Catalysts - AQA GCSE Chemistry Exam Questions

Practise 25 exam-style questions on Catalysts 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.

25 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

What is a catalyst?

Model answer

A catalyst is a substance that changes (increases) the rate of a chemical reaction without being used up during the reaction.

Explanation

Both parts of the definition are needed. Because a catalyst is not used up, it is still there at the end of the reaction and can be used again.

Mark scheme

1 mark
a substance that changes / increases the rate of a chemical reaction
allow speeds up a reaction
1 mark
without being used up (in the reaction)
allow is chemically unchanged at the end / mass is the same at the end

Question 2

2 marks

A student added a small amount of substance Z to a reaction mixture and found that the rate of reaction increased. The student concluded that Z is a catalyst.

Describe the further evidence needed to show that Z is a catalyst.

Model answer

The student needs to show that Z is not used up, for example by recovering it at the end and showing that its mass has not changed. The recovered Z should also still speed up the reaction if it is used again, showing it is chemically unchanged.

Explanation

A substance that speeds up a reaction could be a reactant rather than a catalyst. Only if it is also unchanged at the end can it be called a catalyst.

Mark scheme

1 mark
Z is not used up / the mass of Z is the same at the end of the reaction
1 mark
Z is chemically unchanged / can be reused and still speeds up the reaction / does not appear in the equation for the reaction

Question 3

3 marks

A student wants to show that copper(II) oxide powder is not used up when it catalyses the decomposition of hydrogen peroxide.

Describe how the student could do this.

Model answer

Weigh the copper(II) oxide before adding it to the hydrogen peroxide. When the reaction has finished, filter the mixture to collect the copper(II) oxide, wash it with water and dry it. Weigh it again: if it is a catalyst, its mass will be the same as at the start.

Explanation

A catalyst is chemically unchanged at the end of a reaction, so the same mass can be recovered. Drying is important because any water left on the solid would make its mass seem larger.

Mark scheme

1 mark
weigh the copper(II) oxide before adding it
1 mark
after the reaction, filter off the copper(II) oxide, then wash and dry it
1 mark
reweigh it: the mass should be the same as at the start
allow it can be reused and still speeds up the reaction

Question 4

4 marks

Without a catalyst, the decomposition of hydrogen peroxide produced 12 cm³ of oxygen in 60 seconds. With a catalyst, the same volume and concentration of hydrogen peroxide produced 48 cm³ of oxygen in 20 seconds.

Calculate the mean rate of each reaction and determine how many times faster the catalysed reaction is.

Model answer

Without catalyst: mean rate = 12 ÷ 60 = 0.2 cm³/s
With catalyst: mean rate = 48 ÷ 20 = 2.4 cm³/s
2.4 ÷ 0.2 = 12, so the catalysed reaction is 12 times faster.

Explanation

The two volumes were measured over different times, so you must compare rates, not volumes. Comparing 48 cm³ with 12 cm³ directly would wrongly suggest the catalyst makes the reaction only 4 times faster.

Mark scheme

1 mark
without catalyst: 12 ÷ 60 = 0.2
correct answer with no working scores full marks
1 mark
with catalyst: 48 ÷ 20 = 2.4
1 mark
unit: cm³/s
allow cm³ s⁻¹; must be given at least once
1 mark
2.4 ÷ 0.2 = 12 times faster
allow ecf from calculated rates

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