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A cell with silver and copper

Silver has a higher standard potential than copper, +0.80 V against +0.34 V. Silver therefore becomes the cathode, and copper becomes the anode, even though copper was the cathode when it was paired with zinc. Which electrode is oxidised depends on what it is paired with, not on the metal alone. The difference between the potentials is the cell voltage. The farther apart the metals stand in the electrochemical series, the larger the voltage. Silver ions are reduced to silver, and copper dissolves as copper ions.

E^\circ = E^\circ(\mathrm{Ag}) - E^\circ(\mathrm{Cu})

silver is the cathode

Symbols

$\mathrm{Ag}^+/\mathrm{Ag}$	+0.80	V
$\mathrm{Cu}^{2+}/\mathrm{Cu}$	+0.34	V

Example

Silver is the cathode and copper is the anode.

$E^\circ = 0.80 - 0.34 = 0.46$ V.

Replace silver with a metal below copper, and the roles reverse, so copper becomes the cathode.

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Part of Chemistry 2: Electrochemistry.