Halides : Elements of group 14 react with halogens directly to form tetrahedral and covalent halides except C where its halide is produced by the action of halogens on hydrocarbons. PbBr4
and PbI4
do not exist because Pb4+
is a strong oxidant and
Br- and
I- are strong reductants. Hence
Pb4+ ion is difficult to survive in presence of strong reductants Br-
and I-
and is immediately reduced to Pb2+
.
Carbides : Carbides are binary compounds of carbon with elements of lower or about equal electronegativity.
Preparation : Carbides are generally prepared by heating the elements orits oxide with carbon or hydrocarbon at very high temperatures.
Ca + 2C ——→ BaC2; 2Li + 2C ——→ Li2C2
CaO + 3C ——→ CaC2 + CO
4Li + C2H2 ——→ Li2C2 + LiH
Carbides are classified into three types on the basis of chemical bonding.
(1) Salt like carbides : These carbides are formed by the metals of groups IA, IIA, IIIA (except boron), coinage metals, Zinc, cadmium & some lanthanides.
(i) Acetylides : These are ionic carbides which yield acetylene on hydrolysis. The alkali metals and copper, silver and gold form M2C2 type compounds. These contain C22-
ions.
(ii) Methanides: These carbides evolve methane on hydrolysis. AI4C3, Be2C, Mn3C
etc are some are of methanides. These contains
C4- groups.
(iii) Allylides: These carbides evolve allylene (methyl acetylene) on hydrolysis. This type of the carbides is only Mg2C3
it contains
C34- discrete groups.
(2) Mixed carbides : These carbides yield a mixture of hydrocarbons on hydrolysis, carbides of iron group, UC2
and
ThC2 belong to this group.
(3) Covalent carbides : The only true covalent carbides are those of
SiC (carborundum) and
B4C, B13C2etc. These are chemicallyinert so become hard.
On account of hardness, these are used as abrasives.
(4) Metallic or interstitial carbides : If these carbides possess metallic lustre high electrical conductivity and chemically inert. These are extremely hard like diamond and possess very high melting points. |
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