Chemistry

Key unit competence:

The learner should be able to compare and contrast the chemical properties of the Group 16 elements and their compounds in relation to their position in the Periodic Table.

Learning objectives

By the end of this unit, the learner will be able to:

•Describe the physical properties of Group 16 elements.

•Describe the reactions between sulphur and oxygen.

•Describe the steps and conditions applied in the industrial preparations of sulphuric acid.

•Describe the chemical properties of sulphuric acid.

•Describe the properties of oxoanions.

•State uses of the Group 16 elements and compounds.

11.1. Physical properties of group 16 elements

Activity 11.1

Study carefully the following figure and answer the questions that follow

1. What do you observe on the above figure?

2. Using your own observation and knowledge, predict the physical state of each of the elements in the figure.

3. Which one of the following best defines the word “allotropes”?

a. Different structural forms of an element

b. A pair of substances that differ by H+c. Elements that possess properties intermediate between those of metals and nonmetals

d. Atoms of a given atomic number that have a specific number of neutrons

e. The different phases (solid, liquid or gas) of a substance

11. 1.1. Physical state and metallic character of group 16 elements

This group consists of oxygen (8O), sulphur (16S), selenium (34Se), tellurium (52Te), and polonium (84Po). It is also known as group of chalcogens. The name is derived from the Greek word for brass and points to the association of sulphur and its congeners with copper. Most copper minerals contain either oxygen or sulphur and frequently the other members of the group. Group 16 elements have the valence-shell electron configuration ns2np4.There is a steady change down the group from non-metallic to metallic properties.

Oxygen is a colourless gas, a non-metal. Sulphur is a white and waxy non-metal solid. Selenium has acidic oxides only. It is classified as a semi metal or metalloid. Tellurium has amphoteric oxides, so it is a semi-metal or metalloid element. Polonium is a silvery metallic solid.

The melting and boiling points increase with the increase in atomic number down the group. The large difference between the melting and boiling points of oxygen and sulphur may be explained using their atomicity. Oxygen exists as diatomic molecule (O2) whereas sulphur exists as polyatomic molecule (S8).

Table 11.1: Physical properties of group 16 elements

11.1.2. Allotropes of oxygen and sulphur

a) Oxygen

Oxygen is a gas at room temperature and is colorless, odorless, and tasteless. It is the most abundant element by mass in both the Earth’s crust and the human body. It is second to nitrogen as the most abundant element in the atmosphere (21%).

Oxygen has two allotropes, dioxygen (O2) and trioxygen (O3) (see in Activity 11.1). In general, O2 (or dioxygen) is the form referred to when talking about the elemental or molecular form because it is the most common form of the element. The O2 bond is very strong, and oxygen can also form strong bonds with other elements. However, compounds that contain oxygen are considered to be more thermodynamically stable than O2 itself.

Dioxygen is a colourless and odourless gas. It liquefies at 90 K (-183oC) and freezes at 55 K (-218oC). Oxygen atom has three stable isotopes: 16O, 17O and 18O. Dioxygen directly reacts with nearly all metals and non-metals except some metals (e.g., Au, Pt) and some noble gases. Its combination with other elements is often strongly exothermic which helps in sustaining the reaction. However, to initiate the reaction, some external heating is required as bond dissociation energy of oxygen-oxygen double bond is high (493.4 kJmol–1).

The other allotrope, ozone, O3, is a pale-blue poisonous gas with a strong odor. It is a very good oxidizing agent, stronger than dioxygen, and can be used as a substitute for chlorine in purifying drinking water without giving the water an odd taste. However, because of its unstable nature it disappears and leaves the water unprotected from bacteria. A layer of Ozone at very high altitude in the atmosphere is responsible for protecting the Earth’s surface from ultraviolet radiations; however, at lower altitudes it is poisoneous and becomes a major component of smog.

