8.8 Concentration Cell
We can see the effect of using nonstandard concentrations on the cell potential. Imagine a galvanic cell where both half-cells had identical reactions, but have different concentrations such as
\[\mathrm{Zn}(s) ~ \lvert ~\mathrm{Zn^{2+}}(aq, 0.01~M) ~\rvert\lvert ~ \mathrm{Zn^{2+}}(aq, 1.00~M) ~\rvert ~\mathrm{Zn}(s)\]
This is called a concentration cell. Recognize that the standard cell potential, E°cell, would be 0 V.
We can quickly determine the cell potential using the Nernst equation. At 25 °C,
\[\begin{align*} E_{\mathrm{cell}} &= E_{\mathrm{cell}}^{\circ} - \dfrac{RT}{nF}\ln Q\\ &= (0~\mathrm{V}) - \dfrac{(8.315~\mathrm{J~mol^{-1}~K^{-1}})(298.15~\mathrm{K})}{(2~\mathrm{mol})(96,485~\mathrm{J/V~mol}~e^-)}\ln\dfrac{0.01}{1.0}\\ &= 0.059~\mathrm{V} \end{align*}\]
A simple difference in the concentrations of the solutions allows one to fine tune the produced voltage in a galvanic cell.
Concept Check: What concentrations at the anode and cathode would lead to a nonspontaneous reaction?