Question

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. 

Answer 1

Answer: We have

The given redox reaction can be represented as: 

Cl_2(g)  +  So_2(aq)  +  Cl^-_(aq) +  So₄^2-_(aq)

The oxidation half reaction is: 

S⁺⁴O_2(aq)  →  S⁺⁶O₄^2-_(aq)

The oxidation number is balanced by adding two electrons as: 

So_2(aq)  →  So₄^2-_(aq)  +  2e^-

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

So_2(aq)  →  So₄^2-_(aq)  +  2e^-

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

So_2(aq)  +  2H₂O_(I)  →  sO₄^2-_(aq)  +  4H⁺_(aq)  +  2e^-     ...............(i)

The reduction half reaction is: 

Cl_2(g)  →  Cl^-_(aq)

The chlorine atoms are balanced as: 

Cl⁰_2(s)  →  Cl^-1-_(aq)

The oxidation number is balanced by adding electrons 

Cl_2(g)  +  2e^-  →  2Cl^-_(aq)   ............(i)

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

Cl_2(s)  +  So_2(aq)  +  2H₂O_(l)  →  2Cl^-_(aq)  +  SO₄^2-_(aq)  +  4H⁺_(aq)