Determination of standard molar enthalpies of formation for the two lead borates: Pb4B10O19·2.5H2O and Pb6B11O18(OH)9

Two pure hydrated lead borates, Pb4B10O19·2.5H2O and Pb6B11O18(OH)9, have been synthesized and characterized by XRD, FT-IR, DTA-TG techniques and chemical analysis. The molar enthalpies of solution of Pb4B10O19·2.5H2O and Pb6B11O18(OH)9 in 1 mol dm−3 HNO3(aq) were measured to be −51.44 ± 0.18 kJ mol−1 and −91.70 ± 0.19 kJ mol−1, respectively. With the incorporation of the previously determined enthalpies of solution of H3BO3(s) in 1 mol dm−3 HNO3(aq) and of PbO(s) in (HNO3 + H3BO3)(aq), together with the use of the standard molar enthalpies of formation for PbO(s), H3BO3(s) and H2O(l), the standard molar enthalpies of formation of −8231.4 ± 8.6 kJ mol−1 for Pb4B10O19·2.5H2O and −9967.5 ± 9.8 kJ mol−1 for Pb6B11O18·4.5H2O were obtained on the basis of the appropriate thermochemical cycles.