AQA · GCSE · Chemistry
Course version: 8462 · Version from Sept 2018
Students should be able to: draw simple reaction profiles (energy level…
Full specification wording
Reaction profiles: Students should be able to: draw simple reaction profiles (energy level diagrams) for exothermic and endothermic reactions showing the relative energies of reactants and products, the activation energy and the overall energy change, with a curved line to show the energy as the reaction proceeds use reaction profiles to identify reactions as exothermic or endothermic explain that the activation energy is the energy needed for a reaction to occur.
This practical skill combines drawing and interpreting reaction profiles.
Checked against the AQA 8462 specification (opens in a new tab) on .
- Point 4 of 4
- 4 key terms
Study notes
From the checked teaching notes for this specification point.
The idea
This practical skill combines drawing and interpreting reaction profiles. To draw one, mark energy on the vertical axis and reaction progress on the horizontal axis, place the reactant level, then draw a curved line rising to a peak before falling to the product level. For an exothermic reaction the product level sits below the reactants; for an endothermic reaction it sits above, but the curve still descends from the peak to reach it. Label the activation energy as the gap from the reactants to the peak, and the overall energy change as the gap between reactant and product levels. To identify a reaction type, compare the two levels: lower products mean energy is released, so the reaction is exothermic; higher products mean energy is absorbed, so it is endothermic. Activation energy is the minimum energy colliding particles need for a reaction to occur.
What earns the marks
- Draw axes with energy vertical and reaction progress horizontal, and place reactant and product levels at appropriate heights.
- Use a single curved line that rises from the reactants to a peak and then descends to the products.
- For an exothermic profile, place the product level below the reactant level; for an endothermic profile, place it above.
- Label the activation energy as the vertical gap from the reactant level to the peak.
- Label the overall energy change as the vertical difference between reactant and product levels.
- Identify a reaction as exothermic when products are lower in energy and endothermic when products are higher.
- Explain activation energy as the minimum energy that colliding particles must have for a reaction to occur.
Common mistakes and how to put them right
Drawing the product level below the reactants for an endothermic reaction.
Put it right: Endothermic products must be higher in energy.
Omitting the curved line and joining levels with straight lines, which fails to show the energy barrier.
Labelling the overall energy change as the distance from the peak to the products rather than from reactants to products.
Drawing the curve rising from the peak to the products for an endothermic reaction.
Put it right by ensuring the curve always falls from the peak to the product level.
Exam tips
- Sketch the curve first, then add labels and arrows so the diagram stays clear and uncluttered.
- Use a ruler for the vertical energy arrows but keep the reaction pathway line curved.
- When asked to identify a reaction type, quote the relative heights of the reactant and product levels as your evidence.
Try the lesson
The start of the Reaction profiles: Students should be able to: draw simple reaction profiles (energy level diagrams) for exothermic and endothermic reactions showing the relative energies of reactants and products, the activation energy and the overall energy change, with a curved line to show the energy as the reaction proceeds use reaction profiles to identify reactions as exothermic or endothermic explain that the activation energy is the energy needed for a reaction to occur. lesson, made from checked teaching notes for this specification point.
Have a go first
On a reaction profile, the products are at a lower energy level than the reactants. What type of reaction is this?
Listen
This practical skill combines drawing and interpreting reaction profiles. To draw one, mark energy on the vertical axis and reaction progress on the horizontal axis, place the reactant level, then draw a curved line rising to a peak before falling to the product level. For an exothermic reaction the product level sits below the reactants; for an endothermic reaction it sits above, but the curve still descends from the peak to reach it. Label the activation energy as the gap from the reactants to the peak, and the overall energy change as the gap between reactant and product levels. To identify a reaction type, compare the two levels: lower products mean energy is released, so the reaction is exothermic; higher products mean energy is absorbed, so it is endothermic. Activation energy is the minimum energy colliding particles need for a reaction to occur. A strong answer earns marks for these points. Draw axes with energy vertical and reaction progress horizontal, and place reactant and product levels at appropriate heights. Use a single curved line that rises from the reactants to a peak and then descends to the products. For an exothermic profile, place the product level below the reactant level; for an endothermic profile, place it above. Label the activation energy as the vertical gap from the reactant level to the peak. A common slip: Drawing the curve rising from the peak to the products for an endothermic reaction. Instead, ensuring the curve always falls from the peak to the product level. Exam tip: Sketch the curve first, then add labels and arrows so the diagram stays clear and uncluttered.
Key words
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Energy level diagrams
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Exothermic and endothermic reactions
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Activation energy as an energy barrier
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Interpreting graphical models of reactions
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See every definition
- Energy level diagrams
- Simple reaction profiles showing the relative energies of reactants and products, the activation energy and the overall energy change, with a curved line to show the energy as the reaction proceeds.
- Exothermic and endothermic reactions
- Reactions identified from reaction profiles by comparing the two levels: lower products mean energy is released, so the reaction is exothermic; higher products mean energy is absorbed, so it is endothermic.
- Activation energy as an energy barrier
- The energy needed for a reaction to occur, labelled as the vertical gap from the reactant level to the peak.
- Interpreting graphical models of reactions
- Using reaction profiles to identify reactions as exothermic or endothermic and to explain that the activation energy is the energy needed for a reaction to occur.
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Test your understanding of Reaction profiles: Students should be able to: draw simple reaction profiles (energy level diagrams) for exothermic and endothermic reactions showing the relative energies of reactants and products, the activation energy and the overall energy change, with a curved line to show the energy as the reaction proceeds use reaction profiles to identify reactions as exothermic or endothermic explain that the activation energy is the energy needed for a reaction to occur.
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