Question

State why an aqueous solution of ammonia is used for identifying cations.Anyone answering first will be marked as brainiest.

Answer 1

In the case of the d block metals (all commonly referred to as transition metals, although according to the definition zinc is not a transition metal), the reactions of their aqueous ions with sodium hydroxide and ammonia solutions are due, in some cases, to ligand exchange reactions. The d block metals form metal aquo complexes, like that shown below, in water:


In this complex, water acts as a ‘ligand’, binding to the central metal atom. But water isn’t the only molecule that can act as a ligand; both OH- ions and ammonia can also do so. In the case of hydroxide ions, they can react with the metal aquo complex and take the place of one or more of the ligands. When this happens, the metal hydroxide can be formed. Only group 1 metal hydroxides are soluble in aqueous solution, so hydroxides of any other metals form solid precipitates. Also, only transition metals have the partially filled d orbitals required to appear coloured, so the hydroxides of non-transition metals are colourless.

Ammonia can act as a base as well as a ligand. Added in a small amount, it acts as a base; it reacts with hydrogen ions in water to produce ammonium ions, with the metal aquo complex losing a hydrogen to the solution from one of its water ligands to form the metal hydroxide – so initially the result seen is the same as for with OH- ions.

However, with excess ammonia, we see ligand exchange reactions occur, where ammonia molecules can take the place of the water molecules around the metal ion. This can lead to the drastic colour changes seen in the graphic above. The ligand exchange process works better for some metals than others, which is why we don’t see a drastic change for all of the metals shown.

As was discussed in the previous transition metal graphic, different ligands can alter the colour of transition metal complexes as they affect the splitting in energy of the d orbitals, in turn affecting the wavelengths of light which the complex absorbs.

Answer 2

Answer:

Ammonia is used to identification of cations because it gives deep color with metal due to formation of complex and its solubility.

Explanation:

Ammonia behaves as a Lewis base because of the presence of one lone pair on the nitrogen atom. Therefore, ammonia can form coordinate bond cations or transition metal cations to from respective complex.

When copper salt such as copper sulphate is added in the ammonia.

Ammonia reacts with a solution of Cu²⁺ ions and forms a deep blue colored complex.

CuSO₄  +  NH₄OH  $\longrightarrow$   Cu(OH)₂   + (NH₄)₂SO₄    

Ammonia is a strong ligand. It displaces OH group and forms a stable tetraamine complex.

Cu(OH)₂   + (NH₄)₂SO₄   $\longrightarrow$   [Cu(NH₃)₄]SO₄   +  4H₂O

The deep blue solution is due to formation of the complex ion of copper with ammonia.

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