Dinitrogen (N₂) and dioxygen (O₂) are the main constituents of air but

Dinitrogen (N₂) and dioxygen (O₂) are the main constituents of air but they do not react with each other to form oxides of nitrogen because

[amp_mcq option1=”the reaction requires initiation by a catalyst” option2=”oxides of nitrogen are unstable” option3=”the reaction is endothermic and requires very high temperature” option4=”the stoichiometry of N₂ and O₂ in air is not ideal for the reaction to take place” correct=”option3″]

This question was previously asked in

UPSC NDA-1 – 2019

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Dinitrogen (N₂) and dioxygen (O₂) molecules are very stable due to strong bonds (triple bond in N₂, double bond in O₂). The reaction between them to form oxides of nitrogen, such as nitric oxide (NO), is highly endothermic (requires energy input) and has a high activation energy barrier. Therefore, this reaction requires very high temperatures (e.g., found in lightning strikes or internal combustion engines) to proceed significantly. At normal atmospheric temperatures, there isn’t enough energy to break the strong N≡N bond and initiate the reaction.

– The reaction N₂ + O₂ → 2NO is endothermic (ΔH > 0).
– The strong triple bond in N₂ requires a large amount of energy to break.
– Standard atmospheric temperatures are insufficient to overcome the activation energy barrier for this reaction.

While a catalyst could potentially lower the activation energy, the primary reason the reaction doesn’t occur spontaneously in the air is the high energy requirement. Oxides of nitrogen, once formed, vary in stability, but their stability is not the reason for their lack of formation under normal conditions. The stoichiometry in air is about 4:1 N₂:O₂, which is not ideal for NO formation (1:1), but this ratio doesn’t prevent the reaction from occurring at all; it just affects equilibrium yield.

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