production of sulfuric acid - · PDF fileThe reaction that occurs in the burner is classified as an ... Like all the reactions involved in the production of sulfuric acid, ... H2SO4(l)

production of sulfuric acid unit four – 2014 – chemistry

© The School For Excellence 2014 Unit 4 Chemistry – The Production of Sulfuric Acid Page 1

THE PRODUCTION OF SULFURIC ACID (H2S04)

USES OF SULFURIC ACID

42SOH is one of the most important chemicals produced in industry, as there are very few

consumer goods that do not need 42SOH at some stage in their production. Sulfuric acid is used in the manufacture of paper, detergents, dyes, drugs, car batteries, fertilisers, plastics and in the process of petroleum refining.

PROPERTIES OF SULPHURIC ACID

42SOH is an incredibly diverse molecule in that it exhibits a range of chemical properties that make it the ideal agent in many chemical processes. The acid exhibits acidic, oxidising and dehydrating properties. ACIDIC PROPERTIES In dilute solutions, 42SOH behaves as a diprotic acid (an acid which donates two protons).

MKOHHSOOHSOH aaqaqll9

)(3)(4)(2)(42 10=+→+ +−

2 2

4( ) 2 ( ) 4( ) 3 ( ) 10aq l aq aq aHSO H O SO H O K M− − + −+ + =

Although the first hydrolysis is almost 100% complete, only about 10% of −4HSO ionise.

In the presence of a strong base such as NaOH , the conversion of −4HSO ions to −2

4SO is virtually complete.

OXIDISING PROPERTIES Concentrated 42SOH may also act as a strong oxidant, especially when hot. Hot

concentrated 42 SOH oxidises a number of non metal species such as sulfur, carbon and phosphorus.

)(2)()(42)( 232 lgls OHSOSOHS +→+

Most metals are oxidised to their sulfates, although sulfides may also be formed. As sulfur in 42 SOH is in its highest oxidation state (6), it may also be reduced to 2SO

(oxidation state 4), S (oxidation state 0 ) or SH 2 (oxidation state 2− ), depending upon the strength of the reducing agent, the proportions of reactants and the temperature.


For example: Mg(s) + 2H2SO4(l) → MgSO4(aq) + 2H2O(l) + SO2(g)

3Mg(s) + 4H2SO4(l) → 3MgSO4(aq) + 4H2O(l) + S(g)

4Mg(s) + 5H2SO4(l) → 4MgSO4(aq) + 4H2(l) + H2S(g)

Compare the ratios of metal : acid in each of the above equations. What pattern do you observe? An excess of a reactive metal such as Mg favours the formation of SH 2 , whereas as

excess of acid favours the formation of 2SO .

DEHYDRATING PROPERTIES Concentrated 42 SOH displays a very strong affinity for water, which is clearly reflected by the large amounts of heat evolved when the acid is mixed with excess water.

molkJHSOHSOH aq

OH

l

l

/2.7)(42)(42

)(2

−=Δ→

It is for this reason that great care must be exercised when diluting 42 SOH . The acid should be added slowly to the water in a thin stream, while vigorously mixing the solution to which the acid is being added. Because of its dehydrating properties, concentrated 42 SOH is used as a drying agent for

air, and gases such as 2O , 2CO and 2N . As the acid displays acidic and oxidising

properties, it cannot, however, be used to dry alkaline bases such as 3NH , or reducing

agents such as SH 2 .

42 SOH can also be used to remove hydrogen and oxygen atoms from compounds that do not contain water in a molecular form. Examples include sugars, wood and paper.

For example: The dehydration of glucose.

C6H12O6(s) ⎯⎯ →← 42SOH 6C(S) + 6H2O(L)

For example: The dehydration of glycerol.

C3H8O3(l) + 3HNO(l) ⎯⎯ →← 42SOH C3H5(NO3)3(l) + 3H2O(l)


THE PRODUCTION OF SULFURIC ACID Sulfuric acid is manufactured via the "Contact Process" using sulfur or sulfur dioxide as the starting material. Sulfur is obtained from underground sources using the Frasch mining process. Sulfur dioxide may be obtained from the following sources: • By product of desulfurisation process in petrochemical industries.

