9.8 Sulphur
🎯 Learning outcomes
By the end of this chapter, Students should be able to:
- 9.24 Explain allotropes of sulphur (name only) and uses of sulphur.
- 9.25 Prepare hydrogen sulphide using Kipp’s apparatus.
- 9.26 Explain properties (Acidic nature, reducing nature, analytical reagent) and uses of hydrogen sulphide.
- 9.27 Explain properties of sulphur dioxide (acidic nature, reducing nature, oxidising nature, and bleaching action) and its uses.
- 9.28 Explain sulphuric acid and its properties (acidic nature, oxidising nature, dehydrating nature) and its uses.
- 9.29 Write the formula of sodium thiosulfate and its uses.
Occurrence
Sulphur is present in native form and as sulphide, sulphite, and sulphate ores.
Allotropes of sulphur
- Rhombic or octahedral sulphur (α-sulphur)
- Monoclinic or prismatic sulphur (β-sulphur)
- Plastic sulphur (γ-sulphur)
- Colloidal sulphur (δ-sulphur)
- Milk of sulphur
Uses of sulphur
- One of the essential constituents of the living cell.
- To produce sulphuric acid.
- To prepare gunpowder.
- Sulphur is the most important constituent of sulpha drugs.
- In the preparation of some organo-sulphur compounds which are used in pharmaceuticals and agrochemicals.
Preparation of H₂S gas using Kipp’s apparatus
H₂S gas is formed by the reaction of dilute sulphuric acid with iron sulphide.
Working Principle
- Kipp’s apparatus consists of three bulbs: bulb A, bulb B and bulb C.
- Bulb A communicates with bulb C with a long stem, and bulb C communicates with bulb B with a narrow neck, such that liquids can pass through them. Bulb B is fitted with a stop cock for the delivery of H₂S gas.
- Iron sulphide is kept in bulb B, and H₂SO₄ is passed to bulb C by a long pipe such that the level of acid rises to bulb B.
- When the tap of bulb B is opened, H₂SO₄ reacts with FeS to give H₂S gas.
- But when the tap is closed, the H₂S gas produced exerts pressure, due to which the acid level drops until there is no contact between FeS and H₂SO₄.
Figure: Working mechanism of Kipp’s apparatus
Properties of H₂S
- Colourless gas with a rotten egg smell.
- It is a water-soluble poisonous gas; causes nausea, dizziness and headache in low concentration; in high concentration, it’s deadly.
1. Acidic nature
H₂S is a dibasic acid, so it gives off two series of salts
It combines with oxides to give salts.
2. Action with metals
Some metals such as lead and silver can displace hydrogen from H₂S, forming a sulphide coating on the metal.
3. H₂S as a reducing agent
Since H₂S tends to get oxidised in a chemical reaction, it acts as a reducing agent.
Action with acidified KMnO₄
Action with acidified K₂Cr₂O₇
Action with ferric chloride: H₂S reduces ferric chloride to ferrous chloride
Action with halogens: H₂S reduces halogens into halides.
Action with SO₂: With SO₂, it gives sulphur.
Action with conc. H₂SO₄: With conc. H₂SO₄, it forms sulphur.
With conc. HNO₃, it forms sulphur
4. H₂S as an analytical reagent
H₂S gas is used as an analytical agent to detect group (II) ions and group (IIIB) ions.
When H₂S gas is passed through the solution of group (II) ions in an acidic medium (presence of dil. HCl), the characteristic-coloured ppt. is formed. This shows the presence of metal ions in the given salt.
When H₂S is passed through the solution of the group (III B) metal salts in an alkaline medium (NH₄Cl and NH₄OH), then a characteristic coloured ppt. is formed. This shows the presence of metal ions in the given salt.
Action with lead acetate: When H₂S gas is passed through lead acetate, a black precipitate is formed.
Uses of H₂S
- As a reducing agent
- As an analytical reagent in the laboratory
- To prepare metallic sulphide, which can be used as a pigment.
SO₂ is a colourless gas with a choking smell, highly soluble in water.
1. Acidic nature
SO₂ dissolves in water to form a small amount of sulphurous acid.
