Cracking and Alkenes

This lesson covers cracking and introduces alkenes as required by the AQA GCSE Chemistry specification (5.8.1). Cracking is a vital industrial process that converts less useful long-chain hydrocarbons into more useful shorter-chain hydrocarbons and alkenes. Alkenes are the starting materials for making polymers (plastics).

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Why Is Cracking Needed?

Fractional distillation of crude oil produces fractions in fixed proportions. However, the demand for different fractions does not match the supply:

Situation	Detail
Oversupply	Long-chain hydrocarbons (fuel oil, bitumen) are produced in larger quantities than needed
High demand	Short-chain hydrocarbons (petrol, diesel) and alkenes (for plastics) are in greater demand than supply

Cracking solves this problem by breaking down long-chain hydrocarbons into shorter, more useful molecules.

Exam Tip: Always explain cracking in terms of supply and demand. A common exam question asks "why is cracking carried out?" The answer must include: there is a greater demand for shorter-chain hydrocarbons than the amount produced by fractional distillation.

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What Is Cracking?

Cracking is a thermal decomposition reaction in which long-chain hydrocarbons are broken down into shorter, more useful hydrocarbons. The process always produces at least one alkane and at least one alkene.

Key Points

• Cracking is a chemical reaction (bonds are broken and new bonds are formed)
• It involves breaking C–C bonds in the hydrocarbon chain
• It always produces a mixture of shorter alkanes and alkenes
• The products are more useful as fuels (alkanes) or as feedstock for making plastics (alkenes)

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Methods of Cracking

There are two main methods of cracking:

1. Catalytic Cracking

Feature	Detail
Temperature	Approximately 500–600°C
Catalyst	Zeolite catalyst (an aluminosilicate) or sometimes aluminium oxide
Pressure	Slight pressure
Process	Hydrocarbon vapour is passed over a hot catalyst
Products	Shorter alkanes and alkenes

2. Steam Cracking

Feature	Detail
Temperature	Very high — approximately 800–850°C
Catalyst	No catalyst used
Pressure	High pressure
Process	Hydrocarbon is mixed with steam and heated to a very high temperature
Products	Shorter alkanes and alkenes (including ethene for making plastics)

graph TD
    A["Long-chain Hydrocarbon<br/>(e.g. Decane C10H22)"] --> B["CRACKING"]
    B --> C["Catalytic Cracking<br/>~600°C + zeolite catalyst"]
    B --> D["Steam Cracking<br/>~850°C + steam"]
    C --> E["Shorter Alkane<br/>(e.g. Octane C8H18)"]
    C --> F["Alkene<br/>(e.g. Ethene C2H4)"]
    D --> E
    D --> F

    style A fill:#e74c3c,color:#fff
    style B fill:#8e44ad,color:#fff
    style E fill:#2980b9,color:#fff
    style F fill:#27ae60,color:#fff

Exam Tip: You must be able to describe both methods of cracking. Remember: catalytic cracking uses a lower temperature but requires a catalyst; steam cracking uses a higher temperature but does not require a catalyst.

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Examples of Cracking Reactions

Example 1: Cracking Decane

Decane → Octane + Ethene

Word equation: decane → octane + ethene

Balanced symbol equation: C10H22 → C8H18 + C2H4

Example 2: Cracking Decane (Different Products)

Decane → Pentane + Pentene

C10H22 → C5H12 + C5H10

Example 3: Cracking Hexane

Hexane → Butane + Ethene

C6H14 → C4H10 + C2H4

Checking Cracking Equations

To check a cracking equation is balanced:

1. Count the total number of carbon atoms on each side — they must be equal
2. Count the total number of hydrogen atoms on each side — they must be equal
3. One product must be an alkane (CnH2n+2) and at least one must be an alkene (CnH2n)

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Introduction to Alkenes

Alkenes are unsaturated hydrocarbons with the general formula CnH2n. They contain at least one carbon-carbon double bond (C=C).

The First Three Alkenes

Name	Molecular Formula	Number of Carbons
Ethene	C2H4	2
Propene	C3H6	3
Butene	C4H8	4

Note: There is no "methene" because a double bond requires at least two carbon atoms.

Saturated vs Unsaturated

Term	Meaning	Example
Saturated	Contains only single C–C bonds; each carbon has the maximum number of hydrogen atoms	Alkanes (e.g., ethane C2H6)
Unsaturated	Contains at least one C=C double bond; not all carbons have the maximum number of hydrogens	Alkenes (e.g., ethene C2H4)

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Testing for Alkenes — The Bromine Water Test

You can distinguish between an alkane and an alkene using bromine water (an aqueous solution of bromine, which is orange/brown in colour).

Hydrocarbon	Result with Bromine Water	Explanation
Alkane (saturated)	Bromine water stays orange	No reaction — there is no C=C bond to react with bromine
Alkene (unsaturated)	Bromine water is decolourised (turns colourless)	The bromine reacts with (adds across) the C=C double bond — this is an addition reaction

The reaction of an alkene with bromine is:

ethene + bromine → dibromoethane

C2H4 + Br2 → C2H4Br2

This is called an addition reaction because the bromine atoms add across the double bond without any atoms being lost.

Exam Tip: The bromine water test is a required practical concept. Remember: alkenes decolourise bromine water (it turns from orange to colourless), while alkanes have no effect. Never say the bromine water turns "clear" — all solutions are clear. The correct word is "colourless."

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Uses of Alkenes

Alkenes are extremely important in the chemical industry:

Use	Detail
Making polymers	Alkenes undergo addition polymerisation to form plastics (e.g., ethene → poly(ethene), propene → poly(propene))
Making alcohols	Ethene can be reacted with steam to produce ethanol (an alcohol used as a solvent and fuel)
Chemical feedstock	Alkenes are reactive due to the C=C double bond and are used as starting materials for many chemical products

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Laboratory Cracking

In the laboratory, cracking can be demonstrated by: