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

Explanation

The skeletal equation is:
`Cl_(2)(aq)+SO_(2)(aq)+H_(2)O(l) rarr 2Cl^(Θ)(aq)+SO_(4)^(2-)(aq)`
Reduction half equation:
`Cl_(2)(aq)rarr Cl^(Θ)(aq)`
Balance Cl atoms,
`overset(0)(Cl_(2))(aq) rarr 2overset(-1)(Cl)(aq)`
Balance oxidation number by adding electrons:
`Cl_(2)(aq)+2e^(-)rarr 2Cl^(Θ)(aq)` ...(i)
Oxidation half equation:
`overset(+4)(SO_(2))(aq)rarr overset(+6)(SO_(4)^(2-))(aq)+2e^(-)`
Balance oxidation number adding electrons:
`SO_(2)(aq)rarr SO_(4)^(2-)(aq)+2e^(-)`
Balance charge by adding `H^(o+)` ions:
`SO_(2)(aq)rarr SO_(4)^(2-)(aq)+4H^(o+)(aq)+2e^(-)`
Balance `O` atoms by adding `2H_(2)O`,
`SO_(2)(aq)+2H_(2)O(l) rarr SO_(4)^(2-)(aq)+4H^(o+)(aq)+2e^(-)` ...(ii)
Adding equation (i) and (ii) we have,
`Cl_(2)(aq)+SO_(2)(aq)+2H_(2)O(l) rarr 2Cl^(Θ)(aq)+SO_(4)^(2-)(aq)+4H^(o+)(aq)`
This represented the balanced redox reaction.