Double Award - Science for GCSE

Chapter 23

Ammonia and fertilisers

1.	Reversible reactions and equilibria Reversible reactions can be made to go in either the forward or reverse direction by changing the conditions, or by adding or removing reagents. Irreversible reactions cannot be reversed. Once the reaction has taken place, the products cannot be turned back into the reactants. In some reversible reactions, the reactants are completely changed to the products. In other reversible reactions, the reactants are not completely changed to the products. This produces a mixture in which the concentrations of the reactants and products stay constant. We say the substances are in equilibrium. When equilibrium is reached, both the forward and backward reactions are still taking place, but at the same rate. So there is no change in the amount of any substance. This is described as a dynamic equilibrium with an equilibrium sign, rather than an arrow or arrows, as in the next equation.
2.	Manufacturing ammonia Ammonia is manufactured using the Haber process. The raw materials for the process are: – crude oil which is fractionally distilled to produce naphtha and then this is cracked to give hydrogen; – air which is liquefied and then fractionally distilled to produce nitrogen. These raw materials make the process as cheap as possible. The conditions used in the Haber process ensure a fast reaction: – pressure of 200 atm; – temperature of 450 °C; – iron catalyst. The yield of ammonia increases when: – the pressure is increased; – the temperature is reduced.
3.	Ammonia Ammonia is important in agriculture and industry. It is used to make fertilisers, nitric acid and nylon. Ammonia is a gas at room temperature – colourless, pungent and toxic (poisonous). Ammonia is a base and an alkali. So it neutralises acids to form salts. The reactions with acids are examples of neutralisation. Fertilisers, such as ammonium nitrate and ammonium sulphate, are made by neutralisation. Test for ammonia – ammonia turns damp red litmus paper blue.
4.	Fertilisers After carbon, hydrogen and oxygen, nitrogen is the most important element for plants to grow well. The nitrogen that plants need can be added to the soil in fertilisers as ammonium salts or nitrates. The most widely used fertiliser is ammonium nitrate, Fertilisers increase the yield from crops. The over-use of fertilisers can cause environmental problems: - changes in soil pH harms plants and animals; - pollution of ground water and drinking water; - eutrophication of rivers and streams.