Tabular Representation

The only type of thermochemical database that is free from the approximations ACP = 0 (Fig. 9.8) or ACP = constant (Table 9.3) are listings of ho, so, and go at a series of temperatures for all chemical species for which data is available, including the elements. The inclusion of go in the listing is for convenience in forming AGo from which equilibrium constants can be calculated. The basic thermochemical properties are ho and so, with go determined as ho - Tso. A typical tabular representation is given in Table 9.4, which has been excerpted from the most comprehensive of this kind (the tables for all species included in this database occupy two 3 % inch thick volumes). This compilation, augmented by data obtained since 1977, can be found in commercial software packages (e.g., TAPP, www.esn.software.com/tapp/).

Because of its age, the units of CP and so in Table 9.4 are cal/mole-K and ho and go are in kcal/mole. The only decision that had to be made was the reference temperature at which the molar enthalpy of the elements is specified (i.e., h^). The convention adopted is to set h^ equal to zero for the normal state (i. e., phase) of the elements at 25oC. The reference for the molar entropies of condensed phases has been chosen as zero at 0 K. The following example.

Table 9.3 Selected Standard Free energies of Formation

Reaction

A, J/mole

B, J/mole-K

C, J/mole-K Temp. range, K

4Ag(s)+S2(g)=2Ag2S(s,a)

-187,000

Solar Panel Basics

Solar Panel Basics

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