A student sets up an electrochemical cell using a zinc half-cell and a copper half-cell connected by a salt bridge. The standard electrode potentials are: Zn²⁺/Zn = −0.76 V and Cu²⁺/Cu = +0.34 V. Explain how the electrochemical cell produces a voltage, including the role of the salt bridge.
Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).
An electrochemical cell consists of two half-cells: a zinc electrode in zinc sulfate solution and a copper electrode in copper sulfate solution. The two solutions are connected by a salt bridge soaked in potassium nitrate solution.
Model answer (5 marks)
1. Oxidation occurs at the zinc electrode: Zn(s) → Zn²⁺(aq) + 2e⁻.
2. Reduction occurs at the copper electrode: Cu²⁺(aq) + 2e⁻ → Cu(s).
3. Electrons flow through the external circuit from the zinc electrode to the copper electrode.
4. The difference in standard electrode potentials (ΔE° = +0.34 V – (−0.76 V) = +1.10 V) drives the flow of electrons, giving the cell a voltage of 1.10 V.
5. The salt bridge completes the circuit by allowing K⁺ and NO₃⁻ ions to move between the half‑cells, maintaining electrical neutrality and preventing charge build‑up.
2. Reduction occurs at the copper electrode: Cu²⁺(aq) + 2e⁻ → Cu(s).
3. Electrons flow through the external circuit from the zinc electrode to the copper electrode.
4. The difference in standard electrode potentials (ΔE° = +0.34 V – (−0.76 V) = +1.10 V) drives the flow of electrons, giving the cell a voltage of 1.10 V.
5. The salt bridge completes the circuit by allowing K⁺ and NO₃⁻ ions to move between the half‑cells, maintaining electrical neutrality and preventing charge build‑up.
Examiner tips
- Use the exact wording ‘oxidation at zinc’, ‘reduction at copper’, and ‘difference in electrode potentials drives the voltage’.
- Show the calculation of ΔE° to demonstrate understanding of the driving force.
- Mention the salt bridge’s role in maintaining neutrality, not just ‘completes the circuit’.
- Keep each point concise and separate with numbers.
Mark scheme (5 marks)
- Oxidation occurs at the zinc electrode / zinc loses electrons
- Reduction occurs at the copper electrode / copper ions gain electrons
- Electrons flow through the external circuit from zinc to copper
- The difference in electrode potentials between the two half-cells causes / drives the voltage (e.m.f.)
- The salt bridge completes the circuit by allowing ions to move between the two half-cell solutions, maintaining electrical neutrality
Key terms in this question
electrode potential · salt bridge · electrochemical cell
Related
- All Edexcel A-Level Chemistry (9CH0) revision notes →
- How to answer a "Explain" question →
- Decode the mark scheme abbreviations →
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