iitdreams.com

 Print this page  Email this Page | Discuss Sulphides

SULPHIDES AND THIOSULPHATES

HYDROGEN SULPHIDE, H₂S

It is a colourless, gaseous sulphide and has a characteristic smell of rotten eggs. Other characteristics are

1. Relative density 1.54
2. It is slightly heavier than air
3. Melting point -85.5^OC
4. Boiling point -60.7^OC
5. Moderately soluble in water
6. It is toxic and causes headaches when inhaled.

PREPARATION

In the laboratory, H₂S is usually prepared by the action of dilute HCl or H₂SO₄ on ferrous sulphide in Kipp’s apparatus :

FeS + H₂SO₄ è FeSo₄ + H₂S

CHEMICAL PROPERTIES

1. In the excess of oxygen, the gas burns with a blue flame forming SO₂.

2H₂S + 3O₂ è 2H₂O + 2SO₂

 and in a limited supply, sulphur is deposited :

2H₂S + O₂ è 2H₂O + 2S

 

2. Reducing Action

• It reduces conc. H₂SO₄ to SO₂ and sulphur

H₂SO₄ + H₂S è 2H₂O + SO₂ + S

• It reduces ferric ions and is oxidised to sulphur

2FeCl₃ + H₂S è 2FeCl₂ + 2HCl + S

or 2Fe³⁺ + H₂S è 2Fe²⁺ + 2H⁺ + S

3. It reduces acidified KMnO₄ solution and is oxidised to sulphur

2KMnO₄ + 3H₂SO₄ + 5H₂S è K₂SO₄ + 2MnSO₄ + 8H₂O + 5S

 As a weak acid

H₂ è H⁺ + HS^-

HS^- è H⁺ + S^3-

6. It reacts with alkalies to form sulphides

H₂ + 2NaOH è Na₂S + 2H₂O

TESTS FOR CONFIRMING HYDROGEN SULPHIDE

1. It gives black colour with lead acetate paper

(CH₃COO)₂ Pb + H₂S è PbS ( black) + 2CH₃COOH

USES

• It is used as an important laboratory reagent in qualitative analysis, for the detection of basic radicals
• As a reducing agent
• In the preparation of metallic sulphides

SODIUM THIOSULPHATE

Sodium Thiosulphate, Na₂S₂O₇, it is also called Hypo. It is charecterised by

1. Relative density 1.73
2. Melting point 42^OC
3. Colourless and crystalline
4. Shows efflorescence
5. It is highly soluble in water, it forms a supersaturated solution in water.

PREPARATION

1. It is prepared by boiling Na2SO3 solution with flowers of sulphur in the absence of air.

Na₂SO₃ + S è Na₃S₂O₃

The solution is filtered to remove excess sulphur and the filtrate is evaporated to give the crystals of hypo.

2.SPRINGS REACTION

Hypo is obtained when a mixture of sodium sulphide and sodium sulphite is treated with iodine.

N₂S + I₂ + Na₂S₂O₃ è 2NaI + Na₂S₂O₃

2. On a large scale it is prepared by passing Sulphur dioxide gas into a strong solution of Na₂S.

2Na₂S + 3SO₂ è 2Na₂S₂O₃ + S

CHEMICAL PROPERTIES

1. With dilute HCl or H₂SO₄ it gives a white precipitate of amorphous sulphur and sulphur dioxide gas is evolved.

Na₂S₂O₃ + H₂SO₄ è Na₂SO₄ + SO₂ + H₂O + S

Na₂S₂O₃ + 2HCl è 2NaCl + SO₂ + H₂O + S

2. Action of heat

On heating at 215^OC it completely loses its water of crystallisation.

4Na₂S₂O₇ è Na₂S₃ + 3Na₂SO₄

3. It decolourises a solution of iodine

I₂ + 2Na₂S₂O₃ è Na₂S₄O₆ + 2NaI

NOTE: this property of hypo is used in volumetric analysis in the quantitative estimation of iodine (iodometric titrations).

Discuss Sulphides