The reaction between
ammonia and copper(II) ions
Copper(II) sulphate
solution, for example, contains the blue hexaaquacopper(II) ion - [Cu(H2O)6]2+.
In the first stage of
the reaction, the ammonia acts as a Bronsted-Lowry base. With a small amount of
ammonia solution, hydrogen ions are pulled off two water molecules in the
hexaaqua ion.
This produces a neutral
complex - one carrying no charge. If you remove two positively charged hydrogen
ions from a 2+ ion, then obviously there isn't going to be any charge left on
the ion.
Because of the lack of
charge, the neutral complex isn't soluble in water, and so you get a pale blue
precipitate.
This precipitate is
often written as Cu(OH)2 and called copper(II) hydroxide. The
reaction is reversible because ammonia is only a weak base.
That precipitate
dissolves if you add an excess of ammonia solution, giving a deep blue
solution.
The ammonia replaces
four of the water molecules around the copper to give
tetraamminediaquacopper(II) ions. The ammonia uses its lone pair to form a
co-ordinate covalent bond (dative covalent bond) with the copper. It is acting
as an electron pair donor - a Lewis base.
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