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S6 Chemistry: Electrochemical Cells (Paper 4: misconception drill)
S6 Chemistry: Electrochemical Cells (Paper 4: misconception drill)
Ten drill items on zinc being oxidised, cathode minus anode, the effect of having no salt bridge, the hydrogen zero, zinc-copper at 1.10 V, copper-silver at 0.46 V, lead-silver at 0.93 V, −424.6 kJ for 4 mol of electrons, copper-chlorine at 1.02 V, and a misconception writing anode minus cathode.
Direct answers Answers in order: the zinc electrode, cathode minus anode, charge builds up and the current stops, the hydrogen electrode at 0.00 V, 1.10 V, 0.46 V, 0.93 V, −424.6 kJ, 1.02 V with chlorine positive, and the order is reversed so it must be cathode minus anode.
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: Electrochemical Cells |
| Level | S6 |
02 Questions (10 items)
- FoundationWhich electrode is oxidised in a zinc-copper cell?
- FoundationIn the EMF formula, which electrode comes first?
- FoundationWhat goes wrong in a cell without a salt bridge?
- IntermediateWhat is the reference zero for standard electrode potentials?
- IntermediateWhat is the standard EMF of a zinc-copper cell at −0.76 V and +0.34 V?
- IntermediateWhat is the standard EMF of a copper-silver cell at +0.34 V and +0.80 V?
- ChallengeWhat is the standard EMF of a lead-silver cell (Pb²⁺/Pb at −0.13 V and Ag⁺/Ag at +0.80 V)?
- ChallengeFor a zinc-copper cell with E° = 1.10 V and 4 mol of electrons transferred, what is ΔG° in kJ mol⁻¹?
- ChallengeWhat is the standard EMF of a copper-chlorine cell at +0.34 V and +1.36 V and which electrode is positive?
- Error redoA student writes E°(cell) = E°(anode) − E°(cathode). What is wrong?
03 Answers and one-line notes
| # | Answer | One-line note |
|---|---|---|
| 1 | The zinc electrode, which is the anode | Zinc loses electrons and is oxidised, so zinc is both the anode and the negative electrode. |
| 2 | Cathode minus anode | Reversing the order gives a negative value and can be mistaken for a non-spontaneous reaction. |
| 3 | Charge builds up on both sides and the electron flow soon stops | The bridge lets ions migrate to keep neutrality; without it the circuit is incomplete. |
| 4 | The hydrogen electrode H⁺/H₂, fixed at 0.00 V under standard conditions | Every electrode potential is quoted relative to the hydrogen electrode. |
| 5 | 1.10 V | 0.34 minus (−0.76) gives 1.10 V. |
| 6 | 0.46 V | 0.80 minus 0.34 gives 0.46 V with silver as the cathode. |
| 7 | 0.93 V | 0.80 minus (−0.13) gives 0.93 V with lead as the anode. |
| 8 | −424.6 kJ mol⁻¹ | Minus 4 times 96500 times 1.10 gives −424600 J or −424.6 kJ. |
| 9 | 1.02 V and the chlorine electrode is positive | 1.36 minus 0.34 gives 1.02 V and the higher potential chlorine electrode is the cathode, the positive one. |
| 10 | The order is reversed; it must be cathode minus anode | Reversing the order makes every spontaneous cell negative, so the judgement is wrong throughout. |
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.
Compare the standard electrode potentials first, name the cathode and anode, subtract anode from cathode to get the EMF, then use the free energy change to judge feasibility.Tutor Registration →
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