Chapter 21 - Buffers and neutralisation

Question 1

What is a buffer solution?

Answer 1

A system that minimises pH changes on addition of small amounts of an acid or a base.

Question 2

Buffer solutions contain two components. What are these components?

Answer 2

A weak acid and its conjugate base.

Question 3

Explain how conjugate base removes added acid.

Answer 3

On addition of an acid, H+ (aq):

• [H+] increases, ratio of [products] / [reactants] is greater than Ka
• H+ ions react with the conjugate base, A- (aq)
• Equilibrium position shift to the left, removing most of the H+ (aq) ions. Ratio of [products] / [reactants] becomes equal to Ka.

So [H+] and pH are only changed very slightly.

Question 4

Explain how the weak acid removes added alkali.

Answer 4

On addition of an alkali, OH- (aq):

• [OH-] increases
• OH- ions react with the small concentration of H+ ions.
• Ratio of [products] / [reactants] is lower than Ka
• HA dissociates, shifting the equilibrium position to the right to restore most of the H+ (aq) ions. Ratio of [products] / [reactants] becomes equal to Ka.

So [H+] and pH are only changed very slightly.

Question 5

What is an assumption made in buffer solution calculations?

Answer 5

[Salt] = [A-]˅equilibrium

Because the salt is ionic and dissociates fully.

Question 6

What is the buffer system found in the body to control the pH of blood plasma?
What is its function?

Answer 6

The H2CO3/HCO3 (carbonic acid-hydrogencarbonate) buffer is present in blood plasma.
It maintains a blood pH between 7.35 and 7.45.

Question 7

How does a weak acid and an alkali form a buffer solution?

Answer 7

The weak acid is in excess. The weak acid is partially neutralised by the alkali, forming its conjugate base and leaving some of the weak acid unreacted.

Question 8

How can a buffer solution be prepared?

Answer 8

• By partial neutralisation of a weak acid with a strong alkali
• From a weak acid and its salt (salt completely dissociates in water forming a high concentration of conjugate base)

Question 9

What is a acid-base indicator?

Answer 9

A weak acid that has a different colour from its conjugate base.

An indicator changes colour in response to a shift in equilibrium position (e.g. if equilibrium is far to the right, the colour of the solution will be that of the conjugate base).

At the end-point of a titration:

• [HA] = [A-]
• the colour is between the colour of acid and colour of base (e.g. if acid is red and base is yellow, at the end-point the solution will be orange).

Question 10

REVISE TITRATION CURVES (ACID-BASE AND BASE-ACID)!

Answer 10

REVISE TITRATION CURVES (ACID-BASE AND BASE-ACID)!