Electrolysis is used industrially in two particular circumstances.
- For the preparation of reactive metals. If a metal is above carbon in the reactivity series it cannot be made from its ore by reduction with carbon. Electrolysis of the molten ore is often the best option despite being a very high energy process. Aluminium in particular is made in huge quantities by this method.
- Electrolysis can also be used on solutions of ionic compounds. Here the outcome is complicated as you need to consider the potential of water to be electrolysed instead. On a large scale sodium chloride solution is electrolysed to produce chlorine gas at the anode and hydrogen gas at the cathode (which is usually burnt as a fuel). The chlorine has lots of uses particularly in the manufacture of polymers such as PVC. The residual solution is sodium hydroxide which has a range of uses including reacting with the chlorine made in the electrolysis process to make bleach.