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The rate of reaction is… (Class 12 Chemistry 2023, Q31)

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ChemistryCBSE Class 12 · Board exam 2023, set 56/1/1 · 4 marks · Case-based

CBSE Class 12 Chemistry 2023: question 31

Set in the CBSE Class 12 board exam of 2023, paper 56/1/1, section D, question 31, for 4 marks. Try it first; the hints and our step-by-step solution are below.

Q31 · 4 marks · Case-basedCompetency-basedChemical Kinetics

The rate of reaction is concerned with decrease in concentration of reactants or increase in the concentration of products per unit time. It can be expressed as instantaneous rate at a particular instant of time and average rate over a large interval of time. Mathematical representation of rate of reaction is given by rate law. Rate constant and order of a reaction can be determined from rate law or its integrated rate equation.

  1. (i)

    What is average rate of reaction ?

    1 mark
  2. (ii)

    Write two factors that affect the rate of reaction.

    1 mark
  3. (iii)

    (1) What happens to rate of reaction for zero order reaction ?
    (2) What is the unit of k for zero order reaction ?

    2 marks

  4. OR

    (iii)

    (1) For a reaction Products, Rate = . What is the order of the reaction ?
    (2) Define pseudo first order reaction with an example.

    2 marks

Source: official question paper from cbse.gov.in (PDF). See it in the full 2023 board paper, set 56/1/1.

Hints

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

Hint 1

Average rate is measured over an interval of time, not at an instant.

Hint 2

For zero order, put the exponent 0 in rate = k[R]ⁿ and see what happens.

Hint 3

Order is the sum of the powers in the rate law; pseudo first order involves one reactant in large excess.

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. (i) Average rate is the change in concentration of a reactant or product divided by the time interval over which the change occurs: .

    1 mark

  2. (ii) Any two factors: concentration of reactants, temperature, presence of a catalyst, and surface area of reactants.

    1 mark

  3. (iii)(a)(1) For a zero order reaction, the rate is independent of the concentration of reactants. It stays constant: rate = k.

    1 mark

  4. (iii)(a)(2) Unit of k for a zero order reaction is .

    1 mark

  5. (iii)(b)(1) Order = = 1·5.

  6. (iii)(b)(2) A pseudo first order reaction has a higher molecularity or order but behaves as first order. This happens because one reactant is in large excess, so its concentration stays practically constant. Example: acid hydrolysis of ethyl acetate in excess water, , with rate = .
    (Alternative (iii)(b) carries the same 2 marks: order 1 mark, definition ½, example ½.)

Answer: (i) Δ(concentration)/Δt; (ii) concentration, temperature (also catalyst, surface area); (iii)(a) rate constant, independent of concentration; mol L⁻¹ s⁻¹; (iii)(b) order 3/2; pseudo first order, e.g. acid hydrolysis of ethyl acetate

Where the marks are

  • Definition of average rate (1 mark)
  • Any two factors affecting rate (1 mark)
  • (iii)(a) Rate independent of concentration (1 mark); unit mol L⁻¹ s⁻¹ (1 mark)
  • OR (iii)(b) Order = 3/2 (1 mark); definition of pseudo first order (½ mark) with example (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 paper2023 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.