Chem ch19

Worked solutions to student book questions Chapter 19 Production of ammonia

Heinemann Chemistry 2 4th edition Enhanced Copyright Pearson Australia 2010 (a division of Pearson Australia Group Pty Ltd) 1

Q1. a Write balanced equations for each of the reactions of ammonia with acids to

make fertilisers described on page 320 of the student book. b Suppose each ammonium salt was sold at the same price per kilogram. Which

fertiliser would provide nitrogen most cheaply? c Apart from the result of your calculations in part b, what else might you consider

when choosing the best fertiliser for a particular application? d Why do farmers need to know the exact composition of the fertilisers they use?

A1.

a NH3(aq) + HNO3(aq) NH4NO3(aq) 2NH3(aq) + H2SO4(aq) (NH4)2SO4(aq) 3NH3(aq) + H3PO4(aq) (NH4)3PO4(aq) b Find the n(N) per kilogram:

n(NH4NO3) = 801000 = 12.5 mol, so n(N) = 2 12.5 = 25 mol

n((NH4)2SO4) = 1321000 = 7.6 mol, so n(N) = 2 17.6 = 15 mol

n((NH4)3PO4) = 1491000 = 6.7 mol, so n(N) = 3 6.7 = 20 mol

So ammonium nitrate is the cheapest. c Deficiencies of particular elements in the soil could mean that one type of

fertiliser is preferred over others. For instance, if the soil were deficient in phosphorus, ammonium phosphate might be the most appropriate fertiliser. Plants differ in the amounts of nutrients required to make them grow, so the type of crop to be cultivated is also likely to influence the choice of fertiliser.

d Different crops have different nutrient needs and farmers need to know what amounts of these nutrients they are applying in fertilisers. Too little would result in poor yields and too much would be expensive and wasteful, and possibly detrimental to crop yields and the environment.

Q2.

Construct a flow diagram to show each of the major steps in the production of ammonia from natural gas. Write important equations that occur beside each step in the process.

A2.



E1. Explain the meaning of the terms nitrogen fixing, arbuscular mycorrhiza and symbiosis.

AE1. Nitrogen fixing conversion of atmospheric nitrogen to ammonium and nitrate ions. Arbuscular mycorrihiza an association between the roots of a plant and soil fungi. The fungi absorb nutrients such as phosphates from the soil. These are exchanged for plant sugars which are used by the fungus. Symbiosis an association of two types of organisms that results in a benefit to both.

E2. Legume plants such as peas form a symbiosis with nitrogen-fixing bacteria. What are the benefits of this relationship to: a the plant? b the bacteria?

AE2. a The plant obtains nitrate and ammonium ions from the bacteria. b The bacteria absorbs and uses sugars produced by the plant.

E3. What would be the environmental benefits of using genetically engineered plants to add nitrogen to the soil compared with the use of nitrogenous fertilisers such as ammonia?

AE3.

The use of nitrogen fixing plants will overcome the adverse environmental effects of the manufacture and use of ammonia. For example, the industrial production of ammonia is energy intensive and produces pollutants. The run off into rivers and streams from the use of nitrogenous fertilisers, which are manufactured from ammonia, can result in the formation of toxic algal blooms.

E4. Is the addition of nitrogen to the soil using plants a green chemistry process? Explain your answer.

AE4. Green chemistry considerations that apply include prevention of waste, less hazardous production methods, greater energy efficiency, use of biodegradable materials.

Q3.

Using the information about waste management in the text, construct a table that lists the main by-products of ammonia production from natural gas and describes what becomes of each one.



A3.

Sulfur compounds

Removed during processing of petrochemicals and used to make sulfuric acid.

CO2 Liquefied and sold in limited quantities to the food and beverage markets. It is also used for manufacturing the fertiliser urea. Excess gas is released into the atmosphere.

NOx Released into the atmosphere at carefully controlled levels. NH3 Aqueous solutions of ammonia are purified by passing them through

water. These solutions can also be used in the manufacture of urea. Heat Used to heat incoming gases and converted to electricity.

Q4.

