3.1.6.3 Combustion of alkanes -...

3.1.6.3 Combustion of alkanes

233 minutes

227 marks

of 33

Q1. (a) Gas oil (diesel), kerosine (paraffin), mineral oil (lubricating oil) and petrol (gasoline) are four of the five fractions obtained by the fractional distillation of crude oil within the temperature range 40–400 °C.

Identify the missing fraction and state the order in which the five fractions are removed as the fractionating column is ascended. Give two reasons why the fractions collect at different levels in the fractionating column.

(4)

(b) Thermal cracking of large hydrocarbon molecules is used to produce alkenes. State the type of mechanism involved in this process. Write an equation for the thermal cracking of C

21H

44 in which ethene and propene are produced in a 3:2 molar ratio together with one

other product. (3)

(c) Write equations, where appropriate, to illustrate your answers to the questions below.

(i) Explain why it is desirable that none of the sulphur-containing impurities naturally found in crude oil are present in petroleum fractions.

(ii) The pollutant gas NO is found in the exhaust gases from petrol engines. Explain why NO is formed in petrol engines but is not readily formed when petrol burns in the open air.

(iii) The pollutant gas CO is also found in the exhaust gases from petrol engines. Explain how CO and NO are removed from the exhaust gases and why the removal of each of them is desirable.

(10) (Total 17 marks)

Q2. Many hydrocarbon compounds burn readily in air.

(i) Write an equation to show the complete combustion of C15

H32

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(ii) One of the gaseous products of the incomplete combustion of methane in gas fires is known to be poisonous. Identify this product and write an equation for the reaction in which it is formed from methane.

Identity of product ........................................................................................

Equation ....................................................................................................... (Total 4 marks)

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Q3. (a) (i) Write an equation for the formation of epoxyethane from ethene, showing the structure of the product.

(ii) Explain why the epoxyethane molecule is highly reactive.

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(iii) Give the structure of the product formed by the reaction of one molecule of epoxyethane with one molecule of water. Give one use for this product.

Structure

Use .............................................................................................................. (5)

(b) But-2-ene can exist in two isomeric forms. Give the structures of these two isomers and name the type of isomerism.

Structure 1 Structure 2

Type of isomerism ................................................................................................ (3)

(Total 8 marks)

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Q4. (a) Butane, C4H

10, is a hydrocarbon which is used as a fuel.

(i) Explain what is meant by the term hydrocarbon.

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(ii) Explain what is meant by the term fuel.

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(iii) Write an equation for the complete combustion of butane.

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(iv) Write an equation for the incomplete combustion of butane to produce carbon monoxide and water.

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(v) Under what conditions would you expect incomplete combustion to occur?

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(b) Three different carbocations are formed by breaking C – C bonds in separate molecules of butane during catalytic cracking. One of these structures is shown below. Give the structures of the other two carbocations.

Structure 1 Structure 2 Structure 3

(2)

(c) Ethane can be cracked in the presence of a catalyst to produce ethene and hydrogen.

(i) Write an equation for this reaction.

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(ii) Give a suitable catalyst for this reaction.

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(iii) State one reason why cracking is important.

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(Total 10 marks)

Q5. The burning of fossil fuels can produce atmospheric pollutants.

(a) The combustion of petrol in an internal combustion engine can lead to the formation of carbon monoxide, CO, and nitrogen monoxide, NO.

(i) Write an equation for the incomplete combustion of octane, C8H

18, to produce CO

and water only.

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(ii) State one essential condition for the formation of NO in an engine. Write an equation for the reaction in which NO is formed.

Condition ............................................................................................

Equation .............................................................................................. (3)

(b) All new petrol-engined cars must be fitted with a catalytic converter.

(i) Name one of the metals used as a catalyst in a catalytic converter.

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(ii) Write an equation to show how CO and NO react with each other in a catalytic converter.

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(c) State why sulphur dioxide gas is sometimes found in the exhaust gases of petrol-engined cars. Give one adverse effect of sulphur dioxide on the environment.

Reason for SO2 in exhaust gases .................................................................

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Environmental effect of SO2 ..........................................................................

