Compounds of Aluminium
Aluminium oxide or Alumina (Al2O3)
a)
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b)
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Aluminium chloride: (AlCl3)
a) Anhydrous Aluminium chloride (AlCl3) on heating it sublimes at 180°C and the vapour density corresponds to the formula Al2Cl6.
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The dimeric formula is retained in non polar solvents but it broken into [Al(H2O)6]Cl3 on dissolution in water on account of high heat of hydration.
Aluminium
Aluminium ions crystallize from aqueous solutions, forming double salts. These are called aluminium alums and have the general formula
[M'(H2O)6][Al(H2O)6](SO4)2, M' is a singly charged cation such as Na+, K+ or NH4+
Some M3+ ions other than Al3+ also form alums of formula [M'(H2O)6[ [MIII(H2O)6](SO4)2. Some of important alums are
Potash alum : K2SO4. Al2(SO4)3.24H2O
Ammonium alum : (NH4)2SO4. Al2(SO4)3.24H2O
Sodium alum : Na2SO4.Al2(SO4)3.24H2O
Chrome alum : K2SO4.Cr2(SO4)3.24H2O
Ferric alum : (NH4)2SO4.Fe2(SO4)3.24H2O
Compounds of Boron
1. Boric Acid
a) Orthoboric acid (H3BO3)
a1) Preparation (i) from borax
Na2B4O7 + H2SO4 + 5H2O
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ii) from colemanite
Ca2B6O11 + 2SO2 + 11H2O
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Boric acid forms white needle like crystal. It has a layer structure involving triangular BO3 groups joined by hydrogen bonds.
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a2 Action of heat
4H3BO3
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a3) It behaves as a weak monobasic acid
B(OH)3
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Thus on titration with NaOH, it gives sodium metaborate salt
H3BO3 + NaOH
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a4) B(OH)3 + MO
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Metaloxide
Where M stands for a bivalent metal
a5) B(OH)3
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Ammonium boro fluoride
2. Borax (sodium tetraborate) Na2B4O7. 10H2O Borax occurs naturally and is also called Tinacal or suhaga. Tinacal contains 45% of borax
a) Preparation from Boric Acid
4H3BO3 + Na2CO3
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b) Basic Nature:- aqueous solution of borax is alkaline in nature due to its hydrolysis
Na2B4O7 + 3H2O
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NaBO2 + 2H2O
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c) Action of heat:
Na2B4O7.10H2O
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Anhydrous sodium metaborate anhydride basic Transparent glassy mass
When hot glassy mass is brought in contact with a coloured slat and heated again is the flame, B2O3 displaces the volatile oxides and reacts with basic oxides to form metaborates. Metaborates of basic radicals show characteristic colours. This test is known as borax bead test.
Colour of metaborates of
Cu Fe Co Cr Ni
Blue Green Blue Green Brown
Compounds of Silicon
1. Silicones:
Silicones are organo - silicon polymers containing Si - O - Si linkages. These are formed by the hydrolysis of alkyl or aryl substituted chlorosilanes and their subsequent polymerisation.
R3SiCl
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Silanol
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Silicone
R2SiCl2 gives rise to straight chain linear polymers
RSiCl3 gives a complex cross linked polomer
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These polymers are used in water proofing textiles in glassware, as lubricants and anti foaming agents.
2. Silicon Corbide (Carborundum), SiC
It is obtained when a mixture of sand, carbon common slat and saw dust is strongly heated in an electric furnace
SiO2 + 3C
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It is chemically inert and resists the attack of almost all the reagents
It is used as an abrasive to make grind stones knife sharpness, etc.