SULPHUR

SULPHUR

Sulphur as an element

Sulphur makes up about 0.1% of earth's crust. It is a yellow, non-metallic solid element and second member in group (VIA) of the periodic table. Sulphur occurs freely as deposit in America, Poland, Japan and New Zealand.

It is also found in the combined state as sulphides of iron, zinc, lead, copper and mercury and as the tetraoxosulphate (VI) salts of calcium, magnesium and barium. Small amount of sulphide is also present in some protein building food, plant and animal bodies.

Electronic configuration of sulphur

Sulphur (16) - 1s22s22p63s23p4  or 2,8,6

Method of extraction of Sulphur

 industrially, sulphur is obtained from the deposit by FRASCH PROCESS. In the process, a sulphur pump, made up of 3 concentric steel pipes is sunk to the bottom of the sulphur deposits. Super-heated water at about 17oC and 10 atm pressure is forced through the outermost tube to the sulphur bed to melt the sulphur. Hot compressed air at a pressure of 15atm is then blown down the innermost tube to force the molten sulphur up through the middle tube. The molten sulphur is run into a large tank, where it solidifies and separated from water. The sulphur obtained is about 99.5% pure.

Sulphur occurs as organic sulphur compound such as sulphides and thio-alkanol e.g ethanethiol, C2H5SH. The sulphur contained in petroleum is separated during the preliminary process of petroleum refining. This is referred to as desulphurization of petroleum.

Allotropes of sulphur

Allotropy is the ability of an element to exist in two or more different forms in the same physical state.

The main allotropes of sulphur are:

1.       Rhombic sulphur              2. Monoclinic sulphur       3. Amorphous sulphur     4. Plastic sulphur

Chemical properties of sulphur

1.       Direct combination with other elements: such as metal, oxygen, hydrogen, with carbon and other non-metals e.g. Zn + S                   ZnS;     S + O2                   SO2;     P4 + 3S             P4S3

2.       Action of oxidizing acids: Sulphur is readily oxidized when warmed with concentrated H2SO4 to form sulphur (IV) oxide. S + H2SO4                    2H2O + 3SO2

With conc. HNO3, using bromine as a catalyst, sulphur is oxidized to H2SO4.

S + HNO3                         H2SO4 + 6NO2 + 2H2O

3.       Action of hot conc. Alkalis: Sulphur reacts with hot concentrated alkaline solution to form a mixture of sulphides and trioxosulphate (IV).

3S + 6OH-                           2S2-  + SO32- + 3H2O

In the presence of excess sulphur, a polysulphide and a trioxothiosulphate (VI) are formed respectively. S^2- + nS                             S_n+1^2-

SO₃^2- + S                  S₂O₃^2- where n = 1 to 8

USES OF SULPHUR

1.       Used in the manufacture of matches, fire works and gun powder.

2.       In the manufacture of H2SO4

3.       Used in the vulcanization of rubber

4.       Used to manufacture the bleaching agent used in the pulp and paper industry.

5.       Used in preparing fungicides and insecticide

6.       Used for the production of carbon (IV) sulphide, skin ointment and dyes.

HYDROGEN SULPHIDE H2S

Hydrogen sulphide is found in volcanic gases, sulphur springs, coal gas, and gases formed during the decay of organic matter containing sulphur.

PREPARATION

Hydrogen sulphide is prepared both in the laboratory and commercially by the action of a dilute acid on a metallic sulphide, like Iron (II) sulphide.

Hclaq  +  FeSs                                FeCl2aq  + H2Sg

H2SO4aq  +  FeSs                      FeSO4aq  + H2Sg

The gas is easily liquefied, and may be purchased in this form in steel cylinder.

PHYSICAL PROPERTIES OF H2S

1.       H2S is a colourless gas with a repulsive smell like that of a rotten egg.

2.       Very poisonous and toxic

3.       It is moderately soluble in water

4.       Burns with a pale blue flame

5.       It is denser than air

6.       It turns moist blue litmus paper red.

CHEMICAL PROPERTIES OF H2S

1.       As an acid: Hydrogen sulphide is an acidic gas. It ionize slightly in water to form a weak, dibasic acid.

H₂S  + H₂O                              H₃O⁺   +  HS^- ;  HS^-  +  H₂O                    H₃O⁺  +  S^-

When bubbled through sodium hydroxide solution, a normal salt and water are formed.

2NaOH + H2S                  Na2S + 2H2O

With excess H2S, the corresponding acid salt (sodium hydroxide) is produced

NaOH + H2S            NaHS + H2O

2.       As a precipitating agent: When H2S gas is bubbled through solution of metallic salt, the metallic sulphide are precipitated.

ZnSO4 + H2S                     ZnS(white)  + H2SO4

Pb(CH3COO)2 + H2S                PbS(black) + 2CH3COOH

Paints often change colour because of the reaction of a metallic pigment with the hydrogen sulphide in the surrounding air.

3.       Reaction with oxygen: H2S burns in air with a pale blue flame. In a limited supply of air, sulphur(IV)oxide is formed.

2H2S + O2                2H2O + 2S

In excess supply of air, sulphur(IV)oxide is formed.

2H2S + 3O2                         3H2O  +  2SO2

4.       As a reducing agent: H2S is a powerful reducing agent because the oxidation number of sulphur in this compound is 2, the lowest exhibited by sulphur. During the redox process, H2S itself is oxidized to sulphur, e.g. 2KMnO4 + 3H2SO4 + 3H2S                       K2SO4 + 2MnSO4 + 8H2O + 5S

K2Cr2O7 + 4H2SO4 + 3H2S                     K2SO4 + Cr2(SO4)3 + 7H2O + 3S

2H2S + SO2                         3S + 2H2O

H2S + Cl2                       2HCl + S

TEST FOR H2S

With Lead (II) trioxonitrate (V) : Moisten a piece of filter paper with lead (II) trioxonitrate (V) solution and drop it into a gas jar of the unknown gas if the gas is H2S, the paper turn black due to the formation of black lead (II) sulphide. Lead (II) ethanoate too give the same reaction.

Pb(NO3)2  + H2S                               PbS + 2HNO3

USES OF H2S

H2S is used in the detection of metals in qualitative analysis.