IRON

 

Week:                   FIVE                                       Date:                                                    

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

Subject:                                CHEMISTRY                                                                         Class: S S 3

Topic:                                    TRANSITION METAL

Sub topic: IRON

Reference materials:

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

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

(3) INTERNET

Instructional materials: iron plate

Entry behavior: The students have been taught copper.

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

1.       State the ore of iron.

2.       Explain the extraction of iron.

3.       Mention the physical properties of iron.

4.       Distinguish between the test of iron (ii) and iron (iii)

5.       List some areas how iron can be prevented.

CONTENT

IRON (Fe)

Iron is the second most abundant metal found in the earth’s crust. It is not found in the free metallic state except in meteorites. Its main ores are haematite, Fe2O3; magnetite, Fe3O4, limonite, 2Fe2O3.3H2O, iron pyrites FeS2 and siderite, FeCO3. Iron is also present as trioxosilicate(iv) in clay soil. It is a constituent of the pigment haemoglobin in animals and chlorophyll in plants.

EXTRACTION OF IRON

The ore is first roasted in air so that iron(iii)oxide is produced. Iron (iii) oxide is then mixed with coke and limestone and heated to a very high temperature in a blast furnace. The solid materials are fed into the furnace from the top, while a blast of hot air enters near the base through small air pipes.

In the lower parts, the coke is oxidized by the oxygen in the hot air blast to carbon(iv)oxide. The reaction is exothermic.

C(s) + O2(g)                 CO2(g)    H =-395KJmol-1

On moving up the furnace, the oxygen in the air lessens and carbon(iv)oxide become reduced by reacting with more coke to carbon(ii)oxide in an endothermic reaction.

CO2(s)  +  C(s)                         2CO(g)    H =+175KJmol-1

The carbon(II)oxide then reacts with the iron(III)oxide in an exothermic reaction, reducing it to iron, in the upper part of the furnace which is cooler.

Fe2O3(s) + 3CO(g)                     2Fe(l) + 3CO2(g)      H=-28KJmol-1

The molten iron produced, sink to the bottom of the furnace.

The function of the limestone is to remove impurities, such as silica in the form of a slag. The high temperature causes the limestone to decompose into calcium oxide and carbon(iv)oxide.

CaCO3(g)                CaO(g) + CO2(g)

Silicon(iv)oxide being an acidic oxide reacts with the basic calcium oxide to form calciumtrioxosilicate(iv)

SiO2 + CaO                 CaSiO3(s)

As the molten iron has a much higher densities than the slag. Two layers are formed at the base of the furnace, and they can be tapped separately. The molten iron is run into moulds where it is set as pig iron. It may also be used directly to make steel or further purified to produce cast iron and wrought iron.

Pure iron can also be obtained on the small scale by reducing its oxides or hydroxide with hydrogen.

Fe2O3 + 3H2               2Fe + 3H2O

Or by making its pentacarbonyl and decomposing this at a high temperature.

Fe + 5CO(heat/pressure)             Fe(CO)5above 250oC                 Fe + 5CO

PHYSICAL PROPERTIES OF IRON

I.                    It is malleable and ductile

II.                  It can be magnetized easily

III.                It has a relative density of 7.9gcm-3

IV.                It has a melting point of 1530oC

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

CHEMICAL PROPERTIES OF IRON

1.       REACTION WITH AIR: On exposure to moist air, iron gradually rusts, due to the formation of hydrated iron(iii)oxide.

4Fe(s) + 3O2(g) + 2XH2O              2Fe2O3.XH2O

       When heated in air, finely divided iron burns at high temperature, combining with oxygen to form triirontetraoxide (magnetic oxide)

3Fe(s) + H2O(g)                            Fe3O4(s)

 When steam is passed over red-hot iron filling, triirontetraoxides and hydrogen are produced.

3Fe(s) + H2O(g)                  Fe3O4(s) + H2(g)

2.       REACTION WITH ACIDS: Iron dissolves readily in dilute HCl and conc. HCl and dil. H2SO4 giving the corresponding iron(ii) salt and hydrogen.

Fe(s) + HClaq            FeCl2aq + H2(g)

Fe(s) + H2SO4aq      FeSO4aq   + H2(g)

Hot conc. H2SO4 undergoes reduction with iron giving off sulphur(iv)oxide.

