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S5 Chemistry: Chemical Cells and Fuel Cells (Paper 2: intermediate)
S5 Chemistry: Chemical Cells and Fuel Cells (Paper 2: intermediate)
Ten intermediate items on finding moles of electrons and the mass of zinc consumed, fuel cell efficiency, hydrogen and oxygen stoichiometry, lithium ion cells in series, parallel capacity and the role of the salt bridge.
Direct answers Answers in order: 0.0373 mol, 1.22 g, 69.9 percent, 24 dm3 and 36 g, 11.1 V, 1.5 V with double capacity, 360 C, 143 kJ, ion migration, and the salt bridge is not a source of electrons.
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: S5 Chemistry: Chemical Cells and Fuel Cells |
| Level | S5 |
02 Questions (10 items)
- FoundationA cell discharges 0.50 A for 2.0 hours. How many moles of electrons pass, with the Faraday constant at 96500 C/mol?
- FoundationThe zinc electrode reaction is Zn to Zn2+ plus 2e-. What mass of zinc does 0.0373 mol of electrons consume, relative atomic mass 65.4?
- FoundationIn a hydrogen-oxygen fuel cell each mole of hydrogen can release 286 kJ but only 200 kJ is delivered. What is the energy efficiency?
- IntermediateWhen 4.0 g of hydrogen reacts completely, what volume of oxygen is needed and what mass of water is formed, molar volume 24 dm3/mol?
- IntermediateA lithium ion cell is 3.7 V. What is the total emf of three in series?
- IntermediateWhen two 1.5 V dry cells are connected in parallel, how do the total emf and the available capacity change?
- ChallengeA cell discharges at 0.20 A for 30 minutes. What charge passes?
- ChallengeEach mole of hydrogen releases 286 kJ. How much energy does each gram of hydrogen release in a fuel cell?
- ChallengeWhy must the electrolyte of a fuel cell be kept moist?
- Error redoA student claims that the salt bridge supplies electrons. What is the correct view?
03 Answers and one-line notes
| # | Answer | One-line note |
|---|---|---|
| 1 | 0.0373 mol | The charge of 3600 C divided by 96500 gives 0.0373 mol of electrons. |
| 2 | 1.22 g | Two moles of electrons correspond to one mole of zinc, so 0.01865 mol times 65.4 gives 1.22 g. |
| 3 | 69.9 percent | Dividing 200 by 286 and multiplying by 100 gives 69.9 percent. |
| 4 | 24 dm3 of oxygen is needed and 36 g of water is formed | 4.0 g of hydrogen is 2 mol, which needs 1 mol of oxygen, that is 24 dm3, and gives 2 mol of water, that is 36 g. |
| 5 | 11.1 V | Emfs add in series, so 3.7 times 3 gives 11.1 V. |
| 6 | The emf stays at 1.5 V while the available capacity doubles | Parallel connection only increases the total charge that can be supplied, and the emf is unchanged. |
| 7 | 360 C | Thirty minutes is 1800 seconds, and 0.20 times 1800 gives 360 C. |
| 8 | 143 kJ | One mole of hydrogen is 2 g, so 286 divided by 2 gives 143 kJ per gram. |
| 9 | Ions must migrate between the electrodes to complete the circuit | Protons are carried through the moist electrolyte, and drying out raises the internal resistance until the cell stops working. |
| 10 | The salt bridge lets ions migrate and keeps both solutions electrically neutral, it is not a source of electrons | Electrons travel only in the external circuit, while the salt bridge connects the circuit and balances charge by ion movement. |
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.
For every item state the direction of electron transfer or the formula first, then substitute the numbers and give the unit.Tutor Registration →
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