Question

Chlorine is used to purify drinking wat Excess of chlorine is harmful. The excess chlorine is removed by treating with sulphur dioxide. Present a balanced equation for this reaction taking place in water.

Answer 1

The skeletal equation for the reaction is: 

Cl₂ (aq) + SO₂ (aq) + H₂O (l) → Cl^-(aq) + SO^2-(aq) 

Reduction half cell: Cl₂ (aq) → Cl^- (aq) 

Balance Cl atoms: Cl₂ (aq) → 2Cl^- (aq) 

Balance O.N. by adding electrons : 

Cl₂ (aq) + 2e^- → 2Cl^-(aq)

Oxidation half cell: SO⁺⁴₂ (aq) → SO₂^-4 (aq) + 2e^- + 4H⁺

Balance O.N. by adding electrons : 

SO₂ (aq) →  SO^2-(aq) + 2e^-

Balance charge by adding H+ ions : 

SO₂ (aq) → SO₂^-(aq) + 2e^- + 4H⁺(aq) 

Balance O atom by adding H₂O: 

SO₂ (aq) + 2H₂O → SO₂^-4 (aq) + 2e^- + 4H⁺(aq) 

Adding both the equations we get: 

Cl₂ (aq) + SO₂(aq) + 2H₂O(l) →  2Cl^-(aq) + SO₂^-(aq) + 4H⁺ 

This represents balanced equation.