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CRACKING Some of the heavier fractions obtained from crude oil are not very useful. In the process of cracking, these larger hydrocarbon molecules are broken down ("cracked") to produce smaller, more useful molecules. This process involves heating the hydrocarbons to vaporise them and passing the vapours over a hot catalyst (about 450oC) for a few seconds. A THERMAL DECOMPOSITION reaction then occurs → C10H22 C5H12 + C3H6 + C2H4 Some of the products of cracking are useful as fuels. Other products of cracking are used as feedstock for the petrochemicals industry; these compounds have carboncarbon double bonds (they are unsaturated) and can be used to make plastics (polymers) such as poly(ethene) and poly(propene). ALKENES The alkenes are another group of hydrocarbons which all have the same general formula (CnH2n). Like the alkanes, they contain ONLY carbon and hydrogen atoms. Carbon atoms form the spine of the molecules. Each carbon atom forms four covalent bonds; each hydrogen forms one covalent bond. However, in the alkenes, two of the carbon atoms are joined by a double covalent bond. These two carbon atoms are therefore sharing four electrons. The molecule no longer contains the maximum possible number of hydrogen atoms for its particular number of carbon atoms. The molecule is said to be unsaturated. Alkenes are UNSATURATED hydrocarbons. The first three alkenes are: H C ethene (C2H4) C H C H H H H C C C H H H TOPIC 10.1.5: PRODUCTS FROM OIL 1 C propene (C3H6) H H butene (C4H8) H H H H H C C H H Note that each carbon atom has formed four bonds, but they are not all joined to four other atoms, because of the double bond. Each hydrogen atom has formed one bond. Carbon=carbon double bonds are weaker and are more easily broken than carboncarbon single bonds. When the double bond breaks, the carbon atoms are able to join with more atoms. A double bond in a molecule is therefore a sign of chemical reactivity. Alkenes are REACTIVE. Test for Alkenes The difference in reactivity between alkanes and alkenes provides a useful way of distinguishing between them. If bromine water is added to an alkene, the initial brown colour of the bromine water is lost and the mixture becomes colourless. There is no reaction with an alkane, and the mixture stays brown. This test with bromine water is a test for unsaturation. Polymerisation Alkenes are one of the products obtained from the cracking of crude oil fractions. Alkenes react with lots of other substances to form useful products, but a particularly important reaction occurs when they react with themselves in a process called POLYMERISATION. Polymerisation is the joining together of a large number of small molecules, called monomers, to form one very large molecule, called a polymer. All the common plastics are polymers. When unsaturated monomers join together to form a polymer with no other substance being produced in the reaction, the process is called ADDITION POLYMERISATION. The following are examples of addition polymerisation: 1. Poly(ethene), also called polythene H n H C H ( C H ethene (monomer, unsaturated) H C C H H )n poly(ethene) (polymer, saturated) n is a very large number, often greater than 10,000 Poly(ethene) is used for making plastic bags and bottles. TOPIC 10.1.5: PRODUCTS FROM OIL 2 H 2. Poly(propene) H n CH3 C ( C H H propene (monomer, unsaturated) H CH3 C C H H )n poly(propene) (polymer, saturated) Poly(propene) is used to make crates and rope. 3. Poly(chloroethene), also called p.v.c. H n Cl C H ( C H chloroethene (monomer, unsaturated) H Cl C C H H )n poly(chloroethene) (polymer, saturated) New polymers Shape memory polymers are polymeric smart materials that have the ability to return from a deformed state (temporary shape) to their original (permanent) shape induced by an external stimulus (trigger), such as temperature change. These materials can be used to make stitches to hold a cut together. The polymer tightens as it is warmed by the body and applies just the right amount of pressure. We can make light sensitive polymers that are used on plasters. When this polymer is exposed to light, it becomes less sticky and so is easy to remove. Hydrogels are used as wound dressings since they hold an incredible amount of water so wounds can heal in moist sterile conditions. TOPIC 10.1.5: PRODUCTS FROM OIL 3 Waste plastic We are making more and more waste, and there are fewer holes in the ground to put it. Landfill can be hazardous, if toxic chemicals get into water; Burning waste can give off toxic materials in the waste gases. Recycling can turn waste material into useful raw materials to be made into new useful things, for example rulers made from old coffee cups. The cost of recycling is a lot less than making completely new materials. Many polymer (plastic items) are non-biodegradable and will remain unaffected in landfill for many hundreds of years. They can also be dropped as litter which is unsightly, especially when caught up in fences and trees. This is a kind of pollution. They can be eaten by animals, which can be harmed. They drift about the oceans, causing deaths of many marine creatures. It is far better to recycle. Plastic items have codes on to state what they are, for example: HDPE - high density polyethene; PS - polystyrene; PP - polypropene. Biodegradable materials can be broken down by micro-organisms in the environment. Many polymers are not biodegradable, and can last for many years in land-fill. Others are, and will break down. Scientists have found ways to speed up this decomposition of polymers by adding starch which encourages micro-organisms in the soil to feed on the starch and break down the polymer. Other polymers made using cornstarch biodegrade fully very quickly. Using plant materials to make polymers does mean that these crops are not being used for the food chain as in the case of biofuels mentioned earlier. TOPIC 10.1.5: PRODUCTS FROM OIL 4 ALCOHOLS The alcohols form a homologous series with the general formula CnH2n+1OH. They contain the functional group –OH, which is called a hydroxyl group. It is the presence of this functional group which gives alcohols their characteristic properties. The first members of the series are: H H C OH H methanol H H H C C H H ethanol OH H H H H C C C H H H OH propan-1-ol ETHANOL C2H5OH Ethanol can be produced in two ways: by the fermentation of sugars by the hydration of ethene. 1. Fermentation of Sugars Fermentation is used to produce ethanol as an industrial chemical. The raw material, usually glucose, is dissolved in water to make a solution containing about 15% by mass of glucose. Yeast is then added, and the temperature is maintained at about 25 oC for 3 to 5 days. Yeast is a living organism which contains the enzyme, zymase. Enzymes are biological catalysts. Under these conditions, the sugar reacts to form ethanol and carbon dioxide. C6H12O6 2C2H5OH + 2CO2 The carbon dioxide is allowed to escape and air is prevented from entering the fermentation vessel, often by using a water trap. When the fermentation is complete, the yeast is filtered off, and the ethanol is separated from the reaction mixture by fractional distillation. Fermentation is also used: to produce ethanol in beer and wine to produce the bubbles of CO2 which make bread dough rise fermentation mixture N.B. The simple laboratory test for carbon dioxide is that it turns limewater milky. TOPIC 10.1.5: PRODUCTS FROM OIL 5 2. Hydration of Ethene Ethene undergoes an addition reaction with steam at high temperature and pressure in the presence of a strong acid catalyst (phosphoric acid), to form ethanol. C2H4 + H2O ethene C2H5OH ethanol COMPARISON OF THE TWO PROCESSES Fermentation Disadvantages: a slow reaction (3 to 5 days) the product is a mixture containing only about 10% ethanol, from which the ethanol is separated by distillation the process is a batch process Advantages the raw material (glucose) is renewable the process is low-tech: it needs only inexpensive equipment and unskilled labour Hydration Advantages: a fast reaction (seconds) the product is pure the process is a continuous process Disadvantages the raw material (ethene) is obtained from crude oil, which is nonrenewable the process is high-tech: it requires expensive equipment and semi-skilled labour TOPIC 10.1.5: PRODUCTS FROM OIL 6