21- buffers and neutralisation

Question 1

What is a buffer solution?

Answer 1

• It is a system that minimises pH changes when small amounts of an acid or a base are added.
• Buffer solutions contain 2 components to remove added acid or alkali - a weak acid and its conjugate base.
• The weak acid, HA removes the alkali
• The conjugate base, A- removes added acid

Question 2

How do you prepare a buffer solution from a weak acid and it’s salt? Give an example.

Answer 2

• Can be made by mixing a solution of ethanoic acid CH3COOH with a solution of one of it’s salts e.g. CH3COONa.
• When ethanoic acid is added to water, the acid partially dissociates and the amount of ethanoate ions in solution is very small. Ethanoic acid is the source of the weak acid components in the buffer solution.
• Salts of weak acids provide a source of the conjugate base. When added to water, the salt completely dissolves. Dissociation into ions is complete so the salt is the source of the conjugate base component of the buffer solution.

Question 3

How do you prepare a buffer solution by partial neutralisation of the weak acid?

Answer 3

• Buffer solution can be prepared by adding an aq solution of an alkali, such as NaOH, to an excess of the weak acid.
• The weak acid is partially neutralised by the alkali forming the conjugate base. Some of the weak acid is left over unreacted.
• The resulting solution contains a mixture of the salt of the weak acid and any unreacted weak acid.

Question 4

Explain what happens when an acid is added in terms of how the buffer solution acts.

Answer 4

1. H+ conc. increases
2. H+ ions react with the conjugate base A-
3. The equilibrium position shifts left, removing most of the H+ ions

Question 5

Explain what happens when a weak acid removes an added alkali in a buffer solution.

Answer 5

On addition of alkali; OH-,
1. OH- concentration increases
2. The small concentration of H+ ions reacts with the OH- ions
3. HA dissociates, shifting the equilibrium position to the right to restore most of the H+ ions.

Question 6

When is a buffer most effective?

Answer 6

When there are equal concentrations of the weak acid and it’s conjugate base.
When [HA] = [A-]:
- The pH of the buffer solution is the same as the pKa value of HA
- The operating pH is typically over about 2 pH units, centred at the pH of the pKa value.

Question 7

What is the equation for working out the pH of a buffer solution?

Answer 7

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