The enthalpies of formation and high-temperature thermodynamic functions of As 4S 4 and As 2S 3

Abstract

The energies of combusion of β-As 4S 4 and vitreous As 2S 3 in fluorine were measured in a bomb calorimeter. High-temperature enthalpy increments were also determined by drop calorimetry. The standard enthalpies of formation ΔH f o(298.15 K), were found to be −(134.6±6.7) kJ mol −1 and −(69.6 ± 4.2) kJ mol −1 for β-As 4S 4 and vitreous As 2S 3, respectively. Based on the measured results and data from the literature, the thermodynamic properties of the minerals realgar, α-As 4S 4, and orpiment, As 2S 3(c) were derived. For realgar at 298.15 K, ΔH f o = −(138.1 ± 6.7) kJ mol −1, ΔS f o = −(22.3 ± 3.1) J K −1 mol −1, and ΔG f o = −(131.5 ± 6.8) kJ mol −1 are recommended and, for orpiment at 298.15 K, ΔH f o = −(91.6 ± 4.8) kJ mol −1, ΔS f o = −(3.1 ± 4.3) J K −1 mol −1, and ΔG f o = −(90.7 ± 5.0) kJ mol −1.