Explain what happens at each electrode when a voltaic cell is constructed using a magnesium half-cell (Mg²⁺/Mg, E° = −2.37 V) and a silver half-cell (Ag⁺/Ag, E° = +0.80 V), including identification of the anode and cathode and the direction of electron flow in the external circuit.
Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).
Model answer (4 marks)
Magnesium is the anode – oxidation occurs:
Mg(s) → Mg²⁺(aq) + 2e⁻
Silver is the cathode – reduction occurs:
Ag⁺(aq) + e⁻ → Ag(s)
Electrons leave the magnesium electrode, travel through the external circuit and enter the silver electrode.
The Mg electrode has the more negative E° (−2.37 V) so it is the stronger reducing agent and is oxidised, driving the spontaneous cell reaction.
Mg(s) → Mg²⁺(aq) + 2e⁻
Silver is the cathode – reduction occurs:
Ag⁺(aq) + e⁻ → Ag(s)
Electrons leave the magnesium electrode, travel through the external circuit and enter the silver electrode.
The Mg electrode has the more negative E° (−2.37 V) so it is the stronger reducing agent and is oxidised, driving the spontaneous cell reaction.
Examiner tips
- State the half‑reactions and identify anode/cathode
- Show electron flow direction
- Explain why Mg is the anode using E° values
Common mistakes
- Confusing the direction of electron flow
- Calling the silver electrode the anode
- Ignoring the sign of the standard potentials
Mark scheme (4 marks)
- Magnesium electrode is the anode where oxidation occurs (Mg → Mg²⁺ + 2e⁻)
- Silver electrode is the cathode where reduction occurs (Ag⁺ + e⁻ → Ag)
- Electrons flow through the external circuit from the magnesium electrode (anode) to the silver electrode (cathode)
- The magnesium electrode has the more negative (lower) standard electrode potential, so it has the greater tendency to be oxidised / acts as the stronger reducing agent, driving the spontaneous reaction
Key terms in this question
anode · cathode · electron flow · voltaic cell
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 why zinc displaces copper from a solution of copper(II) sulfate, with re…
- 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…