secondary school chemistry – Hydrogen sulphide

Hydrogen sulphide (H2S)

It is very poisonous
Occurs naturally in some deposits mixed with natural gas
Lab preparation
Reacting a metal sulphide with a dilute acid
Such as hydrochloric acid and iron (II) sulphide

FeS (s) +2HCl (aq)   →    FeCl2 (aq) + H2S (g)

The gas is collected over warm water since it dissolves in cold water
The gas can be dried using anhydrous calcium chloride in a u-tube
Other drying agents react with the gas

3H2S (s) +H2SO4 (aq)   →    4S (s) + 4H2O (l)

Physical properties
• Colourless
• Rotten egg smell
• Very poisonous
• Slightly soluble in water
• Denser than air

Chemical properties
a) Reaction with water
Dissolves in water forming a weak acidic solution H2S (aq)
H2S (aq)     →    2H+ (aq) + S2- (aq)

Aqueous hydrogen sulphide is a weak dibasic acid and forms two types of salts (acid salt and normal salt)
b) Reaction with oxygen (burning)
Burns with a pale blue flame in air
In limited supply of air sulphur and water are formed

2H2S (g) +O2 (g)   →    2S (s) + 2H2O (l)

In excess air sulphur (IV) oxide and water are formed

2H2S (g) + 3O2 (g)   →    2SO2(s) + 2H2O (l)

c) Reactions as a reducing agent
i. Reaction with bromine water
Reddish brown bromine water is reduced to colourless hydrogen bromide solution

H2S (g) +Br2 (aq)   →    S (s) + 2HBr (aq)

ii. Reaction with iron (III) chloride
Yellow iron (III) chloride is reduced to green iron (II) chloride

H2S (g) +2FeCl3 (aq)   →    S (s) + 2HCl (aq) +2FeCl2 (aq)

iii. Reaction with acidified potassium manganate(VII)
Acidified purple potassium manganate (VII) is reduced to colourless manganese (II) ions


iv. Reaction with acidified potassium dichromate(VI)
Acidified orange chromate (VI) is reduced to green chromium (III) ions

v. Reaction with hydrogen peroxide
Hydrogen peroxide is reduced to water

3H2S (g) + H2O2 (aq) + 8H+ (aq)   →    2H2O (l) + S (s)

vi. Reaction with concentrated nitric (V) acid
Concentrated nitric (V) acid is reduced to water

H2S (g) + 2HNO3 (aq) →2NO2 (g) + 2H2O (l) + S (s)

vii. Reaction with concentrated sulphuric (VI) acid
Concentrated sulphuric (VI) acid is reduced to sulphur

3H2S (g) + H2SO4 (aq)   →  4H2O (l) + 4S (s)

Note: in all the cases hydrogen sulphide is oxidised to yellow sulphur powder
d) Reaction with metallic ions
Combines with metallic ions forming black metal sulphides

S2- (aq) + Cu2+ (aq)   →  CuS(s)

S2- (aq) + Zn2+ (aq)   →   ZnS (s)

S2- (aq) + Pb2+ (aq)      →  PbS (s)

S2- (aq) + Fe2+ (aq)     →  FeS(s)

e) Reaction with alkalis
Being a dibasic acid it forms two types of salts
When equal molar solutions of hydrogen sulphide and sodium hydroxide are reacted an acid salt is formed

H2S (g) + NaOH (aq)   → NaHS (aq) + H2S (l)

When excess sodium hydroxide is used a normal salt is formed

H2S (g) + 2NaOH (aq)   → Na2S (aq) + 2H2S (l)