For the first order thermal decomposition reaction, following data was obtained : C_2H_5Cl(g) → C_2H_4(g) + HCl(g) S. No. Time(s) Total Pressure (atm) --- --- --- 1 0 0.30 2 30 0.50 Calculate rate constant. [Given : log 3 = 0.48]

k 3.68×10^-2\ s^-1 (0.0368 s⁻¹)

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

CBSE Class 12 Chemistry 2026: question 22

Set in the CBSE Class 12 board exam of 2026, paper 56/1/1, section C, question 22, for 3 marks. Try it first; the hints and our step-by-step solution are below.

Q22 · 3 marks · Short answerChemical Kinetics

For the first order thermal decomposition reaction, following data was obtained :
$C_{2} H_{5} Cl (g) \to C_{2} H_{4} (g) + HCl (g)$

S. No.	Time(s)	Total Pressure (atm)
1	0	0.30
2	30	0.50

Calculate rate constant. [Given : log 3 = 0.48]

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

One molecule of reactant gives two molecules of product, so the total pressure rises by x.

Hint 2

Express the reactant's pressure at time t in terms of p₀ and p_t.

Hint 3

Use k = (2.303/t) log(p₀/p_A).

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.

Let $p_{0}$ be the initial pressure and $x$ the decrease in the pressure of $C_{2} H_{5} Cl$ at time $t$ :
$C_{2} H_{5} Cl$ $C_{2} H_{4}$ HCl
$t = 0$ $p_{0}$ 0 0
$t$ $p_{0} - x$ $x$ $x$
The total pressure is $p_{t} = p_{0} + x$ , so $x = p_{t} - p_{0}$ and $p_{A} = p_{0} - x = 2 p_{0} - p_{t}$ .
1 mark
With $p_{0} = 0.30$ atm and $p_{t} = 0.50$ atm:
$x = 0.20$ atm and $p_{A} = 0.60 - 0.50 = 0.10$ atm.
First-order equation: $k = \frac{2.303}{t} lo g \frac{p _{0}}{2 p _{0} - p _{t}} = \frac{2.303}{30} lo g \frac{0.30}{0.10}$
1 mark
$k = \frac{2.303 \times 0.48}{30} = \frac{1.105}{30} = 3.68 \times 1 0^{- 2} s^{- 1}$
1 mark

	$C_{2} H_{5} Cl$	$C_{2} H_{4}$	HCl
$t = 0$	$p_{0}$	0	0
$t$	$p_{0} - x$	$x$	$x$

Answer: $k \approx 3.68 \times 1 0^{- 2} s^{- 1}$ (0.0368 s⁻¹)

Where the marks are

Relates the pressure of the reactant to the total pressure: p_A = 2p₀ − p_t (1 mark)
Correct first-order formula with substitution (1 mark)
Correct value k ≈ 3.68 × 10⁻² s⁻¹ with unit (1 mark)

Revise the chapter

Chapter: Chemical Kinetics (notes, formulas, where marks are lost and every past question)
Topic: Rate Law, Order and Molecularity

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.

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CBSE Class 12 Chemistry 2026: question 22

Hints

Step-by-step solution

Revise the chapter

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