UPSC Combined Geo-Scientist, Mains · 2026 · Mains Paper II
Q5(a). Activation energy from the souring of milk
Question
A sample of milk kept at 25°C is found to sour 40 times rapidly as when it is kept at 4°C. Determine the Arrhenius activation energy for this souring process in SI unit.
Answer in brief
Eₐ=1.21×10⁵ J mol⁻¹ (121 kJ mol⁻¹).
The full model answer
Worked out step by step, as it should go in the answer booklet. The solution sheet follows it.
Rate of souring ∝ k, so k₂₉₈/k₂₇₇=40. T₁=277.15 K, T₂=298.15 K.
ln(k₂)/(k₁)=(Eₐ)/(R)(1T₁-1T₂) ⇒ 40=3.689=(Eₐ)/(8.314)(2.541×10⁻⁴)
Eₐ=(8.314)(3.689)2.541×10⁻⁴=1.21×10⁵ J mol⁻¹
CoversArrhenius equationActivation energy from the souring of milk
All parts of Question 5
- 5(a) Activation energy from the souring of milk5 marks
- 5(b) ΔᵣG<0 but Δ^ G not necessarily negative5 marks