Collision Theory
Section: 6. Chemical Reactions | Syllabus: Cambridge IGCSE Chemistry (0620)
What is Collision Theory?
Collision theory explains reaction rates at a particle level. For a reaction to happen, particles must:
- Collide with each other
- Collide with energy ≥ activation energy (Ea)
- Collide with the correct orientation
A collision that meets all three conditions is called a successful collision. Most collisions are unsuccessful - particles simply bounce apart unchanged.
Activation Energy (Ea)
Activation energy is the minimum energy that colliding particles must have for a reaction to occur.
- Energy is needed to start breaking bonds in the reactants before new bonds can form
- Only particles with energy ≥ Ea can react when they collide
- Think of Ea as a hill that must be climbed before the reaction can "roll downhill" to products
Worked Example: Defining Activation Energy
- Question: which statement describes activation energy? (A) the maximum energy given out during a reaction (B) the maximum energy taken in during a reaction (C) the minimum energy colliding particles must have to react (D) the minimum energy particles must have to make bonds
- Answer: C. Activation energy is a minimum energy threshold for a successful collision - not a maximum, and not specifically about bond-making.
What Rate of Reaction Depends On
Collision theory explains rate in terms of:
| Quantity | What it means |
|---|---|
| Number of particles per unit volume | how crowded the particles are - set by concentration (solutions) or pressure (gases) |
| Frequency of collisions | how often particles collide with each other |
| Kinetic energy of particles | how fast particles are moving - set by temperature |
| Activation energy, Ea | the energy threshold a collision must clear to be successful |
rate of reaction is proportional to the number of successful collisions per second
To speed up a reaction, a factor must increase the collision frequency, increase the proportion of particles with energy ≥ Ea, or both. Exactly how each of the five factors (concentration, pressure, surface area, temperature, catalyst) does this is explained topic by topic in the next section.
Common Mistakes
- ❌ Thinking every collision leads to a reaction
- ✓ most collisions fail - either the energy is below Ea, or the orientation is wrong
- ❌ Thinking activation energy is only relevant to endothermic reactions
- ✓ every reaction, exothermic or endothermic, has an activation energy that must be overcome first
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