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Topic: Questions about electrolyzing AgCl in ammonia  (Read 28508 times)

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Offline Borek

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Re: Questions about electrolyzing AgCl in ammonia
« Reply #45 on: December 11, 2012, 08:58:00 AM »
I've been reading up some on reactivity (activity) series.  All the sites seem to say pretty much the same thing, but don't explain WHY one element is more reactive than another.  It seems to me that electronegativity is a physical explanation as to why.

It is not. It happens that both electronegativity and reactivity to some extent depend of the ionization energies, so in some cases there is an obvious correlation between one and the other. But it works only for some cases, so it has no predictive power which makes it useless in this context.

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I can't speak to electronegativities between gold and iodine, but if we are looking only at transition metals, do elect. values not explain why silver reacts as it does relative to hydrogen, and so on with gold?  In other words, why is it incorrect to say that hydrogen has a greater affinity for electrons than silver, and gold has a greater affinity for them than hydrogen, and that this explains their behavior?

Technically electron affinity is defined as amount of energy released when an electron is added to a neutral atom, so it is of no use here, as H+ is not neutral. I guess what you really mean is a "reversed ionization" - that is, amount of energy released when electron is added to a cation, and it is not difficult to address this case. Let's see the numbers:

Hydrogen - 13.6 eV
Silver - 7.58 eV
Gold - 9.22 eV

(see http://www.lenntech.com/periodic-chart-elements/ionization-energy.htm)

So it is incorrect to say that

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hydrogen has a greater affinity for electrons than silver, and gold has a greater affinity for them than hydrogen

because it is not true. Hydrogen cation has the highest affinity for electrons of the three.

Note that part of the problem here is that you have not defined what you mean by the electron affinity - you just throw the term and add some hand waving hoping it will yield a result ;)
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Offline Bushka

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Re: Questions about electrolyzing AgCl in ammonia
« Reply #46 on: December 16, 2012, 03:54:45 PM »
I've been reading up some on reactivity (activity) series.  All the sites seem to say pretty much the same thing, but don't explain WHY one element is more reactive than another.  It seems to me that electronegativity is a physical explanation as to why.

It is not. It happens that both electronegativity and reactivity to some extent depend of the ionization energies, so in some cases there is an obvious correlation between one and the other. But it works only for some cases, so it has no predictive power which makes it useless in this context.

Quote
I can't speak to electronegativities between gold and iodine, but if we are looking only at transition metals, do elect. values not explain why silver reacts as it does relative to hydrogen, and so on with gold?  In other words, why is it incorrect to say that hydrogen has a greater affinity for electrons than silver, and gold has a greater affinity for them than hydrogen, and that this explains their behavior?

Technically electron affinity is defined as amount of energy released when an electron is added to a neutral atom, so it is of no use here, as H+ is not neutral. I guess what you really mean is a "reversed ionization" - that is, amount of energy released when electron is added to a cation, and it is not difficult to address this case. Let's see the numbers:

Hydrogen - 13.6 eV
Silver - 7.58 eV
Gold - 9.22 eV

(see http://www.lenntech.com/periodic-chart-elements/ionization-energy.htm)

So it is incorrect to say that

Quote
hydrogen has a greater affinity for electrons than silver, and gold has a greater affinity for them than hydrogen

because it is not true. Hydrogen cation has the highest affinity for electrons of the three.

Note that part of the problem here is that you have not defined what you mean by the electron affinity - you just throw the term and add some hand waving hoping it will yield a result ;)

Borek - maybe it is a language thing (English your first language?), but you are taking me too literally.  From the context you should realize that I am not talking about "affinity" as in eV's.  I have not defined electron affinity because I wasn't talking about it.  You assumed I was.  I was actually talking about love.  Hydrogen loves it's electrons more than silver, but not as much as gold does.  :-*  OK I think I'm safe because unless they've come up with some new chemical terms in the last few years, I don't think "love" is in the CRC handbook.

Let me get back to the original question, because I would like to understand better.  Rather than you simply telling me what I'm saying wrong, help me understand the right answer.  I originally said that electronegativity explains why sliver and gold react the way they do with hydrogen ions.  You said I was confusing electro. with reactivity.  Are you saying that reactivity is the correct explanation?  If so, what causes one material to be more reactive than another (admittedly I think this can be a complicated answer).

