Account for the effects of NH_3(aq) and HCI(aq) on the CuSO_4 or NiCl_2 solution

Question

Account for the effects of NH_3(aq) and HCI(aq) on the CuSO_4 or NiCl_2 solution. Use equations 16.2-5 in your explanation Metal-Ammonia Ions. Aqueous solutions of copper ions and nickel ions appear sky blue and green, respectively. The colors of the solutions change, however, in the presence of added ammonia. NH_3. Because the metal-ammonia bond is stronger than the metal-water bond, ammonia substitution occurs and the following equilibria shift right, forming the metal-ammonia complex ions:^1 Addition of strong acid, H^+ affects these equilibria by its reaction with ammonia (a base) on the left side of the equations: The ammonia being removed from the equilibria causes the reactions to shift left to relieve the stress caused by the removal of the ammonia, re-forming the aqueous Cu (sky blue) and Nr^2+ (green) solutions. For copper ions, this equilibrium shift may be represented as

Explanation / Answer

In the aqueous solution of the copper sulphate Cu2+ ion is present. The soluble ammonium hydroxide produces ammonium ion and hydroxide ions in solution in equilibrium with ammonia in water. This ammonia molecules react with the copper ion to produce complex ion of Cu2+ coordinated with 4NH3 molecules. As the metal ammonia bond is stronger than metal water bond (due to loan pair donating efficiency of ammonia is higher in alkaline medium) the preferred complex is Cu(NH3)42+ formed and the colour changes from light blue to dark blue.

When HCl is added it introduces H+ ion in the solution which blocks the loan pair of amonia forming NH4+ ion causing release of all the NH3 ligands from Cu(NH3)42+ complex. Blocking of loan pair of ammonia causes huge decrease in its coordination efficiency. But this H+ ions can not block the loan pair of oxygen atom in water molecue (due to higher electronegativity of O-atom) and so in acidic medium water molecule have higher coordination efficiency over ammonia molecule. This means that the addition of HCl nutralizes the previously added ammonia and reverses the reaction and produces the copper-aqueous complex again and so the colour changes back to light blue.

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