Collision theory (AQA GCSE Chemistry Combined Science): Revision Notes
Collision theory
What is collision theory?
Collision theory helps us understand why reaction rates change when we alter the conditions. It explains how things like concentration, temperature, pressure, surface area, and catalysts all affect how fast reactions happen.
The theory is based on a simple idea: for particles to react, they need to bump into each other in the right way.
How particles react
When we have a mixture of reactant particles (let's call them A and B), they're constantly moving around and bumping into each other. Most of these collisions don't actually cause a reaction - only a small number do.
Think of it like this: imagine particles of A (red circles) and particles of B (blue circles) floating around in a container. They're constantly crashing into each other, but most of the time nothing happens.
This constant motion and collision of particles is happening at the molecular level in all chemical reactions, even though we can't see it with our eyes.
Two conditions needed for a reaction
For a collision to actually cause a reaction, two things must happen:
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The reactant particles must collide - they need to physically bump into each other
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The particles must have enough energy - they need a minimum amount of energy called the activation energy
If either of these conditions isn't met, no reaction occurs even if the particles collide.
How to increase the rate of reaction
There are two main ways to make reactions happen faster:
1. Increase collision frequency
Make the particles bump into each other more often. When particles collide more frequently, there are more chances for successful reactions.
Collision frequency is affected by factors like concentration, pressure, and temperature. Higher values of these factors generally mean more collisions per second.
2. Increase particle energy
Give the particles more energy so more collisions have enough energy to overcome the activation energy barrier.
Temperature is the main factor that affects particle energy. Higher temperatures mean particles move faster and have more kinetic energy.
Example: concentration effects
Worked Example: Doubling Concentration Effects
When we double the concentration of reactants, several things happen:
Step 1: More particles present
- The number of particles doubles - there are twice as many particles in the same space
Step 2: Increased collision rate
- Collision frequency doubles - with more particles around, collisions happen twice as often
Step 3: Faster reaction
- The reaction rate doubles - more frequent collisions mean more successful reactions per second
This shows collision theory in action. More particles = more collisions = faster reactions.
Important points about collision theory
Key Insights from Collision Theory:
- Not every collision leads to a reaction - particles need enough energy
- Higher concentrations mean more particles and more frequent collisions
- The theory predicts that doubling concentration should double the rate for many reactions
- Understanding collisions helps us predict and control reaction rates
Key Points to Remember:
- Collision theory explains how reaction conditions affect reaction rates
- Two conditions needed: particles must collide AND have enough energy (activation energy)
- Increase rate by: making particles collide more often or giving them more energy
- Higher concentration = more particles = more collisions = faster reaction
- Only a small fraction of collisions actually cause reactions