[galpinj]

I've been thinking about ammonia and its role in the early atmosphere.

The early atmosphere was apparently reducing which, put another way, meant that the atmosphere was filled with reducing agents (elements/molecules that would donate electrons); however, I'm having trouble fitting this with the ammonia molecule.

Oxidation can be thought of in a few ways, but I'll focus on electron transfer and hydrogen transfer. From what I've learned, oxidation can be thought of as either losing electrons or losing hydrogen. Unfortunately, these two definitions seem to clash with the ammonia molecule, and I haven't been able to reconcile the difference.

NH₃ + H⁺ --> NH₄⁺

In this example there has been no change in the oxidation state for nitrogen (no reduction and no oxidation); however, it has clearly gained a hydrogen (reduction by the second definition).

How can NH₃ be a reducing agent (something that is oxidized) if reactions like the above are either non-redox or reduction reactions? Moreover, how come these definitions can lead to two different answers as to whether something is or isn't a redox reaction?

Any input is greatly appreciated

Edit: I thought I would post the link I used to learn about the different definitions of reduction and oxidation, just in case my explanation wasn't clear: http://www.chemguide.co.uk/inorganic/redox/definitions.html

[Vidya]

It is basically an acid base reaction ...
But N of NH3 is giving its lone pairs (oxidized) to H+ (Reduced)

[galpinj]

Yeah that's what I initially thought as well! It looks like NH₃ is donating two electrons, and thus acting like a base. But this still doesn't explain why I can't consider this reaction (and any other acid-base reaction) as either reduction or oxidation (when looked at from the scope of "hydrogen loss" or "hydrogen gain"). The most common acid, HCl, doesn't have any change in oxidation state, but it certainly loses that hydrogen (which could be defined as oxidation).

So what exactly is going on here? Are all acid-base reactions also redox reactions? Why can different definitions yield different answers? And how is ammonia acting at all like a reducing agent when, by any definition, it is either not part of redox reactions or is in fact being reduced!

[Vidya]

Though N has given its electrons to H but it is not a complete transfer ...N is now sharing these two electrons with H .
so basically no electron transfer has taken place.
Redox reaction by definition are the reactions in which there is a change in oxidation number due to transfer of electrons.

[galpinj]

Though N has given its electrons to H but it is not a complete transfer ...N is now sharing these two electrons with H .
so basically no electron transfer has taken place.
Redox reaction by definition are the reactions in which there is a change in oxidation number due to transfer of electrons.

Yes I would agree that nitrogen is forming a coordinate bond with the hydrogen in the example I posited; however, isn't oxidation states formulated on the assumption that the most electronegative element has complete control of the electrons? As such, we'd still assume that nitrogen has these electrons when calculating the oxidation state (which would remain at +3).

I have read in a myriad of places that redox can be defined by changes not only in electrons, but also in hydrogen or oxygen content. This is outlined pretty well here: http://www.chemguide.co.uk/inorganic/redox/definitions.html

Thank you so much for the quick and insightful responses, very much appreciated.

[Vidya]

Basic definition of redox reaction is in terms of electrons transfer ...rest all are outcome of that mother definition ...
gain of less electronegative atom is reduction
gain of more electronegative atom is oxidation
now in our example NH3 is not gaining H ...it is gaining H+
Oxidation state of N is is not +3  ...it is -3 ,H is +1 ,however N has a formal charge of +1 on it because of the coordinate bond with H

[galpinj]

Sorry about that, it should definitely be -3.

You make a good point that this reaction involves H⁺. The reaction likely wouldn't take place with H₂.

[Arkcon]

I have read in a myriad of places that redox can be defined by changes not only in electrons, but also in hydrogen or oxygen content. This is outlined pretty well here: http://www.chemguide.co.uk/inorganic/redox/definitions.html

That's an older, more simplistic way, of defining reduction-oxidation.  See: oxidation, oxygen?  When something is oxidized, by oxygen, oxygen loses its electrons to the reducing agent.  But we can apply the term oxidation to other electron sources in a reaction.

[Babcock_Hall]

That's an older, more simplistic way, of defining reduction-oxidation.  See: oxidation, oxygen?  When something is oxidized, by oxygen, oxygen loses its electrons to the reducing agent.  But we can apply the term oxidation to other electron sources in a reaction.

If a reducing agent is oxidized by oxygen, the reducing agent is losing its electrons to oxygen, not the other way around.