galvanic cells

Meaning: A galvanic (voltaic) cell converts the energy of a spontaneous redox reaction into electrical work. The two half-reactions occur in separate compartments joined by a wire and a salt bridge. Oxidation occurs at the anode, reduction at the cathode, and electrons flow through the wire from anode to cathode. The salt bridge keeps each half-cell electrically neutral: anions drift toward the anode and cations toward the cathode.

Equation: E°cell = E°cathode − E°anode, and ΔG° = −nFE°cell. E°cell is the standard cell potential (V), n is the moles of electrons transferred (mol), F is Faraday's constant, about 9.65 × 10⁴ C/mol, and ΔG° is the standard free energy change (J/mol).

On the MCAT: You are given a table of standard reduction potentials. Pick the cathode (the half-reaction with the larger, more positive E°), calculate E°cell, and predict the direction of electron flow, the movement of ions in the salt bridge, or which electrode gains or loses mass. A positive E°cell means ΔG° is negative and K is greater than 1.

Common confusion: Students memorize electrode signs and misapply them. The anode is always where oxidation occurs and the cathode always where reduction occurs, but the anode is negative in a galvanic cell and positive in an electrolytic cell. Also, E° is not multiplied by stoichiometric coefficients when you balance the equation.