p Block Elements
14.0 Sulphur
14.0 Sulphur
Occurrence: It occurs in abundance in the combined state as:
(i) Sulphide ores, e,g, Zinc Blend $(ZnS)$, galena$(HgS)$, copper pyrites$(CuFeS_2)$ and iron pyrites $(FeS_2)$
(ii) Sulphate ores, e,g, Gypsum $CaS{o_4}.2{H_2}O$, barytes $(BaS0_4)$, Epsom salt $MgS{o_4}.7{H_2}O$ and celesite $SrS{O_4}$.
(iii) $H_2S$ and $SO_2$ in volcanic gases, $H_2S$ in spring water, coal gas, sewage gas etc.
(iv) Organic compounds of plant and animal origins, e.g. Onion, garlic, mustard, eggs, proteins, hair, wool, etc. contains organic sulphur compounds.
Extraction:
(i) To extract underground sulphur, Franck Process is used. Three concentric pipes are used deep into the ground. Superheated water at $170^\circ C$ is forced down the outer pipes into sulphur which is melted. Compressed air blown through pipe forces the sulphur as a liquid to the surface where itis allowed to solidify 99.5% pure sulphur is obtained by the process.
(ii) Sulphur as a byproduct:
(a) From alkali waste of Lablanase's process: The alkali waste (mainly $CaS$) is suspended in water and carbon dioxide is circulated. Hydrogen sulphide is liberated which is an insufficient supply of air.
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(b) From the spent oxides of gas works: It contains ferric sulphides $(Fe_2S_4)$. It is exposed to no air a few days when sulphur is deposited.
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(c) From iron pyrites: Iron pyrites is a good source of sulphur. It is recovered when pyrites are distilled.
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Pure sulphur is obtained by boiling impure sulphur at $444^\circ C$. The vapours evolved are condensed on the cold walls as a light yellow powder.
Allotropic forms: Sulphur exists in several allotropic forms, the important ones are described below:
1. Rhombic or octahedral or $\alpha $- sulphur: This is the common forms of sulphur. It is pale yellow in color. It melts at $114.5^\circ C$. Its specific gravity is 2.06. It is insoluble in water but readily soluble in carbon disulphide. It is the stable variety at ordinary temperature an all other forms gradually change into this form.
Chemical Properties:
(a) Combustibility: The gas burns with a blue flame in oxygen on air forming sulphur dioxide and water.
In restricted supply of oxygen, sulphur is formed due to incomplete combustion.
(b) Thermal decomposition: The gas is not so stable as water. It decomposes on heating. The decomposition starts at $310^\circ C$ and completes at $1700^\circ C$.
(c) Reducing nature: Hydrogen sulphide acts as a strong reducing agent as it decomposes evolving hydrogen. The following are the important examples of its reducing action:
(i) It reduces halogens into corresponding hydracids.
(ii) It reduces ferric chloride to ferrous chloride.
(iii) Sulphur dioxide is reduced to sulphur in presence of moisture.
The occourence of sulphur in volcanic regions is attributed due to the above reaction as both gases are present in volcanic gases.
(iv) $H_2O_2$ is reduced to water
(v) $O_3$ is reduced to $O_2$.
(vi) $HNO_3$ is reduced to $NO_2$.
(vii) $H_2SO_4$ is reduced to.
(viii) Acidified $KMnO_4$ is decolourised
(ix) Acidfied $K_2Cr_2O_7$ is reduced forming green chromic sulphate.
(d) Acidic nature:
The aqueous solution of $H_2S$ acts as a weak dibasic acid. It ionises as follows:
It forms two series of salts, hydrosulphides and sulphides.
1. Sulphides precipitated in acidic medium: These are sulphides of $Hg,\ Ag,\ Pb,\ Cu,\ Bi,\ Cd,\ As,\ Sb,$ and $Sn$
2. Sulphides precipitated in alkaline medium: These are the sulphides of $Zn,\ Co,\ Ni,\ Mn$ and $Fe$.
$CoS,\ NiS,\ Black,\ ZnS,\ White,\ MnS,\ Flesh\: coloured$
3. Sulphides which are precipitated nither in acidic nor in alkaline medium: These are the sulphides of $Cr,Al,Mg,Ba,Sr,Ca,K$ and $Na$.
(e) Formation of polysulphide: Polysulphide of metals are obtained by passing $H_2S$ through their hydroxides
On passing excess of $H_S$ through ammonium sulphide, yellow ammonium sulphide is formed.
Structure of $H_2S$: The shape of the $H_2S$ molecule is similar to that of the water molecule, i.e. $V-$ shaped structure with bond length $H-S$ $1.35{A^\circ }$ and bond angle $H - S - H\;92.5{A^\circ }$.
