How do temperature, concentration, surface area and catalysts change the rate of a reaction?
Describe and explain the effect of temperature, concentration or pressure, surface area and catalysts on rate using collision theory.
A focused answer to WJEC GCSE Chemistry topic 1.5, explaining how temperature, concentration or pressure, surface area and catalysts each affect the rate of reaction in terms of collision theory, including how catalysts provide a lower activation energy pathway.
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What this topic is asking
WJEC topic 1.5 wants you to describe and explain how four factors change the rate of reaction: temperature, concentration (or pressure for gases), surface area and a catalyst. Each explanation must use collision theory, linking the change to the frequency or energy of successful collisions.
Temperature
A rough rule of thumb is that a rise roughly doubles the rate of many reactions.
Concentration and pressure
This is why a reaction with concentrated acid is faster than the same reaction with dilute acid.
Surface area
This is why powdered solids react faster than large lumps, and why fine dusts can be dangerously explosive.
Catalysts
A catalyst works by providing an alternative reaction pathway with a lower activation energy. Because the energy barrier is lower, a greater proportion of the colliding particles have enough energy to react, so more collisions are successful and the rate increases. Catalysts are important in industry because they speed up reactions and can allow lower temperatures, saving energy and cost. Examples include iron in the Haber process and the metals in catalytic converters.
Following the right variable
When investigating one factor, you must keep the others constant so any change in rate is due only to the factor you are testing. For example, to study concentration you keep temperature, volume and surface area fixed and only change the concentration.
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 sample4 marksExplain, using collision theory, why increasing the temperature increases the rate of a reaction.Show worked answer →
A Unit 1.5 explanation question. Reward: at a higher temperature the particles have more kinetic energy and so move faster. This has two effects: the particles collide more frequently (more collisions per second), and a greater proportion of collisions have at least the activation energy, so more collisions are successful. Both effects increase the rate. Markers credit particles moving faster, more frequent collisions, and more particles having the activation energy. A common error is to give only the "more collisions" point without the "more energetic, successful collisions" point, which is usually the more important one.
WJEC sample3 marksDescribe what a catalyst does and explain how it increases the rate of a reaction.Show worked answer →
A Unit 1.5 explanation question. Reward: a catalyst is a substance that speeds up a reaction without being used up in it. It works by providing an alternative reaction pathway with a lower activation energy. Because the activation energy is lower, a greater proportion of collisions have enough energy to be successful, so the rate increases. Markers credit not being used up, an alternative pathway, lower activation energy, and more successful collisions. A common slip is to say a catalyst makes particles collide more often, when in fact it lowers the energy barrier.
Related dot points
- Define the rate of reaction, describe how it is measured, and use collision theory to explain how reactions occur.
A focused answer to WJEC GCSE Chemistry topic 1.5, covering what the rate of a reaction means, how rate is measured by following mass or gas volume against time, how to read rate graphs, and how collision theory and activation energy explain reactions.
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- Describe the Haber process and the Contact process, including their conditions, and explain why the conditions are a compromise between yield, rate and cost.
A focused answer to WJEC GCSE Chemistry topic 2.6, covering the Haber process for ammonia and the Contact process for sulfuric acid, their reactions and conditions, the use of the products including fertilisers, and why industrial conditions are a compromise.
- Describe reversible reactions and dynamic equilibrium, and predict the effect of changing temperature, concentration and pressure using Le Chatelier's principle.
A focused answer to WJEC GCSE Chemistry topic 2.6, covering reversible reactions and the symbol used, dynamic equilibrium in a closed system, and how changing temperature, concentration and pressure shifts the position of equilibrium using Le Chatelier's principle.
- Explain bond breaking and bond making in terms of energy and calculate the energy change of a reaction from bond energies.
A focused answer to WJEC GCSE Chemistry topic 2.4, explaining why bond breaking is endothermic and bond making is exothermic, and showing how to calculate the overall energy change of a reaction from given bond energies.
Sources & how we know this
- WJEC GCSE Chemistry specification (from 2016) — WJEC (2016)