Revision of the enthalpies and gibbs energies of formation of calcium oxide and magnesium oxide

Abstract

Major revisions in the enthalpies and Gibbs energies of formation of calcium oxide and magnesium oxide are presented. In free-evaporation experiments over the temperature range of 1830 to 2070 K, the kinetics of vaporization of calcium oxide and magnesium oxide have been studied. Conditions of local equilibrium were established. The results lead to new values for the standard enthalpies of formation of CaO and MgO. The ΔfH298 0 of CaO is ≈33 kJ/mol more positive than the currently tabulated value, whereas that for MgO is ≈34 kJ/mol more negative. The currently tabulated values could not account for these observations nor for other recent studies. These significant changes result in a reversal of the relative thermodynamic stabilities of CaO and MgO, with MgO now indicated as the more stable oxide at lower temperatures on Ellingham diagrams. These results are consistent with recent work on the formation of CaC2, solubility product measurements in liquid iron, and with theoretical models based on ionic radii.