High-temperature reaction calorimetry of solid and liquid phases in the quasi-binary system Ag 2S-Sb 2S 3

Abstract

High temperature reaction calorimetry has been applied in a thermochemical study of solid and liquid phases in the quasi-binary system Ag 2S-Sb 2S 3. Direct synthesis calorimetry of stibnite, Sb 2S 3, at 880 K and 768 K, from the elements yields a mean value for the standard enthalpy of formation, ΔH 0 ƒ, stibnite, 298 K = −129.72 ± 3.75 kJ mol −1 ; this value is about 12 kJ mol −1 less exothermic than a previously reported value obtained by fluorine combustion calorimetry. The enthalpies of formation at 298 K of the sulfosalts α-miargyrite (AgSbS 2) and hexagonal pyrargyrite (Ag 3SbS 3) from their endmember component sulfides (Ag 2S and Sb 2S 3) were determined to be −4.1 ± 2.3 kJ mol −1 and −7.8 ± 0.8 kJ mol −1, respectively. The enthalpy of transformation of ordered monoclinic α-miargyrite to disordered cubic β-miargyrite at 298 K was determined to be +8.7 ± 1.4 kJ mol −1. If the equilibrium temperature for this transformation is equal to 650 K, this implies an entropy of transformation of about +13.3 J/K mol. Liquid mixtures of solid Ag 2S and liquid Sb 2S 3 at 880 K were studied by solid-liquid reaction calorimetry, and by dilution calorimetry, with pure liquid Sb 2S 3. Measurements of the solid-liquid enthalpies of mixing yielded the following linear expression ΔH S-L mix X Ag 2S = −6.35 + 13.33X Ag 2S ( kJ mol −1) ), based on experiments with values of X ag 2S ranging from 0.1 to 0.86. This equation is consistent with a symmetrical, regular solution model and implies that the heat of fusion of Ag 2S at 880 K (for X Ag 2 S = 1.00) is 6.98 kJ mol −1. Our data yield an average value for ΔCp s- l , Ag 2 S of about 4 joules K −1 mol −1. The approximate partial enthalpies of solution of liquid Sb 2S 3, obtained from the dilution experiments, are consistent with the expression Δ H ̄ Sb 2S 3 ≈ −13.33(X Ag 2S) 2 kJ mol −1 .