In chemistry, a reaction takes place first by breaking bonds, which is an endothermic process, then by making bonds, which is an exothermic process. This means that for any reaction a little bit of energy is always needed, which is the activation energy, as can be seen in any reaction profile.
When the difference in energy between the bonds created and the bonds destroyed is calculated the enthalpy of the reaction can be found.
For example:
H2 + Cl2 –> 2HCl
| H−H | 436 kJ mol-1 |
| Cl−Cl | 243 kJ mol-1 |
| H−Cl | 432 kJ mol-1 |
H – H 436 kJ mol-1 Cl – Cl 243 kJ mol-1 H – Cl 432 kJ mol-1
Energy in = 436 + 243 = 679 kJ mol-1
Energy out = (2 × 432) = 864 kJ mol-1
Energy change = in – out
= 679 – 864
= -185 kJ mol-1
The reaction has a negative enthalpy, and therefore this reaction would be exothermic.
Average Bond Energy
The energy of each C – H bond in CH4 is not the same as the environment of each C – H bond changes when a bond has been removed. This means that when breaking a C – H bond an average is used, which means that the result obtained in these calculations can vary a little bit from the experimental value.
The total energy required to break all the bonds is 1652 kJ mol-1 which gives an average of 413 kJ mol-1 per C – H bond.