Aqua regia and nascent chlorine

Aqua regia has the famous property of being able to dissolve noble metals, like gold and silver. I read that this has to do with the generation of nascent chlorine. How does this happen and how does it help to dissolve such metals?

Aqua regia is a concentrated mixture of nitric and hydrochloric acids. The two acids react to generate soluble nitrosyl chloride, water and nascent chlorine: $$\ce{HNO3 + 3HCl -> NOCl + 2H2O + 2Cl}$$

Secondly, the gold is oxidised by nitric acid to gold(III) ions: $$\ce{Au <=> Au^{3+} + 3e-}$$

This reaction is normally highly unfavourable and so very few gold(III) ions are formed if you put gold into only concentrated nitric acid. However, the nascent chlorine is highly reactive as it has an unpaired electron in a high energy orbital and it is able to react with the gold(III) ions to form the soluble tetrachloroaurate(III) complex. This removes the gold(III) ions from the equilibrium, driving the reaction to completion.

Overall the reaction is: $$\ce{Au + 3HNO3 + 4HCl -> [AuCl4]- + 3NO2 + H3O+ + 2H2O}$$

• For the most part you are right, but just a minor detail... Chlorine in [AuCl4]- is obviously Cl(-1), so the atomic chlorine(0) cannot react with the gold(III) ions to form [AuCl4-] because there is no reducing agent. I would put it one of two ways: either atomic Cl(0) oxidises the gold to Au(III), itself being reduced to Cl(-1); or HNO3 oxidises Au(0) to Au(III) and the Cl- ions from HCl complex with the gold atoms to form [AuCl4]-. – orthocresol Oct 2 '16 at 16:50
• @orthocresol could you move this into the answer, and delete the comments? Thanks! – Gaurang Tandon Apr 21 '18 at 5:24

Chlorine gas can react with gold powder to generate $\ce{AuCl3}$. In an aqua regia solution, the driving force is the creation of the tetrachloroaurate complex, $\ce{[AuCl4]-}$.

The nascent part describes how the gas chlorine is formed in aqua regia and how it is different from standard chlorine gas. The idea is that the chloride ions from hydrochloric acid don’t all have to reach the oxidising $\ce{HNO3}$ species at the same time but do so one after the other. So that instead of immediately forming a chlorine molecule $\ce{Cl2}$, chlorine radicals $\ce{Cl^.}$ or something close to that are first formed. These then have a much greater reactivity than gaseous chlorine since they have an inherently less stable electronic structure.

A similar phenomenon is observed with e.g. zinc in hydrochloric acid. The reaction is pretty simple and generates hydrogen gas:

$$\ce{Zn + 2 H+ -> Zn^2+ + H2}$$

However, you can think of this as two single electron transfers (each hydrogen ion only gets one electron) with two hydrogen radicals having to meet up to generate the gas; this is why $\ce{Zn + HCl}$ has a better reducing ability than gaseous hydrogen.

As for how aqua regia is able to dissolve otherwise inert metals like gold: it all boils down again to nitrosyl chloride and nascent hydrogen being stronger oxidising agents than simple $\ce{HNO3}$.