where the numerator is a product of reaction product activities, , each raised to the power of a stoichiometric coefficient, , and the denominator is a similar product of reactant activities. All activities refer to a time . Under certain circumstances (see chemical equilibrium) each activity term such as may be replaced by a concentration term, .In an electrochemical cell, the cell potential is the chemical potential available from redox reactions (). is related to the Gibbs energy change only by a constant: