Metals and their compounds: Aluminium

CONTENT

• Aluminum
• Occurrence

Aluminum can be obtained as Kaolin, Al₂O₃.2SiO₃.2H₂O, cryolite. Na₃AlF₆, Corundum Al₂O₃ and mica K₂O.Al₂O₃.6SiO₂.

NB: the main source of aluminum is bauxite Al₂O₃.2H₂O

Extraction

Aluminum can be found in clay and rocks, but due to their high silica content, they can not be used for extraction of aluminum. The extraction of aluminum is carried out by electrolysis of bauxite.

The extraction proceeds in two stages.

1. Purification of Bauxite

Bauxite is first heated with caustic soda solution under pressure to form soluble sodium aluminate(III)

Al2O3 + 2NaOH + 3H2O  →  2NaAl(OH)4

The impurities, iron III oxide and trioxosilicates (iv) can be filtered off as a sludge.

The filterate contains aluminate (III) and then seeded with aluminum hydroxide crystals to induce precipitation of aluminum hydroxide.

NaAl(OH)₄ →   Al(OH)₃  + NaOH

The Al(OH)₃ is then filtered off, washed, dried and heated strongly to yield pure aluminium oxide or alumina while the NaOH is concentrated and used again.

2Al(OH)₃ →      Al₂O₃  + 3H₂O

Chemistry of the Reaction

Alumina  consist of aluminum and oxygen ions

At the Cathode

The aluminum ions gain three electrons each at the cathode to deposit as metallic aluminum.

Al³⁺  + 3e          →Al

At the Anode

The oxygen ions donate two electrons each to form atomic oxygen, which then pair off to form gaseous molecules.

O^2-                  →O + 2e-

O  + O →         O₂

Overall Reaction

4Al₃+ 6O^2-       →       4Al + 3O₂

Physical Properties

1. It is silvery white solid
2. Aluminum has density of 2.7glcm3
3. It is very malleable and ductible
4. It can be rolled into a foil
5. It has melting point of 660o                  
6. It is a very good conductor of  heat and electricity
7. It has moderate tensile strength but high in alloys.

Chemical Properties

1. Reaction with air

    4Al + 3O₂              →    2Al₂O₃(s)

    2Al + N₂                →     2AlN_(s)

2. Reaction with non-metals e.g Sulphur, Nitrogen phosphorus, carbon and halogen

               2Al + 3Cl₂       →      2AlCl₃

3. Reaction with Acids

    2Al + 6HCl  → 2AlCl₃ + 3H₂

    2Al + 6H₂SO₄ →  Al₂(SO₄)₂ + 6H₂O + 3SO₂

NB: Aluminum can not react with either dilute HNO₃ or conc. HNO₃ due to formation of a          protective layer of aluminum oxide.

4.  Reaction with Alkali

       2Al + 2NaOH  + 6H₂O  → 2NaAl(OH)₄  + 3H₂

5.  Reaction with iron III oxide

      2Al + Fe₂O₃  → Al₂O₃ + 2 F_3.

Test for Aluminum Ions

Add drops of sodium hydroxide solution to the unknown salt solution. Formation of a white    gelatinous precipitate which dissolve in an excess of sodium hydroxide solution indicates   the presence of aluminum ions.

Add a few drops of aqueous NH₃ solution to the unknown salt solution. The formation of   white gelatinous precipitate which dissolve in excess of aqueous NH3 confirms the   presence of aluminum ion .

        Al³⁺  + 3OH^–  →           Al(OH)₃

USES

1. Aluminium is used in making cooking utensil
2. It is sued in making overhead electric cables
3. It is used in making alloys e.g duralumin
4. Aluminium powder suspended in oil is used in paints mirrors and cars.

Iron

Occurences

Iron is usually found as haematite, Fe₂O₃, magnetite Fe₃O₄, iron pyrites, FeS₂, siderite or spathic iron ore FeCO₃ and limonite Fe₂O₃.H₂O.

Iron is the second most abundant metal in the earth’s crust after aluminium. It is also present in clay haemoglobin and chlorophyll in plants.

Extraction

The iron e.g haematite is first roasted in air to produce iron (III) oxide.

The iron (III) oxide is then mixed with coke and lime stone and heated to a very high temperature in a blast furnace.

In the lower part of the furnace, the white hot coke is oxidized by the oxygen in the hot air to liberate carbon (Iv) oxide. C + O₂       →   CO₂.

The CO₂ change to carbon(ii) oxide at the top of the furnace and then react with iron (III) oxide and reduce it to iron

Fe₂O₃ + 3CO     →   2F₃ + 3CO₂

The limestone present decomposes at high temperature to yield calcium oxide, which then combine with the silicon(Iv) oxide, impurity, to form calcium trioxosilicate (iv)

Ca CO₃      →     CaO + CO₂

SiO₂ + CaO + CaSiO₃

The molten iron sinks to the bottom of the furnace and is tapped off. It is run into moulds where it sets as pig iron.

Physical Properties

1. Iron is silvery solid with luster
2. It has relative density of 7.9
3. It is very ductile
4. It has high tensile strength
5. It has melting point of 1530oC
6. It is good conductor of heat and electricity

NB: It can be magnetized easily.

Chemical Properties

Reaction with Air

  4Fe + 3O₂ + 2xH₂O     →      2Fe₂O₃.X_H2O

Reaction with steam

   3Fe + 4H₂O         Fe₃O₄ + 4H₂

Reaction with non-metals e.g sulphur, chlorine,

 2Fe + eCl₂ → 2FeCl₃

   Fe + S → FeS

Reaction with Acid

Fe + H₂SO₄  → FeSO₄ + H₂

NB: No reaction is observed when conc. HNO₃ is added to iron.

Uses

NB: Fe²⁺ is used as confirmatory  test for oxidizing agent in the laboratory.

Cast iron is used for making objects which do not require high tensile strength e.g stove, cookers, lamp post radiator etc

Cast iron is used for making nails, chains, iron rods, and sheets of iron, agricultural implements etc.

GENERAL EVALUATION

1. What is the main source of Aluminium in nature?
   1. Using a diagram and equations, explain how pure Aluminium is extracted from the above source.
   1. With the aid of a diagram and the equation of the reaction, show how you would mend a broken iron rod by the thermit process.

READING ASSIGNMENT

New School Chemistry by Osei Yaw Ababio, pages 442 – 478.

WEEKEND ASSIGNMENT

1. Aluminium reacts readily with all common mineral acids except

A. HNO₃ B. HCl  C. H₂SO₄ D. H₃PO₄

2. The great affinity of aluminium for oxygen at high temperatures is employed in the

A. electrolytic process  B. thermit process  C. Haber process  D. lead chamber process

3. Aluminium materials should not be exposed to alkalis because aluminium is A. basic B. acidic  C. an oxidizing agent  D. a reducing agent
4. Wrought iron is the    A. purest form of iron B. most brittle form of iron C. most impure D. form of iron containing no carbon
5. Which one is the most common iron ore? A. Magnetite B. Haematite C. Carnallite D. Dolomite

THEORY

• (a) Write the electronic configuration of iron?

            (b) Iron forms the iron(II) ion, Fe²⁺ and the iron(III) ion, Fe³⁺.

• Write the electronic configurations of these ions.
• Which of them is more stable?
• Give reason for your answer.

• Explain why iron but not aluminium corrode easily on exposure to air despite the fact that aluminium is above iron in the electrochemical series.