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S5 Chemistry Electrolysis, Paper 2: electrode products and plating
S5 Chemistry Electrolysis, Paper 2: electrode products and plating
Ten graded items on products at each electrode, electrode materials, plating and Faraday calculations.
Focus List and rank the ions first; then use Q equals I t, divide by 96 500 for moles of electrons and scale to the product.
01 Paper details (check answers afterwards)
| Item | Detail |
|---|---|
| Questions | 10 |
| Suggested time | 25 |
| Difficulty mix | Foundation 4 · Intermediate 3 · Challenge 2 · Error redo 1 |
| Chapter | Chapter: S5 Chemistry: Electrolysis |
| Level | Four basic, three advanced, two challenge and one error-repair item |
02 Questions (10 items)
- FoundationWhat forms at each electrode when molten lead bromide is electrolysed?
- FoundationDilute sulphuric acid is electrolysed with platinum electrodes. Find the ratio of gas volumes at cathode and anode.
- FoundationFour moles of electrons are transferred during the electrolysis of water. How many moles of hydrogen form at the cathode?
- FoundationIs the cathode of an electrolytic cell the site of oxidation or reduction?
- IntermediateIn silver plating, which electrode holds the object and what is the anode made of?
- IntermediateCopper sulphate solution is electrolysed at 1.0 A for 965 s. Find the mass of copper deposited, with Cu as 63.5.
- IntermediateWhy do concentrated and dilute sodium chloride solutions give different anode products?
- ChallengeSilver nitrate solution is electrolysed at 0.40 A for 4 825 s. Find the mass of silver deposited, with Ag as 107.9.
- ChallengeCompare the electrolysis of copper sulphate solution with a copper anode and with a platinum anode.
- Error redoA student says electrons flow through the solution from anode to cathode. Give the correct statement.
03 Answers and one-line notes
| # | Answer | One-line note |
|---|---|---|
| 1 | Lead at the cathode, bromine at the anode | Lead ions are reduced at the cathode and bromide ions are oxidised to bromine at the anode. |
| 2 | Hydrogen to oxygen is 2 to 1 | Two moles of electrons give one mole of hydrogen but only half a mole of oxygen, a ratio of two to one. |
| 3 | 2 mol | One mole of hydrogen needs two moles of electrons, so four moles of electrons give two moles of gas. |
| 4 | Reduction, because cations gain electrons there | The cathode gains electrons from the supply, so cations are reduced there. |
| 5 | The object is the cathode and the anode is pure silver | Silver ions are reduced onto the object at the cathode while the silver anode dissolves to keep the solution topped up. |
| 6 | About 0.32 g | 965 coulombs is 0.010 mol of electrons, and copper needs two per atom, so about 0.32 grams forms. |
| 7 | Brine is concentrated enough for chloride to discharge as chlorine; in dilute solution water gives oxygen | With concentrated brine the chloride concentration is high enough for chlorine to win; in dilute solution water is oxidised instead. |
| 8 | About 2.16 g | 1 930 coulombs gives 0.020 mol of electrons, and monovalent silver weighs about 2.16 grams. |
| 9 | Copper dissolves at the anode so the colour stays; platinum gives oxygen and the blue fades | A copper anode dissolves and keeps the blue colour, while an inert platinum anode releases oxygen and the blue fades. |
| 10 | Electrons flow only in the external circuit; ions carry the current in solution | Electrons travel only in the external circuit; inside the solution ions carry the current. |
04 What to do after marking
- After marking, redo only the wrong items and write down which step went wrong (formula, unit, or misread number).
- Repeat the same item two days later; move to the next paper only when both attempts are correct.
- Parents only need the answer and note columns; there is no need to explain every item.
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Questions and answers are written originally by the TutorZone team and are not taken from any school, publisher or examination authority paper. Every value is checked by computer.
