5.2.3Additional lessons
Collision Theory — IB Chemistry SL Revision Notes & Topical Questions
5 min read 5 parts Lesson 68 of 98
Part 1 of 5The Theory
Collision theory links the rate of a reaction to what happens when particles meet.
Collision requirement
- Particles must collide in order to react, so a reaction cannot take place unless reactant particles meet.
Sufficient energy
- The colliding particles must have at least the activation energy, so that existing bonds can be broken.
Orientation
- The particles must be aligned so that the reactive parts of the molecules meet, which is why not all energetic collisions succeed.
Successful collisions
- Only collisions which meet both the energy and orientation requirements lead to products.
Part 2 of 5Rate and Collisions
1
The rate depends on the frequency of collisions, which is the number of collisions per second.
2
It also depends on the fraction of those collisions that are successful.
3
More frequent collisions give more successful collisions per second and so a higher rate.
4
A larger fraction with sufficient energy also increases the rate, even when collision frequency is unchanged.
Part 3 of 5Why Collisions Fail
Why Collisions Fail
Insufficient energy
- Most collisions at room temperature involve particles with too little energy and the particles rebound unchanged.
Wrong orientation
- Particles may collide with unreactive parts of the molecules facing each other.
Large molecules
- For large molecules the chance of the correct orientation is lower, so the reaction is slower.
Rare successes
- Only a small fraction of collisions are successful, even in fast reactions.
Part 4 of 5Applying the Theory
Applying the Theory
Concentration
- More particles per unit volume raise the collision frequency.
Temperature
- Faster particles collide more often and more of them reach the activation energy.
Surface area
- A larger exposed area gives more collisions between reactants in different phases.
Catalyst
- Lowers the activation energy so more collisions are successful.
| Change | Collision frequency | Fraction with E ≥ Eₐ |
|---|---|---|
| Higher concentration | Increases | No change |
| Higher temperature | Increases slightly | Increases greatly |
| Catalyst | No change | Increases |
Part 5 of 5Exam Checklist & Tips
5.2.3 – Key Points for the Exam
- Collision theory Reactions occur when particles collide with sufficient energy and the correct orientation.
- Activation energy The minimum energy colliding particles need for a successful collision.
- Collision frequency The number of collisions per unit time.
- Successful collision A collision that results in a chemical reaction.
Common Mistakes
- Saying that all collisions lead to a reaction.
- Ignoring the need for correct orientation.
- Describing activation energy as the energy released by the reaction.
- Stating that a higher temperature increases the rate only because collisions are more frequent.
Last updated:
\n