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Sulphur and its compounds

Facts about Sulphur

Sulphur is the chemical element that has the atomic number 16. It is denoted with the symbol S. It is an abundant, multivalent non-metal. Sulpur, in its native form, is a bright yellow crystalline solid. In nature, it can be found as the pure element and as sulphide and sulphate minerals. It is an essential element for life and is found in two amino acids: cysteine and methionine. Its commercial uses are primarily in fertilizers, but it is also widely used in black gunpowder, matches, insecticides and fungicides. Elemental sulphur crystals are commonly sought after by mineral collectors for their brightly colored polyhedron shapes. In nonscientific contexts, it can also be referred to as brimstone.

Sulphur Crystal

By the end of the lesson the learner should be able to

:-
a) Describe the allotropes of sulphur.
b) Describe extraction of sulphur.
c) State properties of sulphur.
d) State the uses of sulphur.

In this lesson we will discuss the occurrence and extraction of sulphur.

Sulphur deposits

Sulphur occurs as free element in underground deposits.
It also occurs in nature as sulphur containing compounds e.g a) Metallic sulphides- iron pyrites (FeS2), zinc blende (Zns), lead sulphides, copper pyrites (CuFes2).
b) Metallic sulphates e.g. (CaSO4) calcium sulphate. c) Hydrogen sulphide (H2S). The following animation shows some examples of Sulphur containing compound.

The following animation shows how sulphur is extracted using the frasch process.

Super heated water at 1700C is forced down the outermost pipe. This melts sulphur which has a lower melting point of 1130c.
Hot compressed air is down the innermost pipe to force mixture of molten sulphur, water and air out through the middle pipe.
This mixture is then collected in tanks.
In tanks the sulphur cools & separates out with a purity of over 99%.

Sulphur like carbon has two allotropes
(a) Monoclinic or Beta
b) Rhombic or Alpha


Monoclinic sulphur can be prepared by heating sulphur gently in a crucible. Observe the yellow sulphur melting, a crust of sulphur forming on the surface of the molten sulphur, needlelike crystals on the sides of the crucible.Monoclinic sulphur is a yellow crystalline solid with a melting point of 1190 .
It is obtained by allowing molten sulphur to solidify. Long needle like crystals are formed on the walls of the container.
The monoclinic sulphur is stable at temperature above 960 .
Below 960C monoclinic sulphur gradually changes to rhombic sulphur.

Rhombic sulphur consist of relatively large yellow octahedral crystal with melting point of 114 degree celcius.

Rhombic sulphur
The crystals are stable at temperature below 96 degree celcius and above this temperature they slowly change to monoclinic sulphur.
96 degree celcius is the transition temperature of the two allotropes.
 

There are other forms of sulphur (a) A morphous sulphur and plastic sulphur.

Amorphous sulphur

Plastic sulphur

Each sulphur molecule contains 8 atoms arranged in crown-shaped ring
The sulphur atoms are joined by strong covalent bonds.
Weak intermolecular forces exist between the sulphur molecules; hence sulphur has a low melting point.Observe the following animation and note how the eight atoms of sulphur are joined by covalent bond to form a crown shaped ring. Observe how they are joined by weak intermolecular forces.

Sulphur Molecule

 

 

Each sulphur molecule contains 8 atoms arranged in crown-shaped ring
The sulphur atoms are joined by strong covalent bonds.
Weak intermolecular forces exist between the sulphur molecules; hence sulphur has a low melting point.
 

The following animation shows what happens to sulphur molecule when heated.

Sulphur is a yellow non-metallic solid. Observe the following animation and note

the colour of sulphur solid.

Sulphur is insoluble in water .Observe the following animation and note the solubility of sulphur in water.

Sulphur is insoluble in organic solvents like methylbenzene, carbon disulphide

Sulphur has low melting point

Sulphur reacts with most metals to form metal sulphides: Observe the following animation and note yellow colour of sulphur and grey colour of iron powder in the mixture. Black solid formed after heating.

Sulphur burns in air or oxygen with a blue flame to form sulphur (IV) oxide. Observe the following animation and note the colour of the flame.


 

The red glow indicates that the reaction is exothermic.
The yellow sulphur reacts with the grey iron powder to form a black solid called iron (II) sulphide.

