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Hydrogen Sulphide, H₂S

Hydrogen sulphide is a gaseous substance whose molecules contains only the elements of hydrogen and sulphur in a ratio of two atoms of hydrogen to one atom of sulphur.

Preparation of Hydrogen Sulphide, H₂S

The most convenient way of preparing H₂S is by reacting pyrite, i.e. iron(II) sulphide, FeS, with dilute HCl, or with dilute H₂SO₄.

The gas is collected above water. If required dry, it is passed through fused calcium chloride.

FeS_(s) + 2HCl_(aq) → FeCl_2(aq) + H₂S_(g)

FeS_(s) + H₂SO_4(aq) → FeSO_4(aq) + H₂S_(g)

Note: the preparation is done in a fume cupboard because hydrogen sulphide gas is very poisonous.

Other metallic sulphides that may be used are: ZnS and Na₂S.

Solubility of Sulphides

Below is a summary of the solubility of some sulphides in both water and acidified solutions:

Sulphides of K and Na: Soluble in water and acidified solutions

Sulphides of Ca, Mg, Zn, and Fe: Not soluble in water, but form coloured precipitate. However, they are soluble in acidified solutions.

Sulphides of Pb, Cu, and Hg: Not soluble in water, but form coloured precipitate. They are also not soluble in acidified solutions, but form coloured precipitate.

Test for H₂S

Hydrogen Sulphide can easily be identified by bringing a strip of filter paper soaked in Lead(II) ethanoate solution or Lead(II) nitrate(V) solution into the gas.

H₂S turns the paper dark brown or black. The change in colour is as a result of formation of precipitate of black lead(II) Sulphide.

Pb(C₂H₃O₂)_2(aq) + H₂S_(g) → PbS_(s) + 2C₂H₄O_2(aq)

Pb(NO₃)_2(aq) + H₂S_(g) → PbS_(s) + 2HNO_3(aq)

Hydrogen sulphide shows the following physical and chemical properties:

Physical Properties

1. It is colourless and smells like rotten egg - it is actually produced when eggs get rotten and generally from decaying substances composed of sulpur, example, decaying cabbages.

2. It is fairly soluble in water, producing weakly acidic solution.

3. It is denser than air (v.d = 17).

Chemical Properties

1. As a reducing agent

H₂S is a very powerful reducing agent: Note that the usual product of its reducing ability is sulphur (it is oxidized to sulphur).

This is unlike reducing action of SO₂. However, very powerful oxidizing agents, like dilute HNO₃ may convert H₂S to H₂SO₄

(a). 2HNO_3(aq) + H₂S_(g) → 2H₂O(l) +2NO_2(g) + S_(s)

8HNO_3(aq) + H₂S_(g) ® H₂SO_4(aq) + 8NO_2(g) + H₂O_(l)

(b). Reaction of H₂S with iron(III) chloride solution (reddish brown)

The iron(III) is reduced to iron(II) (pale green).

2FeCl_3(aq) + H₂S_(g) → 2FeCl_2(aq) + 2HCl_(aq) + S_(s)

(c). Reaction with concentrated H₂SO₄

H₂S reduces conc. H₂SO₄ to deposit sulphur. This is why concentrated H₂SO₄ is not used in drying H₂S.

3H₂S_(g) + H₂SO_4(aq) → 4H₂O_(l) + 4S_(s)

(d). Reaction with SO₂

H₂S is a more powerful reducing agent than SO₂, hence it reduces SO₂ to S.

2H₂S_(g) + SO_2(g) → 2H₂O_(l) + 3S_(s)

(e). With acidified KMnO₄

H₂S decolorizes KMnO₄, as it reduces it and deposits sulphur.

2MnO^-_4(aq) + 5H₂S_(g) + 6H⁺_(aq) → 2Mn²⁺_(aq) + 8H₂O_(l) + 5S_(s)

Also, with acidified K₂Cr₂O₇

H₂S changes the colour of the K₂Cr₂O₇ from yellow to green (due to its reduction), while sulphur is deposited.

Cr₂O₇^2-_(aq) + 3H₂S_(g) + 8H⁺_(aq) → 2Cr³⁺_(aq) + 7H₂O_(l) + 3S_(s)

2. As an acid

H₂S is dibasic and acts as a weak acid. It forms two possible salts with alkalis .

Example, H₂S_(g) + 2NaOH_(aq) → Na₂S_(aq) + 2H₂O_(l) with excess H₂S, the acid salt is formed.

H₂S_(g) + NaOH_(aq) → NaHS_(aq) + H₂O_(l) 

The NaHS formed is an acid salt.

3. As a precipitating agent

H₂S can be used to precipitate copper or lead in acidified solutions as copper(II) and lead(II) sulphide respectively. This is because the sulphides of metals in acidified solutions are precipitated.

Cu²⁺_(aq) + H₂S_(g) → CuS_(s) + 2H⁺_(aq) 

Note: CuS forms as a black precipitate.

Pb²⁺_(aq) + H₂S_(g) → PbS_(s) + 2H⁺_(aq)

Note: PbS forms as a dark-brown, almost black precipitate.