Chapter 21 Buffers and Neutralisation

Question 1

Buffer solution

Answer 1

A system that minimises pH changes when small amounts of an acid or alkali are added

Question 2

2 components of a buffer solution

Answer 2

1. A weak acid (HA): removes added alkali
2. The conjugate base (A-): removes added acid

Question 3

What happens when an acid or alkali are added to a buffer solution?

Answer 3

The components will eventually be used up -> the solution loses its buffering ability

Question 4

Effect of pH of a buffer when acid or alkali are added

Answer 4

pH changes by very little

Question 5

2 ways of preparing a weak acid buffer solution

Answer 5

1. Mixing the weak acid with its salt
2. Partial neutralisation of the weak acid

Question 6

Preparation of a weak acid buffer solution: Mixing the weak acid with its salt

Answer 6

e.g. ethanoic acid + sodium ethanoate
* When ethanoic acid is added to water, it partially dissociates
* When sodium ethanoate (its ionic) is added to water, it fully dissociates

Question 7

Preparation of a weak acid buffer solution: By partial neutralisation of the weak acid

Answer 7

1. Add an aqueous solution of an alkali (e.g. NaOH) to an excess of the weak acid
2. The weak acid is partially neutralised by the alkali, forming the conjugate base
3. The resulting solution: salt of the weak acid (conjugate base), any unreacted weak acid

Question 8

What does a shift in the position of equilibrium of the weak acid cause?

Answer 8

Change in pH

Question 9

What happens when an acid is added to a buffer solution?

Answer 9

1. [H+] increases
2. H+ ions react with the conjugate base, A-
3. The equilibrium position shifts to the left, which removes most of the H+

Question 10

What happens when an alkali is added to a buffer solution?

Answer 10

1. [OH-] increases
2. The small concentration of H+ reacts with the OH- to form water
3. HA dissociates
4. The equilibrium position shifts to the right, restores much of the H+