Sciencemadness Discussion Board - HCl + AgNO3 = HNO3 + AgCl : How?

HCl + AgNO3 = HNO3 + AgCl : How?

APO - 17-11-2013 at 14:38

I mixed aqueous Hydrochloric Acid with aqueous Silver Nitrate, and got a precipitate of Silver Chloride, so I knew there was Nitric Acid present.

My understanding was that an acid used must be stronger to convert a salt to it's acid, is this not correct?

DJF90 - 17-11-2013 at 14:45

That is correct. HCl is about 6 orders of magnitude stronger than nitric acid. You've also got the precipitation of insoluble silver chloride as a driving factor. So wheres the problem?

APO - 17-11-2013 at 15:37

Wow! I thought Nitric was stronger! Thanks.

bismuthate - 17-11-2013 at 15:48

Kind of on the subject why does HNO3 react with AlCl3? I know that HCl is stronger.

APO - 17-11-2013 at 15:58

From what I understand, in that case it's a matter of being a strong oxidizer, rather than a stronger acid. The Aluminium Chloride is oxidized to Nitrosyl Chloride and Aluminium Nitrate, which I belive relies off of a different mechanism.

bismuthate - 17-11-2013 at 17:43

Ok that makes more sense thanks. Also, how does S replace Cl when H2S passes through CuCl? Sorry for all the questions.

barley81 - 17-11-2013 at 18:06

Cu2S is very insoluble. Ksp = 2.5 x 10^(-48) (http://www.csudh.edu/oliver/chemdata/data-ksp.htm)
CuCl is not that insoluble. Ksp = 1.2 x 10^(-6).

Consider saturated CuCl. Since [Cu+] = [Cl-], we have x^2 = 1.2 x 10^-6 and x = [Cu+] = 1.1 x 10^-3 M.

Consider 0.1M H2S: K<sub>1</sub> = 9.6 x 10^-8 and K2 = 1.3 x 10^-14. Now K1 = [H+][HS-]/[H2S] and K2 = [H+][S2-]/[HS-], so K1K2 = [H+]^2[S2-]/[H2S]. [H+] is roughly 10^-3, so [S2-] is roughly 1.3 x 10^-15.

If these are in the same solution, then [S2-][Cu+]^2 = 1.5 x 10^-21, which is greater than Ksp for Cu2S. Therefore Cu2S will precipitate.

EDIT: metalresearcher is right. It does not have anything to do with acid strength, though acid strength can influence things indirectly. For example, insoluble metal carbonates dissolve in acid because the carbonate ion is protonated as soon as it dissolves, and the CO2 is driven off. Therefore Qsp remains below Ksp, and the solid can continue dissolving.

[Edited on 18-11-2013 by barley81]

metalresearcher - 18-11-2013 at 03:50

Well it has nothing to do with acid strength. HCl and AgNO3 in an aqueous solution *will* precipitate AgCl as Ag+ and Cl- ions cannot co-exist in an aqueous solution due to its very poor solubility.

AJKOER - 18-11-2013 at 16:02

Those acquainted with hydrometallury are acquainted with the use of Citric acid as an apparently more effective, cheaper and environmentally more friendly leaching lixiviant. To quote a recent source (August 2011, Volume 13, Issue 2, Journal of Material Cycles and Waste Management, pp 118-126, "Leaching of heavy metals by citric acid from fly ash generated in municipal waste incineration plants", by Kai Huang,.. at http://link.springer.com/article/10.1007%2Fs10163-011-0001-5... ):

"From the results of screening tests of leaching lixiviants, citric acid was found to be the most effective leaching agent"

This is quite an interesting statement as among the competing lixiviants where the inorganic acids HCl, H2SO4 and even HNO3.

I will let the student do research to undercover the explanation.

APO - 18-11-2013 at 17:21

Metalresearcher, I think it does have to do with acid strength. If HCl wasn't strong enough to react with AgNO3, no reaction would occur, thus no precipitate.