Fe(s) + 2H2SO4           FeSO4aq  + SO2(g) + 2H2O(l)

With dilute HNO3, series of complex reaction occurs leading to the formation of several products such as iron(ii)trioxonitrate(v), ammoniumtrioxonitrate(v) and oxide of nitrogen.

Fe + 10HNO3          4Fe(NO3)2 + NH4NO3 + 3H2O

Concentrated HNO3 renders iron passive.

TEST FOR Fe2+

With NaOH: add few drops of NaOHaq to the unknown salts solution. The formation of dirty-green gelatinous precipitate which is insoluble in excess NaOH, indicate the presence of iron(ii) ion. If the ppt. turns reddish-brown on exposure to air, the presence of iron(iii) is confirmed.

Fe2+ + NaOHaq              Fe(OH)2{dirty-green ppt.} + 2Na2+

Fe(OH)2 + 2H2O + O2                  4Fe(OH)3 {reddish-brown ppt.}

Similar results are obtained if aqueous NH3 is used instead of NaOH.

With hexacyanoferrate(iii): add few drops of potassium hexacyanoferrate(iii) to the unknown salt solution. The formation of dark blue ppt. confirms the presence of iron(ii) ions.

Fe2+ + K3Fe(CN)6                     KFeII[FeIII(CN)6] + 2K+

TEST FOR Fe3+

With NaOH: add few drops of NaOH to the unknown salt solution. The formation of a reddish-brown gelatinous ppt., which is insoluble in excess NaOH, indicate the presence of Fe3+.

Fe3+aq + 3NaOHaq              Fe(OH)3(s){reddish-brown}  + 3Na+

Similar result are obtained with aq. NH3

With hexacyanoferrate(ii): add few drops of K4Fe(CN) to the unknown salt solution. The formation of a blue ppt. confirm the presence of Fe3+.

Fe3+ + K4Fe(CN)6                KFeIII[Fe(CN)6] + 3K+

With thiocyanate: add few drops of potassium thiocyanate solution to the unknown salt solution. The formation of a blood red solution confirms the presence of Fe3+. 

RUSTING OF IRON

The corrosion of iron by destructive process of rusting is essentially electrochemical in nature. When iron is exposed to moist air. It rusts and forms the brown-hydrated iron(iii)oxide.

4Fe(s) + 3O2(g) + 2XH2O(l)                  2Fe2O3.XH2O(s)

This oxide is non-coherent and permeable. It does not protect the metal from further reaction. Impurities in iron, presence of water and oxygen leads to iron rust.

PREVENTION OF RUSTING

I.                    Coating

II.                  Galvanizing

III.                Cathode protection

IV.                Alloying

V.                  Oil and greasing

USES OF IRON

I.                    Cast iron is used for making stoves, radiator, cookers, retort stand, lamp post, railing pipes and certain heavy machinery.

II.                  Wrought iron is used for making chains, nails, iron rod, horse shoe, core of electromagnets, agricultural implement etc

III.                Iron is used in alloying e.g. steel (Fe, C).

PRESENTATION

I.                    The teacher explains the extraction of iron

II.                  The teacher explains the properties of iron

III.                The students state the physical properties of iron.

IV.                The teacher explains test for iron (ii) and iron(iii)

V.                  The teacher explains rusting

VI.                The students state some ways iron is been prevented.

EVALUATION

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

1.  State the ore of iron.

2a. Name the main raw materials used for the extraction of iron in blast furnace

Ans: hametite (iron(III)oxide), limestone and coke

b. write the equations of the reactions taking place in the blast furnace.

Ans: i. C(s) + O2(g)          CO2(g)  ii CO2(s)  +  C(s)           2CO(g)  iii Fe2O3(s) + 3CO(g)       2Fe(l) + 3CO2(g)

 iv CaCO3(g)       CaO(g) + CO2(g)   v. SiO2 + CaO                 CaSiO3(s)

c. what is the name given to the iron obtained directly from the blast furnace? Ans- pig iron

d. state why the iron name in ( c) have a relatively low melting point ans- because it contains impurities/ not pure

ASSIGNMENT

         Iron is manufactured in a blast furnace using iron ore (Fe203), coke and limestone. Write the equation for the reaction) at the:

 (a). Top of the furnace

 (b) Middle of the furnace

 (c). Bottom of the furnace­­­­­­­­­­­­­­­­­­­

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