Fe (s) + S (s) w FeS(s)

3) Sulphur does not react with dilute acids. However, its oxidized by hot concentrated sulphuric acid and nitric acid.
(i) With hot concentrated sulphuric acid
Sulphur is oxidized to sulphur (IV) oxide

S (s) + 2H2SO4(l) w 3SO2(g) + 2H2O(g)
(ii) With hot concentrated Nitric acid
Sulphur is oxidized to sulphuric acid.

S (s) + 2H2SO4(l) w 3SO2(g) + 2H2O(g)
4) Sulphur reacts very slowly with hydrogen gas to form hydrogen sulphide gas.

H2(g) + S(s) w H2S(g)

Sulphur is used in many ways as shown in the following illustrations. Observe the following photographs.

By the end of this topic, you should be able to

:
i. Describe the preparation of sulphur (IV) oxide
ii. State the properties and uses of sulphur (IV) oxide
iii. Explain the manufacture of sulphuric acid

 

In this lesson we will discuss oxides of sulphur.

Sulphur (IV) oxide molecule

Sulphur (IV) oxide is readily generated by reacting sulphite salts with dilute hydrochloric acid.

Click to play the following video clip to observe the preparation of Sulphur (IV) oxide.

 

Sodium sulphite reacts with dilute hydrochloric acid to form sulphur (IV) oxide, sodium chloride and water.

NaSO3 (s)+ HCL (aq) w SO2 (g)+ NaCl (aq)+ H2O (l)
 

The gas is passed through concentrated sulphuric acid to dry it. It is then collected by downward delivery since it is denser than air.
Orange potassium dichromate (VI) solution turns green after some time indicating the gas jar is full of sulphur (IV) oxide.

Note : Sulphur (IV) oxide can also be prepared by heating of concentrated sulphuric acid with copper turnings.

The following are some of the physical properties of the gas i. Is a colourless gas
ii. Has a characteristic irritating and chocking smell
iii. Easily liquidified (at 3 atmospheres)
iv. It is denser than air hence collected by downward delivery
 

Reaction with water

Observe the animation below and note the water level rising in the gas jar, drop in water level in the trough and blue litmus turning red.

Note that the water rises rapidly in the gas jar showing that the gas is highly soluble in water
The gas dissolves in water forming an acidic solution known as sulphurous acid
Sulphurous acid turns blue litmus red

SO2(g) + H2O (l) w  H2SO3(aq)

Sulphurous acid (like carbonic acid, H2CO3) cannot be isolated as it is readily oxidized to sulphuric acid

SO2 2-+ O     SO42-(aq)

Sulphur (IV) Oxide, being acidic, is absorbed by sodium hydroxide solution to form sodium sulphite and sodium hydrogen sulphite.

The following equation shows the products formed.

In the presence of water, sulphur (IV) Oxide bleaches the coloured flower petals by reduction.
It reduces the dye by removing oxygen from it hence flower petals change colour from red to white.
Note that in the absence of water, sulphur (IV) oxide will not bleach the petals.

The following equatons shows how the reaction occurs.

Observe the following animation and note the colour of the flower petals at the beginning. The colour of the flower petals changing white.

Observe the following animation and note the colour change of acidified potassium chromate (VI) solution from orange to green.

Acidified potassium dichromate (VI) solution changes from orange to green i.e.
The orange dichromate (VI) ions are reduced to green chromium (III) ions.The overall reaction is written as follows:

Note that this is the confirmatory test for Sulphur (IV) oxide gas.

Observe the following animation and note magnsesium catching fire on heating, Magnesium continuing to burn in Sulphur (IV) oxide, white powder forming on the spoon and yellow solid deposited on the gas jar.

The following animation shows what happens when purple potassium mangnate (VII) is added to a gas jar containing Sulphue (VI) oxide gas.

Purple potassium mangnate (VII) is decolourised that is it changes to a colourless solution. Purple Mangnate (VII) ions are reduced to colourless Mangnate (II) ions.

This is a test for sulphur (IV) oxide gas.

Magnesium continues to burn in sulphur (IV) Oxide forming white magnesium oxide and yellow sulphur deposits.

Observe the following animation and note the purple colour of Purple potassium manganate (VII) changing to colourless.

Purple potassium manganate (VII) is decolourised i.e. changes to colourless solution Potassium manganate (VII) was are reduced to colourless manganese (II) ions.

