Electrolysis

How electrolysis works. To produce electrolysis, two solid electrical conductors, such as metal or graphite rods, are placed into a liquid. The rods are called electrodes. Wires connect the electrodes to the terminals of a battery or to a direct current generator. The liquid must contain a substance, called an electrolyte, that enables it to carry the current and complete the electrical circuit. For example, distilled water cannot be electrolyzed because it does not conduct electricity well. But it can be electrolyzed if a little table salt (sodium chloride), which is an electrolyte, is added to it. The electrodes, the liquid, and the container that holds them make up an electrolytic cell.

The electrode connected to the battery's negative pole is the cathode. It carries electrons from the battery to the electrolytic cell. The electrode connected to the battery's positive pole is the anode. It carries electrons from the electrolytic cell back to the battery.

As the current flows through the electrolytic cell, chemical changes take place at the surfaces of the electrodes. At the cathode, the electrolyzed liquid combines with electrons supplied by the battery. This process is called reduction. At the anode, the liquid gives electrons to the anode. This process is called oxidation.

In the electrolysis of water, the water combines with electrons at the cathode and is reduced to hydrogen gas. At the anode, water gives up electrons and is oxidized to oxygen gas. The volume of the hydrogen produced is always twice the volume of the oxygen produced, because water contains two atoms of hydrogen for each atom of oxygen. The chemical equation for the processes that occur in the electrolysis of water is

In the electrolysis of solutions containing ions (charged atoms) of such metals as copper and silver, the reduction of the metal at the cathode causes the metal to be deposited, or to plate out, on the cathode.

Uses of electrolysis. Electrolysis plays an important part in industry. Sodium metal is produced by the electrolysis of molten sodium chloride. In this process, chlorine gas is produced at the anode. Both sodium metal and chlorine gas have many important industrial and chemical uses. The electrolysis of sodium chloride dissolved in water forms another important chemical, sodium hydroxide (caustic soda).

Magnesium, aluminum, and certain other metals are produced commercially by electrolysis. Aluminum metal is made by electrolysis of alumina dissolved in the molten mineral, cryolite. Copper and other minerals are purified by electrolysis. A bar of impure copper is made the anode and a bar of pure copper is made the cathode. During electrolysis, the impure copper anode is dissolved into copper ions. The pure copper it contains plates out on the cathode. All impurities fall to the bottom of the electrolytic cell and are removed later.

Important chemicals produced commercially by electrolysis include manganese dioxide, hydrogen peroxide, chlorates, and perchlorates. Hydrogen peroxide and perchlorates are used in rocket fuels. Electrolysis is also used to anodize metals to make their surfaces more decorative and resistant to corrosion.

Laws of electrolysis. The English chemist Michael Faraday was one of the first scientists to investigate electrolysis. After many careful experiments and calculations, he stated the following three "laws":

1. The ability of an electric current to cause electrolysis does not depend on the distance between electrodes.

2. The quantity of a substance that is electrolyzed is proportional to the quantity of the electricity used.

3. The quantity of a substance that is electrolyzed is also proportional to the substance's chemical equivalent. The chemical equivalent of a metal is its atomic weight (in grams) divided by its valence.

Faraday found that approximately 96,500 coulombs of electricity are required to electrolyze one chemical equivalent of any metal. For example, the atomic weight of copper is 63.54, and the valence of copper salts is +2. Therefore, the chemical equivalent of a copper salt solution is 31.77 grams. This much copper will plate out when about 96,500 coulombs pass through the solution.

The number of coulombs that flow in each second is measured in units called amperes. Voltage is like an electrical pressure that pushes the coulombs through the circuit. In electrolysis, voltage is just as important as amperage. A certain minimum voltage is needed to produce electrolysis in any given substance. For example, a minimum of 1.23 volts is needed to electrolyze water to hydrogen and oxygen at 25 ºC.

Contributor: Mark S. Wrighton, Ph.D., Ciba-Geigy Prof. of Chemistry, Massachusetts Institute of Technology.

Related articles include:

Electric Current; Electricity; Electrode; Electrolyte; Faraday, Michael; Ion

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