GCSE Chemistry – Products from Oil

     C1b 4.1 Cracking Hydrocarbons

  • We can break down hydrocarbons in a process called cracking
  • Cracking is normally carried out at high temperatures using a catalyst. This is known as catalytic cracking
  • Catalytic cracking takes place in a cat cracker:

    – The fraction produced from crude oil is heated to form a gas
    – The hydrocarbon gas is passed over a hot catalyst where thermal decomposition takes place
    – The larger molecules split apart to form smaller molecules, which are more useful


    C10H22 (decane)   —-(800oC + catalyst)—> C5H12 (pentane) + C3H6 (propene) + C2H4 (ethene)

    • Some hydrocarbons are unsaturated because they have carbon=carbon double bonds. These are called alkenes The main alkenes are shown in the diagram to the left

    C1b 4.2 Polymers

    • We can make chemicals from crude oil which we use to make plastics
    • Plastics are made from huge molecules, which consist of many smaller molecules joined together. The small molecules are monomers, and the larger molecules are called polymers
    • We are able to make many different plastics which all have different properties
    • Ethene (C2H4) is the smallest unsaturated hydrocarbon molecule, which we can turn into a polymer known as poly(ethene) or polythene
    • Propene (C3H6), another alkene can be used to form the polymer poly(propene) or polypropylene
    • Monomers join together when the double bonds in the alkenes ‘open up’ and are replaced by single bonds of thousands of other molecules joining together
    • This reaction is an addition reaction, and since a polymer is made, we call it addition polymerisation

      C1b 4.3 Plastics

      • The atoms in polymer chains are very strong, but the size of the forces between the molecules differs for different plastics. We call these forces between molecules intermolecular forces, and the size of them depends on:
        – the monomer used

        – the conditions we choose to carry out polymerisation

      • In some plastics, the intermolecular forces waken when heated, and the bonds become strong again when cooled. Plastics which behave in this way are thermosoftening plastics
      • Poly(ethene), poly(propene) and poly(chloroethene) [or polyvinylchloride / PVC for short] are all thermosoftening plastics
      • Some bonds are made to be so strong when formed that they cannot be softened. Plastics like this are useful for things such as kettles, and are called thermosetting plastics