Chem ch22

Worked solutions to student book questions Chapter 22 Production of ethene

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

E1. Use your knowledge of chemical bonding to explain why synthetic oils made from trimer molecules are less viscous than oils made from pentamer molecules.

AE1. Oil molecules made from pentamers are larger and have stronger dispersion forces between, resulting in higher viscosity.

E2. Suggest a suitable catalyst for use in the reaction of the unsaturated oil with hydrogen.

AE2. Nickel or platinum

E3. What is the purpose of a candle wick?

AE3. A candle wick soaks up liquid paraffin for combustion.

E4. When a candle burns, what is reacting with oxygen?

AE4. Paraffin wax vapour and smaller hydrocarbons

Q1. The fractions from a petroleum fractionating tower may be subjected to either steam or catalytic cracking. a Explain the different purposes of the two cracking processes. b What are the differences in the two cracking methods?

A1.

a Steam cracking is the method usually used to produce unsaturated hydrocarbons; catalytic cracking is mainly used to produce light alkane fractions from heavier fractions that are needed as transport fuels.

b Steam cracking does not involve the use of catalysts and involves higher temperatures than those used in catalytic cracking. A zeolite catalyst is used in catalytic cracking.

Q2.

Prepare a simple flow chart showing the main steps involved in producing polyethene from crude oil.



A2. Fractional Addition distillation Gas oil or Cracking polymerisation Crude oil ⎯⎯ →⎯ naphtha fraction ⎯⎯ →⎯ Ethene ⎯⎯ →⎯ Polyethene

Q3. Write a balanced equation to represent the steam cracking of C14H30, from the gas-oil fraction of a fractionating tower, to form ethene and hydrogen as the only products.

A3.

C14H30(g) → 7C2H4(g) + H2(g)

Q4. List the conditions used by industry for the steam cracking process:

C2H6(g) ) → C2H4(g) + H2(g); ∆H = +138 kJ mol–1 Explain how each of these conditions helps to maximise the yield of ethene.

A4. High temperatures increase the value of K, low pressures cause a net forward reaction, short reaction time restricts the extent of secondary reactions.

E5. a Summarise the main reasons why the clustering of companies in the Altona

complex is commercially advantageous. b What compromises would be involved in a decision to organise a group of

industries in such a manner?

AE5. a Commercial reasons for clustering companies at the Altona site include the fact

that gaseous feedstock can be distributed by pipelines; by-products from a particular process can be utilised by a neighbouring company; companies can share some staff and some safety and environmental protection costs, such as the cost of fire protection; and the fact that management of public relations can be shared.

b Unless properly managed and supervised, an accumulation of industries in the one area could lead to increased fire risk, damage to local waterways and localised air pollution. Companies would have to work cooperatively and might have less independence when making decisions about issues such as output, rates of consumption of raw materials, nature and quality of raw materials and expansion of plant.



Q5. 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. Principles Practice 1. Prevent waste . . .

12. Minimise the potential for accidents 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 steam cracking process could be regarded as complying with these principles.

A5. The production of ethene 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 ethene relate to green chemistry principles Principles Practice Prevent waste Unreacted ethane is recycled into

feedstock, byproducts utilised, e.g. propene, coke used as fuel.

Design safer chemicals and products Less hazardous synthesis Renewable raw materials Use catalyst Metal catalyst, Pd or Fe/Ni is used Avoid chemical derivatives Maximise atom economy High 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 sand handling of ethene are in place. Risk of explosion or fire are minimised.



Chapter review

Q6. Ethene can also be produced by the catalytic dehydration of ethanol. This process was done on an industrial scale to produce ethene in the nineteenth century. There is renewed interest in this reaction as an alternative green source of ethene. The process can be written as:

C2H5OH(g) → C2H4(g) + H2O(g) a Use this equation to calculate the percentage atom economy for ethene production

by this process. b Suppose we represent the cracking reaction for the production of ethene from

propane by: C3H8(g) → C2H4(g) + CH4(g)

Use this equation to calculate the percentage atom economy for this process. c Comment on the significance of the percentage atom economies of each method

for producing ethene. d What are some of the issues that would be associated with the widespread

adoption of the production of ethene from ethanol?

A6.

a % atom economy = reactantsallofmassmolar

atoms used of massmolar × 100

= 46

10028×

= 61%

b % atom economy = reactantsallofmassmolar

atoms used of massmolar × 100

= 44

10028×

= 64% c The percentage atom economy for the production of ethene from propane is

slightly greater using propane rather than ethanol. Assuming 100% conversion of reactants to ethene, 61% of the mass of reactants would be present in the final product if ethene was produced from ethanol and 64% if it was produced from propane. The amount of reactant atoms that end up as waste would be 39% and 36% respectively.

d Ethene is produced by cracking of components of crude oil, a finite, non-renewable resource. As the supply of crude oil diminishes, its cost will rise making the production of ethene more expensive. The production of ethene from bioethanol does not rely on the availability of crude oil and would be economically viable if the price of crude oil rose. Factors such as the extent of conversion of ethanol to ethene and the rate of the reaction would also need to be considered. The impact of using agricultural land, fertilisers and water to grow plants for use as fuel also need to be considered.



Q7. In order to obtain ethene from crude oil, the oil is treated by fractional distillation. a Why is the fractional distillation of oil necessary? b Name three fractions obtained from fractional distillation of crude oil and

describe one use for each fraction.

