Do you know what causes acid rain? It is primarily caused by air pollutants; the excess oxides of sulphur and nitrogen are released into the air causing acid rain. To know more about oxides of Sulphur, read the below article.

What are Oxides of Sulphur?

Oxides of Sulphur \(\left({{\text{S}}{{\text{O}}_{\text{x}}}} \right)\) are Sulphur and Oxygen containing compounds such as sulphur monoxide \(\left({{\text{SO}}} \right),\) sulphur dioxide \(\left( {{\rm{S}}{{\rm{O}}_{\rm{2}}}} \right),\) sulphur trioxide \(\left( {{\rm{S}}{{\rm{O}}_{\rm{3}}}} \right),\) etc., Among these, sulphur dioxide and sulphur trioxide are pre-dominant.

Oxides of Sulphur are non-metallic that react with water producing an acidic solution. Hence, non-metallic oxides are called acidic oxides. Oxides of Sulphur also produce acid with water; hence oxides of Sulphur are acidic.

Major Source of Oxides of Sulphur and Nitrogen

Volcanic eruptions are a natural source of \({\text{S}}{{\text{O}}_2}\) discharge into the atmosphere. It makes up around \(67\% \) of the total amount of \({\text{S}}{{\text{O}}_2}\) on the globe. The remaining \(33\% \) is released into the atmosphere as a result of human activity.

Natural bacterial action is the only natural source that releases massive volumes of \({\text{N}}{{\text{O}}_{\text{x}}},\) primarily in the form of NO, into the atmosphere. Furthermore, lightning discharge results in the formation of \({\text{NO}}\) when \({{\text{N}}_2}\) and \({{\text{O}}_2}\) combine.

Human Activity Includes

1. Sulphur oxides are released when sulphur-containing coal and fuel oil are burned in thermal power plants.
2. Sulphide ores, such as iron pyrites \(\left({{\text{Fe}}{{\text{S}}_2}} \right),\) copper pyrites \(\left({{\text{CuFe}}{{\text{S}}_2}} \right),\) copper glance \(\left( {{\text{C}}{{\text{u}}_2}{\text{S}}} \right)\) zinc blende \(\left({{\text{Zns}}} \right),\) galena \(\left({{\text{PbS}}} \right),\) etc., are roasted in the smelting industry.
   \({2\text{C}}{{\text{u}}_2}{\text{S}} + 3{{\text{O}}_2} \to 2{\text{C}}{{\text{u}}_2}{\text{O}} + 2{\text{S}}{{\text{O}}_2}\)
3. In the air, a small amount of \({\text{S}}{{\text{O}}_2}\) undergoes photolytic oxidation to produce \({\text{S}}{{\text{O}}_3.}\)
   \(2{\text{S}}{{\text{O}}_2} + {{\text{O}}_2} \to 2{\text{S}}{{\text{O}}_3}\)
4. In the presence of water vapour, \({\text{S}}{{\text{O}}_3}\) interacts to generate \({{\text{H}}_2}{\text{S}}{{\text{O}}_{4,}}\) which either floats in the air as droplets or falls as acid rain.
   \({\text{S}}{{\text{O}}_3} + {{\text{H}}_2}{\text{O}} \to {{\text{H}}_2}{\text{S}}{{\text{O}}_4}\)

Oxides of Sulphur Effects

1. Both \({\text{S}}{{\text{O}}_2}\) and \({\text{S}}{{\text{O}}_3}\) are highly irritating to the respiratory tract. At a concentration of \(5\,{\text{ppm}},\,{\text{S}}{{\text{O}}_2}\) irritates the throat and eyes. In addition, it affects the larynx, or voice box, and produces dyspnea (upper part of the windpipe). Even at a concentration of \(1\,{\text{ppm}},\,{\text{S}}{{\text{O}}_3}\) is toxic and causes considerable discomfort. The elderly and those with heart and lung illnesses are most severely affected.
2. Even in concentrations, \({\text{S}}{{\text{O}}_2}\) is extremely harmful to plants. When exposed for an extended period, such as a few days or weeks, it can result in Chlorosis, which reduces chlorophyll generation, leading to leaf damage, including loss of green colour. Chlorosis is the medical term for this condition.
3. Sulphur oxide gets converted into sulphuric acid, damaging many materials as listed below.
   a. It can damage construction materials, particularly marble. The marble of the Taj Mahal in Agra, for example, is being damaged by the neighbouring Mathura refinery, thermal power station, and a multitude of foundries.
   b. It can cause rusting of metals, especially iron and steel.
   c. It causes fabric, leather, and paper to fade and deteriorate.
   d. It has an impact on the colour of the paint.

