Organic chemistry

• hydrocarbon = compound of hydrogen and carbon only
• homologous series = series of compounds with the same general formula and similar properties
• functional group = group of atoms responsible for the chemical reactions of a compound
• isomerism = compounds with the same molecular formula exist in different forms due to different arrangements of atoms
• Any molecule with 1 carbon has the prefix of meth-

  • eg methane, methene
• 2 carbons = prefix eth-
• 3 carbons = prop-
• 4 carbons = but-
• 5 carbons = pent-
• 6 carbons = hex-
• Crude oil is a mixture of substances, most of which are hydrocarbons
• crude oil is a finite source and will eventually run out
• air & seawater are other examples of mixtures
• fractional distillation separates a mixture of liquids into two or more fractions

  • as crude oil is a mixture of hydrocarbons, it has to be separated into different fractions
• fractional distillation with crude oil:

  • crude oil heated to make vaporise
  • mixture of vapours go into column at bottom - column is hot at bottom & cooler at top
  • vapours cool as they rise up column
  • different fractions condense & are collected at different temperatures = shorter, lighter gases collect at the top
  • gases condense at their boiling point
• at the bottom of fractioning column = length of hydrocarbons chains increases, boiling point & viscosity also increases

  • flammability & volatility decreases
  • fraction is darker
  • there are longer, heavier chains of carbon
• Fractional distillation

  • shorter, lighter gases collect on top - eg c1, c4
  • heavier, longer carbon chains collect at the bottom

  
• 6 fractions of crude oil:

  • c70 residue = bitumen (used in roads & roofing)
  • c20-c70 = fuel oil (ships, factories, heating)
  • c14-c20 = diesel oils (diesel fuels)
  • c10-c16 = kerosine (jet fuel, paraffin for lighting)
  • c5-c10 = petrol, gasoline
  • c1-c4 = refinery (liquefied petroleum gas)
• as hydrocarbon chain length increases

  • colour gets darker
  • boiling point increases
  • viscosity (thickness) increases
  • flammability decreases
  • volatility (change from liq to gas) decreases
• fuel is a substance that releases heat energy when burned
• Complete combustion happens when there is plenty of air

  • hydrocarbon + oxygen —> carbon dioxide + water + heat energy
• incomplete combustion happens when air is restricted

  • hydrocarbon + oxygen —> carbon monoxide + carbon + water + less heat energy
• Cracking is a process to break large molecules into smaller ones which are more useful

  • due to supply & demand for different fractions
  • it is a thermal decomposition reaction
• products of cracking are alkanes & alkenes
• conditions needed for cracking:

  • heat (600-700C)
  • catalyst is silica (SiO2) or alumina (Al2O2)
• Combustion practical:

  
• limewater turns cloudy when carbon dioxide is produced

  • this shows that complete combustion has taken place
• cracking forms alkanes & alkenes

  
• Cracking paraffin oil practical

  
• hazard of cracking paraffin oil:

  • the delivery tube must be lifted out of the water the moment heating is stopped
  • otherwise the water will be sucked up into the hot test tube & it could break
• Reactant (paraffin oil):

  • appearance = thick colourless liquid
  • smell = no smell
  • flammability = difficult to burn
  • reactions = few chemical reactions
• product of cracking (alkanes & alkenes):

  • appearance = colourless gas
  • smell = pungent smell
  • flammability = burns readily
  • reactions = many chemical reactions
• disadvantages of burning hydrocarbons:

  • when a car engine reaches a high temperature, nitrogen & oxygen from the air react to form oxides of nitrogen (No2, NO) = mix with rain water to form nitric acid (acid rain)
  • sulphur impurities in hydrocarbon fuels react to form sulphur dioxide (SO2) = sulphur dioxide reacts with rain water to form sulphuric acid (acid rain)
  • causes damage to wildlife
• N2 + O2 (in air) —hot engine—> oxides of nitrogen + rain water

  • nitric acid = acid rain (HNO3)
• S + O2 (in fuel)——> SO2 + rain water

  • sulphuric acid = acid rain (H2SO4)
• Alkanes are a homologous series

  • This means they can be represented by the same general formula; have the same functional group & have similar chemical properties
  • Alkane formula = CnH2n+2
• alkanes are classified as saturated hydrocarbons as they have single bonds only
• functional group = a group of atoms that determine how a compound reacts
• alkanes react with halogens in the presence of UV light to form haloalkanes & hydrogen halides

  • this is called a substitution reaction as a chlorine atom takes the place of a hydrogen atom
  • CH4 (methane) + Cl2 —light—> CH3Cl (chloromethane) + HCl
  • if there is enough chlorine, all four hydrogen atoms can be substituted by chlorine atoms
• alkenes are a homologous series

  • represented by formula CnH2n
  • alkenes contain the functional group >C=C<
  • classified as unsaturated hydrocarbons because they can make more bonds - there can be more than one double bond & they can open up