Destruction of the ozone layer

Human activities are rejecting in the atmosphere substances that have the capacity of destroying the ozone layer.The most destructrive substances are the chlorofluorocarbons (CFCs). That is why CFCs have been banned. We can contribute to the protection of the ozone layer by avoiding to use ozone destroying substances in our daily activities.

a. Sulphur

Sulphur occurs in native state, in volcanic lands and some sedimentary lands. Sulphur occurs as free element underground in some countries such as USA and Japan. Sulphur also occurs in many important metallic sulphides (0.05% of earth’s crust), such as lead sulfide, or galena, PbS; zinc blende, ZnS; copper pyrite, (CuFe)S2; cinnabar, HgS; stibnite, Sb2S3; and iron pyrite FeS2. It is also combined with other elements in the form of sulfates such as barite, BaSO4; celestite, SrSO4; and gypsum, CaSO4.2H2O, and it is present in the molecules of many organic substances such as

mustard, eggs, hair, proteins, oil of garlic, in petrol and coal.

Sulfur exists in a variety of forms called allotropes, which consist of several solid varieties, of which the most familiar are rhombic sulfur and monoclinic sulfur.

Rhombic sulphur (α-sulphur) (see figure in Activity 11.1)

Rhombic sulphur consists of yellow, translucent, octahedral crystals. It is stable below 96oC and it slowly changes to monoclinic at temperatures above 96oC. The temperature of 96oC is the transition temperature. It melts at 113oC. It has a density of 2.06 gcm-3

Monoclinic Sulphur (β-sulphur) (see figure in Activity 11.1)

Monoclinic Sulphur is stable above 96oC up to its melting point of 119oC. It consists of needle-shaped amber transparent crystals with density of 1.98gcm-3. Below 96oC, monoclinic sulphur changes to rhombic sulphur. This type of allotropy, in which one form changes to the other depending on the temperature, is called enantiotropy.

The most common naturally occurring allotrope, S8, cyclo-octasulphur, has a zigzag arrangement of the atoms around the ring, also called crown.

All forms of sulfur are insoluble in water, but the crystalline forms are soluble in carbon disulfide.

The most stable variety of the element is rhombic sulfur, a yellow, crystalline solid with a density of 2.06 g/cm3 at 20°C. Rhombic sulfur is slightly soluble in alcohol and ether, moderately soluble in oils and extremely soluble in carbon disulfide. When kept at temperatures above 94.5°C but below 120°C the rhombic form changes into monoclinic sulfur.

When ordinary sulfur melts, it forms a straw-colored liquid that turns darker with additional heating and then finally boils. When molten sulfur is slowly cooled, its physical properties change in accordance with the temperature, pressure, and method of crust formation.

Checking Up 11.1

1. True or false

a. Sulphur is a non-metal element

b. Rhombic sulphur is stable above 96oC

2. List the important sources of sulphur

3. Elements of Group 16 generally show lower value of first ionisation energy compared to the corresponding periods of group 15. Why?

11.2. Comparison of acidity and volatility of group 16 hydrides

Activity 11.2

1. In pairs, carry out research and write a note on the following terms:

a. Hydrides

b. The strength of an acid

d. A weak acid

e. A strong acid

2. With an example, explain what is meant by the term “hydrogen bond” and show how it is formed.

All group 16 elements form hydrides of the type H2E (E = O, S, Se, Te, Po). They are bent in shape. Group 16 hydrides, contray to group 1 & 2 hydrides, are covalent. Their acidic character increases from H2O to H2Te. The increase in acidic character down the group can be explained in term of increase of ionic character of the metallic hydrides, allowing easy dissociation in water to liberate H+ ion. The Acidic nature is in the order, H2O < H2S < H2Se < H2Te.

Owing to the decrease in energy of dissociation of H-E bond down the group, the thermal stability of hydrides also decreases from H2O to H2Po. Thermal stability is in the order: H2O > H2S >H2Se > H2Te > H2Po.

The main hydrides of group 16 are hydrogen oxide (H2O) known as water and hydrogen sulphide (H2S), the rotten egg smelling and poisonous gas.

Hydrogen oxide (H2O) is a liquid at room temperature because of strong intermolecular forces (hydrogen bond). Water is neutral (neither acid nor base).

