Low-Temperature Heat Capacities and Thermodynamic Properties of Triaquabenzoatocalcium Monobenzoate [Ca(Ben)(H2O)3](Ben)(s) (Ben = Benzoate)

Abstract

Triaquabenzoatocalcium monobenzoate [Ca(Ben)(H2O)3](Ben)(s) was synthesized by the method of liquid phase reaction. Chemical and element analyses and X-ray crystallography were used to characterize the composition and structure of the complex. The lattice potential energy of the compound was calculated to be UPOT = 426.61 kJ·mol−1 on the basis of crystal structure information. Low-temperature heat capacities over the temperature range from (78 to 360) K were measured by an automated adiabatic calorimeter, and a polynomial equation was fitted by the least-squares method. On the basis of the fitted polynomial, the smoothed heat capacities and thermodynamic functions of the compound relative to the standard reference temperature 298.15 K were calculated at intervals of 5 K. The thermal decomposition of the complex was studied using TG and DSC analyses. In accordance with Hess’s law, a reasonable thermochemical cycle was designed based on the preparation reaction of the compound. The standard molar enthalpies of dissolution for the reactants and product of the reaction in the selected solvent were measured by an isoperibol solution-reaction calorimeter. The enthalpy change of the liquid phase reaction was calculated to be ΔrHmo = −(46.68 ± 0.40) kJ·mol−1, and the standard molar enthalpy of formation of the complex was determined to be ΔfHmo[Ca(Ben)(H2O)3](Ben),s = −(2088.2 ± 1.1) kJ·mol−1 by means of the enthalpy change of the reaction and other auxiliary thermodynamic quantities.