Nernst equation

Meaning: The Nernst equation gives the cell potential when conditions are not standard. E° applies at 1 M concentrations and 1 atm; real concentrations shift the voltage through the reaction quotient Q. If Q < 1 (reactant-rich), E is greater than E°. If Q > 1, E is less than E°. At equilibrium Q = K and E = 0, like a dead battery.

Equation: E = E° − (RT/nF) ln Q, which at 25 °C becomes E = E° − (0.0592/n) log Q. E is cell potential (V), E° is standard cell potential (V), R is 8.314 J/(mol·K), T is temperature (K), n is moles of electrons transferred in the balanced reaction (mol), F is the Faraday constant, about 96,500 C/mol, and Q is the reaction quotient (unitless).

On the MCAT: Expect questions asking whether the voltage rises or falls when a concentration changes, or a calculation of E using the 0.0592 form. In a concentration cell both half-cells use the same electrode, so E° = 0 and the voltage comes only from the concentration ratio. The cell runs until the two concentrations equalize.

Common confusion: Students invert Q or drop the minus sign. Q is products over reactants, and the term is subtracted, so adding reactant raises E. Also, E° is fixed for a given reaction and does not change with concentration. Only E changes.