Is combustion exothermic or endothermic, and why?

1 Answer
Dec 31, 2015

Combustion reactions are routinely exothermic. Why?

Explanation:

To break bonds, you must expend energy. When bonds are made, energy is released. Let's take the simple combustion of methane as shown:

$C {H}_{4} \left(g\right) + 2 {O}_{2} \left(g\right) \rightarrow C {O}_{2} \left(g\right) + 2 {H}_{2} O \left(g\right)$

Looking at this reaction, I am breaking STRONG $C - H$ and $O = O$ bonds. But at the same time I am making even STRONGER $C = O$, and $O - H$ bonds. The energy balance of bonds made less bonds broken is the energy released by the reaction. These are precisely measured, and an excellent estimate of the energy output of this reaction could be made simply by considering the bond energies of individual $C = O$, and $O - H$ bonds (the bonds formed) versus $C - H$ and $O = O$ bonds, the bonds broken.