Standard molar enthalpies of formation of sodium molybdates (Na 2Mo nO 3 n+1 with n = 1, 2, 3 or 4) at 298.15 K by solution calorimetry

Abstract

Four compounds, namely Na 2MoO 4, Na 2Mo 2O 7, Na 2Mo 3O 10 and Na 2Mo 4O 13, were synthesised from stoichiometric quantities of sodium carbonate and molybdenum trioxide using a pyrometallurgical technique. The products were characterised by chemical and X-ray diffraction analysis and were found to be single-phase compounds. The molar enthalpies of solution of the compounds were measured at 298.15 K using an isoperibol calorimeter. The molar enthalpies of solution at infinite dilution, Δ sol H α m (298.15 K) for Na 2MoO 4, Na 2Mo 2O 7, Na 2Mo 3O 10 and Na 2Mo 4O 13are −11.79 ± 0.51, −55.89 ± 0.69, −135.01 ± 0.71 and −178.94 ± 0.82 kJ mol −1, respectively. These data were used to calculate the standard molar enthalpies of formation at 298.15 K and the values obtained for the compounds are −1465.87 ± 0.98, −2245.02 ± 1.81, −2989.16 ± 2.61 and −3768.58 ± 3.45 kJ mol −1, respectively. The values agree closely with the previously reported values. The standard molar enthalpy of formation of Na 2Mo 3O 10 was determined for the first time.