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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 · 2025 · Mains Paper II

Q11(c). van ’t Hoff isochore and a solution-phase dimerisation

Question

Derive the expression d ln Kc/dT = ΔU°/RT², clearly mentioning assumption(s) and approximation(s) used, and discuss the effect of temperature on the value of Kc (with graphical sketch also) for any solution-phase dimerization reaction.

Answer in brief

d ln Kc/dT = ΔU°/RT²; for an exothermic dimerisation (ΔU° < 0) ln Kc rises linearly with 1/T, so Kc falls on heating.

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Topic notes: chemical equilibrium, ΔG° = −RT ln K, Kp, Kc, Kx and the van ’t Hoff equations, with every Civil Services, IFoS and Geo-Scientist PYQ →

Coversvan 't Hoff isochoretemperature dependence of KcGibbs–Helmholtz equationdimerisationbenzoic acid dimerLe Chatelier's principle

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