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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.
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Solution
The given redox reaction can be represented as:
\[\ce{Cl_{2(s)} + SO_{2(aq)} + H_2O_{(l)} -> Cl-_{(aq)} + SO^{2-}_{4(aq)}}\]
The oxidation half reaction is:
\[\ce{^{+4}SO_{2(aq)} -> ^{+6}SO^{2-}_{4(aq)}}\]
The oxidation number is balanced by adding two electrons as:
\[\ce{SO_{2(aq)} -> SO^{2-}_{4(aq)} + 2e-}\]
The charge is balanced by adding 4H+ ions as:
\[\ce{SO_{2(aq)} -> SO^{2-}_{4(aq)} + 4H+_{(aq)} + 2e-}\]
The O atoms and H+ ions are balanced by adding 2H2O molecules as:
\[\ce{SO_{2(aq)} + 2H_2O_{(l)} -> SO^{2-}_{4(aq)} + 4H+_{(aq)} + 2e-}\] ....(i)
The reduction half reaction is:
\[\ce{Cl_{2(s)} -> Cl-_{(aq)}}\]
The chlorine atoms are balanced as:
\[\ce{^0Cl_{2(s)} -> ^{-1}Cl-_{(aq)}}\]
The oxidation number is balanced by adding electrons
\[\ce{Cl_{2(s)} + 2e- -> 2Cl-_{(aq)}}\] ....(ii)
The balanced chemical equation can be obtained by adding equation (i) and (ii) as:
\[\ce{Cl_{2(s)} + SO_{2(aq)} + 2H_2O_{(l)} -> 2Cl-_{(aq)} + SO^{2-}_{4(aq)} + 4H+_{(aq)}}\]
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