Copper, Cu2+

Most common oxidation states: +1, +2

M.P. 1083o

B.P. 2582o

Density 8.92 g/cm3

Characteristics: Reddish-yellow, rather inactive metal. Dissolves readily in HNO3 and in hot, concentrated H2SO4.

Characteristic reactions of Cu2+:

The +2 oxidation state is more common than the +1. Copper(II) is commonly found as the blue hydrated ion, [Cu(H2O)4]2+.

Aqueous Ammonia:

Copper(II) ion reacts with stoichiometric quantities of aqueous ammonia to precipitate light blue Cu(OH)2. Some basic salts may also form.

Cu2+(aq) + 2NH3(aq) + 3H2O(l) <==> Cu(OH)2(s) + 2NH4+(aq)

The precipitate dissolves in excess ammonia to form a dark blue complex ion:

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

Sodium Hydroxide:

Sodium hydroxide precipitates copper(II) hydroxide:

Cu2+(aq) + 2OH-(aq) <==> Cu(OH)2(s)

The precipitate does not dissolve in excess sodium hydroxide unless the NaOH solution is very concentrated. However, the precipitate will dissolve upon addition of concentrated ammonia solution.

 Potassium Ferrocyanide:

Potassium ferrocyanide precipitates red-brown copper(II) ferrocyanide from Cu2+ solutions:

2Cu2+(aq) + [Fe(CN)6]4-(aq) <==> Cu2[Fe(CN)6](s)

This test is very sensitive. The precipitate is soluble in aqueous ammonia.

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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