The principles of green chemistry can be used to evaluate the environmental impact of a chemical process. Construct a table with two columns headed Principles and Practice, as shown below. In the first column, list the twelve principles of green chemistry (Table 18.3 p. 310). In the second column, indicate the ways a modern plant using the Haber process could be regarded as complying with these principles. Principles Practice 1. Prevent waste . . . . . . . . . .

12. Minimise the potential for accidents

A4.

The production of ammonia is a mature industry established long before the applications of green chemistry practices were considered important. Nevertheless, a number of aspects of the industrial production of ammonia can be related to green chemistry principles.



Principles Practice Prevent waste Unreacted N2 and H2 are recycled

through the converter. Design safer chemicals and products Less hazardous synthesis Renewable raw materials Use catalyst Fe3O4 catalyst is used Avoid chemical derivatives Maximise atom economy 100% atom economy, high yields Use safer solvents and reaction conditions Increase energy efficiency Waste heat recycled Design for degradation Analyse in real time to prevent pollution Continuous monitoring of production and

plant Minimise the potential for accidents Stringent procedures for the storage,

transport sand handling of ammonia are in place.



Chapter review

Q5. Fritz Haber has been credited with developing a process that has helped to feed millions of people. He has also been regarded as responsible for killing 5000 other people. Habers work before the start of World War I allowed Germany to produce sufficient ammonia for its agricultural needs and for production of nitric acid, which wa sneeded to amke explosives. Find out more about his eventful life.

A5. Fritz Haber (18681934) made numerous contributions to chemical research as well as to the development of chemical technology in Germany. He was head of the Kaiser Wilhelm Institute, a leading centre of scientific study, for over twenty years. His award of the 1918 Nobel Prize was bitterly opposed by English and French scientists owing to his role in the advancement of chemical warfare during World War I. Haber developed mustard gas and phosgene for use in chemical warfare; these gases were responsible for the deaths of five thousand soldiers during the first German gas attack on the French in 1915. Habers work on the synthesis of ammonia was also crucial to the German war effort, enabling production of explosives to continue even after supplies of sodium nitrate from Chile were blocked. Among other things, his research led him to investigate many aspects of electrochemistry, and he developed a process for obtaining gold from sea water. Ironically, after all that Haber had done for his country, in 1933 he was forced to leave Germany forever because of his Jewish heritage.

Q6. During ammonia production the nitrogen and hydrogen gas are thoroughly purified before they are admitted to the reactor chamber. Explain the reason for this.

A6. To prevent poisoning of the catalyst.

Q7.

At 200C, nearly twice as much ammonia forms when a 3:1 hydrogen/nitrogen mixture under 250 atm pressure reaches equilibrium as is formed at 400C. Why are temperatures above 400C used to manufacture ammonia?

A7. Temperatures higher than 200oC are required to obtain an acceptably fast rate of reaction.



Q8. In the commercial production of ammonia, nitrogen is reduced by hydrogen gas. a Write a balanced chemical equation for the reaction. b Write an expression for the equilibrium constant for the reaction. c Predict the reaction conditions that would favour the maximum equilibrium yield

of ammonia. Explain your reasoning. d How would these conditions affect the rate of reaction? e Discuss the conflict that arises in the choice of reaction conditions for the Haber

process and the strategies used to deal with this conflict.

A8.

a N2(g) + 3H2(g) 2NH3(g)

b K = 322

23

]H][N[]NH[

c From Le Chateliers principle, since the number of gas particles on the right side of the equation is less than the number on the left side, increased pressures will lead to an increase in the amount of ammonia in equilibrium. Since the forward reaction is exothermic, the equilibrium constant will be higher at low temperatures, so the lower the reaction temperature, the greater will be the equilibrium yield.

d Rates will be fastest at high pressures and high temperatures. e In practice, a catalyst is used to increase reaction rate and a compromise

temperature which gives both an acceptable equilibrium yield and reaction rate is chosen.

Q9. During the Haber process, as gas passes through across the catalyst in the converter its temperature increases. a Why does the temperature of the gas rise? b Why is the gas leaving the converter cooled? c What side-benefit is obtained from the need to cool gases?