A7. a Crude oil is a complex mixtures of hydrocarbons. These can separated into

fractions having similar boiling points by fractional distillation. b

Refinery gas C1–C4 LPG Gasoline C5–C7 petrol Naptha C6–C10 cracked to produced hydrocarbons

used in petrol Kerosesne C10–C16 jet fuel, heating Gas oil C14–C20 diesel Residue > C20 fuel oil, lubricants. bitumen

Q8. Unsaturated hydrocarbons, such as ethene and propene, are produced by steam cracking natural gas or petroleum fractions. a Explain what is meant by the term ‘cracking’. b Why is steam used in this process? c Briefly describe the conditions employed for steam cracking and explain why

these are chosen. d Write equations for two different reactions that could occur during steam

cracking of heptane (C7H16).

A8. a Cracking involves the breaking up of large molecules into smaller ones. b The steam helps to prevent the ethene produced from undergoing further cracking

to produce coke, which prevents the hydrocarbon gases from reaching the correct reaction temperature and lowers the yield. Instead, coke that is formed reacts with the steam and is converted into carbon monoxide and hydrogen gas.

c Since cracking reactions are endothermic, a high equilibrium yield of ethene is favoured by the use of high temperatures and temperatures of 750–900oC are used. Whilst applying the principles of equilibrium to this reaction system has its limitations, the use of hydrocarbon gas pressures of less than one atmosphere is used to drive the reaction forward (Le Chatelier’s Principle). The gas is in the furnace for less than one second to prevent continued cracking of the ethene.

d For example: C7H16(g) → C2H4(g) + C5H12(g) C7H16(g) → C3H8(g) + C4H8(g)



Q9. In 2005 a Dutch chemist reported he had developed a solid containing oxygen that reacts with hydrogen gas to produce water. The solid is referred to as an ‘oxygen sponge’. By referring to reactions that occur during the cracking of ethane gas, explain why it is thought that the oxygen sponge may allow higher yields to be obtained in ethene production.

A9. Ethene can be produced from ethane.

C2H6(g) → C2H4(g) + H2(g) If the hydrogen gas that is produced during cracking reactions is rapidly removed by the oxygen sponge, the reaction will shift in a net forward direction (Le Chatelier’s Principle), increasing the yield of ethene.

Q10. Consider the flow chart for ethene production below.

a Explain why ethane, the feedstock used in the steam cracking process, is present

among the products, and suggest what would be done with this substance. b Name one of the ‘other gases’. c What physical property of the gases does fractional distillation depend on?

A10. a Ethane is present in the products because the cracking process involves

equilibrium reactions. The ethane could be recycled. b hydrogen c differences in boiling points of the gases



Q11. Explain, using a diagram of the energy changes that occur during the course of a reaction (energy profile diagram), the difference between steam and catalytic cracking.

A11. Thermal cracking involves heating a mixture of steam and hydrocarbons, such as ethane or naphtha, to temperatures of 750–900°C. Ethene is one of the products obtained. In catalytic cracking a zeolite catalyst is used to lower the activation energy of the cracking process, as shown in the diagram. This process is used to convert heavier fractions from the fractional distillation of crude oil to lighter transport fuels.

Q12. Under the title ‘Crude oil: An important natural resource’, write a paragraph that uses correctly the words: fractional distillation, fraction, cracking, saturated, unsaturated, alkane, ethene, polymer.

A12.

Sample of suitable response ‘Crude oil is a mixture of many different hydrocarbons. A large proportion of these are saturated hydrocarbons: alkanes. Fractional distillation is used to separate the crude oil mixture into a number of fractions which can be used as fuels for specific purposes. For instance, one fraction is used as aviation fuel, another for domestic heating, and so on. The process of cracking enables greater amounts of the lighter fractions to be produced, as well as producing unsaturated hydrocarbons, such as ethene, which are used by the chemical industry to manufacture polymers, such as polyethene. As a source of fuel and as the starting material for chemical products used widely in the community, crude oil is an important natural resource.’



Q13. Use the information in the text about waste management to construct a table that lists the main by-products of steam cracking of ethane, and describe what becomes of each one.

A13.

Unreacted hydrocarbons

Recycled back to the reactor.

C2H2 Ethyne is converted to ethene by reacting it with hydrogen using a transition metal catalyst

CO2 and H2S Carbon dioxide and hydrogen sulfide are both weakly acidic gases and are removed by treatment with dilute sodium hydroxide solution: Unreacted sodium hydroxide solution is then neutralised by treatment with acid.

Propene Extracted and used for making polypropene. Butadiene Extracted and used to make synthetic rubber. H2 and CH4 Extracted and used as a fuel gas for furnaces. Coke Removed and used in the manufacture of dry cells and electrodes.

Q14. Consider the reaction scheme shown below.

a Write the chemical formulas and names of substances A, B and E. b Name substances C and D needed to produce ethanol from ethene. c Describe the conditions used when converting substance B to ethene.

A14. a A: C2H4Br2, 1,2 dibromomethane; B: for example, C2H6, ethane; E: C2H6, ethane b Water and a catalyst (phosphoric acid) c Steam and the hydrocarbon fraction are passed through coiled metal tubes in a

furnace at 750°C to 900°C.



