ChemistryCBSE Class 12 · Board exam 2026, set 56/1/1 · 2 marks · Very short answer

CBSE Class 12 Chemistry 2026: question 21

Set in the CBSE Class 12 board exam of 2026, paper 56/1/1, section B, question 21, for 2 marks. Try it first; the hints and our step-by-step solution are below.

Q21 · 2 marks · Very short answerChemical Kinetics

Following reaction takes place in one step :
$2 A + B \to 2 C$
How will the rate of above reaction change if the volume of the reaction vessel is decreased to one third of its original volume ? Will there be any change in the order of reaction with the reduced volume ?

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

Hints

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

Hint 1

For a one-step reaction, the rate law exponents equal the stoichiometric coefficients.

Hint 2

How does concentration change when the volume is reduced to one third?

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.

Since the reaction is elementary (one step), the rate law follows the stoichiometry: $Rate_{1} = k [A]^{2} [B]$ .
Reducing the volume to $\frac{1}{3}$ makes each concentration 3 times larger:
$Rate_{2} = k (3 [A])^{2} (3 [B]) = 27 k [A]^{2} [B] = 27 \times Rate_{1}$ .
The rate becomes 27 times the original rate.
1 mark
The order ( $2 + 1 = 3$ ) depends only on the rate law, not on the volume, so it remains unchanged.
1 mark

Answer: The rate increases 27 times; the order remains the same (3).

Where the marks are

Writes rate = k[A]²[B] and shows the concentrations become 3 times, giving rate 27 times (1 mark)
States that the order of reaction does not change (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 56/1/1: 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.

CBSE Class 12 Chemistry 2026: question 21

Hints

Step-by-step solution

Revise the chapter

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