Organic Chemistry

• crude oil is a mixture of hydrocarbons, which are long chains of hydrogen and carbon atoms
• alkenes have double bonds between the carbons
• alkanes have single bonds between the carbons
• hydrocarbon = hydro + carbon
• alkanes have single covalent bonds between the carbons
• hydrocracking breaks down large molecules into smaller ones by using heat and pressure with hydrogen gas
• the cracking process produces alkenes that can be used to make plastics
• hydrocarbon fuels can be used as fuel because they contain lots of energy stored within their chemical bonds
• hydrocarbon fuels can be used as fuel because they contain lots of energy stored within their chemical bonds
• the more complex the molecule, the higher its boiling point
• the general formula for alkanes is CnH2n+2
• the general formula for alkenes is CnH2n
• alkanes can be named using the prefix meth-, eth-, prop- etc.
• alkanes can be represented by the general formula CnH2n+2
• the first four alkanes are methane (CH4), ethane (C2H6), propane (C3H8) and butane (C4H10)
• the cracking process produces alkenes as well as other products such as methane (CH4), ethylene (C2H4) and propene (C3H6)
• Crude oil is an important source of:

  • fuels such as petrol, diesel, kerosene, heavy fuel oil and liquefied petroleum gases
• The alkanes form a homologous series. Like all homologous series, the alkanes:

  • have the same general formula
  • differ by CH2 in molecular formulae from neighbouring compounds
  • show a gradual variation in physical properties, such as their boiling points
  • have similar chemical properties
• The molecular formula for Methane is CH4.
• The molecular formula for ethane is C2H6.
• The molecular formula for propane is C3H8
• The molecular formula for Butane is C4H10
• The alkanes are saturated hydrocarbons:

  • hydrocarbons, because they are compounds containing hydrogen and carbon only
  • saturated, because their carbon atoms are joined by C-C single bonds
• During the fractional distillation of crude oil:

  • heated crude oil enters a tall fractionating column, which is hot at the bottom and gets cooler towards the top
  • vapours from the oil rise through the column
  • vapours condense when they become cool enough
  • liquids are led out of the column at different heights
• Small hydrocarbon molecules have weak intermolecular forces, so they have low boiling points. They do not condense, but leave the column as gases
• Long hydrocarbon molecules have stronger intermolecular forces, so they have high boiling points. They leave the column as hot liquid bitumen.
• The different, useful mixtures are called fractions. This is because they are only part of the original crude oil
• Alkanes have similar: boiling points, melting points, and viscosity
• Hydrocarbon fuels can undergo complete combustion or incomplete combustion, depending on the amount of oxygen available
• Complete combustion of a hydrocarbon fuel happens when there is a good supply of air. Carbon and hydrogen atoms in the fuel react with oxygen in an exothermic reaction:

  • carbon dioxide and water are produced
  • the maximum amount of energy is given out
• hydrocarbon + oxygen → carbon dioxide + water
• Incomplete combustion happens when the supply of air or oxygen is poor. Water is still produced, but carbon monoxide and carbon are produced. Less energy is released than during complete combustion.
• Cracking is a reaction in which larger saturated hydrocarbon molecules are broken down into smaller, more useful hydrocarbon molecules, some of which are unsaturated:

  • the original starting hydrocarbons are alkanes
  • the products of cracking include alkanes and alkenes, members of a different homologous series
• Various methods can be used for cracking, eg catalytic cracking and steam cracking
• • Catalytic cracking uses a temperature of approximately 550°C and a catalyst known as a zeolite which contains aluminium oxide and silicon oxide
• • Steam cracking uses a higher temperature of over 800°C and no catalyst
• Cracking is important for two main reasons:

  1. It helps to match the supply of fractions with the demand for them.
  2. It produces alkenes, which are useful as feedstock for the petrochemical industry
• A saturated hydrocarbon contains no carbon-to-carbon double bonds, only single bonds
• • alkanes are saturated, their carbon atoms are only joined by C-C single bonds
• • alkenes are unsaturated, they contain at least one C=C double bond