walruslover69 - 29-12-2017 at 18:21
There might be another threat about this but I could not find it, but why does nitric acid not dissolve gold and other noble metals. I realize that
the reaction is thermodynamically unfavorable and am aware of the reduction potentials, but When nitric acid acts as an oxidizer it releases NO or NO2
gas. So if the reaction is carried out in the open or under vacuum the equilibrium constant should be trending toward 0 since the NO or NO2
concentration can be conscidered 0. therefore making the reaction thermodynamically favorable. So why does this not happen?
ninhydric1 - 29-12-2017 at 18:36
Nitric acid indeed is able to oxidize gold into +3 ions, but the equilibrium isn't favorable. It produces a minute amount of gold ions. HCl provides a
source of Cl- ions, and since the formation of the complex [AuCl4]- is extremely favorable, the equilibrium is shifted to the right. Because the gold
ions are being consumed by the formation of a complex, the formation of gold ions increases due to the equilibrium shifting to the right. As you can
see, neither nitric acid or hydrochloric acid can do it by itself. The production of NOx gases is the result of the oxidizing reaction. The true
equilibrium is in solution.
EDIT 1: formatting issues.
[Edited on 12-30-2017 by ninhydric1]
walruslover69 - 29-12-2017 at 18:47
but you would take the product of the NO2 concentration and the gold ion concentration, since the NO2 concentration would be 0 if done in an open
container, the product would tend towards 0. To determine if it is favorable you have to take the natural log of the product concentration over the
reactant concentration, since that tends towards 0 the natural log of that will go towards infinity making the reaction favorable.
ninhydric1 - 29-12-2017 at 19:01
The minute amount of gold indeed does produce NOx gas, but it produces so little that it is able to stay dissolved in water, in turn forcing the
equilibrium to the left. The equilibrium isn't shifted enough to the right for the NOx gas to escape, until the HCl forces the equilibrium to the
right, resulting in the evolution of more NOx gas.
walruslover69 - 29-12-2017 at 19:11
if preformed under moderate vacuum as to degass any NO or NO2 gas from solution that the reaction would go forward?
Sulaiman - 29-12-2017 at 19:32
I have been unable to get any sensible (synthsis and/or reactions) for
Au(NO3)3 or HAu(NO3)x
so I guess that could be a reason ?
ninhydric1 - 29-12-2017 at 19:49
That would also make sense, Sulaiman, as the product of this reaction is chloroauric acid.
Good luck degassing a couple nanograms of NOx gas. If you look at the Wikipedia page for agua regia, it states that a "virtually undetectable" amount
of gold ions are produced. But it doesn't mean it isn't there, it's just produced in a tiny quantity.
EDIT 1: Added clarifications.
[Edited on 12-30-2017 by ninhydric1]