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WHY does NaOH + Al produce H2?

WHY does NaOH + Al produce H2? - Chemistry Forum

WHY does NaOH + Al produce H2? - Chemistry Forum. Discuss chemical reactions, chemistry.


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  #1  
Old 02-26-2004, 11:02 PM
Dr F
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Default WHY does NaOH + Al produce H2?



Hello everyone,

When I want to create hydrogen gas without using expensive chemicals
or concentrated acids, I just put aluminum foil in sodium hydroxide
solution. I was wondering, why does this work? What is the reaction
that occurs? What are the biproducts?

Thank you,
B.W.
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  #2  
Old 02-26-2004, 11:47 PM
Oscar Lanzi III
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Default WHY does NaOH + Al produce H2?

Aluminum is a very active metal and thermodynamically it should
decompose water. But with just water you get a passive film of
insoluble aluminum oxide (just as you do in air). With NaOH, however,
the oxide becomes soluble by forming a complex, and when the passive
film is stripped of you corrode the aluminum.

Passive film breakdonwn:

Al2O3 + 2 OH(-) + 3 H2O --> 2 Al(OH)4(-)

Overall aluminum oxidation:

2 Al + 2 OH(-) + 6 H2O --> 2 Al(OH)4(-) + 3 H2

A more controlled version of the redox reaction between aluminum and
alkali may be used to de-tarnish silver. Put the silver in a pan lined
with Al foil, add hot detergent solution (Spic and Span or something
like that), and let it go. Electrons released by the aluminum oxidation
(to form a soluble product again, but more solwly) get conducted to the
silver and reduce the tarnish.

--OL

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  #3  
Old 03-06-2004, 07:54 PM
Dr F
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Default WHY does NaOH + Al produce H2?

Thank you so much for clearing that up. Your explanation makes a lot
of sense. There there is one thing that I don't understand, though. If
aluminum were replacing the hydrogen from water, you would expect it
to form aluminum hydroxide, Al(OH)3. You say that it does not form
aluminum hydroxide, it forms a weird polyatomic anion, Al(OH)4 (-).
Why does aluminum form that weird anion, when other active metals like
sodium and calcium form electrically neutral hydroxides? Also, what is
the name of that polyatomic anion?

Thanks again,

B.W.
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  #4  
Old 03-07-2004, 01:14 AM
Steve Turner
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Default WHY does NaOH + Al produce H2?

[Only registered users see links. ] (Dr F) wrote:


The name of the anion is aluminate.

Steve Turner

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  #5  
Old 03-07-2004, 02:36 AM
Bob
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Default WHY does NaOH + Al produce H2?

On 6 Mar 2004 11:54:48 -0800, [Only registered users see links. ] (Dr F) wrote:


It is ok for you to think of it as forming the hydroxide. It happens
to be a property of aluminum hydroxide that when you put it in base
you get this aluminate ion. That is, think of it as two steps -- one
as you expected, one perhaps new to you.

bob


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  #6  
Old 03-07-2004, 05:31 AM
Mohammed Farooq
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Default WHY does NaOH + Al produce H2?

[Only registered users see links. ] (Dr F) wrote in message news:<bcfb0c2b.0403061154.3a693b7f@posting.google. com>...

The nature of hydrogen which formed by the reaction of metals like
aluminum with bases puzzled many chemists, because it had some
different properties from ordinary hydrogen. It was called nascent
hydrogen, or "atomic" hydrogen for atleast a century, becuase of its
ability to reduce many substances, nitrate was reduced to ammonia,
silver salts reduced to silver metal which ordinary hydrogen gas is
unable to do so in solution. Though these terms are obsolete now.

Read more about amphoteric substances to find why aluminium hydroxide
Al(OH)3 shows this behaviour. The reaction of aluminium with strong
bases is not restricted to aluminum , many other metals for example
zinc, tin dissolve in higlhy alkaline solutions releasing hydrogen
forming zincates Zn(OH)4(2-) and stannates with respective bases. If
you consider Al(OH)4(-) wierd, what would you say about heteropoly
anions which contain upto 40 oxygen atoms in a "single" anion which
you might study about in advanced classes if you wish to pursue
chemistry.
It also happens when the metal itself does not dissolve in alakine
solution but its hydroxide does form "salts" with bases like chromium
hydroxide Cr(0H)3 forms chromite [Cr(OH)6]3(-) with sodium hydroxide.
If all the elements and compounds behaved according to our expections
and wishes, chemistry would have been dull and boring and perhaps no
one would study it.
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  #7  
Old 03-07-2004, 07:26 PM
S. Miller
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Default WHY does NaOH + Al produce H2?

[Only registered users see links. ] (Dr F) wrote in message news:<bcfb0c2b.0403061154.3a693b7f@posting.google. com>...

If there were three mols of hydroxide for every mol of aluminum ion,
then the product of the reaction would be Al(OH)3(s). It is only when
there is an excess of hydroxide ions in the solution that
Al(OH)4(-)(aq) is formed.

(I) Al(3+)(aq) + ex OH(-)(aq) <--> Al(OH)3

(II) Al(OH)3 + ex OH(-) <--> Al(OH)4(-)(aq)

Also, the more technically correct name for the anion is
tetrahydroxoaluminate (III), but aluminate is often used as a more
common name.

S. Miller
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