GCE. Chemistry. Edexcel Advanced Subsidiary GCE in Chemistry (8CH01) Edexcel Advanced GCE in Chemistry (9CH01) Green Chemistry.

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1 GCE Chemistry Context study Edexcel Advanced Subsidiary GCE in Chemistry (8CH01) Edexcel Advanced GCE in Chemistry (9CH01) Green Chemistry October 2007

2 Edexcel, a Pearson company, is the UK s largest awarding body offering academic and vocational qualifications and testing to more than 25,000 schools, colleges, employers and other places of learning here and in over 100 countries worldwide. Our qualifications include GCSE, AS and A Level, GNVQ, NVQ and the BTEC suite of vocational qualifications from entry level to BTEC Higher National Diplomas and Foundation Degrees. We deliver 9.4 million exam scripts each year, with over 3.8 million marked onscreen in As part of Pearson, Edexcel has been able to invest in cutting-edge technology that has revolutionised the examinations system, this includes the ability to provide detailed performance data to teachers. References to third party material made in this specification are made in good faith. Edexcel does not endorse, approve or accept responsibility for the content of materials, which may be subject to change, or any opinions expressed therein. (Material may include textbooks, journals, magazines and other publications and websites.) Authorised by Roger Beard Prepared by Sarah Harrison All the material in this publication is copyright Edexcel Limited 2007

3 Contents Introduction 1 Green chemistry 2 Carbon neutrality 2 Reducing greenhouse emissions 2 Carbon capture 3 Fuels and energy 3 Green chemistry and catalysts 5 The problem of waste 7 Web references 8

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5 Introduction This document is designed to help teachers to understand the contemporary context of green chemistry. It should give teachers information on this context and on how to research it further if they wish. This document could also be given to students as introductory material. 1

6 Green chemistry Switch on the television, read the paper or listen to the radio, and it probably won t be long until you hear or see references to green issues or being greener. Most people know that using energy-saving light bulbs and recycling your rubbish are among green activities, but what is green chemistry? Because of pressures on the environment, there is a move to make human activity more sustainable in other words to reduce or halt depletion of limited resources, and to reduce, halt, or reverse pollution of air, sea and land. Some roles of the chemist involve developing industrial processes based on renewable resources; finding catalysts that allow reactions to proceed at lower temperatures or produce the equilibrium yield more quickly; preventing pollution by using the side products of reactions and reducing waste; identifying hazardous chemicals and finding alternatives; and monitoring what goes on in reactions and in the environment. Carbon neutrality Burning hydrocarbon fuels produces carbon dioxide, which is a greenhouse gas and is believed to contribute to global warming. If hydrogen is burnt as a fuel, the only product is water, so is it greener to burn hydrogen than hydrocarbons? The answer to this question depends on how much energy is needed to produce each fuel. This is where the idea of carbon neutrality comes in. A fuel is carbon neutral if the amount of carbon dioxide absorbed when the raw material is grown equals the amount of carbon dioxide produced when it is manufactured and then burnt. Petroleum fuels come from micro-organisms that absorbed atmospheric carbon dioxide millions of years ago, and they emit carbon dioxide when they are burnt. However, on a sensible time frame, in other words within our life span, the carbon dioxide absorbed cannot be set against the carbon dioxide produced. Reducing greenhouse emissions There are three ways to reduce carbon dioxide emissions. 1 Use renewable energy sources such as photovoltaic cells, wind and wave power and fuels made from crops (bioethanol and biodiesel). All have significant disadvantages. 2 Increase the amount of electricity made from nuclear power. From fission in the short term and from fusion in the long term. This also has certain disadvantages. 3 Conserve energy by being more energy efficient. Examples include using more fuelefficient cars, using energy-saving light bulbs, switching off lights and not leaving television sets on stand-by. Recycling metals such as aluminium can reduce greenhouse emissions by 95 per cent compared with the production of new metal from ores. This reduction does not arise simply from the reduction in fossil fuels used but includes the PFC (perfluorocarbon) gases, which are a byproduct of aluminium smelting by the Hall-Heroult process. PFCs are up to 900 times more potent greenhouse gases than carbon dioxide. Recycling the metal removes this source of atmospheric pollution completely, and it can be recycled with almost no loss of quality. 2

