Electrode potentials and activity. The Nernst equation

The van t Hoff reaction equation expresses the free energy change for a chemical reaction in the form 

the free energy change of a reversible electrode reaction is related to the electrode potential by 

AG and E being the standard free energy change and electrode potential respectively. Relations in Equations (5.9) and (5.10) may be derived simply as follows. Reduction of one mole of M to M requires the passage of n faradays, or a quantity of electricity nF coulombs. A charge nF is thus passed through a potential difference of E volts, so that the electrical work done is nFE joules. This work done by the system, at constant temperature and pressure, is equal to the decrease in free energy of the system, — AG. Hence the equality in Equation (5.9), and, under standard conditions (5.10). 

This is the Nernst equation, in which E eq has been used to emphasize that it is an equilibrium potential referring to the position of dynamic equilibrium between oxidized and reduced forms which is established rapidly at the electrode surface. Only to such a system can this - a thermodynamic equation -be applied. 

The equilibrium at the electrode may be disturbed by making it more oxidizing or reducing by superimposing an external e.m.f. Thus, if a potential, E, is applied such that E E eq some of the oxidized form is reduced until a new equilibrium position is reached where eq = E. Conversely, if J5 eq. some of 

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