Collision Theory

    OCR
    GCSE

    Collision theory dictates that chemical reactions occur only when particles collide with sufficient energy, known as activation energy, and appropriate steric orientation. The rate of reaction is determined by the frequency of successful collisions per unit time, rather than the total number of collisions. Factors including temperature, concentration, pressure, and surface area influence this frequency, while catalysts increase the rate by providing an alternative reaction pathway with a lower activation energy requirement.

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

    What You Need to Demonstrate

    Key skills and knowledge for this topic

    • Award 1 mark for stating that chemical reactions only occur when particles collide with sufficient energy (activation energy)
    • Credit responses that explain increased concentration as 'more particles in the same volume', leading to a higher frequency of collisions
    • Award marks for linking increased temperature to particles moving faster AND having more kinetic energy, resulting in more frequent and more energetic collisions
    • Candidates must identify that catalysts increase the rate by providing an alternative pathway with a lower activation energy

    Example Examiner Feedback

    Real feedback patterns examiners use when marking

    • "You correctly identified that rate increases, but you must use the phrase 'frequency of collisions' to earn the explanation mark"
    • "Good description of the graph. To access higher marks, calculate the gradient of the tangent at t=0 to find the initial rate"
    • "You mentioned particles moving faster. Remember to also explain that more particles now have energy greater than the activation energy"
    • "Excellent definition of a catalyst. Now apply this to the energy profile diagram to show the alternative pathway"

    Marking Points

    Key points examiners look for in your answers

    • Award 1 mark for stating that chemical reactions only occur when particles collide with sufficient energy (activation energy)
    • Credit responses that explain increased concentration as 'more particles in the same volume', leading to a higher frequency of collisions
    • Award marks for linking increased temperature to particles moving faster AND having more kinetic energy, resulting in more frequent and more energetic collisions
    • Candidates must identify that catalysts increase the rate by providing an alternative pathway with a lower activation energy

    Examiner Tips

    Expert advice for maximising your marks

    • 💡In 6-mark extended response questions, structure your answer by factor: State the change → Describe the particle behavior → Explain the effect on collision frequency → Link to rate
    • 💡When calculating rates from a graph, remember that the gradient of the tangent represents the rate at that specific time; draw the tangent carefully
    • 💡Memorize the definition of activation energy: 'the minimum amount of energy that particles must have to react'

    Common Mistakes

    Pitfalls to avoid in your exam answers

    • Referencing 'more collisions' without the critical qualifier 'per second' or 'frequency', which fails to define a rate
    • Describing the effect of temperature only in terms of speed/frequency, neglecting the crucial factor of increased energy relative to activation energy
    • Stating that a catalyst 'does not take part' in the reaction, rather than stating it is 'not used up' or remains 'chemically unchanged'

    Key Terminology

    Essential terms to know

    Activation Energy (Ea) and Reaction Profiles
    Frequency of Successful Collisions
    Factors Affecting Rate (Temperature, Concentration, Pressure, Surface Area)
    Catalysis and Alternative Pathways

    Likely Command Words

    How questions on this topic are typically asked

    Explain
    Describe
    Calculate
    Suggest
    Predict

    Practical Links

    Related required practicals

    • •{"code":"PAG C5","title":"Monitoring the rate of reaction","relevance":"Measuring gas volume (Mg + HCl) or turbidity (Thiosulfate) to generate rate data"}

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