Heat capacities of molten salts with polyatomic anions

Abstract

The molar heat capacities at constant pressure, C P , of molten salts with polyatomic anions, obtained from the literature, are examined. As a rule, the C P values are independent of the temperature T, but the molar heat capacities at constant volume, C V , derived from them, depend on T. The latter were obtained, as far as the required density, expansibility and compressibility data are available, for 1.1 T m, presumed to be the corresponding state, T m being the melting temperature. Their ratio γ = C P / C V is linear with the cation–anion distance in the molten salt, d C–A. The communal, quasi-lattice, heat capacity Δ C P = C P − C P (i.g.) is obtained by subtraction of the sum of published ideal gas heat capacities of the constituent ions at 1.1 T m, C P (i.g.). This communal heat capacity Δ C P is proportional to the packing fraction of the ions in the melt, y = π N A νd C −A 3/6V. Here N A is Avogadro's number, ν the number of ions per formula unit, and V the molar volume at 1.1 T m. Some models for the heat capacities of molten salts are shown not to be well applicable to the set of salts discussed here, but no alternative could be suggested.