Thermodynamics of Cr2O3, FeCr2O4, ZnCr2O4, and CoCr2O4

High-temperature heat capacity measurements were obtained for Cr2O3, FeCr2O4, ZnCr2O4, and CoCr2O4 using a differential scanning calorimeter. These data were combined with previously available, overlapping heat capacity data at temperatures up to 400 K and fitted to 5-parameter Maier-Kelley Cp(T) equations. Expressions for molar entropy were then derived by suitable integration of the Maier-Kelley equations in combination with recent S0(298) evaluations. Finally, a database of high-temperature equilibrium measurements on the formation of these oxides was constructed and critically evaluated. Gibbs free energies of Cr2O3, FeCr2O4, and CoCr2O4 were referenced by averaging the most reliable results at reference temperatures of (1100, 1400, and 1373) K, respectively, while Gibbs free energies for ZnCr2O4 were referenced to the results of Jacob [K.T. Jacob, Thermochim. Acta 15 (1976) 79-87] at T = 1100 K. Thermodynamic extrapolations from the high-temperature reference points to T = 298.15 K by application of the heat capacity correlations gave ΔfG0(298) = (-1049.96, -1339.40, -1428.35, and -1326.75) kJ . mol-1 for Cr2O3, FeCr2O4, ZnCr2O4, and CoCr2O4, respectively