Galvanic Cells

Q1 — Sample Band 6 response (8 marks), annotated

Option A: Zn/Cu cell. Zn has E° = −0.76 V; Cu has E° = +0.34 V. Cu has the more positive E° → cathode. Zn has the more negative E° → anode. E°cell = +0.34 − (−0.76) = +1.10 V per cell. Total voltage (11 cells in series) = 11 × 1.10 = 12.1 V. This meets the 12 V requirement. [2 marks: 1 for correct anode/cathode ID with justification + 1 for correct E°cell and total voltage]

Option B: Fe/Ag cell. Fe has E° = −0.44 V; Ag has E° = +0.80 V. Ag cathode; Fe anode. E°cell = +0.80 − (−0.44) = +1.24 V per cell. Total voltage (8 cells in series) = 8 × 1.24 = 9.92 V. This does not meet the 12 V requirement. [2 marks as above for Option B]

Overall cell equation (Option A chosen): Anode: Zn(s) → Zn²⁺(aq) + 2e⁻. Cathode: Cu²⁺(aq) + 2e⁻ → Cu(s). Electrons balance at 2e⁻ — add directly. Overall: Zn(s) + Cu²⁺(aq) → Zn²⁺(aq) + Cu(s). Charge check: left 0 + 2 = +2; right +2 + 0 = +2. ✓ [2 marks: 1 for balanced half-equations, 1 for correct overall equation with charge check]

Practical limitation (Option A — Zn/Cu): The copper electrode and copper sulfate electrolyte both degrade as the cell operates. Copper metal deposits unevenly on the cathode, increasing internal resistance and reducing efficiency over time. The solution concentration falls, further reducing EMF. In a real engineering backup system, periodic electrolyte replenishment and electrode replacement would be required, adding cost and maintenance complexity. Alternatively for Option B: silver is rare and expensive, making an Fe/Ag system commercially unviable at scale, even if per-cell voltage is higher. [2 marks: 1 for identifying a valid limitation, 1 for explaining it in the context of the engineering application]

Marking criteria.

• 1 mark — Correctly identifies anode and cathode for Option A with E° justification.
• 1 mark — Correctly calculates E°cell for Option A (+1.10 V) and total series voltage (12.1 V), comparing to 12 V requirement.
• 1 mark — Correctly identifies anode and cathode for Option B with E° justification.
• 1 mark — Correctly calculates E°cell for Option B (+1.24 V) and total series voltage (9.92 V), determining it does not meet 12 V.
• 1 mark — Writes balanced oxidation and reduction half-equations for chosen option with electrons shown.
• 1 mark — Writes balanced overall cell equation with charge check.
• 1 mark — Identifies a valid practical limitation of the higher-voltage option.
• 1 mark — Explains that limitation specifically in the context of the Ausgrid/EV engineering application (maintenance, cost, electrode degradation, etc.).

Q2 — Sample Band 6 response (7 marks), annotated

The claim is largely incorrect, containing several significant scientific errors, though it correctly notes that lithium-ion batteries store energy and use a liquid electrolyte rather than a traditional salt bridge. [1 mark — overall evaluative judgement]

What is partially defensible: It is true that lithium-ion batteries function as rechargeable energy storage devices and that their internal ion transport uses a liquid (or gel) electrolyte rather than a KNO₃ salt bridge. This structural difference is real. [1 mark — concedes a correct element]

Refutation 1 — “nothing to do with redox chemistry”: A lithium-ion battery is a galvanic cell during discharge. At the graphite anode, oxidation occurs: LiC₆(s) → Li⁺(aq) + C₆(s) + e⁻ (simplified — lithium is released from graphite). At the LiCoO₂ cathode, reduction occurs: CoO₂(s) + Li⁺(aq) + e⁻ → LiCoO₂(s). This is a spontaneous redox reaction driving electron flow through an external circuit — the defining feature of a galvanic cell. [2 marks: 1 for identifying both oxidation and reduction in discharge; 1 for explicitly linking to galvanic cell definition]

Refutation 2 — E° values do explain voltage differences: The higher cell voltage of a lithium-ion battery (~3.7 V) compared with a zinc-carbon battery (~1.5 V) arises directly from the larger difference in standard reduction potentials between the lithium/graphite anode (highly negative E°, indicating very strong tendency to be oxidised) and the cobalt oxide cathode (more positive E°). E°cell = E°cathode − E°anode applies to all galvanic cells, regardless of their physical design. [1 mark — correct refutation using standard reduction potentials]

Refutation 3 — role of liquid electrolyte: The liquid electrolyte in a lithium-ion battery performs the same function as a salt bridge in a traditional Daniell cell: it allows Li⁺ ions to migrate internally between the two electrodes, maintaining electrical neutrality. Without this ion transport, charge would build up and the cell would stop. The absence of a physical salt bridge tube does not make the cell exempt from the rules of electrochemistry. [1 mark — correct role of electrolyte linked to ion migration / neutrality]

Defensible reformulation: “A lithium-ion battery is a rechargeable galvanic cell in which spontaneous redox reactions during discharge — oxidation at the graphite anode and reduction at the cobalt oxide cathode — drive electron flow through an external circuit. Its higher voltage compared to zinc-carbon batteries is explained by the larger difference in standard reduction potentials between its anode and cathode. The liquid electrolyte performs the same function as a salt bridge, allowing Li⁺ ion migration to maintain electrical neutrality.” [1 mark — scientifically defensible reformulation]

Marking criteria.

• 1 mark — States overall evaluative judgement (e.g. “largely incorrect” or “contains significant scientific errors”).
• 1 mark — Concedes a partially correct element (storage function; structural difference of liquid electrolyte vs salt bridge).
• 1 mark — Identifies oxidation (anode) and reduction (cathode) reactions in a Li-ion cell during discharge (simplified half-equations acceptable).
• 1 mark — Explicitly links Li-ion discharge to the definition of a galvanic cell (spontaneous redox driving electron flow through external circuit).
• 1 mark — Correctly explains that E°cell = E°cathode − E°anode applies to Li-ion cells and explains the higher voltage via a larger E° difference.
• 1 mark — Correctly explains that the liquid electrolyte allows Li⁺ ion migration (equivalent to salt bridge function), maintaining charge neutrality.
• 1 mark — Provides a scientifically defensible reformulation using precise electrochemistry terminology (spontaneous, redox, oxidation, reduction, standard reduction potential, ion migration).