AQA GCSE Chemistry Chemistry Paper 1 (Higher), 2023: Question 7
9 marks · Standard Demand difficulty · Short Answer
This question covers the electrolysis of aluminium oxide and the electrolysis of an aqueous solution, including half-equations, reactivity, mechanisms of ion discharge, experimental apparatus improvements, and gas volume calculations.
Practise this questionQuestion
Question text
07 This question is about electrolysis.
Aluminium is manufactured by electrolysing a molten mixture of aluminium oxide
(Al2O3) and cryolite (Na3AlF6).
07.1 Complete the half equation for the reaction occurring at the negative electrode.
[1 mark]
Al3+ + e− → Al
07.2 Cryolite contains Na+ ions as well as Al3+ ions.
Suggest one reason why sodium is not a product of the electrolysis.
[1 mark]
A student investigated the electrolysis of an aqueous solution of a different compound.
Figure 7 shows the apparatus.
Figure 7
*21* Hydrogen was produced at the negative electrode and oxygen was produced at the
positive electrode.
07.3 Explain how oxygen was produced from water during the electrolysis of this
aqueous solution.
[4 marks]
07.4 The student compared the volumes of the two gases collected.
How can the student change the apparatus in Figure 7 to compare the volumes of the
*22* two gases produced more accurately?
Give one reason for your answer.
[2 marks]
Change
Reason
07.5 The overall equation for the reaction is:
2H2O(l) → 2 H2(g) + O2(g)
What is the volume of oxygen produced when 20 cm3 of hydrogen has been
produced?
[1 mark]
Tick ( ) one box.
10 cm3 20 cm3 30 cm3 40 cm3
Mark scheme
Show the mark scheme
AO /
Question Answers Extra information Mark
Spec. Ref.
Al3+ + 3 e− → Al allow multiples 1 AO2
07.1
4.1.1.1
4.4.3.3
4.4.3.5
AO /
Spec. Ref.
07.2 sodium is more reactive than 1 AO3
aluminium 4.4.3.4
AO /
Spec. Ref.
allow hydroxide ions for OH− AO1
07.3
throughout 4.4.3.4
water (molecules) break down 1
(to) produce (H+ and) OH− (ions)
(so) OH− (ions) are attracted /
move to the positive electrode
(where) OH− (ions) are allow (where) OH− (ions) lose
discharged / oxidised to give electrons to give oxygen
oxygen (molecules) (molecules)
AO /
Spec. Ref.
07.4 allow (inverted) burettes for AO3
measuring cylinders 4.4.3.4
allow gas syringes for
measuring cylinders
(change)
use measuring cylinders 1
(instead of test tubes)
(reason)
because there is a scale (on the allow measuring cylinder(s)–HEMISTRY – 1 –
measuring cylinders) measure volume
AO /
Spec. Ref.
10 cm3 1 AO2
07.5
4.3.5
– HEMISTRY – –
Total Question 7 9
Question 8
How to answer it
Electrolysis: Molten Ores & Aqueous Solutions
- Writing and balancing half equations for the extraction of aluminium.
- Using the reactivity series to predict electrolysis products.
- Explaining the discharge of ions from water in aqueous solutions.
- Improving experimental accuracy when measuring gas volumes.
- Using molar ratios from equations to calculate gas volumes.
Part (07.1) Half Equations
Completing the reduction of aluminium
Correct Answer
Al³⁺ + 3 e⁻ → Al
Key Knowledge
At the negative electrode (cathode), positive ions gain electrons. This is called reduction. The number of electrons must match the positive charge of the ion to make it neutral.
Part (07.2) Predicting Products
Why sodium isn't produced
Correct Answer
- Sodium is more reactive than aluminium.
Exam Technique
When two positive ions are present, the less reactive metal is usually the one that gets discharged. In this molten mixture, aluminium is easier to reduce than sodium.
Part (07.3) Aqueous Electrolysis
Explaining oxygen production from water
The 4-Step Logic
To get full marks on "explain" questions in electrolysis, follow the path of the molecules and ions:
The 4 Marks
- Water molecules break down (dissociate).
- This produces H⁺ and OH⁻ ions.
- OH⁻ ions (hydroxide) are attracted to the positive electrode (anode).
- OH⁻ ions lose electrons (are oxidised) to form oxygen.
Common Errors
- Forgetting that water splits into ions; students often only talk about the dissolved salt.
- Confusing which ion goes to which electrode. Remember: PANIC (Positive Anode, Negative Is Cathode).
Part (07.4) Improving the Experiment
Measuring gas volumes accurately
The Solution
Change: Use measuring cylinders (or burettes/gas syringes) instead of test tubes.
Reason: These have a scale (graduations) to measure the volume.
Examiner Insight
Top-level answers specifically mentioned "graduated" equipment. A test tube can show gas is there, but it cannot tell you how much gas is there without a scale.
Part (07.5) Gas Volume Calculation
Using the balanced equation
Step-by-Step Calculation
The equation is: 2 H₂O(l) → 2 H₂(g) + O₂(g)
- Identify the ratio: Look at the big numbers in front of the gases. The ratio of Hydrogen (H₂) to Oxygen (O₂) is 2 : 1.
- Apply the ratio: This means you always get half as much oxygen as hydrogen.
- Calculate: 20 cm³ (Hydrogen) ÷ 2 = 10 cm³ (Oxygen).
Correct Box to Tick
10 cm³ [✓]
Calculation Traps
Don't be tempted to pick 40 cm³. Students often multiply by the ratio instead of dividing. Always check: which gas has the "2" in the equation? That gas will have the larger volume.
Visual Summary: What to Draw
Diagram Description
If asked to label the movement of ions in 07.3, your diagram should show:
- OH⁻ ions with an arrow pointing towards the (+) electrode.
- H⁺ ions with an arrow pointing towards the (-) electrode.
- Bubbles forming at both electrodes to represent the gases.
Topics
Chemistry
Question and mark scheme from the AQA GCSE Chemistry examination, Chemistry Paper 1 (Higher), 2023. QuestionVault is an independent revision resource; questions remain the copyright of the awarding body.