What factors change the rate of a reaction, and why?
Collision theory; the effects of concentration, pressure, surface area, temperature and catalysts on rate; and how catalysts work.
A focused answer to AQA GCSE Chemistry 4.6.1, covering collision theory and how concentration, pressure, surface area, temperature and catalysts change the rate of a reaction, and how catalysts provide a lower-activation-energy pathway.
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What this dot point is asking
AQA wants you to use collision theory to explain how concentration, pressure, surface area, temperature and catalysts change the rate of a reaction, and to explain how a catalyst speeds up a reaction without being used up. The skill being tested is applying one core model (collision theory) consistently to every factor, distinguishing between collision frequency and collision energy.
Collision theory
This single idea explains every factor: anything that increases how often particles collide, or increases the fraction of collisions that have the activation energy, increases the rate.
The factors
Three of these (concentration, pressure, surface area) work purely by changing the frequency of collisions. Only temperature also changes the proportion of successful collisions, which is why temperature usually has the biggest effect.
How temperature works
Raising the temperature has a double effect: collisions are more frequent because particles move faster, and a greater proportion of collisions are successful because more particles have energy above the activation energy. This is why a small rise in temperature (around 10 degrees) can roughly double the rate of many reactions, an effect far larger than the modest increase in collision frequency alone.
Catalysts
Enzymes are biological catalysts. Catalysts are important in industry (for example iron in the Haber process, or platinum in catalytic converters) because they speed up reactions and let processes run at lower temperatures, lowering energy costs and reducing carbon emissions.
Try this
Q1. Explain why increasing the surface area of a solid speeds up a reaction. [2 marks]
- Cue. More particles are exposed at the surface, so collisions are more frequent.
Q2. Explain why increasing temperature increases the rate of reaction. [2 marks]
- Cue. Particles move faster (more frequent collisions) and more have the activation energy.
Q3. State why a catalyst can be used repeatedly. [1 mark]
- Cue. It is not used up in the reaction.
Exam-style practice questions
Practice questions written in the style of AQA exam questions on this dot point, with worked answer explainers. The year tag is the paper they imitate, not the source.
AQA 20196 marksMarble chips (calcium carbonate) react with hydrochloric acid to produce carbon dioxide. Explain, using collision theory, why the reaction is faster when the marble chips are crushed into a powder and when the acid is more concentrated. [Note: this question targets AO1 and AO2.]Show worked answer →
A 6-mark extended-response question (banded 1 to 6) on collision theory applied to two factors.
For full marks, link each factor to collision frequency. Surface area: crushing the marble into powder exposes many more particles at the surface, so there are more collisions per second between acid particles and carbonate particles, increasing the rate. Concentration: a more concentrated acid has more acid particles in the same volume, so collisions with the marble are more frequent, again increasing the rate.
A top-band answer states that the rate depends on the frequency of successful collisions (those with at least the activation energy) and applies that idea clearly to both factors. Markers penalise the common error of saying concentration gives particles more energy.
AQA 20214 marksA reaction between magnesium and dilute sulfuric acid is carried out at degrees Celsius and repeated at degrees Celsius. Explain, using collision theory, why the rate is greater at the higher temperature. Describe the two separate effects of the temperature increase.Show worked answer →
A 4-mark question requiring both temperature effects.
Effect 1 (2 marks): at the higher temperature the particles have more kinetic energy and move faster, so they collide more frequently. Effect 2 (2 marks): a greater proportion of the particles now have energy equal to or greater than the activation energy, so a larger fraction of collisions are successful.
Markers want both effects, frequency and proportion of successful collisions, stated separately; many candidates give only one and cap at 2 marks.
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Sources & how we know this
- AQA GCSE Chemistry (8462) specification — AQA (2016)