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The SDS for ammonium nitrate says it will react with powdered metals spontaneously. What would be the reaction equation, say with iron or copper?

This mixture is used in various pyrotechnics, and it is intentionally allowed to combust with heat and shock, but how does the reaction proceed spontaneously as described in various SDSs in the absence of any ignition source? Is this due to the mix attracting small amounts of water, allowing the nitrate to attack a metal instead?

Would the theoretical products always just be various oxides of the metal involved?

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closed as too broad by Nilay Ghosh, A.K., Mithoron, Todd Minehardt, Tyberius Jul 6 '18 at 19:40

Please edit the question to limit it to a specific problem with enough detail to identify an adequate answer. Avoid asking multiple distinct questions at once. See the How to Ask page for help clarifying this question. If this question can be reworded to fit the rules in the help center, please edit the question.

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    $\begingroup$ How about your own attempt at an equation? $\endgroup$ – Karl Jul 4 '18 at 17:10
  • $\begingroup$ What powdered metal? $\endgroup$ – Nilay Ghosh Jul 4 '18 at 17:13
  • $\begingroup$ Iron or copper. I'm assuming they end as oxides? But does the reaction proceed differently when it starts spontaneously? $\endgroup$ – crestind Jul 5 '18 at 4:33
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I'm betting they mean that you can inadvertently make an explosive (technically a blasting agent) or highly flammable mixture by mixing powdered metal and ammonium nitrate. I used to be a hard rock miner many centuries ago and was well read on explosives. Aluminum powder and ammonium nitrate is a classic solid rocket fuel and industrial blasting agent/military explosive, for instance ammonal in bombs. Ammonium nitrate/aluminum powder mixes (magnesium works, too) have better explosive properties than the usual mining ANFO (ammonium nitrate/fuel oil) mixtures, but the latter are preferred usually since ANFO is less expensive.

While blasting agents technically need a booster explosive to detonate, they will burn ferociously. I'm guessing when a writer of a SDS sheet talks about powdered metal and AN, they are thinking about the potential of creating an extremely flammable mixture.

I looked at some MSDS for ammonium nitrate, and I see powdered metal listed along with reducing agents as incompatible mixtures. Could you post a link to the MSDS you are reading? I'm interpreting 'incompatible' as referring to explosive potential and not to a slow spontaneous reaction.

EDIT: Well. The usual metal powders that react with AN used in mining and explosives are aluminum and magnesium, so that is all I considered. However, a bit of research shows that powdered zinc does react with ammonium nitrate, as in this video (note that it didn't actually ignite until set afire, but it definitely reacted).

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  • $\begingroup$ sciencelab.com/msds.php?msdsId=9927336 $\endgroup$ – crestind Jul 5 '18 at 1:20
  • $\begingroup$ That SDS has a section saying "can self-ignite/detonate when in contact with powdered metals". I'm assuming that when this particular metal powder and AmNitrate mixture is used intentionally as a pyrotechnic, it's the igniter that sets the whole mix off by decomposing the AmNitrate releasing O2, which sets off the reaction by rapid oxidization of the metal. So I'm a bit confused as to how it could spontaneously occur as described by the SDS, and whether that reaction would be any different to start. My guess is that there would need to be moisture involved? $\endgroup$ – crestind Jul 5 '18 at 1:27
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What would be the reaction equation, say with iron or copper?

It is straightforward stoichiometry to do an equation for a simplified reaction that only considers the main products. For your copper example:

$$\ce{NH4NO3 + Cu -> N2 + 2 H2O + CuO}$$

Copper hydroxides will not form, because they are unstable at the high temperatures of this reaction.

However this oversimplifies things quite a bit for three main reasons.

Firstly, in an accidental fire it is unlikely to be a balanced mixture so one ingredient or other will be in gross excess, which increases the likelihood of side reactions.

Secondly, reactions of this type are often violent, which means they tend not to reach equilibrium. Hence there can be small traces of all sorts of weird things, such as even some free hydrogen from:

$$\ce{H2O_{(g)} + Cu_{(l)} -> H2_{(g)} + CuO}$$

Why doesn't the hydrogen react with more nitrate, or oxygen from the air? Mostly it does, but in a fast, violent reaction a small amount escapes unreacted.

Thirdly, ammonium nitrate disproportionates when it is heated. This releases a complex mixture of nitrogen oxides, water, and nitrogen oxyacids. The reaction starts at fairly mild temperatures but both the rate and the mixture of products depends on the temperature.

Some of these volatiles will react with the hot metal powder, but some will escape the reaction zone. The amount that will escape will depend on how intimately the powders are mixed, the granule size distribution, and the thickness of the pile.

This mixture is used in various pyrotechnics,

Is it? So far as I know, even though ammonium nitrate is a fairly good oxidiser it is avoided in pyrotechnics because it is too hygroscopic. That gives unpredictable performance and makes storage difficult.

how does the reaction proceed spontaneously as described in various SDSs in the absence of any ignition source? Is this due to the mix attracting small amounts of water, allowing the nitrate to attack a metal instead?

Yes, very probably. If fine powders are intimately mixed, quite small amounts of moisture may allow the reaction to start in the liquid phase even without energy input. However there are other ways it could start: direct application of heat; electrostatic discharge; local heating by friction or impact. Depending how sensitive the mixture is, some of these stimuli might be small enough that the event appears spontaneous. This will depend on the particle size distribution (smaller is more sensitive) and the reactivity of the metal: fine zinc powder is likely to be very much more dangerous than coarse copper turnings.

Either way, under appropriate conditions the reaction will accelerate:

  1. If a sufficiently large zone is reacting, then heat is being generated (proportional to volume) faster than it escapes (proportional to area);
  2. This causes the temperature to rise;
  3. This accelerates the reaction rate, increasing rate of heat production;
  4. Feedback loop occurs until the temperature is so high that heat loss and heat production is balanced (or the fuel is consumed, whichever occurs first.) For metal/oxidiser reactions, this can be thousands of degrees.
  5. At some point, the metal may melt (most of them) or even boil (zinc), which will change things from a gas:solid reaction to gas:liquid or gas:gas, which is much faster.

Would the theoretical products always just be various oxides of the metal involved?

I guess you mean the solid products, since gases are also certainly released. In general oxides are likely to be the main product since they are usually thermodynamically favoured at the high temperatures of metal/oxidiser reactions. However it depends on the metal. Other possibilities include peroxides and hydroxides. In a few cases, there may also be some nitride formed.

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