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 Mn 3 O 4 , Mn 2 O 3 , and MnO 2 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 -d O as the reference electrode. The cell can be presented asSince 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 a-Mn and gaseous diatomic oxygen in the temperature range from 875 to 980 K is obtained as:071T From 980 to 1300 K the Gibbs energy of formation of MnO from b-Mn and oxygen gas is given by:¼ À387850 þ 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 -d O 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.

An update on the thermodynamics of Ta2O5

“…Yittria-doped thoria (YDT) was used as the solid electrolyte in the electrochemical cell. It was prepared using the method reported elsewhere [30]. The measuring electrode was prepared by mixing Ta and Ta 2 O 5 in the molar ratio 2:1, compacting the mixture into pellets using a steel die at p = 100 MPa, and sintering at T = 1373 K in the stream of flowing pre-purified argon gas for $30 ks.…”

“…The larger the oxygen fugacity difference between the reference and measuring electrodes, the larger the error caused by electrode polarization [29]. Hence in this study (Mn + MnO), which has oxygen fugacity considerably lower than that of (Fe + ''FeO") [30], but higher than that of (Ta + Ta 2 O 5 ), has been selected as the reference electrode. Gibbs free energy of formation of Ta 2 O 5 has also been calculated by Hong and Lee [31] combining the measured activities of oxygen in (Ta + O) solid solution [32,33] with the solubility limits of oxygen in tantalum at different temperatures [14].…”

Thermodynamic Data for Mn3O4, Mn2O3 and MnO2

“…The alumina powder was dehydrated via vacuum treatment at 1200 K. The difference in the heat flux into the sample and the reference material was integrated during heating at a constant rate (2 K=min) over small temperature steps (10 K) with an isothermal dwell time of 15 min. The accuracy of the measured heat capacity is estimated to be 1% (2 ). (The standard error estimate is denoted as ).…”

“…Since auxiliary thermodynamic functions for nonstoichiometric Mn 1 x O are not available, the best option is to compute standard free energy change for the hypothetical reaction (12) Table 2. From the activity of Mn and oxygen potential at the oxygen-rich boundary, activity of MnO can be computed using accurately known value of Gibbs energy of formation of MnO [2] For the reaction,…”

“…An exhaustive compilation of all published data on phase diagram and thermodynamic properties is presented by Grundy et al They concluded that the Gibbs energy of formation of MnO is known only with large uncertainty (˙5 kJ mol 1 / and this leads to uncertainties in the Gibbs energies of formation of all other oxides since Gibbs energies of higher oxides are based on data for MnO. Subsequently, Gibbs energy of formation of MnO was measured accurately (˙0:25 kJ mol 1 / by Jacob et al [2], consistent with earlier measurements of Alcock and Zador [3]. In this communication, we combine the accurate data for MnO with new measurements on oxygen chemical potentials of three biphasic regions in the phase diagram of the system Mn-O and high-temperature heat capacity of Mn 3 O 4 to derive accurate data for Mn 3 O 4 , Mn 2 O 3 and MnO 2 , taking cognizance of other reliable measurements available in the literature including new low temperature heat capacity measurements.…”