Cold packs are used by athletes to treat injuries. When a cold pack is activated, an endothermic reaction occurs inside it. Explain, using the concept of activation energy and a reaction profile, how a catalyst would affect this endothermic reaction, and describe the overall energy change that would be shown on the reaction profile.
Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).
Cold packs contain chemicals that undergo an endothermic reaction when mixed. Athletes press the pack to activate it and apply it to an injury to reduce swelling. A sports scientist suggests that adding a catalyst to the cold pack would make it work more quickly.
Model answer (5 marks)
A catalyst offers an alternative pathway with a lower activation energy.
Because the activation energy is lower, a larger proportion of the reacting molecules will possess the required energy to reach the transition state, so the rate of the endothermic reaction increases.
The catalyst does not alter the enthalpy change of the reaction; the products still have a higher energy than the reactants.
On the reaction profile the curve rises to a lower peak (lower E‡) but the final product energy level remains above the reactant level.
Thus the cold pack absorbs energy from the surroundings – the skin feels cold because energy is taken in by the reaction.
Because the activation energy is lower, a larger proportion of the reacting molecules will possess the required energy to reach the transition state, so the rate of the endothermic reaction increases.
The catalyst does not alter the enthalpy change of the reaction; the products still have a higher energy than the reactants.
On the reaction profile the curve rises to a lower peak (lower E‡) but the final product energy level remains above the reactant level.
Thus the cold pack absorbs energy from the surroundings – the skin feels cold because energy is taken in by the reaction.
Examiner tips
- Mention the lower activation energy and higher rate; state that ΔH is unchanged; describe the reaction profile with a lower peak but same product energy level.
- Use the words ‘catalyst’, ‘activation energy’, ‘rate’, ‘endothermic’, ‘energy change’ as per the mark scheme.
- Explain that the surroundings lose energy, giving the sensation of cold.
Common mistakes
- Saying the catalyst changes the overall energy change; it only lowers the activation energy.
- Confusing the reaction profile peak with the final product energy; the peak is lower but the product energy remains higher than reactants.
- Using UK spelling incorrectly or omitting the term ‘endothermic’.
Mark scheme (5 marks)
- A catalyst provides a different reaction pathway with a lower activation energy
- A larger proportion of particles will have at least the activation energy, so the rate of reaction increases
- The catalyst does not change the overall energy change of the reaction
- On the reaction profile, the products have more energy than the reactants for an endothermic reaction
- The cold pack absorbs energy from the surroundings / the surroundings (skin/injury) feel cold because energy is taken in by the reaction
Key terms in this question
activation energy · catalyst · reaction profile · endothermic
Related
- All OCR A-Level Chemistry A (H432) revision notes →
- How to answer a "Explain" question →
- Decode the mark scheme abbreviations →
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