Collision Theory Explained Simply

Temperature directly affects the fraction of molecules with enough energy.
This is why heating dramatically speeds up reactions.

Requirement 3: Correct Orientation

Particles must align correctly during collision.

Example:
For a reaction A–B + C → A–C + B

C must collide with the correct side of A–B to displace B.
If the angle is wrong, the collision will not lead to reaction even if energy is sufficient.

Orientation explains why some reactions are slow despite having low Ea.

The Collision Theory Model

Reaction rate depends on:

Rate ∝ (number of collisions) × (fraction with enough energy) × (orientation success)

Even if collisions are frequent, only a small fraction lead to reaction.

The Arrhenius equation mathematically describes the same idea, but collision theory explains it conceptually.

How Temperature Affects Reaction Rate (Using Collision Theory)

Increasing temperature:

• Increases average kinetic energy
• Increases frequency of collisions
• Greatly increases the fraction of particles with energy ≥ Ea

This last effect is the most important and explains why reactions speed up exponentially, not linearly.

For example:

• Raising temperature by 10°C roughly doubles reaction rate for many reactions.

How Concentration Affects Reaction Rate

For solutions:

• Higher concentration → more particles per volume → more collisions → faster rate.

For gases:

• Higher pressure increases particle density, producing the same effect.

In IB exams, concentration is one of the easiest variables to explain using collision theory.

How Surface Area Affects Reaction Rate

For solids:

• Greater surface area (e.g., powdered metal) → more exposed particles for collisions → faster rate.

This is why fine powders react faster than large chunks.

How Catalysts Affect Reaction Rate

Catalysts provide an alternative reaction pathway with a lower activation energy.

Using collision theory:

• More particles now have enough energy to react
• Higher proportion of successful collisions
• Rate increases even at the same temperature

Catalysts do not increase the number of collisions, only the fraction that succeed.

Collision Theory and Reaction Profiles

Reaction coordinate diagrams show:

• Activation energy
• Energy of reactants
• Energy of products

Collision theory explains why the height of the activation energy barrier determines the reaction rate.

A high Ea → slow reaction
A low Ea → fast reaction

Common IB Exam Questions

• Explain why increasing temperature increases rate.
• Describe how catalysts change reaction rate using collision theory.
• Explain why powdered solids react faster than lumps.
• Discuss how pressure affects gaseous reactions.

Collision theory provides the reasoning needed for all of these.

FAQs

Why don’t all collisions lead to reactions?

Most collisions either lack enough energy or occur at the wrong orientation.

Does a catalyst change the number of collisions?

No. It changes how many collisions have enough energy to react.

How does collision theory relate to activation energy?

Activation energy is the energy threshold collisions must exceed for reaction to occur.

Conclusion

Collision theory explains the essential conditions for chemical reactions: collisions, sufficient energy, and proper orientation. By understanding how temperature, concentration, surface area, and catalysts influence these conditions, IB Chemistry students can confidently explain reaction rates and predict how different variables affect chemical systems.

Learn why the mole allows chemists to connect atomic-scale particles to measurable quantities used in real experiments.