5.1 Cracking Hydrocarbons
Large Hydrocarbon molecules can be broken down into smaller molecules by a process called cracking.
· Heating a mixture of hydrocarbon vapours and steam to very high temperatures.
· Passing hydrocarbon vapours over hot catalyst
During cracking thermal decomposition reaction produce a mixture of smaller molecules.
· Alkanes – saturated hydrocarbons with general formula CnH2n+2. (Useful fuels)
· Alkenes – unsaturated hydrocarbons, contain fewer hydrogen atoms with same number of carbon atoms, with general formula CnH2n. Have double bonds making them more reactive.
C10H22 à 800 C + catalyst à C5H12 + C3H6 + C2H4
Decane -------------------à Pentane +Propene + Ethene
5.2 Making Polymers from alkenes
Plastics are made from very large molecules called polymers. Made when very small molecules join together. Small molecules used to make polymers are called monomers. Reaction to make polymers in called polymerisation.
Lots of ethane(C2H4) molecules can join together to form poly(ethene), commonly called polythene. Polymerisation reaction in the double bond in e ach each ethane molecule becomes a single bond and thousands of ethene molecules join together in long chains.
Other Alkenes polymerise in the same way.
PLASTICS WE USE AS BAGS, BOTTLES, CONTAINERS AND TOYS ARE MADE FROM AKLENES
5.3 New and Useful polymers
Uses of Polymers
· Dental fillings that have replace mercury ones.
· Light sensitive polymers are used in sticking plasters to cover wounds.
· Hydrogels are polymers that can trap water and have many uses including dressing for wounds.
· Shape-memory polymers change back to their original shape when temperature or other conditions are changed.
o Smart polymers is material used for stitching wounds that’s changes shape when heated to body temperature.
Fibres used to make fabrics can be coated in polymers to make them waterproof and breathable.
Plastics from many drink bottles can be recycled to make polyester fibres for clothing as well as pillows and duvets.
5.4 Plastic waste
Many plastics are not biodegradable. This means that plastic waste is not broken dwon when left in the environment. Unless disposed of properly.
Biodegradable plastics can be broken down by micro-organisms.
Ethanol C2H6O. Often C2H5OH.
· Ethanol can be made using fermentation of sugar from plants using yeast. Enzymes in yeast convert sugars into ethanol and carbon dioxide.
o Uses renewable resources but produces dilute form of ethanol – fractional distillation has to be used to separate and form pure ethanol.
· Made by hydration of ethene
o Ethene reacted with steam at high temps I presence of catalyst. Ethene obtained from crude oil by cracking.
o Uses non-renewable resources but produces pure ethanol and requires high temps.