Additive single atom values for thermodynamics I: Volumes, entropies, heat capacities of ionic solids

Abstract

In an earlier simple “group contribution” method, molar volumes of organic and inorganic materials were predicted by summing optimised single atom values weighted according to the molecular formula. We here first revisit this procedure for volumes and then apply the method to entropies and heat capacities, providing updated predictive methods. The atom sum method has the unique advantage of working with an essentially complete parameter set because of the finite number of chemical elements but at the expense of omitting the nuance of special interactions as in other more sophisticated and complex group contribution methods. Thus, it does not distinguish among materials with the same chemical formula (that is, among phases or isomers). We here analyse data for nearly 3 500 inorganic materials, both anhydrous and hydrated. On analysing this wealth of data, we note that the optimised atom sum volume data follow the atomic sequence pattern of element volume data quite closely, but with relatively reduced values for the alkali metals and alkaline earths. The entropy atom sum values are similarly dispersed across the atomic sequence. Heat capacity atom sum values have a much reduced range, corresponding to the relatively small range of the ambient heat capacities of inorganic solids as implied by the Dulong-Petit upper limit of 3R per atom. We provide estimates of linear temperature effects for each of the three properties: volume, entropy and heat capacity. The properties of water in hydrates versus pure liquid water are compared for each thermodynamic property yielding information on the effects of incorporation of water into the solid. In summary, summations encompassing inorganic anhydrate and hydrate volumes, entropies and heat capacities over temperature ranges are available from this work, enabling simple first-order thermodynamic predictions and checks.