A student sets up an electrochemical cell using a zinc half-cell and a copper half-cell. The standard electrode potentials are: Zn²⁺/Zn = −0.76 V and Cu²⁺/Cu = +0.34 V. Explain which electrode acts as the negative electrode and why electrons flow in the direction they do through the external circuit.
Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).
The cell is set up under standard conditions, with each half-cell containing a metal electrode in a 1.0 mol/dm³ solution of its metal ions. The two half-cells are connected by a salt bridge and an external wire.
Model answer (5 marks)
Zinc has the more negative standard electrode potential (−0.76 V) compared with copper (+0.34 V), so the zinc electrode is the negative electrode.
At the negative electrode zinc is oxidised: Zn(s) → Zn²⁺ + 2e⁻, losing electrons.
The electrons flow through the external wire from the zinc electrode to the copper electrode.
At the copper electrode the Cu²⁺ ions are reduced: Cu²⁺ + 2e⁻ → Cu(s), gaining the electrons.
The driving force for this electron flow is the difference in standard electrode potentials (the cell EMF).
At the negative electrode zinc is oxidised: Zn(s) → Zn²⁺ + 2e⁻, losing electrons.
The electrons flow through the external wire from the zinc electrode to the copper electrode.
At the copper electrode the Cu²⁺ ions are reduced: Cu²⁺ + 2e⁻ → Cu(s), gaining the electrons.
The driving force for this electron flow is the difference in standard electrode potentials (the cell EMF).
Examiner tips
- State which electrode is negative using the more negative potential.
- Explain oxidation at the negative electrode and reduction at the positive electrode.
- Show the direction of electron flow and link it to the EMF.
- Use the correct symbols for half‑reactions and mention the salt bridge.”]
Mark scheme (5 marks)
- Zinc has a more negative standard electrode potential than copper, so zinc is the negative electrode.
- Zinc is oxidised at the negative electrode, losing electrons.
- Electrons flow from the zinc electrode to the copper electrode through the external circuit.
- Copper ions are reduced at the copper electrode, gaining the electrons.
- The driving force for electron flow is the difference in standard electrode potentials between the two half-cells (the cell EMF).
Key terms in this question
standard electrode potential · negative electrode · external circuit
Related
- All AQA A-Level Chemistry (7405) revision notes →
- How to answer a "Explain" question →
- Decode the mark scheme abbreviations →
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