AQA GCSE Combined Science (8464), Higher tier · Chemistry › The rate and extent of chemical change › Rate of reaction
Practise Catalysts. 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.
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.
Grade by grade
What you need to be able to do, from the first marks up to the top grade.
3
State what a catalyst doesIt increases the rate of a reaction but is not used up.
4
Know that enzymes are biological catalystsEnzymes are the catalysts that speed up reactions in living things.
5
Identify a catalyst from experimental dataIt speeds up the reaction, has the same mass at the end and is not in the equation.
6
Explain how a catalyst increases the rateIt provides a different pathway for the reaction that has a lower activation energy.
6
Draw a reaction profile with a catalystShow a lower peak between the same reactant and product energy levels.
7
Link catalysis to successful collisionsWith a lower activation energy, more collisions have enough energy to react, so successful collisions are more frequent.
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
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.
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.
Cheatsheet
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
How to answer each type of question
Explain how a catalyst increases the rate
2 to 3 marks6
Say it provides a different pathway (route) for the reaction.
Say this pathway has a lower activation energy.
For a third mark: so more collisions have enough energy to react, and successful collisions are more frequent.
Example. Manganese(IV) oxide is a catalyst for the decomposition of hydrogen peroxide. Explain how manganese(IV) oxide increases the rate of this reaction.
Show the model answer
It provides a different pathway for the reaction (1) that has a lower activation energy (1). So a greater proportion of collisions have energy equal to or greater than the activation energy, and successful collisions are more frequent (1).
Identify the catalyst from experimental results
2 to 3 marks5
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. With no powder, it takes 400 s to collect 50 cm3 of oxygen. Powder A: 400 s to collect 50 cm3 of oxygen; mass of powder at the end 0.50 g Powder B: 35 s; mass at the end 0.50 g Powder C: 90 s; mass at the end 0.32 g Which powder is a catalyst for this reaction? Give two reasons for your answer.
Show 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).
Use a reaction profile for a catalysed reaction
2 to 3 marks6
Keep the reactant and product levels the same as for the uncatalysed reaction.
The catalysed peak is lower, so the activation energy is smaller.
Activation energy = peak energy − reactant energy (never measure it from the products).
Example. In a reaction profile for an exothermic reaction, the reactants are at 120 kJ/mol and the products are at 40 kJ/mol. Without a catalyst the peak is at 250 kJ/mol. With a catalyst the peak is at 180 kJ/mol. (a) Calculate the activation energy without the catalyst. (b) Calculate the activation energy with the catalyst. (c) How much energy is released overall by the catalysed reaction?
Show the model answer
(a) 250 − 120 = 130 kJ/mol (1) (b) 180 − 120 = 60 kJ/mol (1) (c) 120 − 40 = 80 kJ/mol, the same as without the catalyst (1)
Shortcuts and 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.
Where marks are lost
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.
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.
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]
A company makes a chemical by reacting two gases. Without a catalyst, the reaction needs a temperature of 450 °C to go at a useful rate. With a solid catalyst, the reaction goes at the same rate at 250 °C.
(a) Explain why the catalyst allows the reaction to go at the same rate at a lower temperature.[2]
(b) Give two advantages of using a lower temperature.[2]
(c) The catalyst is used as a fine mesh or as small pellets, not as large lumps. Explain why.[2]
(d) The catalyst is not used up, but it still needs to be replaced after several years. Suggest one reason why.[1]
Show the answer and mark scheme
(a)Answer: The catalyst lowers the activation energy, so at 250 °C enough particles still have at least the activation energy.
the catalyst provides a pathway with a lower activation energy
so at 250 °C enough particles still have energy greater than or equal to the (lower) activation energy / there are the same number of successful collisions per second
(b)Answer: Less energy is needed, so costs are lower; less fossil fuel is burned, so less carbon dioxide is released.
less energy / fuel is needed, so energy costs are lower
less fossil fuel is burned, so less carbon dioxide is released
it is safer / there is less risk of burns or fire
the equipment does not have to withstand such high temperatures, so it is cheaper to build and maintain
(c)Answer: This gives a larger surface area, so more reactant particles can collide with the catalyst surface at the same time.
this gives a larger surface area
so more reactant particles can collide with / react on the catalyst surface at the same time
(d)Answer: Impurities in the gases coat the surface of the catalyst.
impurities coat / react with the catalyst surface (poisoning it) / the surface becomes damaged or blocked / some catalyst is lost over time