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METAL COUPLING ON RUSTING

OF IRON

NAME:___________________
CLASS:___________________
ROLL .NO:__________________
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INDEX

S .NO TOPIC P .NO

1 Introduction 3

2 Electrochemical 4
Mechanism ( rusting )

3 Rusting of iron 5

4 Prevention 6

5 Treatment 8

6 Coupling Reaction 8

7 Experiment 1 10

8 Experiment 2 12

9 Bibliography 14
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INTRODUCTION:

The process by which some metals when exposed to atmospheric condition i.e.
moist air, carbon dioxide form undesirable compounds on the surface is known
as corrosion..

Rusting is also a type of corrosion but the term is restricted to iron or

products made from it. Iron is easily prone to rusting making it surface rough.

Chemically, rust is a hydrated ferric oxideRust is formed by the redox reaction

of iron and oxygen in the presence of water or air moisture. Rust consists of
hydrated iron (III) oxides Fe2O3.nH2O and iron (III) oxide-hydroxide (FeO(OH),
Fe(OH)3).

Rust is another name for iron oxide, which occurs when iron or an alloy that
contains iron, like steel, is exposed to oxygen and moisture for a long period of
time. Rusting may be explained by an electrochemical mechanism.

In the presence of moist air containing dissolved oxygen or carbon dioxide, the
commercial iron behaves as if composed of small electrical cells. At anode of
cell, iron passes into solution as ferrous ions.

Fe Fe2+ + 2e-

The electrons from the above reaction move towards the cathode and form
hydroxyl ions

H2O + (O) + 2e- 2OH-

Under the influence of dissolved oxygen the ferrous ions and hydroxyl ions
interact to form rust, i.e., hydrated ferric oxide.

2Fe2+ + H2O + (O) 2Fe3+ + 2OH-

2Fe3+ + 6OH- Fe2O3.3H2O or 2Fe(OH) (Rust)

If supply of oxygen is limited the corrosion product may be black anhydrous

magnetite, Fe3
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RUSTING AN ELECTROCHEMICAL MECHANISM:

Rusting may be explained by an electrochemical mechanism.

In the presence of moist air containing

dissolved oxygen or carbon dioxide, the
commercial iron behave as if composed of
small electrical cells. At anode of cell, iron
passes into solution as ferrous ions.

The electron moves towards the cathode and

form hydroxyl ions.

Under the influence of dissolved oxygen the

ferrous ions and hydroxyl ions interact to form rust, i.e., hydrated ferric oxide
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RUSTING OF IRON :

Rusting is the phenomenon of the deposition of a reddish-brown coating on

the surface of iron by the action of moist air, and the reddish-brown
coating is known as rust. In simple words, when an iron object is left in
damp air for a considerable time, it gets covered with a red-brown flaky
substance called rust. This phenomenon is called rusting.

The most common example of metallic corrosion is rusting of iron and

steel. Many well-known examples include the rusting of exhaust systems
and car bodies, water pipes, and various types of structural steelwork.

Iron rusts due to the combined action of air and water on iron.

Rusting does not occur in completely dry air or

pure air free of water.

The precise composition of the rust is determined

by atmospheric conditions and the relative
contributions of the factors that govern rusting.

The following reaction can roughly describe its

formation:

In the presence of moist air, the outer surface of iron rusts first, and a
layer of hydrated ferric oxide (rust) is deposited on the surface.

This layer is soft and porous, and it may fall off if it becomes too thick.
This exposes the lower layers of iron to the environment, causing them to
rust.

The process continues, and iron gradually loses strength.

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PREVENTION:

Rust is permeable to air and water, therefore the interior metallic iron beneath
a rust layer continues to corrode. Rust prevention thus requires coatings that
preclude rust formation.

Some of the methods used to prevent corrosion and rusting are discussed here:

Galvanization:
The metallic iron is covered by a layer of more active metal such as zinc. The
active metal loses electrons in preference to iron.

Zn Zn2+ + 2e-
Thus, protecting iron from rusting and corrosion.

