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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

Q7(a)(ii). Mechanism of heterogeneous catalysis

  • 5 marks
  • Answered
Question

Explain the mechanism of reaction on heterogeneous catalysis.

Answer in brief

Reactants diffuse to the solid surface, are chemisorbed (bonds weakened), react on the surface by a path of lower activation energy, and the products desorb and diffuse away. Two mechanisms: Langmuir–Hinshelwood (both reactants adsorbed) and Eley–Rideal (one adsorbed, the other from the gas).

Key points

  • reactant molecules reach the surface and are chemisorbed on active sites (often with dissociation, e.g. H₂ → 2H on Ni). Bonds within the molecule are weakened.
  • adsorbed species react on the surface. The path has a lower activation energy than the gas-phase reaction; the surface also holds the reactants close together in a suitable orientation.
  • the products are less strongly held, leave the surface and release the sites.
  • reaction between two adsorbed species, v=kθ_Aθ_B=kK_AK_Bp_Ap_B(1+K_Ap_A+K_Bp_B)² (e.g. CO + O₂ on Pt). Eley–Rideal mechanism: an adsorbed species reacts with a gas-phase molecule that strikes it, v=kθ_Ap_B.

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. Diffusion and adsorption: reactant molecules reach the surface and are chemisorbed on active sites (often with dissociation, e.g. H₂ → 2H on Ni). Bonds within the molecule are weakened.
  2. Surface reaction: adsorbed species react on the surface. The path has a lower activation energy than the gas-phase reaction; the surface also holds the reactants close together in a suitable orientation.
  3. Desorption: the products are less strongly held, leave the surface and release the sites.
  4. Langmuir–Hinshelwood mechanism: reaction between two adsorbed species, v=kθ_Aθ_B=kK_AK_Bp_Ap_B(1+K_Ap_A+K_Bp_B)² (e.g. CO + O₂ on Pt). Eley–Rideal mechanism: an adsorbed species reacts with a gas-phase molecule that strikes it, v=kθ_Ap_B.
  5. A good catalyst binds reactants neither too weakly (no activation) nor too strongly (products cannot leave): the Sabatier principle, seen as a volcano curve of activity against adsorption enthalpy. Poisons (S, As compounds) block the active sites.

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Model answer to Q7(a)(ii), 2018 Paper I: solution page 43
Solution page 43 of 55
Read the whole 2018 solution2018 question paper

All parts of Question 7