Thermodynamic properties of manganese oxides

Transposed temperature drop calorimetry and high-temperature drop solution calorimetry in molten 2PbO· B₂O₃ at 977 K were used to study the energetics of some manganese oxides, namely pyrolusite (MnO₂), bixbyite (Mn₂O₃), hausmannite (Mn₃O₄), and manganosite (MnO). The enthalpies of oxidation at 298 K in the manganese-oxygen system, which were determined by appropriate thermodynamic cycles, were (in kJ/mol of oxygen): -441.4 ± 5.8 for the reaction 6MnO + O₂ → 2Mn₃O₄, -201.8 ± 8.7 for the reaction 4Mn₃O₄ + O₂ → 6Mn₂O₃, and -162.1 + 7.2 for the reaction 2Mn₂O₃ + O2 → 4MnO₂. These values agreed very well with previous data that were obtained using equilibrium measurements that were reported in the literature. Thus, direct calorimetric measurements were well suited to obtain reliable enthalpy of formation data for oxides that contain manganese in the 2+, 3+, and 4+ states. Using these new values of enthalpies and reliable standard entropies, the phase-stability diagram of the manganese-oxygen system was constructed.