Ncert Exercises & Solutions

25. Identify the redox reaction out of the following reactions and identify the oxidising and reducing agents in them.

(a) $3 HCl (aq) + {HNO}_{3} (aq) \to {Cl}_{2} (g) + NOCl (g) + 2 H_{2} O (l)$

(b) ${HgCl}_{2} (aq) + 2 KI (aq) \to {HgI}_{2} (s) + 2 KCl (aq)$

(d) ${PCl}_{3} (l) + 3 H_{2} O (l) \to 3 HCl (aq) + H_{2} {PO}_{3} (aq)$

(e) $4 {NH}_{3} (aq) + 3 O_{2} (g) \to 2 N_{2} (g) + 6 H_{2} O (g)$

(a) Writing the O.N. on each atom above its symbol, then

3 \overset{+ 1}{H} \overset{- 1}{Cl} (aq) + \overset{+ 1}{H} \overset{+ 5}{N} {\overset{- 2}{O}}_{3} (aq) \to \overset{0}{C} l_{2} (g) + \overset{+ 3}{N} \overset{- 2}{O} \overset{- 1}{C} l (g) + 2 {\overset{+ 1}{H}}_{2} \overset{- 2}{O} (l)

Here, the O.N. of Cl increases from -1 in HCl to 0 in Cl₂, therefore, Cl^- is oxidised and hence HCl acts as the reducing agent.

The O.N. of N decreases from +5 in HNO₃ to +3 in NOCl, therefore, HNO₃ acts as the oxidising agent.

Thus, this reaction is a redox reaction.

(b) Writing the O.N. of each atom above its symbol, we have,

\overset{+ 2}{Hg} {\overset{- 1}{Cl}}_{2} (aq) + 2 \overset{+ 1}{K} \overset{- 1}{I} (aq) \to \overset{+ 2}{Hg} {\overset{- 1}{I}}_{2} (s) + 2 \overset{+ 1}{K} \overset{- 1}{Cl} (aq)

Here, the O.N. of none of the atoms undergo a change, therefore, this reaction is not a redox reaction.

(c)

{\overset{+ 3}{Fe}}_{2} {\overset{- 2}{O}}_{3} (s) + 3 \overset{+ 2}{C} \overset{- 2}{O} (g) \overset{∆}{\to} 2 \overset{0}{Fe} (s) + 3 \overset{+ 4}{C} {\overset{- 2}{O}}_{2} (g)

Here, O.N. of Fe decreases from +3 in Fe₂O₃ to 0 in Fe, therefore, Fe₂O₃ acts as an oxidising agent. Further, O.N. of C increases from +2 in CO to +4 in CO₂, therefore, CO acts as a reducing agent.

Thus, this reaction is an example of redox reaction.

(d) Writing the O.N. of each atom above its symbol, then

\overset{+ 3}{P} {\overset{- 1}{Cl}}_{3} (l) + 3 {\overset{+ 1}{H}}_{2} \overset{- 2}{O} (l) \to 3 \overset{+ 1}{H} \overset{- 1}{Cl} (aq) + {\overset{+ 1}{H}}_{2} \overset{+ 3}{P} {\overset{- 2}{O}}_{3} (aq)

Here, O.N. of none of the atoms undergo a change, therefore, this reaction is not a redox reaction.

(e) Writing the O.N. of each atom above its symbol, then

4 \overset{- 3}{N} {\overset{+ 1}{H}}_{3} (aq) + 3 {\overset{0}{O}}_{2} (g) \to 2 {\overset{0}{N}}_{2} (g) + 6 {\overset{+ 1}{H}}_{2} \overset{- 2}{O} (g)

Here, O.N. of N increases from -3 to 0 in N₂, therefore, NH₃ acts as a reducing agent. Further, O.N. of O decreases from 0 in O₂ to -2 in H₂O, therefore, O₂ acts as a oxidising agent. Thus, this reaction is a redox reaction.

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