Oxides of Sulphur in Air Pollution

Sulphur dioxide is a significant component of the atmosphere. Oxides of Sulphur are produced when Sulphur containing fossil fuel is burnt. Sulphurous acid is formed when these oxides react with atmospheric moisture. These acids dissolve in water and form acid, which falls as acid rain. Sulphur oxide reacts with other molecules to generate particulate pollution, resulting in foggy skies and reduced visibility. Sulphur dioxide is a severe air contaminant that has serious health consequences.

Furthermore, the amount of sulphur dioxide in the atmosphere impact the suitability of habitat for plant groups and animal life. Sulphur dioxide emissions are precursors of acid rain and harmful particulate matter in the atmosphere.

Sulphur Dioxide \(\left( {{\rm{S}}{{\rm{O}}_2}} \right)\)

The chemical formula of Sulphur dioxide is \({\text{S}}{{\text{O}}_2}.\) It is a poisonous gas that gives off the odour of burned matches. It is produced as a by-product of copper extraction and the combustion of sulphur-contaminated fossil fuels, and it is generated naturally by volcanic activity. The odour of sulphur dioxide is similar to that of nitric acid.

Structure: Sulphur dioxide possesses an angular structure. The \({\text{S}}{{\text{O}}_2}\) molecule is a resonance hybrid of two canonical forms listed below.

Preparation of Sulphur Dioxide

The following methods are used in the preparation of sulphur dioxide:

1. By the action of concentrated sulphuric acid with carbon, sulphur, copper, mercury etc.: When hot and concentrated sulphuric acid is treated with carbon, sulphur, mercury, copper, etc., it produces sulphur dioxide.

\({\text{C}} + 2{{\text{H}}_2}{\text{S}}{{\text{O}}_4} \to {\text{C}}{{\text{O}}_2} + 2{\text{S}}{{\text{O}}_2} + 2{{\text{H}}_2}{\text{O}}\)
\({\text{S}} + 2{{\text{H}}_2}{\text{S}}{{\text{O}}_4} \to 3{\text{S}}{{\text{O}}_2} + 2{{\text{H}}_2}{\text{O}}\)
\({\text{Cu}} + 2{{\text{H}}_2}{\text{S}}{{\text{O}}_4} \to {\text{CuS}}{{\text{O}}_4} + {\text{S}}{{\text{O}}_2} + 2{{\text{H}}_2}{{\text{O}}}{{}}\)
\({\text{Hg}} + 2{{\text{H}}_2}{\text{S}}{{\text{O}}_4} \to {\text{HgS}}{{\text{O}}_4} + {\text{S}}{{\text{O}}_2} + 2{{\text{H}}_2}{\text{O}}\)
\(2{\text{Ag}} + 2{{\text{H}}_2}{\text{S}}{{\text{O}}_4} \cdot \to {\text{A}}{{\text{g}}_2}{\text{S}}{{\text{O}}_4} + {\text{S}}{{\text{O}}_2} + 2{{\text{H}}_2}{\text{O}}\)

2. The decomposition of sulphites or bisulphite: When sulphites or bisulphites such as sodium sulphite or sodium bisulphites are treated with moderately concentrated sulphuric acid, sulphur dioxide is formed.

\({\text{N}}{{\text{a}}_2}{\text{S}}{{\text{O}}_3} + {{\text{H}}_2}{\text{S}}{{\text{O}}_4} \to {\text{N}}{{\text{a}}_2}{\text{S}}{{\text{O}}_4} + {\text{S}}{{\text{O}}_2} + {{\text{H}}_2}{\text{O}}\)
\(2{\text{NaHS}}{{\text{O}}_3} + {{\text{H}}_2}{\text{S}}{{\text{O}}_4} \to {\text{N}}{{\text{a}}_2}{\text{S}}{{\text{O}}_4} + 2{\text{S}}{{\text{O}}_2} + 2{{\text{H}}_2}{\text{O}}\)

3. Laboratory Preparation: Sulphur dioxide is made in the laboratory by treating hot, concentrated sulphuric acid with copper turnings. The copper turnings are first placed in a round bottom flask with a delivery tube and a thistle funnel. The flask is then heated after concentrated sulphuric acid is dropped from the thistle funnel. The sulphur dioxide gas is collected by the upward displacement of air.

