C1.5 Other useful substances from crude oil

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C1.5.1 Cracking

Heavier hydrocarbon fractions (e.g lubricating oils) are in less demand and can be broken down to form smaller, more useful molecules such as alkanes and alkenes. Some of these products are useful as fuels. Smaller hydrocarbons are less viscous, more flammable, and have lower boiling points.

Cracking

  • Thermal decomposition reaction
  • Oil fractions are heated so they vaporise
  • Vapours can be passed over a catalyst which helps speed up the thermal decomposition reaction
  • Otherwise, vapours can be mixed with steam and heated to very high temperatures
  • Example equation: C10H22 -> C8H18 + C2H2
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C1.5.1 Alkenes

Alkenes

  • Unsaturated hydrocarbons (have a double bond between two carbon atoms) 
  • General formula: CnH2n (the number of H atoms is double the number of C atoms) 
  • To test if alkane or alkene: add bromine water. Alkanes: bromine stays orange. Alkenes: bromine turns colourless 
  • Used to make polymers 

ethene C2H4 ethene has 2 carbon atoms and 4 hydrogen atoms (http://www.bbc.co.uk/staticarchive/39df2f1691886e69a85a5fa65b2d89c8dec7a1e5.gif)
propene C3H6 propene has 3 carbon atoms and 6 hydrogen atoms (http://www.bbc.co.uk/staticarchive/3c213f469253d4d2f24b9391bcf026199874566f.gif)

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C1.5.2 Polymers

Polymerisation
Polymers are long chain molecules made by joining together a large number of smaller molecules called monomers. Alkene molecules can act as monomers.
The conditions needed for polymerisation are heat, pressure and a catalyst. 

Displayed structures
Show many (n) monomers can react together to form n units of a polymer. To form the name of a polymer: poly(name of monomer), e.g. poly(ethene)

  • Draw the monomer but with a single bond
  • Draw a long bond either side
  • Draw brackets through the long bonds
  • Write n after the bracket

Uses of polymers

  • new packaging materials
  • waterproof coatings for fabrics (such as for outdoor clothing)
  • fillings for teeth
  • dressings for cuts
  • hydrogels (for example for soft contact lenses and disposable nappy liners)
  • smart materials (for example shape memory polymers for shrink-wrap packaging).
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C1.5.2 Properties of polymers

Thermosoftening/thermoplastics - soften when heated and can be remoulded into new shapes so is much easier to recycle. Relatively weak intermolecular forces between the seperate polymer chains. Its tangled polymer chains can uncoil and slide past each other, so is a flexible material.

Thermosets/thermosetting plastics- do not soften when heated and cannot be reshaped. When heated strongly enough, eventually break down and char. they are hard and rigid so are difficult to recycle. Polymer chains are fixed together by strong bonds called cross links so cannot slide past each other.

LDPE/Low density poly(ethene) - many random side branches on polymer chains, weak relative strength and low maximum useable temperature.

HDPE/High density poly(ethene) - few branches on polymer molecules strong relative strength and high maximum useable temperature.

Problems with polymers
Many polymers are not biodegradale (material that can be broken down by microbes) so can lead to prolems with disposal e.g. litter, but can be useful as polymer materials can last a long time and may be recycled. However, recycling can be difficult and expensive to separate different polymers.
Cornstarch is biodegradable so is used to make biodegradable substances such as carrier bags and plastics.

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C1.5.3 Ethanol

Ethanol is the alcohol in alcoholic drinks but can also be used as a fuel, and has industrial and chemical uses. Its molecular formula is C2H6O or CH3CH2OH. Ethanol can be produced in two ways:

Hydration of ethene
Ethene and steam react together in the prensence of a catalyst to make ethanol. Needs high temperatures and pressures and a phosphoric acid catalyst:

C2H4 + H2O -> C2H5OH

  • Unreacted ethene can e recycled by dehydration
  • Continuous process
  • Pure ethanol
  • Non-renewable (crude oil is limited)
  • Releases CO2 into the atmosphere

Fermentation/bioethanol
Uses yeast in anaeroic respiration to convert sugar from plants to ethanol, then distilled to get close to pure ethanol. Needs water:

C6H12O6 -> 2C2H5OH + 2CO2
sugar/glucose -> ethanol + carbon dioxide

  • Reduces CO2 released- almost carbon neutral because of photosynthesis of plants grown
  • Renewable
  • Only 10-15% ethanol
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