UPSC NDA I 2019 General Ability Solved Paper

Concept:The reaction of dinitrogen ($\mathrm{N}_2$) and dioxygen ($\mathrm{O}_2$) to form nitrogen oxides is highly endothermic and requires extreme thermal energy.Explanation:The triple bond in $\mathrm{N}_2$ and the double bond in $\mathrm{O}_2$ are very strong. Breaking these bonds demands a large input of energy. The reaction $\mathrm{N}_2 + \mathrm{O}_2 \rightarrow 2\mathrm{NO}$ is endothermic, with a positive enthalpy change. It proceeds only at temperatures above $3000^{\circ}\mathrm{C}$ (e.g., inside lightning strikes or internal combustion engines). At ordinary atmospheric temperatures, the kinetic energy of molecules is insufficient to overcome the activation barrier. Thus, even though $\mathrm{N}_2$ and $\mathrm{O}_2$ together make up about 99% of dry air, they do not react spontaneously. The statement "the reaction is endothermic and requires very high temperature" correctly explains this non-reactivity.Answer:C. The reaction is endothermic and requires very high temperature.