Cracking

    OCR
    GCSE

    Cracking is the thermal decomposition of long-chain saturated hydrocarbons into shorter-chain alkanes and alkenes. This endothermic process involves the breaking of strong carbon-carbon covalent bonds to convert low-demand heavy fractions into high-demand fuels and chemical feedstocks. Candidates must distinguish between catalytic cracking, which utilizes a zeolite or silica-alumina catalyst, and thermal cracking, which relies on high temperatures and pressures. The presence of alkenes in the products is confirmed via the decolorization of bromine water, linking the process to polymer manufacturing.

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    Objectives
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    Exam Tips
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    Pitfalls
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    Key Terms
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    Mark Points

    What You Need to Demonstrate

    Key skills and knowledge for this topic

    • Award 1 mark for defining cracking as the thermal decomposition of long-chain hydrocarbons into smaller, more useful molecules
    • Credit responses that specify the conditions as high temperature and use of a catalyst (e.g., aluminium oxide, porous pot, or silica)
    • Award 1 mark for explaining that cracking produces alkenes which are used as feedstock for polymers
    • Award 1 mark for correctly balancing a cracking equation ensuring conservation of C and H atoms (e.g., C10H22 → C8H18 + C2H4)
    • Credit the observation that bromine water changes from orange to colourless in the presence of an alkene

    Example Examiner Feedback

    Real feedback patterns examiners use when marking

    • "You correctly identified the products, but check your atom count—does the number of Hydrogens on the right match the left?"
    • "Avoid using 'clear' for the bromine water test; the examiner requires 'colourless' to distinguish from a transparent solution"
    • "You mentioned 'breaking down', but to secure the mark, use the precise term 'thermal decomposition'"
    • "Good explanation of the process. Now, add the economic context: why do oil companies bother doing this in terms of supply and demand?"

    Marking Points

    Key points examiners look for in your answers

    • Award 1 mark for defining cracking as the thermal decomposition of long-chain hydrocarbons into smaller, more useful molecules
    • Credit responses that specify the conditions as high temperature and use of a catalyst (e.g., aluminium oxide, porous pot, or silica)
    • Award 1 mark for explaining that cracking produces alkenes which are used as feedstock for polymers
    • Award 1 mark for correctly balancing a cracking equation ensuring conservation of C and H atoms (e.g., C10H22 → C8H18 + C2H4)
    • Credit the observation that bromine water changes from orange to colourless in the presence of an alkene

    Examiner Tips

    Expert advice for maximising your marks

    • 💡Memorise the specific conditions: 'heat' alone is insufficient; you must specify 'high temperature' and 'catalyst' to gain full credit
    • 💡Check your equation balancing by summing the subscripts for Carbon and Hydrogen on the right-hand side; they must equal the left-hand side exactly
    • 💡When asked why cracking is important, explicitly link 'supply' of long chains (high supply, low demand) to 'demand' for short chains (low supply, high demand)

    Common Mistakes

    Pitfalls to avoid in your exam answers

    • Confusing physical separation (fractional distillation) with chemical breakdown (cracking) — ensure the distinction between intermolecular forces and covalent bonds is clear
    • Describing the result of the bromine water test as going 'clear' instead of 'colourless' — 'clear' implies transparency, not lack of colour
    • Incorrectly balancing equations by adding hydrogen (H2) as a product when not specified, or failing to conserve total hydrogen atoms across the reaction arrow

    Study Guide Available

    Comprehensive revision notes & examples

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    Key Terminology

    Essential terms to know

    Thermal decomposition mechanisms
    Supply and demand economics of crude oil fractions
    Catalytic versus Thermal cracking conditions
    Identification of unsaturated hydrocarbons (alkenes)

    Likely Command Words

    How questions on this topic are typically asked

    State
    Explain
    Describe
    Write
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    Practical Links

    Related required practicals

    • •{"code":"PAG7","title":"Organic Chemistry / Distillation and Cracking","relevance":"Practical setup for cracking liquid paraffin using mineral wool and porous pot, then testing gas with bromine water"}

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