Chem ch20

Worked solutions to student book questions Chapter 20 Production of nitric acid

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

Q1. TNT is one of the products made from nitric acid. Combustion of TNT (C7H5N3O6) is rapid and releases large amounts of energy. a Write a balanced equation for this reaction if the products are carbon dioxide,

water and nitrogen. b By referring to the equation from part a, explain why TNT is used as an

explosive.

A1. a 4C7H5N3O6(l) + 21O2(g) → 28CO2(g) + 10H2O(g) + 6N2(g) b For each mole of TNT that reacts, 11 moles of gaseous products are formed.

Since the reaction is highly exothermic, the pressure created when the hot gases are generated in a confined space is enormous.

Q2. Prepare a flow chart to show the sequence of processes used to convert ammonia to concentrated nitric acid. Write equations for significant chemical reactions beside the appropriate section of the flow chart.

A2.

E1. Adipic acid (hexanedioic acid), HOOC(CH4)4COOH, and 1,6-diaminohexane, H2N(CH2)6NH2, are monomers used to manufacture nylon 6,6. a Draw a section of the repeating units of this polymer. b What other substance is formed in this polymerisation reaction? c What name is given to the bond formed between the monomer units? d Which polymer of biological importance has the same linkage between its

monomers?

AE1.

a [~NH(CH2)6NHCO(CH2)4CO~]n b water molecule c amide bonds –NHCO d polypeptide bonds formed between the amino acid monomers in proteins have the

same structure as the amide bond formed between the monomers in polyamides such as nylon.



E2. Why is the synthesis pathway developed by Draths and Frost considered to be a green chemistry alternative for the production of adipic acid?

AE2. The adipic acid synthesis using biocatalysts would be considered greener as:

• less hazardous materials are used instead of nitric acid and benzene • environmentally adverse wastes such as N2O are not produced • safer reaction conditions are used. • catalysts (enzymes) are used • process is more energy efficiency • waste materials are biodegradable • glucose, a renewable raw material, is used instead of finite resources.

Q3. Using the information in the text, construct a table that lists the main wastes from nitric acid production, how they are treated, and how the quantities produced are reduced.

A3.

NOx Removed from exhaust gases by reducing it to nitrogen with a fuel over a catalyst. Alternatively, the absorption tower may be modified by increasing its size or operating pressure or an additional absorption tower may be employed.

N2O Methods to reduce emissions of this gas are being tried, including use of an additional catalyst in the converter to decompose it and catalytic reduction of the gas in waste emissions using ammonia.

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 Ostwald process could be regarded as complying with these principles. Principles Practice 1. Prevent waste . . .

12. Minimise the potential for accidents

A4. The production of nitric acid 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 nitric acid can be related to green chemistry principles. Principles Practice Prevent waste Reaction conditions are optimised to

maximise production of NO and to minimise production of other oxides of nitrogen.

Design safer chemicals and products Less hazardous synthesis Renewable raw materials Use catalyst Pt/Rh catalyst is used Avoid chemical derivatives Maximise atom economy 77% atom economy 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 and handling of nitric acid are in place.



Chapter review

Q5. The Arab alchemist Jabir ibin Hayyan is credited with the discovery of nitric acid in about 800 AD. He made it by heating a mixture of sodium nitrate powder with sulfuric acid and condensing the gas produced. The process can be written as:

2KNO3(s) + H2SO4(aq) → K2SO4(aq) + 2HNO3(aq) a Use this equation to calculate the percentage atom economy for nitric acid

production by this process. b In the Ostwald process the overall reaction can be represented by: NH3(g) + 2O2(g) → HNO3(aq) + H2O(l) Use this equation to calculate the percentage atom economy for the Ostwald

process. c Comment on the significance of the difference between the percentage atom

economies of each method for producing nitric acid.

A5. a 2KNO3(s) + H2SO4(aq) → K2SO4 + 2HNO3

molar mass of all atoms in reactants = 2 × 101 + 98 = 300 molar mass of atoms in product (HNO3) = 2 × 63 = 123

% atom economy = reactantsallofmassmolar

atoms used of massmolar × 100

= 300

100126×

= 42% b NH3(g) + 2O2(g) → HNO3(aq) + H2O(l)

molar mass of all atoms in reactants = 17 + 2 × 32 = 81 molar mass of atoms in product(HNO3) = 63

% atom economy = reactantsallofmassmolar

atoms used of massmolar × 100

= 81

10063×

= 77.7% c The percentage atom economy for the production of nitric acid by the Ostwald

process is greater than the process used in 800 AD. Assuming 100% conversion of reactants to nitric acid 78% of the mass of reactants would be present in the final product using the contact process. 22% of the mass of the reactants is waste product. In the earlier process, again assuming all the reactants were converted to products, 42% of the mass of the reactant atoms was used in the sulfuric acid and 58% of the mass of reactant atoms is waste. Equilibrium yields would also need to be taken into account when making these comparisons.

Q6.

During the Ostwald process, nitrogen monoxide is made from ammonia at about 900°C and then cooled to 30°C before being reacted with air to make nitrogen dioxide. Both these reactions are exothermic. Why are the temperatures used for these reactions so different?



A6. A fast rate of reaction is desirable for reactions involved in industrial processes. The oxidation of ammonia to nitrogen dioxide is performed at 900oC, rather than at lower temperatures that would give higher equilibrium yields, because the rate of this reaction increases with increasing temperature. The rate of oxidation of nitrogen oxide to nitrogen dioxide is fast at low temperatures. Since a low temperature also gives a high equilibrium yield for this reaction, a reaction temperature of about 30oC is chosen.

Q7. As the gas passes through the catalyst bed in the converter during nitric acid manufacture, its temperature increases. The gas must be cooled before it is mixed with air. a Why does the temperature of the gas rise? b Why is it necessary to cool the gas? c What side benefit is obtained from the need to cool gases?

