Beatty 19.3 Alkenes WS1 2023 Answers1

1 Beatty Secondary School Science Department (Chemistry Unit) Chemistry 6092 Name: ___________________________________ ( ) Date: _______________ Class: 4E1 TOPIC: ALKENES (WORKSHEET 1) Learning Objectives: (a) Describe the alkenes as an homologous series of unsaturated hydrocarbons with the general formula CnH2n. (b) Draw the structures of unbranched alkenes, C2 to C 4, and name the unbranched alkenes, ethene to butene. (c) Describe the manufacture of alkenes and hydrogen by cracking hydrocarbons and recognise that cracking is essential to match the demand for fractions containing smaller molecules from the refinery process. (d) Describe the difference between saturated and unsaturated hydrocarbons from their molecular structures and by using aqueous bromine. (e) Describe the properties of alkenes (exemplified by ethene) in terms of combustion, polymerisation and the addition reactions with bromine, steam and hydrogen. (f) State the meaning of polyunsaturated when applied to food products. (g) Describe the manufacture of margarine by the addition of hydrogen to unsaturated vegetable oils to form a solid product. Multiple-Choice Questions 1 What feature would show that a compound must be an alkene? A if it only contains carbon and hydrogen B if it reacts only with bromine C if it is a hydrocarbon with a C═C bond D if it burns in air with a smoky flame ( C ) 2 Which of the followin g would be a likely prod uct on cracking the hydrocarbon octadecane, C18H38? A C9H18 B C9H22 C C18H40 D C36H74 ( A ) 3 The cracking o f one mole of X produces two moles of ethene , two moles of prope ne and one mole of propane. How many carbon atoms does one molecule of X contain? A 8 B 10 C 13 D 16 ( C )

2 4 Which statements about the cracking of hydrocarbons are correct? 1 Cracking involves breaking down hydrocarbon molecules. 2 One of the products of cracking is always unsaturated. 3 Cracking is essential because of the demand for fractions containing smaller molecules. A 1 and 2 only B 2 and 3 only C 1 and 3 only D 1, 2 and 3 ( D ) 5 Which structural formula represents an unsaturated hydrocarbon? ( A ) 6 Compounds X and Y are two different alkenes. Which statement about X and Y is not correct? A They both undergo addition reactions. B They have the same empirical formula. C They both contain carbon and hydrogen only. D They have the same boiling point. ( D ) 7 Which one of these equations does not show an addition reaction? A CH3Cl + Cl2 → CH2Cl2 + HCl B C3H6 + Br2 → C3H6Br2 C C4H8 + H2 → C4H10 D nC2H4 → [ CH2-CH2 ]n ( A ) 8 Compound Y has the e mpirical formula CH 2, has an M r of 56 and forms two alco hols that have different structural formulae when reacted with steam. What is compound Y? ( B )

3 9 The diagram shows an apparatus used to test a gas. The bromine solution becomes colourless. What is the structure of the gas? ( C ) 10 Which compound undergoes an addition reaction with bromine? A C2H4 B C2H6 C C2H5OH D CH3CO2H ( A ) 11 What is the structure of the product of the reaction between butene, CH3−CH2−CH=CH2, and bromine? A CH2Br−CH2−CH2−CH2Br B CH2Br−CH2−CHBr−CH3 C CH3−CHBr−CH2−CH2Br D CH3−CH2−CHBr−CH2Br ( D ) 12 A student investigated the reaction of vegetab le oils with hydrogen. 100 cm 3 of hydrogen was bubbled through 1 g samples of four different vegetable oils containing a suitable catalyst. The volume of hydrogen remaining after each experiment was recorded. vegetable oil volume of hydrogen remaining / cm3 P 100 Q 87 R 63 S 0 Which vegetable oils are unsaturated? A P, Q and R B Q and R only C Q, R and S D S only ( C ) 13 How can margarine be made from vegetable oils? A by the addition of hydrogen to a saturated oil B by the addition of hydrogen to an unsaturated oil C by the addition of steam to a saturated oil D by the addition of steam to an unsaturated oil ( B )

4 Structured Questions 14 The hydrocarbon ethene (C2H4) is the first member of the alkenes homologous series. (a) Name and draw the structural formula e of the first three members of the alkene series. ethene propene butene (b) (i) What substance would you expect to be produced when ethene is burned in a plentiful supply of air, other than water vapour? Carbon dioxide (ii) How would you show the presence of the gaseous product other than water vapour? Bubble the gas into limewater. If gas produced forms a white precipitate in limewater, carbon dioxide is present. 15 Alkenes burn in air like alkanes but they produce more soot than alkanes. (a) Write an equation for the complete combustion of butene. C4H8 + 6O2 → 4CO2 + 4H2O (b) Suggest why alkenes produce more soot than alkanes upon combustion. Alkenes have a higher percentage of carbon than correspondi ng alkanes. This leads to a higher proportion of incomplete combustion as carbon particles (soot) may be produced. 16 Write the balanced chemical equation when pro pene undergoes each of the followi ng reactions. (a) addition of bromine, C3H6 + Br2 → C3H6Br2 (b) addition of hydrogen, C3H6 + H2 → C3H8 (c) addition of steam. C3H6 + H2O → C3H7OH

5 17 Kerosene is a mixture of alkanes. The diagram below shows how a mixture of propene and other hydrocarbons can be produced from kerosene in the laboratory. (a) Name the type of reaction shown in the diagram. Catalytic cracking (b) What is the purpose of the pieces of broken pot? Increase the surface area and catalyse the reaction to increase the speed of reaction (c) Why must the end of the delivery tube be taken out of the water before the heating process is stopped? This is to prevent ‘suck-back’ from occurring due to the difference in pressure, where water flows back into th e delivery tube to the boil ing tube, causing it to crack. (d) (i) A typical alkane found in kerosene has the formula C14H30. Construct the equation for one reaction that takes place in the appar atus producing propane from C14H30. C14H30 → C3H8 + C11H22 (ii) Describe a chemi cal test to show that the other product formed is unsaturated. test addition of aqueous bromine observation reddish-brown bromine turns colourless / decolourises 18 Catalytic cracking is used to break big hydrocarbon molecules into smaller ones. (a) State and explain one advantage to vehicles of catalytic cracking of hydrocarbons. Cracking involves breaking down of larger alkanes , which are less useful and lower in demand , into shorter alkane fractions which are higher in demand to be used as fuels but lower in supply.

6 (b) State the conditions for cracking. porous pot / zeolite with finely divided catalyst: aluminium oxide or silicon(IV) oxide and high temperature of 600 °C 19 Explain the meanings of substitution and addition reactions. A substitution reaction is a reaction in which an atom or a group of atoms is replaced by atoms of other elements. An addition reaction is a reaction in which an unsaturated organic compound reacts with another element or compound to form a single new compound. 20 Complete the following boxes with the name of the compounds formed from ethene. Ethene has a wide range of uses. It is used to make organic compounds, solvents and plastics. (a) Bri