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(Total 7 marks)

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Q6. (a) (i) Name the process used to separate petroleum into fractions.

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(ii) Give the molecular formula for an alkane with nine carbon atoms.

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(iii) Write an equation for the complete combustion of the alkane C11

H24

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(iv) Write an equation for the incomplete combustion of C11

H24 to produce carbon

and water only.

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(b) Alkenes can be produced by cracking the naphtha fraction obtained from petroleum.

(i) Write an equation for the thermal cracking of one molecule of C10

H22

to give

one molecule of propene and one molecule of an alkane only.

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(ii) Draw the structure of the chain isomer of but-1-ene.

(2)

(c) The alkanes and the alkenes are examples of homologous series of compounds. One feature of an homologous series is the gradual change in physical properties as the relative molecular mass increases. State two other general features of an homologous series of compounds.

Feature 1 ......................................................................................................

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Feature 2 ......................................................................................................

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(Total 8 marks)

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Q7. Petroleum is separated into fractions by fractional distillation. The petrol fraction (C

4 to C

12) is burned in internal combustion engines and the naphtha

fraction (C7 to C

14) is cracked.

(a) Petroleum is separated into fractions when it is heated and the vapour mixture is passed into a fractionating column.

(i) Explain what is meant by the term fraction as applied to fractional distillation.

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(ii) State a property of the molecules in petroleum which allows the mixture to be separated into fractions.

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(iii) Describe the temperature gradient in the column.

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(b) The fractions from petroleum contain alkane hydrocarbons.

(i) Write an equation for the incomplete combustion of the alkane C8H

18 to produce

carbon monoxide and water only.

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(ii) One isomer of C8H

18 is 2,2,3-trimethylpentane. Draw the structure of this isomer.

(2)

(c) State one economic reason for the cracking of petroleum fractions.

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(d) (i) Give the type of reactive intermediate formed during catalytic cracking.

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(ii) Identify a catalyst used in catalytic cracking.

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(e) (i) Give the type of reactive intermediate formed during thermal cracking. State how this reactive intermediate is formed.

Reactive intermediate ..........................................................................

How intermediate is formed .................................................................

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(ii) Identify the different type of hydrocarbon produced in a high percentage by the thermal cracking of alkanes.

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(Total 11 marks)

Q8. The fractions obtained from petroleum contain saturated hydrocarbons that belong to the homologous series of alkanes.

(a) Any homologous series can be represented by a general formula.

(i) State two other characteristics of homologous series.

Characteristic 1 ...................................................................................

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Characteristic 2 ...................................................................................

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(ii) Name the process which is used to obtain the fractions from petroleum.

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(iii) State what is meant by the term saturated, as applied to hydrocarbons.

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(b) Decane has the molecular formula C10

H22

(i) State what is meant by the term molecular formula.

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(ii) Give the molecular formula of the alkane which contains 14 carbon atoms.

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(iii) Write an equation for the incomplete combustion of decane, C10

H22, to produce

carbon and water only.

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(c) When petrol is burned in an internal combustion engine, some nitrogen monoxide, NO, is formed. This pollutant is removed from the exhaust gases by means of a reaction in a catalytic converter.

(i) Write an equation for the reaction between nitrogen and oxygen to form nitrogen monoxide.

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(ii) Identify a catalyst used in a catalytic converter.

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(iii) Write an equation to show how nitrogen monoxide is removed from the exhaust gases as they pass through a catalytic converter.

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(Total 10 marks)

Q9. (a) Hexane (C6H

14) is a hydrocarbon which is a component of LPG (liquid petroleum gas),

used as a fuel for heating. When burning fuels in boilers it is important to ensure complete combustion.

(i) Give two reasons why boilers are designed to ensure complete combustion.

Reason 1 ............................................................................................

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Reason 2 ............................................................................................

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(ii) Write an equation for the incomplete combustion of hexane.

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(iii) Suggest how an engineer or a chemist could demonstrate that the combustion of hexane in a faulty boiler was incomplete.

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(b) Branched chain alkanes are often preferred as fuels. Draw the structure of two branched chain isomers of hexane and name the first isomer.

