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Author: Subject: Oxidation of iron by ferric nitrate and oxygen
Σldritch
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[*] posted on 29-3-2017 at 11:51
Oxidation of iron by ferric nitrate and oxygen


Im doing homework and i need to know the reaction in the title. I think it would go something like this with the ferric nitrate acting as a catalyst:

4 Fe + 3 O2 + 6 H2O =Fe(NO3)3=> 4 Fe(OH)3

Fe0(s) + 2 Fe+3(aq) = 3 Fe+2
Fe+2(aq) + NO3-(aq) + H2O = Fe+3(aq) + NO2.(aq) + 2 OH-(aq)
2 NO2.(aq) + 2 OH-(aq) = NO2-(aq) + NO3-(aq) + H2O
O2(aq) + 2 NO2-(aq) = 2 NO3-(aq)
Fe+3(aq) + 3 OH-(aq) = Fe(OH)3(s)
When doing the experiment i only observed rust as you would expect from this but i did not exactly have high-tech equipment.

On a related note; does ferric nitrate oxidize copper? When i tried this the copper only got slightly less shiny.


Is it correct? Is it of any use in the preparation of rust for the home chemist?
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AJKOER
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[*] posted on 29-3-2017 at 13:58


If this is an education inspired example, it is not a good one, as one is starting with a net (summary) reaction.

First, the underlying reaction is a spontaneous electrochemical reaction in two stages. First, with Fe acted upon by O2 and H2O in the presence of H+ (which is one the reason for the presence of acidic Fe(NO3)3 ). The reaction continues via a redox with Fe(ll), O2, H2O and H+ and has also been described as electrochemical in nature. The presence of Fe(lll) (from Fe(NO3)3) also provides a source of Fe(ll) from Fe. The ferrous feeds a redox cycle recreating ferric and also radicals.

Apart from the electrochemical depiction, there is also a parallel system of radical based reactions steps involving the creation of the hydroxyl radical (.OH) even without NO3- just H+. The latter reacts with NO3- to form the nitrate radical .NO3 and OH-....The reactions go on and on. The whole complex system is also pH dependent (not to mention other likely important factors as temperature, ionic strength, pressure, surface air interface, the presence any uv light and a small amount of a stable electrolyte).

In my humble studies of such systems, my opinion is to assert that the reaction system listed above possibly accurately depict such a system is the work of the ignorant leading the blind.
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From a source, a possible electrochemical based reaction is given by:

4 Fe(ll) (aq) + 4H+ + O2 → 4 Fe(lll) + 2 H2O (see http://corrosion-doctors.org/Experiments/rust-chemistry.htm )

Some respective half cell reactions (with source):

Cathode: O2 + 4 H+ + 4 e- ---> 2 H2O (see page 20-13 at http://www.public.asu.edu/~jpbirk/CHM-115_BLB/ClassNotes/chp... )

Anode: Fe (s) ---> Fe(ll) + 2 e-

Or, following the consumption of surface iron:

Anode: Fe(ll) ---> Fe(lll) + e-
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Radical based alternate view per my prior comments, to quote:

"First, Ferrous + O2 (assumed present during the boiling part of recovery of HCl) + acid yields a basic Ferric chloride. Here is a similar reaction sequence based on copper:

R24 O2(aq) + Cu+ → Cu2+ + .O2− ( k = 4.6xE05 ) (see Table S-1 at http://www.atmos-chem-phys.net/13/11259/2013/acp-13-11259-20... )
R27 .O2− + Cu+ + 2 H+ → Cu2+ + H2O2 ( k = 9.4xE09 )
R25 H2O2 + Cu+ → Cu2+ + .OH + OH− ( k= 7.0 xE03 )
R23 .OH + Cu+ → Cu2+ + OH− ( k = 3.0×E09 )

Net radical reaction: O2 + 4 Cu+ + 2 H+ → 4 Cu2+ + 2 OH- "

Link: http://www.sciencemadness.org/talk/viewthread.php?tid=70332
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With respect to my comment on the exhaustive nature of possible radical chemistry introduced by nitrate, see my discussion and references at the end of this SM thread: https://www.sciencemadness.org/whisper/viewthread.php?tid=72... .

Also note my comment at the end of the thread on possible surface chemistry coming into the system. I should not leave out atmospheric chemistry as well as performing this experiment by introducing the solution to air as a very fine mist could alter the results (see http://m.pnas.org/content/111/2/623.full ). My explanation is that superoxide radical noted above (R24, R27) is converted to the .HO2 radical in a medium of air as a consequence of the change in dielectric constant altering the effective pKa (see, for example, discussion at https://en.m.wikipedia.org/wiki/Hydroperoxyl ). This apparently, per my provided link, does impact Fenton reaction systems based on iron in the presence of H2O2 (where the hydrogen peroxide could be formed in situ per R27 at a different pH than in bulk solutions).
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I have seem summary reactions of such complex systems and there are clearly characterized generally by more products than expected with decimal coefficients! This would be, of course, unacceptable in most educational settings, as it would generate questions that most teachers and even professionals would be hard pressed to answer.

[Edited on 30-3-2017 by AJKOER]

[Edited on 30-3-2017 by AJKOER]
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