Haber Process and NPK Fertilisers

    OCR
    GCSE

    The Haber process synthesizes ammonia from nitrogen and hydrogen via a reversible reaction, necessitating the application of Le Chatelier's Principle to determine compromise conditions (450°C, 200 atm, iron catalyst) that balance reaction rate against equilibrium yield and economic cost. Ammonia serves as the primary feedstock for the production of nitric acid and ammonium salts, which are essential components of NPK fertilisers alongside potassium salts and treated phosphate rock. Candidates must evaluate the industrial production of fertilisers against laboratory synthesis methods, considering scale, continuous versus batch processing, and the specific chemical treatment of phosphate rock with nitric, sulfuric, or phosphoric acids.

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    Objectives
    3
    Exam Tips
    4
    Pitfalls
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    Key Terms
    5
    Mark Points

    What You Need to Demonstrate

    Key skills and knowledge for this topic

    • Award 1 mark for identifying the raw materials: Nitrogen from fractional distillation of air and Hydrogen from reacting methane (natural gas) with steam.
    • Credit responses that explain 450°C is a compromise: lower temperatures favor yield (exothermic forward reaction) but the rate of reaction would be economically too slow.
    • Award 1 mark for stating that the iron catalyst increases the rate of both forward and reverse reactions equally, having no effect on the position of equilibrium or yield.
    • For NPK production, credit the correct identification of salts formed when phosphate rock reacts with nitric acid (calcium nitrate), sulfuric acid (single superphosphate), or phosphoric acid (triple superphosphate).
    • Award marks for contrasting production methods: Laboratory uses titration/crystallisation (batch, small scale) whereas Industry uses large reaction chambers (continuous, large scale).

    Example Examiner Feedback

    Real feedback patterns examiners use when marking

    • "You correctly identified the conditions, but you need to explain *why* 450°C is used despite the reaction being exothermic."
    • "Be careful with the catalyst—remember it speeds up the reverse reaction too, so it doesn't change the equilibrium position."
    • "Good calculation of atom economy. Now, consider why a company might still choose a process with lower atom economy if the raw materials are cheap."
    • "When comparing lab and industry methods, use comparative terms like 'continuous vs batch' and 'pure reactants vs raw materials'."

    Marking Points

    Key points examiners look for in your answers

    • Award 1 mark for identifying the raw materials: Nitrogen from fractional distillation of air and Hydrogen from reacting methane (natural gas) with steam.
    • Credit responses that explain 450°C is a compromise: lower temperatures favor yield (exothermic forward reaction) but the rate of reaction would be economically too slow.
    • Award 1 mark for stating that the iron catalyst increases the rate of both forward and reverse reactions equally, having no effect on the position of equilibrium or yield.
    • For NPK production, credit the correct identification of salts formed when phosphate rock reacts with nitric acid (calcium nitrate), sulfuric acid (single superphosphate), or phosphoric acid (triple superphosphate).
    • Award marks for contrasting production methods: Laboratory uses titration/crystallisation (batch, small scale) whereas Industry uses large reaction chambers (continuous, large scale).

    Examiner Tips

    Expert advice for maximising your marks

    • 💡In 6-mark 'Level of Response' questions comparing industrial and lab production of fertilisers, ensure you cover three distinct categories: scale, equipment/method, and rate of production.
    • 💡When discussing temperature, use the term 'compromise' and explicitly link it to the opposing effects on rate (kinetics) and yield (equilibrium).
    • 💡Memorise the specific products of phosphate rock reactions; 'calcium salts' is often insufficient—specify calcium nitrate or calcium sulfate.

    Common Mistakes

    Pitfalls to avoid in your exam answers

    • Incorrectly stating that the iron catalyst increases the percentage yield of ammonia rather than just the rate of reaction.
    • Failing to link the high cost of maintaining high pressures (energy and thick-walled equipment) to the choice of 200 atmospheres; students often cite only safety.
    • Confusing the industrial source of hydrogen; candidates often suggest electrolysis of water instead of the reaction of methane with steam.
    • Stating that phosphate rock can be used directly as a fertiliser without acid treatment (it is insoluble).

    Key Terminology

    Essential terms to know

    Reversible reactions and dynamic equilibrium
    Compromise conditions (Temperature, Pressure, Catalyst)
    Industrial vs. Laboratory preparation of ammonium salts
    Chemical treatment of phosphate rock
    NPK formulations and plant nutrients

    Likely Command Words

    How questions on this topic are typically asked

    State
    Explain
    Evaluate
    Predict
    Calculate
    Compare

    Practical Links

    Related required practicals

    • •{"code":"PAG C4","title":"Preparation of salts (Ammonium Sulfate)","relevance":"Comparison of laboratory titration method vs industrial continuous process"}

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