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Indian Forest Service (Mains)

Chemistry optional

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

Paper I covers physical and inorganic chemistry.

Indian Forest Service (Mains), Chemistry optional · 2025 · Paper I

Q6(c). Photochemical hydrogen–bromine reaction

Question

For photoinitiation Br₂ → 2 Br, propagation Br + H₂ → HBr + H (k₂), H + Br₂ → HBr + Br (k₃), inhibition H + HBr → H₂ + Br (k₄), and termination 2 Br → Br₂ (k₅), derive the rate law. Explain a quantum yield as low as 0.01.

Answer in brief

Writing J for the photodissociation-event rate, rHBr = 2k₂[H₂]√(J/k₅)/{1 + (k₄/k₃)[HBr]/[Br₂]}.

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CoversH₂–Br₂ photochemical reactionquantum yieldchain reaction kineticssteady-state approximationHBr inhibition

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