ALUMINIUM AND TIN

 

Week:                   THREE                                   Date:                                                    

Period:                                  Duration: 1 HR 20 MIN.                                                  Average age of learners: 17YEARS

Subject:                                CHEMISTRY                                                                         Class: SS 3

Topic:                                    METALS AND THEIR COMPOUNDS

Sub topic: ALUMINIUM AND TIN

Reference materials:

(1) ESSENTIAL CHEMISTRY, TONALD PUBLISHERS, I. O ODESINA

(2) NEW SCHOOL CHEMISTRY, AFRICAN FIRST PUBLISHERS, OSEI YAW ABABIO

(3) INTERNET

Instructional materials: Milk can and Aluminium pot.

Entry behavior: The students have been taught sodium and calcium.

Behavioural objective: At the end of the lesson the students should be able to:

1.       List the main sources of aluminium.

2.       Explain the extraction of aluminium.

3.       Test for aluminium ions

4.       Itemize some uses of aluminium

5.       State the physical properties of tin

6.       Mention areas tin are used

CONTENT

ALUMINIUM (Al)

Aluminium is the most abundant metal and the third most plentiful element in the earth’s crust. It is not found free in nature because it has great affinity for oxygen. It occur abundantly as trioxosilicate(iv) in rocks and clays. The main source of aluminium is the mineral bauxite,AlO3.2H2O. Other important minerals include cryolite,Na2AlF6; kaolin,Al2Si2O7.2H2O; corundum,Al2O3 and mica,K2Al2Si6O16.

EXTRACTION OF ALUMINIUM

Aluminium is extracted by the electrolysis of solution of aluminium oxide in molten cryolite. The pure aluminium oxide is obtained from the bauxite. The extraction proceed in two stages:

I.                    Purification of bauxite           II. Electrolysis of alumina (reduction).

Chemistry of the reaction

Alumina consists of aluminium and oxygen ions.

At the cathode: The Al3+ gain 3e- each to become deposited at metallic aluminium.

2Al3+ + 6e-                 2Al(s)  (Reduction)

At the anode: The O2- donate 2e- each and become oxidized to atomic oxygen and then pair up to form gaseous oxygen molecules.

O2-            O  +  2e- ;  O + O          O2     or     2O2-           4e- + O2.

PHYSICAL PROPERTIES OF ALUMINIUM

I.                    It is a silvery-white metal.

II.                  It is ductile and malleable.

III.                It is a good conductor of heat and electricity.

IV.                It has a moderate tensile strength.

V.                  It has a melting point of 660oC and boiling point of 2450oC.

CHEMICAL PROPERTIES OF ALUMINIUM

1.       REACTION WITH AIR: When a sheet of aluminium is expose to moist air it forms a thin coating of Al2O3, which prevent further attack of the metal by atmospheric oxygen.

All metal burns in air at a high temperature of about 800oC to form the oxide and the nitride.

4Al  + 3O2             Al2O3;       2Al +N2             2AlN

2.       REACTION WITH ACID: Al react slowly with dilute HCl but more rapidly with the conc. acid to displace hydrogen.    2Al + 6HCl                     2AlCl3 + 3H2

Al does not react with dilute H2SO4 but hot conc. H2SO4 attack Al to liberate SO2.

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

3.       REACTION WITH ALKALI: Al dissolves in both Na and K hydroxide to form a soluble aluminate(iii) with the evolution of hydrogen gas. 2Al + NaOH + 6H2O               NaAl(OH)4 + 3H2

4.       Al + Fe2O3 [iron(iii)oxide]                   Al2O3 + Fe

 

TEST FOR Al3+

I.                    WITH NaOH- Add few drops of NaOHaq to the unknown salt solution. The formation of a white gelatinous ppt. which dissolve in excess NaOH indicate the present of Al3+

Al3+ + NaOH                    Al(OH)3 + 3Na+

Al(OH)3  +  NaOH [excess]             NaAl(OH)4

II.                  WITH aq NH3- Add few drops of aq NH3 to the unknown salt solution. The formation of a white gelatinous ppt. which is insoluble in excess aq. NH3 indicate the presence of Al3+.

