reduction

Meaning: Reduction is a gain of electrons, so the oxidation number of the species decreases. It cannot happen alone: some other species must be oxidized and supply the electrons. The species that is reduced is the oxidizing agent, and the species that is oxidized is the reducing agent. A more positive standard reduction potential (E°) means a species is more easily reduced. Reduction occurs at the cathode in every electrochemical cell.

Equation: ΔG° = −nFE°cell, where ΔG° is the standard free energy change (J/mol), n is moles of electrons transferred (mol), F is the Faraday constant, 96,485 C/mol, and E°cell = E°(cathode) − E°(anode), using reduction potentials (V). A positive E°cell means a spontaneous reaction.

On the MCAT: You may be asked to assign oxidation numbers to find which element is reduced, to name the cathode, or to use a table of reduction potentials to predict which species is reduced and which is oxidized. Electrolysis and plating passages ask which ion gains electrons at the cathode. In biochemistry, NAD⁺ → NADH is a reduction.

Common confusion: The word sounds like losing something, but electrons are gained, which is why the charge or oxidation number goes down. Also, the reducing agent is the species that is oxidized, not the one reduced. The cathode is positive in a galvanic cell and negative in an electrolytic cell, but reduction occurs there in both.