The standard enthalpy and entropy of formation of Rh₂O₃: a third-law optimization

Abstract

The standard Gibbs energy of formation of Rh₂O₃ at high temperature has been determined recently with high precision. The new data are significantly different from those given in thermodynamic compilations. Accurate values for enthalpy and entropy of formation at 298.15 K could not be evaluated from the new data, because reliable values for heat capacity of Rh₂O₃ were not available. In this article, a new measurement of the high temperature heat capacity of Rh₂O₃ using differential scanning calorimetry (DSC) is presented. The new values for heat capacity also differ significantly from those given in compilations. The information on heat capacity is coupled with standard Gibbs energy of formation to evaluate values for standard enthalpy and entropy of formation at 289.15 K using a multivariate analysis. The results suggest a major revision in thermodynamic data for Rh₂O₃. For example, it is recommended that the standard entropy of Rh₂O₃ at 298.15 K be changed from 106.27 J mol^-1 K^-1 given in the compilations of Barin and Knacke et al. to 75.69 J mol^-1 K^-1. The recommended revision in the standard enthalpy of formation is from -355.64 kJ mol^-1 to -405.53 kJ mol^-1.