anode's reduction potential

Meaning: The anode is the electrode where oxidation occurs. Tables list every half-reaction as a reduction, so the anode's reduction potential is the table value for the species that is really being oxidized, with the reaction run backward in the cell. In a galvanic cell the species with the lower (more negative) reduction potential is the one that loses electrons, so it becomes the anode. The species with the higher reduction potential is reduced at the cathode.

Equation: E°cell = E°cathode − E°anode, where E°cell is the standard cell potential, and E°cathode and E°anode are the standard reduction potentials of the two half-reactions. All three are in volts (V). A positive E°cell means the cell is spontaneous, because ΔG° = −nFE°cell.

On the MCAT: A passage gives a table of reduction potentials and a cell diagram or two metals. Pick the half-reaction with the lower E°red as the anode, calculate E°cell, and then predict the direction of electron flow (anode to cathode through the wire), which electrode loses mass, and whether the cell is spontaneous. Do not multiply E° by stoichiometric coefficients, because potential is intensive.

Common confusion: Students flip the sign of the anode's potential and then subtract again, which counts the flip twice. Either subtract the two reduction potentials, or add the cathode's reduction potential to the anode's oxidation potential (−E°red). Also, an electrolytic cell still has oxidation at the anode, but there E°cell is negative, so the anode's E°red is the higher of the two.