Se5 g

No experimental information is available on the entropy and heat capacity of Se5(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. The second law entropies are in favour of the higher of the values calculated from molecular data and the selected entropy value is that in [84PUP/RUS], 

The reported values of the heat capacity of Ses(g) at 298.15 K. are summarised in Table V-11. The heat capacity of ScsCg) assigned to [84PUP/RUS] was calculated as 

02604 X 10 T ) J-K -mol was derived by the review from the mean of the heat capacity expressions for Se4(g) and Sc6(g) (cf Sections V.1.7 and V.1.9). The heat capacity calculated from this expression and that in [74M1L] differ by less than 0.1 J K mor in the temperature range 750 to 1500 K and amounts to a maximum of 0.45 J K mor at 298.15 K. The expression was employed in recalculations and evaluations wherever the thermodynamic properties of Ses(g) were required at other temperatures than the standard temperature. The heat capacity at 298.15 K calculated from the expression is C° ,(Se5, g, 298.15 K) = (100.8 3.0) J-K -mor. No value of the heat capacity is selected because all tentative values are based on estimated quantities only. 

The third law evaluations made in this review reduce the scatter of the enthalpy values by more than 50%. 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-12 because it stems from measurements which result in an enthalpy of fusion of selenium that is twice as large as the correct value. 

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In the present review, the enthalpies of formation at 625 K in Table III of the paper were recalculated to 298.15 K using the selected heat capacities of Se(cr, I), Se2(g), and Se5(g)-Ses(g) to provide for a comparison with other investigations. Similarly, third law enthalpies of formation and second law entropies were evaluated by combining the data of Table 111 in the paper and the total pressure measurements by the Knudsen torsion-effusion method in the same paper with the selected values for the heat capacities and entropies. The results are summarised in the Table A-70. 

The vapour pressures of the species Se2(g), and Se5(g)-Seg(g) in equilibrium with selenium were determined for the temperature range 429 to 575 K using a mass spectrometer and a Knudsen cell. The data were divided into two sets, one for the equilibrium with solid selenium and one for the equilibrium with liquid selenium. In the evaluation of the experimental data in Figure 3 of the paper, the straight lines should intersect at the melting point of selenium. However, the intersection is at 479 K and not at the true... 

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. 

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