Chemistry Allotropic Forms of Sulphur and Sulphur Dioxide
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Class 12 chapter7 p-block elements

Allotropic Forms of Sulphur and Sulphur Dioxide

Topics Covered :

● Allotropic Forms of Sulphur
● Rhombic Sulphur
● Monoclinic Sulphur
● Preparation of Sulphur Dioxide
● Properties and Uses of Sulphur Dioxide

Sulphur — Allotropic Forms :

`=>` Sulphur forms numerous allotropes of which the yellow rhombic (`color{red}(α)`-sulphur) and monoclinic (`color{red}(β)`-sulphur) forms are the most important.

`=>` The stable form at room temperature is rhombic sulphur, which transforms to monoclinic sulphur when heated above `369 K`.

Rhombic sulphur (`α`-sulphur) :

`=>` This allotrope is yellow in colour, m.p. `385.8 K` and specific gravity `2.06`.

`=>` Rhombic sulphur crystals are formed on evaporating the solution of roll sulphur in `color{red}(CS_2)`.

`=>` It is insoluble in water but dissolves to some extent in benzene, alcohol and ether. It is readily soluble in `color{red}(CS_2)`.

Monoclinic sulphur (`β`-sulphur)

`=>` Its m.p. is `393 K` and specific gravity `1.98`.

`=>` It is soluble in `color{red}(CS_2)`.

`=>` This form of sulphur is prepared by melting rhombic sulphur in a dish and cooling, till crust is formed. Two holes are made in the crust and the remaining liquid poured out. On removing the crust, colourless needle shaped crystals of `color{red}(β)`-sulphur are formed.

● It is stable above `369 K` and transforms into `color{red}(α)`-sulphur below it.

● Conversely, `color{red}(α)`-sulphur is stable below `369 K` and transforms into `color{red}(β)`-sulphur above this.

● At `369` K both the forms are stable. This temperature is called transition temperature.

`=>` Both rhombic and monoclinic sulphur have `color{red}(S_8)` molecules. These `color{red}(S_8)` molecules are packed to give different crystal structures.

`=>` The `color{red}(S_8)` ring in both the forms is puckered and has a crown shape.

`=>` The molecular dimensions are given in Fig. 7.5(a).

`=>` Several other modifications of sulphur containing `6-20` sulphur atoms per ring have been synthesised in the last two decades.

`=>` In cyclo-`color{red}(S_6)`, the ring adopts the chair form and the molecular dimensions are as shown in Fig. 7.5 (b).

`=>` At elevated temperatures (`~1000 K`), `color{red}(S_2)` is the dominant species and is paramagnetic like `color{red}(O_2)`.
Q 3080591417

Which form of sulphur shows paramagnetic behaviour ?

Solution:

In vapour state sulphur partly exists as `S_2` molecule which has two unpaired electrons in the antibonding `π **` orbitals like `O_2` and, hence, exhibits paramagnetism

Sulphur Dioxide

Preparation, properties and uses of sulphur dioxide is given below :

Preparation :

`=>` Sulphur dioxide is formed together with a little (`6-8%`) sulphur trioxide when sulphur is burnt in air or oxygen:

`color{red}(S (s) +O_2 (g) → SO_2 (g))`

`=>` In the laboratory it is readily generated by treating a sulphite with dilute sulphuric acid.

`color{red}(SO_3^(2-) (aq) + 2H^(+) (aq) → H_2O (l) +SO_2 (g))`

`=>` Industrially, it is produced as a by-product of the roasting of sulphide ores.

`color{red}(4FeS_2 (s) +11O_2 (g) → 2Fe_2O_3 (s) +8SO_2 (g))`

The gas after drying is liquefied under pressure and stored in steel cylinders.

Properties :

`=>` Sulphur dioxide is a colourless gas with pungent smell and is highly soluble in water.

`=>` It liquefies at room temperature under a pressure of two atmospheres and boils at `263 K`.

`=>` Sulphur dioxide, when passed through water, forms a solution of sulphurous acid.

`color{red}(SO_2 (g) + H_2O (l) ⇌ H_2SO_3 (aq))`

`=>` It reacts readily with sodium hydroxide solution, forming sodium sulphite, which then reacts with more sulphur dioxide to form sodium hydrogen sulphite.

`color{red}(2NaOH + SO_2 → Na_2SO_3 +H_2O)`

`color{red}(Na_2SO_3 +H_2O +SO_2 → 2 NaHSO_3)`

`=>` In its reaction with water and alkalies, the behaviour of sulphur dioxide is very similar to that of carbon dioxide.

`=>` Sulphur dioxide reacts with chlorine in the presence of charcoal (which acts as a catalyst) to give sulphuryl chloride, `color{red}(SO_2Cl_2)`.

`=>` It is oxidised to sulphur trioxide by oxygen in the presence of vanadium(V) oxide catalyst

`color{red}(SO_2 (g) +Cl_2 (g) → SO_2 Cl_2 (l))`

`color{red}(2SO_2 (g) +O_2 (g) overset(V_2O_5)→ 2SO_3 (g))`

`=>` When moist, sulphur dioxide behaves as a reducing agent. For example, it converts iron(III) ions to iron(II) ions and decolourises acidified potassium permanganate(VII) solution; the latter reaction is a convenient test for the gas.

`color{red}(2Fe^(3+) + SO_2+2H_2O →2Fe^(2+) +SO_4^(2-) +4H^(+))`

`color{red}(5SO_2+2MnO_4^(-) +2H_2O → 5 SO_4^(2-) +4H^(+) +2Mn^(2+))`

`=>` The molecule of `color{red}(SO_2)` is angular. It is a resonance hybrid of the two canonical forms as shown in fig.

Uses :

`=>` Sulphur dioxide is used

(a) in refining petroleum and sugar

(b) in bleaching wool and silk and

(c) as an anti-chlor, disinfectant and preservative.

`=>` Sulphuric acid, sodium hydrogen sulphite and calcium hydrogen sulphite (industrial chemicals) are manufactured from sulphur dioxide.

`=>` Liquid `color{red}(SO_2)` is used as a solvent to dissolve a number of organic and inorganic chemicals.
 
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