Sulphur Dioxide – Structure, Preparation, Properties, Uses

The p-block, which spans groups 13 to 18, is located on the right side of the normal periodic table. Their electrical configuration is ns² np^1–6 in general. Despite being the first element in group 18, helium is not part of the p-block. Except for the first row, each row in the table has six p-elements. 

The only block with all three categories of elements: metals, nonmetals, and metalloids is this one. On a group-by-group basis, the p-block elements are group 13, icosagens; 14, crystallogens; 15, pnictogens; 16, chalcogens; 17, halogens; and 18, the helium group, which includes noble gases (except helium) and oganesson. The p-block can also be defined as including post-transition metals, metalloids, reactive nonmetals, such as halogens, and noble gases.

Sulphur Dioxide

When sulphur-containing materials, such as metal, are heated, such as when smelting metal or burning coal or oil, sulphur dioxide is produced as a waste gas. It is a significant contaminant that is presently seen as a threat to the environment.

Structure of Sulphur Dioxide

The polar covalent connections between the sulphur atom and two oxygen atoms give sulphur dioxide its twisted structure. SO₂ is the chemical formula for sulphur dioxide. Sulphur dioxide contains sp² hybridised sulphur. The bond angle should ideally be 120°, whereas the O–S–O bond angle is 119°. The lone pair of electrons on the Sulphur atom cause this. The bond angle is reduced from 120° to 119° due to the lone pair-bond pair repulsion.

A sigma (σ) and (π) connection connects each oxygen atom in the Sulphur dioxide molecule to the Sulphur atom. The sp²–p overlap forms the σ bonds between Sulphur and Oxygen atoms, whereas one of the π links is created by pπ–pπ overlap and the other by pπ–dπ overlap. 

The electronic configuration of S is 1s²2s²2p⁶3s²3p⁴.

One electron from the 3p orbital moves to the 3d orbital during the synthesis of SO₂, and S undergoes sp² hybridization. Two of these orbitals have sigma bonds with two oxygen atoms, whereas the third has a single pair. Each of the p-orbital and d-orbital electrons are unpaired. One electron makes a pπ–pπ link with one oxygen atom, while the other forms a pπ–dπ bond with the other. This is why SO₂ has a bent structure with a 143 pm bond length.

Due to resonance, however, both S–O bonds are identical.

Preparation of Sulphur Dioxide

Sulphur dioxide and traces of sulphur trioxide (6–8%) are generated when sulphur is burned in the air.

S(s)+O₂(g) → SO₂(g)

• Laboratory Preparation of Sulphur Dioxide- Sulphur dioxide is made in the lab by reacting sulphites with dilute sulphuric acid. This process also produces sodium sulphate and water as byproducts.

Na₂SO₃(s)+H₂SO₄(aq) → SO₂(g)+Na₂SO₄(aq)+H₂O(l)

Copper turnings are gently heated in a flask with strong sulfuric acid to produce sulphur dioxide.

Cu(s)+2H₂SO₄(aq)→SO₂(g)+CuSO₄(aq)+2H₂O(l)

• Industrial Preparation of Sulphur Dioxide- Sulphur dioxide is generated commercially as a byproduct of roasting sulphide ores like Iron Pyrites or Zinc blend.

4FeS₂(s)+11O₂(aq)→2Fe₂O₃(s)+8SO₂(g)

Physical Properties of Sulphur Dioxide

• Sulphur dioxide is a colourless, poisonous gas with a terrible, stifling odour.
• It has a lower density than air and is easily soluble in water. At 0°C, one volume of water may dissolve approximately eight litres of gas.
• It is easily liquefied at ambient temperature and at 2 atm pressure.
• It has a boiling temperature of 263K and a freezing point of 197.8 K.
• It’s a non-flammable gas that serves as a solvent for phosphorus, sulphur, and iodine, among other things.

Chemical Properties of Sulphur Dioxide

• Acidic Character of Sulphur Dioxide- Sulphur dioxide reacts with water to generate sulphurous acid. Its acidic aqueous solution turns blue litmus crimson. As a result, SO₂ is classified as a sulphurous acid anhydride.

SO₂(g)+H₂O(l)→H₂SO₃(aq)

• Sulphur dioxide quickly combines with sodium hydroxide solution to produce sodium sulphite, which then reacts with additional sulphur dioxide to produce sodium hydrogen sulphite.

2NaOH(aq)+SO₂(g)→Na₂SO₃(aq)+H₂O(l)

Na₂SO₃(aq)+H₂O(l)+SO₂(g)→2NaHSO₃(aq)

• The solution turns milky when sulphur dioxide gas is bubbled through lime water. The milkiness goes away when you pass too much sulphur dioxide gas because calcium bisulphite forms.

Ca(OH)₂(aq)+SO₂(g)→CaSO₃(aq)+H₂O(l)

CaSO₃(aq)+H₂O(l)+SO₂(g)→Ca(HSO₃)₂(aq)

• Combination with Oxygen- When sulphur dioxide is heated, it interacts with oxygen to generate sulphur trioxide.

This reaction takes place in the presence of catalysts such as platinized asbestos, vanadium pentoxide, and others. When sulphur dioxide is heated, it interacts with oxygen to generate sulphur trioxide.

•  Reaction with Halogens- Sulphur dioxide becomes sulphuryl fluoride, chloride, and bromide when it reacts directly with the halogens (fluorine, chlorine, and bromine). For example, In the presence of charcoal, SO₂ reacts with chlorine to form sulphuryl chloride. The reaction is aided by the use of charcoal as a catalyst.

