Galvanic cells, activity chemical potential

Equation (13-33) is the fundamental equation for the thermodynamic study of galvanic cells. The chemical potential of component i in phase a is related to the corresponding activity by the equation... 

Galvanic cells in which stored chemicals can be reacted on demand to produce an electric current are termed primaiy cells. The discharging reac tion is irreversible and the contents, once exhausted, must be replaced or the cell discarded. Examples are the dry cells that activate small appliances. In some galvanic cells (called secondaiy cells), however, the reaction is reversible that is, application of an elec trical potential across the electrodes in the opposite direc tion will restore the reactants to their high-enthalpy state. Examples are rechargeable batteries for household appliances, automobiles, and many industrial applications. Electrolytic cells are the reactors upon which the electrochemical process, elec troplating, and electrowinning industries are based. 

Concentration cells are a useful example demonstrating the difference between galvanic cells with and without transfer. These cells consist of chemically identical electrodes, each in a solution with a different activity of potential-determining ions, and are discussed on page 171. 

The arrangement shown in Fig. 34.1 represents a simple galvanic cell where two electrodes serve as the interfaces between a chemical system and an electrical system. For analytical purposes, the magnitude of the potential (voltage) or the current produced by an electrochemical cell is related to the concentration (strictly the activity, a, p. 48) of a particular chemical species. Electrochemical methods offer the following advantages ... 

The general conclusion to be drawn from this specific examples is that solid state galvanic cells with solid electrolytes can be used primarily to measure free energies of reactions. From this, it is often possible to deduce the difference in chemical potentials (or the ratios of activities) of components of the participating phases. 

Now that an electrochemical galvanic cell has been described in details, it is convenient at this moment to expand the thermodynamic of electrochemistiy in terms of chemical energy, which in turn, wiU be converted to electrical energy. The subsequent analytical procedure leads to the derivation of the Nemst equation, which is suitable for determining the cell electric potential when ion activities are less than unity as nonstandard conditions. 

While chemical galvanic colls are formed by a combination of two different types of electrodes, by a combination of two identical half cells which differ only in concentration of the electromotively active substances, the so called concentration cells are obtained. In such colls the electrical energy is generated by the spontaneous transfer of the active substance from a higher to a lower potential level (e. g. by transfer from the more concentrated to the more diluted solution). 

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