Ethanol

    OCR
    GCSE

    Ethanol is a primary alcohol with the chemical formula C2H5OH, characterized by the hydroxyl (-OH) functional group which dictates its chemical properties. It is manufactured industrially via two distinct pathways: the hydration of ethene with steam using a phosphoric acid catalyst, and the anaerobic fermentation of glucose by yeast. Chemically, ethanol undergoes complete combustion to release energy, and oxidation to form ethanoic acid using oxidizing agents like acidified potassium dichromate(VI). Candidates must evaluate ethanol's utility as a biofuel, comparing the sustainability and carbon neutrality of bioethanol against fossil fuels.

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    Objectives
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    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 identifying the functional group as -OH (hydroxyl) in displayed or structural formulae
    • Credit responses that specify fermentation conditions: anaerobic, presence of yeast, and temperatures between 25°C and 40°C
    • Award 1 mark for stating that hydration of ethene requires steam and a phosphoric acid catalyst
    • Credit the explanation that fermentation stops when ethanol concentration kills the yeast or sugar is exhausted
    • Award marks for balanced symbol equations of complete combustion producing carbon dioxide and water

    Example Examiner Feedback

    Real feedback patterns examiners use when marking

    • "You correctly identified the raw materials, but you must explain *why* fermentation requires anaerobic conditions (to prevent oxidation to ethanoic acid)"
    • "Good use of the term 'renewable' for fermentation; to improve, contrast this with the 'finite' crude oil used in hydration"
    • "Your displayed formula is nearly correct, but ensure the bond connects the Carbon to the Oxygen, and the Oxygen to the Hydrogen clearly"
    • "Excellent comparison of the methods. For full marks, include the difference in purity and the need for fractional distillation in the fermentation process"

    Marking Points

    Key points examiners look for in your answers

    • Award 1 mark for identifying the functional group as -OH (hydroxyl) in displayed or structural formulae
    • Credit responses that specify fermentation conditions: anaerobic, presence of yeast, and temperatures between 25°C and 40°C
    • Award 1 mark for stating that hydration of ethene requires steam and a phosphoric acid catalyst
    • Credit the explanation that fermentation stops when ethanol concentration kills the yeast or sugar is exhausted
    • Award marks for balanced symbol equations of complete combustion producing carbon dioxide and water

    Examiner Tips

    Expert advice for maximising your marks

    • 💡When evaluating production methods, structure your answer using a comparative table covering: Raw Materials (Renewable vs Finite), Rate of Reaction, Purity of Product, and Energy Requirements
    • 💡For 6-mark extended response questions on synthesis, explicitly link the 'continuous' nature of hydration to its suitability for large-scale industrial demand
    • 💡Always balance combustion equations alphabetically: Carbon first, Hydrogen second, and Oxygen last to avoid arithmetic errors
    • 💡Memorise the specific catalyst for hydration (phosphoric acid) as generic references to 'catalyst' often do not gain credit in higher tariff questions

    Common Mistakes

    Pitfalls to avoid in your exam answers

    • Drawing the O-H bond incorrectly in displayed formulae (e.g., connecting the H to the C instead of the O, or omitting the bond line between O and H)
    • Confusing the purity and rate of reaction between the two production methods (e.g., stating fermentation is fast and produces pure ethanol)
    • Failing to specify 'anaerobic' or 'absence of oxygen' for fermentation, or incorrectly stating that oxygen is required for the yeast to breathe
    • Writing the molecular formula as C2H6O instead of C2H5OH, which fails to explicitly show the alcohol functional group

    Key Terminology

    Essential terms to know

    Functional group chemistry and homologous series
    Industrial production: Fermentation vs. Hydration
    Combustion and oxidation reactions
    Social and economic evaluation of biofuels

    Likely Command Words

    How questions on this topic are typically asked

    State
    Explain
    Evaluate
    Compare
    Draw
    Calculate

    Practical Links

    Related required practicals

    • •{"code":"PAG C5","title":"Monitoring chemical reactions","relevance":"Combustion of alcohols to measure energy release"}
    • •{"code":"PAG 7","title":"Distillation","relevance":"Purification of ethanol produced via fermentation"}

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