[lightarrow (OP)]

Why an acqueous solution of CuSO4 doesn't react with an aluminum foil, but, if we add table salt (NaCl), the reaction starts immediately?

Cu++ + Al +3H2O --> Cu + Al(OH)3 + 3H+

[eric l]

I'm not sure you got your reaction right.  Have you checked the pH ?  You could as well have a reaction giving Al2(SO4)3 !  
Anyway, the contact surface of your aluminium foil with water would be Al2O3 rather than Al.  It is possible that the increased concentration of ions by the addition of table salt makes it possible for the Al2O3 to dissolve either as Al(OH)3 or AlCl3, which would leave a "clean" Al surface for the coppersulphate to react upon.  I've got to check.

[lightarrow (OP)]

quote:Originally posted by eric l

I'm not sure you got your reaction right.  Have you checked the pH ?  You could as well have a reaction giving Al2(SO4)3 !  
Anyway, the contact surface of your aluminium foil with water would be Al2O3 rather than Al.  It is possible that the increased concentration of ions by the addition of table salt makes it possible for the Al2O3 to dissolve either as Al(OH)3 or AlCl3, which would leave a "clean" Al surface for the coppersulphate to react upon.  I've got to check.

I think you are quite right. Probably aluminum forms different kinds of compounds and complexes, together with Al(OH)3: Al2O3, AlOH++, Al(OH)2+, Al(OH)SO4...Maybe, with Cl- ions it forms more soluble complexes?

For what concerns adding ions to the solution, I think it's not this, because I used a much higher amount of CuSO4 than NaCl.

[Mr Andrew]

You probably ran a bit of electrochemistry.  The salt water probably allowed electrons to be transfered between the aluminum foil and the CuSO4. You're also probably at least partially right with the Cl- ions combining with the Al2O3 to give a clean Al surface for the redox reaction to occur on. Try hooking up a ampmeter to it and measure the current.

"His mind is the ultimate weapon!"-MacGyver television series

[daveshorts]

I doubt that it is large scale electrochemistry as he has only put one electrode in the solution. I would have thought it was to do with the Cl-'s breaking down aluminium's Al2O3 protective barrier somehow.

[Mr Andrew]

quote:Originally posted by Mr Andrew

You're also probably at least partially right with the Cl- ions combining with the Al2O3 to give a clean Al surface for the redox reaction to occur on.

I never said that the Cl- ions weren't eroding the Al2O3 barrier (I actually said that is exactly what they did), I focused on what happens after the corundum is gone.

eric 1 is probably right in that aluminum sulphate was formed. You just have to look at the activity series to see that a single replacement reaction would occur. No aluminum chloride could form because it is soluble and when sodium ions are present, evaporating the water to leave a precipitate would only yield common table salt. Aluminum oxide wouldn't form because the only way to get oxide ions would be to strip them from the water or the sulphate and to do that requires more energy than it does to make the aluminum sulphate.

"His mind is the ultimate weapon!"-MacGyver television series