The cell potential, also known as the electromotive force (EMF) or voltage, of an electrochemical cell measures the potential difference between the two half-cells of the cell. At equilibrium, the cell potential is zero because there is no net flow of electrons or current across the cell.
In an electrochemical cell, the cell potential is generated by a redox reaction, where electrons are transferred from the species being oxidized (the anode) to the species being reduced (the cathode). This transfer of electrons creates an electric potential difference between the two electrodes, leading to a flow of electrons through an external circuit.
At equilibrium, the concentrations of the reactants and products involved in the redox reaction have reached a state where their rates of oxidation and reduction are equal. As a result, there is no net change in the concentration of these species over time. In other words, the forward and reverse reactions occur at the same rate.
When the rates of oxidation and reduction are balanced, the flow of electrons in the electrochemical cell ceases, and the potential difference between the two electrodes becomes zero. At this point, the system is in dynamic equilibrium, and the cell potential is said to be zero.
It's important to note that equilibrium in an electrochemical cell does not mean that the cell has reached a state of no reaction. Instead, it indicates that the rates of the forward and reverse reactions are equal, resulting in a steady state with no net flow of electrons or current.