What you'll learn
This revision guide covers the essential organic chemistry topics you need for Edexcel GCSE Chemistry, including hydrocarbons, crude oil fractionation, alkanes and alkenes, addition and condensation polymerisation, and the properties of synthetic materials. You'll develop the skills to name compounds, write balanced equations, and explain how polymers are formed and recycled.
Key terms and definitions
Hydrocarbon — a compound containing only hydrogen and carbon atoms
Homologous series — a family of organic compounds with the same general formula and similar chemical properties
Saturated hydrocarbon — a hydrocarbon containing only single carbon-carbon bonds (alkanes)
Unsaturated hydrocarbon — a hydrocarbon containing at least one carbon-carbon double bond (alkenes)
Monomer — a small molecule that can join with other monomers to form a polymer chain
Polymer — a large molecule made from many repeating monomer units joined together
Addition polymerisation — a process where unsaturated monomers join together without losing any atoms to form a polymer
Condensation polymerisation — a process where monomers join together by eliminating small molecules such as water
Core concepts
Crude oil and fractional distillation
Crude oil is a finite resource formed from the remains of ancient marine organisms subjected to high temperature and pressure over millions of years. It consists of a mixture of hydrocarbons of different chain lengths.
Fractional distillation separates crude oil into useful fractions based on boiling points:
- Crude oil is heated to approximately 350°C and vaporised
- Vapours enter the fractionating column, which has a temperature gradient (hot at bottom, cool at top)
- Hydrocarbons condense at different heights depending on their boiling points
- Short-chain hydrocarbons (low boiling points) exit near the top
- Long-chain hydrocarbons (high boiling points) exit near the bottom
Common fractions and their uses:
- Gases (e.g. methane, propane) — domestic heating and cooking
- Petrol — fuel for cars
- Kerosene — aircraft fuel
- Diesel oil — fuel for diesel engines
- Fuel oil — fuel for large ships and power stations
- Bitumen — surfacing roads and roofs
Properties of hydrocarbons change with chain length:
- Longer chains → higher boiling point, higher viscosity, less flammable
- Shorter chains → lower boiling point, lower viscosity, more flammable
Alkanes: structure and properties
Alkanes are saturated hydrocarbons with the general formula CₙH₂ₙ₊₂. They contain only single covalent bonds between carbon atoms.
The first four alkanes you must know:
- Methane — CH₄
- Ethane — C₂H₆
- Propane — C₃H₈
- Butane — C₄H₁₀
Alkanes undergo complete combustion in excess oxygen, producing carbon dioxide and water:
Methane + oxygen → carbon dioxide + water
CH₄ + 2O₂ → CO₂ + 2H₂O
In limited oxygen, incomplete combustion occurs, producing carbon monoxide (a toxic gas) or carbon (soot), plus water:
2CH₄ + 3O₂ → 2CO + 4H₂O
CH₄ + O₂ → C + 2H₂O
Carbon monoxide is dangerous because it binds irreversibly to haemoglobin in red blood cells, preventing oxygen transport.
Cracking and alkenes
Longer hydrocarbon chains from crude oil can be broken down into shorter, more useful molecules by cracking. This process converts less useful fractions into petrol and produces alkenes for making polymers.
Catalytic cracking:
- Long-chain hydrocarbons are heated to vaporise them
- Vapours are passed over a hot aluminium oxide or silicon dioxide catalyst (approximately 600-700°C)
- Thermal decomposition occurs, breaking C-C bonds
- Products include shorter alkanes and alkenes
Steam cracking:
- Long-chain hydrocarbons are mixed with steam
- Heated to very high temperatures (over 800°C)
- No catalyst required
Example cracking equation:
C₁₀H₂₂ → C₈H₁₈ + C₂H₄
(decane → octane + ethene)
Alkenes are unsaturated hydrocarbons with the general formula CₙH₂ₙ. They contain a carbon-carbon double bond (C=C).
The first four alkenes:
- Ethene — C₂H₄
- Propene — C₃H₆
- Butene — C₄H₈
- Pentene — C₅H₁₀
The C=C double bond makes alkenes more reactive than alkanes. They undergo addition reactions where the double bond opens and atoms add across it.
