1. Varieties of iron: There are three varieties of iron which
differ from each other on the basis of carbon content.
1. Cast or pig iron: It is the impure form of the iron and contain
highest % of carbon. It contains about 2 to 5%of carbon along with
little impurities such as silicon, manganese, phosphorus, sulphur,
etc. The solidified pig iron when re-melted can be cast into shape
thus it is termed as cast iron.
Uses:
1. It is used for the manufacture of steel and wrought iron.
2. It is used in pipes, stoves, building, toys,etc.
2. 2. Wrought iron: It is the purest form of the iron. It contains only 0.12 to
0.25% of carbon along with some little impurities like S, P, Mn etc. Wrought
iron is obtained by heating cast iron in the hearth of reverberatory furnace
lined with haematite (Fe2O3). Impurities like S, P, Mn and P along with carbon
in cast iron are oxidised by Fe2O3.
Fe2O3 + 3C → 2Fe + CO
2Fe2O3 + 3Si → 4Fe + 3SiO2
Fe2O3 + 3Mn → 2Fe + 3MnO
MnO + SiO2 → MnSiO3(slag)
Uses:
1. It is making chains and core of the magnets.
3. 3. Steel: It is an intermediate product containing carbon amount
between cast iron and wrought iron. It contains carbon about 0.1% to
1.5% and little amount of S, P, Si and Mn. Properties and hardness of
steel depend upon its carbon content.
5. Physical properties:
1. Iron is a grey coloured metal.
2. It is malleable, ductile and good conductor of heat and electricity.
3. Pure iron melts at 1536oc and boiling point is around 3000oc.
4. Its density is 7.86 g/cc.
6. Chemical properties:
1. Action of air:-
Dry air has no effect on iron but most air forms a brown layer of
hydrated ferric oxide called Rust.
2Fe +
3
2
O2 + xH2O → Fe2O3.xH2O
Hydrated ferric oxide (Rust)
7. 2. Action of Water:- Cold water does not react with iron but when
steam is passed over red hot iron,H2 gas is displaced.
3Fe + 4H2O → Fe3O4 + 4H2
Ferrosoferric oxide
3.Reaction with carbon monoxide:-
Iron reacts with CO at 1200C forming iron penta carbonyl.
Fe + 5CO → Fe (CO)5
Iron penta carbonyl
8. 4. Reaction with non-metals:-
Iron reacts directly with non-metals like halogens, sulphur carbon etc.
to give respective compounds.
2Fe + 3Cl2 → 2FeCl3
Fe + S → FeS
3Fe + C → Fe3C
Iron carbide or cementite.
9. 5. Reaction with acids:-
With dil. HCl and dil. H2SO4, iron gives ferrous salt along with H2 gas.
Fe + 2HCl → FeCl2 + H2
Fe + H2SO4 → FeSO4 + H2
with hot and conc. H2SO4, iron gas a mixture of ferrous sulphate and ferric sulphate.
Fe + 2H2SO4 → FeSO4 + SO2 + 2H2O
2Fe + 2H2SO4 → Fe2 (SO4)3 + SO2 + 2H2O
with dil HNO3, iron give nitrate and ammonium nitrate
4Fe + 10HNO3 → 4Fe (NO3)2 + NH4NO3 + 3H2O
with mod conc. HNO3, iron gives Ferric nitrate and NO2
Fe + 6HNO3 → Fe (NO3)3 + 3NO2 + 3H2O.
with hot and conc. HNO3, Iron dose not react due to formation of thin layer of Fe3O4 on its surface.
This is called passivity of iron.
3Fe + 8HNO3 → Fe3O4 + 8NO2 + 4H2O
10. Heat treatment of steel:
It is the process of heating and cooling the steel under controlled
condition in order to develop properties like hardness, strength,
ductility, etc. without changing the chemical composition .
The various terms used for the heat treatment of steel and their effects
are described below:
11. 1. Quenching or hardening: When steel is heated to bright redness
and is cooled suddenly by plunging it into water or oil, it becomes
extremely hard, brittle and doesn’t possess elasticity. This process is
known as hardening or quenching. The steel thus obtained is called
quenched steel. The hard and brittle quality of quenched steel is
due to the presence of more carbon in the form of iron carbide or
cementite.
