TOPIC: METALS AND THEIR COMPOUND
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
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.
- 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.
O2- O + 2e-
O + O O2
4Al3+ 6O2- 4Al + 3O2
- It is silvery white solid
- Aluminum has density of 2.7glcm3
- It is very malleable and ductible
- It can be rolled into a foil
- It has 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 aluminum oxide.
- Reaction with Alkali
2Al + 2NaOH + 6H2O 2NaAl(OH)4 + 3H2
- Reaction with iron III oxide
2Al + Fe2O3 Al2O3 + 2 F3.
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
- Aluminium is used in making cooking utensil
- It is sued 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 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.
- Iron is silvery solid with luster
- It has relative density of 7.9
- It is very ductile
- It has high tensile strength
- It has melting point of 1530oC
- It is 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 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.
- 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
- HNO3 B. HCl C. H2SO4 D. H3PO4
- The great affinity of aluminium for oxygen at high temperatures is employed in the
- 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
- (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.