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Zinc, Zn2+

Most common oxidation states: +2

M.P. 420o

B.P. 907o

Density 7.13 g/cm3

Characteristics: Bluish-gray metal. Quite active; burns readily in air to form white ZnO and combines with many nonmetals.

Characteristic reactions of Zn2+:

Zinc(II) ion forms complex ions readily.

Aqueous Ammonia:

Zinc(II) ion reacts with aqueous ammonia to precipitate white gelatinous Zn(OH)2:

Zn2+(aq) + 2NH3(aq) + 2H2O(l) <==> Zn(OH)2(s) + 2NH4+(aq)

The zinc(II) hydroxide precipitate dissolves in excess ammonia:

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

Sodium Hydroxide:

Sodium hydroxide also precipitates zinc(II) hydroxide:

Zn2+(aq) + 2OH-(aq) <==> Zn(OH)2(s)

The zinc(II) hydroxide precipitate also dissolves in excess hydroxide:

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

Potassium Ferrocyanide:

A gray-white precipitate is formed with ferrocyanide ion. The precipitate may be blue-green if traces of iron ions are present:

3Zn2+(aq) + 2K+(aq) + 2[Fe(CN)6]4-(aq) <==> K2Zn3[Fe(CN)6]2(s)

Note: Many metal ions form ferrocyanide precipitates, so potassium ferrocyanide is not a good reagent for separating metal ions. It is used more commonly as a confirmatory test.

No Reaction:

Cl-, SO42-

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