Isomer 1 Isomer 2

Name of isomer 1 ........................................................................................ (3)

(c) Hexane can be cracked in the presence of a catalyst to produce another hydrocarbon, Z, and methane.

(i) Draw a possible structure for Z.

(ii) Give a suitable catalyst for this reaction.

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(iii) Suggest why the product Z has more commercial value than hexane.

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(d) The overall equation for the production of dichloromethane from methane and chlorine is shown below.

CH4 + 2Cl

2 → CH

2Cl

2 + 2HCl

(i) Calculate the % atom economy for the formation of CH2Cl

2 in this reaction.

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(ii) Give one reason why this atom economy of less than 100% is an important consideration for the commercial success of this process and predict how a chemical company would maximise profits from this process.

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(Total 14 marks)

Q10. Petrol contains saturated hydrocarbons. Some of the molecules in petrol have the molecular formula C

8H

18 and are referred to as octanes. These octanes can be obtained from

crude oil by fractional distillation and by cracking suitable heavier fractions.

Petrol burns completely in a plentiful supply of air but can undergo incomplete combustion in a car engine.

(a) State the meaning of both the words saturated and hydrocarbon as applied to the term saturated hydrocarbon.

Name the homologous series to which C8H

18 belongs.

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(b) Outline the essential features of the fractional distillation of crude oil that enable the crude oil to be separated into fractions.

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(c) C8H

18 is obtained by the catalytic cracking of suitable heavy fractions.

State what is meant by the term cracking and name the catalyst used in catalytic cracking.

Write an equation to show how one molecule of C14

H30 is cracked to form one molecule of

C8H

18 and one molecule of another hydrocarbon.

Explain why oil companies need to crack ‘suitable heavy fractions’.

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(d) Write an equation for the incomplete combustion of C8H

18 to form carbon monoxide and

water only.

A catalytic converter is used to remove carbon monoxide from the exhaust gases in a car. Identify a catalyst used in the catalytic converter.

Write an equation to show how carbon monoxide is removed in a catalytic converter.

State why the water produced in the exhaust gases may contribute to global warming.

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(e) When some petrol was accidentally contaminated in 2007, the sensors in the affected cars caused a decrease in the supply of petrol to the engine.

Suggest the effect that the contaminated fuel would have on the performance of the cars.

State how the oil company might have recognised the problem before the petrol was sold.

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(f) The molecular formula C8H

18 represents several structural isomers.

State what is meant by the term structural isomers.

Name the following structural isomer of C8H

18

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(Total 20 marks)

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Q11. (a) Two organic compounds with similar relative molecular masses are shown below.

Ethanol Propane

(i) State the type of bond present between the C and H atoms in both of these molecules. Explain how this type of bond is formed.

Type of bond .......................................................................................

Explanation ......................................................................................... (2)

(ii) State the strongest type of intermolecular force present in each compound.

Liquid ethanol ......................................................................................

Liquid propane …................................................................................. (2)

(b) Ethanol dissolves in water. Draw a diagram to show how one molecule of ethanol interacts with one molecule of water in the solution. Include partial charges and all lone pairs. The ethanol molecule has been drawn for you.

(3)

(c) Ethanol was the fuel used in the first mass-produced car, the Model T Ford.

(i) Write an equation which shows how ethanol burns completely in air to form carbon dioxide and water as the only products.

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(ii) Suggest one environmental problem caused by incomplete combustion of ethanol in a car engine.

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(iii) Suggest one economic problem for the car user caused by incomplete combustion of ethanol in the car engine.

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(d) Propane is also used as a fuel, although sometimes it can be contaminated with sulfur-containing impurities. When this propane burns, these impurities form sulfur dioxide.

(i) State how the sulfur dioxide can be removed from the waste gases produced when this propane is burned on a large scale in industry. Suggest a reason why the method you have stated may not be 100% efficient.

How removed .....................................................................................

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Reason for less than 100% efficiency .................................................

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(ii) Although propane has a boiling point of –42 °C, it is usually supplied as a liquid for use in camping stoves. Suggest why it is supplied as a liquid.

