Fertilisers (Edexcel GCSE Chemistry)

Fertilisers

• Compounds containing nitrogen, potassium and phosphorus are used as fertilisers to increase crop yields

• NPK fertilisers are formulations containing appropriate ratios of all three elements

• From these three essential elements:

  • Nitrogen promotes healthy leaves,

  • Potassium promotes growth, healthy fruit and flowers

  • Phosphorus promotes healthy roots

• A distinct advantage of artificial fertilisers is that they can be designed for specific needs whereas in natural fertilizers, such seaweed or manure, the proportions of elements cannot be controlled

• Fertiliser compounds contain the following water soluble ions:

  • Ammonium ions, NH₄⁺ and nitrate ions, NO₃^–, which are sources of soluble nitrogen

  • Phosphate ions, PO₄^3-, which are a source of soluble phosphorus

• Most common potassium compounds dissolve in water to produce potassium ions, K⁺

• Ammonia is an alkaline substance and neutralises acids producing a salt and water

• The salt it produces contains the ammonium ion, NH₄⁺, which is a component of several fertilisers

• Ammonia also undergoes oxidation to produce nitric acid, HNO₃

• Nitric acid is used as the source of the nitrate ion, NO₃^–, which is another important ion found in fertilisers

• Ammonium nitrate, a fertiliser and one of the most important ammonium salts, is made by reacting ammonia with nitric acid:

NH₃ (aq) + HNO₃ (aq) → NH₄NO₃ (aq)

Making Fertilisers

Preparation of Ammonium Sulfate in the Laboratory

Aim: 

• To prepare ammonium sulfate by titration:

2NH₃ + H₂SO₄ → (NH₄)₂SO₄

Materials:

• Dilute ammonia solution, dilute solution of sulfuric acid, methyl orange indicator

• Clamp and stand, burette and volumetric pipette, conical flask, white tile

Steps in the laboratory preparation of ammonium sulfate

Method:

• Add an exact volume of ammonia to the conical flask and place on the white tile

• Add a few drops of indicator and swirl, it should turn yellow

• Add the acid to the flask solution drop by drop, swirling the flask in between

• Continue until the colour turns red sharply and record the titre

• Repeat by adding exactly the same amount of acid but this time without the indicator which is an impurity

• Pour the reaction mixture in an evaporating dish and gently heat in a water bath to remove some of the water

• Stop heating when the volume has been reduced to roughly one third of its volume

• Leave in a dry place so the remaining water evaporates, allowing crystallisation to occur

• This may take a few days depending on ambient conditions

Analysis of results:

• After a few days ammonium sulfate crystals should appear

• Filter to remove any remaining water

Industrial Preparation

• The industrial preparation of ammonium sulfate is a large scale operation consisting of several stages

• Ammonia is prepared by the Haber process and sulfuric acid by the Contact process

• Both processes require their own supplies of raw materials, energy and equipment

• The most common industrial process of manufacturing ammonium sulfate involves filling a large reactor chamber with ammonia gas.

• Sulfuric acid is sprayed into the chamber from above and ammonium sulfate powder is produced

• Another method involves pumping a mixture of ammonia gas and steam in a reactor which contains some sulfuric acid and a concentrated solution of ammonium sulfate

• The reaction is carried out at 60 ºC and concentrated sulfuric is added gradually

Comparing the laboratory preparation & industrial production of ammonium sulfate

• Comparing the two processes highlights the challenges of scaling up laboratory preparations to industrial levels

Different Ways to Make Ammonium Sulfate