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FahadH. Ahmad (Contact: +92 323 509 4443) ORGANIC CHEMISTRY / IGCSE (0620) / GCE (5070) Combustion Fuels are substances that react with oxygen to release useful energy. Most of the energy is released as heat, but light energy is also released. About 21 per cent of the air is oxygen. When a fuel burns in plenty of air, it receives enough oxygen for complete combustion. Complete combustion Complete combustion needs a plentiful supply of air so that the elements in the fuel react fully with oxygen. Fuels such as natural gas and petrol contain hydrocarbons - which are compounds of hydrogen and carbon only. When hydrocarbons burn completely: the carbon oxidises to carbon dioxide the hydrogen oxidises to water (remember that water, H2O, is an oxide of hydrogen) In general, for complete combustion: hydrocarbon + oxygen → carbon dioxide + water Here are the equations for the complete combustion of propane, used in bottled gas: propane + oxygen → carbon dioxide + water C3H8 + 5O2 → 3CO2 + 4H2O Incomplete combustion Incomplete combustion occurs when the supply of air or oxygen is poor. Water is still produced, but carbon monoxide and carbon are produced instead of carbon dioxide. In general, for incomplete combustion: hydrocarbon + oxygen → carbon monoxide + carbon + water The carbon is released as soot. Carbon monoxide is a poisonous gas, which is one reason why complete combustion is preferred to incomplete combustion. Gas fires and boilers must be serviced regularly to ensure they do not produce carbon monoxide. Here are the equations for the incomplete combustion of propane, where carbon is produced rather than carbon monoxide: propane + oxygen → carbon + water C3H8 + 2O2 → 3C + 4H2O www.fahadsacademy.com Alcohols Page 1 FahadH. Ahmad (Contact: +92 323 509 4443) Alkanes The alkanes are a homologous series of hydrocarbons. This means that they have similar chemical properties to each other and they have trends in physical properties. For example, as the chain length increases, their boiling point increases. The straight chain alkanes share the same general formula: The general formula means that the number of hydrogen atoms in an alkane is double the number of carbon atoms, plus two. For example, methane is CH4 and ethane is C2H6. Alkane molecules can be represented by displayed formulae in which each atom is shown as its symbol (C or H) and the covalent bonds between them by a straight line. Here are the names and structures of five alkanes: www.fahadsacademy.com Alcohols Page 2 FahadH. Ahmad (Contact: +92 323 509 4443) Notice that the molecular models on the right show that the bonds are not really at angles of 90°. Methylpropane Alkanes are saturated hydrocarbons. This means that their carbon atoms are joined to each other by single bonds. This makes them relatively unreactive, apart from their reaction with oxygen in the air - which we call burning or combustion. www.fahadsacademy.com Alcohols Page 3 FahadH. Ahmad (Contact: +92 323 509 4443) Butane Like other homologous series, the alkanes showisomerism. This means that their atoms can be arranged differently to make slightly differentcompounds with different properties. For example, an isomer of butane is methylpropane. Substitution reactions In a substitution reaction, one atom is swapped with another atom. These are very useful reactions in the chemical industry because they allow chemists to change one compound into something more useful, building up designer molecules like drugs. Alkanes undergo a substitution reaction with halogens in the presence of light. For instance, in ultraviolet light, methane reacts with halogen molecules such as chlorine and bromine. For example: methane + bromine → methylbromine + hydrogen bromide CH4 + Br2 → CH3Br + HBr This reaction is a substitution reaction because one of the hydrogen atoms from the methane is replaced by a bromine atom. Alkenes Alkenes are a homologous series of hydrocarbons that contain a carbon-carbon double bond. The number of hydrogen atoms in an alkene is double the number of carbon atoms, so they have the general formula . For example, the molecular formula of ethene is , while for propene it is . Here are the names and structures of four alkenes: www.fahadsacademy.com Alcohols Page 4 FahadH. Ahmad (Contact: +92 323 509 4443) Alkenes are unsaturated, meaning they contain a double bond. This bond is why the alkenes are more reactive than the alkanes. Testing for alkenes The presence of the C=C