Explain why a galvanic cell constructed from a zinc half-cell connected to a copper half-cell produces a positive cell potential, and identify which electrode acts as the anode, giving a reason based on standard electrode potential values.
Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).
Standard electrode potentials (E°) at 298 K: Zn²⁺(aq)/Zn(s): −0.76 V; Cu²⁺(aq)/Cu(s): +0.34 V.
Model answer (4 marks)
The cell potential is calculated as E°(cathode) – E°(anode). For a Zn–Cu cell
E°cell = +0.34 V – (−0.76 V) = +1.10 V, so the potential is positive.
The electrode with the more negative standard potential is the anode; here Zn has –0.76 V, so Zn(s) is oxidised:
Zn(s) → Zn²⁺(aq) + 2e⁻, releasing electrons into the external circuit.
The positive E°cell shows the reaction is spontaneous (ΔG° = –nFE°cell < 0).
E°cell = +0.34 V – (−0.76 V) = +1.10 V, so the potential is positive.
The electrode with the more negative standard potential is the anode; here Zn has –0.76 V, so Zn(s) is oxidised:
Zn(s) → Zn²⁺(aq) + 2e⁻, releasing electrons into the external circuit.
The positive E°cell shows the reaction is spontaneous (ΔG° = –nFE°cell < 0).
Examiner tips
- Show the calculation E°cathode – E°anode explicitly; include the sign of each potential.
- State clearly that the more negative potential is the anode and give the oxidation half‑reaction.
- Mention that a positive cell potential implies ΔG° < 0, indicating spontaneity.
- Use correct symbols (E°, ΔG°, n, F) and units (V).
Common mistakes
- Using the wrong sign for the zinc potential (writing +0.76 V instead of –0.76 V).
- Failing to identify the anode as the electrode with the more negative potential.
- Not showing the full half‑reaction for zinc oxidation or omitting the electron count.
Mark scheme (4 marks)
- The cell potential is positive because it is calculated as E°(cathode) − E°(anode), i.e. +0.34 − (−0.76) = +1.10 V, giving a positive value.
- Zinc is the anode (site of oxidation) because it has the more negative / less positive standard electrode potential (−0.76 V).
- At the anode, zinc is oxidised: Zn(s) → Zn²⁺(aq) + 2e⁻, releasing electrons into the external circuit.
- The positive cell potential indicates a spontaneous reaction / a negative Gibbs energy change (ΔG° < 0), because ΔG° = −nFE°cell.
Key terms in this question
standard electrode potential · galvanic cell · anode · cell potential
Related
- All IB DP Chemistry Higher Level (2023 syllabus) revision notes →
- How to answer a "Explain" question →
- Decode the mark scheme abbreviations →
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