Fertilisers (Edexcel GCSE Chemistry): Revision Notes
📚 Revision Notes
Fertilisers
Essential Elements in Fertilisers
Plants need certain elements from the soil to grow properly. The three main essential elements found in fertilisers are:
- Nitrogen (N)
- Phosphorus (P)
- Potassium (K)
Why are these elements important?
If plants don't get enough of these elements, it affects their growth and life processes.
Fertilisers supply these essential elements when they are missing from the soil, or provide more to:
- Increase crop yield
- Produce bigger crops
- Promote faster growth
Ammonia in Fertilisers
- Ammonia (NH₃) is an important chemical used to produce nitrogen-containing compounds for fertilisers.
- Ammonia can undergo a series of reactions with oxygen and water to produce nitric acid.
- Ammonia can also react with acids, such as nitric acid or phosphoric acid, to produce ammonium salts, which are used in fertilisers.
Producing Ammonium Sulphate (Small Scale in the Lab)
Ammonium sulphate (NH₄)₂SO₄ is a nitrogen-rich fertiliser that can be made in a lab using ammonia and dilute sulfuric acid.
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Equipment needed:
- Ammonia (alkali)
- Dilute sulfuric acid (acid)
- Methyl orange indicator
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Method:
- Add methyl orange to the ammonia solution. The indicator will turn yellow in the alkaline solution.
- Slowly add dilute sulfuric acid from a burette into the ammonia solution, swirling the mixture. When the solution turns red, the ammonia has been fully neutralised, forming ammonium sulfate.
- The first solution contains the indicator, so it is not pure. Repeat the experiment using the exact volume of sulfuric acid needed to neutralise the ammonia, but without the indicator.
- To obtain solid ammonium sulphate crystals:
- Evaporate the solution using a steam bath until only a small volume remains.
- Leave the solution to crystallise.
- Philtre the crystals and leave them to dry on a windowsill.
Raw Materials:
- Ammonia is produced via the Haber Process.
- Sulfuric acid is produced via the Contact Process. The industrial production of ammonium sulphate is much larger and more complex than the lab method.
Industrial Method:
- Ammonia gas is introduced into a large reaction chamber.
- Sulfuric acid is sprayed into the chamber.
- The ammonia reacts with sulfuric acid to produce ammonium sulphate powder.
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Comparison Between Lab and Industrial Production:
- Rates of Reaction: The industrial process is designed for high efficiency and speed, while the lab method is slower and more controlled.
- Reversible Reactions and Equilibrium: The Haber Process and Contact Process involve reversible reactions that are influenced by factors such as temperature and pressure.
- Atom Economy and Yield: The industrial process aims for high atom economy (minimal waste) and maximises yield to ensure cost-efficiency.
- Availability of Raw Materials: On an industrial scale, large amounts of ammonia and sulfuric acid are required, while the lab method only needs small quantities.
- Energy Use: Industrial processes require more energy, especially for the high temperatures and pressures in the Haber and Contact processes.