ELECTROCHEMISTRY. — In any chemical reaction, from the simplest to the most complex, there always occurs a variation in the spatial distribution of the so-called bonding electrons of one or more atoms or atomic groups of the reacting molecules. Consequently, electrochemistry, understood in its etymological sense, should extend over the entire field of chemical reactions. For the sake of convenience in study, however, it has been agreed to assign electrochemistry a more restricted field: today, therefore, the name electrochemistry refers to that branch of physical chemistry which studies, and also exploits, the relations between chemical reactions and electrical work drawn from, or delivered to, an external circuit independent of the reacting system.
In other words: 1) electrolytic (forced) reactions, by means of which, through the consumption of electrical work supplied by an independent external source, a chemical reaction that would not occur spontaneously is made to take place, so as to obtain the desired final products 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 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 elements (cells and accumulators), which convert the system formed by the initial product or products into 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, when the reaction is conducted in a suitable manner, is partly converted into electrical work delivered to and used in an independent external circuit. The fraction of this difference that can be converted into electrical work corresponds to the variation in free energy involved during the transformation. In this case, the nature of the initial products, as well as that 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 with a high degree of purity that is difficult to achieve by other methods: metals, caustic soda, chlorine, hydrogen, oxygen, etc. Some of these products, such as caustic soda and chlorine, are basic products for the entire chemical industry.
The reactions of galvanic elements, as a consequence of the development of electrical engineering in modern times, have lost much of their practical importance: their industrial applications today remain confined to the field of accumulators and so-called dry cells, the latter being used almost exclusively for pocket flashlights and portable radio sets. The reactions of galvanic elements, on the other hand, have assumed great importance as a means of scientific research. Indeed, very many reactions can be conducted in such a way that the variation in free energy involved in the chemical reaction can be converted 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 and by using thermodynamic relationships, many physicochemical data concerning the system, and consequently to calculate others of high theoretical interest: chemical affinity, equilibrium constants, the course of reactions, etc.
Finally, the phenomena included within the field of electrochemistry are
usable (and are also widely used) for solving analytical problems by means of measurements of conductivity (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.