# The Naked Scientists Forum

### Author Topic: Relative Molecular Masses in Thermite  (Read 2672 times)

#### wr2202

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##### Relative Molecular Masses in Thermite
« on: 20/06/2014 15:15:24 »
Hi! I'm new here, so I don't know if I've posted this in the correct place, please tell me if I haven't!

Okay, so I was making some nice homemade thermite for a bit of fun, and I've got my atomised aluminium powder and iron III oxide. Google tells me that the ratio of iron oxide to aluminium most people stick to is 8:3, so I thought I'd do the equations myself to check the chemistry behind this. These are the relative atomic masses that I got from Wikipedia, rounded to 3 decimal places:

Fe: 55.845
O: 15.999
Al: 26.982

The equation for the thermite reaction is Fe2O3 + 2Al --> 2Fe + Al2O3. Which makes the relative formula masses:

Fe2O3: 159.687
2Al: 53.964
Total reactants: 213.651

So if I'm right so far, surely that makes the percentages:

Fe2O3: 74.74%     (159.687 / 213.651)
2Al: 25.26%     (53.994 / 213.651)

This ratio is almost exactly 9:3 (3:1), which makes me wonder why the recognised ratio is 8:3. Have I missed something? I just wondered if anyone could clear this up for me, because I hate not knowing why I'm wrong!

Thanks!
« Last Edit: 20/06/2014 15:18:46 by anonymous_moron »

#### chiralSPO

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##### Re: Relative Molecular Masses in Thermite
« Reply #1 on: 22/06/2014 18:22:48 »
There is a reason that the experimentally determined optimum is more "aluminum rich" than you would find by calculation. The exact best proportion depends on the particle size and purity of the aluminum powder, but it is a pretty safe bet that each particle is not entirely aluminum. In the case of "pure" atomized aluminum, there will be a small coating of aluminum oxide at the surface (where the aluminum has reacted with air)--this increases the mass of the aluminum, and decreases the available fuel, as compared to what you would calculate. One can also find aluminum that does not have an oxide coating, but is instead coated in plastic or wax--this has the same effect on the calculations.

#### fishmicon

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##### Re: Relative Molecular Masses in Thermite
« Reply #2 on: 02/02/2015 10:46:02 »
that does newbielink:http://www.mmovegas.com/fifa-15-coins.html [nonactive] not have an oxide coating

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##### Re: Relative Molecular Masses in Thermite
« Reply #2 on: 02/02/2015 10:46:02 »