Question #95468

1 Answer
MeneerNask
May 13, 2017

This is a comples-forming reaction

Explanation:

At first you will see copper hydroxide precipitating:
Cu^(2+) (aq) +2OH^(-) (aq) ->Cu(OH)_2 (s)
The OH^- coming from the ammonia-solution. There is an equilibrium reaction between ammonia and water:
NH_3+H_2O rightleftharpoonsNH_4^+ +OH^-

On adding more, the ammonia forms a (soluable) complex ion with the copper ions, and even releases them from the precipitate.

What you will see is that the light blue pecipitate, will dissolve into a darker blue, clear solution.

Note : This complex Cu(NH_3)_6^(2+) is easily reduced to a brown-coloured compound, by for instance glucose. That's why it was used, in the form of Fehling's or Benedict's reagens, for testing urine for glucose (diabetes).

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