AQA GCSE Chemistry Chemistry Paper 1 (Higher), 2024: Question 4

14 marks · Standard Demand difficulty · Extended Answer

Analyze the results of a zinc and copper sulfate displacement reaction experiment, including drawing lines of best fit, explaining temperature trends, justifying the use of a polystyrene cup, completing an ionic equation, calculating a mean with uncertainty, and identifying sources of random error.

Practise this question

Question

A multi-part chemistry exam question about an investigation into the reaction between zinc and copper sulfate. It includes a scatter graph (Figure 3) showing the highest temperature reached against the mass of zinc, a table (Table 2) showing four temperature trials, and six sub-questions (04.1 to 04.6) requiring graph analysis, explanation of results, justification of equipment, ionic equation completion, mean calculation with uncertainty, and identification of random errors.
Question text

04 A student investigated the energy change of the reaction between zinc and

copper sulfate solution.

This is the method used.

1. Measure 25 cm3 of copper sulfate solution into a polystyrene cup.

2. Measure the temperature of the copper sulfate solution.

3. Add 0.20 g of zinc powder to the copper sulfate solution.

4. Stir the reaction mixture.

5. Record the highest temperature reached.

6. Repeat steps 1 to 5 with different masses of zinc powder.

Figure 3 shows the results.

Figure 3

04.1 Draw two lines of best fit on Figure 3.

The lines should cross.

[2 marks]

*0104.*2 Explain the results shown in Figure 3.

Do not refer to anomalous points.

Use data from Figure 3.

[4 marks]

04.3 Explain why using a polystyrene cup gives more accurate results than using a

glass beaker.

[2 marks]

04.4 Complete the ionic equation for the reaction between zinc and

copper sulfate solution.

Include state symbols.

[2 marks]

2+ 12

Zn(s) + Cu (aq) → ( ) + ( )

A different student repeated steps 1 to 5 of the method four times using 0.50 g of

zinc powder.

*11* Table 2 shows the results.

Table 2

Trial 1 Trial 2 Trial 3 Trial 4

Highest temperature reached in °C 37.6 37.2 37.8 37.4

04.5 Calculate the mean highest temperature reached.

Include the uncertainty in your answer.

[3 marks]

Mean highest temperature reached = ± °C

04.6 The results show random errors.

The student did not make any measuring errors.

Suggest one reason for the random errors in this experiment.

[1 mark]

Mark scheme

Show the mark scheme The mark scheme for question 4, detailing the marking points for each sub-question. It provides specific instructions for drawing the two lines of best fit, the expected explanation for the temperature rise, the reason for using a polystyrene cup, the correct ionic equation, the calculation for the mean temperature and uncertainty, and acceptable answers for random errors.

Question 4

AO /

Question Answers Extra information Mark

Spec. Ref.

04.1 max 1 mark if the lines do not AO3

intersect 4.5.1.1

line of best fit using the first five 1 RPA4

points

line of best fit using the last four 1

points

AO /

Spec. Ref.

04.2 the temperature rises because 1 AO2

the reaction is exothermic

or

the temperature rises because allow heat for energy

energy is transferred to the

surroundings

until 0.8 g (zinc) is added allow a tolerance of ± ½ a small 1 AO2

square

allow until the temperature

reaches 47 °C

allow a correctly determined

value for mass of zinc or

temperature from the

intersection of drawn lines of

best fit

(so) there is no additional allow (when) the reaction has 1 AO3

reaction finished

(because) zinc is in excess 1 AO3

or

(because) copper sulfate is used – – – 4.5.1.1

up RPA4

AO /

Spec. Ref.

allow converse statements for

04.3 glass AO1

4.5.1.1

polystyrene is a better (thermal) 1 RPA4

14 insulator

(so) there is less energy transfer allow (so) less energy is lost (to 1

to the surroundings the surroundings)

allow heat for energy

AO /

Spec. Ref.

04.4 Zn(s) + Cu2+(aq) → Zn2+(aq) + Cu(s) 2 AO2

4.1.1.1

4.2.2.2

allow 1 mark for Zn2+ + Cu 4.4.1.4

AO /

Spec. Ref.

