Thermodynamic Properties of (NH₄)₂Ca[B₄O₅(OH)₄]₂·8H₂O

Abstract

A pure hydrated calcium double borate, (NH₄)₂Ca[B₄O₅(OH)₄]₂·8H₂O, has been synthesized and identified by\nchemical analysis, elemental analysis, X-ray diffraction, infrared spectra, and thermogravimetric analysis. The\nmolar enthalpy of solution of (NH₄)₂Ca[B₄O₅(OH)₄]₂·8H₂O in 1 mol·dm^-3 HCl(aq) and of NH₄Cl(s) in (HCl +\nH₃BO₃ +CaO)(aq) were measured to be (104.50 ± 0.34) kJ·mol^-1 and (17.71 ± 0.30) kJ·mol^-1, respectively.\nFrom a combination of these results with measured enthalpies of solution of H₃BO₃(s) in 1 mol·dm^-3 HCl(aq)\nand of CaO(s) in (HCl + H₃BO₃) aqueous solution together with the standard molar enthalpies of formation of\nHCl(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\nalso estimated by a group contribution method. The standard molar free energy of formation (Δ_f<i>G</i>_m°) was estimated\nto be −8802.38 kJ·mol^-1. From these values, the standard molar entropy of formation (Δ_f<i>S</i>_m°) was estimated to\nbe −4225.8 J·mol^-1·K^-1. The standard molar entropy (<i>S</i>_m°) was estimated to be 809.3 J·mol^-1·K^-1.