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Consider the graph… (Class 12 Chemistry SQP 2025-26, Q18)

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ChemistryCBSE Class 12 · Sample paper, session 2025-26 · 2 marks · Very short answer

CBSE Class 12 Chemistry sample paper 2025-26: question 18

From the official CBSE sample question paper for 2025-26, section B, question 18, for 2 marks. Try it first; the hints and our step-by-step solution are below.

Q18 · 2 marks · Very short answerCompetency-basedChemical Kinetics

Consider the graph for the reaction

I. Calculate enthalpy of reaction and activation energy for the backward reaction.
II. How will the catalyst affect the rate of this reaction? Explain.

(for visually challenged learners)
I. Define activation energy. What will happen to activation energy if we increase the temperature?
II. How will the catalyst affect the rate of an endothermic reaction?

Figure in the paper: Potential energy (y-axis, marked 100, 110, 120, 130, 140) versus reaction coordinate (x-axis). Reactants H2 + I2 start at about 100; the curve rises to a peak labelled 'Activated complex' at about 140, then falls to products 2HI at a level of about 100 on the right (marked at the baseline near 100). See it in the official paper.

Source: official sample paper from cbseacademic.nic.in (PDF). See it in the full Sample paper 2025-26.

Hints

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

Hint 1

Read three energy levels from the graph: reactants, peak and products.

Hint 2

ΔH is products minus reactants; the backward Ea is measured from the products up to the peak.

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.

  1. I. Reading from the graph: reactants (H + I) at 110 kJ mol, activated complex at 140 kJ mol, products (2HI) at 100 kJ mol.

    kJ mol (magnitude 10 kJ mol; the reaction is exothermic).

    kJ mol.

    1 mark

  2. II. A catalyst increases the rate of reaction. It provides an alternative pathway with a lower activation energy, forming an intermediate complex with the reactants. More molecules then have energy ≥ . It lowers for the forward and backward reactions equally and does not change .

    1 mark

  3. (For visually challenged learners)

    I. Activation energy is the minimum extra energy the reactants must absorb to form the activated complex. Raising the temperature does not change ; it only increases the fraction of molecules with energy ≥ .

    II. A catalyst increases the rate of an endothermic reaction too, by providing a path of lower activation energy.

Answer: I. ΔH = −10 kJ mol⁻¹ (magnitude 10 kJ mol⁻¹); Ea(backward) = 40 kJ mol⁻¹. II. A catalyst increases the rate by lowering the activation energy.

Where the marks are

  • Correct ΔH (10 kJ/mol) and Ea(backward) = 40 kJ/mol from the graph (1 mark)
  • Catalyst increases rate by providing an alternative path of lower activation energy (1 mark)

Revise the chapter

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 2025-26: 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.