Sulphur

Symbol: S
Atomic mass: 32.07 amu
Atomic number: 16
Electronic configuration: 1s 2s 2p 3s 3p
Molecular formula: S₈
Electronegativity: 2.5

Allotropes of sulphur:

1. Crystalline sulphur
   i. Rhombic sulphur
   ii. Monoclinic sulphur

2. Amorphous sulphur
   i. Plastic sulphur
   ii. Milk of sulphur
   iii. Colloidal sulphur

Uses of sulphur:

• It is used to produce sulphuric acid.
• It is used to prepare gun powder. Gun powder is a mixture of carbon, sulphur and niter (KNO₃).
• It is the most important element of sulphur drugs.
• It is used in the vulcanization of rubber. In the vulcanization process, sulphur is incorporated with rubber and the product obtained has desirable resilience and toughness.
• It is used to prepare lime sulphur spray which is used as an insecticide.
• It is used in pharmaceuticals, dyestuff, and agrochemicals.
• It is used as ointments for skin disease (cosmetics).

Compounds of sulphur

A. Hydrogen sulphide gas (H₂S):

Laboratory preparation of H₂S gas:
Hydrogen sulphide gas is prepared in the lab by the action of dilute acid on iron sulphide.

FeS + H₂SO₄ → FeSO₄ + H₂S
FeS + HCl → FeCl₂ + H₂S

• Conc. H₂SO₄ and conc. HNO₃ cannot be used for the preparation of H₂S gas due to their oxidizing power.
• H₂S gas is collected in the gas jar by upward displacement of air.
• H₂S gas is purified by passing magnesia (MgO) and dried with anhydrous CaCl₂ or P₂O₅.

Preparation of H₂S gas by using Kipp’s apparatus:

Hydrogen sulphide gas is used in the lab for qualitative salt analysis. For this purpose, a small quantity of H₂S is required in frequent analysis. For the intermittent supply of H₂S gas, Kipps apparatus is used.

FeS + H₂SO₄ → FeSO₄ + H₂S

Kipp s apparatus contains three bulbs A, B, and C. Upper bulb A has a long stem that reaches the bottom of bulb C. Middle bulb B contains iron sulphide. The apparatus is made air-tight.
Dilute H₂SO₄ is poured from bulb A till the pieces of FeS are just covered. The acid reacts with FeS and evolves H₂S gas which comes out from the tap when it is opened.
When the tap is closed, the pressure of the gas in bulb B increases which forces the acid into bulb A. As a result, the contact of FeS and acid breaks. The gas evolves again until the tap is opened again.

Physical properties:

• It is a colorless gas with a rotten egg smell.
• It is heavier than air.
• It is soluble in water.
• It is a poisonous gas.
• Its b.p. is -61°C and m.p. is -85°C.

Chemical properties:

1. Acidic nature:

The aqueous solution of H₂S is termed hydrosulphuric acid. It is a weak diprotic acid that ionizes in two steps.

H₂S ⇌ H⁺ + HS^– (Bisulphide ion)
HS^– ⇌ H⁺ + S^{– –} (Sulphide ion)

It reacts with a base to give two types of salt.
H₂S + NaOH → NaHS (Sodium bisulphide) + H₂O
H₂S + NaOH → Na₂S (Sodium sulphide) + 2H₂O

More electropositive metals than hydrogen react with H₂S gas.
Zn + H₂S → ZnS + H₂
2Na + H₂S → Na₂S + H₂

It reacts with basic oxide to give salt.
CaO + H₂S → CaS + H₂O
MgO + H₂S → MgS + H₂O

2. H₂S as reducing agent:

i. Action with acidified potassium permanganate
2KMnO₄ + 3H₂SO₄ → K₂SO₄ + 2MnSO₄ + 3H₂O + 5[O]
[O] + H₂S → H₂O + S ] x 5

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2KMnO₄ + 4H₂SO₄ + 5H₂S → K₂SO₄ + 2MnSO₄ + 8H₂O + S

ii. Action with acidified potassium dichromate
K₂Cr₂O₇ + 4H₂SO₄ → K₂SO₄ + Cr₂(SO₄)₃ + 4H₂O + 3[O]
[O] + H₂S → H₂O + S ] x 3

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K₂Cr₂O₇ + 4H₂SO₄ + 3H₂S → K₂SO₄ + Cr₂(SO₄)₃ + 7H₂O + 3S

iii. Action with FeCl₃
2FeCl₃ + H₂S → 2FeCl₂ + 2HCl + S

iv. Action with halogens
Cl₂ + H₂O → 2HCl + [O]
[O] + H₂S → H₂O + S

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Cl₂ + H₂S → HCl + S

Also,

Br₂ + H₂S → 2HBr + S
I₁ + H₂S → 2HI + S

v. Action with SO2
SO₂ + 2H₂S → 2H₂O + 3S

vi. Action with conc. sulphuric acid
H₂SO₄ → H₂O + SO₂ + [O]
[O] + H₂S → H₂O + S
H₂SO₄ + H₂S → 2H₂O + SO₂ + S

vii. Action with conc. nitric acid
2HNO₃ → 2NO₂ + H₂O +[O]
[O] + H₂S → H₂O + S
2HNO₃ + H₂S → 2H₂O + 2NO₂ + S

