Alkane and Alkene


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Alkane and Alkene

  1. 1. Organic Chemistry Homework 2 - Alkanes and Alkenes Name: ………………………………………. Mark: / 100 Class: ……………………………………….. Date: ………………. Paper 2 [Structured and Free Response Questions] Section A [Structured Questions] 1 The table shows some data about the first six members of the alkane homologous series. alkane number of melting boiling heat energy released by carbon atoms point / oC point / oC complete combustion of in one molecule 1 mole of alkane / kJ X Y methane 1 -182 -162 880 ethane 2 -183 -89 1540 propane 3 -188 -42 2200 butane 4 -138 0 2880 pentane 5 -130 36 3500 hexane 6 -95 69 4140 (a) Plot a graph of Y against X on the graph grid below. [3] heat energy released by complete combustion of 1 mole of alkane / kJ number of carbon atoms in one molecule (b) Use the graph to estimate the value of Y for pentane. Write your answer in the table. [2] 1
  2. 2. (c) Calculate the energy released by one gram of methane. Amount of energy released = 1/[12+4(1)] × 880 kJ = 55 kJ [1] (d) Give the molecular formula of an alkane which is a liquid at room temperature and and pressure (25oC and 1 atmosphere). C5H12 / C6H14 [1] (e) Draw the structural formula of four possible isomers of hexane. [Refer to this video on youtube: It gives an excellent explanation to the isomers of hexane! Control-click on the hyperlink above.] [4] (f) What is the difference in the formula of two successive members of an homologous series? Two successive members of an homologous series differ by one unit of CH2. [1] 2 Cracking is used to break big hydrocarbon molecules into smaller ones. (a) (i) State the conditions for cracking. A high temperature and a solid catalyst (such as aluminium oxide or silicon(IV) oxide) (ii) State and explain one advantage to motor vehicles of cracking. Cracking produce small alkane molecules that can be used as fuel for motor vehicles. Cracking of big hydrocarbon molecules from petroleum into smaller alkane molecules that can be used as petrol for cars. Petroleum does not contain enough small alkane molecules to provide all the world’s petrol. (b) A hydrocarbon, C14H30, is cracked to give two products. One of the products is propane. (i) What is the molecular formula of the other product? C11H22 2
  3. 3. (ii) Describe a chemical test to show that the other product is unsaturated. test: Add the liquid product into aqueous bromine in a test tube and shake. observation in test: If the product is unsaturated, the reddish- brown aqueous bromine will turn colourless rapidly. [2] 3 (a) An alkene has the molecular formula C5H10. This alkene reacts with bromine to form a compound with the structural formula shown below. H H H H H H C C C C C H H Br Br H H (i) Draw the structural formula of C5H10. (ii) Write the equation for the reaction of between C5H10 and bromine. C5H10 + Br2 → C5H10Br2 (iii) Give the colours of the alkene: colourless bromine: reddish-brown the product of the reaction: colourless (iv) Explain why this reaction is called an addition reaction. The bromine molecule is added to the alkene molecule to form a single molecule, and a saturated molecule is obtained. [NB: An addition reaction is a reaction in which two or more molecules join together to form a single molecule. A saturated molecule is obtained.] [7] (b) The molecular formulas of the first three members of an homologous series of hydrocarbons are: 3
  4. 4. C4H6 C5H8 C6H10 (i) What is the molecular formula of the next member of this series? C7H12 (ii) Suggest a physical property of these hydrocarbons which is likely to change as the number of carbon atoms increases. How does it change? Boiling point. The boiling points of these hydrocarbon increase as the number of carbon atom increases. [2] 4 The diagram shows how margarine is produced from vegetable oil. H H H H H H H H H H H H H H H2 + C C C C C C C C nickel hydrogen C C powder C C H H H H H H H H part of a vegetable oil molecule part of a margarine molecule (a) What type of reaction takes place between hydrogen and vegetable oil? Addition reaction. [1] (b) (i) What is the purpose of the nickel in this reaction? The nickel acts as the catalyst in this reaction. (ii) Why is the nickel in powder form? Nickel powder has smaller particle size and a larger surface area. Exposing a larger surface area for collisions between reacting particles results in more collision per unit time. With more collisions, the number of effective collisions per unit time increases. The speed of reaction increases. [2] (c) Some vegetable oil molecules contain more than one C=C bond. The number of C=C bonds in a vegetable oil molecule can be found by reacting the oil with iodine. Iodine reacts with C=C bonds in the same way as bromine. 0.05 mole of a vegetable oil was found to increase its mass by 25.4 g when shaken with an excess solution of iodine. 4
