Chapter 19 Ammonia PDF

CHAPTER 19 Ammonia © 2013 Marshall Cavendish International (Singapore) Private Limited Chapter 19 Ammonia 19.1 Reversible Reactions 19.2 Manufacturing Ammonia by the Haber Process 19.3 Displacement of Ammonia from its Salts 2 19.1 Reve...

CHAPTER 19 Ammonia © 2013 Marshall Cavendish International (Singapore) Private Limited Chapter 19 Ammonia 19.1 Reversible Reactions 19.2 Manufacturing Ammonia by the Haber Process 19.3 Displacement of Ammonia from its Salts 2 19.1 Reversible Reactions Learning Outcome At the end of this section, you should be able to: state that some reactions, such as the formation of ammonia, are reversible. 3 19.1 Reversible Reactions Many chemical reactions can proceed in one direction only, i.e. they cannot be reversed. However, some chemical reactions are reversible. One such example is the reaction between nitrogen and hydrogen to form ammonia. N2(g) + 3H2(g) ⇌ 2NH3(g) 4 19.1 Reversible Reactions Forward Reaction Backward Reaction N2(g) + 3H2(g) → 2NH3(g) 2NH3(g) → N2(g) + 3H2(g) Reversible Reaction N2(g) + 3H2(g) ⇌ 2NH3(g) 5  19.1 Reversible Reactions Solid ammonium chloride decomposes upon heating to form ammonia gas and hydrogen chloride gas. ammonium chloride → ammonia + hydrogen chloride NH4Cl(s) → NH3(g) + HCl(g) ammonium chloride Upon cooling, solid ammonium chloride reforms. ammonia + hydrogen → ammonium chloride chloride NH3(g) + HCl(g) → NH4Cl(s) 6  19.1 Reversible Reactions A reversible reaction can go both forward and backward at the same time. A double arrow sign, ⇌, is used to indicate a reversible reaction. NH4Cl(s) ⇌ NH3(g) + HCl(g) The reaction from left to right is called the forward reaction. The reaction from right to left is called the backward reaction. 7 Chapter 19 Ammonia 19.1 Reversible Reactions 19.2 Manufacturing Ammonia by the Haber Process 19.3 Displacement of Ammonia from its Salts 8 19.2 Manufacturing Ammonia by the Haber Process Learning Outcomes At the end of this section, you should be able to: describe the manufacture of ammonia from nitrogen and hydrogen; describe the operating conditions in the manufacture of ammonia in the Haber process. 9 19.2 Manufacturing Ammonia by the Haber Process Conditions Required for Manufacturing Ammonia Raw materials: nitrogen and hydrogen obtained from the fractional produced from the distillation of liquid air cracking of petroleum Conditions: A high pressure and a relatively high temperature needed. Iron is used to speed up the reaction. finely divided iron nitrogen + hydrogen ammonia 10 19.2 Manufacturing Ammonia by the Haber Process Conditions Required for Manufacturing Ammonia The reaction between hydrogen and nitrogen to form ammonia is a reversible process. Some of the ammonia formed may revert to nitrogen and hydrogen. So to achieve the maximum yield of ammonia at the minimum cost, the reaction conditions are very carefully controlled. 11 19.2 Manufacturing Ammonia by the Haber Process How is the optimal pressure for the manufacture of ammonia selected? The higher the pressure, the higher the yield of ammonia. High pressure also increases the speed of the reaction. Maintaining high pressure is costly. Thus, there is a limit to the amount of pressure that can be applied. Yield of ammonia/ % 50 450°C 40 500°C 30 20 600°C 10 Pressure/atm 12 100 200 300 400 19.2 Manufacturing Ammonia by the Haber Process How is the optimal temperature for the manufacture of ammonia selected? The lower the temperature, Yield of the higher the yield of ammonia/% ammonia. This is because the 50 450°C decomposition of ammonia 40 500°C into H2 and N2 is reduced. 30 Lower temperature also 20 600°C results in slower reaction. 10 Thus, a relative high 100 200 300 400 Pressure/atm temperature of 450°C is used. Why is a catalyst used? A catalyst is used to increase the speed of reaction. 13 19.2 Manufacturing Ammonia by the Haber Process What are the operating conditions in the Haber process? It was found that the best conditions for producing ammonia in the Haber process are: a pressure of 250 atm; a temperature of 450°C; the presence of finely divided iron catalyst. 14 19.2 Manufacturing Ammonia by the Haber Process N2 (1 volume) from H2 (3 volumes) from fractional distillation of cracking of petroleum liquid air 1. Nitrogen and hydrogen are mixed in the proportion 1:3 by volume. 2. The mixture of gases is compressed to 250 atm. 3. The gases are heated to 450°C and passed over finely divided iron. Reaction is exothermic. Only 10–15% of nitrogen and hydrogen is converted to ammonia. 15 19.2 Manufacturing Ammonia by the Haber Process 6. Unreacted nitrogen and hydrogen are pumped back into the converter for further reaction (recycled). 5. Ammonia gas condenses to form liquid ammonia. 4. A mixture of ammonia, nitrogen and hydrogen is obtained. The mixture of gases is cooled. 16 Chapter 19 Ammonia 19.1 Reversible Reactions 19.2 Manufacturing Ammonia by the Haber Process 19.3 Displacement of Ammonia from its Salts 17 19.3 Displacement of Ammonia from its Salts Learning Outcome At the end of this section, you should be able to: describe the displacement of ammonia from its salts. 18 19.3 Displacement of Ammonia from its Salts Whenever an ammonium salt is heated with an alkali, ammonia is displaced from the salt. NH4Cl(s) + NaOH(aq) → NH3(g) + H2O(l) + NaCl(aq) Examples of ammonium salts: ammonium nitrate, ammonium sulfate Examples of alkalis: sodium hydroxide, calcium hydroxide 19 Chapter 19 Ammonia Concept Map 20 Chapter 19 Ammonia The URLs are valid as at 15 October 2012. Acknowledgements (slide 1) fertilisers © egiss | iStockphoto.com 21