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Transcript
Chemistry: Unit F322: Chains, Energy and Resources Fuels Fractional Distillation is a method of separating the mixture of hydrocarbons in crude oil according to their boiling points. Fractions which are more volatile, with lower boiling points and shorter carbon chains, rise to the top of the fractionating column and condense where it is cooler. Shorter Hydrocarbon = Fewer Van der Waals’ Forces = less energy required to overcome forces Longer chain hydrocarbons have a higher boiling point and so separate near to the base of the fractionating column. Longer Hydrocarbon = More Van der Waals’ Forces = more energy required to overcome forces A good fuel must have three properties: Volatile (turns to gas easily) Easy to ignite Burns completely in air, no soot produced There is most demand for short chain hydrocarbons because they burn most efficiently. In order to match the demand, long chain hydrocarbons undergo a process called Catalytic Cracking in order to convert them into shorter, more useful alkanes and alkenes. Long chain alkane Heat, Pressure, AlO catalyst shorter chain alkanes and alkenes A catalyst is used because they speed up the rate of reaction and provide an alternative reaction route which has lower activation energy (lower temperatures required, so less fuel and less waste) Petrol contains alkanes with between 4 and 12 carbon atoms. When straight chain alkanes are compressed in the cylinder of a car engine, they pre-ignite before the spark. This is known as knocking. Lead used to be used to prevent this, but now, we use branched alkanes and cyclic hydrocarbons to promote efficient combustion. Producing Branched Hydrocarbons: Isomerisation Unbranched, straight chain hydrocarbons are converted into branched chains under a high temperature with a platinum catalyst. Producing Cyclic Hydrocarbons: Reforming + 2H Reforming turns straight chain alkanes into cyclic alkanes under high temperatures with a platinum catalyst. These are used in petrol and as a feedstock for a wide range of organic chemicals such a pharmaceuticals and dyes. Research Octane Number is used to grade the quality of petrol. If an alkane has a high RON, it burns efficiently. If an alkane has a low RON does not combust efficiently and tends to preignite: Longer chains = Lower RON Increased Branching = Higher RON The ideal fuel is a short chain with lots of branching. Air pollution caused by the combustion of fossil fuels: Carbon Monoxide is a poisonous gas, formed by the incomplete combustion of hydrocarbons. It binds to the haemoglobin in red blood cells, inhibiting the ability to carry oxygen. Oxides of Nitrogen are formed under high temperatures in the car engine, when atmospheric nitrogen is oxidised to form nitrogen oxides. They cause the formation of low level ozone and reacts with water to form nitric acid, which falls as acid rain. Unburnt hydrocarbons are released in car exhausts. They react with nitrogen oxides to form low level ozone, which causes breathing difficulties. The Greenhouse Effect Infrared radiation is absorbed by 3 covalent bonds: C=O O-H C-H This carbon dioxide molecule absorbs Infrared radiation, which causes the molecule to vibrate. The vibrating molecule will eventually emit some of this energy as radiation. This reemitted radiation is absorbed by other gas molecules and at the Earth’s surface, increasing the temperature. Infrared Radiation The greenhouse effect of a given gas is dependent on two things: 1. Its atmospheric concentration 2. Its ability to absorb infrared radiation It is important to control global warming as there can be damaging consequences: Rising Sea levels Melting polar ice caps Unusual weather patterns Increase in desertification Destruction of natural habitats Scientists are trying to minimise climate change caused by the release of greenhouse gases: Providing Scientific Evidence that global warming is taking place. Scientists use ice cores from Antarctica to estimate CO2 levels thousands of years ago – a proxy record. Providing Information to change opinions Educating Society with information on climate change, through the media (books, journals, documentaries) Reducing Global Warming Monitoring progress against agreements such as the Kyoto Protocol, which was an international agreement setting targets to reduce greenhouse gas emissions. Alternative energy resources such as solar power, wind, and development of hydrogen fuel cells. Developing New Technologies More efficient engines for transport such as lean burn engines, hybrid engines or electric cars. Carbon Capture and Storage – turned into a liquid and injected deep into the oceans or stored in geological formations. Waste CO2 can be reacted with metal oxides to make stable carbonates, such as Calcium Carbonate. Sustainability is the development of processes which prevent the depletion of natural resources. There are 5 principles of chemical sustainability: 1. Use industrial processes which will reduce or eliminate hazardous chemicals Phasing out tetraethyl lead as a fuel additive, now using cycloalkanes 2. Minimise the production of waste materials Cracking of distillation residues to gain chemical feedstock 3. Use renewable materials such as plant based substances Fossil fuels being supplemented by biodiesel and bioalcohol 4. Seek alternative energy resources Using solar, wind, tidal energy instead of consuming finite resources 5. Ensure waste products are non toxic Catalytic converters remove NOx, CO and Unburnt hydrocarbons from exhaust gases from an internal combustion engine. The 1992 Rio Declaration on Environment and Development defines the rights of people to be involved in the development of their economies and the responsibilities of human beings to safeguard the environment.