A-Level Chemistry AQA Notes

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Alkanes

Alkanes are a homologous series made up of saturated hydrocarbons with the general formula CnH2n+2.
Carbon atoms have four electrons in their outer shell. Therefore, each carbon atom can form four covalent bonds
alkanes are not polar and only weak London forces of attraction occur, as carbon and hydrogen have similar electronegativities.
Boiling point of alkanes increases with chain length, as there is a greater surface area and number of electrons for stronger London forces.
The more branched the molecules are, the smaller the surface area for contact between molecules and the weaker the London forces resulting in lower boiling points

Fractional Distillation

Crude oil is a fossil fuel formed from the breakdown of plant and animal remains that have been subjected to high pressure over millions of years. It is composed mainly of alkanes.
Fractional distillation will separate crude oil into different fractions
A fraction is a group of hydrocarbons that have a similar boiling point
The crude oil is vaporised in a furnace and passed into the bottom of a fractionating column
In the column, there is a temperature gradient where it is hotter at the bottom and cooler at the top
The vapour passes up the column and the different fractions will condense at different heights onto trays. Longer hydrocarbons have higher boiling points, so condense lower down,

Cracking

Cracking converts longer chain hydrocarbons to more economically valuable shorter chain hydrocarbons
Cracking involves breaking C-C bonds in alkanes
In thermal cracking, alkanes are heated to high temperatures under high pressures. C-C bonds break homolytically and free radicals are formed. These react to form shorter chain hydrocarbons including at least one alkene. It produces a high proportion of alkenes.
In catalytic cracking, alkanes are heated to lower temperatures under lower pressures passed over a zeolite catalyst consisting of silicon dioxide and aluminium oxide. This method is used mainly to produce motor fuels (branched & cycloalkanes) and aromatic compounds.

Combustion of Alkanes

Alkanes can be used as a fuel source
In complete combustion, the alkane burns with a clean blue flame. Water vapour and carbon dioxide are formed (greenhouse gases). E.g.

C3H8(g) + 5O2 → 3CO2 (g) + 4H2O (g)

In incomplete combustion, the alkane burns with a dirty yellow flame. It can produce carbon, carbon monoxide and unburned hydrocarbons as products.

CH4(g) + O2(g) → C (s) + 2H2O (g)
CH4(g) + 1.5O2(g) → CO (g) + 2H2O (g)

Sulfur-containing alkenes can produce sulphur dioxide during combustion that dissolves in rainwater to cause acid rain.
Sulfur dioxide can be removed from flue gases using calciul oxide or calcium carbonate

CaO (s) + 2H2O (l) + SO2 (g) → CaSO3 (s) + 2H2O (l)
CaCO3 (s) + 2H2O (l) + SO2 (g) → CaSO3 (s) + 2H2O (l) + CO2 (g)

Combusting nitrogen-containing alkanes will form nitrogen oxides, which contribute to acid rain and photochemical smog.
Catalytic converters can remove gaseous pollutants from internal combustion engines by using precious metals spread over a mesh to form less harmful products such as N2, CO2, and H2O.

Chlorination of Alkanes

Alkanes react with the halogens, specifically chlorine and bromine, in the presence of UV light to form haloalkanes
Methane reacts with chlorine to form chloromethane and hydrogen chloride:
- CH4 + Cl2 → CH3Cl + HCl
This reaction is a free radical substitution with the steps:
- Initiation- free radicals are formed when exposed to UV Cl2 → 2Cl•
- Propagation- free radicals are used up and created in a chain reaction
  - Cl• + CH4 → •CH3 + HCl
    - CH3 +Cl2 → CH3Cl + Cl•
- Termination- free radicals are removed.
  - 2Cl• → Cl2
  - 2•CH3 → C2H6
  - CH3 + Cl• → CH3Cl

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