Question

Ammonia forms the complex $\left[Cu\right(N{H}_3{)}_4{]}^{2+}$ with copper ions in alkaline solution but not in acidic solution. The reason for this is:

• A. in alkaline solution $Cu(OH{)}_2$ is precipitated which is soluble in excess of alkali
• B. copper hydroxide is amphoteric substance
• C. in acidic solution hydration protects $C{u}^{2+}$ ions
• D. in acidic solution protons are coordinated with ammonia molecules forming $N{H}_4^{+}$ ions

Answer 1

The correct option is D in acidic solution protons are coordinated with ammonia molecules forming $N{H}_4^{+}$ ions

Ammonia acts as a ligand as it donates its lone pair of electrons to $C{u}^{2+}$ ions to form $\left[Cu\right(N{H}_3{)}_4{]}^{2+}$ complex. This is possible in basic medium. In acidic medium, the lone pair of electrons present on ammonia is donated to proton to form ammonium ion which is not a ligand and cannot form a complex with $C{u}^{2+}$ ions.

$C{u}^{2+}\left(aq\right)+4N{H}_3\stackrel{O{H}^{-}}{\rightleftharpoons }\left[Cu\right(N{H}_3{)}_4{]}^{2+}$

$C{u}^{2+}\left(aq\right)+4N{H}_3\stackrel{H^{+}}{\rightleftharpoons }C{u}^{2+}+4N{H}_4^{+}$

Hence, the correct option is $\text{D}$