electrolysis

Meaning: An electrolytic cell has a negative E°cell (positive ΔG), so it will not run on its own. An external source pushes electrons toward the cathode, where reduction occurs, and pulls them from the anode, where oxidation occurs. This makes the cathode negative and the anode positive. Cations migrate to the cathode and anions to the anode. Examples are molten NaCl giving Na at the cathode and Cl₂ at the anode, and electroplating a metal onto an object.

Equation: n(product) = It/(zF), where n(product) is the moles of product (mol), I is the current (A), t is the time (s), z is the moles of electrons transferred per mole of product (unitless), and F is the Faraday constant, about 9.65 × 10⁴ C per mole of electrons. Multiply by molar mass (g/mol) to get mass.

On the MCAT: Passages describe plating or splitting a compound and ask for the mass or moles produced from a given current and time. Other questions ask which species is reduced at the cathode (compare reduction potentials), which electrode is positive, or the direction of ion flow. The applied voltage must exceed the magnitude of the negative E°cell.

Common confusion: Students assume the anode is always negative or always positive. In every cell, oxidation is at the anode and reduction at the cathode. Only the charge signs differ: galvanic has an anode (−) and a cathode (+), while electrolytic has an anode (+) and a cathode (−). Electrons flow from anode to cathode through the wire in both.