Question

Ammonium hydroxide is added in excess to a solution of copper sulphate.

Answer 1

We can answer this question based on the confirmatory test that is done for the detection of the cupric ion in an aqueous solution in accordance with the cation analysis which is a part of the qualitative analysis.

Complete step by step solution:
For solving this question we need to gain some knowledge about the qualitative analysis of the cations. The qualitative analysis deals with the determination of the elemental composition of inorganic salts. Through qualitative analysis, we detect the ions in an aqueous solution of the salt.
We do experimental reactions through which we can see a physical change as well such as precipitation, change in colour of the solution, evolution of a gas etc. in order to detect the type of cation and the anion. For detecting anions we use anion analysis and for detecting cations we use cation analysis.
Let us look into the cation analysis. For detecting cations we have divided them into six groups. Group 2 consists of the cations: ${ Pb }^{ 2+ },{ Cu }^{ 2+ }\quad and\quad { As }^{ 3+ }$
The detection of the cations needs to be done in a stepwise sequence. First, we need to do the tests for Group 0, followed by Group 1, followed by Group 2 and so on.
The preliminary test for Group 2 cations is that on passing ${ H }_{ 2 }S$ gas through the aqueous solution we get precipitates. Both lead and copper ions give a black precipitate while arsenic gives a yellow precipitate. The reaction is shown below:
${ H }_{ 2 }S(g)+{ Cu }^{ 2+ }(aq)\rightarrow CuS(s)\downarrow +2{ H }^{ + }(aq)$
The black precipitate of CuS is dissolved in conc. Nitric acid followed by the addition of an excess of sodium hydroxide solution which gives us a blue solution.
So, if we have a solution of copper sulphate, then in addition to a solution of sodium hydroxide we will first get a pale blue precipitate of copper hydroxide. The reaction is shown below:
${ CuSO }_{ 4 }(aq)+2{ NH }_{ 4 }OH(aq)\rightarrow { Cu(OH) }_{ 2 }(s)\downarrow +{ (NH }_{ 4 }{ ) }_{ 2 }{ SO }_{ 4 }(aq)+4{ H }_{ 2 }O(l)$
To this, if we add ammonium hydroxide in excess, then the precipitate will dissolve to give a deep blue solution of tetraammine copper (II) sulphate. The reaction is given below:
${ Cu(OH) }_{ 2 }(s)+{ (NH }_{ 4 }{ ) }_{ 2 }{ SO }_{ 4 }(aq)+2{ NH }_{ 4 }OH(aq)\rightarrow \left[ Cu{ (NH }_{ 3 }{ ) }_{ 4 } \right] { SO }_{ 4 }(aq)+{ H }_{ 2 }O(l)$
Hence when ammonium hydroxide is added in excess to a solution of copper sulphate we get a deep blue solution of tetraammine copper (II) sulphate.

Note: When the blue solution of tetraammine copper (II) sulphate is treated with potassium ferrocyanide solution, a chocolate brown precipitate is obtained. The chocolate brown precipitate is actually ${ Cu }_{ 2 }\left[ Fe{ (CN) }_{ 6 } \right]$.