Hydrogen sulphide(H2S) is a colorless gas with a repulsive smell similar to that of a rotten egg. It is also fairly soluble in water. It is one of the weakest acids known, this gas is extremely poisonous. H2O and H2S are both polar molecules therefore two molecules of H2O near each other, and two molecules of H2S near each other have dipole-dipole force between them. Yet the dipole-dipole force between two molecules of H2O is a special type of dipole-dipole force called Hydrogen bonding. And since Hydrogen bond is a stronger dipole-dipole force than that of a normal dipole-dipole bond the boiling point of water will be abnormally greater than that of hydrogen sulphide. That is why, water, with a molar mass of 18, is liquid at room temperature, whereas, hydrogen sulfide, with a higher molar mass of 34, is a gas at room temperature.

Checking Up 11.2

1. Give and explain the order of acidic nature of hydrides of group 16 elements.

2. H2S is less acidic than H2Te. Why?

3. Given two hydrides H2S and H2O. Which one is more thermally stable? Justify your answer.

4. H2O and H2S are hydrides of two elements belonging to the same group; yes H2O is liquid at room temperature, whereas H2S is gaseous. Explain why?

11.3 Preparation and properties of sulphuric acid

Activity 11.3

1. Do you know sulphuric acid?

2. If yes, what do you know about it?

3. How will you proceed if you are mixing sulphuricaci with water?

Experiment: Investigating the dehydrating power of Sulphuric acid on sugar

Materials and chemicals:

1. 70 grams granulated sugar, C12H22O11

2. 70 ml of concentrated (18M) sulfuric acid, H2SO4

3. 300 ml tall-form beaker4. 40cm stirring rod5. Paper towels

6. Disposable gloves or

7. 100 ml graduated cylinder

8. Tray9. 1 liter beaker

10. Sodium bicarbonate, NaHCO3

11. Spatula

Safety: Sulfuric acid is a very strong acid and is extremely corrosive to skin. Wear gloves and safety goggles. During the reaction, steam is generated. It is hot. It is recommended to work in a fume cupboard.

Procedure:

Spread some paper towels on the tray.1. Put sugar into 300 ml beaker. 2. Insert stirring rod into center of sugar.3. Put beaker on paper towels on the tray.4. Add 70 ml of sulfuric acid to the sugar and stir briefly.5. Stand about 1 - 2 meters away and wait for reaction to begin and observe what will happen.

Clean Up: You might want to incorporate part of the clean up procedure into the demonstration.

Remove black carbon column from the beaker and put it into a liter beaker with some sodium bicarbonate (hydrogen carbonate). With spatula, break the column of carbon into smaller pieces. Add a little water and set back on the tray. The foaming action is also exciting.

Neutralize any acid spills with sodium bicarbonate and wipe clean. Leave lecture hall clean for the next class.

Rinse all glassware and carbon chunks with lots of water. Carbon can be thrown away in trash.

Study questions

1. Record your observations

2. Write equation that take place in this experiment

11.3.1.Industrial preparation of sulphuric acid and its environmental impact

a. Preparation

Sulphuric acid is manufactured industrially using the contact process which is based on the catalytic oxidation of SO2 to SO3. In this process, there are five steps:

Step 1: Production of sulphur dioxide

Sulphur dioxide is obtained by either burning elementary sulphur or roasting metal sulphides in air in combustion chamber.

Step 2: Purification of gases

The SO2 produced in Step1 contains many impurities (e.g arsenic (III) oxide) and these impurities would poison the catalyst (platinum, Pt) by rendering it ineffective.The mixture is passed through a purifier chamber.

