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 MetalsChapter 20

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Extraction of Metals Metals can be extracted from its compounds or metal

ores by two general methods :

1. By electrolysisFor 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

KNaCaMg Al -----------------------------------------------------------ZnFePbCu

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)


At the anode: 2O2-(l) O2(g) + 4e-

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Quick check 11. 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


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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


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 21. 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 air3. (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



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.


Uses of some common metalsMetal 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 sheetsTin Coating tin-cans, making alloys such as bronze, solder

and pewterMagnesium Making fireworks and flares, alloys

Iron Making motorcar bodies, bridges, building construction and making into steel


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.


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 corrosion2. 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-platingSome 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 protection4. 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 31. 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 water2. Painting, greasing and plastic coating3. Galvanising, cathodic protection4. 4. (i) steel bridge: painting,

(ii) bicycle chain: greasing(iii) water pipes: cathodic protection

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1. http://www.ndt-ed.org/EducationResources/CommunityCollege/Materials/Introduction/metals.htm

2. http://www.school-portal.co.uk/GroupDownloadFile.asp?File=21221

3. http://www.gcsescience.com/ex33.htm

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