Compounds of Sulphur

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In today’s Chemistry class, We will be learning about Compounds of Sulphur. We hope you enjoy the class!

 

 

CONTENT

• H₂S, SO₂ and SO₃: Preparation, Properties and Uses
• Tetraoxosulphate (VI) acid: Industrial Preparation (Contact Process).

 

HYDROGEN SULPHIDE, H₂S

Hydrogen sulphide is found in volcanic gases, sulphur springs, coal gas and natural gas.

LABORATORY PREPARATION

Hydrogen sulphide is prepared in the laboratory by the action of dilute acids on metallic sulphide like Iron (II) sulphide

2HCl_(aq)    +    FeS_(s) → FeCl_2(aq) + H₂S_(g)

The apparatus used for a regular supply of hydrogen sulphide in the laboratory is Kipp’s apparatus.

 

 

PHYSICAL PROPERTIES

1. Hydrogen sulphide is a colourless gas with a smell like that of rotten egg.
2. It is very poisonous.
3. It is about 1.18 times denser than air.
4. It is moderately soluble in water to form a very weak acidic solution.
5. It burns with a pale blue flame.

 

CHEMICAL PROPERTIES

1. As an acid, it reacts with alkali to form a normal salt and water

2NaOH_(aq)     +     H₂S_(g) → Na₂S_(aq)   +    2H₂O_(l)

2. It reacts with excess oxygen to form sulphur (VI) oxide but forms deposit of sulphur with limited oxygen

2H₂S_(g)   + 3O_2(g)        →     2H₂O_(l) + 2SO_2(g)

2H₂S_(g)   +  O_2(g)           →     2H_2(l) + 2S_(s)

 

3. As a reducing agent, it reacts with many oxidizing agents such as acidified KMnO4, acidified K₂Cr₂O₇, chlorine gas, FeCl₂, SO₂, H₂SO₄ and HNO₃

 

 

TEST FOR HYDROGEN SULPHIDE

A piece of filter paper is moistened with lead (II) trioxonitrate (V) solution and dropped it into a gas jar of the unknown gas. If the paper turns black, then the gas is H₂S.

EVALUATION

1. Name the reagents used in the laboratory preparation of hydrogen sulphide.
2. List TWO physical and chemical properties each of hydrogen sulphide.

 

SULPHUR (IV) OXIDE, SO₂

LABORATORY PREPARATION

Sulphur (IV) oxide is prepared in the laboratory by heating sodium or potassium trioxosulphate (IV) with tetraoxosulphate (IV) acid or hydrochloric acid.

Na₂SO_3(aq)    +   2HCl_(aq)       →   2NaCl_(aq) + H₂O_(l) + SO_2(g)

 

PHYSICAL PROPERTIES

1. Sulphur (IV) oxide is a colourless poisonous gas with a smell like that of burning matches.
2. It is very soluble in water.
3. It is about 2.5 times denser than air.

 

CHEMICAL PROPERTIES

1. As an acid, it reacts with alkali to form normal salt of trioxosulphate and water only

2NaOH_(aq)    +     SO_2(g) → Na₂SO_3(aq)  +  H₂O_(l)

2. As a reducing agent, sulphur (IV) oxide reacts with many oxidizing agents such as acidified KMnO₄; acidified K₂Cr₂O_7; FeCl₃, HNO₃, chlorine gas. It decolorizes acidified purple KMnO₄ and turns acidified orange K₂Cr₂O₇ to green.
3. It reacts as bleaching agent decolourising dye by its bleaching action. The bleaching action is similar to that of chlorine in that there must be water. But, while chlorine bleaches by oxidation, sulphur IV oxide bleaches by reduction.
4. Sulphur (IV) oxide reacts as an oxidizing agent in the presence of a strong reducing agent such as hydrogen sulphide.

