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Al + Fe in NH3

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Heiders

Fri May 26 2006, 03:04PM

Registered Member #268 Joined: Tue Feb 28 2006, 02:44AM
Location: Ontario, Canada
Posts: 48

For a grade 12 (senior) Chemistry class, we are making an electrochemical cell using household materials. My group decided on aluminum foil, steel wool, and ammonia, which gave us 1.4-1.5 volts. After looking at the convenient chart of half-reactions in the back of my textbook and googling, I am at a loss. My question for you is this:

What chemical reaction is actually happening in the container with an aluminum electrode, an iron electrode, and ammonia?

My apologies if this is a juvenile question, I have actually tried my best.

Jim

Fri May 26 2006, 05:42PM

Dunce.
Registered Member #28 Joined: Fri Feb 03 2006, 03:29AM
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Posts: 76

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

Eric

Fri May 26 2006, 06:22PM

Registered Member #69 Joined: Thu Feb 09 2006, 07:42AM
Location:
Posts: 116

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 eâ€“ (â€“1.676V)

and

2 H+(aq) + 2 eâ€“ H2(g) (0V)

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

Heiders

Sat May 27 2006, 12:23AM

Registered Member #268 Joined: Tue Feb 28 2006, 02:44AM
Location: Ontario, Canada
Posts: 48

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.

Coyote Wilde

Sat May 27 2006, 01:07AM

Registered Member #175 Joined: Tue Feb 14 2006, 09:32PM
Location: Sudbury, ON
Posts: 111

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.

...

Sat May 27 2006, 02:27AM

Registered Member #56 Joined: Thu Feb 09 2006, 05:02AM
Location: Southern Califorina, USA
Posts: 2445

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.

Eric

Sat May 27 2006, 05:08AM

Registered Member #69 Joined: Thu Feb 09 2006, 07:42AM
Location:
Posts: 116

Heiders wrote ...

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.

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.

Heiders

Sat May 27 2006, 04:39PM

Registered Member #268 Joined: Tue Feb 28 2006, 02:44AM
Location: Ontario, Canada
Posts: 48

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.

Bored Chemist

Sat May 27 2006, 07:25PM

Registered Member #193 Joined: Fri Feb 17 2006, 07:04AM
Location: sheffield
Posts: 1022

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.

Eric

Sat May 27 2006, 08:52PM

Registered Member #69 Joined: Thu Feb 09 2006, 07:42AM
Location:
Posts: 116

Heiders wrote ...

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.

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- â†’ 2 NH3(g) + H2(g)

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