• By product from copper smelting: )(2)( gl OS + )(2 gSO

THE FRASCH PROCESS The Frasch process takes advantage of the following properties of sulfur: • Sulfur has a low melting temperature.

• Sulfur has a low degree of reactivity with water.

(a) Superheated liquid water (at a temperature of Co160 ) is pumped down a pipe to the sulfur deposit. The sulfur melts. (b) A second pipe transfers compressed air into the mixture of molten sulfur and water.

(c) A froth of liquid sulfur, water and air forms, which is forced to the surface through a third pipe. (d) At the surface: The air escapes. The water runs off. The sulfur is collected.


THE CONTACT PROCESS

The stages involved in the Contact process include: Step 1: The oxidation of sulfur to 2SO .

Step 2: Catalytic oxidation of 2SO to 3SO .

Step 3: Absorption of 3SO by impure 42SOH to produce oleum ( 722 OSH ).

Step 4: Dilution of oleum with water, to produce ≈ pure 42SOH .


STEP 1 OF THE CONTACT PROCESS

Oxidation of sulfur to 2SO

Reaction: )(2)(2)( ggl SOOS →+

Molten sulfur is sprayed into the furnace (burner), where it burns in air to produce sulfur dioxide gas. This reaction proceeds easily, and there is almost complete conversion to 2SO . The reaction that occurs in the burner is classified as an oxidation redox reaction, as the oxidation number of sulphur increases from 0 to 4 in the process. The reaction may also be described as a combustion reaction, and as large amounts of heat are evolved, we may also refer to the process as being exothermic. The sulfur dioxide produced is then purified to remove moisture and dust, and any substances which may poison or interfere with the function of the catalyst in the next stage.


Catalytic oxidation of 2SO to 3SO

Reaction: )(2)(22 gg OSO + )(32 gSO molkJH /197−=Δ

Sulfur dioxide gas is oxidised to sulfur trioxide by oxygen, using vanadium (V) oxide ( 52OV )

as a catalyst. This reaction occurs in a vessel referred to as a converter. Like all the reactions involved in the production of sulfuric acid, this process is exothermic in nature. High yields of product can therefore be achieved by using lower temperatures – which results in a compromise in the reaction rates. High temperatures will compensate for the slow reaction rates, but will compromise the yield of product formed. The conflict between rate and yield is resolved by employing a catalyst to increase reaction rates, and enable the reactions to proceed at lower temperatures so as to maximise yields. Rates are further enhanced by: • Maximising the surface area of catalyst by spreading it across multiple beds. This

ensures that there is almost complete oxidation of 2SO to 3SO .

• Adding an excess of the cheaper reactant (air).


Yields are maximised by:

• Using lower temperatures of approximately Co450 . Gases are cooled as they move across each catalyst bed so as to maximise the production of 3SO .

• Although yields would be improved by using higher pressures, reactions are performed

at 1 atmosphere of pressure, as the added costs involved in using high pressures does not justify the improvement in the observed yields.

• Adding an excess of the cheaper reactant (air) to drive the reaction forward. Note: • The heat exchanger removes the heat produced during the exothermic reaction. The

heat collected is recycled and is used to heat the incoming gas. This reduces the costs associated with this process and conserves energy.

• The oxidation of 2SO to 3SO is a redox process. The changes in the oxidation state of

S that occur during this process is from 4 to 6 (an increase in oxidation number).


Absorption of 3SO by impure 42SOH to produce oleum ( 722 OSH )

Reaction: )(722)(3)(42 lgl OSHSOSOH →+

Sulfur trioxide is mixed with concentrated sulfuric acid in an ABSORPTION TOWER. The 3SO formed in the previous step is passed into the base of the absorption tower, and

concentrated 42SOH is applied at the top of the tower. The rising 3SO and 42SOH meet,

and oleum ( 722 OSH ) is formed.

To maximise conversion of 2SO and hence minimise emissions of this gas into the atmosphere, most plants now employ the “double absorption” process. In this process, unreacted gases in the absorption tower are recycled to the converter for one or two additional passes over the catalyst beds, and then returned to the absorption tower.



