OCR GCSE Chemistry A: Electrolysis (J248) – 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.

The eleven questions here cover C3.4 Electrolysis from the J248 specification for OCR GCSE (9–1) Chemistry A (Gateway Science), Version 4.0 (August 2026). Together they test statements C3.4a–C3.4e, mathematical learning outcome CM3.4i and the PAG C2 electrolysis practical. Both tiers are assessed on all of C3.4, so each question suits Foundation and Higher candidates.

If the topic is new to you, read the Electrolysis study guide first; for a quick refresh, use the Electrolysis revision notes. To see how C3.4 sits within the course, open the J248 course hub or the J248 checklist.

Questions

1. (a) State the name given to a positive ion and the electrode it moves towards during electrolysis. [2] (b) With inert electrodes, state the electrode at which non-metals are formed. [1]

2. A student writes: “Solid lithium iodide does not conduct because its electrons cannot move, but when it melts the electrons are free to carry the current.”

Explain what is wrong with this statement and give the correct explanation. [3]

3. Predict the product at each electrode when the following molten compounds are electrolysed with graphite electrodes.

(a) strontium chloride, SrCl₂ [2] (b) nickel(II) bromide, NiBr₂ [2]

4. A sample of a molten metal chloride is electrolysed until it is used up. The cathode collects 2.38 g of tin, and 1.42 g of chlorine is produced at the anode. (Ar: Sn = 119, Cl = 35.5)

(a) Work out the empirical formula of this chloride. [3] (b) Give the half equation for the change at the negative electrode. [1]

5. For each solution below, electrolysed with inert electrodes, predict what forms at each electrode. Give a reason for each cathode product.

(a) copper(II) nitrate solution [2] (b) concentrated calcium chloride solution [2] (c) lithium nitrate solution [2]

6. A student electrolyses concentrated sodium chloride solution with graphite electrodes as part of PAG C2. Gas is collected over each electrode.

(a) Identify the gas collected over each electrode. [2] (b) Describe a test for each gas and give the positive result. [2] (c) Explain, in terms of the ions present, why sodium is not formed at the cathode. [2] (d) Name the compound that builds up in the solution. [1]

7. (a) Complete and balance this half equation for the formation of oxygen at the anode. OH⁻ → O₂ + H₂O + e⁻ [2] (b) Write the half equation for the formation of copper at the cathode. [1] (c) State which of the half equations in (a) and (b) happens at the positive electrode. [1]

8. Copper(II) sulfate solution is electrolysed in two experiments: first with graphite electrodes, then with copper electrodes.

(a) For the graphite electrodes, describe what you would see at each electrode and in the solution. [3] (b) Explain why, with copper electrodes, the blue colour of the solution does not change. [2]

9. A student uses copper electrodes in copper(II) sulfate solution and weighs each electrode before and after.

Electrode	Before (g)	After (g)
cathode	31.85	32.47
anode	30.20	29.56

(a) Calculate the mass change of each electrode. [2] (b) Explain the change at the cathode. Include a half equation. [2] (c) The two changes should be equal. Suggest one practical reason why they are not. [1] (d) Calculate the percentage increase in mass of the cathode. Give your answer to 3 significant figures. [1]

10. Potassium nitrate solution is electrolysed with platinum electrodes and the gases are collected in measuring tubes.

(a) Name the two gases produced, stating which electrode gives each. [2] (b) 54 cm³ of gas is collected at the cathode. Use the half equations to predict the volume of gas at the anode. [1] (c) The measured anode volume is slightly less than predicted. Suggest why. [1]

11. A technician wants to show a class what forms when concentrated lithium chloride solution is electrolysed with inert electrodes.

Describe how the experiment should be set up and carried out safely, and explain the results. Include the products at each electrode, how each would be identified, half equations for both electrode reactions, and what is left in solution. [8]

Answers

1. (a) A cation [1]; it moves to the cathode (negative electrode) [1]. (b) The anode [1]. Examiner insight: Cathode and anode are easily swapped under pressure; linking “cation” to “cathode” by charge (positive to negative) earns both marks reliably.

2. Lithium iodide is ionic; current is not carried by electrons in the compound [1]. Solid: the ions are held in place in the lattice, so they cannot travel to the electrodes [1]. Molten: the ions can now move and carry charge through the liquid [1]. Examiner insight: Any mention of electrons moving through the solid or the melt is a contradiction that usually costs the explanation marks, so the word “ions” must appear in both parts.

3. (a) Cathode: strontium [1]; anode: chlorine [1]. (b) Cathode: nickel [1]; anode: bromine [1]. Examiner insight: Name the element, not the ion; “chloride” at the anode describes what arrives, not what forms.

4. (a) Sn: 2.38 ÷ 119 = 0.02; Cl: 1.42 ÷ 35.5 = 0.04 [1]. Ratio 0.02 : 0.04 = 1 : 2 [1]. Empirical formula SnCl₂ [1]. (b) Sn²⁺ + 2e⁻ → Sn [1] Examiner insight: Show the division by Ar for each element; a formula with no working gives the examiner nothing to credit if the ratio is wrong.

