During electrolysis of an aqueous solution of sulphuric acid between platinum electrodes, two types of anions migrate towards the anode but only one of them is discharged.
(a) Name the two anions.
(b) Name the main product of the discharge of anion at the anode and write the anode reaction.
(c) Name the product at the cathode and write the reaction.
(d) Do you notice any change in colour. Explain.
(e) Why is this electrolysis considered as an example of catalysis?
(a)
- Hydroxide ions (OH-) (from water)
- Sulphate ions (SO42-) (from H2SO4)
(b)
The anion discharged: Hydroxide ion (OH-)
Product at anode: Oxygen gas (O2)
Reaction at anode:
4OH- - 4e- ⟶ 4OH
4OH ⟶ 2H2O + O2
(c)
Product at cathode: Hydrogen gas (H2)
The reaction at cathode:
4H+ + 4e- ⟶ 4H
2H + 2H ⟶ 2H2
(d) No change in colour is observed. The gases produced hydrogen and oxygen are colourless and the solution remains colourless since H2SO4 and water are both colourless.
(e) Water in pure state consists almost entirely of molecules. It is a polar covalent compound and can form ions when traces of dilute sulphuric acid is added. As dilute sulphuric acid catalyses this ionisation, hence this electrolysis of acidified water is considered as an example of catalysis.