A student constructs an electrochemical cell using a standard chromium half-cell (Cr³⁺(aq)/Cr(s), E° = −0.74 V) and a standard tin half-cell (Sn²⁺(aq)/Sn(s), E° = −0.14 V), connected by a salt bridge. Explain how the student can determine which electrode is the negative terminal, describe the direction of electron flow in the external circuit, and explain the role of the salt bridge in maintaining the cell's function.
Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).
The standard electrode potentials for the two half-cells are: Cr³⁺(aq) + 3e⁻ → Cr(s), E° = −0.74 V and Sn²⁺(aq) + Sn(s), E° = −0.14 V.
Model answer (5 marks)
The chromium half‑cell has the more negative standard potential (−0.74 V vs. −0.14 V), so it is the negative terminal (anode). At the anode chromium is oxidised:
Cr(s) → Cr³⁺(aq) + 3e⁻
Electrons therefore leave the chromium electrode and travel through the external circuit to the tin electrode. At the tin electrode tin(II) ions are reduced:
Sn²⁺(aq) + 2e⁻ → Sn(s)
The salt bridge supplies ions to keep both half‑cells electrically neutral; anions move to the anode and cations to the cathode, completing the circuit.
Cr(s) → Cr³⁺(aq) + 3e⁻
Electrons therefore leave the chromium electrode and travel through the external circuit to the tin electrode. At the tin electrode tin(II) ions are reduced:
Sn²⁺(aq) + 2e⁻ → Sn(s)
The salt bridge supplies ions to keep both half‑cells electrically neutral; anions move to the anode and cations to the cathode, completing the circuit.
Examiner tips
- State which half‑cell has the lower E° to identify the anode. Show the oxidation and reduction half‑reactions. Explain electron flow direction. Mention ion movement in the salt bridge.
Mark scheme (5 marks)
- The chromium half-cell has the more negative (lower) standard electrode potential, so it is the negative terminal (anode).
- Chromium is oxidised at the negative electrode (anode), so it loses electrons.
- Electrons flow through the external circuit from the chromium electrode to the tin electrode.
- Tin(II) ions are reduced at the tin electrode (cathode), gaining electrons.
- The salt bridge maintains electrical neutrality in both half-cells by allowing ions to move between them, completing the circuit.
Key terms in this question
salt bridge · external circuit
Related
- All Edexcel A-Level Chemistry (9CH0) revision notes →
- How to answer a "Explain" question →
- Decode the mark scheme abbreviations →
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