What is the rate of a reaction, and how does collision theory explain the factors that change it?
Rates of reaction and collision theory: the meaning of rate, how concentration, pressure, surface area and temperature affect rate, and the explanation in terms of collision frequency and activation energy.
A focused answer to Edexcel GCSE Chemistry topic 7, covering what the rate of a reaction means, collision theory and activation energy, and how concentration, pressure, surface area and temperature change the rate by altering the frequency and energy of collisions.
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What this dot point is asking
Edexcel wants you to define the rate of a reaction, state collision theory and activation energy, and explain how concentration, pressure, surface area and temperature change the rate by altering the frequency and energy of collisions. The collision-theory explanations for each factor are the core marks of the topic.
What rate means
A fast reaction (such as an explosion) finishes in a fraction of a second; a slow reaction (such as rusting) takes years. The rate is largest at the start, when the reactant concentration is highest, and falls to zero as the reactants run out.
Collision theory and activation energy
The rate of a reaction depends on the frequency of successful collisions: more successful collisions per second means a faster reaction. Every factor that speeds up a reaction does so by increasing this frequency.
How the factors affect rate
- Concentration (of a solution)
- A higher concentration means more particles in the same volume, so they collide more frequently, and the rate increases.
- Pressure (of a gas)
- Increasing the pressure squeezes the gas particles closer together, so again they collide more frequently, and the rate increases.
- Surface area (of a solid)
- Breaking a solid into smaller pieces or a powder increases the surface area to volume ratio, exposing more particles, so there are more frequent collisions at the surface, and the rate increases.
- Temperature
- Raising the temperature gives particles more kinetic energy, so they move faster and collide more frequently. More importantly, a greater proportion of collisions have energy greater than or equal to the activation energy, so more collisions are successful. Temperature has a large effect for this reason.
Worked reasoning
Try this
Q1. Define the activation energy. [1 mark]
- Cue. The minimum energy that colliding particles must have for a reaction to occur.
Q2. Explain why increasing the pressure of a gas reaction increases the rate. [2 marks]
- Cue. The particles are squeezed closer together, so they collide more frequently, giving more successful collisions per second.
Q3. State two things, besides enough energy, that a successful collision requires. [2 marks]
- Cue. The particles must actually collide and be in the correct orientation.
Exam-style practice questions
Practice questions written in the style of Pearson Edexcel exam questions on this dot point, with worked answer explainers. The year tag is the paper they imitate, not the source.
Edexcel 20194 marksExplain, using collision theory, why increasing the temperature increases the rate of a reaction.Show worked answer →
A 4-mark collision-theory explanation, a classic Edexcel question.
Increasing the temperature gives the particles more kinetic energy, so they move faster (1 mark). The faster particles collide more frequently, so there are more collisions per second (1 mark). A greater proportion of the collisions have energy greater than or equal to the activation energy (1 mark), so a greater proportion of collisions are successful, and the rate increases (1 mark).
Markers reward both effects (more frequent collisions and more collisions exceeding the activation energy), with the energy effect being the more important one.
Edexcel 20213 marksA student investigates the reaction of marble chips with hydrochloric acid. Explain, in terms of collision theory, why using powdered marble instead of large chips increases the rate of reaction.Show worked answer →
A 3-mark surface-area explanation.
Powdering the marble increases its surface area to volume ratio (1 mark), so more particles of marble are exposed and available to react at the surface (1 mark). This means there are more frequent collisions between the acid particles and the marble per second, so the rate of reaction increases (1 mark).
Markers reward "larger surface area", "more frequent collisions" and the link to a faster rate.
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Sources & how we know this
- Pearson Edexcel GCSE (9-1) Chemistry (1CH0) specification — Pearson Edexcel (2016)