Question

When a hydrocarbon is combusted, clearly we break `C-H` and `C-C` bonds. Why are such reactions exothermic?

Answer 1

Bond-breaking absorbs energy; bond-making releases energy. This is a mantra that you should commit to memory.

Explanation:

Whether a reaction is endothermic or exothermic depends upon the balance between bond-making and bond breaking. For combustion reactions, consider the following:

`CH_4(g) + 2O_2(g) rarr CO_2(g) + 2H_2O(g)`

Now, as you know, if this reaction is performed you get an exotherm. Why? Well, you are breaking strong `C-H` and `O=O` bonds, but at the same time you are making (even stronger) `C=O`, and `O-H` bonds.

Since the bonds made are stronger than the bonds broken, the difference is the heat evolved from the reaction. If you sum up the energy difference in terms of standard bond enthalpies, you get an excellent estimate for the actual experimental value, which is persuasive evidence that this thermodynamic approach is reasonable and rational.