cell potential

Meaning: Cell potential is the voltage between the two electrodes of an electrochemical cell (1 V = 1 J/C). It shows how strongly the redox reaction pushes electrons from the anode, where oxidation occurs, to the cathode, where reduction occurs. Standard cell potential, E°cell, assumes 1 M solutions, 1 atm gases and usually 25 °C. A positive value means a galvanic cell; a negative value means the reaction is nonspontaneous as written, as in an electrolytic cell.

Equation: E°cell = E°cathode − E°anode, where each term is a standard reduction potential in V. Also ΔG° = −nFE°cell, where n is moles of electrons transferred (mol), F is Faraday's constant (9.65 × 10⁴ C/mol) and ΔG° is in J/mol.

On the MCAT: Given a table of reduction potentials, choose the half-reaction with the higher E° as the cathode, calculate E°cell, then state spontaneity, the sign of ΔG° or whether K is above 1. The Nernst equation, E = E° − (RT/nF) ln Q, predicts that E falls as Q rises and reaches zero at equilibrium.

Common confusion: Students multiply E° by stoichiometric coefficients when balancing electrons. E° is intensive, so it stays unchanged when a half-reaction is doubled; only reversing a half-reaction flips its sign.