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Offline aeacfm

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iron +HCl
« on: August 29, 2010, 03:34:04 AM »
it is known that
Fe(s) + HCl (aq)  :rarrow: FeCl3 +H2

let us say that
Fe++ (iron compound in solid state) + HCl (aq)  :rarrow: FeCl3 +product

but what is the product of the following :

 Fe++(iron compound in solid state) + HCl (concentrated solution)  :rarrow:...
will it be Fe++ compound or Fe+++ compound

Offline aeacfm

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Re: iron +HCl
« Reply #1 on: August 29, 2010, 03:39:07 AM »
also if i want to obtain the following state :

Fe++(s) + HCl  :rarrow: FeCl2 + product

how can obtain it ?
i dont mean HCl specifically i want any reagent ro dissolve the Fe++ compound as it to Fe++ liquid phase.
 thanks in advance

Offline MrTeo

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Re: iron +HCl
« Reply #2 on: August 29, 2010, 03:53:02 AM »
To answer you first question:

2H3O+(aq)+2e-→2H2O(l)+H2(g) (E0= 0.000 V)
Fe3+(aq)+e-→Fe2+(aq) (E0=0.771 V)
Fe3+(aq)+3e-→Fe(s) (E0=-0.036 V)
Fe2+(aq)+2e-→Fe(s) (E0=-0.440 V)

just try to use this data, and remember that ∆G=-nF∆E...

Talking about your second question, I don't really understand what you mean with Fe2+(s). If you're talking about a Fe (II) compound dissolved in water and you want to prevent it from taking part in possible redox reactions I think the only way is to complex it using a chelating agent, but I don't know what you could use for your purpose...
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Offline Borek

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Re: iron +HCl
« Reply #3 on: August 29, 2010, 04:28:14 AM »
it is known that
Fe(s) + HCl (aq)  :rarrow: FeCl3 +H2

That's the first time I hear about. As far as I know H+ is not strong enough to oxidize iron to +3. Dissolving iron in acid we get Fe2+, which gets further oxidized to Fe3+ in the presence of air oxygen.

Fe3+(aq)+e-→Fe2+(aq) (E0=0.771 V)
Fe3+(aq)+3e-→Fe(s) (E0=-0.036 V)

There is some inconsistency here. I find it hard to believe that it is easier to oxidize Fe to Fe(III) than Fe(II) to Fe(III).
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Offline aeacfm

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Re: iron +HCl
« Reply #4 on: August 29, 2010, 05:23:55 AM »
To answer you first question:

2H3O+(aq)+2e-→2H2O(l)+H2(g) (E0= 0.000 V)
Fe3+(aq)+e-→Fe2+(aq) (E0=0.771 V)
Fe3+(aq)+3e-→Fe(s) (E0=-0.036 V)
Fe2+(aq)+2e-→Fe(s) (E0=-0.440 V)

just try to use this data, and remember that ∆G=-nF∆E...

Talking about your second question, I don't really understand what you mean with Fe2+(s). If you're talking about a Fe (II) compound dissolved in water and you want to prevent it from taking part in possible redox reactions I think the only way is to complex it using a chelating agent, but I don't know what you could use for your purpose...

many thanks MrTeo
so can i use hydroxyle amine ?i think it will work
the Fe++ i mean ferrous compound
also i will try  H2SO4 as it will form FeSO4 which is satble!!!!!
final thing do you mean by E0 data is to see the reaction  :delta:G and so elucide the rection product
it is known that
Fe(s) + HCl (aq)  :rarrow: FeCl3 +H2

That's the first time I hear about. As far as I know H+ is not strong enough to oxidize iron to +3. Dissolving iron in acid we get Fe2+, which gets further oxidized to Fe3+ in the presence of air oxygen.



i mean by FeCl3 it is the final product 
thanks for your kind reply

Offline FreeTheBee

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Re: iron +HCl
« Reply #5 on: August 29, 2010, 05:39:31 AM »
The numbers are correct, according to the handbook. They also add up when you take the numbers of electrons into account (
3*-0.036-2*-0.44=0.772).

Offline MrTeo

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Re: iron +HCl
« Reply #6 on: August 29, 2010, 06:55:40 AM »
final thing do you mean by E0 data is to see the reaction  :delta:G and so elucide the rection product

Well, if you find out what's the ∆E0 of the reaction you can easily see if it is positive or negative (that tells you if the reaction is spontaneous or not) and eventually compare it with other spontaneous reactions (the lower the ∆G is, the more favored the reaction will be, as $$ K_{eq}=\frac{\Delta G}{RT}/$$).
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Offline Borek

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Re: iron +HCl
« Reply #7 on: August 29, 2010, 07:37:55 AM »
The numbers are correct, according to the handbook.


None of my handbooks (nor other sources that I use) lists this reaction, they all list separately first stage (Fe(0) -> Fe(II)) and second stage (Fe(II) -> Fe(III)).

Quote
They also add up when you take the numbers of electrons into account (
3*-0.036-2*-0.44=0.772).