double bond allows alkenes to react in ways that alkanes cannot. This allows us to tell alkenes apart from alkanes using a simple chemical test. Bromine water is an orange solution of bromine. It becomes colourless when it is shaken with an alkene. Alkenes can decolourise bromine water, but alkanes cannot. The slideshow shows this process. www.fahadsacademy.com Alcohols Page 5 FahadH. Ahmad (Contact: +92 323 509 4443) The reaction between bromine and alkenes is an example of a type of reaction called an addition reaction. The bromine is decolourised because a colourless dibromo compound forms. For example: ethene + bromine → dibromoethane C2H4 + Br2 → C2H4Br2 Other addition reactions of alkenes: Hydrogen can be added to a C=C double bond. This has the effect of ‘saturating’ the molecule, and will turn an alkene into an alkane. For example: C2H4 + H2 → C2H6 If steam (H2O) is added to an alkene, an alcohol is made. For example: C2H4 + H2O → C2H5OH Alcohols The alcohols are a homologous series of organic compounds. They all contain the functional group –OH, which is responsible for the properties of alcohols. The names of alcohols end with ‘ol’, eg ethanol. The first three alcohols in the homologous series are methanol, ethanol and propanol. They are highly flammable, making them useful as fuels. They are also used as solvents in marker pens, medicines, and cosmetics (such as deodorants and perfumes). Ethanol is the alcohol found in alcoholic drinks such as wine and beer. Ethanol is mixed with petrol for use as a fuel. Here are the names and structures of the simplest alcohols: www.fahadsacademy.com Alcohols Page 6 FahadH. Ahmad (Contact: +92 323 509 4443) Ethanol from ethene Structure of ethanol Ethanol molecules contain carbon, hydrogen and oxygen atoms. Ethanol from ethene and steam Ethanol can be manufactured by the hydration of ethene. In this reaction, ethene (which comes from cracking crude oil fractions) is heated with steam in the presence of a catalyst of phosphoric acid (to speed up the reaction): This reaction typically uses a temperature of around 300°C and a pressure of around 60– 70 atmospheres. Notice that ethanol is the only product. The process is continuous – as long as ethene and steam are fed into one end of the reaction vessel, ethanol will be produced. These features make it an efficient process. However, ethene is made from crude oil, which is a non-renewableresource. Ethene from ethanol The reaction of ethene with steam to form ethanol can be reversed. This allows ethanol to be converted into ethene. A catalyst of hotaluminium oxide is used to speed up the reaction. This is called a dehydration reaction. Ethanol from sugars Ethanol can also be made by a process called fermentation. Fermentation www.fahadsacademy.com Ethanol from ethene Page 7 FahadH. Ahmad (Contact: +92 323 509 4443) During fermentation, sugar (glucose) from plant material is converted into ethanol and carbon dioxide. This typically takes place at temperatures of around 30°C. The enzymes found in single-celled fungi (yeast) are the natural catalysts that can make this process happen: Unlike ethene, sugar from plant material is a renewable resource. Hydration of ethene vs fermentation These are some of the advantages and disadvantages of making ethanol by hydration of ethene and by fermentation. The table compares making ethanol by hydration of ethene (ethene and steam) to making ethanol by fermentation (sugar from plant material). Type of raw materials Fermentation Hydration of ethane Renewable (glucose from plants) Non-renewable (ethene from crude oil) Type of process Batch (stop-start) Labour A lot of workers needed Rate of reaction Slow Conditions needed Purity of product Energy needed Continuous (runs all the time) Few workers needed Fast Warm (30°C), normal pressure (1 atm) High temperature (300°C) and high pressure (60-70 atm) Impure (needs treatment) Pure (no by-products made) A little A lot Biofuels With fossil fuels being non-renewable and contributing to global warming, biofuels are increasingly being considered as a possible alternative for the future. Biofuels are produced from natural products, often plant biomass containing carbohydrate. As biofuels are produced from plants, they are renewable and theoretically carbon neutral. www.fahadsacademy.com Hydration of ethene vs fermentation Page 8 FahadH. Ahmad (Contact: +92 323 509 4443) Some biofuels are produced by using microorganisms to anaerobically ferment carbohydrate in the plant material - as is the