Sulphur Dioxide: Do you know what a burning matchstick smells like? It smells like sulphur dioxide. It’s a major contributor to greenhouse gas emissions and a key ingredient in the production of the ‘king of chemicals.’ When we start our car in the morning, we come into contact with sulphur dioxide. We will go over the formula, structure, preparation, properties, and applications of sulphur dioxide in this blog.

Sulphur Dioxide

Sulphur dioxide is created as a waste gas when sulphur-containing materials, such as metal, are heated, such as when smelting metal or burning coal or oil. It’s a major pollutant that’s currently considered a hazard to the environment.

Structure of Sulphur Dioxide

Sulphur dioxide has a bent structure with polar covalent bonds between the sulphur atom and two oxygen atoms. The chemical formula Sulphur dioxide is \({\rm{S}}{{\rm{O}}_2}\). Sulphur in sulphur dioxide is \({\rm{s}}{{\rm{p}}^2}\) hybridised. Ideally, the bond angle should be \({120^{\rm{o}}}\) but the \({\rm{O}} – {\rm{S}} – {\rm{O}}\) bond angle is found to be \({119^{\rm{o}}}\). This happens due to the lone pair of electrons on the Sulphur atom. The lone pair-bond pair repulsion decreases the bond angle from \({120^{\rm{o}}}\) to \({119^{\rm{o}}}\).

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Each oxygen atom in the Sulphur dioxide molecule is attached to the Sulphur atom by a sigma \((\sigma )\) and \((\pi )\) bond. The \(\sigma \) bonds between Sulphur and Oxygen atom is formed by \({\rm{s}}{{\rm{p}}^2}{\rm{ – p}}\) overlap while one of the \(\pi \) bonds is formed by the \({\rm{p\pi – p\pi }}\) overlap and other from \({\rm{p}}\pi – {\rm{d}}\pi \) overlap.

The electronic configuration of \({\rm{S}}\) is \(1{{\rm{s}}^2}2{{\rm{s}}^2}2{{\rm{p}}^6}3{{\rm{s}}^2}3{{\rm{p}}^4}\).

During the formation of \({\rm{S}}{{\rm{O}}_2}\), one electron from the \(3{\rm{p}}\) orbital goes to the \(3{\rm{d}}\) orbital, and \({\rm{S}}\) undergoes \({\rm{s}}{{\rm{p}}^2}\) hybridisation. Two of these orbitals form sigma bonds with two oxygen atoms, and the third contains a lone pair. \({\rm{p}}\)-orbital and \({\rm{d}}\)-orbital have an unpaired electron each. One of these electrons forms \({\rm{p}}\pi – {\rm{ p}}\pi \) bond with one oxygen atom, and the other forms \({\rm{p}}\pi – {\rm{ d}}\pi \) bond with the other oxygen. This is the reason \({\rm{S}}{{\rm{O}}_2}\) has a bent structure with a bond length of \(143\,{\rm{pm}}\).

However, both the \({\rm{S}} – {\rm{O}}\) bonds are identical due to resonance.

Preparation of Sulphur Dioxide

When sulphur burns in the air, sulphur dioxide is formed along with traces of sulphur trioxide \(\left( {6 – 8\% } \right)\). The equation is shown below:

\({\rm{S}}({\rm{s}}) + {{\rm{O}}_2}({\rm{g}}) \to {\rm{S}}{{\rm{O}}_2}({\rm{g}})\)

Laboratory Preparation of Sulphur Dioxide

Sulphur dioxide is prepared in the laboratory by the action of dilute sulphuric acid on sulphites. Sodium sulfate and water are also the byproducts of this reaction.

We can write the chemical reaction as:

\({\rm{N}}{{\rm{a}}_2}{\rm{S}}{{\rm{O}}_3}({\rm{s}}) + {{\rm{H}}_2}{\rm{S}}{{\rm{O}}_4}({\rm{aq}}) \to {\rm{S}}{{\rm{O}}_2}({\rm{g}}) + {\rm{N}}{{\rm{a}}_2}{\rm{S}}{{\rm{O}}_4}({\rm{aq}}) + {{\rm{H}}_2}{\rm{O}}({\rm{l}})\)

Sulphur dioxide is also prepared by gently heating copper turnings in a flask with concentrated sulfuric acid.

We can write the chemical reaction as,

\({\rm{Cu}}({\rm{s}}) + 2{{\rm{H}}_2}{\rm{S}}{{\rm{O}}_4}({\rm{aq}}) \to {\rm{S}}{{\rm{O}}_2}({\rm{g}}) + {\rm{CuS}}{{\rm{O}}_4}({\rm{aq}}) + 2{{\rm{H}}_2}{\rm{O}}({\rm{l}})\)

Industrial Preparation of Sulphur Dioxide

Commercially, sulphur dioxide is produced as a byproduct of the roasting of sulphide ores such as Iron Pyrites or Zinc blende.

