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C20 extraction of metals


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C20 extraction of metals

  1. 1. Chapter 20Extraction of Metals 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 1
  2. 2. Chapter 20Extraction of MetalsExtraction of MetalsMetals can be extracted from its compounds or metalores by two general methods :1. By electrolysis For reactive metals above carbon in the reactivity series E.g. potassium, sodium, calcium, magnesium, aluminium2. By reduction with coke (carbon): For metals below carbon in the reactivity series E.g. zinc, iron, lead, copper 2
  3. 3. Chapter 20Extraction of MetalsThe Reactivity SeriesK Metals are more reactive andNa tightly combined with oxygen,Ca By Electrolysis so more energy is required toMg decompose them.Al-----------------------------------------------------------Zn Metals are less reactive andFePb By Reduction with Coke less tightly combined with oxygen, so less energy isCu required to decompose them. 3
  4. 4. Chapter 20Extraction of MetalsExtraction 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) 4
  5. 5. Chapter 20Extraction of MetalsExtraction 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) At the anode: 2O2-(l)  O2(g) + 4e- Overall reaction: 2Al2O3(l)  4Al(l) + 3O2(g) 5
  6. 6. Chapter 20Extraction of MetalsQuick 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 6
  7. 7. Chapter 20Extraction of MetalsSolution to Quick check 11. 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 7 Return
  8. 8. Chapter 20Extraction of MetalsExtraction 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. 8
  9. 9. Chapter 20Extraction of MetalsExtraction of Iron 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) 9
  10. 10. Chapter 20Extraction of MetalsExtraction of Iron The molten iron sinks to thebottom of the furnace. 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 slag floats on top of the molten iron and can be removed separately.
  11. 11. Chapter 20Extraction of MetalsCast 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. 11
  12. 12. Chapter 20Extraction of MetalsMaking 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). 12
  13. 13. Chapter 20Extraction of MetalsTypes 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. 13
  14. 14. Chapter 20Extraction of MetalsUses 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. 14
  15. 15. Chapter 20Extraction of Metals 15
  16. 16. Chapter 20Extraction of MetalsQuick 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 16
  17. 17. Chapter 20 Extraction of Metals Solution to Quick check 21. Haematite; Iron(III) oxide; Fe2O32. 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 17
  18. 18. Chapter 20Extraction of MetalsUses 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.
  19. 19. Chapter 20Extraction of MetalsUses of some common metals Metal UsesCopper Making electric wires and cables, coins, alloys such as brass and bronzeAluminium Making soft drink cans, cooking pots, alloysZinc Making brass, galvanised iron sheetsTin Coating tin-cans, making alloys such as bronze, solder and pewterMagnesium Making fireworks and flares, alloysIron Making motorcar bodies, bridges, building construction and making into steel
  20. 20. Chapter 20 Extraction of MetalsUses of Some Alloys
  21. 21. Chapter 20Extraction of MetalsCorrosion 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 21
  22. 22. Chapter 20Extraction of MetalsExperimentTo investigate the rusting of iron After a few days, rusting occursonly in Test tube C. Oil This shows that for iron to rust,both air (oxygen) and water must Boiled waterbe present. Iron The part played by water is a nailcomplex one, and we will just Setup to investigateregard rusting as the oxidation of the rusting of ironiron by oxygen in the air to formiron(III) oxide.4Fe(s) + 3O2(g)  2Fe2O3(s) 22
  23. 23. Chapter 20Extraction of Metals Corrosion• 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. 23
  24. 24. Chapter 20Extraction of MetalsPrevention of corrosion1. 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. 24
  25. 25. Chapter 20Extraction of MetalsPrevention 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 shelves3. 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. 25
  26. 26. Chapter 20Extraction of MetalsSacrificial 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. 26
  27. 27. Chapter 20Extraction of MetalsSacrificial protection5. 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. 27
  28. 28. Chapter 20 Extraction of Metals Electroplating• 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. 28
  29. 29. Chapter 20Extraction of MetalsQuick 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 29
  30. 30. Chapter 20Extraction of MetalsSolution to Quick check 31. 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 Return 30
  31. 31. Chapter 20Extraction of Metals To learn more about Extraction of Metals, click on the links below! 1. http://www.ndt- on/metals.htm 2. 3. 31