How Factors Affect the Rate of a Reaction
Learn how temperature, concentration and surface area change reaction rate using collision theory, with a clear GCSE AQA Chemistry example and exam tips.
This article covers 4.6.1.2\text{4.6.1.2}4.6.1.2 and 4.6.1.3\text{4.6.1.3}4.6.1.3 of the AQA GCSE Chemistry 846284628462 specification.
Increasing the temperature, concentration or surface area increases the rate of a reaction. Each change causes more successful collisions between reacting particles per second.
This explanation assumes AQA GCSE Chemistry 846284628462. The content applies to both Foundation and Higher Tier.
Collision theory
A chemical reaction happens only when particles:
- collide with each other;
- collide with enough energy to react.
The minimum energy particles need is called the activation energy. A successful collision is one that results in a reaction.
Temperature
Increasing temperature gives particles more kinetic energy, so they move faster. This means:
- collisions happen more frequently;
- a greater proportion of collisions have energy equal to or greater than the activation energy.
Therefore, there are more successful collisions per second and the reaction is faster.
Concentration
Increasing the concentration of a solution puts more reactant particles into the same volume. The particles are closer together, so collisions happen more frequently. This increases the rate of reaction.
For reacting gases, increasing pressure has a similar effect because the particles are pushed closer together.
Surface area
Breaking a solid reactant into smaller pieces increases its surface area. More particles are exposed and available to collide with particles of the other reactant. This increases collision frequency and therefore increases the reaction rate.
Example
Powdered calcium carbonate reacts with dilute hydrochloric acid faster than the same mass of calcium carbonate in large lumps. The powder has a larger surface area, so acid particles collide with the calcium carbonate more frequently.
Common exam mistake
Do not simply write “there are more collisions.” Explain why the collision frequency increases. For temperature questions, also state that more particles have enough energy to react. Temperature changes both the frequency and the energy of collisions; concentration and surface area mainly change collision frequency.
