Modern inorganic chemistry

GROUP VI 283

Hydrogen sulphide is slightly soluble in water, giving an approxi-

mately 0.1 M solution under 1 atmosphere pressure; it can be

removed from the solution by boiling. The solution is weakly acidic

and dissolves in alkalis to give sulphides and hydrogensulphides.

The equilibrium constants

H 2 S + H 2 O = H 3 O+ + HS~;Ka = 8.9 x 10"^8 moll™^1 at 298 K

HS~ + H 2 O = H 3 O+ + S^2 ~;Ka= 1.2 x 10"^13 moll" * at 298 K

indicate that both normal and acid salts will be hydrolysed

Hydrogen sulphide is a reducing agent in both acid and alkaline

solution as shown by the following examples :

1. Its aqueous solution oxidises slowly on standing in air
   depositing sulphur.


2. It reduces the halogen elements in aqueous solution depositing
   sulphur :
   C1 2 + H 2 S -> 2HC1 + Si


3. It reduces sulphur dioxide, in aqueous solution :

2H 2 S + SOi" + 2H+ -> 3H 2 O + 3Si

4. In acid solution, dichromates(VI) (and also chromates(VI)
   which are converted to dichromates) are reduced to chromium(HI)
   salts:
   Cr 2 O^~ + 8H+ + 3H 2 S -> 2Cr3+ + 7H 2 O

(Hence the orange colour of a dichromate is converted to the green

colour of the hydra ted ehromium(III) ion, Cr3+, and sulphur is

precipitated when hydrogen sulphide is passed through an acid

solution.)

5. In acid solution, the manganate(VII) ion is reduced to the
   manganese(II) ion with decolorisation :

IMnOJ + 5H 2 S + 6H + -> 5S| + 8H 2 O -4- 2Mn^2 +

6. Iron(III) is reduced to iron(II) :

2Fe3+ + H 2 S -^ 2Fe^2 + -f 2H+ -f- S|

Hydrogen sulphide reacts slowly with many metals (more rapidly
if they are heated) to yield the sulphide of the metal and (usually)
hydrogen, for example the tarnishing of silver.
Since most metallic sulphides are insoluble, many are precipitated
when hydrogen sulphide is passed through solutions containing ions
of the metals. Some are precipitated in acid, and others in alkaline