Ncert Exercises & Solutions

Identify the disproportionation reaction among the following:

1. ${CH}_{4} + 2 O_{2} \to {CO}_{2} + 2 H_{2} O$

2. ${CH}_{4} + 4 {Cl}_{2} \to {CCl}_{4} + 4 HCl$

3. $2 F_{2} + 2 {OH}^{-} \to 2 F^{-} + {OF}_{2} + H_{2} O$

4. $2 {NO}_{2} + 2 {OH}^{-} \to {NO}_{2}^{-} + {NO}_{3}^{-} + H_{2} O$

Hint: Same substance is oxidised as well as reduced in a reaction.

Reaction in which the same substance is oxidised as well as reduced is called disproportionation reaction.

Writing the oxidation number of each element above its symbol in the given reactions

(1)

\overset{- 4}{C} {\overset{+ 1}{H}}_{4} + 2 {\overset{0}{O}}_{2} \to \overset{+ 4}{C} {\overset{- 2}{O}}_{2} + 2 {\overset{+ 1}{H}}_{2} \overset{- 2}{O}

(2)

\overset{- 4}{C} {\overset{+ 1}{H}}_{4} + 4 {\overset{0}{Cl}}_{2} \to \overset{+ 4}{C} {\overset{- 1}{Cl}}_{4} + 4 \overset{+ 1}{H} \overset{- 1}{Cl}

(3)

2 {\overset{0}{F}}_{2} + 2 \overset{- 2}{O} \overset{+ 1}{H^{-}} \to 2 \overset{- 1}{F^{-}} + \overset{+ 2}{O} {\overset{- 1}{F}}_{2} + {\overset{+ 1}{H}}_{2} \overset{- 2}{O}

(4)

2 \overset{+ 4}{N} {\overset{- 2}{O}}_{2} + 2 \overset{- 2}{O} \overset{+ 1}{H^{-}} \to \overset{+ 3}{N} \overset{- 2}{O_{2}^{-}} + \overset{+ 5}{N} \overset{- 2}{O_{3}^{-}} + {\overset{+ 1}{H}}_{2} \overset{- 2}{O}

Thus, in reaction (4), N is both oxidised as well as reduced since the O.N. of N increases from +4 in NO₂ to +5 in

{NO}_{2}^{-}

and decreases +4 in NO₃ to +3 in

{NO}_{3}^{-}