Selenium heat capacity

The enthalpies of formation and atomization, the heat capacities, and the entropies of the selenium rings Se (n = 5-12), as derived from mass spectrometric measurements and statistical thermodynamics (8-10,12, 13, 57), are given in Table VI. 

Formation Enthalpies AH Atomization Enthalpies A//°t", Atomization Enthalpies Referred to the Number of Atoms AH°Jna, Heat Capacities Clb, and Entropies Sob of Gaseous Cyclic Selenium Molecules Se (n = 5-12)... 

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. 

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. 

The selection of heat capacities and entropies at 298.15 K for the gaseous species as well as expressions of the temperature dependence of the heat capacity for the same species. Experimental information for the proper calculation of these quantities from statistical mechanics are only available for the species Se(g) and Se2(g). For all the other selenium gas molecules, the published values are based on estimates made by comparison with gaseous sulphur and other selenium molecules. The selected values rely mostly on the work of Pupishev and Rusin [84PUP/RUS] who have made an elaborate analysis of the Se2(g)-Seg(g) and S2(g)-S8(g) molecules using the LCAO method. 

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%. 

The liquid selenium is easily supercooled and undergoes a glass transition at 303.4 K. The heat capacity expression derived by [8IGAU/SHU] for the liquid is based on the work in [73GR02] and [80SHU/GAU] in the temperature ranges 500 to 1000 K and 330 to 520 K, respectively. The expression is selected here,... 

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 ... 

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

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. 

No heat capacity measurements of liquid selenium dioxide are available. 

No heat capacity or heat content measurements have been made for p-SnSe below 796 K and consequently no value can be selected at 298.15K. [96FEU/MAJ] employed the heat capacity expression of a-SnSe to describe the heat capacity of P-SnSe in the thermochemical modelling of the tin-selenium system. 

The enthalpy of formation has been determined from vapour pressure and galvanic cell measurements. Each determination has been re-evaluated as discussed in Appendix A using the second and third laws, the selected heat capacity, the entropy of Ag2Se(cr), the selected properties of selenium, and the CODATA [89COX/WAG] values of silver. The results are summarised in Table V-62. 

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... 

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