Gibbs free energy of formation of calcium rhodite

The Gibbs free energy of formation of CaRh2O4(s) has been determined using two techniques viz., quadrupole mass spectrometer coupled to a Knudsen cell and solid-state cell incorporating CaF2(s) as the solid electrolyte. In the former method, equilibrium O2(g) pressures were measured over the phase field Rh(s)+Rh2O3(s), in the temperature range 793.7–909.1 K and over the three phase mixture CaRh2O4(s)+Rh(s)+CaO(s) was measured from 862.1 to 1022.7 K. The Gibbs free energy of formation of Rh2O3(s) from elements in their standard state can be given by ΔfG°(Rh2O3(s)) (kJ mol−1±2.0)=−363.2+0.241T (K). The Gibbs free energy of formation of CaRh2O4(s) from elements in their standard state can be given by ΔfG°(CaRh2O4(s)) (kJ mol−1±2.0)=−1030.5+0.3437T (K). In the electrochemical technique, the cell configuration employed was (−)Pt/O2(g),{CaO(s)+CaF2(s)}//CaF2//{CaRh2O4(s)+Rh2O3(s)+CaF2(s)},O2(g)/Pt(+). The emf values were measured in the temperature range 879.7–1000 K can be represented by the following expression: E (V)(±7.63×10−4)=0.3928−2.374×10−4T (K). From the measured emf of the cell and requisite ΔfG° values from the literature, ΔfG°(CaRh2O4(s)) from elements in their standard state has been calculated and can be represented by ΔfG°(CaRh2O4(s)) (kJ mol−1±2.0)=−1079+0.390T (K). The uncertainty estimates for ΔfG° include the standard deviation in the emf and uncertainty in the data taken from the literature. The slope and intercept of the above equation gives the entropy and enthalpy of formation of the compound at the average experimental temperature Tav=940 K.