Question

Is chemical bond formation an exothermic or a endothermic process?

Answer 1

Bond formation is an EXOTHERMIC process. Bond breaking (i.e. fission, cleavage) is an ENDOTHERMIC process.

Explanation:

We recognize that the combustion of methane is an exothermic process:

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

(Normally, we would write `DeltaH` `=` `-890` `kJ*mol^-1`; the enthalpy value has been precisely measured.)

In this combustion process, we have to break strong `C-H` and `O=O` bonds. This is an endothermic process. But at the same time we make even stronger `C=O` and `O-H` bonds. The balance, the energy difference between the bonds broken versus the bonds made, is the energy released by the reaction.

WE can go even further than this and approach the problem with individual bond energies. Tables of bond energy data exist that list the energy necessary to break particular bonds, i,e. `C-H`, or `C=O`, or `C-C`. (Of course, these values are all positive because this is a bond-breaking process; if I change their sign I get the corresponding bond-making value).

When you do a summation from these data, with bonds broken positive, and bonds formed negative, you can get an answer that is precisely the same as the experimental value. This same result from experiment and tabulated data persuades us that the treatment is valid.