Steeper = faster. The curve flattens as reactants are used up. 60 s recorded — restart to run again.
Only particles in the shaded area can react. Heating flattens the curve; a catalyst moves Eₐ left.
A catalyst gives a lower hill. Reactants, products and ΔH stay exactly the same.
§1 The big idea
The rate of reaction depends on the number of successful collisions per unit time. Anything that makes successful collisions happen more often makes the reaction faster.
Reactant particles can only react if they actually meet. The more often they collide — the higher the collision frequency — the more chances they have.
A collision only succeeds if it has enough energy. Activation energy, Eₐ, is the minimum energy colliding particles need in order to react.
§2 Factor by factor
Each factor works through one or both conditions. Try it loads that change into the lab above.
The energy effect is the big one — don’t stop at “they collide more often”.
Say “per unit volume”, not just “more particles”. The share of successful collisions doesn’t change.
Only for reactions involving gases. It works exactly like higher concentration.
A catalyst does not give particles more energy, and it is not used up.
| Increase… | Collision frequency | Share with E ≥ Eₐ | Rate |
|---|---|---|---|
| Temperature | ↑ | ↑ big effect | ↑ |
| Concentration | ↑ | no change | ↑ |
| Pressure (gases) | ↑ | no change | ↑ |
| Catalyst | no change | ↑ lower Eₐ | ↑ |
§3 Exam technique
Examiners look for a complete chain: change → particles → collisions → successful collisions per unit time → rate.
Model answer · 3 marks
Explain, in terms of particles, why increasing the temperature increases the rate of reaction.
§4 Quick check
Five questions. Your score is saved on this device and fills in the topic on the syllabus map.
Eₐ is a minimum — a threshold. Collisions with less energy than this just bounce apart.
Concentration only changes how often particles collide. Temperature and catalysts both change the proportion with energy ≥ Eₐ.
Particle energies are unchanged — the energy hill is lower, so more collisions get over it. The catalyst is chemically unchanged at the end.
Pressure only matters when the reactants are gases: squeezing them puts more particles in each unit of volume.
Rate is how quickly product forms — the gradient. The final height depends on how much reactant there was.
Gas syringes, mass loss and drawing tangents are the next lesson in this topic.
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