A7. a The reaction is exothermic. b The gases are cooled to ensure that alternative reactions, the conversion of

ammonia to nitrogen or dinitrogen oxide, do not occur c Heat released by the reaction can be used to heat incoming gases and generate

electricity.

Q8.

In the commercial production of nitric acid, nitrogen monoxide is oxidised to nitrogen dioxide by oxygen 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 maximum equilibrium yield of

nitrogen dioxide. Explain your reasoning. d How do these conditions affect the rate of reaction?

A8. a 2NO(g) + O2(g) → 2NO2(g)

b K = ]O[]NO[

]NO[

22

22

c Applying Le Chatelier’s principle, the equilibrium yield of nitrogen dioxide is increased by low temperatures, since the equilibrium is exothermic, and using high pressures since there are more gas particles on the left-hand side of the reaction (one O2 and two NO molecules) than on the right (two NO2 molecules).

d The reaction rate would be faster at higher pressures. Since the rate of this reaction is quite rapid at low temperatures, in this instance no conflict exists between the conditions needed for high equilibrium yield and economic reaction rate.



Q9. A number of different oxidation states of nitrogen are involved in the industrial production of nitric acid from ammonia. a Give the equation for the reaction in which nitrogen in a compound is oxidised

from the +2 to the +4 oxidation state. b Give the equation for the reaction in which nitrogen in a compound is oxidised

from the –3 to the +2 oxidation state. c Give the equation for the reaction in which nitrogen in a compound is oxidised

from the –3 to the +5 oxidation state.

A9. The oxidation states of nitrogen stages in the conversion of ammonia to nitric acid may be represented as –3 +2 +4 +5

NH3 → NO → NO2 → HNO3 a 2NO(g) + O2(g) → 2NO2(g) b 4NH3(g) + 5O2(g) → 4NO(g) + 6H2O(g) c NH3(g) + 2O2(g) → HNO3(aq) + H2O(l)

Q10. Nitrogen monoxide is present in the gaseous emissions from nitric acid plants. a Explain why the levels of these emissions need to be restricted. b What other human activities cause nitrogen monoxide to be produced?

A10. a Nitrogen oxides contribute to photochemical smog and dinitrogen monoxide,

N2O, is a significant greenhouse gas. Nitrogen dioxide gas must also be carefully monitored since inhalation at relatively low concentrations may cause lung oedema (fluid in the lungs) and excessive exposure may be fatal.

b Nitrogen oxides are produced by motor vehicles and in industry where air is heated to high temperatures.

Q11.

If you were designing a new nitric acid plant, briefly list important factors that you would consider when deciding: a whether to provide further treatment for waste gases containing NOx from the

absorption tower b which catalyst to use, platinum–rhodium or powdered iron c whether to use air or oxygen for the reaction in the converter

A11. a Costs of constructing equipment for further treatment would be weighed against

the increased yield of nitric acid, reduced environmental impact and risks to workers health, as well as government legislation for emissions.

b The higher cost of the platinum-rhodium catalyst would be compared with its higher catalytic efficiency.

c The high cost of oxygen would make its use prohibitive, compared to readily abundant air.



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

A12.

For example: 1 Heat released by the reaction in the converter is removed and used to heat

incoming gases and converted to electricity. 2 The costly platinum catalyst slowly vaporises. Once the gas leaves the

converter it is passed through a filter to recover the metal.

Q13. Under the title ‘Nitric acid: providing fertilisers to feed the world’, write a paragraph that uses the words: fertilisers, Ostwald process, converter, cooling chamber, absorption tower, equilibrium, reaction rate, and catalytic oxidation.

A13.

‘Nitric acid: Providing fertilisers to feed the world’ Modern farming relies on the extensive use of synthetic fertilisers. Nitric acid is widely used for the making of fertilisers. The acid is manufactured by the Ostwald process, which involves catalytic oxidation of ammonia to nitrogen oxide in a vessel called a converter. The nitrogen oxide is then mixed with air in a cooling chamber to form an equilibrium mixture containing nitrogen dioxide. Conditions are chosen so that the position of equilibrium and reaction rate give the most economical yield of nitrogen dioxide. Finally, the nitrogen dioxide is mixed with water in an absorption tower to produce nitric acid.

Q14. Imagine a serious acid spill occurred at a nitric acid plant. As a consequence, the local council invited the plant manager to speak to the local citizens about their health and safety concerns. a Design a flyer advertising the meeting and highlighting the main issues of

concern. b 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 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 company’s safety record, operation to strict government guidelines, provision of employment, incentives for associated industries with employment and the trade spin-offs for local community.



Q15. Construct a concept map that includes the following terms: nitric acid, explosives, fertilisers, Ostwald process, converter, absorption tower, cooling chamber, oxidation, reaction rate, equilibrium yield, and catalyst.

A15.

Q16. Design a safety notice that summarises: a the effects to a person’s health that may result from contact with nitric acid b the first aid treatment that should be used to treat contact with nitric acid c the personal protective equipment that someone working with nitric acid should

use or have access to.

A16. The poster could contain the following information a Harmful if swallowed, burns throat and digestive tract Irritant and corrosive to eyes Corrosive to skin, causes chemical burns Vapour is corrosive to respiratory tract Exposure to high concentration may lead to pulmonary oedema. b Medical treatment should be sought if a person is exposed to concentrated nitric

acid. First aid treatment. (This is a summary only. Refer to MSDS for full details.) If swallowed – rinse mouth for 20 minutes, do not induce vomiting If splashed into eyes – flush eyes for 20 minutes If splashed onto skin – flush skin with water, remove contaminated clothing If inhaled – remove from source of exposure, 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 ch20