Calorimetric measurements of the low-temperature heat capacity, standard molar enthalpy of formation at 298.15 K, and high-temperature molar enthalpy increments relative to 298.15 K of tungsten disulfide (WS 2), and the thermodynamic properties to 1500 K

Abstract

Low-temperature (5 to 350 K) heat capacity, fluorine combustion, and high-temperature (350 to 1500 K) drop-calorimetric measurements have been performed on a pure synthetic specimen of tungsten disulfide, WS 2. The following molar thermodynamic quantities are reported at T o = 298.15 K: the standard enthalpy of formation, Δ f H m o( T o), −(240.8±3.1) kJ·mol −1; the heat capacity, C p,m o( T o), (63.82±0.32) J·K −1·mol −1; the standard entropy, S m o( T o), (67.78±0.34) J·K −1·mol −1; and the standard Gibbs energy of formation, Δ f G m o( T o), −(232.1±3.1) kJ·mol −1. The thermodynamic quantities have been calculated to 1500 K. Standard enthalpies of formation deduced from high-temperature equilibrium and e.m.f. studies in the literature are, in general, not in good agreement with one another or the present result. There have been no previous measurements of the low-temperature heat capacity, but the high-temperature enthalpy increments are in fair agreement with results published for WS 1.97. There is some indication of a γT ( i.e. conduction electrontribution to the heat capacity at low temperatures, but the evidence for this is not strong. The present thermodynamic quantities are consistent with geochemical field observations that molybdenite (MoS 2) and not tungstenite (WS 2) but tungstates and not molybdates are formed in hydrothermal deposits.