Question

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

• A. In acidic solutions hydration protects copper ions.
• B. In acidic solutions protons coordinate with ammonia molecules forming $N{H}_4^{+}$ ions and $N{H}_3$ molecules are not available.
• C. In alkaline solutions insoluble $Cu(OH{)}_2$ is precipitated which is soluble in excess of any alkali.
• D. Copper hydroxide is an amphoteric substance.

Answer 1

Answer: (B)

In acidic solutions protons coordinate with ammonia molecules forming $N{H}_4^{+}$ ions and $N{H}_3$ molecules are not available.

Ammonia is a base. In acidic medium, it reacts with protons to form ammonium ions. Thus free ammonia molecules are not present. So they cannot coordinate with Cu(II) ion to form the $\left[Cu\right(N{H}_3{)}_4{]}^{2+}$ complex ion.