Alkanes

• Alkanes are unreactive
• carbon and hydrogen have similar electronegativities which forma non-polar bonds
• non-polar molecules only have van der waal forces by instantaneous/induced dipole interactions
• What is the hybridisation of orbitals ?

  provide energy to promote one of the S electrons into a P orbital
• Hybridisation of orbitals.

  favourable arrangement

  A) 1s2 2s2 2p2

  B) 1s2 2s1 2p3

  C) repulsion

  D) stable

  
• Hybridisation of orbitals in alkanes

  four orbitals hybridise = four new orbitals (equivalent)

  A) 2s2 2p2

  B) 2s1 2p3

  C) 4 x sp3

  
• What is the structure of alkanes ?

  four sp3 orbitals of carbon repel each other - TETRAHEDRAL bond angle = 109.5

  Each sp3 orbital in carbon overlaps with the 1s orbital of a hydrogen atom
• What are the boiling point trends ?

  greater the chain length, greater the number of electrons, greater the van der waal forces

  if chain length increases, boiling point increases
• What is the combustion of alkanes ?

  occurs when chemicals react with oxygen and give out heat

  only contain carbon and hydrogen

  Products of combustion are their respective oxides
• What is the test for saturation ?

  use bromine water
• Explain the process of halogenation

  substitution reaction - when one functional group is swapped for another

  Caused by photochemical reaction due to UV light

  reaction can repeat itself until all of the hydrogens are halogens
• What are free radicals ?

  species with an unpaired electron
• How does the halogen split ?

  by homolytic fission
• What is the significance of arrows ?

  half curly arrows - movement of one electron

  full curly arrows - movement of pair of electrons
• Explain the mechanism for chlorination

  A) Initiation

  B) CL2

  C) 2CL*

  D) CL*

  E) CH4

  F) *CH3

  G) HCL

  H) *CH3

  I) CL2

  J) CH3CL

  K) CL*

  L) *CH3

  M) CL*

  N) CH3CL

  O) 2CL*

  P) CL2

  Q) 2 *CH3

  R) CH3CH

  S) Propagation

  T) Termination

  
• What are the reactions with other halogens ?

  fluorine - alkanes react explosively

  bromine - same free radical substitution

  iodine - no reaction, activation energy is too high
• What are addition reactions ?

  Two reactant species combine together to form a single product species.

   

  General equation:   A + B —> C
• What are substitution reactions ?

  Two reactant species combine together to form two product species.

  Usually all the species are molecules or ions.

   

  General equation:  A + B —> C + D
• What are oxidation reactions ?

  One organic compound is oxidized usually by an inorganic reagent.

  It can either lose hydrogen or gain oxygen.
• What are reduction reactions ?

  One organic compound is reduced, sometimes by hydrogen gas and a catalyst or by inorganic reagent.

  It can either gain hydrogen or lose oxygen.
• What is homolytic fission ?

  The shared pair of electrons in the covalent bond divide equally between the two atoms.
• What is a heterolytic fission ?

  Both electrons of the shared pair in the covalent bond are kept by one of the atoms.
• What is an electrophile ?

  Refers to a chemical species that ‘likes negative charge’

   

  The positive ion produced by heterolytic fission will be attracted to a region of high electron density in another molecule.
• Explains the process of fractional distillation ?

  Process used to separate a liquid mixture into fractions by boiling + condensing

  A) Individual fractions collected - stored/undergo crackin

  B) larger molecules = longer chains

  C) more imf - higher boiling points

  D) smaller molecules = shorter chains

  E) Less imf - lower boiling points

  F) Some hydrocarbons = dissolved gases

  G) Rise to the top without condensing

  
• Explain the process of cracking

  Break down of molecules into shorter ones by heating with a catalyst

  1. Passing the hydrocarbons in the heavier fractions through a heated catalyst (e.g. zeolite)
  2. Larger molecules break down into smaller ones
• Explain the process of reforming

  conversion of straight - chain hydrocarbons into branched - chain and cyclic hydrocarbons

  vehicle engines - not all hydrocarbons burn the same way

  straight chains = less efficient

  branched chains/cyclic = more efficient

  convert them by heating with a catalyst (usually platinum)
• Complete combustion of alkanes

  Burned in vast quantities to provide the worlds energy

      (E.g propane at high temperatures in containers = home and camping)

   

  Products of combustion

  • Water = adds to the total quantity of global H2O (not an issue)
  • Carbon dioxide = greenhouse gas (correlation to global warming)
• Incomplete combustion

  Not enough oxygen present or very rapid combustion occurred.

