redox reactions

Meaning: A redox reaction transfers electrons between species. Oxidation is loss of electrons (oxidation number increases); reduction is gain (oxidation number decreases). Neither occurs alone, because electrons lost by one species are gained by another. The species oxidized is the reducing agent; the species reduced is the oxidizing agent. Balance by splitting into half-reactions, balancing atoms and charge, then equalizing electrons. In any electrochemical cell, oxidation occurs at the anode and reduction at the cathode.

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 is the standard cell potential (V).

On the MCAT: Expect questions that assign oxidation numbers, identify the oxidizing agent, or balance half-reactions. A passage may give standard reduction potentials; E°cell = E°cathode − E°anode, and a positive value means a spontaneous (galvanic) reaction. Biology passages apply the idea to NAD⁺ being reduced to NADH in metabolism.

Common confusion: Students swap the agents. The oxidizing agent is itself reduced, and the reducing agent is itself oxidized. OIL RIG (oxidation is loss, reduction is gain) settles the direction: reduction lowers the oxidation number because gained electrons make it more negative.