The bond energy also known as bond dissociation energy is the energy required to break a covalent bond. The bond energy is a measure of the strength of a bond and is the enthalpy change for a gas phase reaction in which the bond breaks. These energies are usually expressed in kilojoules per mole of bonds and are compiled in tables like the one below.
The bond energy between a given pair of atoms varies slightly from one compound to another. For example the bond dissociation energy for the C-H bond in methane, CH4 is 435 kJ while in ethane, C2H6 it is 410 kJ. Because these variations in strength are not very great, the average bond energy is a good guide to the strength of a bond in any molecule. It is these average bond energies that are given in tables like the one below.
|C=C||602||C=N||615||C=O||745(799 in CO2|
|N=N||418||N=O||607||S=O (in SO2)||532|
|NN||942||O2||494||S=O (in SO3)||469|
When possible it is preferable to use thermochemical data calculate the enthalpy of a reaction. If the thermochemical data are not known, however, bond energies can be used to give you an estimate of the heat of reaction. To illustrate this consider the following example.
Use bond energies (Table above) to estimate H (kJ) for the following gas-phase reaction:
N2 + 2 F2 ==> NF2-NF2
In the reaction, a NN triple bond and two F-F bonds are broken. One N-N single bond and four N-F bonds are formed.
H = [BE(NN) + 2BE(F-F)] - [BE(N-N) + 4BE(N-F)]
H = [942 + 2(155)] kJ - [167 + 4(283)] kJ = -47 kJ