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UPSC Combined Geo-Scientist

Chemist

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

Mains Paper I covers inorganic chemistry.

UPSC Combined Geo-Scientist, Mains · 2018 · Mains Paper I

Q1(e). Halogen displacement

Question

Iodine and bromine are added to a solution containing I⁻ and Br⁻ ions. What reaction would occur? Explain. The electrode potentials for the reactions are: I₂ + 2e⁻ → 2I⁻, E°=+0.54 V; Br₂ + 2e⁻ → 2Br⁻, E°=+1.08 V.

Answer in brief

Bromine oxidises iodide: Br₂ + 2I⁻ → 2Br⁻ + I₂, with E°_(cell)=+0.54 V.

Key points

  • The more positive reduction potential makes Br₂ the stronger oxidant. Use bromine reduction and reverse the iodine half-reaction.
  • E°_(cell)=1.08-0.54=0.54 V, so ΔG°=-2FE°<0.
  • The reverse displacement, iodine oxidising bromide, has negative standard cell potential. The stated conclusion assumes ordinary comparable activities; the Nernst equation describes the eventual equilibrium.

The full model answer

As written for the answer booklet: the diagram carries the working, the lines stay short. Read here only; it is not offered as a download.

Model answer to Q1(e), 2018 Mains Paper I: solution pages 3 and 4
Solution pages 3 and 4 of 17
Read the whole 2018 solution2018 question paper

CoversHalogen displacementElectrode potentials

All parts of Question 1