04.5 (mean highest temperature =) allow AO2

37.6 + 37.2 + 37.8 + 37.4 (mean highest temperature =) 1 4.3.1.4

4 150

= 37.5 (ºC) 1

37.5 (ºC) ± 0.3 (ºC) 1

AO /

Spec. Ref.

04.6 any one from: 1 AO3

• starting temperature may be ignore room temperature 4.5.1.1

different RPA4

• inconsistent stirring

allow inconsistent use of a lid

Total Question 4 14

How to answer it

Energy Changes: Zinc and Copper Sulfate

What this question tests

This question focuses on Required Practical 4 (Temperature Changes). You need to demonstrate skills in plotting and interpreting graphs, understanding exothermic reactions and limiting reactants, writing ionic equations, and calculating means with uncertainty.

Part 4.1

Graphing Lines of Best Fit

How to draw the lines

You must draw two separate straight lines using a ruler:

  • Line 1: An upward sloping line through the first five points (0.00g to 0.80g).
  • Line 2: A horizontal (or slightly downward) line through the last four points (0.80g to 1.40g).

The two lines must intersect (cross) to get full marks.

Mark Scheme

  • Line 1 through first 5 points [1 mark]
  • Line 2 through last 4 points [1 mark]
Part 4.2

Explaining the Graph Trends

Why does the temperature stop rising?

Key Knowledge

  • Exothermic: Energy is transferred to the surroundings, causing a temperature rise.
  • Limiting Reactant: The reaction stops when one reactant is completely used up.

Exam Technique

When asked to "use data from the figure," you must quote specific numbers. Here, the "turning point" is at 0.8g of zinc.

Model Answer (4 Marks)

  1. The temperature rises because the reaction is exothermic (transfers heat to surroundings).
  2. The temperature stops rising at 0.8g of zinc.
  3. After this point, there is no further reaction.
  4. This is because the copper sulfate has been used up (zinc is now in excess).
Part 4.3

Experimental Equipment

Polystyrene vs. Glass

In energy change experiments, we want to measure the heat from the reaction, not the heat lost to the room.

  • Polystyrene: A good thermal insulator.
  • Glass: A better conductor than polystyrene.

Correct Answer

Polystyrene is a better thermal insulator [1 mark], so there is less energy transfer to the surroundings [1 mark].

Part 4.4

Ionic Equations

What is an ionic equation?

It shows only the particles that actually change during the reaction. In this displacement reaction, Zinc atoms become ions, and Copper ions become atoms.

The Equation

Zn(s) + Cu²⁺(aq) → Zn²⁺(aq) + Cu(s)

1 mark for correct symbols (Zn²⁺ and Cu), 1 mark for correct state symbols.

Part 4.5

Calculating Mean and Uncertainty

Step-by-Step Calculation

Step 1: Calculate the Mean
Sum of trials: 37.6 + 37.2 + 37.8 + 37.4 = 150.0
Mean = 150.0 ÷ 4 = 37.5 °C

Step 2: Calculate the Uncertainty
Uncertainty = Range ÷ 2
Highest value = 37.8, Lowest value = 37.2
Range = 37.8 - 37.2 = 0.6
Uncertainty = 0.6 ÷ 2 = 0.3 °C

Final Answer

Mean highest temperature reached = 37.5 ± 0.3 °C

Common Pitfalls

Don't forget to divide the range by 2 for uncertainty! Many students mistakenly give the full range (0.6) as the uncertainty.

Part 4.6

Random Errors

Identifying Random Errors

Random errors are chance variations. The question says there were no measuring errors, so look for human or environmental inconsistencies.

Accepted Reasons (Choose one)

  • The starting temperature of the solution may have been different each time.
  • Inconsistent stirring (stirring faster/slower affects how quickly heat is distributed).
  • Inconsistent use of a lid.

What to avoid

Do not say "room temperature" on its own. You must specify that the starting temperature of the solution varied.

Topics

Chemistry · Required Practicals · C4: Chemical Changes · C5: Energy Changes · Required Practicals

Question and mark scheme from the AQA GCSE Chemistry examination, Chemistry Paper 1 (Higher), 2024. QuestionVault is an independent revision resource; questions remain the copyright of the awarding body.