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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 · 2022 · Mains Paper II

Q1(e). Reversal of HCl + NH₃ ⇌ NH₄Cl with temperature

Question

HCl(g) + NH₃(g) ⇌ NH₄Cl(s). At lower temperatures the forward reaction is spontaneous while at higher temperatures the reverse reaction is spontaneous. Explain the reversal of the above reaction at different ranges of temperature in the light of thermodynamics (Given: ΔH for the reverse reaction is positive).

Answer in brief

Forward: ΔH < 0 and ΔS < 0; at low T |ΔH| > T|ΔS| so ΔG < 0; at high T T|ΔS| > |ΔH|, ΔG > 0 and the reverse runs; cross-over near 618 K.

Key points

  • At constant T and P a change is spontaneous when ΔG = ΔH − TΔS < 0.
  • Forward: ΔH < 0 (reverse is endothermic, given) and ΔS < 0: 2 mol gas → 1 mol solid, a large loss of entropy.
  • Low T: the −TΔS term is small; ΔH wins ⇒ ΔG < 0: NH₄Cl forms (enthalpy-driven).
  • High T: −TΔS is large and positive ⇒ ΔG(forward) > 0, ΔG(reverse) < 0: NH₄Cl dissociates (entropy-driven).
  • Cross-over (ΔG° = 0, Kp = 1): T* = ΔH°/ΔS° = (−176.2 × 10³)/(−285.1) = 618 K (345 °C).

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), 2022 Mains Paper II: solution pages 7 and 8
Solution pages 7 and 8 of 37
Read the whole 2022 solution2022 question paper

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 →

CoversGibbs free energyspontaneityentropyΔG = ΔH − TΔSammonium chloride

All parts of Question 1