C20 extraction of metals

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LEARNING OUTCOMES
 Relate the principles underlying the extraction
of a metal to its position in the electrochemical
series
 Describe the extraction of aluminium
 Describe the extraction of iron
 Relate the properties of the metals (aluminium,
lead, iron) and their alloys to their uses
 Describe the conditions necessary for the
corrosion of metals
Extraction of Metals
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Metals can be extracted from its compounds or metal
ores by two general methods :
1. By electrolysis
For reactive metals above carbon in the reactivity series E.g.
potassium, sodium, calcium, magnesium, aluminium
2. By reduction with coke (carbon):
For metals below carbon in the reactivity series E.g.
zinc, iron, lead, copper
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The Reactivity Series
K
Na
Ca
Mg
Al
-----------------------------------------------------------
Zn
Fe
Pb
Cu
Metals are more reactive and
tightly combined with oxygen,
so more energy is required to
decompose them.
Metals are less reactive and
less tightly combined with
oxygen, so less energy is
required to decompose them.
By Electrolysis
By Reduction with Coke
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Extraction of metals by reduction with coke
 Metal ores like sulphides and carbonates are first
roasted in air to change them into oxides.
 The metal oxides are then heated strongly with coke
(carbon) in a furnace.
 Reduction takes place and the metal is formed.
E.g. ZnO(s) + C(s)  Zn(l) + CO(g)
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Extraction of metals by electrolysis
 The metal ores are molten and electricity is passed into it. The
metal is deposited on the cathode.
 E.g. The electrolysis of aluminium oxide produces aluminium.
At the cathode: Al3+
(l) + 3e-
 Al(l)
Overall reaction: 2Al2O3(l)  4Al(l) + 3O2(g)
Chapter 20
At the anode: 2O2-
(l)  O2(g) + 4e-

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Quick check 1
1. State the method by which the following metals can be extracted
from their ores:
(a) sodium, (b) aluminium, (c) calcium, (d) iron, (e) copper.
2. Tin is below iron in the reactivity series.
(a) Predict the method to extract tin from its oxide, SnO4.
(b) Write a balanced chemical equation for the extraction of SnO4.
Solution
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1. Sodium, aluminium and calcium by electrolysis;
Iron and copper by reduction with coke.
2. (a) By heating it with coke.
(b) SnO4 + 2C  Sn + 2CO2
Return
Solution to Quick check 1
Chapter 20

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Extraction of Iron
 The main ore from which iron is extracted is called
haematite or iron(III) oxide, Fe2O3.
 Iron is extracted from its ores by reduction with coke in
a blast furnace.
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 A mixture of iron ore, coke and limestone
are added from the top of the furnace.
 Hot air is blown in from the sides of the
furnace.
 The coke burns in the hot air to form
carbon dioxide and heat:
C(s) + O2(g)  CO2(g) + heat
 The carbon dioxide reacts with more
coke to form carbon monoxide:
CO2(g) + C(s)  2CO(g)
 The carbon monoxide then reduces the
iron(III) oxide to iron:
Fe2O3(s) + 3CO(g)  2Fe(l) + 3CO2(g)
Extraction of Iron
Chapter 20

 The limestone breaks down to
form calcium oxide which
reacts with the impurities in iron
ore (SiO2) to form slag,
CaO(s) + SiO2(s)  CaSiO3(l)
 The molten iron sinks to the
bottom of the furnace.
 The slag floats on top of
the molten iron and can be
removed separately.
Extraction of Iron
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Cast iron
 The iron obtained from the blast furnace is impure
as it contains large quantities of carbon and sulphur.
It is called cast iron or pig iron.
 Cast iron is brittle and has few uses.
It is used for making objects which require little
strength such as, drainage covers, support for
stoves and metal gates.
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Making iron into steel
 To make cast iron into steel, pure oxygen is blown
into the hot molten iron.
 The oxygen burns away impurities like carbon and
sulphur into its gaseous oxides.
 The steel formed is called mild steel. It is the most
widely used form of iron (steel).
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Types of steel
 To make iron into different kinds of steel, small
amounts of different metals are added.
 Stainless steel contains chromium and nickel which
make it strong and corrosion resistant.
 To make high speed steel, small amounts of
manganese, chromium and tungsten are added. This
steel is very strong and hard.
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Uses of iron and steel
 Mild steel is used for making the bodies of motorcars,
ships, bridges and building construction.
 Stainless steel is used for making cutlery, surgical
instruments and water pipes.
 High speed steel is used for making drilling and
cutting tools and locks.
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Quick check 2
1. What is the name of the iron ore from which iron is
extracted? What is its chemical name and formula?
2. Name three other raw materials which are added
together with the iron ore to the blast furnace.
3. What is the purpose of adding (a) limestone,
(b) coke, to the iron ore in the blast furnace?
4. When coke is burned, what gas is first formed?
What happens to this gas as it reacts with more
coke? Write chemical equations for both reactions.
5. Write the chemical equation for the reduction of
iron(III) oxide in the blast furnace. Name the
oxidising agent and reducing agent. Solution
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1. Haematite; Iron(III) oxide; Fe2O3
2. Limestone, coke and hot air
3. (a) The limestone decomposes into calcium oxide which then
reacts with the acidic impurities in the iron ore to form slag.
(b) Coke burns to provide the heat for the furnace. It also forms
carbon monoxide which reduces the iron ore to the metal.
4. When the coke burns, carbon dioxide is first produced:
C(s) + O2(g) CO2(g)
The carbon monoxide then reacts with more hot coke to form
carbon monoxide: CO2(g) + C(s)  2CO(g)
5. Fe2O3(s) + 3CO(g)  2Fe(l) + 3CO2(g)
Oxidising agent: iron(III) oxide;
Reducing agent: carbon monoxide Return
Chapter 20

