Question

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

• A. Copper hydroxide is an amphoteric substance.
• B. In acidic solutions, hydration protects copper ions.
• C. In acidic solutions, protons coordinate with ammonia molecules forming $N{H}_4^{+}$ ions and $N{H}_3$ molecules are no longer available for co-ordination to metal ions.
• D. In alkaline solutions, insoluble $Cu(OH{)}_2$ is precipitated out which is soluble in excess of any alkali.

Answer 1

Answer: (C)

In acidic solutions, protons coordinate with ammonia molecules forming $N{H}_4^{+}$ ions and $N{H}_3$ molecules are no longer available for co-ordination to metal ions.

In acidic solutions, ammonia reacts with proton and forms ammonium ion which further cannot act as a ligand. Also, no free ammonia molecules are available for co-ordination. Hence, it does not form a complex with copper ion.
The reaction is as follows:
$N{H}_3+H^{+}\to N{H}_4^{+}$