The Haber Process 1 of 30 Boardworks Ltd 2012

Transcription

1 The Haber Process 1 of 30 Boardworks Ltd 2012

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3 What is ammonia? 3 of 30 Boardworks Ltd 2012 Ammonia is an important compound in the manufacture of fertilizer and other chemicals such as cleaning fluids and floor waxes. It is made industrially by reacting nitrogen with hydrogen in the Haber process. The Haber process is a reversible reaction, so it never ends. Why is this a problem for companies making ammonia? nitrogen + hydrogen ammonia N 2 (g) + 3H 2 (g) 2NH 3 (g)

4 Feedstock 4 of 30 Boardworks Ltd 2012 The nitrogen gas needed for the Haber process comes from the air. The air is 78% nitrogen. Nitrogen can be separated out by fractional distillation. The hydrogen gas needed for the Haber process comes from methane (from natural gas). It is reacted with steam to form hydrogen. CH 4 (g) + H 2 O(g) CO(g) + 3H 2 (g)

5 The Haber process 5 of 30 Boardworks Ltd 2012

6 What is yield? ammonia yield (%) 6 of 30 Boardworks Ltd 2012 The amount of product made in a reaction is called the yield and is usually expressed as a percentage. The yield of ammonia produced by the Haber process depends on the temperature and pressure of the reaction. pressure (atm)

7 What is the Haber compromise? 7 of 30 Boardworks Ltd 2012 The highest yield of ammonia is theoretically produced by using a low temperature and a high pressure. In practice, these conditions are not used. Why? Lowering the temperature slows down the rate of reaction. This means it takes longer for ammonia to be produced. Increasing the pressure means stronger, more expensive equipment is needed. This increases the cost of producing the ammonia. A compromise is used to make an acceptable yield in a reasonable timeframe while keeping costs down.

8 Temperature, pressure and yield 8 of 30 Boardworks Ltd 2012

9 Changing the yield of ammonia 9 of 30 Boardworks Ltd 2012

10 The Haber compromise 10 of 30 Boardworks Ltd 2012 To produce a high yield of ammonia, but with a fast rate of reaction and without the need for overly expensive equipment, the Haber process is carried out at 450 C and 200 atmospheres. The most important factor in deciding what conditions to use is therefore not yield, but total cost. What costs are involved in the industrial production of ammonia? raw materials equipment energy wages.

11 Maximizing productivity 11 of 30 Boardworks Ltd 2012 What else can be done to maximize productivity in the manufacture of ammonia? An iron catalyst is used to increase the rate of reaction. It speeds up both the forward and backward reactions, so the position of equilibrium is not affected. The ammonia is cooled, liquefied and then removed as it is produced. This causes the equilibrium to shift to the right to produce more ammonia. Unreacted nitrogen and hydrogen are recycled and given another chance to react. Gases flow through the reactor too quickly for equilibrium to be reached.

12 Temperature, pressure and yield 12 of 30 Boardworks Ltd 2012

13 Stages of the Haber process 13 of 30 Boardworks Ltd 2012

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15 Environmental impact 15 of 30 Boardworks Ltd 2012 Large industrial plants producing ammonia and its feedstocks consume large amounts of power. This is normally obtained from burning fossil fuels, which releases carbon dioxide and contributes to global warming. A large chemical plant can also affect the environment in other ways. Local residents may experience noise, light and sound pollution from the plant.

16 How do fertilizers work? 16 of 30 Boardworks Ltd 2012 One of the main uses of ammonia is the production of fertilizers. Fertilizers increase crop yield by replacing the nutrients used up by previous crops on an area of land. This allows crops to be grown on the same land year after year. Plants absorb nutrients through their roots. Fertilizers must be dissolved in water before they can be absorbed by plants. Adding fertilizer to water systems can cause environmental problems.

17 Eutrophication 17 of 30 Boardworks Ltd 2012

18 Eutrophication: summary 18 of 30 Boardworks Ltd 2012

19 Reasons for using fertilizers 19 of 30 Boardworks Ltd 2012 Artificial nitrogen-based fertilizers are important, as they increase yield and allow land to be farmed more intensively. Using fertilizers can also allow crops to be grown on poor quality soils. This increases the amount of crops that can be grown. It is estimated that around one-third of the world s population is sustained by the use of artificial fertilizers.

20 Evaluating artificial fertilizers 20 of 30 Boardworks Ltd 2012

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22 How are nitrates produced? 22 of 30 Boardworks Ltd 2012 Nitrates are important because they are a form of nitrogen that plants can absorb. Nitrogen is used by plants to make protein and is passed from plants to animals along food chains. What processes add nitrates to the soil? Decomposers release ammonium compounds from waste (such as urea in urine) and dead matter. Nitrifying bacteria then convert the ammonium compounds into nitrates. Some nitrogen compounds form during lightning strikes and are washed into the soil by rain water.

23 Can plants add nitrogen to the soil? 23 of 30 Boardworks Ltd 2012 This rugged landscape is in Iceland. Deforestation by the original settlers and high levels of volcanic activity have left much of the country as bare lava or sand an environment in which few plants grow. In the 1960s, Iceland began to manage its soil. It decided to drop millions of lupin seeds from the air. Why might lupins survive where nothing else grows?

24 What are legumes? 24 of 30 Boardworks Ltd 2012 Most plants need nitrates from the soil because the nitrogen in air is too unreactive. These plants rely on the presence of nitrifying bacteria in the soil or artificial fertilizers. By contrast, lupins and other legumes, such as clover and peas, are self-sufficient in nitrates. Nitrogen-fixing bacteria living in the root nodules of legumes convert nitrogen gas into nitrates, improving the fertility of poor-quality soils.

25 Enzymes 25 of 30 Boardworks Ltd 2012 Nitrogen-fixing bacteria use nitrogenase enzymes to produce nitrates from the nitrogen in the air. N 2 + 8H + + 8e 2NH 3 + H 2 Why would scientists be interested in producing new catalysts that mimic these natural enzymes? Nitrates could be produced without the high energy requirements of the Haber process. The starting materials would come from the air and not from non-renewable fossil fuels. The reaction goes to completion.

26 Comparing nitrogen fixation methods 26 of 30 Boardworks Ltd 2012

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28 Glossary 28 of 30 Boardworks Ltd 2012

29 Anagrams 29 of 30 Boardworks Ltd 2012

30 Multiple-choice quiz 30 of 30 Boardworks Ltd 2012