Hand warmers contain chemicals that undergo an exothermic reaction when activated. A student claims that adding a catalyst to the hand warmer mixture would make it release heat faster without changing the total amount of heat released. Explain why the student's claim is correct, referring to activation energy and the reaction profile.

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).

Hand warmers are single-use packets used by hikers and athletes. They contain a chemical mixture that reacts when exposed to air, releasing heat that warms the user's hands.

Model answer (5 marks)

A catalyst provides an alternative reaction pathway with a lower activation energy.

Because the activation energy is lower, a greater proportion of the reacting molecules have enough energy to reach the transition state, so the rate of reaction increases and heat is released more quickly.

The catalyst is not consumed; it is regenerated at the end of the reaction.

The overall enthalpy change (ΔH) – the difference in energy between reactants and products – is unchanged, so the total heat released is the same.

On the reaction profile the peak (transition state) is lower with the catalyst, but the energy levels of the reactants and products remain unchanged, giving the same total heat output.

Examiner tips

  • Use the phrase "lower activation energy" and link it to a higher rate. Show that the catalyst is not consumed and that ΔH is unchanged. Mention the reaction profile – lower peak, same reactant/product levels.

Common mistakes

  • Confusing activation energy with overall enthalpy change. Saying the catalyst changes the total heat released. Forgetting to state that the catalyst is regenerated.

Mark scheme (5 marks)

  1. A catalyst provides a different reaction pathway with a lower activation energy
  2. More particles have (at least) the activation energy, so the rate of reaction increases / heat is released faster
  3. The catalyst is not used up / not consumed during the reaction
  4. The overall energy change (difference in energy between reactants and products) remains the same
  5. On the reaction profile, the peak (transition state) is lower with the catalyst but the energy levels of reactants and products are unchanged, so the total heat released is the same

Key terms in this question

activation energy · catalyst · reaction profile · exothermic

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