electrolytic cells

Meaning: An electrolytic cell uses electrical energy from an external source, such as a battery, to force a nonspontaneous redox reaction. Examples are electrolysis of molten NaCl into Na and Cl₂, and electroplating. E°cell is negative and ΔG is positive, so the applied voltage must exceed the magnitude of E°cell. Oxidation occurs at the anode and reduction at the cathode, and electrons flow through the wire from anode to cathode. The anode is positive and the cathode negative.

Equation: n = It/F, where n is moles of electrons (mol), I is current (A), t is time (s), and F is the Faraday constant, about 9.65 × 10⁴ C/mol. Also ΔG° = −nFE°, with ΔG° in J, n in mol, and E° in V.

On the MCAT: Passages describe plating a metal or splitting a compound. Questions ask which electrode gains mass, what forms at each electrode, or how much product forms. Find moles of electrons from It/F, then use the half-reaction ratio to get moles and grams of product. Doubling the current or the time doubles the product.

Common confusion: Students assume the anode is always negative. Anode and cathode are defined by oxidation and reduction in every cell, so the cathode is always where reduction occurs. Only the sign flips: the galvanic anode is negative, the electrolytic anode is positive. Electrons flow from anode to cathode in both.