Calorimetric Study of Nickel Molybdate: Heat Capacity, Enthalpy, and Gibbs Energy of Formation

Abstract

The thermodynamic properties of the α and β polymorphs of NiMoO4 were directly investigated by calorimetry. The standard entropies of formation, ΔfS°T, of α and β were determined from measuring the molar heat capacity, Cp,m, from near absolute zero (2 K) to high temperature (1380 K) by a relaxation method and differential scanning calorimetry. The standard enthalpies of formation, ΔfH°T, of α and β were determined by combining Cp,m with the standard enthalpy of formation, ΔfH°298, at 298 K obtained from drop solution calorimetry in molten sodium molybdate at 973 K. The standard Gibbs energies of formation, ΔfG°T, of α and β were determined from their ΔfS°T and ΔfH°T values. The ΔfG°T values indicate that the polymorphic transformation from α to β occurs at 1000 K, consistent with the observed phase transformation at 1000 K.