P-Block Elements

Compounds of Aluminium
Aluminium oxide or Alumina (Al₂O₃)

a) - Al₂O₃ (rhombic lattice) is dense, hard and resistant to chemical attack.

- Al₂O₃ occurs in the earth's crust as corundum, (Al₂O₃). Transparent coloured crystals of corundum, viz. Ruby (red due to the presence of Cr) and sapphire (blue due to Ti and Re) are used as precious stones (gems)

b) - Al₂O₃ is less dense, soft and has a high surface area. It is used as an excellent selective adsorbent in dehydration, decolourisation and chromatography.

Aluminium chloride: (AlCl₃)

a) Anhydrous Aluminium chloride (AlCl₃) on heating it sublimes at 180°C and the vapour density corresponds to the formula Al₂Cl₆.



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[ [M^III(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 Na₂SO₄ + 4H₃BO₃

ii) from colemanite

Ca₂B₆O₁₁ + 2SO₂ + 11H₂O 2Ca(HSO₃)₂ + 6H₃BO₃

Boric acid forms white needle like crystal. It has a layer structure involving triangular BO₃ groups joined by hydrogen bonds.





a₂ Action of heat

4H₃BO₃


a₃) It behaves as a weak monobasic acid

B(OH)₃ H₃BO₃ H⁺ + H₂O +

Thus on titration with NaOH, it gives sodium metaborate salt

H₃BO₃ + NaOH NaBO₂ + 2H₂O

a₄) B(OH)₃ + MO M - borates

Metaloxide

Where M stands for a bivalent metal

a₅) B(OH)₃ NH₄BF₄ BF₃

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 Na₂B₄O₇ + 6H₂O + CO₂

b) Basic Nature:- aqueous solution of borax is alkaline in nature due to its hydrolysis

Na₂B₄O₇ + 3H₂O NaBO₂ + 3H₃BO₃

NaBO₂ + 2H₂O + H₃BO₃

c) Action of heat:

Na₂B₄O₇.10H₂O Na₂B₄O₇ 2NaBO₂ + B₂O₃

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 R₃SiOH
Silanol



Silicone


R₂SiCl₂ gives rise to straight chain linear polymers

RSiCl₃ gives a complex cross linked polomer








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 SiC + 2CO

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.