Question

Ammonia forms the complex ion $\left(\left[\right. \text{Cu} \left(\right. \left(\text{NH}\right)_{\text{3}} \left.\right)_{\text{4}} \left]\right.\right)^{\text{2} +}$ 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 $NH_{4}^{+}$ ions and $\text{NH}_{\text{3}}$ molecules are not available
• C. In alkaline solutions insoluble $\text{Cu} \left(\right. \text{OH} \left.\right)_{\text{2}}$ is precipitated which is soluble in excess of any alkali
• D. Copper hydroxide is amphoteric substance

Answer 1

Answer: (B)

$\left(Cu\right)^{2 +}\left(\right. aq \left.\right)+\left(NH\right)_{3}\left(\right. l \left.\right)+H^{+}\left(\right. aq \left.\right) \rightarrow \left(\left(NH\right)_{4}\right)^{+}\left(\right. aq \left.\right)+\left(Cu\right)^{2 +}\left(\right. aq \left.\right)$
In acidic solution proton coordinates with ammonia molecules to form $NH_{4}^{+}$ does not have a lone pair and hence cannot form co-ordination compounds.
Whereas in alkaline solution $NH_{3}$ molecules are free to form the complex.