ChemistryCBSE Class 12 · Sample paper, session 2023-24 · 3 marks · Short answer

CBSE Class 12 Chemistry sample paper 2023-24: question 28

From the official CBSE sample question paper for 2023-24, section C, question 28, for 3 marks. Try it first; the hints and our step-by-step solution are below.

Q28 · 3 marks · Short answerChemical Kinetics

The rate constants of a reaction at 200K and 500K are $0.02 s^{- 1}$ and $0.20 s^{- 1}$ respectively. Calculate the value of Ea (Given $2.303 R = 19.15 J K^{- 1} mo l^{- 1}$ )

Source: official sample paper from cbseacademic.nic.in (PDF). See it in the full Sample paper 2023-24.

Hints

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

Hint 1

Use the form of the Arrhenius equation that relates rate constants at two temperatures.

Hint 2

k₂/k₁ = 10, so the log term is simple.

Hint 3

Work out (T₂ – T₁)/(T₁T₂) carefully before solving for Ea.

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.

Arrhenius equation: $lo g \frac{k _{2}}{k _{1}} = \frac{E _{a}}{2.303 R} (\frac{T _{2} - T _{1}}{T _{1} T _{2}})$
1 mark
$lo g \frac{0.20}{0.02} = \frac{E _{a}}{19.15} (\frac{500 - 200}{200 \times 500})$ , so $1 = \frac{E _{a}}{19.15} \times 3 \times 1 0^{- 3}$
1 mark
$E_{a} = \frac{19.15}{3 \times 1 0 ^{- 3}} = 6383.3 J mo l^{- 1} \approx 6.38 kJ mo l^{- 1}$
1 mark

Answer: Ea ≈ 6383 J mol⁻¹ (6.38 kJ mol⁻¹)

Where the marks are

Correct Arrhenius two-temperature equation (1 mark)
Correct substitution with log 10 = 1 and (T₂ – T₁)/T₁T₂ = 3 × 10⁻³ (1 mark)
Ea = 6383 J mol⁻¹ with unit (1 mark)

Revise the chapter

Chapter: Chemical Kinetics (notes, formulas, where marks are lost and every past question)
Topic: Arrhenius Equation and Activation Energy

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 2023-24: every question, by section.

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CBSE Class 12 Chemistry sample paper 2023-24: question 28

Hints

Step-by-step solution

Revise the chapter

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