[Alkrav]

Gold is soluble in royal water: I found the following reaction in the internet:
Reaction equation:
Au(s ) + 3NO3-(aq ) + 6H+(aq ) -------->Au3+(aq ) + 3NO2(g ) + 3H2O(l )
Au3+(aq ) + 4Cl-(aq ) -------->AuCl4-(aq )

But isn't the NOCl (which is formed in aqua regia) the agent which oxidizes the gold?

HNO₃ + 3 HCl  --------->NOCl + Cl₂ + 2 H20
and
NOCl + Au ------------->?

Thanks!

[Borek]

That's not exactly analytical chemistry question

From what I know nitrogen oxychloride (NOCl) is oxidizing agent here. Cl^- are complexing Au³⁺ lowering potential of the Au oxidation.

[Alkrav]

OH I SEE! !
Thanks!
I'm new in this forum..

[Albert]

HNO₃ + 3 HCl  --------->NOCl + Cl₂ + 2 H20
and
NOCl + Au ------------->?

In my opinion, it's just a matter of time. Aqua Regia should be prepared some minutes before you want it to react.
In the beginning:

3HCl + HNO3 -> 4H+ + 3Cl- + NO3-

...then, approximately after 15 minutes,...

4H+ + 3Cl- + NO3- -> Cl2 + NOCl + 2H2O

This is what I was taught to do for using aqua regia in order to dissolve HgS.

[xiankai]

the 3rd reaction is when aqua regia has decomposed, so i dont think its needed in dissolving HgS because HgS is almost insoluble in water

[Borek]

the 3rd reaction is when aqua regia has decomposed, so i dont think its needed in dissolving HgS because HgS is almost insoluble in water

Can you elaborate? I don't get it.

[xiankai]

well i thought that the 3rd equation is the decomposition of aqua regia into its more stable products, and since water is formed as one of the products, that couldnt be used for dissolving HgS.

what i meant to say is that the last reaction was not needed for using aqua regia to dissolve HgS.

[Albert]

well i thought that the 3rd equation is the decomposition of aqua regia into its more stable products, and since water is formed as one of the products, that couldnt be used for dissolving HgS.

what i meant to say is that the last reaction was not needed for using aqua regia to dissolve HgS.

I infer you didn't understand what I meant. I was just trying to explain that aqua regia should be prepared just before adding it.
When you have to dissolve HgS, you simply need 'pure' aqua regia:

3HgS + 8 H+ + 12 Cl- + 2NO3- -> 3[HgCl4]2- + 3S + 2 NO + 4 H2O

Using heat to accelerate the whole process.

Of course you cannot use aqua regia, when Cl2 and NOCl (and water) substituted Cl- and NO3-, for such a reaction. So, in order to use aqua regia for analysing Hg, my professor told me to add HNO3 to HCl just before using it.
I just wanted to highlight the fact that acqua (in Italian that's the right spelling) regia can dissolve gold and HgS, if you pay attention to the same aspects.
I know very well you cannot dissolve HgS if there is water among the reagents.  

[Borek]

I know very well you cannot dissolve HgS if there is water among the reagents.

Aqua regia is made mixing nitric acid with hydrochloric acid in volume ratio of about 1:3. Hydrochloric acid is never more concentrated than about 38-40% and stock nitric is about 68%, so the final mixture is about 50% water. Even if you use 100% HNO₃ you will not get below 40% of water.

[Alkrav]

Hey I've got a further question: "dissolve" doesn't actually mean that a reaction takes place!
e.g. "ZnS is soluble in 2M HCl   ZnS + 2 HCl ------>ZnCl_2(aq) + H₂S_(g)"

[Borek]

Hey I've got a further question: "dissolve" doesn't actually mean that a reaction takes place!
e.g. "ZnS is soluble in 2M HCl   ZnS + 2 HCl ------>ZnCl_2(aq) + H₂S_(g)"

Define "reaction".

[Albert]

Define "reaction".

That's an excellent question.

A (chemical) reaction is a phenomenon, which consists in some species (reagents) reacting for producing other species (products).

That's my opinion. So,

ZnS + 2 HCl ------>ZnCl2(aq) + H2S(g)

is a reaction (I'd call it a reaction of ionic exchange, whose physical consequence is ZnS's dissolution).

[Alkrav]

The dissolution of NaCl in water is: NaCl  +  (n+m)H₂O ---->Na⁺H₂O_n + Cl^-H₂O_m

No exchange of ions at all!

But in the reaction of the dissolution of ZnS in HCl an exange takes place....

[Borek]

Technically speaking in a ZnS dissolution you are moving the reaction equilibrium to the right protonating S^2- anions. Le Chatelier's principle at work.