Question

Give reason why conc. $\mathrm{HCl}$ is used for the preparation of aqua-regia.

Answer 1

Hydrogen chloride gas:

• The chemical formula of Hydrogen chloride gas is $\mathrm{HCl}$.
• It is acidic in nature.

Aqua-regia:

• Aqua-regia is a mixture of three parts of conc. $\mathrm{HCl}$ and one part of conc. ${\mathrm{HNO}}_3$.

Formation of aqua-regia:

A mixture of three parts of conc. $\mathrm{HCl}$ and one part of conc. ${\mathrm{HNO}}_3$ is called aqua-regia.

$$\begin{gathered} 3\mathrm{HCl}\left(\mathrm{aq}\right)+{\mathrm{HNO}}_3\left(\mathrm{aq}\right)\to \mathrm{NOCl}\left(\mathrm{g}\right)+2{\mathrm{H}}_2\mathrm{O}\left(\mathrm{l}\right)+2\left[\mathrm{Cl}\right]\mathrm{nascent} \\ (\mathrm{Conc}.)(\mathrm{Conc}.)(\mathrm{Nitrosyl}\mathrm{chloride}) \end{gathered}$$

Nascent chlorine reacts with noble metals like gold and platinum to give their soluble chlorides.

$$\begin{gathered} \mathrm{Au}\left(\mathrm{s}\right)+3\left[\mathrm{Cl}\right]\to {\mathrm{AuCl}}_{3_{}}\left(\mathrm{s}\right) \\ (\mathrm{Gold}(\mathrm{III})\mathrm{chloride}) \\ \mathrm{Pt}\left(\mathrm{s}\right)+4\left[\mathrm{Cl}\right]\to {\mathrm{PtCl}}_4\left(\mathrm{s}\right) \\ (\mathrm{Platinum}(\mathrm{IV})\mathrm{chloride}) \end{gathered}$$

A mixture of three parts of conc. $\mathrm{HCl}$ and one part of conc. ${\mathrm{HNO}}_3$ is called aqua-regia.
$$\begin{gathered} 3\mathrm{HCl}\left(\mathrm{aq}\right)+{\mathrm{HNO}}_3\left(\mathrm{aq}\right)\to \mathrm{NOCl}\left(\mathrm{g}\right)+2{\mathrm{H}}_2\mathrm{O}\left(\mathrm{l}\right)+2\left[\mathrm{Cl}\right]\mathrm{nascent} \\ (\mathrm{Conc}.)(\mathrm{Conc}.)(\mathrm{Nitrosyl}\mathrm{chloride}) \end{gathered}$$