Evaluation of the Molar Entropies

The lack of values of A S, or of tabulated data necessary for their calculation, would seem at first glance to curtail seriously the application of the third-law method. In reality, for most substances, the absolute values of the entropy for standard conditions (Ar gg) and the corresponding temperature corrections S — Aggg) can be found tabulated in many reference handbooks [4, 9-11]. Nevertheless, for some compounds, even quite common ones, for instance for low-volatility salt molecules in the gas phase, these data are lacking. In these cases, it is sometimes possible to evaluate the entropy from a comparison with the values of Ai.5 available for similar molecules of other metals. This approach was used, in particular, to evaluate the value of A S for the molecules of Li2S04, CaS04, and CUSO4 [12]. 

A more general approach for evaluation of the molar entropy, A S /v, was proposed in [5]. In place of the real values of At-S /p for 20 different reactants their averaged value (148 17 J moP K ) was used. This results only in a slight increase (by 2-3%) of the discrepancy between the final results of the determination of A H and the expected figures. This approach was subsequently refined in [6]. An analysis of the values of A S /p performed for 50 different compounds revealed substantial differences between reactants decomposing with the formation of free metal atoms and those undergoing decomposition to metals bound in molecules. As evident from Tables 4.1 and 

The potential inherent in this approach is far from being exhausted, so that, in particular, in view of the noticeable differences in the entropy of atomic vapours among some metals, one could propose corrections which would further reduce the scatter of the AfS /z/ values from their mean figures. 

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Constancy of the Tapp/E ratio 1975 [2] Evaluation of the molar entropy 1978 [4]... 

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