OxfordAQA International GCSE Chemistry: Electrolysis (9202) – Practice Questions

These are original questions written for Marlbridge, for revision and practice on this content. They are not reproduced past-paper questions, and they do not replicate the exam’s exact structure, question count or mark tariffs – examination boards hold copyright in their own papers. Use these alongside the official past papers from your board or school.

Written for OxfordAQA International GCSE Chemistry (9202), this set follows the OxfordAQA International GCSE Chemistry (9202) specification, Version 6.2, for exams May/June 2018 onwards. It covers section 3.3.2 Electrolysis (statements a to j) and the required practical on copper sulfate solution. The qualification is untiered, so every question suits every candidate. Use relative atomic masses Al = 27, O = 16 and C = 12 where needed.

Links: electrolysis study guide · electrolysis revision notes · course hub · checklist · free diagnostics

Questions

1. This question is about potassium bromide, an ionic compound.

(a) State what is meant by the term electrolyte. [1] (b) Explain why solid potassium bromide does not conduct electricity but molten potassium bromide does. [2]

2. Molten zinc iodide, ZnI₂, is electrolysed using carbon electrodes.

(a) Name the product at the cathode and the product at the anode. [2] (b) Write a half equation for the reaction at each electrode. [2] (c) State the electrode at which oxidation happens. Explain your answer in terms of electrons. [2]

3. Complete and balance each half equation.

(a) Cr³⁺ + …… → Cr [1] (b) 2Br⁻ – …… → Br₂ [1] (c) ……OH⁻ → O₂ + 2H₂O + …… [1]

4. For each solution below, carbon electrodes are used. Predict what forms at each electrode.

(a) concentrated sodium iodide solution [2] (b) silver nitrate solution [2] (c) dilute lithium sulfate solution [2]

5. A student electrolyses copper sulfate solution using two carbon electrodes and a low-voltage direct-current supply. She weighs the cathode at intervals, rinsing and drying it each time. The current is kept constant.

Time / min	0	5	10	15	20	25
Mass gained by cathode / g	0.00	0.20	0.41	0.60	0.70	1.00

(a) Describe what the student would see on the cathode. [1] (b) Bubbles of gas form at the anode. Name the gas and describe a test to confirm it. [2] (c) Identify the anomalous result. Suggest one reason for it. [2] (d) Calculate the mean rate of mass gain over the 25 minutes. Give the unit. [2] (e) Explain why the blue colour of the solution fades during the experiment. [1]

6. A steel door hinge is to be copper plated.

(a) State which electrode the hinge must be. [1] (b) Name a suitable material for the other electrode and a suitable electrolyte. [2] (c) Write a half equation for the reaction at each electrode. [2] (d) Explain why the concentration of copper ions in the electrolyte stays about the same during plating. [2]

7. State whether each change is oxidation or reduction.

(a) Ag⁺ + e⁻ → Ag [1] (b) 2O²⁻ → O₂ + 4e⁻ [1] (c) Cu → Cu²⁺ + 2e⁻ [1]

8. Aluminium is manufactured by electrolysis of a molten mixture of aluminium oxide and cryolite.

(a) Give the reason why heating with carbon is not used to obtain aluminium from its oxide. [1] (b) Explain why cryolite is used. [2] (c) Write the half equations for the changes at the negative and positive electrodes. [2] (d) Explain why the positive electrodes have to be replaced regularly. Include a balanced equation. [2] (e) The overall reaction is 2Al₂O₃ → 4Al + 3O₂. Calculate the maximum mass of aluminium that can be made from 20.4 tonnes of aluminium oxide. [3] (f) Suggest why aluminium is more expensive to extract than iron. [1]

9. Sodium chloride solution is electrolysed using inert electrodes.

(a) Name the four ions present in the solution. [2] (b) Explain, using ideas about reactivity, why hydrogen forms at the cathode rather than sodium. [2] (c) Explain why the solution left at the end is sodium hydroxide solution. [1] (d) Give one use of sodium hydroxide and one use of chlorine in the chemical industry. [2]

10. A solution contains copper(II) ions, magnesium ions, nitrate ions and a low concentration of chloride ions. It is electrolysed with carbon electrodes.

