PbSe cr

References to the heat capacity measurements of PbSe(cr) are found in Table V-37. 

Second law values of the enthalpy of formation calculated as discussed in Appendix A from the electrochemical measurements in [60FIN/WAG], [66SAD/SEM], and [73SHA/MIS] are tabulated in Table V-39 together with the results of direct synthesis calorimetry investigations made by Hajiev [70HAJ] and Boone and Kleppa [92BOO/KLE]. 

The Gibbs energy of formation is calculated from the selected enthalpy of formation and entropy of PbSe(cr), the selected entropy of selenium, and the entropy of lead in [89COX/WAG] to be  

In the aqueous phase, Umland and Wallmeier [80UML/WAL] studied the reduction of selenite at the mercury electrode in the presence of Pb by polarography, see Appendix A. The solubility product of PbSe(s) was obtained from the position of the half-wave potential of the second reduction step, HgSe(s) -i- Pb -i- 2e PbSe(s) -i-Hg(l), to be log,o (PbSe, s, 298.15 K) = -(33.6 2.0). Combined with CODATA for Pb and the selected value for Se p it corresponds to AfG° (PbSe, cr, 298.15 K) = -(87.4 11.8)kJ-mor.  

The value is not in full harmony with the value obtained from an extrapolation of high temperature data. Since the validity of the polarographic method has not been documented, the value of AfG° (PbSe, cr, 298.15 K) obtained from the extrapolation of the high temperature data has been selected. 

---

Figure V-15 The heat capacity of PbSe(cr) as a function of temperature.

Table V-39 Values of the enthalpy of formation of PbSe(cr) at 298.15 K determined using electrochemical cells ([60F1N/WAG], [66SAD/SEM], [73SHA/M1S]) and direct synthesis calorimetry ([70HAJ], [92BOO/KLE]).

Uy and Drowart [69UY/DRO] found PbSe(g) to be the major species in gaseous mixtures of lead and selenium using mass spectrometry. The vaporisation of PbSe(cr) occurs according to the reaction PbSe(cr) PbSe(g). 

Second and third law enthalpies of sublimation were evaluated by the review as discussed in Appendix A from the evaporation studies summarised in Table V-41. The selected value of the enthalpy of sublimation of PbSe(cr) is calculated as the weighted average of the second and third law mean values, (PbSe, cr,... 

K) = (228.0 + 4.2) kJ mol. The selected value of the enthalpy of formation of PbSe(g) is calculated from the selected value of the sublimation enthalpy and the enthalpy of formation of PbSe(cr) yielding ... 

Table V-41 Second and third law enthalpies of sublimation of PbSe(cr) at 298.15 K evaluated by the review as discussed in Appendix A.

The Gibbs energy of formation of PbSe(cr) was measured in the temperature range 473 to 573 K using an electrochemical cell. The reaction studied was... 

The evaporation of PbSe(cr) was studied in the temperature range 936 to 1016 K using the Knudsen weight loss technique. The original evaluation employed auxiliary data... 

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]. 

Table A-95 Enthalpies and entropies of formation of PbSe(cr) at the mean reaction temperatures.

Table A-96 Standard enthalpies and entropies of formation and the standard entropy of PbSe(cr). ...

The evaporation of PbSe(cr) was studied in the temperature range 835 to 1047 K using the torsion effusion technique. The original evaluation employed auxiliary data different from those used by the review and the vapour pressure expression was therefore reevaluated using the selected values of the entropies and heat capacities of PbSe(cr) and PbSe(g). The second law entropy of PbSe(g) and the second law enthalpy of sublimation of PbSe(cr) were calculated to be S°(PbSe, g, 298.15 K) = (260.6 + 7.0)... 

---