For a cell reaction characterized by the chemical equation: at constant temperature and pressure, the thermodynamic voltage (minimum voltage required to drive the reaction) is given by the Nernst equation: where
The cell reaction is generally endothermic: i.e. it will extract heat from its environment.
[citation needed] The Gibbs energy calculation generally assumes an infinite thermal reservoir to maintain a constant temperature, but in a practical case, the reaction will cool the electrode interface and slow the reaction occurring there.
[1] For water at standard temperature (25 C) the net cell reaction may be written: Using Gibbs potentials (
kJ/mol),[2][3] the thermodynamic voltage at standard conditions is Just as the combustion of hydrogen and oxygen generates heat, the reverse reaction generating hydrogen and oxygen will absorb heat.