AQA GCSE Chemistry Chemistry Paper 1 (Foundation), June 2022: Question 8
9 marks · Low Demand difficulty · Short Answer
Answer questions on the electrolysis of aqueous sodium sulfate and molten compounds, including gas volume measurement and atom economy calculation.
Practise this questionQuestion
Question text
08 This question is about electrolysis.
Ionic compounds decompose when they are electrolysed.
A student electrolyses sodium sulfate solution.
Figure 13 shows the apparatus used.
Figure 13
08.1 Sodium sulfate solution contains:
• hydrogen ions
• hydroxide ions
• sodium ions
• sulfate ions.
Oxygen is produced at the positive electrode.
Which ions are discharged at the positive electrode to produce oxygen?
[1 mark]
Tick ( ) one box.
Hydrogen ions
Hydroxide ions
Sodium ions
Sulfate ions 29
08.2 Figure 14 shows one of the measuring cylinders during the electrolysis.
*28* Figure 14
What is the volume of gas in the measuring cylinder?
[1 mark]
Volume of gas = cm3
08.3 Ionic compounds can be electrolysed when molten or dissolved in water.
Why can ionic compounds not be electrolysed when solid?
You should answer in terms of ions.
[1 mark]
08.4 Table 7 shows the products of electrolysis of two molten compounds.
Table 7
Molten
Product at negative electrode Product at positive electrode
compound
Potassium iodide Potassium
*29* …
Zinc bromide Bromine
Complete Table 7.
[2 marks]
08.5 The electrolysis of molten sodium chloride is used to extract sodium metal.
Why is sodium metal extracted by electrolysis instead of by reduction with carbon?
[1 mark]
Tick ( ) one box.
Carbon conducts electricity.
Carbon is less reactive than sodium.
Carbon reduction uses more energy.
08.6 What is the state symbol for molten sodium chloride?
[1 mark]
Tick ( ) one box.
(aq) (g) 31 (l) (s)
08.7 Titanium can be produced from titanium oxide by electrolysis.
The equation for the reaction is:
TiO2 → Ti + O2
Calculate the percentage atom economy for the production of titanium from
titanium oxide by electrolysis.
Use the equation:
Relative atomic mass of desired product
Percentage atom economy = × 100
Relative formula mass of reactant
Relative atomic mass (Ar): Ti = 48
Relative formula mass (Mr): TiO2 = 80
[2 marks]
Percentage atom economy = %
Mark scheme
Show the mark scheme
Question 8
AO /
Question Answers Extra information Mark
Spec. Ref.
08.1 hydroxide ions 1 AO1
4.4.3.4
AO /
Spec. Ref.
27 (cm3) 1 AO2
08.2
4.4.3.4
AO /
Spec. Ref.
08.3 ions cannot move (freely in a allow ions are fixed in place (in a 1 AO1
solid) solid) 4.2.2.1
4.2.2.3
4.4.3.1
AO /
Spec. Ref.
08.4 AO2
Molten Product at Product at 4.4.3.2
compound negative electrode positive electrode
Potassium iodide Potassium Iodine 1
Zinc bromide Zinc Bromine 1
AO /
Spec. Ref.
08.5 carbon is less reactive than 1 AO1
sodium 4.4.1.2
4.4.1.3
4.4.3.3
AO /
Spec. Ref.
08.6 (l) 1 AO1
4.2.2.2
AO /
Spec. Ref.
(percentage atom economy =)
08.7 48 1 AO2
× 100 4.3.3.2
= 60 (%) 1
Total Question 8 9
How to answer it
Electrolysis, Molten Compounds & Atom Economy
This question assesses fundamental understanding of Electrolysis and quantitative chemistry:
- Predicting products discharged at electrodes during aqueous electrolysis.
- Reading inverted laboratory apparatus (measuring cylinders collecting gas).
- Explaining conductivity in terms of mobile ions across different states of matter.
- Predicting products from the electrolysis of simple molten ionic salts.
- Comparing extraction methods (electrolysis vs. reduction with carbon) using reactivity.
- Applying standard state symbols and calculating percentage atom economy.
Discharge of Ions at the Positive Electrode (Anode)
Identifying which negative ion forms oxygen gas in aqueous solution
✅ Correct Answer
Tick: Hydroxide ions
💡 Key Knowledge: Anode Rules
In aqueous electrolysis, negative ions (anions) are attracted to the positive electrode:
- If a halide ion (Cl⁻, Br⁻, I⁻) is present, the halogen is produced.
- If no halide is present (e.g. sulfate SO₄²⁻, nitrate NO₃⁻), hydroxide ions (OH⁻) from water are discharged to produce oxygen gas:
4OH⁻ → O₂ + 2H₂O + 4e⁻
🧠 Exam Technique
Always eliminate positive ions immediately. Hydrogen (H⁺) and sodium (Na⁺) are cations, so they travel to the negative electrode (cathode). Between OH⁻ and SO₄²⁻, sulfate is polyatomic and not discharged here.
