# Why is the combustion of Nitrogen endothermic? [closed]

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

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

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.

• – Mithoron Dec 12 '18 at 19:24