We define the ratio in the last term as the reaction quotient : Q r = ∏ ∏ j a j ν ν j ∏ ∏ i a i ν ν i ≈ ≈ [ Z ] z [ Y ] y [ A ] a [ B ] b , where the numerator is a product of reaction product activities, a j, each raised to the power of a stoichiometric coefficient, ν j, and the denominator is a similar product of reactant activities. All activities refer to a time t. Under certain circumstances (see chemical equilibrium) each activity term such as a j may be replaced by a concentration term, [A].In an electrochemical cell, the cell potential E is the chemical potential available from redox reactions (E = ⁠ μ c / e ⁠). E is related to the Gibbs free energy change Δ G only by a constant: Δ G = − zFE, where n is the number of electrons transferred and F is the Faraday constant. There is a negative sign because a spontaneous reaction has a negative Gibbs free energy Δ G and a positive potential E. The Gibbs free energy is related to the entropy by G = H − TS, where H is the enthalpy and T is the temperature of the system. Using these relations, we can now write the change in Gibbs free energy, Δ Δ G = Δ Δ H − − T Δ Δ S = Δ Δ G ⊖ ⊖ + k T ln ⁡ ⁡ Q r , and the cell potential, E = E ⊖ ⊖ − − k T z e ln ⁡ ⁡ Q r .