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(Total 13 marks)

Q12. Alkanes are saturated hydrocarbons which can be obtained from crude oil. Pentane is an example of an alkane. A molecule of pentane contains five carbon atoms.

(a) (i) State the meaning of the term saturated and of the term hydrocarbon as applied to alkanes.

Saturated ............................................................................................

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Hydrocarbon .......................................................................................

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(ii) Give the general formula for the alkanes.

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(b) Pentane burns completely in oxygen.


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(ii) State how the products of this reaction may affect the environment.

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(c) Give the name of a solid pollutant which may form when pentane burns incompletely in air.

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(d) One molecule of C9H

20 can be cracked to form one molecule of pentane and one other

product.

(i) Write an equation for this cracking reaction.

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(ii) Suggest a type of compound that can be manufactured from the other product of this cracking reaction.

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(iii) State why a high temperature is needed for cracking reactions to occur.

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(e) Pentane can react to form the following haloalkane Q.

(i) Name Q.

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(ii) State the type of structural isomerism shown by Q and the haloalkane shown below.

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(Total 11 marks)

Q13. The alkane butane is used as a fuel.

(a) (i) Write an equation for the complete combustion of butane.

............................................................................................................. (1)

(ii) State a condition which may cause carbon to be formed as a product in the combustion of butane.

............................................................................................................. (1)

(b) Butane obtained from crude oil may contain trace amounts of an impurity. When this impurity burns it produces a toxic gas that can be removed by reacting it with calcium oxide coated on a mesh.

(i) Suggest the identity of the toxic gas.

............................................................................................................. (1)

(ii) Suggest why calcium oxide reacts with the toxic gas.

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(iii) Suggest why the calcium oxide is coated on a mesh.

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(Total 5 marks)

Q14. There are several oxides of nitrogen.

(a) An oxide of nitrogen contains 25.9% by mass of nitrogen. Determine the empirical formula of this oxide.

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(b) Give one reason why the oxide NO is a pollutant gas.

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(c) The oxide NO reacts with oxygen to form nitrogen dioxide. Write an equation for this reaction.

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(d) Explain how NO is produced in the engine of a motor vehicle.

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(e) Write an equation to show how NO is removed from the exhaust gases in motor vehicles using a catalytic converter.

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(Total 8 marks)

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Q15. Cetane (C16

H34) is a major component of diesel fuel.

(a) Write an equation to show the complete combustion of cetane.

...................................................................................................................... (1)

(b) Cetane has a melting point of 18 °C and a boiling point of 287 °C. In polar regions vehicles that use diesel fuel may have ignition problems. Suggest one possible cause of this problem with the diesel fuel.

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...................................................................................................................... (1)

(c) The pollutant gases NO and NO2 are sometimes present in the exhaust gases of vehicles

that use petrol fuel.

(i) Write an equation to show how NO is formed and give a condition needed for its formation.

Equation .............................................................................................

Condition ............................................................................................. (2)

(ii) Write an equation to show how NO is removed from the exhaust gases in a catalytic converter. Identify a catalyst used in the converter.

Equation ..............................................................................................

Catalyst ............................................................................................... (2)

(iii) Deduce an equation to show how NO2 reacts with water and oxygen to form nitric

acid (HNO3).

............................................................................................................. (1)

(d) Cetane (C16

H34) can be cracked to produce hexane, butene and ethene.

(i) State one condition that is used in this cracking reaction.

............................................................................................................. (1)

(ii) Write an equation to show how one molecule of cetane can be cracked to form hexane, butene and ethene.

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(iii) State one type of useful solid material that could be formed from alkenes.

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(Total 10 marks)

Q16. Pentane is a member of the alkane homologous series.

(a) Give the general formula for the homologous series of alkanes.

..................................................................................................................... (1)

(b) One of the structural isomers of pentane is 2,2-dimethylpropane.

Draw the displayed formula of 2,2-dimethylpropane.

State the type of structural isomerism shown.

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(c) A molecule of hydrocarbon Y can be thermally cracked to form one molecule of pentane and two molecules of ethene only.

