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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. 

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Answer: We have

The given redox reaction can be represented as: 

Cl2(g)  +  So2(aq)  +  Cl-(aq) +  So42-(aq)

The oxidation half reaction is: 

S+4O2(aq)  →  S+6O42-(aq)

The oxidation number is balanced by adding two electrons as: 

So2(aq)  →  So42-(aq)  +  2e-

The charge is balanced by adding 4H+ ions as: 

So2(aq)  →  So42-(aq)  +  2e-

The O atoms and H+ ions are balanced by adding 2H2O molecules as: 

So2(aq)  +  2H2O(I)  →  sO42-(aq)  +  4H+(aq)  +  2e-     ...............(i)

The reduction half reaction is: 

Cl2(g)  →  Cl-(aq)

The chlorine atoms are balanced as: 

Cl02(s)  →  Cl-1-(aq)

The oxidation number is balanced by adding electrons 

Cl2(g)  +  2e-  →  2Cl-(aq)   ............(i)

The balanced chemical equation can be obtained by adding equation (i) and (ii) as: 

Cl2(s)  +  So2(aq)  +  2H2O(l)  →  2Cl-(aq)  +  SO42-(aq)  +  4H+(aq)

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