Determination of standard molar enthalpies of formation for the two lead borates: Pb 4B 10O 19·2.5H 2O and Pb 6B 11O 18(OH) 9

Abstract

Two pure hydrated lead borates, Pb 4B 10O 19·2.5H 2O and Pb 6B 11O 18(OH) 9, have been synthesized and characterized by XRD, FT-IR, DTA-TG techniques and chemical analysis. The molar enthalpies of solution of Pb 4B 10O 19·2.5H 2O and Pb 6B 11O 18(OH) 9 in 1 mol dm −3 HNO 3(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 H 3BO 3(s) in 1 mol dm −3 HNO 3(aq) and of PbO(s) in (HNO 3 + H 3BO 3)(aq), together with the use of the standard molar enthalpies of formation for PbO(s), H 3BO 3(s) and H 2O(l), the standard molar enthalpies of formation of −8231.4 ± 8.6 kJ mol −1 for Pb 4B 10O 19·2.5H 2O and −9967.5 ± 9.8 kJ mol −1 for Pb 6B 11O 18·4.5H 2O were obtained on the basis of the appropriate thermochemical cycles.