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Author Topic: Which oxidises first, iron or silver?  (Read 6236 times)

Offline miss_maple

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Which oxidises first, iron or silver?
« on: 16/02/2009 12:04:47 »
 :o
heys .
it was during my chemistry lesson that when our teacher was showing us this C-D
and there was this part where we had to number the reactions given from 1 -4 depending on the rate of reaction.(fast to slow reaction )
2 of the options were -
- rusting of iron
- tarnishing of silver

ans by us & our teacher was that the rusting would take place earlier than that of tarnishing .
However we're wrong .
our teacher has not explaination !!!
pls help !
thank you very much .
miss maple
« Last Edit: 18/02/2009 09:30:33 by chris »


 

Offline lightarrow

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Re: Which oxidises first, iron or silver?
« Reply #1 on: 16/02/2009 16:11:57 »
:o
heys .
it was during my chemistry lesson that when our teacher was showing us this C-D
and there was this part where we had to number the reactions given from 1 -4 depending on the rate of reaction.(fast to slow reaction )
2 of the options were -
- rusting of iron
- tarnishing of silver

ans by us & our teacher was that the rusting would take place earlier than that of tarnishing .
However we're wrong .
our teacher has not explaination !!!
pls help !
thank you very much .
miss maple
miss maple, those reaction speeds are extremely dependent on the reaction conditions; you should specify them, or an answer is impossible (and however, it's difficult the same).
Rusting of iron with what air or water? Air? Then how is the humidity? Water? How much ohygen and CO2 dissolved in there? Temperature, pressure? Tarnishing silver with what? Where and how?
« Last Edit: 16/02/2009 16:14:48 by lightarrow »
 

Offline Chemistry4me

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Re: Which oxidises first, iron or silver?
« Reply #2 on: 17/02/2009 02:49:51 »
and there was this part where we had to number the reactions given from 1 -4 depending on the rate of reaction
And what were the other two?
 

Offline lancenti

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Re: Which oxidises first, iron or silver?
« Reply #3 on: 18/02/2009 03:01:18 »
Tarnished Silver: Ag2S

Rusted Iron: Fe(OH)2 or Fe2O3

Rate of Reaction would depend on a lot of things, as lightarrow has said. But if it were a proper experiment, we would assume that it was 1 atmosphere of H2S and Oxygen for Silver and Iron respectively, at 25 degrees celsius. However, if this were performed in the open atmosphere then you'd have to know the amount of Hydrogen Sulfide in the air at your place.

The Equation for the Tarnishing of Silver would probably look like:

2 Ag(s) + H2S(g) → Ag2S(s) + H2(g)

... and the Equation for the Oxidation of Iron would look like:

2 Fe(s) + O2(g) + 2 H2O(l) → 2 Fe(OH)2(s)

Since I'm being lazy, and since it's fair anyway since we always see green rust before red rust (ie Fe2+ before Fe3+), we're going to use these two equations to calculate the Ecello of the two reactions.

Half Equations:

Fe2+(aq) + 2 e- → Fe(s); - 0.44 eV
Ag+(aq) + e- → Ag(s); + 0.80 eV
2 H+(aq) + 2 e- → H2(g); 0.00 eV
O2(g) + 2 H2O(l) + 4 e- → 4 OH-(aq); + 0.40 eV

Since Ecello is calculated by subtracting the oxidation half-cell's reduction potential from the reduction half-cell's reduction potential, we get:

Tarnishing of Silver: - 0.80 eV
Rusting of Iron (Green): + 0.84 eV

Under standard conditions then the formation of green rust should be faster than the tarnishing of silver. Of course, rusting to red rust may be a different story but I don't have enough time to do it now so I'll leave it as this first.
 

Offline lancenti

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Re: Which oxidises first, iron or silver?
« Reply #4 on: 18/02/2009 03:08:54 »
Now that I think a bit more about it, maybe an energetics approach would be clearer. Though that'd have to assume that the kinetics are roughly the same.
 

Offline Chemistry4me

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Re: Which oxidises first, iron or silver?
« Reply #5 on: 18/02/2009 03:23:51 »
What ever happened to E values not giving information about the rate of reaction?
 

Offline lancenti

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Re: Which oxidises first, iron or silver?
« Reply #6 on: 18/02/2009 06:18:16 »
Well, they don't but we see one being a feasible reaction while the other one being a not feasible reaction. That itself kind of says one goes faster than the other since, well, the other one isn't going.
 

