Question

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

Answer 1

Skeletal equation would be

${ Cl }_{ 2 }(aq)+{ SO }_{ 2 }(aq)+{ H }_{ 2 }O(l)\longrightarrow{ Cl }^{ - }(aq)+{ SO }_{ 4 }^{ -2 }(aq)$

Reducing the half eqn is ${ Cl }_{ 2 }(aq)\longrightarrow{ Cl }^{ - }(aq)$

Balance the Cl atoms , ${ { Cl }_{ 2 } } {\longrightarrow2Cl }^{ - }(aq)$

Balance the oxidation number by adding the electrons.

${ Cl }_{ 2 }(aq)+2{ e }^{ - }\longrightarrow2{ Cl }^{ - }(aq).....(i)$

Oxidation of half equation is $\quad \overset { +4 }{ { SO }_{ 2 }(aq) } \longrightarrow { SO }_{ 4 }^{ -2 }(aq)+\overset { +6 }{ 2{ e }^{ - } } \quad \quad \quad$

Balance the oxidation number by adding the electrons.

${ SO }_{ 2 }(aq)\longrightarrow{ SO }_{ 4 }^{ -2 }(aq)+2{ e }^{ - }$

Balancing the charge by adding H^{ + }ions

${ SO }_{ 2 }(aq)\longrightarrow{ SO }_{ 4 }^{ -2 }(aq)+4H^{ + }(aq)+2{ e }^{ - }\quad.....(ii)$

Balancing the oxygen atoms by adding the 2 { H }_{ 2 }O

Adding both the eqn (i) and eqn (ii), we get

${ Cl }_{ 2 }(aq)+{ SO }_{ 2 }(aq)+{ 2H }_{ 2 }O(l)\longrightarrow{ 2Cl }^{ - }(aq)+{ SO }_{ 4 }^{ -2 }(aq)+4H^{ + }(aq)$

This is the balanced redox reaction.