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Dependence of the… (Class 12 Chemistry SQP 2024-25, Q29)

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ChemistryCBSE Class 12 · Sample paper, session 2024-25 · 4 marks · Case-based

CBSE Class 12 Chemistry sample paper 2024-25: question 29

From the official CBSE sample question paper for 2024-25, 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

Dependence of the rate of reaction on the concentration of reactants, temperature, and other factors is the most general method for weeding out unsuitable reaction mechanisms. The term mechanism means all the individual collisional or elementary processes involving molecules (atoms, radicals, and ions included) that take place simultaneously or consecutively to produce the observed overall reaction. For example, when hydrogen gas reacts with bromine, the rate of the reaction was found to be proportional to the concentration of and to the square root of the concentration of . Furthermore, the rate was inhibited by increasing the concentration of HBr as the reaction proceeded. These observations are not consistent with a mechanism involving bimolecular collisions of a single molecule of each kind. The currently accepted mechanism is considerably more complicated, involving the dissociation of bromine molecules into atoms followed by reactions between atoms and molecules:

It is clear from this example that the mechanism cannot be predicted from the overall stoichiometry.
(source: Moore, J. W., & Pearson, R. G. (1981). Kinetics and mechanism. John Wiley & Sons.)

  1. a

    Predict the expression for the rate of reaction and order for the following:

    1 mark
  2. a (cont.)

    What are the units of rate constant for the above reaction?

    1 mark
  3. b

    How will the rate of reaction be affected if the concentration of is tripled?

    1 mark

  4. OR

    b

    What change in the concentration of will triple the rate of reaction?

    1 mark
  5. c

    Suppose a reaction between A and B, was experimentally found to be first order with respect to both A and B. So the rate equation is: Rate = k[A][B]

    Which of these two mechanisms is consistent with this experimental finding? Why?

    Mechanism 1: A → C + D (slow); B + C → E (fast)
    Mechanism 2: A + B → C + D (slow); C → E (fast)

    1 mark

Figure in the paper: A mechanism scheme (dissociation of bromine into atoms followed by atom-molecule reactions) is referred to in the passage but not shown in the text. See it in the official paper.

Source: official sample paper from cbseacademic.nic.in (PDF). See it in the full Sample paper 2024-25.

Hints

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

Hint 1

Read the passage: how does the rate depend on [H₂] and on [Br₂]?

Hint 2

For units, divide mol L⁻¹ s⁻¹ by (mol L⁻¹) raised to the overall order.

Hint 3

The rate law comes from the slowest step. Which mechanism's slow step involves both A and B?

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. From the passage, rate ∝ [H₂] and ∝ [Br₂]^½:
    Rate ; order

    1 mark

  2. a (cont.)
    Units of k:

    1 mark

  3. b (first alternative). Rate′ , so the rate becomes times.

    1 mark

  4. b (OR alternative). gives . Since the rate is first order in H₂, tripling [H₂] triples the rate.

  5. c. The slow step is rate-determining. In Mechanism 1 the slow step involves only A, giving Rate = k[A]. In Mechanism 2 the slow step is A + B → C + D, giving Rate = k[A][B]. So Mechanism 2 is consistent with the experimental finding.

    1 mark

Answer: a. Rate = k[H₂][Br₂]^½, order 3/2; units mol^−½ L^½ s^−1; b. rate increases √3 (≈1.73) times / [H₂] must be tripled; c. Mechanism 2

Where the marks are

  • Rate law and order 3/2 (1 mark)
  • Units of k: mol^−½ L^½ s^−1 (1 mark)
  • Rate becomes √3 times, OR [H₂] tripled (1 mark)
  • Mechanism 2, because its slow step involves both A and B (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 paperSample paper 2024-25: 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.