Question

Which of the following is a redox reaction?
(A) ${H_2}S{O_4}$ with NaOH
(B) In atmosphere, ${O_3}$ from ${O_2}$ by lightning
(C) Nitrogen oxides from nitrogen and oxygen by lightning
(D) Evaporation of ${H_2}O$

Answer 1

Hint: A redox reaction is a reaction where oxidation and redox reaction occur simultaneously. So, there is a change in the oxidation number of the atoms during the reaction.

Complete step by step solution:
We will analyze all the reactions to check whether they are redox reactions or not.
- Redox reactions are the reactions where both oxidation and reduction reactions occur simultaneously. So, there is a change in the oxidation number of atoms during the reaction.
A)
\[{H_2}S{O_4} + 2NaOH \to N{a_2}S{O_4} + 2{H_2}O\]
Sulphuric acid is a mineral acid and NaOH is a base. So, a neutralization reaction takes place when they react together. Salt and water are the products of the reaction. Here, we can see that there is no change in the oxidation number of any of the atoms in the reaction. So, this reaction is not a redox reaction.
B)
\[{O_2} \to {O_3}\]
Here, the oxidation number of oxygen in both of the given compounds is the same and it is zero. So, during the reaction, there is no change in the oxidation number of the atoms. So, we can say that this reaction is not a redox reaction.
C)
\[N{O_2} \to {N_2} + {O_2}\]
Here, we will check the oxidation number of oxygen and nitrogen during the reaction.
We can say that
Overall charge on $N{O_2}$= Oxidation number of N + 2(Oxidation number of O)
0 = Oxidation number of N + 2(-2)
Oxidation number of N = +4
- And the oxidation number of nitrogen in ${N_2}$ and oxygen in ${O_2}$ is zero. So, the oxidation number of nitrogen and oxygen atoms changes during the reaction. So, we can say that this reaction is a redox reaction.
D)
\[{H_2}{O_{(l)}} \to {H_2}{O_{(g)}}\]
This change is just a physical change. So, there is no change in oxidation number of the atoms during the reaction. So, this reaction is not a redox reaction.

So, we can conclude that option (C) is the correct answer.

Note: Note that the oxidation number of the atom in the molecule where only one type of atoms are present is always zero. e.g. oxidation number of oxygen in ${O_3}$ and ${O_2}$ is always zero as both are the elemental forms of oxygen.