secondary school chemistry – Sulphur

SULPHUR AND ITS COMPOUND

SULPHUR

Introduction
Atomic number 16
Period 3 of the periodic table
Group VI in the periodic table
Electronic configuration of 2.8.6
Has a variable oxidation states such as II, IV and VI



Sources
Underground deposits in Texas and Louisiana U.S.A, Sicily in Italy and in Japan
In combined form occurs as
1. Copper pyrites (CuFeS2)
2. Iron pyrites (FeS2)
3. Petroleum gas and coal mines contain sulphur in form of hydrogen sulphide

Extraction from underground deposit (Frasch process)
Based on low melting point of sulphur (115°C) its low density and being insoluble in water.
Three concentric pipes of diameters 15, 8 and 2cm are drilled into the sulphur deposit underground

Super-heated water at 170°C and under pressure of about 10 atmospheres is forced down the outermost pipe
The pressure ensures the water is in liquid state and not in vapour form at 170°C

The water reaches the underground and melts the sulphur in the sulphur deposits

Hot compressed air at 15 atmospheres is forced down through the innermost pipe. It produces a light froth of molten sulphur and water
The high pressure forces the mixture up through the middle pipe
The mixture is run into large tanks on the surface where sulphur solidifies at 115°C from water and is stored
Sulphur obtained is about 99% pure




Allotropes of sulphur
Crystalline forms
They are
1. Monoclinic or Prismatic or β sulphur
2. Rhombic or octahedral or sulphur
Rhombic
Preparation
Place 10cm³ of carbon (IV) sulphide in a clean boiling tube
Add two spatulas of powdered sulphur and stir
Filter the contents in a clean beaker
Place the mixture in an open place for the solvent to evaporate slowly
Examine the crystals using a hand lens

rhombic

Caution
Carbon (IV) sulphide is highly poisonous and flammable

Packing of sulphur molecules in rhombic sulphur

Monoclinic sulphur
Preparation
Place powdered sulphur in a crucible and heat it under gentle heat until it melts
Continue adding powdered sulphur until the crucible is almost full with molten sulphur
Allow it to cool
A solid crust forms at the top of the molten sulphur
Piece the crust at two separate far ends and quickly out the molten sulphur below the solid crust
Using a knife to remove the crust and observe the crystals using a hand lens

Observations
Needle like crystals are observed

Packing of sulphur molecules in monoclinic sulphur

Properties
Rhombic
Bright yellow
Octahedral in shape
Melts at 113°C and has a density of 2.06g/cm³
Stable at temperatures of below 96oC
Above this temperature it slowly changes to monoclinic sulphur

Monoclinic
Pale yellow
Hexagonal prism
Melting point of 119°C
Density 1.98 g/cm³
Stable at temperature of above 96°C
Below this temperature it slowly changes to rhombic sulphur
This temperature of 96°C is known as transition temperature for rhombic and monoclinic sulphur
Transition temperature is the temperature at which one allotrope of an element changes to another



Amorphous forms of sulphur (non crystalline forms)
Plastic sulphur
Prepared by pouring boiling sulphur in cold water
It is yellow long elastic solid
It is insoluble in carbon (IV) sulphide
Turns to rhombic sulphur when left for a few days

Colloidal sulphur
Formed when sodium thiosulphate reacts with dilute hydrochloric acid

Na2S2O3 (aq) + 2HCl (aq)   →    2NaCl (aq) + H2O (l) + SO2 (g) + S (s)
It is a yellow precipitate which is almost white

Powdery sulphur
Prepared by saturating distilled water with hydrogen sulphide then exposing it to air

2H2S (g) + O2 (g)     →   2H2O (l) + S (s)

It is a white powder

Physical properties of sulphur
A molecule of sulphur is made of 8 sulphur atoms bonded using strong covalent bonds in a crown shape

• Low melting and boiling point
The S8 molecules are joined using van der Waal forces which are easily broken
• Yellow in colour
• Soluble in organic solvents such as benzene, methyl benzene and carbon (IV) sulphide but not soluble in water
• Does not conduct heat or electricity

Effects of heat on sulphur
When sulphur is heated in a test tube it melts at about 115°C to a liquid that flows easily and can be poured out of the test tube.

Explanation
The weak van der Waal forces between the molecules have been broken hence the solid melts and flows easily

On further heating at about 160°C the liquid darkens and becomes viscous and cannot be easily poured out of the test tube even when the test tube is inverted

Explanation
The free S8 molecules break and join to form long chains which are entangled with each other hence the viscosity of the liquid increases

On further heating above 160°C the liquid darkens and becomes almost black
When it nears its boiling point the liquid becomes mobile and can be poured out of the test tube

Explanation
The long chains break and form shorter chains which are less tangled with each other hence the liquid becomes more mobile

When heated further the liquid finally boils at 444°C forming reddish brown vapour which consists of mainly S8, S6 and S2 molecules
The vapour deposits on the cooler parts forming a yellow sublimate known as flowers of sulphur which consists of S8 rings




Chemical properties
Burning
It burns in air with a pale blue flame forming misty fumes of sulphur (IV) oxide with a chocking smell
Small amounts of sulphur (VI) oxide are also formed

S (s) + O2 (g)    →     SO2 (g)

2S (s) + 3O2 (g)    → 2 SO3 (g)

The fumes turn wet blue litmus to red indicating that they are acidic

Reaction with metals

When a mixture of sulphur and iron is heated, the mixture glows red hot and the reaction proceeds without further heating. This is because the reaction is exothermic and the heat energy produced maintains the temperature necessary for the reaction to take place.
Iron (II) sulphide which is a black solid is finally formed

Fe (s) + S (s)      →     FeS (s)

 

Hot powdered copper similarly combines with heated sulphur forming black copper (II) sulphide

Cu (s) + S (s)    →   CuS (s)

Reaction with non-metals

It forms non-metallic sulphides when reacted with non-metals

Combines with carbon at high temperature forming carbon (IV) sulphide

C (s) + 2S (s)     →     CS2 (s)

Combines with hydrogen to form hydrogen sulphide

H2 (g) + S (s)      →      H2S (g)

Reduces hot concentrated nitric (V) acid to nitrogen (IV) oxide

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

Reduces hot concentrated sulphuric (VI) acid to sulphur (IV) oxide

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

Sulphur is oxidised to sulphur (IV) oxide

Uses of sulphur
a) Industrial manufacture of sulphuric (VI) acid
b) As a fungicide
c) Manufacture of bleaching agent used to wood bleach pulp in paper industry
d) Vulcanisation (hardening) of rubber
e) Manufacture of dyes and fire works