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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 · 2018 · Paper I

Q8(b)(ii). The photochemical reaction of hydrogen and chlorine

  • 5 marks
  • Answered
Question

Discuss the photochemical reactions between hydrogen and chlorine.

Answer in brief

A mixture of H₂ and Cl₂ reacts slowly in the dark but explosively in sunlight or blue light. Light below 494 nm splits Cl₂ into atoms, which start a chain: Cl + H₂ → HCl + H, H + Cl₂ → HCl + Cl. One photon gives 10⁴ to 10⁶ molecules of HCl.

Key points

  • Cl₂ + hν → 2Cl. H₂ does not absorb visible light; Cl₂ does, and blue light carries enough energy to break the Cl–Cl bond.
  • Cl + H₂ → HCl + H (Δ H=436-431=+5 kJ mol⁻¹); H + Cl₂ → HCl + Cl (Δ H=242-431=-189 kJ mol⁻¹). Each step regenerates the atom the other needs: a chain.
  • Cl + Cl + M → Cl₂ + M, or Cl atoms removed at the wall.
  • Steady-state treatment (wall termination) gives (d[HCl])/(dt)=(4k₂)/(k₄)Iₐ[H₂]: the rate is proportional to the absorbed light intensity and the yield Φ=4k₂[H₂]/k₄ is very large (10⁴ to 10⁶).

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. Initiation (the only light step): Cl₂ + hν → 2Cl. H₂ does not absorb visible light; Cl₂ does, and blue light carries enough energy to break the Cl–Cl bond.
  2. Propagation: Cl + H₂ → HCl + H (Δ H=436-431=+5 kJ mol⁻¹); H + Cl₂ → HCl + Cl (Δ H=242-431=-189 kJ mol⁻¹). Each step regenerates the atom the other needs: a chain.
  3. Termination: Cl + Cl + M → Cl₂ + M, or Cl atoms removed at the wall.
  4. Steady-state treatment (wall termination) gives (d[HCl])/(dt)=(4k₂)/(k₄)Iₐ[H₂]: the rate is proportional to the absorbed light intensity and the yield Φ=4k₂[H₂]/k₄ is very large (10⁴ to 10⁶).
  5. Traces of O₂ inhibit the reaction (H + O₂ → HO₂ removes carriers). This large Φ does not violate the Einstein law: one photon still dissociates one Cl₂; the many HCl come from the dark chain steps.

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Model answer to Q8(b)(ii), 2018 Paper I: solution page 51
Solution page 51 of 55
Read the whole 2018 solution2018 question paper

All parts of Question 8