alkanes

• combustion 


  combustion is when a substance burns in oxygen to produce heat energy
• incomplete combustion

  when a substance burns with a limited supply of oxygen and forms carbon (soot), carbon monoxide, and water vapour
• complete combustion 


  when a substance burns completely in oxygen and forms carbon dioxide and water
• What are alkanes?

  • main component of natural gas and crude oil
  • one of the most stable and least reactive amongst organic compounds
  • mainly used as fuels, reacting with oxygen to generate heat
• what is the general formula for an alkane

  CnH2n+2
• Bonding in alkanes

  -saturated hydrocarbons, the carbon and hydrogen atoms are joined by single covalent bonds

  -Sigma bonds (each carbon connected to 4 other atoms in a single covalent bond)
• Covalent bonding in alkanes

  • a sigma bond is the result of two overlapping orbitals from each bonding atom
  • each overlapping orbital has an electron, meaning the sigma bond has two electrons shared between the bonding atoms
  • sigma line positioned directly between bonding atoms
• Shapes of alkanes

  • each carbon atom is surrounded by 4 electron pairs in four sigma bonds
  • repulsion between these pairs results in a 3D tetrahedral arrangement around each carbon atom
  • each bond angle is approximately 109.5 degrees
  • the sigma bonds acts as axes around with the atoms can rotate freely, they are not rigid
• Boiling points of alkanes

  • crude oil is separated into fractions by fractional distillation in a distillation tower
  • boiling points increase as the chain length increases
  • the greater the intermolecular forces- the greater the boiling points
• effect of chain length on boiling point

  • London forces occur between molecules that are in close surface contact
  • In longer chains, there is a higher surface area so more surface contact is possible between molecules
  • London forces between molecules will be greater and more energy will be required to overcome those forces
• Effect of branching on boiling point

  • Isomers of alkanes have the same molecular mass
  • branched alkanes have lower boiling points as there are fewer surface points of contact needed to make london forces
  • branches get in the way of branched molecules, preventing them from being closer together, further decreasing intermolecular forces