Collision Theory and Activation Energy
Build confidence in Collision Theory and Activation Energy 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. Reactions happen when particles collide with at least the activation energy; more frequent or more energetic collisions speed the reaction up.
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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
Collision theory explains how the factors affecting rate actually work. Chemical reactions can only occur when reacting particles collide with each other and with sufficient energy.
The minimum amount of energy that particles must have to react is the activation energy. A collision with less than this energy will not result in a reaction, however many collisions occur.
So increasing the rate means either making collisions more frequent, or making a greater proportion of them successful. Concentration, pressure and surface area work by the first mechanism; temperature works by both; and catalysts work by lowering the activation energy so more collisions succeed.
Revision notes
The two conditions
Particles must collide with each other, and they must collide with sufficient energy.
Both conditions are necessary. A collision with insufficient energy simply bounces the particles apart without a reaction occurring.
Activation energy
The activation energy is the minimum energy particles must have for a collision to be successful.
It varies from reaction to reaction. Reactions with a low activation energy proceed readily; those with a high one need more energy input to get started.
Explaining the factors
Concentration, pressure and surface area increase the frequency of collisions.
Temperature increases both the frequency and the proportion with sufficient energy. Catalysts lower the activation energy, so a greater proportion of existing collisions become successful.
Key points
- Particles must collide to react.
- Collisions must have sufficient energy.
- Activation energy is the minimum energy needed.
- Insufficient energy means no reaction occurs.
- Some factors increase collision frequency.
- Catalysts increase the proportion of successful collisions.
Worked examples
Example 1
State the two conditions needed for a reaction to occur between particles. [2 marks]
Model answer
The particles must collide with each other (1 mark), and the collision must have sufficient energy (1 mark).
Example 2
Define activation energy. [1 mark]
Model answer
The activation energy is the minimum amount of energy that particles must have in order to react (1 mark).
Example 3
Explain, using collision theory, why increasing concentration increases the rate of reaction. [2 marks]
Model answer
A higher concentration means there are more particles in the same volume, so they are closer together (1 mark). This means collisions between reacting particles occur more frequently, so the rate increases (1 mark).
Common mistakes
Saying every collision causes a reaction.
Only collisions with energy at or above the activation energy are successful.
Giving only collision frequency.
Energy is equally important, and both conditions must be stated.
Saying particles react when they are close.
They must actually collide, not merely be nearby.
Confusing activation energy with the overall energy change.
Activation energy is the barrier; the energy change is the difference between reactants and products.
Exam tips
- Always state both conditions: collision and sufficient energy.
- Use collision theory to explain every rate factor.
- Say more frequent collisions rather than more collisions.
- Keep activation energy and overall energy change distinct.
Key terms
- Collision theory
- The explanation that particles must collide with enough energy to react.
- Successful collision
- A collision that results in a reaction.
- Activation energy
- The minimum energy needed for a successful collision.
- Frequency
- How often collisions occur.
Related topics
Written and reviewed against the current AQA, Edexcel and OCR specifications. Spotted an error? Let us know.