SELECTED SELENIUM DATA

This chapter presents the chemical thermodynamic data set for selenium species which has been selected in this review. Table lll-l contains the reeommended thermodynamic data of the selenium species, Table III-2 the recommended thermodynamic data of chemical equilibrium reactions by which the selenium compounds and complexes are formed, and Table III-3 the temperature coefficients of the heat capacity data of Table lll-lwhere available (see Appendix E for additional selenium data, cf. Section 11.7). 

The species and reactions in the tables appear in standard order of arrangement. Table III-2 contains information only on those reaetions for whieh primary data selections are made in Chapter V of this review. These seleeted reaetion data are used, together with data for key selenium species and auxiliary data seleeted in this review, to derive the eorresponding formation data in Table lll-l. The uneertainties associated with values for key selenium species and the auxiliary data are in some cases substantial, leading to comparatively large uneertainties in the formation quantities derived in this manner. 

The values of A G° for many reactions are known more accurately than would be calculated directly from the uneertainties of the AfG° values in Table 111-1 and auxiliary data. The inelusion of a table for reaction data (Table 111-2) in this report allows the use of equilibrium constants with total uncertainties that are based directly on the experimental aeeuraeies. This is the main reason for including both Table 111-1 and Table I1I-2. 

The selected thermal functions of the heat eapacities, listed in Table 111-3, refer to the relation 

A detailed discussion of the selection procedure is presented in Chapter V. It may be noted that this chapter contains data on more species or compounds than are present in the tables of Chapter 111. The main reasons for this situation are the lack of information for a proper extrapolation of the primary data to standard conditions in some systems and lack of solid primary data in others. 

---

The total vapour pressure of liquid selenium at equilibrium was measured by a static method in the temperature range 673 to 958 K and given as log o (/ /bar) = 5.217 - 4991 T. A third law evaluation of the enthalpy of formation of Se2(g) at 298.15 was made by this review using the vapour pressure at 940 K and the selected data for the heat capacities and entropies of Se(l) and Se2(g). A mole fraction of Sc2(g), - sej(g) 0.63 was estimated from the selected selenium data. The enthalpy of formation derived... 

New density and total pressure measurements of selenium vapour are presented. A previous investigation by the same author, [67RAU], is re-evaluated and combined with the new data in an evaluation of the thermodynamic properties of the molecules Se2(g)-Seg(g). The enthalpies of formation and entropies were varied in a procedure to obtain a good fit to the observed densities. The procedure contains many parameters that can be varied and different sets may fit the experimental data almost equally well. The thermodynamic properties derived for the species Se2(g)-Seg(g) are too dependent on the choices made by the author in order to be considered by the review. However, the new measurements of the total vapour pressure in the temperature range 1073 to 1373 K were evaluated by the review for the thermodynamic properties of Sc2(g) using the third law and mole fractions of Se2(g) estimated from the selected selenium data. The enthalpy of formation derived was Af//°(Se2, g, 298.15 K) = (138.9 1.9) kJmoP. ... 

---