Reaction Profiles
Get to grips with Reaction Profiles for GCSE Chemistry with this free worksheet and full mark scheme — Foundation and Higher exam-style questions with worked answers for AQA, Edexcel and OCR. A reaction profile shows the activation energy — the minimum energy needed to react — and whether the reaction is exothermic or endothermic.
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These worksheets and mark schemes are original, written for Virtus Academy and checked against the current AQA, Edexcel and OCR specifications. Every worksheet comes with a full mark scheme.
Topic overview
A reaction profile shows the relative energies of the reactants and products, and how the energy changes during the reaction.
In an exothermic profile the products are lower than the reactants, because energy has been released. In an endothermic profile the products are higher, because energy has been taken in.
Every profile has a hump between the reactants and products. The height of that hump from the reactants is the activation energy — the minimum energy that colliding particles need for a reaction to occur. The overall energy change is measured from the reactants to the products, not from the top of the hump.
Revision notes
Exothermic profiles
The products are at a lower energy level than the reactants.
The difference between them is the energy released to the surroundings. The curve rises to a peak, then falls below the starting level.
Endothermic profiles
The products are at a higher energy level than the reactants.
The difference is the energy taken in from the surroundings. The curve rises to a peak and then falls, but ends above the starting level.
Activation energy
The activation energy is the minimum energy particles must have to react.
On the profile it is the height from the reactants up to the top of the hump. Catalysts lower the activation energy by providing a different pathway, but do not change the overall energy change.
Key points
- A reaction profile shows relative energies.
- In exothermic reactions the products are lower.
- In endothermic reactions the products are higher.
- Activation energy is the minimum energy needed to react.
- It is measured from reactants to the top of the hump.
- A catalyst lowers the activation energy.
Worked examples
Example 1
On a reaction profile, how can you tell a reaction is exothermic? [2 marks]
Model answer
The products are at a lower energy level than the reactants (1 mark), showing that energy has been released to the surroundings (1 mark).
Example 2
Define activation energy. [2 marks]
Model answer
Activation energy is the minimum amount of energy that colliding particles must have (1 mark) for a reaction to take place (1 mark).
Example 3
Explain the effect of a catalyst on a reaction profile. [2 marks]
Model answer
A catalyst lowers the activation energy by providing an alternative reaction pathway (1 mark), but it does not change the overall energy change between reactants and products (1 mark).
Common mistakes
Measuring the energy change from the top of the hump.
The overall change is measured from the reactants to the products.
Saying a catalyst changes the energy change.
It lowers only the activation energy.
Drawing an exothermic profile with products higher.
Exothermic means products are lower, because energy was released.
Forgetting to label the axes.
The vertical axis is energy and the horizontal is progress of reaction.
Exam tips
- Label reactants, products, activation energy and overall energy change.
- Measure the overall change between reactants and products only.
- Show a catalyst as a lower hump with the same start and end points.
- Say minimum energy in your definition of activation energy.
Key terms
- Reaction profile
- A diagram showing energy changes during a reaction.
- Activation energy
- The minimum energy needed for a reaction to occur.
- Catalyst
- A substance lowering the activation energy without being used up.
- Energy change
- The difference in energy between reactants and products.
Related topics
Written and reviewed against the current AQA, Edexcel and OCR specifications. Spotted an error? Let us know.