# 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.

> IB DP Chemistry Higher Level (2023 syllabus) — R3.2 Electron transfer reactions · Explain · 4 marks

> 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.

## Mark scheme (4 marks)

1. As the cell operates, the concentration of Zn²⁺ increases and the concentration of Cu²⁺ decreases, so conditions are no longer standard.
2. 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.
3. When E°cell reaches zero the cell is at equilibrium, so no net current flows.
4. At equilibrium (E = 0), Q = K, so the ratio [Zn²⁺]/[Cu²⁺] equals the equilibrium constant K for the cell reaction.

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