AQA GCSE Combined Science: Trilogy Chemistry Paper 1 (Higher), 2021: Question 5

8 marks · Standard Demand difficulty · Short Answer

Explain and complete half-equations for the electrolysis of aqueous sodium chloride and sodium sulfate, identifying reduction at the negative electrode and oxidation producing chlorine and oxygen at the positive electrode.

Practise this question

Question

The question page is headed with question number 05 and states that it is about the electrolysis of aqueous solutions. Part 05.1 says hydrogen gas and chlorine gas are produced when sodium chloride solution is electrolysed, gives the half-equation at the negative electrode as 2H+ + 2e− → H2, and asks for the type of reaction at the negative electrode and the reason, worth 2 marks, with answer lines provided. Part 05.2 says chloride ions are attracted to the positive electrode and asks students to complete the half-equation for producing chlorine gas, shown as blank, Cl−, arrow, blank plus blank, worth 2 marks. Part 05.3 states that hydrogen gas and oxygen gas are produced when sodium sulfate solution is electrolysed and asks students to explain how oxygen gas is produced in this electrolysis, worth 4 marks, with several writing lines and no diagrams or graphs.
Question text

05 This question is about the electrolysis of aqueous solutions.

Hydrogen gas and chlorine gas are produced when sodium chloride solution

is electrolysed.

Hydrogen ions (H+) are attracted to the negative electrode.

05.1

The half equation for the reaction at the negative electrode is:

2 H+ + 2 e– → H

What type of reaction happens at the negative electrode?

Give the reason for your answer.

[2 marks]

Type of reaction

Reason

05.2 Chloride ions are attracted to the positive electrode.

Complete the half equation for the production of chlorine gas (Cl2).

[2 marks]

Cl– →17 +

05.3 Hydrogen gas and oxygen gas are produced when sodium sulfate solution

is electrolysed.

Explain how oxygen gas is produced in the electrolysis of sodium sulfate solution.

[4 marks]

Mark scheme

Show the mark scheme The mark scheme is a table with columns for Question, Answers, Extra information, Mark, and AO/Spec. Ref. For 05.1 it awards one mark for 'reduction' and one mark for stating that H+ ions gain electrons; for 05.2 it gives the answer 2Cl− → Cl2 + 2e−, allows the rearranged form 2Cl− − 2e− → Cl2, and notes one mark can be given for Cl2 plus e− or for e− on the left and Cl2 on the right. For 05.3 it awards marks for stating that water molecules break down or dissociate to produce OH− ions, that these ions are attracted to the positive electrode, and that OH− ions are oxidised or lose electrons; the total shown is 8 marks, with references including 5.4.1.4, 5.4.3.1, 5.4.3.4, 5.4.3.5, and RPA9.

AO /

Question Answers Extra information Mark

Spec. Ref.

05.1 reduction ignore electrolysis 1 AO2

(as H+ ions) gain electrons 1 AO1

5.4.1.4

5.4.3.1

5.4.3.4

RPA 9

05.2 2 Cl– → Cl + 2 e– allow 2 Cl– – 2 e– → Cl 2 AO2

5.4.1.4

ignore state symbols 5.4.3.1

5.4.3.4

allow 1 mark for Cl + e– RPA9

allow 1 mark for

– e– (on lhs) and Cl (on rhs)

05.3 water molecules 1 AO1

5.4.1.4

break down to produce OH– ions allow dissociate to produce OH– 1 5.4.3.1

ions 5.4.3.4

5.4.3.5

(which are) attracted to the 1 RPA9

positive electrode

(where OH– ions are) oxidised 1

or

(where OH– ions) lose electrons ignore discharged

ignore oxygen is produced as no

halide is present

Total 8

How to answer it

Electrolysis of Aqueous Solutions: Sodium Chloride & Sodium Sulfate

What this question tests

You need to know what happens at the electrodes during electrolysis of aqueous solutions: reduction at the negative electrode, oxidation at the positive electrode, how to write correct half-equations, and how water can be involved when no halide ions are present.

