Electrolysis

Electrolysis is the process of causing a chemical reaction to occur by passing an electric current through a substance or mixture of substances, most often in liquid form. Electrolysis frequently results in the decomposition of a compound into its elements. To carry out an electrolysis, two electrodes, a positive electrode (anode) and a negative electrode (cathode), are immersed into the material to be electrolyzed and connected to a source of direct (DC) electric current.

The apparatus in which electrolysis is carried out is called an electrolytic cell. The roots -lys and -lyt come from the Greek lysis and lytos, meaning to cut or decompose; electrolysis in an electrolytic cell is a process that can decompose a substance.

The substance being electrolyzed must be an electrolyte, a liquid that contains positive and negative ions and therefore is able to conduct electricity. There are two kinds of electrolytes. One kind is a ion compound solution of any compound that produces ions when it dissolves in water, such as an inorganic acid, base, or salt. The other kind is a liquefied ionic compound such as a molten salt.

In either kind of electrolyte, the liquid conducts electricity because its positive and negative ions are free to move toward the electrodes of opposite charge—the positive ions toward the cathode and the negative ions toward the anode. This transfer of positive charge in one direction and negative charge in the opposite direction constitutes an electric current, because an electric current is, after all, only a flow of charge, and it does not matter whether the carriers of the charge are ions or electrons. In an ionic solid such as sodium chloride, for example, the normally fixed-in-place ions become free to move as soon as the solid is dissolved in water or as soon as it is melted.

During electrolysis, the ions move toward the electrodes of opposite charge. When they reach their respective electrodes, they undergo chemical oxidation-reduction reactions. At the cathode, which is pumping electrons into the electrolyte, chemical reduction takes place-a taking-on of electrons by the positive ions. At the anode, which is sucking electrons out of the electrolyte, chemical oxidation takes place-a loss of electrons by the negative ions.

In electrolysis, there is a direct relationship between the amount of electricity that flows through the cell and the amount of chemical reaction that takes place. The more electrons are pumped through the electrolyte by the battery, the more ions will be forced to give up or take on electrons, thereby being oxidized or reduced. To produce one mole's worth of chemical reaction, one mole of electrons must pass through the cell. A mole of electrons, that is, 6.02 × 10²³ of electrons, is called a faraday. The unit is named after Michael Faraday (1791-1867), the English chemist and physicist who discovered this relationship between electricity and chemical change. He is also credited with inventing the words anode, cathode, electrode, electrolyte, and electrolysis.

Various kinds of electrolytic cells can be devised to accomplish specific chemical objectives.