Given that electrochemical half reactions can occur in either direction, they are often written using chemical equilibrium notation* as follows:
$latex O + ne^- \rightleftharpoons R&s=3$
Each half reaction has an associated standard electrode potential (
) which is a thermodynamic quantity related to the free energy associated with the equilibrium. Like many other standard thermodynamic quantities, the standard electrode potential corresponds to a given standard state. The standard state corresponds to a thermodynamic system where the chemical activities of O and R are unity (i.e., when all solution concentrations are
, all gases are present at
partial pressure, and other materials are present as pure phases with unity activity).