QUESTION 133 (6 marks)
An unknown metal M forms M²⁺ ions. A galvanic cell uses M/M²⁺ and Cu/Cu²⁺ half-cells under standard conditions. Electrons flow from M to Cu and the cell potential is 1.10 V. Use the supplied reduction potentials.
| Reduction couple | E° / V |
|---|---|
| Mg²⁺/Mg | −2.37 |
| Zn²⁺/Zn | −0.76 |
| Fe²⁺/Fe | −0.44 |
| Cu²⁺/Cu | +0.34 |
(a) Identify M using the measured cell potential. [2 marks]
(b) Write the balanced net ionic equation. [1 marks]
(c) Sketch a labelled cell. Show the electrode polarities, electron flow, both ion solutions and the direction of NO₃⁻ migration from a KNO₃ salt bridge. [3 marks]
Practice marking scheme
Answer
M is zinc: . . Zn is the negative anode, Cu the positive cathode; electrons and salt-bridge anions travel away from and towards the Zn half-cell respectively.
Working
Electron flow identifies M as the anode. Subtract its reduction potential from the copper reduction potential to calculate the cell voltage. The required value is −0.76 V, identifying zinc. Zinc oxidation produces Zn²⁺, so nitrate ions migrate towards the zinc solution to maintain charge balance. Copper ions are reduced at the positive copper electrode.
Marking criteria
- Uses E°cell = 0.34 − E°M to obtain −0.76 V. [1 mark]
- Identifies zinc from the supplied table. [1 mark]
- Writes the balanced Zn/Cu²⁺ net ionic equation with states. [1 mark]
- Draws and labels Zn/Zn²⁺ and Cu/Cu²⁺ half-cells connected by an external circuit and salt bridge. [1 mark]
- Shows negative Zn anode, positive Cu cathode and electron flow Zn to Cu. [1 mark]
- Shows nitrate migration towards the Zn half-cell. [1 mark]
Practice question aligned to the current QCAA syllabus; review the worked solution and marking criteria.
View the QCAA syllabusCompare your working with the guide above.