Q15. When the present supplies of crude oil and natural gas are depleted, an alternative source of ethene may be the production of ethanol by fermentation of plant material, followed by the dehydration of ethanol:

C2H5OH(g)) → C2H4(g) + H2O(g); ∆H = +45 kJ mol–1 You have been asked to design a pilot plant to test the feasibility of this process. Describe how you might maximise the yield of ethene.

A15. Use low pressure, high temperature, a suitable catalyst and employ a continuous flow process so products are continually removed.

Q16. A simplified flow chart involving ethene is shown below.

a Describe the composition of crude oil. b Name processes X and Y. c Describe the composition of mixture Z. d Name another raw material from which ethene can be manufactured. e Identify substances A, B and C and write equations for the reactions that produce

them. f What type of reaction is involved in the production of substances A and B?

A16. a Crude oil is a mixture of hydrocarbons. Most of them are alkanes, with formulae

ranging from CH4 to C70H142. b X: fractional distillation; Y: thermal cracking c Mixture Z contains saturated hydrocarbons of similar masses and similar boiling

temperatures. d natural gas e A: ethanol; C2H4(g) + H2O(g) ⎯⎯ →⎯ 43POH CH3CH2OH(g) B: polyethene; nC2H4(g) ⎯⎯ →⎯catalyst (CH2CH2)n(s) C: carbon dioxide; C2H4(g) + 3O2(g) → 2CO2(g) + 2H2O(g) f addition reactions



Q17. The world has become very dependent on the products of the petrochemical industry, but the raw materials of crude oil and natural gas are likely to be virtually exhausted by 2100. Assuming the current production process of ethene stays unchanged, suggest the impact of the lack of raw materials on our lifestyle.

A17. Almost all use of plastics, such as packaging, plastic goods and components, would be affected by a lack of raw petrochemical materials. There would also be restrictions in many other areas, including pharmaceuticals and solvents.

Q18. A group calling itself the ‘Concerned Residents Group’ has been formed in the suburb next to a major petrochemical plant with the intention of lobbying the state government to have the plant moved. a Design a flyer advertising the first public meeting of the group to highlight the

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

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

A18. a The flyer might mention safety issues, such as the risk of explosions from

flammable materials and concerns about emissions. It might also mention traffic volume and visual pollution. (A flyer advertising a public meeting would also give the date, time and venue.)

b The public relations manager might describe the safety precautions in place, operation to strict government guidelines, provision of employment, incentives for associated industries with employment and trade spin-offs for local community, ways traffic movement is minimised by supply of raw materials and distribution of products to associated industries.



Q19. Construct a concept map that includes the following terms: addition polymerisation, alkanes, alkenes, cracking, fractional distillation, fractionating tower, monomers, petroleum and polymers.

A19.



Unit 4 Area of Study 1 review

Multiple-choice questions

Q1. Nickel metal reacts with carbon monoxide according to the equation:

Ni(s) + 4CO(g) → Ni(CO)4(s) Which one of the following would decrease the activation energy of the reaction? A addition of a catalyst B increasing the temperature C increasing the pressure of CO D crushing the nickel into a fine powder

A1. A. Catalysts lower the activation energy Questions 2 and 3 refer to the following information. An energy profile diagram shown below is for the reaction

CO2(g) + NO(g) → CO(g) + NO2(g)

Q2.

The ∆H of the forward reaction, in kJ mol–1, is A –170 B +130 C +230 D +260

A2.

D. The reaction is endothermic, i.e. ∆H > 0, since the energy of the products is greater than the energy of reactants. The energy difference between products and reactants is 260 kJ mol–1.



Q3. The activation energy of the reverse reaction, in kJ mol–1, is A 40 B 130 C 230 D 360

A3. B. The activation energy for the reverse reaction is 390 kJ mol–1 – 260 kJ mol–1 = 130 kJ mol–1.

Q4. Consider the following reactions I A(g) + B(g) → 2C(g); ∆H = +180 kJ mol–1 II D(g) + 3E2(g) → 2F(g); ∆H = –90 kJ mol–1 III 2G(g) → H(g) + I(g); ∆H = –180 kJ mol–1 From a comparison of the enthalpy change, ∆H, it can be deduced that: A The activation energy of equation I > activation energy equation II. B The activation energy of equation I < activation energy equation II. C The activation energy of equation I = activation energy equation III. D No information about activation energy can be deduced from ∆H.

A4. D. The activation energy depends on the configuration of intermediate complexes and is independent of ∆H.

Q5. A reaction is found to have an activation energy of 960 kJ mol–1 and an increase in enthalpy of 120 kJ mol–1. The reaction: A requires a source of energy to start B will occur spontaneously at room temperature C requires a catalyst to occur D can be used as a source of energy

A5. A. An activation energy of 960 kJ is quite high and will not occur spontaneously at room temperature. The positive enthalpy shows that the reaction is endothermic and so cannot be used as a source of energy.

Q6. Which of the following statements about a catalyst is true? A Only a small amount of a catalyst is consumed in a reaction. B A catalyst can occur as a reactant or product in the overall equation. C A catalyst decreases the ∆H of the reaction. D The proportion of molecules with sufficient energy to react is increased by a

catalyst.



A6. D. A catalyst has no effect on the enthalpy of the reactants and products. A catalyst is not consumed in a reaction so it does not appear in the reaction equation.

Q7.

