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Define molecularity of the… (Class 12 Chemistry 2024, Q18)

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ChemistryCBSE Class 12 · Board exam 2024, set 56/1/1 · 2 marks · Very short answer

CBSE Class 12 Chemistry 2024: question 18

Set in the CBSE Class 12 board exam of 2024, paper 56/1/1, section B, question 18, for 2 marks. Try it first; the hints and our step-by-step solution are below.

Q18 · 2 marks · Very short answerChemical Kinetics

Define molecularity of the reaction. State any one condition in which a bimolecular reaction may be kinetically of first order.

Source: official question paper from cbse.gov.in (PDF). See it in the full 2024 board paper, set 56/1/1.

Hints

Open one hint at a time and try again before the next.

Hint 1

Molecularity is defined only for an elementary step — think about collisions.

Hint 2

Consider the acid hydrolysis of ethyl acetate: what happens to the concentration of water during the reaction?

Step-by-step solution

Each step shows the marks it earns, as an examiner would award them. Checked by an independent re-solve from the question paper.

  1. Molecularity: the number of reacting species (atoms, ions or molecules) that must collide simultaneously in an elementary reaction to bring about the chemical change.

    1 mark

  2. A bimolecular reaction becomes kinetically first order when one of the reactants is present in large excess (e.g. water in the acid hydrolysis of an ester), so its concentration stays practically constant — a pseudo first order reaction.

    1 mark

Answer: Molecularity = number of species colliding simultaneously in an elementary step; a bimolecular reaction is first order when one reactant is in large excess.

Where the marks are

  • Correct definition of molecularity for an elementary reaction (1 mark)
  • Condition: one reactant in large excess / pseudo first order (1 mark)

Revise the chapter

Practise this chapterBoard-style questions from Chemical Kinetics, one at a time, marked against the answer key and marking points.Back to the full paper2024 board paper, set 56/1/1: every question, by section.

The question is transcribed from the official paper; figures are described in words. The solution, hints and chapter tag are ours. Spotted an error? Write tosupport@questionbuddy.in.