Question

Ammonia forms the complex ion ${\left([\text{Cu}({\left(\text{NH}\right)}_{\text{3}}{)}_{\text{4}}]\right)}^{\text{2}+}$ with copper ions in alkaline solution but not in acidic solution. The probable reason for this is

• A. In acidic solution, hydration protects ${\text{Cu}}^{\text{2}+}$ ions
• B. In acidic solution, proton co-ordinates with ammonia molecules to form $N{H}_4^{+}$ ions and ${\text{NH}}_{\text{3}}$ molecules are not available
• C. In alkaline solutions insoluble $\text{Cu}(\text{OH}{)}_{\text{2}}$ is precipitated which is soluble in excess of any alkali
• D. Copper hydroxide is amphoteric substance

Answer 1

Answer: (B)

${\left(Cu\right)}^{2+}(aq)+{\left(NH\right)}_3(l)+H^{+}(aq)\to {\left({\left(NH\right)}_4\right)}^{+}(aq)+{\left(Cu\right)}^{2+}(aq)$
In acidic solution proton coordinates with ammonia molecules to form $N{H}_4^{+}$ does not have a lone pair and hence cannot form co-ordination compounds.
Whereas in alkaline solution $N{H}_3$ molecules are free to form the complex.