TaylorS
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HELP: Reduction of Silver (I) Sulfide
As highschool student who has just finished my first year of chemistry, I am trying to run a redox reaction on Ag2S (silver tarnish) with elemental
aluminum (al) to form silver (Ag) and Al2S3 for the basic purpose of demonstrating the restoration of tarnished silver back into elemental silver.
This reaction is shown below:
3Ag2S (s) + 2Al (s) --> 6Ag (s) + Al2S3 (s)
In my Internet research, I have learned of a rather ingenious method in which this may be accomplished. That is to collect some aluminum foil, your
tarnished piece of silver, boiling water, and baking soda (Na2CO3) and place the tarnished silver in contact with the aluminum foil to insure proper
electron transfer from the aluminum (which is oxidized) to the silver (which is reduced). Then pour boiling (or nearly boiling) water over this
apparatus, and dissolve the Na2CO3 in the water to provide proper balance to the electrical charges and serve as a basic "salt bridge" so that the
reaction may proceed until the reactants are used up and all silver tarnish is converted back to silver.
Silver tarnishes by reacing with H2S to form Ag2S and H2 (g). This reaction is shown below:
2Ag (s) + H2S (g) --> Ag2S (s) + H2 (g)
I learned of this reaction rather early on in my research. But then I ran across a scientist who said that the aluminum sulfide (Al2S3) that forms
from the restoration of the silver sulfide quickly hydrolyzes into Al(OH)2 and H2S. I have attempted as best I can to find the balanced chemical
equation for this reaction, but all I have been able to come up with is the following, which is obviously unbalanced:
Al2S3 (s) + H2O (l) --> Al(OH)2 + H2S (s)
Upon attempting to balance the aluminum and sulfur atoms, it seems impossible to balance the other atoms of hydrogen and oxygen. Does anyone know how
to balance this equation? If so, I would be most appreciative if you could post it here. Thanks so much!
Taylor
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The_Davster
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Its Al(OH)3; that will make balancing easy  .
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TaylorS
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Ah, thank you rogue chemist! Balancing it will be, as you said, easy now.
Taylor
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woelen
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Also keep in mind that H2S is not a solid, but a gas. Under the alkaline conditions with Na2CO3 in solution, however, the H2S is converted to HS(-)
ions and S(2-) ions in solution. You certainly will not see bubbles of H2S, but you _might_ be able to notice the smell of rotten eggs.
Btw, I also have seen descriptions of this, but these were using NaHCO3. You mentioned baking soda, but baking soda is not Na2CO3, but NaHCO3. So, I
think that you are confusing washing soda (Na2CO3) with baking soda (NaHCO3). If I were you, I first would try the latter. It is less corrosive and if
it does not work, then at least it will do no damage at all to the silver ornaments.
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TaylorS
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Thanks woelen, for your input. No, I didn't use Na2CO3 as my salt bridge, though I did say that in my previous post. I used baking soda, NaHCO3. I
must have gotten confused with the chemical formulae. Ahhh.
I ran the redox reaction with the silver sulfide and aluminum yesterday. The reaction proceeded quite slowly, though the tarnish did disappear. I
smelled the hydrogen sulfide being produced from the hydrolysis of the aluminum sulfide, which was an indication that the reaction was taking place.
Further evidence that the reaction was successful was that I saw bubbles forming on the aluminum foil beneath the baking soda/water solution. As you
said, woelen, the hydrogen sulfide is a gas, not a solid, and so these bubbles add further evidence.
The silver piece I used was a silver spoon of my great-grandmothers, or perhaps even earlier. It had tarnish blotches on it of black and grey. It
looked terrible. After the reaction, though, it was very shiney, though some tarnish did not react, as I was impatient and did not give it enough time
to finish.
If anyone else has done this or a similar reaction, I have a question to ask. Does the elemental silver that is produced from this reaction appear
exceptionally shiney, looking almost fake after restoration? I compared the restored piece with another silver piece that was not tarnished, and the
restored piece looked so bright it looked fake. Is this normal or did the reaction actually plate the sliver tarnish with a layer of aluminum?
