Gibbs free energy of formation of rhodium sulfides

• Gibbs energies of formation of RhS0.882, Rh3S4 and Rh2S3 are accurately measured.
• Employed a novel solid-state cell based on single crystal CaF2 as an electrolyte.
• Auxiliary electrodes of (CaS + CaF2) convert S2 potential into F2 potential.
• Measuring electrodes consists of two adjacent phases in the system (Rh + S).
• Evaluated S298.15Ko and ΔfH298.15Ko for three rhodium sulfides at T = 298.15 K.

Using a solid-state electrochemical technique, thermodynamic properties of three sulfide phases (RhS0.882, Rh3S4, Rh2S3) in the binary system (Rh + S) are measured as a function of temperature over the range from (925 to 1275) K. Single crystal CaF2 is used as the electrolyte. The auxiliary electrode consisting of (CaS + CaF2) is designed in such a way that the sulfur chemical potential converts into an equivalent fluorine potential at each electrode. The sulfur potentials at the measuring electrodes are established by the mixtures of (Rh + RhS0.882), (RhS0.882 + Rh3S4) and (Rh3S4 + Rh2S3) respectively. A gas mixture (H2 + H2S + Ar) of known composition fixes the sulfur potential at the reference electrode. A novel cell design with physical separation of rhodium sulfides in the measuring electrode from CaS in the auxiliary electrode is used to prevent interaction between the two sulfide phases. They equilibrate only via the gas phase in a hermetically sealed reference enclosure. Standard Gibbs energy changes for the following reactions are calculated from the electromotive force of three cells:2.2667Rh (s) + S2 (g) → 2.2667RhS0.882 (s), ΔrGo±2330/(J·mol-1)=-288690+146.18(T/K),4.44RhS0.882 (s) + S2 (g) → 1.48Rh3S4 (s),ΔrGo±2245/(J·mol-1)=-245596+164.31(T/K),4Rh3S4 (s) + S2 (g) → 6Rh2S3 (s),ΔrGo±2490/(J·mol-1)=-230957+160.03(T/K).Standard entropy and enthalpy of formation of rhodium sulfides from elements in their normal standard states at T = 298.15 K are evaluated.

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