Electric Occurrence: Electrolysis
Peter Joshua Atkins · Reactions · 2011
Abstract Electrolysis makes use of electric currents, a stream of electrons, to bring about chemical change. It puts electricity to work by using it to break or form bonds by forcing electrons on to molecules or sucking electrons out of them. Electrolysis is an application of the redox processes I described in Reaction 5, where I showed that reduction is the gain of electrons and that oxidation is their loss. All that happens in electrolysis is the use of an external supply of electrons from a battery or other direct-current (DC) source to push them on to a species and so bring about its reduction, or the use of the electron-sucking power of a battery to remove them from a species to bring about its oxidation. Electrolysis, in other words, is electrically driven reduction and oxidation. In fact, the process is rather broader than just forcing species to accept or give up electrons because, as I have hinted, molecules might respond to the change in their number of electrons by discarding or rearranging atoms. For instance, when water is electrolysed, the H–O bonds of the H2O molecules are broken and hydrogen and oxygen gases are formed. When an electric current is passed through molten common salt (sodium chloride, NaCl), metallic sodium and gaseous chlorine, Cl2, are formed. Electrolysis is a major technology in the chemical industry, for among other applications it is used to make chlorine, to purify copper, and to extract aluminium. To bring about electrolysis, two metal or graphite rods, the ‘electrodes’, are inserted into the molten substance or solution and connected to a DC electrical supply. The electrons that form the electric current enter the substance through one electrode (the ‘cathode’) and leave it through the other electrode (the ‘anode’). A molecule or ion close to the cathode is forced to collect one or more electrons from that electrode and be reduced. A molecule or ion close to the anode is forced to release them to that electrode and thereby become oxidized. A reasonably simple first example is the purification of copper.