Collision theory and activation energy - AQA GCSE Chemistry Exam Questions

Practise 30 exam-style questions on Collision theory and activation energy for AQA GCSE Chemistry. Write your answer as you would in the exam and get it marked in seconds against the mark scheme, point by point, with an AI tutor on hand if you get stuck.

30 questions
1–6 marks each
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20 questions, marked as you go

Sample questions with model answers

Here are 4 of the questions in this topic, each with a model answer, an explanation of why it earns the marks and the mark scheme an examiner would use.

Question 1

2 marks

Increasing the concentration of a reactant increases the frequency of collisions. However, it does not change the proportion of collisions that are successful.

Explain why the proportion of successful collisions does not change.

Model answer

Changing the concentration does not change the temperature, so the particles have the same range of energies as before. The same fraction of collisions therefore have energy equal to or greater than the activation energy; there are simply more collisions every second.

Explanation

Concentration, pressure and surface area only change how often particles collide. Only temperature (or a catalyst, which lowers the activation energy) changes the proportion of collisions that are successful.

Mark scheme

1 mark
the temperature is the same, so the particles have the same energy
allow concentration does not affect the energy of the particles
1 mark
so the same fraction of collisions have energy greater than or equal to the activation energy
allow the activation energy is not changed

Question 2

2 marks

In a car engine, the fuel is sprayed into the cylinders as a fine mist of tiny droplets.

Explain why the fuel is sprayed as tiny droplets.

Model answer

Tiny droplets have a much larger surface area to volume ratio than larger drops, so more fuel molecules are exposed to oxygen. Collisions between fuel and oxygen molecules are more frequent, so the fuel burns very quickly.

Explanation

Breaking up a liquid into droplets has the same effect as grinding a solid into a powder. In an engine the fuel must burn in a fraction of a second, so a large surface area is essential.

Mark scheme

1 mark
tiny droplets have a larger surface area (to volume ratio)
1 mark
so collisions between fuel and oxygen molecules are more frequent and the fuel burns faster
allow burns more quickly / more completely

Question 3

3 marks

In a flour mill, fine flour dust in the air can explode if it is ignited by a spark. A bag of flour only burns slowly.

Explain why.

Model answer

Fine flour dust spread through the air has a huge surface area to volume ratio compared with flour packed in a bag. Many more flour particles are exposed to oxygen, so collisions between flour and oxygen particles are far more frequent. The reaction is so fast that the energy is released almost instantly, causing an explosion.

Explanation

In a bag, only the flour on the outside is in contact with air. As a fine dust, every tiny grain is surrounded by oxygen, so the rate of combustion becomes extremely fast.

Mark scheme

1 mark
flour dust has a much larger surface area (to volume ratio)
1 mark
more flour particles are exposed to oxygen (in the air)
1 mark
so collisions with oxygen are much more frequent, giving a very fast reaction
do not accept 'more collisions' alone

Question 4

4 marks

Copper oxide powder is heated in a stream of hydrogen gas:
CuO(s) + H₂(g) → Cu(s) + H₂O(g)

Suggest two changes that would increase the rate of this reaction. Explain each change using collision theory.

Model answer

Increase the temperature: the particles gain energy, so they collide more frequently and a greater proportion of collisions have energy equal to or greater than the activation energy.
Use finer copper oxide powder: this increases the surface area to volume ratio, so more copper oxide particles are exposed to hydrogen and collisions are more frequent.

Explanation

This reaction involves a gas and a solid, so the factors that apply are temperature, the pressure of the gas and the surface area of the solid. Each change must be linked to how it affects the frequency or energy of collisions.

Mark scheme

1 mark
increase the temperature
max 4: up to 2 changes, each with its explanation
1 mark
particles have more energy, so collisions are more frequent and a greater proportion have energy ≥ activation energy
1 mark
increase the pressure / concentration of the hydrogen
allow increase the flow of hydrogen
1 mark
more hydrogen molecules in the same volume, so collisions with copper oxide are more frequent
1 mark
use a finer powder of copper oxide
1 mark
larger surface area (to volume ratio), so more copper oxide particles are exposed and collisions are more frequent

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