Sulphur and its Compounds: Comprehensive Study Guide

Extraction of Sulphur by the Frasch Process

Sulphur is extracted from underground deposits using three concentric pipes sunk down into the beds. Each pipe serves a distinct purpose in the extraction process:

1. Pipe J: Superheated water at a temperature of approximately $170^{\circ}C$ and high pressure is passed down through this pipe to melt the sulphur in the underground deposits.

2. Middle Pipe: Molten sulphur, being less dense than the water-sulphur mixture and pushed by external pressure, flows out through this pipe.

3. Inner Pipe: Hot compressed air is blown down through this pipe to create a foam of molten sulphur that is light enough to be forced to the surface.

During this process, the molten sulphur flows specifically through the middle pipe as opposed to the outer pipe because the outer pipe is reserved for the descending superheated water, and the compressed air forces the liquid sulphur upward through the alternate path.

Allotropes of Sulphur

Sulphur exists in several different forms known as allotropes, which are different physical forms of the same element in the same physical state. The two primary crystalline allotropes of sulphur are:

1. Rhombic Sulphur (also known as alpha-sulphur).
2. Monoclinic Sulphur (also known as beta-sulphur).

These two allotropes are stable at different temperatures. The transition temperature between them is $95.5^{\circ}C$. Below $95.5^{\circ}C$, rhombic sulphur is the stable form. When heated above $95.5^{\circ}C$, it transforms into monoclinic sulphur. This relationship is represented as:

$\text{Rhombic sulphur} \rightleftharpoons \text{monoclinic sulphur}$

Another form is plastic sulphur, which is formed when boiling sulphur is poured into cold water. In terms of molecular structure, rhombic sulphur (often labeled X) and monoclinic sulphur (often labeled Y) have distinct crystal habits, such as octahedrons or needle-like shapes.

Properties and Concentrations of Sulphuric Acid

Commercial sulphuric acid has a density of $1.8\,g/cm^3$. To determine the molarity based on this density, calculations involve the mass of the solute per unit volume of the solution. For instance, to prepare $500\,cm^3$ of $0.2\,M$ $H_2SO_4$ solution from commercial acid, the required volume must be calculated using the dilution formula $M_1V_1 = M_2V_2$.

Concentrated sulphuric acid is considered a weak acid compared to its dilute form. This is because concentrated sulphuric acid contains very little water and exists primarily as molecules with very few ions. In contrast, dilute sulphuric acid is a strong acid because it ionizes completely in water to produce a high concentration of hydrogen ions:

$H_2SO_4(aq) \rightarrow 2H^+(aq) + SO_4^{2-}(aq)$

The Contact Process for the Manufacture of Sulphuric Acid

The industrial production of sulphuric acid occurs via the Contact Process, which involves several stages:

1. Production of Sulphur (IV) Oxide: Sulphur or solid A (such as Iron Pyrites) is burned in air to produce sulphur (IV) oxide ($SO_2$). The equation is $S(s) + O_2(g) \rightarrow SO_2(g)$.

2. Purification: The $SO_2$ and air mixture is passed through a purifier to remove impurities like dust and arsenic compounds. It is necessary to remove these impurities because they poison the catalyst, making it ineffective.

3. Catalytic Converter: The pure $SO_2$ and oxygen are passed over a catalyst at a temperature of approximately $450^{\circ}C$ and a pressure of $2-3\,atm$. The catalyst used can be Vanadium (V) oxide ($V_2O_5$) or platinized asbestos. The reaction is exothermic:

$2SO_2(g) + O_2(g) \rightleftharpoons 2SO_3(g) \quad \Delta H = -196\,kJ$

A heat exchanger is used in this stage to heat the incoming gases using the heat generated by the exothermic reaction, while cooling the outgoing gases.

4. Absorption Tower: Sulphur (VI) oxide ($SO_3$) is not dissolved directly into water because the reaction is highly exothermic and produces a dense mist of sulphuric acid droplets that are difficult to condense. Instead, $SO_3$ is dissolved in concentrated sulphuric (VI) acid to form oleum ($H_2S_2O_7$):

$SO_3(g) + H_2SO_4(l) \rightarrow H_2S_2O_7(l)$

5. Dilution: Oleum is then carefully reacted with a calculated amount of water to produce sulphuric (IV) acid (specifically sulphuric (VI) acid concentration):

$H_2S_2O_7(l) + H_2O(l) \rightarrow 2H_2SO_4(l)$

Sulphur (IV) Oxide ($SO_2$) and Sulphur (VI) Oxide ($SO_3$)

Sulphur (IV) oxide is a colourless gas with several key characteristics:

1. Preparation: In the laboratory, it can be prepared by reacting dilute hydrochloric acid with a sulphite reagent, such as sodium sulphite ($Na_2SO_3$):

$Na_2SO_3(s) + 2HCl(aq) \rightarrow 2NaCl(aq) + H_2O(l) + SO_2(g)$

2. Bleaching Action: The bleaching action of $SO_2$ is by reduction and requires moisture. It is temporary because the original colour can be restored by oxidation in air. This differs from chlorine ($Cl_2$), which bleaches by oxidation and is permanent.

