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

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    $\begingroup$ consider the strength of the Nitrogen-Nitrogen triple bond $\endgroup$ – Waylander Dec 12 '18 at 8:27
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    $\begingroup$ And, not all reactions with oxygen are exothermic - the ones that are tend to be called combustion reactions since, well, they actually combust. $\endgroup$ – Jon Custer Dec 12 '18 at 14:10
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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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