Cell Potentials
High-Yield Summary
- Reduction potential measures a species' tendency to gain electrons, defined relative to the standard hydrogen electrode (SHE), set at 0 V.
- More positive E°red = greater tendency to be reduced; less positive/more negative E°red = greater tendency to be oxidized.
- Standard reduction potential (E°red) is measured at 25°C, 1 atm, 1 M — lets you directly compare half-cells.
- Galvanic cell: more-positive-E°red electrode = cathode (spontaneous, ΔG<0). Electrolytic cell: external power forces the more-positive electrode to be the anode instead (nonspontaneous, ΔG>0).
- E°cell = E°cathode − E°anode — using each electrode's reduction potential directly, never scaled by moles of electrons transferred (it's an intensive property).
- Daniell cell: E°cell = (+0.34 V) − (−0.76 V) = +1.10 V, matching its known emf.
Key Terms
- Reduction potential
- A species' tendency (in volts) to gain electrons and be reduced, measured relative to the standard hydrogen electrode.
- Standard hydrogen electrode (SHE)
- Reference electrode arbitrarily assigned a reduction potential of exactly 0 V; all other potentials are reported relative to it.
- Standard reduction potential (E°red)
- Reduction potential measured under standard conditions: 25°C, 1 atm, 1 M concentration for all reactants/products.
- Oxidation potential
- The reverse of a reduction half-reaction, with the sign of its potential flipped.
- Intensive property
- A property (like E°red) that depends only on chemical identity, not on the amount of material present or how the reaction is balanced.
Galvanic vs. Electrolytic: Which Electrode Is Which
| Galvanic cell (spontaneous) | Electrolytic cell (nonspontaneous) |
|---|---|
| More positive E°red → cathode (reduction) | More positive E°red → forced to be anode (oxidation) |
| Less positive E°red → anode (oxidation) | Less positive E°red → forced to be cathode (reduction) |
| ΔG negative, reaction proceeds on its own | ΔG positive, external power source required |
Standard Cell Potential (E°cell)
E°cell = E°cathode − E°anode
- E°cathode = standard reduction potential of the cathode half-cell
- E°anode = standard reduction potential of the anode half-cell (used as-is, not converted to oxidation potential)
- Never multiply either potential by moles of electrons transferred — E°red is intensive, independent of how the half-reaction is balanced.
- Worked example, Daniell cell: Cu²⁺+2e⁻→Cu E°red=+0.34 V (cathode); Zn²⁺+2e⁻→Zn E°red=−0.76 V (anode). E°cell=(+0.34)−(−0.76)=+1.10 V.
Getting an Oxidation Potential from a Reduction Potential
- 1Start with the known reduction half-reaction and its E°red.
- 2Reverse the half-reaction (flip reactants and products).
- 3Flip the sign of the potential — e.g. E°red = +0.34 V becomes oxidation potential −0.34 V.
Common MCAT Trap
- Don't scale E°cell by the number of electrons transferred — a tempting mistake right after balancing a half-reaction, but standard potentials are intensive and never multiplied by n in the E°cell formula (n only appears later, in ΔG=−nFE°cell).
- You don't need to convert the anode's value to an oxidation potential before using E°cell=E°cathode−E°anode — the subtraction already accounts for the reversal.
Quick Recall
What reduction potential is the standard hydrogen electrode assigned?
In a galvanic cell, does the more-positive-E°red electrode become the anode or cathode?
For the Daniell cell (Cu E°red=+0.34 V, Zn E°red=−0.76 V), what is E°cell?