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In a Daniell cell, $\mathrm{Zn} | \mathrm{Zn}^{2+}(aq) \,||\, \mathrm{Cu}^{2+}(aq) | \mathrm{Cu}$, the standard EMF is calculated as:
A$E^\circ_{\mathrm{Cu}^{2+}/\mathrm{Cu}} + E^\circ_{\mathrm{Zn}^{2+}/\mathrm{Zn}}$
B$E^\circ_{\mathrm{Zn}^{2+}/\mathrm{Zn}} - E^\circ_{\mathrm{Cu}^{2+}/\mathrm{Cu}}$
C$E^\circ_{\mathrm{Cu}^{2+}/\mathrm{Cu}} - E^\circ_{\mathrm{Zn}^{2+}/\mathrm{Zn}}$
D$\dfrac{1}{2}\left(E^\circ_{\mathrm{Cu}^{2+}/\mathrm{Cu}} + E^\circ_{\mathrm{Zn}^{2+}/\mathrm{Zn}}\right)$
Answer & Solution
Correct answer: C. $E^\circ_{\mathrm{Cu}^{2+}/\mathrm{Cu}} - E^\circ_{\mathrm{Zn}^{2+}/\mathrm{Zn}}$
$E^\circ_\text{cell} = E^\circ_\text{cathode} - E^\circ_\text{anode}$ where both standard electrode potentials are written as reduction potentials. Copper is the cathode (reduction $\mathrm{Cu}^{2+} + 2e^- \to \mathrm{Cu}$) and zinc is the anode in a Daniell cell.
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