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Copper(II), Cu2+

Some blue crystals of copper(II) sulphate-5-water

The hexaquo copper(II) ion, [Cu(H2O)6]2+, gives the familiar blue colour to copper(II) sulfate solution.

This ion can be deprotonated by the addition of sodium hydroxide solution to give a gelatinous, blue precipitate of copper(II) hydroxide:

Gelatinous blue precipitate of copper(II) hydroxide

Cu2+(aq) + 2OH-(aq) ⇒ Cu(OH)2(s)

or [Cu(H2O)6]2+(aq) + 2OH-(aq) ⇒ Cu(H2O)4(OH)2(s)

No further reaction takes place on addition of excess alkali so the blue precipitate remains insoluble.

The royal blue tetrammino copper(II) complex

The same blue precipitate forms on addition of ammonia solution (ammonia solution is alkaline as it produces hydroxide ions in aqueous solution). However, in excess ammonia it dissolves due to the formation of the tetrammino copper(II) ion by ligand exchange. The complex is royal blue in colour.

Cu(H2O)4(OH)2(s) + 4NH3(aq) ⇒ [Cu(NH3)4(H2O)2]2+(aq) + 2H2O(l) + 2OH-(aq)

On addition of concentrated hydrochloric acid to the blue, aqueous solution of hexaquo copper(II), the solution first turns yellow, and then green. This is the result of a ligand exchange reaction which produces the blue tetrachloro cuprate(II) ion, [CuCl4]2-. The intermediate yellow colour being a mixture of the blue and green complex ions:

Lime green tetrachlorocuprate(II) complex

[Cu(H2O)6]2+(aq) + 4Cl-(aq)[CuCl4]2-(aq) + 6H2O(l)

Brown iodine solution is present after reaction with a white precipitate of copper(I) iodide settling to the bottom

An interesting redox reaction occurs when a solution of iodide ions is added to an aqueous copper(II) salt. The iodide is oxidized to brown iodine and the copper(II) is reduced to a white precipitate of copper(I) iodide:

2Cu2+(aq) + 4I-(aq)2CuI(s) + I2(aq)