Understanding an allotrope of sulphur

Sulphur is classified in group 16 of the periodic table. It makes up 0.17 % of Earth’s crust. It is a non-metallic compound produced as a byproduct of natural gas production.

The General Properties Sulphur 

Consider the important physical and chemical features of sulphide related to various applications.

Physical Characteristics

• Sulphur appears to be yellow.
• Water is unable to dissolve this chemical. It is also highly soluble in toluene (methylbenzene) and carbon disulfide.
• Because it is not made of metal, sulphur is a poor conductor of electricity and heat.
• When we reach a certain point in condensing sulphur vapour, we obtain a fine powder that forms a pattern that resembles a flower. The ‘Flower of Sulphur’ is the name given to this flower.

Chemical Characteristics

• Sulphur reacts with most metals and nonmetals when certain conditions are fulfilled.
• Sulphur burns with a bright blue flame when exposed to too much air, resulting in the production of sulphur (IV) oxide and a small amount of sulphur (VI) oxide.

S + O2  → SO2

2 S + 3 O2  → 2 SO3

• This chemical interacts with hydrogen at extremely high temperatures, resulting in the formation of hydrogen sulphide.
• Additionally, sulphur vapour combines with hot coke to generate a fluid known as carbon disulphide vapour.

C + S2 → CS2

Different allotropic forms of sulphur

Sulphur creates a variety of allotropes, but let us focus on the two most essential allotropes of sulphur for the time being. The yellow rhombic sulphur (α-sulphur) and the monoclinic sulphur (β-sulphur) are sulphur types. 

The most exciting aspect is that the thermal stability and allotrope of sulphur compounds are interconvertible, which means that they may be converted into one another. It signifies that rhombic sulphur transforms into monoclinic sulphur when heated to a temperature greater than 369K. Let us consider the following two allotropes of sulphur in greater depth.

Rhombic Sulphur or α-sulphur

(i) It is the most common type of sulphur and is made by slowly evaporating a solution of roll sulphur in CS2 until octahedral sulphur crystals form.

(ii) 2.06 g cm3 is its specific gravity.

(iii) It melts at 385.80Celsius.

(iv) This is the most stable allotrope of sulphur at room temperature, and all other conditions of the sulphur shift to this state when left standing.

(v) It is insoluble in water and also soluble in benzene, alcohol, and ether to a greater or lesser extent. However, it is soluble in CS2.

(vi) It is a poor conductor of heat and electricity.

(vii) It has a specific gravity of 2.07 grams per cubic centimeter.

(viii) The colour is bright yellow.

(ix) Rhombic sulphur exists in the form of S8 molecules. Sulphur atoms are grouped in a puckered ring configuration.

Monoclinic sulphur or β-sulphur

(i) This sulphur is made by melting rhombic sulphur in a dish and cooling until a crust forms. Two wells are created in the crust, and the remaining liquid is drained. When the crust is removed, colourless needle-shaped crystals of β-sulphur develop.

(ii) It is a dull yellow crystalline substance that, like rhombic sulphur, is soluble in carbon disulfide.

(iii) It has a specific gravity of 1.98 grams per cubic centimeter.

(iv) It has a melting point of 393 degrees Kelvin.

(v) Like rhombic sulphur, monoclinic sulphur exists as S8 molecules with puckered ring configurations. However, the two forms differ in terms of the symmetry of their crystals.

(vi) It is stable above 369K but degrades to α-sulphur below that point. α-sulphur, on the other hand, is stable below 369 K and converts into β-sulphur above this point. Thus, both forms are durable and coexist at 369 K. This point in time is referred to as the changeover temperature.

As a result, rhombic and monoclinic sulphur both contain S8 molecules. These S molecules are packed so that they form a variety of crystal structures. The S8 ring is puckered in both variants and has a crown shape.

Plastic sulphur 

(i) Plastic sulphur is created by pouring molten sulphur into cold water, forming a soft rubber-like mass known as plastic sulphur.

(ii) It is a sulphur compound that is amorphous.

(iii) It is first soft and elastic but progressively hardens and transforms into rhombic sulphur.

(iv) It does not have a well-defined melting point.

(v) Its specific gravity is 1.95 grams per cubic centimeter.

(vi) Carbon disulphide is insoluble in it. Plastic sulphur is classified as a supercooled liquid, which means that it was rapidly cooled below its freezing point and had no chance to crystallize.

Colloidal sulphur

(i) This form of sulphur is durable and can be synthesized by passing hydrogen sulphide through a saturated and cooled solution of sulphur dioxide in water. The other way is to dilute the water with an alcohol and sulphur solution.

(ii) Additionally, it serves as a solvent for the carbon disulphide.

(iii) We can incorporate this compound into medications.

Sulphur milk

(i) This form of sulphur can be produced by the action of mild hydrochloric acid on ammonium sulphide. This milk of sulphur is generated similarly by heating sulphur with calcium hydroxide (which becomes an aqueous solution). This combination can be filtered, and the weak hydrochloric acid is added to generate sulphur milk.

(ii) This substance is supplied as a non-crystalline powder with a white appearance.

(iii) When this compound is heated, it transforms into the traditional yellow colour generated of sulphur, which can be used in medicines.

(iv) Carbon disulphide dissolves it.

Applications of sulphur

Consider the following significant uses of sulphur:

• Sulphur compounds can promote the growth of particular fungi in vines.
• Sulphur is a frequent component in synthesizing tetraoxosulphate (VI) acid. Additionally, this is the primary application of sulphur.
• Sulphur can be used to make calcium hydrogen tetraoxosulphate (IV), Ca(HSO3)2. This compound is also used as a wood pulp bleacher in the papermaking business.
• Sulphur is a critical and frequently used component in the vulcanization of rubber. This process hardens and toughens the rubber by clustering the rubber molecules together.
• Sulphur is used in the manufacture of colours.
• Sulphur is used extensively to manufacture sulphur compounds, such as CS2 and sulphur monochloride and carbon disulphide, S2Cl2.
• It is often used in ointments.
• Sulphur is also a necessary component of sulphides, including phosphorus sulphide. This can be used to make gunpowder, matches, and firecrackers.

Conclusion

Sulphur can be found in various structures in the same physical form. However, the most effective crystal forms are rhombic or octahedral sulphur (α– sulphur) and monoclinic sulphur (β– sulphur). At temperatures less than 96oC, we observe the formation of rhombic sulphur. On the other hand, monoclinic sulphur occurs at temperatures greater than 96oC.

96oC is the critical temperature for the transition between the two crystalline forms. There is another sulphur allotrope known as polymeric sulphur (S8). It is a ring particle composed of eight constituents. This substance is insoluble in organic solvents and synthetic and natural rubber. Additionally, it is insoluble in carbon disulphide.