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UPSC Civil Services (Mains)

Chemistry optional

  • 2 papers
  • 250 marks each
  • 3 h per paper
  • 5 of 8 questions, Q1 and Q5 compulsory

Paper I covers physical and inorganic chemistry.

UPSC Civil Services (Mains), Chemistry optional · 2026 · Paper I

Q1(g). Why the cell potential is measured at zero current

  • 5 marks
  • Answered
Question

Why is it necessary to measure the cell potential using potentiometer under zero-current conditions?

Answer in brief

Only at zero current does the cell work reversibly at fixed composition, so the measured value is the true EMF that obeys Δ G=-nFE. Any current lowers the reading by IR drop and polarisation.

Key points

  • Δ G=-nFE holds only for a reversible change. A cell is reversible only when the current is infinitesimally small, with its reaction poised but not running.
  • when a current I flows, the terminal voltage is V=E-IRᵢₙₜ, smaller than E.
  • current changes the ion concentrations at the electrode surfaces (concentration polarisation) and needs an activation overpotential; it also uses up reactants, so the reading falls and drifts.
  • the driver cell sets a uniform potential drop along wire AB. The contact D is moved until the galvanometer shows no deflection. The drop over AD then exactly opposes Eₓ and no current is drawn. Repeating with a standard…

The full model answer

Written out in full, as it should go in the answer booklet. The solution page with its diagrams follows it.

  1. Thermodynamics: Δ G=-nFE holds only for a reversible change. A cell is reversible only when the current is infinitesimally small, with its reaction poised but not running.
  2. Internal resistance: when a current I flows, the terminal voltage is V=E-IRᵢₙₜ, smaller than E.
  3. Polarisation: current changes the ion concentrations at the electrode surfaces (concentration polarisation) and needs an activation overpotential; it also uses up reactants, so the reading falls and drifts.
  4. Potentiometer: the driver cell sets a uniform potential drop along wire AB. The contact D is moved until the galvanometer shows no deflection. The drop over AD then exactly opposes Eₓ and no current is drawn. Repeating with a standard (Weston) cell gives Eₓ=Eₛ AD/AD'.
  5. An ordinary voltmeter draws current and reads low; a high-impedance digital voltmeter (pH meter) draws almost none.

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Model answer to Q1(g), 2026 Paper I: solution page 6
Solution page 6 of 55
Read the whole 2026 solution2026 question paper

All parts of Question 1