Water reacts with chlorine according to the equation 2H2O(g) + 2Cl2(g) → 4HCl(g) + O2(g)

At a particular temperature the value of the equilibrium constant for this reaction is 4.0 × 10–4. At the same temperature, the value of the equilibrium constant for the reaction

2HCl(g) + ½O2(g) → H2O(g) + Cl2(g) is equal to A 2 × 10–4 B 2 × 10–2 C 2.5 × 103 D 50

A7. D. When an equation is reversed the new equilibrium constant is the inverse of the original. When the coefficients are halved the new equilibrium constant is the square root of the original. In this case the original equation has been both reversed and the coefficients halved. The new equilibrium constant is thus the square root of the inverse of the original constant.

i.e. K2 = √1/K1 = √1/4 × 10–4 = 50

Q8.

Consider the following equation: A + 3B → 2C + 4D. The correct expression for the equilibrium constant is:

A 42

3

]D4[]C2[]B3][A[

B 3

42

[A][B]]D[[C]

C 42

3

]D[]C[]B][A[

D ]B3][A[]D4][C2[

A8.

B. For the general equation, aA + bB → cC + dD, the equilibrium expression is

ba

dc

]B[]A[]D[]C[ .



Q9. The concentration–time diagram could represent the concentrations of A and B in the equation:

A A → B B B → A C 2A → B D A → 2B

A9. C. Initially, there is no B present, so B must be a product. The amount of A consumed and B produced is 0.4 and 0.2 mol, respectively, a ratio of A:B = 2:1.

Q10. In a sealed vessel, nitrogen oxide, oxygen and nitrogen dioxide form the equilibrium

2NO(g) + O2(g) → 2NO2(g); ∆H = –114 kJ mol–1 Which one of the following sets of conditions is likely to lead to the highest yield of nitrogen dioxide gas? A 200oC and 1 atm pressure B 200oC and 2 atm pressure C 300oC and 1 atm pressure D 300oC and 2 atm pressure

A10.

B. Since the reaction is exothermic a lower temperature would favour the forward reaction. As there are fewer molecules on the product side of the equation, a high pressure would favour the forward reaction.



Q11. A sample of NOCl is allowed to come to equilibrium according to the following equation:

2NOCl(g) → 2NO(g) + Cl2(g) The volume of the mixture is halved and allowed to come to a new equilibrium, the temperature remaining constant. At the new equilibrium the chlorine has: A decreased in amount and decreased in concentration B increased in amount and decreased in concentration C decreased in amount and increased in concentration D increased in amount and increased in concentration

A11. C. The equilibrium shifts to the left, the side of the equation with fewer molecules, so the amount of chlorine decreases. The concentration of chlorine is still higher than in the first equilibrium due to the halving in volume.

Q12.

The reaction between nitrogen monoxide and ozone is: NO(g) + O3(g) → NO2(g) + O2(g); K = 6 × 1034 at 25°C

An equal amount of NO and O3 are used. Which of the following statements cannot be inferred from the magnitude of the equilibrium constant? A [NO2][O2] >> [NO][O3] B The equilibrium of the reaction lies well to the right. C The reaction has a low activation energy. D Very little ozone will remain at equilibrium.

A12. C. The equilibrium constant does not give information about the activation energy or the rate of the reaction.

Q13. Carbon dioxide gas dissolves to a small extent in water, forming carbonic acid in an exothermic reaction:

CO2(g) + H2O(l) → H2CO3(aq); ∆H negative H2CO3(aq) + H2O(l) → HCO3

–(aq) + H3O+(aq) This is the reaction involved in forming fizzy drinks. Which of the following strategies would not be effective at increasing the amount of dissolved carbon dioxide? A Decreasing the pH of the solution B Decreasing the temperature of the solution C Increasing the concentration of carbon dioxide in the gas D Increasing the pressure of the carbon dioxide gas



A13. A. A decrease in pH or an increase in H3O+ ions in the solution will set up a competing equilibrium which will force H2CO3 out of solution, i.e. an increase in H3O+ forces this reaction to the left.

H2CO3(aq) + H2O(l) → H3O+(aq) + HCO3–(aq)

The consequential increase in HCO3–(aq) forces this reaction to left.

CO2(g) + H2O(l) → H2CO3(aq)

Q14.

The pH of a tomato juice drink is 5.3 at 25°C. The hydroxide ion concentration in the drink, in mol L–1, is A 105.3 B 10–5.3 C 108.7 D 10–8.7

A14. D. If pH = 5.3 then [H30+] = 10–5.3 [H30+][OH–] = 10–14

[OH–] = 3.5

14

1010

−

−

= 10–8.7

Q15. The self-ionisation of water is affected by temperature:

H2O(l) + H2O(l) →H3O+(aq) + OH–(aq), Kw = [H3O+][OH–]

Temperature (°C) Ionisation constant for water, Kw 5 1.85 × 10–15 15 4.51 × 10–15 25 1.00 × 10–14 35 2.09 × 10–14

From the data it can be inferred that: A the pH of pure water at 35°C is less than 7 B self-ionisation of water is an exothermic reaction C equilibrium for the reaction lies well to the left with mainly reactants present D [OH–] of pure water at 15°C is less than [H3O+]

A15. A. The self-ionisation of water is an endothermic reaction. As the temperature increases, both [H3O+] and [OH–] increase and pH decreases.