case with bioethanol and biogas production (each process uses different microorganisms). Bioethanol When ethanol is made by fermentation, sugar is converted into ethanol and carbon dioxide if conditions are anaerobic. Single-celled fungi, called yeast, contain enzymes that are natural catalysts for making this process happen. In some countries, such as Brazil, the source of sugar is sugar cane - which yeast can directly ferment into ethanol. In other countries, plants such as maize are used. Because maize contains starch rather than sugar, the enzyme amylase must first break down the starch into sugar before the yeast can ferment it into ethanol. The ethanol produced by yeast only reaches a concentration of around 15 per cent before the ethanol becomes toxic to the yeast. In order to make it sufficiently concentrated to be burnt as a fuel, the ethanol must be distilled. Disadvantages of bioethanol There are some disadvantages to growing biofuel crops (such as sugar cane and maize) to be used as bioethanol: The demand for biofuel crops means greater demand on rainforest land. Crops grow slowly in parts of the world that have lower light levels and temperatures, so growing biofuel crops in these countries would not satisfy the demand for fuel. For bioethanol to be burnt in a car engine, some engine modification is needed. Modern petrol engines can use petrol containing up to 10 per cent ethanol without needing any modifications, and most petrol sold in the UK contains ethanol. Although biofuels are in theory carbon neutral, this does not take into account the carbon dioxide emissions associated with growing, harvesting and transporting the crops, or producing the ethanol from them. Therefore, overall, more carbon dioxide is emitted than is absorbed, which means that it contributes to global warming. Some people morally object to using food crops to produce fuels. For example, it could cause food shortages or increases in food prices. Biodiesel Biodiesel is produced by reacting vegetable oils with methanol. The main product is a methyl ester of a long chain fatty acid, and this is used as biodiesel. Glycerol is produced as a by-product. www.fahadsacademy.com Page 9 FahadH. Ahmad (Contact: +92 323 509 4443) Biodiesel can be used as a replacement fuel in most modern diesel engines - unlike raw vegetable oil, which can only be used in converted or old-fashioned diesel engines. Carboxylic acids Carboxylic acids are a group of important organic chemicals. Vinegar contains ethanoic acid, which is a carboxylic acid. All carboxylic acids have a –COOH functional group, and have similar reactions as a result. They are weak acids because this functional group is only partly ionised in solution. The carboxylic acids are a homologous series of organic compounds. They all contain the same functional group –COOH. The names of carboxylic acids end in ‘-oic acid’ – eg ethanoic acid. In the exam, you will need to be able to recognise the following carboxylic acids from their names and formulae. Carboxylic acid Number of C atoms Structural formula Displayed formula Methanoic acid 1 HCOOH Ethanoic acid 2 CH3COOH Propanoic acid 3 CH3CH2COOH You are not expected to remember the names and formulae of other carboxylic acids. Ethanoic acid from ethanol Vinegar is an aqueous solution containing ethanoic acid. Ethanoic acid is formed from the mild oxidation of the ethanol (which is an alcohol). This can be achieved through: The addition of chemical oxidising agents - such as acidified potassium dichromate. www.fahadsacademy.com Carboxylic acids Page 10 FahadH. Ahmad (Contact: +92 323 509 4443) The action of microbes in aerobic conditions (in the presence of oxygen). This happens on a small scale when a bottle of wine is left open and exposed to air. On a commercial scale, it is achieved in a fermenter using acetic acidbacteria. Properties of carboxylic acids Carboxylic acids have the following properties: 1. They dissolve in water to produce acidic solutions (pH less than 7). 