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Question

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 HT[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 25C is 1.53 V and at 50C is 1.55 V. The value of ΔS for the overall reaction is

A
154.4JK
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B
154.4JK
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C
154.4KJ K
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D
77.2 JK
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Solution

The correct option is A 154.4JK
[δEδT]P=0.0225VK1
ΔS for the reaction =nF[δEδT]P=2×96500×0.0225
=154.4JK1

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