GROUP 13: ALUMINIUM

Learning outcomes:

-Extraction of Aluminium

-Bonding in Aluminium Compounds.

-Acidic Characteristics of Aluminium Salts

-Resistance of Aluminium to Corrosion

-Uses of Aluminium and Its Compounds

2.1 The extraction of Aluminium

Reaction with Oxygen

- Due to its strong affinity towards oxygen, aluminium does not exist as free metal in nature.

- Aluminium is extracted from bauxite ( hydrated aluminium oxide, Al2O3.2H2O)

The Electrolysis of Aluminium Oxide.- Aluminium is so reactive that the electrolytic process would have to use molten solids instead of

aqueous solution. This in turn require very high temperature in order to maintain the oxide in the molten state.

- Pure aluminium oxide has a very high melting point which is 2500 degree Celcius.- The electrolysis process can be performed by dissolving the oxide in molten cryolite which

enables the electrolysis to be carried out at 900 degree Celcius. This temperature is maintained by electric current flowing through the electrolyte .

RX AT CATHODE (graphite) : Al3+(l) + 3e- Al (l)

RX AT THE ANODE (carbon): 2O2- (l) O2 (g) + 4e-

Physical Property Of Aluminium

- Silvery metal, good conductor of electricity and heat

- Strong but light, does not corrode, strong reducing agent.

- Versatile metal, ductile, malleable.

- Soft, and usually alloyed with other metals to increase its tensile strength and also to increase its resistance to corrosion

2.2 Bonding and Properties of Aluminium Oxide and Chloride Compounds.

• Aluminium forms ionic compounds with covalent characteristics due to the aluminium(III) ions having high charge density which enables it to polarise the electron cloud of an anion to give it a high degree of covalent character in the ionic bond.

• Magnitude of covalent characteristics present depends on the polarising power of the cation and the polarisability of the electron cloud surrounding the negative ion.

• For halide ions, the ability of the cation to polarise the electron cloud of the anion increase in the following order

F- < Cl- < Br- < I-

The tendency of the halide to form a covalent compound increase in the following order:

F- < Cl- < Br- < I-

Therefore aluminium floride has ionic characteristics, aluminium chloride is partially ionic and partially covalent whereas aluminium bromide shows covalent characteristics.

Aluminium Oxide (Alumina)

1.)White solid, very high melting point that is 2050 degree Celcius.2.) The bond between Al3+ and O2- ion is strong. The ionic bond between the two ions shows significant

covalent characteristics making Al2O3 amphoteric.3.)Lattice energy of Al2O3 is high. Therefore it is insoluble in water but dissolves in both hot acid and alkali.

Uses of aliminium oxide includes:1.) As a catalyst in cracking of alcohol and dehydration of alcohols.2.) As the stationary phase in column or thin layer of chromatography.3.) As refractory furnace bricks

Thermite Process1.) Aluminium oxide has a strong affinity for oxygen, forming aluminium oxide, Al2O3, a very stable

compound.

4Al (s) +3O2 (g) 2Al2O3 (s).

2.) The Thermite process is a redox reaction. Thermite is a mixture of aluminium powder and iron (III) oxide, Fe2O3.

3.) When the thermite mixture mixture is ignited, a violent reaction which is highly exorthemic will take place.

4.) The aluminium will be oxidised and iron (III) oxide will be reduce to white hot molten iron. This molten iron will sink to the bottom.

Fe2O3 (s) + 2Al Al2O3 (s) + 2Fe (l)

5.) The Thermite process is used in welding steel objects and for making bombs.

Aluminium Chloride

1.) Aluminium chloride (AlCl3) is a white solid.2.) The type of bonding in aluminium chloride is covalent with van der Waals forces of attraction

acting between molecules. This cause AlCl3 toa.) sublime at low temperature (180 degree Celcius)b.) dissolve easily in organic solvents,c.) form fumes in moist air

3.) In vapor state or when dissolved in organic solvent, aluminium chloride exists as a dimer (Al2Cl6). The monomers are bonded by dative covalent bonds. At temperature above 400 degree celcius, the dimer will dissociate to form monomer, AlCl3.

4.) In dimer, the chlorine atoms contain lone pair electrons which can be donated to the empty d-orbitals in aluminium. In this way, the aluminium atom ( in AlCl3) which has six electrons will achieve octet configuration.

5.) Aluminium chloride can be prepared by passing dry chlorine has or dry hydrogen chloride has over aluminium powder which has been heated.

2Al + 3Cl2 Al2Cl6

2Al + 6HCl Al2Cl6

6.)The solubility of aluminium chloride is an extremely exothemic process.AlCl3 + 6H2O [Al(H2O)6] 3+ + 3Cl-

7.) Aluminium chloride easily undergo hydrolysis when exposed to moisture in air.Al2Cl6 + 6H2O 2Al(OH)3 + 6 HCL

8.) The hydration energy is also large due to the high charge density of Al3+ . The Al-Cl bond can be easily broken and this causes the standard electrode potential to be high for aluminium.

3.3 Acidic Character of Aqueous Aluminium Salts.

1.) Aluminium salts from hexaaquaaluminium (III) ions in water.

2.) The charge density of the central aluminium ion withdraws electron density from the O-H bonds in the water molecules, thus weakening the bonds.

3.) The hydrated complex ion is therefore likely to lose a proton, H+ and behaves as an acid. The aluminium ion acts as a Bronsted-Lowry acid.

Al3+ + 3H2O Al(OH)3 + 3 H+

2.4 Resistance of Aluminium to Corrosion

1.) Aluminium is a reactive metal and therefore it should react very actively with oxygen and dilute acid.

2.) Aluminium is exposed to air (oxygen) or reacted with dilute acid, a thin, non-porous oxide layer (Al2O3 ) is formed on the surface of metal.

3.) The oxide layer protects the aluminium from further attack by oxygen or dilute acid.

4.) This oxide layer can be further thickened using the anodisation process, where sulphuric acid is electrolyed with aluminium as the anode.

Rx at the anode: 2Al + 3H2O Al2O3 + 6H+ + 6e- Rx at the cathode: 6H+ + 6e- 3H2

5.) The thickness of the aluminium oxide can be controlled by changing the flow of electric current during the electrolytic process.

6.) The oxide later can also trap dyes which enables aluminium to be coloured.

The Uses of Aluminium and its Compounds in Industry.

1.) Aluminium is used to make cans and good and drinks and also as aluminium foil for baking or packaging because it can be heated or chilled easily( good thermal conductivity)

2.)Aluminium is used in high-voltage transmission towers because aluminium is light and also a good conductor of electricity.

3.) Alloying of aluminium such as duralumin and magnalium to make it stronger and harder.

4.) Aluminium are useful in water purification.