Cell Potential Formula
Find galvanic/voltaic cell voltage from half-reactions
Standard Cell Potential
E°cell = E°cathode - E°anode
Use tabulated standard reduction potentials (V). Identify which half-reaction is reduction (cathode) and which is oxidation (anode).
Non-Standard Conditions (Nernst)
Ecell = E°cell - (RT/nF) ln Q
At 25 C: Ecell = E°cell - (0.0592/n) log Q
- n = moles of electrons transferred
- Q = reaction quotient (activities or concentrations)
- R = 8.314 J/mol·K, T in K, F = 96485 C/mol
Example
Cell: Zn(s) | Zn2+ (1.0 M) || Cu2+ (1.0 M) | Cu(s)
E°(Cu2+/Cu) = +0.34 V (cathode)
E°(Zn2+/Zn) = -0.76 V (anode)
E°cell = 0.34 - (-0.76) = +1.10 V
Answer: E°cell = +1.10 V (spontaneous)
Common Mistakes
Sign confusion
Do not flip the sign of tabulated reduction potentials when identifying the anode; use reduction values and subtract.
Wrong n value
Balance electrons; n is required for Nernst calculations.
Unit mix-ups
Use log base 10 only with 0.0592/n; use natural log with RT/F.
Ignoring phase/activities
Pure solids/liquids have activity 1; only species in Q with concentration/pressure appear.
FAQ
Does multiplying a half-reaction change E°?
No, potentials do not scale with stoichiometric multiplication.
How to tell spontaneity?
Positive Ecell indicates a spontaneous galvanic cell under stated conditions.
What if concentrations are not 1 M?
Use the Nernst equation with the appropriate Q value.