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UPSC Combined Geo-Scientist

Chemist

  • 3 descriptive papers
  • 200 marks each
  • 3 h per paper

Mains Paper I covers inorganic chemistry.

UPSC Combined Geo-Scientist, Mains · 2024 · Mains Paper I

Q1(e). Spontaneity of Fe²⁺ oxidation

Question

Given that the standard electrode potentials for the reactions (i) 2Fe²⁺(aq) ⇌ 2Fe³⁺(aq) + 2e⁻ and (ii) H₂O₂ + 2H⁺(aq) + 2e⁻ ⇌ 2H₂O are −0.77 V and 1.77 V respectively, determine whether the oxidation of Fe²⁺ to Fe³⁺ is spontaneous or not.

Answer in brief

Yes, under standard conditions with H₂O₂ as oxidant: E°cell = +1.00 V and ΔG° = −193 kJ mol⁻¹ of reaction.

Key points

  • Equation (i) is written as an oxidation, so its supplied potential is −0.77 V. Do not reverse its sign a second time.
  • Add the half-reactions: 2Fe²⁺ + H₂O₂ + 2H⁺ → 2Fe³⁺ + 2H₂O.
  • E°cell = E°oxidation + E°reduction = −0.77 + 1.77 = +1.00 V. Electrode potentials are not multiplied by stoichiometric coefficients.
  • ΔG° = −nFE° = −2 × 96485 × 1.00 = −192970 J mol⁻¹ ≈ −193 kJ mol⁻¹.
  • A positive cell potential and negative ΔG° establish standard-state spontaneity; they do not specify the reaction rate.

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.

Model answer to Q1(e), 2024 Mains Paper I: solution pages 5 and 6
Solution pages 5 and 6 of 33

Coversstandard electrode potentialFe2+ oxidationH2O2 oxidantΔG = −nFEcell EMF spontaneity