electrochemical cell's potential

Meaning: The cell potential is the voltage between the two electrodes, the driving force pushing electrons through the external circuit from the anode (oxidation) to the cathode (reduction). It is found from the standard reduction potentials of the two half-reactions. A positive E°cell means a galvanic cell, with a reaction that is spontaneous as written. A negative value means the reaction is nonspontaneous and needs an applied voltage, as in an electrolytic cell.

Equation: E°cell = E°cathode − E°anode, and ΔG° = −nFE°cell, where E is in volts (V = J/C), n is the moles of electrons transferred (mol), F is the Faraday constant, about 96,500 C/mol, and ΔG° is in J/mol.

On the MCAT: A passage gives a table of standard reduction potentials. You identify the cathode (the more positive E°), calculate E°cell, and decide whether the cell is spontaneous. For nonstandard concentrations, use the Nernst equation, E = E° − (RT/nF) ln Q. A higher reactant concentration or a lower product concentration lowers Q and raises E. At equilibrium Q = K and E = 0, like a dead battery.

Common confusion: Students multiply E° by the coefficients when they scale a half-reaction. Potential is intensive (a voltage per electron), so it does not change. Only n changes, and that scales ΔG°. Also, always subtract reduction potentials; do not flip a sign and then subtract again.