Standard Molar Enthalpies of Formation for the Two Hydrated Calcium Borates xCaO·5B2O3·yH2O (x = 2 and 4, y = 5 and 7)

Abstract

Two pure calcium borates of 2CaO·5B2O3·5H2O (I) and β-4CaO·5B2O3·7H2O (II) have been synthesized under hydrothermal condition and characterized by X-ray diffraction, infrared spectra, and thermogravimetric analysis as well as by chemical analysis. All the enthalpies of solution were measured at 298.15 K by using a heat conduction microcalorimeter. The molar enthalpies of solution of (I) and (II) in 2.00 cm3 of 1 mol·dm-3 HCl(aq) were measured to be −(60.30 ± 0.78) kJ·mol-1 and −(158.63 ± 0.72) kJ·mol-1, respectively. The molar enthalpies of solution of H3BO3(s) in 2.00 cm3 of 1 mol·dm-3 HCl(aq) and of CaO(s) in 2.00 cm3 of (HCl + H3BO3) aqueous solution were measured to be (21.84 ± 0.05) kJ·mol-1 and −(188.59 ± 0.42) kJ·mol-1, respectively. The enthalpies change of borates (I) and (II) formation from the reaction of reagents in the solid phase, −(98.3 ± 1.3) kJ·mol-1 and −(377.2 ± 1.9) kJ·mol-1 were calculated respectively on the basis of the appropriate thermochemical cycles. From these data and the stan...