Collision theory says a reaction happens only when particles collide with enough energy and in the right orientation. A factor speeds up a reaction when it makes effective collisions happen more often per second.
This lesson belongs to SPM Chemistry rate of reaction. It builds on calculating average and instantaneous rates from graphs.
What three links does a full explanation need?
Every factor explanation follows the same chain. Name the change, say what it does to the particles, then link it to the frequency of effective collisions and the rate.
- Change: what was altered, for example a higher concentration.
- Particles: what this does to them, for example more particles per unit volume.
- Result: a higher frequency of collisions, so a higher frequency of effective collisions, so a higher rate.
Worked example: marble chips and acid
An original question: 5 g of marble chips reacts with excess hydrochloric acid. The same mass of powdered marble reacts faster. Explain why.
The change is that the marble is powdered, so the total surface area exposed to the acid is larger. The particles effect is that more marble particles are at the surface where acid particles can hit them.
The result is that the frequency of collisions, and of effective collisions, between acid particles and marble particles increases, so the rate of reaction is higher.
The mass of marble is the same, so the total volume of gas collected at the end is the same. Only the speed changes.
Strong and weak answers side by side
Question: Explain why 1.0 mol/dm³ acid reacts faster than 0.2 mol/dm³ acid with zinc.
| Answer | Why it scores |
|---|---|
| Weak: “Higher concentration means a faster reaction.” | Restates the question and adds no particle idea. |
| Strong: “1.0 mol/dm³ acid has more hydrogen ions per unit volume. The frequency of collisions between hydrogen ions and zinc atoms is higher, so the frequency of effective collisions is higher and the rate is higher.” | Names the particles and completes all three links. |
How does each factor fit the pattern?
- Concentration or pressure (gases): more particles in the same volume, so collisions are more frequent.
- Total surface area: more particles of a solid exposed, so collisions are more frequent.
- Temperature: faster particles collide more often, and a larger fraction have energy at or above the activation energy.
- Catalyst: an alternative pathway with lower activation energy, so a larger fraction of collisions are effective.
The temperature and catalyst answers are different because they change the energy condition, not only the number of collisions. Mention this difference when a question asks for a full explanation.
The mistake that costs marks
Writing “the particles collide more” for a temperature question gives half of the answer. A full answer also needs the energy idea: more particles reach or exceed the activation energy.
Another slip is claiming that a catalyst “gives the particles more energy”. It does not. It lowers the activation energy, so the same particles are more likely to be energetic enough.
Check yourself
A student warms the same acid from 30 °C to 50 °C before adding magnesium ribbon. Explain why the rate increases.
Answer
The particles gain kinetic energy and move faster, so the frequency of collisions between acid particles and magnesium atoms increases.
A larger fraction of the particles have energy equal to or greater than the activation energy, so the frequency of effective collisions increases.
Therefore the rate of reaction increases. Both the collision frequency and the energy idea are needed for full marks.
What to study next
The catalyst explanation uses the energy profile, which is covered in interpreting activation-energy diagrams. To practise writing explanations under exam conditions, use the rate of reaction practice set.
If you want a teacher to read your written explanations and show where a link is missing, see online one-to-one Chemistry tuition. The mole and stoichiometry steps tool helps with the amounts side of rate questions.