Dilution of oleum to produce 42SOH The oleum is reacted with water to form > 98% pure 42SOH . Reaction: )(42)(2)(722 2 lll SOHOHOSH →+

Note:

3SO cannot be directly reacted with OH 2 to produce 42SOH . As this reaction is highly

exothermic, a net back reaction is favoured, reducing the yields of sulfuric acid. Furthermore, at higher temperatures, the acid forms a mist which is difficult and expensive to collect.

COMPROMISING CONDITIONS IN THE CONTACT PROCESS

• Choice of temperature: A rate vs yield problem.

• Choice of catalyst: Efficiency vs cost problem.

• Choice of pressure: Cost vs yield problem.

OPERATING CONDITIONS • Increase equilibrium yield by:

Decreasing temperature. Adding excess reactant ( the cheaper one).

• Increase reaction rates by:

Using catalysts. Using as high temperatures as possible without affecting yields.


WASTES AND MANAGEMENT

• The contact process is very energy efficient and produces little waste. In addition, the process helps reduce the environmental impact of industries that produce sulfur or sulfur dioxide by using these by-products as raw materials.

• To minimise 2SO emissions and hence the formation of acid rain, most plants use a double absorption processes. In this way the percentage of sulfur dioxide converted can be increased from 98% to better than 99.6%.

• The amount of sulfuric acid mist emitted from the process is minimised by controlling

the operating temperature of the absorber, gas flow rates, and concentrations. • Small amounts of caesium are added to the vanadium(V) oxide catalyst to increase its

efficiency (and hence maximise yields of 3SO ) and allow it to operate at lower

temperatures. • Mildly toxic vanadium from spent catalyst, the catalyst is disposed of in landfill sites. • Cooling water is usually recycled. • The heat exchanger removes the heat produced during the exothermic reaction. The

heat collected is recycled and is used to heat the incoming gas. This reduces the costs associated with this process and conserves energy.

• The oxidation of sulfur dioxide to sulfur trioxide is highly exothermic, generating

sufficient heat energy to meet the energy needs of the rest of the plant. • Surplus energy is produced and converted to electricity. • Correct treatment of waste water from cooling process.

ECONOMIC FACTORS TO CONSIDER • Plants should be located close to sulfur sources in order to minimise transportation

costs. • Maximise conversion of 2SO to 3SO to reduce costs.

• Use cheaper catalysts. • Purify 2SO to avoid catalytic poisoning.


HEALTH AND SAFETY Sulfuric acid is highly corrosive and can: • Cause severe burns to the skin and eyes.

• Cause blindness.

• Damage plants Exposure to sulfuric acid mist can result in a build-up of fluid in the lungs (pulmonary oedema). Sulfur dioxide and sulfur trioxide are respiratory irritants, damage plants, and contribute to a major extent to acid rain. Oleum is a highly corrosive oily liquid that produces sulfur trioxide fumes. Industries must therefore engage in strict safety procedures to minimise the escape of gases and trap fumes. • Work areas must be well ventilated and employees must wear protective clothing. • Acid spills are contained using materials such as earth, clay or sand, and then slowly

diluted with water (sulfuric acid releases large amounts of energy when added to water) before being neutralised with a base such as limestone ( 3CaCO ) or sodium carbonate.

• There are strict protocols in place for the transport and loading/unloading of the acid to

minimise accidents.


MIXED QUESTIONS QUESTION 1 A student carelessly dropped M18 sulfuric acid onto some paper. He noticed that, within a few seconds, the paper became blackened. In this reaction, the sulfuric acid is acting as: A A dehydrating agent B An acid C A base D An oxidant QUESTION 2 Concentrated sulfuric acid reacts with glucose. One of the chemical reactions that can occur may be represented as:

−+ ++→+ )(4)(3)()(42)(6126 6666 aqaqslaq HSOOHCSOHOHC

This reaction is best described as being: A Reduction B Halogenation C Hydration D Dehydration QUESTION 3 Concentrated sulfuric acid reacts with sodium nitrate according to the following equation.

