Fertilizers and Ammonia

N, K, P fertilizers

• Fertilisers contain nitrogen, potassium and phosphorus

  • Nitrogen makes chlorophyll and protein and promotes healthy leaves

  • Potassium promotes growth and healthy fruit and flowers

  • 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+

• Common fertiliser compounds include:

  • Ammonium nitrate, NH4NO3

  • Ammonium phosphate, (NH4)3PO4

  • Potassium sulfate, K2SO4

• Different fertilisers contain different amounts of fertiliser compounds so each contain different proportions of nitrogen, potassium and phosphorous

Displacement of Ammonia

• Ammonia can be displaced from its salts by the addition of an alkali substance

• Farmers regularly add basic substances such as calciumhydroxide 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

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 beds 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 the 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

Conditions

• As the reaction is reversible and the forward reaction is exothermic, the amount of ammonia produced (the yield) can be favoured by altering the conditions

Temperature: 450 ºC

• 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 made

• However at a lower temperature the rate of reaction is very slow

• So 450 ºC 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

Catalyst: Iron

• The iron catalyst does not increase the yield of ammonia produced but it does increase the rate of reaction, meaning that more ammonia can be made in a given time