3Fe + C → Fe3C (endothermic reaction)
12. 2. Annealing: When steel is heated to bright redness and cooled slowly,
it becomes soft, weak, ductile, less brittle and elastic. This process is
known as annealing and the steel obtained is called annealed steel. At
high temperature, iron and carbon in steel combines to form iron
carbide known as cementite.
3Fe + C ⇌ Fe3C (endothermic reaction)
This reaction is reversible and endothermic. At high temperature, the
forward reaction is favoured, when steel containing cementite is cooled
slowly, the above equilibrium shifts to left and carbon separates out as
small particles of graphite and steel becomes soft, less brittle and
elastic.
13. 3. Tempering: When the hardened or quenched steel is heated to a
temperature below redness and then cooled slowly, it neither becomes
too hard nor too brittle. This process is known as tempering and the
steel thus obtained is called tempered steel. The tempered steel is used
for making razor blades, knives, etc.
14. Steel Composition Properties Uses
Stainless Steel Fe=75%
Cr=18%
Ni=8%
C=1%
Have very high
chemical resistance.
in using ornamental
pieces, utensils, etc.
Manganese Steel Fe=86%
Mn=135
C=1%
Very hard and
resistant to wear
and tear.
Railway tracks,
safes, safety
helmets.
Tungsten Steel Fe=94%
W=5%
Very hard. Many high speed
cutting tools.
15. Invar Fe=64%
Ni=36%
Have very small
coefficient of linear
expansively.
in making clock hands ,
pendulum rods, meter
scales, etc.
Nickel Steel Fe=96-98%
Ni=2-4%
Resistant to corrosion
and is elastic.
wire cables, gears, drive
shafts, etc.
Perm alloy Fe=21%
Ni=78%
C=1%
Strongly magnetized by
electric field and gets
demagnetized when
current is switched off.
used in making
electromagnet.
16. Rusting of iron:
Formation of amorphous brown scale of hydrated oxide of iron on its
surface in the presence of moisture is called rusting of iron.
Electrochemical theory of rusting: The theory is called electrochemical
theory because it explains the formation of rust on the basis of the
formation of electrochemical cell on the surface of the metal. It is
believed that non-uniform surface of metal or impurities present in iron
behave like small galvanic cell in the presence of water containing
dissolved oxygen along with presence or absence of carbon dioxide.
In the small cell, pure iron acts as anode while cathodes are impure
portions. Oxidation occurs at anode. Therefore at anode iron atoms are
oxidised to ferrous ion.
17. At anode: Fe(S) → Fe++(aq) + 2e- ………………..(i)
Thus the metal atom in the lattice pass into the solution as ions,
leaving electrons on the metal itself. These electrons move towards the
cathode region through the metal.
At cathodes, the electrons ate taken up by gydrogen ions (reduction
takes place). The H+ ions are obtained either from water or from acid
substance(e.g. CO2) in water.
H20 ⇌ H+ + OH-
H20 + CO2 ⇌ H+ + HCO3
--
18. At cathode: H+ + e- → H
The hydrogen atoms on the surface reduced dissolved oxygen.
4H + O2 → 2H2O
Therefore, overall reaction at cathode is
4H+ + 4e- + O2 → 2H2O
19. Now,
Oxidation half reaction
Fe(s) → Fe++(aq) + 2e-]× 2 Eo = -0.44 V
Reduction half reaction
4H+ + O2 + 4e- → 2H2O Eo = +1.23 V
Overall cell reaction: 2Fe(s) + 4H+ + O2 → 2Fe++(aq) + 2H2O Eo
cell= +1.67V
The ferrous ions are oxidised further by atmospheric oxygen to form rust.
4Fe++(aq) + O2 (g) + 4H2O → 2Fe2O3 + 8H+
Ferric oxide absorbs moisture from air and give hydrated ferric oxide called
rust.
Fe2O3 + xH2O → Fe2O3xH2O(rust)
