Alkanes

• alkanes general formula: CnH2n+2
• alkanes are hydrocarbons (carbon + hydrogen only)
• alkanes are saturated (single bonds only)
• what is a cycloalkanes?


  have a ring of carbon atoms with two hydrogens attached to each carbon and have different general formula
• what is petroleum?


  a mixture of hydrocarbons, mostly made up of alkanes
• fractional distribution splits long crude oil into separate bits to make it more useful
• Fractional distribution


  1. Crude oil is vaporised at 350°C

  2. Vaporised crude oil goes into a fractionating column and rises up through the trays

  3. Largest hydrocarbons don't vaporise at all because boiling points are too high - form residue

  4. As crude oil vapour goes up the column, it gets cooler

  5. Alkene molecules have different chain lengths but different boiling points, so each fraction condenses at different temperature

  6. Fractions are drawn off at different levels of the column

  7. Hydrocarbons with lowest boiling points do not condense, they're drawn off as gases at the top of the column
• hydrocarbons with lowest boiling points do not condense. They've drawn off as gases at the top of the column
• fractional distillation

  
• cracking?


  breaking long chain alkanes into smaller hydrocarbons and also involves breaking the C=C bonds
• what is thermal cracking?

  • takes place at high pressure and high temperature
  • produces lots of alkenes - used to make heaps of valuable products, like polymers
• what is catalytic cracking?

  • takes place at a slight pressure and high temperature
  • mostly produces aromatic hydrocarbons and motor fuels (aromatic compounds contain benzene rings which has six carbon atoms with three double bonds
  • using catalysts cuts cost, since reaction done in lower pressure and lower temperature. Using catalysts saves time.
• combustion reaction?


  if alkanes oxidise (burn) with oxygen, carbon dioxide and water is produced
• complete combustion?


  only products are water and carbon dioxide
• alkanes make great fuels-burning releases a lot of energy but produces lots of pollutants
• incomplete combustion?


  not enough oxygen around so hydrocarbons combust incompletely which changes products of reaction and lead to side effects
• when a hydrocarbon undergoes incomplete combustion, carbon monoxide is produced - poisonous. carbon monoxide molecules bind to the same sited on haemoglobin molecules in red blood cells as oxygen molecules. so oxygen can not be carried around the body. they can be removed from exhaust gases by catalytic converters on cars.
• carbon particles (soot) can also be formed from incomplete combustion which causes breathing problems. it can also build up in engines, meaning they do not work properly.
• global warming:

  • burning fossil fuels produces carbon dioxide - greenhouse gases
  • greenhouse gases in our atmosphere absorbs infrared energy (heat). They emit some of the energy they absorb back towards the Earth, keeping it warm - greenhouse effect
  • by increasing the amount of carbon monoxide in out atmosphere, the Earth gets warmer
• engines do not burn all the fuel molecules, some will come out as unburnt hydrocarbons. when pressure and temperature in engine in high, the nitrogen and oxygen atoms from the air react together to produce oxides of nitrogen. hydrocarbons and nitrogen oxides react in presence of sunlight to form ground-level ozone (smog), which irritates peoples eyes, aggravations respiratory problems and cause lung damage. Catalytic converters on cars remove unburnt hydrocarbons + oxides of nitrogen from exhaust
• when fossil fuels that contain sulfur are burnt, it reacts to form sulfur dioxide (SO2), which when into the atmosphere, it dissolves the moisture and is converted into sulfuric acid which causes acid rain which destroys trees, vegetation, corroding buildings, statues and kills fish in lakes.
• sulfur dioxide can be removed from power station flue gases. powered calcium carbonate/calcium oxide mixed with water to make an alkaline slurry when sulfur dioxide gas reacts with calcium compounds to form a harmless salt
• free radical?


  a particle with an unpaired electron
• free radicals form when a covalent bond splits eventually, giving one electron to each atom
• unpaired electron in free radical atoms makes then very reactive
• halogen react with alkanes in photochemical reactions (reactions that are started by ultraviolet light) to form halogenoalkanes
• free radical substitution?

  a hydrogen atom is substituted (replaced) by chlorine/bromine
• Reaction mechanisms


  1. Initiation reactions

  2. Propagation reactions

  3. Termination reactions
• Initiation reactions


  Free radicals are produced
• Initiation reactions


  • Sunlight gives energy to break C=C bond (photodissociation)
  • Bond splits equally and each atom gets one electron
  • Atom becomes highly reactive free radical because of its unpaired electron
• Propagation reactions


  Free radicals are used up and created in a chain reaction
• Propagation reactions


  1. Atom attacks a methane molecule

  2. The new methyl free radical attacks another atom

  3. Which attacks another methane molecule

  4. And carries on until all atoms + molecules are used up
• Termination reactions


  Free radicals are mopped up
• Termination reactions


  1. If two free radicals join together, they make a stable molecule

  2. The two unpaired electrons form a covalent bond

  3. There are heaps of possible termination reactions
• • if Cl's in excess, Cl` free radical will start attacking chloromethane, producing dichloromethane, trichloromethane and tetrachloromethane.
  • but if methane's in excess, product will be chloromethane
• chlorofluorocarbons (CFC)?


  are halogenoalkane molecules where all hydrogen atoms are replaced by Cl + F atoms
• ozone (O3)?


  a chemical sunscreen in the upper atmosphere. It absorbs ultraviolet radiation from the sun, stopping it reaching us. Ultraviolet radiation can cause sunburn/skin cancer
• • Ozone's formed naturally when oxygen is broken down into two free radicals by ultraviolet radiation. the free radicals attack other oxygen molecules forming ozone.
  • O2 --UV--> O' + O' then O2 + O' ---> O3
• CFC's create holes in in the ozone layer:

  • chlorine free radicals (Cl') are formed in the upper atmosphere when C-Cl bonds in CFCs are broke down by ultraviolet radiation
  • these free radicals are catalysts. they react with ozone to form an intermediate (ClO') and an oxygen molecule
  • Cl' is the catalyst
• CFCs are unreactive, non-flammable and non-toxic, used to be used as coolant gas in fridges, as solvents and as propellants in aerosols