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From Russian test problem 4301:

$$\ce{3NaNO3 + 8Al + 5NaOH + 18H2O -> 8Na[Al(OH)4] + 3NH3(g)}$$

How does ammonia evolve here? Is it that we get hydrogen gas evolving in the reaction between Al and NaOH, and this gas reacts with the NO3 anion?

I there a logical way to deduce this, or should one just memorize that "Al and Zn reduce $\ce{NO3-}$ to ammonia in basic solutions"?

P.S.

I learned that the reaction is called "a nitrate test using Devarda's alloy", but there is no specific description of the process in Wikipedia. All I learned is that the reason is certainly not the freshly-minted hydrogen:

Nascent hydrogen was supposed to be responsible for the reduction of arsenate or nitrate in arsine or ammonia respectively. Nowadays, isotopic evidence[8] has closed the nascent hydrogen debate, presently considered to be a Gedanken artifact of romanticism.

And I wonder why NaOH is needed. Probably to activate the aluminium (to break the oxide layer).


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    $\begingroup$ In the test problem linked you are not expected to know this is the case, but to guess from other data with more common knowledge. And no, neither guessing, nor remembering such corner-cases is expected and/or possible. Some should be remembered if they are part of your field of work, and that's it. $\endgroup$
    – permeakra
    May 24 '16 at 11:19
  • $\begingroup$ @permeakra - thank you! So I was told on a Russian chem forum. Still I'm curious to know how exactly that ammonia evolves. ^_^ P.S. I'm finding that part of the USE exam the hardest yet. Too many reactions for rote memorization. $\endgroup$ May 24 '16 at 11:28
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    $\begingroup$ You should use reduction potential to deduce whether it will yield ammonia or higher nitrogen oxidation states compound. But in this case, Al and Zn are quite strong reductive metal, so both can strongly reduce nitrate ion to ammonia. $\endgroup$
    – lambda23
    May 24 '16 at 12:56
  • $\begingroup$ That's true, but the reduction potential alone is not enough. Thermodynamically, nitrate can oxidize pretty many things (including nearly any metal), but kinetics would sometimes forbid that. $\endgroup$ May 24 '16 at 19:10
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    $\begingroup$ Does this answer your question? Testing for nitrate ions with NaNO3 in Devarda's test $\endgroup$ Jun 29 '21 at 5:17

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