Aluminum and Potassium Hydroxide
Hello, I am currently taking grade 11 chemistry and
doing rather well in the class. I have ambitions to enter the field
of chemistry when I am older and finished University. Now lets get
down to business.
I have recently found out that adding a piece of Al to a solution of
KOH causes a reaction in which hydrogen is produced. This particular
reaction confuses me because I do not not know how an element with a
reactivity such as Al is displacing hydrogen. What I am asking for
is a simple chemical equation for this reaction because I am having
trouble coming up with my own.
The equation that you are looking for is:
2Al(solid) + 6H2O + 2KOH ---> 2K[Al(OH)4] + 3H2 (gas)
This is typically an intermediate step in the production of alum by
further addition of sulfuric acid.
Hope this helps.
This is a famous reaction.
Aluminum is so reactive, so energetic, it is the most common metal
added to rocket fuel.
It would always react with water, if it did not tend to grow an oxide film
that blocks off the surface and stops the reaction.
Aluminum oxides are soluble in basic water solutions,
so KOH enables the reaction to go continuously.
Basically, it is:
2 Al(0)+ 6 H2O --(KOH)--> 2 Al(+3) + 6 OH(-) + 3 H2
There are many slightly different ways to express it
which more nearly describe the exact state of the stuff in water
after the reaction. This is where the KOH gets involved a little,
and where one might understand why precipitates form.
2 Al(0) + 6 H2O + 2 K(+) + 2 OH(-) -->
--> K2Al2O4.3H20 + 3 H2 + OH(-)
or 2 K+ + 2 (Al(OH)4)- + 3 H2O + + 3 H2 + OH(-)
or a mix of these and similar things.
This reaction consumes some OH- as well as Al and water.
I would not assume that K2Al2O4.3H20 is exactly what is there,
but at least the equation is balanced.
The hydrogen evolved is flammable and explosive.
Please only do this with supervision and use no glass containers.
Use ventilation. The alkaline water-mist is bad for lungs eyes and skin, too.
Click here to return to the Chemistry Archives
Update: June 2012