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.

Coversstandard electrode potentialFe2+ oxidationH2O2 oxidantΔG = −nFEcell EMF spontaneity
All parts of Question 1
- 1(a) Minerals and their compositions5 marks
- 1(b) Magnetic moment of titanium(III)5 marks
- 1(c) Choosing indicators5 marks
- 1(d) Silica by volatilisation gravimetry5 marks
- 1(e) Spontaneity of Fe²⁺ oxidation5 marks
- 1(f) Oxidising power and charge transfer5 marks
- 1(g) pH dependence of cyanide masking5 marks
- 1(h) Terminal and bridging carbonyl bands5 marks
- 1(i) Identifying radioactive decay5 marks
- 1(j) Why ordinary lanthanoid carbonyls are unstable5 marks