Crude Oil

    OCR
    GCSE

    Crude oil is a finite resource formed from the remains of ancient biomass, consisting primarily of a complex mixture of hydrocarbons called alkanes. Separation of this mixture is achieved through fractional distillation, a physical process relying on differences in boiling points to isolate useful fractions. Candidates must understand the trends in physical properties—viscosity, flammability, and boiling point—as carbon chain length increases, linking these to the strength of intermolecular forces. Furthermore, the topic covers the chemical processes of combustion and cracking, the latter being essential to meet demand for smaller chain hydrocarbons and alkene feedstocks.

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    Objectives
    3
    Exam Tips
    4
    Pitfalls
    5
    Key Terms
    5
    Mark Points

    What You Need to Demonstrate

    Key skills and knowledge for this topic

    • Award 1 mark for stating that crude oil is a mixture of hydrocarbons
    • Credit responses that explain separation occurs because fractions have different boiling points
    • Award 1 mark for stating the fractionating column is hottest at the bottom and coolest at the top
    • For Higher Tier: Award marks for linking larger molecular size to stronger intermolecular forces requiring more energy to overcome
    • Award 1 mark for identifying that gases condense when they reach a region where the temperature is below their boiling point

    Example Examiner Feedback

    Real feedback patterns examiners use when marking

    • "You correctly identified the trend in boiling points, but you must mention 'intermolecular forces' to get the explanation mark"
    • "Check your definition of hydrocarbon — did you specify it is a compound containing hydrogen and carbon 'only'?"
    • "Good description of the column temperature, but ensure you explicitly state where condensation happens"
    • "Your cracking equation is unbalanced; count the hydrogens on the left and ensure they match the total on the right"

    Marking Points

    Key points examiners look for in your answers

    • Award 1 mark for stating that crude oil is a mixture of hydrocarbons
    • Credit responses that explain separation occurs because fractions have different boiling points
    • Award 1 mark for stating the fractionating column is hottest at the bottom and coolest at the top
    • For Higher Tier: Award marks for linking larger molecular size to stronger intermolecular forces requiring more energy to overcome
    • Award 1 mark for identifying that gases condense when they reach a region where the temperature is below their boiling point

    Examiner Tips

    Expert advice for maximising your marks

    • 💡In 6-mark Level of Response questions on distillation, structure your answer chronologically: Evaporation -> Rising/Cooling -> Condensation -> Collection
    • 💡Never use the word 'break' regarding bonds when discussing boiling points; use 'overcome' regarding intermolecular forces
    • 💡When predicting properties, remember the rule: as chain length increases, boiling point and viscosity increase, but flammability decreases

    Common Mistakes

    Pitfalls to avoid in your exam answers

    • Stating that 'covalent bonds' break when crude oil boils (examiners reject this; only intermolecular forces are overcome)
    • Confusing the temperature gradient of the column, incorrectly stating it is hotter at the top
    • Defining a hydrocarbon as a 'mixture' of hydrogen and carbon rather than a 'compound' containing hydrogen and carbon only
    • In cracking equations, failing to ensure the number of carbon and hydrogen atoms is conserved on both sides

    Key Terminology

    Essential terms to know

    Composition and origin of crude oil
    Fractional distillation and separation techniques
    Properties of alkanes (trends in viscosity, boiling point, flammability)
    Combustion of hydrocarbons (complete and incomplete)
    Cracking and thermal decomposition

    Likely Command Words

    How questions on this topic are typically asked

    Describe
    Explain
    Predict
    Calculate
    Evaluate
    Write

    Practical Links

    Related required practicals

    • •{"code":"PAG C4","title":"Distillation","relevance":"Technique used to separate mixtures based on boiling points, modelling fractional distillation"}

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