Cracking- a chemical reaction which produces smaller hydrocarbons, including alkanes and alkenes.
Hydrocarbons- a group of compounds containing hydrogen and carbon.
Polymers- large molecules formed from many identical smaller molecules (monomers)
- Feuls made from oil mixtures containing large hydrocarbons are often not efficient because they are difficult to ignite or do not flow easily.
- Often, crude oil contains too many large hydrocarbon molecules and not enough small ones to meet demand.
- Cracking allows large molecules to be broken down into smaller and more useful hydrocarbon molecules.
- Fractions large hydrocarbon molecules are heated to vapourise them, then the vapour is either passed over a hot catalyst or mixed with steam and heated to a very high temperature.
- This breaks chemical bonds in the molecules (thermal decomposition). Cracking produces smaller alkanes and alkenes.
- These smaller hydrocarbons can now be used as feuls and the alkenes are used to make polymers in plastics manufacture.
- The products of cracking include alkenes (eg ethene and propene).
- The alkenes are a family of hydrocarbons that share the same general formula:CnH2n
- The number of hydrogen atoms in an alkene is double the number of carbon atoms. For example, ethene is C2H4 and propene is C3H6.
- Alkenes are unsaturated hydrocarbons. They contain a double covalent bond, which is shown as two lines between two of the carbon atoms.
- The presence of this double bond allows alkenes to react in ways that alkanes cannot.
- They can react with oxygen in the air, so they could be used as fuels. But they are more useful than that: they can be used to make ethanol and polymers (plastics)