Large hydrocarbon molecules can be broken down into smaller molecules by a process caled cracking.
Cracking can be done in two ways:
- By heating a mixture of hydrocarbon vapours and steam to a very high temperature.
- By passing hydrocarbon vapours over a hot catalyst.
During cracking thermal decomposition reactions produce a mixture of smaller molecules. Soem of the smaller molecules are alkanes, which are saturated hydrocarbons with the general formula CnH2n + 2. These alkanes with smaller molecules are more useful as fuels.
Some of the other molecules formed are hydrocarbons with the general formula of CnH2n. These are called alkenes. Alkenes are unsaturated hydrocarbons because they contain fewer hydrogen atoms than alkanes with the same numberof carbon atoms.
Alkenes have a double bond between two carbon atoms and this makes them more reactive than alaknes. Alkenes react with bromine water turning it from orange to colourless.
Making Polymers from Alkenes
Plastics are made of very large molecules called polymers. Polymers are made from many small molecules joined together. The samll molecules used to amke polymers are called monomers. the reaction to make a polymer is called polymerisation.
Monomers -------------------- Polymer
Ethene -------------------- Poly(ethene)
Propene -------------------- Poly(propene)
Lots of ethene molecules can join together to form polyethene. In the polymerisation reaction the double bond in each ethene molecule becomes a single bond and thousands more ethene molecules join together in long chains.
Many of the commonly used plastics, such as bags, bottles and containers are made from alkenes.
We can make polymers from alkenes but not alkanes because alkenes are unsaturated and alkanes are.
New and Useful Polymers
Scientists have developed many new polymer materials with special properties for particular uses.
For example, new polymer materials for dental fillings have been developed to replace fillings that contain mercury. light sensitive polymers are used in sticking plasters to cover wounds so the plasters are easily removed.
Hydrogels are polymners that absorb alot of water and turn into a gel. One popular use of this is in nappies to prevent nappy rash.
Shape memory polymers change back to their original shape when temperture or other conditions are changed. An example of this is a material used for stitching wound that cahnged shape when heated to body temperature.
Waterproof and breathable fabrics have been coated with polymers.
The plastic used to make drinks bottles can eb recycled to make polyester fibres for clothing.
Ethanol can be produced by fermentation of sugar from plants using yeast.
Enzymes in the yeast cause the sugar to convert to ethanol and carbon dioxide .This method is used to make alcoholic drinks.
This uses are renewable source and sugar from plants. This is done at room temp however it can onbly produce a dilute aqueos solution of ethanol, this then must be seperated from the solution to give pure ethanol.
Athother way of making it is by the hydration of ethene.
Ethene is reacted with steam at a high temperature in the presence of a catalyst. The ethene is obtaned from crude oil by cracking.
This uses a non renewable source and requires a high temp but the reaction can be run continuously and produces pure ehanol.
Pant oils are substances taken from plants instead of products extracted from animal fats. Plants make glucose from carbon dioxide and water by photosynthesis and store glucose as starch or vegetable oils. Some plants store enough oil to amke extraction worthwhile, in nuts, fruits or seeds.
Methods of extraction:
First plant material is crushed to break open the oils containing cells, then there are a number of methods to extract the oil; Pressing, solvent extraction and steam distillation.
Steam Distillation -
Vapours from the lavender are extracted and travel through a tube. The water jacket helps the vapours to cool and condense. It then drops into a beaker at the bottom.
Unsaturated - Double Bond (Liquid at room temp)
Saturated - Single Bonds (Solid at room temp)
Oil droplets in water (milk, ice-cream, salad cream, mayonaise)
Water droplets in oil (margarine, butter, skin cream, moisturising lotion)
Mayonaise is a stable emulsion of vegatable oil and vinegar with egg yolk. Egg yolk contains a natural emulsifier.
Emulsifier molecules have two different ends:
- A hydrophillic end - 'water loving'. Forms chemical bonds with water not oils.
- A hydrophobic end - 'water hating'.Forms bonds with oils not water.
In this way, the water and oil droplets become unable to seperate out. The mixture formed is called an emulsion.