VKC ScienceChemistry for written exams Telegram

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 · 2026 · Paper I

Q1(h). Activation energy of food spoilage

  • 5 marks
  • Answered
Question

Everyday experience demonstrates that food will spoil outside in a hot summer day much faster than it would be in a refrigerator. Estimate the activation energy of these decomposition processes by assuming that food will spoil about 40 times faster at a temperature of 30°C than at a temperature of 8°C.

Answer in brief

Eₐ≈1.19×10² kJ mol⁻¹ (118.8 kJ mol⁻¹)

Key points

  • Two-temperature form of the Arrhenius equation: ln(k₂)/(k₁)=(Eₐ)/(R)((1)/(T₁)-(1)/(T₂))
  • (1)/(281.15)-(1)/(303.15)=2.581×10⁻⁴ K⁻¹; 40=3.689
  • Eₐ=8.314×3.6892.581×10⁻⁴=1.188×10⁵ J mol⁻¹, Eₐ≈119 kJ mol⁻¹

The full model answer

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

Given: T₁=8°C =281.15 K, T₂=30°C =303.15 K, k₂/k₁=40 (rate ∝ k at the same composition).

  1. Two-temperature form of the Arrhenius equation: ln(k₂)/(k₁)=(Eₐ)/(R)((1)/(T₁)-(1)/(T₂))
  2. (1)/(281.15)-(1)/(303.15)=2.581×10⁻⁴ K⁻¹; 40=3.689
  3. Eₐ=8.314×3.6892.581×10⁻⁴=1.188×10⁵ J mol⁻¹, Eₐ≈119 kJ mol⁻¹

Reading the answer: A is taken as independent of T. With this Eₐ the rate rises about 5 times per 10°C near room temperature, which is why refrigeration slows spoilage so much.

Practising this answer? Write it in a Mains test and get it checked by VKC. Civil Services Mains test series · Telegram

Model answer to Q1(h), 2026 Paper I: solution page 7
Solution page 7 of 55
Read the whole 2026 solution2026 question paper

All parts of Question 1