H₂S as an analytical reagent

Salt analysis involves qualitative as well as quantitative salt analysis. Qualitative involves the detection of acidic and basic radicals. In the detection of basic radicals by wet ways, H2S plays a key role. So it acts as an analytical reagent in wet test of basic radicals of group II and group IIIB.

i. H₂S as an analytical reagent for the detection of group II basic radicals: Group II basic radicals are precipitated in the form of sulphide in an acidic medium.

ii. H₂S as an analytical reagent for the detection of group IIIB basic radicals:
Group IIIB basic radicals are precipitated in the form of sulphide in an alkaline medium.

Test of H₂S:

H₂S + (CH₃COO)₂Pb (lead acetate) → PbS(black ppt.) + CH₃COOH

Uses of H₂S:

• As a reducing agent.
• As an analytical agent in the laboratory.
• To prepare sulphides used as a pigment.

Sulphur dioxide (SO₂):

Laboratory preparation of SO₂ gas:

In the laboratory, sulphur dioxide gas is prepared by heating copper turnings with concentrated sulphuric acid.

H₂SO₄ → H₂O + SO₂ + [O]
Cu + [O] → CuO
CuO + H₂SO₄ → CuSO₄ + H₂O

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Cu + 2H₂SO₄ → CuSO₄ + 2H₂O + SO₂

SO₂ gas is dried by conc. sulphuric acid.

Physical properties:

• It is a colorless gas with an irritating odour.
• It is soluble in water.
• It is heavier than air.

Chemical properties:

1. Acidic nature:

It gets dissolved in water to form sulphurous acid.
SO₂ + 2H₂O → H₂SO₃

Sulphurous acid is diprotic acid and ionizes in two steps.
H₂SO₃ ⇌ H⁺ + HSO3^–(Bisulphite ion)
HSO3^– ⇌ H⁺ + SO3^{– –} (Sulphite ion)

It reacts with base to give two types of salt.
H₂SO₃ + NaOH → NaHSO₃(Sodium bisulphite) + H₂O
NaHSO₃ + NaOH → Na₂SO₃(Sodium sulphite) + H₂O

2. Reducing nature:

i. Action with halogens
Cl₂ + H₂O → 2HCl + [O]
[O] + SO₂ + H₂O → H₂SO₄

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SO₂ + 2H₂O + Cl₂ → 2HCl + H₂SO₄

ii. Action with acidified potassium permanganate
2KMnO₄ + 3H₂SO₄ → K₂SO₄ + 2MnSO₄ + 3H₂O + 5[O]
[O] + SO₂ + H₂O → H₂SO₄ ] x 5

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

iii. Action with acidified potassium dichromate
K₂Cr₂O₇ + 4H₂SO₄ → K₂SO₄ + Cr₂(SO₄)₃ + 4H₂O + 3[O]
[O] + SO₂ + H₂O → H₂SO₄ ] x 3

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

iv. Action with ferric sulphate:
Fe₂(SO₄)₃ + SO₂ + 2H₂O → 2H₂SO₄ + 2FeSO₄

v. Action with potassium iodate:
2KIO₃ + 5SO₂ + 4H₂O → K₂SO₄ + 4H₂SO₄ + I₂

3. Oxidizing nature:
2H₂S + SO₂ → 2H₂O + S
SO₂ + 2Mg → 2MgO + S
SO₂ + 2Fe → 2FeO + S
SO₂ + 4HI → 2H₂O + 2I₂ + S

4. Bleaching nature:

i. Bleaching by reduction: In presence of moisture, sulphur dioxide gas bleaches vegetable colouring matter.
SO₂ + 2H₂O → H₂SO₄ + 2[H]
Colouring substance + [H] → Colourless substance

ii. Bleaching by addition reaction
Colored compound + SO₂ → Colourless compound

Comparison of bleaching nature of SO₂ and Cl₂:

Sulphur dioxide	Chlorine
– Due to reduction or addition product formation	– Due to oxidation.
– Temporary	– Permanent
– Reduced product regains its original color exposed to air.	– No regain of original colour exposed to light.
– Mild bleaching action.	– Strong bleaching action
– Bleaches wool, silk, sugar	– Bleaches paper, wood, textile

Uses of SO₂:

• Manufacture of sulphuric acid.
• As bleaching agent.
• As a disinfectant, fungicides, and preservation of food.
• As antichlor (removing chlorine).

Some Important Questions

1. List the allotropes of sulphur. Which is more stable?
2. How is H₂S gas prepared by Kipp’s apparatus?
3. Write the action of H₂S with moist Cl₂, SO₂, KMnO₄/H⁺.
4. How does H₂S gas is used as an analytical agent?
5. What happens when SO₂ gas
   a. reacted with moist Cl₂
   b. passed into K₂Cr₂O₇/H⁺
   c. passed into a moist red rose?