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S6 Chemistry: Electrolysis and Cells (Paper 1: basics)
S6 Chemistry: Electrolysis and Cells (Paper 1: basics)
Ten items on the energy change in electrolysis and cells, Q = I t, the cathode on the negative terminal, 600 C from 2.0 A for 5 minutes, 0.00622 mol of electrons from 0.50 A for 20 minutes, copper deposited from dilute copper(II) chloride, oxidation at the anode, the object as cathode in plating, the 2 : 1 gas ratio in water electrolysis, and the misconception that electrolysis needs no supply.
Direct answers Answers in order: electrolysis drives a non-spontaneous reaction with electrical energy, Q = I t, the negative terminal because the cathode receives electrons, 600 C, about 0.00622 mol, red-brown copper deposited, oxidation, the object is the cathode, 2 : 1, and electrolysis is non-spontaneous so a supply is required.
01 Paper details (check answers afterwards)
| Item | Detail |
|---|---|
| Questions | 10 |
| Suggested time | 20 |
| Difficulty mix | Foundation 3 · Intermediate 3 · Challenge 3 · Error redo 1 |
| Chapter | Chapter: S6 Chemistry: Electrolysis and Cells |
| Level | S6 |
02 Questions (10 items)
- FoundationHow do electrolysis and a chemical cell differ in energy conversion?
- FoundationWhat is the relation between charge Q, current I and time t?
- FoundationWhich terminal of the power supply must the cathode of an electrolytic cell be connected to, and why?
- IntermediateWhat charge passes when a current of 2.0 A flows for 5 minutes?
- IntermediateHow many moles of electrons pass when 0.50 A flows for 20 minutes?
- IntermediateWhat is seen at the cathode when dilute copper(II) chloride solution is electrolysed with carbon electrodes?
- ChallengeDoes oxidation or reduction happen at the anode of an electrolytic cell, and which species loses electrons?
- ChallengeDuring electroplating, which electrode holds the object being plated and what ions must the bath contain?
- ChallengeWhat is the volume ratio of the gases released at the cathode and anode during the electrolysis of water?
- Error redoA student says electrolysis is spontaneous and needs no power supply. What is wrong?
03 Answers and one-line notes
| # | Answer | One-line note |
|---|---|---|
| 1 | Electrolysis uses electrical energy to drive a non-spontaneous reaction, while a cell turns a spontaneous reaction into electrical energy | The two run in opposite directions, which is why a cell and an electrolytic cell are described as reverse processes. |
| 2 | Q = I t, with current in amperes and time in seconds, giving charge in coulombs | Charge is current multiplied by time, so the time must be converted to seconds first. |
| 3 | The negative terminal, because the cathode receives electrons and reduction happens there | The negative terminal supplies electrons, which enter the cathode so that cations are reduced. |
| 4 | 600 C | Five minutes is 300 seconds, and 2.0 times 300 gives 600 C. |
| 5 | About 0.00622 mol | The charge is 0.50 times 1200 seconds, that is 600 C, divided by 96 500 C per mole gives 0.00622 mol. |
| 6 | Red-brown copper is deposited on the cathode | Copper(II) ions gain electrons and are reduced to copper, so a red-brown deposit appears. |
| 7 | Oxidation, with anions or water molecules losing electrons at the anode | The anode is joined to the positive terminal, electrons are drawn away, so anions or water are oxidised. |
| 8 | The object is the cathode and the bath contains ions of the plating metal | Metal ions are reduced to metal on the cathode, so the object has to be the cathode. |
| 9 | 2 : 1, hydrogen at the cathode to oxygen at the anode | Every two moles of electrons give 1 mol of hydrogen and 0.5 mol of oxygen, a volume ratio of 2 to 1. |
| 10 | Electrolysis is non-spontaneous and needs an external supply | A spontaneous reaction would discharge by itself in a cell, so electrolysis must be driven by electrical energy. |
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.
Decide the electrodes and the ion discharge order first, then use Q = It, divide by the Faraday constant 96 500 C per mole to get moles of electrons, and finally convert with the half equation to mass or moles of gas.Tutor Registration →
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