Fertilisers
Fertilisers are substances that contain the minerals needed for plants to grow. Artificial fertilisers contain ammonia which is industrially manufactured using the Haber process.
Please note that this topic is only included on the AQA triple science specification.
The Haber Process
The Haber process is an industrial process used to manufacture ammonia, which is a major component of fertilisers. It involves reacting nitrogen and hydrogen in the presence of an iron catalyst. Nitrogen is obtained from the air whereas hydrogen is obtained from natural gas (a fossil fuel). The purified gases are passed over an iron catalyst at high temperatures (around 450oC) and high pressures (around 200 atm). Some of the hydrogen and nitrogen react to form ammonia. This reaction is reversible so some ammonia will break down to reform hydrogen and nitrogen. The ammonia can be removed from the reaction vessel by cooling and allowing the ammonia to liquefy. The liquid ammonia can then be removed and any remaining hydrogen and nitrogen is recycled.
This reaction can reach dynamic equilibrium when it is in a closed container (revise equilibrium reactions here). According to Le Chatelier’s principle, to maximise the yield of ammonia we need to use high pressures (because there are fewer gas molecules on the right) and low temperatures (because the forward reaction is exothermic). However, if low temperatures are used the reaction would take place extremely slowly. In practice, a compromise is reached where high temperatures and high pressures are used. Pressures aren’t increased any further than 200 atm due to cost and safety reasons.
NPK fertilisers
Fertilisers are substances which contain the mineral ions plants need to grow. These mineral ions include nitrogen compounds (needed for proteins and DNA), phosphorous (for DNA) and potassium (for enzymes). As plants absorb these ions from the soil, the mineral ion content of the soil decreases. Fertilisers replenish the mineral ions in the soil and can be natural fertilisers (such as manure and garden waste) or chemical fertilisers (made using the Haber process). Chemical fertilisers are formulations (mixtures of compounds) containing the appropriate proportions of nitrogen, phosphorous and potassium compounds.
Fertilisers that contain all three elements (nitrogen, phosphorous and potassium) are called NPK fertilisers. Industrial production of NPK fertilisers can be achieved using a variety of raw materials in several integrated processes. The compounds in NPK fertilisers are obtained from the following substances:
Ammonia is used as a source of ammonium salts and nitric acid
Potassium chloride and potassium sulfate can be extracted from the ground by mining
Phosphate rock can also be obtained by mining but it is insoluble so cannot be absorbed by plant. It first needs to be treated with acid to produce soluble potassium salts.
Different potassium salts are formed depending on the acid used:
Reacting phosphate rock with nitric acid produces calcium nitrate and phosphoric acid
Reacting phosphate rock with sulfuric acid produces single superphosphate (a mixture of calcium sulfate and calcium phosphate)
Reacting phosphate rock with phosphoric acid produces triple superphosphate (aka calcium phosphate)
Did you know…
Farmers add fertilisers to the soil to increase crop growth. A major ingredient of fertilisers, ammonium nitrate is also used in bombs due to its explosive nature. When mysterious reports of exploding trousers emerged in New Zealand in the 1930s, it was discovered that the farmers had accidentally sprayed ammonium nitrate onto their clothes, causing their trousers to explode when they were near a fire or in direct sunlight.