Step 3: Oxidation of SO2 to SO3

The purified gases (mixture of SO2 and O2) are dried and passed through converters containing a catalyst, vanadium (V) oxide V2O5 or fine divided platinum and at temperature about

The reaction is exothermic and once the reaction has started no external heat is re-quired. The evolved heat maintains the temperature of the reaction

Step 4: Absorption of SO3

The SO3 is an anhydride of sulfuric acid, direct absorption in water gives a very vigorous reaction and forms droplets of sulphuric acid. To overcome this problem, SO3 is absorbed in the sulfuric acid in which it dissolves with ease in the absorption tower to form oleum or pyrosulphuric acid

Step 5: Dilution of oleum

The oleum formed is diluted with the correct quality of water to give concentrated sulphuric acid 98%.

b. Environmental impact of the industrial manufacture of sulphuric acid

Industrial emissions of sulfur oxides result into the increase of the concentration of sulfuric acid (H2SO4) and sulfurous acid (H2SO3) in the atmosphere. Those acids are present as particles or droplets which dissolve in clouds, fog, rain, dew, or snow, resultingin very dilute acid solutions. This constitutes a kind of pollution called acid rain. Acid rains cause acidification of soils and lakes, and destruction of some buildings, infrastructures and statues made in marble or other carbonaceous rocks.

11.3.2. Oxidising and dehydrating properties of sulphuric acid

a. Oxidizing properties

Hot concentrated Sulphuric acid is an oxidizing agent:

In other conditions, sulphuric acid is not considered as an oxidizing acid. When dilute, sulphuric reacts with other metals to form metal sulphates, and hydrogen gas, but it does’t attack copper.

b. Dehydrating properties

Concentrated sulphuric acid has a high affinity for water. It shows this property when it reacts with sugars, hydrated copper (II) sulphate, ethanol, oxalic acid and other organic matter. For example:

Because of this, concentrated sulphuric acid is used as a drying agent in laboratories.It is also this property that makes sulphuric acid destroy or burn any object containing cellulosic matter such as paper, clothes, skin, etc...). Due to this avidity for water, it is recommended, when mixing sulphuric acid and water, to pour slowly the acid in water, not the opposite.

Checking Up 11.3

1. a) Describe the Haber or Contact process for the manufacture of sulphuric acid.

b) Why is sulphur trioxide formed in this process not absorbed directly in water?

2. Concentrated sulphuric acid acts as a dehydrating agent. What does it mean?

3. Write equations to show how concentrated sulphuric acid reacts with:

a. Zinc

b. Magnesiumc. Carbon

11.4 Properties of oxoanions of sulphur

Activity 11.4 (a)

1. Use the library and/or internet to explain the following:

a. Oxidation

i) In terms of oxidation state

ii) In terms of electron transfer

b. Reduction

i) In terms of oxidation state

ii) In terms of electron transfer

c. Oxidizing agent

d. Reducing agent

Activity 11.4 (b)

2. An experiment for Heating hydrated copper(II) sulfate

Objectives:

Students remove the water of crystallisation from hydrated copper (II) sulfate by heating. Condensing in a test-tube collects the water. The white anhydrous copper (II) sulfate can then be rehydrated, the blue colour returns.

Apparatus and equipment (per group)

1. Two test-tubes

Activity questions

Record any observations made during the process and when the water was poured back onto the white copper (II) sulfate.

11.4.1. Reducing action of S2O32-

The thiosulfate anion reacts with iodine in aqueous solution, reducing it to iodide as it is itself oxidized to tetrathionate:

Thiosulfate ion reduces Iron (III) to iron (II) according to the following equation:

Thiosulfate ion is also used in analytical chemistry. It can, when heated with a sample containing aluminium cations, produce a mixture of whitish precipitate:

11.4.2. Reaction of SO32- with acids

The sulphites of Na+, K+, and NH4+ are soluble in water. Most of other sulphites are insoluble in water. However, due to the basic nature of SO32-, all sulphites dissolve in acidic solution.

In general, sulphite ion reacts with acids liberating sulphur dioxide according to the

ionic equation:

Examples:

11.4.3.Action of heat on sulphates (SO42-)The sulphates are normal salts of sulphuric acid and contain the sulphate ion, SO42-. Soluble sulphates are prepared by action of dilute acid on the metal, metal oxide, metal hydroxide,metal carbonate or hydrogencarbonate:

11.4.3.Action of heat on sulphates (SO42-)

The sulphates are normal salts of sulphuric acid and contain the sulphate ion, SO42-. Soluble sulphates are prepared by action of dilute acid on the metal, metal oxide, metal hydroxide,metal carbonate or hydrogencarbonate:

Insoluble sulphates (BaSO4 and PbSO4) are obtained by precipitation method:

Most sulphate salts do not decompose on heating. For instance, sodium sulphate, potassium sulphate, and calcium sulphate are not decomposedby heat.