4. Manufacturing of sulphur dioxide on a commercial scale: Sulphur dioxide is produced commercially by burning sulphur or roasting sulphide ore such as iron pyrites \(\left( {{\text{Fe}}{{\text{S}}_2}} \right)\) and zinc blend \(\left({{\text{ZnS}}} \right),\) etc. The sulphur dioxide obtained is dried by using concentrated sulphuric acid. Calcium oxide is not used to dry sulphur dioxide gas because it reacts with sulphur dioxide and forms calcium sulphite.

Properties of Sulphur Dioxide

The physical and chemical properties of sulphur dioxide are explained below:

Physical Properties of Sulphur Dioxide

1. Sulphur dioxide is a colourless gas having a pungent and suffocating smell.
2. It is poisonous and causes breathing problems.
3. It is \(2.2\) times heavier than air.
4. It is very soluble in water. Its aqueous solution is sulphurous acid and is known as sulphur dioxide water.
5. It readily liquefies at room temperature under a pressure of two atmospheres.

Chemical Properties of Sulphur Dioxide

I. Combustibility: It neither burns nor aids in the burning process. Burning magnesium and potassium, on the other hand, continue to burn in it.

\(2\,{\text{Mg}} + {\text{S}}{{\text{O}}_2} \to 2\,{\text{MgO}} + {\text{S}}\)
\(4{\mkern 1mu} {\rm{K}} + 3{\mkern 1mu} {\rm{S}}{{\rm{O}}_2} \to {{\rm{K}}_2}{{\rm{S}}_2}{{\rm{O}}_3} + {{\rm{K}}_2}{\rm{S}}{{\rm{O}}_3}\)

II. Thermal decomposition: When sulphur dioxide is heated strongly, it dissociates to give sulphur and sulphur trioxide.

\(3{\rm{S}}{{\rm{O}}_2} \to \,{\rm{S}}\,{\rm{ + }}\,2{\rm{S}}{{\rm{O}}_3}\)

III. Acidic nature: Sulphur dioxide, when dissolved in water, gives sulphurous acid because it is an acidic oxide. As a result, it turns blue litmus to red.

\({\text{S}}{{\text{O}}_2} + {{\text{H}}_2}{\text{O}} \to {{\text{H}}_2}{\text{S}}{{\text{O}}_3}\)

Following are the reactions that indicate the acidic nature of Sulphur dioxide.

a) Reaction with alkalis: It reacts with alkalis and produces sulphites and bisulphite salts. For example, when sulphur dioxide is treated with sodium hydroxide, it produces sodium sulphite and sodium bisulphite.

\({\rm{NaOH}} + {\rm{S}}{{\rm{O}}_2} \to \mathop {{\rm{NaHS}}{{\rm{O}}_3}}\limits_{{\rm{Sodium}}\,\,{\rm{bisulphite}}} \)
\({\rm{2NaOH}} + {\rm{S}}{{\rm{O}}_2} \to \mathop {{\rm{N}}{{\rm{a}}_2}{\rm{S}}{{\rm{O}}_3}}\limits_{{\rm{Sodium}}\,{\rm{sulphite}}}  + {{\rm{H}}_2}{\rm{O}}{\mkern 1mu} \)

b) Reaction with carbonates: It decomposes carbonates to form carbon dioxide gas.

For example, sulphur dioxide decomposes sodium carbonate to sodium bisulphite with the liberation of carbon dioxide gas.

