UPSC Civil Services (Mains), Chemistry optional · 2023 · Paper I

Q2(d). Dissociation equilibrium A₂(g) ⇌ 2A(g)

10 marks
Topic: Thermodynamics
Answered

Question

Consider the equilibrium reaction A₂(g) ⇌ 2A(g), in which A₂ gas is 18.5% dissociated at 25 °C and 1 bar. (i) Calculate Keq at 25 °C. (ii) Calculate Keq at 100 °C, given that ΔH° = 57.2 kJ mol⁻¹ over this temperature range. (iii) What is the effect of compression on this reaction?

Answer in brief

(i) K = 4α²P/(1 − α²) = 0.142; (ii) K at 100 °C = 14.6; (iii) compression shifts the equilibrium towards A₂: α falls, K is unchanged.

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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 →

Coversequilibrium constantdegree of dissociationvan't Hoff equationLe Chatelier's principleN₂O₄ ⇌ 2NO₂

All parts of Question 2

2(a) Variation of the Gibbs energy with temperature and pressure20 marks
2(b) Continuous energy of a free particle, quantised energy in a box10 marks
2(c) Primitive cubic lattice: points, voids and the sphere at the centre10 marks
2(d) Dissociation equilibrium A₂(g) ⇌ 2A(g)10 marks