Topic 10.2: Alkanes

10.2.1 Explain the low reactivity of alkanes in terms of bond enthalpies and bond polarity

Alkanes contain only carbon - carbon and carbon - hydrogen bonds. These are both strong bonds so these molecules will only react in the presence of strong source of energy, strong enough to break these bonds. As a result, alkanes are stable under most conditions and can be stored, transported and even compressed safely.

Both carbon - carbon and carbon - hydrogen bonds are also charateristically non-polar, so these molecules are not susceptible to attack by most common reactants. These two factors taken together mean that alkanes are generally of very low reactivity.

10.2.2 Describe, using equations, the complete and incomplete combustion of alkanes

Combustion are highly exothermic. This is mainly because of the large amount of energy released in forming the double bonds in carbon dioxide and the bonds in water.

Alkanes burn in the presence of excess oxygen produces carbon dioxide and water. When oxygen supply is limited, carbon monoxide is produced instead of carbon dioxide.

10.2.3 Describe, using equations, the reactions of methane and ethane with chlorine and bromine

Substitution reactions of alkanes are known as halogenalkanes. The reaction cannot take place in the dark as the energy of UV light is necessary to break the covalent bond in the chlorine molecule. This splits the chlorine molecule into chlorine atoms, which each have an unpaired electron and are known as free radical.

10.2.4 Explain the reactions of methane and ethane with chlorine and bromine in terms of a free-radical mechanism

Free-radical mechanism has three parts, this includes initiation, propagation and termination.

Initiation

This process occurs in the presence of ultra-violet light. It is known as homolytic fission because the bonds between the chlorine atoms is broken, splitting the shared pair of electrons between the two atoms.


Propagation

These reactions are called propagation because they both use and produce free radicals, and so allow the reaction to continue. This is why the reaction is often called a chain reaction.


Termination

These reactions remove free radicals from the mixture by cause them to react together and pair up their electrons.