Controversy on the free energy of formation of CaO-additional evidence in support of thermochemical data

Abstract

The standard free energies of formation of CaO derived from a variety of high-temperature equilibrium measurements made by seven groups of experimentalists are significantly different from those given in the standard compilations of thermodynamic data. Indirect support for the validity of the compiled data comes from new solid-state electrochemical measurements using single-crystal CaF₂ and SrF₂ as electrolytes. The change in free energy for the following reactions are obtained: CaO + MgF₂ → MgO + CaF₂ ΔG° = -68,050 - 2.47 T(±100)J mol^-1 SrO + CaF₂→ SrF₂ + CaO ΔG° = -35,010 + 6.39 T(±80) J mol^-1 The standard free energy changes associated with cell reactions agree with data in standard compilations within ±4 kJ mol^-1. The results of this study do not support recent suggestions for a major revision in thermodynamic data for CaO.