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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 · 2025 · Paper I

Q5(d). Showing that a decomposition is first order

Question

From the following data for the decomposition of ammonium nitrite in aqueous solution, show that the reaction is of the first order. Time (minutes): 10, 15, 20, 25, ∞; Volume of N₂ (cc): 6.25, 9.00, 11.40, 13.65, 35.05.

Answer in brief

k = (1/t) ln[V∞/(V∞ − Vt)] has the same value, 1.97 × 10⁻² min⁻¹, at every time, so the reaction is first order.

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

Coversfirst-order reactionintegrated rate lawammonium nitrite decompositionhalf-lifegasometric method

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