A student constructs an electrochemical cell using a silver half-cell (Ag⁺(aq)/Ag(s)) and a nickel half-cell (Ni²⁺(aq)/Ni(s)) connected by a salt bridge. The standard electrode potential of the silver half-cell is +0.80 V and that of the nickel half-cell is −0.25 V. Explain how the cell produces a voltage, identifying which electrode acts as the anode and which acts as the cathode, and describe the role of the salt bridge.
Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).
Standard electrode potentials: Ag⁺(aq) + e⁻ → Ag(s), E° = +0.80 V; Ni²⁺(aq) + 2e⁻ → Ni(s), E° = −0.25 V
Model answer (5 marks)
The nickel electrode is the anode because Ni is oxidised:
Ni(s) → Ni²⁺(aq) + 2e⁻
The silver electrode is the cathode because Ag⁺ is reduced:
Ag⁺(aq) + e⁻ → Ag(s)
The cell generates a voltage because the two half‑cells have different standard potentials; the more negative Ni²⁺/Ni half‑cell drives electrons to the more positive Ag⁺/Ag half‑cell. Electrons flow through the external circuit from the nickel anode to the silver cathode.
The salt bridge completes the circuit by allowing ions to move between the two half‑cells, keeping each half‑cell electrically neutral as the reaction proceeds.
Ni(s) → Ni²⁺(aq) + 2e⁻
The silver electrode is the cathode because Ag⁺ is reduced:
Ag⁺(aq) + e⁻ → Ag(s)
The cell generates a voltage because the two half‑cells have different standard potentials; the more negative Ni²⁺/Ni half‑cell drives electrons to the more positive Ag⁺/Ag half‑cell. Electrons flow through the external circuit from the nickel anode to the silver cathode.
The salt bridge completes the circuit by allowing ions to move between the two half‑cells, keeping each half‑cell electrically neutral as the reaction proceeds.
Examiner tips
- Show the half‑cell reactions and identify anode/cathode using E° values
- Explain the direction of electron flow and the role of the salt bridge
- Use correct terminology: anode, cathode, oxidation, reduction, EMF
Common mistakes
- Confusing which electrode is anode/cathode by using the sign of E° alone
- Forgetting to mention ion flow in the salt bridge
- Using the wrong half‑cell reaction (e.g. writing reduction at the anode)
Mark scheme (5 marks)
- The nickel electrode is the anode (negative electrode) because nickel is oxidised / loses electrons
- The silver electrode is the cathode (positive electrode) because silver ions are reduced / gain electrons
- The cell produces a voltage / EMF because there is a difference in electrode potentials between the two half-cells (accept: the more negative half-cell drives electrons towards the more positive half-cell)
- Electrons flow through the external circuit from the nickel (anode) to the silver (cathode)
- The salt bridge completes the circuit by allowing ions to flow between the two half-cells, maintaining electrical neutrality in each half-cell
Key terms in this question
standard electrode potential · anode · cathode · salt bridge
Related
- All Edexcel A-Level Chemistry (9CH0) revision notes →
- How to answer a "Explain" question →
- Decode the mark scheme abbreviations →
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