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Physical and chemical properties of Aluminium

Answer Text:
Properties of Aluminium:
Physical properties
- Is a silvery white metal which quickly becomes dulled with a thin oxide layer.
- Has very low density (#2.7gcm^(-3)#), with ability to be rolled into wires / foil.
- Is a good conductor of heat and electricity.
Chemical properties.
1. Reaction with air;
- In air it acquires a continuous very thin coating of oxide, which resists further reaction.
- Removal of this protective cover renders the metal reactive.
- Consequently steel wool or wood ash should NOT be used in aluminium utensils.
- Usually, salty water attacks the oxide film allowing the aluminium to corrode and for this reason, ordinary aluminium is not used for marine purposes.
- Aluminium will burn in air at #800^oC# to form is oxide and nitrate.
Equations:
#4Al(s) +3O_2(g) to 2Al_2O_3(s)#
#2Al(s) + N_2(g) to 2AlN(s)#
2. Reaction with Acids.
Note:
- The protective Aluminium oxide (being covalent and insoluble) layer makes its reactivity with acids less than expected.
-With nitric (V) acid;
- Has hardly any effect on the metal, at any concentration.
Reason:
- Being a powerful oxidizing agent, it simply thickens the oxide layer thereby preventing further reaction.
-With sulphuric (VI) acid;
- Only hot concentrated sulphuric (VI) acid breaks down the oxide layer and reacts with the metal.
Equation:
#2Al(s) + 6H_2SO_4(l) to Al_2(SO_4)_3(aq) + 6H_2O(l) + 3SO_2(g)#
-With Hydrochloric acid;
- Dilute HCl dissolves aluminium slowly; liberating hydrogen.
Equation:
#2Al(s) + 6HCl(l) to 2AlCl_3(aq) + 3H_2(g) #
With concentrated HCl the rate of reaction is increased.
3. Reaction with chlorine;
- Hot aluminium burns in chlorine gas with a white light, forming dense white fumes of Aluminium (III) Chloride.
-The white fumes cool and collect on the cooler parts of the apparatus as a white solid.
Equation:
#2Al(s) + 3Cl_2(g) to 2AlCl_3(g)#
Note:
- The apparatus for the preparation of #AlCl_3# is kept very dry.
Reason:
- Aluminium chloride is readily/easily hydrolysed by water/moisture, and so it fumes in damp air with the evolution of hydrogen chloride gas.
Equation:
#AlCl_3(s) + 3H_2O(l) to Al(OH)_3(s)+3HCl(g)#
4. Reaction with water.
- Aluminium does not react with cold water, due to the formation of an insoluble coating of Aluminium oxide.
Note;
- If the oxide film is removed, the metal reacts slowly with cold water.
5. Reaction with caustic soda.
- The metal, especially in powder form, reacts with caustic soda solution, liberating hydrogen and leaving sodium aluminate in solution.
- The reaction is exothermic and once started, it is very vigorous.
Equation:
#2NaOH(aq) + 2Al(s) + 2H_2O(s) 2NaAlO_2(aq) + 3H_2(g)#
Ionically:
#2Al(s) + 2OH^(-) (aq) + 2H_2O(l) 2AlO^(-2) (g) + 3H_2(g)#
Note:
- Thus aluminium has an amphoteric nature as it reacts with both acids and alkalis.