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In chemistry, Carbide is a compound of carbon with a less electronegative element. Carbides are important industrially; for example calcium carbide is a feedstock for the chemical industry and iron carbide, Fe3C (cementite), is formed in steels to improve their properties.
In addition to the carbides there are other groups of binary carbon compounds i.e.
See Category:Carbides for a bigger list.
Types of carbides
Salt like carbides are formed by the metals of
Most commonly they are salts of C22− and are called acetylides, ethynides, acetylenediides or very rarely, percarbides.
The naming of ionic carbides is not consistent and can be quite confusing.
The polyatomic ion C22− contains a triple bond between the two carbon atoms. Examples are the carbides of the alkali metals e.g. Na2C2, some alkaline earths, e.g. CaC2 and lanthanoids e.g. LaC2. The C-C bond distance ranges from 109.2pm in CaC2 (similar to ethyne), to 130.3 pm in LaC2 and 134pm in UC2. The bonding in LaC2 has been described in terms of LaIII with the extra electron delocalised into the antibonding orbital on C22−, explaining the metallic conduction.
The monatomic ion C4− is a very strong base, and will combine with four protons to form methane. Methanides commonly react with water to form methane, however reactions with other substances are common.
The polyatomic ion C34− is found in e.g. Li4C3, Mg2C3. The ion is linear and is isoelectronic with CO2. The C-C distance in Mg2C3 is 133.2 pm. Mg2C3 yields methylacetylene, CH3CCH, on hydrolysis which was the first indication that it may contain C34−.
Silicon and boron form covalent carbides. Silicon carbide has two similar crystalline forms, which are both related to the diamond structure. Boron carbide, B4C, on the other hand has an unusual structure which includes icosahedral boron units linked by carbon atoms. In this respect boron carbide is similar to the boron rich borides. Both silicon carbide, SiC, (carborundum) and boron carbide, B4C are very hard materials and refractory. Both materials are important industrally. Boron also forms other covalent carbides, e.g. B25C.
The carbides of the group 4, 5 and 6 transition metals (with the exception of chromium) are often described as interstitial compounds. These carbides are chemically quite inert, have metallic properties and are refractory. Some exhibit a range of stoichiometries, e.g. titanium carbide, TiC. Titanium carbide and tungsten carbide are important industrially and are used to coat metals in cutting tools.
The longheld view is that the carbon atoms fit into octahedral interstices in the metal lattice when the metal atom radius is the greater than 135 pm. If the metal atoms are cubic close packed, (face centred cubic), then eventually all the interstices could be filled to give a 1:1 stoichiometry, with the rock salt structure, e.g. tungsten carbide. When the metal atoms are hexagonal close packed then only half of the interstices are filled, giving a stoichiometry of 2:1, e.g. divanadium carbide, V2C. The following table shows actual structures of the metals and their carbides, the notation "h/2" refers to the V2C type structure described above, which is an approximate description of the actual structures. The simple view that the lattice of the pure metal "absorbs" carbon atoms is only true for the monocarbides of vanadium, VC and niobium, NbC.
For a long time the non stoichiometric phases were believed to be disordered with a random filling of the interstices, however short and longer range ordering has been detected.
Intermediate transition metal carbides
In these the transition metal ion is smaller than the critical 135 pm and the structures are not interstitial but are more complex. Multiple stoichiometries are common, for example iron forms a number of carbides, Fe3C, Fe7C3 and Fe2C. The best known is cementite, Fe3C, which is present in steels. These carbides are more reactive than the interstitial carbides, for example the carbides of Cr, Mn, Fe, Co and Ni all are hydrolysed by dilute acids and sometimes by water, to give a mixture of hydrogen and hydrocarbons. These compounds share fetaures with both the inert interstitals and the more reactive salt-like carbides.
|This article is licensed under the GNU Free Documentation License. It uses material from the Wikipedia article "Carbide". A list of authors is available in Wikipedia.|