11.3 Sulphur | Nitrogen And Sulphur | Chemistry 11

Sulphur is a non-metal element located below oxygen in Group 16 (VIA) of the periodic table. At room temperature, elemental sulphur is a bright yellow, crystalline solid. Sulphur atoms typically form stable, cyclic octatomic molecules with the chemical formula $S_{8}$ .

Pasted image 20251010215654 — Figure 11.5: Crystal Structure of sulphur (S8 ring)

Electronic Configuration and Stability

Sulphur has 6 electrons in its valence shell. According to the Rules of Electronic Configuration→, its valence shell configuration is $3 s^{2} 3 p^{4}$ . This leads to possible oxidation states of -2, +2, +4, and +6.

Most Stable Oxidation State: The +6 oxidation state is the most stable for sulphur. This is due to its ability to expand its octet using vacant 3d-orbitals, allowing it to form six bonds (e.g., in $H_{2} S O_{4}$ or $S F_{6}$ ).
Examples: Sulphur in the +6 state is found in important compounds like sulphuric acid ( $H_{2} S O_{4}$ ) and sulphate ions ( $S O_{4}^{2 -}$ ).
Molecular Stability: The elemental $S_{8}$ molecule is relatively inert because the single S-S bonds are strong and require significant energy to break.

11.3.1 Allotropes of Sulphur

Sulphur exists in several allotropic forms. The two main crystalline allotropes are:

Allotrope	Also Called	Stable Temperature Range	Structure
Rhombic sulphur	Alpha ( $α$ ) sulphur	Below $95. 5^{\circ} C$	$S_{8}$ puckered rings, orthorhombic crystal
Monoclinic sulphur	Beta ( $β$ ) sulphur	Above $95. 5^{\circ} C$ (up to m.p.)	$S_{8}$ puckered rings, monoclinic crystal

The temperature $95. 5^{\circ} C$ is the transition temperature at which the two forms are in equilibrium. Rhombic sulphur is the thermodynamically stable form at room temperature.

11.3.2 Properties of Sulphur

Sulphur is a versatile element that can function as both an oxidising agent and a reducing agent, depending on the chemical reaction.

As an Oxidising Agent: Sulphur can gain two electrons to achieve a stable octet (like a noble gas), forming the sulphide ion ( $S^{2 -}$ ). Its oxidation state decreases from 0 to -2.
As a Reducing Agent: Sulphur can also lose electrons to exist in higher oxidation states like +4 and +6. Its oxidation state increases from 0 to +4 or +6.

11.3.3 Sulphur as an Oxidising Agent

In these reactions, sulphur's oxidation state is reduced from 0 to -2.

1. Reaction with Hydrogen

Sulphur combines directly with hydrogen gas upon heating to form hydrogen sulphide.

$S + H_{2} \to H_{2} S$

2. Reaction with Metals

Sulphur reacts with active metals like zinc and iron on heating to form metal sulphides.

$Z n + S \to Z n S$ $F e + S \to F e S$

In these reactions, the oxidation state of sulphur decreases from 0 to -2, while the oxidation state of the metals (Zn and Fe) increases from 0 to +2.

11.3.4 Sulphur as a Reducing Agent

In these reactions, sulphur's oxidation state increases from 0 to +4 or +6.

1. Reaction with Oxygen (Combustion)

Sulphur burns in air with a characteristic blue flame to produce sulphur dioxide gas.

$S + O_{2} \to S O_{2}$

In the presence of moist air, sulphur can be slowly oxidised to sulphuric acid.

$2 S + 2 H_{2} O + 3 O_{2} \to 2 H_{2} S O_{4}$

2. Reaction with Oxidising Agents

When treated with strong oxidising agents like concentrated nitric acid ( $H N O_{3}$ ) or concentrated sulphuric acid ( $H_{2} S O_{4}$ ), sulphur is oxidised.

Reaction with Nitric Acid: $S + 6 H N O_{3} \to H_{2} S O_{4} + 6 N O_{2} + 2 H_{2} O$ Here, sulphur is oxidised from 0 to +6, while nitrogen in nitric acid is reduced from +5 to +4.
Reaction with Sulphuric Acid: $S + 2 H_{2} S O_{4} \to 3 S O_{2} + 2 H_{2} O$ In this reaction, elemental sulphur is oxidised from 0 to +4, while the sulphur in sulphuric acid is reduced from +6 to +4.

11.3.5 Uses of Sulphur and its Compounds

Sulphur and its compounds have a wide range of important industrial and chemical applications.

Fertiliser Production: Sulphuric acid and ammonium sulphate are crucial components in fertilisers, providing essential sulphur for the synthesis of amino acids and vitamins in plants.
Gunpowder: Gunpowder is a mixture of sulphur, charcoal, and potassium nitrate ( $K N O_{3}$ ). Sulphur acts as a fuel, lowers the ignition temperature, and increases the combustion rate.
Rubber Industry: Sulphur is used in the vulcanization of rubber, a process that cross-links polymer chains to improve the rubber's strength, elasticity, and durability.
Synthetic Organic Chemistry: Sulphur-containing functional groups are vital in organic synthesis. For example, thiols (-SH) and sulfonyl chlorides are used to create pharmaceuticals, including a class of antibiotics known as sulfonamides.
Dyes and Pigments: Sulphur compounds are used in the manufacturing of various dyes and pigments.
Food Preservatives: Sulphur dioxide ( $S O_{2}$ ) and sulphites (e.g., $N a_{2} S O_{3}$ ) act as reducing agents that inhibit oxidation, preventing spoilage and preserving the color of foods and beverages like wine and dried fruits.

