Alkanes (OCR AS Chemistry A): Exam Questions

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.

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.

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.

Give the IUPAC name for alkane C.

Alkynes are hydrocarbons with the general formula C_nH_2n-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.

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.

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.

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. 

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.

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.

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

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.

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.

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. 

i) Give the IUPAC name for isooctane,

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

Write an equation for the complete combustion of isooctane.

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.

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.

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.

Write an overall equation for formation of dichloromethane from chloromethane.

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.

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 .

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.

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

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

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. 

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

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.

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

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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. 

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

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

i) Draw the 3D structure for pentane.

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ii) State and explain the bond angles between adjacent atoms in pentane.  

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Pentane undergoes free radical substitution to form 3-chloropentane.

i) Write two equations for the propagation steps involved in this mechanism.

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ii) Name an impurity that could be formed from the radicals in part i) that might be found in the final mixture.

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iii) Write an equation for the reaction between pentane and chlorine to form a disubstituted product. 

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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. 

i) Draw the skeletal formula for three other branched alkanes formed by heptane. 

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ii) Give the IUPAC name of compound C.

[1]

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

• 2,3-dimethylpentane

• 3-methylhexane

• 2,4-dimethylpentane

Octane and one of its isomers are shown below:

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

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ii) Give the IUPAC name for isooctane. 

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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.

Describe the bonding and structure of octane. 

Explain how its bonding and structure affects the following properties:

• Reactivity

• Boiling Point 

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. 

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ii) Explain why this is an example of homolytic fission. 

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

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

i) Write an equation to show how this product is formed. 

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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]

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