electrolysis

electrolysis

[ih-lek-trol-uh-sis, ee-lek-]
electrolysis, passage of an electric current through a conducting solution or molten salt that is decomposed in the process.

The Electrolytic Process

The electrolytic process requires that an electrolyte, an ionized solution or molten metallic salt, complete an electric circuit between two electrodes. When the electrodes are connected to a source of direct current one, called the cathode, becomes negatively (-) charged while the other, called the anode, becomes positively (+) charged. The positive ions in the electrolyte will move toward the cathode and the negatively charged ions toward the anode. This migration of ions through the electrolyte constitutes the electric current in that part of the circuit. The migration of electrons into the anode, through the wiring and an electric generator, and then back to the cathode constitutes the current in the external circuit.

For example, when electrodes are dipped into a solution of hydrogen chloride (a compound of hydrogen and chlorine) and a current is passed through it, hydrogen gas bubbles off at the cathode and chlorine at the anode. This occurs because hydrogen chloride dissociates (see dissociation) into hydrogen ions (hydrogen atoms that have lost an electron) and chloride ions (chlorine atoms that have gained an electron) when dissolved in water. When the electrodes are connected to a source of direct current, the hydrogen ions are attracted to the cathode, where they each gain an electron, becoming hydrogen atoms again. Hydrogen atoms pair off into hydrogen molecules that bubble off as hydrogen gas. Similarly, chlorine ions are attracted to the anode, where they each give up an electron, become chlorine atoms, join in pairs, and bubble off as chlorine gas.

Commercial Applications of Electrolysis

Various substances are prepared commercially by electrolysis, e.g., chlorine by the electrolysis of a solution of common salt; hydrogen by the electrolysis of water; heavy water (deuterium oxide) for use in nuclear reactors, also by electrolysis of water. A metal such as aluminum is refined by electrolysis. A solution of aluminum oxide in a molten mineral decomposes into pure aluminum at the cathode and into oxygen at the anode. In these examples the electrodes are inert.

Electroplating

In electroplating, the plating metal is generally the anode, and the object to be plated is the cathode. A solution of a salt of the plating metal is the electrolyte. The plating metal is deposited on the cathode, and the anode replenishes the supply of positive ions, thus gradually being dissolved. Electrotype printing plates, silverware, and chrome automobile trim are plated by electrolysis.

The English scientist Michael Faraday discovered that the amount of a material deposited on an electrode is proportional to the amount of electricity used. The ratio of the amount of material deposited in grams to the amount of electricity used is the electrochemical equivalent of the material. Actual electric consumption may be as high as four times the theoretical consumption because of such factors as heat loss and undesirable side reactions.

Electric Cells

An electric cell is an electrolytic system in which a chemical reaction causes a current to flow in an external circuit; it essentially reverses electrolysis. A battery is a single electric cell (or two or more such cells linked together for additional power) used as a source of electrical energy. Metal corrosion can take place by electrolysis in an unintentionally created electric cell. The Italian physicist Alessandro Volta discovered the principle of the electric cell (see voltaic cell) in 1800. Within a few weeks William Nicholson and Sir Anthony Carlisle, English scientists, performed the first electrolysis, breaking water down into oxygen and hydrogen.

electrolysis, cosmetic, method of permanently removing superfluous or unwanted hair. A fine needle is inserted into the hair follicle; the application of an electric current through the needle destroys the hair root, or papilla, and the hair is removed.

Process in which electric current passed through a substance causes a chemical change, usually the gaining or losing of electrons (see oxidation-reduction). It is carried out in an electrolytic cell consisting of separated positive and negative electrodes (anode and cathode, respectively) immersed in an electrolyte solution containing ions or in a molten ionic compound. Reduction occurs at the cathode, where electrons are added that combine with positively charged cations in the solution. Oxidation occurs at the anode, where negatively charged anions give up electrons. Both thus become neutral molecules. For historical reasons, electric current is defined to flow in the opposite direction to the flow of electrons. Thus, current is said to flow from the cathode to the anode, even though electrons flow in the opposite direction. Electrolysis is used extensively in metallurgy to extract or purify metals from ores or compounds and to deposit them from solution (electroplating). Electrolysis of molten sodium chloride yields metallic sodium and chlorine gas; that of a strong solution of sodium chloride in water (brine) yields hydrogen gas, chlorine gas, and sodium hydroxide (in solution); and that of water (with a low concentration of dissolved sodium chloride or other electrolyte) yields hydrogen and oxygen.

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

History

Overviews

Electrolysis involves the passage of an electric current through, in general, an ionic substance that is either molten or dissolved in a suitable solvent, resulting in chemical reactions at the electrodes. The positive electrode is called the anode, and the negative electrode is the cathode. To be useful for electrolysis, the electrodes need to be able to conduct electricity, and metal electrodes are generally used. Graphite electrodes and semiconductor electrodes are also used. An ionic compound, or a compound that reacts with the solvent to produce protons (such as an acid) is dissolved in an appropriate solvent, or an ionic compound is melted by heat. Then some free ions exist in the liquid. An electrical potential is applied between a pair of electrodes immersed in the liquid. Each electrode attracts ions that are of the opposite charge. Therefore, positively-charged ions (called cations) move towards the electron-emitting (negative) cathode, whereas negatively-charged ions (termed anions) move toward the positive anode. The energy required to separate the ions, and cause them to gather at the respective electrodes, is provided by an electrical power supply. At the electrodes, electrons are absorbed or released by the ions, forming a collection of the desired element or compound.