\(4{\rm{Fe}}{{\rm{S}}_2}({\rm{s}}) + 11{{\rm{O}}_2}({\rm{aq}}) \to 2{\rm{F}}{{\rm{e}}_2}{{\rm{O}}_3}({\rm{s}}) + 8{\rm{S}}{{\rm{O}}_2}({\rm{g}})\)

Physical Properties of Sulphur Dioxide

1. Sulphur dioxide is a colourless, toxic gas with a suffocating and pungent odour.
2. It is heavier than air and readily soluble in water. One volume of water can dissolve about eight volumes of the gas at \(0\;^\circ {\rm{C}}\).
3. Under the pressure of \(2{\rm{ atm}}\) and at room temperature, it can be easily liquified.
4. Its boiling point is \(263\, {\rm{ K}}\), and itszing point is \(197.8\, {\rm{ K}}\).
5. It is a non-flammable gas that acts as a solvent for phosphorus, sulphur, iodine, etc.

Chemical Properties of Sulphur Dioxide

The chemical properties of Sulphur Dioxide are explained below

1. Acidic Character of Sulphur Dioxide

When Sulphur dioxide dissolves in water, sulphurous acid is formed. Its aqueous solution is acidic that turns blue litmus red.

Hence, \({\rm{S}}{{\rm{O}}_2}\) is considered an anhydride of sulphurous acid.

\({\rm{S}}{{\rm{O}}_2}({\rm{g}}) + {{\rm{H}}_2}{\rm{O}}({\rm{l}}) \to {{\rm{H}}_2}{\rm{S}}{{\rm{O}}_3}({\rm{aq}})\)

Because of its acidic character:

(a) Sulphur dioxide reacts readily with sodium hydroxide solution to form sodium sulphite solution, which reacts with more sulphur dioxide to form sodium hydrogen sulphite.

\(2{\rm{NaOH}}({\rm{aq}}) + {\rm{S}}{{\rm{O}}_2}({\rm{g}}) \to {\rm{N}}{{\rm{a}}_2}{\rm{S}}{{\rm{O}}_3}({\rm{aq}}) + {{\rm{H}}_2}{\rm{O}}({\rm{l}})\)

\({\rm{N}}{{\rm{a}}_2}{\rm{S}}{{\rm{O}}_3}({\rm{aq}}) + {{\rm{H}}_2}{\rm{O}}({\rm{l}}) + {\rm{S}}{{\rm{O}}_2}({\rm{g}}) \to 2{\rm{NaHS}}{{\rm{O}}_3}({\rm{aq}})\)

(b) When Sulphur dioxide gas is bubbled through lime water, the solution turns milky. On passing excess sulphur dioxide gas, the milkiness disappears due to the formation of calcium bisulphite.

\({\rm{Ca}}{({\rm{OH}})_2}({\rm{aq}}) + {\rm{S}}{{\rm{O}}_2}({\rm{g}}) \to {\rm{CaS}}{{\rm{O}}_3}({\rm{aq}}) + {{\rm{H}}_2}{\rm{O}}({\rm{l}})\)

\({\rm{CaS}}{{\rm{O}}_3}({\rm{aq}}) + {{\rm{H}}_2}O({\rm{l}}) + {\rm{S}}{{\rm{O}}_2}({\rm{g}}) \to {\rm{Ca}}{\left( {{\rm{HS}}{{\rm{O}}_3}} \right)_2}({\rm{aq}})\)

2. Combination with Oxygen

Sulphur dioxide reacts with oxygen on heating to form sulphur trioxide.

The above reaction is slow and reversible. Therefore, the reaction is carried out in the presence of some catalysts such as platinised asbestos, vanadium pentoxide, etc.

3. Reaction with Halogens

Sulphur dioxide combines directly with the halogens (fluorine, chlorine, and bromine) to form sulphuryl fluoride, chloride, and bromide.

For example: \({\rm{S}}{{\rm{O}}_2}\) combines with chlorine in the presence of charcoal to give sulphuryl chloride. The charcoal acts as a catalyst for the reaction.

