Heats of Formation and Atomization

Heats of reaction may be obtained from molar enthalpies by using the equation  

Molar enthalpies are given by Eq. (22), which shows that, in principle, heats of reaction can be theoretically calculated at any temperature from 

Some years ago, Pople and co-workers (Hehre et al., 1970) introduced the interesting concept of isodesmic reaction. By definition, this is a reaction in which all the bonds are conserved (in number and nature). The corresponding theoretical heat of reaction is generally reliable due to approximate cancellation of correlation errors. Moreover, in that case, ZPE and thermal corrections are often negligible. Thus, we may write  

This allows us to use a semiempirical procedure for estimating the heat of formation of any chemical species. For that purpose, we introduce the experimental heats of formation of reference compounds in the following equation  

Furthermore, the heat of atomization of the species under consideration can be determined from the experimental heats of formation of gaseous atoms, using the equation  

---

Table 3. Heats of formation and atomization of gaseous germanium dihalides, and stabilities...

To illustrate this procedure, we estimate below the heats of formation and atomization of the ON radical. We choose the isodesmic reaction ... 

The calorimetric measurement of heats of formation and atomization energies is a classical discipline of chemistry and a number of compilations of such data exist. In Table 15.20, we have listed the experimental atomization energies for the molecules in Table 15.1. 

---