Molar reaction entropy exchanged

In principle, the free energy change and in turn the equilibrium constant for such reactions can be calculated from conventional thermodynamic data (molar enthalpy, entropy, volume data) on the end-member isotopic species denoted in the reaction. This approach, however, is generally not practicable because of the paucity of thermodynamic data on isotopically pure end-members. Moreover, even if such data were widely available, the Gibbs free energy changes associated with most isotope exchange reactions... 

As shown by Helgeson et al. (1978), satisfactory estimates of standard state molar entropy for crystalline solids can be obtained through reversible exchange reactions involving the compound of interest and an isostructural solid (as for heat capacity, but with a volume correction). Consider the generalized exchange reaction... 

Table 3.6 Comparison of predictive capacities of various equations in estimating standard molar entropy T = 298.15 K, P = 1 bar). Column I = simple summation of standard molar entropies of constituent oxides. Column II = equation 3.86. Column III = equation 3.86 with procedure of Holland (1989). Column IV = equation 3.85. Values are in J/(mole X K). Lower part of table exchange reactions adopted with equation 3.85 (from Helgeson et al., 1978) and Sj finite differences for structural oxides (Holland, 1989). 

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