Re: [ATM] Need a Chemistry guru! OT

[Mark Holm <mdholm@telerama.com>]

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

Thomas Janstrom wrote:
> Hi All,
> 
>  
> 
> I need a chemistry guru to help explain something that I'm observing in a
> hydroxide reaction. 
> 
>  
> 
> The basic reaction is: 6NaOH(aq) + 3Al(s) + 3H2O --> 2Al(OH)3(aq) + 3 Na2O
> + 3H2(g).
> 
>  
> 
> However I'm observing the production of two gas streams, one is composed of
> very fine (submicron) bubbles the other larger (1-2mm) bubbles. I should
> mention that there are two spectator metals present, platinum and cobalt. 
> 
>  
> 
> So my question (well one of them anyway!) is what is the second gas stream
> (I suspect O2), but more importantly why is it being produced?
> 
>  
> 
> As this is quite off topic I'd prefer that respondents respond off list.
> Unless the moderators want to entertain a segway into the world of the
> atoms, that is! 
> 
>  
> 
> Cheers, Thomas.
> 
>  
> 
> _______________________________________________
> ATM mailing list http://www.atmlist.net/
> 
> 

-- 
Mark Holm
mdholm@telerama.com

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