Alkanes (AQA A Level Chemistry)

Alkanes are saturated hydrocarbons which can be used as fuels, lubricants and as starting materials for a wide range of other compounds. The main source of alkanes is petroleum. Petroleum is a mixture of alkanes with different chain lengths, which are
separated in a process called fractional distillation.

Describe the process of fractional distillation.

Once the mixture of saturated hydrocarbons has been separated, they can be used as fuels. The high temperatures in a car engine with the presence of oxygen, means that the alkanes burn (combust) to give out heat energy.

Figure 1 below shows the structure of three alkanes.

Figure 1

The boiling points of the alkanes from Figure 1 are summarised in Table 1 below.

Table 1

Use Table 1 to explain the effect that chain length and branching of molecules has on boiling point.

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.

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.

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

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.

A fuel is a substance which has stored energy. During combustion, a fuel is burned in oxygen, to release this energy. A car engine is a common example of where a fuel is combusted.

The internal combustion of the fuels used in a car’s engine produces a number of pollutants, including oxides of nitrogen. This is despite nitrogen itself being generally unreactive.

Table 1 shows a summary of other pollutants which may be produced during the combustion of fuels.

Table 1

Complete Table 1 by filling in the gaps.

Many power stations generate electricity by burning fossil fuels such as coal or natural gas. The gases given out by these power stations are called flue gases.

Fossil fuels often contain small amounts of impurities. Two of these are shown in Table 2 below.

Table 2

Suggest how each impurity can result in the formation of a pollutant and explain their damaging effects. Use equations to aid your answer.

Due to the damaging effect it causes, sulfur dioxide is removed from these flue gases.
There are two methods used for this removal of sulfur dioxide.

Describe one method that can be used to remove sulfur dioxide from flue gases.
Use equations to aid your explanation.

The combustion of petrol in cars can cause a number of pollutant gases, including sulfur dioxide gas. Most sulfur is removed from petrol so that sulfur dioxide has become less of a problem. However, a car’s engine also produces other pollutants.

Most cars’ engines are now equipped with a catalytic converter in their exhaust system to reduce pollutant gases. The catalytic converter is a honeycomb made of a ceramic material, coated with a thin layer of platinum and rhodium metals.

The pollutant gases will pass over the thin metal layer inside the catalytic converter, resulting in less harmful products being formed.

The pollutants produced by the car’s engine and found in exhaust gases include carbon monoxide, nitrogen oxides and unburnt hydrocarbons. Sometimes, the hydrocarbon isooctane is added to petrol in order to increase its octane rating.

Isooctane which is added to petrol to increase the octane rating burns smoothly in a car engine. It is a branched chain isomer of octane, with the skeletal formula shown in figure 1 below. 

Figure 1

Many alkanes which are used as fuels (such as octane) are obtained by the cracking of suitable heavy fractions, which can be achieved thermally or catalytically.

Crude oil contains a mixture of hydrocarbons. Most crude oil has more of the longer chain fractions than is wanted and not enough smaller fractions. Many of the larger ones are broken into shorter lengths in a process called cracking.

A hydrocarbon with molecular formula C₁₃H₂₈ is catalytically cracked to form C₇H₁₄ and one other compound A.

Table 1 below shows information on four different fractions found in crude oil.

Table 1

“Cracking long chain hydrocarbons into short chain hydrocarbons increases their economic value, unless they are cracked to form very small hydrocarbons.”

Justify the statement above using the data provided in Table 2.

The alkane tetradecane (C₁₄H₃₀) is commonly found in crude oil. This long chain hydrocarbon can be cracked to form hydrocarbons with smaller chains, as shown below:

C₁₄H₃₀ → C₆H₁₄ + compound X + 2C₂H₄

Another hydrocarbon, C₁₂H₂₆, is cracked to form propane and one other product.

Crude oil is a complex mixture of hydrocarbons. Figure 1 shows a general oil trap that could be drilled for in the North Sea.

Figure 1

Explain the position of the oil in the trap.

Fractional distillation is used to separate crude oil into fractions depending on their boiling point.

Explain how volatility and thermal energy determine the position of the fractions in the fractional distillation column.

Figure 2 shows the relative amounts of some fractions from the fractional distillation of crude oil.

Figure 2 

The estimated market demand for diesel is shown on Figure 2. Plot the estimated market demand for each of the remaining fractions on Figure 2.

Swimmers of the English Channel are often covered in jellies containing paraffin wax, a mixture of long chain hydrocarbons derived from crude oil products.

Give two reasons why Channel swimmers use paraffin wax.

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.

Fuels often contain impurities which lead to environmental pollution. One impurity could be diethyl sulfide, C₄H₁₀S₂, as shown in Figure 1.

Figure 1

A petrol based fuel containing an isomer of octane has the structural formula (CH₃)₂CHCH₂C(CH₃)₂CH₃.

What volume of oxygen, in cm³, would be required to completely combust 0.5 dm³ of the octane isomer?

When fuels combust there are various chemicals released.

Identify four products of combustion which have a negative effect on human body and explain how they affect the human body.

The estimated composition of North Sea crude oil is shown in Figure 1.

Figure 1

The refinery gases, petrol and naphtha fractions are in the highest demand but have the lowest supply. The longer chains from crude oil can be used to produce the high demand shorter chains in the process of cracking.

Compare the two types of cracking.

The 3-D structure of a compound involved in catalytic cracking is shown in Figure 2.

Figure 2

A sample of decane can be cracked in various ways to form three different compounds. Analysis of the product mixture shows that the smallest product decolourises bromine water.

Use the information to write balanced symbol equations for the cracking of decane.

Using your answers to part (c), explain which type of cracking was performed on the decane sample.

Alkanes are relatively unreactive and require harsh conditions to crack them including temperatures of 700 - 1200 K and pressures of up to 7 MPa. When butane is reacted under these conditions, a number of products can be formed.

Identify the possible intermediates formed.

In the free radical chlorination of methane, the chlorine and the methane molecules absorb one quantum of ultraviolet light.

Figure 1

Figure 1 shows the bond enthalpy values for the chlorine and methane molecules.

Estimate the energy, in kJ mol^-1, in one quantum of ultraviolet light. Justify your answer including chemical equations where applicable.

Figure 1 shows the reactants in one propagation step for the chlorination of methane.

Figure 1

Outline the mechanism and identify the products for this propagation step.

The free radical chlorination of methane can form four different products.

Write an overall, balanced symbol equation for the free radical chlorination of methane forming all of the wanted and unwanted products.

The majority of cars have an alkane fueled internal combustion engine as shown in Figure 1.

Figure 1

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

A student read a magazine article that stated “catalytic converters are devices that use redox reactions to reduce the pollutants a car makes”.

The student wrote a reply to the magazine including the following equation:

2 CO (g) + 2 NO (g) → N₂ (g) + 2 CO₂ (g)

The student explained that both of the pollutants had not been reduced, therefore the article was incorrect.

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 C. Suggest whether the boiling point of its straight chain isomer is higher or lower. Explain your answer.

Removing nitrogen oxides from the exhaust fumes of diesel engines can be hard. Ammonia can be injected into exhaust gases before they reach the catalytic converter as a method to help with the removal of nitrogen oxides.

Calculate the volume of oxygen that is needed to fully react with 200 cm³ of nitrogen (II) oxide at 20 C and 100 kPa.