K from mass spectrometric investigations

The third law evaluations made in this review reduce the scatter of the enthalpy values by more than 65%. The scatter of the values obtained from the second law is an effect of the small temperature ranges used in most of the studies. The second law value in [75HOA/REY] for the equilibrium with liquid selenium is not included in the average in Table V-15 because it stems from measurements which result in an enthalpy of fusion of selenium that is twice as large as the correct value. 

It was observed in the mass spectrometric studies that SesCg) and Sc4(g) were formed due to the fragmentation of Ses(g) and Se6(g) in the spectrometer and in most of the investigations Se3(g) and Se4(g) were omitted from evaluation because of this problem. However, in all studies the partial pressures of Ses(g) and Sesfg) were evaluated without compensating for fragmentation. The partial pressures of these species are therefore too low and the enthalpies of formation should be somewhat lower than the values calculated from the experimental data. As discussed in the previous sub-section 

No experimental information is available about the entropy and heat capacity of Se7(g). The published values are based on estimates made by comparison with gaseous sulphur and other selenium molecules. 

Second law entropies were calculated by the review as discussed in Appendix A from those investigations in which the temperature dependence could be measured for equilibria involving Se7(g). Such evaluations were not made in the original papers. The values in [66BER/CHU] were derived in two steps. First the entropy of Sefi(g) was determined from the equilibria Se6(g)/liquid and Se6(g)/solid and then the entropy of Se7(g) was evaluated from the reaction 7Sc6(g) 6Sc7(g). The first step resulted in two different values for Se6(g) and hence also in two separate entropies of Se7(g). The entropy values are summarised in Table V-16. 

The difference between the average of the second law values and those calculated from molecular data is not large, but the second law values are widely scattered due to the small temperature ranges of the measurements. The entropy value in [84PUP/RUS] is selected since it is the result of the best documented molecular calculations, 

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Table V-15 Re-evaluated second and third law values of the enthalpy of formation of Se6(g) at 298.15 K. from mass spectrometric investigations.

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