Offline Bushka

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Re: Questions about electrolyzing AgCl in ammonia
« Reply #47 on: December 16, 2012, 04:19:10 PM »
TWW if you're still reading:  How much concentrated nitric acid do you need to dissolve silver?  I read somewhere that it takes about 150 ml of acid to dissolve 1 ounce of silver.  Does that sound about right, and what concentration do you use when you aren't "making your own"?

Also, I've taken all of my "dirty" silver chloride and soaked it in muriatic (HCl) acid.  The solvent turned green/olive, which I presume might be copper sulfate.  Interestingly, if I add ammonia to the acidic green solvent, and raise the pH above 7, the green turns bright blue.  I assume it has something to do with copper valence. (?)  Anyway, originally there were dark green flakes and crystals in the AgCl, and these are now all dissolved.  I'm at the point where flushing the AgCl with additional acid does not yield a visibly green/yellowish liquid.

So now I have AgCl mixed in with some bits of silver.  Some of it is a bit lumpy, so here's my plan:  Dry out the AgCl and crush everything as finely as possible in a mortar and pestle.  Resoak in muriatic acid to flush out any copper salts.  Rinse and then add ammonia and soak.  The idea is to dissolve anything else in the AgCl that might be soluble in ammonia.  I would then reacidify the ammonia to drop out any AgCl that dissolved in the ammonia.  I would discard the solvent, thinking that it might contain some contaminants that went into solution with the ammonia, and then did not precip. out like the AgCl does.  Seems like this ammonia step wouldn't hurt anything.

Now the interesting part.  I would like to add nitric acid to the pulverized AgCl powder in order to dissolve any free silver.  If I poured off the solvent, I would have silver nitrate, which could then be converted to silver via the acidify/syrup method.  Dissolving the silver with nitric acid in the first place would attempt to remove impurities from the silver.  The next question is dealing with the remaining AgCl.  I would like to add lye directly to the AgCl to see if it can be converted to silver, again, via the syrup method.  The question is whether the AgCl is too "hard" to respond to the lye.  If I can at least get all my silver chloride/silver powder turned into an ingot, then maybe I can analyze purity and go from there.

Any comments?

Offline Borek

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Re: Questions about electrolyzing AgCl in ammonia
« Reply #48 on: December 16, 2012, 05:52:12 PM »
Borek - maybe it is a language thing (English your first language?), but you are taking me too literally.  From the context you should realize that I am not talking about "affinity" as in eV's.  I have not defined electron affinity because I wasn't talking about it.  You assumed I was.  I was actually talking about love.  Hydrogen loves it's electrons more than silver, but not as much as gold does.  :-*  OK I think I'm safe because unless they've come up with some new chemical terms in the last few years, I don't think "love" is in the CRC handbook.

There is a reason why "love" is not in the CRC handbook - it is not precisely defined, it can't be measured, it can't be used to predict properties of elements, so in this context it is useless.

It is similar problem like with dissolving silver in acids - you need to be precise with what you really mean, otherwise things you say don't mean anything.

English is my second language, but the way I see it is not the problem here. Problem is you are trying to use hand waving for the explanation. Sorry, it won't work. You need to be precise with the meanings, otherwise sooner or later we will run into communication problems - as we already did.

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Let me get back to the original question, because I would like to understand better.  Rather than you simply telling me what I'm saying wrong, help me understand the right answer.  I originally said that electronegativity explains why sliver and gold react the way they do with hydrogen ions.  You said I was confusing electro. with reactivity.  Are you saying that reactivity is the correct explanation?  If so, what causes one material to be more reactive than another (admittedly I think this can be a complicated answer).

Reactivity is a property that can be checked - basically it says which metal replaces which metal in the solution. As these reactions are in a way similar to the reaction between metals and H+ cations, H+ is also on the list. This is checked experimentally and the series can be used to predict what will happen when you add piece of a metal to the solution containing H+ or other metal cations. Does it explain "why"? Depends on how you look at it, I would say it doesn't.

But there is really no simple explanation. Basically reactivity of the metal in water solutions depends on its ionization energy and solvation effects. The lower the ionization energy, the more reactive the metal, the higher the solvation enthalpy, the more reactive the metal, but in some cases both can counteract each other, plus often we are talking about several ionization steps, each with its own ionization energy and each cation with its own solvation enthalpy. This can become quite complicated. Reactivity series ignores all these considerations, just gives a final (and because of the fine details not always precise) answer.
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Offline Bushka

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Re: Questions about electrolyzing AgCl in ammonia
« Reply #49 on: December 16, 2012, 06:23:25 PM »
Reactivity series ignores all these considerations, just gives a final (and because of the fine details not always precise) answer.

