CIE IGCSE Chemistry

Revision Notes

14.2.2 Alkenes

Alkenes: Catalytic Cracking & Distinguishing from Alkanes

  • A homologous series of hydrocarbon compounds with carbon-carbon double bonds (C = C)
  • General formula:
CnH2n
  • All alkenes contain a double carbon bond, which is shown as two lines between two of the carbon atoms
  • This is the alkene functional group and is what allows alkenes to react in ways that alkanes cannot
  • Compounds that have a C=C double bond are also called unsaturated compounds

The first three alkenes

The-first-three-Alkenes table, IGCSE & GCSE Chemistry revision notes

 

Manufacture of alkenes and hydrogen

  • Although there is use for each fraction obtained from the fractional distillation of crude oil, the amount of longer chain hydrocarbons produced is far greater than needed
  • These long chain hydrocarbon molecules are further processed to produce other products
  • A process called catalytic cracking is used to convert longer-chain molecules into short-chain and more useful hydrocarbons
  • Alkenes and hydrogen are produced from the cracking of alkanes
  • Kerosene and diesel oil are often cracked to produce petrol, other alkenes and hydrogen

Catalytic cracking explanation:

  • Cracking allows large hydrocarbon molecules to be broken down into smaller, more useful hydrocarbon molecules
  • Fractions containing large hydrocarbon molecules are heated at 600 – 700°C to vaporise them
  • Vapours will then pass over a hot catalyst of silica or alumina
  • This process breaks covalent bonds in the molecules, causing thermal decomposition reactions
  • As a result, cracking produces smaller alkanes and alkenes. The molecules are broken up in a random way which produce a mixture of alkanes and alkenes
  • Hydrogen and a higher proportion of alkenes are formed at temperatures of above 700ºC and higher pressure

 

Cracking-Decane, IGCSE & GCSE Chemistry revision notesThe 10 carbon molecule decane is catalytically cracked to produce octane for petrol and ethene for ethanol

 

Distinguishing between alkanes and alkenes

  • 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:

 

Bromine-Test Alkenes, IGCSE & GCSE Chemistry revision notesDiagram showing the result of the test using bromine water with alkanes and alkenes

 

Explanation:

  • Bromine water is an orange coloured solution of bromine
  • When bromine water is shaken with 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 shaken with an alkene, the alkene will decolourise the bromine water and turn colourless as alkenes do have double carbon bonds (C=C)
  • The bromine atoms add across the C=C double bond hence the solution no longer contains the orange coloured bromine
  • This reaction between alkenes and bromine is called an addition reaction

 

Bromine-Addition-to-Ethene, IGCSE & GCSE Chemistry revision notesEach carbon atom of the double bond accepts a bromine atom, causing the bromine solution to lose its colour

Addition Polymerisation

  • Addition polymers are formed by the joining up of many small molecules called monomers
  • Addition polymerisation only occurs in monomers that contain C=C bonds
  • One of the bonds in each double bond breaks and forms a bond with the adjacent monomer
  • There are many types of polymers that are synthesized from alkene monomers
  • A common example is poly-ethene (polythene) which is the addition of many ethene monomers

 

Forming_Polythene, IGCSE & GCSE Chemistry revision notesPolymerisation of ethene monomers to produce polythene

Extended Only

Further Addition Reactions

  • 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

Bromine-Addition-to-Ethene, IGCSE & GCSE Chemistry revision notesBromine atoms add across the C=C in the addition reaction of ethene and bromine

 

  • Alkenes also undergo addition reactions with hydrogen in which an alkane is formed
  • These are hydrogenation reactions and occur at 150ºC using a nickel catalyst
  • Hydrogenation reactions are used to change vegetable oils into margarine to be sold in supermarkets

 

Addition- Hydrogen to Ethene, IGCSE & GCSE Chemistry revision notesHydrogen atoms add across the C=C in the hydrogenation of ethene to produce an alkane

 

  • Alkenes also undergo addition reactions with steam in which an alcohol is formed. Since water is being added to the molecule it is also called a hydration reaction
  • The reaction is very important industrially for the production of alcohols and it occurs using the following conditions:
    • Temperature of around 330ºC
    • Pressure of 60 – 70 atm
    • Concentrated phosphoric acid catalyst

 

Addition- Water to Ethene, IGCSE & GCSE Chemistry revision notesA water molecule adds across the C=C in the hydration of ethene to produce ethanol

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.
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