EMF or Ecell

Meaning: EMF (electromotive force) is the cell potential, the voltage between the two electrodes of a redox cell. Despite the name it is not a force; it is energy per unit charge. A positive Ecell means the reaction is spontaneous as written (a galvanic cell). A negative Ecell means it is nonspontaneous and needs an external source, as in an electrolytic cell.

Equation: E°cell = E°cathode − E°anode, where each E° is a standard reduction potential in volts (V = J/C). It links to free energy by ΔG° = −nFE°cell, where n is moles of electrons transferred (mol), F is 96,485 C/mol and ΔG° is in J/mol.

On the MCAT: A passage gives a table of reduction potentials and asks which half-cell is the cathode, whether Ecell is positive, or whether a reaction is spontaneous. Under nonstandard conditions, use the Nernst equation, E = E° − (RT/nF) ln Q. Raising reactant concentration or lowering product concentration raises E. As the cell runs, Q rises toward Keq. At equilibrium Q = Keq and Ecell = 0, which is a dead battery.

Common confusion: Students multiply E° by the coefficients when they scale a balanced equation. Potential is intensive, so it stays the same. Only n, and therefore ΔG, changes. Also, use reduction potentials for both half-cells and subtract anode from cathode. Do not flip the sign of one and then subtract again.