Abstract
A pure hydrated calcium double borate, (NH4)2Ca[B4O5(OH)4]2·8H2O, has been synthesized and identified by chemical analysis, elemental analysis, X-ray diffraction, infrared spectra, and thermogravimetric analysis. The molar enthalpy of solution of (NH4)2Ca[B4O5(OH)4]2·8H2O in 1 mol·dm-3 HCl(aq) and of NH4Cl(s) in (HCl + H3BO3 +CaO)(aq) were measured to be (104.50 ± 0.34) kJ·mol-1 and (17.71 ± 0.30) kJ·mol-1, respectively. From a combination of these results with measured enthalpies of solution of H3BO3(s) in 1 mol·dm-3 HCl(aq) and of CaO(s) in (HCl + H3BO3) aqueous solution together with the standard molar enthalpies of formation of HCl(aq), NH4Cl(s), CaO(s), H3BO3(s), and H2O(l), the standard molar enthalpy of formation of (NH4)2Ca[B4O5(OH)4]2·8H2O was found to be −(10062.3 ± 6.5) kJ·mol-1. Thermodynamic properties of the compound were also estimated by a group contribution method. The standard molar free energy of formation (ΔfGm°) was estimated to be −8802.38 kJ·mol-1. From these values, the standard molar entropy of formation (ΔfSm°) was estimated to be −4225.8 J·mol-1·K-1. The standard molar entropy (Sm°) was estimated to be 809.3 J·mol-1·K-1.
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