Carbide. Carbon binary compound with a less electronegative element than it. Carbon – hydrogen compounds are excluded . Carbides are metal- carbon compounds , if boron and silicon are among the normal metals . Volatile compounds (except A1C or its dimer) are essentially not known as they decompose at high temperatures.
Most carbides can be prepared by heating a mixture of the powdered metal and carbon, almost always at high temperatures, but not always as high as its melting point. The same result is usually obtained by heating a mixture of the metal oxide with carbon (CaO + 3C → CaC2 + CO). Some can be prepared by passing a hydrocarbon in a vapor state over the hot metal, in the form of an electrically heated filament.
Summary
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- 1 General
- 2 Ionic carbides
- 3 Covalent carbides
- 4 Metal carbides
- 5 Sources
general
The synthesis of carbides is generally carried out from elemental carbon with the element, its oxide or its carbonate at high temperatures in a solid reaction. Thus , calcium carbide is the best known carbide. It can be obtained from calcium carbonate, which is unstable at high temperatures and is transformed into quicklime, losing a molecule of carbon dioxide or calcium oxide and coke at temperatures between 2000 and 2500 ºC. CaCO3 → CaO + CO2 CaO + 3 C → CaC2 + CO
Ionic carbides
Ionic carbides have a strong salt character and are formed above all from the elements of groups I and II of the periodic table. All of these elements are highly electropositive and carbon is therefore negatively charged. Typical examples are lithium carbide (Li4C), beryllium carbide (Be2C), magnesium carbide (Mg2C3), calcium carbide (CaC2), aluminum carbide (Al4C3), and iron carbide (Fe3 C ) known as cementite.
These, according to the anionic species present in the solid, can be differentiated into methanes (derived from methane with formally the C4 ion – such as Li4C or Al4C3), acetylides (derived from acetylene such as CaC2) and alenides with the C34 ion – = 2 – C = C = C2–. In contact with water these carbides give the oxide or hydroxide of the element and the corresponding hydrocarbon (methane, acetylene or propadiene). Thus in the laboratory they are easy to handle sources for these gases that would otherwise require a pressurized cylinder.
Covalent carbides
Covalent carbides form between carbon and elements with approximately the same electronegativity. The most important examples of this group are silicon carbide or carborundum (SiC) with a diamond structure and a Mohs scale hardness of between 9 and 9.5 and boron carbide (B4C4). These substances are usually very hard due to covalent bonds formed in all three dimensions. They are used for example as abrasive materials or as coatings on parts that have to resist mechanical abrasions. The silicon carbide is also used as a support for catalysts because of their high strength and good thermal conductivity.
Metal carbides
These carbides are formed with transition metals such as tungsten or titanium . They often do not have a defined stoichiometry. This is because carbon occupies tetrahedral free positions in the metal structure. The substances formed are characterized by their high mechanical and thermal resistance (melting points typically on the order of about 3,000 to 4,000 ºC) and are used in the manufacture of ceramic utensils and machinery.
