Collision Theory & Rate Factors

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CognitoCollision theory and rate of reaction

Key facts & methods

Collision theory

Reactions only occur when particles collide with sufficient energy (activation energy).
Rate of reaction depends on: frequency of collisions AND proportion with enough energy.
Increasing rate = increasing the number of successful collisions per second.
Activation energy: the minimum energy particles need to collide and react.
A catalyst lowers the activation energy, providing an alternative reaction pathway.

Effect of concentration

Higher concentration → more particles in the same volume → more frequent collisions → faster rate.
If you double the concentration, collisions happen roughly twice as often.
Pressure (for gases): higher pressure = more particles in same volume → same effect as higher concentration.
Graphs: higher concentration → steeper gradient, same final amount of product.

Effect of temperature

Higher temperature → particles move faster → collide more frequently AND with more energy.
More collisions exceed the activation energy → greater proportion of successful collisions.
A 10°C rise roughly doubles the rate of most reactions.
Graphs: higher temperature → steeper gradient, same final amount.

Surface area and catalysts

Smaller particle size → greater surface area to volume ratio → more particles exposed → more collisions → faster rate.
Powders react faster than lumps of the same mass.
Catalyst: increases rate without being used up. Provides alternative reaction pathway with lower activation energy.
Homogeneous catalyst: same state as reactants. Heterogeneous: different state.
Catalysts do not change the amount of product — they only change the rate.

Exam questions — 4 questions · 10 marks · AQA 8462 style

1 markCollision theoryAQA 8462 style

According to collision theory, for a reaction to occur, particles must:

A Simply collide with each other

B Collide with sufficient energy to overcome the activation energy

C Collide at exactly 180° to each other

D Have identical sizes and shapes

3 marksEffect of temperatureAQA 8462 P2 style

Explain in terms of collision theory why increasing temperature increases the rate of a reaction. (3 marks)

Hint: Cover: more energy → faster movement → more frequent collisions → more exceed activation energy → more successful collisions.

+30 XP

1 markSurface area effectAQA 8462 style

The same mass of calcium carbonate is used in two experiments: one as a large lump, one as a powder. The powder reacts faster. Explain why.

A The powder has less mass than the lump

B The powder has a greater surface area — more particles are exposed to acid — more frequent collisions

C The powder has a different chemical composition from the lump

D The powder produces more heat which speeds up the reaction

4 marksCatalyst explanationAQA 8462 style

Manganese dioxide (MnO₂) acts as a catalyst in the decomposition of hydrogen peroxide: 2H₂O₂ → 2H₂O + O₂. Explain what a catalyst is and how it increases the rate of reaction. (4 marks)

Hint: Define catalyst (increases rate, not used up). Explain mechanism: lower activation energy → more successful collisions. State amount of product is unchanged.

+40 XP

1 markRate graph interpretationAQA 8462 style

A rate experiment produces a graph of volume of gas vs time. A steeper gradient at the start indicates:

A More product is produced overall

B A faster initial rate of reaction

C The reaction takes longer to complete

D The activation energy is higher

Collision Theory & Rate Factors

Collision theory

Effect of concentration

Effect of temperature

Surface area and catalysts

Module complete! 🎉