CONTROVERSY ON THE FREE-ENERGY OF FORMATION OF CAO-ADDITIONAL EVIDENCE IN SUPPORT OF THERMOCHEMICAL DATA

The standard free energies of formation of CaO derived from a variety of high-temperature equilibrium measurements made by seven groups of e xperimentalists are significantly different from those given in the st andard compilations of thermodynamic data. Indirect support for the va lidity 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 Delta G degrees = -68,050 -2.47 T(+/-100) J mol(-1 ) SrO + CaF2 --> SrF2 + CaO Delta G degrees = -35,010 + 6.39 T (+/-80) J mol(-1) The standard free energy changes associated with cell react ions agree with data in standard compilations within +/- 4 kJ mol(-1). The results of this study do not support recent suggestions for a maj or revision in thermodynamic data for CaO.