Silicon and its compounds
Silicon, being a second member of group – 14, has a much larger size and lower electronegativity than that of carbon. As a result silicon does not form double bond with itself or with oxygen. Thus SiO bonds are much stronger than Si – Si and Si –H bonds. Silicon has vacant 3d-orbitals in its valence shell due to which it can extend its covalency from four to five and six.
(1) Occurrence : Silicon is the second most abundant element (27.7%) in earth’s crust next to oxygen .It does not occur in free state. It occurs mainly in the form of Silica and silicates. Silicates are formed in rocks and clay as silicates of Mg, Al, K or Fe. e.g. Feldspar ; K2Al2O3.6SiO2, Kaolinite; Al2O3.2SiO2.2H2O.
(2) Preparation : Elemental silicon is obtained by reduction of silica with high purity coke in an electric furnace using excess of silica e.g.
SiO2 + 2C → Si + 2CO
Very high purity silicon required for making semiconductors is obtained by reduction of highly purified SiCl4 form (SiHCl3) with hydrogen followed by purification by zone refining eg.
SiCl4 + 2H2 → Si + 4HCl; SiHCl3 + H2 → Si + 3HCl
(3) Properties : Silicon exists in three isotopes 14Si29 (most common), 14Si30 with air at high temperature form,
Si + O2 → SiO2.
With steam, Si reacts when heated to redness to liberate hydrogen,
Si + 2H2O SiO2+ 2.
With halogens, Si reacts at elevated temperature forming except fluorine which reacts at room temperature.
Silicon combines with C at 2500K forming Silicon Carbide (SiC) known as carborundum (an extremely hard substance),
Si + C SiC.
It reacts with metals like Ca, Mg etc in an electric arc furnace to form Silicides (Ca2Si, Mg2Si etc.)
Silicon dissolves in hot aqueous alkalies liberating hydrogen,
Si + 4NaOH Na4SiO4 + 2H2 ↑
It also dissolves in fused Na2CO3 displacing carbon Na2SiO3 + C.
(4) Uses of Silicon : It is added to steel as ferrosilicon ( an alloy of Fe and Si) to make it acid resistant. It is used in the pure form as a starting material for production of silicon polymers (Silicones).
(5) Compounds of silicon
Silica or silicon dioxide (SiO2)
It occurs in nature in various forms such as sand, quartz and flint .It is also a constituent of various rocks. It is solid at room temperature. It is insoluble in water.
Silica has a three dimensional network structure in which each Si is bonded to four oxygen atoms which are tetrahedrally disposed around silicon atom. Each O atom is shared by two Si atoms. It may be noted that CO2 is a gas, while SiO2 is hard solid with very high melting point.
SiO2 + 4HF → SiF4 + 2H2O; Si+ 2HF → H2SiF6
Hydro flouro silica acid
HF readily dissolves Silica, therefore HF can not be store in glass bottles which contain Silica.
It is used in large amount to form mortar which is a building material. It is also used in the manufacture of glass and lenses.
Silicates
Almost all rocks and their products (Soil, clay and sand) are made up of silicate minerals and Silica. The basic unit of all silicates is tetrahedral SiO4–4 ion. Some of the important silicates are quartz, mica, asbestos, felspar and zeolites.
Silica gel
When a mineral acid (Such as HCl) is added to a concentrated solution of a silicate, gelatinous white ppt. of hydrated silica (silicic acid) separate out.
Na2SiO3 + 2HCl → 2NaCl + SiO2. xH2O
The white ppt. thus obtained is heated to lose water. When the water content is very low, the solid product is called silica gel. It possesses excellent absorptive properties due to its porous nature and is used for absorbing moisture and an adsorbent in chromatography.
Glass
Glass is an amorphous and transparent solid which is obtained by solidification of various silicates and borates of potassium and calcium.
(1) Preparation : Ordinary glass is a mixture of sodium and calcium silicates and is produced by fusing together a mixture of sodium carbonate, calcium oxide and silicon dioxide ( Silica) in a furnace at about 1700K
Na2CO3 + SiO2 → Na2SiO3 + CO2 ↑;
CaO + SiO2 → CaSiO3
On continuously heating the entire amount of CO2 is driven out and clear viscous fused mass is obtained. It is poured into moulds to get different types of articles, which are allowed to cool gradually.
This typed of glass is called soda glass or soft glass which has the approximate composition, Na2SiO2, CaSiO3, 4SiO2.
(2) Various varieties of glass : The different varieties of glasses and their special constituents are given below,
Type of glass | Constituents | Special use |
Soft glass | Na2CO3.CaCO3.SiO2 | Ordinary glass for window panes, test tubes, bottles, etc. |
Hard glass | K2CO3.CaCO3.SiO2 | For combustion tubes and chemical glassware |
High refractive index glass | Lead oxide, K2CO3 | For making lenses cut glasses |
Pyrex glass | Na2CO3.Al2CO3. B2O3 or borax, sand | For high quality glass apparatus cooking utensils |
Crook’s glass | K2CO3.PbCO3.CeO2, sand | Absorbs ultra violet rays, for making lenses |
(3) Coloured glass : Addition of transition metal compounds to glass give coloured glasses . Small amounts of Cr(III), Mn(IV), Co(II) and Fe(III) compounds impart green, violet blue or brown colour respectively
Compound added – Colour imparted | Compound added – Colour imparted |
Cobalt axide ( CoO) – Blue | Chromium oxide (Cr2O3)– Green |
Cuprous oxide (Cu2O) – Red | Auric chloride (AuCl3)– Ruby |
Cadmium sulphide (CdS) – Lemon yellow | Manganese dioxide (MnO2) – Purple |