chemistry C1 (5)

Cracking hydrocarbons

• the smaller hydrocarbons fractions obtained from crude oil are the most useful and are in heavy demand, particularly the petrol fractions
• the larger fractions are less usefuk as fuels and are not in such high demand and the supply exceeds demand
• it is possible to break down larger, less useful alkane molecules into smaller more useful ones
• the process uses heat and a catalyst and is known as catalytic cracking
• the large molecules are heated until they vapourise and passed over a hot catalyst
• this splits the molecule in a reaction known as thermal decomposition
• as well as producing a smaller more useul alkane molecule, other products called alkenes are formed
• decane to octane + ethene
• C10H22 to C8H18 + C2H4
• alkenes are another homologous series of hydrocarbons
• they have a different general formula to that of the alkanes

Alkenes

• alkenes are unsaturated hydrocarbons- they do not contain as many hydrocarbons atoms as possible because they have a double bond between two of the carbon atoms
• the name of every alkenes ends in -ene
• methene doesn't exist as alkenes need a double bond so therefore they must have at least 2 carbon atoms
• the general formula is CnH2n
• bromine water can be used to test between alkenes and alkanes
• when it is alkenes the bromine water turns from yellow to colourless as the double bonds breaks and bonds to the bromine
• alkenes burn with a smoky flame, so do not make such good fuels

Key terms about crude oil

• distillation: separation method used to get a pure liquid from a mixture
• cracking: the use of heat and a catalyst to break down a long chain alkane to form an alkane and an alkene
• refining: to remove impurities
• saturated: a molecule with only single bonds
• unsaturated: a molecule with a C=C bond
• hydrocarbon: contains only carbon and hydrogen
• propene: a molecule with 3 carbon atoms that would decolourise bromine water
• propane: a molecule with 3 carbon atoms that would not decolorise bromine water

Making polymers from alkenes

• Alkenes produced from the catalytic cracking of large alkane molecules have many uses
• One of the most important uses is in manufacture of plastic
• There are many different types of plastics, but most have one thing in common- they are polymers made from alkenes
• A polymer is a huge, long chain molecule made by joining lots of small reactive alkene molecules together
• The small molecules are called monomers
• Different types of plastics have different properties because they are made from different monomers
• A plastic bag is made from a polymer called poly(ethene)
• Huge numbers of ethene monomers join together when they are heated, under pressure, in the presence of a catalyst
• The double bond in each ethene monomer breaks, and they join together forming a long chain polymer
• This is known as addition polymerisation because the reaction is an addition reaction
• Poly(ethene) is used for bags, bottles, buckets, washing up bowls
• …