Bond energy

The bond dissociation energy (enthalpy) is also referred to as bond disruption energy, bond energy, bond strength, or binding energy (abbreviation: BDE, BE, or D). It is defined as the standard enthalpy change of the following fission: R—X → R + X. The BDE, denoted by Dº(R—X), is usually derived by the thermochemical equation,

${\displaystyle \begin{array}{l}{\mathrm{D}}^{\circ }(\mathrm{R}-X)=\mathrm{\Delta }{H}_f^{\circ }(\mathrm{R})+\mathrm{\Delta }{H}_f^{\circ }(X)-\mathrm{\Delta }{H}_f^{\circ }(\mathrm{R}X)\end{array}}$

This equation tells us that the BDE for a given bond is equal to the energy of the individual components that make up the bond when they are free and unbonded minus the energy of the components when they are bonded together. These energies are given by the enthalpy of formation ΔH_fº of the components in each state.

The enthalpy of formation of a large number of atoms, free radicals, ions, clusters and compounds is available from the websites of NIST, NASA, CODATA, and IUPAC. Most authors use the BDE values at 298.15 K.

For example, the carbon–hydrogen bond energy in methane BE_(C–H) is the enthalpy change (∆H) of breaking one molecule of methane into a carbon atom and four hydrogen radicals, divided by four. The exact value for a certain pair of bonded elements varies somewhat depending on the specific molecule, so tabulated bond energies are generally averages from a number of selected typical chemical species containing that type of bond.

Bond energy versus bond-dissociation energy

Bond energy (BE) is the average of all bond-dissociation energies of a single type of bond in a given molecule. The bond-dissociation energies of several different bonds of the same type can vary even within a single molecule.

For example, a water molecule is composed of two O–H bonds bonded as H–O–H. The bond energy for H₂O is the average energy required to break each of the two O–H bonds in sequence:

Although the two bonds are the equivalent in the original symmetric molecule, the bond-dissociation energy of an oxygen–hydrogen bond varies slightly depending on whether or not there is another hydrogen atom bonded to the oxygen atom. Thus, the bond energy of a molecule of water is 461.5 kJ/mol (110.3 kcal/mol).

When the bond is broken, the bonding electron pair will split equally to the products. This process is called homolytic bond cleavage (homolytic cleavage; homolysis) and results in the formation of radicals.