Offline Chemistry4me

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Re: Which oxidises first, iron or silver?
« Reply #7 on: 18/02/2009 06:31:25 »
Oh sorry lancenti [:I]! I wasn't even paying attention to the signs [:I][:I]
 

Offline miss_maple

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Which oxidises first, iron or silver?
« Reply #8 on: 18/02/2009 10:02:15 »
the
and there was this part where we had to number the reactions given from 1 -4 depending on the rate of reaction
And what were the other two?
the other two were ... i cant remember but they were correctly placed though !
 

Online Bored chemist

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Which oxidises first, iron or silver?
« Reply #9 on: 18/02/2009 20:02:42 »
The values you have for reaction potentials are correct for the case where all the components are in their standard states in an aqueous solutions and at unit activity. The oxygen concentration is only roughly 0.2 bar and the partial pressure of H2S is very low and there's no water.
Also ,as has been pointed out, the potentials tell you nothing about the raates of teh reactions and that's what the question is about.
If you apply these assumptions then you will show that the reaction of silver with H2S is not spontaneous at anything like normal conditions (it might work under some high pressure conditions).
However, in the real world, silver goes black.
there is, therefore clearly something wrong with one or more of the assumptions.

Did anyone consider the idea that "The answer in the book" is just plain wrong?
 

Offline techmind

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Which oxidises first, iron or silver?
« Reply #10 on: 19/02/2009 00:31:08 »
Does the Ag2S eventually "protect" the underlying silver from further sulphation? After all, it's a fairly hard black layer.

Rust tends to just get worse and worse - and in real-world outdoor conditions the flaky rust traps more water and if anything accelerates the decay.


It really does all depend on the conditions and whether you're looking at initial signs or the point of serious structural failure.

A car brake-disc shows rust just a few hours after the car has been parked up, or a well-used railway track top-surface goes noticeably rusty after a day with no trains.

We've all seen iron which falls apart with rust, but I've never seen or heard of domestic silverware getting to that state!!!


If the original question was as the title of this thread, ie which oxidises first then surely it has to be iron as we've said silver doesn't oxidise but reacts with sulphur.
« Last Edit: 19/02/2009 00:39:43 by techmind »
 

Offline lancenti

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Which oxidises first, iron or silver?
« Reply #11 on: 19/02/2009 03:18:00 »
The silver oxidizes from a zero oxidation state to a 1+ oxidation state. The half-equation was in my earlier post. Just because it doesn't react with oxygen doesn't mean it doesn't oxidize.

The layer of Ag2S indeed protects the inner layers. Very much the same way Al2O3 protects the inner Aluminium.

We're probably looking at surface decay, since structural failure depends very much on the structure itself and most iron structures do not have a silver equivalent.
 

Offline lightarrow

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Which oxidises first, iron or silver?
« Reply #12 on: 19/02/2009 09:29:28 »
The layer of Ag2S indeed protects the inner layers. Very much the same way Al2O3 protects the inner Aluminium.
Are you sure of this?

About the reaction

2Ag + H2S --> Ag2S

(but miss_maple still haven't clarified the reaction conditions), I'd say it could be faster then what one could say from ΔGf, because I've oserved it: in the island "Vulcano" in Sicily, near a hot sulfuric pot, a silver bracelet I had became dark in a few minuts.
« Last Edit: 19/02/2009 09:38:51 by lightarrow »
 

Online Bored chemist

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Which oxidises first, iron or silver?
« Reply #13 on: 19/02/2009 20:40:30 »
Does anyone happen to know if the reaction 2Ag + S -> Ag2S is spontaneous ie what's the formation energy of silver sulphide?

H2S is unstable in air- it should oxidise to S then to SO2 then (particularly in wet air) to SO3  or H2SO4.

Silver is used as a catalyst for some oxidation reactions.
Perhaps it catalyses the oxidation of H2S to S and then the S reacts with the Ag.
 

Offline lightarrow

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Which oxidises first, iron or silver?
« Reply #14 on: 19/02/2009 22:39:42 »
Does anyone happen to know if the reaction 2Ag + S -> Ag2S is spontaneous ie what's the formation energy of silver sulphide?
ΔG0f(Ag2S) = -40.67 kJ/mol from these tables:
http://publishing.yudu.com/Aqmh7/NomenclaturaChimicae/resources/53.htm?skipFlashCheck=true
 

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Which oxidises first, iron or silver?
« Reply #14 on: 19/02/2009 22:39:42 »

 

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