AQA GCSE Combined Science Foundation (8464), Foundation tier · Chemistry › The rate and extent of chemical change › Rate of reaction
Practise Catalysts. 8 exam-style questions on this subtopic, at up to four difficulty levels, with full mark schemes and a progress tracker. Free, no account needed.
What catalysts do, how they speed up reactions by providing a pathway with a lower activation energy, and how to show this on a reaction profile. Questions ask you to identify a catalyst from data, explain catalytic action and draw or use reaction profiles.
Key facts
Catalyst: speeds up a reaction, is not used up, same mass at the end
Catalysts are not included in the chemical equation
Enzymes = catalysts in biological systems
Different reactions need different catalysts
A catalyst provides a different pathway with a lower activation energy
A catalyst does not change the energy change or the amount of product
Reaction profile: activation energy = from the reactants' level up to the peak
Notes
What a catalyst is
A catalyst changes the rate of a chemical reaction but is not used up during the reaction.
At the end of the reaction the catalyst is chemically unchanged and has the same mass, so it can be used again.
Catalysts are not included in the chemical equation for the reaction (sometimes the name is written above the arrow).
Different reactions need different catalysts.
Enzymes act as catalysts in biological systems.
You can identify a catalyst from data: it makes the reaction faster, its mass is the same at the end, and it is not a reactant or product in the equation.
How a catalyst works
A catalyst provides a different pathway for the reaction that has a lower activation energy.
So a greater proportion of particles have energy equal to or greater than the activation energy, and successful collisions are more frequent.
A catalyst does not change the overall energy change of the reaction or the amount of product made.
With a catalyst, a reaction can be fast enough at a lower temperature, which saves energy.
Reaction profiles
diagram
A reaction profile shows energy (y-axis) against the progress of the reaction (x-axis).
Activation energy is the energy difference from the reactants' level up to the peak of the curve.
With a catalyst, the reactants and products are at the same energy levels, but the peak is lower.
Catalysed path (blue, dashed): lower peak, same reactants, products and overall energy change.
Draw the catalysed curve starting at the reactants and finishing at the products, with a lower hump. Label its activation energy with an arrow from the reactant level up to the new peak.
How to answer each type of question
Identify the catalyst from experimental results
2 to 3 marksGrade 5
Find the substance that makes the reaction faster than with nothing added.
Check its mass at the end: a catalyst has the same mass as at the start.
Check the equation: a catalyst is not a reactant or a product.
Example. A student added 0.50 g of each of three powders, A, B and C, to separate samples of the same hydrogen peroxide solution.
Powder added
Time to collect 50 cm3 of oxygen in s
Mass of powder at the end in g
none
400
–
A
400
0.50
B
35
0.50
C
90
0.32
Which powder is a catalyst for this reaction? Give two reasons for your answer.Show the model answerHide the model answer
B (1). It made the reaction faster than with no powder (1). Its mass did not change, so it was not used up (1).
Don’t lose marks
Writing only 'it lowers the activation energy'. The full answer is that it provides a different pathway with a lower activation energy.
Saying a catalyst gives the particles more energy or makes them move faster. It does neither.
Saying a catalyst is used up in the reaction.
Moving the product level on a catalysed profile, or measuring activation energy from the products.
Saying a catalyst increases the amount of product. It only makes the product form faster.
More tips
Memory tricks
A catalyst is like a tunnel through a hill: same start and finish, but a lower climb.
Catalyst check: faster? same mass at the end? not in the equation? Then it is a catalyst.
On a reaction profile, a catalyst only lowers the hump. The start and end levels never move.
Enzyme = biological catalyst: the same idea you meet in Biology.
Exam technique
Learn the key phrase: 'provides a different pathway with a lower activation energy'.
If asked how you know a substance is a catalyst, use evidence from the data: the rate increased and its mass stayed the same.
On reaction profiles, draw activation energy arrows from the reactant line up to the peak, with the arrowhead at the peak.
You do not need to learn the names of particular catalysts for this topic: the question will tell you which one is used.
What each grade needs
What you need to be able to do, from the first marks up to the top grade.
Grade 3
State what a catalyst doesIt increases the rate of a reaction but is not used up.
Grade 4
Know that enzymes are biological catalystsEnzymes are the catalysts that speed up reactions in living things.
Grade 5
Identify a catalyst from experimental dataIt speeds up the reaction, has the same mass at the end and is not in the equation.
Quick recall
Cover the answers and test yourself. The app has these as flashcards that come back just before you'd forget them.
What is a catalyst?
A substance that increases the rate of a reaction without being used up.
What name is given to the catalysts found in living things (biological systems)?
Enzymes
How does the equation show that catalase is acting as a catalyst?
Catalase does not appear in the equation.
Complete the sentence. A catalyst changes the rate of a chemical reaction, but is not changed ________ or used up by the reaction.
chemically
Sample questions
Written for this site in the style of AQA exam questions. They are not taken from real past papers.
Question 1Easy4 marks
(a) What is a catalyst?[2]
(b) What name is given to the catalysts in living things?[1]
(c) How does a catalyst increase the rate of a reaction? Tick (✓) one box.[1]
It gives the particles more energy.
It provides a different reaction pathway with a lower activation energy.
It increases the temperature of the mixture.
It increases the concentration of the reactants.
Show the answer and mark scheme
(a)Answer: A substance that increases the rate of a reaction without being used up.
a substance that increases / changes the rate of a reaction
without being used up (in the reaction)
(b)Answer: Enzymes
enzymes
(c)Answer: It provides a different reaction pathway with a lower activation energy.
Question 2Medium3 marks
A student compared two catalysts for the decomposition of hydrogen peroxide solution. The student used the same volume and concentration of hydrogen peroxide solution and the same mass of powdered catalyst each time, and collected the oxygen produced in a gas syringe for 60 s.
With manganese(IV) oxide: 48 cm3 of oxygen collected in 60 s.
With copper(II) oxide: 4.0 cm3 of oxygen collected in 60 s.
(a) Calculate the mean rate of reaction with each catalyst. Give your answers in cm3/s.[2]
(b) Calculate how many times faster the reaction was with manganese(IV) oxide than with copper(II) oxide.[1]