3.02 alkanes

• what are alkanes?

  saturated hydrocarbons.
• what does saturated mean?

  no double bonds.
• what does hydrocarbon mean?

  only contains hydrogen and carbon.
• What's the general formula for alkanes?

  CnH2n+2
• how does the boiling point of alkanes change as the carbon chain increases and why?

  boiling point increasesthis is due to more electrons causing more VDW forces between the molecules.
• how does the boiling point of alkanes change as branching increases and why?

  boiling point decreases because the molecules aren't as close together therefore there are less VDW forces.
• How does viscosity in alkenes change as the carbon chain increase and why?

  viscosity increases due to VDWs.
• how does volatility and flammability change in alkanes as the carbon chain increases and why?

  volubility and flammability decreases due to VDWs.
• how are akanes obtained?

  crude oil is extracted from earth via drilling processes and transported to an oil refinery to be separated into fractions by fractional distillation as raw crude oil has limited use.
• what is the method of fractional distillation?

  crude oil is heated in a furnace, vaporised, and passed through a column structured in a temperature gradient (hotter at the bottom) where it rises, cools, and condenses to be collected with other compounds of similar BPTs.
• will smaller chained alkanes with lower Mrs collect higher or lower in the column of fractionating distillation?

  higher as less thermal energy is needed to overcome the forces.
• what further process is used for longer hydrocarbon alkanes after fractional distillation and why?

  longer hydrocarbon chains are less in demand/useful so are broken into smaller chains via cracking.
• what is cracking?

  a chemical process that uses harsh conditions to break covalent bonds.
• what are the two types of cracking?

  thermal and catalytic.
• what conditions are needed for thermal cracking?

  • 7000kpa (high pressure) • 400-900 degrees (high temp)• only maintained for 1s
• why is the resistant time for thermal cracking 1s?

  because if it was any longer the compound would completely decompose.
• what is the method of thermal cracking?

  long chained alkanes are heated under pressure in the absence of air (usually a steam cracker), they vibrate vigorously and the bonds break.
• what is produced from thermal cracking?

  short chain alkanes and alkenes.
• evaluate thermal cracking.

  lots of energy is required but it can be used on all fractions.
• what are the conditions for catalytic cracking?

  • 450 degrees (high temp) • slight pressure • zeolite catalyst
• what does catalytic cracking produce?

  branched and cyclic alkanes.
• evaluate catalytic cracking.

  much cheaper than thermal as less temperature and pressure is needed.
• why are alkanes used as fuels?

  they are readily available and very exothermic.
• what is produced for the combustion of alkanes?

  water is always produced but the form of carbon depends on the amount of oxygen reacting.
• what is the formula for complete combustion?

  hydrocarbon + oxygen = carbon dioxide + water
• what is the formula for combustion in limited oxygen?

  hydrocarbon + oxygen = carbon monoxide + water
• what is the formula for combustion in very limited oxygen?

  hydrocarbon + oxygen = carbon + water
• why is incomplete combustion less desirable?

  because less energy is released.
• name 7 pollutants from alkane combustion.

  • co2 • co• c particulates • hydrocarbons• h2o• nox • so2
• where does the pollutant co2 come from and what is its affect?

  the carbon from the fuel

  its a GHG that contributes to global warming
• where does the pollutant co come from in combustion and what is its affect?

  carbon from the fuel

  it is toxic, odourless, and colourless and irreversibly binds to haemoglobin so less oxygen is delivered to cells
• where do C particulates come from in combustion and and what is its affect?


  carbon from the fuel

  
  

  forms smog that causes global dimming and can cause respiratory issues e.g asthma
• where does the pollutant hydrocarbons come from during combustion and what is their affect?

  unburnt alkanes in fuel they react with nox to form ground level ozone and can cause respiratory issues.
• where does the pollutant h2o come from during combustion and what is its affect?

  H from fuel

  
  

  it is a GHG and contributes to global warming
• where does the pollutant nox come from during combustion and what is its affect?

  N2 in air creates smog which causes global dimming and respiratory issues
• where does the pollutant so2 come from and what is its affect?

  S from fuel impuritiescauses acid rain and respiratory issues
• how do sulfur impurities in fuel turn into acid rain during combustion?

  sulfur impurities produce sulfur dioxide when burnt, this reacts with oxygen to produce sulfur trioxide. this dissolves in water to produce acid rain.
• why isn't all sulfur impurity removed from fuel before use?

  because it is not economically viable.
• at what point is sulfur removed from fuel during combustion?

  it is removed from the emissions through flue gas desulfurisation.
• what is the method of flue gas desfulferisation?

  gases are passed through a wet semi-solid mixture containing either calcium oxide or calcium carbonate. the bases neutralise the acidic SO2 to form calcium sulphite. this has little commercial use to it is oxidised to calcium sulphate which is used in the building industry. CaO + SO2 = CaSO3 or CaCO3 + SO2 = CaSO3 + CO2CaSO3 + [O] = CaSO4