igcse-extraction-and-uses-of-metals.pdf, Lecture notes of Chemical Processes

Download igcse-extraction-and-uses-of-metals.pdf and more Lecture notes Chemical Processes in PDF only on Docsity! Extracting and using metals Only the most unreactive metals such as gold and platinum are found as native metals. All the other metals we use are extracted from their ores by chemical processes. iron ore ores native Definition: An ore is a rock containing enough of a metal compound for it to be worth extracting the metal from the rock. The metal compounds in rocks are often metal oxides, sometimes metal sulphides or carbonates. Bauxite contains aluminium oxide, from which we extract aluminium. Galena is lead sulphide from which we obtain lead. Iron pyrites (Fools Gold) is actually iron sulphide, and contains no gold. We normally obtain iron from an ore called haematite which contains iron oxide. The process happens in a blast furnace. Raw materials: Iron ore – contains the iron to be extracted Coke – contains the carbon to reduce the iron ore Air – provides oxygen to burn the carbon, which also heats the blast furnace Limestone – is mainly calcium carbonate, which removes the impurities (sand) in the iron ore by reacting to form slag, which floats on the molten iron The chemistry: Firstly the carbon in the coke is oxidised by the oxygen in the air to form carbon dioxide. This exothermic reaction heats the blast furnace: C(s) + O2(g) CO2(g) The carbon dioxide reacts with more oxygen to form the reducing agent, carbon monoxide: C(s) + CO2(g) 2CO(g) The carbon monoxide then reduces the iron oxide in the iron ore to iron: Fe2O3(s) + 3CO(g) 2Fe(l) + 3CO2(g) Meanwhile the calcium carbonate is thermally decomposed to calcium oxide. This is a base which neutralises the sandy impurities (silicon dioxide, an acidic non-metal oxide) to make calcium silicate: CaCO3(s)  CaO(s) + CO2(g) CaO(s) + SiO2(s) CaSiO3(l) calcium oxide sand slag What comes out: Molten iron – this is cast iron and contains carbon impurities. It is brittle and not yet very useful. Slag – calcium silicate is a waste product, but can be used for road surfacing. Carbon dioxide – produced in several of the reactions in the blast furnace. Nitrogen – left over from the air going into the blast furnace, after the oxygen has been used in reactions. The lead oxide is then heated with coke (carbon) in a blast furnace to cause a displacement reaction which produces lead and carbon dioxide. This works because carbon is more reactive than lead, so carbon can reduce the lead oxide to lead. 2PbO(s) + C(s)  2Pb(l) + CO2(g) Extraction and Uses of Aluminium Electrolysis is used to extract aluminium from aluminium oxide; this is obtained by purifying its ore (bauxite). The ionic aluminium oxide is split into its elements, aluminium (a metal) and oxygen (a gas). aluminium oxide  aluminium + oxygen 2 Al2O3(l)  4 Al(l) + 3 O2(g) The aluminium oxide is mixed with cryolite, which acts as a solvent to dissolve the aluminium oxide, and allows the mixture to melt at a lower temperature (about 950˚C rather than nearly 2000˚C). This saves some energy costs – although the current required is very high, so the cost of electricity is a major factor. At the cathode, molten aluminium is collected: Al3+ (l) + 3e-  Al(l) At the anode, oxygen is formed: 2O2- (l) - 4e-  2O2(g) The anodes are made of graphite (carbon) and react with the oxygen, releasing carbon dioxide. This means they get eaten away, and have to be replaced sometimes. Aluminium resists corrosion because it forms a thin but tough layer of aluminium oxide over the surface of the metal. This prevents the metal underneath from reacting further with oxygen or moisture from the air. Titanium also forms a protective layer like this, so it too resists corrosion – it is also less reactive than aluminium. By growing the oxide layer on the surface of titanium to different thicknesses, it can be made to reflect different colours of light. Extracting titanium from rutile Carbon can’t be used to extract titanium from its ore because the titanium reacts further with carbon, so a more reactive metal, e.g. sodium or magnesium, is used to reduce the titanium ore instead. £ For both titanium and aluminium: Extraction of these metals from their ores is expensive. - because there are many stages in the processes - large amounts of energy are needed rutile – titanium ore Titanium is used in bodies of high-performance aircraft and in some jet engine parts • light weight (low density) • corrosion resistant • Keeps strength at high temperature Titanium is used in hip-replacements. • strength • resistance to corrosion