)(3)(4)(3)(42 gssl HNONaHSONaNOSOH +→+

In this particular reaction sulfuric acid is behaving as: A An acid B An oxidant C A reductant D A dehydrating agent QUESTION 4 Which one of the following is NOT a use of sulfuric acid? A Manufacture of detergents B Preparation of diethyl ether C Production of fertilisers D As electrolyte in batteries E As rocket fuel


QUESTION 5 Concentrated 42 SOH is often used as a dehydrating agent for gases. For which one of the following gases would this method not be suitable? A 2O

B 2N

C 3NH

D 2CO QUESTION 6 Why can 42SOH act as a dehydrating agent, but HCl cannot? Solution QUESTION 7 Describe two uses of sulfuric acid and give details of how each use is related to the properties of the substance. Solution


QUESTION 8 Sulfuric acid is used by a large proportion of the world’s chemical industry. It has the ability to react in several different ways, depending upon the nature of the reactants and reaction conditions. (a) Write balanced chemical equations to show the action of sulfuric acid: (i) as an acid in aqueous solutions. (ii) as an oxidising agent. (iii) as a dehydrating agent. (iv) in the preparation of a volatile acid. (b) Describe how concentrated sulfuric acid may be safely diluted with water.


QUESTION 9 Classify each of the reactions involved in the Contact Process by placing a tick in the appropriate boxes.

Condensation

Exothermic

Combustion

Redox

)(2)(2)( ggl SOOS →+

)(2)(22 gg OSO + )(32 gSO

)(722)(3)(42 lgl OSHSOSOH →+

)(42)(2)(722 2 lll SOHOHOSH →+

QUESTION 10 Which one of the following statements relating to the production of sulfuric acid is incorrect? A In the combustion chamber, the reaction that takes place is the oxidation of sulfur. B In the converter, beds of catalyst ensure efficient conversion of the incoming reactants. C In the converter, the pressure is normally several hundred atmospheres.

D In the converter, the temperature is normally between Co400 and Co500 .

QUESTION 11 The four stages involved in the preparation of sulfuric acid can be summarised as follows:

43214272232 SOHOSHSOSOS →→→→

In which of these steps does the oxidation number of sulfur not change? A 1 B 2 and 3 C 2 and 4 D 3 and 4


QUESTION 12 A catalyst is used in the industrial production of 3SO from 2SO . The best description of the

effect of the catalyst is that it: A Increases the proportion of 3SO in the mixture at equilibrium

B Increases the rate of oxidation of 2SO

C Increases both rate of oxidation of 2SO and the rate of reduction of 3SO

D Decreases the rate of reduction of 3SO

QUESTION 13 The oxidation of sulfur dioxide to sulfur trioxide in the reaction:

2SO2(g) + O2(g) ⎯⎯ →← 52OV 2SO3(g)

is part of the Contact Process for the formation of sulfuric acid. This reaction has a value of

HΔ which is: A Positive B Negative C Zero QUESTION 14 Why is excess oxygen added in the converter?

2SO2(g) + O2(g) ⎯⎯ →← 52OV 2SO3(g) Δ H = -198kJ/mol Solution


QUESTION 15 State the conditions that will favour a high yield of SO3. Solution QUESTION 16 Even though high pressures will improve the yield of product, why do industries carry out the Contact Process at normal atmospheric conditions? Solution QUESTION 17 Why is it important to ensure almost complete conversion of SO2 to SO3? Solution


QUESTION 18 Why is the catalyst spread over beds in the converter? Solution QUESTION 19 Why don’t we react SO3 directly with H2O to produce H2SO4? Solution QUESTION 20 Why can’t 3SO be produced via the reaction )(3)(2)( 232 ggs SOOS →+ ?

Solution


QUESTION 21 The diagram below represents one way by which sulfuric acid can be produced industrially. (a) Name a suitable catalyst for the reaction at Step II. (b) Identify substance X. (c) Describe how 3SO is converted into sulfuric acid in Step III.

(d) Name two major uses for sulfuric acid in Australia. Solution


QUESTION 22 The Contact Process for the manufacture of sulfuric acid did not come into widespread use until the importance of the following points was realised: • There must be excess air in the air/ 2SO mixture.

• The temperature of the reacting gases must be kept as low as is practical. Explain the significance of these two points in terms of the reactions involved in the manufacture of sulfuric acid. Solution


QUESTION 23 During the production of sulfuric acid the reaction in the converter is given by the equation:

)(2)(22 gg OSO + )(32 gSO

(a) Explain the effect of the following changes on the rate and the yield of the reaction if the given mixture is initially at equilibrium.

