How do temperature, concentration, surface area and catalysts change the rate of a reaction?
The effects of temperature, concentration, pressure, surface area and catalysts on the rate of reaction, explained by collision theory.
A focused answer to the WJEC GCSE Science Double Award Unit 2 topic on the factors affecting reaction rate, covering temperature, concentration, pressure, surface area and catalysts, each explained using collision theory.
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What this dot point is asking
WJEC Double Award Unit 2 wants you to describe the effects of temperature, concentration, pressure, surface area and catalysts on the rate of a reaction, and explain each using collision theory.
Temperature
A useful rule of thumb is that a 10 degree Celsius rise roughly doubles the rate of many reactions.
Concentration and pressure
Increasing the concentration of a solution means there are more particles in the same volume, so collisions are more frequent and the rate increases. For reactions involving gases, increasing the pressure squeezes the particles closer together, which has the same effect: more frequent collisions and a higher rate.
Surface area
For a reaction involving a solid, breaking it into smaller pieces (or a powder) increases its surface area. More of the solid is then exposed to the other reactant, so collisions are more frequent and the rate increases. This is why powders react faster than lumps, and why fine dusts can be dangerously explosive.
Catalysts
Because the activation energy is lower, a greater proportion of collisions have enough energy to react, so the rate increases. Catalysts are very important in industry because they make reactions faster and cheaper without being consumed. Enzymes are biological catalysts.
Why catalysts matter in industry
In industry, a faster reaction means more product made in the same time, which saves money. Because a catalyst is not used up, the same catalyst can be used over and over, so only a small amount is needed. Catalysts also let reactions run at lower temperatures, which saves energy and reduces costs and carbon dioxide emissions. For these reasons many large-scale processes rely on catalysts, even though the catalyst can be expensive to buy at first. Linking the lower activation energy to lower costs and energy savings is a common application question.
Summarising the factors
It helps to keep the four main factors clear and to know which type of reactant each applies to. Temperature affects all reactions (more energetic and more frequent collisions). Concentration applies to reactions in solution, and pressure is the equivalent for gases (more particles per volume). Surface area applies to solid reactants (more exposed surface). A catalyst can be added to lower the activation energy. In a question, first decide what state the reactants are in, then choose the factors that apply.
Try this
Q1. State two factors that increase the rate of a reaction. [2 marks]
- Cue. Any two of: higher temperature, higher concentration/pressure, larger surface area, a catalyst.
Q2. Does a catalyst get used up in a reaction? [1 mark]
- Cue. No, it is not used up and can be reused.
Exam-style practice questions
Practice questions written in the style of WJEC exam questions on this dot point, with worked answer explainers. The year tag is the paper they imitate, not the source.
WJEC style4 marksExplain, using collision theory, why increasing the temperature increases the rate of a reaction.Show worked answer →
A Unit 2 explain question worth 4 marks. Reward: at a higher temperature the particles have more kinetic energy and move faster (1); so they collide more often (more frequent collisions) (1); and a greater proportion of collisions have at least the activation energy (1); so more collisions are successful and the rate increases (1). Markers credit faster movement, more frequent collisions and more collisions with enough energy. A common error is to give only "more collisions" without the energy point.
WJEC style3 marksWhat is a catalyst, and how does it increase the rate of a reaction?Show worked answer →
A Unit 2 explain question. A catalyst is a substance that speeds up a reaction without being used up itself (1); it works by providing an alternative pathway with a lower activation energy (1), so more collisions have enough energy to react, increasing the rate (1). Markers credit not being used up and lowering the activation energy. A common error is to say a catalyst is used up or that it makes the particles move faster.
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