Why do the rate factors work, and how do catalysts speed reactions up?
Collision theory, activation energy, how concentration, temperature and surface area change the frequency and energy of collisions, and how catalysts work by lowering activation energy.
A CCEA GCSE Chemistry answer on collision theory, covering how successful collisions need enough energy (the activation energy), how each rate factor changes the frequency or energy of collisions, and how catalysts speed reactions by providing a lower-activation-energy pathway.
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What this dot point is asking
CCEA wants you to use collision theory to explain why reactions happen and why the rate factors work, define activation energy, and explain how a catalyst speeds a reaction up by lowering the activation energy.
Collision theory and activation energy
So a reaction's rate depends on two things: how often particles collide, and what proportion of those collisions have enough energy. Anything that increases either of these increases the rate. This single idea explains all the rate factors.
Why each factor works
Temperature is the most powerful factor because it does both jobs at once: more collisions and a much greater share of them successful.
Catalysts
Catalysts are vital in industry because they let reactions run faster, or at lower temperatures, saving energy and money. Enzymes are biological catalysts that speed up reactions in living things and in processes such as brewing.
Worked example
Examples in context
Example 1. Catalytic converters. A car's catalytic converter uses metal catalysts to speed up the conversion of harmful exhaust gases into less harmful ones, without the metals being used up. The catalyst lowers the activation energy so the reactions proceed quickly in the exhaust, a direct industrial use of catalysis.
Example 2. Enzymes in washing powders. Biological washing powders contain enzymes that catalyse the breakdown of food and grease stains at low temperatures. This lets clothes be washed cooler, saving energy, because the enzyme lowers the activation energy of the stain-removing reactions.
Try this
Q1. State what is meant by the activation energy. [1 mark]
- Cue. The minimum energy particles need when they collide for a reaction to occur.
Q2. Explain how a catalyst increases the rate of a reaction. [2 marks]
- Cue. It provides a pathway with lower activation energy, so more collisions are successful.
Exam-style practice questions
Practice questions written in the style of CCEA exam questions on this dot point, with worked answer explainers. The year tag is the paper they imitate, not the source.
CCEA 20184 marksUsing collision theory, explain why increasing the temperature increases the rate of a reaction.Show worked answer →
Markers want both effects of temperature on collisions.
When the temperature increases, the particles gain more kinetic energy and move faster. This has two effects.
First, faster particles collide more frequently, so there are more collisions each second.
Second, and more importantly, more of the colliding particles now have energy greater than or equal to the activation energy, so a greater proportion of collisions are successful (lead to a reaction).
Together these mean more successful collisions per second, so the rate increases.
Markers reward more frequent collisions and, crucially, more particles having the activation energy so more collisions succeed.
CCEA 20213 marksExplain what a catalyst is and how it increases the rate of a reaction.Show worked answer →
The marks are for the definition and the activation-energy mechanism.
A catalyst is a substance that speeds up a reaction without being used up in the reaction, so it can be reused.
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 react, so more collisions succeed and the rate increases.
Markers reward catalyst speeds up the reaction without being used up, and lowers the activation energy so more collisions are successful.
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Sources & how we know this
- CCEA GCSE Chemistry specification (1110) — CCEA (2017)