3.2.2Physical Chemistry

Explaining Rates — Edexcel International GCSE Science Double Award 4SD0 - Chemistry Revision Notes & Topical Questions

5 min read 11 parts Lesson 10 of 19

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Part 1 of 5Collision theory in brief

Particles must collide

Particles must collide A reaction can only happen when reactant particles collide with each other.
Not every collision works. A collision is successful only if the particles hit with enough energy to react.

What rate depends on

What rate depends on The rate of reaction depends on how often successful collisions occur, that is, on the number of successful collisions per second.
Any factor that raises this number makes the reaction faster.

Factors to explain

Factors to explain You need to explain the effect of concentration, pressure (for gases), temperature, surface area of a solid, and catalysts.

1Change made

Change made A solution is made more concentrated, or a gas is compressed to a higher pressure.

2Particles crowded

Particles crowded There are more reactant particles in the same volume, because the same number are squeezed into a smaller space for gases.

3More frequent collisions

More frequent collisions Particles are closer together, so they collide more often each second.
- Doubling the number of particles doubles the collisions per second.

4Rate increases

Rate increases More frequent collisions mean more successful collisions per second, so the reaction is faster.

1Heat the reactants

2Collisions more frequent

Collisions more frequent Faster particles meet more often, so collisions per second rise.

3Collisions more energetic

Collisions more energetic A bigger proportion of particles now has enough energy to react when they collide.

4Rate increases a lot

Rate increases a lot A small rise in temperature gives a large rise in rate.
- As a rough rule, the rate of many reactions doubles for every 10 °C increase.

Other ways to speed up a reaction

Larger surface area

Larger surface area Crushing a solid exposes more of its particles to the other reactant.
- More collisions per second take place, and doubling the surface area doubles the collision rate.

Adding a catalyst

Adding a catalyst A catalyst speeds up a reaction without being used up.
- It provides an alternative pathway with a lower activation energy, so more collisions are successful.

Same products

Same products A catalyst changes only the rate, not the products or the amount made, and it can be recovered unchanged at the end.

Factor increased	Effect on particles	Effect on rate
Concentration	More particles per unit volume, more collisions per second	Faster
Pressure (gases)	Same particles in a smaller volume, more collisions per second	Faster
Temperature	Higher kinetic energy: more frequent and more energetic collisions	Much faster
Surface area	More particles exposed to the other reactant	Faster
Catalyst	Lower activation energy, more successful collisions	Faster

Term	Meaning
Collision theory	The idea that reactions happen when particles collide with enough energy.
Successful collision	A collision in which the particles react to form products.
Activation energy	The minimum energy colliding particles need for a reaction to occur.
Kinetic energy	The energy a particle has because of its movement.
Catalyst	A substance that speeds up a reaction without being used up.

Writing 'more collisions' without 'per second'. Rate is about collision frequency.

Saying temperature only makes particles collide more. It also gives them more energy, and this is the more important effect.

Stating that a catalyst is used up or gives more product. It is unchanged at the end and does not alter the amount of product.

Claiming that a higher concentration means the particles move faster. Their speed is the same; there are just more of them in each cm³.

Saying pressure affects the rate of reactions of solids or liquids. It matters for gases.

Last updated: 20 October 2026