Uses of Metals
• Metals are used for making many objects.
• The choice of using a metal depends on
the following factors:
1. its physical properties, such as its
strength, density and melting point;
2. its cost;
3. its chemical properties, such as
corrosion-resistance.
Chapter 20

Uses of some common metals
Metal Uses
Copper Making electric wires and cables, coins,
alloys such as brass and bronze
Aluminium Making soft drink cans, cooking pots, alloys
Zinc Making brass, galvanised iron sheets
Tin Coating tin-cans, making alloys such as bronze, solder
and pewter
Magnesium Making fireworks and flares, alloys
Iron Making motorcar bodies, bridges, building construction
and making into steel
Chapter 20

Uses of Some Alloys
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Corrosion of Iron & steel
 Iron and steel are cheap and the most widely used
forms of metal.
 However, iron and steel rust or corrode easily.
 Large sums of money have to be spent on the
prevention of corrosion of iron and steel objects and
structures, and to replace items which have corroded.
Chapter 20
Corrosion of vehicle
parts made of mild steel

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Experiment
 After a few days, rusting occurs
only in Test tube C.
 This shows that for iron to rust,
both air (oxygen) and water must
be present.
 The part played by water is a
complex one, and we will just
regard rusting as the oxidation of
iron by oxygen in the air to form
iron(III) oxide.
Oil
Boiled
water
Iron
nail
Setup to investigate
the rusting of iron
4Fe(s) + 3O2(g)  2Fe2O3(s)
To investigate the rusting of iron
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• Iron and steel corrode rapidly under normal atmospheric
conditions to form a reddish-brown solid called rust.
• Both water and oxygen play a part in the corrosion of
metal.
• When a water droplet comes into contact with a piece of
iron, it becomes a voltaic cell and iron starts to be
oxidised.
• Corrosion of iron is an electrochemical process, which
means that rusting is accelerated by the presence of
electrolytes in the water.
Corrosion
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Prevention of corrosion
1. Painting or greasing
 This the most common method of rust
prevention. The layer of paint protects the iron
from contact with air and water and hence
prevents it from rusting.
 For objects which cannot be painted e.g. moving
parts of a machine, a layer of grease is applied.
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Prevention of corrosion
2. Plastic coating
Some iron objects are covered or coated with a layer of
plastic to prevent it from corrosion.
E.g. paper clips, clothes hangers and cabinet shelves
3. Tin-plating
Some iron objects are covered or coated with a layer of
tin to prevent it from corrosion. Steel cans are coated
with a thin layer of tin on the inside of the cans.
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Sacrificial protection
4. Galvanising
 The iron is covered with a layer of zinc which protects
the iron both physically and chemically from corrosion.
Zinc being more reactive than iron in the reactivity
series, corrodes in preference over iron and hence is
“sacrificed” to protect the iron from rusting.
 Hence this method of protection is also called “sacrificial
protection”.
 Objects made from galvanised iron include “zinc roofs”,
dust-bins and fences.
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Sacrificial protection
5. Cathodic protection
 This is also a form of sacrificial protection. The iron
object is connected by a wire to a block of a more
reactive metal, such as magnesium or zinc.
 The more reactive metal acts as an anode and is
oxidised (corroded) in preference over the iron
which becomes the cathode.
 This method of protection is used to protect the
hull of ships and underground pipe lines.
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• This process is similar to tin plating.
• Carried out by electrolysis.
• The metal that is to be protected will be made the cathode
while the less reactive metal will be made the anode.
• Electroplating thus prevents corrosion and beautifies the
object.
Electroplating
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Quick check 3
1. State the conditions for the rusting of iron.
2. Name three methods of preventing the rusting of iron and
steel by placing a physical barrier around the metal.
3. Name two methods of preventing the rusting of iron by
electrochemical means.
4. State the method by which you would prevent corrosion of
the following:
(i) steel bridge, (ii) bicycle chain, (iii) water pipes.
Solution
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1. Presence of both air (oxygen) and water
2. Painting, greasing and plastic coating
3. Galvanising, cathodic protection
4. 4. (i) steel bridge: painting,
(ii) bicycle chain: greasing
(iii) water pipes: cathodic protection
Return
Chapter 20

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1. http://www.ndt-
ed.org/EducationResources/CommunityCollege/Materials/Introducti
on/metals.htm
2. http://www.school-portal.co.uk/GroupDownloadFile.asp?File=21221
3. http://www.gcsescience.com/ex33.htm
To learn more about Extraction of
Metals, click on the links below!
Chapter 20

References
• Chemistry for CSEC Examinations
by Mike Taylor and Tania Chung
• Longman Chemistry for CSEC by
Jim Clark and Ray Oliver
32

C20 extraction of metals

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C20 extraction of metals