Question

Is the chemical bond formation an exothermic or a endothermic process? If it is an exothermic reaction, why energy is released again during bond breaking?

Answer 1

Bond breaking requires energy; bond making releases energy. This is a mantra you simply must learn.

Explanation:

We recognize that hydrocarbon combustion is an exothermic reaction:

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

Looking at this reaction, clearly we have to BREAK strong `C-H` and `C-C` bonds in the hydrocarbon, and strong `O=O` bonds in the oxidant. Nevertheless, strong `C=O` and `O-H` bonds are FORMED; the balance between bond formation and bond breaking is the enthalpy associated with the reaction, and here the balance results in the RELEASE of energy.

Of course, by convention, if energy is released a negative sign is associated the term; `DeltaH` is negative.