electrochemical cell

Meaning: An electrochemical cell separates an oxidation half-reaction from a reduction half-reaction so electrons travel through an external wire. Electrons flow from anode to cathode. A salt bridge lets ions migrate to keep each half-cell neutral: anions move toward the anode, cations toward the cathode. Galvanic (voltaic) cells are spontaneous, with ΔG < 0 and E°cell > 0. Electrolytic cells need an applied voltage to force a nonspontaneous reaction.

Equation: ΔG° = −nFE°cell, where E°cell = E°cathode − E°anode (V), n = moles of electrons transferred (mol), F = 96,485 C/mol, and ΔG° is in J/mol.

On the MCAT: Given a table of standard reduction potentials, identify the anode and cathode, calculate E°cell, and predict electron flow, salt-bridge ion movement, or which electrode gains mass by plating. Electrolysis questions ask for moles of product from current and time (mol e⁻ = It/F). Nernst equation questions ask how concentration changes shift the voltage.

Common confusion: Students assume the anode is negative. That is true in a galvanic cell, but in an electrolytic cell the anode is positive. The definition that holds in both is that oxidation occurs at the anode. Also, do not multiply E° by stoichiometric coefficients; potential does not scale with the amount of reaction.