(i) Increased applied pressure at constant temperature. (ii) Increased temperature at constant volume. (iii) The addition of excess 2O at constant temperature and volume.

(b) Sulfur dioxide gas is passed over a catalyst. What is the catalyst and what is its purpose?


(c) Name two industrial products which are produced using sulfuric acid? (d) Write a chemical equation which demonstrates 42SOH acting as: (i) A strong acid (ii) A dehydrating agent (iii) An oxidant


QUESTION 24 This figure shows a flow diagram of the industrial production of sulfuric acid.

(a) Write down chemical equations for the following processes that occur during the industrial production of sulfuric acid: (i) In the furnace. (ii) In the converter. (iii) In the absorption tower. (iv) In the production of sulfuric acid from oleum.


(b) The chemical process in the converter takes place in the presence of a catalyst. (i) Explain briefly what the catalyst does. (ii) Name a suitable catalyst. (iii) Explain the reason for the recycling process in the converter. (c) Give a brief explanation of the factors that help determine the exact conditions of temperatures and pressure chosen for the reaction occurring in the converter.


QUESTION 25 The oxidation of sulfur dioxide to sulfur trioxide is an equilibrium process described by the reaction )(2)(22 gg OSO + )(32 gSO , molkJH /198−=Δ .

(a) Draw a potential energy diagram for the reaction given that the activation energy requirement for the reaction is molkJ /250 and the enthalpy of 1 mole of sulfur trioxide

is molkJ /396− .

3 marks (b) (i) Sulfuric acid is produced via the Contact Process. Where does the conversion of sulfur dioxide to sulfur trioxide occur?

__________________________________________________________________

__________________________________________________________________

1 mark

(ii) Suggest 1 reason for using excess air in the conversion of 2SO to 3SO .

__________________________________________________________________

__________________________________________________________________

1 mark

(iii) What conditions of temperature and pressure favour the production of sulfur trioxide at equilibrium?

__________________________________________________________________

__________________________________________________________________

1 mark


(iv) In what way(s) and why do the conditions used in the manufacture of sulfuric acid differ from those described above?

__________________________________________________________________

__________________________________________________________________

__________________________________________________________________

__________________________________________________________________

__________________________________________________________________

__________________________________________________________________

2 marks (c) Given: )(2)(22 gg OSO + )(32 gSO , predict the effects on the equilibrium mass of 2SO

when the following changes are introduced. In each case, circle the correct response. (i) The pressure is increased by halving the volume at constant temperature. Higher Lower The same (ii) 2O is added to the reaction mixture at constant temperature and volume. Higher Lower The same 2 marks

(d) (i) Oxygen is added to the equilibrium system at constant volume. As a result, the temperature of the reaction vessel should:

Rise Fall Remain constant

(ii) 3SO is removed from the equilibrium system at constant temperature and volume.

As the system re-establishes equilibrium, the rate of the forward reaction will: Increase Decrease Remain the same 2 marks

Total = 11 marks


QUESTION 26 Sulfuric acid is one of the most extensively used chemicals in industry, and is primarily produced by the Contact Process. A student in a laboratory decides to construct a small scale 42 SOH production plant, that is modelled on the Contact Process. (a) Stage 1 - Reaction: )(2)(2)( ggg SOOS →+

(i) Would air or pure oxygen be used in the contact process? Explain.

__________________________________________________________________

__________________________________________________________________

__________________________________________________________________

__________________________________________________________________

__________________________________________________________________

__________________________________________________________________

2 marks (ii) What type of a reaction is occurring at this stage?

__________________________________________________________________

__________________________________________________________________

__________________________________________________________________

1 mark


(b) Stage 2

(i) What is the purpose of this stage?

__________________________________________________________________

__________________________________________________________________

1 mark (ii) Why is this stage important?

__________________________________________________________________

__________________________________________________________________

__________________________________________________________________

1 mark (c) Stage 3: Catalytic Oxidation of 2SO Reaction: )(2)(22 gg OSO + )(32 gSO

When the reaction is carried out at C200 curve B is obtained and at C300 curve C is obtained. (i) If the maximum amount of product that could be produced occurs at the point A on the above graph, explain why the maximum theoretical amount is NOT obtained.