❌ Common Error
Selecting sulfate ions. Students often assume that because sodium sulfate was added, sulfate must react. Remember that water molecules also ionise into H⁺ and OH⁻.
Reading Inverted Apparatus
Determining the collected volume of gas from an inverted measuring cylinder
✅ Correct Answer
Volume of gas = 27 cm³
🧠 Exam Technique: Reading Upside-Down Scales
Because the measuring cylinder is inverted over an electrode, the gas collects at the closed top (marked 0):
- Find the major numbered marks: the liquid level is between 20 and 30.
- Determine scale subdivisions: there are 10 tick marks between 20 and 30, so each subdivision = 1 cm³.
- Count downwards from 20 towards 30: five ticks = 25 cm³, plus two more = 27 cm³.
❌ Common Error: Reading from the Wrong Direction
A frequent mistake is reading upwards from the 30 line or reading it as if the cylinder were upright (e.g. giving 33 cm³ or 23 cm³). Always trace the numbers sequentially from 0 downwards.
Conductivity in Solid vs. Molten/Aqueous States
Explaining why solid ionic compounds cannot be electrolysed
✅ Correct Answer
Ions cannot move (they are held in fixed positions in the giant ionic lattice).
💡 Key Knowledge
- Electrolysis requires ions to carry charge and migrate to electrodes.
- In a solid: strong electrostatic forces hold ions in fixed positions—they can only vibrate.
- When molten or in solution: the lattice breaks down and ions are free to move.
❌ Examiner Trap: "Electrons Cannot Move"
The question specifies: "You should answer in terms of ions." Mentioning "delocalised electrons cannot move" earns 0 marks. Ionic compounds do not conduct electricity via free electrons!
Electrolysis of Molten Binary Compounds
Completing Table 7 for products at the cathode and anode
✅ Completed Table
| Molten compound | Product at negative electrode | Product at positive electrode |
|---|---|---|
| Potassium iodide | Potassium | Iodine |
| Zinc bromide | Zinc | Bromine |
💡 Key Rule: Molten Binary Salts
Molten salts contain only two elements (no water):
- Negative electrode (Cathode): attracts positive metal ions → forms the metal.
- Positive electrode (Anode): attracts negative non-metal ions → forms the non-metal.
Metal Extraction: Reactivity and Carbon Reduction
Why sodium is extracted by electrolysis instead of carbon reduction
✅ Correct Answer
Tick: Carbon is less reactive than sodium.
💡 Key Knowledge: Reactivity Series
Extraction method depends on position relative to carbon:
- More reactive than carbon (e.g. K, Na, Ca, Mg, Al): must be extracted using electrolysis.
- Less reactive than carbon (e.g. Zn, Fe, Cu): can be extracted cheaply by reduction with carbon.
Carbon cannot displace sodium because it cannot take oxygen away from a metal that holds it more strongly.
State Symbols
Identifying the state symbol for molten sodium chloride
✅ Correct Answer
Tick: (l)
💡 Understanding State Symbols
- (s) = solid
- (l) = liquid (melted/molten pure substance)
- (g) = gas
- (aq) = aqueous (dissolved in water)
❌ Common Misconception: Confusing (l) and (aq)
Many students tick (aq) whenever a substance is in fluid form. (aq) means dissolved in water! Molten salt is pure liquid substance that has melted at high temperature, so its state is (l).
Percentage Atom Economy Calculation
Calculating atom economy using given relative masses
📐 Step-by-Step Calculation
Given reaction: TiO₂ → Ti + O₂
Formula provided:
Percentage atom economy = (Relative atomic mass of desired product / Relative formula mass of reactant) × 100
- Identify the desired product mass: Aᵣ(Ti) = 48
- Identify the reactant mass: Mᵣ(TiO₂) = 80
- Substitute into formula:
(48 / 80) × 100
- Calculate final value:
= 60 %
🧠 Exam Technique
All numbers needed are explicitly supplied in the question (Aᵣ = 48, Mᵣ = 80). Show your substitution clearly to secure the method mark even if you make an arithmetic error on your calculator.
❌ Common Error
Inverting the fraction (e.g. 80 / 48 = 166.7%). Atom economy for a single reactant breaking down cannot be greater than 100%!
- 1 mark for substitution: (48 / 80) × 100
- 1 mark for answer: 60 (%)
Topics
Chemistry · Required Practicals · Required Practicals · C2: Bonding, Structure and the Properties of Matter · C3: Quantitative Chemistry · C4: Chemical Changes
Question and mark scheme from the AQA GCSE Chemistry examination, Chemistry Paper 1 (Foundation), June 2022. QuestionVault is an independent revision resource; questions remain the copyright of the awarding body.