Explaining Rates Using Collision Theory
Section: 6. Chemical Reactions | Syllabus: Cambridge IGCSE Chemistry (0620)
Explaining Each Factor Using Collision Theory
Exam answers should explain the effect of a factor in terms of collisions: state what happens to the particles, explain the effect on collision frequency and/or energy, then link this to the rate.
| Factor increased | Effect on particles | Effect on collisions | Why rate increases |
|---|---|---|---|
| Concentration | more particles in the same volume | particles are closer together - more frequent collisions | more collisions overall → more successful collisions per second |
| Pressure (gases) | same number of particles in a smaller volume | particles closer together - more frequent collisions | more collisions overall → more successful collisions per second |
| Surface area (solids) | more surface exposed | more reactant particles available at the surface | more collisions possible → more successful collisions per second |
| Temperature | particles move faster (more kinetic energy) | more frequent collisions, AND a greater proportion have energy ≥ Ea | more successful collisions per second - this is why rate is so sensitive to temperature |
| Catalyst | reaction follows an alternative pathway | collision frequency is unchanged | a greater proportion of collisions now have energy ≥ the new, lower Ea |
Key distinction: concentration, pressure and surface area only increase collision frequency. Temperature increases both frequency and the proportion of particles with sufficient energy - which is why a 10°C rise can roughly double the rate. A catalyst does not change collision frequency at all; it only increases the proportion of collisions with enough energy.
Catalysts Decrease Activation Energy
A catalyst provides an alternative reaction pathway with a lower activation energy.
- More particles now have energy ≥ the new, lower Ea
- A greater proportion of collisions are successful, so the rate increases
- The catalyst itself is regenerated at the end of the reaction, so it is not used up
Worked Example: Explaining the Effect of Temperature
- Question: use collision theory to explain why increasing temperature increases the rate of a reaction. [3]
- Answer: particles have more kinetic energy and move faster [1]; this causes more frequent collisions, and a greater proportion of particles now have energy greater than or equal to the activation energy [1]; so there are more successful collisions per second, and the rate increases [1]
Worked Example: Explaining the Effect of Surface Area
- Question: powdered zinc reacts faster with dilute acid than the same mass of zinc lumps. Explain why. [3]
- Answer: powdered zinc has a much larger surface area than lumps of the same mass [1]; more zinc particles are exposed and available for collisions with acid particles, so collisions are more frequent [1]; more successful collisions per second means a faster rate of reaction [1]
Key Exam Phrases
- "More frequent collisions" - for concentration, pressure, surface area, and temperature
- "A greater proportion of particles have energy ≥ activation energy" - for temperature and catalysts
- "More successful collisions per second" - the link to rate, needed for ALL factors
- "Lower activation energy / alternative pathway" - for catalysts specifically
Common Mistakes
- ❌ "Higher concentration gives particles more energy"
- ✓ concentration does not change particle energy - it changes how many particles are packed into a given volume, so collisions are more frequent
- ❌ "A catalyst gives particles more energy"
- ✓ a catalyst does not add energy to particles - it lowers the activation energy needed, so more of the existing particles already qualify
- ❌ Writing "the reaction is more successful" without saying what increased
- ✓ always be precise: "more successful collisions per second", not just "more successful"
Interactive revision notes, videos and practice questions load below.