Electronic Configuration and Stability

Most Stable Oxidation State: The +6 oxidation state is the most stable for sulphur. This is due to its ability to expand its octet using vacant 3d-orbitals, allowing it to form six bonds (e.g., in $H_{2} S O_{4}$ or $S F_{6}$ ).
Examples: Sulphur in the +6 state is found in important compounds like sulphuric acid ( $H_{2} S O_{4}$ ) and sulphate ions ( $S O_{4}^{2 -}$ ).
Molecular Stability: The elemental $S_{8}$ molecule is relatively inert because the single S-S bonds are strong and require significant energy to break.

11.3.1 Allotropes of Sulphur

Sulphur exists in several allotropic forms. The two main crystalline allotropes are:

Allotrope	Also Called	Stable Temperature Range	Structure
Rhombic sulphur	Alpha ( $α$ ) sulphur	Below $95. 5^{\circ} C$	$S_{8}$ puckered rings, orthorhombic crystal
Monoclinic sulphur	Beta ( $β$ ) sulphur	Above $95. 5^{\circ} C$ (up to m.p.)	$S_{8}$ puckered rings, monoclinic crystal

The temperature $95. 5^{\circ} C$ is the transition temperature at which the two forms are in equilibrium. Rhombic sulphur is the thermodynamically stable form at room temperature.

11.3.2 Properties of Sulphur

Sulphur is a versatile element that can function as both an oxidising agent and a reducing agent, depending on the chemical reaction.

As an Oxidising Agent: Sulphur can gain two electrons to achieve a stable octet (like a noble gas), forming the sulphide ion ( $S^{2 -}$ ). Its oxidation state decreases from 0 to -2.
As a Reducing Agent: Sulphur can also lose electrons to exist in higher oxidation states like +4 and +6. Its oxidation state increases from 0 to +4 or +6.

11.3.3 Sulphur as an Oxidising Agent

In these reactions, sulphur's oxidation state is reduced from 0 to -2.

1. Reaction with Hydrogen

Sulphur combines directly with hydrogen gas upon heating to form hydrogen sulphide.

$S + H_{2} \to H_{2} S$

2. Reaction with Metals

Sulphur reacts with active metals like zinc and iron on heating to form metal sulphides.

$Z n + S \to Z n S$ $F e + S \to F e S$

In these reactions, the oxidation state of sulphur decreases from 0 to -2, while the oxidation state of the metals (Zn and Fe) increases from 0 to +2.

11.3.4 Sulphur as a Reducing Agent

In these reactions, sulphur's oxidation state increases from 0 to +4 or +6.

1. Reaction with Oxygen (Combustion)

Sulphur burns in air with a characteristic blue flame to produce sulphur dioxide gas.

$S + O_{2} \to S O_{2}$

In the presence of moist air, sulphur can be slowly oxidised to sulphuric acid.

$2 S + 2 H_{2} O + 3 O_{2} \to 2 H_{2} S O_{4}$

2. Reaction with Oxidising Agents

When treated with strong oxidising agents like concentrated nitric acid ( $H N O_{3}$ ) or concentrated sulphuric acid ( $H_{2} S O_{4}$ ), sulphur is oxidised.

Reaction with Nitric Acid: $S + 6 H N O_{3} \to H_{2} S O_{4} + 6 N O_{2} + 2 H_{2} O$ Here, sulphur is oxidised from 0 to +6, while nitrogen in nitric acid is reduced from +5 to +4.
Reaction with Sulphuric Acid: $S + 2 H_{2} S O_{4} \to 3 S O_{2} + 2 H_{2} O$ In this reaction, elemental sulphur is oxidised from 0 to +4, while the sulphur in sulphuric acid is reduced from +6 to +4.

11.3.5 Uses of Sulphur and its Compounds

Sulphur and its compounds have a wide range of important industrial and chemical applications.

Fertiliser Production: Sulphuric acid and ammonium sulphate are crucial components in fertilisers, providing essential sulphur for the synthesis of amino acids and vitamins in plants.
Gunpowder: Gunpowder is a mixture of sulphur, charcoal, and potassium nitrate ( $K N O_{3}$ ). Sulphur acts as a fuel, lowers the ignition temperature, and increases the combustion rate.
Rubber Industry: Sulphur is used in the vulcanization of rubber, a process that cross-links polymer chains to improve the rubber's strength, elasticity, and durability.
Synthetic Organic Chemistry: Sulphur-containing functional groups are vital in organic synthesis. For example, thiols (-SH) and sulfonyl chlorides are used to create pharmaceuticals, including a class of antibiotics known as sulfonamides.
Dyes and Pigments: Sulphur compounds are used in the manufacturing of various dyes and pigments.
Food Preservatives: Sulphur dioxide ( $S O_{2}$ ) and sulphites (e.g., $N a_{2} S O_{3}$ ) act as reducing agents that inhibit oxidation, preventing spoilage and preserving the color of foods and beverages like wine and dried fruits.