Explain why the standard cell potential, E°cell, of a galvanic cell decreases to zero as the cell operates under standard conditions, and state what is true of the concentrations of the two half-cell solutions when E°cell reaches zero.
Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).
A galvanic cell is constructed using a standard zinc half-cell (Zn²⁺/Zn) and a standard copper half-cell (Cu²⁺/Cu). The cell is allowed to operate until no further current flows.
Model answer (4 marks)
1. In a galvanic cell the reaction Zn(s)+Cu²⁺(aq)→Zn²⁺(aq)+Cu(s) proceeds spontaneously, giving a positive standard cell potential E°cell.
2. As the cell runs, Zn²⁺ is produced and its concentration rises, while Cu²⁺ is consumed and its concentration falls, so the reaction quotient Q=[Zn²⁺]/[Cu²⁺] becomes greater than 1.
3. The actual cell potential is given by the Nernst equation E=E°cell−(RT/nF)lnQ; as Q increases, the second term grows and E falls below E°cell.
4. When the cell potential reaches zero, the reaction is at equilibrium (E=0), so Q=K and the ratio [Zn²⁺]/[Cu²⁺] equals the equilibrium constant for the cell reaction.
2. As the cell runs, Zn²⁺ is produced and its concentration rises, while Cu²⁺ is consumed and its concentration falls, so the reaction quotient Q=[Zn²⁺]/[Cu²⁺] becomes greater than 1.
3. The actual cell potential is given by the Nernst equation E=E°cell−(RT/nF)lnQ; as Q increases, the second term grows and E falls below E°cell.
4. When the cell potential reaches zero, the reaction is at equilibrium (E=0), so Q=K and the ratio [Zn²⁺]/[Cu²⁺] equals the equilibrium constant for the cell reaction.
Examiner tips
- Use the Nernst equation to link E to Q; show the sign change as Q>1.
- State that E=0 means equilibrium and give the Q=K relationship.
- Mention that concentrations change away from standard values.
Mark scheme (4 marks)
- As the cell operates, the concentration of Zn²⁺ increases and the concentration of Cu²⁺ decreases, so conditions are no longer standard.
- The actual cell potential is governed by the Nernst equation / depends on the reaction quotient Q, so as Q increases above 1 the cell potential falls below E°cell.
- When E°cell reaches zero the cell is at equilibrium, so no net current flows.
- At equilibrium (E = 0), Q = K, so the ratio [Zn²⁺]/[Cu²⁺] equals the equilibrium constant K for the cell reaction.
Related
- All IB DP Chemistry Higher Level (2023 syllabus) revision notes →
- How to answer a "Explain" question →
- Decode the mark scheme abbreviations →
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