Question

Which of the following chemical reactions correctly shows Aqua-regia dissolving in gold?
(A) $Au+2[Cl]\to AuC{{l}_{2}}\overset{HCl}{\mathop{\to }}\,{{H}_{2}}[AuC{{l}_{4}}]$
(B) $Ag+3[Cl]\to AgC{{l}_{3}}\overset{HCl}{\mathop{\to }}\,H[AgC{{l}_{4}}]$
(C) $Au+3[Cl]\to AuC{{l}_{3}}\overset{HCl}{\mathop{\to }}\,H[AuC{{l}_{4}}]$
(D) $Ag+2[Cl]\to AgC{{l}_{2}}\overset{HCl}{\mathop{\to }}\,{{H}_{2}}[AgC{{l}_{4}}]$

Answer 1

Hint: The extraction of metals involves the refining of metals that are found in the gangue. There are three major stages involved in the extraction of metals- concentration of ore, isolation of metal from the concrete ore, and purification of metal. Aqua regia is used to refine the highest quality of gold.

Complete step by step answer:
Aqua regia is a mixture of hydrochloric acid and nitric acid in the molar ratio of 3:1, which is yellow-orange fuming liquid. This mixture is named aqua regia because it dissolves the only gold, platinum, and noble metals.
$HN{{O}_{3}}(aq)+3HCl(aq)\to NOCl(g)+C{{l}_{2}}(g)+2{{H}_{2}}O(l)$

The volatile products in this mixture of aqua regia are nitrosyl chloride and chlorine gas. If these volatile products escape from the solution, then aqua regia losses its nature of dissolving metals like gold, platinum, etc.
The process of refining the highest quality of gold by using the Aqua regia mixture is known as the Wohlwill process. Chloroauric acid which is produced from aqua regia is used as an electrolyte in this process.

Dissolving gold: When aqua regia dissolves gold, nitric acid is a strong oxidizer that dissolves an undetectable amount of gold, and hydrochloric acid supply chloride ions, which react with gold forms chloroauric anions. The reaction of dissolving gold in aqua regia is,
$Au+3[Cl]\to AuC{{l}_{3}}\overset{HCl}{\mathop{\to }}\,H[AuC{{l}_{4}}]$
So, the correct answer is “Option C”.

Note: Aqua regia is also used in specific analytic procedures. This is used in laboratories to clean glassware of organic compounds and metal particles. Sometimes chromic acid is used for this purpose, but chromium is highly toxic and potential for explosions. So aqua regia is preferred to replace with chromic acid, because it is highly corrosive.