Empirical Values of Single-Bond Energies

It is probable that in general the postulate of the geometric mean leads to somewhat more satisfactory values for the energy of normal covalent bonds between unlike atoms than does the postulate of additivity. The postulate of the geometric mean is more difficult to apply than the postulate of additivity, however, since values of A can be obtained directly from heats of reaction, whereas knowledge of individual bond energies is needed for the calculation of values of A, and in the following sections of this chapter wre shall sometimes use the postulate of additivity. 

Empirical values of bond energies in diatomic molecules are given directly by the energies of dissociation into atoms, which may be determined by thermochemical or spectroscopic methods. In the case of a polyatomic molecule thermochemical data provide a value for the total energy of dissociation into atoms, that is, for the sum of the bond 

Values for single-bond energies, defined in this way, for many bonds can be found by this process—for the S—S bond from the Sg molecule (an eight-membered ring containing eight S—S bonds), for N—H, P—H, S—H, and so forth from NH, PHS, H S, and so forth. These values are given m Table 3-4. 

The methods used in obtaining the remaining values in the table are described in the following paragraphs. 

The thermochemical values used in this work have been taken for the most part from the compilation Selected Values of Chemical Thermo- 

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