the work required to break up a molecule into two parts (atoms, groups of atoms) and separate them an infinite distance from each other. For example, when considering the bond energy of H3C—H in a methane molecule, the two parts are the methyl group CH3 and the hydrogen atom H; when considering the bond energy of H—H in a hydrogen molecule, each hydrogen atom constitutes a part.
The bond energy is a specific case of binding energy and is generally expressed in kilojoules per mole (kJ/mole) or kilocalories per mole (kcal/mole). The value of the bond energy ranges from 8–10 to 1,000 kJ/mole, depending on the particles forming the chemical bond, the nature of the interaction between the particles (such as a covalent bond or hydrogen bond), and the multiplicity of the bond (for example, a double or triple bond). For molecules containing two or more identical bonds, the bond energy of each bond (bond dissociation energy) is distinguished from the mean bond energy, which is equal to the averaged value of the dissociation energy of the bonds. For example, the dissociation energy of the HO—H bond in a molecule of water, that is, the heat effect reaction H2O = HO + H, is equal to 495 kJ/mole; the dissociation energy of the H—O bond in a hydroxyl group is equal to 435 kJ/mole, and the mean bond energy is thus equal to 465 kJ/mole. The difference between the values of the bond dissociation energy and the mean bond energy is due to the fact that, during partial molecular dissociation (the breaking of one bond), a change occurs in the electron configuration and in the mutual arrangement of the remaining atoms in the molecule, thereby altering their interaction energy. The value of the bond energy depends on the initial molecular energy, which is sometimes referred to as the temperature dependence of bond energy. The bond energy is usually considered for cases where the molecules are in the standard state or at 0°K. These are the values of the bond energy commonly given in handbooks.
The bond energy is an important characteristic that determines the reactivity of a substance and is used in the thermodynamic and kinetic calculations of chemical reactions. It may be determined indirectly from given calorimetric measurements; it may also be determined by direct calculations and by means of mass spectroscopy and spectral analysis.
REFERENCESEnergiia razryva khimicheskikh sviazei: Potentsialy ionizatsii i srodstvo k elektronu. Moscow, 1974.
Kireev, V. A. Kurs fizicheskoi khimii, 3rd ed. Moscow, 1975.