3. Reaction with Water: When moist blue litmus paper is placed in $SO_2$, it turns red and is then bleached white. The acidity is due to the formation of sulphurous acid:

$SO_2(g) + H_2O(l) \rightarrow H_2SO_3(aq)$

4. Combustion: A piece of burning magnesium continues to burn in a gas jar of $SO_2$ because the heat of the burning magnesium is sufficient to decompose the gas into sulphur and oxygen, which then supports continued combustion:

$2Mg(s) + SO_2(g) \rightarrow 2MgO(s) + S(s)$

The product is a mixture of white magnesium oxide and yellow sulphur. The MgO can be removed by adding an acid to dissolve it, leaving the solid sulphur residue.

Hydrogen Sulphide ($H_2S$)

Hydrogen sulphide is a gas often prepared by the reaction of solid iron (II) sulphide (Solid H) and dilute hydrochloric acid:

$FeS(s) + 2HCl(aq) \rightarrow FeCl_2(aq) + H_2S(g)$

The gas is collected by downward delivery (upward displacement of air) as it is denser than air. Warm water may be used in certain setups to facilitate the reaction or collection.

When hydrogen sulphide is bubbled through a solution of Iron (III) chloride ($FeCl_3$), the following observations are made:

• The solution changes from reddish-brown to green due to the reduction of $Fe^{3+}$ ions to $Fe^{2+}$ ions.
• A yellow solid (sulphur) is deposited as $H_2S$ is oxidized.

Reaction Equation: $2FeCl_3(aq) + H_2S(g) \rightarrow 2FeCl_2(aq) + S(s) + 2HCl(aq)$

In the reaction between hydrogen sulphide and sulphur (IV) oxide, yellow deposits of sulphur and water are formed, where $SO_2$ acts as an oxidizing agent:

$2H_2S(g) + SO_2(g) \rightarrow 3S(s) + 2H_2O(l)$

Environmental Impact and Industrial Applications

1. Environmental Pollution: Sulphur (IV) oxide and Nitrogen (II) oxide are released from internal combustion engines. These gases contribute to the formation of acid rain, which corrodes buildings, acidifies water bodies, and damages vegetation. $SO_2$ also causes respiratory ailments in humans. Pollution from the Contact process is controlled by scrubbing the waste gases (effluents) to remove residual $SO_2$ before they are released into the atmosphere.

2. Vulcanization of Rubber: Natural rubber is heated with sulphur to improve its physical properties. Vulcanized rubber is harder, has higher tensile strength, and is more resistant to heat and chemical wear than non-vulcanized rubber.

3. Uses of Sulphur: Sulphur is used in the manufacture of sulphuric acid, the vulcanization of rubber, the production of matches and fireworks, and as a fungicide in agriculture.

4. Uses of Sulphuric Acid: It is used in the manufacture of fertilizers (like ammonium sulphate), as an electrolyte in lead-acid batteries, in the manufacture of paints and plastics, and in the refining of petroleum.

Laboratory Observations and Reactions

• Iron and Sulphur Mixture: When iron powder and sulphur are heated, they react to form Iron (II) sulphide. The reaction is highly exothermic and the mixture glows. Iron powder glows more brightly than iron filings because the powder has a larger surface area, increasing the rate of reaction.

• Copper and Concentrated Sulphuric Acid: Heating copper turnings with concentrated $H_2SO_4$ produce $SO_2$ gas. The solid mixture remaining contains white anhydrous copper (II) sulphate and black copper (II) oxide. Upon adding water, the white solid dissolves to form a characteristic blue solution of hydrated copper (II) sulphate.

• Barium Carbonate and Sulphuric Acid: When dilute sulphuric acid reacts with barium carbonate, the yield of $SO_2$ is negligible. This is because the reaction forms insoluble barium sulphate ($BaSO_4$), which forms a coating around the carbonate, preventing further reaction with the acid.

Questions & Discussion

1. Name the process represented for sulphur extraction.

   • The Frasch process.

2. What is passed down through pipe J?

   • Superheated water.

3. Name the two allotropes of sulphur.

   • Rhombic and monoclinic sulphur.

4. Differentiate the bleaching action of chloride and sulphur (IV) oxide.

   • Chlorine bleaches by oxidation and is permanent; sulphur (IV) oxide bleaches by reduction and is temporary.

5. Is concentrated sulphuric acid weak or strong?

   • It is a weak acid because it has low ionization/dissociation into ions in its concentrated form.

6. State the observations made when iron powder and sulphur are heated.

   • The mixture glows red-hot and forms a black solid (Iron (II) sulphide).

7. Calculate the volume of 6.0 dm3 of sulphur (IV) oxide oxidized by oxygen to sulphur (VI) oxide at R.T.P.

   • Equation: $2SO_2(g) + O_2(g) \rightarrow 2SO_3(g)$.
   • Moles of $SO_2 = \frac{6.0}{24.0} = 0.25\,moles$.
   • Moles of $O_2 = \frac{1}{2} \times \text{moles of } SO_2 = 0.125\,moles$.
   • Volume of $O_2 = 0.125 \times 24.0 = 3.0\,dm^3$.