Testing for alkenes:
Alkenes decolourise bromine water (orange/brown) to colourless. The bromine adds across the double bond:
C₂H₄ + Br₂ → C₂H₄Br₂
Alkanes do not react with bromine water under normal conditions.
Addition polymerisation
Addition polymerisation involves many small alkene monomers joining together to form a long polymer chain. The C=C double bond opens up and monomers link together.
Common addition polymers:
Poly(ethene) — made from ethene monomers
- Low-density poly(ethene): flexible, used for plastic bags and bottles
- High-density poly(ethene): rigid, used for water pipes and containers
Poly(propene) — made from propene monomers
- Used for rope, carpets, food containers
Poly(chloroethene) or PVC — made from chloroethene monomers
- Used for window frames, water pipes, insulation on electrical wires
Poly(tetrafluoroethene) or PTFE (Teflon) — made from tetrafluoroethene
- Non-stick coating for pans, low-friction applications
To draw the repeating unit of an addition polymer:
- Draw the monomer with its C=C double bond
- Open the double bond to a single bond
- Add continuation bonds on each side
- Place the structure in square brackets with subscript n
Example: ethene → poly(ethene)
Monomer: H₂C=CH₂
Polymer repeating unit: [—CH₂—CH₂—]ₙ
Condensation polymerisation
Condensation polymerisation occurs when monomers join together with the elimination of a small molecule, usually water. Monomers must have two functional groups to form long chains.
Polyesters form when dicarboxylic acids react with diols:
- Dicarboxylic acid — molecule with two -COOH groups
- Diol — molecule with two -OH groups
- Each condensation reaction releases one water molecule
- Forms an ester link (-COO-)
Example: formation of polyester from ethanedioic acid and ethanediol
The ester link connects the monomers: —OC(=O)—
Polyamides (nylon, Kevlar) form when dicarboxylic acids react with diamines:
- Diamine — molecule with two -NH₂ groups
- Each condensation reaction releases one water molecule
- Forms an amide link (-CONH-)
Nylon is used for clothing, ropes, parachutes and carpets due to its strength and flexibility.
Properties and uses of polymers
Polymer properties depend on:
Chain length — longer chains mean stronger intermolecular forces, higher melting points
Branching — unbranched chains pack together more closely, creating stronger materials
Cross-linking — covalent bonds between chains create rigid, heat-resistant structures
Crystallinity — regular arrangement of chains creates stronger, less flexible materials
Thermosoftening polymers (thermoplastics):
- Weak intermolecular forces between polymer chains
- Soften when heated, harden on cooling
- Can be remoulded repeatedly
- Examples: poly(ethene), poly(propene), PVC
Thermosetting polymers (thermosets):
- Strong covalent cross-links between polymer chains
- Do not soften when heated
- Cannot be remoulded once set
- Examples: melamine resin, epoxy resins
Environmental issues and recycling
Problems with polymer waste:
- Most polymers are non-biodegradable (do not break down naturally)
- Accumulate in landfills and oceans
- Burning produces carbon dioxide (greenhouse gas) and may release toxic gases
- Made from crude oil, a finite resource
Recycling strategies:
Mechanical recycling:
- Polymers sorted by type
- Washed, shredded and melted
- Remoulded into new products
- Only works for thermosoftening polymers
Chemical recycling:
- Polymers broken down into monomers or other useful chemicals
- Monomers can be repolymerised
- Works for some condensation polymers through hydrolysis
Advantages of recycling:
- Conserves crude oil resources
- Reduces landfill waste
- Less energy required than making new polymers
- Reduces carbon dioxide emissions
Challenges:
- Collection and sorting is expensive
- Contaminated polymers difficult to recycle
- Recycled polymers sometimes lower quality
- Some polymers cannot be recycled economically
Biodegradable and compostable polymers:
Some modern polymers are designed to break down:
- Biodegradable polymers — broken down by microorganisms
- Made from plant materials (starch) or specially designed synthetic polymers
- Reduce environmental persistence
- May require specific composting conditions
Worked examples
Example 1: A student cracks decane (C₁₀H₂₂) and produces octane and one other product.
