Percentage Yield and Atom Economy

    OCR
    GCSE

    Percentage yield quantifies the efficiency of a specific reaction instance by comparing the actual mass obtained against the theoretical maximum derived from stoichiometry. Atom economy evaluates the inherent efficiency of a reaction pathway, determining the proportion of reactant atoms converted into the desired product versus waste. Mastery of these concepts enables the critical assessment of industrial processes, balancing economic profitability with the principles of Green Chemistry and waste minimization.

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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 correct substitution into the atom economy equation: (Relative formula mass of desired product / Sum of relative formula masses of all reactants) × 100.
    • Credit responses that identify specific reasons for yield being less than 100%, such as incomplete reactions (equilibrium), side reactions producing by-products, or mechanical loss during separation (filtration/transfer).
    • Award marks for calculating the theoretical yield using stoichiometric ratios before substituting into the percentage yield formula.
    • Candidates must link high atom economy to sustainable development principles, specifically the reduction of waste products and conservation of raw materials.

    Example Examiner Feedback

    Real feedback patterns examiners use when marking

    • "You have correctly calculated the yield, but you must explain *why* it is not 100% using specific chemical reasons like 'side reactions'."
    • "Check your atom economy calculation: did you include the big balancing numbers (coefficients) when summing the reactant masses?"
    • "Good definition of atom economy. To improve, explicitly link this to the concept of 'Green Chemistry' and waste minimization."
    • "You identified the limiting reactant correctly; now ensure you use its molar ratio to find the theoretical yield before calculating the percentage."

    Marking Points

    Key points examiners look for in your answers

    • Award 1 mark for correct substitution into the atom economy equation: (Relative formula mass of desired product / Sum of relative formula masses of all reactants) × 100.
    • Credit responses that identify specific reasons for yield being less than 100%, such as incomplete reactions (equilibrium), side reactions producing by-products, or mechanical loss during separation (filtration/transfer).
    • Award marks for calculating the theoretical yield using stoichiometric ratios before substituting into the percentage yield formula.
    • Candidates must link high atom economy to sustainable development principles, specifically the reduction of waste products and conservation of raw materials.

    Examiner Tips

    Expert advice for maximising your marks

    • 💡Always show the calculation of Relative Formula Mass (Mr) separately; this ensures access to Error Carried Forward (ECF) marks if the final answer is incorrect.
    • 💡When asked to evaluate a reaction pathway for industry, consider both atom economy (waste minimization) and percentage yield (process efficiency) together, not in isolation.
    • 💡In atom economy calculations, remember that the total mass of reactants equals the total mass of products; you can use the product side to check your denominator if the equation is balanced.

    Common Mistakes

    Pitfalls to avoid in your exam answers

    • Confusing atom economy with percentage yield; treating them as interchangeable concepts rather than distinct measures of theoretical vs. practical efficiency.
    • Neglecting the stoichiometric coefficients (balancing numbers) when calculating the total relative formula mass of reactants for atom economy calculations.
    • Stating 'human error', 'spillage', or 'wrong measurements' as a reason for low yield without further qualification; examiners require specific procedural descriptions like 'loss of product on filter paper'.

    Key Terminology

    Essential terms to know

    Calculation of theoretical and percentage yield
    Calculation of atom economy from balanced equations
    Factors affecting yield (reversibility, side reactions, product loss)
    Industrial application and Green Chemistry principles

    Likely Command Words

    How questions on this topic are typically asked

    Calculate
    Explain
    Suggest
    Evaluate
    Compare

    Practical Links

    Related required practicals

    • •{"code":"PAG C3","title":"Preparation of crystals of a pure salt","relevance":"Calculation of percentage yield based on the mass of dry crystals obtained vs theoretical mass."}

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