Alkanes (OCR A Level Chemistry)

Exam Questions

3 hours38 questions
1a1 mark

This question is about the alkane family of hydrocarbons.

State the meaning of the term hydrocarbon.

1b3 marks

A molecule of heptane contains seven carbon atoms.

i)
State the molecular formula of hexane
[1] 

ii)
State the empirical formula of hexane
[1] 

iii)
Draw the skeletal structure of hexane 
[1] 
1c3 marks

The table shows a list of alkanes and their respective boiling points.

Alkane  Boiling Point / °C
Heptane 98
Octane 126
Nonane  
Decane  174

i)
Predict the boiling point of nonane.
[1] 
ii)
State and explain the trend in boiling points of the alkanes.
[2] 
1d1 mark

State the bond angle in an alkane molecule. 

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2a2 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 products formed in your answer.

2b2 marks

Write a balanced symbol equation for the complete combustion of pentane, C5H12. State symbols are not required.

2c1 mark

Write a balanced symbol equation for the incomplete combustion of pentane, C5H12forming carbon monoxide as one product. State symbols are not required.

2d2 marks

Decane is a major component in diesel fuel.

i)
Write a balanced symbol equation for the incomplete combustion of decane, C10H22, forming carbon as one product. State symbols are not required.
[1]
ii)
Suggest what might be observed to show that carbon is being formed. 
[1] 
2e1 mark

Carbon monoxide is a colourless, odourless and poisonous gas.  

State why carbon monoxide can cause dizziness, fainting, loss of consciousness and death in humans. 

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3a2 marks

State two reasons why alkanes are unreactive.

3b3 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 conditions required to form chlorine free radicals from Cl2.
[2]
3c1 mark

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

3d3 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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4a2 marks

This question is about the isomers of hexane.

Table 1 shows three of the five possible structural isomers of C6H14.

 
Table 1
4-2_q4a-ocr-a-as--a-level-easy-sq

State the IUPAC name for isomers B and C.

4b4 marks

Draw the skeletal structures and state the IUPAC name of isomers D and E.

4c3 marks

Explain why the boiling point of isomer A is higher than that of isomer B

4d1 mark

Isomer A can react with chlorine at high temperatures and in the presence of UV light to form C6H13Cl. Write an overall equation for this reaction.

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1a7 marks

Alkanes are saturated hydrocarbons that can be used as fuels, lubricants and as starting materials for a wide range of other compounds.

In the high temperatures in a car engine with the presence of oxygen, alkanes burn (combust) to give out heat energy.

The image below shows the structure of three alkanes.

2

i)
Define the terms complete and incomplete combustion.

ii)
Name the alkane in the image which is most likely to undergo incomplete combustion.
Explain your answer.
1b5 marks

The boiling points of alkanes A, B and C are 196 °C, -42 °C and 107 °C respectively.

Explain the effect that chain length and branching of molecules has on boiling point.

1c1 mark

Give the IUPAC name for alkane C.

1d2 marks

Alkynes are hydrocarbons with the general formula CnH2n-2. The alkyne containing 2 carbon atoms is ethyne, C2H2. 

State, with a reason, whether the boiling point of ethyne would be expected to be higher or lower than the boiling point of ethane, C2H6.

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2a5 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)
State the name of this type of reaction mechanism.

ii)
A student places a small amount of methane and pale green chlorine gas into two different test-tubes. One test tube is left in the dark, whereas the other is placed in direct sunlight.

State and explain the observations that the student would see in each test-tube and give an equation for the reaction that has occurred if applicable.

2b4 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)

State and explain the type of bond breaking that occurs in the initiation step.

ii)
Define the term free radical.

iii)
Explain why the C-H bond in the alkane does not break in the initiation step instead of the Cl-Cl bond.
2c4 marks

Give the equations for each step of the reaction between methane and chlorine. 

Label each equation with the name of the correct step. 

You only need to provide one equation for the final step. 

2d2 marks

Chlorofluorocarbons (CFCs) are organic compounds consisting of carbon, chlorine and fluorine atoms.

They are believed to destroy the ozone layer in a similar reaction as described in part (a).

Ozone can be broken down by radicals formed in the atmosphere. The ozone layer is important to protect the Earth from harmful exposure to too much ultraviolet light. Without this ozone layer, life on Earth would be very different.

An example of a CFC which can damage the ozone layer is CCl3F. 

Draw the 3D structure of CCl3F and name the shape.

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3a3 marks

Ethane is mainly used to produce ethene, which is then used by the petrochemical industry to produce a range of intermediate products, most of which are converted into plastics.

i)
Draw the dot cross diagram for ethane.

ii)
State the bond angle in the ethane molecule.
3b2 marks

Explain in terms of bond strength and polarity why ethane is an unreactive molecule.

3c3 marks

Excess bromine, Br2, will react with ethane in the presence of UV light to produce bromoethane, CH3CH2Br.

By use of equations, explain how the product will become contaminated with CH3CHBr2.

