[joenhermans]

I encountered a strange phenomenon the other day. I was trying to dissolve some fine iron powder (about 1 gram) in nitric acid (should have used sulphuric acid, I know). I took about 15 mL of 65% nitric acid, added about 10 mL of water and added the fine powder. Nothing happened, even when the beaker was stirred by hand, no temperature rising, nothing. I let it stand for about 20 minutes, but when I came back, still no sign of dissolving. The iron powder just stayed in an even layer on the bottom of the beaker.
Then I thought of heating the mixture, so I took a magnetic stirring plate with heater and put the beaker on top of it. Within a second, a huge cloud of NO₂ formed in the fume hood, the mixture boiled heavily and drops of acid splashed everywhere. What happened there?

I have put this question to my fellow students, but no one has a clue what happened. I tried to reproduce the effect (being a scientist and all), but without succes. I think it has something to do with the magnet in the stirring plate, because the iron powder forms spikes with a big surface when I held a magnet under the beaker in my second try. But that doesn't explain why my second try failed. Can someone explain to me?

[covek11]

I doubt that magnet has to do anything to it.
Let me tell you about my experience and answer could be there.
I tried to dissolve Fe turnings in 500 cc of HNO3 in a 4L beaker. I think I used 70% conc HNO3 at 50 C. For a while nothing was happening. Turnings were stirred by a lab stirrer.
At one poing (after 30-40 min) a few bubbles started to come out. That moment is crucial.Within one minute mixture almost boiled, so I had to add water to cool it down quickly. Dissolution continued but it took forever to dissolve it and amount of NO2 was unbelievable. In the end I found small amount of probably Fe3O4 which couldn't be dissolved by HNO3.
I still wonder how they make Fe(NO3)3 on a large scale ? Any ideas ?

[joenhermans]

One of my friends did tell me that dissolution can happen quite sudden and extreme. At least, his supervisor had a similar effect once, but with other chemicals. It makes me wonder what circumstances lead to this sudden reaction. I did let my second try at the experiment stand for about half an hour, to see if the reaction would occur without a magnet being present.

[zxt]

I encountered a strange phenomenon the other day. I was trying to dissolve some fine iron powder (about 1 gram) in nitric acid (should have used sulphuric acid, I know). I took about 15 mL of 65% nitric acid, added about 10 mL of water and added the fine powder. Nothing happened, even when the beaker was stirred by hand, no temperature rising, nothing. I let it stand for about 20 minutes, but when I came back, still no sign of dissolving. The iron powder just stayed in an even layer on the bottom of the beaker.
Then I thought of heating the mixture, so I took a magnetic stirring plate with heater and put the beaker on top of it. Within a second, a huge cloud of NO₂ formed in the fume hood, the mixture boiled heavily and drops of acid splashed everywhere. What happened there?

I have put this question to my fellow students, but no one has a clue what happened. I tried to reproduce the effect (being a scientist and all), but without succes. I think it has something to do with the magnet in the stirring plate, because the iron powder forms spikes with a big surface when I held a magnet under the beaker in my second try. But that doesn't explain why my second try failed. Can someone explain to me?

The key is heat. And consider whether this dissolving reaction is endo/exothermic. Dissolving speed can change dramatically by changing temp.

[joenhermans]

It sounds plausible that dissovation is much faster at higher temperatures, which explains why the reaction was so sudden. But what triggered it then? Is the movement of the beaker onto a plate enough to begin the reaction?

[zxt]

Another cause is rotation. When you didn't use a magnetic stir, the powders stayed agglomerated in the bottom of the beaker and the surface of the powders was not big enough for them to dissolve fast,thus the temp of the solution would not increase immediately(I think this dissolution is exothermal because you told that NO₂ was released which indicates the decomposition of HNO₃ which absorbs the heat (given by the dissolving reaction in this case) to conduct.).So when stir was used, the powders were separated to a great extent that caused the surface increased and dissolution speed was greatly accelerated meanwhile it helped to raise the temp to helping dissolving the powders furthermore.You know I mean it forms a reaction circulation.So that may be the reason why a sudden dissolution happened but not any related to magnetic field.

[joenhermans]

But the stirrer wasn't turned on when I put the beaker onto the plate... The magnet does increase the surface area of the iron powder, maybe that is enough to increase the temperature.
Still a bit weird (and dissapointing) that I failed to reproduce the effect.

[billnotgatez]

When you say the stirrer was not turned on do you mean the heater or the stirring or both?

[joenhermans]

When you say the stirrer was not turned on do you mean the heater or the stirring or both?

Both, I was about to turn on the heater when the drops of acid started flying around.

[zxt]

Since you didn't turn on the stirring and heater, neither would be the magnetic field.It has nothing to do with the magnet or eventhough it does,magnet may not is the only reason attrbutes to this phenomenon at least,otherwise your second trial would not fail.But this phenomenon is really weird,could you tell more details?

[joenhermans]

Most details are already mentioned in my first post.
I don't know exactly what grain size the iron powder was, but it was impossible to see individual grains of iron, it was really like a dust. I weighed one gram of it, rinsed the plastic weighing cup with concentrated nitric acid to get all the iron in the beaker. Then the beaker was filled to about 15 mL with the same 65% nitric acid. I also added about 10 mL of distilled water, I am not exactly sure why.
In the next 20 minutes, I stirred the liquid occasionally by turning the beaker. Then I put the beaker in a stirring plate with heater. Underneath the heating plates is a rotating bar with two magnets glued on top of it, which spins when you turn the stirrer on. Within a second after I put the beaker onto that plate, the reaction occured, producing a cloud of NO₂ that filled about half of the fume hood. The mixture was too hot to touch, but I did remove the beaker from the plate, in a reflex. After the reaction, the mixture was of a dark reddish brown colour, with black specks of iron still present.
The second time, I had only around 0.25 gram of iron left, which was put in 10 mL nitric acid. But this time, nothing happened whatsoever. I stirred it, put it on the stirring plate, held a neodymium magnet under it, waited 15 minutes and tried everything once more.
Maybe the different amount of iron in the second try was fatal, maybe it was just a lucky coincidence the first time. Although I am not really a fan of lucky coincidences in chemistry...

[zxt]

Now one thing can be sure that the huge heat comes from the oxidation reaction:Fe + 6HNO₃ :rarrow:Fe（NO₃)₃ + 3NO₂ + 3H₂O. There may be a coincidence that when you put the beaker on the plate, the temp had raised to a certain acute reaction point. Yes, with different amount of reactant, the heat given is different makes the temp not so high.

[joenhermans]

Yeah, I was thinking something like that, it seems to be the only reasonable explanation that I (and some other people who I have questioned) can think of. Not entirely satisfactory, but reasonable.

[BluRay]

Maybe it has just to do with the beaker's movement, made exactly in a moment in which the reaction was about to start. I remember a similar experiment with iron wool in diluted HNO₃ (probably not diluted enough to prevent passivation); I left it in a beaker but nothing happened for some minutes. I then added H₂O₂ with the idea of accelerating the oxydation, but none. Then, suddenly, a great amount of NO₂, heat, vapour and liquid spilling around; I didn't even have the time to move the beaker in a more suitable place.

About getting solid, crystalline Fe(NO₃)₃ (hydrated) it's not possible in that way because it hydrolizes during concentration. I don't know how they make it.

[billnotgatez]

Could you tell us what happens when you turn on the stirring and not the heat