[ATM] Need a Chemistry guru! OT

[Thomas Janstrom]

Hi Dominic-Luc and Mark,

Here is the reply I sent (off list) to James Lerch....

I'm not too sure about that, because I have a white precipitate that is
floating on the surface (the only insoluble product possible, from the
reactants present, is Al(OH)3) and the reaction is progressing at @20'C
(varies between 20 and 23'C).

XXX supposition follows XXX
I think that the "standard" reaction producing dissolved sodium aluminate is
taking place, but that the platinum is catalysing a side reaction that is
producing the floating precipitate, but as I see it there can only be more
H2 from this process, unless it's a straight decomposition reaction and the
resulting Al is recycled and O2 released. 
XXX end supposition XXX

The reactant mix is stichometric 3:1 NaOH:Al in 2L of distilled water. The X
factor really is the platinum, I see the large bubbles ONLY on the platinum
containing areas, and the submicron ones are only off the freshly
exposed/reacting aluminium.

Well back to reading what I can find online as I have gone through the three
university chemistry texts I have here to no avail.

.....

So now that you both have some more information what do you think? The
distilled water was freshly produced, i.e. had not been stored for any
period of time, I made it and then used with only a few minutes between the
two tasks.

Well I'll wish you both a safe and happy New Year and get back to my
reading.

Cheers, Thomas.

-----Original Message-----
From: Dominic-Luc Webb [mailto:dlwebb at canit.se] 
Sent: Monday, 31 December 2007 7:35 AM
To: Thomas Janstrom
Cc: ATM Superheros
Subject: Re: [ATM] Need a Chemistry guru! OT


# The basic reaction is: 6NaOH(aq) + 3Al(s) + 3H2O -->
# 2Al(OH)3(aq) + 3 Na2O + 3H2(g).

No! I don't think so, and don't think a guru is needed just yet.
This is not a balanced equation, and even if it was, the
product Na2O makes no sense.

Try this:

2NaOH + 2Al + 2H2O  --->   2Na + 2AlO2- + 3H2

These reactions are quite exothermic. You might be getting
steam as a gas. You might also be getting some more
complicated species like aluminate ion, which is considerably
more complicated. Be careful with these reactions. Some reaction
of aluminum (e.g., thermite) are quite explosive. I can
appreciate why some ATMs would be interested in these reactions,
but really one should lean on side of safety first.

Dominic 

[Thomas Janstrom] Another response appended below......

-----Original Message-----
From: atm_free at yahoogroups.com [mailto:atm_free at yahoogroups.com] On Behalf
Of Mark Holm
Sent: Monday, 31 December 2007 8:08 AM
To: 'ATM'; atm_free at yahoogroups.com
Subject: [atm_free] Re: [ATM] Need a Chemistry guru! OT

It's not all that OT. ATM's make a lot of use of aluminum, as a 
reflective coating, as a structural material (in alloys), as a thermal 
conductor and as an electrical conductor. Doesn't hurt an ATM to know 
some of the most basic aluminum chemistry.

First, a minor point. The equation, as you wrote it, isn't balanced. 
Looks like it should be 2Al(s) on the left side.

Second, if this is going on in an excess of water, you can take the 
sodium hydroxide and sodium oxide out of the equation altogether. Why? 
because sodium oxide and water are not going to hang around long 
together without reacting to sodium hydroxide. The sodium hydroxide is 
acting as a catalyst in this reaction. In net, it is neither consumed 
nor produced. Aluminum metal and water are being consumed. Aluminum 
hydroxide and dihydrogen (molecular hydrogen) are produced.

Now, you see that you are consuming twice as much water
2Al + 6 H2O --> 2Al(OH)3 + 3H2

I don't think there is a chance you are making oxygen gas. Not when 
there is aluminum around to eat the oxygen.

For the composition of the two gas stream, I see two possibilities:

1. Hydrogen and dissolved air (mostly nitrogen).

2. Hydrogen and more hydrogen.

The first possibility is kind of dull, but testable. Make up a fresh 
batch of everything but sparge the solution first with helium before you 
put the aluminum in. That will remove most of the dissolved gas. 
(Helium has low solubility in water.)

The second possibility is more interesting. It says hydrogen is being 
evolved in two different situations, one leading to large bubbles and 
another leading to small. I think there may be more than one possible 
explanation for this, and I don't know how to go about testing them.

One possible explanation is fairly prosaic: as the aluminum reacts, 
small particles of aluminum, still metallic, are broken away from the 
surface, drift out with the flow induced by the larger bubbles and make 
smaller bubbles as they react.

Mark Holm