Can you elaborate on what you are doing here, as I am not getting the idea behind.
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Offline MrTeo

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Re: iron +HCl
« Reply #8 on: August 29, 2010, 08:07:37 AM »
Erratum



@Borek:

Actually I got that value from the Net, I'll try to retrieve the site and post the URL here...
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Offline FreeTheBee

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Re: iron +HCl
« Reply #9 on: August 30, 2010, 03:32:20 AM »
None of my handbooks (nor other sources that I use) lists this reaction, they all list separately first stage (Fe(0) -> Fe(II)) and second stage (Fe(II) -> Fe(III)).

These potentials are listed in the electrochemical series in the CRC Handbook of Chemistry and Physics (90th ed.). Among colleagues we usually refer to it as 'the handbook', hence my lack of specification.

Can you elaborate on what you are doing here, as I am not getting the idea behind.

I just checked if the numbers add up when converting them to free energy, using :delta: G=-nFE. I left out F since it gets divided out anyway, as well as some double minus signs.

Fe3+ + 3e-  :rarrow: Fe(s)  :delta: G = -3*F*-0.036
Fe2+ + 2e-  :rarrow: Fe(s) :delta: G = -2*F*-0.44

Subtracting the two gives :delta: G for Fe3+ + e-  :rarrow: Fe2+.
And E = - :delta: G/nF then gives, -(-3F*-0.036-(-2F*-0.44))/F = 0.772V.

Offline Borek

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Re: iron +HCl
« Reply #10 on: August 30, 2010, 06:30:05 AM »
OK, I see. But there is something fishy here, IMHO just because these numbers add up doesn't mean they are correct. Unfortunately my thermodynamics is so rusty I don't see what is wrong with this approach, but it is not that difficult to show that something _is_ wrong.

Imagine you have a piece of iron in an acid solution. For the sake of simplicity let's assume activities of all substances involved are 1, so we can deal with standard potentials. Iron dissolves and hydrogen evolves. In such solution, as suggested by the potential given, iron gets oxidized to Fe3+. So, what we do have now? H+ and H2 - that means half cell of potential 0, and Fe3+. But if you look at standard potentials table Fe3+ can't exist in such mixture, with its standard potential at 0.77 V it will immediately oxidize hydrogen to H+ and get reduced to Fe2+. So there will be no Fe3+ in the solution (well, some traces), only Fe2+.

I have a feeling that potential given for Fe/Fe3+ is calculated using approach similar to the one you have used to check the numbers (so in fact you have just reversed the procedure getting back to the original data - no wonder numbers add up). There is something wrong this approach, as it gives values that won't survive in solution.
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Offline FreeTheBee

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Re: iron +HCl
« Reply #11 on: August 30, 2010, 08:19:16 AM »
I agree it looks strange. I looked it up in "Standard Potentials in Aqueous Solutions" by Bard, Parsons and Jordan. The standard potential for Fe(III)/Fe is indeed calculated from the other two half reactions. They refer to the Pourbaix book "Atlas d'équilibres électrochimiques".

Why would iron form Fe(III) based on the potentials? The standard reduction potential of Fe(II) to Fe is -0.44V, and thus 0.44V for the reverse process, which occurs upon dissolution. For oxidation to Fe(III) it is only 0.036V. The dissolution of iron in acid should then give mostly Fe(II) and some Fe(III). The Fe(III)/Fe(II) standard potential also pushes the system in that direction. I think it is consistent even though the numbers look a bit weird.




Offline aeacfm

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Re: iron +HCl
« Reply #12 on: August 30, 2010, 09:27:27 AM »
i can see that you Forgot heart of the matter and every body want to prove some thing to him self

Offline Borek

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Re: iron +HCl
« Reply #13 on: August 30, 2010, 09:53:55 AM »
The standard reduction potential of Fe(II) to Fe is -0.44V, and thus 0.44V for the reverse process, which occurs upon dissolution.


Where this idea of reversed potential comes from? Reaction doesn't care which way it goes, for given activities of all substances involved it goes in both directions at exactly the same potential, as given by the Nernst equation. Take a look at the CV curve - http://en.wikipedia.org/wiki/Cyclic_voltammetry - one scan is reduction, other is oxidation, both go at (almost) the same potential; difference is 0.059/n and (for reversible processes) doesn't depend on standard reduction potential.
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Offline Borek

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Re: iron +HCl
« Reply #14 on: August 30, 2010, 09:59:25 AM »
i can see that you Forgot heart of the matter and every body want to prove some thing to him self


It is not that easy. You assumed that Fe dissolves producing Fe3+ - as I have pointed out as far as I know this assumption is wrong. However, according to CRC handbook quoted by others you are right - but numbers shown in the book lead to absurd conclusions. We are trying to find out what is really going on.

FeCl2 solution acidified with HCl will not miraculously convert itself to FeCl3 solution. Also dissolving ferrous salt in HCl will not lead to its oxidation, you will be still left with Fe2+.
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