Sciencemadness Discussion Board

Aluminium Oxide

kyro8008 - 13-6-2006 at 11:17

I have read that if you heated a solution of aluminium chloride to dryness, that you would obtain the oxide and not the chloride.

How is this possible; where does the chloride ion go? At a guess I would say that the description is not acurate and you obtain some sort of mixed product or maybe that HCl is generated as the solution is evaporated and leaves the system.

However, if the latter was the case then consider the following:

If aluminium was dissolved in hydrochloric acid, then you would be left with a solution containing aluminium ions and chloride ions. If evaporated to dryness and the second point was true then HCl would be reformed and leaves the system. That would be very strange as that was one of the initial reactants...

Anyway, the point of this post is that I am trying to make some aluminium oxide and if this method works then great as it would be very easy.

I have made aluminium hydroxide but apparently need too high temperatures to dehydrate it to the oxide so this method was out.

I tried heating a solution of aluminium nitrate to dryness strongly but this refuses to stop giving off loads of thick white clouds and takes ages to solidify for some reason and I am unsure of what I was left with afterwards.

Bearing in mind that I cannot get very high temperatures, does anyone know any good methods of obtaining aluminium oxide?

Any suggestions would be appreciated, thanks!

Organikum - 13-6-2006 at 11:51

IIRC aluminium added to mercury on air, say the mercury should just be enough to amalgamate the aluminium not cover it, produces over some time aluminium oxide and - mercury.

/ORG

12AX7 - 13-6-2006 at 13:44

Quote:
Originally posted by kyro8008
I have read that if you heated a solution of aluminium chloride to dryness, that you would obtain the oxide and not the chloride.

How is this possible; where does the chloride ion go?


Off, canoodling with the hydrogen ion.

When aluminum dissolves in water, it forms the favorable hydrate ion, a small aluminum atom missing three electrons and thus attracting the oxygen end of um, six? water molecules. You can crystallize such a solution, yielding aluminum chloride hexahydrate -- more aptly phrased hexa-aquo-aluminum chloride, but this cannot be dehydrated further: one way or another, the aluminum is going to have its oxygen, be it also bonded to hydrogen as hydrates, or oxygen alone. It's a selfish atom and the hydrogen goes off to play nicely with the chlorine, who isn't as selfish as aluminum or oxygen, but unfortunately for hydrogen, still doesn't play well.
Quote:
If aluminium was dissolved in hydrochloric acid, then you would be left with a solution containing aluminium ions and chloride ions. If evaporated to dryness and the second point was true then HCl would be reformed and leaves the system. That would be very strange as that was one of the initial reactants...


Not really, you just get back what you started with.
Quote:
Anyway, the point of this post is that I am trying to make some aluminium oxide and if this method works then great as it would be very easy.


Bleh, buy it! Aluminum is worth a whole lot more for the electrons it contains (something like $1.10 at the moment).
Quote:
I tried heating a solution of aluminium nitrate to dryness strongly but this refuses to stop giving off loads of thick white clouds and takes ages to solidify for some reason and I am unsure of what I was left with afterwards.


Hmm, ought to be nitric acid. I'm suprised it wasn't brown fumes. Not good to breathe in either case.

Tim

kazaa81 - 13-6-2006 at 16:02

If you need alumina, Al2O3, you can buy it or make it.
Assuming you're starting from AlCl3 (aq), add NH3 (aq) to it.
You will obtain a gel, probably Al(OH)3.
Heat this to dryness and you'll get alumina.

Hope this helps

kyro8008 - 14-6-2006 at 15:07

Sorry, I was being very stupid there; I completly forgot about the hydrolysis of the aluminium ions!

Ive just realised, I think the example of heating aluminium hydroxide to obtain the oxide is exactly the same as evaporating a solutuion of aluminium chloride to dryness, because:

[Al(H2O)6]^3+ --> [Al(H2O)5(OH)]^2+ + H+
[Al(H2O)5(OH)]^2+ --> [Al(H2O)4(OH)2]^+ + H+
[Al(H2O)4(OH)2]^+ --> [Al(H2O)3(OH)3] + H+

This should happen as HCl leaves the system, which means that hydrogen ions are removed from the system and drives the equilibrium's to the right.

Of course then you are left with the solid hydroxide as you are evaporating to dryness. Heating the hydroxide dehydrates it to the oxide, which is back at square one. Maybe it will work at low temps...

Im just working this out, but is it right?

hodges - 15-6-2006 at 14:04

Quote:
Originally posted by kyro8008
If aluminium was dissolved in hydrochloric acid, then you would be left with a solution containing aluminium ions and chloride ions. If evaporated to dryness and the second point was true then HCl would be reformed and leaves the system. That would be very strange as that was one of the initial reactants...


You could think of the HCl as simply a catalyst that allows aluminum and water to react to form hydrogen and aluminum oxide. Aluminum is a very active metal and it would react with water on its own if it were not for the protective layer of aluminum oxide that forms on the surface. When you add HCl, it reacts with the oxide layer to form soluable aluminum chloride, thus exposing more aluminum surface and allowing the reaction to continue.

A similar reaction that comes to mind where one of the original reactants is re-created is the conversion of acetic acid to acetone by calcium carbonate. Vinegar is added to calcium carbonate to produce calcium acetate (and water). When calcium acetate is heated, it decomposes to acetone and calcium carbonate. The calcium carbonate theoretically can be re-used indefinitely Just like with your reaction where the hydrogen chloride can theoretically be re-used indefinitely.

franklyn - 2-7-2006 at 15:07

Just a thought

2AlCl3 + 6CH3OH -> Al2O3 + 3H2O + 6CH3Cl

Aluminum Chloride and Methanol gives

Alumina and Monochloromethane

or is it _ 2Al(OCH3)3 + 6 HCl

Aluminum Methoxide and Hydrogen Chloride



CH3F + NaOCH3 -> NaF + CH3OCH3

Monoflouromethane and Sodium Methoxide gives
Sodium Flouride and Methyl ether

CH3F + NaCOOCH3 -> NaF + CH3COCH3

Monoflouromethane and Sodium Acetate gives
Sodium Flouride and Acetone

.

[Edited on 3-7-2006 by franklyn]

The_Davster - 2-7-2006 at 18:24

Judging by the reactions of other anhydrous inorganic chlorides, I figure the methoxide is much much more likely to form.

YT2095 - 3-7-2006 at 10:25

Quote:
Originally posted by kazaa81
If you need alumina, Al2O3, you can buy it or make it.
Assuming you're starting from AlCl3 (aq), add NH3 (aq) to it.
You will obtain a gel, probably Al(OH)3.
Heat this to dryness and you'll get alumina.

Hope this helps


Interesting that you should mention the Gel, I also have over the last few days made Alu Chloride soln, upon evaporation you get no hexhydrate crystals form (or at least I didn`t anyway), the liquid becomes very viscous like honey almost, and leaves white rings around the droplets of the soln when put under a fan at room temp, but no crystals.

however upon the addition of KOH soln a Gel instantly forms (looks alot like Mg hydroxide in many respects).
this WILL dry after much water decanting and later filtering, the product put into an evaporating tray and left under a fan at room temp.
the powder is SOoooo fine like dust and pure white, I`m wondering now if additional heating of this powder will produce my Oxide (Alumina).
I want part of it as an abbrasive powder for polishing with my dremel brushes.

if not, it will certainly be interesting to see how the Hydroxide performs as a polishing agent ;)