Sulphur occurs in the free state as well as in combined state.
Contents
Allotropes of Sulphur
Sulphur exists in numerous allotropic forms of which three forms are the most important. These three main allotropic forms are:
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(i) Rhombic sulphur
(iii) Plastic sulphur
(ii) Monoclinic sulphur
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(i) Rhombic sulphur or α-Sulphur
(i) It is the common form of sulphur and is formed by slowly evaporating the solution of roll sulphur in CS2, when octahedral crystals of sulphur appear.
(ii) Its specific gravity is 2.06 g cm3.
(iii) Its melting point is 385.8 K.
(iv) This is the most stable form of sulphur at room temperature and all other varieties of sulphur change into this form on standing.
(v) It is insoluble in water but dissolves to some extent in benzene, alcohol and ether. However, it is readily soluble in CS2.
(vi) It has low thermal and electrical conductivity.
(vii) Its specific gravity is 2.07 g cm-3.
(viii) It is bright yellow in colour.
(ix) Rhombic sulphur exists as S8 molecules. The sulphur atoms are arranged in a puckered ring.
(ii) Monoclinic sulphur or β-Sulphur
(i) This form of sulphur is prepared by melting rhombic sulphur in a dish and cooling till a crust is formed. Two holes are made in the crust and the remaining liquid poured out. On removing the crust, colourless needle shaped crystals of B-sulphur are formed.
(ii) It is dull yellow in colour and like rhombic sulphur, it is also soluble in carbon disulphide.
(ii) It has specific gravity of 1.98 g cm-3.
(iv) Its melting point is 393 K.
(v) Monoclinic sulphur also exists as S8 molecules with puckered ring structures like rhombic sulphur. However, the two forms differ in the symmetry of their crystals.
(vi) It is stable above 369K and transforms into α-sulphur below this temperature. Conversely, α-sulphur is stable below 369 K and transforms into β-sulphur above this temperature. Thus, at 369 K both the forms are stable and coexist. This temperature is called the transition temperature.
Thus, both rhombic and monoclinic sulphur have S8 molecules. These S8 molecules are packed to give different crystal structure . The S8 ring in both the forms is puckered and has a crown shape.
(iii) Plastic sulphur or δ-sulphur
(i) Plastic sulphur is obtained by pouring molten sulphur into cold water when a soft rubber like mass called plastic sulphur is formed.
(ii) It is an amorphous form of sulphur.
(iii) It is soft and elastic in the beginning but hardens on standing and gradually changes to rhombic sulphur.
(iv) It has no sharp melting point.
(v) It has specific gravity of 1.95 g cm-3.
(vi) It is insoluble in carbon disulphide. Plastic sulphur is regarded as super cooled liquid i.e. a liquid which due to rapid cooling below its freezing point had no time to settle in a crystalline form.
Sulphur Dioxide
It contains sulphur in + 4 oxidation state.
Preparation
Sulphur dioxide is formed together with a little (6-8%) sulphur trioxide when sulphur is burnt in air or oxygen.
S(s) + O2(g) → SO2(g)
Laboratory Preparation of Sulphur Dioxide
In the laboratory
(1) Sulphur dioxide is prepared by treating a sulphite with dilute sulphuric acid.
Na2SO3(s) + H2SO4(aq) → SO2 (g) + Na2SO4 (aq) + H2O
Sodium sulphite
(2) By heating copper turnings with concentrated sulphuric acid.
Cu + 2 H2SO4 → CuSO4 + SO2 + H2O
Industrially, it is produced as a by-product of roasting of sulphide ores such as iron pyrites or zinc blende.
Cu + 2H2SO4 → 2Fe2O3 + 8SO2
Iron pyrites
2ZnS + 3O2 → 2ZnO + 2SO2
The gas is dried, liquefied under pressure and stored in steel cylinders.
Properties
Physical Properties of Sulphur Dioxide
(i) It is a colourless, toxic gas with a pungent and suffocating odour.
(ii) It is heavier than air.
(iii) It is readily soluble in water. At 0°C, 1 volume of water can dissolve about 8 volumes of the gas.
(iv) It can be easily liquefied at room temperature under a pressure of 2 atmosphere. The liquid is colourless, boils at 263 K and freezes at 197.5 K, giving a white snow-like mass.
(u) Liquid sulphur dioxide is a good solvent for sulphur, phosphorus, iodine, etc.
Chemical Properties of Sulphur Dioxide
(i) Acidic character
Sulphur dioxide dissolves in water giving sulphurous acid.
SO2(g) + H2O(l) → H2SO4 (aq)
Therefore, SO2 is regarded as anhydride of sulphurous acid. Its aqueous solution is acidic turning blue litmus red.
