AQA GCSE Chemistry Chemistry Paper 1 (Higher), 2022: Question 6
9 marks · Standard Demand difficulty · Short Answer
Analyze the electrolysis of molten and aqueous sodium chloride, including writing half equations, identifying ions, and explaining the formation of sodium hydroxide.
Practise this questionQuestion
Question text
06 This question is about electrolysis.
Molten sodium chloride is electrolysed in an industrial process to produce sodium.
Figure 8 shows a simplified version of the electrolysis cell used.
Figure 8
06.1 Which is the correct half equation for the production of sodium?
[1 mark]
Tick ( ) one box.
Na + e− → Na+
Na → Na+ + e−
Na+ + e− → Na
Na+ → Na + e− 19
A mesh is used to keep the products of the electrolysis apart.
06.2 Suggest one reason why the products of the electrolysis must be kept apart.
[1 mark]
06.3 Which type of particle passes through the mesh in the electrolysis of
molten sodium chloride?
*18* [1 mark]
Tick ( ) one box.
Atom
Electron
Ion
Molecule 20
Aqueous sodium chloride solution is electrolysed in a different industrial process.
Two gases and an alkaline solution are produced.
06.4 Which two ions are present in aqueous sodium chloride solution in addition to
sodium ions and chloride ions?
[2 marks]
06.5 Name the alkaline solution produced.
[1 mark]
06.6 Explain how the alkaline solution is produced.
You should refer to the processes at the electrodes.
[3 marks]
Mark scheme
Show the mark scheme
Question 6
AO /
Question Answers Extra information Mark
Spec. Ref.
Na+ + e− → Na 1 AO2
06.1
4.4.3.2
4.4.3.5
AO /
Spec. Ref.
06.2 so the products do not react (to 1 AO3
reform sodium chloride) 4.4.3.2
AO /
Spec. Ref.
06.3 ion 1 AO3
4.4.3.2
AO /
Spec. Ref.
hydrogen / H+ (ions) 1 AO1
06.4
4.4.3.4
hydroxide / OH− (ions) 1
AO /
Spec. Ref.
06.5 sodium hydroxide allow NaOH 1 AO2
4.4.3.4
AO /
Spec. Ref.
06.6 sodium ions and hydroxide ions 1 AO2
are left (in solution) 4.4.3.4
(because) hydrogen ions are allow (because) hydrogen ions 1
discharged / reduced (at the gain electrons (at the negative
negative electrode to form electrode to form hydrogen)
hydrogen)
allow (because at the negative
electrode)
2 H+ + 2 e− → H
(and because) chloride ions are allow (and because) chloride 1
discharged / oxidised (at the ions lose electrons (at the
positive electrode to form positive electrode to form
chlorine) chlorine)
allow (and because at the
positive electrode)
2 Cl− → Cl + 2 e−
Total Question 6 9
How to answer it
Electrolysis of Sodium Chloride (Molten vs. Aqueous)
What this question tests
This exam question assesses your understanding of electrolysis in two different states: molten (melted) sodium chloride and aqueous (dissolved in water) sodium chloride. You need to be able to:
- Identify and write ionic half-equations for reactions at the electrodes.
- Understand the practical setup of industrial electrolysis (Down's Cell mesh).
- Identify the mobile charge carriers in molten electrolytes.
- Predict the products of aqueous electrolysis using selective discharge rules.
- Explain step-by-step how an alkaline solution (sodium hydroxide) is formed during aqueous electrolysis.
Half Equation for Sodium Production
Selecting the correct reduction half-equation
✅ Correct Answer
Na⁺ + e⁻ → Na
💡 Key Knowledge
- Sodium ions ( Na⁺ ) are positive cations.
- They are attracted to the negative electrode (cathode).
- At the cathode, they gain electrons (reduction) to form neutral sodium atoms ( Na ).
🧠 Exam Technique
Remember the mnemonic OIL RIG:
- Oxidation Is Loss of electrons
- Reduction Is Gain of electrons
Since sodium ions are positive, they must gain a negative electron to become neutral. Gaining an electron is written on the left-hand side of the arrow: Na⁺ + e⁻ .
❌ Common Errors
Avoid choosing Na + e⁻ → Na⁺ . This equation suggests a neutral sodium atom gains an electron to become positive, which is chemically impossible because electrons have a negative charge!