2H₂S_(g)   +    SO_2(g) →           2H₂O_(l)    +    3S_(s)

C_(s) +   SO_2(g) →     CO_2(g)    +    S_(s)

 

TEST FOR SO₂

1. If an unknown gas bleaches a coloured flower, SO₂ can be suspected
2. The unknown gas bubbled through a solution of either acidified potassium heptaoxodichromate (VI) or potassium tetraoxomanganate (VII). If the orange colour of the acidified K₂Cr₂O₇ turn green or the purple colour of the acidified KMnO₄ turns colourless, then the unknown gas is SO₂

 

USES

1. It is used in the manufacture of tetraoxosulphate (VI) acid.
2. It is used as a germicide and a fumigant especially for destroying termites.
3. It is used as a bleaching agent for straw, silt and wood.
4. It is used as a preservative in some liquid e.g orange juice.
5. Liquid sulphur (IV) oxide is used as a refrigerant.

 

EVALUATION

1. Give one method of preparing sulphur (IV) oxide in the laboratory.
2. State THREE physical and chemical properties of sulphur (IV) oxide.

 

SULPHUR (VI) OXIDE, SO₃

Sulphur (VI) oxide is prepared by reacting sulphur (IV) oxide and oxygen under special conditions which are:

1. Presence of platinized asbestos or vanadium (V) oxide as catalyst
2. Pressure of 1atm
3. Temperature range of 400^oC – 450^o

2SO_2(g)   +   O_2(g)                 2SO_3(g)

 

PHYSICAL PROPERTIES OF SO₃

1. It exists as white needle-like crystals at room temperature.
2. It has a low boiling point and vaporizes on gentle heating.
3. It dissolves readily in water to give tetraoxosulphate (VI) acid.

 

TRIOXOSULPHATE IV ACID, H₂SO₃

Trioxosulphate (IV) acid is a dibasic acid with a molecular formula H₂SO₃

LABORATORY PREPARATION OF H₂SO₃

It is prepared by the action of dilute hydrochloric acid on heated sodium trioxosulphate (IV) to produce sulphur (IV) oxide, which is then dissolved in water.

Na₂SO_3(s)  +   2HCl_(aq)     →   2NaCl_(aq) + H2O_(l) + S0_2(g)

H₂O_(l)  +  SO_2(g)                 H₂SO_3(aq)

Sulphur (IV) Oxide is the acid anhydride of trioxosulphate (IV) acid.

 

PHYSICAL PROPERTIES OF H₂SO₃

• It turns blue litmus paper red.
• It is a colourless liquid.
• It mixes readily with water.
• It has an irritating and choking smell.

 

CHEMICAL PROPERTIES OF H₂SO₃

• It reacts with alkalis to form salt and water.

2NaOH_(aq) + H₂SO_3(aq)    →    Na₂SO_3(aq)  + 2H₂O_(l)

• It is oxidized in air to tetraoxosulphate (VI) acid

2H₂SO_3(aq)  +  O_2(g)   →  2H₂SO_4(aq)

• It reduces oxidizing agent such as potassium tetraoxomanganate (VII) and potassium heptaoxodichromate (VI).
• It bleaches dyes in the presence of water.

 

TEST FOR SO₃^2-

Barium chloride solution is added to the solution of the unknown substance. A white precipitate soluble in dilute hydrochloric acid confirms the presence of a trioxosulphate (IV) ion.

 

USES OF H₂SO₃

1. It is used for bleaching straw and other fabrics.
2. It is used as a germicide.

 

EVALUATION

1. Write an equation of reaction for the laboratory preparation of trioxosulphate IV
2. State two physical and two chemical properties of trioxosulphate IV

 

TETRAOXOSULPHATE VI ACID, H₂SO₄

Tetraoxosulphate VI acid is one of the most important chemical compounds known. It is used in almost every manufacturing process; hence it is mostly prepared industrially.

 

INDUSTRIAL PREPARATION OF H₂SO₄

Industrially, tetraoxosulphate VI acid is manufactured by Contact process. The following steps are involved in the Contact process.

