Collision theory (AQA GCSE Chemistry): Revision Notes
Collision theory
What is collision theory?
Collision theory helps us understand why reaction speed changes when we alter different conditions. It explains how things like concentration, temperature, surface area, and catalysts all affect how fast reactions happen.
The theory focuses on what happens when reactant particles (the substances that are reacting) move around and bump into each other.
The particle model of reactions
When substances react, their particles are constantly moving and bumping into each other. Imagine tiny balls bouncing around in a container - some will crash into each other, and some of these crashes will cause a chemical reaction to happen.
Only a small number of collisions actually result in a reaction. Most particles just bounce off each other without reacting.
Two conditions needed for a reaction
According to collision theory, two things must happen for particles to react:
Two Essential Conditions for Chemical Reactions:
- The reactant particles must collide - they need to bump into each other
- The particles must have enough energy - they need at least the minimum amount of energy called activation energy
Think of it like this: particles need to crash into each other hard enough to break the bonds and form new ones.
How to increase the rate of reaction
There are two main ways to make reactions happen faster:
- Increase the frequency of collisions - make particles bump into each other more often
- Increase the energy of the particles - give particles more energy so more collisions are successful
These changes can be achieved by altering conditions like temperature, concentration, surface area, or adding a catalyst.
Effect of concentration - worked example
Worked Example: How Concentration Affects Reaction Rate
When we double the concentration of reactants:
- The number of particles doubles in the same space
- This means collision frequency doubles - particles bump into each other twice as often
- Therefore, the rate of reaction doubles
Real-world application: If magnesium ribbon reacts with dilute hydrochloric acid, increasing the concentration of the acid will increase the number of acid particles available to collide with the magnesium. More collisions mean a faster reaction.
Remember!
Key Points to Remember:
- Collision theory explains how reaction conditions affect reaction speed
- Two conditions needed: particles must collide AND have enough energy (activation energy)
- Rate increases when particles collide more often or with more energy
- Higher concentration means more particles, so more frequent collisions and faster reactions
- Only a small fraction of all collisions actually lead to a reaction