Re: What exactly is going on when iron is placed in vinegar?
Posted By: Richard Oldroyd, Post-doc/Fellow Davy-Faraday Research Laboratory
Date: Thu Apr 24 04:21:13 1997
Area of science: Chemistry
Well, Mark, an interesting problem and I must apologise for taking so
long to answer this question.
I have done quite a bit of reading about this matter as I was rather
surprised that the gas liberated did not test positive for hydrogen.
I therefore investigated the possibility that the Fe centres, somehow
caused the decomposition of acetate (from the acetic acid in vinegar)
to produce carbon dioxide. However, nothing in the literature would
seem to support this hypothesis.
My conclusions are as follows:
Although the test for hydrogen proved negative, I still believe the
hydrogen hypothesis for the following reasons.
- The initial reaction (as you suggested) is a straight oxidation of metallic iron to a mixture of FeO and some Fe2O3, due to the moist
air environment and the acetic acid that is covering the steel wool.
2 Fe + O2 ---> 2 FeO
2 Fe + 3 O2 ---> 2 Fe2O3
(the top reaction being most predominant)
The next phase is, I think, nothing more complex than acetic acid
residue reacting with the iron metal that isn't oxidised to produce
hydrogen - i.e. iron dissolution into a dilute acetic acid solution
to form iron acetate and hydrogen.
Fe + 2 CH3COOH(aq) ---> Fe(CH3COO)2 + H2
I know that it was not me who actually did the experiment, and I am
perfectly willing to be corrected on any of the above. However, there
are no literature precedents for acetate decomposition when coordinated
to iron - in fact both iron(II) and iron(III) acetates are rather stable
Although the acetic acid may be dilute, there would still be quite
a bit left on the steel wool, even after drying with a paper towel, as
the wool does have a high surface area.
It would not take much hydrogen liberation to start to counter the
effects of the oxygen uptake, which would be rather slow in comparison -
especially after most of it has been used up in the oxidation process.
In my own experience of testing for hydrogen by holding a flame near
the mouth of the test-tube and waiting for a "squeaky pop", a good
deal of hydrogen is necessary. Hydrogen escapes from the test-tube
very quickly and, if it is only there in small quantities, it would
be very difficult to ignite it in time. The other complicating factor
is that the test-tube is initially full of air, so a lot would need
to be displaced to get a decent amount of hydrogen in the test-tube.
My only suggestion when repeating this experiment is to try and let the
gas evolution continue for as long as possible so that any gas liberated
fills up the whole test-tube. Then repeat the tests for the gases.
My only warning (as may be obvious, and almost certainly doesn't need
reiterating to yourself being a high-school teacher) is that hydrogen
is explosive! If there is a substantial amount of oxygen
remaining in the test-tube, then you may get rather more than a
"squeaky pop" if the liberated gas is hydrogen!!!
If you do end up repeating these experiments, I would be extremely
interested in the outcome. Please let me know (mail firstname.lastname@example.org)
All the best, and happy experimenting.
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