Why is the combustion of Nitrogen endothermic? [closed]








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Other combustion reactions are exothermic but this combustion reaction endothermic. Why ? Please explain..




As said in the comments a reaction with oxygen doesn't have to be exothermic it can be endothermic - as of this we only really say the reaction with oxygen is combustion if the reaction is exothermic.


In the example of the reaction of $N\_2$ with $O\_2$ several nitrogen oxides can be formed $N₂O$, $NO$, $N₂O₃$, $NO₂$, and $N₂O₅$. But well do an example of $NO$ formation as this one of most common products. 


For a reaction the enthalpy change of the reaction using bond enthalpies is given by $$\Delta H=\sum{ broken}-\sum{ formed}$$


So are reaction for $NO$ formation is 
$$N\_2 + O\_2\rightarrow 2NO$$ and the bond enthapy of $N\equiv N$ is 891 kJ/mol, $O=O$ is 495kJ/mol, and $NO$ is around 90.2 kJ/mol


Therefore $$\sum{ broken}=1386 kJ/mol$$
$$\sum{ formed}=180.4 kJ/mol$$
so $$\Delta H = 1389-180.4=1205.6 kJ$$
This means that $\Delta \_f H[NO]=602.8 kJmol^{-1}$ this shows that the reaction is endothermic as $\Delta H$ is positive despite being a combustion reaction.


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