_____________________________________________________________________

_____________________________________________________________________

_____________________________________________________________________

1 mark

Amount of SO3


(ii) Explain the differences in the reaction rates at the different temperatures.

_____________________________________________________________________

_____________________________________________________________________

_____________________________________________________________________

1 mark (iii) In industrial plants, sulfur dioxide is passed over several catalyst beds. The first bed usually contains very little catalyst. Each subsequent bed below contains increasing amounts of catalyst. Explain why this occurs.

_____________________________________________________________________

_____________________________________________________________________

_____________________________________________________________________

_____________________________________________________________________

_____________________________________________________________________

_____________________________________________________________________

2 marks (d) Stage 4: Dilution of Sulfur Trioxide With Water to Produce Sulfuric Acid

Explain why this stage is not performed in the contact process. In your answer, include the equations that describe the reactions that do occur.

_____________________________________________________________________

_____________________________________________________________________

_____________________________________________________________________

_____________________________________________________________________

_____________________________________________________________________

_____________________________________________________________________

2 marks

Total 11 Marks


QUESTION 27 How do industries minimise 2SO emissions into the atmosphere? Solution QUESTION 28 Why is no external heating supplied in the Contact Process? Solution


QUESTION 29 (a) Calculate the atom economy of ethylene oxide, created in the following reaction:

(b) Would this method of production of ethylene oxide be considered as a “Green” process? Give a reason for your answer.

(c) Recently, a method of synthesising ethylene oxide from ethene and oxygen using a silver catalyst was developed. What’s the atom economy of this alternative reaction?

SOLUTIONS


FOR ERRORS AND UPDATES, PLEASE VISIT WWW.TSFX.COM.AU/MC-UPDATES

QUESTION 1 Answer is A QUESTION 2 Answer is D QUESTION 3 Answer is A QUESTION 4 Answer is E QUESTION 5 Answer is C QUESTION 6

Why can 42SOH act as a dehydrating agent, but HCl cannot?

QUESTION 7


QUESTION 8 (a) (i) as an acid in aqueous solutions.

(ii) as an oxidising agent

(iii) as a dehydrating agent

(iv) in the preparation of a volatile acid

(b) Describe how concentrated sulfuric acid may be safely diluted with water.


QUESTION 9

Condensation

Exothermic

Combustion

Redox

)(2)(2)( ggl SOOS →+

YES YES YES

)(2)(22 gg OSO + )(32 gSO

YES YES YES

)(722)(3)(42 lgl OSHSOSOH →+

YES

)(42)(2)(722 2 lll SOHOHOSH →+

YES

QUESTION 10 Answer is C QUESTION 11 Answer is D QUESTION 12 Answer is C QUESTION 13 Answer is B QUESTION 14

QUESTION 15

QUESTION 16


QUESTION 17

QUESTION 18

QUESTION 19

QUESTION 20


QUESTION 21

QUESTION 22


QUESTION 23 (a)

(b)

(c)


(d)

QUESTION 24

(b) (c)


QUESTION 25

(a) (b) (i) (ii) (iii) (iv)


(c)

(i)

(ii)

(d) (i) (ii)


QUESTION 26 (a) (i) (ii) (b) (i) (ii) (c) (i) (ii)


(iii) (d)

QUESTION 27

QUESTION 28

QUESTION 29

(a) C2H4O = 44g/mol CaCl2 = 111g/mol H2O = 18g/mol

(2 44) 100% 37.4%

(2 44 111 2(18))Atom Economy × ×= =

× + +

(b) An atom economy of 37.4% is particularly poor, and this is a very wasteful process. This would not be considered a green process, as one the key principles of green chemistry is that it is better to develop reactions with fewer waste products than to have to clean up the waste (eg. achieve high atom economy). (c) All atoms in the reactants used in products, therefore 100% atom economy

Documents

production of sulfuric acid - · PDF fileThe reaction that occurs in the burner is classified as an ... Like all the reactions involved in the production of sulfuric acid, ... H2SO4(l)