Cadmium, Cd2+

Most common oxidation state: +2

M.P. 321o

B.P. 767o

Density 8.65 g/cm3

Characteristics: Silvery, crystalline metal, resembling zinc. Moderately active. Cd2+ is colorless in solution and forms complex ions readily.

Characteristic reactions of Cd2+:

Aqueous Ammonia:

Aqueous ammonia reacts with cadmium ion to precipitate white cadmium hydroxide, which dissolves in excess ammonia:

Cd2+(aq) + 2NH3(aq) + 2H2O(l) <==> Cd(OH)2(s) + 2NH4+(aq)

Cd(OH)2(s) + 4NH3(aq) <==> [Cd(NH3)4]2+(aq) + 2OH-(aq)

Addition of 6 M NaOH to a solution of Cd(NH3)42+ precipitates a white basic salt of unknown formula. This salt is not soluble in ammonia.

Sodium Hydroxide:

Sodium hydroxide produces a precipitate of Cd(OH)2, but the precipitate does not dissolve in excess hydroxide:

Cd2+(aq) + 2OH-(aq) <==> Cd(OH)2(s)

 

Hydrogen Sulfide:

Hydrogen sulfide reacts with cadmium ion to precipitate yellow-orange cadmium sulfide from basic, neutral, or weakly acidic solutions:

Cd2+(aq) + H2S(aq) <==> CdS(s) + 2H+(aq)

Cd2+(aq) + HS-(aq) <==> CdS(s) + H+(aq)

Cd2+(aq) + S2-(aq) <==> CdS(s)

Cadmium sulfide is soluble in hot dilute nitric acid:

3CdS(s) + 8H+(aq) + 2NO3-(aq) <==> 3Cd2+(aq) + 2NO(g) + 4H2O(l) + 3S(s)

Cadmium sulfide is also soluble in 3 M HCl and in hot, dilute H2SO4.

 No Reaction:

Cl-, SO42-

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