7 Carbon capture Alternatively, the impact of carbon dioxide produced in the combustion of fossil fuels could be lessened by carbon capture. The waste gases from the power station can be cooled and compressed. The carbon dioxide is liquefied and can be separated from the other gases (mostly nitrogen and water vapour). This liquid carbon dioxide can then be pumped down old gas wells where it evaporates and is trapped in the geological formations that previously held the hydrocarbon gases. Power stations burning fossil fuels will remain major contributors of carbon dioxide for the indefinite future. Norway, a major exporter of natural gas, is planning to bury large amounts of carbon dioxide one kilometre below the North Sea. The waste gas will be compressed and injected into the porous rocks of the depleted Sleipner natural gas field. Other ideas include the storage of liquefied carbon dioxide under the great pressures found in the deep ocean. There remain serious concerns about the security of such measures and the consequences of the rapid venting to the atmosphere of enormous volumes of carbon dioxide. Figure 1 Capture and storage of CO 2 (Source: Carbon Capture and Storage) Fuels and energy Burning hydrogen produces no carbon dioxide, but to be a carbon-neutral fuel no carbon dioxide should have been produced in its manufacture. Hydrogen is usually made by the reaction of methane and steam, which produces hydrogen and carbon monoxide. The latter can then be converted to more hydrogen and carbon dioxide by further reaction with steam. This means that methane is used as a reactant, and also in combustion as a source of energy to maintain the temperatures required. The carbon dioxide given off when hydrogen is produced in this way should be counted in the comparison of CO 2 absorbed and emitted by the different fuels. 3

8 In the USA and Brazil there has been a shift in agricultural production to give crops that can be converted to ethanol as an alternative liquid fuel. Biofuels like bioethanol can be made from crops such as sugar cane, sugar beet, or corn. If the crop was grown in the last few years, then use of bioethanol comes close to being carbon neutral. Doubts have arisen owing to the energy needed to produce these biofuels, for example in the distillation and transportation of the products. Carbon dioxide would almost certainly be produced in the process of harvesting, extracting sugar from the crop, converting it to ethanol and purifying the fuel, so there is a carbon footprint when we use these. Growing crops for biofuels may have other environmental impacts, as in Malaysia where forests are cut down to provide land for the growing of oil palms, or where they would produce a monoculture in a particular area. In overpopulated countries this is very problematic as a solution to the greenhouse effect, as growing biofuels would replace food crops. In the UK, extra oil seed rape, corn and sugar beet for conversion to biodiesel and ethanol could be grown on land set aside under EU regulations. Figure 2 Sugar cane harvesting (Source: Wikipedia) Energy can be produced from fuel cells and solar cells. In a fuel cell, fuel and oxygen react in a cell without actually being burnt. Yet again, the process is not carbon neutral as carbon dioxide will be produced during the manufacture of the cell itself and the fuel used in the cell. Solar panels are made from thin layers of silicon with an anti-reflective coating and strips of metal leading to a circuit. The manufacturers design them with an expected life span of about twenty years. To reduce atmospheric carbon dioxide, the CO 2 produced during manufacture must be less than the CO 2 that would have been emitted by burning fuel to release the same amount of energy as the solar panel. If wind farms are used to produce energy, there is a carbon footprint caused by the energy required to build them, the electricity transmission lines, that the fuel burnt by the stand-by power stations for when the wind strength is too low to generate enough energy. 4

9 Green chemistry and catalysts Catalysts speed up reactions and are not used up in the process. They range from enzymes, which work at body temperature, to transition metals and their compounds operating at several hundred degrees celsius. One use of enzymes is in detergents where proteases can be used to break down protein, lipases to break down fats and even cellulases to break down matted fibres on the surface of clothing. Enzyme detergents have to be used at relatively low temperatures, so they save energy in use when compared to detergents used at high temperatures. Other applications for enzymes have been developed such as breaking up oil spillages, breaking down toxic substances like cyanide ions, and making test strips for glucose for diabetics. Enzymes working in solution are homogeneous catalysts, and therefore are difficult to separate so that they can be reused. Chemists can immobilise enzymes by trapping them on inert materials such as clays or ion exchange resins, so that they can be filtered off and reused. This can easily be shown with the enzyme lactase which breaks down the lactose in milk to glucose and galactose. It is used to prepare milk for lactose-intolerant people, or to make cows milk suitable for cats, who they cannot digest much lactose. Our entire transport system depends on the use of catalysts, as the alkanes produced by fractional distillation of crude oil are not suitable for direct use in petrol vehicles. Straight chain alkanes explode prematurely and cause an effect called knocking, and large alkanes burn too slowly. Alkanes from crude oil are cracked by one of two processes. Thermal cracking produces alkenes and smaller alkanes including some branched alkanes. Catalytic cracking takes place at a lower temperature and produces mostly branched chain alkanes and some arenes. The catalysts used can be bonded onto natural minerals called zeolites. These contain tiny interconnecting pores and have a huge surface area per gram. Chemists can grow crystals of zeolites with pores of precise dimensions and hence control the products of catalytic cracking. Figure 3 The micro-porous molecular structure of a zeolite, ZSM-5 (Source: Wikipedia) 5

