VKC ScienceChemistry for written exams Telegram

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 · 2022 · Mains Paper II

Q1(a). Postulates of kinetic theory and the two that fail

Question

What are the postulates of kinetic theory of gases? Out of these which ones are not valid for real gases?

Answer in brief

Point-mass molecules in random motion, elastic collisions, no forces between them, mean kinetic energy ∝ T; not valid for real gases: negligible molecular volume and no intermolecular attraction.

Key points

  • Postulates: tiny molecules of mass m in continuous, random, straight-line motion; perfectly elastic collisions; pressure due to collisions with the walls; ½mu² = (3/2)kBT.
  • Postulate 2 fails at high pressure: molecules have a finite volume, so the space available is V − nb; repulsion makes Z > 1.
  • Postulate 5 fails at low temperature and moderate P: attraction lowers the wall pressure by an²/V², making Z < 1; liquefaction itself proves attraction.
  • The two corrections give the van der Waals equation: (P + an²/V²)(V − nb) = nRT.

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(a), 2022 Mains Paper II: solution pages 3 and 4
Solution pages 3 and 4 of 37
Read the whole 2022 solution2022 question paper

Topic notes: kinetic theory of gases, Maxwell distribution and effusion, with every Civil Services, IFoS and Geo-Scientist PYQ →

Coverskinetic theory of gasespostulatesreal gasescompressibility factorvan der Waals equation

All parts of Question 1