Potential, Electrical Work, and Free Energy

Cell Potential, Electrical Work, and Free Energy... 

Relating the free energy change to electrical work and cell potential (701) ... 

Ion-selective electrodes including pH electrodes The transport of ions across either a solid or liquid membrane is, in the case of different ion activities in the solutions on the two sides of the membrane, accompanied by a change in Gibbs free energy, which results in the production of electrical work and therefore the establishment of an internal potential difference, named transmembrane potential,... 

Redox Potential and Free Energy. The concept of redox potential, derived from the above experimental setup, has been an invaluable aid in chemistry. The concept is intimately associated with the free energy of an oxidation-reduction reaction, because the reaction in a galvanic cell is reversible and electric energy is made available for useful work. Thus the redox potential becomes a direct measure of the free energy (cf. Chapt. V-2), except that it is expressed in different units. It must always be remembered, however, that the redox potential invariably refers to the reaction with gaseous hydrogen. That is the zero point of the redox scale. 

We now consider briefly the matter of electrode potentials. The familiar Nemst equation was at one time treated in terms of the solution pressure of the metal in the electrode, but it is better to consider directly the net chemical change accompanying the flow of 1 faraday (7 ), and to equate the electrical work to the free energy change. Thus, for the cell... 

Since the reactant and/or the product are charged species, the fact that the electrical potential in the solution and at the reaction site is not the same leads to the introduction of work terms wr = z.a(P2 for the reactants and w p = (za + n) for products (n = 1 for oxidations, n = — 1 for reductions), which measure the free energy required to bring the reactant and the product, respectively, from the bulk of the solution to the reaction site. 

The papers in the second section deal primarily with the liquid phase itself rather than with its equilibrium vapor. They cover effects of electrolytes on mixed solvents with respect to solubilities, solvation and liquid structure, distribution coefficients, chemical potentials, activity coefficients, work functions, heat capacities, heats of solution, volumes of transfer, free energies of transfer, electrical potentials, conductances, ionization constants, electrostatic theory, osmotic coefficients, acidity functions, viscosities, and related properties and behavior. 

To relate these expressions to Gibbs free energy changes, and thus to ion activities, we recall that AG is equivalent to electrical work under constant-T, P conditions. We therefore integrate dwemf [cf. (3.16), where we used the symbol E for electrical potential O] to obtain... 

An electrochemical cell generates a potential difference E. (The symbol E, commonly used in electrochemistry, refers to electromotive force, an archaic term for potential difference.) The electrical work done when n moles of electrons is passed by the cell can be found using Eq. (15-1), w = -nFE. It can be shown that the electrical work done by an electrochemical cell, at constant temperature and pressure, is equal to the change in Gibbs free energy of the cell components,... 

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