Back to: CHEMISTRY SS3
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In today’s class, we will be talking about the extraction of metals. Enjoy the class!
Extraction of Metals
Aluminium can be obtained as Kaolin, Al2O3.2SiO3.2H2O, cryolite. Na3AlF6, Corundum Al2O3 and mica K2O.Al2O3.6SiO2.
NB: the main source of aluminium is bauxite Al2O3.2H2O
Aluminium can be found in clay and rocks, but due to their high silica content, they can not be used for extraction of aluminium. The extraction of aluminium is carried out by electrolysis of bauxite.
The extraction proceeds in two stages.
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 filtrate contains aluminate (III) and then seeded with aluminium hydroxide crystals to induce precipitation of aluminium 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 aluminium and oxygen ions
At the Cathode:
The aluminium ions gain three electrons each at the cathode to deposit as metallic aluminium.
Al3+ + 3e → Al
At the Anode:
The oxygen ions donate two electrons each to form atomic oxygen, which then pairs off to form gaseous molecules.
O2- → O + 2e-
O + O → O2
4Al3+ 6O2- → 4Al + 3O2
- It is silvery-white solid
- Aluminium has a density of 2.7glcm3
- It is very malleable and ductile
- It can be rolled into a foil
- It has a melting point of 660o
- It is a very good conductor of heat and electricity
- It has moderate tensile strength but high in alloys.
- Reaction with air:
4Al + 3O2 → 2Al2O3(s)
2Al + N2 → 2AlN(s)
- Reaction with non-metals: e.g. Sulphur, Nitrogen phosphorus, carbon and halogen
2Al + 3Cl2 → 2AlCl3
- 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 aluminium oxide.
- Reaction with Alkali:
2Al + 2NaOH + 6H2O → 2NaAl(OH)4 + 3H2
- Reaction with iron (III) oxide:
2Al + Fe2O3 → Al2O3 + 2 F3.
Test for aluminium ions
Add drops of sodium hydroxide solution to the unknown salt solution. Formation of a white gelatinous precipitate which dissolves in an excess of sodium hydroxide solution indicates the presence of aluminium ions.
Add a few drops of aqueous NH3 solution to the unknown salt solution. The formation of a white gelatinous precipitate which dissolves in excess of aqueous NH3 confirms the presence of aluminium ion.
Al3+ + 3OH– → Al(OH)3
- Aluminium is used in making cooking utensil
- It is used in making overhead electric cables
- It is used in making alloys e.g. duralumin
- Aluminium powder suspended in oil is used in paints mirrors and cars.
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.
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 limestone 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 combines with the silicon(Iv) oxide, impurity, to form calcium trioxosilicate (iv)
Ca CO3 → CaO + CO2
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.
- Iron is silvery solid with lustre
- It has a relative density of 7.9
- It is very ductile
- It has a high tensile strength
- It has a melting point of 1530oC
- It is a good conductor of heat and electricity
NB: It can be magnetized easily.
- Reaction with air:
4Fe + 3O2 + 2xH2O → 2Fe2O3.XH2O
- 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.
NB: Fe2+ is used as a confirmatory test for an 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.
- What is the main source of Aluminium in nature?
- Using a diagram and equations, explain how pure Aluminium is extracted from the above source.
- 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.
New School Chemistry By Osei Yaw Ababio, pages 442 – 478.
- Aluminium reacts readily with all common mineral acids except A. HNO3 B. HCl C. H2SO4 D. H3PO4
- 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
- Aluminium materials should not be exposed to alkalis because aluminium is A. basic B. acidic C. an oxidizing agent D. a reducing agent
- 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
- Which one is the most common iron ore? A. Magnetite B. Haematite C. Carnallite D. Dolomite
- Explain why iron but not Aluminium corrode easily on exposure to air even though Aluminium is above iron in the electrochemical series.
- (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 a reason for your answer.
In our next class, we will be talking about Introduction to Qualitative Analysis. We hope you enjoyed the class.
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