Sulphur

Sulphur occurs naturally in three forms:

1. Free form in Japan, Texas and Louisiana.

Extraction of sulphur

In order to extract this underground sulphur (the Frasch process), three concentric pipes are sunk deep into the ground as show in figure 1. Superheated water at 170⁰C is forced down the outer pipe into the sulphur which is melted; compressed air is blown through the inner pipe forcing sulphur as a liquid to the surface, where it is allowed to solidify. Sulphur of about 99.5% purity is obtained by this process.

2. In the form of sulphides of zinc, lead, copper, iron, hydrogen and mercury.

Sulphur dioxide is obtained as a by-product in the course of extraction of these metals.  From hydrogen sulphide, sulphur can be obtained by first, burning hydrogen sulphide to form sulphur dioxide and this is then reacted with more hydrogen sulphide.

                 2H₂S (g) + 3O₂ (g)  → 2H₂O (g) + 2SO₂ (g)

                 2H₂S (g) + SO₂ (g)  → 2H₂O (l) + 3S (s)

3. As metal sulphate, e.g., gypsum, CaSO₄.2H₂O, and epsom salt, MgSO₄.7H₂O.

The allotropy of sulphur

Sulphur exists in a number of allotropic varieties depending on the method of preparation and temperature (fig.2).

The differences between Rhombic and monoclinic sulphur

Rhombic sulphur	Monoclinic sulphur
Stable below 960C	Stable below 960C
Octahedral crystals	Needle-shaped crystal
Bright yellow	Pale yellow
M.pt 1130C	M.pt 1190C
Density higher (2.06g/cm3)	Density lower (1.98 g/cm3)

Facts to prove that Rhombic and monoclinic sulphur are allotropes of sulphur

1. One gram of monoclinic slowly changes at room temperature into one gram of rhombic sulphur.
2. One gram of either form will burn in oxygen to yield the same mass (2g) of sulphur dioxide.

Uses of sulphur

1. Manufacture of sulphuric acid
2. For dusting wines to prevent the growth fungus
3. For the vulcanization of rubber
4. For manufacture of dye,
5. Sulphur dioxide bleaches sugar

Chemical properties

1. Sulphur burns in air to form sulphur dioxide.

S(s) + O₂ (g)     →      SO₂(g)

• It combines with metals and non-metal when heated to give sulphides.

                 Mg (s) + S (s)  → MgS (s)

                 Fe (s) + S(s) →     FeS(s)

                 S (s) + 3F₂ (g)   → SF₆ (g)

                 2S (s) + Cl₂ (g)  → S₂Cl₂ (g) 

                  C (s) + 2S (s)   → CS₂ (l)

                 S (s) + O₂ (g)  → SO₂ (g)

                  H₂ (g) + S (l)   ↔    H₂S (g)

• Sulphur is oxidized by concentrated nitric and sulphuric acid to sulphuric acid and sulphur dioxide respectively.

                 S (s) + 6HNO₃ (l)  → 2H₂O (l) + H₂SO₄ (aq) + 6NO₂ (g)

                 S (s) + 2H₂SO₄ (l)   → 2H₂O (l) + 3SO₂ (g)

Compounds of sulphur

Hydrogen sulphide

It is a bad smelling covalently bonded gas.

Preparation

By reaction between metal sulphides with aqueous hydrochloric acid.

         FeS (s) + 2HCl (aq)  →FeCl₂ (aq) + H₂S (g)

Or    FeS(s)  + H₂SO₄(s)   →    FeSO₄(aq) + H₂S (g)

Testing for hydrogen sulpide

Turns a strip of paper dipped in lead ethanoate solution brown or black

Pb²⁺ (aq) + S^2- (aq) →     PbS(s)

Properties of hydrogen sulphide

• Has rotten egg smell

Sulphur dioxide or sulphur (IV) oxide

Preparation

By reacting copper with hot concentrated sulphuric acid

Cu(s) + 2H₂SO₄ (aq)     →      CuSO₄(aq) + 2H₂O (l) + SO₂ (g)

Or

By action of concentrated sulphuric or hydrochloric acid on sodium sulphite or hydrogen sulphite

                Na₂SO₃ (s) + H₂SO₄ (aq)     →             Na₂SO₄(aq) + H₂O(l) + SO₂(g)

Setup of apparatus

Testing for sulphur dioxide

1. It   turns the colour of acidified potassium dichromate from orange to green
2. Decolorizes  acidified potassium permanganate

Uses

1.  For the manufacture of sulphuric acid.

2.  For bleaching silk, straw and sponges.

3.  As a preservative for fruits, fruit juices and grains.

4. As an antichlor for removing chlorine from fabrics after bleaching.

5. For the preparation of NaHSO₃ and Ca(HSO₃)₂.

6. For refining sugar and petroleum.

Sulphuric acid, H₂SO₄

Sulphuric acid is mainly manufactured by the contact process, in which sulphur dioxide is oxidised to sulphur trioxide, which is then reacted with water to give sulphuric acid.