Because of its acidic character :
(a) It reacts readily with sodium hydroxide solution forming sodium sulphite, which then reacts with more sulphur dioxide to form sodium hydrogen sulphite.
2NaOH + SO2 → Na2SO4 + H2O
Sod. sulphite
Na2SO3 + H2O + SO2 → 2NaHSO3
Sod. hydrogen sulphite
(b) When the gas is bubbled through lime water, it becomes milky. On passing the excess of the gas the milkiness disappears due to formation of calcium bisulphite.
Ca(OH)2 + SO2 → CaSO3 + H2O
CaSO3 + SO2 +H2O → Ca(HSO3)2
(ii) Combustibility
The gas is non-combustible and does not support combustion. However, certain substances such as carbon, magnesium, etc. extract oxygen from the gas and burn in it when ignited.
SO2 + C → CO2 + S
SO2 + 2Mg → 2 MgO + S
(iii) Combination with oxygen
Sulphur dioxide reacts with oxygen when heated forming sulphur trioxide.
2SO2 + O2 ⇔ 2 SO3
The above reaction is reversible and slow. Therefore, the reaction is carried out in the presence of some catalyst such as platinized asbestos, vanadium pentoxide, etc.
(iv) Combination with halogens
Sulphur dioxide combines directly with the halogens (fluorine, chlorine and bromine) forming sulphuryl fluoride, chloride and bromide respectively.
For example: SO2 combines with chlorine in the presence of charcoal (which acts as a catalyst) to give sulphuryl chloride (SO2Cl2).
2SO2 (g) + Cl2 (g) → SO2Cl2 (l)
Sulphuryl chloride
(v) Combination with hydrogen sulphide
Sulphur dioxide reacts with hydrogen sulphide when the two gases are brought into contact with each other.
Therefore, it acts as a mild oxidising agent.
SO2 + 2H2S → 2H2O + 3S
(vi) As a reducing agent
Sulphur dioxide acts as a fairly strong reducing agent in the presence of moisture.
It reduces
(a) potassium dichromate to chromium sulphate.
K2Cr2O7 + H2SO4 + 3SO2→ K2SO4 + Cr2(SO4)3 + H2O
or Cr2O72- + 3SO2 + 2 H+→ 3SO42- + 2Cr3+ + H2O
(b) potassium permanganate to manganese sulphate.
2KMnO4 + 5SO2 + 2H2O → K2SO4 + MnSO4 + 2 H2SO4
or2MnO4– + 5SO2 + 2H2O → 2 SO42- + 2Mn2+ + 4H+
Therefore, it decolourises pink violet colour of acidified potassium permanganate solution. The reaction serves as a convenient test for the gas.
(c) potassium iodate to iodine.
2KIO3 + 5SO2 + 4H2O
(d) halogens to halide ions.
SO2 + H2O + Cl2→ H2SO4 + 2HCl
or
SO2 + 2H2O + Cl → SO42- + 2Cl– + 4H+
SO2 + H2O + Br2→ H2SO4 + 2HBr
SO2 + 2H2O + Br2→ SO42- + 2Br¯ + 4H+
(e) ferric sulphate to ferrous sulphate
Fe2(SO4)3 + SO2 + 2H2O → 2 FeSO4 + 2 H2SO4
or
2Fe3+ + SO2 + 2H2O → 2Fe2+ + SO42- + 4 H+
(f) lead dioxide to lead sulphate.
PbO2 + SO2→ PbSO4
(g) sodium peroxide to sodium sulphate.
Na2O2 + SO2→ Na2SO4
(vii) Bleaching action
In the presence of moisture, sulphur dioxide acts as a bleaching agent.
SO2 + 2H2O → H2SO4 + 2H
The nascent hydrogen bleaches vegetable colouring matter to colourless matter.
Colourless matter + [H] → Colouring matter
On exposing the bleached article to air, the reverse process takes place, i.e. colour is restored due to the atmospheric oxygen.
Uses of Sulphur Dioxide
(1) It is used in the manufacture of important chemicals such as sulphuric acid , sodium hydrogen sulphite, calcium hydrogen sulphite etc. These bisulphites are used for preservatives for jams, pickles, jellies.
(2) It is used for refining of petroleum and sugar.
(3) It is used for bleaching delicate articles such as wool, silk , straw etc.
(4) It is used as a disinfectant and germicides.
(5) It is as an antichlor i.e. Foe removing excess chlorine from bleached articles.
(6) Liquid SO2 is used as a solvent to dissolve a number of organic and inorganic chemicals.
(7) Liquid SO2 is used as a refrigerant because it can be liquified and re-evaporated easily.
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