Fertilisers IGCSE Chemistry

Ammonia Ammonia is a colourless gas. It is highly soluble in water. It has choking smell. Ammonia is an alkaline gas. It turns the litmus colour from red to blue. Ammonia is a weak alkali.

Ammonium compounds Formula for ammonia is NH3 Compounds of ammonia are called ammonium NH4+ Examples of ammonium compounds: Ammonium chloride, ammonium sulphate, ammonium nitrate, ammonium phosphate etc. Ammonium ion is tested by heating it with sodium hydroxide and test using a litmus paper. The litmus colour changes to blue. This is due to the formation of ammonia gas when ammonium compound is heated with alkali

Fertilisers Fertilisers are used to improve the quality of soil for proper plant growth. The three important elements plants require for proper growth are: Nitrogen, Phosphorus and Potassium (NPK) Plants are not able to absorb or use nitrogen in the air (about 78-80%) (nitrogen in the air is inert and so not useful for plants directly). Nitrogen should be converted to nitrogen containing compounds before it is absorbed by plants. Nitrogen is important for plants because it is a must when they make proteins.

Nitrates NO3- Nitrates are nitrogen containing compounds So many nitrates are used as fertilisers. Test for nitrates: Heat a little of the nitrate with a some aluminium powder (or foil) and sodium hydroxide solution. Show a litmus paper at the mouth of the test tube. It should turn blue (because ammonia gas is produced in the test)

Nitrogen fixation Nitrogen fixing bacteria living in the root nodules of certain plants (pea, bean etc) convert atmospheric nitrogen to ammonia and nitrates.

Nitrogen fixation The conversion of nitrogen to useful nitrogen compounds is called nitrogen fixation. During thunder-storms, nitrogen and oxygen in the air react to for nitrogen oxides. Nitrogen oxides dissolve in rain water to form dilute nitric and nitrous acids. These acids help to make nitrogen salts when reacting with compounds in the soil.

Compounds important for fertiliser manufacture Ammonia, Sulphuric acid and Nitric acid are important compounds to manufacture fertilisers. Ammonia is manufactured by Haber process N2 + 3H2   2NH3 (reversible reaction) This reaction needs Iron as catalyst. Sulphuric acid is manufactured by Contact process. First sulphur is changed to sulphur dioxide. S + O2  SO2

Then sulphur dioxide is oxidised to sulphur trioxide SO2 + O2   SO3 (reversible reaction) This reaction needs Vanadium (V)oxide as catalyst. Sulphur trioxide is then dissolved in water to get sulphuric acid SO3 + H2O  H2SO4

Nitric acid First, ammonia is oxidised to nitrogen dioxide. Ammonia + Oxygen  Nitrogen dioxide 4NH3 + 7O2  4NO2 + 6H2O This reaction needs platinum catalyst at 850 C Nitrogen dioxide is mixed with water with more oxygen produces nitric acid. Nitrogen dioxide + water + Oxygen  Nitric acid 4NO2 + O2 + 2H2O  4HNO3

Leaching leaching is the removal of a important minerals from soil by rain water. Nitrate is important for the plant growth and so leaching of nitrate is a big problem for soil. Leaching of potassium or phosphates is not problematic because these can be easily come back to soil by natural means

Important fertilisers Ammonium sulphate (NH4)2SO4 Ammonium nitrate NH4NO3 Potassium sulphate K2SO4 Ammonium phosphate (NH4)3PO4 Find out which fertilizer contains highest amount of nitrogen THE END