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.
Thermodynamics: every question, 2021 to 2025
17 parts worth 155 marks have been set on this unit in UPSC CSE Chemistry Paper I from 2021 to 2025. Each one opens on its own page with the question, the answer in brief and the key points.
Work, heat and internal energy; first law of thermodynamics. Second law of thermodynamics; entropy as a state function, entropy changes in various processes, entropy–reversibility and irreversibility, free energy functions; thermodynamic equation of state; Maxwell relations; temperature, volume and pressure dependence of U, H, A, G, Cp and Cv, α and β; J-T effect and inversion temperature; criteria for equilibrium, relation between equilibrium constant and thermodynamic quantities; Nernst heat theorem, introductory idea of third law of thermodynamics.
Notes: Second and third laws: entropy, Gibbs and Helmholtz energies, Maxwell relations (full notes) · short notes
Notes: Chemical potential, partial molar quantities, Gibbs–Duhem equation, fugacity and activity (full notes) · short notes
Notes: Chemical equilibrium: ΔG° and K, Kp, Kc and Kx, van ’t Hoff isochore and isobar (full notes) · short notes
Notes: First law of thermodynamics: work, heat, enthalpy, adiabatic changes, Joule–Thomson effect, thermochemistry (full notes) · short notes
2025
- Q1(g) Heat of combustion of naphthalene from bomb calorimetry5 marks Answered
- Q1(j) Maximum work of a steam turbine5 marks Answered
- Q3(d) Water condensed while ice melts and warms10 marks Answered
- Q4(d) ΔG°f with the standard state moved to 2 atm10 marks Answered
2024
- Q1(c) Normal boiling point of mercury10 marks Answered
- Q3(c)(i) Change in Helmholtz energy on reversible isothermal compression10 marks Answered
- Q3(c)(ii) Why a tyre gets hot when it is pumped5 marks Answered
- Q3(c)(iii) Pressure of O₂ over NiO at 25 °C5 marks Answered
- Q3(c)(iv) Final temperature after Joule–Thomson expansion5 marks Answered
2023
- Q2(a) Variation of the Gibbs energy with temperature and pressure20 marks Answered
- Q2(d) Dissociation equilibrium A₂(g) ⇌ 2A(g)10 marks Answered
- Q7(e) Reversible isothermal expansion of an ideal gas10 marks Answered
2022
- Q1(d) ΔG for the formation of ozone in smog10 marks Answered
- Q2(a) Adiabatic expansion of tyre air against a constant external pressure10 marks Answered
- Q3(b) Entropy changes when an ice cube melts in the sea at 0 °C10 marks Answered
2021
- Q1(c) Rubber band: change of length on heating10 marks Answered
- Q3(c)(ii) Expansion of an ideal gas into a vacuum in an insulated box10 marks Answered
