Combustion

    OCR
    GCSE

    Combustion is a rapid oxidation reaction wherein a fuel reacts with oxygen to release significant thermal energy, characterizing it as an exothermic process. Candidates must differentiate between complete combustion, yielding carbon dioxide and water, and incomplete combustion, resulting in carbon monoxide and carbon particulates due to oxygen limitation. Mastery requires analyzing the environmental impact of atmospheric pollutants, including the greenhouse effect, acid rain formation from sulfur impurities, and the toxicity of carbon monoxide.

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    Objectives
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    Exam Tips
    4
    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 identifying combustion as an oxidation reaction that releases thermal energy to the surroundings
    • Credit responses that correctly balance the equation for complete combustion, ensuring oxygen atoms are balanced last
    • Award 1 mark for stating that incomplete combustion produces carbon monoxide (CO) and/or carbon particulates (soot) due to insufficient oxygen
    • In bond energy calculations, award marks for calculating 'energy in' (breaking bonds) and 'energy out' (making bonds) separately before finding the difference
    • Credit the explanation that combustion is exothermic because more energy is released making bonds in products than is needed to break bonds in reactants

    Example Examiner Feedback

    Real feedback patterns examiners use when marking

    • "You correctly identified the products, but check your balancing of the oxygen atoms again"
    • "Good definition of exothermic, but to get full marks you must reference bond breaking and bond making energies"
    • "You have confused the pollutants here; remember that sulfur impurities cause acid rain, while CO2 causes global warming"
    • "Excellent calculation of the bond energies. For the next step, explain what the negative sign represents in terms of energy transfer"

    Marking Points

    Key points examiners look for in your answers

    • Award 1 mark for identifying combustion as an oxidation reaction that releases thermal energy to the surroundings
    • Credit responses that correctly balance the equation for complete combustion, ensuring oxygen atoms are balanced last
    • Award 1 mark for stating that incomplete combustion produces carbon monoxide (CO) and/or carbon particulates (soot) due to insufficient oxygen
    • In bond energy calculations, award marks for calculating 'energy in' (breaking bonds) and 'energy out' (making bonds) separately before finding the difference
    • Credit the explanation that combustion is exothermic because more energy is released making bonds in products than is needed to break bonds in reactants

    Examiner Tips

    Expert advice for maximising your marks

    • 💡When asked to explain why a reaction is exothermic, you must explicitly compare bond breaking energy vs. bond making energy; do not just say 'it releases heat'
    • 💡For questions on evaluating fuels, structure your answer to compare energy density, ease of ignition, pollution produced, and state of matter for transport
    • 💡Memorise the specific tests for combustion products: limewater turning milky for CO2 and white anhydrous copper sulfate turning blue for water
    • 💡In calculation questions, always show the subtraction step (Breaking - Making) clearly to secure method marks even if the final arithmetic is incorrect

    Common Mistakes

    Pitfalls to avoid in your exam answers

    • Confusing the environmental impacts of combustion products, such as attributing acid rain to carbon dioxide instead of sulfur dioxide or nitrogen oxides
    • Omitting the negative sign for the final energy change value when calculating the enthalpy change of combustion
    • Failing to account for the oxygen atom already present in the fuel (e.g., ethanol) when balancing combustion equations
    • Stating that energy is 'created' during combustion rather than transferred from the chemical store to the thermal store

    Key Terminology

    Essential terms to know

    Complete versus Incomplete Combustion
    Exothermic Energy Changes
    Atmospheric Pollutants (CO, C, SO2, NOx)
    Greenhouse Effect and Climate Change
    Balancing Chemical Equations

    Likely Command Words

    How questions on this topic are typically asked

    State
    Explain
    Calculate
    Write
    Evaluate
    Describe

    Practical Links

    Related required practicals

    • •{"code":"PAG C7","title":"Energy changes","relevance":"Investigating the energy released by the combustion of different alcohols using a spirit burner and calorimeter"}

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