A student is investigating two different reactions. Reaction A is exothermic and Reaction B is endothermic. Using your knowledge of reaction profiles and activation energy, explain how energy changes during a chemical reaction and how a catalyst affects the rate of reaction.
Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).
A student notices that one reaction releases heat to the surroundings whilst another reaction takes in heat from the surroundings. Both reactions can be sped up by adding a suitable catalyst.
Model answer (5 marks)
A reaction profile shows the relative energies of reactants and products and the activation energy of the reaction.
Activation energy is the minimum amount of energy that particles must collide with to react.
In an exothermic reaction the products have less energy than the reactants, so energy is released to the surroundings.
In an endothermic reaction the products have more energy than the reactants, so energy is absorbed from the surroundings.
A catalyst increases the rate of reaction by providing a different reaction pathway with a lower activation energy, and it is not used up during the reaction.
Activation energy is the minimum amount of energy that particles must collide with to react.
In an exothermic reaction the products have less energy than the reactants, so energy is released to the surroundings.
In an endothermic reaction the products have more energy than the reactants, so energy is absorbed from the surroundings.
A catalyst increases the rate of reaction by providing a different reaction pathway with a lower activation energy, and it is not used up during the reaction.
Examiner tips
- Use the exact phrases from the mark scheme; include all five points.
- Show the reaction profile concept first, then activation energy, then exothermic/endothermic energy changes, finally catalyst effect.
- Keep each point concise and separate with line breaks or bullets.
Common mistakes
- Confusing activation energy with overall enthalpy change.
- Saying the catalyst is consumed or that it changes the enthalpy of the reaction.
- Omitting the statement that the products have higher/lower energy than reactants.
Mark scheme (5 marks)
- A reaction profile shows the relative energies of reactants and products and the activation energy of the reaction
- Activation energy is the minimum amount of energy that particles must collide with to react
- In an exothermic reaction the products have less energy than the reactants, so energy is released to the surroundings
- In an endothermic reaction the products have more energy than the reactants, so energy is absorbed from the surroundings
- A catalyst increases the rate of reaction by providing a different reaction pathway with a lower activation energy, and it is not used up during the reaction
Key terms in this question
reaction profile · activation energy · exothermic · endothermic · catalyst
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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