SO₂(g)+Cl₂(g)→SO₂Cl₂(l)

• As an Oxidising Agent- SO₂ oxidises powerful reducing agents like H₂S, HI, Mg, Fe, etc., and itself gets reduced to sulphur or sulphide.

SO₂(g)+2H₂S(g)→2H₂O(l)+3S

• SO₂ as a Reducing Agent- Aqueous SO₂ shows a reducing character and itself gets oxidised to H₂SO₄.

Acidified orange potassium dichromate solution is reduced to light green chromium sulphate.

K₂Cr₂O₇+H₂SO₄+3SO₂→K₂SO₄+Cr₂(SO₄)₃+H₂O

It lowers the acidified potassium permanganate solution, releasing the pink colour of KMnO₄.

2KMnO₄+5SO₂+2H₂O → K₂SO₄+2MnSO₄+2H₂SO₄

• As a Bleaching Agent- In the presence of moisture, SO₂ functions as a bleaching agent. It may bleach coloured wool, silk, flowers, and hair, among other things. The bleaching effect of sulphur dioxide is accounted for by the creation of nascent hydrogen, which converts the colouring ingredient to a colourless reduced result.

SO₂+2H₂O → H₂SO₄+2[H]

SO₂‘s bleaching effect is quite transient. Due to oxidation by air, the bleached colourless chemical will gradually recover its original colour standing in the air.

Test of Sulphur Dioxide

The existence of this gas can be detected by reducing an acidified potassium permanganate solution. The pink colour of KMnO₄ is released, indicating that sulphur dioxide gas is present.

2KMnO₄+5SO₂+2H₂O → K₂SO₄+2MnSO₄+2H₂SO₄

Uses for Sulphur Dioxide

1. Sulphur dioxide is the main component of sulphuric acid, one of the most widely used chemicals on the planet.
2. It is used to make a variety of sulphites, including sodium hydrogen sulphite, calcium hydrogen sulphite, and others. These sulphites are used to keep jams, pickles, and jellies fresh.
3. Sulphur dioxide is a common pesticide that may also be used to sterilise materials like wood or straw.
4. It kills insects and other pests by acting as a fumigating agent.
5. It’s used to refine petroleum and sugar, as well as bleach sensitive fabrics like wool.
6. Sulphur dioxide is also employed as an antichlor or a substance that removes excess chlorine from bleached materials.
7. A variety of organic and inorganic compounds are dissolved using liquid SO₂.
8. It’s also utilised to keep things cool.

Sample Questions

Question 1: How does Sulphur dioxide form acid rain?

Answer:

When molecules like sulphur dioxide and nitrogen oxides are released into the air, a chemical reaction occurs, resulting in acid rain. Sulfuric and nitric acids are formed when these substances combine with water, oxygen, and other chemicals. Acid rain is formed when these acids fall to the earth.

Question 2: What are the uses of sulphur dioxide?

Answer:

• Sulfur dioxide is the primary component of sulfuric acid, which is one of the world’s most commonly used chemicals.
• It’s utilised to create sulphites like sodium hydrogen sulphite and calcium hydrogen sulphite, among other things. Sulphites are employed to preserve the freshness of jams, pickles, and jellies.
• Sulfur dioxide is a widely used insecticide that may also be used to sterilise materials such as wood or straw.
• It acts as a fumigating agent, killing insects and other pests.
• It’s utilised in the refinement of petroleum and sugar, as well as the bleaching of delicate materials such as wool.
• Antichlor, or a chemical that eliminates excess chlorine from bleached materials, is also used with sulphur dioxide.

Question 3: What are the physical properties of sulphur dioxide?

Answer:

1. Sulphur dioxide is a colourless, toxic gas having a foul, suffocating odour.
2. Its density is lower than that of air, and it is easily soluble in water. One litre of gas may be dissolved in one litre of water at 0°C.
3. At 2 atm pressure and ambient temperature, it is quickly liquefied.
4. It has a boiling point of 263K and a freezing point of 197.8K .
5. It’s a non-flammable gas that may be used to dissolve phosphorus, sulphur, and iodine, among other substances.

Question 4: What will happen when sulphur dioxide reacts with chlorine?

Answer:

Sulphuryl chloride is formed when SO₂ interacts with chlorine in the presence of charcoal. The use of charcoal as a catalyst aids the process.

SO₂(g) + Cl₂ (g) → SO₂Cl₂ (l)

Question 5: What will happen when sulphur dioxide reacts with sodium hydroxide?

Answer:

Sulphur dioxide reacts fast with sodium hydroxide solution to form sodium sulphite, which is then combined with more sulphur dioxide to form sodium hydrogen sulphite.

2NaOH (aq) + SO₂ (g) → Na₂SO₃ (aq) + H₂O(l)

Na₂SO₃ (aq) + H₂O (l) + SO₂ (g) → 2NaHSO₃ (aq)

Question 6: What is the confirmatory test to detect the presence of Sulphur dioxide gas?

Answer

The existence of this gas can be detected by reducing an acidified potassium permanganate solution. The pink colour of KMnO₄ is released, indicating that sulphur dioxide gas is present.

2KMnO₄+5SO₂+2H₂O → K₂SO₄+2MnSO₄+2H₂SO₄