AQA GCSE Combined Science (8464), Higher tier · Chemistry › The rate and extent of chemical change › Rate of reaction
Practise Collision theory and activation energy. 14 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.
Grade by grade
What you need to be able to do, from the first marks up to the top grade.
3
State that particles must collide to reactA reaction can only happen when reactant particles collide with each other.
4
Define activation energyThe minimum amount of energy that particles must have to react.
5
Explain concentration and pressure effectsThere are more particles in the same volume, so collisions are more frequent.
5
Explain the effect of surface areaMore particles of the solid are exposed to the other reactant, so collisions are more frequent.
6
Explain the effect of temperature fullyParticles move faster, so collisions are more frequent and more energetic, and more of them have at least the activation energy.
7
Calculate surface area to volume ratioFor a cube, surface area = 6 × side2 and volume = side3; smaller pieces have a larger ratio.
8
Use proportionality to predict changes in rateE.g. doubling the concentration doubles the frequency of collisions, so the rate doubles.
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
Higher concentration (solutions) or higher pressure (gases): more reactant particles in the same volume, so collisions are more frequent.
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
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.
The larger the ratio, the more particles are exposed, so the more frequent the collisions and the faster the reaction.
Proportionality
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.
To check data for direct proportionality: doubling one quantity should double the other, and a graph of one against the other is a straight line through the origin. grade 8+
Cheatsheet
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)
How to answer each type of question
Explain the effect of concentration, pressure or surface area
2 marks5
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 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).
Explain the effect of temperature
3 marks6
Particles have more energy and move faster.
So collisions are more frequent.
And a greater proportion of collisions have energy equal to or greater than the activation energy.
Example. A student found that a reaction was faster at 40 °C than at 20 °C. Explain why, in terms of particles.
Show the model answer
At 40 °C the particles have more energy and move faster (1). So they collide more frequently (1). And a greater proportion of the collisions have energy equal to or greater than the activation energy, so more collisions are successful (1).
Calculate surface area to volume ratios and explain
3 to 5 marks7
Surface area of a cube = 6 × side × side.
Volume of a cube = side × side × side.
Write the ratio surface area : volume and simplify it to 'something : 1'.
Link: larger ratio → more particles exposed → more frequent collisions → faster reaction.
Example. A 3 cm cube of calcium carbonate is cut into 1 cm cubes. (a) Calculate the surface area to volume ratio of the 3 cm cube. (b) Give the surface area to volume ratio of one 1 cm cube. (c) Explain why the small cubes react faster with hydrochloric acid than the large cube.
Show the model answer
(a) surface area = 6 × 3 × 3 = 54 cm2 and volume = 3 × 3 × 3 = 27 cm3 (1) ratio = 54 : 27 = 2 : 1 (1) (b) 6 : 1 (1) (c) The small cubes have a larger surface area to volume ratio, so more particles are exposed to the acid (1), so collisions are more frequent (1).
Use proportionality to predict a change in rate
2 marks8
Say how the number of particles in the same volume changes (e.g. halves).
Say the frequency of collisions changes by the same factor, so the rate does too.
Example. A student repeated a reaction with the concentration of the acid halved and everything else kept the same. Predict how the initial rate would change. Explain your prediction using collision theory.
Show the model answer
The initial rate would halve (1). There are half as many acid particles in the same volume, so the frequency of collisions halves (1).
Shortcuts and 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.
Where marks are lost
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.
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.
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
so there are more frequent collisions
Question 3Hard7 marks
A company produces a gas by reacting lumps of a solid with an acid at room temperature.
(a) What is meant by a successful collision?[1]
(b) The company wants to increase the rate of production of the gas. Explain how the company could change the conditions to increase the rate of reaction. Use collision theory in your answer.[6]
Show the answer and mark scheme
(a)Answer: A collision between reacting particles with at least the activation energy, so that they react.
a collision between reacting particles with energy greater than or equal to the activation energy (so that a reaction happens)
(b)Answer: See indicative content.
increase the temperature: particles move faster, so they collide more frequently
at a higher temperature collisions are more energetic, so a greater proportion have energy greater than or equal to the activation energy
use smaller lumps or a powder: larger surface area to volume ratio, so more particles are exposed and collisions are more frequent
increase the concentration of the acid: more acid particles in the same volume, so collisions are more frequent
add a catalyst: provides a different pathway with a lower activation energy, so more collisions are successful
changing the pressure has no effect because the reactants are not gases
each change gives more successful collisions per second, so a faster rate
Marked with levels of response: the full level descriptors are in the app.