AQA GCSE Combined Science: Trilogy Chemistry Paper 2 (Foundation), 2023: Question 4

10 marks · Standard Demand difficulty · Short Answer

Investigate the rate of decomposition of hydrogen peroxide using different catalysts by plotting data and interpreting graphs.

Practise this question

Question

A multi-part exam question about the rate of decomposition of hydrogen peroxide. It includes a diagram of a flask containing a catalyst and hydrogen peroxide solution connected to a gas syringe. A table shows time in seconds and volume of oxygen in cubic centimeters for a manganese dioxide catalyst. A graph plots volume of oxygen against time for copper oxide and zinc oxide catalysts, with space for the student to plot the manganese dioxide data.
Question text

04 A catalyst is used to increase the rate of decomposition of hydrogen peroxide.

04.1 Hydrogen peroxide decomposes to produce water and oxygen.

Write a word equation for the reaction.

[1 mark]

→ +

A student investigated the rate of decomposition of hydrogen peroxide using three

different catalysts:

• manganese dioxide

• copper oxide

• zinc oxide.

Figure 7 shows the apparatus.

Figure 7

This is the method used.

1. Measure 25 cm3 of hydrogen peroxide solution into a flask.

2. Add 0.5 g of manganese dioxide catalyst to the flask.

3. Attach a gas syringe to the flask.

4. Measure the volume of oxygen produced every 20 seconds for 120 seconds.

5. Repeat steps 1 to 4 two more times.

6. Repeat steps 1 to 5 using copper oxide catalyst.

7. Repeat steps 1 to 5 using zinc oxide catalyst.

04.2 Which two control variables are used in the investigation?

[2 marks]

Tick ( ) two boxes.

Mass of catalyst

Shape of flask

Time of taking readings

Volume of hydrogen peroxide solution

Volume of oxygen produced 22

Table 3 shows the results with manganese dioxide catalyst.

Table 3

Time in seconds 0 20 40 60 80 100 120

Volume of oxygen in

30 20 32 40 45 49 50

cm

Figure 8 shows a graph of the results with copper oxide catalyst and with

zinc oxide catalyst.

Figure 8

04.3 Complete Figure 8.

You should:

• plot the data for the manganese dioxide catalyst from Table 3

• draw a line of best fit.

The first two points have been plotted for you.

[3 marks]

04.4 Which is the best catalyst?

Give one reason for your answer.

Use the completed Figure 8.

[2 marks]

Tick ( ) one box.

*22* Copper oxide

Manganese dioxide

Zinc oxide

Reason

04.5 The student repeated the investigation with manganese dioxide catalyst.

The results were all close to the true value.

Complete the sentence.

Choose the answer from the box.

[1 mark]

accurate approximate random

Results that are close to the true value are24 .

04.6 The student repeated the investigation using manganese dioxide catalyst at a

higher temperature.

Complete the sentence.

Choose the answer from the box.

[1 mark]

decrease stay the same increase

At a higher temperature, the rate of reaction will .

Mark scheme

Show the mark scheme The mark scheme for question 4 lists the correct answers: the word equation 'hydrogen peroxide → water + oxygen', control variables 'mass of catalyst' and 'volume of hydrogen peroxide solution', plotting instructions for the graph, 'manganese dioxide' as the best catalyst due to its steeper curve, 'accurate' for results close to the true value, and 'increase' for the effect of temperature on rate. It specifies a total of 10 marks.

Question 4

AO /

Question Answers Extra information Mark

Spec. Ref.

04.1 hydrogen peroxide → water + oxygen allow H2O2 for hydrogen 1 AO2

peroxide 5.6.1.4

allow H2O for water

allow O2 for oxygen

AO /

Spec. Ref.

04.2 mass of catalyst 1 AO3

5.6.1.4

volume of hydrogen peroxide 1

solution

AO /

Spec. Ref.

