Even though the thermochemical database need contain only AGo (or, equivalently, AHo and ASo), the number of reactions that would have to be included in such a compilation is intractably large. The key to reducing data requirements to manageable size is to provide the standard free energy changes of forming the individual molecular species from their constituent elements. Particular reactions are constructed from these so-called formation reactions.
For molecular compounds containing two or more elements, the basic information is the Free energy change for reactions by which the compound is created from its constituent elements, the latter in their normal state at the particular temperature. These reaction free energy changes are called standard free energies of formation of the compound. For example, the methane combustion reaction of Eq (9.1) involves one elemental compound (O2) and three molecular compounds (CH4, CO2, and H2O). The formation reactions for the latter and their associated free energy changes are:
No formation reaction is needed for the O2 reactant because this species, although molecular in form, is in its elemental state. The above formation reactions contain species not included in Eq (9.1), namely C(s) and H2(g). However, when the above reactions are combined algebraically to produce Eq (9.1) by the formula:
The extraneous species cancel out. Similarly, the free energies of formation are combined to produce the free energy change of Eq (9.1):
In this fashion, a universal chemical thermodynamic database can be constructed from formation free energies of those compounds that have been investigated experimentally.
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