USES OF ALUMINIUM

i.                     Al foils are used as packaging materials.

ii.                   Al3+ are used as coagulating agent i.e water treatment plants to remove colloidal organic matters.

iii.                  Used in making cooking utensil.

iv.                 Used in making overhead electric cable.

v.                   Making alloy such as duralumin.

vi.                 Al powder suspended in oil is used in paints and mirror because of its high reflectivity.

 

TIN (Sn)

Tin occur naturally as cassiterite or tinstone SnO2. Large deposit of the ore is found in Malaysia, Bolivia, and Indonesia. Some quantity is found in the granite rocks and alluvial bed in Jos Plateau in Nigeria.

EXTRACTION OF TIN

The ore, SnO2 is crushed and washed free of impurities by strong water current. It is then roasted in air to remove impurities such as sulphur, arsenic and antimony as volatile oxides. The tin(iv) oxide so obtained is then reduced by heating with coke in a reverberatory furnace. Molten tin is then tapped off. SnO2 + 2C2            Sn + 2CO

The molten tin is obtained is impure. It is purified by heating it gently on a sloping surface. As it flows down the surface, impurities are exposed to air and become converted to oxides which are left behind as scum.

PHYSICAL PROPERTIES OF Sn

I.                    Tin is silvery-white solid with metallic luster.

II.                  A good conductor of heat and electricity.

III.                It is not ductile enough to be drawn into wires.

IV.                It has fairly strong tensile strength.

V.                  It has melting point of 230oC and boiling point of 2270oC

VI.                It exist in 3 allotropic form with different densities.

Grey tin,5.76gcm-3,                      White tin,7.28gcm-3,                           Rhombic tin,6.6gcm-3

VII.              Tin exist as white tin at room temperature.

CHEMICAL PROPERTIES OF TIN

1.       REACTION WITH OXYGEN: Sn + O2(1200Oc)                             SnO2

2.       REACTION WITH NON-METAL: Sn +Cl2                       SnCl4 (Tin(iv)chloride). It does not combine with nitrogen and carbon.

3.       REACTION WITH ACID: Sn + 2HCl        SnCl2 + H2; Sn + H2SO4 (hot conc.)             SnSO4 + SO2 + 2H2O

4.       REACTION WITH ALKALI: it reacts with concentrated solution of alkalis to give trioxostannate (iv) salt and hydrogen. Sn + 2NaOH +H2O                  Na2SnO3 + 2H2

USES OF TIN

I.                    It is widely in coating steel to prevent it from corrosion.

II.                  It is used in making sheet glass.

III.                It is used in making alloys such as bronze ( Sn, Cu)

PRESENTATION

I.                    The teacher states the main sources of aluminium and the extraction process.

II.                  The teacher explains the properties of aluminium.

III.                The students mention uses of aluminium

IV.                The teacher states the main sources of tin and the extraction process.

V.                  The teacher explains the properties of tin.

VI.                The students mention uses of tin.

EVALUATION

 The teacher evaluates the lessons by asking the following questions:-

1.       List the main sources of aluminium.

2.       Explain the extraction of aluminium.

3.       Test for aluminium ions

4.       Itemize some uses of aluminium

5.       State the physical properties of tin

6.       Mention areas tin are used

ASSIGNMENT

1.            Aluminum is extracted from its ore by electrolysis.

(a)          Name the ore from which the metal is extracted

(b)          State the role of molten cryolite in the extraction.

(c.)         Describe in outline how the ore is purified before electrolysis.

(d)          Give the reason why aluminum, which is a reactive metal, is  resistance to corrosion.

2a           Give two reasons why aluminum is preferred to copper for making over head electric cables.

 (bi).       Describe briefly how tin can be extracted from its ore.

(ii).         State two uses of tin.

(iii).        Make one property that makes tin suitable for the use stated     in 2 (b ii).

 

 

 

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