E°cell = E°cathode − E°anode
The electromotive force (EMF) of an electrochemical cell is the potential difference between the cathode (reduction) and anode (oxidation): E°cell = E°cathode − E°anode. Standard reduction potentials are measured relative to the Standard Hydrogen Electrode (SHE, 0.00 V). A positive E°cell indicates a spontaneous reaction (galvanic cell), while negative means non-spontaneous (electrolytic cell). The relationship to Gibbs energy: ΔG° = −nFE°cell, where n = electrons transferred and F = 96,485 C/mol. Common cells: Daniell cell (Zn|Zn²⁺||Cu²⁺|Cu, E° = 1.10 V), lead-acid battery (Pb|PbSO₄||PbO₂|Pb, E° = 2.05 V per cell). Non-standard conditions require the Nernst equation to adjust E.