Coatings and Paintings:

Rust formation can be controlled with coatings, such as paint, lacquer, or
varnish that isolate the iron from the environment.

Inhibitors:
Corrosion inhibitors, such as gas-phase or violate inhibitors, can be used to
prevent corrosion inside sealed systems. They are not effective when air
circulation disperses them, and brings in fresh oxygen and moisture.
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Humidity control:
Rust can be avoided by controlling the moisture in the use of silica gel packets
to control humidity in equipment shipped by sea.

1 Rusting of iron can be prevented by painting: The most

common method of preventing the rusting of iron is to coat its surface
with paint.
 When the paint is applied to the surface of an iron object, air and
moisture are prevented from coming
into contact with the iron object, and
thus rusting does not occur.Window
grills, railings, iron bridges, steel
furniture, railway coaches, and the
bodies of cars, buses, and trucks,
among other things, are all painted
regularly to keep them from rusting.
2 Rusting of iron can be prevented
by applying grease or oil: When
some grease or oil is applied to the surface of an iron object, then air
and moisture cannot come in contact with it, and hence rusting is
prevented. For example, to avoid corrosion, iron and steel tools and
machine parts are rubbed with grease or oil.
3 Rusting of iron can be prevented by galvanisation: Articles
that are exposed to extreme moisture, such as roof sheets and pipes, are
protected from Rusting by galvanising. Galvanization is the process of
coating steel and iron with a thin layer of zinc to prevent rusting.
 The iron coated with zinc is called galvanised iron. As zinc is more
reactive than iron, it reacts with oxygen in the presence of moisture to
form an invisible layer of zinc oxide which protects it from further
Rusting.
 It is interesting to note that galvanised iron articles remain protected
from rusting even if zinc coating is broken.
 This is because zinc is more reactive than iron.
4 Rusting of iron can be prevented by electroplating: Another
technique used to prevent articles from rusting is electroplating.
 Metals that do not corrode, such as tin, nickel, and chromium, are
electroplated on iron in this process.
 This process not only prevents rusting but also improves the
appearance of the articles. Some of the articles that are chromium -
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plated are bathroom fittings and vehicle parts such as bicycle

handlebars, car bumpers, and so on.
5 Rusting of iron can be prevented by alloying it to make
stainless steel: When iron is alloyed with chromium and nickel,
stainless steel is formed. Stainless steel does not rust at all.
 Cooking utensils, scissors, surgical instruments, etc., are made of
stainless steel and do not rust at all.
 However, stainless steel is too expensive to be used in large
quantities .
6 Rusting of iron can be prevented by tinning:
 Tin is non-toxic and has a lower reactivity than iron.
 Food cans are tinned, which means they are thinly coated with tin.
 So, when a thin layer of tin metal is deposited on iron and steel
objects by electroplating, then the iron and steel objects are protected
from rusting. Tin-plated tiffin boxes are used because they are non-
poisonous and do not contaminate the food stored inside.
7 Rusting of iron can be prevented by Enameling: Enameling is a
process by which powdered glass is fused to a metal substrate at high
heat. Enamels can be applied to glass-ceramics.

TREATMENT :
Rust removal from small iron or steel objects by electrolysis can be done in a
home workshop using simple materials such as plastic bucket, tap water,
lengths of rebar, washing soda, baling wire, and a battery charger.

Rust may be treated with commercial products known as rust converter which
contain tannic acid which combines with rust

COUPLING REACTION :
A coupling reaction in organic chemistry is a general term for a variety of
reactions where two fragments are coupled with an aid of metal catalyst. In one
important reaction type a main group organometallic compound of the type
RM(R= organic fragment, M= main group centre) reacts with an organic halide
of the type R’X with formation of a new carbon-carbon bond in the product R-
R’.
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Catalysts involved in metal coupling reaction:

The most common catalyst is palladium, but an increasing number of
reactions use nickel. Other catalysts include copper, platinum, iron, cobalt,and
amines.In organic chemistry, a coupling reaction is a type of reaction in which
two reactant molecules are bonded together. Such reactions often require the
aid of a metal catalyst. In one important reaction type, a main
group organometallic compound of the type R-M (where R = organic group, M
= main group centre metal atom) reacts with an organic halide of the type R'-X
with formation of a new carbon - carbon bond in the product R-R'. The most
common type of coupling reaction is the cross coupling reaction broadly
speaking, two types of coupling reactions are recognized:

 Homocouplings joining two identical partners. The product is

symmetrical R−R
 Heterocouplings joining two different partners. These reactions are also
called cross coupling reactions. The product is unsymmetrical, R−R'.

