voltaic cells

Meaning: A voltaic (galvanic) cell splits a spontaneous redox reaction into two half-cells joined by a wire and a salt bridge. Oxidation occurs at the anode (the negative electrode) and reduction at the cathode (the positive electrode). Electrons travel through the wire from anode to cathode. The salt bridge lets ions migrate so each half-cell stays neutral: anions move toward the anode, cations toward the cathode. A metal anode loses mass, and the cathode often gains mass as metal plates onto it. The voltage falls to zero at equilibrium.

Equation: E°cell = E°cathode − E°anode, and ΔG° = −nFE°cell, where E° is the standard reduction potential (V), n is moles of electrons transferred (mol), F is the Faraday constant, about 96,500 C/mol, and ΔG° is in J/mol.

On the MCAT: A passage gives a table of standard reduction potentials and a cell diagram. You pick the cathode (the more positive E°), calculate E°cell, and give the direction of electron flow, the salt bridge ion movement, or which electrode gains mass. The Nernst equation, E = E° − (RT/nF) ln Q, predicts how changing concentrations shifts the voltage.

Common confusion: Students reverse the electrode signs when switching between cell types. Oxidation is always at the anode and reduction always at the cathode. In a voltaic cell the anode is negative; in an electrolytic cell it is positive. Also, E° is an intensive property, so you do not multiply it by stoichiometric coefficients when balancing half-reactions.