution that resists changes inpH Weak acid + Conjugate Base (Acidic buffer) Weak base + Conjugate Acid (Alkaline buffer)

Week 10 CHEM 1310 - Sections L and M 8

Buffers Are Found in Many Places

Human Blood

Foods

Shampoos

Georgia Aquarium

5

Week 10 CHEM 1310 - Sections L and M 9

How Do Buffers Work?

Week 10 CHEM 1310 - Sections L and M 10

Henderson-Hasselbalch Eqn

˜̃¯

ˆÁÁË

Ê-=

˜̃¯

ˆÁÁË

Ê--=

=

-

-+

-+

][A

[HA]log pKpH

][A

[HA]log]Klog[]Olog[H-

][A

[HA]K]O[H

a

a3

a3

˜̃¯

ˆÁÁË

Ê+=

-

[HA]

][Alog pKpH a

Two useful forms of the Henderson-Hasselbalch equation

6

Week 10 CHEM 1310 - Sections L and M 11

Making Buffer Solutions

From Wikipedia