Homo-coupling types :

Coupling reactions are illustrated by the Ullmann reaction:

Reaction Year Reactant A Reactant B Reagent Remark

Wurtz 1855 R-X sp3 R-X sp3 Na as
reaction reducing
agent
Pinacol 1859 R-HC=O R-HC=O various requires
coupling or or metals proton donor
reaction R2(C=O) R2(C=O)
Glaser 1869 RC≡CH sp RC≡CH sp Cu O2 as H-
coupling acceptor
Ullmann 1901 Ar-X sp2 Ar-X sp2 Cu high
reaction temperatures
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Cross-coupling types

EXPERIMENT 1 :

Aim:
To study the effect of metal coupling on rusting of iron.

Requirements:
Two petridishes, four test tubes, four iron nails, beaker, sand paper, wire
gauge. Gelatin, copper, zinc, magnesium strips, potassium ferricyanide
solution, phenolphthalein.

Procedure:
Clean the surface of iron nails with the help of sand paper. Wash them with
carbon tetrachloride and dry on filter paper.

Wind a clean zinc strip around one nail, a clean copper wire around the
second and clean magnesium strip around the third nail. Put all these third
and a fourth nail in petridishes so that they are not in contact with each other.

Preparation of agar agar solution:

Heat about 3gm of agar agar in 100ml of water taken in a beaker until
solution becomes clear. Add about 1ml of 0.1M potassium ferri-cyanide
solution, 1ml of phenol-phthalein solution and stir well the contents.

Fill the petridishes with hot agar agar solution in such a way that only
lower half of the nails are covered with the liquids.

Keep the covered petridishes undisturbed for one day or so.

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The liquid sets to a gel on cooling. Two types of patches are observed around
the rusted nail, one is blue and the other is pink. Blue patch is due to the
reaction between ferrous ions and potassium ferricyanide, to form potassium
Ferro-ferricyanide, KFe [Fe(CN)6] whereas pink patch is due to the formation of
hydroxyl ions which turns colorless phenolphthalein to pink.

Observations:

S.No. Metal pair Colour of the Nail rusts or

patch not
1. Iron-zinc pink No
2. Iron- Pink No
magnesium
3. Iron-copper blue Yes
4. Iron -nail blue Yes

Concl
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usion:
It is clear from the observations that coupling of iron with more
electropositive metal such as zinc and magnesium resists corrosion and
rusting of iron Coupling of iron with less electropositive metal such as copper
increases rusting

EXPERIMENT 2 :

Experiment to Prove that Air and Moisture are Essential for

Rusting

Procedure to demonstrate that rusting requires moisture and air:

1. Place clean iron nails in each of three test tubes labelled A,B, and C.
2. Pour some tap water into test tube AA and cork it.
3. Pour boiled distilled water into test tube B, then add about 1ml1ml of oil
and cork it. The oil will float on the water, preventing the air from
dissolving.
4. Place some anhydrous calcium chloride in test tube C and cork it. If there
is any moisture in the air, anhydrous calcium chloride will absorb it.
5. Allow these test tubes to sit for a few days before observing.
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Observation:
It is observed that iron nails rust in test tube aa but not in test
tubes B and C. Because the nails in test tube aa are exposed to both air and
water, therefore, the nails become rusted. The nails in test tube bb are only
exposed to water, while the nails in test tube C are exposed to dry air .

Conclusion:
This experiment shows that both air (oxygen) and moisture are essential
for rusting to take place.
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Bibliography:

 NCERT Chemistry – XII

 Comprehensive Practical Chemistry – XII
 Wikipedia