Se4 g

No experimental information is available about the entropy and heat capacity of Se4(g). All published values are based on estimates made by comparison with gaseous sulphur and other selenium molecules. 

The values in [70KEL] and [74MIL] seem to be based on the same set of molecular parameters although those in [74MIL] are undocumented. Although not decisive, the second law entropies are in favour of the lower of the values calculated from molecular data. The entropy value in [84PUP/RUS] is selected, 

All available heat capacity values of Se4(g) at 298.15 K are based on estimated quantities and are summarised in Table V-8. The heat capacity value attributed to [84PUP/RUS] was calculated by the review from the estimated molecular parameters reported in the paper. 

Se4(g) is a minor species in selenium vapour and the situation is even worse than for Se3(g) with respect to fragmentation problems. The evaluation of thermodynamic data for Se4(g) has therefore been avoided except in three studies, summarised in Table V-9, where the investigators thought the fragmentation was under control. The study of Keller [70KEL] is the only one in which the fragmentation has been monitored properly and the review selects from this study  

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Table V-7 Entropy of Se4(g) at 298.15 K as (1) calculated from estimated molecular constants and (2) evaluated using the second law.

Table V-11 Heat capacity values of Ses(g) at 298.15 K. The value attributed to [84PUP/RUS] was not explicitly stated in the paper and was therefore calculated as the mean of their values for Se4(g) and Se6(g) in agreement with their heat content calculations.

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... 

Enthalpies of formation at 0 K for the species Se2(g)-Seg(g) were derived using Gibbs energy functions calculated from estimated molecular parameters. In the present review, the experimental results presented in Tables 11 and 15 to 20 of the paper were re-evaluated using the second and third laws and the selected data for the heat capacities and entropies. The results are summarised in the Table A-78. No evaluation using the second law can be made for Se4(g) because the measurements were made at a single temperature only. 

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