I read that ammonium is the "fuel" and nitrate is the "oxidizer". The nitrate oxidizes because it's a nitro group, but what's the mechanism of the ammonium?
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1$\begingroup$ Will it help if you know NH3 is flammable ? $\endgroup$– PoutnikCommented Nov 9, 2021 at 10:04
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$\begingroup$ I was thinking more along the lines of nitrous oxide being an oxidizer. The ammonium would then be a what combines with the nitrate to produce said nitrous oxide. $\endgroup$– Oscar LanziCommented Nov 9, 2021 at 10:30
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$\begingroup$ @OscarLanzi I see. But N2O is created mainly during peaceful decomposition. It decomposes AFAIK as well during explosion of the pure nitrate, even if it goes better when extra flammable stuff like Al is present. $\endgroup$– PoutnikCommented Nov 9, 2021 at 10:42
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1 Answer
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When heated slowly, ammonium nitrate gets decomposed by the reaction $$\ce{NH4NO3 -> N2O + 2 H2O}$$ This is the industrial way of synthetizing the gas $\ce{N2O}$. The nitrate group is oxidizing and the nitrogen atom in the ion ammonium is oxidized. Both atoms are changing their oxidizing number by the same amount : $4$ units. $\ce{N}$ from $\ce{NO3^-}$ goes down from $+5$ to $+1$. $\ce{N}$ from $\ce{NH4^+}$ goes from $-3$ to $+1$.
Nevertheless in case of overheating, a further reaction takes place : $$\ce{2 N2O -> 2 N2 + O2}$$ and this reaction is exothermic (+$82$ kJ/mol) and exoergic (+$104$ kJ/mol).
