Sulphur its Compounds and Properties

Sulphur (S): Sulphur is the 2^nd element of the oxygen family. Sulphur forms a large number of allotropes. Among these Yellow Rhombic (α-Sulphur) and Monoclinic (β -Sulphur) forms are the most important. The stable form at room temperature is rhombic sulphur, which transforms into monoclinic sulphur when heated above 369 K.

(i) Rhombic Sulphur (a – Sulphur): This allotrope is yellow in colour, m.p. 385.8 K and specific gravity 2.06. Rhombic sulphur crystals are formed on evaporating the solution of roll sulphur in CS₂. It is insoluble in water but dissolved to some extent in benzene, alcohol and ether. It is readily soluble in CS₂.

(ii) Monoclinic (β -Sulphur): It 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 is poured out. On removing the crust, colourless needle-shaped crystals of β-sulphur are formed. It is stable above 369 K and transforms into α-sulphur below it. At 369 K both the forms are stable. This temperature is called transition temperature.

Both rhombic and monoclinic sulphur have S₈ molecules. The S₈ ring in both forms is puckered and has a crown shape.

Sulphur its Compounds and Properties

Sulphur Dioxide (SO₂):
(i) Preparation:
Sulphur dioxide is formed when sulphur is burnt in air or oxygen: S(s) + O₂(g) → SO₂ (g)
In the laboratory, it is obtained by treating a sulphite with dilute sulphuric acid.
SO₃^2-(aq) + 2H⁺ (aq) → H₂O(l) + SO₂ (g)
Industrially, it is produced by roasting of sulphide ores.
4 FeS₂(s) + 11 O₂(g) → 2 Fe₂O₃(s) + 8 SO₂(g)

(ii) Properties:

• Sulphur dioxide is a colourless gas with a pungent smell and is highly soluble in water.
• With water, it forms a solution of sulphurous acid which is a dibasic acid and forms two types of salts with alkalies – normal salt (sulphite) and acid salt (bisulphate or hydrogen sulphite).
  SO₂(g) + H₂O(l) → H₂SO₃(aq)
• Neither burns nor helps in burning but burning magnesium and potassium continue to burn in its atmosphere.

3Mg + SO₂ → 2 MgO + MgS
4K + 3SO₂ → K₂SO₃ + K₂S₂O₃

• With sodium hydroxide solution, it forms sodium sulphite, which then reacts with more sulphur dioxide to form sodium hydrogen sulphite.

2NaOH + SO₂ → Na₂SO₃ + H₂O
Na₂SO₃ + H₂O + SO₂ → 2NaHSO₃

• SO₂is oxidised to sulphur trioxide by oxygen in the presence of vanadium pentoxide (V₂O₅) catalyst. 2SO₂ + O₂ → 2SO₃
• Moist sulphur dioxide behaves as a reducing agent. It converts iron (III) ions to iron (II) ions and decolourises acidified potassium permanganate(VII) solution (This used as a test for SO₂).

     2Fe³⁺ + SO₂ + 2H₂O → 2Fe²⁺ + SO₄^2- + 4H⁺
     5 SO₂ + 2MnO₄^– + 2H₂O → 5 SO₄^2- + 4H⁺ + 2Mn²⁺
Acidified K₂Cr₂O₇ → Cr³⁺ (green coloured solution)

Bleaching Action: SO₂ + 2H₂O → H₂SO₄ + 2H
This is due to the reducing nature of SO₂
Coloured matter + H ↔ Colourless matter. Therefore, the bleaching is temporary.
 
Uses: Sulphur dioxide is used:

• Used in the manufacture of H₂SO₄& paper from wood pulp.
• As a bleaching agent for delicate articles like wool, silk and straw.
• Used in the refining of petroleum and sugar.
• Liquid SO₂is used as a solvent to dissolve a number of organic and inorganic chemicals.

