Question

Which among the following are intramolecular redox reactions?
The question has multiple correct answer
A. ${\text{PC}}{{\text{l}}_{\text{5}}} \to {\text{PC}}{{\text{l}}_{\text{3}}}{\text{ + C}}{{\text{l}}_{\text{2}}}$
B. ${\text{CHO - CHO + }}{\text{OH }} \to {\text{ CO}}{{\text{O}}^{\text{ - }}}{\text{ - C}}{{\text{H}}_{\text{2}}}{\text{OH}}$
C. ${\text{2KCl}}{{\text{O}}_{\text{3}}} \to {\text{2KCl + 3}}{{\text{O}}_{\text{2}}}$
D. ${\text{N}}{{\text{H}}_{\text{4}}}{\text{N}}{{\text{O}}_{\text{2}}} \to {\text{ }}{{\text{N}}_{\text{2}}}{\text{ + 2}}{{\text{H}}_{\text{2}}}{\text{O}}$

Answer 1

Check for both the oxidation and reduction reaction in these reactions. Also look for the species which are undergoing the oxidation and reduction for finding the answer.

Complete step by step answer:
The phosphorus pentachloride is being decomposed and such reaction is an intramolecular redox reaction. As for the reason, the oxidation state phosphorus of $PC{l_5}$ is $+ 5$ and is changed to $+ 3$ when it converts to $PC{l_3}$. Similarly the oxidation state of $Cl$ in $PC{l_5}$ is $- 1$ and is changed to $0$ in $C{l_2}$. Hence, the reaction is an Intramolecular redox reaction.
In case 2,
One aldehyde group $\left( { - CHO} \right)$ is oxidized to carboxylic group $\left( {CO{O^ - }} \right)$ whereas other aldehyde group is reduced to alcohol group $\left( {OH} \right)$. Hence this reaction is also an Intramolecular redox reaction.
In case 3,
The oxidation number of Chlorine in $KCl{O_3}$ is changing from $+ 5$ to $- 1$ in $KCl$, means Chlorine is being reduced. Similarly the oxidation number of oxygen is changing from $- 2$ in $KCl{O_3}$ to $- 1$ in ${O_3}$ which means Oxygen is oxidized. Hence, this reaction is also an Intramolecular redox reaction.
In case 4,
The oxidation number of nitrogen in ammonia is changing from $- 3$ to $0$ in ${N_2}$ which indicates that nitrogen is undergoing oxidation. Similarly, the oxidation number of nitrogen in $N{O_2}$ is changing from $+ 4$ to $0$ in ${N_2}$ which means it is undergoing reduction. Hence this reaction is also an Intramolecular redox reaction.
Hence,

$\therefore$ The answer to this question is option A, B, C, D.

Note:
Intramolecular Redox reactions are the reactions in which a single molecule is undergoing both the oxidative and reductive changes. You can consider all the above reactions as an example of such. It is to be noted that most of the redox reactions are intermolecular and not intramolecular.