Carbon Compounds As Fuels
Visão Geral
Somewhere under the ground beneath you, or under a sea floor a long way from you, the crushed remains of organisms that died before there were dinosaurs have been sitting under heat and pressure for tens of millions of years. What comes out of the drill is a dark, sticky liquid that will not burn cleanly, will not flow through an engine and cannot be sold as anything. Yet almost every journey you have ever taken began with that liquid, and so did the plastic in the device you are reading this on.
This lesson is about the chemistry that turns a useless mixture into a shelf of specialised fuels. You will see how a single tall steel tower sorts thousands of different molecules without a single chemical reaction taking place, why a molecule with eight carbon atoms makes a good petrol and one with twenty-two does not, what actually leaves the exhaust pipe when a fuel burns and why some of it is dangerous, and how refiners deliberately smash big molecules apart to manufacture the small ones the world keeps running out of.
Objetivos
- Crude oil is a mixture of a very large number of compounds.
- Most of the compounds in crude oil are hydrocarbons, which are molecules made up of hydrogen and carbon atoms only.
- The many hydrocarbons in crude oil may be separated into fractions, each of which contains molecules with a similar number of carbon atoms, by evaporating the oil and allowing it to condense at a number of different temperatures. This process is called fractional distillation. Students should know and understand the main processes in continuous fractional distillation in a fractionating column. Knowledge of the names of specific fractions or fuels is not required.
- Most of the hydrocarbons in crude oil are saturated hydrocarbons called alkanes. The general formula for the homologous series of alkanes is CnH2n+2 Students should know that in saturated hydrocarbons all the carbon-carbon bonds are single covalent bonds.
- Alkane molecules can be represented in the following forms: C2H6 or H H | | H C C H | | H H Students should know that in displayed structures represents a covalent bond. Students should be able to recognise alkanes from their formulae in any of the forms, but do not need to know the names of specific alkanes other than methane, ethane and propane.
- Some properties of hydrocarbons depend on the size of their molecules. These properties influence how hydrocarbons are used as fuels. Knowledge and understanding of trends in properties of hydrocarbons is limited to: boiling points; viscosity; flammability.
- Most fuels, including coal, contain carbon and/or hydrogen and may also contain some sulfur. The gases released into the atmosphere when a fuel burns may include carbon dioxide, water (vapour), carbon monoxide, sulfur dioxide and oxides of nitrogen. Solid particles (particulates) may also be released. Solid particles may contain soot (carbon) and unburnt fuels. Sulfur dioxide and oxides of nitrogen cause acid rain, an increase in carbon dioxide may result in climate change, and solid particles cause global dimming. Students should be able to relate products of combustion to the elements present in compounds in the fuel and to the extent of combustion (whether complete or incomplete). No details of how the oxides of nitrogen are formed are required, other than the fact that they are formed at high temperatures.
- The combustion of hydrocarbon fuels releases energy. During combustion, the carbon and hydrogen in the fuels are oxidised.
- Ethanol can be made by reacting ethene with steam: C2H4 + H2O → C2H5OH Phosphoric acid is used as a catalyst to increase the rate of the reaction. This is efficient because it is a continuous process. However, ethene is derived from crude oil, a non- renewable resource which may run out one day. Ethanol can also be made by fermenting sugar: C6H12O6 → 2 C2H5OH + 2CO2 Enzymes in yeast are natural catalysts for this reaction, and this is a batch process. Unlike ethene, sugar is a renewable resource. Students should be able to compare the two ways of making ethanol in terms of type of raw material, type of process, labour, rate of reaction, conditions needed, purity of product and energy requirements.
- Biofuels, including biodiesel and ethanol, are produced from plant material, and are possible alternatives to hydrocarbon fuels. Students should know and understand the benefits and disadvantages of biofuels in terms of: use of renewable resources; their impacts on land use; their carbon footprint. Students should know that ethanol for use as a biofuel is produced from a dilute solution of ethanol obtained by the fermentation of plant materials at a temperature between 20 °C and 35 °C. Detailed knowledge of the methods used to produce other biofuels is not required.
- Hydrocarbons can be broken down (cracked) to produce smaller, more useful molecules. This process involves heating the hydrocarbons to vaporise them. The vapours are either passed over a hot catalyst or mixed with steam and heated to a very high temperature so that thermal decomposition reactions then occur.
- The products of cracking include alkanes and unsaturated hydrocarbons called alkenes. The general formula for the homologous series of alkenes is CnH2n Required practical: Test for the presence of a double bond in an unknown hydrocarbon. Students should know that in unsaturated hydrocarbons some of the carbon-carbon bonds are double covalent bonds.
- Unsaturated hydrocarbon molecules can be represented in the following forms: C3H6 or H H H | | | H C C C | | H H Students should know that in displayed structures represents a double bond. Students should be able to recognise alkenes from their names or formulae, but do not need to know the names of individual alkenes other than ethene and propene.
- Alkenes react with bromine water, turning it from orange to colourless.
- Some of the products of cracking are useful as fuels.
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- What is the general formula of the alkanes? A. CnHn B. CnH2n C. CnH2n+2 D. CnH2n-2 Answer: C
- An alkane molecule contains 6 carbon atoms. How many hydrogen atoms does it contain? A. 6 B. 12 C. 14 D. 16 Answer: C
- Which gas released when a fuel burns is a cause of acid rain? A. carbon dioxide B. carbon monoxide C. sulfur dioxide D. water vapour Answer: C
- Bromine water is shaken with an unknown hydrocarbon and changes from orange to colourless. What does this show? A. The hydrocarbon is saturated. B. The hydrocarbon contains a carbon to carbon double bond. C. The hydrocarbon contains sulfur. D. The hydrocarbon has a high viscosity. Answer: B
- Compared with the fraction collected near the bottom of a fractionating column, the fraction collected near the top has: A. larger molecules and a higher boiling point B. smaller molecules and a higher viscosity C. smaller molecules and a lower boiling point D. larger molecules and a greater flammability Answer: C
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