How can I know that a given reaction is endothermic or exothermic? I know that all combustion reactions are exothermic. But what about other reactions?

In my school exam I was asked about whether the reaction

$$\ce{N2 + O2 -> 2 NO}$$

is exothermic or endothermic. I didn't know that it is endothermic reaction and left that question. My teacher hence suggested me to learn all exothermic and endothermic reactions! Is it possible to do so? (Obviously not.) Please help me.


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    $\begingroup$ Well, you may learn just the endothermic reactions. The rest are exothermic. $\endgroup$ – Ivan Neretin Sep 4 at 8:06
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    $\begingroup$ If the product looks unfamiliar or you have trouble writing a nice all-octet Lewis structure (like you would have for NO), chances are the reaction is endothermic. You could memorize a table of bond dissociation energies for a more quantitative estimate, but maybe there are other things more worthwhile. $\endgroup$ – Karsten Theis Sep 4 at 10:04
  • $\begingroup$ Your teacher is being sarcastic. I am sure you have discussed this exact reaction in class, because you have to avoid that it runs in opposite direction during the Ostwald process when producing nitric acid. Because it is endothermic. Classic high school chemistry. $\endgroup$ – Karl Sep 4 at 21:12
  • $\begingroup$ Another hint is, if you need high temperature for the reaction it is probably endothermic. $\endgroup$ – Fabian Sep 4 at 21:21