Bond dissociation energy (BDE or D0) is one measure of the strength of a chemical bond. It can be defined as the standard enthalpy change when a bond is cleaved by homolysis, with reactants and products of the homolysis reaction at 0 K (absolute zero). For instance, the bond-dissociation energy for one of the C–H bonds in ethane (C2H6) is defined by the process:
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CH3CH2–H → CH3CH2• + H• D0 = ΔH = 101.1 kcal/mol = 423.0 kJ/mol = 4.40 eV (per bondDefinitions and related parameters
The bond-dissociation energy is sometimes called the bond-dissociation enthalpy (or bond enthalpy), but these terms may not be strictly equivalent. Bond-dissociation enthalpy usually refers to the above reaction enthalpy at 298 K (standard conditions) rather than at 0 K, and differs from D0 by about 1.5 kcal/mol (6 kJ/mol) in the case of a bond to hydrogen in a large organic molecule. Nevertheless, the term bond-dissociation energy and the symbol D0 have been used for the reaction enthalpy at 298 K as well.
Bond energy
Except for diatomic molecules, the bond-dissociation energy differs from the bond energy. While the bond-dissociation energy is the energy of a single chemical bond, bond energy is the average of all the bond-dissociation energies of the bonds in a molecule.
For example, dissociation of HO–H bond of a water molecule (H2O) requires 493.4 kJ/mol. The dissociation of the remaining hydroxyl radical requires 424.4 kJ/mol. The bond energy of the covalent O–H bonds in water is said to be 458.9 kJ/mol, the average of these values.
In the same way for removing successive hydrogen atoms from methane the bond-dissociation energies are 104 kcal/mol (435 kJ/mol) for D(CH3–H), 106 kcal/mol (444 kJ/mol) for D(CH2–H), 106 kcal/mol (444 kJ/mol) for D(CH–H) and finally 81 kcal/mol (339 kJ/mol) for D(C–H). The bond energy is, thus, 99 kcal/mol or 414 kJ/mol (the average of the bond-dissociation energies). None of the individual bond-dissociation energies equals the bond energy of 99 kcal/mol.
Homolytic versus heterolytic dissociation
Bonds can be broken symmetrically or asymmetrically. The former is called homolysis and is the basis of the usual BDEs. Asymmetric scission of a bond is called heterolysis. For molecular hydrogen, the alternatives are:
H2 → 2 H• ΔH = 104 kcal/mol (see table below) H2 → H+ + H− ΔH = 66 kcal/mol (in water)The data tabulated above shows how bond strengths vary over the periodic table. There is great interest, especially in organic chemistry, concerning relative strengths of bonds within a given group of compounds.