DETERMINATION OF STANDARD GIBBS ENERGIES OF FORMATION OF MGO, SRO, AND BAO

The standard Gibbs energy of formation of MgO was determined from the measurement of the DELTAG-degrees for the reaction Mg (l) + O(in Nb) = MgO; by equilibrating Mg and Nb with a magnesia crucible and is expressed as follows: Mg (l) + 1/2O2 (g) = MgO (s) DELTAG-degrees = -657,000(+/-5000) + 141(+/-13)T[J/mol] (973 to 1323 K) The standard Gibbs energies of formation of SrO and BaO were determined by equilibrating silver and MO(M: Sr, and Ba) in a graphite crucible on the basis of the reaction M(in Ag) + CO (g) = MO (s) + C (s), yielding the following results: Sr (l) + 1/2O2 (g) = SrO (S) DELTAG-degrees = -645,000(+/-3000) + 133(+/-7)T[J/mol] (1373 to 1773 K) Ba (l) + 1/2O2 (g) = BaO (s) DELTAG-degrees = -610,000(+/-2000) + 137(+/-6)T[J/mol] (1373 to 1673 K) The present results for SrO and BaO confirm the JANAF values. The present results for MgO suggest that there are some errors in the JANAF table or in the DELTAG-degrees for dissolution of oxygen into niobium.