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Two questions: what reaction forms between ammonium nitrate, black iron oxide, and sucrose; and a story related question.

So here's the whole story:

So I was playing around a few months ago and stumbled upon a mixture that generated something similar to Black Snakes (firework). I recently decided to demo this mixture to someone, but I could not find my notes about the experiment, so I decided to try to replicate the mixture from what I remembered.

I am certain that the mixture contained ammonium nitrate, black iron oxide, and some sort of fuel (probably sucrose, impure in the form of powdered sugar). Combining these three ingredients (one part by volume, whatever particle sizes I have), and igniting with a lighter, the mixture does form black snaking ash, and turns red and crumbles as it cools.

Q1: So, what kind of reaction has taken place between the black iron oxide, ammonium nitrate, and sucrose?

If I leave out any one of the ingredients, the ash snake does not form, so all three compounds are definitely involved. If we leave out either the ammonium nitrate or sucrose, little happens with the application of heat.

Here's a picture after the fact.

I haven't had a very good batch since I lost my notes, so maybe I am missing an ingredient, or maybe I just haven't got the right ratios. Q2: Any Suggestions?^

Edit: I have been having more success with this mixture (by volume): 1 part ammonium nitrate 1 part potassium nitrate 2 parts black iron oxide 2 parts sucrose

Mix contents and melt over medium indirect heat (skillet is fine); once it is all melted, immediately remove from heat and form into pellet as it cools. The mixture will reduce in volume by several times. I suggest doing this in small batches since it can self ignite if you are not careful, or do not heat it at the right rate.

If the ammonium nitrate is omitted or substituted by potassium nitrate, the ash does not expand or cohere. So the ammonium nitrate is doing something more than just providing oxygen here. The reduction of iron oxide seems to make the ash less monolithic, and moderate reductions in iron oxide forms snakes that fall apart quickly during production.

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  • $\begingroup$ I'd say that's a quasi-dehydration reaction with Iron oxide as a catalyst forming nitric acid from the ammonium nitrate which removes the water equivalents from the sucrose. $\endgroup$ – user2617804 Jun 20 '14 at 2:29
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Disclaimer: what I offer here is an educated guess.

$\ce{NH4NO3}$ is known to decompose quite easily, especially if proper catalyst is present. Thus, one of the reactions is

$\ce{NH_4NO_3 = N_2 + 2H_2O + [O]}$

generated oxygen may be used to oxidize sucrose or black iron oxide. I doubt that $\ce{N2O}$ is formed, since it is quite strong oxidizer and decomposes quite easily, especially if redox catalyst is present

$\ce{C_12H_22O_11 + 24[O] = 12 CO_2 + 11 H_2O}$

$\ce{2Fe3O4 +[O] = 3 Fe2O3}$

However, as you said, the snake is black, so the second reaction does not occur

the final process is dehydration of sugar, producing carbon (actually, the carbon may contain significant amount of hydrogen and oxygen)

$\ce{C_12H_22O_11 = 12 C + 11 H_2O}$

at the same time numerous other processes, producing gaseous reducing organics, happens, so I advise to use a fume hood.

When cooled, reducing atmosphere from sugar decomposition vanishes, and black iron oxide reacts with air

$\ce{4Fe3O4 +O2 = 6 Fe2O3}$

Since black iron oxide acts as a catalyst and does not melt on reaction conditions, it is good to use as fine one as you can get and maybe it may be used in smaller amounts. Since ammonium nitrate acts as main energy source here, it is likely that it can be used in larger amounts. Sucrose forms the body of the 'snake', so it may be used in smaller amount.

The 'snake' consists of (partially) carbonized/caramelized sucrose binding other ashes together and leftovers from iron oxide. The expansion of volume results from gases emitted in the processes of decomposition of ammonium nitrate and decomposition of sucrose, though decomposition of ammonium nitrate is probably the main source here.

BTW, thanks for recipe, I think it would come handy for me this year.

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  • $\begingroup$ If you make it, I suggest melting it together over low indirect heat. It will shrink in volume considerably. $\endgroup$ – novellus Jun 20 '14 at 18:23

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