Carbon Compounds As Fuels
Overview
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.
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, how refiners deliberately smash big molecules apart to manufacture the small ones the world keeps running out of, and how one shake of a test tube tells you which of the two hydrocarbon families you are holding.
Objectives
- 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.
- 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.
Mind map
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Lesson Note
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Lesson Evaluation
Congratulations on completing the lesson on Carbon Compounds As Fuels. Now that youve explored the key concepts and ideas, its time to put your knowledge to the test. This section offers a variety of practice questions designed to reinforce your understanding and help you gauge your grasp of the material.
You will encounter a mix of question types, including multiple-choice questions, short answer questions, and essay questions. Each question is thoughtfully crafted to assess different aspects of your knowledge and critical thinking skills.
Use this evaluation section as an opportunity to reinforce your understanding of the topic and to identify any areas where you may need additional study. Don't be discouraged by any challenges you encounter; instead, view them as opportunities for growth and improvement.
- 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 statement about cracking is correct? A. It separates crude oil into fractions without any reaction taking place. B. It joins small hydrocarbon molecules together to make larger ones. C. It breaks large hydrocarbon molecules into smaller, more useful ones. D. It sorts the hydrocarbons in crude oil according to their boiling points. 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 has a very high boiling point. 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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