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ASChemistryCIEExam Questions14. Hydrocarbons14.1 Alkanes

Alkanes (CIE AS Chemistry)

Exam Questions

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14.1 Alkanes

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14.1 Alkanes

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1a2 marks

State two reasons why alkanes are unreactive.

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1b2 marks

Methane reacts with chlorine to produce chloromethane, CH3Cl. The reaction is initiated by the formation of chlorine radicals.

i)
State what is meant by a radical.
[1]
ii)
State the condition required to form chlorine free radicals from Cl2.
[1]

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1c1 mark

State the type of bond fission involved in the reaction in part (b).

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1d3 marks

Chloromethane will be formed via several steps.

i)
Write the equations for the two propagation steps.
[2]

ii)
Write an equation to show how ethane can be formed in this reaction.
[1]

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2a4 marks

Alkanes can undergo both complete and incomplete combustion.

State the difference between complete and incomplete combustion of a hydrocarbon fuel, such as octane. Include the balanced symbol equation for each in your answer.

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2b1 mark

Octane is found in petrol.

Carbon monoxide is a colourless, odourless and poisonous gas which is emitted in the exhaust fumes of motor vehicles that use a petrol internal combustion engine.

 
i)
State why carbon monoxide can cause dizziness, fainting, loss of consciousness and death in humans. 
[1]

ii)
State another pollutant that is also released in cars' exhaust fumes. Explain the effect of this pollutant in the atmosphere.

[2]

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2c2 marks

Octane can be cracked to form pentane and one other product.

i)
Give the balanced symbol equation for this reaction.

[1]

ii)
Describe how catalytic cracking is carried out.

[2]

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2d3 marks

Describe and give the result of a chemical test that would distinguish between the two products.

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1a1 mark

A reaction scheme involving cyclohexane is shown in Fig. 1.1.

fig-4-1-9701-y22-sp-2-cie-ial-chem

Fig. 1.1.

Reaction 1 involves a free radical substitution mechanism.

State the essential condition required for reaction 1 to occur.

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1b5 marks

Complete Table 1.1 to give details of the mechanism in reaction 1. Include curly arrows to show the movement of electrons occurring in the termination step.

Table 1.1

GZyGrUb2_table-4-1-9701-y22-sp-2-cie-ial-chem

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1c1 mark

Deduce the type of reaction that occurs in reaction 2.

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1d2 marks

Hex-3-ene is an isomer of cyclohexane. Hex-3-ene can be converted into propanoic acid.

hex-3-ene rightwards arrow with reaction space 3 on top propanoic acid


Deduce the reagents and conditions for reaction 3.

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2a2 marks

Crude oil contains a mixture of hydrocarbons which are separated using fractional distillation. 

Further processes can then be carried out, including cracking. 

i)
What is meant by the term 'cracking'. 
[1]
ii)
Explain why cracking is carried out.
[1]

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2b3 marks
i)
State two conditions required for the cracking of alkanes. 

[2]

ii)
Construct a balanced equation for the formation of heptane, C7H16 and two other products, by cracking tetradecane, C14H30.

[1]

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2c4 marks

Another alkane, C8H18 is used in gasoline. It undergoes combustion to produce a mixture of products including carbon monoxide, oxides of nitrogen and unburnt hydrocarbons. 

i)
Explain why carbon monoxide is dangerous if inhaled.
[2]

ii)
Describe the environmental effect of oxides of nitrogen and unburnt hydrocarbons.

[2]

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2d2 marks

Catalytic converters are found in the exhaust system of cars. 

Use equations to show how a catalytic converter reduces the levels of carbon monoxide and oxides of nitrogen released into the atmosphere.

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3a5 marks

Alkanes are generally unreactive and do not react with acids, bases, or with oxidising or reducing agents. However, they will react with halogens under suitable conditions, to form halogenoalkanes.

Methane reacts with chlorine in this way to form chloromethane.

i)
Deduce the type of mechanism that occurs in this reaction.
 
[1]
 
ii)
Two test tubes are set up, each containing a small amount of methane and pale green chlorine gas. One test tube is left in the dark, whereas the other is placed in direct sunlight.
 
Explain the observations that would be observed in each test tube and give an equation for the reaction that has occurred if applicable.
 
[4]

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3b4 marks

The reaction described in part (a) consists of three steps. The first step is the initiation step in which the Cl-Cl bond is broken to form two chlorine free radicals.

i)
Explain the type of bond breaking that occurs in the initiation step.
 
[2]
 
ii)
Define the term free radical.
 
[1]
 
iii)
Explain why the C-H bond in the alkane does not break in the initiation step instead of the Cl-Cl bond.
 
[1]

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3c3 marks

Name each step of the reaction between methane and chlorine as described in part (a). Include relevant equations for each step.

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1a2 marks

This question is about free radical substitution. 

1,2-dibromoethane reacts with bromine in UV light to produce a mixture of further substituted haloalkanes. 

i)
Write an equation for the initiation step. 
[1]
ii)
Explain why this is an example of homolytic fission. 
[1]

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1b2 marks

Write two equations showing the propagation of this chain reaction to produce 1,1,2-tribromoethane. 

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1c2 marks

Traces of 1,2,3,4-tetrabromobutane are found in the reaction mixture. 

i)
Write an equation to show how this product is formed. 
[1]
ii)
Write a balanced symbol equation to show the overall reaction between 1,1,2-tribromoethane with bromine in UV light to form hexabromoethane. 
[1]

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1d2 marks

Using the information from this question and your own knowledge, suggest the limitations of free radical substitution.

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