\({\rm{S}}{{\rm{O}}_2}({\rm{g}}) + {\rm{C}}{{\rm{l}}_2}({\rm{g}}) \to {\rm{S}}{{\rm{O}}_2}{\rm{C}}{{\rm{l}}_2}({\rm{l}})\)

4. As an Oxidising Agent

\({\rm{S}}{{\rm{O}}_2}\) oxidises powerful reducing agents like \({{\rm{H}}_2}\;{\rm{S}},\,{\rm{HI}},\,{\rm{Mg}},\,{\rm{Fe}}\), etc., and itself gets reduced to sulphur or sulphide. For example:

\({\rm{S}}{{\rm{O}}_2}({\rm{g}}) + 2{{\rm{H}}_2}\;{\rm{S}}({\rm{g}}) \to 2{{\rm{H}}_2}{\rm{O}}({\rm{l}}) + 3\;{\rm{S}}\)

5. \({\rm{S}}{{\rm{O}}_2}\) as a Reducing Agent

Aqueous \({\rm{S}}{{\rm{O}}_2}\) shows reducing character and itself gets oxidised to \({{\rm{H}}_2}{\rm{S}}{{\rm{O}}_4}\).

1. It reduces acidified orange potassium dichromate solution to light green chromium sulphate.
   \({{\rm{K}}_2}{\rm{C}}{{\rm{r}}_2}{{\rm{O}}_7} + {{\rm{H}}_2}{\rm{S}}{{\rm{O}}_4} + 3{\rm{S}}{{\rm{O}}_2} \to {{\rm{K}}_2}{\rm{S}}{{\rm{O}}_4} + {\rm{C}}{{\rm{r}}_2}{\left( {{\rm{S}}{{\rm{O}}_4}} \right)_3} + {{\rm{H}}_2}{\rm{O}}\)
2. It reduces acidified potassium permanganate solution and pink colour of \({\rm{KMn}}{{\rm{O}}_4}\) is discharged.
   \(2{\rm{KMn}}{{\rm{O}}_4} + 5{\rm{S}}{{\rm{O}}_2} + 2{{\rm{H}}_2}{\rm{O}} \to {{\rm{K}}_2}{\rm{S}}{{\rm{O}}_4} + 2{\rm{MnS}}{{\rm{O}}_4} + 2{{\rm{H}}_2}{\rm{S}}{{\rm{O}}_4}\)
3. It reduces halogens \(\left( {{\rm{C}}{{\rm{l}}_2},\,{\rm{B}}{{\rm{r}}_2},\,{{\rm{l}}_2}} \right)\) in aqueous solution. To respective halides. Eg.
   \({\rm{C}}{{\rm{l}}_2} + {\rm{S}}{{\rm{O}}_2} + 2{{\rm{H}}_2}{\rm{O}} \to 2{\rm{HCl}} + {{\rm{H}}_2}{\rm{S}}{{\rm{O}}_4}\)
4. It reduces ferric salt to ferrous salt, and colour changes from yellow to light green.
   \(2{\rm{FeC}}{{\rm{l}}_3} + {\rm{S}}{{\rm{O}}_2} + 2{{\rm{H}}_2}{\rm{O}} \to 2{\rm{FeC}}{{\rm{l}}_2} + {{\rm{H}}_2}{\rm{S}}{{\rm{O}}_4} + 2{\rm{HCl}}\)
5. It reduces potassium iodate \(\left( {{\rm{KI}}{{\rm{O}}_3}} \right)\) solution to iodine.
   \(2{\rm{KI}}{{\rm{O}}_3} + 5{\rm{S}}{{\rm{O}}_2} + 4{{\rm{H}}_2}{\rm{O}} \to {{\rm{K}}_2}{\rm{S}}{{\rm{O}}_4} + {{\rm{I}}_2} + 4{{\rm{H}}_2}{\rm{S}}{{\rm{O}}_4}\)

6. As a Bleaching Agent

\({\rm{S}}{{\rm{O}}_2}\) acts as a bleaching agent in the presence of moisture. It can bleach coloured wool, silk, flower, hair, etc. The formation of nascent hydrogen accounts for the bleaching action of sulphur dioxide, which reduces colouring substance to colourless reduced product.

\({\rm{S}}{{\rm{O}}_2} + 2{{\rm{H}}_2}{\rm{O}} \to {{\rm{H}}_2}{\rm{S}}{{\rm{O}}_4} + 2[{\rm{H}}]\)

\({\rm{Colouring}}\,{\rm{matter}} + [{\rm{H}}] \to {\rm{Colourless}}\,{\rm{matter}}\)

The bleaching action of \({\rm{S}}{{\rm{O}}_2}\) is temporary. The bleached colourless compound will gradually regain its original colour standing in the air due to oxidation by air.