Deduce the molecular formula of Y.

State why high temperatures are necessary for cracking reactions to occur.

Give one reason why thermal cracking reactions are carried out in industry.

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(d) Write an equation for the incomplete combustion of pentane to form a solid pollutant.

Suggest why this solid pollutant is an environmental problem.

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(Extra space) ..............................................................................................

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(e) Pentane can react with chlorine as shown in the following equation.

C5H

12 + Cl

2 → C

5H

11Cl + HCl

Calculate the percentage atom economy for the formation of C5H

11Cl

Deduce how many straight-chain isomers of C5H

11Cl could be formed.

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(Extra space)...............................................................................................

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(f) Consider the following compound.

Name this compound.

Deduce the empirical formula of this compound.

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(Total 13 marks)

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Q17. (a) There is a risk of gas explosions in coal mines. This risk is mainly due to the presence of methane. If the percentage of coal-mine methane (CMM) in the air in the mine is greater than 15%, the explosion risk is much lower. CMM slowly escapes from the mine into the atmosphere.

Write an equation to show the complete combustion of methane.

Suggest one reason why there is a much lower risk of an explosion if the percentage of CMM is greater than 15%.

State why it is beneficial to the environment to collect the CMM rather than allowing it to escape into the atmosphere.

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(b) Methane can be obtained from crude oil. Some of this crude oil contains an impurity called methanethiol (CH

3SH). This impurity causes environmental problems when burned.

Write an equation to show the complete combustion of methanethiol.

State why calcium oxide can be used to remove the sulfur-containing product of this combustion reaction.

State one pollution problem that is caused by the release of this sulfur-containing product into the atmosphere.

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(Extra space)................................................................................................

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(Total 6 marks)

Q18. Alkanes are used as fuels. A student burned some octane (C8H

18) in air and found that the

combustion was incomplete.

(a) (i) Write an equation for the incomplete combustion of octane to produce carbon monoxide as the only carbon-containing product.

............................................................................................................... (1)

(ii) Suggest one reason why the combustion was incomplete.

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(b) Catalytic converters are used to remove the toxic gases NO and CO that are produced when alkane fuels are burned in petrol engines.

(i) Write an equation for a reaction between these two toxic gases that occurs in a catalytic converter when these gases are removed.

............................................................................................................... (1)

(ii) Identify a metal used as a catalyst in a catalytic converter. Suggest one reason, other than cost, why the catalyst is coated on a ceramic honeycomb.

Metal .....................................................................................................

Reason .................................................................................................

............................................................................................................... (2)

(c) If a sample of fuel for a power station is contaminated with an organic sulfur compound, a toxic gas is formed by complete combustion of this sulfur compound.

(i) State one environmental problem that can be caused by the release of this gas.

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(ii) Identify one substance that could be used to remove this gas. Suggest one reason, other than cost, why this substance is used.

Substance ............................................................................................

Reason why used ................................................................................

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(Total 8 marks)

Q19. Compound X is shown below. It is a member of a homologous series of hydrocarbons.

(a) (i) Deduce the general formula of the homologous series that contains X.

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(ii) Name a process used to obtain a sample of X from a mixture containing other members of the same homologous series.

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(b) There are several isomers of X.

(i) Give the IUPAC name of the position isomer of X.

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(ii) Draw the structure of a functional group isomer of X.

(1)

(c) At high temperatures, one molecule of C15

H32 can be converted into two molecules of X

and one molecule of another compound.


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(ii) State the name of the process used to obtain a high yield of X from C15

H32

Give one reason why this process is used in industry.

Name ....................................................................................................

Reason .................................................................................................

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(iii) State why high temperatures are needed for this process.

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(d) Compound X can be converted into compound Y. Compound Y is shown below.

(i) Suggest the formula of a reagent that could be added to X in order to convert it into Y.

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(ii) Give one use of Y.

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(iii) Write an equation to show the reaction of Y in a limited supply of air to produce a solid and water only.

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(iv) When a sample of Y, contaminated with CH3SH, is burned completely in air, a toxic

gas is formed. Identify this toxic gas and suggest a compound that could be used to remove the toxic gas from the products of combustion.

