The compounds of aluminium are mainly covalent. In ionic compounds, such as aluminum fluoride, it exits as a trivalent aluminum ion, A13+. The hydrated aluminum ion is found in many compound e.g. A12(SO4)3 . 18H2O, and in aqueous solution Hydrated aluminum ions are usually acidic in aqueous solution. This post as also explains the industrial applications and uses of the various compounds of aluminium
Compounds of Aluminium
1. Aluminium oxide, A12O3:- This compound is commonly known as alumina. It occurs naturally as bauxite, corundum and white sapphire. It also occurs as ruby amethyst, topaz blue sapphire and emerald, the characteristic colour of which are due to other metallic oxide present as impurities. It is prepared by heating aluminum hydroxide or other aluminum compounds.
2A1(OH)3(s)→ A12O3(s) + 3H2O(g)
Aluminium oxide is a white crystalline powder which is almost insoluble in water. It reacts readily with dilute acids and strong alkalis to form salts, i.e it is amphoteric. Its reactions with acids and alkalis depend on the oxide being prepared at the lowest temperature possible, otherwise it will exits in a form which is insoluble in both acids and alkalis.
Aluminium oxide is used as a source of aluminum and in making furnace linings, ceramics and synthetic gem stones. It is also used as an abrasive, for absorption in chromatography, and as a catalyst in the catalytic cracking of hydrocarbons. This compound is one of the most widely used compounds of aluminium in industrial production processes.
2. Aluminium hydroxide:- One of the most important compound of aluminium is Aluminium hydroxide. it is an amphoteric compound. In excess alkali, it gives tetrahydroxoaluminate(III) ion, and in acids, it forms salts.
A1(OH)3(s) + NaOH(aq)→ NaA1(OH)4(aq)
2A1(OH)3(aq) + 3H2SO4(aq)
→ A12(SO4)3(aq) + 6H20(1)
Aluminium hydroxide is used in the dyeing industry. It is called a mordant as it helps the dye to stick to the cloth.
3. Aluminium chloride, A1C13:- Aluminium chloride is prepared by heating aluminum foils in a stream of dry chlorine or hydrogen chloride. The ease with which the anhydrous salt hydrolyzes explains why hydrogen chloride fumes are evolved from it in damp air.
Note: Sodium chloride and calcium chloride are electrovalent compounds which dissolve in water to give ions. Aluminium chloride, however, is a covalent compound which reacts with water.
4. Aluminium tetraoxosulphate(VI), A12(SO4)3:- This is also one of the important compounds of aluminium that occurs naturally but can be manufactured by the action of hot concentrated tetraoxosulphae(VI) acid on aluminum oxide.
A12O3(s) + 3H2SO4(aq)→ A12(SO4)3(aq) + 3H2O(1)
It is a white crystalline solid which is moderately soluble in water. It forms double salt (alums) with other tetraoxosulphates(VI), and is used in the treatment of water and sewage, and in paper making.
5. Alums: Alums are double traoxosulphates(VI) which ionize in solution to yield a monovalent cation (usually Na+=, K+ or NH4+), a trivalent cation (usually Fe3+, A13+ or Cr3+) and tetraoxosulphate(VI) ions. Their general formula can be written as follows.
M21SO4M2III(SO4)3, 24H2O or M1MIII(SO4)2.12H2O
Where M1and MIII are the monvalane and trivalent cations respectively.
All alums can be made by mixing equimolar masses of the tetraoxosulphates(VI) concerned. The alums crystallize out, each cation being associated with six molecules of water of crystallization. All alums have similar octahedral crystalline shapes.
Aluminium potassium tetraoxosulphate(VI)-dodlecahydrate, KA1(SO4)2. 12H2O, or potash alum, is one of the commonest alums known. It is obtained by mixing hot solutions containing equimolar masses of potassium and aluminium tetraoxosulphates(VI). When the mixture is cooled, colourless crystals of the alum separate out.
Potash alum is usually used as a mordant in dyeing, i.e. it combines with and fixes the dyestuff onto the fibre in cases where the fibre cannot by dyed directly. Potash alum is also commonly used as a coagulant in water purification for removing colloidal suspension particles.
6. Aluminium trioxonitrate(V), A1(NO3)3: Aluminium nitrate may be prepared in the laboratory by dissolving aluminum oxide or aluminum hydroxide in dilute trioxonitrate(V) acid. the salt crystallizes out as nonahydrate crystals, A1(NO3)39H2O.
A12O3(s) + 6HNO3(aq) → 2A1(NO3)3,+ 3H2O(1)
A1(OH)3(s) + 3HNO3(aq) → A1(NO3)3,(aq)+ 3H2O(1)
7. Aluminium hydride, A1H3:– This is a white covalent solid. It can be prepared by reacting excess aluminum chloride with lithium hydride.
6LiH + A12C16 → 2A1H3 + 6LiCI
With excess lithium hydride, lithium tetrahydriod aluminate(III) is obtained.
8LiH + A12C16 → 2LiAIH + 6LiC1
Lithium tetrahydridoaluminate(III), LiA1H4, and sodium terahydridobroate(III), NaBH4, are strong reducing agents used in organic chemistry.