half reactions

Meaning: A half reaction is the oxidation or the reduction part of a redox reaction, written alone with electrons shown. Oxidation loses electrons (Zn → Zn²⁺ + 2e⁻); reduction gains them (Cu²⁺ + 2e⁻ → Cu). In an electrochemical cell they occur at separate electrodes: oxidation at the anode, reduction at the cathode. Add the two after multiplying so the electrons cancel.

Equation: E°cell = E°cathode − E°anode, where each E° is the standard reduction potential of a half reaction and E°cell is the standard cell potential, all in volts (V).

On the MCAT: A table of standard reduction potentials is given. The half reaction with the higher E° is the cathode; the other runs in reverse as the anode. You calculate E°cell, and E°cell > 0 means a spontaneous (galvanic) cell. Electrons flow from anode to cathode through the external wire.

Common confusion: Students multiply E° by the coefficient when scaling a half reaction to balance electrons. Potential is an intensive property, so it does not change when you scale; only n, the number of electrons transferred, changes. Reversing a half reaction, however, does flip the sign of its potential.