3.3.2 Alkanes

• As the number of carbon atoms increases the boiling point increases. As the hydrocarbons get longer there are more/stronger Van Der Waals forces between the molecules. This means more energy is required to separate the hydrocarbon molecules so the boiling point increases.
• Thermal Cracking involves breaking the C-C bond so that long alkanes are broken down to produce shorter alkanes and a high percentage of alkenes
• Name the fractions and their uses starting from the smallest molecules

  Refinery Gases - Bottled Gas

  Gasoline - Petrol for cars

  Naphtha - Chemicals

  Kerosene - fuel for planes

  Diesel oil - fuel for buses/lorries

  Fuel oil - fuel for ships

  Bitumen - tarmac
• Thermal cracking happens at high temperatures (450-900 degrees) and high pressure (7000kPa)
• Thermal cracking

  C14H30 --> C7H16 + C3H6 + 2C2H4
• Ethene is a valuable product of cracking and it can be used to make polymers such as poly(ethene) which is used to make plastics
• The demand for the shirt hydrocarbons is greater than the longer hydrocarbons. There is a large supply of long hydrocarbons but low demand so cracking is used to break the long hydrocarbons into shorter hydrocarbons
• Catalytic Cracking takes place at a slight pressure (100kPa) and high temperatures (450 degrees) in the presence of a zeolite catalyst
• Catalytic Cracking :

  It is mainly used to produce motor fuels and aromatic hydrocarbons
• Catalytic cracking produces branched, cyclic and aromatic hydrocarbons. Branched hydrocarbons are used to produce motor fuels are they burn more smoothly than unbranched.
• Complete combustion:

  Hydrocarbon + oxygen -> carbon dioxide + water

  Incomplete combustion:

  Hydrocarbon + oxygen -> carbon monoxide + water

  Incomplete combustion:

  Hydrocarbon + oxygen -> carbon + water
• Sulfur Impurities :

  Combustion of hydrocarbons containing sulphur leads to sulfur dioxide that causes air pollution. Sulfur dioxide leads to acid rain damaging the environment (plants, buildings)

  The sulfur dioxide dissolves in water vapour to form sulfurous acid

  SO2 + H2O -> H2SO3

  The sulfurous acid is then oxidised by a series of reactions to form sulfuric acid. The sulfuric acid makes the rain become acidic
• Sulfur Impurities:

  In power stations, the acidic sulfur dioxide is removed from the chimney gases by reacting it with an alkaline substance such as quickly (CaO).

  CaO + SO2 -> CaSO3

  The product of the reaction is calcium sulphite (CaSO3) which can be oxidised to make calcium sulphate (CaSO4) which can be used to make plasterboard
• Combustion engines:

  The temperature of the burning petrol car can reach 2500 degrees which provides sufficient activation energy for nitrogen to react with oxygen to form nitrogen monoxide

  N2 + O2 -> 2NO

  Nitrogen monoxides reacts further with oxygen to make NO2

  2NO + O2 -> 2N02

  Nitrogen dioxide reacts with water vapour and more oxygen to form nitric acid which leads to acid rain.

  2NO2 + H2O + O2 -> 2HNO3
• Combustion engines:

  The unburnt hydrocarbons can react with nitrogen oxides in the presence of sunlight to producer an irritating photochemical smog
• Combustion engines:

  Exhausts in cars have been fitted with catalytic converters to remove carbon monoxide. The catalytic converter contains transition metals such as Pd, Pt and Rh which are spread over a thin layer

  2CO + 2NO -> 2CO2 + N2
• Chlorination of alkanes:

  Homolytic fission is when the covalent bond between 2 atoms breaks, one electron is transferred to each atom producing 2 free radicals
• Chlorination of alkanes:

  A radical is a species with an unpaired electron.
• Chlorination of alkanes:

  UV light provides a high enough activation energy to break the Cl-Cl covalent bond.
• Chlorination of alkanes:

  >Reaction conditions

  Formation of chloromethane is favoured by an excess of methane

  More chance of Cl radical reaction with methane and used up before further substitution

  formation of tetrachloromethane is favoured by an excess of chlorine
• A fraction is a group of hydrocarbons / compounds with similar boiling points
• Crude oil is separated into fractions by fractional distillation. Outline how different fractions are obtained by this process.

  Crude oil is heated to vaporise it

  Vapour passed into fractionating tower / column which has a temperature gradient cooler at the top.

  So smaller / lighter molecules with lower boiling points condense at the top where it is cooler