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Polymerisation and Cracking

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Introduction

Polymerisation Polymerisation is when you join together lots of little monomers to form a long chain, which is called a polymer. Polymerisation can only be done with alkenes. This is because alkenes are unsaturated, meaning they have at least one c=c. When they are 'added' together, the double bond opens up and joins together the many monomers. However, polymerisation doesn't just happen at room temperature and pressure, and it the conditions depend on what alkene you are polymerising. For the polymerisation of ethane, you require a temperature of 200oC, a pressure of 2000 atmospheres. Each polymerisation reaction also requires something to start the process, called an initiator. It is like a catalyst, except it gets used up in the reaction. The chain length of the polymer varies from about 2000 to 40,000 carbon atoms. Example process of polymerisation: The top layer shows the monomers of ethane. This consists of two carbon atoms and four hydrogen atoms (C2H4). ...read more.

Middle

In the molecular level, certain chains may lie close to each other, packed together, these parts are called crystalline. Parts where the chains are jumbled, not closely packed are called amorphous. Many types of polymers are formed with the many alkenes. I will talk about the plastic poly(ethene): There are two types of poly(ethane)--> Low density polythene (LDPE) and high density polythene (HDPE) LDPE * This has quite a lot of branching chains, thus creating jumbled chains, so it is amorphous * Amorphous areas means that the chains are inefficiently packed, so there is less of the Van der Waals attraction force (as the chains may be farther away) * This reduces the strength of the bond between chains, thus lowering the strength of the material and the melting point. * So LDPE is used for light, flexible and weak material such as plastic bags HDPE * Unlike LDPE, the structure of HDPE is 95% or more crystalline * This means that there are greater Van der Waals forces, so the attraction between the chains is higher ...read more.

Conclusion

at 500oC Example: You can see that hexane (C6H14) is being cracked to form ethane (C2H4) and butane (C4H10). There are two ways of cracking: Catalytic cracking Thermal cracking There are many types of catalysts, some common ones are ZEOLITES and a mixture of silicon dioxide and aluminium oxide. Zeolites are basically crystal lattices consisting of aluminium, silicon, and oxygen. It works when the crystal is able to remove a hydrogen and the two electrons in the covalent bond from the alkane. This leaves the carbon with a positive charge, which causes the other atoms to be attractedrepelled. This reorganizes the structure, resulting in smaller molecules. This involves temperatures of upto 750oC and a pressure of 70 atmospheres. This breaks the c-c bonds, giving a mixture of smaller molecules. Cracking is really useful as it splits to give alkenes, which are used to make plastics (after being polymerised), and short-length hydrocarbons, such as petrol which are essential for everyday life. ?? ?? ?? ?? Jagan Annamaraju 10AN ...read more.

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