AQA GCSE Chemistry Foundation (8462), Foundation tier · The rate and extent of chemical change › Rate of reaction
Practise Collision theory and activation energy. 7 exam-style questions on this subtopic, at up to four difficulty levels, with full mark schemes and a progress tracker. Free, no account needed.
Collision theory explains why concentration, pressure, surface area and temperature change the rate of a reaction. Expect 2 to 4 mark 'explain' questions using the ideas of collision frequency and activation energy, plus surface area to volume ratio calculations and simple proportionality.
Key facts
Particles must collide with at least the activation energy to react
Activation energy = the minimum energy particles must have to react
Higher concentration or pressure → more particles in the same volume → more frequent collisions
Larger surface area → more exposed particles → more frequent collisions
Higher temperature → faster particles → more frequent AND more energetic collisions
Cube: surface area = 6 × side2; volume = side3
Smaller pieces → larger surface area to volume ratio → faster reaction
Double the concentration → double the collision frequency → double the rate (in simple cases)
Notes
Collision theory
Chemical reactions can only happen when reacting particles collide with each other with enough energy.
The activation energy is the minimum amount of energy that particles must have to react.
Collisions with less energy than the activation energy do not lead to a reaction: the particles just bounce apart.
The rate depends on the frequency of successful collisions: the number of successful collisions per second.
Explaining each factor
diagram
Higher concentration (solutions) or higher pressure (gases): more reactant particles in the same volume, so collisions are more frequent.
Same volume, more reactant particles, so collisions happen more often.
Larger surface area: more particles of the solid are exposed to the other reactant, so collisions are more frequent.
Higher temperature: particles move faster, so collisions are more frequent. The collisions are also more energetic, so a greater proportion of them have energy equal to or greater than the activation energy.
Temperature is the only one of these factors that changes the energy of the collisions as well as their frequency.
Surface area to volume ratio
diagram
Cutting a solid into smaller pieces does not change its total volume but increases its total surface area, so the surface area to volume ratio increases.
A 2 cm cube: surface area = 6 × 2 × 2 = 24 cm2, volume = 2 × 2 × 2 = 8 cm3, ratio = 24 : 8 = 3 : 1.
Cut it into eight 1 cm cubes: total surface area = 8 × 6 = 48 cm2, volume still 8 cm3, ratio = 6 : 1.
Cutting the cube up doubles the surface area but keeps the volume the same.
The larger the ratio, the more particles are exposed, so the more frequent the collisions and the faster the reaction.
Proportionality
diagram
Doubling the concentration of a solution gives twice as many reactant particles in the same volume.
In simple cases this doubles the frequency of collisions, so the rate doubles: the rate is directly proportional to the concentration.
Double the concentration, double the rate: a straight line through (0, 0).
How to answer each type of question
Explain the effect of concentration, pressure or surface area
2 marksGrade 5
Say what happens to the particles: more in the same volume, or more exposed.
Link to collisions: collisions are more frequent (more collisions per second).
Conclude that the rate increases.
Example. Explain why magnesium reacts faster with concentrated hydrochloric acid than with dilute hydrochloric acid.
Show the model answerHide the model answer
In the concentrated acid there are more acid particles in the same volume (1), so collisions between acid particles and magnesium are more frequent (1).
Don’t lose marks
Writing 'more collisions' instead of 'more frequent collisions' or 'more collisions per second'. The time idea is needed for the mark.
Saying particles move faster when the concentration increases. Only a temperature rise makes particles move faster.
Saying a higher temperature lowers the activation energy. It does not: more particles have enough energy to overcome it.
Saying particles 'collide harder' or 'with more force' instead of 'with more energy'.
Saying larger pieces have more surface area. For the same mass, smaller pieces have the larger total surface area.
More tips
Memory tricks
Every rate explanation follows one chain: particles → collisions → frequency (and energy) → rate.
Temperature is the only factor with two effects: more frequent collisions AND more energetic collisions.
Picture activation energy as a hurdle: only particles with enough energy get over it and react.
For a cube, surface area to volume ratio = 6 ÷ side length, so halving the side doubles the ratio.
Exam technique
Use the key words examiners look for: frequency, collisions per second, energetic, activation energy.
For 3-mark temperature questions, give the speed/energy point, the frequency point and the activation energy point.
When asked to 'use collision theory', write about particles and collisions, not about the reaction 'getting excited'.
In ratio calculations, show the surface area (cm2) and volume (cm3) before the ratio, so you can earn method marks.
What each grade needs
What you need to be able to do, from the first marks up to the top grade.
Grade 3
State that particles must collide to reactA reaction can only happen when reactant particles collide with each other.
Grade 4
Define activation energyThe minimum amount of energy that particles must have to react.
Grade 5
Explain concentration and pressure effectsThere are more particles in the same volume, so collisions are more frequent.
Grade 5
Explain the effect of surface areaMore particles of the solid are exposed to the other reactant, so collisions are more frequent.
Quick recall
Cover the answers and test yourself. The app has these as flashcards that come back just before you'd forget them.
Complete the sentence. Chemical reactions can only happen when reacting particles ________ with each other with enough energy.
collide
Sample questions
Written for this site in the style of AQA exam questions. They are not taken from real past papers.
Question 1Easy5 marks
(a) Complete the sentence. Chemical reactions can only happen when reacting particles ________ with each other with enough energy.[1]
(b) What is the name for the minimum amount of energy that particles must have to react?[1]
(c) Why does increasing the concentration of an acid increase the rate of its reaction with a metal? Tick (✓) one box.[1]
The acid particles move faster.
There are more acid particles in the same volume, so collisions are more frequent.
The activation energy is lower.
Each collision has more energy.
(d) Increasing the temperature increases the rate of a reaction. Give two reasons why, in terms of particles.[2]
Show the answer and mark scheme
(a)Answer: collide
collide
(b)Answer: Activation energy
activation energy
(c)Answer: There are more acid particles in the same volume, so collisions are more frequent.
(d)Answer: Particles move faster so collide more often; collisions are more energetic, so more particles have at least the activation energy.
particles move faster, so they collide more frequently
collisions are more energetic / more particles have energy greater than or equal to the activation energy
Question 2Medium6 marks
A student compared a single 2 cm cube of calcium carbonate with another 2 cm cube that had been cut into eight 1 cm cubes.
(a) Calculate the surface area to volume ratio of the single 2 cm cube.[2]
(b) Calculate the total surface area of the eight 1 cm cubes.[1]
(c) Both samples were added to equal volumes of the same hydrochloric acid. Explain why the eight small cubes reacted faster than the single cube.[3]
(c)Answer: The small cubes have a larger surface area to volume ratio (6 : 1 instead of 3 : 1), so more particles are exposed to the acid and collisions are more frequent.
the small cubes have a larger surface area to volume ratio (6 : 1 compared with 3 : 1)
so more calcium carbonate particles are exposed to the acid