Alkenes (Cambridge (CIE) IGCSE Co-ordinated Sciences (Double Award)): Revision Note

Catalytic cracking

• Alkenes are unsaturated hydrocarbons

  • This means that they contain at least one carbon-carbon double bond (C=C)

• The bonding in alkenes is covalent, as they consist only of non-metal atoms

• Their general formula is C_nH_2n

Table of alkenes

Displayed formula	Name	Molecular formula
	ethene	C2H4
	propene	C3H6
	but-1-ene	C4H8
	pent-1-ene	C5H10

The first four members of the alkene homologous series

Why alkenes are more reactive than alkanes

• The carbon–carbon double bond can open up and form new bonds.

• This allows alkenes to react more easily than alkanes, which only contain strong single covalent bonds.

Manufacture of alkenes and hydrogen (cracking)

Extended tier only

• Although fractional distillation produces useful fractions, it produces:

  • Too many long chain hydrocarbons

  • Not enough short-chain hydrocarbons, such as petrol and alkenes

• To solve this, long-chain alkane molecules are broken down into smaller, more useful molecules in a process called cracking

What is cracking?

• Cracking is the thermal decomposition of long-chain alkanes into:

  • Shorter-chain alkanes

  • One or more alkenes

  • Hydrogen

• Kerosene and diesel oil are often cracked to produce petrol, other alkenes and hydrogen

Conditions for cracking

• Cracking requires:

  • A high temperature (around 600–700 °C)

  • A catalyst, usually aluminium oxide (alumina) or silicon dioxide (silica)

What happens during cracking

• Long-chain alkane vapours are heated

• The vapours pass over the hot catalyst

• Carbon–carbon bonds break

• A mixture of smaller alkanes, alkenes, and hydrogen is formed

Example of cracking

• A long-chain alkane can be cracked to form an alkene and a shorter alkane:

decane → octane + ethene

C₁₀H₂₂ → C₈H₁₈ + C₂H₄

Why cracking is important

• Alkenes are used to make polymers

• Hydrogen is used in processes such as ammonia manufacture

• It increases the supply of useful fuels like petrol

Distinguishing from alkanes

• The presence of a C=C double bond allows alkenes to be identified using aqueous bromine

• Aqueous bromine, Br₂ (aq), is an orange solution of bromine dissolved in water

  • It is often called bromine water

Testing with aqueous bromine

• A few drops of aqueous bromine are added to the sample

• The sample is then shaken

Results

• Alkane + aqueous bromine

  • No colour change

  • The solution remains orange

• Alkene + aqueous bromine

  • Aqueous bromine is decolourised

  • Colour change: orange → colourless

• This happens because bromine reacts with the C=C double bond and is no longer present in solution.