Q16. The dihydrogen phosphate ion, H2PO4

–(aq), is an amphiprotic ion formed after the first dissociation step of phosphoric acid in water. The equilibrium constants of the dihydrogen phosphate ion acting as an acid and as a base at 25°C are:

H2PO4–(aq) + H2O(l) → HPO4

2–(aq) + H3O+(aq) K = 6.3 × 10–8 M H2PO4

–(aq) + H2O(l) → H3PO4(aq) + OH–(aq) K = 1.6 × 10–7 M From these data, dihydrogen phosphate will: A form an acidic solution in water B accept a proton and donate a proton to an equal extent in water C more readily accept a proton than donate a proton in water D readily lose two protons in water

A16. C. The equilibrium constant for the second reaction, H2PO4

– acting as a base, is larger than for the reaction of H2PO4

– as an acid. H2PO4– will accept rather than donate in

water, giving a slightly basic solution.

Short-answer questions

Q17. A student investigated the factors affecting the rate of reaction between a solution of sodium thiosulfate and hydrochloric acid.

Na2S2O3(aq) +2HCl(aq) → 2NaCl(aq) + SO2(g) + S(s) + H2O(l) The reaction was carried out in a conical flask placed on top of a piece of white paper with a dark cross marked on it. The rate of reaction was determined by measuring the time taken for the cross to be masked by the suspension of sulfur formed during the reaction, as shown.

The rate was determined for different concentrations of sodium thiosulfate and for different temperatures. The volume of each solution and the concentration of hydrochloric acid were kept constant. The results are summarised in the table below.

Experiment no.

[Na2S2O3(aq)] (M)

Temperature(°C)

Time taken for the cross to be masked

1 0.1M 20 36 seconds 2 0.2M 20 20 seconds 3 0.1M 25 28 seconds



a Explain in terms of collision theory why the rate in Experiment 2 is faster than the rate in Experiment 1.

b Explain in terms of collision theory why the rate in Experiment 3 is faster than the rate in Experiment 1 would be the effect on the rate of if the concentration of hydrochloric is doubled?

c What other factors, other than the two investigated in this experiment, may affect the rate of a reaction?

A17. a The [Na2S2O3(aq)] is higher in Experiment 2 than in Experiment 1. There are a

greater number of particles in solution in Experiment 2 therefore the number of collisions with particles having the minimum activation energy needed for a reaction to occur is greater.

b Since Experiment 3 is carried out a higher temperature, a larger proportion of the colliding molecules will have the minimum activation energy needed for a reaction to occur than in Experiment 1.

c The rate of reaction is also dependent on surface area; the greater the surface area the greater the rate. Rate of a reaction can also be increased by using a suitable catalyst.

Q18. The activation energy for the decomposition of hydrogen peroxide to oxygen was measured under two different conditions.

2H2O2(aq) → 2H2O(l) + O2(g) I The activation energy for the decomposition reaction when an enzyme was added

was found to be 36.4 kJ mol–1. The temperature of the reaction mixture increased. II When platinum was added to another sample of the hydrogen peroxide solution

the activation energy was 49.0 kJ mol–1. The temperature of the reaction mixture increased.

a What is the function of the enzyme and platinum in each these reactions? b Sketch, on the same energy profile diagram, the energy profiles for the

decomposition of hydrogen peroxide with the enzyme and the decomposition using the platinum.

c Which reaction system, I or II, would be faster? Explain your answer.

A18. a Both the enzyme and the platinum are catalysts, increasing the rate of the reaction

by lowering the activation energy.



b

c The decomposition reaction catalysed by the enzyme has a lower activation

energy and would be faster.

Q19.

Dinitrogen tetroxide, N2O4, and nitrogen dioxide, NO2, coexist according to the equilibrium reaction: N2O4 → 2NO2. A concentration–time graph for the system coming to equilibrium is shown.

a Identify A and B. b Write an expression for the equilibrium constant of the decomposition of N2O4. c Sketch on the graph the line showing the effect on A if a catalyst were added to

the mixture. d Calculate K for the reaction at equilibrium according to this concentration graph.



A19. a After reaction, B is reduced by 0.05 mol while A is increased by 0.10 mol.

Therefore twice the amount of product is formed as reactant consumed. A is NO2 and B, N2O4.

b Equilibrium expression ]O[N

][NO

42

22

c

d Equilibrium [NO2] = 0.01 M; equilibrium [N2O4] = 0.005 M

K = 0.1 × 05.01.0 = 0.2 M

Q20. Carbon monoxide and iodine pentoxide react to form iodine and carbon dioxide in the equilibrium reaction:

5CO(g) + I2O5(s) → I2(g) + 5CO2(g); ∆H = –1175 kJ Use your knowledge of Le Chatelier’s principle to predict the effect of the change (column 1) on the designated quantity (column 2). Complete the effect on the equilibrium (column 3) by using the terms decrease, increase, no change. Assume that the change listed is the only one taking place, e.g. if I2 is added, the volume and the temperature are kept constant.

Change Quantity Effect Increase T K Decrease T amount I2O5(s) Add I2(g) K Add CO2(g) amount I2(g) Double volume concentration CO2 Remove CO amount CO2 Add catalyst I2(g) Add inert gas Ar K



A20.

