Monitoring and controlling chemical reactions

    OCR
    GCSE

    This topic necessitates the quantitative and qualitative analysis of reaction kinetics and system dynamics. Candidates must investigate methods for monitoring reaction progress—measuring gas volumes, mass loss, or turbidity—and apply collision theory to explain how temperature, concentration, pressure, surface area, and catalysts alter reaction rates. The study extends to reversible reactions, requiring an understanding of dynamic equilibrium and, for higher-tier assessment, the application of Le Chatelier’s Principle to predict shifts in equilibrium position in response to changing conditions.

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

    What You Need to Demonstrate

    Key skills and knowledge for this topic

    • Award 1 mark for calculating the rate of reaction at a specific time by drawing a tangent and determining its gradient (change in y / change in x)
    • Credit responses that explain increased rate in terms of 'increased frequency of successful collisions', not just 'more collisions'
    • Award 1 mark for stating that a catalyst increases the rate by providing an alternative reaction pathway with a lower activation energy
    • For Higher Tier, credit explanations that link a shift in equilibrium position to the system opposing the change imposed (Le Chatelier's principle)

    Example Examiner Feedback

    Real feedback patterns examiners use when marking

    • "You correctly identified that rate increases, but you must specify 'frequency of successful collisions' to earn the explanation mark"
    • "Your tangent is drawn correctly, but your gradient calculation needs to use a larger triangle to ensure accuracy"
    • "Good recall of the factors affecting rate; now try to apply this to the equilibrium context—how does temperature affect the yield?"
    • "You have confused rate and yield here—remember that a catalyst increases the rate but does not change the position of equilibrium"

    Marking Points

    Key points examiners look for in your answers

    • Award 1 mark for calculating the rate of reaction at a specific time by drawing a tangent and determining its gradient (change in y / change in x)
    • Credit responses that explain increased rate in terms of 'increased frequency of successful collisions', not just 'more collisions'
    • Award 1 mark for stating that a catalyst increases the rate by providing an alternative reaction pathway with a lower activation energy
    • For Higher Tier, credit explanations that link a shift in equilibrium position to the system opposing the change imposed (Le Chatelier's principle)

    Examiner Tips

    Expert advice for maximising your marks

    • 💡When drawing a tangent to calculate rate, ensure your construction lines cover at least half the graph paper to maximise accuracy
    • 💡In 6-mark Level of Response questions regarding practicals, explicitly name the apparatus (e.g., 'gas syringe' or 'inverted measuring cylinder') and control variables
    • 💡For reversible reactions, clearly distinguish between the 'forward' and 'reverse' reactions when explaining the effect of temperature or pressure changes

    Common Mistakes

    Pitfalls to avoid in your exam answers

    • Omitting the word 'frequency' when describing collisions, leading to the vague and uncreditworthy phrase 'more collisions'
    • Confusing the 'rate of reaction' (speed) with the 'yield' (amount of product), particularly in equilibrium questions
    • Drawing tangents with triangles that are too small, resulting in gradient calculations outside the allowed tolerance range
    • Stating that catalysts are 'not involved' in the reaction rather than 'not used up' or 'chemically unchanged at the end'

    Key Terminology

    Essential terms to know

    Experimental methods for monitoring reaction rates
    Collision theory and activation energy
    Factors affecting rate: temperature, concentration, surface area, catalysts
    Reversible reactions and dynamic equilibrium
    Le Chatelier's Principle and equilibrium shifts

    Likely Command Words

    How questions on this topic are typically asked

    Calculate
    Explain
    Describe
    Predict
    Suggest

    Practical Links

    Related required practicals

    • •{"code":"PAG C5","title":"Monitoring the rate of reaction","relevance":"Measuring gas evolution (Mg + HCl) or precipitate formation (Thiosulfate)"}

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