AQA · Chemistry · 8462
GCSE Chemistry (AQA): Cracking & Alkenes
Topic 7 — Organic Chemistry. Generated from the caption track of Cognito's lesson video "GCSE Chemistry - Cracking & Alkenes - Catalytic & Steam Cracking | Equations (2027/28 exams)"; each card deep-links to the moment in the video it came from. Lesson 7.07 of Cognito's AQA GCSE Combined Science (Trilogy) and Chemistry course (Higher tier), filed under AQA Chemistry 8462. Teaching by Cognito — https://cognitoedu.org/coursesubtopic/c2-gcse-aqa-h-c_7.07.
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Which catalyst is used in catalytic cracking?
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Hot powdered aluminium oxide, over which the hydrocarbon vapour is passed.
Why can alkenes be used to make polymers?
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Their double bond can break to form two single bonds, letting each molecule bond to adjacent molecules.
What are the conditions for steam cracking?
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The vaporised hydrocarbons are mixed with steam and heated to a very high temperature.
Write the balanced equation for cracking decane (C10H22) into ethene (C2H4) and one other hydrocarbon.
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C10H22 → C8H18 + C2H4. The carbons (10 − 2 = 8) and hydrogens (22 − 4 = 18) left over give the alkane C8H18.
What type of reaction is cracking?
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Thermal decomposition — molecules are broken down by heating.
Why are long-chain hydrocarbons cracked?
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To convert them into shorter, more flammable hydrocarbons, which are far more useful (e.g. as fuels).
What is the first step in both catalytic and steam cracking?
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Heating the long-chain hydrocarbons to vaporise them (turn them into a gas).
What is the chemical test for an alkene, and its positive result?
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Add bromine water: it is decolourised from orange to colourless.
What is the structural difference between alkanes and alkenes?
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Alkenes contain a carbon–carbon double bond (unsaturated); alkanes have only single bonds (saturated).
Why must one product of cracking contain a double bond?
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There are not enough hydrogen atoms for both products to be saturated, so one must be an unsaturated alkene.
Cracking an alkane produces which two types of product?
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A shorter alkane and an alkene.
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