Question

The temperature of the nickel equilibrium changed when acid was added. Explain the source of the change.

Answer 1

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, NH3. 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:

[Cu(H2O)4]2+(aq) + 4NH3(aq) ? [Cu(NH3)4]2+(aq) + 4H2O(l)
[Ni(H2O)6]2+(aq) + 6NH3(aq) ? [Ni(NH3)6]2+(aq) + 6H2O(l)

Addition of strong acid, H+, affects these equilibria by its reaction with ammonia (a base) on the left sides of the equations.
NH3(aq) + H1+(aq) ? NH41+(aq)

The ammonia is removed from the equilibria, and the reactions shift left to relieve the stress caused by the removal of the ammonia, re-forming the aqueous Cu2+ (sky blue) and Ni2+(green) solutions. For copper ions, this equilibrium shift may be represented as
?
[Cu(H2O)4]2+(aq) + 4NH3(aq) ? [Cu(NH3)4]2+(aq) + 4H2O(l)
? 4H+(aq)
4NH4+(aq)