AQA · A-Level · Chemistry
Course version: 7405 · Version from Sept 2016
Students should be able to: define the term activation energy explain why…
Full specification wording
Collision theory: Students should be able to: define the term activation energy explain why most collisions do not lead to a reaction.
Two related things are being asked for.
Checked against the AQA 7405 specification (opens in a new tab) on .
- Point 3 of 3
- 4 key terms
Study notes
From the checked teaching notes for this specification point.
The idea
Two related things are being asked for. First the definition: activation energy is the minimum energy colliding particles must possess for a reaction to occur. Second the explanation of failure, which is a statement about a distribution. At any temperature the particles have a wide spread of kinetic energies, and only the small area under the tail of that distribution beyond Eₐ represents particles energetic enough to react. Most collisions therefore involve particles below the barrier, so the particles simply bounce apart unchanged. Orientation adds a second filter: even an energetic collision can fail if the reacting groups are pointing the wrong way. The proportion of collisions that succeed, not the sheer number of collisions, controls the rate.
What earns the marks
- The definition, the minimum energy required for colliding particles to react.
- Stating that most particles have energy below the activation energy.
- Concluding that those collisions leave the particles unchanged, so no reaction occurs.
- Citing incorrect orientation as an additional reason why an energetic collision may fail.
Common mistakes and how to put them right
Explaining failure by saying the particles do not collide, rather than that they collide without enough energy.
Saying the particles do not have enough activation energy, treating Eₐ as something a particle carries.
Answering in terms of chance or luck rather than the energy distribution.
Assuming all particles at a given temperature have the same energy.
Leaving out orientation when the question asks for two reasons.
Exam tips
- Structure a two mark answer as definition then consequence: the minimum energy, and most particles being below it.
- Refer to the proportion of successful collisions rather than the number of collisions when the temperature is unchanged.
- Avoid the words chance and probability; the mark scheme ignores them.
Try the lesson
The start of the Collision theory: Students should be able to: define the term activation energy explain why most collisions do not lead to a reaction. lesson, made from checked teaching notes for this specification point.
Have a go first
Which definition of activation energy is correct?
Listen
Two related things are being asked for. First the definition: activation energy is the minimum energy colliding particles must possess for a reaction to occur. Second the explanation of failure, which is a statement about a distribution. At any temperature the particles have a wide spread of kinetic energies, and only the small area under the tail of that distribution beyond Eₐ represents particles energetic enough to react. Most collisions therefore involve particles below the barrier, so the particles simply bounce apart unchanged. Orientation adds a second filter: even an energetic collision can fail if the reacting groups are pointing the wrong way. The proportion of collisions that succeed, not the sheer number of collisions, controls the rate. A strong answer earns marks for these points. the definition, the minimum energy required for colliding particles to react. stating that most particles have energy below the activation energy. concluding that those collisions leave the particles unchanged, so no reaction occurs. citing incorrect orientation as an additional reason why an energetic collision may fail. Exam tip: Structure a two mark answer as definition then consequence: the minimum energy, and most particles being below it.
Key words
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activation energy
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spread of molecular energies
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unsuccessful collisions
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orientation
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See every definition
- activation energy
- The minimum energy colliding particles must possess for a reaction to occur.
- spread of molecular energies
- At any temperature the particles have a wide spread of kinetic energies, with only the small area under the tail beyond Eₐ representing particles energetic enough to react.
- unsuccessful collisions
- Most collisions involve particles below the barrier, so the particles simply bounce apart unchanged.
- orientation
- Even an energetic collision can fail if the reacting groups are pointing the wrong way.
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