Only certain sulphate salts are decomposed by heat when heated strongly. On heating, some sulphates decompose to give either sulphur trioxide or sulphur dioxide or both.

For instance, strong heating of green crystal iron (II) sulphate will release steam, sulphur dioxide, sulphur trioxide and leave behind a reddish solid iron (III) oxide residue. The steam released comes from the hydrated water of the crystallize salt:

Zinc sulphate, copper (II) sulphate, and iron (III) sulphate decompose when heated strongly to produce sulphur trioxide gas and a metal oxide.

Examples:

When ammonium sulphate is heated strongly, this white solid sublimate and is decomposed into ammonia gas and sulphuric acid vapour.

On heating, hydrated crystals of metal sulphates lose their water of crystallization and turn into a powder. They are then said to be anhydrous. Sometimes they may also lose their colour. Hydrated copper (II) sulphate is heated according to the following equation:

Checking Up 11.4

1. Write equations to show how thiosulfate ions reduce the following substances:

a. Iodine

b. Iron (III) ion

c. Aluminium ion

2. Write equations to show the action of heat on the following sulphates:

a. Zinc (II) sulphate

b. Iron (III) sulphate

c. Copper (II) sulphate

3. When hydrated copper II sulphate solid is heated in a boiling tube, a white solid Q and droplets of a colourless liquid P are observed.

a. Identify substances; liquid P and solid Q.

b. Explain the observation above.

Explain what would be observed if water is added to white solid Q.

11.5 Identification of sulphite and sulphate ions

Activity 11.5

Given a substance Y which contains one cation and one anion, identify the cation and the anion in Y. Carry out the following tests on Y and record your observations and deductions in the table below.

11.5.1. Identification of sulphate ion (SO42-)

a. Addition of barium nitrate solution

When an aqueous solution of Ba2+ ion is added to a solution containing SO42- ion, a white precipitate of barium sulphate insoluble in dilute nitric acid or hydrochloric acid is formed.

Use of barium chloride solution also gives a white precipitate insoluble in dilute hydrochloric acid. This reaction serves as the test for the sulphate ion in solution and also differentiates sulphate ions, SO42-, from sulphite ions, SO32-.

b. Addition of lead (II) nitrate solution

When an aqueous solution of Pb2+is added to a solution containing SO42-, a white precipitate of lead (II) sulphate is formed:

c. Addition of silver nitrate solution

When an aqueous solution of Ag+ is added to a solution containing SO42-, a white precipitate of silver sulphate:

11.5.2. Identification of sulphite ion (SO32-)

To an aqueous solution of any sulphite, dilute nitric acid is added followed by a few drops of barium nitrates or barium chloride solution. A white precipitate of barium sulphite forms which dissolves in the nitric acid or hydrochloric acid. The sulphite dissolves in acid with evolution of sulphur dioxide (seen as bubbles of a colourless gas).

Hence, sulphites give a white precipitate of barium sulphite soluble in acid while sulphates give a white precipitate insoluble in acid.

Checking Up 11.5

1. Describe a chemical test you would use to distinguish between the pairs of compounds below. In each case give the reagent, conditions if any and the ex-pected observation.

a. BaSO3 and BaSO4

b. BaSO4 and PbSO4

2. Carry out the following experiment to identify the cation and the anion in substance X provided by the teacher.

11.6 Uses of group 16 elements and compounds

Activity 11.6

1.What are the most known applications of Sulphur and Oxygen?

2.The following figures show how some elements play a big role in our daily lives. Observe carefully and answer the following questions:

a. What can you say about figure A, B, C and D?

b. Research in library or internet to find out the substance used in each of the figure.

c. Research in library or internet to find different uses of sulfuric acid.

