Explain why zinc displaces copper from a solution of copper(II) sulfate, with reference to the standard electrode potentials of the two half-reactions involved.
Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).
The standard electrode potentials for the relevant half-reactions are: Cu²⁺(aq) + 2e⁻ → Cu(s), E° = +0.34 V and Zn²⁺(aq) + 2e⁻ → Zn(s), E° = −0.76 V.
Model answer (4 marks)
Zinc is oxidised: Zn(s) → Zn²⁺(aq) + 2e⁻, while copper(II) ions are reduced: Cu²⁺(aq) + 2e⁻ → Cu(s). The standard potential for the Zn²⁺/Zn couple (−0.76 V) is more negative than that for the Cu²⁺/Cu couple (+0.34 V), so Zn is the stronger reducing agent and will lose electrons.
The cell potential is E°cell = E°cathode – E°anode = +0.34 V – (−0.76 V) = +1.10 V. A positive E°cell shows the reaction is spontaneous.
Because the reaction is spontaneous, zinc displaces copper from copper(II) sulfate solution.
The cell potential is E°cell = E°cathode – E°anode = +0.34 V – (−0.76 V) = +1.10 V. A positive E°cell shows the reaction is spontaneous.
Because the reaction is spontaneous, zinc displaces copper from copper(II) sulfate solution.
Examiner tips
- Show the two half‑reactions and state which is anode/cathode; use the correct sign for each E°; calculate E°cell and note it is positive; link a positive E°cell to spontaneity.
- Use the phrase "oxidised" for Zn and "reduced" for Cu to demonstrate electron flow.
- Include the numerical value +1.10 V to show you have performed the calculation.
Common mistakes
- Writing the wrong direction for electron transfer (e.g. saying Cu is oxidised); forgetting to subtract the anode potential; not calculating E°cell or giving the wrong sign; not explicitly stating that a positive E°cell means the reaction is spontaneous.
Mark scheme (4 marks)
- Zinc is oxidised (loses electrons) and copper(II) ions are reduced (gain electrons), identifying the correct direction of electron transfer.
- The standard electrode potential of zinc (−0.76 V) is more negative than that of copper (+0.34 V), so zinc is the stronger / better reducing agent.
- The overall cell potential (E°cell) is positive, calculated as E°(cathode) − E°(anode) = +0.34 − (−0.76) = +1.10 V, indicating a spontaneous reaction.
- Because E°cell is positive, the reaction is thermodynamically feasible / spontaneous, so zinc does displace copper from solution.
Key terms in this question
Related
- All IB DP Chemistry Standard Level (2023 syllabus) revision notes →
- How to answer a "Explain" question →
- Decode the mark scheme abbreviations →
More Electron transfer reactions questions
- Explain what happens at each electrode when a voltaic cell is constructed using …
- Explain how electrolysis of molten lead(II) bromide produces lead and bromine, i…
- Explain how the sign and magnitude of a standard cell potential (E°cell) can be …
- Explain why a standard hydrogen electrode is assigned a standard electrode poten…