      Some of the carbon atoms form gaseous carbon monoxide/solid carbon.

   
• What are the fatal products of incomplete combustion ?

   

  • Carbon monoxide

  Toxic gas = causes many deaths per/year

  Prevents the transport of oxygen around the body

  Colorless and odorless

   

  Equation = C3H8 + 402 —> 2C02 + C02 + 4H20

   

  • Unburned hydrocarbons (UHC)

  Hydrocarbons do not burn at all

  Small proportion in a fuel are released in the atmosphere 
• Oxides of sulfur

  some crude oil molecules = atoms of sulfur = not removed during fractional distillation/cracking/reforming.

  1. Combustion = form sulfur dioxide gas                        
  2. Reacts with the atmosphere to form sulfur trioxide gas  
  3. Both are acidic oxides = dissolve in water (atmosphere) to form sulfurous acid/sulfuric acid.

  • Sulfurous acid = SO2 + H20 —> H2SO3
  • Sulfuric acid = SO3 + H20 —> H2SO4

   
• Oxides of nitrogen


  very few alkane molecules contain atoms of nitrogen = combustion occurs at very high temperatures.

  High temperatures causes nitrogen molecules in the air to react with oxygen molecules

  Main oxides = Nitrogen monoxide (NO) and Nitrogen dioxide (NO2)

   
• Catalytic converters 


  Small quantities of metals (e.g platinum) fitted to exhaust system = reducing pollution

   

  Process

  1. Exhaust gases passes through the catalytic converter
  2. Oxidation of the carbon monoxide
  3. Oxidation of unburned hydrocarbons

   

  Not great at removing sulfur compounds, therefore, some countries remove them before the fuel is burned.

   

  Other types are: rhodium, palladium

           Often found in the d-block
• What are biofuels (alternative fuels)? 


  obtained from living matter that has died recently, rather than a million of years ago.
• Explain the concept of carbon neutrality

  considered net zero effect on the amount of carbon dioxide in the atmosphere.

   

  example -

  • Growing tree = absorbs carbon dioxide from the atmosphere
  • Tree is burned = carbon dioxide released is equal to CO2 absorbed.

   

  but they are not considered carbon neutral = carbon dioxide absorbed from the atmosphere was way higher (millions of years ago) than the release nowadays.
• What are bioalcohols ?

  fuel made from plant matter, often using enzymes of bacteria

   

  Process of harvesting/transporting/processing releases more carbon dioxide than plant has absorbed and then released.

   

  not carbon neutral
• How do you compare fuels ?

  1. Land use - how much land is used to grow the crop? Should the land be used for other purposes, like to grow food to feed people?

   

  1. Yield - how much of a crop can be grown on a given piece of land, how quickly will it grow? What % of the carbon and hydrogen atoms in the crop ends up in the fuel?

   

  1. Manufacture and transport - how much energy is used in growing, processing and transporting the crop ?

   

  1. Carbon neutrality - how close if the fuel to being carbon neutral ?

   
• How is hydrogen used as a fuel ?

  considered as an ideal alternative fuel (not a biofuel though).

   

  Two main ways -

  1. Burned instead of a fossil fuel such as petrol or natural gas
  2. It can be used in a fuel cell to generate electricity that powers and electric motor

   

  No carbon dioxide is produced = no greenhouse effect.

  However…

  • Requires energy to obtain it
  • Electricity use
  • Power stations
  • That burns fossil fuels
• What are the limitations to storing hydrogen as a fuel ?

  1. Very low density = stored under very high pressure
  2. Could be stored as a liquid = boiling temperature is -253 so container needs to be very cold and insulated.
• What is a hazard ?

  something that could cause harm to a user
• What is a risk ?

  the chance of a hazard causing harm