ChemistryCBSE Class 12 · Sample paper, session 2024-25 · 1 mark · MCQ

CBSE Class 12 Chemistry sample paper 2024-25: question 10

From the official CBSE sample question paper for 2024-25, section A, question 10, for 1 mark. Try it first; the hints and our step-by-step solution are below.

Q10 · 1 mark · MCQChemical Kinetics

A first-order reaction is found to have a rate constant, $k = 5.5 \times 1 0^{- 14} s^{- 1}$ . The time taken for completion of the reaction is:

(A) $1.26 \times 1 0^{13}$ s
(B) $2.52 \times 1 0^{13}$ s
(C) $0.63 \times 1 0^{13}$ s
(D)It never goes to completion

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

Write [R] as a function of t for a first order reaction.

Hint 2

Can $e^{- k t}$ ever become exactly zero?

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.

For a first order reaction, $[R] = [R]_{0} e^{- k t}$ .
[R] = 0 needs $e^{- k t} = 0$ , which happens only as $t \to \infty$ . So the reaction never goes to completion. Option (D).
1 mark

Answer: Option (D)

Where the marks are

Uses the exponential decay law to show completion needs infinite time (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 paperSample paper 2024-25: every question, by section.

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CBSE Class 12 Chemistry sample paper 2024-25: question 10

Hints

Step-by-step solution

Revise the chapter

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