Standard Cell Potential from Half-Cells
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Every half-cell potential in the standard tables is quoted as a reduction potential against the standard hydrogen electrode, arbitrarily pinned at 0.000 V. To assemble a cell, pick which electrode is reduced (the cathode) and subtract the other's tabulated value: the Daniell cell puts Cu²⁺/Cu at +0.34 V against Zn²⁺/Zn at −0.76 V, giving 0.34 − (−0.76) = 1.10 V. A positive result means the cell runs spontaneously as written; a negative one means you have the electrodes backwards.
Two traps catch almost everyone. First, do not flip the sign of the anode value before subtracting — the minus in the formula already does that, and doing it twice cancels the reaction. Second, never multiply a half-cell potential by its stoichiometric coefficient: potential is energy per coulomb, an intensive property, so balancing 2Ag⁺ + Cu → 2Ag + Cu²⁺ leaves silver's +0.80 V untouched and the cell delivers 0.80 − 0.34 = 0.46 V. It is ΔG = −nFE°, not E° itself, that scales with the amount of reaction.
- = Standard cell potential
- = Cathode standard reduction potential
- = Anode standard reduction potential
- Standard cell potential — Nernst Equation, Ohm's Law
- Cathode standard reduction potential — Nernst Equation, Ohm's Law
- Anode standard reduction potential — Nernst Equation, Ohm's Law