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In chemistry, a carbide is a compound composed of carbon and a less electronegative element. Carbides can be generally classified by chemical bonding type as follows: (i) salt-like, (ii) covalent compounds, (iii) interstitial compounds, and (iv) "intermediate" transition metal carbides. Some examples include calcium carbide, CaC2, important industrially, silicon carbide (SiC), carborundum, a covalent compound that is a useful abrasive, tungsten carbide (often called simply carbide), an interstitial compound widely used for cutting tools, and cementite, Fe3C) an important constituent of steel.[1]
[edit] Types of carbides
[edit] Salt-like materialsSalt-like carbides are composed of highly electropositive elements such as the the alkali metals, alkaline earths, and group 3 metals including (scandium, yttrium and lanthanum. Aluminium from group 13 forms a carbide, but gallium, indium and thallium do not. Some compounds contain other formally anionic species:[1] C4−, sometimes called methanides (or methides) because they hydrolyse to give methane gas. The naming of ionic carbides is not consistent and can be quite confusing. [edit] AcetylidesThe hypothetical polyatomic ion C22− is described by a triple bond between the two carbon atoms. Alkali and alkaline earth metals form acetylides, e.g., Na2C2 and CaC2. Lanthanoids are similar, e.g. LaC2.[1] The C-C bond distance ranges from 109.2pm in CaC2 (similar to ethyne), to 130.3 pm in LaC2 and 134pm in UC2.[1] 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.[1] Metals from group 11 tend to form acetylides, such as copper(I) acetylide and silver acetylide. lanthanides when forming MC2 and M2C3 carbides. Actinides, which form materials of the stoichiometry MC2 and M2C3, are described as salt-like derivatives of C22−, although percarbides are also known. [edit] MethanidesThe monatomic ion C4− would be highly basic, but is unknown except in under extreme conditions. Methanides in principle would react with water to form methane.
Examples of compounds that are described as methanides include C4− are Be2C and Al4C3.[1] [edit] SesquicarbidesThe polyatomic ion C34− is found in Li4C3, Mg2C3.[1] The ion is linear and is isoelectronic with CO2.[1] The C-C distance in Mg2C3 is 133.2 pm.[2] Mg2C3 yields methylacetylene, CH3CCH, on hydrolysis which was the first indication that it may contain C34−. The ion C34− is sometimes called sesquicarbides, they hydrolyse to give methylacetylene. [edit] Covalent carbidesThe carbides of silicon and boron are described as "covalent carbides", although virtually all compounds of carbon exhibit some covalent character.[1] Silicon carbide has two similar crystalline forms, which are both related to the diamond structure.[1] 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 industrially. Boron also forms other covalent carbides, e.g. B25C. [edit] Interstitial carbidesThe carbides of the group 4, 5 and 6 transition metals (with the exception of chromium) are often described as interstitial compounds.[1] 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.[3] The longheld view is that the carbon atoms fit into octahedral interstices in a close packed metal lattice when the metal atom radius is greater than approximately 135 pm:[1]
The following table[1][3]shows actual structures of the metals and their carbides. (N.B. the body centred cubic structure adopted by vanadium, niobium, tantalum, chromium, molybdenum and tungsten is not a close packed lattice.) The notation "h/2" refers to the M2C type structure described above, which is only an approximate description of the actual structures. The simple view that the lattice of the pure metal "absorbs" carbon atoms can be seen to be untrue as the packing of the metal atom lattice in the carbides is different from the packing in the pure metal.
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.[4] [edit] Intermediate transition metal carbidesIn these the transition metal ion is smaller than the critical 135 pm and the structures are not interstitial but are more complex.[1] Multiple stoichiometries are common, for example iron forms a number of carbides, Fe3C, Fe7C3 and Fe2C.[1] The best known is cementite, Fe3C, which is present in steels.[1] 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.[1] These compounds share features with both the inert interstitals and the more reactive salt-like carbides.[1] [edit] Related materialsIn addition to the carbides, other groups of related carbon compounds exist, i.e.[1]
[edit] References
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