ScienceSquirrel - 18-11-2013 at 17:36

Quote: Originally posted by AJKOER

Those acquainted with hydrometallury are acquainted with the use of Citric acid as an apparently more effective, cheaper and environmentally more friendly leaching lixiviant. To quote a recent source (August 2011, Volume 13, Issue 2, Journal of Material Cycles and Waste Management, pp 118-126, "Leaching of heavy metals by citric acid from fly ash generated in municipal waste incineration plants", by Kai Huang,.. at http://link.springer.com/article/10.1007%2Fs10163-011-0001-5... ):

"From the results of screening tests of leaching lixiviants, citric acid was found to be the most effective leaching agent"

This is quite an interesting statement as among the competing lixiviants where the inorganic acids HCl, H2SO4 and even HNO3.

I will let the student do research to undercover the explanation.

Citric acid is a very effective lixiviant because it is a complexing agent as well as an acid.
Look at Benedict's Solution, the copper is complexed by the .citrate anion.
http://en.wikipedia.org/wiki/Benedict%27s_reagent

AJKOER - 19-11-2013 at 11:54

Quote: Originally posted by ScienceSquirrel

Quote: Originally posted by AJKOER

Yes, Citric acid complexing ability is usually cited.

The acid's solubility (related no doubt to its ability to form complexes) is another plus. The dry acid does not decompose until heated over 300 C (good thermal stability). Also, an aqueous solution of Critic acid is less volatile (or harder to be boiled off) probably adds to its performance as an effective lixiviant.

[Edited on 19-11-2013 by AJKOER]

MrHomeScientist - 20-11-2013 at 10:35

Quote: Originally posted by APO

Metalresearcher, I think it does have to do with acid strength. If HCl wasn't strong enough to react with AgNO3, no reaction would occur, thus no precipitate.

I don't think that's quite correct, because everything is dissociated in solution. When you mix a solution of hydrochloric acid and silver nitrate, you get a soup containing H₃O⁺, Cl^-, Ag⁺, and NO₃^-. The silver and chloride ions very readily combine to form a precipitate of AgCl because it is nearly insoluble in solution. This is the main driver of the reaction, and leaves H₃O⁺ and NO₃^- in solution to form nitric acid. If done stroichiometrically, I would think this should yield quite pure, or at least chloride-free, acid!

APO - 20-11-2013 at 17:27

Oh, thanks. It would be different with a covalent bond correct?

MrHomeScientist - 21-11-2013 at 13:25

I think so, yes. If, say, HCl wasn't dissociated then the chlorine atom would be less readily available to bond to a silver atom.

Thinking about all this, I know less about covalent bonds than I thought I did. I'll look into it more, and if I'm incorrect please someone correct me.

vulture - 22-11-2013 at 01:46

Quote:

I think so, yes. If, say, HCl wasn't dissociated then the chlorine atom would be less readily available to bond to a silver atom.

If HCl was covalently bonded, it wouldn't be an acid.

HCl is a stronger acid than HNO₃ so it will liberate HNO₃ from nitrate salts.

In the case that HCl would be a weaker acid than HNO₃, this reaction could still be possible because of the very low solubility of the AgCl. Then you would have to compare the Ksp with the Ka.

Dany - 22-11-2013 at 02:04

Quote: Originally posted by vulture

If HCl was covalently bonded, it wouldn't be an acid.

if HCl were in the gas phase (not dissociated) you wouldn't call it acid???

Saying that if HCl was covalently bonded, it wouldn't be an acid is not true. another example is the active hydrogen compounds like malonate esters. in the presence of base they will act as acid by giving the covalently bonded hydrogen forming the enolate form which is useful in organic synthesis, or for example the formation of acetylide from acetylene when reacted with liquid ammonia at -33°C.
I will add one more thing. The concept of acid and bases is relative. Even nitric acid will act as a base when mixed with sulfuric acid (because the pKa of H₂SO₄ is less than the HNO₃) which will lead to the protonation of nitric acid and the formation of the nitronium cation (NO₂+) after the elimination of water molecule.
so depending on the situation a species may act as an acid or base

Dany.

[Edited on 22-11-2013 by Dany]

vulture - 22-11-2013 at 06:04

Generally I classify acids as acids when they are capable of protonating water. I find this a useful distinction in most situations, but you are right, in different media the lines are blurred.

Quote:

if HCl were in the gas phase (not dissociated) you wouldn't call it acid???

You are right, I was wrong.