Carbide

A carbide is a compound composed of carbon and a less electronegative element, usually a metal or a metalloid.

Types

Salt-like

Salt-like carbides are composed of highly electropositive elements such as the alkali metals, alkaline earth metals. They are split in three main categories, based on the hydrocarbon released from their reaction with water: methanides (aluminium carbide Al₄C₃, magnesium carbide Mg₂C and beryllium carbide Be₂C), acetylides (calcium carbide CaC₂, lanthanum carbide LaC₂), sesquicarbides (Li₄C₃ and Mg₂C₃).

Covalent compounds

The carbides of silicon and boron are described as "covalent carbides", although virtually all compounds of carbon exhibit some covalent character. 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.

Interstitial compounds

The carbides of the group 4, 5 and 6 transition metals (with the exception of chromium) are often described as interstitial compounds. These carbides have metallic properties and are refractory. Some exhibit a range of stoichiometries, e.g. titanium carbide, TiC. Titanium carbide and tungsten carbide (WC) are important industrially and are used to coat metals in cutting tools.

"Intermediate" transition metal carbides

In these carbides, 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.

Molecular carbides

Metal complexes containing C are known as metal carbido complexes. They can contain metals that normally do not form carbides, like gold.

General properties

Salt-like caribdes readily hydrolyze in water, releasing methane, acetylene or methylacetylene and propadiene. Covalent carbides resist hydrolysis and most acids but can be attacked by oxidizing mixtures, though the reaction is slow. Transition metal carbides resist hydrolysis, but will be slowly attacked by acids.

Carbides from alkaline and alkaline-earth metals are brittle solids. Transition metal carbides are very tough and hard, but are brittle.

Availability

Many carbides, such as calcium carbide or tungsten carbide can be found in hardware stores. Hard carbides can be found in many tools or can be bought online.

Preparation

Calcium carbide can be made in a furnace, by reducing calcium oxide with carbon.

Projects

• Make acetylene
• Make methane
• Gas torch
• Make acetylides

References

Relevant Sciencemadness threads

• mechanisms of calcium carbide reactivity