Question

sulphur powder is boiled with milk of lime and then resulting solution is acidified

Answer 1

The abundance of sulphur in earth's crust is about 0.6%. Free sulphur occurs in large natural deposits. In Mexico and Sicily, it is being mined and used directly without further purification. Some useful sources of sulphur are sulphide ores such as copper pyrites (CuFeS2), iron pyrites (FeS2) also known as Fool's gold, galena (PbS), zinc blende (ZnS), cinnabar (HgS) etc.

Sulphur is an important element because of the industrial importance of sulphuric acid and agricultural importance of sulphate fertiliser. Sulphur is also important in living systems.

Position of Sulphur in The Periodic Table

Sulphur is the second member of oxygen family which belongs to group VIA in the periodic table. The family members in order of increasing atomic number are Oxygen (O), Sulphur (S), Selenium (Se), Tellurium (Te) and Polonium (Po).

The electronic configuration of sulphur is 1s2 2s2 2p6 3s2 3p4 and general valence shell electronic configuration of this family is ns2 np4. The members of the family contain 6 valence electronics and so all of them are placed in group VIA. Sulphur similar to other members is the p-block element. It is a solid non-metallic element. It belongs to the second period between phosphorous and chlorine in the periodic table.

Allotropy of Sulphur

The element can exist in different allotropic forms and all of them are in the solid state. Different allotropes of sulphur arise due to the difference in crystal structures, one form differs from its another form in density, outward crystal appearance, colour, thermal stability and intrinsic energy. The followings are the allotropic modification of sulphur.

a. Crystalline form

i) Rhombic or Octahedral or α-sulphur

ii) Monoclinic or Prismatic or β-sulphur

b. Noncrystalline or amorphous form

i) Plastic or γ-sulphur

ii) Milk of sulphur or δ-sulphur

iii) Colloidal sulphur

1. Rhombic or Octahedral sulphur (α-Sulphur)

Preparation

Rhombic sulphur is the most stable among all varieties. The ordinary roll sulphur or flowers of sulphur belong to this variety. It can be obtained by the slow evaporation of a solution of roll sulphur in carbon disulphide (CS2) when the crystals obtained are octahedral i.e. eight sided. CS2 is a volatile liquid and as such, it soon evaporates.

Properties

It is transparent and pale yellow in colour.It is soluble in carbon disulphide, benzene, chloroform etc. But it is insoluble in water.It is non-conductor of heat and electricity.Its melting point is 113°C and boiling point is 445°C and specific gravity is 2.05.As the name signifies, it crystallises in the octahedral rhombic form.When slowly heated to 95.5°C, it gets converted to monoclinic sulphur.

2.Monoclinic or Prismatic (β-sulphur)

It is a needle-shaped crystalline allotrope of sulphur having melting point 119°C. It is insoluble in water but soluble in organic solvents like carbon disulphide (CS2). It changes into Rhombic form at 96°C.

3. Plastic ( γ-sulphur)

It is an amorphous form of sulphur having the rubber-like mass and gradually changes into rhombic form. It is insoluble in both water and carbon disulphide (CS2) and has no sharp melting point. It is also called as the supercooled sulphur liquid in which sulphur atoms are randomly arranged.

4. Colloidal sulphur

It is another variety of the amorphous form of sulphur which is prepared by passing H2S through the oxidising agent like HNO3, KMnO4 etc. It can also be prepared by treating the solution of sodium thayoSulphatewith dil. HCl to get white turbid mass. On heating, it changes into rhombic form.

H2S+2HNO3⟶2NO2+2H2O+S↓H2S+2HNO3⟶2NO2+2H2O+S↓

KMnO4+3H2SO4+5H2S⟶K2SO4+2MnSO4+8H2O+5SKMnO4+3H2SO4+5H2S⟶K2SO4+2MnSO4+8H2O+5S

5. Milk of Sulphur

It is another amorphous form of sulphur which can be obtained by boiling milk of lime with sulphur followed by decomposing with HCl.

3Ca(OH)2+12S⟶3CaS5+CaS2O3+3H2O3Ca(OH)2+12S⟶3CaS5+CaS2O3+3H2O

2CaS5+CaS2O3+6HCl⟶3CaCl2+3H2O+12S↓2CaS5+CaS2O3+6HCl⟶3CaCl2+3H2O+12S↓

It is insoluble in water but soluble in CS2 and used as a medicine.

Manufacture of Oil of Vitrol (H2SO4) by Contact process

Principle

(1) Production of SO2

Sulphur dioxide gas is produced by burning sulphur with air or from iron-pyrites.

S+O2−→ΔSO2S+O2→ΔSO2

4FeS2+11O2⟶2Fe2SO3+8SO24FeS2+11O2⟶2Fe2SO3+8SO2

(2) Catalytic Oxidation of SO2

Hence formed SO2 gas is catalytically oxidised into sulphur trioxide gas in the presence of air and finely divided V2O5 as the catalyst. This is the key step in the manufacture of H2SO4 because as much as SO3 gas is formed the high amount of H2SO4 is obtained.

2SO2+O2⇌2SO3+Heat2SO2+O2⇌2SO3+Heat

(3) Formation of Oleum / absorption of SO3

Hence, formed SO3 gas is absorbed in the presence of conc. sulphuric acid to get fuming acid called oleum.

SO3+H2SO4⟶H2S2O7SO3+H2SO4⟶H2S2O7

(4) Dilution of oleum

The oleum is diluted with required amount of water in order to obtain desired concentration of sulphuric acid.

H2S2O7+H2O⟶2H2SO4H2S2O7+H2O⟶2H2SO4