Hydrocarbons

• Marine animals such as plankton died millions of years ago. They were buried by sediment. Over millions of years they were compressed over high pressure and temperature to create crude oil.
• To get to crude oil we drill into the Earth's crust
• Crude oil is a mixture of hydrocarbons C1 - 50
• A hydrocarbon is a compound made up of carbon and hydrogen only
• We separate crude oil by a process called fractional distillation
• To begin fractional distillation, crude oil is heated and vapourises
• After it is vaporised, the vapour enters the bottom of the column and ruses up until it is cool enough to condense at its boiling point
• In the fractional distillation column, smaller molecules are collected at the top because they have lower boiling points
• In the fractional distillation column, the groups of similar sized compounds that are collected at the different levels are called fractions
• The smaller hydrocarbons are paler in colour, more flammable. burn with a cleaner flame, are more volatile, have a lower boiling point and are less viscous
• The compounds obtained from fractional distillation are alkanes
• Alkanes are saturated compounds meaning each carbon is joined to 4 other atoms by single covalent bonds
• Unsaturated compounds are compounds that contain one or more double/triple bond
• A homologous series is a family of compounds that have the same functional group, general formula, similar chemical properties and differ by CH2 unit
• Alkanes have many uses. Short chained alkanes are useful as fuels. Larger alkanes are used for cracking.
• Combustion is a reaction in which fuels burn to produce heat energy and oxides. Its an oxidation reaction. The formula for the complete combustion of methane is:

  $CH_4^{ }\ +$$\ O_2\ \rightarrow\ 2H_2O\ +$$\ CO_2$
• The general formula for alkanes is:

  $C_nH_{2n+2}$
• We crack larger alkanes because there is a high supply of them but not a lot of demand. On the other hand smaller alkanes are in high demand but have a limited supply. By cracking the larger alkanes we can get rid of larger alkanes and create smaller alkanes
• Cracking is a thermal decomposition reaction, meaning it uses heat to break down a compound
• Cracking breaks down a long hydrocarbon into a small alkane and an alkene
• Cracking must be done in a high temperature and with either a pass over catalyst or while being mixed with steam
• The general formula for Alkenes is:

  $C_nH_{2n}$
• Alkenes are a family of hydrocarbons (homologous series) with double bonds. This means the molecules are unsaturated. They burn with a smoky flame, are reactive, decolourise bromine or iodine water and are named the same way as alkanes
• Polymers are long molecules. They have strong covalent bonds within chains. The chains aren't joined together but are often tangled up. There are strong intermolecular forces between chains
• A polymer is a long chain molecule made from lots of small molecules joined together. A monomer is the small molecule that joins together to make the polymer
• Addition polymers are long chain molecules joining together with no other products formed. Under heat or pressure, the double bond in the monomer breaks open and the other monomers can join the chain. The monomers are alkenes
• Condensation polymerisation is the formation of long chain molecules from lots of small molecules joining together with another small molecule produced as well. Monomers have 2 functional groups
• Low density polythene is created using high pressure and a trace of oxygen. Randomly branched chains that cannot pack closely are compressed together. They have weak intermolecular forces, low density and are soft. They are used for plastic bags
• High density polyethene is made with a catalyst at 50 degrees. They have straighter chains which pack more closely so they have stronger intermolecular forces and higher density. They are used for bottles and storage tubs
• How do we get crude oil

  Drilling into the Earth's crust
• Incomplete combustion occurs in an environment with limited oxygen. The equation for the incomplete combustion of methane is:

  $CH_4+$$O_2\ \rightarrow\ 2H_2O\ +$$\ CO_{ }^{ }$