Question

How does gold react with aqua regia?

Answer 1

Hint: It is known that the mixture in which there is one part of nitric acid and three parts of hydrochloric acid is termed as aqua regia. It dissolves noble metals like gold and palladium.

Complete answer:
We know that gold is considered as a noble metal and is not soluble in either acid when taken alone. It is soluble in aqua regia because it is the mixture of two acids and both the acids perform their characteristic function. We also know that, the ${\mathrm{HNO}}_3$ is a strong oxidizing agent and therefore, it able to oxidize solid gold into gold ions whereas, $\mathrm{HCl}$ in the solution provides the ample amount of ${\mathrm{Cl}}^{-}$ ions which on reaction with gold ions results to the formation of coordination complex ion. Therefore, the IUPAC name of the complex so formed is tetrachloroaurate $\text{ (III) }$ anion.

The equilibrium reaction takes place between $\mathrm{HCl}$ and gold which leads to the production of chloroaurate anions in the solution. As the result of this the removal of gold ions takes place followed by its oxidation. The formation of chloroauric acid takes place once the gold gets completely soluble in aqua regia due to the presence of free chlorine. The chemical equations taking place between gold and aqua regia solution are shown below.

$\mathrm{Au}+3{\mathrm{HNO}}_3+4\mathrm{HCl}\rightleftharpoons {\mathrm{AuCl}}_4^{-}+3{\mathrm{NO}}_2+{\mathrm{H}}_3{\mathrm{O}}^{+}+2{\mathrm{H}}_2\mathrm{O}$

$\mathrm{Au}+{\mathrm{HNO}}_3+4\mathrm{HCl}\rightleftharpoons {\mathrm{AuCl}}_4^{-}+\mathrm{NO}+{\mathrm{H}}_3{\mathrm{O}}^{+}+{\mathrm{H}}_2\mathrm{O}$

Note: It is known that, the removal of excessive aqua regia solution from the reaction mixture of gold and aqua regia results to the formation of solid tetrachloroauric acid by repeated boiling of solution with $\mathrm{HCl}$ as all the residual ${\mathrm{HNO}}_3$ gets boils off.