THERMOCHEMISTRY OF DOUBLE CARBONATES IN THE K2CO3-CACO3 SYSTEM

Abstract

Enthalpies of formation, ΔHf, of the double carbonates K2Ca(CO3)2 (with two polymorphs, bütschliite and fairchildite) and K2Ca2(CO3)3 from the components K2CO3 and CaCO3 were determined at 298 K by drop solution calorimetry in molten 2PbO·B2O3 at 974 K. The enthalpies of formation were -38.7 ± 3.2 kJ/mol for bütschliite, -5.1 ± 3.3 kJ/mol for fairchildite and -7.2 ± 5.8 kJ/mol for K2Ca2(CO3)3. The entropy changes for the formation, ΔSf, were also evaluated for the double carbonates. Assuming ΔSf(bütschliite) = 0, because of its ordered structure, the entropy of formation of fairchildite is 40.9 ± 5.1 J/mol·K and of K2Ca2(CO3)3 is 39.4 ± 7.3 J/mol·K.