Unit 23 Solution

Unit 23 Sulphuric acid and sulphur dioxide Suggested answers to in-text activities Internet Search and Presentation (page 21)

Uses of sulphuric acid Fertilizer production The most important use of sulphuric acid is the manufacture of nitrogenous and phosphate fertilizers. An important fertilizer, ammonium sulphate, is produced by neutralizing ammonia with sulphuric acid according to the following equation: 2NH3(aq) + H2SO4(aq)

(NH4)2SO4(aq)

Ammonium phosphate is a common phosphate fertilizer. It is manufactured by reacting calcium phosphate first with sulphuric acid and then with ammonia. sulphuric acid ammonia Calcium phosphate Phosphoric acid

Ammonium phosphate

Titanium(IV) oxide production Another important industrial use of sulphuric acid is the production of titanium(IV) oxide TiO2 from ilmenite FeTiO3. Titanium(IV) oxide is widely used as a white pigment in paints, paper, plastics, synthetic fibres, cosmetics and sunscreen. In the paint industry it has largely replaced lead-based compounds which posed significant health risks, especially to young children. Ilmenite, one of the main titanium-bearing ores, is treated with sulphuric acid to form a soluble sulphate according to the following equation: FeTiO3(s) + 2H2SO4(aq)

Fe2+ (aq) + TiO2+ (aq) + 2SO42- (aq) + 2H2O(l)

Further treatment of the mixture yields titanium(IV) oxide. Cleaning iron Before galvanizing or electroplating iron or steel, it is essential that the surface be free of its

oxide layer and any materials such as grease and dirt that could reduce the ability of the plating metal to bind strongly with the iron. Sulphuric acid is used for this purpose because it is very corrosive and readily removes these coatings. Manufacture of detergents Sulphuric acid is also used to manufacture some types of detergents. An important group of detergents called alkylbenzene sulphonates have a SO3- group attached to an alkylbenzene molecule. This group is attached to the molecule by reacting an alkylbenzene with concentrated sulphuric acid as shown in the following equation: R

+ H2SO4

R

SO3H + H2O

Other uses Other uses of sulphuric acid include: • as a raw material or catalyst in many manufacturing processes; • certain wood pulping processes for paper; • some textile and fibres processes (such as rayon and cellulose manufacture); • leather tanning; • effluent treatment; • manufacture of food acids such as citric acid and lactic acid. Check Your Understanding (page 24) 1

Burning sulphur in air / roasting iron sulphide ores (iron pyrite or zinc blende) S(s) + O2(g) 4FeS2(s) + 11O2(g) 2ZnS(s) + 3O2(g)

2

SO2(g) 2Fe2O3(s) + 8SO2(g) 2ZnO(s) + 2SO2(g)

The oleum is diluted with water to give 98% sulphuric acid. H2S2O7(l) + H2O(l)

2H2SO4(l)

3

Ammonia

4

As a fertilizer

Discussion (page 29) Case 1 • Turning blue litmus paper red

Concentrated sulphuric acid shows Acidic property

• Turning litmus paper black 2 Preparing sulphur dioxide by the reaction between concentrated sulphuric acid and copper 3 Adding concentrated sulphuric acid to glucose crystals, producing a lump of charcoal-like substance

Dehydrating property Oxidizing property Dehydrating property

Debate (page 33) Student may find the following questions helpful. • Ease of availability of resources 1

What raw materials are needed for the production?

2

From where can the raw materials be obtained?

3

Are the raw materials readily available in Hong Kong?

4

Are there sufficient land resources for the plant?

• Demand of products 1

Is Hong Kong a heavily industrialized city?

2

How much sulphuric acid is consumed in Hong Kong each year?

3

Who are the main consumers of the product?

4

Are there any side products made during the production process?

5

What products can be produced from sulphuric acid?

• Environmental impact 1

What are the problems associated with leakage, spillage and emission of raw materials and wastes? 2

Is there a suitable location for the plant?

3

What precautions have to be taken during the production and transportation

processes? • Economic impact 1

Jobs created by the plant. What is the present unemployment rate in Hong

2

Tax paid to the government by the company.

3

Is it profitable and possible to sell the sulphuric acid produced to nearby

Kong?

countries and cities? Your Decision Making (page 34) Students may choose location A due to the following reasons: • It is near to the industrial area and the demand for the acid should be great. • It is close to the power station and thus there is a reliable supply of electricity for the production process. • It is near to the residential area and labour is readily available. • Water required for the production can be obtained from the lake. However, the prevailing wind would blow air pollutants produced towards the residential area nearby. Therefore, measures should be taken to reduce air pollutants. Students may choose location B due to the following reasons: • It is near to the sulphur deposits and thus easy to obtain the raw material.

