The electrochemical cells we have looked at so far have all involved spontaneous redox reactions. In such galvanic cells electron transfer occurs during chemical change. When the electrons are diverted to some kind of device electrical work may be done. Under completely reversible conditions (i.e., if the work is done verrrrrrrrry slowly), all of the energy in the reaction is released as free energy.
But there is another kind of electrochemical process in which electrical work must be done on a chemical mixture in order to make a reaction occur. If the electrical work is stopped, the reaction stops. Such a process is non-spontaneous. The free energy change is positive and the Eo is negative. Cells in which reactions like this take place are called electrolytic.
Although at first such processes might not seem very useful they are sometimes the only way to obtain certain elements. Some very useful commercial and ornamental processes such as plating are also electrolytic in nature. Electrolysis can be roughly divided into three categories.