Metals and their compound, extraction of metals (e.g. aluminium, copper, tin and iron) their properties and their reaction, their uses, alloys, composition and uses

TOPIC: METALS AND THEIR COMPOUND

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.

O2-                 O + 2e-

O  + O           O2

Overall Reaction

4Al3+ 6O2-              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)

 

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

2Al + 3Cl2             2AlCl3

  1. 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.

  1. Reaction with Alkali

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

 

  1. 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

 

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.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.

 

 

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?
  2. Using a diagram and equations, explain how pure Aluminium is extracted from the above source.
  3. 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
  1. HNO3 B. HCl            C. H2SO4          D. H3PO4
  1. The great affinity of aluminium for oxygen at high temperatures is employed in the
  1. electrolytic process B. thermit process  C. Haber process  D. lead chamber process
  1. Aluminium materials should not be exposed to alkalis because aluminium is A. basic                         B. acidic        C. an oxidizing agent       D. a reducing agent
  2. 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
  3. Which one is the most common iron ore? A. Magnetite    B. Haematite C. Carnallite             D. Dolomite

 

THEORY

  1. (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.
  1. Explain why iron but not Aluminium corrode easily on exposure to air despite the fact that Aluminium is above iron in the electrochemical series.
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