(a) Predict the product at the cathode. Explain your answer. [2] (b) Predict the product at the anode. Explain your answer. [2] (c) State how the anode product would change if the chloride ion concentration were made much higher. [1]

11. In aluminium manufacture, assume that all the oxygen formed reacts with the carbon positive electrodes.

(a) Write the equation for the reaction of carbon with oxygen. [1] (b) Using 2Al₂O₃ → 4Al + 3O₂, calculate the mass of carbon dioxide released for every 1.00 tonne of aluminium made. Give your answer to 3 significant figures. [3]

Answers

1. (a) A molten or dissolved ionic substance that is broken down by electrolysis [1]. (b) Solid: the ions are locked in the lattice and cannot travel [1]. Molten: the lattice has broken down, so ions can drift to the electrodes, carrying charge [1]. Examiner insight: Answers that say electrons move through the molten compound usually lose the mark; the explanation must be about ions.

2. (a) Cathode: zinc [1]. Anode: iodine [1]. (b) Zn²⁺ + 2e⁻ → Zn [1]; 2I⁻ → I₂ + 2e⁻ [1]. (c) The anode (positive electrode) [1], because iodide ions lose electrons there [1]. Examiner insight: “Oxidation at the anode” on its own is a statement, not an explanation; you need the words “lose electrons” to earn the second mark.

3. (a) Cr³⁺ + 3e⁻ → Cr [1] (b) 2Br⁻ – 2e⁻ → Br₂ [1] (c) 4OH⁻ → O₂ + 2H₂O + 4e⁻ [1] Examiner insight: Check the charge on each side before moving on; in part (c) both the 4 in front of OH⁻ and the 4e⁻ are needed for the equation to balance.

4. (a) Cathode: hydrogen [1]. Anode: iodine [1]. (b) Cathode: silver [1]. Anode: oxygen [1]. (c) Cathode: hydrogen [1]. Anode: oxygen [1]. Examiner insight: Naming “sodium” or “lithium” at the cathode for a solution is a common error; the metal only forms if it is less reactive than hydrogen.

5. (a) A pink-brown (copper-coloured) coating forms [1]. (b) Oxygen [1]. It relights a glowing splint [1]. (c) The 20 minute reading (0.70 g) [1]. Suggestion: a one-off weighing error, such as misreading the balance or recording the wrong value; the 25 minute reading is back on the trend, so no copper was lost for good [1]. (d) 1.00 ÷ 25 [1] = 0.040 g/min [1]. (e) Copper ions (which make the solution blue) are removed from the solution as copper is deposited on the cathode [1]. Examiner insight: For an anomalous result, quote the specific reading and compare it with the trend (about 0.04 g each minute predicts 0.80 g at 20 minutes); “the one that doesn’t fit” is too vague.

6. (a) The cathode (negative electrode) [1]. (b) Anode: copper [1]. Electrolyte: copper sulfate solution, or any solution containing copper ions [1]. (c) Anode: Cu → Cu²⁺ + 2e⁻ [1]. Cathode: Cu²⁺ + 2e⁻ → Cu [1]. (d) Copper ions are removed from solution at the cathode [1], and the same number are added at the copper anode as it dissolves [1]. Examiner insight: An electrolyte that contains no copper ions, such as sodium chloride solution, cannot plate copper, so read the electrolyte you name before you move on.

7. (a) Reduction [1] (b) Oxidation [1] (c) Oxidation [1] Examiner insight: Look at which side the electrons are on: electrons on the left are being gained (reduction), electrons on the right are being lost (oxidation).

