Re: pinhole a chemical reaction

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From: Karen Kalumuck (karenk@exploratorium.edu)
Date: Tue Nov 26 2002 - 15:46:38 PST 

Message-Id: <v01540b14ba09b78fe6ef@[192.174.3.119]>
Date: Tue, 26 Nov 2002 15:46:38 -0800
From: karenk@exploratorium.edu (Karen Kalumuck)
Subject: Re: pinhole a chemical reaction

Hello, Joe!

I passed along your question about soap making in aluminum pie pans, to my
husband, who is a biochemist. It seems that the students in question were
reproducing an infamous event in the history of flight on a very small
scale. Here's what Rob had to say:

The gas produced by the reaction of
aluminum and sodium hydroxide is Hydrogen gas, so they were doing a
mini-Hindenberg type of experiment. What happens is that the sodium hydroxide
reacts first with the aluminum oxide (Al2O3) that coats the surface of the
aluminum metal (which is there simply by virtue of being constantly exposed to
oxygen in the atmosphere) to expose the metallic aluminum beneath. Hence:

Al2O3 + 2 NaOH + 3 H2O -> 2 Na+ + 2 [Al(OH)4]-

Now once that is gone the following happens with the generation of gas and heat:

   First 2 Al + 6 H2O > 2 Al(OH)3 + 3 H2

   Then Al(OH)3 + NaOH > Na+ + [Al(OH)4]-

Fortunately for the class, the amount of heat produced is not so great to ignite
the Hydrogen gas (like metallic sodium or potassium do), so they didn't have an
explosion. Hydrogen itself does not smell, so my GUESS here is that something
in the soap mixture got cooked up by the heat of this reaction and that the
volatilizing Hydrogen gas helped to carry it off into the room. It is good that
they evacuated the room.

By the way, this is the reason that Drano contains little pieces of aluminum in
addition to the NaOH; the Hydrogen gas evolution helps to mechanically loosen
the gook clogging the drains.

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