Standard Absolute Entropy, S°298 , Values from Volume or Density. 1. Inorganic Materials

Standard absolute entropies of many inorganic materials are unknown; this precludes a full understanding of their thermodynamic stabilities. It is shown here that formula unit volume, Vm, can be employed for the general estimation of standard entropy, S°298, values for inorganic materials of varying stoichiometry (including minerals), through a simple linear correlation between entropy and molar volume. Vm can be obtained from a number of possible sources, or alternatively density, (rho), may be used as the source of data. The approach can also be extended to estimate entropies for hypothesized materials. The regression lines pass close to the origin, with the following formulas: For inorganic ionic salts, S°298/J K^-1 mol^-1 = 1360 (Vm/nm^3 formula unit^-1) + 15 or = 2.258 [M/((rho)/g cm^-3)] + 15. For ionic hydrates, S°298/J K^-1 mol^-1 = 1579 (Vm/nm^3 formula unit^-1) + 6 or = 2.621 [M/((rho)/g cm^-3)] + 6. For minerals, S°298/J K^-1 mol^-1 = 1262 (Vm/nm^3 formula unit^-1) + 13 or = 2.095 [M/((rho)/g cm^-3)] + 13. Coupled with our published procedures, which relate volume to other thermodynamic properties via lattice energy, the correlation reported here complements our development of a predictive approach to thermodynamics and ultimately permits the estimation of Gibbs energy data. Our procedures are simple, robust, and reliable and can be used by specialists and nonspecialists alike.