The potential applied to an electrode sets the energy of its electrons, the Fermi level. Whether electrons flow from the electrode into a redox couple in solution (reduction) or from the couple into the electrode (oxidation) depends on whether the Fermi level lies above or below the energy level of the couple. Work through the three stages in order.
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Predict, then see: the zinc electrode is set to
Or explore freely:
−0.76 V
Choose a couple:
1 M
−0.76 V
Dilute the solution tenfold and watch the redox level: it rises, because Eeq becomes more negative (Nernst).
Set E = Eeq for each couple: the Fermi level and the redox level line up.
Remember
The Fermi level, EF, is the energy of the highest-energy electrons in the electrode.
Potential and electron energy run in opposite directions: EF = −4.44 eV − eE, with E in V vs SHE. Making the potential 1 V more negative raises EF by 1 eV.
A redox couple in solution has its own level, −4.44 eV − eEeq, where Eeq = E° + (RT/nF) ln[Mn+] from the Nernst equation.
EF above the redox level (E more negative than Eeq, η < 0): electrons flow into solution; reduction; cathodic current.
EF below the redox level (E more positive than Eeq, η > 0): electrons flow into the electrode; oxidation; anodic current.
Overpotential: η = E − Eeq.
The absolute potential of the SHE, 4.44 V at 298 K, is the IUPAC recommended value. The redox level is drawn as a single line; in reality it is broadened by the solvent. How fast the electrons cross is the subject of the next lessons.