Question

Why does gold and platinum dissolve in aqua regia?

Answer 1

Hint: Aqua regia is a mixture which consists of 3:1 molar ratio of hydrochloric acid and nitric acid respectively.

Complete step by step answer:
We know that, Gold and Platinum, both are completely inert solutions and hence do not react with the solution of aqua regia. Aqua regia consists of two components in definite proportion and each of the components has a definite function which together help in dissolving metals such as gold and platinum. Aqua regia, as we know, consists of nitric acid, i.e., $HN{O_3}$ which acts as a strong oxidizing agent. This nitric acid converts the gold or platinum metal to their corresponding ions by oxidizing them. These so formed metal ions, for e.g., $A{u^{3 + }}$then forms coordination complexes with the $C{l^ - }$ ions released from the second counterpart of aqua regia, i.e., HCl. This results in the removal of Gold and platinum ions from the solution and their dissolution. To make this clearer, let us consider the reaction of gold metal in aqua regia.
As we know, the first step would involve the oxidization of Gold metal atoms to gold ions, i.e., $A{u^{3 + }}$ in the presence of nitric acid as a strong oxidizing agent. The reaction taking place is as shown below-
$Au(s) + 3NO_3^ - (aq) + 6{H^ + }(aq) \rightleftharpoons A{u^{3 + }}(aq) + 3N{O_2}(g) + 3{H_2}O(l)$
The next step involves the formation of coordination complex with the Cl- released from the hydrochloric acid. This reaction takes place as follows –
$A{u^{3 + }}(aq) + 4C{l^ - }(aq) \rightleftarrows AuCl_4^ - (aq)$
The product which is formed is called tetrachloroaurate. This tetrachloroaurate present in aqua regia then quickly dissolves to form chloroauric acid, i.e., $HAuC{l_4}$. In this way metals like gold and platinum dissolves aqua regia without chemically reacting with it.

Note: It must be noted that the properties shown by aqua regia is a result of its combining counterparts and not just because of any one species. Hence, one must know the composition of aqua regia for better understanding of the concept.