Dinitrogen and dioxygen are main constituents of air but these do not react with each other to form oxides of nitrogen because _________.
(i) the reaction is endothermic and requires very high temperature.
(ii) the reaction can be initiated only in presence of a catalyst.
(iii) oxides of nitrogen are unstable.
(iv) N2 and O2 are unreactive.

Answer: Option (i)

Solution: The correct answer is option (i), which states that the reaction between dinitrogen and dioxygen to form oxides of nitrogen is endothermic and requires very high temperature.
This means that the reaction needs to absorb heat in order to occur, and the temperature required is very high, typically around 2000°C.
This is why the reaction does not occur under normal atmospheric conditions, and instead, oxides of nitrogen are formed through other processes such as lightning or combustion of fossil fuels.
The reaction between dinitrogen and dioxygen to form oxides of nitrogen can be represented by the following equations:
N2(g) + O2(g) → 2NO(g) ΔH = +180.6 kJ/mol
N2(g) + 2O2(g) → 2NO2(g) ΔH = +67.6 kJ/mol
As you can see, both reactions are endothermic, meaning they require energy input to occur. The high positive values of enthalpy change indicate that a significant amount of energy is required to break the strong triple bond in N2 and the double bond in O2. This is why the reaction requires very high temperature to proceed.

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