11.6.1.Uses of oxygen

The first use of oxygen is in breathing and metabolism processes of all living organisms.

There are many other commercial uses for oxygen gas, which is typically obtained through fractional distillation of air. It is used in all operations involving combustion as the active component of air.

It is used in the manufacture of iron, steel, and other chemicals. Oxygen is also used as an oxidizer in rocket fuel, and for medicinal purposes. Mixture of oxygen and ethyne (oxyacetylene) is used for welding and metal cutting.

11.6.2. Uses of sulphur

The main use of Sulphur is the manufacture of sulphuric acid.Sulphur is also used in vulcanization of rubber, a chemical process for converting natural rubber or related polymers into more durable and pressure resisting materials by heating them with sulfur or other equivalent curatives or accelerators. These additives modify the polymer by forming cross-links (bridges) between individual polymer chains, making the final product very hard and resistant to pressure and other conditions.Sulphur is an ingredient in the manufacture of dyes, fireworks and other sulphur compounds.

11.6.3. Uses of sulphuric acid

Sulphuric acid is a very important industrial chemical. It used to be called the giant of chemical industry. It is used in the manufacture of hundreds of other compounds in many industrial processes.

•The bulk of sulphuric acid produced is used in the manufacture of fertilisers (e.g., ammonium sulphate, superphosphate).

•Sulfuric acid is also used in many other applications such as in: metallurgical industry, storage batteries, chemistry laboratories, etc....

11.6.4. Uses of hydrogen sulphide (H2S)

Some nuclear power plants use hydrogen sulphide to produce heavy water. Farmers use H2S as an agricultural disinfectant. In Analytical Chemistry, H2S is used in selective precipitation of metal sulfides

Checking Up 11.6

1. Give at least 3 uses of oxygen

2. Mention three areas in which H2SO4 plays an important role

3. Briefly explain what is meant by “vulcanization”

4. What property makes sulphuric acid a dangerous corrosive product?

5. How do you proceed when you mix sulphuric acid with water

11.7. End unit assessment

Multiple choice questions

1. Which one(s) of the following substancs is not easily soluble in water and why?

(A) H2        (B)O2       (C) SO2         (D) CO2

2. Select the molecular formula of sulphur in the following list at room temperature:

(A) S      (B) S2            (C)S4        (D) S8

3. Choose the correct answer. All the elements of Group 16 are:

(A) Non - metal         (B) Matalloids       (C) Radioactive        (D) All have allotropic forms

4. The triatomic species of element oxygen is known as:

(A) Azone      (B) Polyzone       (C) Trizone      (D) Ozone

5. Which of the following acts as pickling agent:

(A) HNO3     (B) H2SO4     (C) HNO2

6. Which of the following is not suitable for use in desiccators to dry substance and why?

(A) ConcH2SO4      (B) Na2SO4      (C) CaCl2     (D) P4O10

7. Which of the following methods are you going to use to test presence of water in a liquid:

(A)Taste      (B) Smell      (C) Use of litmus paper       (D) Use of anhydrous CuSO4

8. When sulphur is boiled with Na2SO3 solution, the compound formed is:

(A) Sodium Sulphide    (B) Sodium Sulphate    (C) Sodium persulphate     (D) Sodium thiosulphate

Short and long answer open questions

9. State two ways in which oxygen differs from other group 16 elements.

10. State two sources and two uses of oxygen.

a. Name two crystalline allotropes of sulphur.

b. Write equation to show how sulphur reacts with oxygen, iron and sulphu-ric acid.

c. Write two equations to illustrate the reducing property of hydrogen sul-phide

12. Sulphur is used to make sulphuric acid. The reactions in the manufacture of sulphuric acid by the Contact Process are shown below.

a. Give a large scale source of the element sulphur

b. State another use of sulphur dioxide

c. How is sulphur changed into sulphur dioxide?

d. Write a complete chemical equation for reaction 3.

e. Write a chemical equation for reaction 4.