A reaction profile shows relative energy as a reaction proceeds. Its horizontal axis represents progress through a pathway, not clock time.
Compare the reactant and product levels to classify the overall change. Lower-energy products correspond to an exothermic forward reaction; higher-energy products correspond to an endothermic one.
Activation energy is the barrier from the reactant level to the highest point on the forward pathway. It is different from the overall reactant-to-product change.
A catalyst gives the reaction a different pathway with lower activation energy. The reactant and product energy levels stay the same. The diagram uses a made-up energy scale to show the differences. It does not tell you how fast the reaction happens.
Worked example
If reactants lie above products, energy is transferred to the surroundings overall, even though reacting particles must first overcome a barrier.
A lower catalysed peak does not move the products downwards or change the overall energy difference.
Specification references
AQA 4.5.1.2 and 4.6.1.4; Edexcel 7.7, 7.15–7.16. These questions ask you to interpret a profile. Practise drawing your own profiles separately.
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Explain why a catalysed profile can have a lower peak but the same overall energy change.
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Distinguish barrier and endpoint difference.
Explain the alternative pathway.
Keep the same reactants and products.
Example answer
The catalyst changes the pathway and lowers the activation barrier. The same reactants and products have the same relative endpoint energies, so the overall change between them remains the same.
An answer to improve
The lower peak means the reaction releases less energy.
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Name the system, direction and controlled conditions. Replace the unsupported claim with the causal link or evidence that is missing.
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