                     2SO₂ (g) + O₂ (g)    ↔    2SO₃ (g)

                 SO₃ (g) + H₂O (l)  → H₂SO₄ (l)

Sulphur dioxide is obtained by burning sulphur in the air. Alternatively, it is obtained as a by-product in the extraction of metals from sulphide ores.

                 2ZnS (s) + 3O₂ (g)  → 2ZnO (s) + 2SO₂ (g) (During roasting)

The contact process

In sulphur dioxide is reacted with oxygen in the presence of Vanadium (V) oxide (catalyst) to form sulphur trioxide. The conversion of sulphur dioxide and oxygen into sulphur trioxide (sulphur (VI) oxide) is an exothermic reaction with a decrease in gas volume.

                 2SO₂ (g) + O₂ (g)     ↔  2SO₃ (g)   ΔH^θ_m (298 K) = -197.6 kJ mol^-1.

Condition for production of sulphur trioxide

Temperature: 420⁰C

Catalyst: vanadium (V) Oxide

Preparation of sulphuric acid from sulphur trioxide

Sulphur trioxide is dissolved in 98% sulphuric acid to form oleum.

                 H₂SO₄ (aq) + SO₃ (g)    ↔      H₂S₂O₇ (l) (oleum)

The oleum is later diluted with water to produce sulphuric acid.

                 H₂S₂O₇ (l) + H₂O (l)  → 2H₂SO₄ (l)

Note that sulphur trioxide is not dissolved in water directly because this produces a lot of heat and corrosive fumes.

Physical properties of sulphuric acid

Pure sulphuric acid is a viscous liquid with a density of 1.85 g cm^-3 and a freezing point of 10.5⁰C.  In the absence of water, it does not turn litmus red nor does it react with metals to form hydrogen. It decomposes on boiling (b.pt. 270 ⁰C) to form sulphur trioxide and water, and a constant boiling mixture (azeotrope) is formed, containing 98.3% of acid.

Its high viscosity and boiling point are due to strong intermolecular hydrogen bonding.

Fig. 5 Extensive intermolecular hydrogen bonds in sulphuric acid

Chemical properties

a) Acidity

Sulphuric acid ionises in water to form a strong dibasic acid.

                 H₂SO₄ (l) + H₂O (l)    ↔    H₃O⁺ (aq) + HSO₄^– (aq)

                 HSO₄^– (aq) + H₂O (l)     ↔   H₃O⁺   (aq) + SO₄^2- (aq)

It neutralizes bases to form hydrogen sulphate and sulphate. It displaces carbon dioxide from carbonates and reacts with strong electropositive metals to form salts and hydrogen.

                   H₂SO₄ (l) + OH^– (aq)  → H₂O (l) + HSO₄^– (aq)

                   HSO₄^– (aq) + OH^– (aq) → H₂O (l) + SO₄^2- (aq)

                   CO₃^2- (aq) + 2H⁺ (aq)  → CO₂ (g) + H₂O (l)

                   Ca (s) + 2H⁺ (aq) → Ca²⁺ (aq) + H₂ (g)

b) As a dehydrating agent

Concentrated sulphuric acid has such an affinity for water that is, it removes water from mixtures and compounds with the evolution of much heat and because of this, the acid should always be diluted by pouring it into water while stirring and not the other way round.

The concentrated acid absorbs water vapour from moist air and other moist substances (i.e., it is hygroscopic).  It is used as a drying agent for gases (except those that react with the acid) and in desiccators. When left exposed to air, it absorbs water and increases in volume

It dehydrates glucose and sugar to a black mass of carbon

It dehydrates ethanol to ethene

Being hygroscopic, it absorbs water from atmosphere and increases in volume.

(c) Oxidizing properties.

Hot sulphuric acid oxidizes hydrogen sulphide to sulphur.

                 3H₂S (g) + H₂SO₄ (l)  → 4H₂O (l) + 4S (s)

It oxidizes carbon and sulphur to carbon dioxide and sulphur dioxide respectively

                 C (s) + 2H₂SO₄ (l)  → 2SO₂ (g) + CO₂ (g) + 2H₂O (l)

                 S (s) + 2H₂SO₄ (l)  → 3SO₂ (g)  + 2H₂O (l)

It oxidizes copper to blue copper (II) sulphate.

                 Cu (s) + 2H₂SO₄ (l)  → CuSO₄ (aq) +  SO₂ (g) + 2H₂O (l)

                 Zn (s) + 2H₂SO₄ (l)  → ZnSO4 (aq) +  SO₂ (g) + 2H₂O (l)

                               (98% acid)

d) Displacement reaction.

Because sulphuric acid is a strong acid and not easily vaporized, it displaces other more volatile acids on warming with their salts.

                 NaNO₃ (s) + H₂SO₄ (l)  → NaHSO₄ (aq)  + HNO₃ (g)                  

                 NaCl (s) + H₂SO₄ (l)  → NaHSO₄ (aq)  + HCl (g)          

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      sulphur and its compounds

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