2. They react with carbonates to produce carbon dioxide and a salt and water. For example: calcium carbonate + ethanoic acid → calcium ethanoate + water + carbon dioxide 3. They all react with alcohols, in the presence of an acid catalyst, to form esters. For example: ethanol + ethanoic acid → ethyl ethanoate + water Carboxylic acids and esters Carboxylic acids and esters are organic chemicals that occur naturally and can also be made from alcohols. The uses of vegetable oils are extended using additives and chemical treatments. Carboxylic acids The carboxylic acids are a homologous series of organic compounds. Carboxylic acids contain the carboxyl functional group (-COOH). Carboxylic acids end in 'oic acid'. The carboxyl group will never have a position number in a carboxylic acid, as it is always on the end of the carbon chain. The basic rules of naming apply. Carboxylic acids take their names from their ‘parent’ alkanes. For example, ethane is the ‘parent’ alkane of ethanoic acid. Ethanoic acid has the formula CH3COOH and this structure: Properties of carboxylic acids Short carboxylic acids are liquids and are soluble in water. Longer carboxylic acids are solids and are less soluble in water. www.fahadsacademy.com Properties of carboxylic acids Page 11 FahadH. Ahmad (Contact: +92 323 509 4443) The boiling point of a carboxylic acid is higher than that of the alkane with the same number of carbon atoms because the intermolecular forces are much stronger. Carboxylic acids are weak acids, so they can donate a hydrogen ion(H+) in acidbase reactions: This means that they will react with carbonates to produce a salt, water and carbon dioxide: They will also react with reactive metals to produce a salt and hydrogen. Making a carboxylic acid Ethanoic acid can be made by oxidising ethanol (which is an alcohol). In this case, oxidation involves adding an oxygen atom and removing two hydrogen atoms. This can happen: during fermentation if air is present when ethanol is oxidised by an oxidising agent, such as acidified potassium manganate(VII) Making an ester Esters occur naturally - often as fats and oils - but they can be made in the laboratory by reacting an alcohol with an organic acid. A little sulfuric acid is needed as a catalyst. The general word equation for the reaction is: alcohol + organic acid → ester + water For example: methanol + butanoic acid → methyl butanoate + water The diagram shows how this happens, and where the water comes from: www.fahadsacademy.com Making a carboxylic acid Page 12 FahadH. Ahmad (Contact: +92 323 509 4443) So, to make ethyl ethanoate, you would need to react ethanol with ethanoic acid. What esters smell like Different esters have different smells. These smells are often fruity. Take a look at the following four examples: Alcohol Organic acid Ester made Smell of ester Pentanol Ethanoic acid Pentyl ethanoate Pears Octanol Ethanoic acid Octyl ethanoate Bananas Pentanol Butanoic acid Pentyl butanoate Strawberries Methanol Butanoic acid Methyl butanoate Pineapples Fats and oils Fats and oils are naturally-occurring esters. Fats are solid at room temperature, whereas oils are liquids. Vegetable oils Vegetable oils are natural oils found in seeds, nuts and some fruit. The oil can be extracted. The plant material is crushed and pressed and the oil, eg olive oil, is squeezed out. Sometimes the oil is more difficult to extract and has to be dissolvedin a solvent. Once the oil is dissolved, the solvent is removed bydistillation and impurities (such as water) are also removed. This leaves pure vegetable oil, eg sunflower oil. Structure of vegetable oils Molecules of vegetable oils consist of glycerol and fatty acids. The diagram shows how three long chains of carbon atoms are attached to a glycerol molecule to make one molecule of vegetable oil. www.fahadsacademy.com What esters smell like Page 13 FahadH. Ahmad (Contact: +92 323 509 4443) The structure of a vegetable oil molecule Plant oils and their uses Vegetable oils in cooking Vegetable oils have higher boiling points than water - so foods can be cooked or fried in vegetable oils at higher temperatures than they can be if they are cooked or boiled in water. Food cooked in vegetable oils: cook faster than if they were boiled have different flavours than if they were boiled www.fahadsacademy.com Plant oils and their uses Page 14 FahadH. Ahmad (Contact: +92 323 509 4443) Vegetable oils are a source of energy in the diet. Food cooked in vegetable oils releases more energy when it is eaten than food cooked in water. This can have an impact on our health and cause excess weight. Some vegetable oils can be converted to biodiesel by reacting them with methanol. This allows certain crops to be grown and used to make fuels for cars and lorries without needing to use fossil fuels. This can help to make biodiesel carbon neutral. Saturated and unsaturated fats and oils The fatty acids in some vegetable oils are saturated - they only havesingle bonds between their carbon atoms. Saturated oils tend to be