Oxidation of ions or neutral molecules can take place at the anode, and the reduction of ions or neutral molecules at the cathode. For example, it is possible to oxidize ferrous ions to ferric ions at the anode:

mathrm{Fe^{2+}_{aq} longrightarrow Fe^{3+}_{aq} + e^- } .
It is also possible to reduce ferricyanide ions to ferrocyanide ions at the cathode:
mathrm{Fe(CN)_6^{3-} + e^- longrightarrow Fe(CN)_6^{4-} }

Neutral molecules can also react at either electrode. For example: p-Benzoquinone can be reduced to hydroquinone at the cathode:

mathrm{+ 2 e^- + 2 H^+ longrightarrow } In the last example, H^{+} ions (hydrogen ions) also take part in the reaction, and are provided by an acid in the solution, or the solvent itself (water, methanol etc). Electrolysis reactions involving H^{+} ions are fairly common in acidic solutions. In alkaline solutions, reactions involving OH^- (hydroxide ions) are common.

The substances oxidised or reduced can also be the solvent (usually water) or the electrodes. It is possible to have electrolysis involving gases. For instance, fuel cells often use oxygen and hydrogen gases as reactants.

The amount of electrical energy that must be added equals the change in Gibbs free energy of the reaction plus the losses in the system. The losses can (in theory) be arbitrarily close to zero, so the maximum thermodynamic efficiency equals the enthalpy change divided by the free energy change of the reaction. In most cases, the electric input is larger than the enthalpy change of the reaction, so some energy is released in the form of heat. In some cases, for instance, in the electrolysis of steam into hydrogen and oxygen at high temperature, the opposite is true. Heat is absorbed from the surroundings, and the heating value of the produced hydrogen is higher than the electric input.

The following technologies are related to electrolysis:

Electrolysis of water

One important use of electrolysis of water is to produce hydrogen.

2H2O(l) → 2H2(g) + O2(g)

This has been suggested as a way of shifting society toward using hydrogen as an energy carrier for powering electric motors and internal combustion engines. (See hydrogen economy.)

Electrolysis of water can be observed by passing direct current from a battery or other DC power supply through a cup of water (in practice a salt water solution increases the reaction intensity making it easier to observe). Using platinum electrodes, hydrogen gas will be seen to bubble up at the cathode, and oxygen will bubble at the anode. If other metals are used as the anode, there is a chance that the oxygen will react with the anode instead of being released as a gas, or that the anode will dissolve. For example, using iron electrodes in a sodium chloride solution electrolyte, iron oxides will be produced at the anode. With zinc electrodes in a sodium chloride electrolyte, the anode will dissolve, producing zinc ions (Zn2+) in the solution, and no oxygen will be formed. When producing large quantities of hydrogen, the use of reactive metal electrodes can significantly contaminate the electrolytic cell - which is why iron electrodes are not usually used for commercial electrolysis. Electrodes made of stainless steel can be used because they will not react with the oxygen.

The energy efficiency of water electrolysis varies widely. The efficiency is a measure of what fraction of electrical energy used is actually contained within the hydrogen. Some of the electrical energy is converted to heat, a useless by-product. Some reports quote efficiencies between 50% and 70% This efficiency is based on the Lower Heating Value of Hydrogen. The Lower Heating Value of Hydrogen is total thermal energy released when hydrogen is combusted minus the latent heat of vaporisation of the water. This does not represent the total amount of energy within the hydrogen, hence the efficiency is lower than a more strict definition. Other reports quote the theoretical maximum efficiency of electrolysis as being between 80% and 94%. The theoretical maximum considers the total amount of energy absorbed by both the hydrogen and oxygen. These values refer only 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 likely to be between 25% and 40%.

About four percent of hydrogen gas produced worldwide is created by electrolysis, and normally used onsite. Hydrogen is used for the creation of ammonia for fertilizer via the Haber process, and converting heavy petroleum sources to lighter fractions via hydrocracking.

Experimenters

Scientific pioneers of electrolysis included:

Pioneers of batteries:

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.

Faraday's laws of electrolysis

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 is proportional to the quantity of electric charge passed through the circuit. This became the basis of the first law of electrolysis:

m = k cdot q

Second law of electrolysis

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

Industrial uses

Electrolysis has many other uses:

  • Electrometallurgy is the process of reduction of metals from metallic compounds to obtain the pure form of metal using electrolysis. For example, sodium hydroxide in its molten form is separated by electrolysis into sodium and oxygen, both of which have important chemical uses. (Water is produced at the same time.)
  • Anodization is an electrolytic process that makes the surface of metals resistant to corrosion. For example, ships are saved from being corroded by oxygen in the water by this process. The process is also used to decorate surfaces.
  • A battery works by the reverse process to electrolysis. Humphry Davy found that lithium acts as an electrolyte and provides electrical energy.
  • Production of oxygen for spacecraft and nuclear submarines.
  • Electroplating is used in layering metals to fortify them. Electroplating is used in many industries for functional or decorative purposes, as in vehicle bodies and nickel coins.
  • Production of hydrogen for fuel, using a cheap source of electrical energy.
  • Electrolytic Etching of metal surfaces like tools or knives with a permanent mark or logo.

Electrolysis is also used in the cleaning and preservation of old artifacts. Because the process separates the non-metallic particles from the metallic ones, it is very useful for cleaning old coins and even larger objects.

See also

References

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