Electrolysis

This article is about the chemical process. Electrolysis is also a method of depilation.

In chemistry and manufacturing, electrolysis is a method of separating bonded elements and compounds by passing an electric current through them.

An apparatus called a Hoffman voltameter is being used for the electrolysis of water. It is connected to a direct current power source converter.

Overview

In general terms, the process of electrolysis can be described as follows. An ionic compound is dissolved in a solvent (which may be water or some other liquid) or melted by heating. As a result, ions from the compound become available in the liquid form, which is called the electrolyte. A pair of inert electrodes are immersed in the liquid, and an electrical voltage is applied to them. One electrode, called the cathode, becomes negatively charged; the other, called the anode, becomes positively charged.

Each electrode attracts ions of opposite charge. Positively charged ions (cations) move toward the cathode, while negatively charged ions (anions) move toward the anode. The ions are carriers of electric current through the liquid (electrolyte). The cathode supplies electrons to the cations, and the anode takes up electrons from the anions. As a result, the cations and anions are neutralized at the electrodes, producing the desired elements or compounds.

In most cases, some energy is released in the form of heat. In some cases, such as the high-temperature electrolysis of steam to produce hydrogen and oxygen, heat is absorbed from the surroundings.

A technique called gel electrophoresis uses the process of electrolysis to separate molecules of different sizes. For instance, if a sample contains a mixture of DNA strands of different lengths, the DNA is taken in an aqueous solution and made to pass through a porous gel, under the influence of an electric field. The DNA strands, being negatively charged, move toward the anode a

where the solvent is a gel: it is used to separate substances, such as DNA strands, based on their electrical charge.

Electrochemical cells, including the hydrogen fuel cell, use the reverse of this process.

Electrolysis of water

File:Hoffman voltameter.jpg

Diagram of a Hoffman voltameter used for the electrolysis of water to produce hydrogen and oxygen gases.

One important use of electrolysis is to produce hydrogen gas from water. The reaction that occurs is as follows.

2H₂O → 2H₂ + O₂

This has been suggested as a way of shifting society towards using hydrogen as an energy carrier for powering electric motors and internal combustion engines. (See hydrogen economy.) Electrolysis of water can be achieved in a simple hands-on project, where electricity from a battery or low-voltage DC power supply (e.g. computer power supply 5 volt rail) is passed through a cup of water (in practice a saltwater solution or other electrolyte will need to be used otherwise no result will be observed). Using platinum electrodes, hydrogen gas will be seen to bubble up at the cathode, and oxygen will bubble at the anode. Using any other electrode for the anode however, the oxygen will react with the anode instead of being released as a gas. For example using iron electrodes in a sodium chloride solution electrolyte, iron oxide will be produced at the anode, which will react to form iron hydroxide. When producing large quantites of hydrogen, this can significantly contaminate the electrolytic cell - which is why iron is not used for commercial electrolysis.

The energy efficiency of water electrolysis varies widely. Some report 50–70%[1], while others report 80–94%.[2] These values only refer to the efficiency of converting electrical energy into hydrogen's chemical energy. The energy lost in generating the electricity is not included. For instance, when considering a power plant that converts the heat of nuclear reactions into hydrogen via electrolysis, the total efficiency is more like 25–40%.[3]

Experimenters

Scientific pioneers of electrolysis included:

Humphry Davy
Michael Faraday
Paul Héroult
Svante Arrhenius
Adolph Wilhelm Hermann Kolbe

More recently, electrolysis of heavy water was performed by Fleischmann and Pons in their famous experiment, resulting in anomalous heat generation and the controversial claim of cold fusion.

Laws of electrolysis

Faraday's law of electrolysis states that:

The mass of a substance produced at an electrode during electrolysis is proportional to the number of moles of electrons (the quantity of electricity) transferred at that electrode
The number of Faradays of electric charge required to discharge one mole of substance at an electrode is equal to the number of "excess" elementary charges on that ion

These two statements are often considered as separate laws: Faraday's 1st and 2nd laws of electrolysis. 7777777777777

First law of electrolysis

In 1832, Michael Faraday reported that the quantity of elements separated by passing an electrical current through a molten or dissolved salt was proportional to the quantity of electric charge passed through the circuit. This became the basis of the first law of electrolysis.

Second law of electrolysis

Faraday also discovered that the mass of the resulting separated elements was directly proportional to the atomic masses of the elements when an appropriate integral divisor was applied. This provided strong evidence that discrete particles of electricity existed as parts of the atoms of elements.

Industrial uses

Manufacture of aluminum, lithium, sodium, potassium, aspirin.
Manufacture of hydrogen for hydrogen cars and fuel cells.
High-temperature electrolysis is also being used for this.
Coulometric techniques can be used to determine the amount of matter transformed during electrolysis by measuring the amount of electricity required to perform the electrolysis.
Manufacture of chlorine and sodium hydroxide.
Manufacture of sodium and potassium chlorate.
Manufacture of perfluorinated organic compounds like trifluoroacetic acid.

Military uses

As well as producing hydrogen, electrolysis also produces oxygen. Nuclear submarines are able to generate breathing oxygen from the water around them, so can remain underwater for as long as their fuel lasts.

Space stations can also use electrolysis to produce amounts of extra oxygen from waste water or surplus water produced from the Space Shuttle fuel cells.

Both these applications depend on having an abundant electrical supply, from either the reactor or solar panels.

Examples

Electrolysis of an aqueous solution of table salt (NaCl, or sodium chloride) produces aqueous sodium hydroxide and chlorine, although usually only in minute amounts. NaCl(aq) can be reliably electrolysed to produce hydrogen. In order to produce chlorine commercially, molten sodium chloride is electrolysed to produce sodium metal and chlorine gas. These will react violently, so a mercury cell is used to ensure they do not come into contact with each other.

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