Chemistry C1 (part 2)

• Long-chain hydrocarbons form thick gloopy (viscous) liquids eg. tar  > not that useful
• Most longer molecules produced from fractional distillation are turned into smaller ones by the process of cracking
• Some products from cracking are useful as fuels, eg. diesel > cracked to form petrol, paraffin 
• Ethene can also be produced > needed for making plastics

• Cracking is a thermal decomposition reaction (breaking molecules down with heat)
• First the long-chain hydrocarbon is heated to vaporise it > passed over a powdered catalyst (aluminium oxide) at around 400 - 700ºC
• The long-chain molecules split apart or 'crack' on the surface of the catalyst
• Most products of cracking are alkanes and alkenes.

• Alkenes > hydrocarbons > double bond between 2 carbon atoms in their chain C=C
• They're unsaturated because they can make more bonds - double bond can open, allows the 2 carbon atoms to bond with other atoms
• Alkenes: ethene (2 Cs), propene (3 Cs), butene (4 Cs)    general formula: CnH2n
• Test for alkenes > add to bromine water > the alkene will decolourise the bromine water > orange to colourless (double bond is opened and forms bonds with bromine)

• Ethene (C2H4) can be hydrated with steam (H2O) with a catalyst > produce ethanol
• atm. it's a cheap process > ethene is fairly cheap, not much wasted. Ethene's from crude oil (non-renewable) > will run out > therefore the process will get more expensive

• Ethanol can also be produced by fermentation of sugar (renewable) + yeast
• sugar -> carbon dioxide + ethanol
• Needs a lower temp. and simpler equipment. It's cheap fuel
• Disadvantages: the ethanol produced isn't very concentrated, it has to be distilled and purified

Alkenes > polymerisation > polymers

• Joining together lots of small alkene molecules (monomers) to form very large molecules called polymers
• Eg. join up lots of ethene molecules > produce polyethene

• Physical properties depend on: what it's made from and temperature & pressure of polymerisation
• eg. Polyamides are stronger than polyethene. Polyethene made at 200ºC with 2000 atmospheres pressure > flexible, low density. Polyethene made at 60ºC and low pressure + catalyst > rigid, dense
• Light, stretchable polymers > plastic bags. Elastic polymer fibres > LYCRA fibre > tights. Waterproof coatings for fabrics (polymers). Dental polymers > resin tooth fillings. Polymer hydrogel wound dressings > keep moist. Biodegrable packaging > polymers & cornstarch. Memory foam (smart material) > polymer gets softer when warmer.

• Most polymers arn't biodegradable > hard to get rid of > contribute to landfill or recycle
• Polymers are cheaper than metal, but as crude oil runs out > get more expensive

Some fruits and seeds contain lots of oil > extracted for food/fuel

• Plant material crushed > pressed between metal plates > oil squeezed out
• OR use a centrifuge to separate oil from crushed plant OR use solvents
• Distillation refines oil and removes water, solvents & impurities

• Vegetable oils (used in food) have a very high energy content and contain nutrients & essential fatty acids (metabolic processes)
• Benefical:
  • higher boiling point than water, cook foods at higher temp. and faster
  • gives a different flavour
  • get more energy from foods cooked in it

• Vegetable oils (eg. rapeseed & soybean oil) can be turned into fuels (provide lots of energy)
• Useful fuel from vegetable oils > biodiesel.
• Similar properties to ordinary diesel - burns in the same way, so it can be used to fuel a diesel engine.