Properties of Solids at Elevated T and

Thus far all equations have been derived for conditions of elevated temperature but with the pressure fixed at Pr, normally one bttr. For conditions of elevated pressure as well as temperature, an additional term must be added. Equation (7.21) thus becomes, for compound i, 

However, while the temperature integral can be evaluated using A/a, A/6j, and A/Cj for the reaction forming i from its elements, resulting in equation (7.25), the pressure integral (which involves both i and its elements) can only be conveniently evaluated for solids. Thus for compounds in which some of the elements are gases at one bar, the evaluation of the pressure integral in (7.28) becomes a decided nuisance, if not impossible, and in fact it is never used. Even for those cases where the reference elements are carried up to T from 298 K, they are left at one bar, and only the compound 

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