Chemistry · Using resources. The idea in plain words, the rules to remember, the mistake to avoid and a worked example, written and checked by Mega Lecture.
The Haber process makes ammonia from nitrogen and hydrogen. The purified gases pass over an iron catalyst at a high temperature and a high pressure. The reaction is reversible, so only some of the gases react. The ammonia is cooled until it turns to liquid and is removed, and the unreacted nitrogen and hydrogen are recycled.
The conditions are a compromise (Higher Tier). A lower temperature gives a higher yield, but the reaction is too slow. A higher pressure gives a higher yield and a faster rate, but the equipment costs too much and is less safe.
Ammonia is used to make NPK fertilisers, which are formulations that supply nitrogen, phosphorus and potassium to plants.
Students say 450 °C is chosen because it gives the best yield. A lower temperature gives a better yield; 450 °C is a compromise that gives a reasonable yield at a fast enough rate.
Answer: 21.2% nitrogen by mass (to 3 significant figures).
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