Question

Which of the following is/are not intermolecular redox reactions?
A.$Pb{O_2} + {H_2}O \to PbO + {H_2}{O_2}$
B.$2KCl{O_3} \to 2KCl + 3{O_2}$
C.${(N{H_4})_2}C{r_2}{O_7} \to {N_2} + C{r_2}{O_3} + 4{H_2}O$
D.$N{H_4}N{O_2} \to {N_2} + 2{H_2}O$

Answer 1

A redox reaction is in which both oxidation and reduction occurs. There are two types of redox reactions- intramolecular and intermolecular redox reactions.

Complete step by step answer: Oxidation is loss of electrons or increase in oxidation state. Gain of electrons or decrease in oxidation state is called reduction. A reaction which involves both oxidation and reduction is called redox reaction.
When one molecule is oxidised and another molecule is reduced it is called intermolecular redox reaction. When one atom of a molecule is oxidised and another atom of the same molecule is reduced it is called intramolecular redox reaction.
Now let us discuss each of the options.
In option a, Pb changes its oxidation state from $+ 4$ to $+ 2$. Hence Pb is reduced. Oxygen changes its oxidation state from $- 2$ to $- 1$. Hence oxygen gets oxidised. Since oxidation and reduction occurs to different atoms in different molecules this is an intermolecular redox reaction.
In option b, Cl changes its oxidation state from $+ 5$ to $- 1$. Hence Cl is reduced. Oxygen changes its oxidation state from $- 2$ to $0$. Hence oxygen is oxidised. Since oxidation and reduction occurs to different atoms in the same molecule this is an intramolecular redox reaction.
In option c, N changes its oxidation state from $- 3$ to $0$. Hence N is oxidised. Cr changes its oxidation state from $+ 6$ to $+ 3$. Hence Cr is reduced. Since oxidation and reduction occurs to different atoms in the same molecule this is an intramolecular redox reaction.
In option c, N in $N{H_4}^ -$ changes its oxidation state from $- 3$ to $0$ in ${N_2}$. Hence N is oxidised. Also N in $N{O_2}^ -$ changes its oxidation state from $+ 3$ to $0$ in ${N_2}$. Hence N is also reduced. Therefore this is also not an intermolecular redox reaction.
Hence the answers are options B,C and D. They are not intermolecular reactions.

Note:
Redox reactions can be broken down to oxidation half reaction and reduction half reaction. When doing this, the charge and number of electrons should also be balanced besides the number of atoms.