Domain: inorganic chemistry.
A 1:3 mixture of concentrated nitric acid and 37% (conc.) hydrochloric acid (royal water) is a highly oxidizing mixture. Many times stronger oxidizing than both components separately. How is this explained?
Does the complexation of the metal ion with the chloride ion drive off the reaction so much? (via Le Châtelier) (despite the good solubility of chloride complexes in aq.)
So does the same effect exist when adding NaCl (table salt) to concentrated nitric acid?
Answer
Dear Eric,
Royal water (aqua regia) is the trivial name for a 3:1 mixture of concentrated hydrochloric acid (HCl) and concentrated nitric acid (HNO3). This mixture is called aqua regia because it is able to dissolve precious metals such as gold and platinum.
When both acids are mixed, reactions occur that result in the formation of the volatile products NOCl and Cl2.
HNO3(aq) + 3 HCl (aq) → NOCl (g) + Cl2(g) + 2H2O (l)
These products are responsible for the smoking of the solution and the typical orange-yellow color of aqua regia. When the volatile components disappear from the solution, the aqua regia loses its strength. NOCl can further dissociate to form nitric oxide (NO) and chlorine gas (Cl2).
2 NOCl (g) → 2 NO (g) + Cl2(g)
The formed Cl2 is even more oxidizing than HNO3 (this can be derived from tables of standard redox potentials)
If we apply this for gold:
At the beginning of the reaction, HNO3 a tiny bit of gold in solution:
Au(s) + NO3–(aq) + 4 H+(aq) → Au3+(aq) + NO (g) + 2 H2O (l).
HCl provides a continuous supply of chloride ions (Cl-) which react with the gold ions to form the tetrachloroaurate anion (AuCl4-).
au3+(aq) + 4 Cl–(aq) → AuCl4–(aq).
The reaction with HCl is an equilibrium. Due to the presence of large concentrations of Cl–
According to Le Châtelier’s principle, the equilibrium is shifted to the right and AuCl . becomes4– formed. Gold ions are thus continuously removed from the solution by complexation with Cl- causing the oxidation of the gold by HNO3 and Cl2 can continue until the gold is completely dissolved. Mixing NaCl with HNO3 will not achieve the same effect as NOCl and Cl2 can only be formed in the presence of both acids. I hope this is a clear answer to your questions.
Sincerely,
Frederick
Answered by
Frederik Dhooghe
Chemistry
Ringlaan 3 1180 Brussels
http://www.aeronomie.be/
.