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State a condition under… (Class 12 Chemistry 2025, Q18)

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ChemistryCBSE Class 12 · Board exam 2025, set 56/7/1 · 2 marks · Very short answer

CBSE Class 12 Chemistry 2025: question 18

Set in the CBSE Class 12 board exam of 2025, paper 56/7/1, 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

State a condition under which a bimolecular reaction is kinetically first order reaction. Give an example. For which type of reactions, do order and molecularity have the same value ?

Source: official question paper from cbse.gov.in (PDF). See it in the full 2025 board paper, set 56/7/1.

Hints

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

Hint 1

If one reactant's concentration hardly changes, can it still appear in the rate law?

Hint 2

Think of a hydrolysis reaction in which water is the solvent.

Hint 3

Order and molecularity can match only when the reaction has no separate mechanism steps.

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. Condition: a bimolecular reaction behaves as first order (pseudo first order) when one of the two reactants is present in large excess. Its concentration then stays practically constant during the reaction.

    1 mark

  2. Example: acid hydrolysis of ethyl acetate, . Water is in large excess, so rate . Inversion of cane sugar is another example.

    ½ mark

  3. Order and molecularity have the same value for elementary reactions, i.e. reactions that occur in a single step.

    ½ mark

Answer: One reactant in large excess (pseudo first order), e.g. acid hydrolysis of ethyl acetate; order = molecularity for elementary (single-step) reactions.

Where the marks are

  • Condition: one reactant present in large excess (1 mark)
  • Correct example such as hydrolysis of an ester or sucrose (½ mark)
  • Elementary / single-step reactions (½ 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 paper2025 board paper, set 56/7/1: 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.