Alkanes

    AQA
    A-Level

    Alkanes are a homologous series of saturated hydrocarbons characterized by single carbon-carbon bonds and the general formula CnH2n+2. Their physical properties, including boiling point and viscosity, exhibit predictable trends based on chain length and the resulting strength of intermolecular forces. While generally unreactive, alkanes undergo exothermic combustion to release energy and thermal decomposition (cracking) to produce alkenes, linking them directly to global fuel consumption and polymer synthesis.

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    Objectives
    3
    Exam Tips
    4
    Pitfalls
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    Key Terms
    5
    Mark Points

    What You Need to Demonstrate

    Key skills and knowledge for this topic

    • Award 1 mark for defining a hydrocarbon as a compound consisting of hydrogen and carbon atoms only
    • Credit the correct initiation step showing homolytic fission of the halogen bond requiring UV light
    • Award marks for explaining that larger molecules have stronger van der Waals forces requiring more energy to break
    • Candidates must identify Zeolite catalyst and slight pressure as specific conditions for catalytic cracking
    • Credit balanced equations for the reaction of SO2 with CaO or CaCO3 to form CaSO3 or CaSO4

    Example Examiner Feedback

    Real feedback patterns examiners use when marking

    • "You correctly identified the products of cracking, but check your conditions — thermal cracking requires high pressure"
    • "Your mechanism initiation is correct; ensure the propagation steps show the radical attacking the neutral molecule"
    • "Good definition of fractional distillation, but you must mention the temperature gradient in the column"
    • "Remember to specify 'van der Waals forces' rather than just 'intermolecular forces' for higher marks"

    Marking Points

    Key points examiners look for in your answers

    • Award 1 mark for defining a hydrocarbon as a compound consisting of hydrogen and carbon atoms only
    • Credit the correct initiation step showing homolytic fission of the halogen bond requiring UV light
    • Award marks for explaining that larger molecules have stronger van der Waals forces requiring more energy to break
    • Candidates must identify Zeolite catalyst and slight pressure as specific conditions for catalytic cracking
    • Credit balanced equations for the reaction of SO2 with CaO or CaCO3 to form CaSO3 or CaSO4

    Examiner Tips

    Expert advice for maximising your marks

    • 💡In mechanism questions, the position of the dot is critical; it must be on the C or Cl atom, not the H or the bond
    • 💡Distinguish clearly between the conditions for thermal cracking (high T, high P) and catalytic cracking (Zeolite, slight P)
    • 💡For combustion analysis, remember that SO2 causes acid rain and is removed by neutralisation, not by catalytic converters

    Common Mistakes

    Pitfalls to avoid in your exam answers

    • Placing the radical dot on the wrong atom or ambiguously in space during mechanism questions
    • Stating that thermal cracking uses a catalyst or confusing the products of thermal vs catalytic cracking
    • Incorrectly balancing propagation steps by failing to regenerate the free radical
    • Describing fractional distillation as 'breaking bonds' rather than overcoming intermolecular forces

    Study Guide Available

    Comprehensive revision notes & examples

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    Key Terminology

    Essential terms to know

    Likely Command Words

    How questions on this topic are typically asked

    Define
    Explain
    Write
    Name
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    Practical Links

    Related required practicals

    • •{"code":"Technique","title":"Distillation","relevance":"Separation of alkanes based on boiling point differences"}

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