5. (a) Cathode copper, because copper is less reactive than hydrogen [1]; anode oxygen [1]. (b) Cathode hydrogen, because calcium is more reactive than hydrogen [1]; anode chlorine [1]. (c) Cathode hydrogen, because lithium is more reactive than hydrogen [1]; anode oxygen [1]. Examiner insight: The reason must compare the metal with hydrogen; “because it is a metal” or “because it is reactive” does not explain the choice.

6. (a) Cathode: hydrogen [1]; anode: chlorine [1]. (b) Hydrogen: lighted splint gives a squeaky pop [1]. Chlorine: blue litmus paper, moistened, goes red and is then bleached [1]. (c) The solution contains Na⁺ and H⁺ ions (H⁺ from water) [1]. Because sodium is above hydrogen in reactivity, it is the H⁺ ions that take electrons and are discharged as hydrogen [1]. (d) Sodium hydroxide [1]. Examiner insight: For the chlorine test, “damp” and “bleached” are both expected; “turns litmus white” alone may not be enough.

7. (a) 4OH⁻ → O₂ + 2H₂O + 4e⁻: correct species and 4OH⁻, 2H₂O [1]; 4e⁻ so charges balance [1]. (b) Cu²⁺ + 2e⁻ → Cu [1] (c) The half equation in (a) (oxygen forms at the anode) [1]. Examiner insight: Check the charge totals on each side before moving on; a half equation with balanced atoms but unbalanced charge is incomplete.

8. (a) Cathode: pink-brown solid (copper) forms [1]. Anode: bubbles of gas (oxygen) [1]. Solution: blue colour fades [1]. (b) The copper anode dissolves, forming Cu²⁺ ions (Cu → Cu²⁺ + 2e⁻) [1]. Cu²⁺ ions are added at the anode at the same rate as they are removed at the cathode, so their concentration stays the same [1]. Examiner insight: “Describe what you would see” asks for observations, so “bubbles” earns credit where naming oxygen alone may not.

9. (a) Cathode: 32.47 − 31.85 = +0.62 g [1]. Anode: 29.56 − 30.20 = −0.64 g [1]. (b) Cu²⁺ ions from the solution gain electrons at the cathode and copper is deposited [1]: Cu²⁺ + 2e⁻ → Cu [1]. (c) Any one: some copper fell off the cathode before weighing; the anode was still wet when first weighed; the anode copper contained impurities that dropped off [1]. (d) 0.62 ÷ 31.85 × 100 = 1.95% [1] Examiner insight: A sign on each mass change shows gain or loss clearly; a percentage is easier to credit when the calculation is written out before the rounded answer.

10. (a) Cathode: hydrogen [1]; anode: oxygen [1]. (b) Hydrogen and oxygen form in a 2 : 1 ratio, so 54 ÷ 2 = 27 cm³ [1]. (c) Some oxygen dissolves in the solution (oxygen is slightly more soluble than hydrogen) [1]. Examiner insight: The ratio comes from the electrons in the half equations (2e⁻ per H₂, 4e⁻ per O₂); stating 1 : 1 because “both are gases” loses the mark.

11. Indicative points, one mark each:

• Two graphite (or platinum) electrodes in a beaker of the solution, not touching, connected to a low-voltage d.c. supply [1].
• Test tubes filled with solution inverted over each electrode to collect gas [1].
• Chlorine is toxic: use small volumes, a fume cupboard or good ventilation, and eye protection [1].
• Cathode: hydrogen, because lithium is more reactive than hydrogen; identified by a squeaky pop with a lighted splint [1].
• Anode: chlorine, because chloride ions are present; identified by bleaching damp blue litmus paper [1].
• Cathode half equation: 2H⁺ + 2e⁻ → H₂ [1].
• Anode half equation: 2Cl⁻ → Cl₂ + 2e⁻ [1].
• Li⁺ and OH⁻ ions remain, so lithium hydroxide solution forms and the solution becomes alkaline [1]. Examiner insight: Extended answers are judged on a logical, linked account; a list of products with no tests, safety or reasoning limits the credit however accurate it is.

Where marks are usually lost

• Saying electrons, not ions, move through a molten or dissolved compound.
• Giving a reactive metal such as sodium, calcium or lithium as a product of an aqueous solution.
• Naming the ion (chloride, bromide) instead of the element formed (chlorine, bromine).
• Leaving electrons out of a half equation, or putting them on the wrong side.
• Assuming a 1 : 1 ratio for hydrogen and oxygen.
• Saying the colour of copper sulfate fades when copper electrodes are used.
• Omitting the sign on a mass change, or using the after mass as the base in a percentage.
• Forgetting the word “damp” in the chlorine test.

Next steps

• Go back over weak spots in the Electrolysis study guide.
• Keep the Electrolysis revision notes for last-minute recall.
• When C3.4 feels secure, record it on your printable checklist, then choose the next topic from the J248 course hub.
• Book a free trial class.
• See all free 10-minute diagnostics.

Official syllabus

Source: OCR’s specification for GCSE (9–1) Chemistry A (Gateway Science), J248, Version 4.0 (August 2026), for first assessment in 2018. Content tested here: C3.4 Electrolysis (statements C3.4a–C3.4e), mathematical learning outcome CM3.4i and the PAG C2 practical suggestions.