Sulphur its Compounds and Properties

Oxoacids of Sulphur: Sulphur forms a large no. of oxoacids. Sulphurous acid (H₂SO₃), Sulphuric acid (H₂SO₄), Dithionic acid (H₂S₂O₆), Peroxomonosulphuric acid (Caro’s acid, H₂SO₅), Peroxodisulphuric acid (Marshell’s acid, H₂S₂O₈) etc. The structure of some oxoacids are

Sulphur its Compounds and Properties

Sulphuric acid (H₂SO₄): One of the most important oxoacids of sulphur is H₂SO₄. It is also known as the king of all acid.

• Manufacture:  Sulphuric acid is manufactured by the Contact Process which involves three steps:
• Manufacture: Sulphuric acid is manufactured by the Contact Process which involves three steps:

Step 1: Burning of sulphur or sulphide ores in the air to generate SO₂.
S(s) + O₂(g) → SO₂ (g)
OR
4FeS₂(s) + 11 O₂(g) → 2 Fe₂O₃(s) + 8 SO₂(g)
Step 2: Conversion of SO₂ to SO₃ by the reaction with oxygen in the presence of a catalyst (V₂O₅).
2SO₂ + O₂ → 2SO₃
Step 3: Absorption of SO₃ in H₂SO₄ to give Oleum (H₂S₂O₇).
SO₃ + H₂SO₄ → H₂S₂O₇
Dilution of oleum with water gives H₂SO₄ of the desired concentration.
H₂S₂O₇ + H₂O → 2H₂SO₄.
Lead Chamber Process (Industrial method):
2SO₂ + O₂ (air) + 2H₂O + [NO] (catalyst) ¾® 2H₂SO₄ + [NO] (catalyst)
Acid obtained is 80% pure and is known as Brown Oil of Vitriol.

Properties of Sulphuric acid:

• Sulphuric acid is a colourless, dense, oily liquid.
• It dissolves in water with the evolution of a large quantity of heat. Hence, for diluting the acid, the concentrated acid must be added slowly into the water with constant stirring.
• It fumes strongly in moist air and is highly corrosive in nature.
• Thermal decomposition: It decomposes at 440 ⁰C

H₂SO₄ ↔H₂O+SO₃

• The chemical reactions of sulphuric acid are due to

(i) Its low volatility
(ii) Strong acidic character
(iii) Strong affinity for water and
(iv) Its ability to act as an oxidising agent.

In aqueous solution, sulphuric acid ionises in two steps.
H₂SO₄(aq) + H₂O(l) → H₃O⁺(aq) + HSO₄^–
HSO₄^–(aq) + H₂O(l) → H₃O⁺(aq) + SO₄^2-
So, it is dibasic and forms two series of salts: Normal Sulphates and Acid   
Sulphates.

• Decomposes carbonates and bicarbonates into carbon dioxide

Na₂CO₃ + H₂SO₄ → Na₂SO₄ + H₂O + CO₂
NaHCO₃ + H₂SO₄ → NaHSO₄ + H₂O + CO₂
Concentrated sulphuric acid is a strong dehydrating agent and drying agent. Many wet gases can be dried by passing them through sulphuric acid. Sulphuric acid removes water from organic compounds
e.g.: C₁₂H₂₂O₁₁ + H₂SO₄ → 12C + 11H₂O

• Hot concentrated sulphuric acid is a moderately strong oxidising agent. It oxidises both metals and non-metals and the acid itself reduces to SO₂.

Cu + 2 H₂SO₄(conc.) → CuSO₄ + SO ₂ + 2H₂O
S + 2H ₂SO₄(conc.) → 3SO₂ + 2H₂O
C + 2H₂SO₄(conc.) → CO₂ + 2 SO₂ + 2 H₂O

Uses: The important uses of Sulphuric acid are:

• In the manufacture of fertilizers
• In petroleum refining
• In the manufacture of pigments, paints and dyestuff intermediates.
• In the detergent industry
• In metallurgical applications
• As electrolyte in storage batteries.
• In the manufacture of nitrocellulose products and
• As a laboratory