Crude oil is split up into many useful groups of hydrocarbons, called fractions, by fractional distillation. This can be done because long chains have higher boiling points than short chains.
Long chain molecules come off near the bottom of the column where it is hottest.
Short chain molecules come out of the column near the top where it is coolest.
Butane and light gases out
Fractionating Column
400C Petrol out
1750C
Kerosene out
2500C
Diesel out
Crude oil in
4000C
Cracking
Far more diesel is obtained than is needed. Its molecules are usually about 20 carbon atoms long. This excess diesel is turned into more useful short molecules by a process called catalytic cracking which breaks the long molecules up into short molecules containing double bonds.
The conditions used for this cracking are high temperature (800oC) and in the presence of a catalyst (porous pot).
Polymerisation
The small molecules produced by cracking can be used to make polymers (plastics).
For this to happen the little molecule(called a monomer) must have a double bond.
The monomers undergo a reaction called polymerisation when they are heated with a catalyst under high pressure. Thousands of monomers join up to make a polymer (plastic molecule).
Types of Plastic
There are two main groups of plastics: thermoplastics and thermosetting plastics
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Thermoplastics go soft every time that they are heated and can be moulded into new shapes.
Thermosetting plastics go soft and can be moulded into shape the first time that they are heated. They do not go soft after that no matter how much they are heated.
Properties and Uses of Plastics
- Plastics do not conduct electricity (they are used as coatings on the outside of wires)
- Plastics are cheap to make
- Plastics are bad conductors of heat (they can be used to make panhandles)
- Plastics are waterproof (they can be used as washing up bowls)
- Plastics do not corrode or react with water (they can be used to make pipes)
- Plastics can be strong (polythene is used to make shopping bags).
- Some new plastics are made biodegradable (they rot away) to avoid damaging the environment
Other Organic Chemicals and their uses
Alcohols : used to dissolve things (as solvents). Eg aftershave is perfume dissolved in alcohol.
Sugars: Used as sweeteners in foods.. eg Glucose Can be used as preservatives. Eg in jam.
Carboxylic acids : Used as preservatives for food. Eg the vinegar that stops pickled onions going off is a carboxylic acid.
Esters : Have a sweet smell and are used as flavourings in food.
Fermentation
When yeast is placed in warm glucose solution then fermentation occurs. The glucose is converted into alcohol (ethanol) and carbon dioxide gas. The yeast contains enzymes which are living catalysts which speed up the fermentation.
Word Equation : Glucose Ethanol + Carbon Dioxide
Symbols : C6H12O6 2C2H5OH + 2CO2
Factors which change the rate of fermentation are :
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Temperature : The yeast is living therefore it works best when warm (about 300C). If it is heated much above this then it dies and the fermentation stops.
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Amount of yeast. The more yeast present the faster the fermentation will go.
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Concentration of glucose . The glucose is the food of the yeast . The higher its concentration the faster the fermentation will go.
Fermentation is used in industry to brew beer and also for bread-making.
You can test for the carbon dioxide made because if it is bubbled through limewater then the limewater changes from colourless to milky white colour.
Covalent bonding
When a non-metal atom bonds to another non-metal atom it does so by sharing electrons. This is called covalent bonding and is done so both atoms can have a full outside shell of electrons.
It is covalent bonding that holds together hydrocarbons.
Electronic structures of atoms
Hydrogen Carbon Oxygen
