JEE Main 2022 · 28 July · evening · Single correct · +4 / −1
Dinitrogen and dioxygen, the main constituents of air do not react with each other in atmosphere to form oxides of nitrogen because :
- (a) N₂ is unreactive in the condition of atmosphere.
- (b) Oxides of nitrogen are unstable.
- (c) Reaction between them can occur in the presence of a catalyst.
- (d) The reaction is endothermic and require very high temperature.
Answer and solution
Answer (d) The reaction is endothermic and require very high temperature.
Solution
Chemistry ma’am · 4 steps
Making NO from air absorbs heat, so it needs lightning-level temperatures, not ordinary air.
- The reaction. Nitrogen and oxygen can combine to give nitric oxide. Check the sign of Δ H: heat has to be supplied, so the reaction is endothermic.
N₂(g) + O₂(g) ⇌ 2NO(g) Δ H = +180.5 kJ mol⁻¹
- Why so costly. The N≡N triple bond is very strong, with a bond enthalpy of about 941 kJ mol⁻¹. Breaking it takes far more energy than the new bonds in NO give back.
- Where it happens. At ordinary temperatures the equilibrium lies almost completely on the left. Noticeable NO forms only near 2000 K, which is reached in lightning flashes and inside car engines.
- So. Air does not turn into oxides of nitrogen because the reaction is endothermic and needs a very high temperature. The answer is (d).
The idea to keep. When a reaction does not happen under ordinary conditions, look for the energy reason. For N₂ it is the strong triple bond, which makes forming its oxides endothermic and in need of high temperature.
Answer from the NTA final answer key. Solution by PrepWiser.