Aluminum Compounds

Aluminium compounds

ANHYDROUS ALUMINIUM CHLORIDE, AlCl3 (or Al2Cl6)

PREPARATION

It can not be prepared by heating AlCl3. 6H2O because of its hydrolysing tendency by its own water as below.

2AlCl3. 6H2O  2Al(OH)3 + 6HCl

2Al(OH)3  Al2O3 + 3H2O

However, it can be prepared by following methods:

 

By passing dry chlorine or HCl gas over heated Al.

2Al + 3Cl2  2AlCl3

2Al + 6HCl 2AlCl3 + 3H2

 

By heating a mixture of alumina and carbon in a current of dry chlorine.

Al2O3 + 3C + 3Cl2  2AlCl3 + 3CO

 

PROPERTIES

It fumes in moist air due to hydrolysis

AlCl3 + 3H2O   Al(OH)3 + 3HCl

The resulting solution is acidic due to the formation of HCl.

It behaves as lewis acid.

It is a covalent solid and dissolves in organic solvents like C6H6 etc.     

 

STRUCTURE

It exists as dimer Al2Cl6 in which each Al atom is tetrahedrally surrounded by four Cl atoms as below.  

 

USES

·                     As a catalyst in Friedel - Craft reactions

·                     As a mordant in dyeing

ALUMINIUM OXIDE OR ALUMINA, Al2O3

It is the most stable compound of aluminium and occurs in nature as colourless corundum and several coloured oxides, (when present in combination with different metal oxides) like ruby (red), topaz (yellow), sapphire (blue), amethyst (violet) and emerald (green), which are used as precious stones (gems).

THERMITE

A mixture of aluminium powder and ferric oxide in the ratio 1: 3.

ALUMINIUM SULPHATE, Al2(SO4)3

It is used for obtaining H2S in pure form and for making fire proof clothes.

ALLOYS OF ALUMINIUM

 

Alloy

Composition

Properties

Uses

Duralumin

Light, tough, ductile

Aeroplanes and automobile parts

Aluminium Bronze

Light, strong, golden lustre

Coins, jewellery

Alcald

Duralumin coated with pure aluminium

corrosion resistant, strong

aircraft industry

Magnalium

Light, tough and strong

Balance beams and machinery

Alnico

Highly Magnetic

Permanent magnets

ALUMS

The term alum is given to double sulphates of the type X2SO4, Y2(SO4)3. 24H2O where X represents a monovalent cation such as Na+,K+ and NH Rb+, Cs+, Ag+ while Y is a trivalent cation such a Al3+, Cr3+ and Fe3+, Co3+, Ga3+, V3+, Ti3+. (Li+ is too small to be accommodated in the lattice)

 

General formula :

or

 

Some important alums are :

·                     Potash alum K2SO4. Al2(SO4)3. 24H2O

·                     Sodium alum Na2SO4. Al2(SO4)3. 24H2O

·                     Ammonium alum (NH4)2SO4. Al2(SO4)3. 24H2O

·                     Ferric alum (NH4)2SO4. Fe2(SO4)3. 24H2O

·                     Chrome alum K2SO4. Cr2(SO4)3. 24H2O

 

Out of these, potash alum is the most important which is prepared in the laboratory by mixing hot solutions of equimolar quantities of K2SO4 and Al2(SO4)3. The resulting solution on  concentration and crystallization gives potash alum (emperical formula is KAl(SO4)2.12H2O).

 

Pseudo alums : When monovalent element of ordinary alums is replaced by a bivalent element eg Mn2+, Fe2+, Mg2+, Cu2+ or Zn2+, the alums are called pseudo alums.

Examples : FeSO4, Al2(SO4)3.24H2O

Ferrous aluminium pseudo alum

MnSO4.Al2(SO4)3.24H2O

Manganese aluminium pseudo alum

 

PROPERTIES

Potash alum is a white crystalline compound.

The aqueous solution of all alums is acidic due to hydrolysis of Al2(SO4)3, Cr2(SO4)3 or Fe2(SO4)3 as given below

Al2(SO4)3 + 6H2O 2Al(OH)3 + 3H2SO4

On heating all alums lose water of crystallization
and swell up. The anhydrous alum is known as burnt alum.

Ionisation of aqueous solution of a double salt is as

K2SO4. Al2(SO4)2. 24H2O K+ + 2Al3+ + 3SO + 24H2O

 

USES

·                     In purification of water

·                     For sizing of paper

·                     As a styptic to stop bleeding

·                     As a mordant in dyeing and tanning of leather