hodges
National Hazard
Posts: 525
Registered: 17-12-2003
Location: Midwest
Member Is Offline
|
|
Fe(2) vs Fe(3) - I'm confused
I made iron chloride a while back through the electrolysis of NaCl in water with a membrane between the plates. I wasn't sure which chloride of
iron I ended up with, but the solution was dark orange which would indicate FeCl3. I let this dry and ended up with an orange powder.
Recently, I took some of the (presumed) FeCl3 and made a solution of it. I added NaOH until the solution was slightly basic. The result was a thick
orange precipitate, which I presume was Fe(OH)3. After repeatedly filtering and washing this precipitate, I dried it for several hours in an oven at
400 degrees F. I expected to get Fe2O3, which is orange. Instead, I got a powder that was very dark - it was orange, but so dark that it looks
closer to black unless viewing very small amounts of it. I successfully used this iron oxide to make a small amount of thermite.
Confused as to why the product was not rust colored, I thought maybe I really didn't have FeCl3 to start with. So I bought some PC board etching
product that the label said contained FeCl3. Again I added NaOH until the solution was slightly basic, filtered and washed the precipitate multiple
times, and dried in an oven at 400 degrees F for a couple hours. Again, I ended up with a product that was very dark orange - closer to black - in
color.
Then I dissolved some steel bolts in HCl, resulting in a dark green solution. The green color would indicate that it was FeCl2 instead of FeCl3.
After precipitating using NaOH, washing, and drying in the oven as before - surprise - the resulting product this time was rust-colored! That would
indicate Fe2O3, right?
Presumably then the iron oxide I got from FeCl3 was either FeO or Fe3O4 based on the nearly black color. And the oxide I get using FeCl2 is
presumably Fe2O3, again based on color. So why does FeCl3, where iron is +3, give FeO where iron is +2? And conversely, why does FeCl2, where iron
is +2, give Fe2O3 where iron is +3? This switching of oxidation numbers doesn't make sense to me. I could see if Fe remained the same or if it
always switched to one value or another - but why would it always switch to the opposite value?
I made thermite using the orange oxide. This thermite reacted much faster than the thermite made with the black oxide (in both cases I used 25% Al
and 75% iron oxide by weight). The faster reaction would indicate to me that the orange product is Fe2O3, since Fe2O3 thermite is more energetic than
FeO or Fe3O4 thermite. Of course, it might also be because the orange product is softer and easier to grind into small particles than the black
product.
I then did a test for magnetic properties, which again yielded a surprise. The black product shows practially no attraction to a magnet. Even a
strong neodynium magnet only produced a slight hint of attraction. But the orange product is strongly magnetic! But neither FeO or Fe2O3 is
magnetic, right? And Fe3O4 is magnetic, but its not orange, right?
Can someone explain what I am seeing and which iron oxide is produced in each case?
Thanks,
Hodges
|
|
|
guaguanco
Hazard to Others
Posts: 216
Registered: 26-11-2003
Member Is Offline
Mood: heterocyclic
|
|
Without diving too deeply into the reactions between Iron, H+, OH- etc:
I would be very cautious about trying to deduce your results purely based on the colors of your solution or precipitate:
1) Are you starting with chemically pure Iron, or a steel alloy? There might be a variety of other metals in your initial Iron source(s).
2) Iron itself forms a wide variety of oxides, hydroxides and amorphous mixtures of the two. Trying to deduce exactly what you have from the color of
a precipitate is going to be tough.
If you could get some pure Iron, that would be a big step forward. There are specific tests for ferrous and ferric ions; these tests will be your best
bet for determining the course of your reactions.
|
|
|
Al Koholic
Hazard to Self
Posts: 98
Registered: 2-12-2002
Member Is Offline
Mood: Seeking ligand
|
|
A reasonably pure source of iron would be the cheap brands of fine steel wool. I don't know if you've all seen those stainless steel pads
for cleaning but those are usually more expensive and cannot be used as iron.
As a side note, I have used this type of steel wool for Fe + S --> FeS with good results. I was able to make a nice H2S generator with the
product. Additionallly, if the wool is made wet with tap water, it is usually heavily rusted 24 hours later with crumbling to orange powder (Fe2O3)
within days. It also burns when ignited with a match.
[Edited on 19-1-2004 by Al Koholic]
|
|
|
hodges
National Hazard
Posts: 525
Registered: 17-12-2003
Location: Midwest
Member Is Offline
|
|
An Update
I was able to buy some "99% pure" Fe2O3. It has a red color, similar to what I got from what I made by dissolving steel bolts in HCl and
precipitating/baking Fe(OH)2. It is only very slightly attracted by a magnet (just like the black stuff I got from precipitating Fe(OH)3 from FeCl3).
Given the high purity of the purchased FeCl3, I expected a very fast thermite reaction so I only made about 1/2 gram. Surprisingly, the thermite
reaction was significantly slower than the one with the iron oxide made from the bolts. I'm guessing that the Fe2O3 I purchased might have more
moisture in it than the stuff I made. Which is fine - I don't need the thermite reaction to be any faster than it is already (I have a picture
of it in Misc under the topic Where to Experiment).
Hodges
|
|
|
|
![[*]](images/xpblue/default_icon.gif){kind=icon}
