Collision Theory and Factors Affecting Rate

Collision theory is the model used to explain why different factors change the rate of a chemical reaction. This lesson covers the core ideas of collision theory, activation energy, and the five main factors that affect rate. Understanding these concepts is essential for the AQA GCSE Combined Science Trilogy specification (8464).

Collision Theory

For a chemical reaction to occur, two conditions must be met:

Reactant particles must collide with each other.
The collision must have sufficient energy (equal to or greater than the activation energy) and the particles must collide with the correct orientation.

A collision that meets both conditions is called a successful collision. Collisions that do not meet both conditions are unsuccessful — the particles simply bounce apart.

Key Definition: The activation energy is the minimum amount of energy that colliding particles need for a reaction to take place.

Factors That Affect the Rate of Reaction

There are five main factors that affect rate. Each can be explained using collision theory.

graph TD
    A["Factors Affecting Rate of Reaction"] --> B["Temperature"]
    A --> C["Concentration"]
    A --> D["Surface Area"]
    A --> E["Pressure (gases)"]
    A --> F["Catalyst"]
    B --> B1["Particles move faster → more frequent collisions and more energy per collision"]
    C --> C1["More particles per unit volume → more frequent collisions"]
    D --> D1["More particles exposed at surface → more frequent collisions"]
    E --> E1["Particles closer together → more frequent collisions"]
    F --> F1["Provides alternative pathway with lower activation energy"]

1. Temperature

Increasing temperature:

Particles gain more kinetic energy and move faster.
Collisions become more frequent.
A greater proportion of collisions have energy ≥ activation energy.

Both effects increase the rate. The second effect (more particles exceeding activation energy) is the more important one.

Exam Tip: When explaining how temperature affects rate, you must mention BOTH more frequent collisions AND more collisions with energy equal to or greater than the activation energy. Stating only one will not gain full marks on a 3-mark question.

2. Concentration (solutions)

Increasing the concentration of a reactant in solution:

There are more reactant particles in the same volume.
Collisions between reactant particles are more frequent.
The rate increases.

3. Surface Area (solids)

Breaking a solid into smaller pieces increases its surface area:

More particles are exposed at the surface.
There are more collisions per second between the solid and the solution or gas.
The rate increases.

Form of Solid	Relative Surface Area	Relative Rate
Large lump	Low	Slow
Small chips	Medium	Moderate
Fine powder	High	Fast

4. Pressure (gases)

Increasing the pressure of gaseous reactants:

Particles are pushed closer together in the same volume.
Collisions become more frequent.
The rate increases.

This is analogous to increasing concentration for solutions.

5. Catalysts

A catalyst increases the rate of reaction without being used up. It works by providing an alternative reaction pathway with a lower activation energy.

More particles now have energy ≥ the (lower) activation energy, so more collisions are successful.

Key Point: Catalysts are not used up. They can be recovered chemically unchanged at the end of the reaction.

Energy Profile Diagrams

An energy profile diagram shows the energy changes during a reaction.

Feature	Without Catalyst	With Catalyst
Activation energy	Higher peak	Lower peak
Overall energy change	Same	Same
Energy of reactants	Same	Same
Energy of products	Same	Same

The catalyst lowers the activation energy but does not change the overall energy change of the reaction (the difference between reactants and products).

Worked Example: Explaining a Rate Change

Question: A student reacts magnesium ribbon with hydrochloric acid at 20°C and then repeats the experiment at 40°C. Explain why the rate is faster at 40°C. (3 marks)

Answer:

At 40°C the particles have more kinetic energy (1 mark).
They collide more frequently (1 mark).
A greater proportion of collisions have energy equal to or greater than the activation energy, so more collisions are successful (1 mark).

Common Mistake: Writing "the particles have more energy so they react faster" is too vague and will not gain full marks. You must refer to collision frequency AND the proportion of collisions exceeding the activation energy.

Comparison Table: All Five Factors

Factor	How It Works (Collision Theory)	Effect on Rate
↑ Temperature	Faster particles; more frequent and more energetic collisions	Increases
↑ Concentration	More particles per volume; more frequent collisions	Increases
↑ Surface area	More particles exposed; more frequent collisions	Increases
↑ Pressure (gases)	Particles closer together; more frequent collisions	Increases
Catalyst added	Lower activation energy; more successful collisions	Increases

Summary

Collision theory: particles must collide with sufficient energy and correct orientation
Activation energy is the minimum energy needed for a successful collision
Five factors affect rate: temperature, concentration, surface area, pressure, catalysts
Each factor can be explained by its effect on collision frequency or the proportion of successful collisions
Energy profile diagrams show how catalysts lower the activation energy without changing overall energy change

Exam Tip: AQA often asks you to explain rate changes using collision theory. Structure your answer: state the factor → describe the effect on particles → link to collision frequency and/or activation energy.

Collision Theory and Factors Affecting Rate

Collision Theory and Factors Affecting Rate