Part 05.1 — Reaction at the negative electrode

Question focus: What type of reaction happens, and why?

✅ Correct answer

Type of reaction: reduction

Reason: the H+ ions gain electrons to form H₂.

Full marks come from naming the reaction and giving the electron-transfer reason.

💡 Key knowledge

  • The negative electrode is the cathode.
  • Reduction = gain of electrons.
  • Here: 2H+ + 2e− → H₂.

🧠 Exam technique

  • State the reaction type first: reduction.
  • Then explain it using the word electrons.
  • If you write “electrolysis” instead of reduction, you do not get the mark.

❌ Common errors

  • Confusing negative electrode with oxidation.
  • Saying “hydrogen gains protons” instead of H+ gains electrons.
  • Writing “discharge” without explaining electron gain.

📐 Calculation / equation check

There is no calculation here, but the half-equation shows the key idea:

2H+ + 2e− → H₂

Why this is reduction: H+ ions gain electrons.

Part 05.2 — Half-equation for chlorine gas

Question focus: Complete the equation for chlorine at the positive electrode

✅ Correct answer

2Cl− → Cl₂ + 2e−

State symbols are not needed.

💡 Key knowledge

  • The positive electrode is the anode.
  • Oxidation = loss of electrons.
  • Two chloride ions join to make one chlorine molecule, Cl₂.

🧠 Exam technique

  • Remember chlorine is diatomic, so write Cl₂, not Cl.
  • Balance both the atoms and the charge.
  • For 2Cl−, you need 2 electrons on the right.

❌ Common errors

  • Writing Cl− → Cl₂ + e− without balancing.
  • Forgetting the 2 in front of Cl−.
  • Putting electrons on the wrong side.

📐 Step-by-step balance

  1. Start with chlorine ions: Cl−.
  2. Make chlorine gas: Cl₂, so you need 2 chloride ions.
  3. Charge on left = −2, so add 2e− to the right to balance charge.

Final equation: 2Cl− → Cl₂ + 2e−

Part 05.3 — Why oxygen gas is produced in sodium sulfate solution

Question focus: Explain oxygen production in aqueous electrolysis

✅ Correct answer

Water molecules break down to produce OH− ions. These ions are attracted to the positive electrode, where they are oxidised, meaning they lose electrons. Oxygen gas is produced.

💡 Key knowledge

  • In sodium sulfate solution, there are no halide ions such as Cl−.
  • So water is involved in making oxygen.
  • At the positive electrode, OH− ions are oxidised.
  • Oxidation means loss of electrons.

🧠 Exam technique

  • For 4 marks, give the process in a chain: water → OH− → positive electrode → oxidation → oxygen.
  • Use the words attracted, oxidised, and lose electrons.
  • Top answers explain why water is used: there are no halide ions present.

❌ Common errors

  • Saying chlorine is made instead of oxygen — there is no chloride in sodium sulfate solution.
  • Writing only “water breaks down” without saying OH− ions form.
  • Confusing oxidation with reduction.
  • Not linking the ions to the positive electrode.

📐 Clear 4-mark answer structure

  1. Water molecules break down / dissociate to produce OH− ions.
  2. The OH− ions are attracted to the positive electrode.
  3. At the positive electrode, the OH− ions are oxidised.
  4. They lose electrons, and oxygen gas is produced.
Examiner point: students lost marks when they mentioned oxygen but did not explain the role of OH− ions or the electron loss.

Mark scheme summary

What gets the marks

  • 05.1: reduction + H+ gains electrons
  • 05.2: 2Cl− → Cl₂ + 2e−
  • 05.3: water forms OH−, OH− goes to positive electrode, is oxidised / loses electrons, oxygen produced

🧠 Final exam tip

When answering electrolysis questions, always think: Which ion moves? Which electrode? Gain or lose electrons? What gas forms?

Topics

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

Question and mark scheme from the AQA GCSE Combined Science: Trilogy examination, Chemistry Paper 1 (Higher), 2021. QuestionVault is an independent revision resource; questions remain the copyright of the awarding body.