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Electrolysis of Water

Definition: electrolysis is the passage of a direct electric current through an ion-containing solution (for us, water and electrolyte). Electrolysis produces chemical changes at the electrodes.



When an electric current passes across a solid conductor, a magnetic field is created around the conductor and the conductor is heated by the passage of the current. Both the magnetic field and the heat bear a definite relationship to the magnitude of the current passing; the stronger the current the stronger the magnetic field; the stronger the current the more heat. Some liquids are conductors of electricity; mercury for one. The passage of a current through such a conductor produces identical results with those produced in solid conductors. Other liquids are also conductors, but besides passing of current creating a magnetic field and a heating effect, a portion of the liquid is split into two parts which may each be a chemical element, or one, or either may be a chemical group.

In the image above, two platinum plates are placed as shown, one plate is connected to the positive pole of the battery and the other to the negative. If a strong aqueous solution of hydrochloric acid is added, decomposition of the liquid will take place: hydrogen will be given off at the negative plate or cathode, and chlorine at the positive or anode. If the solution of hydrochloric acid is replaced by one of caustic soda, the caustic soda is split up by the current into oxygen, which is liberated at the anode, positive, and metallic sodium which is deposited on the cathode; but since metallic sodium cannot exist in contact with water, the following reaction takes place at the cathode:  2Na + 2 H 2O = 2NaOH + H2   Thus, by a secondary reaction, hydrogen is liberated at the cathode, or, in other words, water is split into its constituents, while the caustic soda is reformed.

Now, let the caustic soda solution be replaced by an aqueous solution of sulphuric acid. In this case hydrogen will be liberated at the cathode and the group SO4 at the anode, but the group SO4 cannot exist in contact with water, as the following reaction takes place:  2SO4 + 2H2O = 2H2SO4 + O2   Thus by a secondary reaction, oxygen is liberated at the anode, or, in other words, water is split into its constituents while the sulphuric acid is reformed.

Liquids which, under the influence of the electric current, behave in the manner of the above are termed "Electrolytes",

The laws relating to this decomposition of liquids by the electric current were enunciated by Faraday as follows:

  1. The quantity of an electrolyte decomposed is proportional to the quantity of electricity which passes.
  2. The mass of any substance liberated by a given quantity of electricity is proportional to the chemical equivalent weight of the substance.

By the chemical equivalent weight of a substance is meant, in the case of elements, the figure which is obtained by dividing its atomic weight by its valency, which in the case of compounds, it is the molecular weight divided by the valency of the compound. However, many elements have more than one valency, therefore they have more than one chemical equivalent weight.


Electrolysis is a process in which an electric current is passed through a liquid, causing a chemical reaction to take place. If the liquid is water, electrolysis "breaks up" the water into two gases--hydrogen and oxygen. If the liquid is a solution that contains a metal, electrolysis breaks up the solution so that the metal is removed. The electrolysis of metallic solutions is useful in putting metal coatings on objects, and in refining, or purifying, metals.

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 electrolysed because it does not conduct electricity well. But it can be electrolysed 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 electrolysed 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.

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.


The reduction of metals from their compounds by the process of electrolysis.
Examples : 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, aluminium, and certain other metals are produced commercially by electrolysis. Aluminium metal is made by electrolysis of alumina dissolved in the molten mineral, cryolite.

Electro refining

The process of purifing metals extracted by checmical processes by the process of electrolysis. In this anode is made from the impure metal which has to be purified . The cathode is made of sheets of pure metal. The electrolyte is of a compound of the metal being refined. The cathode increases in mass and anode decreases. The impurities either get dissolved in the electrolyte or get deposited at the bottom of the vessel as anode mud.

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 from the anode 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" governing electrolysis:

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

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

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

Faraday found that approximately 96,500 coulombs of electricity are required to electrolyse 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 amount of copper will plate out onto the cathode when 96,500 coulombs are passed 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 electrolyse water to hydrogen and oxygen at 25 C.

Reference:  http://www.beta-theta.com/Chemistry/Electrolysis.html


    Copyright 2003   All rights reserved.   Revised: 04/03/22.                                             Web Author, David Biggs
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