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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 · 2018 · Paper I

Q1(c)(i). ΔᵣG° for the formation of MgO at 0°C

  • 5 marks
  • Answered
Question

Consider the formation of MgO(s). Assume that Δ H°ᵣ and Δ S°ᵣ are independent of temperature.
[Mg(s) + 1/2 O₂(g) → MgO(s) Δ H°ᵣ=-602 kJ/mol, Δ S°ᵣ=-108 J/K/mol]
(i) Calculate Δ G°ᵣ for the formation of MgO(s) at 0°C. Is this reaction spontaneous or non-spontaneous at 0°C.

Answer in brief

ΔᵣG°=-572.5 kJ mol⁻¹ at 273.15 K: negative, so the formation of MgO is spontaneous at 0°C.

Key points

  • T=0°C=273.15 K; Δ S°ᵣ=-108 J K⁻¹ mol⁻¹=-0.108 kJ K⁻¹ mol⁻¹
  • TΔ S°=273.15×(-0.108)=-29.5 kJ mol⁻¹
  • Δ G°ᵣ=Δ H°ᵣ-TΔ S°ᵣ=-602-(-29.5) ΔᵣG°=-572.5 kJ mol⁻¹
  • Δ G°<0: the reaction is spontaneous at 0°C. Magnesium burns in air once ignited; at room temperature only a thin oxide film forms because the reaction is slow (kinetics), not because it is unfavourable.

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. T=0°C=273.15 K; Δ S°ᵣ=-108 J K⁻¹ mol⁻¹=-0.108 kJ K⁻¹ mol⁻¹
  2. TΔ S°=273.15×(-0.108)=-29.5 kJ mol⁻¹
  3. Δ G°ᵣ=Δ H°ᵣ-TΔ S°ᵣ=-602-(-29.5) ΔᵣG°=-572.5 kJ mol⁻¹
  4. Δ G°<0: the reaction is spontaneous at 0°C. Magnesium burns in air once ignited; at room temperature only a thin oxide film forms because the reaction is slow (kinetics), not because it is unfavourable.

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Model answer to Q1(c)(i), 2018 Paper I: solution page 4
Solution page 4 of 55
Read the whole 2018 solution2018 question paper

All parts of Question 1