soap problem

For a high school Chemistry course I made soap for a project. Before doing this, thank god, i learned that one should never use an aluminum container to mix the lye (NaOH) and water. Now, I need to know why.
I know it eats away at the container but i can't figure out why. It goes against everything I've learned so far. At first i thought it was because Aluminum was, for some strange reason :o , more reactive than Sodium therefore replacing it in the reaction but that turned out not to be true. Does anyone have an idea of why?


Comment viewing options

Select your preferred way to display the comments and click "Save settings" to activate your changes.

I think because aluminium reacts with lyes. I know it sounds strange but it is true.

2Al + 2NaOH + 6 H20 --> 2Na (Al (OH)4 )2 + 3H2

The reason is because Al is a "heavy metal"; that is its oxides do not form bases in aqueous solution but behave as acids and thus combine with "light" metals. As the quotation marks indicate, this nomenclature is old fashioned and has nothing to do with the actual density of the elements in question.

use something with a teflon coating
its what i did

I believe the real reason for aluminum's reaction with lye is due to the fact that the lye removes the protective aluminum oxide coating on the surface of aluminum metal. With the oxide gone, the very reactive aluminum will then act like an alkali metal in water. It will remove the hydrogen from the water and form aluminum hydroxide and hydrogen gas. The reaction proceeds slowly at first since the Al still has the oxide coating on it, but in a short while it really speeds up as all the protective coating has dissolved away.

No, Al doesn't behave like an alkali, and if it did, it would react with water not displace a more reactive metal like Na.

It is true that one the protective oxide coating is removed, Al is quite reactive. If you are foolhardy, you can cut a bar of Al under Hg and the exposed Al will form an amalgam which will absorb O2 from the air with the evolution of a lot of heat (and poisonous Hg vapor), but Al2O3 is combines with metals to form aluminates much as lead forms plumbates and platinum forms platinates when metals combine with its oxides. Specific gravities not withstanding, this is the chemistry of a "heavy metal."

Traditionally, soap was made in iron vessels, but glass or teflon works as well. Aluminum is highly reactive with lye and also ammonia.

The mixture of lye and Al can be quite reactive; Drano solid is basically lye and aluminum chips which heats and melts grease; the excess lye combines with the grease to form soluble soap.

If you wish, washing soda Na2CO3, hydrolyzes giving a slightly less basic solution that substitutes for lye; bits of aluminum (foil will do) can be boiled in the solution and H2 gas will evolve.

WebElements: the periodic table on the WWW []

Copyright 1993-20010 Mark Winter [The University of Sheffield and WebElements Ltd, UK]. All rights reserved.