Alkenes and Addition Polymers

This lesson covers alkenes, the C=C double bond, testing with bromine water, and addition polymerisation as required by the Edexcel GCSE Combined Science specification (1SC0). You need to understand the properties of alkenes, how to test for unsaturation, and how polymers such as polyethene and PVC are formed.

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What Are Alkenes?

Alkenes are a homologous series of hydrocarbons that contain at least one carbon–carbon double bond (C=C). This makes them unsaturated hydrocarbons.

General Formula

The general formula for alkenes is:

CₙH₂ₙ

The First Three Alkenes

Name	Molecular Formula	Structure
Ethene	C₂H₄	H₂C=CH₂
Propene	C₃H₆	CH₃CH=CH₂
Butene	C₄H₈	CH₃CH₂CH=CH₂ (but-1-ene)

Key Differences Between Alkanes and Alkenes

Feature	Alkanes	Alkenes
General formula	CₙH₂ₙ₊₂	CₙH₂ₙ
Bonding	Single C–C bonds only	Contains C=C double bond
Saturated / Unsaturated	Saturated	Unsaturated
Reactivity	Relatively unreactive	More reactive (due to C=C)
Bromine water test	Stays orange	Turns colourless

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Testing for Alkenes — The Bromine Water Test

This is an important test that distinguishes alkenes from alkanes.

Method

1. Add a few drops of orange bromine water to the substance being tested.
2. Shake the mixture.

Results

Substance	Observation	Conclusion
Alkene	Bromine water turns from orange to colourless	Unsaturated — C=C double bond present
Alkane	Bromine water stays orange	Saturated — no C=C double bond

The decolourisation occurs because bromine adds across the C=C double bond in an addition reaction:

C₂H₄ + Br₂ → C₂H₄Br₂ (1,2-dibromoethane)

Exam Tip: The bromine water test is the standard test for unsaturation (the presence of a C=C double bond). If the orange colour disappears, the compound is unsaturated.

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Addition Reactions

The C=C double bond in alkenes makes them much more reactive than alkanes. The double bond can "open up" and allow other atoms to add across it. This is called an addition reaction.

In an addition reaction:

• The C=C bond breaks open.
• Two new bonds form (one to each carbon).
• Only one product is formed (no other by-products).

Example: Ethene + Bromine

C₂H₄ + Br₂ → C₂H₄Br₂

Example: Ethene + Hydrogen (Hydrogenation)

C₂H₄ + H₂ → C₂H₆ (ethane)

This is used industrially to convert vegetable oils (unsaturated) into margarine (saturated).

Example: Ethene + Steam (Hydration)

C₂H₄ + H₂O → C₂H₅OH (ethanol)

This is how ethanol is produced industrially.

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Addition Polymerisation

Polymerisation is the process of joining many small molecules (monomers) together to form a very large molecule called a polymer.

In addition polymerisation, many alkene monomers join together. The C=C double bonds open up and the molecules link together in a long chain.

How It Works

1. Many alkene monomers (e.g. ethene) are placed under high pressure and high temperature with a catalyst.
2. The C=C double bond in each monomer opens up.
3. The monomers join end-to-end to form a long chain polymer.
4. No other molecule is produced — the only product is the polymer.

Polyethene (Polythene)

The most common addition polymer. The monomer is ethene (C₂H₄):

n(C₂H₄) → (–CH₂–CH₂–)ₙ

or written as:

n CH₂=CH₂ → (–CH₂–CH₂–)ₙ

graph LR
    M1["CH₂=CH₂"] --> M2["CH₂=CH₂"] --> M3["CH₂=CH₂"]
    M3 --> P["—CH₂—CH₂—CH₂—CH₂—CH₂—CH₂—<br/>Poly(ethene)"]

    style M1 fill:#3498db,color:#fff
    style M2 fill:#3498db,color:#fff
    style M3 fill:#3498db,color:#fff
    style P fill:#e74c3c,color:#fff

Poly(chloroethene) — PVC

The monomer is chloroethene (CH₂=CHCl):

n(CH₂=CHCl) → (–CH₂–CHCl–)ₙ

PVC is used for window frames, pipes, electrical cable insulation and clothing.

Other Common Addition Polymers

Polymer	Monomer	Uses
Poly(ethene) / polyethene	Ethene (C₂H₄)	Plastic bags, bottles, packaging
Poly(propene) / polypropene	Propene (C₃H₆)	Ropes, crates, carpets
Poly(chloroethene) / PVC	Chloroethene (C₂H₃Cl)	Pipes, window frames, clothing
Poly(tetrafluoroethene) / PTFE	Tetrafluoroethene (C₂F₄)	Non-stick pans (Teflon)

Exam Tip: To draw a polymer from a monomer: (1) Open the C=C double bond to make two single bonds. (2) Show the repeating unit in brackets with bonds extending out of each side. (3) Put 'n' outside the brackets.

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Problems with Polymers

Most addition polymers are not biodegradable — they do not decompose naturally. This causes environmental problems:

• Landfill — polymers take up space and persist for hundreds of years.
• Litter — plastic waste pollutes land and oceans, harming wildlife.
• Incineration — burning plastics releases toxic gases (e.g. HCl from PVC).

Solutions

• Recycling — reduces the need for new raw materials and reduces landfill.
• Developing biodegradable polymers — made from plant-based materials that can be broken down by microorganisms.
• Reducing use — using fewer single-use plastics.

Exam Tip: You may be asked why disposal of addition polymers is difficult. The answer is that their C–C backbone is very strong and they are chemically inert, so microorganisms cannot break them down — they are not biodegradable.

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Summary

• Alkenes are unsaturated hydrocarbons with the general formula CₙH₂ₙ containing a C=C double bond.
• The bromine water test distinguishes alkenes (decolourises) from alkanes (stays orange).
• Alkenes undergo addition reactions — the double bond opens and atoms add across it.
• In addition polymerisation, many alkene monomers join to form a long-chain polymer.
• Common polymers: poly(ethene), poly(propene), PVC, PTFE.
• Most addition polymers are not biodegradable, causing disposal and environmental problems.

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Worked Examples

Worked Example 1 — Distinguishing an alkane from an alkene

Question: A student has two colourless liquids, A and B. They add a few drops of each to orange bromine water and shake.

• A decolourises the bromine water to colourless.
• B stays orange.

Which is the alkane and which is the alkene? Explain.

Answer: A is the alkene, B is the alkane.

Reason: Alkenes contain a C=C double bond (functional group) which undergoes an addition reaction with bromine: Br₂ adds across the double bond, producing a colourless dibromoalkane and removing the orange Br₂ colour. Alkanes are saturated (no double bond) and do not react with bromine under these conditions.

Worked Example 2 — Writing an addition reaction

Question: Write a balanced equation for the reaction of ethene with hydrogen.