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