Sulphates ans Sulphites can be tested using the following Apparatus and chemicals
(i) Test tubes
(ii) Droppers
(iii) Barium nitrate or Barium chloride solutions
(iv) Dilute nitric acid
(v) 10cm3 measuring cylinder

Click to play the following video clip and observe how to test for the presence of sulphates and sulphites.


 

When aqueous solution of barium nitrate or barium chloride solution is added to a solution of sodium sulphate, a white precipitate of Barium sulphate is formed.
When dilute nitric acid is added to the precipitate it does not dissolve.

When aqueous solution of Barium nitrate or barium chloride solution is added to a solution of sodium sulphite, a white precipitate of Barium sulphite is formed.
When dilute nitric acid is added to the precipitate in excess. The precipitate decomposes and sulphur (IV) Oxide is evolved
Note: This distinguishes the precipitate of Barium sulphate from barium sulphite which does not decompose . This test therefore distinguishes Sulphate and Sulphite ions.
 

Sulphur (IV) oxide is used to manufacture some products whose photographs are shown below. Bleaching agent, fumigant,food preservative, disinfectant, petroleum product.

The process by which sulphuric acid is produced is known as contact process.Observe the following animation showing the contact process.



 

Stage 1: PRODUCTION OF SULPHUR (IV) OXIDE
Sulphur is burnt in a furnace in presence of purified air to form sulphur (IV) Oxide.

S(s) + O2(g) w SO2(g)
Stage 2: CONVERSION OF SULPHUR (IV) OXIDE TO SULPHUR VI OXIDE (CATALYTIC CHAMBER)
Any dust particles and impurities are removed from the SO2 produced.

These clean gas are heated a temperature of about 4500c and are passed over vanadium (V) oxide .
This catalyses the reaction between sulphur (IV) oxide and oxygen to produce sulphur (VI) oxide (SO3).

2SO2 (g) + O2(g)w 2SO3(g)
Note Vanadium V oxide is used as a catalyst because
(i) It is cheaper
(ii) Less easily poisoned by impurities

STAGE III: CONVERSION OF SULPHUR VI OXIDE TO SULPHURIC ACID (ABSORPTION TOWER)
The sulphur (VI) Oxide is passed into an absorption tower (chamber where it is dissolved in concentrated Sulphuric acid to produce an oily liquid called OLEUM
(In the chamber animate the oily liquid formation)
STAGE IV: DILUTION OF OLEUM TO PRODUCE SULPHURIC ACID (DILUTION CHAMBER)
The Oleum is then passed to the dilution chamber which is then diluted with water to form concentrated sulphuric acid.
Note:
The following are the optimum conditions for the contact process
a) Catalyst ' vanadium (V) oxide (V2O5)
b) Temperatures of 4500c
c) Pressure of about 2-3 atmospheres

STAGE III: CONVERSION OF SULPHUR VI OXIDE TO OLEUM (ABSORPTION TOWER)
The sulphur (VI) Oxide is passed into an absorption tower (chamber where it is dissolved in concentrated Sulphuric acid to produce an oily liquid called OLEUM

SO3 (g) + H2SO4 (l) w H2S2O7(I)
STAGE IV: DILUTION OF OLEUM TO PRODUCE SULPHURIC ACID (DILUTION CHAMBER)
The Oleum is then passed to the dilution chamber where it is then diluted with water to form concentrated sulphuric acid.

H2S2O7(I) + H2O w 2H2SO4(I)
 

Note:
The following are the optimum conditions for the contact process
a) Catalyst vanadium (V) oxide
b) Temperatures of 450 degree celcius
c) Pressure of about 2-3 atmospheres

Vanadium (V) oxide

By the end of the lesson, the learner should be able to

:-
a) Distinguish between the reactons of dilute and concentrated sulphuric acid
b) State the uses of sulphuric acid
c) Describe the preparation of hydrogen sulphide
d) State the properties of hydrogen sulphide
e) Explain environmental pollution caused by sulphur containing compounds
 

In this lesson we will discuss properties of sulphuric acid and Hydrogen sulphide gas.

The following are the steps in the manufacture of sulphuric acid (VI) acid. Type in the correct number that show the correct sequence.

Concentrated suphuric (VI) acid is colourless liquid.
It is a viscous dense oily liquid whose density is 1.84g/cm3.
It is a non-volatile acid compared to nitric (V) and hydrochloric acid
It has a high boiling point of 3380c.

