ALKENE molecules (e.g. ethene) — very important as feedstock for making POLYMERS and other chemicals.
Cracking also produces HYDROGEN GAS in some processes.
Cracking increases the proportion of petrol available from crude oil and produces the ethene needed for the plastics industry.
Methods of Cracking
THERMAL CRACKING:
High temperature (400–900°C) and high pressure.
Produces a high proportion of ALKENES.
The long-chain hydrocarbon vapour is passed through a hot tube — heat breaks the C-C bonds.
CATALYTIC CRACKING:
High temperature (~500°C) but LOWER pressure.
Uses a ZEOLITE (aluminium silicate) catalyst.
More economical — lower pressure needed because catalyst lowers activation energy.
Produces more BRANCHED ALKANES useful for high-performance petrol.
General equation:
Large alkane → smaller alkane + alkene
Example:
C₁₀H₂₂ → C₈H₁₈ + C₂H₄
(decane → octane + ethene)
Condition: high temperature, catalyst (or high pressure for thermal cracking).
Alkenes — Structure and Reactions
ALKENES are hydrocarbons containing at least one C=C DOUBLE BOND — they are UNSATURATED.
General formula: CₙH₂ₙ
Ethene: C₂H₄
Propene: C₃H₆
Butene: C₄H₈
The double bond makes alkenes MUCH MORE REACTIVE than alkanes.
TEST FOR ALKENES — bromine water test:
Shake with BROMINE WATER (orange/brown).
Alkene: bromine water DECOLOURISES (turns colourless) — the C=C double bond reacts with Br₂.
Alkane: NO CHANGE — single bonds don't react with bromine water at room temperature.
ADDITION REACTIONS of alkenes:
UNSATURATED molecules add atoms across the C=C double bond.
Hydrogen: C₂H₄ + H₂ → C₂H₆ (hydrogenation — used to make margarine)
Bromine: C₂H₄ + Br₂ → CH₂BrCH₂Br (dibromoethane)
Water: C₂H₄ + H₂O → C₂H₅OH (hydration — makes ethanol)
POLYMERISATION:
Ethene molecules join together to form poly(ethene): n CH₂=CH₂ → [−CH₂−CH₂−]ₙ
⚠️ Common Mistake
Cracking always produces at least one ALKENE — alkenes are ALWAYS one of the products. You cannot crack a hydrocarbon to produce only alkanes. Also: alkenes DECOLOURISE bromine water (C=C reacts with Br₂). Alkanes do NOT decolourise bromine water.
📐 Key Equations
C₁₀H₂₂ → C₈H₁₈ + C₂H₄ (cracking example)
General formula alkenes: CₙH₂ₙ
C₂H₄ + H₂O → C₂H₅OH (hydration)
n CH₂=CH₂ → [−CH₂−CH₂−]ₙ (polymerisation)
📌 Key Note
Cracking: large alkane → smaller alkane + alkene. Needed because demand for petrol > supply from crude oil. Thermal: high T + pressure. Catalytic: ~500°C + zeolite catalyst. Alkenes: CₙH₂ₙ, C=C double bond, unsaturated, decolourise bromine water, undergo addition reactions, polymerise.
🎯 Matching Activity — Cracking and Alkenes
Match each term to its correct description. — drag the symbols on the right to match the component names on the left.
Cracking
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Alkene test
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Alkane test
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Addition reaction
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Polymerisation of ethene
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Breaking long-chain alkanes into smaller alkanes + alkenes using heat/catalyst
Decolourises orange bromine water — C=C reacts with Br₂
Many ethene molecules join to form poly(ethene) — n CH₂=CH₂ → [−CH₂−CH₂−]ₙ
No change with bromine water — no double bond to react
Two molecules join across the C=C double bond to form one product
🎯 Test Yourself
Question 1 of 2
1. Why is cracking important for the petrochemical industry?
2. A student adds bromine water to two hydrocarbons. With compound A, the bromine water decolourises. With compound B, there is no change. What does this show?
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