We have seen how to construct standard reaction enthalpies by combining standard
enthalpies of formation. The question that now arises is whether we can construct
standard enthalpies of formation from a knowledge of the chemical constitution of the
species. The short answer is that there is no thermodynamically exact way of expressing
enthalpies of formation in terms of contributions from individual atoms and bonds.
In the past, approximate procedures based on mean bond enthalpies, ΔH(A–B), the
average enthalpy change associated with the breaking of a specific A–B bond,