Re: What exactly is going on when iron is placed in vinegar?

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:

1. 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)

2. 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

1. 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.

2. 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.

3. 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 richie@ri.ac.uk)

All the best, and happy experimenting.

Richie....