8. (a) Aluminium is more reactive than carbon [1]. (b) On its own, aluminium oxide would need a temperature above 2000 °C to melt [1]. Cryolite lowers the temperature needed, so less energy is used and costs are lower [1]. (c) Negative: Al³⁺ + 3e⁻ → Al [1]. Positive: 2O²⁻ → O₂ + 4e⁻ [1]. (d) The carbon electrodes react with the oxygen formed and burn away [1]: C + O₂ → CO₂ [1]. (e) Mᵣ Al₂O₃ = 102, so amount = 20.4 ÷ 102 = 0.200 million mol (0.2 × 10⁶ mol) [1]. Moles of Al = 2 × 0.200 = 0.400 million mol [1]. Mass = 0.400 × 27 = 10.8 tonnes [1]. (f) Large amounts of energy (electricity) are needed for melting and electrolysis [1]. Examiner insight: Saying cryolite is a catalyst, or that it “makes the reaction faster”, does not answer part (b); the answer must link a lower melting temperature to saving energy.

9. (a) Na⁺, Cl⁻, H⁺ and OH⁻ [2] (all four needed for both marks; two or three correct earns one). (b) Hydrogen is less reactive than sodium [1], so H⁺ ions accept electrons more easily than Na⁺ ions and hydrogen gas is released [1]. (c) Hydrogen ions and chloride ions are removed, leaving sodium ions and hydroxide ions in solution [1]. (d) Sodium hydroxide: making soap [1]. Chlorine: making bleach or plastics [1]. Examiner insight: In part (b), a bare “hydrogen is less reactive” without comparing it with sodium is incomplete; name both elements in the comparison.

10. (a) Copper [1]. Copper is less reactive than hydrogen (and magnesium is more reactive), so copper ions are discharged [1]. (b) Oxygen [1]. The chloride ions are at low concentration and nitrate is not discharged, so hydroxide ions lose electrons [1]. (c) Chlorine would form instead [1]. Examiner insight: At the anode this specification asks you to use the relative concentrations of the ions, so refer to the low chloride concentration rather than only listing ion types.

11. (a) C + O₂ → CO₂ [1] (b) 3 mol O₂ gives 3 mol CO₂, so 4 mol Al goes with 3 mol CO₂; mass ratio 4 × 27 = 108 : 3 × 44 = 132 [1]. Mass of CO₂ = 1.00 × 132 ÷ 108 [1] = 1.22 tonnes [1]. Examiner insight: Show the mole ratio taken from the equations; a correct final figure is far easier to credit when the 4 : 3 link between aluminium and carbon dioxide is written down.

Where marks are usually lost

• Using single atoms in half equations, such as I or O, instead of I₂ or O₂.
• Electrons on the wrong side of a half equation, which turns reduction into oxidation.
• Naming a reactive metal (sodium, lithium, potassium, magnesium) as a cathode product from a solution.
• Ignoring concentration at the anode, so a dilute chloride solution is wrongly said to give chlorine.
• In the practical, not giving the result of the gas test (“relights”) or describing the copper only as “a solid”.
• In electroplating, making the object the anode, or naming an electrolyte without ions of the plating metal.
• For aluminium, calling cryolite a catalyst, or saying the anodes “dissolve” rather than react with oxygen.
• Leaving out the unit on a rate, or losing track of tonnes and moles in reacting-mass work.

Next steps

• Recap fast with the electrolysis revision notes.
• Re-learn anything you missed in the electrolysis study guide.
• Practise more reacting-mass questions with the quantitative chemistry practice set.
• See the whole course on the OxfordAQA IGCSE Chemistry hub and tick off topics on the printable checklist.
• Try one of the free 10-minute diagnostics.
• Book a free trial class.

Official syllabus

OxfordAQA International GCSE Chemistry (9202) specification, Version 6.2, for exams May/June 2018 onwards, published by OxfordAQA. This practice set covers section 3.3.2 Electrolysis and its required practical.