Oxides of Sulphur

Oxides are chemical compounds composed of oxygen atoms combined with atoms of another element. When oxygen reacts with sulfur, it forms sulfur dioxide (SO₂) or sulfur trioxide (SO₃), depending on the conditions of the reaction.

These compounds play crucial roles in industrial processes, environmental chemistry, and atmospheric reactions. In this article, we will discuss these sulfur oxides in detail.

1.0What Are Oxides of Sulphur?

Sulfur forms various oxides, including sulfur monoxide (SO), sulfur sesquioxide (S₂O₃), sulfur heptoxide (S₂O₇), and sulfur tetroxide (SO₄). 

Sulfur monoxide, an unstable colorless gas, is produced by electric discharge in a mix of Sulfur dioxide and sulfur vapor under low pressure. Upon cooling, it transforms into an orange-red solid that gradually breaks down into sulfur and sulfur dioxide. Sulfur sesquioxide, created by dissolving sulfur in sulfur trioxide, appears as a blue-green solid and remains stable only at temperatures below 15°C (59°F). Both sulfur heptoxide and sulfur tetroxide are unstable, melting around 0°C (32°F), and can be synthesized through electric discharge in sulfur dioxide or trioxide and oxygen mixtures.

The oxides of sulfur consist of inorganic compounds composed of sulfur and oxygen atoms. In the Earth’s lower atmosphere, sulfur dioxide (SO₂) and sulfur trioxide (SO₃) are the most prevalent sulfur oxides. 

Other significant classes of sulfur oxides include lower sulfur oxides with the general formula S_mO_n (m > 2n), such as sulfur monoxide (SO) and disulfur dioxide (S₂O₂), formed through the dimerization of sulfur monoxide, and disulfur monoxide (S₂O). Additionally, higher sulfur oxides with sulfur in the +6 oxidation state are noteworthy.

Sulfur oxides typically result from the combustion of sulfur-containing substances in oxygen-rich air. They are generated during processes like roasting sulfide ores, burning fossil fuels, and coal combustion. Vehicle emissions are a common source of sulfur dioxide, and it can also be naturally produced through volcanic activity. Industrially, sulfur trioxide, known as sulfuric anhydride, is prepared as a precursor to sulfuric acid. The lower sulfur oxides are formed as intermediates during the combustion of elemental sulfur but are less stable compared to SO₂ and SO₃. Understanding these sulfur oxides is crucial for addressing environmental impacts and industrial applications.

Let’s discuss oxides of sulphurs in detail

2.0Sulphur Dioxide (SO₂)

Properties of Sulphur Dioxide (SO₂):

Uses of Sulphur Dioxide

1. Preservative: It is used as a preservative in the food and wine industry.

2. Bleaching Agent: SO₂ is employed in bleaching and disinfecting activities.

3. Chemical Industry: It serves as a precursor in the production of sulfuric acid and other chemicals.

Reactions involving Sulphur Dioxide

• 1. Formation of Sulfurous Acid: 

     SO₂ + H₂O  →  H₂SO₃

• 2. Reaction with Oxygen: 

2SO₂ + O₂  →  2SO₃

• 3. Acid-Base Reactions: 

SO₂ reacts with bases to form sulfites, 

e.g., SO₂  +  2NaOH   →   Na₂SO₃ + H₂O

Preparation of Sulphur Dioxide (SO₂)

Sulphur dioxide (SO₂) can be prepared by various methods, primarily involving the combustion or reduction of sulfur-containing compounds.

• 1. Burning Sulfur: 
• The most common method involves burning elemental sulfur in the presence of oxygen.

                    S   +   O₂       →      SO₂

• 2. Roasting Metal Sulfides: 
• Sulfur dioxide is produced as a byproduct during the roasting of metal sulfide ores.

                      MS  +   3O_2​      →      MO  +  2 SO₂ (where MS is a metal sulfide)

• 3. Catalytic Oxidation: Sulfur dioxide can be obtained by catalytic oxidation of sulfur dioxide.

                      2 H₂S  +  O₂       →      2 H₂O  +  2 SO₂

3.0Sulphur Trioxide

Properties of Sulphur Trioxide

Uses of Sulphur Trioxide

• Sulfuric Acid Production: The primary use of sulfur trioxide is in the production of sulfuric acid  (H₂SO₄) .
• Chemical Synthesis: It is employed in various chemical syntheses, including the production of detergents and pharmaceuticals.

Reactions of Sulphur Trioxide

• Reaction with Water:

SO₃   +  H₂O    →      H₂SO₄   (Sulfuric Acid Formation)

• Reaction with Ammonia:

SO₃  +  2 NH₃  →   (NH₄)₂SO₄ (Formation of Ammonium Sulfate)

• Reaction with Alcohols:

SO₃     +  ROH    →    RSO₃H  (Formation of Sulfuric Esters)

Methods of Preparation of Sulfur Trioxide (SO₃):

• Oleum Process :
• • Oleum (H₂S₂O₇), also known as fuming sulfuric acid or pyrosulfuric acid, can be reacted with water to produce sulfuric acid and sulfur trioxide:

                  H₂S₂O₇    +   H₂O   →    2 H₂SO₄

• Laboratory Synthesis :
• • In laboratory settings, sulfur trioxide can be prepared by treating sulfuric acid (H₂SO₄) with phosphorus pentoxide (P₄O₁₀):

                 2 H₂SO₄  +  P₄O₁₀  →  4SO₃  +  P₄O₉  +  4 H₂O

• Catalytic Combustion :
• • Sulfur dioxide (SO₂) can be catalytically oxidized by oxygen (O₂) in the presence of a suitable catalyst to produce sulfur trioxide:

$S{O}_2(g)+\frac{1}{2}{O}_2(g)\stackrel{catalyst}{\to }S{O}_3(g)$

• Electrochemical Method:
• • An electrochemical cell can be employed to convert sulfur dioxide (SO₂) to sulfur trioxide using a solid electrolyte cell with a vanadium(V) oxide (V₂O₅) cathode:

$2S{O}_2(g)+O_2(g)\stackrel{V_2{O}_5}{\to }2S{O}_3(g)$

Contact Process:

• This is the most common industrial method, these alternative methods may find use in specific applications or laboratory settings. Here are important steps of Contact process-

Steps:

• 1. Combustion of Sulfur:
• • Elemental sulfur (S₈) is burned in air to produce sulfur dioxide (SO₂):

           S₈  +  8O₂  →  8 SO₂

• 2. Conversion to Sulfur Trioxide:
• • The SO₂ is then passed over a vanadium(V) oxide (V₂O₅) catalyst at an elevated temperature (around 450 °C):

$2S{O}_2(g)+O_2(g)\stackrel{V_2{O}_5}{\to }2S{O}_3(g)$

• 3. Absorption:
• • The sulfur trioxide is absorbed into a solution of water to form sulfuric acid (H₂SO₄):

         SO₃  +  H₂O  →  H₂SO₄