Selenium, heat capacity 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 selection of entropy and heat capacity data for trigonal selenium in the temperature range 298.15 to 494.2 K and for the liquid in the temperature range 494.2 to 1500 K. The thermodynamic properties of the metastable monoclinic phase and the supercooled liquid are assessed for use as auxiliary data. The major source of information is the review by Gaur, Shu, Mehta, and Wunderlich [81GAU/SHU] which has been combined with other information for phase transformations. 

Chapter III contains a table of selected thermodynamic data for individual compounds and complexes of selenium (Table III-l), a table of selected reaction data (Table III-2) for reactions concerning selenium species and a table containing selected thermal functions of the heat capacities of individual species of selenium (Table III-3). The selection of these data is discussed in Chapter V. 

The selection of enthalpies of formation for the gaseous species by reevaluating previous investigations using the data selected in the previous steps. This evaluation order allows the third law to be used with a consistent set of heat capacities and entropies in deriving enthalpies of formation. The third law has rarely been applied in selenium gas phase studies and it reduces the scatter between derived experimental enthalpy values from various investigations by 50 to 80%. 

No experimental values are available for the heat capacity of liquid selenium above 1000 K. For this work a constant value equal to the value at 1000 K was used in evaluations of data above 1000 K ... 

Monoclinic selenium is metastable and its enthalpy of formation and entropy are needed as auxiliary data for some evaluations. They were derived from a thermodynamic cycle involving the enthalpy and entropy of fusion at the melting point 413 K, the selected data for trigonal and liquid selenium, and the heat capacity of monoclinic selenium. The selected enthalpy of fusion is that in [81GAU/SHU] ... 

The major contribution to the error limits is the uncertainty in the molar mass of Se(g) due to the natural variation in the isotopic composition of selenium. The selected value of the heat capacity was calculated from the same data as the entropy yielding ... 

The enthalpy of formation of Seg(g) has been determined from mass spectro-metric investigations of selenium vapour using Knudsen cells. The measurements can be separated into the approximate temperature ranges 420 to 494 K for equilibria with solid selenium and 494 to 700 K for equilibria with liquid selenium. The various investigations have used different methods and auxiliary data for deriving enthalpies of formation at 298.15 K from the measurements. For the purpose of this review the measurements were recalculated (see Appendix A) using the adopted values for heat capacities and entropies of the gaseous species and condensed phases. The values of the enthalpies of formation of Seg(g) are summarised in Table V-21. 

The saturated vapour pressure of a-CdSe was measured in the temperature range 1016 to 1170 K using a gas flow method. Values for the entropy and the enthalpy of formation of a-CdSe at 298.15 were derived using the second law and an estimated heat capacity of a-CdSe. The experimental results were therefore re-evaluated by the review using both the second and the third law, the selected thermodynamic functions of selenium, the data for Cd in [89COX/WAG], the selected heat capacity of a-CdSe, and the selected entropy of a-CdSe in the case of the third law. The vaporisation was assumed to occur according to the reaction a-CdSe Cd(g) + The results were... 

The total vapour pressure of liquid selenium at equilibrium was measured by a static method in the temperature range 573 to 1167 K and given as logn, (p/bar) = 5.722 - 5468 T A third law evaluation of the enthalpy of formation of Sc2(g) at 298.15 K was made by this review using the vapour pressure at 1150 K and the selected data for the heat capacities and entropies of Se(l) and Sc2(g). A mole fraction of Se2(g),... 

Two compositions, approximately corresponding to the limits of the homogeneity range of Coi jSe(cr), were considered. The recalculation employed the selected properties of selenium, the data of cobalt in [98CHA], and the estimated mean heat capacity of CO] vSe(cr) in the temperature range 298.15 to 700 K. The calculated values are tabulated in Table A-91. 

The Gibbs energy of formation of PbSe(cr) was measured in the temperature ranges 490 to 600 K and 650 to 858 K using electrochemical cells. The determined enthalpies and entropies of reaction at the mean temperatures of the temperature intervals are given in Table A-95. The quantities were recalculated to 298.15 K by the review and are tabulated in Table A-96. In addition, the corresponding values of the absolute entropy of PbSe(cr) at 298.15 K were calculated. All calculations employed the selected heat capacity of PbSe(cr), the selected thermodynamic properties of selenium, and the data for Pb(cr, I) in [89COX/WAG]. 

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