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UPSC Combined Geo-Scientist

Chemist

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

Mains Paper II covers physical chemistry.

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

Q1(j). Primary kinetic salt effect

Question

For I-(aq) + H₂O₂(aq) → IO-(aq) + H₂O(l), predict how increasing ionic strength affects the reaction rate and explain.

Answer in brief

There is no primary kinetic salt effect in the limiting dilute-solution model, because one reactant is neutral.

Key points

  • The Brø nsted–Bjerrum relation is log₁₀ (k/k₀)≈2AzAz_B√I at low ionic strength.
  • Here zA=-1 for iodide and zB=0 for hydrogen peroxide, so zAz_B=0 and the predicted primary change of k is zero.
  • The conclusion assumes the rate-determining encounter involves the stated ion and neutral molecule.

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(j), 2018 Mains Paper II: solution pages 3 and 4
Solution pages 3 and 4 of 17
Read the whole 2018 solution2018 question paper

CoversPrimary kinetic salt effectIonic kinetics

All parts of Question 1