1.4.1 Electrolysis of Water
The electrolysis of water is achieved by conducting electrolysis of dilute sulfuric acid. What are the ions present?
cations anions
From sulfuric acid: H+(aq) SO42-(aq)
From water: H+(aq) OH-(aq)
Hydrogen ions are the only cations in the solution. They are discharged at the cathode to give hydrogen gas:
2H+ (aq) + 2e- → H2 (g)
The anode attracts sulfate ions from the acid and hydroxide ions from the water. However, only hydroxide ions are discharged, producing oxygen gas:
4OH- (aq) → O2 (g) + 2H2O (l) + 4e-
From the water
Combining the two half-equations above gives the overall equation:
4H+ (aq) + 4OH- (aq) → O2 (g) + 2H2 (g) + 2H2O (l)
4H2O
That is:
2H2O (l) → 2H2 (g) + O2 (g)
The number of H2, molecules produced is double the number of O2 molecules. This is observed in the experiment, in which the volume of hydrogen produced is double the volume of oxygen (Figure 1.49).