\({\text{NaC}}{{\text{O}}_3} + 2{\text{S}}{{\text{O}}_2} + {{\text{H}}_2}{\text{O}} \to 2{\text{NaHS}}{{\text{O}}_3} + {\text{C}}{{\text{O}}_2}\)

IV. Reducing Nature: It acts as a reducing agent because it liberates nascent hydrogen in the presence of moisture.

\({\text{S}}{{\text{O}}_2} + 2{{\text{H}}_2}{\text{O}} \to + {{\text{H}}_2}{\text{S}}{{\text{O}}_4}{\text{ + 2}}\left[{\text{H}} \right]\)

  Some of the reducing properties of sulphur dioxide are as follows:

a) It decolourises acidified potassium permanganate.

\(\mathop {2{\rm{KMn}}{{\rm{O}}_4}}\limits_{\left( {{\rm{purple}}} \right)}  + 5{\rm{S}}{{\rm{O}}_2} + {{\rm{H}}_2}{\rm{O}} \to \mathop {{{\rm{K}}_2}{\rm{S}}{{\rm{O}}_4}}\limits_{\left( {{\rm{colourless}}} \right)}  + 2{\rm{MnS}}{{\rm{O}}_4} + 2{{\rm{H}}_2}{\rm{S}}{{\rm{O}}_4}\)

b) It turns acidified potassium dichromate green.

\({{\rm{K}}_2}{\rm{C}}{{\rm{r}}_2}{{\rm{O}}_7} + 3{\rm{S}}{{\rm{O}}_2} + {{\rm{H}}_2}{\rm{S}}{{\rm{O}}_4} \to {{\rm{K}}_2}{\rm{S}}{{\rm{O}}_4} + {\rm{C}}{{\rm{r}}_2}{\left( {{\rm{S}}{{\rm{O}}_4}} \right)_3} + {{\rm{H}}_2}{\rm{O}}{\mkern 1mu}\)

v. Oxidising Properties: Sulphur dioxide acts as an oxidising agent. It oxidises hydrogen sulphide to sulphur, iron to ferrous oxide and tin to stannous.

The chemical reactions are as follows:

1. \(2{{\text{H}}_2}{\text{S}} + {\text{S}}{{\text{O}}_2} \to 3{\text{S}} + 2{{\text{H}}_2}{\text{O}}\)
2. \(3{\text{Fe}} + {\text{S}}{{\text{O}}_2} \to 2{\text{FeO}} + {\text{FeS}}\)
3. \(3{\text{Sn}} + {\text{S}}{{\text{O}}_2} \to 2{\text{SnO}} + {\text{SnS}}\)

VI. Bleaching property: The bleaching action of sulphur dioxide is due to the liberation of nascent hydrogen gas, reducing the vegetable colouring matter to a colourless reduction product. It can bleach coloured flowers, leaves and fabric dyed with vegetable colours.

\({\text{S}}{{\text{O}}_2} + 2{{\text{H}}_2}{\text{O}} \to {{\text{H}}_2}{\text{S}}{{\text{O}}_4} + 2\left[{\text{H}} \right]\)

Vegetable Colouring matter \( + \left[{\text{H}} \right] \to \) Colourless matter (in the presence of air)

The colourless reduction product is re-oxidised in the presence of atmospheric air, and the original colour reappears. Therefore, the bleaching action of sulphur dioxide is temporary.

Uses of Sulphur Dioxide

1. Sulphur dioxide is used to manufacture sulphuric acid and calcium bisulphite.
2. It is used as a bleaching agent for delicate articles like wool, silk, etc.
3. It is used for refining sugar.
4. It is used as a refrigerant, antichlor, organic solvent in the liquified form, disinfectant and household fumigant.

Sulphur Trioxide \(\left({{\text{S}}{{\text{O}}_3}} \right)\)

Sulphur trioxide \(\left({{\text{S}}{{\text{O}}_3}} \right)\) is a colourless white crystalline solid chemical compound that fumes in the air. It has a greater reactivity than sulphuric acid.

Structure of Sulphur Trioxide

The molecule of sulphur trioxide is found to be planar and symmetrical and is supposed to be a resonance hybrid of the following canonical forms. Solid sulphur trioxide can be found in polymeric form, with either a cyclic trimeric or chain structure.