Change Quantity Effect Increase T K decrease Decrease T amount I2O5(s) increase Add I2(g) K no change Add CO2(g) amount I2(g) decrease Double volume concentration CO2 decrease Remove CO amount CO2 decrease Add catalyst I2(g) no change Add inert gas Ar K no change

Q21. The reaction between hydrogen and iodine in the gaseous phase to produce hydrogen iodide is described by the equation:

H2(g) + I2(g) → 2HI(g); ∆H negative The diagram shows the change in concentration of gaseous hydrogen, iodine and hydrogen iodide as the equilibrium is reached and disturbed.

a At point A, a sudden change occurs to the system. What has occurred? b At point B, another change has occurred. What has happened? c Mark with an ‘X’ on the time axis, a point when the system as at equilibrium. d Extend the graph past point C to show what would happen to the concentration of

the gases, if the volume of the reaction vessel was doubled, until equilibrium is obtained. Assume the temperature remains constant.



A21. a Iodine vapour has been added to the mixture. b The temperature of the mixture has been increased. c, d

Q22. Hydrocyanic acid, HCN, is a weak acid. It ionises in water as follows:

HCN(aq) + H2O(l) → CN–(aq) + H3O+(aq) A 0.1 M HCN solution has a pH of 5.15 at 25°C. a Write an expression showing how pH is related to the concentration of H3O+(aq). b Write an expression for the acidity constant, Ka, of hydrocyanic acid. c What is the concentration of H3O+(aq) and CN–(aq) ions in a 0.1 M HCN

solution? d What assumption can you make about the equilibrium concentration of HCN(aq)? e Determine the value of the acidity constant, Ka, of hydrocyanic acid. f Explain why a 0.1 M solution of HCl has a pH of 1 while a 0.1 M solution of

HCN has a pH of 5.15.

A22. a pH = –log10[H3O+(aq)]

b Ka = ]HCN(aq)[

](aq)O(aq)][H[CN 3+−

c pH = –log10[H3O+(aq)]

5.15 = –log10[H3O+(aq)]

[H3O+ (aq)] = 10–5.15

= 7 × 10–6 M

from equation

[CN–(aq)] = [H3O+(aq)] = 7 × 10–6 M



d [HCN(aq)] ≈ 0.1 M since only a very small amount of HCN(aq) will be ionised.

e Ka = ]HCN(aq)[

](aq)O(aq)][H[CN 3+−

= 1.0

10710 7 66 −××× -

= 4.9 × 10–10 M

f 0.1 M HCl(aq) is a strong acid and is completely ionised, i.e. [H3O+(aq)] = 0.1 M and hence pH = 1. As 0.1 M HCN is weak acid and is only partly ionised, the [H3O+(aq)] will be much less than 0.1 M. As shown in part c, [H3O+(aq)] = 7 × 10–6 M and hence pH = 5.15.

Q23. A mixture of 0.040 M Na2HPO4 and 0.040 M KH2PO4 acts as a buffer, maintaining a pH of approximately 7. Use equations to show how the mixture can resist change of pH when a small amount of acid or base is added.

A23. HPO4

2– can act as a base, accepting protons if an acid is added. HPO4

2–(aq) + H3O+(aq) → H2O(l) + H2PO4–(aq)

H2PO4– can act as an acid, donating protons if a base is added.

H2PO4–(aq) + OH–(aq) → H2O(l) + HPO4

2–(aq)

Q24. Both thymol blue and methyl orange are acid–base indicators that are red in the acidic form and yellow in the basic form. The acidity constants, Ka, of the two indicators are: thymol blue Ka = 0.020 M, methyl orange Ka = 0.00040 M. Both indicators are weak monobasic acids. The protonated form of the indicator can be represented as HIn and the unprotonated form as In–:

HIn + H2O(l) → H3O+(aq) + In–(aq) a Determine the pH at which [In–] = [HIn] for both indicators. b A weak acid and strong base are titrated and found to have an equivalence point

of pH ≈ 4.3. Which of the indicators would be more suitable to use in this titration? Justify your answer.

c Calculate the value of the ratio [In–]/[HIn] when thymol blue is in a solution of pH = 5. What is the colour of the indicator in this solution?

A24.

[HIn]]O][H[In 3

+-

= Ka

When [In–] = [HIn], then [H3O+] = Ka a Thymol blue [H3O+] = 0.020 M; pH = 1.7 Methyl orange [H3O+] = 0.00040 M; pH = 3.4 b Methyl orange would be the most suitable indicator. An acid–base indicator can

be used approximately 1 pH unit either side of the pH of its 50% dissociation value.



c At pH = 5, [H3O+] = 1 × 10–5 M

HIn][

M 101][In 5−− ×× = 0.020 M

[HIn]

][In -

= M 101

M 02.05-×

= 2000

The indicator is present almost entirely in the base form, so will appear yellow.

Q25.

A solution of barium hydroxide, Ba(OH)2, is found to have a pH of 12.60 at 25°C. Assuming that it fully ionises in water, what was the concentration of the barium hydroxide solution?

A25.

