ELECTROCHEMISTRY. — In any chemical reaction, from the simplest to the most complex, there is always a change in the spatial distribution of the so-called bonding electrons of one or more atoms or atomic groups of the reacting molecules. Consequently, electrochemistry, taken in its 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 narrower field: today, therefore, under the name of electrochemistry is understood that part of physical chemistry which studies, and also exploits, the relationships between chemical reactions and electrical work drawn from an external circuit independent of the reacting system or supplied to II.
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 take place 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 higher energy content than that of the system formed by the initial product or products, and for this reason the transformation cannot occur spontaneously; 2) spontaneous reactions occurring in galvanic elements (cells and accumulators), which bring the system from the initial products, with a higher energy content, to the final products, which in this case have 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 transformed into electrical work given off and utilized in an independent external circuit. The fraction of this difference which can be transformed into electrical work corresponds to the change in free energy involved in the transformation. In this case, the nature of the initial products, as well as those resulting from the chemical transformation, is of relatively minor interest.
Electrolytic reactions are now widely exploited industrially for the preparation of a large number of products, generally obtained in 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 elements, as a result of the present development of electrical engineering, have lost much of their practical importance: their industrial applications are today confined to the field of accumulators and the so-called dry cells, the latter being used almost exclusively for flashlights and portable radio sets. The reactions of galvanic elements have, however, assumed great importance as a means of scientific research. Indeed, many reactions can be conducted in such a way that the change in free energy involved in 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 way, by using thermodynamic relations, many physicochemical data of interest concerning the system and to calculate others of high theoretical interest: affinity, equilibrium constants, course of reactions, etc.
Finally, the phenomena included in the field of electrochemistry are used (and are also widely used) to solve 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.