CIE IGCSE Chemistry

Revision Notes

11.2.1 Nitrogen & Fertilisers

Fertilisers & Ammonia

NPK Fertilisers

  • Fertilisers contain nitrogen, potassium and phosphorus
  • Nitrogen promotes healthy leaves, potassium promotes growth and healthy fruit and flowers and phosphorus promotes healthy roots
  • Fertiliser compounds contain the following water-soluble ions:
    • Ammonium ions, NH4+ and nitrate ions, NO3, are sources of soluble nitrogen
    • Phosphate ions, PO43- are a source of soluble phosphorus
    • Most common potassium compounds dissolve in water to produce potassium ions, K+

Displacement of ammonia

  • Ammonia can be displaced from its salts by the addition of an alkali substance
  • Farmers regularly add basic substances such as calcium hydroxide to their soil to neutralise any excess soil acidity
  • If too much of the basic substance is added or if it has been added too soon after fertiliser has been added, then an ammonia displacement reaction may occur
  • This involves the loss of nitrogen from the fertiliser, nullifying its effectiveness as a fertiliser
  • For example, the salt ammonium chloride is used extensively in fertilisers and reacts with calcium hydroxide:
2NH4Cl + Ca(OH)2 → CaCl2 + 2NH3 + 2H2O
Extended Only

Manufacture of Ammonia

  • Ammonia is manufactured using The Haber Process which occurs in five stages
  • Stage 1: H2 and N2 are obtained from natural gas and the air respectively and are pumped into the compressor through pipe
  • Stage 2: the gases are compressed to about 200 atmospheres inside the compressor
  • Stage 3: the pressurised gases are pumped into a tank containing layers of catalytic iron beads at a temperature of 450°C. Some of the hydrogen and nitrogen react to form ammonia:
N2(g) + 3H2(g) ⇌ 2NH3(g)
  • Stage 4: unreacted H2 and N2 and product ammonia pass into a cooling tank. The ammonia is liquefied and removed to pressurised storage vessels
  • Stage 5: the unreacted H2 and N2 gases are recycled back into the system and start over again

 

, IGCSE & GCSE Chemistry revision notesThe production of ammonia by the Haber Process

 

Conditions

Temperature: 450ºC

  • A higher temperature would favour the reverse reaction as it is endothermic (takes in heat) so a higher yield of reactants would be made
  • If a lower temperature is used it favours the forward reaction as it is exothermic (releases heat) so a higher yield of products will be mad
  • However at a lower temperature the rate of reaction is very slow
  • So 450ºC is a compromise temperature between having a lower yield of products but being made more quickly

Pressure: 200 atm

  • A lower pressure would favour the reverse reaction as the system will try to increase the pressure by creating more molecules (4 molecules of gaseous reactants) so a higher yield of reactants will be made
  • A higher pressure would favour the forward reaction as it will try to decrease the pressure by creating less molecules (2 molecules of gaseous products) so a higher yield of products will be made
  • However high pressures can be dangerous and very expensive equipment is needed
  • So 200 atm is a compromise pressure between a lower yield of products being made safely and economically

 

Conditions for Haber process, IGCSE & GCSE Chemistry revision notesChoosing the conditions for the Haber Process

Exam Tip

The reaction conditions chosen for the Haber process are not ideal in terms of the yield but do provide balance between product yield, reaction rate and production cost. These are called compromise conditions as they are chosen to give a good compromise between the yield, rate and cost.

Author: Morgan

Morgan’s passion for the Periodic Table begun on his 10th birthday when he received his first Chemistry set. After studying the subject at university he went on to become a fully fledged Chemistry teacher, and now works in an international school in Madrid! In his spare time he helps create our fantastic resources to help you ace your exams.
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