A9. a The reaction is exothermic. b The gas mixture leaving the reactor is cooled to liquefy the ammonia so it can be

removed. c The heat is used to raise the temperature of the incoming gas mixture, saving

costs.

Q10. Ammonia is normally liquefied to isolate it from the reaction mixture obtained from the Haber process. a Suggest another way that ammonia could be separated from nitrogen and

hydrogen gas. b What are the advantages and disadvantages of the method you have suggested?



A10. a Since ammonia is very soluble in water, ammonia gas could be separated from

the reaction mixture by passing the mixture through water. b Advantages: uses less energy than liquefaction of ammonia; method is relatively

straightforward. Disadvantages: less ammonia might be removed by this process; the exit gases

may need to be dried prior to recycling to the reactor; transportation costs might be higher as water is also involved.

Q11. Suppose that Fritz Haber did not invent a method to synthesise ammonia and that no economical method had ever been discovered. What do you think the world would be like today?

A11.

When answering this question, you should consider the consequences of less fertiliser having been available to societies around the world during the twentieth century.

Q12. Describe two ways that expenses are minimised in the Haber process.

A12. Recycling of nitrogen and hydrogen gases; use of heat exchangers to recover energy released by the reaction between nitrogen and hydrogen.

Q13. Under the title Ammonia: providing fertilisers to feed the world, write a paragraph which uses correctly the words: fertilisers, urea, Haber process, steam reforming, synthesis gas, converter, equilibrium, reaction rate and condenser.

A13. Ammonia: Providing fertilisers to feed the world Modern farming relies on the extensive use of synthetic fertilisers. Ammonia is widely used for making fertilisers such as urea. It is manufactured by the Haber process. This process involves the reaction between nitrogen and hydrogen. Hydrogen can be obtained from synthesis gas, which is produced in a process called steam reforming. In the Haber process, nitrogen and hydrogen are mixed in a reactor. Reaction conditions are carefully controlled to ensure that the reaction rate and equilibrium position give optimum yields. The gas mixture from the reactor is then passed to a condenser where the ammonia is liquefied and extracted.



Q14. A citizens action group calls a meeting of residents living near a major ammonia manufacturing plant with the intention of lobbying the government to have the plant moved because of health, safety and environmental concerns. a Design a flyer advertising the first public meeting of the group, highlighting the

main issues of concern. b The public relations manager of the ammonia company has been invited to speak

at the meeting. List the points that the manager might wish to make to the concerned residents.

A14. a The flyer might mention safety issues, such as the risk of spillage and concerns

about gas emissions. It might also mention the safety record of the company, traffic issues and visual pollution. (A flyer advertising a public meeting would also give the date, time and venue.)

b The plant manager might describe the safety precautions in place, the companys safety record, operation to strict government guidelines, provision of employment, incentives for associated industries with employment and the trade spin-offs for local community and traffic management plan.

Q15. Construct a concept map that includes the following terms: ammonia, hydrogen bonds, fertilisers, Haber process, converter, condenser, waste heat, desulfurisation, steam reforming and synthesis gas.

A15.



Q16. Design a safety notice that summarises: a the effects to a persons health that may result from exposure to ammonia fumes b the first aid treatment that should be used to treat exposure to ammonia c the personal protective equipment that someone working with ammonia should

use or have access to.

A16. a Effects on health Harmful if swallowed, burns throat Irritable to eyes Corrosive to skin, causes chemical burns Irritant to the respiratory system Exposure to high concentration (5000 ppm) may cause death b Medical treatment should be sought if a person is exposed to ammonia. First aid treatment. (This is a summary only. Refer to MSDS for full details.) If swallowed rinse mouth for 20 minutes If splashed into eyes flush eyes for 20 minutes If splashed onto skin flush skin with water If inhaled take patient into fresh air c Protective equipment to prevent skin and eye contact and the inhalation of

vapours Respiratory masks Eye goggles or face mask Protective clothing, including gloves and boots Access to eye bath and safety shower Access to MSDS

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Chem ch19