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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 · 2025 · Paper I

Q4(d). ΔG°f with the standard state moved to 2 atm

Question

Suppose we redefine the standard state as pressure P = 2 atm. Find the new standard ΔG°f values of each substance: (i) HCl(g) (ii) N₂O(g). Explain the results in terms of the relative entropies of reactants and products of each reaction. Given: Standard free energy of formation at 25 °C: (i) ΔG°(HCl) = −95.3 kJ mol⁻¹; (ii) ΔG°(N₂O) = +103.7 kJ mol⁻¹.

Answer in brief

(i) HCl(g): unchanged, −95.3 kJ mol⁻¹. (ii) N₂O(g): +102.8 kJ mol⁻¹, lower by 0.86 kJ mol⁻¹.

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Coversstandard stateGibbs energy of formationpressure dependence of Gibbs energyentropy of ideal gasLe Chatelier's principle

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