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 given redox reaction can be represented as :
`Cl_(2 (s)) + SO_(2 (aq)) + H_(2)O_((l)) to Cl_((aq))^(-) + SO_(4 (aq))^(2-)`
The oxidation half reaction is :
`overset(+4)(S) O_(2 (aq)) to overset(+6)(S) O_(4 (aq))^(2-)`
The oxidation number is balanced by adding two electrons as :
`SO_(2 (aq)) to SO_(4 (aq))^(2-) + 2e^(-)`
The charge is balanced by adding `4H^(+)` ions as :
`SO_(2 (aq)) to SO_(4 (aq))^(2-) + 4H_((aq))^(+) + 2e^(-)`
The O atoms and `H^(+)` ions are balanced by adding `2H_(2)O` molecules as :
`SO_(2 (aq)) + 2 H_(2)O_((l)) to SO_(4 (aq))^(2-) + 4H_((aq))^(+) + 2e^(-) ........(i)`
The reduction half reaction is :
`Cl_(2 (s)) to Cl_((aq))^(-)`
The chlorine atoms are balanced as :
`overset(0)(C)l_(2 (s)) to overset(-1)(C) l_((aq))^(-)`
The oxidation number is balanced by adding electrons
`Cl_(2 (s)) + SO_(2 (aq)) + 2H_(2)O_((l)) to 2 Cl_((aq))^(-) + SO_(4 (aq))^(2-) + 4 H_((aq))^(+)`