[H3O+] = 2.51 × 10–13 M

[OH–] = 13

14

1051.2101

−

−

×× = 0.0398 ≈ 4.0 × 10–2 M

[Ba(OH)2] = 2104 2−× = 2.0 × 10–2 M

Q26. Methanol is used as a fuel for some race cars. The reaction conditions used to manufacture methanol must be carefully adjusted in order to ensure efficient production. The synthesis of methanol from methane involves three major steps: I reaction of methane with steam to yield carbon monoxide and hydrogen II an exothermic reaction between carbon monoxide and hydrogen to produce

methanol III separation of the methanol from the reaction mixture a Draw a flow chart of this process, showing the chemicals involved in each stage. b Write equations for the two reactions that are described above. c Suggest how methanol might be separated from other gases in the reaction

mixture. In practice the reaction of carbon monoxide with hydrogen is performed at about 250oC and 100 atm pressure. Copper, zinc oxide and alumina, are also present and about 10 per cent of the reactants are converted to methanol as the reactant gases pass through the reactor. d i What is the likely function of the copper, zinc oxide and alumina in the

reactor? ii How do the copper, zinc oxide and alumina acid allow the process to operate

more efficiently? e A much higher conversion of carbon monoxide to methanol than 10% is obtained

in industry. Suggest how this is achieved. f For the reaction between carbon monoxide and hydrogen, what would be the

effect on the yield of methanol if you were to: i increase the pressure? ii increase the temperature?



g With reference to reaction rates and equilibrium, discuss why the particular temperatures and pressures described above would have been chosen.

h Suggest two waste management which might be involved in this method of methanol production.

i Suggest two health and safety issues which might be involved in this method of methanol production.

A26.

a

b CH4(g) + H2O(g) ) → CO(g) + 3H2(g); CO(g) + 2H2(g) → CH3OH(g) c liquefy the gases followed by fractional distillation d i They act as a catalyst.

ii A faster reaction rate is obtained at the temperature used for the process than would otherwise be possible, thus maximising the rate of production of the methanol.

e Unreacted gases are recycled into the reaction chamber. f i increase ii decrease g Whereas the reaction rate is faster at high temperatures, for an exothermic

reaction, the equilibrium yield is more favourable at low temperatures. The temperature chosen represents a compromise between these two considerations. Applying Le Chatelier’s Principle, in the equation for methanol production there are more reactant gas particles than product gas particles, so high pressures will drive the reaction forward and increase the equilibrium yield. High pressures are therefore used for this reaction.

h Waste management issues include: need to monitor emissions of CO, H2 and NOx (NOx is likely to be formed in the course of both reactions); disposal of excess H2; and use of the excess heat produced in the methanol production reaction.

i Health and safety issues include avoiding exposure to toxic CO and CH3OH gases; the risk of fire and explosions; and hazards associated with using elevated temperatures and pressures.

Q27. Select one of the following chemicals of industrial importance: ammonia, ethene, sulfuric acid or nitric acid. a Name and write the formulas of the raw materials from which the chemical is

made. b Write chemical equations for the main reactions that occur during its production. c Sketch and label a flow diagram of the process, indicating the reactants and

products at each stage. d The synthesis of each chemical involves a step in which conditions must be

carefully controlled in order for the production process to be efficient. i What pressures and temperatures would be used in this step if a high

equilibrium yield were desired?



ii What pressures and temperatures would be used if a high reaction rate were desired?

iii State the pressures and temperatures actually used for this step and, where appropriate, explain why they vary from those given for parts d i and d ii.

e Apart from careful selection of pressure and temperature, describe one other way that the efficiency of the production of the chemical is improved.

f Name three wastes generated in the production process and where appropriate describe how these wastes are treated and reduced.

g Name three health and safety risks associated with the process and describe the precautions taken.

h List three commercial uses for the chemical.

A27.

For ammonia: a nitrogen (N2) from air and hydrogen (H2) b N2(g) + 3H2(g) → 2NH3(g) c Refer to Figure 19.9 in the textbook. d i high pressure; low temperature

ii high pressure; high temperature iii pressure of 100–250 atm; temperature of 350–550°C; the temperature is a

compromise between the need for a high reaction rate and high equilibrium yield.

e For example: a catalyst is used to increase the reaction rate; unreacted gases are recycled

f Aqueous ammonia which can be used for urea manufacture; carbon dioxide produced in steam reforming can be liquefied and sold for beverage production; nitrogen oxides

g Ammonia is a toxic gas, so areas are well ventilated and workers are appropriately dressed; fires and explosions are minimised by careful plant design and operation; levels of toxic carbon monoxide are carefully monitored

h manufacture of fertilisers, explosives and nylon For nitric acid: a ammonia (NH3) oxygen (O2) from air and water (H2O) b 4NH3(g) + 5O2(g) ) → 4NO(g) + 6H2O(g)

2NO(g) + O2(g) → 2NO2(g) 3NO2(g) + H2O(l) → 2HNO3(aq) + NO(g) c Refer to Figure 20.5 in the textbook. d For the oxidation of nitrogen monoxide to nitrogen dioxide:

i high pressure; low temperature ii high pressure; low temperature iii higher pressure is sometimes used; temperature of 30°C

e For example, the gas mixture in the converter is in contact with the catalyst for only a short time; nitrogen monoxide formed in the absorption tower is reacted with air to form nitrogen dioxide.

