Question

Why are addition reactions of alkenes exothermic?

Answer 1

Well, you are making `"carbon-carbon"` or `"carbon-heteroatom"` bonds, and given this it is inevitably exothermic.

Explanation:

It is generally observed that bond formation is exothermic, and bond-breaking is endothermic. Olefin reactions generally involve the formation of `2xxC-X` bonds of some sort. Of course, we must break an `X-X` bond of some sort, but formation of `2xxC-X` bonds is energetically downhill.