UPSC Combined Geo-Scientist, Mains · 2020 · Mains Paper II
Q1(j). Open-circuit emf and equilibrium
Question
A potentiometer measures a galvanic cell at zero current, yet -ΔG=nFE with E>0, whereas equilibrium at fixed T,p requires ΔG=0. Explain.
Answer in brief
Zero current in an open circuit is constrained electrode equilibrium, not complete chemical equilibrium of the overall cell reaction.
Key points
- A high-resistance or null measurement prevents net charge transfer. Each electrode interface can be locally reversible while the separated reactants retain a chemical driving force.
- The emf is the reversible potential difference that would drive reaction if the external circuit allowed charge to pass.
- At true overall chemical equilibrium, the reaction quotient equals K, ΔᵣG=0 and the reversible emf for that cell reaction is zero.
The full model answer
As written for the answer booklet: the diagram carries the working, the lines stay short. Read here only; it is not offered as a download.
CoversOpen-circuit emfequilibriumElectrochemistry
All parts of Question 1
- 1(a) Critical-point inflection5 marks
- 1(b) Boyle and inversion temperatures5 marks
- 1(c) Accuracy at high diffraction angle5 marks
- 1(d) Composition change and non-expansion work5 marks
- 1(e) Van't Hoff plots5 marks
- 1(f) Potassium sulfate activities5 marks
- 1(g) Reaction order and rate constant5 marks
- 1(h) Catalytic rate enhancement5 marks
- 1(i) Electrochemical potential of zinc ions5 marks
- 1(j) Open-circuit emf and equilibrium5 marks
- 1(k) Hermitian operators5 marks
- 1(l) Probabilities of quantum states5 marks
- 1(m) Hydrogen rotational spectra5 marks
- 1(n) Wavelength and the einstein5 marks
- 1(o) Uranyl-sensitised decomposition5 marks
- 1(p) HI photolysis yield5 marks