Rate of Reaction and Collision Theory
How fast a reaction goes. Every explanation on this page uses the same three-part structure — and answers that give only part of it lose marks.
1. Collision theory
For a reaction to occur, particles must COLLIDE, with enough energy (at least the activation energy) and in the correct orientation.
Rate depends on the FREQUENCY of successful collisions.
The explanation template — use all three parts:
1. The particles have more energy / are closer together / there is more surface exposed 2. So they collide MORE FREQUENTLY (and/or with more energy) 3. So the RATE INCREASES
Incomplete explanations were a recorded error. Naming the factor is not enough — you must say what happens to the collisions and then to the rate.
2. The factors
Concentration
Higher concentration → more particles in the same volume → MORE FREQUENT collisions → faster rate.
Pressure (gases only)
Higher pressure → particles pushed closer together → more frequent collisions → faster rate.
Pressure applies to GASES; concentration applies to SOLUTIONS. Confusing the two was recorded — they work the same way but describe different systems.
Temperature
Higher temperature → particles gain KINETIC ENERGY and move FASTER → they collide MORE FREQUENTLY and MORE of the collisions have energy above the ACTIVATION ENERGY → faster rate.
Temperature changes BOTH the frequency and the energy of collisions. This is the one factor with two effects, and the energy part is the one students omit. Incomplete explanations of why rate differs with temperature were recorded directly.
Surface area
Smaller pieces (powder) → LARGER SURFACE AREA → more particles exposed for collisions → faster rate.
Powder reacts faster than lumps for the same mass. Confusion about the effect of surface area was recorded — cutting a solid into smaller pieces increases the total surface area.
Catalyst
A catalyst speeds up a reaction by providing an alternative route with a LOWER ACTIVATION ENERGY, and is not used up in the reaction.
More collisions now have enough energy to react, so the rate increases.
A catalyst does not change the amount of product — only how fast it arrives.
3. Measuring rate
Two standard methods:
| Method | What you measure | Suits |
|---|---|---|
| Gas collection | volume of gas over time, with a gas syringe or measuring cylinder over water | reactions producing a gas |
| Mass loss | mass on a balance over time | reactions releasing a gas (CO₂, H₂) |
| Disappearing cross | time for a mark to become invisible | precipitate-forming reactions |
Mass decreases because GAS ESCAPES, not because of surface area. A recorded error linked the mass loss to surface area — the mass falls because the gas produced leaves the flask.
Check the gas collection setup for leaks — a recorded error missed an apparatus fault. Gas escaping before it reaches the syringe is the usual problem.
Rate = change in quantity ÷ time taken.
4. Rate graphs
A typical graph plots volume of gas (or mass lost) against time.
The STEEPER the initial gradient, the FASTER the reaction. The graph levels off when the reaction is complete — one reactant has been used up.
Comparing two experiments:
A faster reaction gives a STEEPER curve that levels off SOONER — but at the SAME final height, provided the same amounts of reactants were used.
This is the most-tested graph point, and it was got wrong directly. A student expected the faster curve to finish higher. It doesn’t: a catalyst or higher temperature changes how fast, not how much. The final volume depends only on the amount of limiting reactant.
A more concentrated solution gives a steeper curve AND a higher final volume — but only because more reactant was present, not because it was faster.
Finding the rate at a point: draw a tangent to the curve and find its gradient.
Levelling off means the reaction has finished — not that it has reached equilibrium. Confusing completion with equilibrium was recorded twice.
5. Mistakes that cost marks
Giving only part of the explanation — factor, collisions, rate.
Omitting the energy effect for temperature.
Saying larger pieces have more surface area.
Confusing concentration with pressure.
Saying a catalyst is used up, or that it increases the yield.
Expecting a faster reaction to reach a higher final volume.
Confusing “levelling off” with equilibrium.
Attributing mass loss to anything but escaping gas.
Frequently asked questions
What is collision theory? Particles must collide with enough energy and correct orientation to react.
What is activation energy? The minimum energy particles need for a successful collision.
Why does higher concentration increase rate? More particles per volume → more frequent collisions.
Why does temperature increase rate? Particles move faster (more frequent collisions) and more collisions exceed the activation energy.
Why does powder react faster than lumps? Larger surface area exposes more particles.
How does a catalyst work? It provides an alternative route with lower activation energy, and is not used up.
Does a catalyst change the amount of product? No — only the rate.
What does a steeper graph mean? A faster reaction.
Why do two curves level off at the same height? Because the same amount of reactant was used — the rate differed, not the quantity.
What does levelling off mean? The reaction is complete, not at equilibrium.
Quick revision checklist
- I can state collision theory with all three requirements
- I use the three-part explanation structure every time
- I can explain concentration, pressure, temperature, surface area and catalysts
- I include the energy effect for temperature
- I know a catalyst lowers activation energy and isn’t used up
- I can describe gas collection and mass loss methods
- I know mass falls because gas escapes
- I can read a rate graph and compare gradients
- I know faster curves level off sooner at the same height
- I can find a rate from a tangent
- I don’t confuse completion with equilibrium
These notes cover rate of reaction and collision theory in the Cambridge IGCSE Chemistry (0620) syllabus and are written for Grade 9–11 / Year 10–11 students. They are based on teaching patterns observed across a large set of one-to-one IGCSE Chemistry lessons, with particular attention to the errors students make most often and the wording examiners reward. Always check the current syllabus for your own exam series.
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