Standard cell potential
The standard cell potential is E°cell = E°cathode − E°anode, using standard reduction potentials in volts. A positive E°cell means the reaction is spontaneous. ΔG° = −nFE°, where F = 96,485 C/mol.
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Analogy
Voltage is the push electrons feel, like water pressure behind a tap.
Example
E°(Cu²⁺/Cu) = +0.34 VE°(Zn²⁺/Zn) = −0.76 VE°cell = 0.34 − (−0.76)= +1.10 V
In practice
Reduction potential tables rank metals: those with very negative values, like lithium, make powerful battery anodes.
Practice
Flashcards
- How do you calculate E°cell?E°cathode − E°anode.
Interview questions
- Describe a Zn/Cu galvanic cell, label the electrodes, and calculate E°cell.
- Zn anode: oxidation
- Cu cathode: reduction
- Electrons flow from Zn to Cu
- E°cell = 0.34 − (−0.76) = 1.10 V
Questions people ask
Do I multiply E° by the number of electrons?
No. Cell potential is an intensive property, like temperature, so it does not change when you multiply a half-reaction by a number. Only the free energy change depends on how many electrons move: ΔG° = −nFE°. Never scale E° when balancing.
How do I know which half-cell is the cathode?
The half-cell with the higher (more positive) reduction potential is the cathode, because it is more strongly pulled toward being reduced. The other becomes the anode and runs in reverse as an oxidation. That choice always gives a positive E°cell for a spontaneous cell, which is a quick way to check your work.