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The rate of a chemical… (Class 12 Chemistry 2025, Q29)

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ChemistryCBSE Class 12 · Board exam 2025, set 56/1/1 · 4 marks · Case-based

CBSE Class 12 Chemistry 2025: question 29

Set in the CBSE Class 12 board exam of 2025, paper 56/1/1, section D, question 29, for 4 marks. Try it first; the hints and our step-by-step solution are below.

Q29 · 4 marks · Case-basedCompetency-basedChemical Kinetics

The rate of a chemical reaction is expressed either in terms of decrease in the concentration of reactants or increase in the concentration of a product per unit time. Rate of the reaction depends upon the nature of reactants, concentration of reactants, temperature, presence of catalyst, surface area of the reactants and presence of light. Rate of reaction is directly related to the concentration of reactant. Rate law states that the rate of reaction depends upon the concentration terms on which the rate of reaction actually depends, as observed experimentally. The sum of powers of the concentration of the reactants in the Rate law expression is called order of reaction while the number of reacting species taking part in an elementary reaction which must collide simultaneously in order to bring about a chemical reaction is called molecularity of the reaction.
Answer the following questions :

  1. (a)

    (i) What is a rate determining step ?
    (ii) Define complex reaction.

    2 marks
  2. (b)

    What is the effect of temperature on the rate constant of a reaction ?

    1 mark

  3. OR

    (b)

    Why is molecularity applicable only for elementary reactions whereas order is applicable for elementary as well as complex reactions ?

    1 mark
  4. (c)

    The conversion of molecule X to Y follows second order kinetics. If concentration of X is increased 3 times, how will it affect the rate of formation of Y ?

    1 mark

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

Hints

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

Hint 1

The overall speed of a multi-step process is limited by its slowest step.

Hint 2

Think of the Arrhenius equation for the temperature effect.

Hint 3

For rate = k[X]², replace [X] by 3[X].

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. (a)(i) The rate determining step is the slowest step in the mechanism of a reaction. It controls the overall rate.

    1 mark

  2. (a)(ii) A complex reaction takes place through a series of two or more elementary steps rather than in a single step.

    1 mark

  3. (b) The rate constant increases with increase in temperature, roughly doubling for every 10 K rise ().

    1 mark

  4. (b) OR: Molecularity is the number of species colliding in a single elementary step, so it has meaning only for an elementary reaction. In a complex reaction each step may have a different molecularity. Order is found experimentally from the rate law, which is decided by the slowest step, so it applies to both elementary and complex reactions. (Alternative to (b) above; carries the same 1 mark.)

  5. (c) For second order, Rate . If becomes , Rate. The rate of formation of Y becomes 9 times.

    1 mark

Answer: (a)(i) Slowest step; (ii) reaction proceeding in two or more elementary steps; (b) k increases with temperature / molecularity defined only for elementary steps, order is experimental; (c) 9 times

Where the marks are

  • Rate determining step = slowest step (1 mark)
  • Complex reaction = series of elementary steps (1 mark)
  • k increases with temperature, OR correct reasoning on molecularity vs order (1 mark)
  • Rate increases 9 times (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 paper2025 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.