Organic chemistry: hydrocarbons, alkenes, alcohols and polymers — Edexcel GCSE Chemistry revision

Crude oil and hydrocarbons

• Crude oil is a mixture of hydrocarbons, mostly alkanes, formed from ancient organic matter over millions of years.
• Fractional distillation separates crude oil into fractions based on boiling point, with shorter chain molecules having lower boiling points.
• Alkanes are saturated hydrocarbons containing only single carbon-carbon bonds, with the general formula CnH2n+2.
• Longer chain hydrocarbons are less flammable, more viscous and have higher boiling points than shorter chain hydrocarbons.

Cracking and alkenes

• Cracking breaks down long-chain, less useful hydrocarbons into shorter, more useful ones, often including alkenes.
• Cracking can be done using heat and a catalyst (catalytic cracking) or high temperature and pressure without a catalyst (steam cracking).
• Alkenes are unsaturated hydrocarbons containing at least one carbon-carbon double bond, with general formula CnH2n.
• Bromine water is used to test for alkenes: it turns from orange to colourless in the presence of a carbon-carbon double bond.

Key terms

Hydrocarbon

A compound made of only hydrogen and carbon atoms.

Alkane

A saturated hydrocarbon with only single bonds, general formula CnH2n+2.

Alkene

An unsaturated hydrocarbon with at least one carbon-carbon double bond, general formula CnH2n.

Fractional distillation

A separation process that splits crude oil into fractions by boiling point.

Cracking

The process of breaking long-chain hydrocarbons into shorter, more useful molecules.

Addition polymerisation

A reaction where many alkene monomers join together to form a polymer with no other products.

Common exam traps

• Alkanes are saturated (single bonds only); alkenes are unsaturated (contain a double bond) — don't mix these up.
• The alkane general formula is CnH2n+2, while alkenes are CnH2n — a common exam slip-up.
• Bromine water turns from orange to colourless with alkenes, not the other way round.

Practice questions with answers

1. 1. What elements make up a hydrocarbon?

   • • Carbon and oxygen
   • • Hydrogen and carbon
   • • Hydrogen and nitrogen
   • • Carbon and chlorine

   Answer: Hydrogen and carbon

   Hydrocarbons are compounds containing only hydrogen and carbon atoms.

2. 2. What is the general formula for alkanes?

   • • CnH2n
   • • CnH2n+2
   • • CnH2n-2
   • • CnHn

   Answer: CnH2n+2

   Alkanes follow the formula CnH2n+2 as they are saturated hydrocarbons.

3. 3. What does the term 'saturated' mean for a hydrocarbon?

   Answer: It contains only single carbon-carbon bonds

   Saturated hydrocarbons have no double or triple bonds between carbon atoms.

4. 4. What separation method is used to divide crude oil into fractions?

   • • Filtration
   • • Fractional distillation
   • • Chromatography
   • • Crystallisation

   Answer: Fractional distillation

   Fractional distillation separates crude oil by differences in boiling point.

5. 5. Why do shorter hydrocarbon chains have lower boiling points?

   Answer: They have weaker intermolecular forces, so less energy is needed to separate the molecules

   Boiling point increases with chain length due to stronger intermolecular forces.

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