AP® Chemistry · Unit 9 · Topic 9.9
Standard Reduction Potentials
A standard reduction potential measures how strongly a half-reaction pulls in electrons compared to every other half-reaction, all referenced to hydrogen at 0.00 V. Subtracting the anode's potential from the cathode's gives the cell's overall voltage.
Every reduction potential needs a common zero point, because you can only ever measure a voltage difference between two electrodes, never the absolute pull of one electrode alone. Chemists picked the standard hydrogen electrode, 2H+ + 2e- → H2, and set its potential to exactly 0.00 V at standard conditions. Every other half-reaction's E deg is really a measurement of how it performs when paired against that hydrogen reference. Once you have that whole table built on the same zero, you can compare any two half-reactions directly without re-measuring them together.
A table of reduction potentials ranks half-reactions by how badly they want electrons. The half-reaction with the higher E deg runs as written, as a reduction, and becomes the cathode. The one with the lower E deg gets forced backward, into oxidation, and becomes the anode. That is the whole logic behind a metal displacing another: a metal higher on the reduction list will pull electrons away from a metal lower down, dissolving the lower metal and plating out the higher one. The cell's standard potential is then E deg cell = E deg cathode - E deg anode, always using the tabulated reduction values for both, even though one is actually being oxidized.
This sets up the free energy connection: a positive E deg cell means a negative delta G, a reaction that runs forward on its own, tying voltage directly to spontaneity. It also explains why potentials never scale with coefficients, since E deg is energy per electron, not total energy, while delta G does scale with the number of moles of electrons transferred.
Key terms
- Standard reduction potential (E deg)
- The voltage of a half-reaction run as a reduction, measured against the hydrogen electrode at standard conditions.
- Standard hydrogen electrode
- The reference half-cell, 2H+ + 2e- → H2, defined to have a potential of exactly 0.00 V.
- Cathode
- The electrode where reduction occurs, taken as the half-reaction with the higher reduction potential.
- Anode
- The electrode where oxidation occurs, taken as the half-reaction with the lower reduction potential, run in reverse.
- Intensive property
- A quantity that does not depend on the amount of substance, so it stays the same when a reaction's coefficients are scaled.
Watch it explained
- How it worksHow does a table of reduction potentials predict which metal will displace which?
- Worked exampleFind the standard cell potential for a zinc-copper cell from the two half-cell potentials.
- Common mistakeWhy isn't the standard potential doubled when you double the coefficients of the half-reaction?
- Going furtherWhat happens when you try to build a cell from two half-reactions with identical potentials?
Check yourself
End of topic.