AQA · GCSE · Chemistry
Course version: 8462 · Version from Sept 2018
Collision theory explains how various factors affect rates of reactions.
Full specification wording
Collision theory and activation energy: Collision theory explains how various factors affect rates of reactions.
Collision theory is the model used to explain why reaction rates change when conditions change.
Checked against the AQA 8462 specification (opens in a new tab) on .
- Point 1 of 7
- 4 key terms
Study notes
From the checked teaching notes for this specification point.
The idea
Collision theory is the model used to explain why reaction rates change when conditions change. It states that particles must collide with sufficient energy for a reaction to occur. Factors such as concentration, pressure, surface area, temperature and the presence of a catalyst affect how often particles collide or how many collisions have enough energy. For example, increasing the concentration of hydrochloric acid increases the number of acid particles in a given volume, so collisions with magnesium atoms become more frequent and the rate increases. Raising temperature increases both the frequency of collisions and the proportion of collisions with energy greater than or equal to the activation energy, so the rate increases more sharply. A catalyst provides an alternative pathway with a lower activation energy.
What earns the marks
- States that reactions occur when particles collide with sufficient energy.
- Explains that increasing concentration or pressure increases the frequency of collisions because there are more particles in a given volume.
- Explains that increasing surface area increases the frequency of collisions because more particles are exposed.
- Explains that increasing temperature increases both collision frequency and the proportion of collisions with energy greater than or equal to the activation energy.
- Explains that a catalyst increases the rate by providing an alternative pathway with a lower activation energy.
Common mistakes and how to put them right
Saying that increasing concentration makes particles collide harder.
Put it right: It increases the frequency of collisions because there are more particles per unit volume.
Saying that a catalyst increases the energy of particles.
Put it right: It lowers the activation energy by providing an alternative pathway.
Saying that increasing surface area increases the energy of collisions.
Put it right: It increases the frequency of collisions by exposing more particles.
Exam tips
- Name the factor and then explain its effect using collision frequency or energy, not both interchangeably.
- Use the phrase 'frequency of collisions' for concentration, pressure and surface area, and 'proportion of collisions with energy greater than or equal to the activation energy' for temperature.
- When explaining a catalyst, refer to an alternative pathway and a lower activation energy.
Try the lesson
The start of the Collision theory and activation energy: Collision theory explains how various factors affect rates of reactions. lesson, made from checked teaching notes for this specification point.
Have a go first
Which statement best describes how collision theory explains the effect of increasing the concentration of a solution on the rate of reaction?
Listen
Collision theory is the model used to explain why reaction rates change when conditions change. It states that particles must collide with sufficient energy for a reaction to occur. Factors such as concentration, pressure, surface area, temperature and the presence of a catalyst affect how often particles collide or how many collisions have enough energy. For example, increasing the concentration of hydrochloric acid increases the number of acid particles in a given volume, so collisions with magnesium atoms become more frequent and the rate increases. Raising temperature increases both the frequency of collisions and the proportion of collisions with energy greater than or equal to the activation energy, so the rate increases more sharply. A catalyst provides an alternative pathway with a lower activation energy. A strong answer earns marks for these points. States that reactions occur when particles collide with sufficient energy. Explains that increasing concentration or pressure increases the frequency of collisions because there are more particles in a given volume. Explains that increasing surface area increases the frequency of collisions because more particles are exposed. Explains that increasing temperature increases both collision frequency and the proportion of collisions with energy greater than or equal to the activation energy. A common slip: Saying that increasing concentration makes particles collide harder. Instead, it increases the frequency of collisions because there are more particles per unit volume. Exam tip: Name the factor and then explain its effect using collision frequency or energy, not both interchangeably.
See it
Key words
Say what each word means before you turn the card.
Collision theory
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Frequency of collisions
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Activation energy
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Catalyst
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See every definition
- Collision theory
- The model used to explain why reaction rates change when conditions change, stating that particles must collide with sufficient energy for a reaction to occur.
- Frequency of collisions
- How often particles collide, increased by raising concentration or pressure because there are more particles in a given volume, and by increasing surface area because more particles are exposed.
- Activation energy
- The energy that collisions must have at least for a reaction to occur; raising temperature increases the proportion of collisions with energy greater than or equal to it.
- Catalyst
- A substance that increases the rate by providing an alternative pathway with a lower activation energy.
Label the diagram
Some boxes have lost their names. Pick the right name for each number.
1Box 1
2Box 2
3Box 3
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