AQA · GCSE · Combined Science
Course version: 8464 · Version from Sept 2018
Collision theory and activation energy: predict and explain using collision theory the effects of changing conditions of concentration, pressure and temperature on the rate of a reaction
For concentration, increasing the amount of dissolved reactant in a given volume means more particles in the same space, so collisions become more frequent and the rate increases.
- 4 key terms
Study notes
From the checked teaching notes for this specification point.
The idea
For concentration, increasing the amount of dissolved reactant in a given volume means more particles in the same space, so collisions become more frequent and the rate increases. For pressure, increasing the pressure of reacting gases compresses the same number of particles into a smaller volume, again increasing collision frequency. For temperature, particles move faster, so collisions are more frequent and a greater proportion of collisions have energy greater than or equal to the activation energy. A student might predict that raising the temperature of sodium thiosulfate solution with dilute hydrochloric acid makes the cross disappear sooner, then explain this using collision theory.
What earns the marks
- Predicts that increasing concentration increases the rate of reaction.
- Explains that increasing concentration means more reactant particles in the same volume, so collisions are more frequent.
- Predicts that increasing the pressure of reacting gases increases the rate of reaction.
- Explains that increasing pressure brings gas particles closer together, so collisions are more frequent.
- Predicts that increasing temperature increases the rate of reaction.
- Explains that increasing temperature increases particle speed, so collisions are more frequent and a greater proportion of particles have energy greater than or equal to the activation energy.
- Uses the term successful collisions to describe collisions that lead to reaction.
Common mistakes and how to put them right
Claiming that increasing concentration increases the activation energy.
Put it right: Concentration changes collision frequency, not the activation energy.
Saying that increasing pressure increases the number of gas particles.
Put it right: Pressure increases because the same particles occupy a smaller volume, increasing collision frequency.
Explaining temperature only by saying particles move faster without mentioning activation energy.
Put it right: Include both collision frequency and the proportion of collisions with sufficient energy.
Exam tips
- Structure each explanation as condition changed, effect on particles, effect on collisions, effect on rate.
- Use the phrase energy greater than or equal to the activation energy when explaining temperature effects.
- For pressure questions, make clear that the change applies to gases and that volume decreases if pressure increases at constant temperature.
Try the lesson
The start of the Collision theory and activation energy: predict and explain using collision theory the effects of changing conditions of concentration, pressure and temperature on the rate of a reaction lesson, made from checked teaching notes for this specification point.
Have a go first
A student repeats an experiment reacting magnesium ribbon with hydrochloric acid, but uses a more concentrated solution of the acid. Using collision theory, why does the rate of reaction increase?
Listen
For concentration, increasing the amount of dissolved reactant in a given volume means more particles in the same space, so collisions become more frequent and the rate increases. For pressure, increasing the pressure of reacting gases compresses the same number of particles into a smaller volume, again increasing collision frequency. For temperature, particles move faster, so collisions are more frequent and a greater proportion of collisions have energy greater than or equal to the activation energy. A student might predict that raising the temperature of sodium thiosulfate solution with dilute hydrochloric acid makes the cross disappear sooner, then explain this using collision theory. A strong answer earns marks for these points. Predicts that increasing concentration increases the rate of reaction. Explains that increasing concentration means more reactant particles in the same volume, so collisions are more frequent. Predicts that increasing the pressure of reacting gases increases the rate of reaction. Explains that increasing pressure brings gas particles closer together, so collisions are more frequent. A common slip: Claiming that increasing concentration increases the activation energy. Instead, concentration changes collision frequency, not the activation energy. Exam tip: Structure each explanation as condition changed, effect on particles, effect on collisions, effect on rate.
See it
Key words
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Collision theory
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Activation energy
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Concentration and pressure
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Temperature and reaction rate
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See every definition
- Collision theory
- Used to predict and explain how changing conditions of concentration, pressure and temperature affect the rate of a reaction.
- Activation energy
- Collisions with energy greater than or equal to this value are needed for reaction when temperature increases.
- Concentration and pressure
- More reactant particles in the same volume, or gas particles brought closer together, make collisions more frequent and increase the rate.
- Temperature and reaction rate
- Particles move faster, so collisions are more frequent and a greater proportion have energy greater than or equal to the activation energy.
Label the diagram
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1Box 1
2Box 2
3Box 3
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