Sulphur

Occurrence

Sulphur occurs in the earth’s crust to the extent of 0.05%. In occurs in nature in the free (native) state as well as in the combine state.

(a) In free state: Sulphur occurs in the free state in the volcanic regions of Italy, Japan, South America and Russia. Largest deposits of sulphur occur in the states of Louisiana and Texas in USA. Small deposits of sulphur are also found in Himachal Pradesh (at Jawalamukhi in Kangra district) in India and Baluchistan in Pakistan.

(b) In combined state: Sulphur occurs mainly as sulphides and sulphates. For example,

(i) Iron pyrites, FeS2

(ii) Galena, PbS

(iii) Zinc blende, ZnS

(iv) Copper pyrites, CuFeS2

(v) Cinnabar, HgS

(vi) Gypsum, CaSO4.2H2O

(vii) Epsom salt, MgSO4.7H2O

(viii) Baryte or Heavy spar, BaSO4

Another major source of sulphur in H2S present in natural gas and crude oil.

Preparation

Isolation from natural gas: Hydrogen sulphide, present as an impurity in natural gas, is first separted by absorbing in monoethanolamine (HOCH2CH2NH2) and then oxidized to sulphur by passing oxygen at low temperature.

H2S(g) + ½O2(g) 1/8S8(s) + H2O(l)

A certain amount of SO2 may also be formed during the above oxidation of process.

Hg2S(g) + 3/2O2(g) →  SO2(g) + H2O(l)

SO2 thus produced is converted into sulphur by passing more of H2S at about 303 K in presence of ferric oxide (Fe2O3) as catalyst.

2H2S(g) + SO2(g) 3/8S8(s) + 2H2O(l)

Properties

(i) Action of air or oxygen: Sulphur burns in air or oxygen with a blue flame forming sulphur dioxide along with a small amount of sulphur trioxide.

S8(s) + 8O2(g) →  8SO2(g)2SO2(g) + O2(g) →  2SO3(g)

(ii) Combination with non−metals

(a) With hydrogen: When H2 gas is passed through boiling sulphur, hydrogen sulphide is formed.

8H2(g) + S8(l) 8H2S(g)

(b) With halogens: Sulphur burns in F2 giving sulphur hexafluoride.

S8(s) + 24F2(g) →  8SF6(g)

When Cl2 is passed through boiling sulphur, sulphur monochloride (S2Cl2) is formed.

S8(l) + 4Cl2(g) →  4S2Cl2(g)

(c) With carbon and phosphorus: When sulphur vapours are passed over red hot carbon, carbon disulphide is formed and with phosphorus, it forms tetraphosphorus trisulphide.

4C + S8 4CS2

 (carbon disulphide)

8P4+ 3S8 8P4S3

(tetraphosphorus trisulphide)

(iii) Reducing character: Sulphur acts as a reducing agent and thus reduces nitric acid to nitrogen dioxide and sulphuric acid to sulphur dioxide.

S8 + 48HNO3 →  8H2SO4 + 16H2O + 48NO2

S8 + 16H2SO4 →  24SO2 + 16H2O

(iv) Action of alkalies: On heating with NaOH solution, it forms a mixture of sodium sulphide and sodium thiosulphate.

S8 + 12NaOH →  4Na2S+  2NaS2O3 + 6H2O

sodium sulphide       sol. thiosulphate

If sulphur is present in excess, sodium sulphide further reacts with it to give sodium pentasulphide.

2Na2S + S8 →  2Na2S5

(sodium pentasulphide)