Chapter 19 – Production of Ammonia

Properties of Ammonia

Uses of Ammonia The greatest use of ammonia is in fertilisers. Plants needs ammonia to make protein and other growth compounds, however, most plants cannot use the N 2 gas directly from the atmosphere. Instead they have to absorb soluble nitrogen compounds from the soil. The conversion of atmospheric nitrogen into soluble nitrogen compounds that can be used by plants is called nitrogen fixation.

Ammonia Fertilisers ammonium nitrate fertilizer … NH 3(aq) + HNO 3(aq)  NH 4 NO 3(aq) ammonium phosphate fertilizer 3NH 3(aq) + H 3 PO 4(aq)  (NH 4 ) 3 PO 4(aq) ammonium sulfate fertilizer … 2NH 3(aq) + H 2 SO 4(aq)  (NH 4 ) 2 SO 4(aq) urea fertilizer … 2NH 3(aq) + CO 2(g)  (NH 2 ) 2 CO (aq) + H 2 O (l)

Other uses… Ammonia is also used in the production of:- – nitric acid [4NH 3(g) + 5O 2(g)  4NO (g) + 6 H 2 O (g) ] – synthetic fibres [eg. nylon] – explosives [NH 4 NO 3 ] – pharmaceuticals [sulfonamides]

Production of Ammonia – Raw Materials Ammonia is formed from the reaction of nitrogen with hydrogen. The following is an example of production: STEP 1: CH 4 (g) + H 2 O(l) CO(g) + 3H 2 (g) STEP 2: CO(g) + H 2 O(l) CO 2 (g) + H 2 (g) Carbon dioxide is then removed so the H 2 can reaction with nitrogen.

Production of Ammonia – Haber Process Ammonia is manufacture using the Haber process, which employs an iron/iron oxide catalyst. The reaction is exothermic. N 2 (g) + 3H 2 (g)2NH 3 (g); ΔH = -91kJ mol -1 By using Le Chatelier’s principle you can predict the way in which temperature and pressure will alter the equilibrium yield of ammonia.

Haber Process cont… If the pressure exerted increases the system moves to decrease the pressure by moving the position of the equilibrium to the side with fewer particles. Therefore increased pressure will lead to an increase in the amount of ammonia. Since the forward reaction is exothermic, the position of equilibrium will lie further to the right and the equilibrium yield of ammonia will increase at low temperatures. The rate of reaction will be faster as the temperature increases, as the pressure increases or if a catalyst is used.

Waste Management The Haber process maximises the conversion of nitrogen and hydrogen into ammonia by recycling unreacted gases back into the converter for further passes over the catalyst. In this way, almost complete conversion is achieved. Because the hydrogen is recycled, the amount of hydrogen feedstock required from the hydrogen generation process is reduced. Consequently the amount of raw materials, energy and waste materials involved in the production of the hydrogen used in the Haber process is reduced. In some section of the production process, aqueous solutions of ammonia are produced when gases being released to the atmosphere are purified by passing them through water. These solutions can also be used in the manufacture of urea.

Health and Safety In gaseous form it is an extreme irritant to the eyes, respiratory system and other parts of the body. Areas need to be well ventilated and safety clothing must be worn. Because ammonia is a gas at room temperature, in the event of a spill, it can usually be allowed to disperse after ensuring safety of people who are downwind.