Organic

• What are alkenes


  a homologous series with general formula CnH2n
• What is a homologous series

  a series of compounds with the same general formula and similar chemical properties
• How are alkanes different to alkenes

  Alkenes are unsaturated molecules. They contain a carbon to carbon double covalent bond C=C
• How can we chemically distinguish between alkenes and alkanes

  Alkenes will turn orange bromine water colourless. There is no visible reaction with alkanes
• How are alkenes formed

  Long chain alkanes are broken down under heat and pressure (cracked) to form smaller, more useful, alkanes and alkenes
• What are the conditions for cracking

  high temperature and steam for steam cracking or high temperature and an aluminium oxide catalyst for catalytic cracking
• Why does industry crack alkanes

  There is a greater demand than supply for the smaller, more useful hydrocarbon molecules but a greater supply than demand for the less useful larger molecules. Industry breaks down the larger alkanes to match supply with demand
• What type of reaction is cracking

  a thermal decomposition reaction
• Name a use of alkenes

  making plastics / polymers
• Why do polymers generally have a high melting point

  polymers consist of very long chains

  the strength of forces between the chains increases with chain length
• what is the difference between thermosetting and thermosoftening polymers?

  there are weak forces between thermosetting polymers which can be easily overcome: thermosetting polymers can be reshaped as they melt easily as polymer chains can slide over each other

  
  

  thermosetting polymers have strong covalent bonds cross-linking the polymer chains holding them in place. When heated, thermosetting polymers char and do not melt

  
• What are the properties of low-density polyethene LDPE?


  LDPE has a branched structure so the polymer chains cannot get close to each other

  
  

  the polymer is flexible and unreactive and can be made into films, so can be used for plastic bags and cling-film
• What are the properties of high-density polyethene HDPE?

  HDPE consists of mostly linear chains (little branching) which means the chains can line up and can be close together

  
  

  the polymer is strong but flexible and resists chemical attack and so is used for plastic bottle, pipes and buckets

  
• What is crude oil

  It is a mixture of a very large number of hydrocarbons
• What is a hydrocarbon


  molecules made up of only hydrogen and carbon atoms
• State how crude oil is separated

  By fractional distillation
• Describe how crude oil is separated

  1, the crude oil is vapourised

  2. the vapour rises up the fractionating column and cools and condenses at their boiling points

  3. the column is hot at the bottom and cool at the top

  4. small molecules with low boiling points condense at the top of the column, whilst large molecules with high boiling points condense at the bottom of the column

  5. the molecules are thus separated by their boiling point in fractions
• What are fractions


  a range of molecules with similar boiling points
• What are alkanes


  hydrocarbons that belong to a homologous series, with general formula CnH2n+2
• What is a homologous series

  compounds that have the same general formula and similar chemical properties
• What is a saturated compound


  one that contains only single covalent bonds
• What is an unsaturated compound


  one that contains at least one double covalent bond
• What is the formula of methane

  CH4
• What is the formula of ethane

  C2H6
• What is the formula of propane

  C3H8
• What is the formula of butane

  C4H10
• Why are alkanes deemed to be saturated molecules


  they only contain single covalent bonds
• Describe the structure and bonding in alkanes

  molecular covalent
• State and explain the trend in boiling point as hydrocarbons increase in size

  boiling point increases as molecular size increases.The molecule's electron cloud is larger, hence the strength of forces between the molecules is greater. More energy is required to overcome these forces
• State and explain the trend in viscosity as hydrocarbons increase in size

  viscosity increases as molecular size increases.The molecule's electron cloud is larger, hence the strength of forces between the molecules is greater. More energy is required to overcome these forces, hence viscosity increases
• State and explain the trend in flammability as hydrocarbons increase in size

  with increasing carbon number, more oxygen is required for complete combustion [flame becomes smokier as incomplete combustion occurs) hence flammability decreases