Sulphurous acid is a diprotic acid; therefore, it produces two series of salts if reacted with a base.
2. SO₂ as a reducing agent
i. Action with halogens: With halogens, respective halides are formed.
ii. Action with acidified KMnO₄ solution
iii. Action with acidified K₂Cr₂O₇
iv. With ferric sulphate
v. Action with potassium iodate
3. SO₂ as an oxidising agent
Sulphur dioxide acts both as oxidizing as well as reducing agent.
a) Action with H₂S: SO₂ oxidizes H₂S to Sulphur
b) Action with iron: Sulphur dioxide oxidises the iron to iron oxide and iron sulphide.
4. SO₂ as a bleaching agent
Bleaching action of SO₂ is due to:
- Bleaching by Reduction: In the presence of moisture, SO₂ produces nascent hydrogen, which bleaches the colouring matter.
- Bleaching by addition reaction: In some cases, the bleaching action of SO₂ is due to the formation of the addition product.
Comparison of the bleaching action of SO₂ and Cl₂
| Bleaching action of SO₂ | Bleaching action of Cl₂ |
|---|---|
| The bleaching action of SO₂ is due to the reduction or addition of product formation. | The bleaching action of Cl₂ is due to an oxidation reaction. |
| Action with moisture takes place as follows: SO₂ + H₂O → H₂SO₄ + [H] This nascent hydrogen reduces the colouring matter to colourless matter. | Action with moisture takes place as follows: Cl₂ + H₂O → HCl + [O] This nascent oxygen oxidises colouring matter to colourless matter. |
| The bleaching action of SO₂ is temporary. | The bleaching action of Cl₂ is permanent. |
| The original colour can be regained by adding dil. H₂SO₄, if the colour change is due to the addition product. | The original colour cannot be regained by adding dil. H₂SO₄. |
Uses
- In the manufacture of sulphuric and sulphurous acid
- Bleaching agent for delicate articles like wool, silk, straw, hair, etc
- As a disinfectant for killing germs, fungi, moulds, etc.
Chemical properties
1. As a strong dibasic acid, it gives a series of two salts
2. Action with salt
a) Action of salt with dilute sulphuric acid
b) Action of salt with concentrated sulphuric acid
3. As an oxidising agent
i. Action with metals: Metals lying above hydrogen in the electrochemical series can displace hydrogen from dilute acids
With conc. H₂SO₄, however, SO₂ gas is produced.
ii. Action with non-metals
a) Action with C: Sulphuric acid oxidises carbon to carbon dioxide, and itself is reduced to sulphur dioxide.
b) Action with S
c) Action with P
iii. Action with H₂S
iv. Action with HI
4. Sulphuric acid as a dehydrating agent
a. Due to the high affinity of sulphuric acid towards water molecules, it absorbs water of crystallisation from hydrated salts.
b. Conc. H₂SO₄ dehydrates compounds such as formic acid and oxalic acid
c. Concentrated sulphuric acid dehydrates glucose, sucrose, etc., strongly, leaving behind only carbon
d. Conc. H₂SO₄ dehydrates ethyl alcohol, whereby Ethene or ether is formed depending upon the applied temperature.
e. Conc. H₂SO₄ dehydrates HNO₃ giving nitronium ion.
The nitronium ion is used to nitrate benzene to form nitrobenzene. The nitronium ion is an electrophile.
5. Test for sulphuric acid
With barium chloride, it gives a white precipitate of barium sulphate, which is insoluble in dilute HCl.
This reaction is also used for the confirmatory test of sulphate ions (as an acid radical) in salt solutions.
Uses of sulphuric acid
- In the manufacture of fertilisers
- In petroleum refining
- In the manufacture of lead-acid batteries
- In chemical laboratories
- as dehydrating agent
- as an oxidising agent
- as precipitant
Na₂S₂O₃ · 5H₂O
Uses of sodium thiosulfate
- In medicine, as an antidote for cyanide poisoning
- In the estimation of iodine.
- In photography.
- To remove excess chlorine in the bleaching industry (an antichlor).
Test your understanding
Take a short quiz on sulphur and its compounds.