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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 · 2024 · Paper I

Q4(c)(i). Time of maximum concentration of B in A → B → C

Question

For the sequential reaction A → B → C (rate constants kA and kB), the rate constants are kA = 5 × 10⁶ s⁻¹ and kB = 3 × 10⁶ s⁻¹. Determine the time when the concentration of B is at a maximum.

Answer in brief

tmax = ln(kA/kB)/(kA − kB) = 2.55 × 10⁻⁷ s.

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Topic notes: rate laws, integrated rate equations and half-lives, with every Civil Services, IFoS and Geo-Scientist PYQ →

Coversconsecutive reactionsfirst-order kineticsmaximum concentration of intermediatesteady-state approximationrate constants

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