Chemistry SS 2 Week 8
Topic: Sulphur
Introduction
Sulphur is a yellow, brittle non-conducting solid with a fairly low melting point (115oC). It is insoluble in water but soluble in other solvents such as carbon disulphide or methylbenzene. In the lab, sulphur can be found as two forms: roll sulphur and flowers of sulphur. (Note: these are not allotropes but just different forms – polymorphs.
Sulphur is found in metal ores, for example zinc blende (ZnS) and copper pyrites (CuFeS2). It also occurs as the free element in sulphur beds found in Poland, Iraq, USSR and USA. These beds occur about 100-200m below ground level and in between the sulphur deposits there are layers of clay and sand. So, a special process is needed to extract this Sulphur. Sulfur is a non-metallic element, which is very important in the chemical industry.
Sources
It is found as the element in sulphur beds 200m below the ground in volcanic areas and also in metal ores such as copper pyrites (iron sulphides) and zinc blende and also in volcanic areas.
Allotropes Of Sulphur
Sulphur exists in many allotropic forms however the important three allotropic forms of sulphur are as under.
- Rhombic Sulphur: It is an allotropic form of sulphur which is stable below 96oC one molecule of rhombic sulphur contains 8-atoms i-e 8g. The crystal of rhombic sulphur has octahedral structure.
Preparation
Rhombic sulphur is prepared by dissolving ordinary powdered sulphur in carbon disulphide (cs2) the saturated solution is then filtered and the filtrate is allowed to evaporate at room temperature. After evaporating Cs2, crystals of rhombic sulphur are obtained crystal of rhombic sulphur.
Properties:
- It is consist of pale yellow crystals.
- It melts at 110℃.
- It is insoluble in water and soluble in carbon disulphide.
- It is stable at room temperature.
- Its specific gravity is 208g/cm3.
- Monoclinic Sulphur: It is the allotropic form of sulphur which is stable between 96 to 119oC a molecule of monoclinic sulphur consists of eight sulphur atoms i-e 8g, but is different from rhombic sulphur in the arrangement of atoms.
Preparation:
Ordinary sulphur is mulled in a china dish with slow heating. The molten sulphur is allowed to cool down until a crest is formed at the surface, the holes are made in the crest by glam rod and molten sulphur is powered out, the crest is also removed and at the bottom, needle shaped crystals of monoclinic sulphur are left.
Properties:
- It is stable from 96℃-119℃.
- Its melting point is 119℃.
- It is soluble in carbon disulphide.
- Its one molecule consist of 8 atoms.
- It is found as pale yellow needle shaped crystals.
- Plastic Sulphur: It is a non crystalline allotropic form of sulphur, it can be stretched like a rubber, it is unstable and changes into rhombic sulphur on slight heating even at room temperature it also changes.
Preparation:
Heat some powdered sulphur in a test tube, it will melt into a yellow liquid, on further heating it changes into a dark red molten mars. Throw this molten maxs into cold water. On solidification the plastic sulphur is obtained.
Extraction of Sulphur
Sulphur is extracted using the Frasch Process.
Superheated water at 170°C and hot compressed air are pumped into layers of sulphur through pipes. This forces water and molten sulphur to the surface. Sulphur is insoluble in water and so the two substances emerging from the pipes are easily separated. The sulphur is about 99.5% pure and can be used directly.
Physical Properties of Sulphur
The Frasch Process, as a result of some of the following properties, extracts sulphur:
- brittle yellow solid
- melting point of 119°C and a boiling point of 444oC
- non-conductor of heat and electricity
- burns with blue flame when lit
- insoluble in water
Chemical Properties of Sulphur
- Sulphur burns readily in air forming sulphur dioxide
S + O2 ==> SO2
- Sulphur reacts with hydrogen gas when the latter is bubbles through molten sulphur near its boiling point.
S + H2 ==> H2S
- Sulphur dissolves in caustic alkali solutions forming sulphides and thiosulphates.
4S + 6 NaOH ==> Na2S2O3 + 2 Na2S + 3H2O
Uses of Sulphur
- For the manufacture of sulphuric acid.
- Fruit trees are sprayed with sulphur to prevent insect and fungal attack.
- For the manufacture of calcium hydrogensulphite Ca(HSO3)2 which is in turn used to bleach wood pulp in the manufacture of paper.
- For the vulcanisation of rubber, i.e. converting soft rubber into the hard, tough substance of which motor tyres and similar substances are made of.
- In the manufacture of dyes, fireworks, matches, sulphur compounds such as CS2 and sodium thiosulphate (Na2S2O3) used in photography, sulphur ointment, and tablets and drugs used in medicine.
