Question

Which of the following is a redox reaction?

• A. $\mathrm{NaOH}+\mathrm{HCl}\to \mathrm{NaCl}+{\mathrm{H}}_2\mathrm{O}$
• B. ${\mathrm{CaCO}}_3\to \mathrm{CaO}+{\mathrm{CO}}_2$
• C. ${\mathrm{N}}_2+3{\mathrm{H}}_2\to 2{\mathrm{NH}}_3$
• D. ${\mathrm{NH}}_4\mathrm{CNO}\to {\mathrm{NH}}_2{\mathrm{CONH}}_2$

Answer 1

The correct option is C

${\mathrm{N}}_2+3{\mathrm{H}}_2\to 2{\mathrm{NH}}_3$

Explanation for the correct option:

Redox reaction:

1. Redox reaction means a reaction in which oxidation and reduction takes place simultaneously.
2. In reaction ${\mathrm{N}}_2+3{\mathrm{H}}_2\to 2{\mathrm{NH}}_3$, here ${\mathrm{H}}_2$ is getting oxidized with rise in oxidation state from $0\mathrm{to}\left(+1\right)$ and ${\mathrm{N}}_2$ is getting reduced with decrease in oxidation state from $0\mathrm{to}\left(-3\right)$. Therefore, this is an redox reaction.
3. $\underset{\underset{\left(Oxidation\right)}{\underbrace{Increaseinoxidationstate}}}{\underbrace{H_2^0\to H^{+1}}}and\underset{\underset{\left(Reduction\right)}{\underbrace{Decreaseinoxidationstate}}}{\underbrace{N_2^0\to N^{-3}}}$

Explanation for the incorrect options:

1. In the very first reaction, a base reacts with an acid to form salt and water, therefore, it is an example of neutralization reaction, and there is no change in oxidation states of the elements, so it is not a redox reaction. So, option (A) is incorrect.
2. The second reaction given in option (B) is an example of decomposition reaction, and here in this reaction also there is no change of oxidation state is seen in the elements present, so, option (B) is also incorrect since it is not a redox reaction.
3. Option (D) is not a reaction but it is an another way of writing Ammonium cyanate, hence here also there is no change in oxidation state, so it is also not a redox reaction. So, option (D) is an incorrect option.

Hence, option (C) is correct. i.e., the redox reaction given is ${\mathrm{N}}_2+3{\mathrm{H}}_2\to 2{\mathrm{NH}}_3$.