Energetics of Manganese Oxides

Abstract

ABSTRACTThe energetics of some manganese oxides : pyrolusite ΜnO2, bixbyite Μn2O3, hausmannite Μn3O4 and manganosite MnO were studied by transposed temperature drop calorimetry and high temperature drop solution calorimetry in molten 2PbOB2O3 at 977 K. The enthalpies of oxidation at 298 K in the Mn-O system, determined by appropriate thermodynamic cycles, are (in kJ/mol O2) : −441.4 ± 5.8 for the reaction 6 MnO + O2 -> 2 Mn3O4, −201.8 ± 8.7 for 4 Mn3O4 + O2 -> 6 Mn2O3 and −162.1 ± 7.2 for 2 Mn2O3 + O2 -> 4 ΜnO2 These values agree very well with previous data obtained by equilibrium ρθ2 measurements and indicate that direct calorimetrie measurements are well suited to obtain reliable enthalpy of formation data for oxides containing manganese in the 2+, 3+ and 4+ states. The phase diagram of the Mn-O system was calculated using these new values of enthalpies.