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What happens at one electrode of a concentration cell?

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Chemical and Physical Foundations

A concentration cell contains two M(s)/M2+(aq)M(s)/M^{2+}(aq) half-cells with ion concentrations of 0.12 M0.12\ \mathrm{M} and 0.72 M0.72\ \mathrm{M}. Which of the following occurs at the metal electrode in the half-cell with the lower M2+M^{2+} concentration as the cell operates?

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Answer: D (Oxidation occurs, and the electrode mass decreases.)

In a concentration cell, the reaction proceeds in the direction that decreases the difference in ion concentration between the half-cells. The half-cell with the lower M2+M^{2+} concentration is the anode, where oxidation produces additional ions: M(s)M(s) →M2+(aq)+2e−{}\rightarrow M^{2+}(aq)+2e^-. Because atoms from the metal electrode enter the solution as ions, the electrode mass decreases.

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