AQA · GCSE · Chemistry
Course version: 8462 · Version from Sept 2018
Chemical reactions can occur only when reacting particles collide with…
Full specification wording
Reaction profiles: Chemical reactions can occur only when reacting particles collide with each other and with sufficient energy.
This is the collision theory explanation of reaction rates.
Checked against the AQA 8462 specification (opens in a new tab) on .
- Point 1 of 4
- 4 key terms
Study notes
From the checked teaching notes for this specification point.
The idea
This is the collision theory explanation of reaction rates. For a reaction to happen, reactant particles must collide, and the collision must involve at least a certain minimum energy called the activation energy. A collision with less energy than the activation energy does not lead to reaction; the particles simply bounce apart. For example, in the reaction between hydrogen and oxygen, most collisions at room temperature lack sufficient energy, which is why the mixture can remain unreactive until a spark provides activation energy. Reaction profiles show the energy of reactants and products and the activation energy as the height from the reactants' energy level to the peak of the curve. Increasing temperature, concentration, pressure or surface area, or adding a catalyst, increases the frequency or success of collisions and so increases the rate.
What earns the marks
- States that particles must collide for a reaction to occur, and that not every collision is successful.
- Explains that a collision must have at least the activation energy for a reaction to take place.
- Describes activation energy as the minimum energy that colliding particles need to react.
- Uses reaction profiles to identify the energy of reactants, the energy of products and the activation energy.
- Links successful collisions to reaction rate, explaining that more frequent or more energetic collisions increase the rate.
Common mistakes and how to put them right
Claiming that any collision causes a reaction.
Put it right: Only collisions with at least the activation energy are successful.
Confusing activation energy with the overall energy change of the reaction.
Put it right: Activation energy is the energy needed to start the reaction, while overall energy change is the difference between reactants and products.
Saying that a catalyst increases the energy of the particles.
Put it right: A catalyst provides an alternative pathway with a lower activation energy.
Exam tips
- Use the phrase 'collide with sufficient energy' when explaining why a reaction occurs, and refer to activation energy by name.
- On a reaction profile, mark the activation energy as the vertical distance from the reactants' energy level to the peak of the curve.
- When explaining rate changes, state whether collision frequency, collision energy or both are affected.
Try the lesson
The start of the Reaction profiles: Chemical reactions can occur only when reacting particles collide with each other and with sufficient energy. lesson, made from checked teaching notes for this specification point.
Have a go first
According to collision theory, what two conditions are needed for a chemical reaction to occur?
Listen
This is the collision theory explanation of reaction rates. For a reaction to happen, reactant particles must collide, and the collision must involve at least a certain minimum energy called the activation energy. A collision with less energy than the activation energy does not lead to reaction; the particles simply bounce apart. For example, in the reaction between hydrogen and oxygen, most collisions at room temperature lack sufficient energy, which is why the mixture can remain unreactive until a spark provides activation energy. Reaction profiles show the energy of reactants and products and the activation energy as the height from the reactants' energy level to the peak of the curve. Increasing temperature, concentration, pressure or surface area, or adding a catalyst, increases the frequency or success of collisions and so increases the rate. A strong answer earns marks for these points. States that particles must collide for a reaction to occur, and that not every collision is successful. Explains that a collision must have at least the activation energy for a reaction to take place. Describes activation energy as the minimum energy that colliding particles need to react. Uses reaction profiles to identify the energy of reactants, the energy of products and the activation energy. A common slip: Claiming that any collision causes a reaction. Instead, only collisions with at least the activation energy are successful. Exam tip: Use the phrase 'collide with sufficient energy' when explaining why a reaction occurs, and refer to activation energy by name.
See it
Key words
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Collision theory
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Activation energy
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Reaction profiles
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Factors affecting reaction rate
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See every definition
- Collision theory
- Chemical reactions can occur only when reacting particles collide with each other and with sufficient energy.
- Activation energy
- The minimum energy that colliding particles need to react; a collision with less energy does not lead to reaction.
- Reaction profiles
- These show the energy of reactants and products and the activation energy as the height from the reactants' energy level to the peak of the curve.
- Factors affecting reaction rate
- Increasing temperature, concentration, pressure or surface area, or adding a catalyst, increases the frequency or success of collisions and so increases the rate.
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