The following video clip shows what happens when concentrated Sulphuric acid is added to sucrose crystals. Click to play the video and observe carefully.

The following video clip shows what happens when concentrated Sulphuric acid is added to Ethanol. Click to play the video and observe carefully.

The following video clip shows what happens when concentrated Sulphuric acid is added to Copper turnings. Click to play the video and observe carefully.

This is an extremely corrosive acid in concentrated form. It should be handled with a lot of care. This is because it has a high affinity for water. The video clip below shows some tests carried out on conc. sulphuric acid.



 

When concentrated Sulphuric (VI) acid is added to blue hydrated copper (II) sulphate crystals, a white powder of the anhydrous copper (II) sulphate is formed.

CuSO4.5H2O(s) w CuSO4(s) + 5H2O
When concentrated Sulphuric (VI) is added white sucrose crystal are charred to a black mass.

C12H22O11(s) w 12C(s) + 11H2O(l)

When concentrated Sulphuric (VI) acid is added to ethanol, the ethanol is dehydrated to ethene gas formed which decolourises purple acidified potassium manganate (VII)

C2H5OH(l) w C2H4(g) + H20(l)

Concentrated sulphuric (VI) acid dehydrates copper (II) sulphate crystals, sucrose and ethanol as shown in the following equations.
 

The following video clip shows some tests carried out using concentrated sulphuric acid. Observe what happens carefully.

Place 2cm3 of concentrated sulphuric (VI) acid in a boiling tube.
Add 2 to 3 pieces of copper turnings to the boiling tube and warm gently
Test the gas produced with acidified potassium chromate (VI) solution and observe
Place 2cm3 of concentrated sulphuric (VI) acid in a test tube
Step 2: Add a spatula of powdered charcoal (carbon)and warm gently
Test the gas produced with lime-water and observe
 

The observations show that concentrated sulphuric acid is an oxidizing agent. It reacts with copper to form sulphuric oxide, copper (II) sulphate and water.
Similarly concentrated sulphuric (VI) acid oxidizes carbon to carbon (IV) oxide, water and sulphur (IV) oxide.

The following are some uses of Sulphuric acid.

In this lesson we will discuss preparation and properties of Hydrogen sulphide gas.

Hydrogen Sulphide is a poisonous gas.

Hydrogen Sulphide is prepared in the laboratory as follows. Observe the following animation carefully.

Hydrogen Sulphide is formed when dilute hydrochloric acid is added to Iron (II) Sulphide.

FeS(s) + 2HCl(aq) w FeCl2(aq) + H2S(g)

The gas dissolves in cold water hence passed over warm water.

The gas is also very poisonous and is therefore not prepared in the school laboratory.

Hydrogen sulphide as a reducing agent. The following animation shows what happens when Iron (III) Chloride reacts with Hydrogen sulphide gas. Note that Hydrogen syphide reacts with iron(III) chloride solution to give iron (II) chloride. The solution of iron (III) chloride changes to pale green due to formation of iron (II) chloride.

2Fe3+(aq) + S2-(aq) w 2Fe2+(aq) + S (s)

Hydrogen sulphide decolourises acidified potassium manganate(VII) solution and bromine water. Similarly acidifies orange potassium dichromate (VI) TURNS GREEN WHEN HYDROGEN SULPHIDE IS BUBBLED THROUGH IT.

To test for the presence of Hydrogen Sulphide gas a piece of damp lead acetate paper is inserted into a gas jar containing the gas to be tested.The following animation shows the chemical test for Hydrogen Sulphide gas.

A shiny brown-black colour is formed on the paper. This is due to the formation of lead (II) sulphide. This is a chemical test for hydrogen sulphide. The ionic equation for the reaction is: Pb2+(aq) + S2-(aq)w PbS(s) 

 

Sulphur compounds are responsible for three types of pollution. Odour, acid rain and health effects. The following are some effects of acid rain.

Gaseous sulphur compounds such as sulphur(w) oxide and hydrogen sulphide are pollutants in the atmosphere Hydroden sulphide is toxic. Sulphur (IV) oxide dissolves in moisture to form sulphurous and sulphuric acid. These acids cause acid rain when washed from air by rain.
Acid rain destroys vegetation and affects aquatic life in water bodies such as lakes

 




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