Chemistry_Grade_10-12 (1).pdf

A what is the structural formula of methane b write

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(a) What is the structural formula of methane? (b) Write an equation to show the reaction that takes place when methane is burned as a fuel. (c) Explain what is meant by the statement that methane ’has a greater global warming potential than carbon dioxide’. 4. Chlorine and ethane react to form chloroethane and hydrogen chloride. (a) Write a balanced chemical equation for this reaction, using molecular for- mulae. (b) Give the structural formula of chloroethane. (c) What type of reaction has taken place in this example? 5. Petrol (C 8 H 18 ) is in fact not pure C 8 H 18 but a mixture of various alkanes . The ’octane rating’ of petrol refers to the percentage of the petrol which is C 8 H 18 . For example, 93 octane fuel contains 93% C 8 H 18 and 7% other alkanes. The isomer of C 8 H 18 referred to in the ’octane rating’ is in fact not octane but 2,2,4-trimethylpentane. (a) Write an unbalanced equation for the chemical reaction which takes place when petrol (C 8 H 18 ) burns in excess oxygen. (b) Write the general formula of the alkanes . (c) Define the term structural isomer . (d) Use the information given in this question and your knowledge of naming organic compounds to deduce and draw the full structural formula for 2,2,4-trimethylpentane. (IEB pg 25) 165
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9.7 CHAPTER 9. ORGANIC MOLECULES - GRADE 12 9.7.5 The alkenes In the alkenes, there is at least one double bond between two carbon atoms. This means that they are unsaturated and are more reactive than the alkanes. The simplest alkene is ethene (also known as ethylene), which is shown in figure 9.14. (a) C C H H H H (b) CH 2 CH 2 (c) C 2 H 4 Figure 9.14: The (a) structural, (b) condensed structural and (c) molecular structure represen- tations of ethene As with the alkanes, the elkenes also form a homologous series. They have the general formula C n H 2 n . The second alkene in the series would therefore be C 3 H 6 . This molecule is known as propene (figure 9.15). Note that if an alkene has two double bonds, it is called a diene and if it has three double bonds it is called a triene . (a) H C C C H H H H H CH 3 CHCH 2 (b) C 3 H 6 (c) Figure 9.15: The (a) structural, (b) condensed structural and (c) molecular structure represen- tations of propene The elkenes have a variety of uses. Ethylene for example is a hormone in plants that stimulates the ripening of fruits and the opening of flowers. Propene is an important compound in the petrochemicals industry. It is used as a monomer to make polypropylene and is also used as a fuel gas for other industrial processes. 9.7.6 Naming the alkenes Similar rules will apply in naming the alkenes, as for the alkanes. Worked Example 42: Naming the alkenes Question: Give the IUPAC name for the following compound: 166
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CHAPTER 9. ORGANIC MOLECULES - GRADE 12 9.7 H C (1) H H C (2) H C (3) H C (4) H H H Answer Step 1 : Identify the functional group The compound is an alkene and will have the suffix -ene.
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