Ncert Exercises & Solutions

8.23 : 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.

NEETprep Answer:

The given redox reaction can be represented as:

{Cl}_{2}_{(s)} + {SO}_{2 (aq)} + H_{2} O_{(l)} \to {Cl}^{-}_{(aq)} + {SO}_{4}^{2 -}_{(aq)}

The oxidation half reaction is:

\overset{+ 4}{S} O_{2 (a q)} \to \overset{+ 6}{S} {O_{4}^{2 -}}_{(aq)}

The oxidation number is balanced by adding two electrons as:

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

The charge is balanced by adding 4

H^{+}

ions as:

{SO}_{2 (aq)} \to {SO}_{4}^{2 -}_{(aq)} + 4 {H^{+}}_{(aq)} + 2 e^{-}

The O atoms and

H^{+}

ions are balanced by adding

2 H_{2} O

molecules as:

{SO}_{2 (aq)} + 2 H_{2} O_{(l)} \to {SO}_{4}^{2 -}_{(aq)} + 4 {H^{+}}_{(aq)} + 2 e^{-} . . . . (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)} + 2 e^{-} \to 2 {Cl}^{-}_{(aq)} . . . . . . . (ii)

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

{Cl}_{2 (s)} + {SO}_{2 (aq)} + 2 H_{2} O_{(l)} \to 2 {Cl}^{-}_{(aq)} + {SO}_{4}^{2 -}_{(aq)} + 4 {H^{+}}_{(aq)}

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