# Ammonium Nitrate

Discussion in 'Chemistry' started by exchemist, Aug 15, 2020.

1. ### exchemistValued Senior Member

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Seems topical to post the two competing decomposition reactions that can lead to explosion:

1) NH4NO3 -> N2O + 2H2O , predominant at lower temperatures

2) 2NH4NO3 -> 2N2 + 4H2O + O2 , at higher temperatures

Both are exothermic.

In the second mode, it generates free oxygen, capable or reacting with another oxidisable substance, as well. Hence its use as an ingredient of explosives when mixed with other things, most notoriously, in the case of the IRA, sugar.

The Beirut explosion was notable for its orange cloud, which is due to NO2, a secondary reaction product which absorbs mainly in the blue/green region of the spectrum, leading to a red/orange colour.

Unrelated to its explosive potential, ammonium nitrate is also interesting because it dissolves in water very endothermically. You stir it in and the water becomes instantly cold! It is a nice demonstration of how a reaction can occur spontaneously, even though it requires significant energy input, due to the increase in entropy associated with dissolution. Reactions take place when there is a reduction in the free energy, G, i.e. a negative value for ΔG = ΔH - TΔS. In this case even though the enthalpy, H change is +ve (i.e. you have to put energy in), ΔS is sufficiently +ve that the -TΔS term wins.

As to why the entropy change is so +ve in the case of ammonium nitrate, I am don't have the answer. It may be that less of a solvent cage is formed when these ions dissolve than is the case with most ionic compounds, so there is not much of an ordering process among the solute molecules to offset again the disordering due to the break up of the crystal structure. But I have never seen this explained so I am speculating.

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3. ### James RJust this guy, you know?Staff Member

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I'm not a chemist, so can't really comment. The information about how ammonium nitrate explodes is interesting, though. Thanks.

About that orange $NO_2$. From memory, there's some kind of equilibrium between $NO_2$ and $N_2 O$, isn't there? Does that mean the the initial decomposition of ammonium nitrate produces $N_2 O$, some of which then spontaneously transforms to $NO_2$?

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5. ### exchemistValued Senior Member

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The equilibrium you are thinking of is between NO2 and its dimer, N2O4, which is colourless, i.e. rather trivially: 2 NO2 <-> N2O4.

I think the appearance of NO2 is via further reactions: oxidation of N2O, or even N2, by the free O2 generated.

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7. ### James RJust this guy, you know?Staff Member

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Oh, yes. That's what I was thinking of.

That makes sense. Thanks.

8. ### TraverseRegistered Member

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Yes, nascent (freshly-produced) dioxygen molecules are more chemically reactive than 'aged' dioxygen molecules (just like freshly-generated dihydrogen molecules are more chemically reactive).

9. ### TraverseRegistered Member

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Hydrogen bonding on steroids, haha.

10. ### exchemistValued Senior Member

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You mean the oxygen molecules are formed in an electronically excited state, that subsequently relaxes to the ground state?

11. ### TraverseRegistered Member

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Yes, the electronic E.S. is only initial, then the molecule relaxes and we get the O2 reactivity that we're used to. Same is true for freshly-generated ozone (O3), and for many other small molecules as well, especially those generated at electrodes.

Last edited: Jan 18, 2021 at 5:30 PM
12. ### Xelasnave.1947Valued Senior Member

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I only heard of this recently.
Surprised not mentioned when the blast in Lebanon occurred.

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13. ### DaveC426913Valued Senior Member

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That's a 46 minute video!

14. ### Xelasnave.1947Valued Senior Member

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I will get you a shorter one...
Alex

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16. ### exchemistValued Senior Member

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There is also a Wiki article that you can read in a few minutes:https://en.wikipedia.org/wiki/Texas_City_disaster. (I don't know why people persist in linking effing videos, that take 10 times as long, to tell you generally less.

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)

Indeed, a humungous explosion, in 1947, of NH4NO3, loaded in bags aboard a freighter. Killed nearly 600 people.

When I first saw the post I thought it was a reference to a different, more recent Texas City disaster, involving a hydrocarbon vapour cloud at a BP refinery (not Shell, thank God!). That one is in all the safety textbooks in the oil industry, which is where I came across it.https://en.wikipedia.org/wiki/Texas_City_Refinery_explosion

17. ### Xelasnave.1947Valued Senior Member

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Well I post them because I like videos... you can be watching a video and practice guitar for example...
Heck dont you find watching explosions just a little bit better than reading about it?

Alex

18. ### exchemistValued Senior Member

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I'm more interested in what happened and why, to be honest. That's what comes of being a scientist and an oil industry man, I suppose.

And having limited patience with videos. I always find myself thinking: "Get the F on with it, can't you". They always seem to be aimed at a 12yr old.

19. ### TraverseRegistered Member

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Yes indeed, time's a precious commodity.

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20. ### DaveC426913Valued Senior Member

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Traverse and exchemist like this.
21. ### TraverseRegistered Member

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Makes one proud to be a Canadian, haha.