10.2 – Alkanes
10.2.1 – Explain the low reactivity of alkanes in terms of bond enthalpies and bond polarity
Alkanes are saturated hydrocarbons. They are non-polar molecules, so they are insoluble in water, along with being less dense that water. In their pure form, alkanes are colourless.
The C-C and C-H bonds are quite strong and stable, making alkanes fairly unreactive. In addition, they are non-polar, so they do not attract polar or ionic compounds. This makes them valuable lubricants.
Since they are so unreactive, reactions with alkanes have a high activation energy to break the bonds. Once this has been overcome, the reaction is able to proceed rapidly.
10.2.2 – Describe, using equations, the complete and incomplete combustion of alkanes
Alkanes react exothermically with oxygen when the temperature is high enough. This makes them useful as fuel. The main products of this react are water and carbon dioxide.
Butane and Oxygen:-
Methane and Oxygen:-
The general formula for the combustion of a hydrocarbon is:
If there is not enough oxygen available, incomplete combustion will occur, causing carbon and poisonous carbon monoxide to form. This can cause health problems and pollution.
The incomplete combustion in a car can represented:
The supply of oxygen can be limited in a Bunsen burner by closing the air holes. This means that any test tube heated on it will blacken from the carbon:
10.2.3 – Describe, using equations, the reaction of methane and ethane with chlorine and
bromine
Halogens can be added to an alkane by a substitution reaction, where the halogen replaces a hydrogen atom. For this to happen, the bond between the hydrogen and carbon atoms must break. The bond between the halogen atoms will also need to break in a process called homolytic fission to form free radicals. Radicals do not have a charge, but have an unpaired electron in their outer shell, making them very reactive.
When the halogen radical reacts with the alkane, the hydrogen will then form a radical.
10.2.4 – Explain the reactions of methane and ethane with chlorine and bromine in terms of free-radical mechanism
UV light provides sufficient energy to break the Br-Br bond to form radicals, a process called initiation.
These bromine radicals then react with the methane molecules, called the propagation step:
Then there is another propagation step:
The final stage is called the termination stage, in which three reactions may take place:
Depending on the supply of bromine, other reactions may occur. If there is ample supply, the molecules may continue to react according to the equation:
The same type of reactions can take place with chlorine. With ethane, there are more steps because the molecule is larger.