electromotive force

Meaning: Emf is the energy per coulomb of charge that a redox reaction can deliver, measured as the cell potential when no current flows. It is found from standard reduction potentials: E°cell = E°cathode − E°anode. Reduction occurs at the cathode and oxidation at the anode, and electrons flow through the wire from anode to cathode. A positive emf means a spontaneous (galvanic) cell. A negative emf means the reaction needs an applied voltage, as in an electrolytic cell.

Equation: ΔG° = −nFE°cell, where ΔG° is the standard free energy change (J/mol), n is the moles of electrons transferred per mole of reaction (mol), F is the Faraday constant, about 9.65 × 10⁴ C/mol, and E°cell is the standard emf (V).

On the MCAT: You are given a table of reduction potentials and asked to pick the cathode and anode, calculate E°cell, and decide whether the reaction is spontaneous. Under nonstandard concentrations, the Nernst equation, E = E° − (RT/nF) ln Q, predicts the direction of change: a larger Q lowers the emf.

Common confusion: Students multiply E° by the stoichiometric coefficients when balancing electrons. E° is intensive, so it does not change; only n does. Also, the voltage measured while a cell delivers current is lower than the emf because of internal resistance.