How do reaction profiles show energy changes and activation energy?
Reaction profiles for exothermic and endothermic reactions; activation energy; and how a catalyst lowers the activation energy.
A focused answer to AQA GCSE Chemistry 4.5.1, covering reaction profiles for exothermic and endothermic reactions, reading the overall energy change and activation energy from them, and how a catalyst lowers the activation energy.
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What this dot point is asking
AQA wants you to draw and interpret reaction profiles (energy level diagrams) for exothermic and endothermic reactions, identify the overall energy change and the activation energy, and explain how a catalyst lowers the activation energy. The profile is a visual summary of the whole energy story of a reaction, linking the bond-energy idea to the rate of reaction through the activation energy barrier.
Reading a reaction profile
- Exothermic: the products are lower in energy than the reactants, so the overall change is negative and energy is released to the surroundings.
- Endothermic: the products are higher in energy than the reactants, so the overall change is positive and energy is taken in from the surroundings.
The hump in the middle represents an unstable, high-energy arrangement of atoms partway through the reaction; reactant bonds are breaking and product bonds are forming there. The reaction can only proceed if colliding particles bring enough energy to climb to the top of this hump.
Activation energy
A reaction with a high activation energy is slow at room temperature because only a small fraction of collisions are energetic enough to react. This is why many fuels are stable in air until a spark provides the activation energy.
Catalysts and the profile
Because the catalyst is not used up and does not appear in the overall equation, it speeds the reaction without changing how much product can form or how much energy is released or absorbed overall.
Try this
Q1. On a reaction profile, where are the products for an exothermic reaction compared with the reactants? [1 mark]
- Cue. Lower in energy than the reactants.
Q2. Explain what a catalyst does to the activation energy. [2 marks]
- Cue. It provides an alternative pathway with a lower activation energy, so more particles can react.
Q3. A profile shows reactants at kJ, products at kJ, and a hump peak at kJ. State the activation energy. [1 mark]
- Cue. kJ.
Exam-style practice questions
Practice questions written in the style of AQA exam questions on this dot point, with worked answer explainers. The year tag is the paper they imitate, not the source.
AQA 20194 marksSketch and describe the reaction profile for an exothermic reaction. Label the reactants, the products, the activation energy and the overall energy change, and explain what each shows.Show worked answer →
A 4-mark Paper 1 question marked on the profile and its interpretation.
The profile (1 mark) starts at the reactant energy on the left, rises over a hump, then falls to a lower product energy on the right (products below reactants because it is exothermic). Activation energy (1 mark): the height from the reactant level up to the top of the hump, the minimum energy for colliding particles to react. Overall energy change (1 mark): the vertical drop from the reactant level to the product level, which is negative (energy released). Description (1 mark): because the products are lower in energy, energy is transferred to the surroundings.
Markers reward the products being drawn below the reactants and both energy quantities labelled from the correct reference levels.
AQA 20223 marksA catalyst is added to a reaction. Explain, with reference to a reaction profile, how the catalyst increases the rate of reaction, and state what happens to the overall energy change.Show worked answer →
A 3-mark question linking the profile to rate.
The catalyst provides an alternative reaction pathway with a lower activation energy (1 mark), shown as a lower hump on the profile. This means a greater proportion of colliding particles have at least the activation energy, so more collisions are successful per second and the rate increases (1 mark). The overall energy change is unchanged because the reactant and product energy levels are not affected (1 mark).
Markers want the lower-barrier and the more-successful-collisions ideas, plus the explicit statement that the energy change stays the same.
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Sources & how we know this
- AQA GCSE Chemistry (8462) specification — AQA (2016)