Question

Aqua regia is used for dissolving noble metals $(\mathrm{Au},\mathrm{Pt},\mathrm{etc}).$The gas evolved in this process is;

• A. ${\mathrm{N}}_2{\mathrm{O}}_2$
• B. ${\mathrm{N}}_2$
• C. ${\mathrm{N}}_2{\mathrm{O}}_5$
• D. $\mathrm{NO}$

Answer 1

The correct option is D

$\mathrm{NO}$

Answer : (D)

Explanation for correct option:

Step 1:

Aqua regia $=3$ parts of $\mathrm{HCl}$ $+$1 part of ${\mathrm{HNO}}_3$

Used as a solvent for inert metals $(\mathrm{Au},\mathrm{Pt})$

Step 2:

The below reaction represent the reaction of gold and aqua regia to release Nitric oxide.

Step 3:

This is an oxidation-reduction (redox) reaction:

$$\begin{gathered} \mathrm{Au}°-3{\mathrm{e}}^{-}\to {\mathrm{Au}}^{\mathrm{III}} \\ \left(\mathrm{oxidation}\right) \\ {\mathrm{N}}^{{}^{\mathrm{V}}}+3{\mathrm{e}}^{-}\to {\mathrm{N}}^{\mathrm{II}} \\ \left(\mathrm{reduction}\right) \end{gathered}$$

$\mathrm{Au}$ is a reducing agent, ${\mathrm{HNO}}_3$ is an oxidizing agent

Step 4:

$$\begin{gathered} 3\mathrm{Pt}\left(\mathrm{s}\right)+4{\mathrm{HNO}}_3\left(\mathrm{aq}\right)+18\mathrm{HCl}\left(\mathrm{aq}\right)\to 3{\mathrm{H}}_2\left[{\mathrm{PtCl}}_6\right]\left(\mathrm{aq}\right)+4\mathrm{NO}\left(\mathrm{g}\right)+8{\mathrm{H}}_2\mathrm{O}\left(\mathrm{l}\right) \\ (\mathrm{Hexachloroplatinic}\mathrm{acid}) \end{gathered}$$

Step 5:

This is an oxidation-reduction (redox) reaction:

$$\begin{gathered} 3{\mathrm{Pt}}^{\left(0\right)}-12{\mathrm{e}}^{-}\to 3{\mathrm{Pt}}^{\mathrm{IV}} \\ \left(\mathrm{oxidation}\right) \\ 4{\mathrm{N}}^{\left(\mathrm{V}\right)}+12{\mathrm{e}}^{-}\to 4{\mathrm{N}}^{\left(\mathrm{II}\right)} \\ \left(\mathrm{reduction}\right) \end{gathered}$$

$\mathrm{Pt}$ is reducing agent, ${\mathrm{HNO}}_3$ is an oxidizing agent.

Hence option (D) is correct