Electrical energy produced by a reversible electrochemical cell is given by the free energy decrease (–Δ G) of the reaction occurring in the cell. According to Gibbs- Helmholtz equation, decrease in free energy is given by –Δ G=–Δ H–T[δ(Δ G)δ T]P where –Δ H is the decrease in enthalpy of the cell reaction at constant pressure. EMF of the cell, E=−ΔHnF+T[δ Eδ T]P. By measuring the emf of the cell and its temperature co-efficient, thermodynamic quantities like Δ H,Δ G and Δ S can be determined. Standard emf of the cell is related to equilibrium constant of the cell reaction as E0=2.303RTlog knF.
The standard emf of the cell, ZnZn+2Fe+3 Fe+2(S)(aq)(aq) (aq) Pt at 25∘C is 1.53 V and at 50∘C is 1.55 V. The value of ΔS∘ for the overall reaction is