AQA GCSE Combined Science: Trilogy Chemistry Paper 1 (Higher), 2023: Question 5

Core knowledge

• Why ionic solids don’t conduct (ions fixed in a lattice).
• When ionic compounds do conduct (molten / aqueous).
• Choosing inert electrodes (e.g. graphite).
• Writing a correct anode half-equation for chlorine.
• Explaining hydrogen at the cathode and pH increases in brine electrolysis.

Skills

• Use key electrolysis vocabulary: ion, discharge, oxidation, reduction.
• Balance atoms and charges in half-equations.
• Link cause → effect (which ions form, which are discharged, what changes in solution).

✅ Correct answer (1/1)

Ions are not free to move (they are fixed in the lattice).

💡 Key knowledge

• Electric current in ionic substances is carried by moving ions.
• In a solid ionic lattice, ions are in fixed positions → no charge flow.

❌ Common errors

• Saying “it has no electrons” (ionic compounds don’t use free electrons to conduct).
• Just writing “it is solid” with no explanation about ion movement.

🧠 Exam technique

For 1-mark “why” questions, aim for a single sharp sentence using the marking-point phrase: ions are not free to move .

✅ Correct answer (1/1)

…when it is molten (or liquid).

Mark scheme allows “liquid or molten”.

💡 Key knowledge

• Molten ionic compounds conduct because ions can move freely.
• Aqueous solutions also conduct (already stated in the question), but this blank is testing the other case: molten.

❌ Common errors (examiner insight)

• Writing “when dissolved” or “when in solution”. The question already gave that—this mark is for molten.
• Writing “when heated” (not precise; must state molten/liquid).

🧠 Exam technique

Watch for the word also: it signals “a different condition than the one already mentioned”.

✅ Correct answers (1/1)

Carbon (graphite)

Also allowed by mark scheme: platinum / gold / silver.

💡 Key knowledge

• Inert means the electrode does not react with the electrolyte/products.
• Graphite is commonly used because it conducts and is relatively unreactive (and cheaper than platinum).

❌ Common errors

• Choosing reactive metals (e.g. iron) that could react and change products.
• Writing “graphene” or “charcoal” loosely—GCSE expected answer is graphite (a form of carbon).

🧠 Exam technique

For 1 mark, give one clear element name. Best: carbon (graphite) .

✅ Correct answer (2/2)

2 Cl⁻ → Cl₂ + 2 e⁻

State symbols are ignored.

📐 How to build it (step-by-step)

1. Chlorine is diatomic: product must be Cl₂.
2. Balance chlorine atoms: need 2 Cl⁻ on the left.
3. Balance charge: left has −2, so add 2 e⁻ on the right to make charges match.

💡 Key knowledge

• At the anode (+), negative ions lose electrons: this is oxidation.
• Mnemonic: OIL RIG (Oxidation Is Loss, Reduction Is Gain).

❌ Common errors (and how marks are lost)

• Not balancing electrons (e.g. writing 2 Cl⁻ → Cl₂ + e⁻ ).
• Forgetting chlorine is diatomic (writing Cl⁻ → Cl + e⁻ ).
• Putting electrons on the wrong side (would change oxidation to reduction).

Mark scheme note: 1 mark can be earned for something like Cl₂ + e⁻ , but to get full marks you must have the fully balanced half-equation.

✅ Full-mark explanation (4/4)

Water molecules break down (effectively producing equal numbers of) H⁺ and OH⁻ ions. The H⁺ ions are attracted to the negative electrode (cathode) and are discharged/reduced to form hydrogen gas. Hydrogen is produced rather than sodium because hydrogen is less reactive than sodium. Removing H⁺ means the solution has a lower relative amount of H⁺ (or a higher relative amount of OH⁻), so the pH increases.

💡 Key knowledge you’re expected to use

• In aqueous NaCl you have ions: Na⁺, Cl⁻, plus from water: H⁺ and OH⁻.
• Cathode (−): positive ions go here and are reduced.
• Competition of ions: for the cathode, H⁺ is discharged instead of Na⁺ because sodium is very reactive.
• If H⁺ decreases, pH goes up (solution becomes more alkaline due to relatively more OH⁻).

🧠 Exam technique (how to secure 4 marks)

• Write a linked chain: water provides H⁺/OH⁻ → H⁺ goes to cathode → H₂ formed → H⁺ decreases / OH⁻ relatively increases → pH rises.
• Use the mark-scheme words: break down, attracted, discharged/reduced, less reactive, increase in OH⁻ / decrease in H⁺.
• Keep it about ions (not “acid/base chemicals” being added).

What distinguished top answers: They explicitly mentioned both (1) why H₂ forms (H⁺ discharged; hydrogen vs sodium reactivity) and (2) why pH increases (H⁺ decreases / OH⁻ increases).

❌ Common errors (examiner insight)

• Saying “Na⁺ turns into hydrogen” (wrong ion).
• Missing the pH link: students describe hydrogen formation but don’t explain why pH increases.
• Saying “OH⁻ is produced at the cathode” without linking to removal of H⁺ (the mark scheme rewards the relative increase in OH⁻ or decrease in H⁺).
• Vague statements like “it becomes alkaline” with no ion explanation.

💡 Must-know facts

• Solid ionic compounds don’t conduct: ions not free to move.
• They do conduct when molten or in aqueous solution.
• Inert electrodes: graphite (carbon) is the standard answer.
• Anode chlorine half-equation: 2 Cl⁻ → Cl₂ + 2 e⁻ .
• In brine, H₂ forms at the cathode; pH rises because H⁺ decreases / OH⁻ increases.

🧠 9-mark strategy

• Use the electrode names and charges: anode (+), cathode (−).
• For half-equations: balance atoms first, then charge.
• For “explain” (4 marks): aim for 4 linked points, not 4 separate facts.