Yr10 Energy from fuels and incomplete combustion Combustion is the rapid oxidation of a substance accompanied by a flame. One example is the combustion of magnesium. Q1. Write the word equation for the reaction between magnesium and oxygen. A hydrocarbon is a compound made up only of carbon and hydrogen. One example is methane, CH4. When hydrocarbons undergo complete combustion they produce carbon dioxide and water. As long as the fuel is volatile (forms gas easily) and there is a good supply of oxygen, complete combustion will occur. The following reaction represents the complete combustion of methane: CH4(g) + 2 O2(g) CO2(g) + 2 H2O(g) + energy Q2. Write the word equation for the complete combustion of ethane, C2H6. Extension- write the chemical equation for this reaction. Many fuels such as petrol and natural gas are hydrocarbons. Hydrocarbons with small molecules make better fuels than hydrocarbons with large molecules. This is because small hydrocarbon molecules: are more volatile - turn into a gas more easily are less viscous - they flow more easily are more easily ignited burn with a cleaner, less sooty flame Complete and incomplete combustion Incomplete combustion is what occurs when there is not a good supply of oxygen OR if the fuel molecules are too big to vaporise easily. If there is not enough oxygen available for all the carbon to turn into carbon dioxide (complete combustion), then some or all of the carbon turns to carbon monoxide or soot. This happens with any hydrocarbon - we shall take methane as an example. During incomplete combustion methane gas burns with a yellow flame. (unlike the clear blue flame seen in complete combustion). Carbon particles (sooty marks) may also be seen. methane + oxygen 4CH4(g) + 5O2(g) carbon monoxide + soot + water. 2CO(g) + 2C(s) + 8H2O(l) Carbon monoxide is a very poisonous gas. It cannot be seen or smelt. Faulty gas fires or boilers may produce carbon monoxide and poison the air in a room without anyone knowing. Carbon monoxide acts as a poison by combining with haemoglobin in the blood. Haemoglobin normally reacts with oxygen from the air and transports the oxygen to the parts of the body which need it. Carbon monoxide is much more reactive with haemoglobin than oxygen is. It combines to form a stable compound with haemoglobin, preventing the transport of oxygen around the body. The person dies by suffocating from the inside (nasty!). Soot is a health hazard too. The fine particles can get trapped in lungs and cause respiratory problems. The main sign of incomplete combustion is a “dirty” flame. The smoke is the soot being produced. The flame burns yellow because all the tiny soot particles glow red hot. Q3. Which is more likely to undergo incomplete combustion, methane or octane, C8H18? Explain your answer. FOSSIL FUELS Fossil fuels are non-renewable resources formed over millions of years from the remains of living things. They include coal, oil (petroleum) and natural gas. Since the Industrial Revolution (a time of rapid technological advancement in the 18th and 19th centuries), we have used fossil fuels in ever-increasing amounts, primarily to run vehicles and for electricity generation. The combustion of fossil fuels has resulted in an increase in the carbon dioxide concentration in the atmosphere. This has caused global warming due to the enhanced greenhouse effect. Combustion of fossil fuels also causes photochemical smog and acid rain. Coal is formed from plant matter in ancient swamps. Crude oil and natural gas are formed from the remains of marine organisms, mainly plankton. The organic matter is buried in sediment and then compressed and heated over millions of years to form fossil fuels. Q4. Watch the brainpop videos on fossil fuels and the greenhouse effect and add your notes in the spaces below and on the back of this page.