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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(k). Heat of spontaneous adsorption

Question

Why is spontaneous adsorption of a gas on a solid at constant temperature and pressure exothermic? Use thermodynamic principles.

Answer in brief

Gas adsorption usually reduces entropy; spontaneity then requires a sufficiently negative enthalpy change.

Key points

  • At constant T,p, a spontaneous change has ΔG=ΔH-TΔS<0.
  • On adsorption, gas molecules lose much of their translational freedom and become localised at surface sites. For the usual gas-on-solid case, ΔS<0.
  • Consequently ΔH<TΔS<0: heat must be released to overcome the unfavourable entropy contribution. Attractive adsorbate–surface interactions provide this energy lowering.
  • This argument assumes the usual negative adsorption entropy; coupled solvent release or surface reactions can require a fuller entropy balance.

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(k), 2018 Mains Paper II: solution pages 5 and 6
Solution pages 5 and 6 of 17
Read the whole 2018 solution2018 question paper

CoversHeat of spontaneous adsorptionSurface thermodynamics

All parts of Question 1