solid at room temperature, and are sometimes called vegetable fatsinstead of vegetable oils. Lard is an example of a saturated oil. The fatty acids in some vegetable oils are unsaturated - they havedouble bonds between some of their carbon atoms. Unsaturated oils tend to be liquid at room temperature, and are useful for frying food. They can be divided into two categories: monounsaturated fats have one double bond in each fatty acid polyunsaturated fats have many double bonds Unsaturated fats (rather than saturated fats) are thought to be a healthier option in the diet. Emulsions Vegetable oils do not dissolve in water. If oil and water are shaken together, tiny droplets of one liquid spread through the other liquid, forming a mixture called an emulsion. Emulsions are thicker than the oil or water they contain. This makes them useful in foods such as salad dressings and ice cream. Emulsions are also used in cosmetics and paints. There are two main types of emulsion: oil droplets in water (milk, ice cream, salad cream, mayonnaise) water droplets in oil (margarine, butter, skin cream, moisturising lotion) Emulsifiers If an emulsion is left to stand, eventually a layer of oil will form on the surface of the water. Emulsifiers are substances that stabilise emulsions, stopping them separating out. Egg yolk contains a natural emulsifier. Mayonnaise is a stable emulsion of vegetable oil and vinegar with egg yolk. Emulsifier molecules have two different ends: a hydrophilic (water-loving) ‘head’ that forms chemical bonds with water but not with oils a hydrophobic (water-hating) ‘tail’ that forms chemical bonds with oils but not with water Lecithin is an emulsifier commonly used in foods. It is obtained from oil seeds and is a mixture of different substances. A molecular model of one of these substances is seen in the diagram: www.fahadsacademy.com Saturated and unsaturated fats and oils Page 15 FahadH. Ahmad (Contact: +92 323 509 4443) Emulsifier molecules The hydrophilic 'head' dissolves in the water and the hydrophobic 'tail' dissolves in the oil. In this way, the water and oil droplets become unable to separate out. Hydrogenation Bromine water test Unsaturated vegetable oils contain carbon-carbon double bonds. They can be detected using bromine water, just as alkenes can be detected in this way. Bromine water becomes colourless when shaken with an unsaturated vegetable oil, but it stays orange-brown when shaken with a saturated vegetable fat. Bromine water can also be used to determine the level of saturation of a vegetable oil. Hydrogenation Saturated vegetable fats are solid at room temperature, and have a higher melting point than unsaturated oils. This makes them suitable for making margarine or for commercial use in the making of cakes and pastry. Unsaturated vegetable oils can be ‘hardened’ by reacting them with hydrogen, a reaction called hydrogenation. The structure of part of a fatty acid During hydrogenation, vegetable oils are reacted with hydrogen gas at about 60°C. A nickel catalystis used to speed up the reaction. The double bonds are converted to single bonds in the reaction. In this way, unsaturated fats can be made into saturated fats – they are hardened. www.fahadsacademy.com Hydrogenation Page 16 FahadH. Ahmad (Contact: +92 323 509 4443) Fractional distillation of crude oil Fractional distillation separates a mixture into a number of different parts, called fractions. www.fahadsacademy.com Fractional distillation of crude oil Page 17 FahadH. Ahmad (Contact: +92 323 509 4443) A tall fractionating column is fitted above the mixture, with several condensers coming off at different heights. The column is hot at the bottom and cool at the top. Substances with high boiling pointscondense at the bottom and substances with lower boiling points condense on the way to the top. Crude oil is a mixture of hydrocarbons. The crude oil is evaporatedand its vapours condense at different temperatures in the fractionating column. Each fraction contains hydrocarbon molecules with a similar number of carbon atoms and a similar range of boiling points. Oil fractions The diagram below summarises the main fractions from crude oil and their uses, and the trends in properties. Note that the gases leave at the top of the column, the liquids condense in the middle and thesolids stay at the bottom. The fractionating column As you go up the fractionating column, the hydrocarbons have: 1. lower boiling points 2. lower viscosity (they flow more easily) 3. higher flammability (they ignite more easily). www.fahadsacademy.com Oil fractions Page 18 FahadH. Ahmad (Contact: +92 323 509 4443) Other fossil fuels Crude oil is not the only fossil fuel. Natural gas mainly consists of methane. It is used in domestic boilers, cookers and Bunsen burners, as well as in some power stations. Coal was formed from the remains of ancient forests. It can be burned in power stations. Coal is mainly carbon but it may also contain sulfur compounds, which produce sulfur dioxide when the coal is burned. This gas is a cause of acid rain. Also, as all fossil fuels contain carbon, the burning of any fossil fuel will contribute to global warmingdue to the production of carbon dioxide. Cracking Fuels made from oil mixtures containing large hydrocarbon molecules are not efficient as they do not flow easily and are difficult to ignite. Crude oil often contains too many large hydrocarbon molecules and not enough small hydrocarbon molecules to meet demand. This is where cracking comes in. Cracking allows large hydrocarbon molecules to be broken down into smaller, more useful hydrocarbon molecules. Fractions containing large hydrocarbon molecules are heated to vaporise them. They are then either: heated to 600-700°C passed over a catalyst of silica or alumina These processes break covalent bonds in the molecules, causing thermal decomposition reactions. Cracking produces smaller alkanes and alkenes (hydrocarbons that contain carbon-carbon double bonds). For example: hexane → butane + ethene C6H14 → C4H10 + C2H4 The slideshow shows this process: Some of the smaller hydrocarbons formed by cracking are used as fuels, and the alkenes are used to make polymers in plastics manufacture. Sometimes, hydrogen is also produced during cracking. www.fahadsacademy.com Other fossil fuels Page 19 FahadH. Ahmad (Contact: +92 323 509 4443) Combustion of fuels Complete combustion Fuels are substances that react with oxygen to release useful energy. Most of the energy is released as heat, but light energy is also released. About 21 per cent of air is oxygen. When a fuel burns in plenty of air, it receives enough oxygen for complete combustion. Complete combustion needs a plentiful supply of air so that the elements in the fuel react fully with oxygen. Fuels such as natural gas and petrol contain hydrocarbons. These are compounds of hydrogen and carbon only. When they burn completely: the carbon oxidises to carbon dioxide the hydrogen oxidises to water (remember that water, H 2O, is an oxide of hydrogen) In general, for complete combustion: hydrocarbon + oxygen → carbon dioxide + water Here are the equations for the complete combustion of propane, used in bottled gas: propane + oxygen → carbon dioxide + water C3H8 + 5O2 → 3CO2 + 4H2O Incomplete combustion Incomplete combustion occurs when the supply of air or oxygen is poor. Water is still produced, but carbon monoxide and carbon are produced instead of carbon dioxide. In general for incomplete combustion: hydrocarbon + oxygen → carbon monoxide + carbon + water The carbon is released as soot. Carbon monoxide is a poisonous gas, which is one reason why complete combustion is preferred to incomplete combustion. Gas fires and boilers must be serviced regularly to ensure they do not produce carbon monoxide. Carbon monoxide is absorbed in the lungs and binds with the haemoglobin in our red blood cells. This reduces the capacity of the blood to carry oxygen. Here are the equations for the incomplete combustion of propane, where carbon is produced rather than carbon monoxide: propane + oxygen → carbon + water C3H8 + 2O2 → 3C + 4H2O Nitrogen oxides When fuels are burned in vehicle engines, high temperatures are reached. At these high temperatures, nitrogen and oxygen from the air combine to produce nitrogen monoxide. www.fahadsacademy.com Combustion of fuels Page 20 FahadH. Ahmad (Contact: +92 323 509 4443) nitrogen + oxygen → nitrogen monoxide N2(g) + O2(g) → 2NO(g) When this nitrogen monoxide is released from vehicle exhaust systems, it combines with oxygen in the air to form nitrogen dioxide. nitrogen monoxide + oxygen → nitrogen dioxide 2NO(g) + O2(g) → 2NO2(g) Nitrogen dioxide is a cause of acid rain. Nitrogen monoxide and nitrogen dioxide are jointly referred to as NOx. Sulfur dioxide and acid rain Many fossil fuels contain sulfur impurities. When these fuels are burned, the sulfur is oxidised to form sulfur dioxide. S(s) + O2(g) → SO2(g) This sulfur dioxide then dissolves in droplets of rainwater to form sulfurous