Gibbs energy of formation of MnO: measurement and assessment

Abstract

Based on the measurements of Alcock and Zador, Grundy et al. estimated an uncertainty of the order of ±5 kJ mol⁻¹ for the standard Gibbs energy of formation of MnO in a recent assessment. Since the evaluation of thermodynamic data for the higher oxides Mn₃O₄, Mn₂O₃, and MnO₂ depends on values for MnO, a redetermination of its Gibbs energy of formation was undertaken in the temperature range from 875 to 1300 K using a solid-state electrochemical cell incorporating yttria-doped thoria (YDT) as the solid electrolyte and Fe + Fe_1−δO as the reference electrode. The cell can be presented as Pt, Mn+MnO/YDT/Fe+Fe_1−δO,pt Since the metals Fe and Mn undergo phase transitions in the temperature range of measurement, the reversible emf of the cell is represented by the three linear segments. Combining the emf with the oxygen potential for the reference electrode, the standard Gibbs energy of formation of MnO from α-Mn and gaseous diatomic oxygen in the temperature range from 875 to 980 K is obtained as: ΔG°_f/Jmol⁻¹(±250)=−385624+73.071T From 980 to 1300 K the Gibbs energy of formation of MnO from β-Mn and oxygen gas is given by: ΔG°_f/Jmol⁻¹(±250)=−387850+75.36T The new data are in excellent agreement with the earlier measurements of Alcock and Zador. Grundy et al. incorrectly analyzed the data of Alcock and Zador showing relatively large difference (±5 kJ mol⁻¹) in Gibbs energies of MnO from their two cells with Fe + Fe_1−δO and Ni+NiO as reference electrodes. Thermodynamic data for MnO is reassessed in the light of the new measurements. A table of refined thermodynamic data for MnO from 298.15 to 2000 K is presented.