10 When the world s oil supplies run out, an alternative fuel may be methanol, as it burns very cleanly. Methanol can be manufactured from carbon monoxide and hydrogen with a metal catalyst. The process works at pressure of around 100 atmospheres and a temperature of lower than 300 C. However, the carbon monoxide is currently made from coal or methane, so it is still dependent on fossil fuels. Catalysts are also important in treating the fumes from car exhausts, which contain unburnt hydrocarbons, carbon monoxide and oxides of nitrogen. A three-way catalyst is needed to clean up this mixture and the annual MoT test on cars over three years old ensures it is working. Transition metals like iron are well known for reactions like the Haber process, but chemists have discovered uses for many other scarcer metals. Ethanoic acid can be manufactured by the reaction of methanol with carbon monoxide. CH 3 OH + CO CH 3 COOH This process has an atom economy of 100 per cent. The atom economy of a reaction is the percentage of reactant atoms in the desired product. In other words there are no unwanted products in this reaction. It was first carried out using a cobalt-based catalyst at 700 atmospheres pressure and a temperature of 200 C, when it still produced 65 tonnes of byproducts per 100 tonnes of ethanoic acid. Chemists investigated other catalysts and the latest process uses a rhodium and iridium based catalyst at a temperature of 180 C and pressure of 30 atmospheres. This gives 99 per cent selectivity for ethanoic acid, so money and energy are saved in carrying out the process and purifying the product. Chemists now know a great deal about how catalysts work, including: their electronic structure must allow a bond to form with the reactants the shape of their surface must allow the reactant to attach there must be a large number of active sites where the reactant can attach, and the bonding must be of appropriate strength for the product to detach from the catalyst surface. Metals are not necessarily used as elements in ethanoic acid manufacture the catalyst is a 2-phosphanethylcyclopentadienyl complex. Catalysts can be used to control the structure of polymers. Poly(ethene) has a low-density form with few crystalline regions and a much stronger, more crystalline high-density form. Organometallic catalysts cause polymers to grow out from the catalyst surface like hairs, and chemists now have control over the structure and hence the properties of the catalysts they are making. 6

11 The problem of waste Plastics were a miracle chemical of the twentieth century, but the fact that many are nonbiodegradable has caused problems with long-lasting rubbish in streets and in landfill sites. Chemists have sought various solutions to the problem. Biodegradable plastics are now being developed from lactic acid (2-hydroxypropanoic acid), and 3-hydroxypropanoic acid which forms a polymer known as biopol. If certain monomers containing carbonyl groups are incorporated into polymer chains, they degrade more easily in sunlight. If a plastic is mixed with starch granules and disposed of by burying in soil, microorganisms break down the starch granules and the structure of the plastic object collapses. Soluble polymers such as poly(ethenol), are used to make liquitabs (detergents in water soluble packets) and laundry bags for use in hospitals, to avoid handling of contaminated laundry. The use of bacteria to make polymers like poly(3-hydroxypropanoic acid) reduces our dependence on crude oil as a source of monomers. At present it is still a very expensive option. Figure 4 Biodegradable plastic cutlery (Source: United States Department of Agriculture) To keep our water supply pure, we need to prevent toxic waste entering drainage water. Pesticides kill insects but can damage us and other organisms so they should be as specific as possible, and not last for long in soil, to prevent contamination of ground water. Chemists have made new pesticides by identifying the molecular structures of natural pesticides and synthesising modified versions, by changing groups on the molecule. This can improve the pesticide in use by changing its solubility or altering its stability in the environment. Nowadays there are organisations that track the use of dangerous substances and develop clean technology. There are degrees in Clean Technology and learners can study topics such as using energy, water and raw materials efficiently and productively, creating less waste or toxicity while delivering equal or superior performance, and improving customer profitability. There is legislation on waste disposal, which regulates the ph, metal-ion content and organic content of effluents. This legislation is of no use unless we can measure the concentrations of possible contaminants, and again chemists have analytical methods, from spectroscopy to chromatography, to detect minute quantities of a wide variety of substances. So chemists can help to sustain the resources and environment on our planet; they have techniques to detect when things go wrong, and the potential to repair some of the damage. 7

12 Web references BBC Climate Change Carbon Footprint Ethanoic acid manufacture Carbon capture and storage Greener Industry Intelligent Energy Renewable Energy Access The International Aluminium Institute 1578sb291007S:\LT\PD\SUPPORT\GCE in Chemistry Green Chemistry CS.doc.1-13/1 8

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