Re: Al + Fe in NH3
Jim, Fri May 26 2006, 05:42PM

Uh Fe going to Fe 3+ or something, and Al going to Al something - I don't know.

Re: Al + Fe in NH3
Eric, Fri May 26 2006, 06:22PM

Aluminum is much more reactive than iron so it's being oxidized to Al+3 which will precipitate out as aluminum hydroxide after it builds up a little. It doesn't form amines like copper and zinc. At the iron some reduction is occuring, i'd guess 2H+ to H2(g).

So:

Al(s) -> Al3+(aq) + 3 e– (–1.676V)

and

2 H+(aq) + 2 e– H2(g) (0V)

But then I'm not a chemist. I don't even play one on TV.

Re: Al + Fe in NH3
Heiders, Sat May 27 2006, 12:23AM

Sorry, I think I missed something. Where are the H+ ions coming from? The NH3 doesn't dissociate to N and 3H...

As far as I know, it is NH4OH <==> NH3 + H2O in the commercial ammonia.

My chemistry teacher didn't even know what was going on, off the top of his head, so any help is appreciated.

Eric, how can copper or zinc form amines? It involves carbon, not metals. I'm not a big fan of organic chem, but I remember that.

Re: Al + Fe in NH3
Coyote Wilde, Sat May 27 2006, 01:07AM

By "amines" he ment in a more general sense as "ammonia compound" and, in fact, one definition of an amine is such--we just learn it in Organic Chem as a carbon-centered set; it can also be viewed as an ammonia-centered one-- you probably still need carbon, if I recall, but its an easier mistake to make. But I digress.
H3 and N are definately not happening, you're right. But maybe NH3 + H+ <---> NH4? Er, yearg, that's not working if it's NH4OH.
Nope, don't have a clue. Googling "ammonium battery" gets one page admitting that this sort of thing used to exist, but no electrochem. My textbooks draw a blank, too, as does my more-chemically-minded luv...
so, I'll give the archtypical answer on this part of the forum: Wait for Bored Chemist. He's smarter than me.

Re: Al + Fe in NH3
..., Sat May 27 2006, 02:27AM

Beats me, we didn't learn about cells with an electrolyte other than a salt of the electrode... But my best guess would be that you have Al -> Al3+ + 3 e- (which forms 1.6v in a 1M solution), which is then reacting with the free OH- in the solution (from the NH3). The Fe is not reacting at all, and is just acting like a platinum or carbon one would and giving a way to get electrons out.

Re: Al + Fe in NH3
Eric, Sat May 27 2006, 05:08AM


Like CW said, ammoniacal metal complexes are called amines. Anyway, aluminum doesn't form one which might have changed the electrode potential.

The H+ just comes from the water, not from the ammonia. At the same time you'd have OH- generated at the iron as the H+ is reduced and bubbles away. Did you see any bubbles at the iron? You'd have to have current flowing to see that. If not, maybe something else is happening there, though I can't imagine what... I guess we'll see what BC says.

Re: Al + Fe in NH3
Heiders, Sat May 27 2006, 04:39PM

My chem teacher suggested that the iron was just conducting and not reacting, but my voltage changed when I used something like copper (wire with the coating sanded off) or zinc (galvanized nail) with the aluminum.

I didn't notice any bubbles, but we didn't leave it for more than 3-4 min at a time. It was rolled up steel wool, so bubbles may have formed inside the electrode without us seeing.

Re: Al + Fe in NH3
Bored Chemist, Sat May 27 2006, 07:25PM

Just a quick pointer.
Amines are carbon containing thingies.
Ammines (2 "m"s) are complexes with ammonia.
The currents through the cell are very small so you won't see any bubbles to speak of.
Also, since the currents are small the total quantity of metal disolved is small.
Aluminiumhydroxide is just slightly soluble and it will still form, even if it doesn't disolve.

Re: Al + Fe in NH3
Eric, Sat May 27 2006, 08:52PM


Copper and zinc are both attacked by ammonia solutions so there would have been another reaction (and voltage generated) there. If you had the battery 'short circuited' the current flow should protect the copper and zinc from attack since aluminum is more active. You'd have to have the battery short circuited in order to generate enough current to get the H bubbles on the iron I think. I do think that the iron wouldn't react but there's got to be some kind of reduction reaction occuring at the iron, electrons can't just float off into the solution. This one is possible:

2 NH4+(aq) + 2 e- → 2 NH3(g) + H2(g)

Re: Al + Fe in NH3
Heiders, Sun May 28 2006, 06:33PM

So from what I can see, the general consensus is that the iron is just conducting, and the half reactions are

3 [2 NH4+(aq) + 2 e- → 2 NH3(g) + H2(g)]
2 [Al(s) → Al3+(aq) + 3 e-]
6 NH4+(aq) + 2 Al (s) → 6 NH3(g) + H2(g) + 2 Al3+(aq)

This looks logical, and it makes sense. Corrections?

Re: Al + Fe in NH3
Bored Chemist, Mon May 29 2006, 08:08AM

The 6 NH 3 in the last equation will be (aq) rather than (g). Otherwise I think it's OK.

(In reality, it will be more complicated- CO2 and O2 from the air will probably be involved and some of the Al will form various hydrated oxides and these may, in turn, incorporate NH3.)

The story with the different counter-electrode (Zn, Cu) is also more complicated due to overvoltage effects, which should be small, and the real effects of the different metals.
Granted that, in this case, the other electrode isn't doing anything but conducting. On the other hand, it doesn't know that- it will still produce a potential when placed in the solution.
With the ammonia there, the concentration of H+ is very small, I guess about 10^-8M. Nevertheless the zinc will still react with it to give Zn++ and H2 (very slowly). In the cell you have set up this reaction is prevented from taking place, so that affects the cell voltage overall. The effect of Fe would be similar, but smaller because Fe is less reactive.

The trouble with electrochemistry is that it looks nice and simple but really it's quite complicated.

Re: Al + Fe in NH3
Heiders, Tue May 30 2006, 02:29AM

Ah, BC, you are awesome! To the whole lot of you, thanks from my group and me!