Answer the following. (a) Define order of a reaction. (b) The rate for the following reaction is given by : A + B → C Rate = k[A][B]² (i) How is the rate of reaction affected if we double the concentration of B ? (ii) Write the overall order of a reaction if ‘A’ is present in large excess.

ChemistryCBSE Class 12 · Board exam 2026, set 56/5/1 · 3 marks · Short answer

CBSE Class 12 Chemistry 2026: question 23

Set in the CBSE Class 12 board exam of 2026, paper 56/5/1, section C, question 23, for 3 marks. Try it first; the hints and our step-by-step solution are below.

Q23 · 3 marks · Short answerChemical Kinetics

Answer the following.

(a)
Define order of a reaction.
1 mark
(b)
The rate for the following reaction is given by : $A + B \to C$
Rate = $k [A] [B]^{2}$
(i) How is the rate of reaction affected if we double the concentration of B ?
(ii) Write the overall order of a reaction if ‘A’ is present in large excess.
2 marks

Source: official question paper from cbse.gov.in (PDF). See it in the full 2026 board paper, set 56/5/1.

Hints

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

Hint 1

Order comes from the exponents in the rate law.

Hint 2

A reactant in large excess hardly changes in concentration, so its term merges into the rate constant.

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.

(a) Order of a reaction is the sum of the powers to which the concentrations of the reactants are raised in the experimentally determined rate law.
1 mark
(b)(i) Rate $\propto [B]^{2}$ . When [B] is doubled, rate $= k [A] (2 [B])^{2} = 4 k [A] [B]^{2}$ , so the rate becomes 4 times.
1 mark
(b)(ii) With A in large excess, [A] stays practically constant. Then rate $= k^{'} [B]^{2}$ with $k^{'} = k [A]$ , so the overall order is 2 (pseudo second order).
1 mark

Answer: (a) Definition; (b)(i) rate becomes 4 times; (ii) order = 2

Where the marks are

Correct definition of order (1 mark)
Rate becomes 4 times (1 mark)
Order is 2 (1 mark)

Revise the chapter

Chapter: Chemical Kinetics (notes, formulas, where marks are lost and every past question)
Topic: Integrated Rate Equations and Half-Life

Practise this chapterBoard-style questions from Chemical Kinetics, one at a time, marked against the answer key and marking points.Back to the full paper2026 board paper, set 56/5/1: every question, by section.

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CBSE Class 12 Chemistry 2026: question 23

Hints

Step-by-step solution

Revise the chapter

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