Bond dissociation energy

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In chemistry, bond dissociation energy, D₀, is one measure of the bond strength in a chemical bond. It is defined as the standard enthalpy change when a bond is cleaved by homolysis, with reactants and products of the homolysis reaction at 0K (absolute zero). For instance, the bond dissociation energy for one of the C-H bonds in ethane (C₂H₆) is defined by the process:

CH₃CH₂-H → CH₃CH₂ + H

D₀ = ΔH = 101.1 kcal/mol

The bond dissociation energy is sometimes also called the bond dissociation enthalpy (or bond enthalpy), but these terms are not strictly correct, as they refer to the above reaction enthalpy at standard conditions, and may differ from D₀ by more than 3 kcal/mol.

The bond dissociation energy is usually different from the bond energy, which is calculated from the sum of the bond dissociation energies of all bonds in a molecule. ^[1]

For example, an O-H bond of a water molecule (H-O-H) has 493.4 kJ mol^-1 of bond dissociation energy, and 424.4 kJ mol^-1 is needed to cleave the remaining O-H bond. The bond energy of the O-H bonds in water is 458.9 kJ mol^-1, which is the average of the values.

In the same way for removing successive hydrogen atoms from methane the bond dissociating energies are 104 kcal/mole for D(CH₃-H), 106 kcal/mole for D(CH₂-H), 106 kcal/mole for D(CH-H) and finally 81 kcal/mole for D(C-H). The bond energy which is average is 99 kcal/mole.

Note that following dissociation, if new bonds are formed at lower enthalpy, then there is a net loss of energy, and thus an overall exothermic process.

Heterolytic bond dissociation energy is involved in chemical bond breaking by heterolysis rather than homolysis.