acid. SO2(g) + H2O(l) → H2SO3(aq) Effects of acid rain Acid rain reacts with metals and rocks such as limestone. Buildings and statues are damaged as a result. Acid rain damages the waxy layer on the leaves of trees and makes it more difficult for trees to absorb the minerals they need for healthy growth. They may die as a result. Acid rain also makes rivers and lakes too acidic for some aquatic life to survive. The carbon cycle Most of the chemicals that make up living tissue contain carbon. When organisms die, the carbon is recycled so that it can be used by other organisms. The model that describes the processes involved is called the carbon cycle. Stages in the carbon cycle www.fahadsacademy.com Sulfur dioxide and acid rain Page 21 FahadH. Ahmad (Contact: +92 323 509 4443) Carbon enters the atmosphere as carbon dioxide from respiration and combustion. 1. Carbon dioxide is absorbed by producers to make carbohydrates during the process of photosynthesis. 2. Animals feed on the plant passing the carbon compounds along the food chain. Most of the carbon they consume is exhaled as carbon dioxide (formed during respiration). The animals and plants eventually die. 3. The dead organisms are eaten by decomposers and the carbon is returned to the atmosphere as carbon dioxide. In some conditions decomposition is blocked. The plant and animal material may then be available as fossil fuel in the future for combustion. Note that throughout the processes, carbon is always being recycled. Methane Methane, CH4, is a gas that can be produced by: decomposition of vegetation waste gases from digestion in animals Methane is a powerful greenhouse gas and therefore contributes toglobal warming. Polymers Polymers are long chain molecules that occur naturally in living things and can also be made by chemical processes in industry. Plastics are polymers, so polymers can be extremely useful. Addition polymers Alkenes can be used to make polymers. www.fahadsacademy.com Methane Page 22 FahadH. Ahmad (Contact: +92 323 509 4443) Polymers are very large molecules made when many smaller molecules join together, end to end. The smaller molecules are called monomers. In general: lots of monomer molecules → a polymer molecule The polymers formed are called addition polymers. This slideshow shows how several chloroethene monomers can join end to end to make poly(chloroethene), also called PVC: Alkenes can act as monomers because they are unsaturated: ethene can polymerise to form poly(ethene), also called polythene propene can polymerise to form poly(propene), also called polypropylene chloroethene can polymerise to form poly(chloroethene), also called PVC Repeating units Polymer molecules are very large compared with most other molecules, so the idea of a repeat unit is used when drawing a displayed formula. When drawing one, you need to: 1. change the double bond in the monomer to a single bond in the repeat unit 2. add a bond to each end of the repeat unit www.fahadsacademy.com Repeating units Page 23 FahadH. Ahmad (Contact: +92 323 509 4443) Addition polymerisation It can be tricky to draw the repeat unit of poly(propene). Propene is usually drawn like this: It is easier to construct the repeat unit for poly(propene) if you redraw the monomer like this: You can then see how to convert this into the repeat unit. Condensation polymers Some polymers are made via condensation polymerisation. In condensation polymerisation, a small molecule is formed as a by-product each time a bond is formed between two monomers. This small molecule is often water. An example of a condensation polymer is nylon. www.fahadsacademy.com Condensation polymers Page 24 FahadH. Ahmad (Contact: +92 323 509 4443) Uses of polymers Different polymers have different properties, so they have different uses. The table gives some examples: Polymer Typical use Poly(ethene) Plastic bags and bottles Poly(propene) Crates and ropes Poly(chloroethene) Water pipes and insulation on electricity cables Polymers have properties that depend on the chemicals they are made from and the conditions in which they are made. For example, there are two main types of poly(ethene) - LDPE, low-density poly(ethene), and HDPE, high-density poly(ethene). LDPE is weaker than HDPE and becomes softer at lower temperatures. Modern polymers are very useful. For instance, they can be used as: new packaging materials waterproof coatings for fabrics (eg for outdoor clothing) fillings for teeth dressings for cuts hydrogels (eg for soft contact lenses and disposable nappy liners) smart materials (eg shape memory polymers for shrink-wrap packaging) www.fahadsacademy.com Uses of polymers Page 25 FahadH. Ahmad (Contact: +92 323 509 4443) Example of the Homologous Series of Alkanes, Structure: CnH2n+2 Number Name of Carbon Chemical Simple Structure Alkane atoms Formula (Molecular Diagram) Methane 1 C H4 Ethane 2 C2H6 Propane 3 C3H8 Butane 4 C4H10 Pentane 5 C5H12 Hexane 6 C6H14 Heptane 7 C7H16 Octane 8 C8H18 Nonane 9 C9H20 www.fahadsacademy.com Uses of polymers Page 26 FahadH. Ahmad (Contact: +92 323 509 4443) Decane 10 C10H22 Glossary 1. Alkane Saturated hydrocarbon. A compound of hydrogen and carbon only, with no C=C bonds. 