Se6 g

No experimental information is available about the entropy and heat capacity of Se6(g). The published values are based on estimates made by comparison with gaseous sulphur and other selenium molecules. 

The difference between values calculated from molecular data is small. Despite the fact that the average value calculated from the second law is slightly in favour of the larger entropy values calculated from molecular data, the entropy value in [84PUP/RUS] is selected since it is the result of the best documented molecular calculations, 

All values of the heat capacity of See(g) at 298.15 K are based on estimated quantities and are summarised in Table V-14. The heat capacity value attributed to [84PUP/RUS] was calculated by the review from the estimated molecular parameters reported in the paper. Similarly, the heat capacity expression C° ,(Se6, g, (298.15 -1500) K) = (132.511 -H 0.7288 X 10 T - 0.2731 x 10 6.36682 x 10 T ) 

J K -mor was obtained by fitting the expression to heat capacity values calculated from the molecular parameters reported in [84PUP/RUS], The difference between the heat capacity calculated from this expression and that calculated from the expression in [74M1L] amounts to 0.05 J K. mor in the temperature range 1000 to 1500 K and reaches a maximum of 0.7 J K mor at 298.15 K. The derived heat capacity expression was employed by the review in all recalculations and evaluations wherever the thermodynamic properties of Sc6(g) were required at other temperatures than the standard temperature. The heat capacity at 298.15 K calculated from the expression is C° , (Se6, g, 298.15 K) = (125.5 +2.0) J-K -mor. No value of the heat capacity is selected because all tentative values are based on estimated quantities only. 

The heat capacities calculated from the expressions in [74MIL] and [84PUP/RUS] differ by less than 0.6 J K -mol in the whole temperature range 298.15 to 1500 K. 

---

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.

Table V-13 Entropy of Se6(g) at 298.15 K as (1) calculated from estimated molecular constants and (2) evaluated using the second law.

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.

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... 

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 partial pressures of the species Sc5(g), Se6(g), and Se7(g) in equilibrium with trigonal selenium were determined for the temperature range 430 to 490 K using a mass spectrometer and a Knudsen cell. The results obtained at 473 K were recalculated to 298.15 K by this review using the second and third laws and the selected heat capacities and entropies of Se(trig), Se5(g), Sc6(g), and Se7(g). The results are summarised in Table A-111. 

---