A student uses a reaction profile to compare two reactions. Reaction X has a small difference in energy between the reactants and the transition state, while Reaction Y has a large difference in energy between the reactants and the transition state. Both reactions release energy overall. Explain what the reaction profile tells us about the activation energy of each reaction and how this affects the rate at which each reaction proceeds at the same temperature.

OCR A-Level Chemistry A (H432) — 5.2 Energy · Explain · 5 marks · View as Markdown

Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).

Reaction profiles show the relative energies of reactants and products, the activation energy, and the overall energy change of a reaction. Reaction X and Reaction Y are both exothermic and are carried out at the same temperature.

Model answer (5 marks)

The activation energy is the minimum energy that must be supplied for the reaction to proceed. Reaction X has a smaller energy difference between the reactants and the transition state, so its activation energy is lower than that of Reaction Y. At the same temperature a larger proportion of the molecules in Reaction X will possess energy equal to or greater than its lower activation energy. Consequently Reaction X will proceed faster than Reaction Y. Both reactions are exothermic, so the products lie below the reactants on the profile for both X and Y.

Examiner tips

  • Use the term "activation energy" explicitly; link the smaller energy gap to a lower Ea. Show the kinetic consequence: higher proportion of molecules above Ea → faster rate. Mention that both are exothermic to satisfy the last point. Keep answer concise and to the point.

Common mistakes

  • Confusing activation energy with overall ΔE; writing that the lower Ea means lower overall energy change. Mixing up the direction of the energy change (products lower than reactants). Not explicitly stating that a lower Ea gives a higher rate at the same temperature.

Mark scheme (5 marks)

  1. The activation energy is the minimum amount of energy that particles must collide with in order to react
  2. Reaction X has a lower activation energy than Reaction Y because the energy difference between the reactants and the transition state is smaller
  3. At the same temperature, a larger proportion of particles in Reaction X will have energy equal to or greater than the activation energy
  4. Therefore Reaction X has a faster rate of reaction than Reaction Y
  5. Both reactions are exothermic, so on a reaction profile the energy of the products is lower than the energy of the reactants for both X and Y

Key terms in this question

activation energy · reaction profile

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