Collision Model

Learning Objective 5.5.A Explain the relationship between the rate of an elementary reaction and the frequency, energy, and orientation of particle collisions.

Quick Notes

For a reaction to occur, particles must collide.
Only a small proportion of total collisions are successful.
For a collision to cause a reaction it must have:
- Enough energy to overcome activation energy (E_a)
- The correct orientation of particles for bond rearrangement
Maxwell–Boltzmann distribution shows the spread of particle energies.

Only particles with energy ≥ activation energy can react.
Increasing temperature shifts the distribution → more particles exceed the activation energy.

Higher temperature = more effective collisions = faster reaction rate.

Full Notes

The Collision Theory of Reactions

In an elementary reaction, particles must collide to react. However, not every collision leads to a reaction.

For a collision to result in product formation, it must have both sufficient energy and the correct orientation between particles.

Diagram showing collisions with correct and incorrect orientation and the resulting reaction outcomes.

Sufficient Energy

The collision must provide at least the activation energy (E_a) required to break bonds in the reactants.
If the energy is too low, the particles bounce off each other without reacting.

Correct Orientation

The reacting parts of the molecules must face each other correctly.
Even high-energy collisions can fail if the orientation is wrong.

Why Most Collisions Fail

Most collisions in a gas sample do not result in a chemical reaction.

Only a small fraction of molecules have enough energy and the right alignment. This is why reaction rates are usually much slower than the total number of collisions per second might suggest.

The Maxwell–Boltzmann Distribution

This curve shows the distribution of kinetic energies in a sample of particles at a given temperature.

Maxwell–Boltzmann distribution curve showing fraction of molecules with sufficient energy to overcome activation energy.

Most particles have moderate energy.
The area under the curve beyond E_a represents the fraction of particles that can successfully react.
Only a small proportion of particles have high enough energy to react.

Effect of Temperature on the Distribution

Increasing temperature shifts the curve to the right and lowers the peak, meaning more molecules have energy ≥ E_a. The total number of molecules stays the same (area under the curve is unchanged).

Effect of increased temperature on the Maxwell–Boltzmann distribution: peak shifts right, more particles exceed activation energy.

The number of molecules with energy ≥ E_a increases.
The curve flattens and shifts right.
Reaction rate increases significantly.

Matt’s exam tip

Be able to describe how the Maxwell–Boltzmann distribution changes with temperature. A higher temperature shifts the peak to the right and flattens it — this increases the proportion of particles with energy ≥ E_a.

Orientation and Activation Energy Together

Even when particles have enough energy, orientation of collisions matters.

For complex molecules, there's often only one way they can align to react properly. This is why molecules with complex structures may react slowly, even with high-energy collisions.

Example: During nucleophilic substitutoin reactions, a nucleophile must approach the correct part of a molecule for the reaction to occur.

Diagram showing nucleophile approaching with correct and incorrect orientation.

Wrong orientation = no reaction, even with enough energy.

Summary

The collision model explains reaction rates in terms of how often and how effectively particles collide.

Particles must collide for a reaction to occur.
Collisions must have sufficient energy (≥ E_a).
Particles must have the correct orientation.

The Maxwell–Boltzmann distribution helps explain why increasing temperature increases the reaction rate — more particles have enough energy to overcome the activation barrier.