Chapter 4:-
B.sc 1st year Book
(Page 12)
CARBIDES
Carbides are the binary compounds of carbon(C) with more electropositive elements than carbon. Depending upon the nature of bonding between electropositive elements and carbon, carbides are classified as :
(a) Ionic Carbides:
Ionic carbide are formed by heating the more electropositive metals or their oxides with carbon, CO, or acetylene, They are non-conductors of electricity. These are easily hydrolyzed by water or dilute acids to give different hydrocarbons. Depending upon the nature of hydrocarbons produced as a result of hydrolysis, the ionic carbides are further classified as :
(i) Acotylides :
These acetylides give acetylene on hydrolysis and contain [C≡C]2– ions: Few examples are: BeC2,MgC2,CaC2, etc.
(ii) Methanides :
These carbides give methane on hydrolysis and contain C4, ions. The common examples of such carbides are Be2C, N4C3, etc.
(iii) Allylldes:
These carbide give allylene (H2C=C=CH2) on hydrolysis and contain [C=C−c]4−ions The only example of this type of carbide is Mg2C3.
(iv) Mixed carbides:
This carbide on hydrolysis gives a mixture of hydrocarbons Including acetylene, olefins, and hydrogen. The common examples of this type of carbide are UC2and ThC2.
(b) Covalent Carbides:
This carbide is obtained by reducing the oxides of S and B with carbon in an electric furnace. These carbide are extremely very hard and decompose at very high temperatures. Due to their hardness, they were used as abrasives.
(c) Interstitial Carbides:
These carbide are prepared by a direct combination of transition metals of the groups IVB, VB, and VIB with carbon or by reducing their oxides with carbon in an electric furnace at high temperatures. Transition metals of these groups form two types of carbides ⋅MC type(where M=T, Zr, H, V, Nb, Ta, Mo, and W ) and MC2 type(where M=V, Mo, and W ).
Interstitial carbide are extremely hard like diamonds and are chemically inert. In this carbide, small C atoms occupy an interstitial position in the crystal lattice of the metals, hence the name “interstitial carbides”.
(d) Iron Carbides:
These carbide are given by those transition metals which have atomic radii less than 1.30 A∘. Such transition metals are- Cr and Mn. Fe, Co, and NL. They are less stable and are easily decomposed by water and acids.
General methods of preparation of carbides:
Metal carbide are prepared by the following methods:
(3) By heating metal carbonates with carbon in the presence of an oxidizable metal:
(4) Bypassing CO over active metals like Li1, Al :
(5) By heating hydroxides, sulfates, nitrides, or phosphates with carbon at high temperatures:
(6) Bypassing acetylene or ethylene overheated metals or their oxides :
(7) Bypassing acetylene ammoniacal solution of Ag(D) and Cu(l) Salts:
Uses of carbides:
(i) Tungston carbide, WC is used in the manufacturing of high-speed tools.
(ii) Boron carbide is used for cutting diamonds, drilling holes in rocks, and for making electrodes for electric furnaces.
(iii) Silicon carbide is used in glass cutting, making metal melting crucibles, cutting tools, and grinding wheels.
(iv) Calcium carbide is used in the preparation of acetylene which is employed in oxy-acetylene flame for welding. It is also used in the preparation of nitrol(CaCN (C+C) which is used as fertilizer.
(ii) Boron carbide is used for cutting diamonds, drilling holes in rocks, and for making electrodes for electric furnaces.
(iii) Silicon carbide is used in glass cutting, making metal melting crucibles, cutting tools, and grinding wheels.
(iv) Calcium carbide is used in the preparation of acetylene which is employed in oxy-acetylene flame for welding. It is also used in the preparation of nitrol(CaCN (C+C) which is used as fertilizer.