Al and H2SO4... not what i expected.

[kingjewel1]

Hi there!

I'm just wondering why my nice lil equation isn't working in reality:

2Al+3H₂SO₄=>Al₂(SO₄)₃+3H₂ excess sulphuric is always used.

The Sulphuric acid is battery grade, so I'm guessing it must be contaminated, but isn't it usually just some Pb?

 

Even when I concentrated the acid, still nothing appeared to happen a d no effervence was noted.

 

I know the Al has an oxide coating, but it should still go to completion.even if

i do get a drop of water, shouldn't it still continue on with evolving H2.

Al2O3+3H2SO4=>2Al2(SO4)3+3H20

 

What am i doing wrong here? It seems pretty simple, but hmmm (I haven't done formal chem for some months now so maybe i'm just a bit rusty)

so

[woelen]

Indeed, aluminium is remarkably inert in sulphuric and nitric acids. This has to do with the nature of the process for breakdown of the oxide layer. Both the sulphuric acid and nitric acid have to eat throughthe oxide layer by means of the equation you mention: Al2O3+3H2SO4=>2Al2(SO4)3+3H20

This process is very slow. Many oxides are remarkably inert, especially when they have been heated. The sulfate ion and nitrate ion of these acids do not coordinate to the aluminium ion.

 

With hydrochloric acid, the situation is quite different. The oxide coating now is quickly eaten away, due to the coordinating properties of the chloride ion. A complex ion AlCl4(-) is formed and apparently, there is a fast mechanistic pathway for formation of this ion from the oxide and hydrochloric acid.

 

Just try it. Add some Al-foil to 30% HCl or to your battery acid, to which quite a lot of table salt is added. You'll see that within a few tens of seconds the reaction with the aluminium becomes very vigorous.

 

Things become even more striking, when a combination of copper (II) ions and chloride ions is present. Then the acid is not even needed anymore. The CuCl4(2-) ion in a concentrated solution of a chloride, with some copper (II) added is capable of instant destruction of the oxide layer.

 

If you have copper sulfate, then try dissolving some of this in your battery acid and also add quite some table salt. You get a green solution. You can also dissolve some copper sulfate in 30% HCl if you wish. If you add the Al-foil to this, then you get a very violent reaction (be careful!) in which a lot of H2 is produced, a lot of steam and a small amount of copper metal. But even in a plain solution of table salt, to which some copper sulfate is added, the Al-foil already dissolves quite vigorously.

[kingjewel1]

Yeah! that sounds good. I hadn't thought about the size of the cation which you mentioned in the complexing part.

So that's also why sulphuric acid is prefered as a catalyst over HCl, apart from the fact that H2SO4 is a dehydrating agent!

 

Thanks very much!