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UPSC Civil Services (Mains)

Chemistry optional

  • 2 papers
  • 250 marks each
  • 3 h per paper
  • 5 of 8 questions, Q1 and Q5 compulsory

Paper I covers physical and inorganic chemistry.

UPSC Civil Services (Mains), Chemistry optional · 2017 · Paper I

Q5(c). Signs of Δ G, Δ S and Δ H for adsorption of a gas

  • 10 marks
  • Answered
Question

At constant pressure and temperature, adsorption of a gas on solid surface occurs spontaneously. For this natural process, what will be the signs of (i) Δ G, (ii) Δ S and (iii) Δ H? Give reason(s) in support of your answer.

Answer in brief

(i) Δ G<0, (ii) Δ S<0, (iii) Δ H<0: adsorption is spontaneous, the gas loses freedom, so it must be exothermic.

Key points

  • (i) Δ G negative. At constant T and p a spontaneous process lowers the Gibbs energy of the system: Δ G<0
  • (ii) Δ S negative. Gas molecules that move freely in three dimensions are bound to the surface; they lose translational freedom (at best they move in two dimensions). The system becomes more ordered: Δ S<0
  • (iii) Δ H negative. Δ G=Δ H-TΔ S. With Δ G<0 and -TΔ S>0, the only way is Δ H<0 with |Δ H|>T|Δ S|: Δ H<0, adsorption is exothermic.
  • Consequences. Raising T makes -TΔ S larger, so Δ G rises: physical adsorption decreases with temperature (Le Chatelier). Above T=Δ H/Δ S the gas desorbs.

The full model answer

Written out in full, as it should go in the answer booklet. The solution page with its diagrams follows it.

  1. (i) Δ G negative. At constant T and p a spontaneous process lowers the Gibbs energy of the system: Δ G<0
  2. (ii) Δ S negative. Gas molecules that move freely in three dimensions are bound to the surface; they lose translational freedom (at best they move in two dimensions). The system becomes more ordered: Δ S<0
  3. (iii) Δ H negative. Δ G=Δ H-TΔ S. With Δ G<0 and -TΔ S>0, the only way is Δ H<0 with |Δ H|>T|Δ S|: Δ H<0, adsorption is exothermic.
  4. Consequences. Raising T makes -TΔ S larger, so Δ G rises: physical adsorption decreases with temperature (Le Chatelier). Above T=Δ H/Δ S the gas desorbs.
  5. Size of Δ H: physisorption (van der Waals) about -20 kJ mol⁻¹; chemisorption (bond formation) of the order of -200 kJ mol⁻¹.

Application. because adsorption is exothermic, gas masks and charcoal traps work best cold (charcoal cooled in liquid nitrogen adsorbs gases for high vacuum), and adsorbents are regenerated by heating.

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Model answer to Q5(c), 2017 Paper I: solution page 21
Solution page 21 of 38
Read the whole 2017 solution2017 question paper

All parts of Question 5