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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 · 2021 · Mains Paper II

Q1(e). Electrophoretic effect in the Debye–Hückel–Onsager theory

Question

Explain the electrophoretic effect in Debye-Hückel-Onsager theory of strong electrolytes.

Answer in brief

The ionic atmosphere and its solvent move opposite to the central ion, so the ion travels against a counter-current of solvent: extra viscous drag lowers its mobility and Λm.

Key points

  • Each ion is surrounded by an ionic atmosphere of net opposite charge, thickness 1/κ (about 3 nm in 0.01 M 1:1 solution at 25 °C).
  • The atmosphere, with its solvation water, moves the other way; the ion swims against a counter-current of solvent ⇒ extra viscous drag ⇒ mobility falls.
  • The second retarding effect is the relaxation (asymmetry) effect: the atmosphere lags behind and pulls the ion back.
  • DHO equation: Λm = Λ°m − (A + BΛ°m)√c; A ≈ 60 (electrophoretic), B = 0.229 (relaxation); Kohlrausch's square-root law.
  • 0.001 M KCl (Λ° = 149.9): electrophoretic lowering 1.9, relaxation lowering 1.1 S cm² mol⁻¹ ⇒ Λm ≈ 146.9 S cm² mol⁻¹.

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), 2021 Mains Paper II: solution pages 5 and 6
Solution pages 5 and 6 of 36
Read the whole 2021 solution2021 question paper

Topic notes: electrolytic conductance, Kohlrausch's law, transport numbers and the Debye–Hückel limiting law, with every Civil Services, IFoS and Geo-Scientist PYQ →

Coverselectrophoretic effectDebye–Hückel–Onsager equationrelaxation effectionic atmospherestrong electrolytesmolar conductivity

All parts of Question 1