Preparation of Sulphur trioxide

1. Sulphur trioxide can be prepared by heating ferric sulphate.

2. When concentrated sulphuric acid is distilled with phosphorus pentoxide, sulphur trioxide is formed.
\(2{{\text{H}}_2}{\text{S}}{{\text{O}}_4} + {{\text{P}}_4}{{\text{O}}_{10}} \to 4{\text{HP}}{{\text{O}}_3} + 2{\text{S}}{{\text{O}}_3}\)

Properties of Sulphur Trioxide

i. It forms long, transparent, ice-like needles which melts at \(290\,{\text{k}}.\)
ii. It dissolves in water to form sulphuric acid.
iii. It is an acidic oxide and reacts with basic oxides to form sulphates.
\({\text{CaO}} + {\text{S}}{{\text{O}}_3} \to {\text{CaS}}{{\text{O}}_4}\)
\({\text{BaO}} + {\text{S}}{{\text{O}}_3} \to {\text{BaS}}{{\text{O}}_4}\)
iv. It dissolves in concentrated sulphuric acid to give oleum which is also known as fuming sulphuric acid.
\(\begin{gathered} {{\text{H}}_2}{\text{S}}{{\text{O}}_4} + {\text{S}}{{\text{O}}_3} \to {{\text{H}}_2}{{\text{S}}_2}{{\text{O}}_7} \hfill \\ \,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,{\text{Oleum}} \hfill \\ \end{gathered} \)

Summary

Oxides of Sulphur \(\left({{\text{S}}{{\text{O}}_{\text{x}}}} \right)\) are sulphur and oxygen containing compounds such as sulphur monoxide \(\left({{\text{SO}}} \right),\) sulphur dioxide \(\left({{\text{S}}{{\text{O}}_{\text{x}}}} \right)\) and sulphur trioxide. Sulphur oxides are released when sulphur-containing coal and fuel oil are burned in thermal power plants.

FAQs

Q.1. What are the main sources of sulphur oxides?
Ans: Volcanic eruptions are a natural source of \({\text{S}}{{\text{O}}_2}\) emissions. It accounts for approximately \(67\) per cent of the total amount of \({\text{S}}{{\text{O}}_2}\) on the planet. The remaining \(33\% \) is released into the atmosphere as a result of human activity. 

Q.2. How are Sulphur oxides formed?
Ans: The combustion of sulphur-containing fossil fuels produces sulphur oxides.

Q.3. How do oxides of sulphur affect the environment?
Ans: Oxides of Sulphur are a source of air pollution and causes acid rain. Exposure of plants to a higher concentration of sulphur dioxide causes leaf damage, including green colour loss, known as chlorosis. These also damage the construction materials, buildings, etc.

Q.4. What is the difference between sulphur oxide and sulphur dioxide?
Ans: Sulphur oxides are inorganic compounds that contain a molecule of sulphur and oxygen, whereas sulphur dioxide is the most common form of sulphur oxide in the lower atmosphere. 

Q.5. Are oxides of sulphur acidic or basic?
Ans. The oxides of sulphur are acidic in nature. 

Q.6. What are the harmful effects of oxides of sulphur?
Ans: Following are the harmful effects of oxides of Sulphur;
1. Both \({\text{S}}{{\text{O}}_2}\) and \({\text{S}}{{\text{O}}_3}\) are highly irritating to the respiratory tract. At a concentration of \(5\,{\text{ppm}},{\text{S}}{{\text{O}}_2}\) irritates the throat and eyes. In addition, it affects the larynx, or voice box, and produces dyspnea (upper part of the windpipe). Even at a concentration of \(1\,{\text{ppm}},{\text{S}}{{\text{O}}_3}\) is highly toxic and causes discomfort. The elderly and those with heart and lung illnesses are most severely affected.
2. Even low concentrations of \({\text{S}}{{\text{O}}_2}\) are extremely harmful to plants. When exposed for an extended period, such as a few days or weeks, it can result in Chlorosis, which reduces chlorophyll generation, leading to leaf damage, including loss of green colour. Chlorosis is the medical term for this condition.

Q.7. What is the chemical formula of sulphur oxide?
Ans: The general formula of sulphur oxide is \({\text{S}}{{\text{O}}_{\text{x}}}{\text{.}}\) Sulphur oxide is a sulphur and oxygen containing compound. \({\text{S}}{{\text{O}}_2}\) (sulphur dioxide) and \({\text{S}}{{\text{O}}_3}\) (sulphur trioxide) are the oxides of Sulphur.

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