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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) → FeCl2(aq) + H2S(g)

FeS(s) + H2SO4(aq) → FeSO4(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) + 2C2H4O2(aq)

Pb(NO3)2(aq) + H2S(g) → PbS(s) + 2HNO3(aq)

Properties of H2S

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

8HNO3(aq) + H2S(g) ® H2SO4(aq) + 8NO2(g) + H2O(l)

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

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

2FeCl3(aq) + H2S(g) → 2FeCl2(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) + H2SO4(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) + SO2(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.

Cr2O72-(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.   

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