Metals and Non-Metals - Part XIX

Sulphur
Sulphur has chemical symbol S. It has 16 protons and 16 neutrons. An atom of S is represented as ³²₁₆S. Sulphur is a non-metal and exists in the earth’s crust either as pure sulphur or as a metal-sulphide.

Since S has 16 protons, it also has 16 electrons. The electronic configuration of S is K-shell – 2 electrons, L-shell – 8 electrons and M-shell has 6 electrons. S is placed in Group VI A of the periodic table, just after phosphorus, and below oxygen. The reaction of S is similar to oxygen. Sometimes sulphur can have hybridized orbits similar to carbon and it forms covalent bonds.  

In India sulphur mineral deposits are found in the Kangra district of Himachal Pradesh.

Extraction of pure sulphur : Since sulphur in free state is found at depths of more than 150 to 300 meters below the earth’s surface, the method of extraction of sulphur differs from other metal or non-metal extractions. Sulphur’s relatively low melting point (115°C) is utilized in this process. This is known as the Frasch process. Here compressed super heated water (at 170°C)  is pressed into a pipe which reaches upto the sulphur deposits. The sulphur here melts. Introducing hot compressed air through another pipe brings it up. The molten sulphur and water mixture is forced up and is collected in a settling tank. The sulphur is cooled and water is evaporated. The sulphur extracted in this way is more than 99% pure.  

The sulphur obtained by Frasch process is a yellow and brittle solid or powder.

Physical properties of sulphur : Since S has 6 electrons in its outermost shell, it needs 2 more electrons to complete its shell. But S combines with 7 other atoms to make a sulphur molecule that has a total of 8 sulphur atoms. Thus each S atom shares 2 electrons with its neighboring atom. The bonds are covalent in nature. A molecule of sulphur is represented as S₈. It is a ringed molecule. The structure is shown below.  

Sulphur is a yellow crystalline solid. It is tasteless and odourless. The melting point of S is 115°C. Sulphur is an insulator and is a poor conductor of heat and electricity. S is insoluble in water but is soluble in CS₂. Sulphur forms covalent bonds and shows allotropic forms. The allotropes have different crystalline shapes such as rhombic and monoclinic. There is another allotrope which has no shape and is called plastic sulphur. Vapours of sulphur are pungent and although not poisonous, they can cause health problems to humans.

Chemical properties of sulphur :  
1. Valence : Since S has 6 electrons in its M-shell, the M-shell is more than half filled. Hence S does not give off its electrons easily. It readily forms covalent bonds to complete its M-shell. It shows variable valence of 2 or 6. S is quite a reactive element and forms oxides, chlorides and sulphides readily.

2. Action of oxygen :  Sulphur  reacts with oxygen and burns with a blue flame. It forms sulphur dioxide which is a colourless gas having a pungent smell.  

3. Reaction with chlorine : S  reacts with Cl to produce sulphur monochloride when Cl gas is passed through boiling S.  

S reacts with other non-metals also. In all cases sulphur has to be heated or boiled for the reaction to take place.  

4. Reaction with metals : Heated S reacts with metals like Fe, Cu, Zn, Sb directly to give metal-sulphide. A few reactions are shown below.  

5. Reaction with acids : S is oxidized by strong concentrated oxidizing acids such as sulphuric acid and nitric acid.

In both the reactions S acts as a reducing agent.

Effect of heat on sulphur :  
A sulphur molecule consists of 8 atoms in a ring form. When heated, S melts at 115°C and a pale yellow liquid is formed. The S₈ ringed molecules are connected to other molecules in a long chain. On heating, the long chain breaks up. The individual molecules can slip over each other when melted. On further heating, the liquid becomes dark brown and viscous. When the temperature rises beyond 160°C, the intra-molecular bonds break. Sulphur boils at 444°C. At this temperature the large molecule breaks up into pieces of  S₂ molecule. This molecule is pale yellowish-brown in colour. The vapours of S contains S₂ molecules.

Vulcanization of rubber : Natural rubber is a soft and sticky solid. Rubber is a long chain polymer made out of isoprene (2 – methyl butadiene) monomer. The long molecule forms a coil like structure.  

The unique property of rubber is that it is elastic. When rubber is stretched, the molecular bonds can be extended out. When released, the molecules coil back to their original shape.

Natural rubber looses its rubber-like properties at temperatures above 60°C. Also its wear resistance and tensile strengths are low. The process of vulcanization can improve the quality of rubber. Raw rubber is heated with sulphur during vulcanization. This makes the rubber hard, more elastic and strong. During the process of vulcanization, the sulphur atoms attach themselves to extra loose bonds in the rubber molecule and also cross-link the molecules. The cross-linking locks the molecules in place and prevents slipping. Thus making the vulcanized rubber more strong. Vulcanized rubber is non-sticky and has higher elasticity. It does not loose its properties easily and can be used in a temperature range of – 40°C to 100°C.  

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