F.5 Chemistry Question 3

Question

Using the electrolysis of potassium bromide solution as an example, briefly discuss the factors affecting the discharge of ions in electrolysis.

Answer 1

There are three factors that affects the discharge of ion : the nature of anode, the concentration of the ion, and the position of the ion in electrochemical series (ECS).

Assume that inert electrodes (graphite or platinum) are used in electrolysis of KBr(aq) solution.
At the anode : Br-(aq) and OH-(aq) ions migrate to the anode. Since the concentration of Br-(aq) ion is higher, it will be preferentially discharged.
2Br-(aq) → Br2(aq) + 2e-
At the cathode : K+(aq) and H+(aq) ions migrate to the cathode. Since H+(aq) ion is in a higher position in ECS, it will be preferentially discharged.
2H+(aq) + 2e- → H2(g) or 2H2O(l) → H2(g) + 2OH-(aq)

If a reactive metal (e.g. magnesium) is used as the anode, the anode reaction would be the oxidation (dissolving) of the metal instead because the metal is oxidized more readily than the anions migrated to it (i.e. the metal is higher in ECS).
Mg(s) → Mg2+(aq) + 2e-