So we all know that iron nails don't burn, but iron wool will smolder and
iron filings will spark in a Bunsen flame. This is a surface area effect
maximising the contact between air and the metal. If we can get extremely small
pieces though, is it possible that iron could burn even without a flame? Let's find out.
When I was first training to teach and looking for good ideas for
chemistry lessons in terms of practicals and demos, there were two reliable texts
that I could always go to: Classic chemistry practicals and Classic
chemistry demonstrations. While much of the health and safety guidance is now
out of date, they're still worth dipping into for ideas. There was one demo I
always wanted to try for showing surface area effects, but I was always a little
bit put off by not having had iron ethanedioate in stock, also known as iron oxalate.
If I had read on I would have found out it's actually quite easy to
make from chemicals you're more likely to have in the cupboard. So today we're
going to make some of this and decompose it to produce an extremely fine powder
of iron, which will spontaneously combust in air. Obviously you can make as much as
you like, but in the instructions I've gone for quantities that will yield
enough for around four demonstrations. Simply make up some solutions of sodium
ethanedioate and iron sulfate. For further details, as always, check out the
accompanying article on the EiC website. These two chemicals can be combined to
precipitate out the iron ethanedioate. Once you have the two solutions, combine
them, filter off the lovely yellow precipitate you get,
and leave it to dry overnight.
It's then ready for demonstrating with. Once you're ready, simply take 2 g
of your iron ethanedioate, load them into a borosilicate boiling tube
and use some mineral wool to loosely plug the end so the fumes can escape,
but any powder will be retained in the tube. Work in a well-ventilated area,
because carbon monoxide is going to be released in the reaction. As the sample
is heated, water of crystallisation is first released. You'll see condensation
further up the tube and the sample will begin to turn black.
So, although when I first filmed this I did so using the tube held in a clamp
stand for the ease of focusing, I actually recommend that you do this
using a handheld test tube holder, a little bit like this one and that just
allows you to agitate the the mixture and make sure that you've got some even heating.
That allows the reaction to proceed a little bit better.
As we reach these sort of higher temperatures and the water of crystallisation is finished
coming off, what starts to happen is that the evolved carbon monoxide begins to
reduce magnetite that's in our mixture here and that reduces it down to these
very ultrafine particles of iron. Also iron carbide thermally decomposes to
produce more iron and we get these like... this really fine powder here. And as I'm
agitating this mixture, I'm able to sort of determine when the gases are sort
of finished coming off, because if the viscosity - if that's the sort of correct
word to use here - of the powder sort of increases, it stops flowing around quite
so much I get the sense that it's pretty much finished reacting so I can take off
the heat. I'm only using a blowtorch for again for the ease of filming. Of course
there's no reason why you can't use a Bunsen burner.
And give your iron powder a little bit of time to sort of cool down.
Any sort of carbon monoxide in here will maintain this in a reducing environment
so you'll be able to sort of keep the powder for your demonstration. And then
when you're ready... I recommend obviously sort of working in
a slightly dimmer room so I might actually go and pull the blind across
momentarily. Take out the mineral wool at the top, pour out your powder from a
reasonable height onto heat-resistant mats and you get these lovely
smoldering particles of iron demonstrating the importance of high
surface area in kinetics.
So there we go, very simple. All you've got to do is heat up
some yellow powder, pour it out into a heat-resistant mat and you're golden.
A really good, quick, simple demonstration of surface area effects. Give it a try
and if you do, let us know in the comments how you got on with it.
See you next time.