3d4 marks

Propane undergoes the same reaction as ethane in order to form 1-bromopropane, suggest two different way this reaction will become contaminated with impurities, other than those from multiple bromine propagation steps.

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4a
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4 marks

Isooctane which is added to petrol to increase the octane rating burns smoothly in a carengine. It is a branched chain isomer of octane, with the skeletal formula shown below. 

DMx1-6ea_3-2

i)
Give the IUPAC name for isooctane,

ii)
Suggest whether octane or isooctane would have the higher boiling point. Explain your answer. 
4b
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1 mark

Write an equation for the complete combustion of isooctane.

4c
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2 marks

When insufficient oxygen for the complete combustion to occur, the toxic gas carbon monoxide can be produced. Explain why carbon monoxide is dangerous if inhaled.

4d
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1 mark

A diesel fuel containing the hydrocarbon tetradecane, C14H30, burns with a sooty yellow flame. 

Write a balanced symbol equation, including state symbols, for the reaction described.

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5a4 marks

Trichloromethane is one of the products formed when dichloromethane reacts with excess chlorine in UV light.

i)
Name this mechanism.

ii)
Write equations for the initiation and propagation steps.

5b1 mark

Write an overall equation for formation of dichloromethane from chloromethane.

5c
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2 marks

A compound was found to have 20.3 % carbon, 48.1 % fluorine, 30.0 % chlorine and 1.6 % hydrogen. Calculate the empirical formula for this compound.

5d
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2 marks

Suggest the structural formula of two bromine containing compounds that are formed during the reaction of bromine and dibromomethane in the presence of UV light .

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1a1 mark

This question is about combustion.

A diesel fuel containing the hydrocarbon tetradecane, C14H30, burns with a yellow flame.

Write a balanced symbol equation, including state symbols, for the reaction described.

1b4 marks

The majority of cars have an alkane fueled internal combustion engine as shown below. 

3-2-q1-combustion-engine

Identify four pollutant products of combustion and describe how they are formed.

1c2 marks

Petrol vapour and air are compressed in a car engine. This can cause pre-ignition which can lead to an undesirable knocking sound. Branched chain alkanes, such as 2,5-dimethylhexane, can be added to fuels as they are less likely to cause pre-ignition and therefore improve fuel efficiency.

The boiling point of 2,5-dimethylhexane is 109.1 oC.

Explain the difference in boiling point between 2,5-dimethylhexane with its isomer 4-methylheptane. 

1d5 marks

Explain why the production of carbon monoxide from a faulty heater in someone's home can have deadly consequences.

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2a2 marks

Methane reacts with chlorine in the presence of UV light to form several products. 

Two of these products are chloromethane and hydrogen chloride. 

i)
Write an equation for the reaction between methane and chlorine to form chloromethane and hydrogen chloride.

[1]

ii)
Name one other chlorine-containing organic product which can be formed during this reaction.

[1]

2b7 marks

The mechanism for this reaction is free radical substitution. 

Describe this mechanism for the free radical substitution of methane by chlorine. 

Also suggest why a small amount of ethane is formed in the reaction. 

2c3 marks

Explain the effect of increasing the intensity of UV light would have on the rate of reaction between methane and chlorine.

2d4 marks

Pentane is another saturated hydrocarbon which can undergo free radical substitution.  

i)
Draw the 3D structure for pentane.
[1]
ii)
State and explain the bond angles between adjacent atoms in pentane.  
[3]
2e4 marks

Pentane undergoes free radical substitution to form 3-chloropentane.

i)
Write two equations for the propagation steps involved in this mechanism.
[2]
ii)
Name an impurity that could be formed from the radicals in part i) that might be found in the final mixture.
[1]
iii)
Write an equation for the reaction between pentane and chlorine to form a disubstituted product. 
[1]

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3a4 marks

This question is about isomers of different alkanes.

Heptane can be produced by the fractional distillation of crude oil.

There are a total of eight branched isomers of heptane, four of which are shown below. 

4-2_q3a-ocr-a-as--a-level-hard-sq

i)
Draw the skeletal formula for three other branched alkanes formed by heptane. 
[3]
ii)
Give the IUPAC name of compound C.
[1]
3b2 marks

Explain which of the following isomers will have the highest boiling point.

  • 2,3-dimethylpentane
  • 3-methylhexane
  • 2,4-dimethylpentane
3c2 marks

Octane and one of its isomers are shown below:

 

4-2_q3c-ocr-a-as--a-level-hard-sq

i)
Write an equation for the complete combustion of octane. 

[1]
ii)
Give the IUPAC name for isooctane. 
[1]
3d3 marks

The two isomers in part (c) can be separated in the lab. 

Name and outline a technique that could be used to separate isooctane from a mixture of octane and isooctane.

3e6 marks

Describe the bonding and structure of octane. 

Explain how its bonding and structure affects the following properties:

    • Reactivity
    • Boiling Point 

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4a2 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]

4b2 marks

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

4c2 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]

4d3 marks

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

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