ChemistryCBSE Class 12 · Board exam 2024, set 56/5/1 · 3 marks · Short answer

CBSE Class 12 Chemistry 2024: question 27

Set in the CBSE Class 12 board exam of 2024, paper 56/5/1, section C, question 27, for 3 marks. Try it first; the hints and our step-by-step solution are below.

Q27 · 3 marks · Short answerChemical Kinetics

The rate constant of a reaction quadruples when the temperature changes from 300 K to 320 K. Calculate the activation energy for this reaction.
[log 2 = 0.30, log 4 = 0.60, 2.303 R = 19.15 J $K^{- 1} mo l^{- 1}$ ]

Source: official question paper from cbse.gov.in (PDF). See it in the full 2024 board paper, set 56/5/1.

Hints

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

Hint 1

'Quadruples' means $k_{2} / k_{1} = 4$ .

Hint 2

Use $lo g (k_{2} / k_{1}) = \frac{E _{a}}{2.303 R} \cdot \frac{T _{2} - T _{1}}{T _{1} T _{2}}$ and solve for $E_{a}$ .

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.

Arrhenius equation in two-temperature form:
$lo g \frac{k _{2}}{k _{1}} = \frac{E _{a}}{2.303 R} (\frac{T _{2} - T _{1}}{T _{1} T _{2}})$
1 mark
With $k_{2} / k_{1} = 4$ , $T_{1} = 300$ K and $T_{2} = 320$ K:
$0.60 = \frac{E _{a}}{19.15} \times \frac{20}{300 \times 320}$
1 mark
$E_{a} = \frac{0.60 \times 19.15 \times 96000}{20} = 55152 J mol^{- 1} \approx 55.15 kJ mol^{- 1}$
1 mark

Answer: $E_{a} \approx 55.15$ kJ mol $^{- 1}$ (55152 J mol $^{- 1}$ )

Where the marks are

Arrhenius two-temperature equation (1 mark)
Correct substitution with log 4 = 0.60 (1 mark)
$E_{a}$ = 55.15 kJ 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 paper2024 board paper, set 56/5/1: every question, by section.

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CBSE Class 12 Chemistry 2024: question 27

Hints

Step-by-step solution

Revise the chapter

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