Cracking and Alkenes

Crude oil is a complex mixture of hydrocarbons. Two types of hydrocarbons are alkanes and alkenes.

  • Alkanes are saturated hydrocarbons, which means that there are no double bonds in the structure. There are only single covalent bonds between the carbon atoms. The general formula for alkanes is CnH2n+2
  • Alkenes are unsaturated hydrocarbons, which means they have at least one double bond between two carbon atoms. The general formula for alkenes is CnH2n

Shorter hydrocarbons tend to be more useful than longer ones; therefore, longer-chain hydrocarbons undergo a process called cracking to produce shorter chains.

Cracking is the process of breaking down long-chain saturated hydrocarbon molecules into smaller, more useful molecules. One example of this is the cracking of decane into octane and ethene.

A decane molecule with an arrow labelled "heat" going downwards to an octane molecule and an ethene molecule.

Two methods for carrying out cracking are:

  • Catalytic cracking – This involves using a high temperature and a catalyst to speed up the reaction.
  • Steam cracking – In this method, hydrocarbons are mixed with steam and heated to a high temperature, but without the use of a catalyst.

Cracking is important due to the higher demand for shorter-chain hydrocarbons. These shorter hydrocarbons, being more flammable, serve as more effective fuels than longer-chain hydrocarbons. For example, cars require fuel that burns quickly, allowing them to change speeds rapidly.

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