These twelve questions move from easy to harder. They practise the five lessons in electrolysis reasoning: electrode roles, molten products, half-equations, aqueous cases and electroplating.
Write your own answer first, then open the explanation. All numerical data below is invented for practice. Use Ar: Cu = 64, Al = 27, and 24 dm³ for one mole of gas at room temperature and pressure.
Questions
1. An electrode is joined to the positive terminal of a supply. Name it and say which ions move to it.
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It is the anode. Negative ions (anions) move to it, because opposite charges attract. See electrode roles.
2. Explain why solid lead(II) bromide does not conduct but the molten compound does.
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In the solid, Pb²⁺ and Br⁻ ions are fixed in the lattice. When molten, the ions are free to move and carry the current through the liquid.
3. Molten lead(II) bromide is electrolysed with inert electrodes. State the product at each electrode.
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Ions: Pb²⁺ and Br⁻. Lead at the cathode, bromine at the anode. See molten products.
4. Molten magnesium chloride, MgCl₂, is electrolysed. Give the products and the overall equation.
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Mg²⁺ goes to the cathode, Cl⁻ to the anode. Products: magnesium and chlorine. Overall: MgCl₂ → Mg + Cl₂. Atom check: 1 Mg and 2 Cl on each side.
5. Write the half-equation at each electrode for question 4 and state which is oxidation and which is reduction.
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Cathode: Mg²⁺ + 2e⁻ → Mg, reduction (electrons gained). Anode: 2Cl⁻ → Cl₂ + 2e⁻, oxidation (electrons lost). Both involve 2 electrons, so they add to give MgCl₂ → Mg + Cl₂. See half-equations.
6. Molten aluminium oxide has the half-equations Al³⁺ + 3e⁻ → Al and 2O²⁻ → O₂ + 4e⁻. Combine them into one overall equation.
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Make the electrons equal. The lowest common multiple of 3 and 4 is 12. Multiply the first by 4: 4Al³⁺ + 12e⁻ → 4Al. Multiply the second by 3: 6O²⁻ → 3O₂ + 12e⁻. Add and cancel the electrons: 4Al³⁺ + 6O²⁻ → 4Al + 3O₂.
That is 2Al₂O₃ → 4Al + 3O₂. Check: 4 Al and 6 O on each side.
7. Copper(II) sulfate solution is electrolysed with inert electrodes. Give the product at each electrode and explain why these and not the others.
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Cathode: copper, because copper is less reactive than hydrogen, so Cu²⁺ is discharged rather than H⁺. Anode: oxygen, because no halide ion is present, so OH⁻ gives up electrons. See aqueous and molten cases.
8. (Invented data.) In the cell from question 7, 0.64 g of copper is deposited. How many moles of copper is that, how many moles of electrons were transferred, and what volume of oxygen forms at the anode at room temperature and pressure?
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Moles of Cu = 0.64 ÷ 64 = 0.01 mol.
Each Cu²⁺ gains 2 electrons, so electrons = 2 × 0.01 = 0.02 mol.
At the anode, 4 electrons release 1 O₂. So O₂ = 0.02 ÷ 4 = 0.005 mol. Volume = 0.005 × 24 = 0.12 dm³ = 120 cm³.
Check by ratio: Cu : O₂ = 2 : 1, so 0.01 mol Cu matches 0.005 mol O₂. The mole and equation-ratio tutor shows the same pattern.
9. (Invented data.) In the aluminium cell of question 6, 0.40 mol of aluminium forms. How many moles of oxygen gas form, and what is the mass of aluminium?
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From 2Al₂O₃ → 4Al + 3O₂, Al : O₂ = 4 : 3. Moles of O₂ = 0.40 × 3/4 = 0.30 mol.
Mass of Al = 0.40 × 27 = 10.8 g.
Check: 0.30 ÷ 0.40 = 0.75 = 3/4, matching the ratio.
10. Concentrated sodium chloride solution is electrolysed with inert electrodes. Name the gas at each electrode and the substance left in solution.
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Cathode: hydrogen, because sodium is more reactive than hydrogen. Anode: chlorine, because chloride is a halide ion in a concentrated solution. Left in solution: sodium hydroxide, from the Na⁺ and OH⁻ ions not discharged.
11. An iron gate is to be nickel-plated. Name the cathode, anode and a suitable electrolyte, and write the cathode half-equation.
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Cathode: the iron gate. Anode: a nickel electrode. Electrolyte: a solution containing nickel(II) ions, such as nickel(II) sulfate solution. Cathode: Ni²⁺ + 2e⁻ → Ni. See electroplating diagrams.
12. A student writes: “In the electrolysis, electrons flow through the electrolyte from the cathode to the anode.” Identify the error and correct it.
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Electrons do not flow through the electrolyte. They move through the wires and electrodes, and ions carry the current through the electrolyte. Also, electrons leave the anode and travel round the external circuit towards the cathode, so the stated direction is reversed.
If you got these wrong
| What went wrong | Questions | Go to |
|---|---|---|
| Mixed up anode and cathode, or where ions move | 1, 2, 12 | roles of electrodes and electrolyte |
| Wrong molten product or added hydrogen | 3, 4 | predict a molten product |
| Electrons on the wrong side, or unbalanced equations | 5, 6 | half-equations |
| Wrong aqueous product or left-over solution | 7, 10 | aqueous and molten cases |
| Mole or ratio slips | 8, 9 | mole and equation-ratio tutor and equation balance trainer |
| Plating diagram confusion | 11 | electroplating diagrams |
Record each miss in the mistake log and retest queue, then retry a similar question after a few days.
When a pattern keeps returning, our teachers can trace it in online one-to-one Chemistry tuition.