Using Resources 2 1. Corrosion and its prevention Corrosion is the destruction of materials by chemical reactions with substances in the environment, eg rusting is the reaction of iron with both air and water. Corrosion can be prevented by applying a coating that acts as a barrier eg greasing, painting or electroplating. Aluminium forms an oxide coating on its surface that protects the metal from further corrosion. Sacrificial protection is coating a less reactive metal with a more reactive one. The more reactive metal in the coating reacts to protect the metal underneath, eg Galvanised iron has a coating of zinc which stops corrosion of the iron. 2. Alloys Most metals in common use are alloys. An alloy is a mixture of 2 metals or a metal with another element such as carbon. Alloys are usually stronger than pure metals because the atoms of the element stop the layers of metal atoms sliding over each other. Some examples include: Bronze = copper + tin Brass = copper + zinc Gold used in jewellery is usually an alloy with silver, copper and zinc. The proportion of gold in the alloy is measured in carats. 24 carat is 100% pure gold, and 18 carat is 75% gold. Steels are alloys of iron with carbon and other metals. Different steels have different properties and uses. Eg High carbon steel is strong but brittle. Use = cutting tools. Low carbon steel is soft and easily shaped. Use = car body panels. Stainless steel (alloy of iron with chromium and nickel) is hard and resistant to corrosion. Use = cutlery. 3. Ceramics and composites Soda lime glass – made by heating a mixture of sand, sodium carbonate and limestone. This is the glass we commonly use for bottles, window pains, etc. Borosilicate glass – made from sand and boron trioxide. Melts at higher temperatures than soda lime glass. It resits expansion when heated and is used in glass ovenware. Clay ceramics – eg pottery and bricks, are made by shaping wet clay and heating in a furnace. A composite is made of 2 materials : one forms a matrix which binds fibres or fragments of the other. This provides reinforcement. An example is concrete. Metal atom 4. Polymers Polymers are long chain molecules made by joining many small molecules (monomers). Different polymers are made from different monomers. The properties of the polymer depend on the monomer they are made from and the conditions under which they are made. Eg. Low density polythene (LDPE) has side branches and is on-crystalline, whereas high density polythene (HDPE) is lined up in a crystalline structure. Thermosoftening polymers can be heated and shaped many times. Poly(ethene) is an example. Its tangles polymer chains can uncoil and slide past each other. Thermosetting polymers can only be heated and shaped once. Vulcanised rubber is an example. Once set, its polymer chains are joined by cross-links and so they cannot slide past each other easily. Other element atom 5. The Haber process and NPK fertilisers The Haber process is used to make ammonia (NH3), which is used to make nitrogen-based fertilisers. The raw materials and nitrogen (from the air) and hydrogen (from water or methane). 2N2 + 3H2 2NH3 High Temp (450oC) and pressure (200 atm) The ammonia is collected by cooling and liquefying. Any unreacted nitrogen and hydrogen is recycled. The temperature is not too high as it would reverse the equilibrium (reduce the yield of ammonia), but not too low as the rate would be too slow. The pressure is high enough to increase the yield and rate, but not too high as this would become dangerous. Iron catalyst Key words: corrosion, alloy, composite, ceramic, polymer, thermosoftening, thermosetting, Haber process, ammonia