Applications of Aqueous Equilibrium Chapter 15

Common Ion Effect & Buffers

Sections 1-3

NaF Na+ + F-

HF H+ + F-

What effect does the NaF have on the dissociation of HF?

HF H+ + F-

Common Ion Effect

• a shift in equilibrium that occurs because of the addition of an ion already in the equilibrium

• Important in polyprotic acids

– The production of H+ in the first step inhibits the production of H+ in subsequent steps

Solutions of Acids or Bases Containing a Common Ion

add F-

from NaF

fewer H+

In Section 14.5 we found the equilibrium concentration of H+ in a 1.0 M HF solution is 2.7 x 10-2 M, and the percent dissociation of HF is 2.7%. Calculate the [H+] and the percent dissociation of HF in a solution containing 1.0 M HF (Ka = 7.2 x 10-4) and the 1.0 M NaF.

Common Ion Sample Problem 15.1 p. 682

Compare the percent dissociation of 0.100 M propanoic acid (HC3H5O2, Ka = 1.3 x 10-5 with the percent dissociation of a mixture containing 0.100M HC3H5O2 and 0.100M NaC3H5O2. Explain the large difference in precent dissociation of the acid.

Common Ion Practice Exercise #25 page 740

Resists changes in its pH when H+ or OH- are added

A weak acid and its salt

Or

A weak base and its salt

Buffers

A buffered solution contains 0.50 M acetic acid (HC2H3O2, Ka = 1.8 x 10-5) and 0.50 M sodium acetate (NaC2H3O2). Calculate the pH of this buffer.

Buffer Calculations Sample Problem 15.2 p. 684

Buffer Calculations

Calculate the change in pH that occurs when 0.010 mol solid NaOH is added to 1.0L of the buffered solution described in Sample Exercise 15.2. Compare this pH change with that which occurs when 0.010 mol solid NaOH is added to 1.0 L of water.

Buffer Calculations Sample Problem 15.3 p. 685

HA H+ + A-

When OH- is added to the system it reacts completely with the HA to produce water and A-.

OH- + HA H2O + A-

For the original buffering system Ka = [H+][A-]/[HA]

Rearrange [H+] = Ka x [HA]/[A-]

Ka will not change so..

[H+] (and pH) is governed by [HA]/[A-]

When OH- is added the HA is converted to A-

[HA]/[A-] decreases so [H+] decreases

The change in [HA]/[A-] is small so the change in [H+] and pH is small

The essence of buffering is that [HA] & [A-] are large compared to [OH-] added

How does buffering work when base is added?

Try

[HA]

[A]

[HA]

[A]

[HA]

[A]

How does a buffer work when OH- is added?

How does a buffer work when OH- is added?

HA H+ + A-

When H+ is added to the system it reacts completely with the A- to produce HA.

H+ + A- HA

For the original buffering system Ka = [H+][A-]/[HA]

Rearrange [H+] = Ka x [HA]/[A-]

Ka will not change so..

[H+] (and pH) is governed by [HA]/[A-]

When OH- is added the A- is converted to HA

[HA]/[A-] increases so [H+] decreases

The change in [HA]/[A-] is small so the change in [H+] and pH is small

The essence of buffering is that [HA] & [A-] are large compared to [H+] added

How does buffering work when acid is added?

Try

Useful for calculating the pH of systems where [HA]/[A-] is known.

Henderson Hasselbalch Equation

Try

[H+] = Ka [HA]

[A-]

-log [H+] = -log Ka [HA]

[A-] - log

pH = pKa [HA]

[A-] - log

pH = pKa [HA]

[A-] + log

pH = pKa [acid]

[base] + log Henderson Hasselbalch

Calculate the pH of a solution containing 0.75 M lactic acid (Ka = 1.4 x 10-4) and 0.25 M sodium lactate. Lactic acid, HC3H5O3, is a common constituent of biological systems. For example, it is found in milk and is present in human muscle tissue during exertion.

Buffer Calculations Sample Problem 15.4 p. 689

A buffered solution contains 0.25 M NH3 (Kb = 1.8 x 10-5) and 0.40 M NH4Cl. Calculate the pH of this solution.

Buffer Calculations Sample Problem 15.5 p. 690

Calculate the pH of the solution that results when 0.10 mole gaseous HCl is added to 1.0 L of the buffered solution from exercise 15.5. (A buffered solution contains 0.25 M NH3 (Kb = 1.8 x 10-5) and 0.40 M NH4Cl.)

Buffer Calculations Sample Problem 15.6 p. 691

• Contains large concentrations of weak acid and weak base

– weak acid, HA & conjugate base, A

– Weak base B & conjugate acid, BH+

• When H+ is added, it reacts to completion with the weak base.

– H+ + A- HA or H+ + B BH+

• When OH- is added, it reacts to completion with the weak acid.

– OH- + HA A- + H2O or OH- + BH+ B + H2O

• The pH of the buffered solution is determined by the ratio of [HA] to [A-] or [B] to [BH+] these concentrations are large compared to the amount of H+ or OH- added.

Summary of Most Important Characteristics of Buffered Solutions

The amount of H+ that can be absorbed with out significant change in pH

The pH of a buffer is determined by the ratio [A-]/[HA].

The capacity of the buffer is determined by the magnitudes of [HA] & [A-].

Buffering Capacity

Calculate the change in pH that occurs when 0.010 mole gaseous HCl is added to 1.0L of

Solution A: 5.00 M HC2H3O2 and 5.00 M NaC2H3O2

Solution B: 0.050 M HC2H3O2 and 0.050 M NaC2H3O2

Ka for HC2H3O2 = 1.8 x 10-5

Buffer Calculations Sample Problem 15.7 p. 693

Comparing Solution A and Solution B

Comparing Solution A and Solution B

A chemist needs a solution buffered at a pH 4.30 and can choose from the following acids and their sodium salts.

a. chloroacetic acid, Ka = 1.35 x 10-3

b. propanoic acid, Ka = 1.3 x 10-5

c. benzoic acid, Ka = 6.4 x 10-5

d. hypochlorous acid, Ka = 1.35 x 10-8

Calculate the ratio [HA]/[A-] required for each system to yield a pH of 4.30. Which system will work best?

Buffer Calculations Sample Problem 15.8 p. 695