f NOx which can be reduced to nitrogen; vaporised metal from the catalyst is filtered and recovered; dinitrogen monoxide



g Nitric acid is harmful and leaks and spills are carefully monitored and disposed of; ammonia to air ratios are controlled to avoid explosions; nitrogen dioxide inhalation can be fatal, so levels are also carefully monitored

h Manufacture of fertilisers, explosives and laughing gas For sulfuric acid: a sulfur (S) or sulfur dioxide (SO2) oxygen (O2) from air and water (H2O) b S(l) + O2(g) → SO2(g) 2SO2(g) + O2 → 2SO3(g) SO3(g) + H2SO4 → H2S2O7(l) H2S2O7(l) + H2O(l) → 2H2SO4(l) c Refer to Figure 21.5 in the textbook. d i high pressure; low temperature

ii high pressure; high temperature iii atmospheric pressure; temperature of 400–500oC; the temperature is a

compromise between the need for a high reaction rate and high yield and high pressures are not required for good rates and yields.

e For example, a catalyst is used to increase the reaction rate; unreacted gases are recycled.

f Sulfur dioxide which is recycled to the converter; cooling water is recycled; spent catalyst is disposed of in landfill sites.

g Sulfuric acid is highly corrosive and strict protocols exist for its handling and transport; sulfur dioxide is a respiratory irritant and methods are in place to trap fumes; oleum is highly corrosive and workers wear protective clothing.

h Manufacture of fertilisers, paper and detergents For ethene: a ethane (C2H6) or propane (C3H8) from natural gas or naptha (e.g. C8H18) or gas-

oil (e.g. C16H34) fractions from crude oil b For example, C3H8(g) → C2H4(g) + CH4(g) c Refer to Figure 22.7 in the textbook. d i low pressure; high temperature

ii high pressure; high temperature iii low pressure (through the use of steam); temperature of 750–900oC; the

pressures employed give a high reaction rate and high yield and avoid unwanted side reactions.

e Unreacted alkanes are recycled back to the furnace. f Sulfur from oil and natural gas is used to make sulfuric acid; unreacted alkanes

are recycled back to the furnace; carbon dioxide is removed by treatment with dilute sodium hydroxide solution

g Butadiene may cause damage to the nervous system and emissions are carefully monitored; fires and explosions are minimised by careful plant design and operation; attention is given to the prevention of burning and freezing injuries because high temperature and low temperature stages are involved.

h manufacture of polyethene, ethanol and polyvinyl chloride



Q28. Ethanol is a major product of the chemical industry, with many applications as a solvent and raw material for other products. With reference to a MSDS for ethanol (this can be down loaded from the Internet): a Explain why large storage tanks should not be located next to residential areas. b What is the LD50 for rats and what does this term mean? c What steps should you take in making a risk assessment for using ethanol in a

school laboratory experiment? d What personal protection should be used when working with ethanol? e Should waste ethanol be flushed down the sink? Explain.

A28. a Ethanol is flammable and toxic. b LD50 = 5860 mg kg–1, the lethal dose for 50% rats (based on laboratory tests) c Identify hazardous chemicals, assess risk, control risk, document risk d Safety glasses, gloves, respirator or ventilation, remove sources of ignition e Ethanol is miscible (mixes with water) and not toxic in very low concentrations

so small quantities can be flushed down a sink. Large quantities should be stored for proper disposal. Undiluted ethanol is flammable and high concentrations are toxic to aquatic life (for example, LC50 fish = 96 h 5 540 mg L–1).

Q29. ‘Money makes the world go round’ or so the saying goes. Explain the impact of finances on the production and use of chemicals in the following situations. a The effect of high retail prices on the acceptable yield of a product. b The effect of a carbon tax on the cost of producing electricity by burning coal c The effect of toxicology testing for all chemical products d The effect of rehabilitating polluted land charged against the emitter of the

pollutant. e The effect of waste disposal charged against the manufacturer of a product f The effect of costly start-up procedures on a decision to produce small quantities

of a chemical by batch or continuous processing.

A29.

a Low yields become more profitable → increased production b Increased cost of electricity → reduced production and use c Increased cost of products → reduced production and use d Increased cost use → reduced production and use e Increased cost of disposal → reduced production and use f Costly start-up favours continuous production but small quantities favours batch

processing



Q30. Victoria has struggled to find a suitable site for a proposed High Temperature Incinerator (HTI) to treat chemical wastes, with the State government and locals repeatedly failing to reach agreement on where it should be located. a List two considerations for each of technical, social, environmental and economic

factors involved in locating a HTI (or a chemical production plant such as an oil refinery). i technical factors ii social factors iii environmental factors iv economic factors

b Explain how the following decision-making tools could assist in finding a suitable location for a High Temperature Incinerator (or a chemical production plant such as an oil refinery). i cost–benefit analysis ii environmental effects statement

c Which of the above decision-making tools is most likely to give the ‘fairest’ answer? Explain.

A31. a i Completeness of combustion, availability of alternative waste management

processes ii Proximity of human populations, public knowledge of waste management

issues, attitudes and politics iii Toxicity of combustion products, environmental effects of alternative waste

management processes iv Cost of HTI, cost alternative waste management processes including waste

minimisation b i Cost–benefit analysis helps minimise dollars lost or maximise profits.

ii EES considers social, technical, economic and environmental factors. c An EES considers multiple factors and is arguably fairer than a cost–benefit

analysis that focuses on economic factors only.

Chem ch22

Documents

b steam cracking

steam cracking process

cracking processes

cracking methods

steam cracking of c14h30

oil molecules

unsaturated oil

gasoil fraction