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Author Topic: Reaction between aluminium and sodium hydroxide solution to produce hydrogen  (Read 24504 times)

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bigbob789

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I have the chemical equation for this reaction and I want to calculate the amount of hydrogen produced, but I get much more hydrogen in practice than on paper.

Equation:
2Al ... -> 3H2
(see http://www1.eere.energy.gov/hydrogenandfuelcells/pdfs/aluminium_water_hydrogen.pdf)

I used 0.5g aluminium.

Mass of one mole of Al: 26.98g
Mass of one mole of H2: 1g*2 = 2g

aluminium:
n=m/Mr
=0.5/26.98
=0.018532 mole of aluminium

hydrogen:
m=n*Mr
=(3/2)*0.018532 * 2g
=0.055596g hydrogen produced

Volume of H2=mass/density
=0.055596g/0.08988 g/L
=0.618558 L
=618.558cm3 hydrogen gas produced

However, in my experiments using 0.5g aluminium I got up to 1100cm3 hydrogen gas.  How can this be?
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Yggdrasil

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The density of hydrogen will be dependent on both temperature and pressure.  Check to see whether the temperature and pressure used to calculate the density of hydrogen is close to the temperature and pressure at which you measured the volume.

Is it possible that other gasses are generated during the reaction?  These could account for the increased volume of gas collected.  Water vapor could also be another source (esp. if the reaction produced heat or heat was added to the reaction).
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Borek

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Seems to me like twofold difference is a little bit too large to be explained by the pressure/temperature difference.

Unless bigbob has his lab somehere on mountain top in tropics ;)
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billnotgatez

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I wonder which equation to use

The following is from the document you mention.

Quote
Hydroxide Promoters:
A number of U.S. patents describe the use of hydroxides, primarily sodium hydroxide (NaOH), to promote aluminum-water reactions (4-8). There are two pertinent technical references on this subject. The first is by Belitskus (9). Here, experiments were described in which aluminum specimens including a cylindrical block, uncompacted powders, and pellets of various densities were reacted with aqueous solutions of sodium hydroxide at different concentrations to produce hydrogen gas at temperatures near room temperature. The formation of sodium aluminate was observed, as well as the regeneration of sodium hydroxide via the precipitation of aluminum hydroxide. Stockburger et.al. (10) described hydrogen generators in which aluminum was reacted with an aqueous solution of 5.75 M sodium hydroxide.
The types of reactions found to occur between aluminum, sodium hydroxide, and water are shown below:
2Al + 2NaOH + 2H2O = Na2Al2O4 + 3H2 (4)
2Al + 6NaOH + xH2O = Na6Al2O6 + xH2O + 3H2 (5)
2Al + 2NaOH + 6H2O = 2NaAl(OH)4 + 3H2 (6)
2NaAl(OH)4 = 2NaOH + 2Al(OH)3 (7)
One of the problems with the use of aqueous NaOH solutions is the corrosive nature of the liquid, which can lead to corrosion of system equipment.
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Borek

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Doesn't matter which equation you use - 2Al -> 3H2
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bigbob789

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Water vapour and sodium hydroxide fumes may be given off, but the gas was bubbled through cold water into an upside-down measuring cylinder so I think any water vapour should have condensed back into liquid water.  I'm not sure about the sodium hydroxide fumes, but surely it should not have had such a great effect?
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billnotgatez

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borek - i see now
bigbob - almost 2 times what you calculated?
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bigbob789

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Yes I noticed that the amount of hydrogen produced was about double the amount I calculated on paper, but I can't seem to find the reason for it.  Can someone confirm if my calculations are correct?
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Borek

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0.5 g Al should give 0.623L of hydrogen at STP - you are close, so the difference will be not large.

Just in case - such things are easily checked using my stoichiometry calculator - see signature.
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DrCMS

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Your calculations are approximately correct - I get ~667ml per 0.5g Aluminium at room temp.

Either you did not use Aluminium or Sodium Hydroxide.  
Al + HCl gives about twice as much gas as NaOH.
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bigbob789

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I'm pretty sure the chemicals I used are correct.  I used various types of aluminium: powder, aluminium foil, scrap aluminium turnings, a spray can and others.  I used the sodium hydroxide from our school's lab, but later I bought other caustic soda with the same results.  I also tested the pH of the remaining solution with red litmus paper after the reaction and it tested alkaline.

DrCMS: How can Al+HCl give twice as much gas?  I searched for the equation on the internet and got the same result as with NaOH:
2Al ... -> ... 3H2
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DrCMS

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Yeh i forgot to divide by 2 for the HCl.
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Borek

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Your calculations are approximately correct - I get ~667ml per 0.5g Aluminium at room temp.

I should probably explain that by STP I meant 0 deg C.

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bigbob789

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I don't know if this means anything, but I noticed that when I used aluminium with a large surface area the reaction was fast, from under a minute for powder under the right conditions to 42min for aluminium turnings, and for these reactions I usually got between 800cm3 and 1000cm3 gas.  However, I had three longer reactions which stood out:
1.  Spray can (sanded to remove paint etc):  1h30min, 365cm3 H2 produced
2.  Aluminium wire (diameter ~3mm):  ~3 days, 370cm3 H2 produced
3.  A piece of aluminium plate (thickness ~ 2mm):  ~2.5days, 415cm3 H2 produced
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