Buffers and neutralisation

Question 1

Describe the pH titration curve of a weak acid and strong base.

Answer 1

• Weak curve at the bottom indicates weak acid

- Strong spike at the point of neutralisation indicates strong acid

Question 2

How is an indicator chosen using a pH titration curve?

Answer 2

Indicator is chosen at the point where the vertical section (sharp rise) overlap with its pH range.

Question 3

Describe the pH titration curve.

Answer 3

The bottom represents the presence of excess acid when the base is firstly added- therefore pH is low.

Vertical section:
sharp rise in pH on addition of a small volume of base. The middle of this section is the ‘equivalence point’. This is the exact point of neutralisation (where the volume of the solution reacts exactly with another).

Question 4

What is a buffer and how does it work? (7)

Answer 4

A buffer minimises the change in pH on the addition of small amounts of H+ or OH- ions.

It is made from a weak acid and its conjugate base.

The weak acid dissociates into its equilibrium:
HA ~ H+ + A-

On addition of H+ ions, it reacts with A- :
H+ + A- = HA
This shifts the equilibrium to the left in order to minimise increase of H+

On addition of OH-, it reacts with H+:
OH- + H+=H2O
This removes H+, therefore shifting equilibrium to the right in order to overcome loss of H+.

Question 5

Describe the system that acts as a buffer in the body. (3)

Answer 5

Dissociation of H2CO3 in the blood forms an equilibrium:
H2CO3~ H+ + HCO3-

On addition of H+, it reacts with HCO3-, which shifts equilibrium to the left, to minimise rise in H+.

On addition of OH-, it reacts with H+ in a neutralisation reaction, which shifts the equilibrium to the right.
H+ + OH-= H2O

Question 6

Explain how an indicator changes colour. (5)

Answer 6

Indicators are weak acids, therefore they dissociate to form an equilibrium.

The dissociated form has a different colour compared to the undissociated form.

Hln (colourless) ~ H+ + Ln- (red)

On addition of H+ ions, the solution will shift to the right, therefore turning colourless.
On addition of OH- ions, equilibrium shifts to the right, therefore causing a colour change to red.