Explain why a cell made from a magnesium half-cell and a silver half-cell produces a greater voltage than a cell made from a magnesium half-cell and a nickel half-cell, and state which electrode acts as the negative electrode in the magnesium–silver cell.
Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).
Standard electrode potentials (E°) relative to the standard hydrogen electrode: Mg²⁺/Mg = −2.37 V; Ni²⁺/Ni = −0.25 V; Ag⁺/Ag = +0.80 V.
Model answer (5 marks)
The voltage of a galvanic cell is the difference between the standard electrode potentials of the cathode and the anode.
1. The magnesium–silver cell uses Mg²⁺/Mg (−2.37 V) and Ag⁺/Ag (+0.80 V).
2. The potential difference is 0.80 V – (−2.37 V) = +3.17 V.
3. The magnesium–nickel cell uses Mg²⁺/Mg (−2.37 V) and Ni²⁺/Ni (−0.25 V).
4. The potential difference is (−0.25 V) – (−2.37 V) = +2.12 V.
5. Because 3.17 V > 2.12 V, the Mg–Ag cell produces a greater voltage.
The more positive electrode (Ag⁺/Ag) is the cathode; the more negative electrode (Mg²⁺/Mg) is the anode and therefore the negative electrode in the Mg–Ag cell.
1. The magnesium–silver cell uses Mg²⁺/Mg (−2.37 V) and Ag⁺/Ag (+0.80 V).
2. The potential difference is 0.80 V – (−2.37 V) = +3.17 V.
3. The magnesium–nickel cell uses Mg²⁺/Mg (−2.37 V) and Ni²⁺/Ni (−0.25 V).
4. The potential difference is (−0.25 V) – (−2.37 V) = +2.12 V.
5. Because 3.17 V > 2.12 V, the Mg–Ag cell produces a greater voltage.
The more positive electrode (Ag⁺/Ag) is the cathode; the more negative electrode (Mg²⁺/Mg) is the anode and therefore the negative electrode in the Mg–Ag cell.
Examiner tips
- Show the calculation of the potential difference for each cell; use the formula E°cell = E°cathode – E°anode.
- Explain that the larger the difference, the higher the cell voltage.
- State clearly which half‑reaction is the anode (Mg) and that it is the negative electrode.
Common mistakes
- Using the wrong sign for the potential difference (adding instead of subtracting).
- Failing to identify the anode as the more negative electrode and therefore the negative electrode.
Mark scheme (5 marks)
- The cell voltage depends on the difference between the two standard electrode potentials
- Magnesium–silver cell has a larger difference in electrode potentials than magnesium–nickel cell
- Silver has a more positive/higher standard electrode potential than nickel
- Magnesium has the more negative/lower standard electrode potential so it acts as the negative electrode
- Magnesium is oxidised / loses electrons / is the anode, so it is the negative electrode
Key terms in this question
Related
- All AQA A-Level Chemistry (7405) revision notes →
- How to answer a "Explain" question →
- Decode the mark scheme abbreviations →
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