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Inorganic Chemistry |
SILICON |
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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.
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(1) Occurrence of Silicon : 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
.
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(2) Preparation of Silicon : 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
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SiCl4 + 2H2 ——> Si + 4HCl
; SiHCl3 + H2 ——> Si + 3 HCl
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(3) Properties of Silicon: (i) Silicon exists in three isotopes
14Si29 (most common), 14Si30
with air at high temperature
SiO2 form, |
| (ii) With steam, Si reacts when heated to redness to liberate hydrogen, Si + 2
H2O
Si
+ ———> 2H2
. |
(iii) With halogens, Si reacts at elevated temperature forming
except fluorine which reacts at room temperature. |
| (iv) Silicon combines with C at 2500K forming Silicon Carbide (SiC) known as carborundum (an extremely hard substance), 2500 K |
| Si + C ————>
SiC. |
| (v) It reacts with metals like Ca, Mg etc in an electric arc furnace to form Silicides (Ca2Si, Mg2Si
etc.) |
| (vi) Silicon dissolves in hot aqueous alkalies liberating hydrogen, Si + 4NaOH ——> Na4 SiO4 + 2H2 |
| (vii) It also dissolves in fused Na2CO3
displacing carbon Na2SiO3+C
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| (4) Uses of silicon : (i) It is added to steel as ferrosilicon ( an alloy of Fe and Si) to make it acid resistant. |
(ii) It is used in the pure form as a starting material for production of silicon polymers (Silicones). |
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