04.3 all points plotted correctly allow a tolerance of ± ½ a small 2 AO2

square 5.6.1.4

allow at least 3 points plotted

correctly for 1 mark

line of best fit 1

AO /

Spec. Ref.

04.4 manganese dioxide allow ecf from question 04.3 1 AO3

5.6.1.4

any one from: 1

• steepest curve

• reaction finishes first

• greatest volume of oxygen (in

given time)

AO /

Spec. Ref.

04.5 accurate 1 AO1

– OMBINED SCIENCE: TRILOGY – 2F – 5.6.1.4

AO /

Spec. Ref.

04.6 increase 1 AO1

5.6.1.2

14 5.6.1.3

5.6.1.4

Total Question 4 10

How to answer it

Investigating Catalysts and Rates of Reaction

What this question tests

  • Chemical Equations: Recalling the decomposition of hydrogen peroxide.
  • Practical Skills: Identifying control variables in a rate of reaction experiment.
  • Data Handling: Plotting accurate graphs and drawing smooth lines of best fit.
  • Analysis: Using graph gradients to determine the most effective catalyst.
  • Scientific Vocabulary: Understanding the definition of "accuracy".
  • Theory: Predicting how temperature changes affect the rate of reaction.
Part 4.1

Word Equations

Correct Answer

hydrogen peroxide → water + oxygen

Key Knowledge

Decomposition means a single reactant breaks down into two or more products. Hydrogen peroxide (H₂O₂) is unstable and naturally breaks down, but catalysts speed this up.

Exam Technique

Always use an arrow (→) in equations, never an equals sign (=). If the question asks for a word equation, do not use symbols unless you are 100% sure they are correct, as a mistake in a formula will lose the mark.
Part 4.2

Identifying Variables

Correct Answers

  • Mass of catalyst
  • Volume of hydrogen peroxide solution

How to find them

Look at the Method provided in the question.
Step 1 says "Measure 25 cm³..."
Step 2 says "Add 0.5 g..."
Because these specific numbers are used for every catalyst tested, they are the control variables.

Common Error

Students often confuse "Time of taking readings" with a control variable. While we take readings at the same intervals, the time is the independent variable on the x-axis, not a condition we are keeping constant to ensure a fair test.
Part 4.3

Graph Skills

Plotting the Data

You must plot the points from Table 3 for Manganese Dioxide:
  • (40, 32)
  • (60, 40)
  • (80, 45)
  • (100, 49)
  • (120, 50)

Line of Best Fit

For rate experiments, the line of best fit is almost always a smooth curve that starts at (0,0). Do not use a ruler to "dot-to-dot" the points.

Examiner Insight

Marks are awarded for accuracy within ± ½ a small square. Use a sharp pencil! If you misplot more than two points, you lose the plotting marks.
Part 4.4

Evaluating Catalysts

Best Catalyst

Manganese dioxide

The Reason

Any one of:
  • It has the steepest curve.
  • The reaction finishes first.
  • It produced the greatest volume of oxygen in the given time.

Exam Technique

When a question says "Use the completed Figure 8", your reason must refer to the data on the graph (e.g., the gradient or the final volume).
Parts 4.5 & 4.6

Terminology & Trends

4.5 Accuracy

Results close to the true value are: accurate.

4.6 Temperature

At a higher temperature, the rate of reaction will: increase.

Why does rate increase?

Higher temperature means particles have more kinetic energy. They move faster, leading to:
  1. More frequent collisions.
  2. More energetic collisions (more particles have the Activation Energy).

Accuracy vs Precision

Accurate: Close to the "true" or "target" value.
Precise: Multiple measurements are close to each other (consistent).

Topics

Chemistry · C6: The Rate and Extent of Chemical Change

Question and mark scheme from the AQA GCSE Combined Science: Trilogy examination, Chemistry Paper 2 (Foundation), 2023. QuestionVault is an independent revision resource; questions remain the copyright of the awarding body.