Cracking Hydrocarbons

  • Saturated molecules contain single bonds only whereas unsaturated molecules contain double bonds between their carbon atoms.
  • Alkanes are saturated compounds and alkenes are unsaturated compounds.
  • Alkenes are generally more desirable than alkanes as they are more reactive due to the presence of the carbon carbon double bond, so they can take part in reactions in which alkanes cannot, making them more desirable than alkanes.
  • They are used to make polymers and are the starting materials for the production of many other chemicals.
  • Long chain alkane molecules are further processed to produce other products consisting of smaller chain molecules.
  • A process called cracking is used to convert them into short chain molecules which are more useful.
  • Small alkenes and hydrogen are produced using this process.
  • Kerosene and diesel oil are often cracked to produce petrol, other alkenes and hydrogen.
  • There are two methods used to crack alkanes: catalytic cracking and steam cracking.
  • As the names suggest, one method uses a catalyst and the other uses steam.

Catalytic & Steam Cracking

Cracking-Decane, IGCSE & GCSE Chemistry revision notes

Decane is cracked to produce octane for petrol and ethene for ethanol

  • Catalytic cracking involves heating the hydrocarbon molecules to around 600 – 700°C to vaporise them.
  • The vapours then pass over a hot powdered catalyst of aluminium oxide.
  • This process breaks covalent bonds in the molecules as they come into contact with the surface of the catalyst, causing thermal decomposition reactions.
  • The molecules are broken up in a random way which produces a mixture of smaller alkanes and alkenes.
  • Hydrogen and a higher proportion of alkenes are formed at temperatures of above 700ºC and higher pressure.
  • In steam cracking the same process is used.
  • The vaporised hydrocarbons are mixed with steam and heated to a high temperature which induces cracking.

Alkenes

  • Alkenes are a homologous series of hydrocarbon compounds with at least one double bond between two of the carbon atoms on the chain.
  • The double bond can be written as carbon carbon double bond or as C=C.
  • The general formula for alkenes is:

CnH2n

  • E.g. a straight chain alkene with 8 carbons has 8 x 2 = 16 hydrogen atoms, so it has twice as many hydrogens as carbons.

Combustion of Alkenes

  • These compounds undergo complete and incomplete combustion but because of the carbon carbon double they tend to undergo incomplete combustion, producing a smoky flame in air.
  • Complete combustion occurs when there is excess oxygen so water and carbon dioxide form e.g:

C4H8+ 6O2→ 4CO2+ 4H2O

  • Incomplete combustion occurs when there is insufficient oxygen to burn so a mixture of products can form, e.g:

C4H8+ 5O2→ 2CO + 4H2O + 2CO2

CH4+ 3O2→ 2C + 2H2O + 2CO

Bromination of Ethene

  • Alkenes undergo addition reactions in which atoms of a simple molecule add across the C=C double bond.
  • The reaction between bromine and ethene is an example of an addition reaction .
  • The same process works for any halogen and any alkene in which the halogen atoms always add to the carbon atoms involved in the C=C double bond.

Bromine-Addition-to-Ethene, IGCSE & GCSE Chemistry revision notes

Bromine atoms add across the C=C in the addition reaction of ethene and bromine

Bromine Water Test

  • Alkanes and alkenes have different molecular structures.
  • All alkanes are saturated and alkenes are unsaturated.
  • The presence of the C=C double bond allows alkenes to react in ways that alkanes cannot.
  • This allows us to tell alkenes apart from alkanes using a simple chemical test called the bromine water test.

Bromine-Test Alkenes, IGCSE & GCSE Chemistry revision notes

Diagram showing the result of the test using bromine water with alkanes and alkenes

  • Bromine water is an orange coloured solution.
  • When bromine water is added to an alkane, it will remain as an orange solution as alkanes do not have double carbon bonds (C=C) so the bromine remains in solution.
  • But when bromine water is added to an alkene, the bromine atoms add across the C=C bond, hence the solution no longer contains bromine so it loses its colour.

Writing Equations for Cracking

  • We can use the general formulae for alkanes and alkenes to check that we have correctly written out equations for cracking.
  • Hexane for example, can be cracked to form butane and ethene, both of which are very useful molecules.
  • Ethene as the starting material for the production of alcohol and butane as a fuel.
  • The equation for this reaction is:

C6H14 ⟶ C4H10 + C2H4

  • Note that the starting compound for this reaction is an alkane and thus the general formula CnH2n+2 applies.
  • Butane is also an alkane and so the same rule applies.
  • Ethene is an alkene and so its formula will follow the C2H2n rule.

AQA GCSE Chemistry Notes

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Morgan Curtin Chemistry

Author: Morgan

Morgan’s passion for the Periodic Table begun on his 10th birthday when he received his first Chemistry set. After studying the subject at university he went on to become a fully fledged Chemistry teacher, and now works in an international school in Madrid! In his spare time he helps create our fantastic resources to help you ace your exams.