Electrochemistry: laws governing processes

In the article "Electrochemistry. Preliminaries to the study of electrolysis". that an electrochemical process involves two major phenomena:

• the electrochemical reaction taking place at the electrode surface, resulting from the transfer of electrical charge across the interface between the electrode and the electrolyte (in either direction);

• transport of the matter involved in the reaction, either towards the interface in the case of chemical species consumed by the reaction, or from the interface in the case of species produced by the reaction: this is the phenomenon of diffusion, which may be convective if it occurs in a moving liquid medium. But in the case of ions, this diffusional transport phenomenon is combined with general ionic transport, which, under the influence of the electric field established in all the phases present, ensures charge transport within them, i.e. the passage of electric current through the electrolyte(s) of the electrolysis cell.

These various phenomena are governed by kinetic laws (the thermodynamic laws governing steady-state electrode phenomena - no matter transport in this case - were described in the article ) which are the subject of this article, and whose knowledge is necessary for understanding the morphology of the current-potential characteristics on which the design of electrochemical processes is based.

It's worth remembering that the intensity of the current flowing through an electrolytic cell (or the current density calculated for each electrode interface) is an experimentally accessible measure of the overall speed of the processes involved, both at each electrode surface and within the electrolyte.

We'll start by describing the laws of matter transport (electromigration of ions under the effect of an electric field, pure or convective diffusion of electroactive species at the electrodes and, more generally, their participation in electrochemical reactions), before moving on to the highly complex phenomena occurring at the surface of electrodes (or in their immediate vicinity).