3.02 Alkanes

• What is an alkane?

  Saturated hydrocarbon containing C-H bonds only
• What is the general formula of an alkane?

  CnH2n+2
• Are their bonds polar? Why/why not?

  Non-polar - carbon and hydrogen have similar electronegativities
• Which intermolecular forces do they have? Why?

  Only van der Waals forces of attraction as bonds are non-polar
• Are they soluble in water? Why?

  Insoluble because hydrogen bonds in water are stronger than alkanes' VDW forces of attraction
• How reactive are alkanes?

  Very unreactive
• Which reactions will alkanes undergo?

  • Combustion
  • Reactions with halogens
• What is crude oil? How is it formed? Is it renewable? Why?

  • Mixture of fractions (hydrocarbons with similar boiling points and properties)
  • Formed at high temperatures and pressures deep below earth's surface over millions of years therefore non-renewable
• What is fractional distillation, and how does it work?

  • Crude oil is heated until mostly vapourised
  • It is passed into a fractionating column that is cooler at the top than the bottom
  • Liquid fractions are piped off at the bottom
  • Vapour rises up the column and condense when temperature is less than their Tb.
  • Shortest chain hydrocarbons condense at the top as they have the lowest boiling points
• What is fracking and how is it done?

  Natural gas held within shale rock

  • Drill into shale, force pressurised water and sand into rock to fracture it, and collect gas
  • HCl and methanol added to break up shale and prevent corrosion
• What are the pros and cons of fracking?

  • Advantages - gas supply for many years, reduces imported gas and electricity
  • Disadvantages - lots of traffic to local area, concern about amount of water used, chemical additives can pollute water supplies, can cause small earthquakes, can combust methane into carbon dioxide causing global warming
• Why are alkanes cracked?

  To turn a long chain alkane, which is not very economically valuable, into a shorter chain alkane (more economically valuable due to usage as a fuel) and an alkene (more reactive and can be used as a starting point for many products)
• What are the conditions for thermal cracking?

  • 700-1200K temperature
  • Up to 700kPa pressure
• What is the intermediate for thermal cracking?

  Free radicals
• What are the main products of thermal cracking?

  Alkenes
• What are the conditions for catalytic cracking?

  • Lower temp. - 720K
  • Lower pressure (but above atmospheric/1 atm)
  • Zeolite catalyst (SiO2 and Al2O3) with a honeycomb structure to give it a large surface area
• What are the main products of catalytic cracking?

  • Cycloalkanes
  • Aromatic hydrocarbons
  • Branched alkanes
• What is a fuel?

  Something which releases heat energy when combusted
• What are the five main fuels comprising of alkanes?

  • Methane
  • Butane
  • Propane
  • Petrol (about 8 carbons in the chain)
  • Paraffin (10-18 carbons in the chain)
• What is incomplete combustion and what products are formed in the case of alkanes?

  Combustion in a limited supply of oxygen

  • CO - carbon monoxide, which is dangerous
  • C - carbon - particulates+soot, which contributes to global dimming
• Which type of hydrocarbons are most likely to undergo incomplete combustion?

  Longer chains
• What is the environmental impact of carbon monoxide?

  It is toxic/poisonous
• What is the environmental impact of nitrogen oxides?

  • Forms nitric acid, which turns into acid rain
  • Photochemical smog
• What is the environmental impact of sulfur impurities/sulfur dioxide?

  Forms sulfuric acid, which turns into acid rain
• What is the environmental impact of soot (carbon)?

  • Asthma
  • Cancer
  • Global dimming
• What is the environmental impact of unburnt hydrocarbons?

  Photochemical smog
• What is the environmental impact of carbon dioxide?(+ water vapour)

  greenhouse gas therefore contributes to global warming, increasing global temperatures and speeds up climate change
• What are flue gases?

  Gases given out by power stations
• What is an equation for a way to desulfurise flue gases?

  CaCO3(s) + SO2(g) + 1/2 O2(g) -> CaSO4(s) + CO2(g)
• What are catalytic converters made up of?

  Ceramic honeycomb coated with platinum, palladium, and rhodium
• What do catalytic converters catalyse?

  They catalyse these reactions of products from car exhausts:

  • 2CO(g)+2NO(g) -> N2(g)+2CO2(g)
  • Hydrocarbons+NO(g) -> N2(g)+CO2(g)+H2O(g)
• What are greenhouse gases?

  Gases which trap infared radiation, making the earth act like a greenhouse
• What is the greenhouse effect and how does it contribute to global warming?

  Greenhouse gases trap infared radiation in the atmosphere, atmosphere heats up therefore global warming occurs
• Define carbon neutral activities

  Activities that produce no net/overall CO2 emissions
• How are halogenoalkanes formed from alkanes?

  Free radical substitution reaction
• What are the 3 stages of free radical substitution?

  • Initiation - breaking halogen bond to form free radicals
  • Propagation - chain part of the reaction where products are formed but free radicals remains
  • Termination - free radicals removed, stable products formed
• What are the conditions needed for the formation of a free radical chlorine atom?

  Presence of UV light
• Write equations for the reaction of methane with chlorine to form chloromethane:

  • Initiation - Cl2 -> 2Cl•
  • Propagation - Cl• + CH4 -> HCl + •CH3
  • Termination - •CH3 + Cl• -> CH3Cl or 2Cl• -> Cl2 or •CH3 + •CH3 -> CH3CH3
• What is the ozone layer's function?

  Protects the earth from harmful exposure to too many UV rays
• How do CFCs break the ozone layer down?

  Free radical substitution