Alkanes

• Saturated compounds - have molecules in which all carbon-carbon bonds are single bonds

  • e.g. alkanes
  • saturated means that they only have single carbon-carbon bonds, there are no double bonds
• Unsaturated compounds - consist of molecules in which one or more carbon-carbon bonds are not single bonds

  • They contain carbon-carbon double bonds (C=C), or carbon -carbon triple bonds
  • e.g. alkenes
• Alkanes - a homogolous series of saturated hydrocarbons with the general formula CnH2n+2

  • They are colourless compounds which have a gradual change in their physical properties as the number of carbon atoms in the chain increases
• Alkanes are generally unreactive compounds but they do undergo combustion reactions - can be cracked into smaller molecules and react with halogens in the presence of light in substitution reactions
• Substitution reaction - one atom (or group of atoms) is swapped with another atom (or group of atoms)
• Photochemical reaction - alkanes undergo a substitution reaction with halogens in the presence of ultraviolet radiation (sunlight is a source of UV radiation)

  • The UV light provides the activation energy, Ea, for the reaction
  • A hydrogen atom is replaced with the halogen atom
  • More than one hydrogen atom can be substituted depending on the amount of ultraviolet radiation there is
• Small alkane properties

  • low boiling points
  • high flammability
  • high cleanliness of flame
  • low viscosity
• Big alkane properties

  • high boiling point
  • low flammability
  • low cleanliness of flame
  • high viscosity
• Wax is a hydrocarbon

  • its gas is flammable
• When a hydrogren burns with oxygen it creates water
• When a hydrocarbon burns w/ oxygen:

  - little oxygen → C

  - more oxygen → CO

  - lots of oxygen - CO2
• Complete combustion - good supply of oxygen

  • alkanes produces CO2 and water
• Incomplete combustion - poor supply of oxygen

  • alkanes produces CO or C and water
• All combustion with alkanes give water & either CO2, CO or C
• Alkanes burn well and release a lot of energy which is why they are used as fuels
• You can use fractions for balancing equations
• Volatile substances are easy to catch on fire which is why they turn into gas easily
• Orange flame means incomplete combustion and blue flame means complete combustion
• Alkane experiment

  A) alkane burning

  B) water liquid

  C) ice water

  D) pump

  E) limewater

  F) cloud (CO2)

  
• Methane is the main constituent of natural gas

  • produced when vegetation decomposes or as waste gases during digestion
• Alkanes are generally unreactive but will burn in oxygen and react with halogens
• Halogent reactions:

  • needs UV light to work
  • turns colourless when light shines on it
• Free Radical - unpaired electrons are highly reactive
• Photochemical reactions with just Cl

  A) UV

  B) Unpaired electron aka free radical

  
• Photochemical reaction with unpaired Cl and Alkanes

  A) new free radical formed

  B) methyl free radical

  
• Chain reaction - chlorine atoms react further and the photochemical process continues until there are no more free radicals
• Substitution reaction - where one atom or group of atoms is replaced by another atom or group of atoms

  • involves light aka photochemical reaction
  • light provides activation energy for reaction
• Substitution reaction involving a chlorine atom also happens in the ozone with the chlorine coming from CFC

  • CFC - chlorofluorocarbon
  • reacts with the ozone and causes holes
• Heaveir alkanes are used as lubricants, waxes/polishes and alkenes
• Sources of methane:

  • found underground
  • produced by decomposition of materials
  • waste gases from digestion in animals
• Nitrogen is the most common gas in our atmosphere

  • doesn't react at lower temperatures
  • reacts at higher temperatures where it will combine with oxygen - forms nitric oxide gas aka NO
  • nitric oxide reacts instantly in contact with air to form nitrogen dioxide
• Nitric oxide is formed in the car engine
• Nitric dioxide (NO2) is a brown poisonous gas
• Catalyst converter - reduces pollution caused by nitrogen oxide by using a catalyst and high temperatures to remove oxide by reacting it with carbon monoxide
• Catalytic converter

  A) unburned hydrocarbons

  B) water vapour

  C) carbon monoxide

  D) nitric oxide

  E) air

  F) catalytic converter

  G) carbon dioxide and water

  H) nitrogen

  I) carbon dioxide

  J) catalyst

  K) platinum, palladium, rhodium

  
• Carbon monoxide is produced in a car engine through incomplete combustion of fossil fuels
• Carbon Monoxide Properties:

  • poisonous
  • colourless
  • odourless
  • tasteless
• The exhaust system of a car rusts more rapidly than the car body as hot gases are being released
• Alkanes have the general formula: HnC2n+2
• Define cracking

  Cracking is the breakdown of alkanes using heat or catalysts to form short-chained molecules