  5. 5. How many C=C bonds are contained in one molecule of the oil? Number of moles of iodine, I2, reacted = 25.4 / 2(127) = 0.1 mol 0.05 mole of vegetable oil reacts with 0.1 mole of iodine, I2 Thus, 1 mole of vegetable oil will react with 0.1/0.05 = 2 mole of iodine, I2 One molecule of the oil contained 2 C=C bonds. [2] 5 Many useful products can be made from crude oil (petroleum). This is shown in the flow chart below. Useful fuels process X Petroleum e.g. butane, ‘cylinder gas’ reaction Y reaction Z Ethene, C2H4 Ethanol, C2H5OH (a) Name (i) process X: fractional distillation (ii) reaction Y: cracking [2] (b) For reaction Z, (i) what type of reaction takes place? Addition reaction (ii) write the equation for the reaction: C2H4 + H2O → C2H5OH [2] (c) Butane forms two isomers. Draw the structural formula of both isomers. [Refer to this website for the isomers of butane: Again, control-click on the link!] [2] (d) Butane is used as ‘cylinder gas’ for cooking. (i) Construct the equation for the complete combustion of butane in air. 2C4H10(g) + 13O2(g) → 8CO2(g) + 10H2O(l) 5
  6. 6. (ii) Why is good ventilation important when ‘cylinder gas’ is being used? Good ventilation ensures that enough oxygen is available for complete combustion of butane. If insufficient oxygen is present, incomplete combustion of butane occurs and poisonous carbon monoxide gas will be produced. 6 The following questions refer to compounds with the structure drawn below. Using the letters A to F as appropriate, answer the following questions. (a) Which compound is an unsaturated hydrocarbon? D (b) Which compounds are members of the same homologous series? A and F (c) Which compounds are isomers of each other? C and E (d) Which compound is a product of the reaction of ethene with steam? B (e) C16H34 undergoes cracking to produce C13H26 and another compound. What is the other compound? F [5] Section B [Free Response Questions] 1 (a) Give the structure of methane. 6
  7. 7. H H C H H [1] (b) Methane undergoes a reaction with chlorine. (i) State the type of reaction that takes place and one important condition for the reaction. Substitution reaction. The important condition is that the mixture of methane and chlorine gas is exposed to diffused sunlight or ultra-violet light. (ii) Draw the structures of any two organic products of this reaction and write an equation for the reaction producing one of them. H H H Cl H C H H C Cl Cl C Cl Cl C Cl Cl Cl Cl Cl [any 2] CH4 + Cl2 → CH3Cl + HCl CH3Cl + Cl2 → CH2Cl2 + HCl CH2Cl2 + Cl2 → CHCl3 + HCl CHCl3 + Cl2 → CCl4 + HCl [any 1 equation] (c) Methane is the first member of a homologous series. Another homologous series has the general formula CnH(2n - 6), where n is a number greater than 5. Give the formulas of the first two members of this series. C6H6, C7H8 [2] 2 One of the compounds in crude oil is dodecane, C12H26. (a) One of the reactions of dodecane is shown in the equation below. C12H26  C8H18 + C4H8 (i) What is the name of this type of reaction? Cracking (ii) State two conditions required for this reaction. 7
  8. 8. A high temperature and a solid catalyst (such as aluminium oxide or silicon(IV) oxide) (iii) In the reaction, 1.00 mol of dodecane, C12H26, gives only 22.8 g of octane, C8H18. Calculate the percentage yield. 1 mole of dodecane, C12H26, produce 1 mole of octane, C8H18 Theoretical mass of octane produced = 1 × [8(12) + 18(1)] = 114 g Thus, percentage yield = 22.8/114 × 100% = 20 % [5] (b) (i) Draw the structural formulas of two isomers of C4H8. [Refer to this website: ] (ii) Suggest one use of these isomers, other than as fuels. The isomers can be polymerised to make plastic. / The isomers can react with steam to make alcohol. [3] (c) Octane is a saturated hydrocarbon and butene, C4H8, is an unsaturated hydrocarbon. Explain how aqueous bromine can be used to distinguish between these two compounds. Add the liquid octane into aqueous bromine in a test tube and shake. Since octane is saturated, no visible change will be observed and the reddish-brown aqueous bromine is not decolourised. Bubble the gaseous butene into aqueous bromine in a test tube. Since butene is unsaturated, the reddish-brown aqueous bromine will decolourise rapidly. [2] 3 An organic liquid X has the following composition by mass: carbon – 82.76% hydrogen – 17.24% Its relative molecular mass is 58. (a) Determine the empirical formula of X. C H Percentage by mass 82.76 17.24 8
  9. 9. Ar of element 12 1 Divide percentage by 82.76/12 = 6.89667 17.24/1 = 17.24 Ar Divide by smallest 6.89667/6.89667 = 1 17.24/6.89667 = number 2.5 Simplest ratio 2 5 Thus, the empirical formula of X is C2H5 (b) Hence, determine the molecular formula of X. molecular formula of X = (C2H5)n n = 58 / [2(12) + 5(1)] =2 Thus, the molecular formula of X is (C2H5)2 = C4H10 (c) Draw the full structural formula and name two possible isomers of X. [Refer to this website for the isomers of butane: Again, control-click on the link!] (d) Write a balanced equation for the complete combustion of X. 2C4H10(g) + 13O2(g) → 8CO2(g) + 10H2O(l) (e) Describe what you would observe when a sample of X is shaken with aqueous bromine in a test tube. When X is shaken with aqueous bromine in a test tube, no visible change will be observed and the reddish-brown aqueous bromine is not decolourised. [7] 4 Artificial rubber is made from the compound below. (a) (i) What is the mass of 1 mole of the compound? Mass of 1 mole of the compound = 1 × Mr of compound = 1 × [5(12) + 8(1)] = 68 g 9
  10. 10. (ii) What maximum volume of hydrogen, measured at r.t.p., would you expect to react with 1.36 g of the compound? Number of moles of the compound = 1.36/68 = 0.02 mol One mole of the compound reacts with 2 moles of hydrogen gas, H2 Thus, number of mole of hydrogen that would react = 2 × 0.02 = 0.04 mol Volume of hydrogen that would react = 0.04 × 24 dm3 = 0.96 dm3 [4] (b) Draw the structure of one isomer of the compound. One possible isomer: [1] 10