ELECTROCHEMISTRY. - In any chemical reaction, from the simplest to the most complex, there always takes place a variation in the spatial distribution of the electrons, the so-called bonding electrons, of one or more atoms or atomic groups of the reacting molecules. Consequently, electrochemistry, taken in the etymological sense, should extend to the entire field of chemical reactions. For convenience of study, however, it has been agreed to assign to electrochemistry a more restricted field: today, therefore, by the name of electrochemistry is meant that part of physical chemistry which studies, and also exploits, the relations between chemical reactions and electrical work drawn from an external circuit independent of the reacting system or supplied to it.
In other words: 1) electrolytic (forced) reactions by means of which, consuming electrical work supplied by an independent external source, a chemical reaction is made to occur which would not occur spontaneously, so as to obtain the desired final products starting from certain initial products. A characteristic of this transformation is that the system formed by the final product or products has a greater energy content than that of the system formed by the initial product or products, and for this reason the transformation cannot occur spontaneously; 2) the reactions taking place spontaneously in galvanic cells (batteries and accumulators), which lead the system formed by the initial product or products to the system formed by the final product or products, which in this case has a lower energy content. The difference between the energy contents of the initial and final systems, by conducting the reaction in a suitable manner, is transformed in part into electrical work delivered and utilized in an independent external circuit. The fraction of this difference that can be transformed into electrical work corresponds to the variation of free energy brought into play during the transformation. In this case the nature of the initial products, as well as of those resulting from the chemical transformation, is of relatively lesser interest.
Electrolytic reactions are today widely exploited industrially for the preparation of a large number of products, generally obtained at a high degree of purity difficult to achieve by other methods: metals, caustic soda, chlorine, hydrogen, oxygen, etc. Some of these products, such as, for example, caustic soda and chlorine, are basic products for the entire chemical industry.
The reactions of galvanic cells, as a consequence of today's development of electrical engineering, have lost very much of their practical importance: their industrial applications remain today confined to the field of accumulators and of so-called dry batteries, the latter used almost exclusively for pocket lamps and for portable radio sets. The reactions of galvanic cells have instead assumed great importance as a means of scientific research. Very many reactions can in fact be conducted in such a way that the variation of free energy brought into play by the chemical reaction can be transformed into electrical energy measurable by very simple means in an external electrical circuit independent of the reacting system. This makes it possible to measure in a very simple manner, availing oneself of thermodynamic relations, many physicochemical data of interest concerning the system and to calculate therefrom others of high theoretical interest: chemical affinity, equilibrium constants, course of reactions, etc.
Finally, the phenomena comprised within the field of electrochemistry are usable (and are also widely used) for solving analytical problems by means of conductivity measurements (conductometric analysis), of potential differences (potentiometric analysis and pH measurement), of current intensity as a function of electrode potential (polarographic and polarometric analysis), by means of electrolysis (electrolytic analysis), etc.
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Giulio Milazzo