2. Alkene Unsaturated hydrocarbon with a double bond between the carbon atoms. 3. Atom All elements are made of atoms. An atom consists of a nucleus containing protons and neutrons, surrounded by electrons. 4. complete combustion Burning in a plentiful supply of oxygen or air. Complete combustion of a hydrocarbon produces water vapour and carbon dioxide. 5. Compound A substance formed by the chemical union of two or more elements. 6. covalent bond A bond between atoms formed when atoms share electrons to achieve a full outer shell of electrons. 7. double bond A covalent bond resulting from the sharing of four electrons (two pairs) between two atoms. 8. Element A substance made of one type of atom only. 9. Halogen An element placed in Group 7 of the period table, which starts with fluorine and ends with astatine. 10.homologous series A 'family' of organic compounds that have the same functional group and similar chemical properties. 11.Hydrocarbon A compound that contains hydrogen and carbon only. 12.Isomer Chemicals that have the same molecular formula but different arrangements of atoms. 13.Molecule A collection of two or more atoms held together by chemical bonds. 14.Oxidation The gain of oxygen, or loss of electrons, by a substance during a chemical reaction. 15.ultraviolet light Electromagnetic radiation with a greater frequency than visible light but less than X-rays. Humans cannot see it but it can damage eyes and skin in high doses. 1. 2. 3. 4. 5. 6. 7. 8. Glossary amylaseAn enzyme found in yeast, which can break down starch into simple sugars. anaerobicWithout oxygen. atmosphereA unit of pressure. biomassThe dry mass of an organism. carbon neutralA carbon neutral fuel is one in which the amount of carbon dioxide released when it is used is equal to amount taken in when it formed. catalystChanges the rate of a chemical reaction without being changed by the reaction itself. crackingThe breaking down of large hydrocarbon molecules into smaller, more useful hydrocarbon molecules by vaporising them and passing them over a hot catalyst. distillationThe process of separating two liquids with different boiling points. www.fahadsacademy.com Uses of polymers Page 27 FahadH. Ahmad (Contact: +92 323 509 4443) 9. enzymeProteins which catalyse or speed up chemical reactions. 10.esterA type of organic compound formed in the reaction between an alcohol and a carboxylic acid. 11.fermentFermentation is an anaerobic process (it takes place in the absence of oxygen). Fermentation by yeast produces carbon dioxide and ethanol. 12.flammableAble to ignite and burn. 13.Fraction In fractional distillation, such as that of crude oil, the different parts of the original mixture are called fractions. The substances in each fraction have similar boiling points to each other. 14.functional group An atom, or group of atoms, that determines the main chemical properties of an organic compound. 15.global warming The rise in the average temperature of the Earth's surface. 16.glycerolPropane-1,2,3-triol. It reacts with fatty acids to form esters, found in natural as fats and oils. 17.homologous series A 'family' of organic compounds that have the same functional group and similar chemical properties. 18.Methanol The simplest alcohol. 19.Microorganism Another name for a microbe. It is microscopic and is an organism, such as a virus or bacteria. 20.non-renewable A resource that cannot be replaced when it is used up, such as oil, natural gas or coal. 21.organic compound Compounds that contain carbon atoms, joined by covalent bonds to other atoms (including other carbon atoms). 22.Solvent The liquid in which the solute dissolves to form a solution. USE THIS LINK FOR FURTHER INFORMATION file:///C:/Users/Parasu/Desktop/ORGANIC%20CHEMISTRY/BCCSTUDY%20MATERIALS/BBC%20-%20GCSE%20Chemistry.htm www.fahadsacademy.com Uses of polymers Page 28