AQA · GCSE · Combined Science
Course version: 8464 · Version from Sept 2018
Reaction profiles: 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
A reaction profile is a graph of energy against reaction progress.
- 4 key terms
Study notes
From the checked teaching notes for this specification point.
The idea
A reaction profile is a graph of energy against reaction progress. Draw a horizontal axis labelled progress of reaction and a vertical axis labelled energy. Mark the reactant energy with a short horizontal line and the product energy with another. For an exothermic reaction the product line is lower than the reactant line; for an endothermic reaction it is higher. Join the levels with a single curved line that rises to a peak then falls. The peak represents the transition state. Activation energy is the vertical distance from the reactant level to the peak. Overall energy change is the vertical difference between reactant and product levels, negative for exothermic and positive for endothermic. Label all three features with arrows and ΔH notation.
What earns the marks
- Axes correctly labelled: progress of reaction on the horizontal axis and energy on the vertical axis, with no numerical scale required.
- Reactant and product energies shown as short horizontal lines at different heights, lower products for exothermic and higher products for endothermic.
- A single curved line connects the reactant level to the product level via a maximum, showing energy changing continuously as the reaction proceeds.
- Activation energy labelled as the energy difference from the reactant level to the peak of the curve.
- Overall energy change labelled as the energy difference between reactant and product levels, with the correct sign convention.
- Exothermic profile shows products at lower energy than reactants; endothermic profile shows products at higher energy than reactants.
Common mistakes and how to put them right
Drawing the curve as a straight diagonal line.
Put it right by using a smooth curve that rises to a peak and then falls to the product level.
Labelling activation energy from the product level to the peak.
Put it right by measuring activation energy from the reactant level to the peak.
Showing products higher than reactants for an exothermic reaction.
Put it right by placing the product line below the reactant line for exothermic reactions.
Exam tips
- Sketch both profiles side by side and annotate each with reactant, product, activation energy and overall energy change before writing your final answer.
- Use arrows with clear arrowheads for activation energy and overall energy change so the examiner can see exactly which gap you are labelling.
- Check the sign of the overall energy change: exothermic reactions release energy so ΔH is negative, endothermic reactions absorb energy so ΔH is positive.
Try the lesson
The start of the Reaction profiles: 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 lesson, made from checked teaching notes for this specification point.
Have a go first
On a reaction profile for an exothermic reaction, how does the energy of the products compare with the energy of the reactants?
Listen
A reaction profile is a graph of energy against reaction progress. Draw a horizontal axis labelled progress of reaction and a vertical axis labelled energy. Mark the reactant energy with a short horizontal line and the product energy with another. For an exothermic reaction the product line is lower than the reactant line; for an endothermic reaction it is higher. Join the levels with a single curved line that rises to a peak then falls. The peak represents the transition state. Activation energy is the vertical distance from the reactant level to the peak. Overall energy change is the vertical difference between reactant and product levels, negative for exothermic and positive for endothermic. Label all three features with arrows and ΔH notation. A strong answer earns marks for these points. Axes correctly labelled: progress of reaction on the horizontal axis and energy on the vertical axis, with no numerical scale required. Reactant and product energies shown as short horizontal lines at different heights, lower products for exothermic and higher products for endothermic. A single curved line connects the reactant level to the product level via a maximum, showing energy changing continuously as the reaction proceeds. Activation energy labelled as the energy difference from the reactant level to the peak of the curve. A common slip: Drawing the curve as a straight diagonal line. Instead, using a smooth curve that rises to a peak and then falls to the product level. Exam tip: Sketch both profiles side by side and annotate each with reactant, product, activation energy and overall energy change before writing your final answer.
Key words
Say what each word means before you turn the card.
Reaction profile
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Activation energy
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Overall energy change
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Transition state
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See every definition
- Reaction profile
- A graph of energy against reaction progress, joining the reactant and product levels with a single curved line that rises to a peak then falls.
- Activation energy
- The vertical distance from the reactant level to the peak of the curve.
- Overall energy change
- The vertical difference between reactant and product levels, negative for exothermic and positive for endothermic.
- Transition state
- The peak of the curved line, representing the transition state.
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Test your understanding of Reaction profiles: 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
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