In the Interactive Student Tutorial section of Chapter 7.1,
the H2 bond-formation movie illustrated how the H2 molecule
is lower in energy than the two separate hydrogen atoms. When one mole
of hydrogen atoms forms a bond, 436 kJ of energy is released. We can also
consider another point of view: 436 kJ of energy must be added per mole
of H2 molecules to split them apart into their component atoms.
The amount of energy required to break a chemical bond in an isolated molecule in the gas phaseand thus the amount of energy that is released when a bond formsis called the bond dissociation energy, abbreviated D. Bond dissociation energies are always positive because energy must always be supplied to break a bond. Every bond in every molecule has a unique bond dissociation energy, but bonds between pairs of the same atoms usually have similar dissociation energies. Average bond dissociation energies are listed in Table 7.1