Explain how an electrochemical cell produces a voltage, including the role of the two half-cells and the salt bridge.
Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).
An electrochemical cell can be made by connecting two different metal/metal ion half-cells. For example, a zinc half-cell (zinc rod in zinc sulfate solution) can be connected to a copper half-cell (copper rod in copper sulfate solution).
Model answer (5 marks)
Each half‑cell contains a metal electrode in a solution of its ion. At the zinc electrode the oxidation reaction
Zn(s) → Zn²⁺(aq) + 2e⁻
occurs, while at the copper electrode the reduction reaction
Cu²⁺(aq) + 2e⁻ → Cu(s)
takes place. Electrons flow through the external circuit from the more reactive zinc (negative electrode) to the less reactive copper (positive electrode). The difference in electrode potentials of the two metals creates a potential difference, i.e. a voltage, across the cell.
The salt bridge connects the two half‑cells and allows ions to move between the solutions. It supplies cations to the copper half‑cell and anions to the zinc half‑cell, thereby maintaining electrical neutrality and completing the internal circuit.
Thus the cell produces a voltage because of the differing tendencies of the two metals to lose or gain electrons, and the salt bridge keeps the charges balanced so the reaction can continue.
Zn(s) → Zn²⁺(aq) + 2e⁻
occurs, while at the copper electrode the reduction reaction
Cu²⁺(aq) + 2e⁻ → Cu(s)
takes place. Electrons flow through the external circuit from the more reactive zinc (negative electrode) to the less reactive copper (positive electrode). The difference in electrode potentials of the two metals creates a potential difference, i.e. a voltage, across the cell.
The salt bridge connects the two half‑cells and allows ions to move between the solutions. It supplies cations to the copper half‑cell and anions to the zinc half‑cell, thereby maintaining electrical neutrality and completing the internal circuit.
Thus the cell produces a voltage because of the differing tendencies of the two metals to lose or gain electrons, and the salt bridge keeps the charges balanced so the reaction can continue.
Examiner tips
- Use the exact half‑reactions for Zn and Cu, show electron flow, mention reactivity difference, explain salt bridge role and neutrality
- Keep answer concise, use correct terminology (oxidation, reduction, electrode potential, external circuit, internal circuit)
Common mistakes
- Confusing which electrode is positive/negative, or writing the wrong half‑reaction; forgetting to mention ion movement in the salt bridge; not explaining that the salt bridge maintains neutrality
Mark scheme (5 marks)
- Each half-cell involves a different oxidation or reduction reaction (half-reaction) occurring at the electrode
- Electrons are transferred through the external circuit from the more reactive metal (negative electrode) to the less reactive metal (positive electrode)
- The difference in reactivity (or tendency to lose electrons / electrode potential) between the two metals causes the voltage / potential difference
- The salt bridge completes the electrical circuit by allowing ions to move between the two solutions
- The salt bridge maintains electrical neutrality in each half-cell by balancing the build-up of charge
Key terms in this question
electrochemical cell · half-cell · salt bridge
Related
- All AQA A-Level Chemistry (7405) revision notes →
- How to answer a "Explain" question →
- Decode the mark scheme abbreviations →
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