SAINIK SCHOOL
KAZHAKOOTAM
Effect of metal coupling on rusting
of iron
CHEMISTRY PRACTICAL
Index
TOPIC PAGE NO.
Certificate 1
Acknowledgement 2
Introduction 3
Prevention 5
Treatment 7
Coupling Reaction 8
Experiment 9
Bibliography 12
Certificate
It has been certified that Siddharth U studying in XII-A, during
the academic year 2017-2018 has completed the project on the
study of the effect of metal coupling on the rusting of iron and
has given satisfactory account of this in her project report.
Mr Arunkumar MS MR Sohan IS
(PGT Chemistry) (lab assistant )
Signature Signature
Acknowledgement
I would like to express my gratitude to Mr.Arunkumar MS, our
chemistry teacher, whose help and encouragement made this
project possible.
I would also like to thanks to Mr. Sohan IS, the lab assistant for
his help during the completion of this project.
Siddharth U
XII- A
Introduction
Metals and alloys undergo rusting and corrosion. 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. The compounds formed are
usually oxides. 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 oxide.
Rust 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. Over time, the oxygen
combines with the metal at an atomic level, forming a new
compound called an oxide and weakening the bonds of the
metals itself.
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)3
(Rust)
If supply of oxygen is limited the corrosion product may be
black anhydrous magnetite, Fe3
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.
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.
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’.
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.
Experiment
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 Petri dishes with hot agar agar solution in such
a way that only lower half of the nails are covered with
the liquids.
Keep the covered Petri dishes undisturbed for one day
or so.
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 Nail rusts
the patch or not
1. Iron-zinc pink No
2. Iron- Pink No
magnesium
3. Iron-copper blue Yes
4. Iron -nail blue Yes
Conclusion:
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
Bibliography:
NCERT Chemistry – XII
Comprehensive Practical Chemistry – XII
www.scribd.com
www.google.com
Wikipedia