(a) Write a balanced equation for this cracking reaction. [2 marks]
(b) State one condition needed for catalytic cracking. [1 mark]
(c) Describe a chemical test to distinguish the products. [2 marks]
Solution:
(a) C₁₀H₂₂ → C₈H₁₈ + C₂H₄ [1 mark for correct products, 1 mark for balancing]
(b) High temperature (600-700°C) OR aluminium oxide/silicon dioxide catalyst [1 mark]
(c) Add bromine water to both products [1 mark]. Ethene decolourises bromine water from orange to colourless, octane does not react [1 mark].
Example 2: Poly(propene) is made by addition polymerisation.
(a) Draw the displayed formula of propene. [1 mark]
(b) Draw the repeating unit of poly(propene). [2 marks]
(c) Explain why poly(propene) is non-biodegradable. [2 marks]
Solution:
(a) H₂C=CH—CH₃ with all bonds shown [1 mark]
(b) [—CH₂—CH(CH₃)—]ₙ with continuation bonds and square brackets [2 marks: 1 for correct structure, 1 for brackets and bonds]
(c) The carbon-carbon bonds in the polymer chain are very strong/stable [1 mark]. Microorganisms do not produce enzymes that can break down these bonds [1 mark].
Example 3: A polyester is formed from a dicarboxylic acid and a diol.
(a) Name the type of polymerisation. [1 mark]
(b) Name the small molecule eliminated during polymerisation. [1 mark]
(c) State one use of polyester. [1 mark]
Solution:
(a) Condensation polymerisation [1 mark]
(b) Water / H₂O [1 mark]
(c) Clothing / fabrics / bottles (any one sensible use) [1 mark]
Common mistakes and how to avoid them
Confusing addition and condensation polymerisation: Remember that addition uses alkene monomers and nothing is lost; condensation uses monomers with two functional groups and eliminates small molecules (usually water).
Incorrectly drawing polymer repeating units: Always include continuation bonds extending from both sides, use square brackets, and add subscript n. Don't draw the C=C double bond in addition polymer repeating units.
Forgetting to balance cracking equations: The number of carbon and hydrogen atoms must be the same on both sides. Count carefully and check your working.
Stating alkanes react with bromine water: Only alkenes react with bromine water at room temperature. Alkanes are unreactive under these conditions due to their saturated structure.
Mixing up thermosoftening and thermosetting polymers: Thermosoftening polymers soften when heated (weak intermolecular forces); thermosetting polymers do not soften (strong covalent cross-links between chains).
Not explaining environmental issues fully: When discussing polymer disposal, explain both the problem (non-biodegradable, landfill accumulation) and the chemical reason (strong C-C bonds cannot be broken down by microorganisms).
Exam technique for "Organic Chemistry, Polymers and Materials"
Command word awareness: "State" requires a simple fact (1 mark); "Explain" requires a reason or mechanism (2+ marks); "Compare" requires you to discuss both similarities and differences between substances.
Drawing organic structures: Use displayed formulae when asked to show bonds. Ensure all bonds are shown clearly. For polymers, always include continuation bonds, square brackets and subscript n to gain full marks.
Linking structure to properties: When explaining polymer properties, explicitly connect molecular structure (chain length, branching, cross-linking) to physical properties (melting point, strength, flexibility). Use comparative language for higher marks.
Extended response questions: Structure answers logically with one point per sentence. For 6-mark questions on polymers and the environment, cover production, use, disposal problems, and recycling/alternatives for balanced coverage.
Quick revision summary
Crude oil is separated by fractional distillation into useful hydrocarbon fractions. Alkanes (CₙH₂ₙ₊₂) are saturated; alkenes (CₙH₂ₙ) are unsaturated and decolourise bromine water. Cracking breaks long chains into shorter alkanes and alkenes. Addition polymerisation joins alkene monomers without loss of atoms. Condensation polymerisation joins monomers with two functional groups, eliminating water. Thermosoftening polymers soften when heated; thermosetting polymers do not. Polymer disposal creates environmental problems, but recycling and biodegradable alternatives offer solutions.