• It has a good transportation network as it is located along the railway and near to the pier. This is essential for the transportation of raw materials, products and labour. • It is very far away from the residential areas. A plant located there should have the least environmental impact on the residents. However, labour is less readily available. Location C is probably not a good choice as it is very near to the farm and residential area. The environment of the farm and residential area is likely to be affected by a plant in location C. It is far away from the industrial area where the demand for the acid should be great. It is also very far away from the power station where there is a reliable supply of electricity for production.

Suggested answers to exercise 1 • Making fertilizers • Making soapless detergents • Making paint additives 2

3

4

5 Sulphur dioxide Bleaches by reduction Bleaching action is slower and milder Beaching effect is not permanent

6 A

Chlorine Bleaches by oxidation Bleaching action is faster and more vigorous Bleaching effect is permanent

Option C- Atmospheric pressure is used. Option D- 98% sulphuric acid is used to absorb the sulphur trioxide.

7 C 8 D 9 B

Option A - This is no reaction between copper and iron(II) nitrate solution. Option B - The red flower would be bleached. When sulphur dioxide dissolves in water, it reacts to form sulphurous acid. SO2(g) + H2O(l) H2SO3(aq) sulphurous acid The bleaching action of aqueous sulphur dioxide is due to the reduction of the sulphite ion in sulphurous acid. Option C - The blue copper(II) sulphate crystals turn white due to the dehydrating property of concentrated sulphuric acid. Option D - When carbon dioxide dissolves in water, it reacts to form

carbonic acid. There is neutralization reaction between the acid and sodium hydroxide solution. 10

D

11

A

12

A

13

A 14 C

15

D

16

a)

X may be sulphur, iron pyrite or zinc blende.

S(s) + O2(g) SO2(g) 4FeS2(s) + 11O2(g) 2ZnS(s) + 3O2(g)

2Fe2O3(s) + 8SO2(g) 2ZnO(s) + 2SO2(g)

b)

To remove any impurities which would poison the catalyst used in the next

c)

Platinum or vanadium(V) oxide

d)

2SO2(g) + O2(g)

step.

2SO3(g)

e) Sulphur trioxide reacts violently with water and a large amount of heat is involved. This would lead to the formation of sulphuric acid mist in the absorption tower which would take a long time to settle out. f)

• Making fertilizers • Making soapless detergents • Making paint additives

g)

Because of its great attraction for water, concentrated sulphuric acid reacts

vigorously with water, giving out a great amount of heat. If water is added to the acid, the heat given out will change the water into steam. This will cause the acid droplets to spurt out

from the container. 17 18 19 20

a)

I - The blue dye becomes colourless. Sulphite ions are formed when sulphur

dioxide dissolves in water. The sulphite ions remove oxygen from the blue dye, making it colourless. SO32-(aq) + dye coloured

SO42-(aq) + (dye - O) colourless

II - The potassium dichromate solution turns from orange to green. Sulphite ions are formed when sulphur dioxide dissolves in water. The sulphite ions reduce the dichromate ions to chromium(III) ions. b)

The blue dye would restore its colour as it is reoxidized slowly in the air.

c)

i)

Sulphate ion

ii)

3SO32-(aq) + Cr2O72- (aq) + 8H+ (aq)

3SO42- (aq) + 2Cr3+(aq) +

4H2O(l) 21 Students should be able to justify their answers with sensible reasons. Students may choose site B as it is near to the industrial area and the demand for ammonia should be great. It is also close to the power station and thus there is a reliable supply of electricity for the production process. Site B is close to the main road and thus the transportation of the raw materials, products and labour is easier and faster. Site B is quite close to the residential area where labour is readily available. However, the environment of the residential area is likely to be affected by the factory. Furthermore, the prevailing wind would carry air pollutants produced by the factory to the residential area. Therefore, site B may not be the best choice due to the environmental impact mentioned. Students may choose site C as it is near to the pier. This makes the transportation of the raw materials, products and labour easy. It is also far away from the residential area. The prevailing wind would blow air pollutants produced by the factory away from the residential area, leaving the area unaffected. Furthermore, the water current would carry liquid waste

of the factory away from the residential area. However, site C is far away from both the power station and the industrial area. Labour is less readily available as it is far away from the residential area. Site A is probably not a good choice as it is close to the playground. Furthermore, the prevailing wind would carry air pollutants produced by the factory to the residential area.