JEE Main · 2020 · 07 Jan 2020 · Shift 2 · Redox Reactions and Electrochemistry

A redox reaction is one in which oxidation states actually change. When dinitrogen combines with dioxygen at $2000$ K, $\mathrm{N}{}_2+\mathrm{O}{}_2\to 2\mathrm{NO}$, nitrogen moves from $0$ to $+2$ and oxygen from $0$ to $-2$, so this is the redox change. The others are not: the reaction with $\mathrm{AgNO}{}_3$ is a precipitation, the $\mathrm{H}{}_2\mathrm{SO}{}_4$ with $\mathrm{NaOH}$ is a neutralisation, and the formation of ozone is only an allotropic rearrangement of oxygen with no change in oxidation number.