Chemical reactions

    OCR
    GCSE

    Chemical reactions involve the rearrangement of atoms to form new substances, strictly adhering to the Law of Conservation of Mass where no atoms are created or destroyed. This process necessitates energy transfer, categorized as exothermic or endothermic, driven by the breaking and making of chemical bonds. Candidates must represent these transformations using balanced symbol equations and state symbols to demonstrate stoichiometric relationships between reactants and products.

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    Objectives
    4
    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 mass is conserved because no atoms are lost or made during a chemical reaction
    • Credit responses that correctly balance symbol equations using coefficients, not by changing subscripts
    • Award 1 mark for identifying a reaction as exothermic if the temperature of the surroundings increases
    • Award 1 mark for drawing a reaction profile where product energy is lower than reactant energy for exothermic reactions
    • For bond energy calculations (Higher), award 1 mark for: Energy required to break bonds - Energy released making bonds

    Example Examiner Feedback

    Real feedback patterns examiners use when marking

    • "You correctly identified the reaction type, but you need to reference the 'surroundings' when discussing temperature changes"
    • "Your balancing is correct, but remember that changing a subscript changes the substance identity—only use coefficients"
    • "Good calculation of the energy change. To secure the top marks, explicitly state that the negative sign indicates an exothermic reaction"
    • "You have described the profile correctly; now explain *why* the activation energy hump exists in terms of particle collisions"

    Marking Points

    Key points examiners look for in your answers

    • Award 1 mark for stating that mass is conserved because no atoms are lost or made during a chemical reaction
    • Credit responses that correctly balance symbol equations using coefficients, not by changing subscripts
    • Award 1 mark for identifying a reaction as exothermic if the temperature of the surroundings increases
    • Award 1 mark for drawing a reaction profile where product energy is lower than reactant energy for exothermic reactions
    • For bond energy calculations (Higher), award 1 mark for: Energy required to break bonds - Energy released making bonds

    Examiner Tips

    Expert advice for maximising your marks

    • 💡When asked to 'Explain in terms of bond breaking and making', you must state that breaking bonds is endothermic and making bonds is exothermic
    • 💡Always check if the question asks for state symbols (s, l, g, aq) – omitting these often results in lost marks on OCR papers
    • 💡For 6-mark 'Level of Response' questions on energy changes, structure your answer: Define reaction type → Cite temperature evidence → Explain using bond energies
    • 💡Use a tally chart for atoms on the left and right side to ensure accuracy when balancing complex equations

    Common Mistakes

    Pitfalls to avoid in your exam answers

    • Altering chemical formulae subscripts (e.g., changing O2 to O) instead of using coefficients when balancing equations
    • Confusing the activation energy arrow on reaction profiles; often drawing it from the top of the curve to the products rather than from reactants to peak
    • Stating that energy is 'created' or 'destroyed' rather than transferred to/from the surroundings
    • In bond energy calculations, subtracting 'bonds broken' from 'bonds made' resulting in a sign error

    Key Terminology

    Essential terms to know

    Law of Conservation of Mass
    Balancing symbol equations and state symbols
    Exothermic and endothermic energy changes
    Reaction profiles and activation energy
    Oxidation and reduction (Redox)

    Likely Command Words

    How questions on this topic are typically asked

    Calculate
    Explain
    Draw
    Describe
    Suggest

    Practical Links

    Related required practicals

    • •{"code":"PAG C3","title":"Energy Changes","relevance":"Measuring temperature changes in neutralisation or displacement to determine exo/endothermic nature"}

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