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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 · 2020 · Paper I

Q8(a). Electrode reactions and Nernst equation of a Zn | calomel cell

  • 10 marks
  • Answered
Question

Write down the respective electrode reactions, cell reaction and Nernst equation for cell EMF of the following galvanic cell:
Zn(s) | ZnCl₂(aq) | Hg₂Cl₂(s) | Hg(l)(Pt)

Answer in brief

Left (anode): Zn → Zn²+ + 2e⁻. Right (cathode): Hg₂Cl₂ + 2e⁻ → 2Hg + 2Cl⁻. Cell: Zn(s) + Hg₂Cl₂(s) → Zn²+(aq) + 2Cl-(aq) + 2Hg(l), with E=E°-(RT)/(2F)ln(a_(Zn²+) a_(Cl⁻)²).

Key points

  • Zn(s) → Zn²+(aq) + 2e⁻ E°(Zn²+/Zn)=-0.7628 V
  • the calomel electrode, mercury covered with Hg₂Cl₂ in a chloride solution: Hg₂Cl₂(s) + 2e⁻ → 2Hg(l) + 2Cl-(aq) E°=+0.2676 V. The Pt wire only makes contact with the mercury.
  • Zn(s) + Hg₂Cl₂(s) → Zn²+(aq) + 2Cl-(aq) + 2Hg(l)
  • E°=E°ᵣight-E°ₗeft=0.2676-(-0.7628)=1.0304 V; ΔᵣG°=-nFE°=-199 kJ mol⁻¹ (spontaneous as written).

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. Left electrode (anode, oxidation): Zn(s) → Zn²+(aq) + 2e⁻ E°(Zn²+/Zn)=-0.7628 V
  2. Right electrode (cathode, reduction): the calomel electrode, mercury covered with Hg₂Cl₂ in a chloride solution: Hg₂Cl₂(s) + 2e⁻ → 2Hg(l) + 2Cl-(aq) E°=+0.2676 V. The Pt wire only makes contact with the mercury.
  3. Cell reaction (sum, 2 electrons): Zn(s) + Hg₂Cl₂(s) → Zn²+(aq) + 2Cl-(aq) + 2Hg(l)
  4. Standard EMF: E°=E°ᵣight-E°ₗeft=0.2676-(-0.7628)=1.0304 V; ΔᵣG°=-nFE°=-199 kJ mol⁻¹ (spontaneous as written).
  5. Nernst equation: solids and liquid mercury have unit activity, so 3pt] E=E°-(RT)/(2F)ln(a_(Zn²+) a_(Cl⁻)²) at 298 K: E=1.0304-0.02958log(a_(Zn²+) a_(Cl⁻)²) V
  6. In terms of molality b of ZnCl₂: b_(Zn²+)=b, b_(Cl⁻)=2b, so a_(Zn²+)a_(Cl⁻)²=4b³γ_±³: 3pt] E=E°-(3RT)/(2F)ln(4^(1/3) b γ_±)

Application. the cell measures the mean activity coefficient of ZnCl₂. For b=0.0050 mol kg⁻¹ the measured EMF is 1.2272 V; the equation then gives γ_±=0.763 (with γ_±=1 the EMF would be 1.2168 V). The calomel half-cell is the usual reference electrode of pH meters and potentiometric titrations.

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Model answer to Q8(a), 2020 Paper I: solution page 40
Solution page 40 of 45
Read the whole 2020 solution2020 question paper

All parts of Question 8