1.9.1Physical Chemistry
Collision Theory and Rates — Edexcel AS Chemistry Revision Notes & Topical Questions
5 min read 5 parts Lesson 38 of 42
Part 1 of 5Collision Theory
Collisions are needed
- A reaction can only happen when particles collide with each other.
Sufficient energy
- Colliding particles must have at least the activation energy for bonds to break.
Correct orientation
- Particles must collide in a suitable orientation so that the reactive parts meet.
Successful collisions
- Only a small fraction of collisions are successful, and the rate depends on how often these occur.
Rate and time
- Reaction rate is the change in amount of reactant or product divided by the time taken, so a faster reaction takes less time.
Part 2 of 5Factors That Change Rate
Factors That Change Rate
Concentration or pressure
- More particles in a given volume mean more frequent collisions, so more successful collisions per second.
Temperature
- Particles move faster and a greater proportion have energy at or above the activation energy.
Surface area
- Smaller pieces of a solid expose more particles to the other reactant, increasing collision frequency.
Catalyst
- Provides an alternative route with lower activation energy so a larger proportion of collisions succeed.
Pressure of gases
- Raising the pressure of a gas forces the same number of particles into a smaller volume,
- which increases the frequency of collisions.
| Factor | Frequency of collisions | Proportion with E ≥ Ea |
|---|---|---|
| Higher concentration | Increases | Unchanged |
| Higher temperature | Increases slightly | Increases greatly |
| Catalyst added | Unchanged | Increases |
Part 3 of 5Explaining a Rate Change
1
State the change, for example a rise in temperature or an increase in concentration.
2
Say what happens to the particles: they move faster or are closer together.
3
Link this to the frequency of collisions and the proportion with enough energy.
4
Conclude that there are more successful collisions per unit time.
5
Finish by saying the rate of reaction increases.
Part 4 of 5Practical Applications
Food storage
- Refrigeration lowers temperature, slowing the reactions that spoil food.
Powdered fuels
- Fine dust has large surface area and can burn explosively.
Industry
- High pressure increases the concentration of gases and speeds up gaseous reactions.
Safety
- Understanding rate factors helps control reactions that could be dangerous if too rapid.
Part 5 of 5Exam Checklist & Tips
1.9.1 – Key Points for the Exam
- Activation energy Minimum energy particles need for a successful collision.
- Rate of reaction Change in concentration of a reactant or product per unit time.
- Catalyst Substance that increases rate without being used up.
- Successful collision A collision with enough energy and correct orientation.
Common Mistakes
- Saying that a catalyst provides the particles with extra energy; it lowers the activation energy.
- Stating that temperature only increases collision frequency;
- the main effect is more particles with energy above Ea.
- Writing only that there are more collisions instead of more successful collisions.
- Confusing surface area with the amount of solid used.
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