Toxic gas ..............................................................................................

Compound used to remove toxic gas ..................................................

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(v) Suggest the name of the process that occurs when the toxic gas in part (d)(iv) is removed.

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(e) Explain why the boiling points of X and Y are similar.

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(Total 16 marks)

Q20. The following table shows the boiling points of some straight-chain alkanes.

(a) State a process used to separate an alkane from a mixture of these alkanes.

........................................................................................................................ (1)

CH4

C2H

6

C3H

8

C4H

10

C5H

12

Boiling point / °C −162 −88 −42 −1 36

(b) Both C3H

8 and C

4H

10 can be liquefied and used as fuels for camping stoves.

Suggest, with a reason, which of these two fuels is liquefied more easily.

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(c) Write an equation for the complete combustion of C4H

10

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(d) Explain why the complete combustion of C4H

10 may contribute to environmental problems.

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(e) Balance the following equation that shows how butane is used to make the compound called maleic anhydride.

..........CH3CH

2CH

2CH

3 + .......... O

2 ..........C

2H

2(CO)

2O + .......... H

2O

(1)

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(f) Ethanethiol (C2H

5SH), a compound with an unpleasant smell, is added to gas to enable

leaks from gas pipes to be more easily detected.

(i) Write an equation for the combustion of ethanethiol to form carbon dioxide, water and sulfur dioxide.

............................................................................................................... (1)

(ii) Identify a compound that is used to react with the sulfur dioxide in the products of combustion before they enter the atmosphere.

Give one reason why this compound reacts with sulfur dioxide.

Substance .....................................................................................................

Reason ..........................................................................................................

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(iii) Ethanethiol and ethanol molecules have similar shapes.

Explain why ethanol has the higher boiling point.

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(g) The following compound X is an isomer of one of the alkanes in the table on above.

(i) Give the IUPAC name of X.

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(ii) X has a boiling point of 9.5 °C.

Explain why the boiling point of X is lower than that of its straight-chain isomer.

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...............................................................................................................

...............................................................................................................

............................................................................................................... (2)

(iii) The following compound Y is produced when X reacts with chlorine.

Deduce how many other position isomers of Y can be formed. Write the number of other position isomers in this box.

(1)

(h) Cracking of one molecule of an alkane Z produces one molecule of ethane, one molecule of propene and two molecules of ethene.

(i) Deduce the molecular formula of Z.

............................................................................................................... (1)

(ii) State the type of cracking that produces a high proportion of ethene and propene. Give the two conditions for this cracking process.

Type of cracking ...................................................................................

Conditions .............................................................................................

............................................................................................................... (2)

(Total 17 marks)

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Q21. (a) The hydrocarbon but-1-ene (C4H

8) is a member of the homologous series of alkenes.

But-1-ene has structural isomers.

(i) State the meaning of the term structural isomers.

...............................................................................................................

...............................................................................................................

...............................................................................................................

............................................................................................................... (2)

(ii) Give the IUPAC name of the position isomer of but-1-ene.

............................................................................................................... (1)

(iii) Give the IUPAC name of the chain isomer of but-1-ene.

............................................................................................................... (1)

(iv) Draw the displayed formula of a functional group isomer of but-1-ene. (1)

(b) But-1-ene burns in a limited supply of air to produce a solid and water only.


............................................................................................................... (1)

(ii) State one hazard associated with the solid product in part (b)(i).

............................................................................................................... (1)

(c) One mole of compound Y is cracked to produce two moles of ethene, one mole of but-1-ene and one mole of octane (C

8H

18) only.

(i) Deduce the molecular formula of Y.

............................................................................................................... (1)

(ii) Other than cracking, give one common use of Y.

............................................................................................................... (1)

(d) In cars fitted with catalytic converters, unburned octane reacts with nitrogen monoxide to form carbon dioxide, water and nitrogen only.


............................................................................................................... (1)

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(ii) Identify a catalyst used in a catalytic converter.

............................................................................................................... (1)

(Total 11 marks)

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3.1.6.3 Combustion of alkanes -...

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