Collision Theory – IGCSE Chemistry Definition

Collision theory explains why reactions occur at different rates. It is the underlying framework for every rate of reaction question on the 0620 syllabus. Whenever a question asks “explain why the rate increases/decreases,” collision theory is the expected answer framework.

The 0620 definition

Collision theory states that for a chemical reaction to occur:

1. Reactant particles must collide with each other
2. The collision must have sufficient energy (at least the activation energy)
3. A collision that meets these conditions is called a successful collision

The rate of reaction depends on the frequency of successful collisions per unit time.

Factors affecting rate explained by collision theory

Factor	Change	Effect on collisions	Effect on rate
Temperature	Increase	Particles move faster, collide more often and with more energy; more collisions exceed Ea	Increases
Concentration	Increase	More particles per unit volume; more frequent collisions	Increases
Surface area	Increase (smaller pieces)	More particles exposed to react; more frequent collisions	Increases
Catalyst	Add catalyst	Lower activation energy; more collisions are successful	Increases
Pressure (gases)	Increase	Particles closer together; more frequent collisions	Increases

Temperature — the double effect

Temperature is unique because it increases the rate in two ways:

1. Particles move faster, so they collide more frequently
2. A greater proportion of collisions have energy equal to or greater than the activation energy

The second effect is more important. Even a small temperature rise significantly increases the fraction of particles with enough energy to react.

Concentration and surface area

These factors increase the frequency of collisions but do not change the energy of the collisions:

• Higher concentration: more particles in the same volume means collisions happen more often
• Greater surface area: smaller pieces expose more particles to the other reactant

Worked exam question

Zinc reacts faster with hydrochloric acid at 50 degrees C than at 20 degrees C. Explain why, using collision theory. (3)

Mark scheme

At higher temperature, particles have more kinetic energy / move faster [1]; particles collide more frequently [1]; a greater proportion of collisions have energy greater than or equal to the activation energy / more successful collisions [1]

Common exam mistakes

• Saying “particles collide more” without specifying more frequently or with more energy. Be precise — examiners want both ideas for temperature questions.
• Forgetting the activation energy when explaining temperature effects. It is not enough to say particles “collide harder” — state that more collisions exceed the activation energy.
• Explaining surface area by saying “there is more substance”. The amount of substance is the same — what changes is the area exposed to the other reactant.