Carbides

Carbides are usually a group of binary chemical compounds composed of an element (E) and carbon (C) with the general formula E _x C _y . This group includes salt-like (e.g. calcium carbide , CaC ₂ ) as well as metallic compounds (e.g. tantalum carbide , TaC); an exception is z. B. Tantalum Hafnium Carbide (Ta ₄ HfC ₅ ).

General

Carbides are usually made from elemental carbon, which is reacted with the corresponding element, element oxide or element carbonate in a solid-state reaction at high temperatures. The production of calcium carbide from coke and calcium carbonate, for example, takes place at 2000 ° C in an electric arc furnace .

{\ displaystyle \ mathrm {CaCO_ {3} \ rightarrow \! \ CaO + CO_ {2}}}

{\ displaystyle \ mathrm {CaO + 3 \, C \ rightarrow CaC_ {2} + CO}}

Depending on the electronegativity difference between the element in question and carbon, carbides are one of the following three types.

Ionic carbides

Ionic carbides (salt-like carbides) have a strong salt character and are typically formed by the strongly electropositive elements of the alkali, alkaline earth and earth elements. They contain carbon as the more electronegative component.

Typical examples are lithium carbide Li ₂ C ₂ , beryllium carbide Be ₂ C, magnesium carbide Mg ₂ C ₃ , calcium carbide CaC ₂ or aluminum carbide Al ₄ C ₃ . In their ion lattice they contain the respective metal cation as well as the carbide ion, which is formally derived from various hydrocarbons. This leads to a further differentiation of the ionic carbides into:

Methanides, e.g. B. Be ₂ C and Al ₄ C ₃ , contain C ⁴⁻ , derived from methane CH ₄
Acetylides , e.g. B. Li ₂ C ₂ , CaC ₂ , contains C ₂²⁻ , derived from ethyne (acetylene) C ₂ H ₂
Allenide, e.g. B. Mg ₂ C ₃ , contains C ₃⁴⁻ , derived from Allen C ₃ H ₄

The ionic carbides also include the fullerides. These are compounds with the composition MC ₆₀ , M ₂ C ₆₀ and M ₃ C ₆₀ (M = Na, K). They are created by reducing Buckminster fullerene C ₆₀ with elemental alkali metals.

Ionic carbides, which are derived from hydrocarbons, react with water to form the corresponding metal hydroxide and the hydrocarbon, which is formed by multiple protonation of the anion with water.

The most famous representative is the u. a. Calcium carbide (CaC ₂ ) used in the carbide lamp , popularly called carbide , which releases ethine in the presence of water:

{\ displaystyle \ mathrm {CaC_ {2} +2 \ H_ {2} O \ rightarrow Ca (OH) _ {2} + C_ {2} H_ {2}}}

The gray color of the calcium carbide, which is actually white, results from contamination by elemental carbon from the manufacturing process (see above). The typical odor of calcium carbide is attributable to the monophosphane PH ₃ , which is formed from calcium phosphide by hydrolysis in an analogous manner to acetylene . Calcium phosphide is formed in the manufacturing process when the calcium carbonate used contains traces of calcium phosphate as an impurity.

The hydrolysis of calcium carbide was until the 1930s, the only method for the industrial production of acetylene as fuel gas (gas welding) in gas welding . Calcium carbide was also an important starting material for the development of acetylene chemistry (see Reppe chemistry ). However, with the advent of the petrochemical industry, this source of ethyne has lost its importance.

Covalent carbides

Silicon carbide

Covalent carbides are formed between carbon and elements with approximately the same electronegativity. The two most important examples are SiC ( silicon carbide , “carborundum”) and B ₄ C( Boron carbide ). They have covalent bonds between carbon and the respective element. The very strong covalent bonds, combined with a crystal structure, which are very similar to those of other hard materials (SiC has a structure similar to diamond) leads to a high mechanical stability. Accordingly, these carbides are usually used as hard materials (coatings, abrasives ) and for reinforcing plastics.

Silicon carbide is also of interest as a carrier material for catalysts in the chemical industry, as it has a high thermal conductivity and practically no abrasion.

Metal-like carbides

These carbides are derived from the elements of the 4th – 6th centuries. Subgroup formed, typical examples are titanium , tantalum and tungsten . As a rule, they do not have a precisely defined stoichiometry . Rather, the carbon atoms are in the tetrahedral sites and depending on the size ratio to the metal in the octahedral incorporated the metal grid and form inclusion compounds or interstitial compounds . These substances are characterized by high mechanical and thermal stability and high melting points (3000 to 4000 ° C) and are used as hard materials and ceramics in chemical apparatus and plant construction, to sprinkle grinding tools and to manufacture hard metal inserts for cutting tools . Cementite (Fe ₃ C) is a component of steel .

For example, the ball of a ballpoint pen is made of tungsten carbide .

In common parlance, carbide is usually equated with calcium carbide. This reacts with water to form acetylene, which can be used for various applications. For the use of calcium carbide, see there.

Individual evidence

^ ^A ^b ^c Brockhaus ABC Chemie , VEB FA Brockhaus Verlag Leipzig 1965, pp. 643-644.
^ AF Holleman, E. Wiberg, N. Wiberg: Inorganische Chemie . 103rd edition. tape 1 . Walter de Gruyter, Berlin 2017, ISBN 978-3-11-026932-1 , p. 1022 .

[ABC_Chemie-1] A ^b ^c Brockhaus ABC Chemie , VEB FA Brockhaus Verlag Leipzig 1965, pp. 643-644.

[2] AF Holleman, E. Wiberg, N. Wiberg: Inorganische Chemie . 103rd edition. tape 1 . Walter de Gruyter, Berlin 2017, ISBN 978-3-11-026932-1 , p. 1022 .