1. Burning sulphur in dry air to obtain sulphur (IV) oxide, SO₂

S_(s) +  O_2(g)     → SO₂(g)

2. The sulphur (IV) oxide produced is mixed with excess air and passed through an electric chamber to remove impurities and dust which may poison the catalyst. The gaseous mixture is then passed through concentrated H₂SO₄ to dry it before passing it into the reaction chamber.
3. The dried gaseous mixture is delivered to the contact tower (reaction chamber) where the sulphur (IV) oxide and oxygen combine in the presence of pellets of catalyst, vanadium (V) oxide, V₂O₅ to yield sulphur (VI) oxide. The reaction takes place at atmospheric pressure and temperature of 450-500^o

2SO_2(g)  +  O_2(g)                             2SO_3(g) + heat

4. The sulphur (VI) oxide is cooled and then dissolved in concentrated H₂SO₄ to produce a thick liquid called Oleum.

H₂SO_4(aq) +  SO_3(g)  → H₂S₂O_7(aq)

5. The Oleum is then diluted with water appropriately to produce 98% tetraoxosulphate (VI) acid.

H₂O_(l)  +  H₂S₂O_7(aq)  →  2H₂SO_4(aq)

 

NOTE: Sulphur (VI) oxide is not dissolved directly in water to produce the acid because of the large amount of heat that is evolved in the process. The heat is capable of boiling the acid formed to produce a mist of droplets which can spread throughout the factory and cause acid burns.

 

PHYSICAL PROPERTIES

1. It is a colourless, viscous liquid with a density of 1.84gcm^-3
2. It is corrosive and causes burns when in contact with the skin.
3. It is highly soluble in water evolving a large amount of heat.

 

CHEMICAL PROEPERTIES

1. As an acid, it reacts with metal which are above hydrogen in the electrochemical series to liberate hydrogen gas

Zn_(s)  +  H₂SO_4(aq)    →         ZnSO_4(aq)  +  H_2(g)

2. It reacts with bases to form salts and water

MgO_(s)  +       H₂SO_4(aq) →  MgSO_4(aq)  +  H₂O_(l)

3. It reacts with alkali to form normal and acidic salt

H₂SO_4(aq)    +    NaOH_(aq)  → NaHSO_4(aq)  + H₂O_(l)

H₂SO_4(aq)    +    NaOH_(aq)   → Na₂ SO_4(aq) + 2H₂O_(l)

4. As an acid, it reacts with trioxocarbonate (IV) to liberate carbon (IV) oxide

H₂SO_4(aq)   + CuCO_3(aq)     →  CuSO_4(aq)  +  H₂O_(l) + CO_2(g)

5. As an oxidizing agent, concentrated H₂SO₄ oxidize metals, non –metals and hydrogen sulphide in the following ways.

Zn_(s) +  2H₂SO_4(aq)    →  ZnSO_4(aq)  +  2H₂O_(l)     +  SO_2(g)

C_(s) + 2H₂SO_4(aq)      →  2H₂O_(l)  +  CO_2(g)  +      2SO_2(g)

H₂SO_4(aq) +  H₂S_(g)    →  S_(s)  +  H₂O_(l)  +  SO_2(g)

6. Concentrated tetraoxosulphate (VI) acid is a dehydrating agent, removing water from compounds like sugar, ethanol, methanoic acid and ethanedioic acid

C₁₂H₂₂O_11(s)       →      12C_(s)      +    11H₂O_(l)

^{sugar                                          charcoal}

7. Concentrated tetraoxosulphate (VI) displaces volatile acids from their salts

KCl_(s) +  H₂SO_4(aq)     → KHSO_4(aq) + HCl_(g)

 

TEST FOR SO₄^2-

Barium chloride solution is added to the solution of the unknown substance. A white precipitate insoluble in excess dilute hydrochloric acid confirms the presence of a tetraoxosulphate (VI) ion.

USES OF H₂SO₄

• It is used in the production of fertilizers e.g. ammonium tetraoxosulphate (VI).
• It is used in the purification of crude oil.
• It is used as an electrolyte in a lead-acid accumulator.
• It is used as a drying agent for many gases except NH₃ and H₂S gas.
• It is used to clean metals before electroplating.

 

USES OF TETRAOXOSULPHATE (VI) SALTS

1. Ammonium tetraoxosulphate (VI) used as fertilizers
2. Sodium tetraoxosulphate (VI) is used in paper manufacture and as a purgative
3. Calcium tetraoxosulphate (VI) is mined as gypsum and when heated forms plaster of Paris used to set broken bones.
4. Aluminium tetraoxosulphate (VI) is used to coagulate precipitate in the purification of water
5. Iron II tetraoxosulphate (VI) is used to treat anaemia.

 

 

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