Chemically, I cannot see how this would happen, but then again, I am knew to this stuff.
Taylor
[Edited on 22-6-2006 by TaylorS]
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Odyssèus
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Just tried this reaction:
I used a screwcap lid for a sterling silver salt/pepper shaker, it was pretty ugly and blackened with tarnish. I dissolved about 2 tablespoons (Didnt
measure it) of NaHCO3 in about 100-150 mL water. Took a folded up piece of Al foil and set the silver on it. I then heated it on a hot plate on my
patio.
The reaction proceeded slowly untill the water got hot. I noticed small black specks on my Al foil. As the water neared boiling the Al foil started
fizzing with CO2 coming off (No H2S smell yet). I thought that I might suddenly run out of NaHCO3 and it would start releasing H2S so I seperated the
silver/Al and started to add more NaHCO3. I was suprised when it almost overflowed due to CO2 gas being released. Anyone have any ideas on why this
occured? IIRC H2S isnt that soluble in water (And being a gas, less in hot water than cold).
NaSH + NaHCO3 => Na2S + CO2 + H2O perhaps?
My silver was still a little tarnished when I was done, dull and slightly darker than silver with a few black spots on it. Never smelled any H2S...
EDIT: Have a slight headache, but I'm pretty sure thats from my hot plate burning insulation or something rather than H2S... I'm always nervious about
toxic gasses.
[Edited on 23-6-2006 by Odyssèus]
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12AX7
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Ah yes... didn't you know NaHCO3 is unstable at warm temperatures?
Heh. I've boiled bicarb solution myself, to attempt to make plain washing soda. It goes really slowly because it takes a lot of heat to drive out
the CO2- it's endothermic (as removing gasses tends to be).
Suddenly adding fresh bicarb to a hot solution could easily cause it to bubble off I think.
Now that your silver is reduced to metal, you have to buff it, because the reduced surface is porous or something like that.
Tim
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Odyssèus
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Ah, thought that required higher temperatures. Though, this was pretty fast for a decomposition (Especially an endothermic one), like pouring NaHCO3
into dilute acid.
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12AX7
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Yeah, that I don't know about. I would've thought the endotherm would be enough to keep it under control.
*Shrug* can try it with boiling water and a box of baking soda some other time to see if it does do it.
Tim
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Nerro
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| Quote: | Originally posted by Odyssèus
Just tried this reaction:
I used a screwcap lid for a sterling silver salt/pepper shaker, it was pretty ugly and blackened with tarnish. I dissolved about 2 tablespoons (Didnt
measure it) of NaHCO3 in about 100-150 mL water. Took a folded up piece of Al foil and set the silver on it. I then heated it on a hot plate on my
patio.
The reaction proceeded slowly untill the water got hot. I noticed small black specks on my Al foil. As the water neared boiling the Al foil started
fizzing with CO2 coming off (No H2S smell yet). I thought that I might suddenly run out of NaHCO3 and it would start releasing H2S so I seperated the
silver/Al and started to add more NaHCO3. I was suprised when it almost overflowed due to CO2 gas being released. Anyone have any ideas on why this
occured? IIRC H2S isnt that soluble in water (And being a gas, less in hot water than cold).
NaSH + NaHCO3 => Na2S + CO2 + H2O perhaps?
My silver was still a little tarnished when I was done, dull and slightly darker than silver with a few black spots on it. Never smelled any H2S...
EDIT: Have a slight headache, but I'm pretty sure thats from my hot plate burning insulation or something rather than H2S... I'm always nervious about
toxic gasses.
[Edited on 23-6-2006 by Odyssèus] |
I suffer from the same thing, my nerves are usually what caused the
headaches rather than any gases.
Are you sure the NaHCO3 is there to serve as a saltbridge? I always thought it was there to remove the Al2O3 on the surface of the foil to "activate"
it as it were.
#261501 +(11351)- [X]
the \"bishop\" came to our church today
he was a fucken impostor
never once moved diagonally
courtesy of bash
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