Synthesis, characterization and standard molar enthalpies of formation of two zinc borates: 2ZnO·2B2O3·3H2O and ZnB4O7

The s tandard mole enthalpy of formation is an essential feature of chemical compounds, which is critical for assessing the thermal effects of chemical reactions with their participation. In this paper, two pure phases of zinc borates, 2ZnO·2B 2 O 3 ·3H 2 O and ZnB 4 O 7 , have been synthesized by hydrothermal and boric acid flux methods, respectively. They were characterized by XRD, FT-IR and TGA. The molar enthalpies of solution of 2ZnO·2B 2 O 3 ·3H 2 O (s) and ZnB 4 O 7 (s) in 1 mol·dm −3 HCl (aq) were measured at T  = 298.15 K, respectively, by the use of an isothermal microcalorimeter. In accordance with Hess’ law, the standard molar enthalpies of formation of the two zinc borates 2ZnO·2B 2 O 3 ·3H 2 O (s) and ZnB 4 O 7 (s) were calculated to be −(4177.02 ± 3.3) kJ·mol −1 and −(2971.1 ± 3.3) kJ·mol −1 , respectively, by the use of standard molar enthalpies of formation for ZnO (s), H 3 BO 3 (s) and H 2 O (l) based on the two designed thermochemical cycles.