cathode

Meaning: The cathode is the electrode at which reduction takes place, meaning a species gains electrons. Electrons move through the external wire toward the cathode, and cations in solution migrate toward it. In a galvanic (voltaic) cell, where the reaction is spontaneous, the cathode is the positive electrode. In an electrolytic cell, where an external power source drives a nonspontaneous reaction, the cathode is the negative electrode, because the source pushes electrons onto it. When metal ions are reduced and plate out, the cathode gains mass.

Equation: E°cell = E°cathode − E°anode, where E°cell is the standard cell potential, and E°cathode and E°anode are the standard reduction potentials of the two half-reactions, all in volts (V). A positive E°cell means a spontaneous galvanic cell.

On the MCAT: A passage gives a table of standard reduction potentials and a cell diagram. You identify the cathode (in a galvanic cell, the half-cell with the more positive reduction potential), write its half-reaction, and predict the direction of electron flow, which electrode gains mass, and where salt-bridge cations travel. Electrolysis and plating questions ask which species is reduced at the cathode, sometimes with a Faraday's-law calculation.

Common confusion: Students memorize "cathode = positive" and then miss electrolytic cells. The sign depends on the cell type, but the definition does not: reduction always occurs at the cathode. Use "red cat" and "an ox" (anode, oxidation) to anchor it.