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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 · 2023 · Paper I

Q8(d). Transition-state theory and the pre-exponential factor

Question

Explain the transition-state theory of reaction rates. How is this theory considered superior to collision theory in providing a much more complete interpretation of the pre-exponential factor A in the Arrhenius equation?

Answer in brief

k = κ(kBT/h)K‡ = κ(kBT/h)eΔS‡/Re−ΔH‡/RT (Eyring equation); the theory fixes A through the entropy of activation, which collision theory covers only with an empirical steric factor.

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Topic notes: Arrhenius equation, collision theory and transition-state theory, with every Civil Services, IFoS and Geo-Scientist PYQ →

Coverstransition-state theoryEyring equationactivated complexentropy of activationcollision theorysteric factor

All parts of Question 8