I understand better.  However, it seems your original answer isn't correct, either.  I'm not trying to nitpick, just trying to get it right in my mind.  I said electronegativity explains, or let's say predicts, the behavior of gold and silver in HCl.  The electronegativity of gold is higher than hydrogen, and it does not dissolve.  The elect. of silver is lower than that of hydrogen, and silver DOES dissolve in HCl.  Apparently it is the hard shell of AgCl that prevents any appreciable dissolution of Ag.  On the other hand, the reactivity, to which you referred me, does not explain this same behavior.  Since both gold and silver are shown lower than hydrogen on the activity series, this would predict that silver does NOT dissolve in HCl.

So maybe electronegativity just happens to predict what happens by chance, but by the same token reactivity does not predict what happens in this case, either.  It seems that silver is the culprit, or "odd-man out."  Do I have this right?  According to activity, silver does not dissolve, but in reality it does (until it is blocked by AgCl).  There must be a physical explanation for that, no?

Offline Borek

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Re: Questions about electrolyzing AgCl in ammonia
« Reply #50 on: December 16, 2012, 06:53:14 PM »
Sigh. Silver doesn't dissolve in HCl. It dissolves in HCl after adding an oxidizing agent. When we speak about dissolving in acid we speak about oxidizing a metal with H+, not with anything that you happen to add to the solution.
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Offline Bushka

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Re: Questions about electrolyzing AgCl in ammonia
« Reply #51 on: December 16, 2012, 08:31:20 PM »
Sigh. Silver doesn't dissolve in HCl. It dissolves in HCl after adding an oxidizing agent. When we speak about dissolving in acid we speak about oxidizing a metal with H+, not with anything that you happen to add to the solution.

No need to sigh.  I understand the difference you talk about, and if you read my previous posts I defended you on that.  However, I may be guilty of believing things I read on the internet.  ::) 

From this source: http://www.ehow.com/how_8254040_dissolve-silver.html I got the following:

Acids react with and dissolve most metals, but to achieve full dissolution, the resulting compounds must also exhibit solubility in water. Silver, for example, will dissolve in hydrochloric acid, or HCl, to form silver chloride, or AgCl. Silver chloride, however, is insoluble in water, which means a white solid of AgCl crystals will form in the resulting solution. The full dissolution of silver requires nitric acid, or HNO3, which reacts with silver to form silver nitrate, a water-soluble compound

This guy wrote the article so you'd think he'd have some knowledge.  It wasn't just an anonymous person responding to a post.  I could swear I found a post by a chemist saying about the same thing, but I can't seem to find it. 

In googling more and more sources, it seems the vast majority say that silver does not dissolve in dilute HCl.  Given that, I no longer see any discrepancy in the activity series for silver vs hydrogen.

I think this case is closed, amigo.  Thanks for the refresher course.



Offline Bushka

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Re: Questions about electrolyzing AgCl in ammonia
« Reply #52 on: December 24, 2012, 04:22:07 PM »
Can anyone help me explain what is going on here?  I'm mostly curious about the color change.  I washed some AgCl that had some green flakes in it in water, and a little ammonia.  The solution turned dark blue.  I assume this is due to copper sulfate or other copper salts.  Can silver turn a solution blue?  See photo 1.

Then I added HCl and the solution turned milky white/bluish.  The milkiness I'm sure is caused by AgCl precipitating out.  When the precip. settles , the solvent is clear.  What causes the blue color to change to clear?

Thanks!

Offline Borek

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Re: Questions about electrolyzing AgCl in ammonia
« Reply #53 on: December 25, 2012, 04:20:49 AM »
Copper (Cu2+) is light blue in the solution (unless really concentrated). Plus ammonia it created a complex - dark blue tetraamminecopper. When you added strong acid ammonia got protonated, which is equivalent of removing the ammonia from the solution, and complexation reaction equilibrium shifted far to the left (reactants side) - so you got the Cu2+ back.
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Offline Bushka

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Re: Questions about electrolyzing AgCl in ammonia
« Reply #54 on: December 25, 2012, 10:53:54 AM »
OK, thanks!

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