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
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)
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)
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•
- Cl• + CH4 → •CH3 + HCl
- Termination- free radicals are removed.
- 2Cl• → Cl2
- 2•CH3 → C2H6
- CH3 + Cl• → CH3Cl