In chemistry, bond dissociation is a process in which a specific bond is cleaved by homolysis.
Bond dissociation is an endothermic process, and energy that is necessary to the process is called bond dissociation energy. 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.
Strictly, the bond dissociation energy is defined as the standard reaction enthalpy of a bond fission process at 0 K (absolute zero). This standard reaction enthalpy is also called the bond dissociation enthalpy (or bond enthalpy).
Heterolytic bond dissociation energy is involved in chemical bond breaking by heterolysis rather than homolysis.
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.
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.
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