Keeping the Products Apart
Why the mesh barrier is essential in industrial electrolysis
✅ Correct Answer
- So the products (sodium and chlorine) do not react with each other.
- Also accept: To prevent them from reforming sodium chloride.
💡 Key Knowledge
Sodium is a highly reactive alkali metal, and chlorine is a highly reactive halogen gas. If they come into contact at high temperatures, they will react violently and explosively to reform sodium chloride ( NaCl ), reversing the entire electrolysis process.
Charge Carriers in Molten Electrolytes
Identifying what carries the current through the liquid
✅ Correct Answer
Ion
💡 Key Knowledge
In a molten ionic compound, the giant ionic lattice has broken down. The ions ( Na⁺ and Cl⁻ ) are free to move and carry the electrical charge through the liquid.
❌ Common Errors
Do not choose "Electron"! Electrons only flow through the metallic wires and electrodes. They cannot travel freely through the liquid electrolyte. In the liquid, the current is carried entirely by moving ions.
🧠 Quick Recall Rule
Remember this distinction for exams:
- In metals and graphite: free/delocalised electrons conduct electricity.
- In molten/aqueous ionic compounds: free-moving ions conduct electricity.
Ions in Aqueous Solutions
Identifying the extra ions introduced by water
✅ Correct Answer
Any order:
- Hydrogen ions / H⁺
- Hydroxide ions / OH⁻
💡 Key Knowledge
The word aqueous means dissolved in water. Water molecules ( H₂O ) naturally dissociate (split up) into a small concentration of two ions:
H₂O ⇌ H⁺ + OH⁻
Therefore, aqueous sodium chloride contains four types of ions: Na⁺ , Cl⁻ , H⁺ , and OH⁻ .
🧠 Exam Technique
Read the question carefully! It asks for the two ions present in addition to sodium ions and chloride ions. If you write "sodium" or "chloride" here, you will lose marks.
Naming the Alkaline Product
Identifying the chemical left in solution
✅ Correct Answer
Sodium hydroxide (or NaOH )
💡 Key Knowledge
An alkali is a soluble base that releases hydroxide ions ( OH⁻ ) in solution. When Na⁺ and OH⁻ ions are left behind in the water, they form sodium hydroxide, which has a high pH (around 13-14).
Explaining the Aqueous Process
A step-by-step explanation of electrode reactions
📐 Step-by-Step Mechanism (How to build a 3-mark answer)
To explain why the solution becomes alkaline, you must describe what happens to the other ions at the electrodes, leaving Na⁺ and OH⁻ behind:
- At the Negative Electrode (Cathode): Hydrogen ions ( H⁺ ) are discharged (reduced/gain electrons) to form hydrogen gas ( H₂ ).
2 H⁺ + 2 e⁻ → H₂ - At the Positive Electrode (Anode): Chloride ions ( Cl⁻ ) are discharged (oxidised/lose electrons) to form chlorine gas ( Cl₂ ).
2 Cl⁻ → Cl₂ + 2 e⁻ - In the Solution: Because H⁺ and Cl⁻ have been removed as gases, only sodium ions (Na⁺) and hydroxide ions (OH⁻) remain in the solution, forming alkaline sodium hydroxide.
✅ Mark Scheme Requirements
- Mark 1: State that sodium ions and hydroxide ions are left in solution.
- Mark 2: Explain that hydrogen ions are discharged/reduced at the negative electrode to form hydrogen gas (or write the half-equation).
- Mark 3: Explain that chloride ions are discharged/oxidised at the positive electrode to form chlorine gas (or write the half-equation).
❌ Common Errors & Pitfalls
- Missing one electrode: Many students only explain what happens at one electrode and lose the third mark. You must mention both the cathode (-) and anode (+) processes.
- Confusing discharge rules: Students often incorrectly state that sodium is produced at the negative electrode. Remember: Hydrogen is discharged instead of sodium because sodium is more reactive than hydrogen.
Topics
Chemistry · C4: Chemical Changes
Question and mark scheme from the AQA GCSE Chemistry examination, Chemistry Paper 1 (Higher), 2022. QuestionVault is an independent revision resource; questions remain the copyright of the awarding body.