Difference Between Sulphur Dioxide and Chlorine

Sulphur dioxide \({\rm{S}}{{\rm{O}}_2}\)	Chlorine \({\rm{C}}{{\rm{l}}_2}\)
1. Sulphur dioxide reacts with water to yield nascent hydrogen, which bleaches coloured substances to the colourless product by reduction. \({\rm{S}}{{\rm{O}}_2} + 2{{\rm{H}}_2}{\rm{O}} \to {{\rm{H}}_2}{\rm{S}}{{\rm{O}}_4} + 2[{\rm{H}}]\)	1. Chlorine reacts with water to yield nascent oxygen, which bleaches coloured substances to the colourless product by oxidation. \({\rm{C}}{{\rm{l}}_2} + {{\rm{H}}_2}{\rm{O}} \to 2{\rm{HCl}} + [{\rm{O}}]\)
2. Bleaching by \({\rm{S}}{{\rm{O}}_2}\) may sometimes be due to the formation of colourless addition products	2. No such colourless addition product is formed with chlorine.
3. Bleaching by \({\rm{S}}{{\rm{O}}_2}\) is reversible, i.e., a temporary process.	3. Bleaching by \({\rm{C}}{{\rm{l}}_2}\) is irreversible, i.e., a permanent process.
4. \({\rm{S}}{{\rm{O}}_2}\) is a mild bleaching agent.	4. \({\rm{C}}{{\rm{l}}_2}\) is a strong bleaching agent.
5. It is used to bleach delicate articles such as wool, silk, cane sugar, etc.	5. It is used to bleach wood, textiles, paper, etc.

Test of Sulphur Dioxide

The reduction of acidified potassium permanganate solution is a convenient test to detect the presence of this gas. The pink colour of \({\rm{KMn}}{{\rm{O}}_4}\) is discharged, which confirms the presence of sulphur dioxide gas.

\(2{\rm{KMn}}{{\rm{O}}_4} + 5{\rm{S}}{{\rm{O}}_2} + 2{{\rm{H}}_2}{\rm{O}} \to {{\rm{K}}_2}{\rm{S}}{{\rm{O}}_4} + 2{\rm{MnS}}{{\rm{O}}_4} + 2{{\rm{H}}_2}{\rm{S}}{{\rm{O}}_4}\)

Uses for Sulfur Dioxide

1. Sulfur dioxide is the primary ingredient for the most commonly used chemicals across the globe, i.e., sulfuric acid.
2. It is the precursor to the synthesis of many sulphites such as sodium hydrogen sulphite, calcium hydrogen sulphite, etc. These sulphites are used in the preservation of jams, pickles, and jellies.
3. Sulfur dioxide is a widely-used pest control product and can be used to disinfect materials, such as wood or straw.
4. It acts as a fumigating agent to eliminate insects and other pests.
5. It is used for refining petroleum and sugar and for bleaching delicate articles such as wool.
6. Sulphur dioxide is also used as antichlor, i.e., for removing excess chlorine from bleached articles.
7. Liquid \({\rm{S}}{{\rm{O}}_2}\) is used as a solvent to dissolve a number of organic and inorganic chemicals.
8. It is also used as a refrigerant.

Summary

Sulphur dioxide is an important greenhouse gas and a major chemical ingredient used to synthesise sulphuric acid. Similar to chlorine, sulphur dioxide also acts as a bleaching agent. In this article, we learnt the formula, structure, preparation, and properties of sulphur dioxide. We also learned some of its uses and how sulphur dioxide’s bleaching action is different from that of chlorine.

FAQs on Sulfur Dioxide

Q.1. Is sulfur dioxide a compound?
Ans: Yes, sulphur dioxide is an inorganic compound. It has a bent structure with polar covalent bonds between the sulphur atom and two oxygen atoms.

Q.2. What is the confirmatory test to detect the presence of Sulphur dioxide gas?
Ans: The reduction of acidified potassium permanganate solution is a convenient test to detect the presence of this gas. The pink colour of \({\rm{KMn}}{{\rm{O}}_4}\) is discharged, which confirms the presence of sulphur dioxide gas.
\(2{\rm{KMn}}{{\rm{O}}_4} + 5{\rm{S}}{{\rm{O}}_2} + 2{{\rm{H}}_2}{\rm{O}} \to {{\rm{K}}_2}{\rm{S}}{{\rm{O}}_4} + 2{\rm{MnS}}{{\rm{O}}_4} + 2{{\rm{H}}_2}{\rm{S}}{{\rm{O}}_4}\)

Q.3. How does Sulphur dioxide form acid rain?
Ans: Acid rain is caused by a chemical reaction that takes place when compounds like sulfur dioxide and nitrogen oxides are released into the air. These compounds react with water, oxygen, and other chemicals to form sulfuric and nitric acids. These acids fall to the ground as acid rain.

Q.4. Is sulfur dioxide harmful to humans?
Ans: Sulphur dioxide is a major greenhouse gas. It causes wheezing, shortness of breath and chest tightness, and other respiratory-related problems.

Q.5. Does sulfur dioxide contribute to global warming?
Ans: Sulfur dioxide, along with carbon emissions, contributes to the formation of aerosols. These compounds directly or indirectly affect the warming and cooling of the earth’s atmosphere. Beyond sulfur dioxide’s environmental impact, this gas also is detrimental to human health.

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