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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 · 2019 · Paper I

Q8(d). Electrode reactions of the fuel cell and the lead–acid cell

  • 10 marks
  • Answered
Question

Write electrode reactions for the following:
(i) H₂–O₂ fuel cell under acidic condition.
(ii) Lead acid battery during discharge.

Answer in brief

(i) Anode H₂ → 2H⁺ + 2e⁻; cathode O₂ + 4H⁺ + 4e⁻ → 2H₂O; E°=1.229 V. (ii) Anode Pb + SO₄²- → PbSO₄ + 2e⁻; cathode PbO₂ + 4H⁺ + SO₄²- + 2e⁻ → PbSO₄ + 2H₂O; E°=2.04 V.

Key points

  • Anode (−): H₂(g) → 2H+(aq) + 2e⁻ E°=0.00 V
  • Cathode (+): O₂(g) + 4H+(aq) + 4e⁻ → 2H₂O(l) E°=+1.229 V
  • 2H₂(g) + O₂(g) → 2H₂O(l); E°_(cell)=1.229 V; Δ G°=-nFE°=-237.2 kJ per mol H₂O. H⁺ crosses the acid electrolyte (phosphoric acid or a proton-exchange polymer) from anode to cathode; Pt catalyses both electrodes.
  • Pb(s) + SO₄²-(aq) → PbSO₄(s) + 2e⁻ E°(PbSO₄/Pb)=-0.356 V

The full model answer

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

(i) Hydrogen–oxygen fuel cell, acid electrolyte

  1. Anode (−): H₂(g) → 2H+(aq) + 2e⁻ E°=0.00 V
  2. Cathode (+): O₂(g) + 4H+(aq) + 4e⁻ → 2H₂O(l) E°=+1.229 V
  3. Overall: 2H₂(g) + O₂(g) → 2H₂O(l); E°_(cell)=1.229 V; Δ G°=-nFE°=-237.2 kJ per mol H₂O. H⁺ crosses the acid electrolyte (phosphoric acid or a proton-exchange polymer) from anode to cathode; Pt catalyses both electrodes.

(ii) Lead–acid cell, discharge

  1. Anode (−), Pb plate: Pb(s) + SO₄²-(aq) → PbSO₄(s) + 2e⁻ E°(PbSO₄/Pb)=-0.356 V
  2. Cathode (+), PbO₂ plate: PbO₂(s) + 4H+(aq) + SO₄²-(aq) + 2e⁻ → PbSO₄(s) + 2H₂O(l) E°=+1.685 V
  3. Overall: Pb + PbO₂ + 2H₂SO₄ → 2PbSO₄ + 2H₂O; E°_(cell)=1.685-(-0.356)=2.04 V (about 2 V per cell; six cells make a 12 V battery).
  4. Sulfuric acid is consumed and water formed, so the density of the acid falls on discharge: a hydrometer reading shows the state of charge. On charging, both reactions run in reverse.

Application. proton-exchange-membrane fuel cells power buses and submarines with water as the only product; lead–acid batteries remain the standard starter battery because they deliver large currents cheaply and can be recharged many times.

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Model answer to Q8(d), 2019 Paper I: solution page 39
Solution page 39 of 40
Read the whole 2019 solution2019 question paper

All parts of Question 8