Sciencemadness Discussion Board

Aluminum foil not reacting with CuSO4 solution

Upsilon - 24-3-2014 at 13:32

I made a fairly concentrated solution of copper(ii) sulfate, and threw some aluminum foil into it. After 2 days, basically nothing has happened. I added some sulfuric acid to help speed it along, and it greatly sped up the process but only for a minute or so, then it went back to its inactive state. What is the reason for this?

DraconicAcid - 24-3-2014 at 13:34

Add a chloride source to prevent passivation of the aluminum.

Upsilon - 24-3-2014 at 13:53

Quote: Originally posted by DraconicAcid  
Add a chloride source to prevent passivation of the aluminum.


Ah, thanks, some NaCl really got things going. Why does the presence of chloride ions prevent the aluminum from passivating?

AJKOER - 26-3-2014 at 18:09

OK, this actually is an electrochemical reaction and you need an electrolyte. Salt, NaCl, is a good electrolyte. It can participate in the reaction but must remain available to aid in ion transfers. In the current context,

2 Al + 3 CuSO4 ----) 3 Cu + Al2(SO4)3

where the Al2(SO4)3 can coat the Aluminum hindering the continuation of the reaction. But, it is dissolved by salt as:

6 NaCl + Al2(SO4)3 = 2 AlCl3 + 3 Na2SO4

and:

2 AlCl3 + 3 H2O = Al2O3 (s) + 6 HCl

so the Sodium and Chloride ions remain in the solution.

[Edited on 27-3-2014 by AJKOER]

Steam - 26-3-2014 at 18:19

I know you got it to work but just so you know when doing chemistry with Al foil to use the cheapest thinnest foil you can find. The thick heavy duty stuff often contains a thin layer of coating to make it more "non-stick". It also makes it difficult to use as a reducing agent.

macckone - 27-3-2014 at 07:56

Quote: Originally posted by Steam  
I know you got it to work but just so you know when doing chemistry with Al foil to use the cheapest thinnest foil you can find. The thick heavy duty stuff often contains a thin layer of coating to make it more "non-stick". It also makes it difficult to use as a reducing agent.


Heavy duty generic brands are seldom non-stick. That non-stick coating adds
cost and generic brands simply don't add it. At least in my experience anything
with a non-stick coating will be clearly labelled as non-stick. Of course your
mileage may vary.

Steam - 27-3-2014 at 08:13

Quote: Originally posted by macckone  
Quote: Originally posted by Steam  
I know you got it to work but just so you know when doing chemistry with Al foil to use the cheapest thinnest foil you can find. The thick heavy duty stuff often contains a thin layer of coating to make it more "non-stick". It also makes it difficult to use as a reducing agent.


Heavy duty generic brands are seldom non-stick. That non-stick coating adds
cost and generic brands simply don't add it. At least in my experience anything
with a non-stick coating will be clearly labelled as non-stick. Of course your
mileage may vary.


sorry about that, I wasn't clear. The more expensive "Heavy-duty" foils don't react as well has the generic foil does. I am sure there are generic heavy duty foils that are just fine, but just as a good rule of thumb buy the most simple, plain, generic foil you can find.

In my experience "heavy duty" reynolds has been notorious for giving inconsistent results.

[Edited on 27-3-2014 by Steam]