## Analytic Representation

Some databases give free energies of formation by equations of the type:

where A, B and C are constants. The second term on the right hand side originates in the difference in the heat capacities between products and reactants. The effect of phase changes is not part of Eq (9.57); these are accounted for by restricting the coefficients to temperature intervals where all species maintain the same phase. Neglecting phase changes, substituting Eqs (9.8) and (9.12) into Eq (9.53) gives Eq (9.57), with the coefficients identified as:

Note that if ACP is assumed to be zero, the coefficients A and C reduce to the constant standard enthalpy and entropy changes, B vanishes, and Eq (9.58) reduces to Eq (9.53) with constant coefficients.

Table 9.3 gives an abbreviated example of a compilation based on Eq (9.57). Note that many of the reactions give no value for B. These are reactions for which the experimental data is not sufficiently accurate to detect curvature in the AGo Vs T plot.

Other representations similar to Table 9.3 are available. The JANAF (for Joint Army, Navy, Air Force) Thermochemical Tables contain analytic equations for the free energies of formation of thousands of compounds. This compilation can be accessed on the internet at WWW.ipt.arc.nasa.gov/database2.html

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