Compounds Carbon

Carbides. They are binary compounds of carbon with more electropositive elements - metals, silicon, boron.

Carbides are formed only at high temperature interaction of carbon with elements (but technologically more frequent of carbon with oxides). All without exception they are very hard substances. Conventionally they can be divided into three large classes according the structure, character of bond and chemical properties:

• ionic – are formed by active metals (they are easily decomposed by water or acids);

• covalent – have an atomic crystalline lattice, are extraordinarily hard and chemically inert (SiC, B₄C);

• inclusion are metal-like, are conductors of electric current, they are very hard and refractory, extraordinarily chemically inert. The atoms of carbon in them are located between the knots of crystalline lattice of metals. Their composition often does not answer the ordinary valence states of atoms of elements (Mn₄C, Mn₃C, Mn₈C₃; Cr₃C₂, Cr₇C₃; MoC, Mo₂X, WC, W₂C; VC; NbC; Nb₂C; TaC; Ta₂C). Sometimes they have no definite stoichiometry and belong to bertollides, for example, TiC_0,6-1,0, VC_0,58-1,0.

Ionic carbides. This name is only conditional. They are divided on 1) acetylenides (the most widespread class), 2) methanides and 3) those which are decomposed by acids with formation of mixture of hydrocarbons and hydrogen.

Acetylenides. Are formed by most active metals (for example, Li₂C₂), have in composition bivalent ion C₂^2-. They are easily decomposed by water with exothermic heat effect:

СаС₂ + 2Н₂О = Са(ОН)₂ + С₂Н₂ Н₂₉₈ = –125,5 kJ/mol

More frequently, worldwide is used and prepared CaC₂ (~ 5 million of ton annually). It is used for preparation of C₂H₂ in gas welding and as reductant is in metallurgy. The carbide of calcium is preprec in electric furnaces at fusion of calcium oxide with coal:

СаО + 3С = СаС₂ + СО Н₂₉₈ = 464,4 kJ/mol

Methanides. Are known only for the beryllium and aluminium (Be₂C and Al₄C₃). With hot water or diluted acids they are decomposed with the formation of methane:

Al₄C₃ + 12H₂O = 4Al(OH)₃ + 3CH₄

Carbides of the third type. These are Fe₃C, Co₃C and some other. Are decomposed by acids:

Fe₃C + 6HCl = 3FeCl₃ + CH₄ + H₂.

At the same time are formed other gaseous hydrocarbons.

To the separate type belongs carbide Mg₂C₃, which is formed from MgC₂ subject to the condition of breaking of carbon off at 500. At interaction with water, it gives pure propine:

Mg₂C₃ + H₂O = 2Mg(OH)₂ + CH₃–CCH

The covalent carbides. Most widespread are SiC, V₄C. They are inorganic polymers, and widely used in industry. A carborundum SiC has high hardness and wearproof, chemically is very inert. V₄C is very hard (~diamond) and chemically stable.

Carbides of inclusion. NbC and TaC are melted at 3500 and 3900 and are most refractory from all known substances. Carbides of tungsten and tantalum are extraordinarily hard and are used for creation of superhard alloys.

Such carbides are decomposed by oxidizing alkaline melts only:

2WC + 5O₂ + 8NaOH 2Na₂WO₄ + 2Na₂CO₃ + 4H₂O

WC is oxidized in this reaction. In this case, it is possible conditionally to adopt the oxidation states of W and C equal to zero (0!). Then W gives 6 electrons, C - 4 electrons, and together WC gives 10 electrons to the molecule O₂, which takes four electrons from WC. Basic coefficients are 2 before WC and 5 before O₂.